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Full text of "Characterization of low thermal conductivity PAN-based carbon fibers"

lAEROSPACE REPORT NO. 
|aTR-91(6819)-1 



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Characterization of Low Thermal Conductivity 
PAN-Based Carbon Fibers 



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Prepared by 

H. A. KATZMAN, P. M. ADAMS, T D. LE, and C. S. HEMMINGER 

Mechanics and Materials Technology Center 

Technology Operations 



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15 March 1992 



Prepared for 

National Aeronautics and Space Administration 

Marshall Space Flight Center 

Huntsville, AL 35812 



Engineering and Technology Group 



THE AEROSPACE CORPORATION 



El Segundo, California V^ 




PUBLIC RELEASE IS AUTHORIZED 



TECHNOLOGY OPERATIONS 



The Aerospace Corporation functions as an "architect-engineer" for national security programs, 
specializing in advanced military space systems. The Corporation's Technology Operations supports the 
effective and timely development and operation of national security systems through scientific research 
and the application of advanced technology. Vital to the success of the Corporation is the technical staffs 
wide-ranging expertise and its ability to stay abreast of new technological developments and program 
support issues associated with rapidly evolving space systems. Contributing capabilities are provided by 
these individual Technology Centers: 

Electronics Technology Center: Microelectronics, solid-state device physics, VLSI 
reliability, compound semiconductors, radiation hardening, data storage technoic^es, 
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development, optical resonators, beam control, atmospheric propagation, and laser 
effects and countermeasures; atomic frequency standards, applied laser spectroscopy, 
laser chemistry, laser optoelectronics, phase conjugation and coherent imaging, solar 
cell physics, battery electrochemistry, battery testing and evaluation. 

Mechanics and Materials Technology Center: Evaluation and characterization of new 
materials: metals, alloys, ceramics, polymers and their composites, and new forms of 
carbon; development and analysis of thin films and deposition techniques; 
nondestructive evaluation, component failure analysis and reliability; fracture 
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Aerospace Report No. 
ATR-91(6819)-1 



CHARACTERIZATION OF LOW THERMAL CONDUCTIVITY 
PAN-BASED CARBON FIBERS 



Prepared by 

H. A. Katzman, R M. Adams, T D. Le, and C. S. Hemminger 

Mechanics and Materials Technology Center 

Technology Operations 



15 March 1992 



Engineering and TechnoloCT Group 

THE AEROSPACE CORPORATION 

El Segundo, CA 90245-4691 



Prepared for 

National Aeronautics and Space Administration 

Marshall Space Flight Center 

Huntsville, AL 35812 



PUBLIC RELEASE IS AUTHORIZED 



Aerospace Report No. 
ATR-91(6819)-1 



CHARACTERIZATION OF LOW THERMAL CONDUCTIVITY 
PAN-BASED CARBON FIBERS 



Prepared 




T D. Le 

Structural Materials Department 



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P. M. Adams 

Materials Evaluation and Survivability 
Department 



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C. S. Hemmmger 
Materials Evaluation and Survivability 
Department 



Approved 




W. H. Kao, Director 

Structural Materials Department 

Mechanics and Materials Technology Center 



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R. W. Fillers, Principal Director 
Mechanics and Materials Technology Center 
Technology Operations 



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D. G. Sutton, Director 

Materials Evaluation and Survivability 

Department 
Mechanics and Materials Technology Center 



PRECED\rW PAGE BLAfm NOT FiLWEL 



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ABSTRACT 



The microstructure and surface chemistry of eight low thermal conductivity (LTC) 
PAN-based carbon fibers were determined and compared with PAN-based fibers heat 
treated to higher temperatures. 

Based on wide-angle X-ray diffraction, the LTC PAN fibers all appear to have a simi- 
lar turbostratic structure with large 002 d-spacings, small crystallite sizes, and moder- 
ate preferred orientation. All the LTC fibers are slightly less ordered than T300 
fibers, which are much less ordered than T50 fibers. This observation is consistent 
with the relative heat-treatment temperatures (HTTs) of the fibers. 

Limited small-angle X-ray scattering (SAXS) results indicate that, with the exception 
of LTC fibers made by BASF, the LTC fibers do not have well developed pores. This 
suggests that they have not been subjected to a high enough HTT to drive off all of 
their volatile constituents (mainly nitrogen). The BASF fibers produce SAXS patterns 
intermediate between T300 and the other LTC fibers. 

Transmission electron microscopy shows that the texture of the two LTC PAN-based 
fibers studied (Amoco T350/23X and /25X) consists of multiple sets of parallel, wavy, 
bent layers that interweave with each other forming a complex three-dimensional net- 
work oriented randomly around the fiber axis. In the cases of higher HTT fibers 
(T300 and T50), the texture consists of two distinct regions with the core's texture 
being similar to that of the LTC fibers, whereas the outer region is better ordered and 
oriented. No differences between the textures of the outer and inner regions of the 
LTC fibers were observed. Crystallite size and the extent of graphitization, the latter 
based on lattice d-spacing, were both observed to increase with increased HTT. These 
results are consistent with those from X-ray diffraction. 

X-ray photoelectron spectroscopy (XPS) analysis finds correlations between the HTT 
and the surface composition and chemistry of the carbon fiber samples. The concen- 
tration of surface oxygen and nitrogen for PAN-based fibers decreases with increasing 
HTT, which is consistent with increased volatilization of nitrogen and greater extent of 
graphitization. The XPS data did not have adequate resolution to distinguish differ- 
ences in the extent of graphitization of the LTC fibers. Comparative analyses were 
also hindered by significant variability in surface contamination levels, particularly of 
silica, and by variability in surface composition between different samples taken from 
the same batch of fibers. 



PRECEDING PAGE BLANK NOT FILMEQ 



ACKNOWLEDGMENTS 

The authors would hke to thank NASA Marshall Space Flight Center (Ms. Ann 
Puckett and Dr. R. G. Clinton) and USAF Ballistic Missile Office (Lt. R. Einhorn) for 
funding this program. Ms. Gloria To performed the pH and fiber sizing level mea- 
surements. Mr. Joe Uht was responsible for the scanning electron microscopy. 
Drs. W. T. Barry and J. J. Mallon provided many helpful suggestions. Dr. G. S. 
Rellick reviewed the manuscript, and Ms. J. Naiditch and Mr. J. Shaffer edited the 
manuscript. 

Funding for this effort was processed through SSD contract No. F04701-88-C-0089 
under an Interagency Agreement from NASA. 



PP^CED'^!^i PAGE BLAfVK NOT FILMED 

vii ' 

JfOINTIOfiAl^y 91AH 



CONTENTS 



ABSTRACT '^ 

ACKNOWLEDGMENTS ^^ 

I. INTRODUCTION ^ 

II. X-RAY DIFFRACTION ^ 

A. Wide- Angle X-Ray Diffraction ^ 

1. Background 

2. Experimental 

3. Results ^^ 

IS 

B. Preferred Orientation 

1 S 

1. Background 

2. Experimental 

3. Results ^^ 

C. Small-Angle X-Ray Scattering 19 

in 
1 Background 

22 

2. Experimental 

3. Results ^^ 

a. Qualitative Observations ^^ 

O 1 

b. Ouantiative Measurements -'^ 

D. Summary ^^ 

III. TRANSMISSION ELECTRON MICROSCOPY 37 

37 

A. Background 

37 

B. Experimental 

C. Results and Discussion 

L Macroscopic Texture 

a. Longitudinal 

b. Transverse 



IX 



PR^CE0^^K5 PAGP BLANK NOT FILMED 



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CONTENTS (Continued) 

2. Microscopic Texture 50 

a. Selected Area Diffraction 50 

b. Dark Field Study 55 

c. High-Resolution (HR) Lattice Fringe Study 61 

d. PlOO Fibers 70 

D. Conclusions 70 

IV. FIBER SURFACE CHEMISTRY 75 

A. X-ray Photoelectron Spectroscopy (XPS) 75 

1. Background and Experimental 75 

2. Results and Discussion 76 

3. Summary 80 

B. Acid/Base Character 81 

V. FIBER SIZING LEVELS 83 

A. Background 83 

B. Experimental 83 

C. Results 83 

VI. CONCLUSIONS 85 

REFERENCES 87 

APPENDIX A - WIDE-ANGLE X-RAY DIFFRACTION SCANS 91 

APPENDIX B - PREFERRED-ORIENTATION SCANS 107 

APPENDIX C - SMALL- ANGLE X-RAY SCATTERING PLOTS 113 



FIGURES 



1. Effect of heat- treatment temperature on the thermal conductivity 

and tensile strength of PAN-based carbon fibers ^ 

2. Structure of three-dimensional and two-dimensional graphitic carbon 6 

3 Theta two-theta X-ray diffraction scans of ATJS graphite, 

DuPont E130 fibers, and Amoco T50 and T300 fibers » 

4 Flat-plate photographs (Mo radiation, X = 5 cm) of (a) ATJS 
graphite, (b) DuPont E130 fibers, (c) Amoco T50 fibers, 

(d) Amoco T300 fibers ^ 

5. Schematics of various types of X-ray diffractometers H 

6. Theta two-theta X-ray diffraction scans of DuPont E130 fibers 12 

7. Low-angle and 002 reflection X-ray diffraction scans of low 

thermal conductivity PAN fibers 

8. 10 and 1 1 reflection X-ray diffraction scans of low thermal 

conductivity PAN fibers 

9. (a) Effect of heat treatment temperature on the preferred 
orientation of PAN- and pitch-based carbon fibers. 

(b) Effect of pitch-based carbon fiber preferred orientation on modulus ... ib 

10. Relationship between preferred orientation and tensile modulus 

for ETC and standard PAN fibers ^^ 

11. Schematic of pore system in carbon fibers 20 

12. Qualitative small-angle X-ray scattering photographs of (A) Amoco 

T50, (B) Amoco T300, and (C) CCA3 Rayon fibers ^^ 

13. Qualitative small-angle X-ray scattering photographs of 
(A) Amoco T350/23X, (B) Amoco T350/25X, (C) Hercules 

R879-01, and (D) Hercules R879-02 fibers ^^ 

14. Qualitative small-angle X-ray scattering photographs of 
(A) Textron Avcarb B-2, (B) Textron Avcarb G, (C) BASF 

DG Rayon 1, and (D) BASF DG Rayon 2 fibers ^f> 

15 Scanning electron microscope photographs of the surfaces of 

Amoco T50 and T300 fibers ^« 

16. Scanning electron microscope photographs of surfaces of 

Amoco T350/23X and Textron Avcarb B-2 fibers ^^ 



XI 



FIGURES (Continued) 



17. Scanning electron microscope photographs of surfaces of 

Hercules R879-02 and BASF DG Rayon 1 fibers 30 

18. Characteristic curves determined for Kodak direct exposure film 32 

19. Gray scale/optical density calibration curves measured 
from photographic step tablet with different CCD 

shutter speeds 33 

20. Equatorial lines traces of SAXS of carbon fibers obtained 

by digitizing SAXS photographs 33 

21. SAXS intensity contour maps for (a) Amoco T50 and (b) T300 fibers 34 

22. SAXS intensity contour maps for (a) BASF DG Rayon 1 and 

(b) Amoco T350/23X fibers 34 

23. The proposed ribbon microtexture model 38 

24. A proposed model of fiber microtexture 39 

25. BF image showing the macrotexture of Amoco T350/23X 

fiber (longitudinal section) 42 

26. BF image showing the macrotexture of Amoco T350/25X 

fiber (longitudinal section) 43 

27. BF image showing the macrotexture of Amoco T300 

fiber (longitudinal section) 44 

28. Sketch showing the macrotexture 45 

29. BF image of Amoco T350/23X fiber (transverse section) 46 

30. BF image of Amoco T350/25X fiber (transverse section) 47 

31. BF image of Amoco T300 fiber (transverse section) 48 

32. BF image of Amoco T50 fiber (transverse section) 49 

33. Transverse section of Amoco T50 fiber showing two-phase structure 50 

34. Sketch showing that the observed SAD pattern is 
composed of two different patterns resulting from 

"face-on" and "edge-on" grains 5I 



xn 



FIGURES (Continued) 

35. Sketch showing reflections of planes misoriented by an angle of ± a 52 

36. Diffraction patterns resulting from tilting the basal planes 54 

37. Diffraction patterns resulting from tilting the perpendicular basal planes 55 

38. 002 DF image of Amoco T350/23X fiber (transverse section) 56 

39. 002 DF image of Amoco T350/25X fiber (transverse section) 57 

40. 002 DF image of Amoco T300 fiber (transverse section) 58 

41. 002 DF image of Amoco T50 fiber (transverse section; no tilt angle) 59 

42. 002 DF images of Amoco T50 fiber (transverse section; 

different tilt angles) "^ 

43. Lattice-fringe image of Amoco T350/23X fiber (transverse section) 62 

44. Lattice-fringe image of Amoco T350/25X fiber (transverse/outer region) ... 63 

45. Lattice-fringe image of Amoco T350/25X fiber (transverse/core) 63 

46. Lattice-fringe image of Amoco T300 fiber (transverse/outer region) 64 

47. Lattice-fringe image of Amoco T300 fiber (transverse/core) 64 

48. Lattice-fringe image of Amoco T50 fiber (transverse) 65 

49. Lattice-fringe image of Amoco T350/23X fiber (longitudinal) 66 

50. Lattice-fringe image of Amoco T350/25X fiber (longitudinal) 67 

51. Lattice-fringe image of Amoco T300 fiber (longitudinal) 68 

52. Lattice-fringe image of Amoco T50 fiber (longitudinal) 69 

53. BF image and SAD pattern of Amoco PlOO fiber 71 

54. DF image of Amoco PlOO fiber 72 

55. XPS Cls spectra ^^ 



xin 



TABLES 

1. Fiber Suppliers and Product Codes for Low Thermal Conductivity 
PAN-based Fibers 2 

2. Tensile Properties of Fibers 3 

3. Summary of Wide-Angle X-Ray Diffraction Data 14 

4. Fiber Orientation Parameters 17 

5. Summary of Structural Information Derived from 

Small-Angle X-ray Scattering (SAXS) 35 

6. XPS Surface Composition Data 77 

7. XPS Curve-Fit Data for Cls Photoelectron Peaks 80 

8. Acid/Base Character of Unsized Fiber Surfaces 81 

9. Fiber Sizing Levels 83 



XIV 



I. INTRODUCTION 



Low thermal conductivity (LTC) polyacrylonitrile (PAN)-based carbon fibers are being 
developed as a substitute for rayon-based carbon fibers currently used in solid rocket 
motor (SRM) nozzles and exit cones. This development is being driven by the poten- 
tial lack of availability of domestic rayon precursor fiber as a result of environmental 
restrictions on the manufacturing process and the declining commercial market for 
rayon. PAN-based carbon fibers are desirable because they are produced by a num- 
ber of domestic suppliers by a relatively clean process, they can be manufactured with 
reproducible mechanical properties, and they are used extensively for other applica- 
tions so that a relatively large property data base exists. Up until now, however, PAN- 
based carbon fibers have been optimized for structural applications where their 
mechanical properties are more important than their thermal properties. SRM abla- 
tive applications, on the other hand, require low fiber thermal conductivity to mini- 
mize composite char depth and backface temperature rise. There is a need, therefore, 
to produce PAN-based carbon fibers with thermal conductivity less than that of stan- 
dard PAN-based carbon fibers (-13 W/mK), which is considerably higher than that of 
rayon-based carbon fibers (-4 W/mK). 

During the late 1970s, the Defense Nuclear Agency funded a joint program at The 
Aerospace Corporation and BASF Structural Materials, Inc. (Celanese). This pro- 
gram demonstrated the feasibility of producing low thermal conductivity fibers by car- 
bonizing PAN precursor fibers at somewhat lower temperatures (900-1 100 °C) than 
those used for standard processing (1250-1500° C). Figure 1 presents the unpublished 
results of that program. The thermal conductivity and tensile strength of the fibers 
produced in that program are plotted as a function of heat-treatment temperature 
(HTT). The lower thermal conductivity of the fibers heat treated to lower tempera- 
tures is due primarily to their retention of a large amount of nitrogen. At higher tem- 
peratures, most of this nitrogen is volatilized, leaving a purer carbon fiber with higher 
thermal conductivity. 

Eight LTC PAN fibers are currently being evaluated by NASA. These fibers were 
manufactured from commercial precursor PAN fibers by Amoco Performance Prod- 
ucts, Inc., Hercules Graphite Fibers, Textron Specialty Materials, and BASF Structural 
Materials, Inc. (two fibers each). A list of these fibers according to product code des- 
ignated by the manufacturers is shown in Table 1. Their properties have been mea- 
sured at Lockheed Research and Development Division, Palo Alto, California, and 
the results are reported in Reference 1. Variations in the mechanical and thermal 



14 



I 10 



O Thermal Conductivity 
• Tensile Strength 




-1 700 



- 600 



Rayon Thermal 
Conductivity 



Rayon Tensile Strength 



500 



400 



300 



200 



So 
w 

c 



- 100 



1000 



1300 



1400 



1100 1200 

Heat Treatment Temperature ('C) 

Figure 1. Effect of heat-treatment temperature on the thermal 
conductivity and tensile strength of PAN-based carbon fibers. 



Table 1. Fiber Suppliers and Product Codes for Low Thermal 
Conductivity PAN-Based Fibers 



Fiber Supplier 



Amoco Performance Products, Inc. 
BASF Structural Materials, Inc. (Celanese) 
Hercules Graphite Fibers 
Textron Specialty Materials 



Product Code 



T350/23X 
T350/25X 

DG Rayon 1 
DG Rayon 2 

R879-01 (LF1) 
R879-02 (LF2) 

Avcarb G 
Avcarb B-2 



properties of the LTC PAN fibers are due to variations in (1) precursor PAN fibers 
(each manufacturer starts with a shghtly different precursor), (2) oxidation stabihza- 
tion parameters (time, temperature, environment, stress state), and (3) heat treatment 
schedule (final temperature, time at temperature, heating, and cooling rates). The 
tensile mechanical properties^ are shown in Table 2. Included for comparison are 
properties of fibers that are heat treated at higher temperatures. 

Table 2. Tensile Properties of Fibers 



Fiber 


Tensile Strength^ (psi) 


Tensile Modulus^ (Msi) 


Strain-to-Failurea (%) 


R879-01 


339,000 


21.0 


1.53 


R879-02 


41 1 ,000 


24.9 


1.57 


Avcarb B-2 


317,000 


21.1 


1.48 


Avcarb G 


190,000 


10.5 


1.77 


T350/25X 


320,000 


24.2 


1.28 


T350/23X 


348,000 


22.8 


1.44 


DG Rayon 1 


462,000 


25.3 


1.54 


DG Rayon 2 


454,000 


23.8 


1.61 


Reference Data 








Shuttle-Grade Rayon 


50-100,000 


8-9 


0.6-1.1 


Fabric 








Amoco T300 PAN 


530,000 


33.5 


1.58 


Hercules AS4 PAN 


590,000 


36.0 


1.65 


Amoco T50 PAN 


350,000 


57 


0.61 


Amoco PI 00 Pitch 


325,000 


105 


0.30 


DuPont E130Pitch 


350,000 


130 


0.27 


^ASTM- D4018. 









The Aerospace Corporation task focused on the determination of the microstructures 
of the LTC PAN fibers utiHzing X-ray diffraction (XRD) and transmission electron mi- 
croscopy (TEM) techniques. The LTC PAN fibers were compared with the higher 
fired PAN fibers, T300 and T50, and with high-modulus, pitch-based fibers, PlOO and 
E130. In addition, the surface chemistry of each LTC PAN fiber was investigated with 
X-ray photoelectron spectroscopy (XPS). The goal of these efforts was to determine 
selected microstructural parameters in order to help understand the basis of the ther- 
mal and mechanical properties of the LTC PAN fibers. As a separate task, an evalua- 
tion of the sizing levels on the LTC PAN fibers was undertaken to help resolve a dis- 
crepancy in this measurement. The results of each of these facets of the study are re- 
ported in the following sections. 



II. X-RAY DIFFRACTION 



A. WIDE-ANGLE X-RAY DIFFRACTION 

1. Background 

X-ray diffraction (XRD) has been widely used in the structural characterization of 
carbons and graphites, and wide-angle XRD (WAXD) has been used in the study of 
carbon fibers.^-^ Reviews of the structure of polyacrylonitrile (PAN)-based carbon 
fibers are given in References 7 and 8, and the correlation of the microstructure of the 
fibers with mechanical properties is discussed in References 9-11. 

Diffraction effects arise when X rays interact with crystalline matter because the X-ray 
wavelengths are of the same order as the interatomic distances between the repeat- 
able lattice planes in crystals. In the simplified model of XRD, diffraction occurs 
when the conditions of Bragg's Law are satisfied 

r\X = 2dsin(6>) (1) 

where n is an integer, \ is the wavelength of the X rays, d is the interatomic spacing of 
the crystal lattice planes, and is the angle between the incident X-ray beam and the 
lattice planes. The crystal structure of hexagonal graphite is portrayed in Figures 2(a) 
and (b). In this crystal, a high degree of three-dimensional order exists, and XRD 
reflections are produced by the various lattice planes. The most prominent reflections 
(001) are associated with the basal layer planes, which are perpendicular to the c-axis 
and have an interatomic spacing of 3.35 A. The structure of lower heat-treatment 
temperature (HTT) carbons, however, is considerably different since only two- 
dimensional (turbostratic) order is present [Figure 2(c)]. In this structure, there is no 
registry between atoms in successive layers, the distance between layers is greater than 
3.4 A, considerable disorder exists within the layers, and the size of domains with 
similar structure (crystallites) is small ( < 100 A). As a result, the XRD pattern of 
low HTT carbon is much different from that of crystalline (three-dimensional) graph- 
ite. High-temperature heat treatment improves the structure of the disordered car- 
bon, but the extent to which the structure approaches that of three-dimensional graph- 
ite strongly depends on the starting material. 




(a) 




(b) 




(c) 



Figure 2. Structure of three-dimensional and two-dimensional 
graphitic carbon, (a) Graphite crystal lattice, (b) graphite crys- 
tal unit cell, (c) turbostratic graphitic carbon. 



The effect of these structural changes on the XRD pattern is illustrated in Figures 3 
and 4. These figures compare the XRD patterns of well-crystallized bulk graphite 
(ATJS), a high-modulus, highly oriented, pitch-based fiber (E130 from DuPont), and 
PAN-based fibers (Amoco T50 and T300). The XRD patterns in Figures 3 and 4 were 
obtained with a powder diffractometer (Cu radiation) and a flat plate camera (Mo 
radiation), respectively. As order increases, the spacing between the basal layer 
planes decreases and approaches 3.35 A, and the crystallite size increases. These 
changes influence the XRD pattern by shifting the position of the 001 reflections [002 
in Figure 3(b)] to higher angles and by decreasing the peak full-width at half- 
maximum (FWHM), respectively. An estimate of the crystallite size perpendicular to 
the layer planes can be obtained from the Scherrer equation 

Lc = ia/Bcos0 , (2) 

where U is the crystallite size along the c-axis of the crystals, K is a constant (0.89 for 
the 002 reflection of graphite), \ is the X-ray wavelength, B is the FWHM (m 
radians), and 9 is the Bragg angle. The crystallite size along the a-axis (U) of the 
crystals can also be estimated with the Scherrer equation; for that estimate, K m 
Eq. (2) is 1.84. 

The change from turbostratic to three-dimensional order can be monitored by the 
appearance of hOl and hkl reflections, which result from the formation and alignment 
of lattice planes. This effect can be seen in Figures 3(c) and (d) for the 101 and 112 
reflections, respectively. In turbostratic carbons, only a broad 10 reflection is 
observed [Figure 3(c)] as a result of the interatomic spacings (-2.1 A) within indi- 
vidual layer planes. With the appearance of three-dimensional order, this single 
broad peak is replaced by a pair of reflections (100-101), as can be seen for highly 
graphitic ATJS. The development of three-dimensional order and its effect on the 
XRD pattern is also shown in Figure 3(d) for the 110-112 reflections. In turbostratic 
material, only a broad 11 reflection is observed as a result of an approximately 1.2 A 
interatomic spacing within the layer planes, while three-dimensional graphite exhibits 
a splitting of this peak into the 110 and 112 reflections. These effects are seen quali- 
tatively in Figure 4. 

2. Experimental 

Wide-angle 9-26 XRD scans were performed using copper ka radiation on a com- 
puter-controlled vertical powder diffractometer supplied by Philips Electronics Instru- 
ments. The diffractometer was equipped with a 6-compensating slit, a diffracted-beam 
graphite crystal monochromator, and a scintillation detector. In an attempt to ran- 
domize the fibers, each type of low thermal conductivity (LTC) fiber and samples of 






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ORIGINAL PAGE 
BLACK AND WHITE PHOTOGRAPH 




Figure 4. Flat-plate photographs (Mo radiation. X = 5 cm) ot 
(a) ATJS graphite, (b) DuPont E130 fibers, (c) Amoco T5() li- 
bers, (d) Amoco '1300 libers. 



ORIGINAL PAGE IS 
OF POOR QUALITY 



well-characterized higher HTT PAN-based fibers (Amoco T50 and T300) were ground 
in a mortar with a pestle until they passed through a 200-mesh sieve. The powder 
samples were pressed into a 100 iim deep cavity in a standard Philips sample holder 
that was backed with a zero background plate. This plate consists of a single crystal 
of quartz, cut 6° from the c-axis, which produces no XRD reflections and very little 
background scattering. Survey scans of the powder samples were run, varying 26 
between 2° and 100° in 0.02° steps. In order to obtain more accurate measurements 
of the d-spacings of the graphite 002 reflections, a small amount of an internal stan- 
dard was mixed with a portion of the powders, and 9-29 scans were repeated between 
15° and 35°. National Institute of Standards and Technology (NIST) standard refer- 
ence material No. 674 zinc oxide (ZnO) was used as the internal standard, and the 
position of the ZnO 100 reflection was used to calibrate the 9-26 scans. The XRD 
reflections of turbostratic carbons are often very broad and may be asymmetric, 
thereby making it difficult to define or measure peak locations. For simplicity, the 
location of the 002 reflection was defined as the bisector of the full width at three- 
fourth intensity (peak minus background). 

Typical XRD patterns of ground carbon fibers display a high degree of preferred 
orientation because of the orientation of graphite layer planes along the fiber axes 
and the extreme difficulty of randomizing the ground fiber fragments during the 
mounting of samples. This can be seen qualitatively in the flat plate photographs in 
Figure 4. The randomly oriented ATJS specimen produces an XRD pattern that con- 
sists of complete concentric rings whereas the patterns of the carbon fibers exhibit 
arcs with varying lengths with the shorter arc indicating greater degree of preferred 
orientation. As a result, in 9-29 scans, which represent an equatorial trace in the pho- 
tographs in Figure 4, the 001 reflections are usually very strong; on the other hand, 
hOl and hkl reflections, which give an indication of the in-plane and three-dimensional 
order/crystallite size, respectively, are very weak. It is possible to take advantage of 
the preferred carbon fiber orientation, however, to study the hOl and hkl reflections by 
analyzing fiber tows directly in a symmetric transmission geometry. This method max- 
imizes the signal from the hkO and hkl reflections, removes interferences from 001 
reflections, and represents the equivalent of a meridional trace in Figure 4. This 
experimental arrangement is compared with the usual reflection geometry in Figure 5. 
To record the hkl reflections, the fibers are mounted with their axes rotated 20° [3 = 
20° (refer to Figures 4 and 5c)]. ^ The selection of this orientation is based on the 
angle hOl/hkl planes (such as 101 and 112) make with the basal 001 planes, on the gen- 
eral alignment of the 001 planes parallel with the fiber axis, and on the fact that only 
lattice planes that satisfy the symmetric geometry will be recorded in the 9-26 scans. 
Samples were analyzed in this geometry with a simple fixture consisting of a micro- 
scope slide with a narrow (2 mm) slot cut in it. This slot served as an aperture that 
defined a narrow beam of X rays along the diffractometer axis. Fiber bundles were 



10 




, FOCUS 




PRIMARY BEAM 



(c) 



Figure 5. Schematics of various types of X-ray diffractome- 
ters. (a) reflection, (b) symmetric transmission, (c) transmission 
(perspective). 



11 



positioned and aligned manually and secured with a collodion adhesive solution. In 
this geometry, with a narrow X-ray beam and thin (-100 fim] poorly graphitic sam- 
ples, it was necessary to run 6-26 scans for 12 hr to obtain a reasonable signal-to- 
noise ratio for the weak hOl/hkl reflections. An identical scan was recorded from the 
empty sample fixture and subtracted from the scans of the carbon fibers to remove the 
effects of amorphous scattering from the glass microscope slide. 

In Figure 6, the effect of the sample geometry on the 6-26 XRD scans is illustrated 
for highly graphitic mesophase pitch-based EDO fibers. In the scan of ground E130 
powder taken in the reflection geometry, most reflections are seen. However, the 001 
reflections are extremely strong because of the highly graphitic and oriented nature of 
the fibers and, therefore, tend to obscure some of the hOl/hkl reflections. In the 
transmission geometry, with the fiber axes at 3 = 0°, the 001 reflections are ehmi- 
nated, but hkl reflections are missing because the correct alignment conditions are not 
satisfied. When the fiber axes are positioned at 3 = 20°, the 001 reflections are also 
not seen, but the 101 and 112 reflections are readily observable. 




c 
u 



C 
<1> 



c 
O 



2.5 





r~ 


2 


- 


1.5 


_ 


1 


_ 


0.5 


_ 








40 60 

Two Theta (degrees) 



(100) 



J. 



20 



40 60 

Two Theta (degrees) 



1000 
800 U 
600 
400 
200 




(100) 



(101) 



''^"***'*'***>l»**«* 



»»l< H *'»»»<l 




(a) 



J 

100 



(110) 



(b) 



ILi. 



80 



100 



(110) 



(112) 



(C) 



20 



60 80 




40 

Two Theta (degrees) 
Figure 6. Theta two-theta X-ray diffraction scans of DuPont 
E130 fibers, (a) reflection/powder, (b) transmission/fiber (3 = 
0°), (c) transmission/fiber (3 = 20°). 



100 



12 



3. Results 

The complete 0-29 XRD scans from all of the samples are included in Appendix A. 
Comparative plots of the 002, 10, and 11 reflections, along with low-angle scattering 
curves from the LTC fibers, are presented in Figures 7 and 8. Measurements from 
these scans are summarized in Table 3. All of the fibers examined are turbostratic, 
and the XRD patterns give no indication of three-dimensional order (they display no 
hOl or hkl reflections). The XRD patterns are similar in appearance, and only subtle 
differences exist between the various LTC fibers. The d-spacings (d) of the 002 reflec- 
tions for the LTC fibers ranged from 3.48 to 3.53 A, with the Hercules and Textron 
fibers exhibiting the smallest and largest values, respectively. For comparison, 
d-spacings of 3.51 and 3.43 A were recorded from the Amoco T300 and T50 fibers. 
The L^ values calculated from the FWHMs of the 002 reflections are all very similar 
and very small (12.0-13.9 A), with the two Textron fibers displaying the smallest crys- 
tallite sizes. These values, calculated using the Scherrer equation, are not very mean- 
ingful because the unit cell in the c-axis direction for single-crystal graphite 



ff> 



2.5 ^ 




0.5 



B2 

J L 



4 6 8 10 12 

TW THETA (d»9r««t) 



14 



IS 



2.5 ^ 



002 




15 



20 25 30 

1W0 THETA (dognet) 



2.5 



o 2 



1.5 _ 



0.5 _ 



T350/23X 




6 8 10 12 

TWO THETA (degrees) 



2.5 



? " 



o 



1.5 _ 



1 _ 



0.5 



002 




_L 



_L 



I 



15 20 25 30 

TWO THETA (degrees) 



Figure 7. Low-angle and 002 reflection X-ray diffraction scans of low 
thermal conductivity PAN fibers. Intensities have been scaled so that 
the heights of the 002 reflections are all equal. 



35 



13 



10 



0.4 



0.2 



S 0.1 _ 






8 0.3 |_"'->'VA-' J*- 

DGl , DG2 



G 
B2 



0.39 

0.3 

^ 0.25 

I 0.2 



^ 0.1S 




33 



40 



49 

TWOTHETA (« 



90 



99 



0.1 _ 
0.05 




11 



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DG1,DG2 



"^^" 



70 79 80 89 

1W0 THETA (dagrMt) 



BO 




0.35 



0.3 



40 



45 SO 

TWO THETA (dagrM*) 



55 



J 0.25 

I 

i 0.2 
u 

* 0.15 



0.1 

0.05 





M^ai*'''*^^^ 



23,25,Lri,IJ-2 



70 



75 



_L 



80 



85 



TWO THETA (degra«a) 



Figure 8. 10 and 11 reflection X-ray diffraction scans of low 
thermal conductivity PAN fibers. Intensities have been scaled 
so that the heights of the 002 reflections are all equal. 

Table 3. Summary of Wide-Angle X-ray Diffraction Data 



90 



Fiber 


d 002 (A) (±0.01) 
3.50 


FWHM 002 (° 26) 


Lc{A) 


Amoco T350/23X 


6.08 


13.3 


Amoco T350/25X 


3.50 


6.07 


13.3 


BASF DG Rayon 1 


3.53 


6.15 


13.1 


BASF DG Rayon 2 


3.52 


6.24 


12.9 


Hercules R879-01 


3.49 


5.79 


13.9 


Hercules R879-02 


3.48 


5.92 


13.6 


Textron Avcarb G 


3.53 


7.10 


11.3 


Textron Avcarb B-2 


3.53 


6.73 


12.0 


CCA3 Rayon 


3.83 


8.60 


9.3 


Amoco T300 


3.51 


4.93 


16.3 


Amoco T50 


3.43 


1.65 


48.9 



14 



(Co) is 6.7 A. Since the fibers are turbostratic, these values represent the thicknesses 
of domains that have their basal planes aligned, rather than actual crystallite sizes. 

Based on the wide-angle 9-26 XRD analyses, the majority of the LTC fibers have sim- 
ilar turbostratic structures, with the exception of the Textron fibers, which consistently 
have the largest 002 d-spacings and smallest crystallite sizes, indicating that they are 
the least graphitic and most disordered of the fibers examined. 

B. PREFERRED ORIENTATION 

1. Background 

The crystal lattice of graphite is highly anisotropic (Figure 2) and consists of sheets of 
strong, covalently bonded carbon atoms held together by weak van der Waals forces. 
As a result, the orientation of basal planes in carbon fibers is expected to directly 
affect the fiber mechanical and physical properties, such as tensile modulus and ther- 
mal conductivity. Therefore, the greater the fraction of basal planes aligned parallel 
to the fiber axis, the higher the fiber modulus and thermal conductivity. To deter- 
mine the degree of orientation using X-ray diffraction, the angle (3) between the fiber 
axis and the X-ray scattering vector is varied, and the 002 reflection intensity is mea- 
sured as a function of p. The orientation parameter, Z, is defined as the FWHM of 
the intensity profile as a function of sample rotation angle (3). Approximately two- 
thirds of the basal planes are oriented within this angle. Greater orientation (higher 
order) of the basal planes results in a decrease in the FWHM. Figure 9 shows 
textbook examples of how the FWHM varies as a function of HTT for PAN and meso- 
phase pitch fibers and how the modulus of mesophase pitch fibers varies as a function 
of FWHM. 12 

2. Experimental 

The degree of preferred orientation of the 001 planes in the carbon fibers can be seen 
qualitatively in flat-plate photographs (Figure 4). However, quantitative measure- 
ments were obtained using a computer-controlled sample spinning attachment in the 
symmetric transmission geometry. The intensity of the 002 reflection was measured 
as a function of azimuth from 3 = ±90° [refer to Figure 5(c)] at 1° intervals. The 
raw data were fit to a Gaussian function, and the FWHM of this function is reported 
as the orientation parameter. With this technique, it was necessary to cover the 



15 



Heat treatment temperature, 

3000 3500 4000 

x 



4500 




(a) 



1400 1600 1800 2000 2200 2400 
Heat treatment temperature, "C 




(b) 



5 10 15 20 25 30 

Preferred orientation, (full width 
at half maximum), degrees 



35 



Figure 9. (a) Effect of heat-treatment temperature on the pre- 
ferred orientation of PAN- and pitch-based carbon fibers, 
(b) Effect of pitch-based carbon fiber preferred orientation on 
modulus (from Ref. 12). 



16 



l-in.-diam opening of the sample spinner with a large number of parallel fiber tows. 
As a result, the precision and minimum detectable orientation parameter angle is a 
function of how parallel and reproducible the fiber tows were manually mounted. 

3. Results 

Table 4 presents our data on the orientation parameter for the eight LTC PAN fibers 
as well as some other fibers for comparison. The azimuthal intensity scans are pro- 
vided in Appendix B. Increasing FWHM corresponds to decreasing preferred orienta- 
tion. The orientation parameter (Z) for most of the LTC fibers is similar (38.5-42.4°) 
and almost within the measurement error of the analysis (±1.5°) [with the exception 
of the Textron fibers, which were much less oriented (Z = 49.3-54.0°)]. One would 
expect the lower degree of preferred orientation in the LTC fibers to influence their 
mechanical properties, which it does. All of the LTC PAN fibers exhibited a lower 
degree of preferred orientation than standard PAN fibers, and this is reflected in the 
lower tensile moduli of the LTC fibers. Figure 10 shows the relationship between 
preferred orientation (from Table 4) and tensile modulus (from Tkble 2) for LTC and 
standard PAN fibers. In addition to lower modulus, the lower degree of preferred 
orientation of the LTC PAN fibers is also partially responsible for their lower thermal 
conductivities. 

Table 4. Fiber Orientation Parameters 



Fiber 


FWHM (°)a 


Hercules R879-01 


41.9 


Hercules R879-02 


38.5 


BASF DG Rayon 1 


42.4 


BASF DG Rayon 2 


39.2 


Amoco T350/25X 


39.5 


Amoco T350/23X 


41.1 


Textron Avcarb B-2 


49.3 


Textron Avcarb G 


54.0 


Amoco T300 


37.0 


BASF Ceiion 


37.0 


Amoco T50 


20.8 


Amoco P-55 


17.1 


Amoco PI 00 


10.4 


DuPont El 30 


10.1 



^Measurement Error = ±1.5°. 



17 



60 



50 



=- 40 

V) 

CO 



"8 

0) 



30 



20 



10 



•T50 


















T300, Celion 

• 








LF2. 25X DG1 

• • 23X Ro 

DG2 , S2 ^ 








LF1 


G 

• 











20 



30 



40 



50 



60 



Preferred Orientation (FWHM) (Deg) 



Figure 10. Relationship Between Preferred Orientation and 
Tensile Modulus for LTC and Standard PAN Fibers. 



18 



C. SMALL-ANGLE X-RAY SCATTERING 

1. Background 

Both wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) 
are produced by the scattering of X rays by electrons in materials. In the case of 
WAXD from a crystalline material, the electrons are confined to a periodic array 
determined by the positions of atoms in the crystal lattice. This produces constructive 
and destructive interference of the scattered X rays, resulting in discrete reflections. 
In the absence of a periodic crystalline array, X rays may still be scattered at small 
angles by differences in electron density resulting from the presence of more than one 
phase or from other differences in mass density. This scattering is more diffuse and is 
sensitive to differences in electron density within a material on the scale of 10 to 
1000 A. As a result, this scattering can detect pores of these dimensions in carbon 
fibers and can provide semiquantitative information on their size, shape, and 
orientation. 

A variety of analytical models have been developed for deriving structural information 
from SAXS data and have been applied to carbon fibers. ^3-22 x^ese models are often 
dependent on the type of instrumentation used (e.g., slit vs pinhole collimation) and 
the form of the samples (e.g., random powder vs fiber bundles). Comparisons of 
results from one model to another can also be difficult because of differing definitions 
for structural parameters. Reference 23 presents an excellent review of SAXS meth- 
ods and attempts to show how the various models are related, to reconcile differences, 
and to systematize nomenclature. 

The method of Debye^^'^^ for deriving the sizes of voids in materials and the internal 
surface area has been applied to carbon fibers^^.i^ and is relatively simple to imple- 
ment. According to the Debye method, if the distribution of voids is random, a corre- 
lation length (a) that is a measure of the grain size of the material is related to the dif- 
fracted intensity (I) by 

(1^^0/1)1/2 = 1 + i6^2^aVA2 (3) 

The correlation length (a) can then be derived from the slope (m)-to-intercept (b) ra- 
tio of a plot of r^'2 vs 9^ by 

m/b = 167r2aVA2 . (4) 



19 



provided the data fall on a straight line. Deviation from a straight line in these plots 
is an indication that the distribution of voids is nonrandom. The specific inner sur- 
face, Sy, of the voids, defined in terms of the overall volume in the system, can be esti- 
mated from the correlation length, a, using the expression 

Sv = 4(1 - C)/a , (5) 

where C is the fraction of the total volume occupied by carbon, which can be approxi- 
mated from the density ratio of the carbon fiber to solid graphite. Other characteris- 
tics of the fibers, such as the mean chord intercept lengths in the crystallites (Ic) and 
pores (Ip), are given by 



Ic = 4C/Sv = a/(l - C) 



and 



Ip = 4(1 - C)/Sv = a/C 



(6) 



These parameters, more easily visualized in Figure 11, have also been referred to as 
transversal or inhomogeneity lengths.^^ They are related to the correlation length by 



1/a = 1/le + 1/lp 



(7) 




Figure 11. Schematic of pore system in carbon fibers. The av- 
erage lengths of the dark areas of the arrow shafts represent l^, 
and the average lengths of the light areas of the arrow shafts 
represent Ip. 



20 



A distance of heterogeneity (Lp) has also been defined^^.u ^^at relates the probability 
that a line of length r will have both its ends situated in pores. Lp is related to a by 

Lp = 2a . (8) 

Lp may be more meaningful than Ic and lp since it is independent of the packing frac- 
tion C and does not assume any particular form for the statistical distribution of pore 
size.^'^ 

The method of Porod23'26 has also been applied to carbon fibers^^"^^ and can be used 
to obtain another measure of the internal specific surface and other structural param- 
eters. This method is more difficult to implement, however, and requires numerical 
integration. In Porod's theory, the tail end of the scattering curve should follow an 
asymptotic course of s"'* for coUimation with small pinholes, where s = (2 sine)/K. As 
a result, plots of s'*! vs s"^ reach a limiting value at large angles. To determine struc- 
tural parameters, it is also necessary to evaluate a quantity referred to as the invariant, 
Qs^ given by 



Qs 



= f s2l(s) ds . (9) 

Jo 



The experimental determination of the invariant, however, is often difficult because 
the scattered intensities at both very small and very large angles must be determined 
with acceptable accuracy.^^ In addition, the intensities and their measurement are a 
function of the samples and the method of recording the data. 

The method of Guinier, which applies to dilute systems,^^ may also be applied to car- 
bon, where it can be used to approximate the electronic radius of gyration, R. If 
Guinier's law is obeyed, a plot of logel(s) vs s^ will be linear over a large range, with 
an ordinate intercept of logel(O) and a slope of -R^/B. Converting to logarithms to 
the base 10, 

R = (l/27r) (3 X 2.303 x slope)!/^ (10) 



21 



2. Experimental 

The small-angle X-ray scattering from the fibers was recorded using nickel-filtered 
copper radiation with a flat-plate camera having pinhole collimation (0.020 in. pin- 
holes). The camera was purchased from Blake Industries and could accommodate 
sample-to-film distances of 3 to 31 cm. At the largest distance, a range of 0.3° to 3.0° 
26 could be recorded on the film, which can be correlated with pore features as large 
as 320 A. The distance, r, of a feature on the film from the direct beam can be re- 
lated to the diffraction angle, 26, using simple geometry with the equation 

tan(20) = r/X , (H) 

where X is the sample-to-film separation distance. This camera has the advantage 
that it is relatively inexpensive and the film can easily record SAXS intensity in two 
dimensions, which is important for studies of preferred orientation of nonspherical 
particulates or voids. It is difficult, however, to obtain good quantitative measure- 
ments of intensity from the film, as opposed to systems with electronic detectors, and 
the pinhole collimation system drastically reduces the intensity of the incident X-ray 
beam. 

Qualitative survey photographs of one tow of each of eight LTC PAN fibers, along 
with Amoco T300 and T50, and CCA3 Rayon, were recorded at 45 kV/30 mA with a 
sample-to-film separation distance of 17 cm and an exposure time of 3 hr. The photo- 
graphs were taken with Kodak direct exposure film (DEF). Samples were analyzed 
under a vacuum produced by a mechanical pump in order to eliminate the effects of 
scattering from air in the camera. 

To make the conditions of analysis more reproducible for quantitative measurements, 
a sample fixture that fit over the collimator assembly was designed with a 0.050 in. 
wide channel to accept the fiber tows. Two fiber tows consisting of - 3000 fibers each 
were placed in the channel and compacted with a microscope slide while an adhesive 
was applied to the ends of the tows. The exposure times and the power to the X-ray 
generator were selected to produce a maximum optical density (OD) for the SAXS 
pattern of less than approximately 3.0. For these analyses, a sample-to-film distance 
of 31 cm was used, and, with the exception of the Amoco T50 fibers, generator set- 
tings of 45 kV/30 mA and exposure times of 30 min were employed. Much lower set- 
tings (30 kV/20 mA; 30 min) were used for the Amoco T50 fibers because of their 
intense SAXS. It was estimated that the intensity of the X-ray beam was 35% of that 
used for the other fibers. 



22 



The characteristic curve, a plot of optical density vs log exposure, was generated for 
the DBF by exposing an Amoco T300 sample (X = 5 cm) for times ranging from 5 to 
900 min and measuring the maximum darkening (optical density) of the 002 reflection 
using a densitometer with a 1-mm aperture. This curve is necessary for extracting 
quantitative information about structural parameters, such as pore sizes, from the 
SAXS photographs since it relates film darkening (optical density) directly to relative 
X-ray flux (which correlates with relative exposure time). 

Quantitative measurements were obtained from the SAXS photographs of Amoco 
T50, T300, T350/23X, T350/25X, and BASF DG Rayon 1 fibers by digitizing both the 
images and a Kodak photographic step tablet (No. 2). The digitizing was accom- 
plished by backlighting the SAXS photographs and step tablet with a high-intensity 
light source. The optical system, consisting of a Pulnix TM 745 charge-coupled device 
(CCD) camera equipped with a Nikon 50-mm macro lens, was interfaced to a Macin- 
tosh II FX computer. The computer was operated with an NIH Image version 1.22 
image analysis software package. The interface was accomplished by an 8-bit Data 
Translation DT2255 60-Hz frame grabber board. Each SAXS photograph and the 
step tablet were digitized using three shutter speeds for the CCD in order to extend 
the limited (0-255) gray scale range over a greater spread of optical density. Equato- 
rial line profiles were obtained from the same pixel locations of the SAXS images at 
each shutter speed setting. These data were then transferred to a VAX 4000 com- 
puter for data processing. A composite OD equatorial line profile was constructed 
from the files taken at the three shutter speeds and the corresponding calibration 
curves obtained from the digitized step tablet images. The characteristic curve for the 
X-ray film was then used to calibrate the OD to the relative X-ray exposure. 

3. Results 

a. Qualitative Observations 

The qualitative (with X = 17 cm) and quantitative (with X = 31 cm) SAXS photo- 
graphs of the LTC PAN fibers, along with Amoco T300 and T50, and CCA3 Rayon, 
are presented in Figures 12-14. They cover a range of approximately 0.5° to 4.8° 20. 
The white disc in the center of the photographs is the shadow of the direct beam stop 
supported by a thin wire. Note that the qualitative photographs have not been 
recorded under completely reproducible conditions. In particular, the SAXS from the 
Amoco T300 and T50 fibers was probably greater than the SAXS from the LTC fibers 
since the camera was better aligned during the exposures for the T300 and T50 fibers, 
and the X-ray beam was more intense. 



23 



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26 



The CCA3 Rayon displays very little SAXS [Figure 12(c)] , indicating that very few 
pores in the range of 10 to 320 A are present. The Amoco T50 and T300 fibers, on 
the other hand, display considerable SAXS in lobe-shaped patterns. These patterns 
have been attributed to the presence of needle-shaped pores aligned along the fiber 
axis. This interpretation is based on the observation that the SAXS does not extend 
very far in the direction of the fiber axis, which indicates the presence of a large-size 
scattering dimension. That is, r is small, 6 is small [from Eq. (11)], and, therefore, d is 
large [from Eq. (1)]. Conversely, since the SAXS extends further in the direction per- 
pendicular to the fiber axis, the size of the scattering dimension must be smaller, 
resulting in an elongate or needle-shaped feature. The fan-shaped lobes in the SAXS 
pattern from the T50 fibers are narrower than the lobes in the SAXS pattern from the 
T300 fibers. The narrower lobes have been attributed to a greater alignment of the 
pores, which, in PAN-based fibers, directly relates to the alignment of the graphitic 
basal planes in the fiber.i5.i6 since the basal planes have a greater preferred orienta- 
tion in the T50 fibers (see Tkble 4), it follows that the pores are also more highly 
aligned. 

With the exception of the BASF DG Rayon 1 and DG Rayon 2 fibers, the SAXS pat- 
terns from the LTC fibers are very similar and show very little SAXS. The SAXS pat- 
terns from the BASF fibers are intermediate between T300 and the other LTC fibers, 
and display fan-shaped lobes in the longer exposure (3 hr) photographs. These lobes 
are less evident in the shorter exposure (0.5 hr) photographs. The intensities of SAXS 
from the BASF fibers are consistent with the low-angle portion of the WAXD 6-20 
scans (Figure 7). 

The SAXS photographs of the LTC fibers display oriented and intense scattering at 
very low angles (< 0.4°e ) that may not be related to scattering from pores in the 
fibers. SAXS studies have been conducted where PAN-based fibers have been ana- 
lyzed before and after soaking in glycerin. It was found that this intense scattering at 
very low angles was greatly diminished after soaking with glycerin, while other SAXS 
was minimally affected. i6'i^'22 From this observation, it has been concluded that the 
majority of the pores in the fibers were inaccessible to glycerin. However, the cause 
for the intense scattering at very low angles is unresolved. It has been suggested that 
multiple scattering within fibers with highly irregular cross sections is responsible, ■ 
but a similar effect has been observed in fibers with nearly circular cross sections and 
minimal surface texture. In the latter case, it was thought that external reflection^of 
X rays from the fiber surfaces produced the intense scattering at very low angles. 

Samples of T50 and T300 fibers and one LTC fiber from each of the manufacturers 
were examined in the scanning electron microscope (SEM) to determine whether sur- 
face irregularities were presem that might contribute to the SAXS profiles. The SEM 
images are presented in Figures 15-17. Large axial grooves were observed on the 



27 



ORIGINAL PAGE 
BLACK AND V;H1TE PHOTOGRAHh 




Figure 15. Scanning electron microscope photographs of the 
surfaces of Amoco T50 and T300 fibers. 



28 



■r,m:HM PAGE 
BLACK AND VvHlTE PHOTOGRAPH 




Figure 16. Scanning electron microscope photographs of sur- 
faces of Amoco T350/23X and Textron Avcarb B-2 fibers. 



29 



ng 



"R,s^ 




Figure 17. Scanning electron microscope photographs of sur- 
faces of Hercules R879-02 and BASF DG Rayon 1 fibers. 



30 



surfaces of the Amoco T50, T300, T350, and Textron fibers. The widths of these 
grooves were on the order of 0.1 to 1.0 ^lm. In contrast, the surface of the Hercules 
fiber (R879-02) was much smoother. However, those differences could not be easily 
correlated with the SAXS patterns. 

b. Quantitative Measurements 

Figure 18 presents the characteristic curve measured for Kodak DBF showing the 
near-linear relationship between OD and X-ray exposure (exposure time) over a range 
of OD = 0-3, as shown in Figure 18(b). The gray scale/OD calibration curves 
obtained by digitizing the Kodak step tablet at various CCD shutter speeds are pres- 
ented in Figure 19. The calibration curves represent either linear or polynomial fits 
to the data. Composite equatorial line traces of the SAXS data were assembled from 
individual traces at the three shutter speeds. Calibration for the OD ranges of 0-1, 
1-2, and 2-3 were obtained from the digitized images taken with shutter speed set- 
tings of 2, 4, and 7, respectively (see Figure 19). 

Comparative equatorial line traces of SAXS intensity (OD) as a function of 6 for five 
fibers are presented in Figure 20. Intensity (OD) contour maps of these SAXS images 
are shown in Figures 21 and 22. Complete sets of SAXS data plots including OD vs 6, 
Debye (r^^^ ^^ q^), Guinier (log I vs S^) and Porod (IS^ vs S^) plots are presented in 
Appendix C. 

In Figure 20, it can be seen that the SAXS intensity from the Amoco T50 and T300 
fibers is considerably greater than the intensity from the three LTC fibers that were 
examined. This is consistent with the model of pore formation in carbon fibers. As 
the fibers are heat treated, volatiles such as nitrogen are released, which contribute to 
the formation of pores. At higher HTTs, a greater fraction of the volatiles is expected 
to have been evolved, thereby resulting in larger and more numerous voids. In Fig- 
ure 20, the strong SAXS intensity at very low angles (< 0.4°e) is the most prominent 
feature of the profiles of the LTC fibers. As mentioned earlier, this SAXS may not be 
related to pores in the material. The SAXS differences between the LTC fibers seen 
in the qualitative photographs are not obvious in Figure 20 because they are lost in 
the noise in the tails of the profiles. This was a result of the much shorter exposure 
times used to record the quantitative photographs. The more lobe-shaped nature of 
the SAXS from the BASF DO Rayon 1 sample, however, may be seen in the digitized 
image of the quantiative SAXS photographs [Figure 22(a)]. 

A summary of the structural parameters derived from the SAXS data using the 
method of Debye is given in Table 5. The values of Ic Ip, and Lp for the T50 and 
T300 fibers are similar to those reported for high-modulus carbon fibers graphitized 
to 2800° C^-^'i^ and are consistent with the trend that these values increase with 



31 



Characteristic Curve for DEF Film 



Q 



a 
O 




Log Exposure (Time) 



Characteristic Curve for DEF Film 




250 



50 100 150 200 

Exposure Time (mln) 

Figure 18. Characteristic curves determined for Kodak direct 
exposure film (DEF). 



300 



32 



3.5 



2.5 



c 

0) 

Q 



O 



2 _ 



1.5 _ 



0.5 _ 



Gray Scale/Optical Density Calibration 




150 

Gray Scaie 
Figure 19. Gray scale/optical density calibration curves measured from photo- 
graphic step tablet with different CCD shutter speeds. 



300 



2.5 _ 



(0 

c 
<1> 
Q 



Sl.5 



Q. 
O 




DG Rayon 1 
Amoco T50 
Amoco T300 



Amoco T350/23X 
Amoco T350/25X 



0.5 _ 



-1.5 



-0.5 

THETA (degrees) 

Figure 20. Equatorial lines traces of SAXS of carbon fibers obtained by digi- 
tizing SAXS photographs. Intensity of T50 x 0.35. 



1.5 



33 




ij.Tiai^ J ^=^ TJ'?*i 






■■^=<i*& ^-^6B^ 




(a) (b) 

Figure 21. SAXS intensity contour maps for (a) Amoco T50 and (b) T300 fibers. 




"%,iy' 







(a) 



(b) 



Figure 22. SAXS intensity contour maps for (a)BASF DG 
Rayon 1 and (b) Amoco T350/23X fibers. 



34 



Table 5. Summary of Structural Information Derived from Small-Angle X-ray Scattering 

(SAXS) 



Sample 


(a). A 


Sv, A-1 


Ic.A 


lp,A 


^,A 

23.8 


R,A 


Amoco T50 


11.9 


0.049 


67.2 


14.5 


20.8 


Amoco T300 


10.8 


0.056 


58.1 


13.3 


21.6 


14.0 


Amoco 
T350/23X 


32.2 


0.024 


168.6 


39.8 


64.4 


* 


Amoco 
T350/25X 


33.4 


0.023 


174.9 


41.4 


66.8 


* 


BASF DG 
Rayon 1 


12.0 


0.063 


62.8 


14.8 


24.0 


* 


*Not determined. 













increasing HTT.^^ Perret and Ruland^^ have published values of Ic and Ip for PAN- 
based fibers heat treated from 2000° to 3000 °C that are smaller than those in Table 5 
by approximately a factor of 2. However, they used the Porod model for deriving 
these values. In addition, their instrumentation and experimental conditions, while 
requiring different data treatment, were capable of easily measuring a greater range 
(3 to 4 orders of magnitude) of scattering intensity than the film technique employed 
by us. Therefore, the values for the structural parameters in Table 5 convey the cor- 
rect magnitude for the various features and for the relative differences between sam- 
ples. However, it is difficult to compare these values directly with other data in the 
literature. 

The Ic, Ip, and Lp values derived for the two Amoco LTC fibers (T350/23X and 
T350/25X) are similar to each other and considerably greater than those for the T50 
and T300 fibers. This is "out of trend" since these values are usually expected to 
increase with increasing HTT. It was previously noted that the LTC fibers exhibit 
intense SAXS at very low angles (0.4 "6), which may not be related to the pore struc- 
ture of the fiber, and that the associated pore-related SAXS is very weak. As a result, 
the structural parameter values derived from the SAXS data may be erroneous. The 
qualitative SAXS photographs for both BASF DG Rayon fibers display weak lobe- 
shaped features that more closely resemble the photograph of T300. As a result, the 
SAXS is intermediate between the other LTC fibers and T300. The structural param- 
eters for DG Rayon 1 are more similar to those for the T50 and T300 fibers, but do 
not follow the correct trend in that lp and Lp are greater than in the two higher heat 
treatment fibers. There is a reasonable amount of scatter in the data from the BASF 
DG Rayon 1 sample (Appendix C), and as a result, the uncertainties in the structural 
parameter measurements are greater for this sample than for T50 or T300. While the 



35 



structural parameters derived for the BASF DG Rayon 1 fibers are more similar to 
T50 and T300, it should be noted that the overall SAXS intensity is much lower (Fig- 
ure 20), indicating that while the dimensions of pores in those fibers may be similar, 
they are more prevelant in the T50 and T300 fibers. 

From Guinier plots of the SAXS data (Appendix C), the radius of gyration for the T50 
and T300 fibers was calculated to be 20.8 and 14.0 A, respectively (Ikble 5). The 
Guinier plots of the data from the Amoco LTC and BASF DGl fibers exhibited sig- 
nificant deviations from linearity. Therefore, an estimation of the radius of gyration 
was not possible. Porod plots (Appendix C) of the SAXS data from the T50 and T300 
fibers show the general trend of reaching a limiting value at large s values. The noise 
in the Porod plots of the LTC fiber data, however, was too large for any conclusions 
to be drawn. 

D. SUMMARY 

Based on the wide-angle 6-26 XRD scans, the LTC PAN fibers all appear to have a 
similar turbostratic structure with large 002 d-spacings, small "crystallite sizes," and 
moderate preferred orientation. The Textron Avcarb fibers (B-2 and G), however, 
consistently have the least-ordered graphitic structure as measured by the preceding 
parameters, whereas the Hercules fibers exhibit the most order of the LTC fibers. 
Limited SAXS results indicate that the LTC PAN fibers have a poorly developed pore 
structure, with the exception of the BASF fibers (DG Rayon 1 and DG Rayon 2), 
which produce SAXS patterns intermediate between T300 and the other LTC fibers. 



36 



III. TRANSMISSION ELECTRON MICROSCOPY 



A. BACKGROUND 

There are several proposed models for the microstructure of PAN-based fibers based 
on transmission electron microscopy (TEM). Several studies ^^'^^'^^ have shown that 
these fibers have a ribbon structure with a high degree of crystallite orientation along 
the fiber axis. The undulating ribbons lie predominantly parallel to the fiber axis, are 
connected together through a network of branched fibrils, and are separated by elon- 
gated pores (see Figure 23). Johnson et al.^'*-^^'^'^ suggested that the structure consists 
of arrays of misoriented turbostratic graphitic crystallites stacked approximately end to 
end in a columnar manner, forming tilt and twist boundaries between them. In later 
work with exceptionally high-modulus PAN-based carbon fibers, Johnson et al.^'' sug- 
gested a model of interlinked crystallites arranged perpendicular to each other, con- 
sisting of a complex arrangement of subgrains. Guigon et al.-'^ recently proposed that 
high-modulus and high-strength PAN-based fibers consist of sheets of aromatic layers 
folded with various concave radii along the fiber axis (see Figure 24). 

Our TEM study was undertaken to determine the microstructure of the two Amoco 
ETC PAN fibers and to compare them with those of Amoco fibers that utilize the 
same PAN precursor and experience the same oxidation stabilization, but are heat 
treated to higher temperatures, T300 (HTT = 1300-1500° C) and T50 (HTT = 
2000-2200° C). The heat-treating schedule for the ETC fibers is also expected to be 
different from those for T300 and T50. 

B. EXPERIMENTAL 

Amoco T350/23X, T350/25X, T300, and T50 PAN-based and PlOO pitch-based fibers 
were examined. Various techniques have been developed for preparation of thin sam- 
ples of fibers for TEM examination. One is grinding with a mortar and pestle,2^-29 
another is ultramicrotomy.-'O-^^ However, these two techniques have been criticized 
because they can cause mechanical damage to the fibers and alter their microstruc- 
ture. Therefore, in this study we used the technique of mechanical dimpling the sam- 
ple to a certain thickness and then ion-milling until perforation. This technique has 
been widely used in preparing TEM specimens of ceramic materials, and since it 
works by removing atoms layer by layer, mechanical damage is minimized. 



37 




Figure 23. The proposed ribbon microtexture model (from 
Ref. 17). 



38 




external 
suiface 



Figure 24. A proposed model of microtexture (from Ref. 38). 



39 



In our preparation technique for longitudinal samples, a bundle of fibers was cut to a 
length of approximately 2 cm and laid flat at the bottom of a mold. Epoxy resin was 
gently poured over the fibers, and the mold was put in a vacuum chamber to remove 
dissolved air and gas bubbles. After the resin was cured, samples were cut with a dia- 
mond saw blade to 2-mm squares with fibers oriented in the longitudinal direction. 
The bottom surfaces were lightly polished on a 3-p.m diamond sheet to remove a thin 
layer of epoxy. This polishing was carefully monitored with an optical microscope 
until the bottom fibers were barely touched. The polished sample was then turned 
over and ground on the opposite surface until a final thickness of about 150 ^.m was 
reached. The samples were then dimpled with 3-M.m AI2O3 paste and again carefully 
monitored by optical microscopy until the bottom fibers were barely touched. The 
dimpled samples were mounted on single-hole Nb rings using a quick-setting resin, 
and then dried and argon-ion milled. Both surfaces of the samples were simulta- 
neously ion milled on a cold stage with 5 kV accelerating voltage, 1 mA current, and 
the argon-ion beam inclined at an angle of 12° to the sample surface. The samples 
were milled until the middle section of the fibers, where the ion beam was concen- 
trated, were electron transparent. 

Because of the significant difference in the milling rates of graphite fibers and the 
epoxy matrix, the transverse thin sections tended to disintegrate before the fibers were 
thin enough to be electron transparent. The transverse TEM samples were, therefore, 
prepared differently. Several bundles of fibers were tightly packed in a heat- 
shrinkable tube. They were then impregnated under vacuum with a mixture of epoxy 
and 0.1 ^im AI2O3 powder. The AI2O3 powder increases the hardness of the matrix, 
thereby reducing the disparity of the milling rates. After being cured, the fully packed 
tube was sliced transversely and went through the same preparation process as the 
longitudinal samples. 

The Philips EM420 electron microscope used in this study was equipped with a LaBg 
filament, and operated at 120 keV. The images were recorded in the bright field (BF), 
dark field (DP), and high-resolution (HR) mode. In BF mode, the image is formed by 
allowing only the incident beam through the objective aperture. DF images were 
obtained by tilting the 002 reflection to center with the aperture. The aperture was 
chosen so that all undesirable transmissions were excluded. Stereo pairs were pro- 
duced by taking the fiber images at the same location after tilting 30°. Each of these 
techniques is described in more detail in the following sections. 

C. RESULTS AND DISCUSSION 

This section is divided into two parts. In the first part, the macrotextures of the fibers 
obtained by the BF and stereographic images are presented and discussed. In the sec- 
ond part, microstructure is analyzed based on the results of BF and DF observations 



40 



of the fibers at various tilt angles and lattice fringe images. A model for the structure 
of the fibers studied is developed and compared with previous models. ' - 

1. Macroscopic Texture 

a. Longitudinal 

Figures 25 to 27 show BF electron micrographs of Amoco T350/23X, T350/25X, and 
T300 fibers, respectively. T50 fibers were not examined in the longitudinal direction. 
The three micrographs have similar features. In addition, the selected area diffraction 
(SAD) patterns of these fibers show basically the same features: 100, 110, and 002 
reflections. A typical SAD pattern of T300 is shown in Figure 27. Figure 25 shows 
that the texture of the T350/23X fiber consists of three distinctive features: a solid 
core representing the fiber axis, extending along the whole length of fiber; and two 
sets of striation features, one set (marked as I) running obliquely to the fiber axis and 
the other set (marked as II) with edges running almost normal to the core. It was 
seen in the stereo viewer that Set I represents layers stacked one on top of the other 
in an arrangement parallel to the core. The observed striations are the edges of these 
layers cut at a slant angle and forming steps. Set II has distinctive layers also stacked 
parallel to the core with wider, uneven spacing, but arranged at a different tilt angle 
with respect to the core axis. They interweave with Set I, forming a three-dimensional 
network with the core as the zone axis. This three-dimensional structure is depicted in 
Figure 28. Figures 26 and 27 show the texture of T350/25X and T300 fibers, also 
consisting of stacking layers, extending along the length of the fiber, and oriented par- 
allel to the fiber axis. Stereomicroscopy shows that these stacking layers are also 
arranged at different tilt angles with respect to the fiber axis. As shown in Figure 27, 
the left set, tilted further away from the plane of view, shows more closely spaced 
edges than the one on the right, which is less tilted. These two sets are interconnected 
by an oblique, slightly curved layer in the middle, forming a wide step. 

b. Transverse 

The TEM cross-section images of T350/23X, T350/25X, T300, and T50 fibers are 
shown in Figures 29-32. In all cases, a complex arrangement of interwoven, wavy, 
twisted features were revealed. As expected, these features are more prominent and 
coarser in the higher HTT fibers (T300 and T50). In the cases of the T300 and T50 
fibers, two distinctive regions are seen. A typical two-phase structure is shown in Fig- 
ure 33. In both T300 and T50, the core shows a highly complex, irregular, turbulent 
arrangement similar to the observed overall texture of the two LTC fibers (T350/23X 
and /25X). However, the texture of the outer regions of T300 and T50 exhibits better 
order and is more oriented than the T350/23X and T350/25X fibers. The thickness of 



41 




BASAL 
PLANES 



0.1 \im 



Figure 25. BF image showing the macrotexture of Amoco 
T350/23X fiber (longitudinal section). 



42 




BASAL 
PLANES 



Figure 26. BF image showing the macrotexture of Amoco 
T350/25X fiber (longitudinal section). 



43 




0.5 nm 



Figure 27. BF image showing the macrotexture of Amoco T300 
fiber (longitudinal section). 



44 




Figure 28. Sketch showing the macrotexture. 



45 




Figure 29. BF image of Amoco T350/23X fiber (transverse section). 



46 




Figure 30. BF image of Amoco T350/25X fiber (transverse section). 



47 



CORE 



OUTER 
REGION 




Figure 31. BF image of Amoco T300 fiber (transverse section). 



48 



CORE 



OUTER 
REGION 




Figure 32. BF image of Amoco T5() fiber (transverse section). 



49 



ORIGIN AL PAGE 
SLACK AND WHITE PHOTOGRAPH 




Figure 33. Transverse section of Amoco T50 fiber showing two- 
phase structure. 



the outer region is dependent on the preoxidation stage in fiber processing, which can 
result in a fully stabilized outer region and a partially stabilized core. The core tex- 
ture inherits more of the structure of the precursor. The four fibers basically have the 
same original texture inherited from their precursors. Depending on the preoxidation 
condition, the texture evolves into two distinctive phases during heat treatment. How- 
ever, higher HTTs than those used for the LTC PAN fibers are necessary for the outer 
layers to order. 

2. Microscopic Texture 

a. Selected Area Diffraction 

Before discussing the microtexture of the fibers, it is necessary to point out some fea- 
tures of the selected area diffraction (SAD) patterns and their significance. Electrons 
produce diffraction effects similar to X rays when they pass through crystalline materi- 
als. The lattice planes that are properly oriented for diffraction can be thought of as 
being aligned parallel with the electron beam, and only lattice planes satisfying the 



50 



Bragg condition contribute to the SAD pattern. The SAD patterns from longitudinal 
sections of four PAN-based fibers actually represent a combination of two superim- 
posed SAD patterns, ring patterns from the 100 and 1 10 reflections and arc features 
associated with 002 reflections (Figure 34). These result from the reflections of two 
distinct sets of basal planes oriented at different angles with respect to the beam 
direction. The orientation of turbostratic crystallites that produce the 002 arcs is dif- 
ferent from those producing the complete 100 and 110 rings. Crystallites whose basal 
planes lie perpendicular to the beam direction, i.e., "face-on," give rise to 100 and 110 
rings. The diffuse ring pattern that we observed indicates that the structure is not 
highly crystallized, the crystallized grains are ultrafine, and their basal planes are not 
well ordered. Basal planes oriented approximately parallel to the beam direction, i.e., 
"edge-on," produce 002 and 004 reflections. Rotating the SAD pattern to a proper 
angle with respect to the associated BF image shows that the edge-on basal planes are 
aligned predominately parallel to the fiber axis in all cases (marked by triple lines in 
Figures 25-27). 

The arc features of 002 reflections are a characteristic of the basal planes. Figure 35 
schematically illustrates three sets of basal planes. They are oriented such that the 
Bragg condition is satisfied for all three and emit 002 reflections; i.e., the basal planes 
are parallel to the electron beam. However, Sets II and III are twisted by an angle 
±0! relative to Set I. Each of the basal planes in Sets 11 and III will produce its own 
002 diffracted beam at an angle a relative to the one diffracted by Set I. This is true 
for all intermediate situations where the planes are twisted at an angle from to ± a. 
An arc fanning out from the 002 spot of Set I at an angle of ± a is therefore 
observed. In the case of T300, it is seen in Figure 27 that the 002 reflection shows an 
arc feature with the measured angle - ±29°. T350/23X and T350/25X fibers are 
similar. This indicates that the aromatic planes in these fibers are not perfectly ori- 
ented. In fact, they undulate with a deflection angle that varies from to ±29° 

110 



002 





+ { O 



"Face-on" grains "Edge-on" grains 

Figure 34. Sketch showing that the observed SAD pattern is composed of 
two different patterns resulting from "face-on" and "edge-on" grains. 



51 




Figure 35. Sketch showing reflections of planes misoriented by 
an angle of ± oc. 

relative to the principal direction. This is confirmed by lattice fringe images of the 
basal planes, which will be shown and discussed in a later section and agrees with pre- 
vious studies, which have shown the wavy nature of the basal planes in PAN-based 
fibers.^ 

The SAD patterns observed result from crystallites arranged in a particular orienta- 
tion. It is, therefore, difficult to arrive at a complete three-dimensional understanding 
of the microstructure of the fibers from just one SAD pattern. Electron diffraction at 
various tilt angles around the fiber axis is necessary. Before further discussing our 
results, we will continue our review of the characteristic electron diffraction of 



52 



aromatic graphite layers. This has been extensively described before.'^^-'^'' Imagine a 
thin layer of basal planes of fine graphite grains oriented randomly relative to each 
other. The reciprocal space of this layer is represented by concentric hollow 100 and 
110 cylinders as depicted in Figure 36. The diffraction pattern seen in TEM results 
from the intersection of the Ewald sphere with the reciprocal lattice. In the case of 
"face-on" orientation of the layer planes, the resultant diffraction pattern is repre- 
sented by concentric 100 and 110 rings as depicted in Figure 36(a). Tilting the layer 
by an angle around a particular axis results in tilting the reciprocal lattice an equal 
angle around the same axis. The resulting diffraction pattern is now elliptical, extend- 
ing along the direction normal to the tilting axis as shown in Figure 36(b). Upon fur- 
ther tilting of the layer until it is oriented parallel to the beam direction (i.e., the 
grains are seen edge-on), the diffraction pattern consists of 100, 110 streaks due to the 
intersection of Ewald sphere with the horizontal reciprocal lattice, in addition to 002, 
004, etc., reflections from edge-on planes [Figure 36(c)]. 

Returning to the case of PAN-based fibers, by tilting the fiber around the fiber axis, 
the edge-on basal planes start to move away from the incident beam, and their 002, 
004 reflections are expected to become progressively weaker and completely disap- 
pear when the Bragg condition is no longer fulfilled. The resultant SAD pattern of 
the face-on grains is expected to become elliptical, elongated along the direction 
normal to the fiber axis, depending on the tilt angle, as depicted in Figure 37(b). In 
this study, the longitudinal sections were tilted around the fiber axis to angles of 20°, 
30°, and 45°. The corresponding SAD patterns do not evolve into elliptical patterns, 
and the 002 reflections do not disappear, but the same features as shown in the dif- 
fraction pattern in Figure 27 remain through large tilt angles. These observations are 
true for all fibers studied. This indicates that at any tilt angle, there exists similar sets 
of normal basal planes satisfying the Bragg condition and emitting consistent diffrac- 
tion patterns. 

At this point, the results of the electron diffraction study conforms to two models. 
First, the microstructure can be viewed as multiple randomly distributed crystallites 
with basal planes oriented parallel to and tilted at different angles around the fiber 
axis. These crystallites could be interlinked by either tilt or twist grain boundaries 
forming long strips of graphite along the fiber axis. We have found that when the 
sample is tilted to other orientations around the fiber axis, additional stacks of basal 
planes appear. This implies that the observed individual strips of graphite are present 
at all radial angles throughout the fiber and might crosslink in a complex three- 
dimensional manner to structure the whole fiber. Second, it might be fitted to a mod- 
el proposed by Guigan^s that crystallites are not randomly distributed but arranged in 
an orderly manner edge to edge, forming multifolded aromatic layers (see Figure 24). 



53 





/" 



(( 



J) 



(a) 




,.^ 



'( ^ )) (b) 



V, 



^, 



"^ — 



/ 



o s » X 



(c) 



Figure 36. Diffraction patterns resulting from tilting the basal planes. 



54 





(a) 




(b) 



Figure 37. Diffraction patterns resulting from tilting the perpendicular basal planes. 

To further explore which model is more suitable, we supplemented the electron 
diffraction investigation with DF and high-resolution lattice fringe image studies m 
both the longitudinal and transverse directions of the fiber. 

b. Dark Field Study 

In the DF mode, the objective aperture was set paraxial, and the incident beam was 
tilted to let the scattered beam through the aperture. The aperture was chosen small 
enough to allow only a given hkl diffracted beam to go through. Those regions that 
reflect that particular chosen diffracted beam appear bright in the DF image and pro- 
vide the projection images of the lattice planes along the lens optical axis while the 
other regions are completely dark. With the selection of the 002 reflection for DF 
imaging the bright regions represent locations where the basal planes are approxi- 
mately parallel to the electron beam. (The Bragg angle is very small due to the short 
electron wavelength emitted by the LaB6 filament.) In other words, 002 DF will 
image the whole domain of aromatic layer stacks oriented edge-on, providing infor- 
mation on their qualitative distribution and the crystallite thicknesses. Figures 38 to 
40 show the transverse 002 DF images of the T350/23X, T350/25X, and T300 fibers. 



55 



BLACK AND WHiTE PiiOTOGRAhh 




Figure 38. 002 DF image of Amoco T350/23X fiber (transverse section). 



56 



BLACK Ar<U V/nrf r.iQ^'OCRAPH 




Figure 39. 002 DF image of Amoco T350/25X fiber (transverse section). 



57 



8LACK Ar^D V.'H;rr P''— v^^o 




Figure 40. 002 DF image of Amoco T300 fiber (transverse section). 



58 



respectively. The DF images reveal an abundance of randomly distributed crystallites, 
or some short extended sheet-like features. As expected, in T50 fibers, the bright 
domains are much better oriented, as shown in Figure 41. However, long-range fea- 
tures were not observed. 

In order to take into account the possibility that the adjacent crystallites might be ori- 
ented at slightly different angles, such that they do not all satisfy the Bragg condition 
to appear in the DF image, transverse samples of T5() fibers were examined in the DF 
mode at tilt angles from 0° to A\ in 1° increments, with the aperture set at a fixed 
location of the 002 ring. The purpose of the tilting was to bring the crystallites that 
happen to be slightly misoriented into view. This was done to determine whether the 
crystallites, when matched together, would line up in a continuous multifolded sheet 
of graphite. However, as seen in Figure 42, this is not the case. The DF images at 
different tilt angles show the in and out of contrast of multiple randomly distributed, 
isolated crystallites, or in an extreme case, an arrangement of shortly extended fea- 
tures. It was noted that the large, chunky, or shortly extended bright regions were 
split into many smaller segments due to tilting. This indicates that the crystallites are 




Fiuure 41. 002 DF image of Amoco T.SO fiber (transverse section; no tilt angle). 



BLACK !^vu; \- 



...RAPH 



59 




(a) Tilting 1 ' 



(b) Tilting 2° 




(c) Tilting 3°. 



(d) Tilting 4'" 



Figure 42. 002 DF images of Amoco T50 fiber (transverse sec- 
tion; different lilt angles). 



60 



arranged side by side to form short, thick strips of basal planes. In certain areas, 
there is a high density of bright areas due to overlapping images of superimposed 
domains. Thus, the dark field study does not support Oberlin's model. 

In areas where the sample is thin enough such that individual domains can be dis- 
cerned, size measurements can be estimated. By comparing these DF images, it is 
seen that the crystallite size increases in the order of T350/23X, /25X, T300, and T50 
corresponding to HTT. The crystallite thicknesses (Ic) of each fiber were measured as 
approximately 10 to 20 A (T350/23X, T350/25X), 25 to 40 A(T300), and 60 to 120 A 
(T50). The associated SAD patterns of the transverse sections show that the 002 arc 
reflection is extended into a complete ring. This indicates that there is no preferred 
orientation in the transverse sections; i.e., the basal planes have preferred orientation 
along the fiber axis, but their c-axes are randomly oriented throughout the fiber's 
cross section. 

c. High-Resolution (HR) Lattice Fringe Study 

To confirm the DF results, high-resolution (HR) lattice fringe images of the graphite 
planes composing the observed crystallites in DF images were obtained for the PAN 
fibers in both longitudinal and transverse sections. Figures 43 to 52 show lattice 
fringe images of T350-23X, -25X, T300, and T50 fibers in both the transverse and 
longitudinal directions. These HR micrographs were obtained in the beam-tilted con- 
dition such that individual planes could be brought into sharp contrast and identified 
as black and white fringes running in almost parallel stacks. Overall, the transverse 
HR images show several domains of stacking fringes, representing individual, isolated 
crystallites. The thickness of crystallites varies over a wide range in each case, but 
clearly show the trend of increasing crystallite size with HTT. For lower HTT fibers 
(T350/23X, /25X, and T300), the fringe patterns are not well developed, graphite 
planes undulate, and are not well ordered. In the T50 fiber, the wavy turbostratic- 
planes are more planar and stacked in a better order. Figures 44 to 47 show that lat- 
tice fringe images of transverse sections of T350/25X and T300 fibers are more com- 
plicated. In the core region, the lattice planes are arranged randomly in a turbulent 
fashion throughout the core. This observation is consistent with the macrotexture of 
the core of the fiber and again suggests that the core structure is not radially oriented. 
This type of structure is expected to also exist in the core of the T50 fiber, but unfor- 
tunately, the core of the T50 specimen was too thick for effective imaging of the lat- 
tice fringes. In contrast, lattice fringe images of the outer region of T50 fibers show 
that the lattice planes are well oriented and arranged in better order. This indicates a 
strong relation between the micro- and macro-texture of the fibers. It could not be 
discerned if the basal planes in the outer region are arranged radially, circumferential- 
ly, or randomly. The lattice spacings measured directly from these images are =3.5 to 



61 



BLACK 



ORIGINAL 
AND WH.TE 



PAGC 



PHOTOGRAPH 




Figure 43. Lattice-fringe image of Amoco T350/23X fiber (transverse section). 



62 



'-;P 



BLACK AND VVh'TL PHOTOGRAPH 




Figure 44. Lattice-fringe image of Amoco T350/25X fiber (transverse/outer region) 




Figure 45. Lattice-fringe image of Amoco T35()/25X fiber (transverse/core). 



63 



8UCK AND WHITE PhGiObKMrri 




Figure 46. Lattice-fringe image of Amoco T300 fiber (transverse/outer region). 




Figure 47. Lattice-fringe image of Amoco T300 fiber (transverse/core). 



64 



lU.'-i^i. ■■ 



£ FHCTQGRAPH 




• > -*'*' 'iM'&k-miu^mw^'' 



Figure 48. Lattice-fringe image of Amoco T50 fiber (transverse). 



65 



BLACK AiSO WHITE FHOTOGRAFh 




100 A , k-| 



Figure 49. Lattice-fringe image of Amoco T350/23X fiber (longitudinal). 



66 



Dl .' 



■ '^rt^ 



HGTOGRAPH 




Figure 50. Lattice-fringe image of Amoco T350/25X fiber (longitudinal). 



67 



BLACK AND WHiT£ f'iiOTOGRAPh 




Figure 51. Lattice-fringe image of Amoco T30() fiber (longitudinal). 



68 



BLACK IK 



■^ ^'^0]OGf^;APH 




Figure 52. Lattice-fringe image of Amoco T50 fiber (longitudinal). 



69 



3.7 A for all lower HTT fibers and «3.45 A for T50 fibers. In the longitudinal direc- 
tion, all features are consistent with those seen in the transverse direction, except that 
the single crystallites are longer. The lattice fringes indicate that multiple crystallites 
are interconnected or interwoven smoothly, continuously forming strips of basal 
planes extending over a long distance along the fiber axis. The evidence of a large 
number of lattice defects can be discerned along the fiber axis. The evidence of a 
large number of lattice defects can be discerned from the faults in the fringe patterns. 
In particular, some nonbasal edge dislocations were observed forming low-angle grain 
boundaries separating individual crystallites. 

d. PlOO Fibers 

In order to compare our results on PAN fibers with a more ordered type of fiber, the 
structure of P-100 pitch-based fiber was examined (Figures 53 and 54). This fiber is 
known to have a high HTT and a high modulus. The structure of this fiber has been 
extensively discussed in many previous studies.'*'^"''^ It is evident in the DF image that 
the graphite crystallites are much larger and more highly oriented along the fiber 
direction. The SAD pattern shows sharp, higher-order diffractions (e.g., 004, 006), 
indicating that the P-100 fiber has a higher degree of graphitization. It is also noted 
that the arc associated with the 002 reflection is not as wide as the one of the PAN- 
based fibers. This implies that the crystallites are more perfect; i.e., the basal planes 
are less undulated and appear straight when observed edge-on. As expected, these 
fibers exhibit a higher degree of graphitization and better oriented, larger, and more 
perfect graphite crystallites. Treatment at higher temperatures induces grain growth, 
causing the disappearance of tilt and twist boundaries, leading to flat, more perfect 
carbon layers. 

D. CONCLUSIONS 

The texture of the LTC PAN-based fibers studied (T350/23X and /25X) consists of 
multiple sets of parallel, wavy, bent layers that interweave with each other forming a 
complex three-dimensional network oriented randomly around the fiber axis. In the 
cases of higher HTT fibers (T300 and T50), the texture consists of two distinct 
regions, with the core's texture being similar to low HTT fiber's, whereas the outer 
region is better ordered and oriented. The two-phase structure was not observed in 
the LTC fibers. 

Based on our observations, the following is postulated: under tension during preoxi- 
dation and at low HTT, ordered domains, consisting of a few parallel basal planes, 
develop from an initially amorphous polymer matrix. These isolated domains, which 
could be envisioned as short strip-like segments, form randomly and tend to grow 
preferentially along the fiber axis. As the HTT increases, two simultaneous 



70 



BLACK mu VVhlTt PHOTOGRAPH 




(a) BF Image 




(b) SAD Pattern 



Figure 53. BF image and SAD pattern of Amoco PlOO fiber. 



71 



ORIGINAL PAGE 
BLACK AND WHITE PHOTOGRAPH 




Figure 54. DF image of Amoco PKX) fiber. 



72 



phenomena occur, more domains form and grow concurrently with those already exist- 
ing. At even higher HTT, the domains grow wider and longer until they merge into 
each other. Depending on their original orientation, they could be separated by 
either tilt or twist low-angle grain boundaries, forming a long string of edge-to-edge 
interconnecting regions of certain thicknesses. This sequence of events is expected to 
develop further at higher temperature, arising in a structure similar to the proposed 
interlinking ribbon model ^'^•^^-^^ with which the results of this study are in good agree- 
ment. The microstructure of the fibers develops from short individual strips of basal 
planes which grow both normal and parallel to the formed layers. Finally, the small 
basal-plane segments join to form large, thick stacks of near-perfect basal planes. 
Depending on the HTT, the observed crystalline defects in the structure may be 
annealed out, leading to a more perfect, better-ordered morphology of basal planes. 
The evidence of consistent diffraction patterns over a large tilt angle indicates that this 
type of structure is present at any radial angle of the fiber. This confirms the belief 
that these ribbons crosslink in a highly complex three-dimensional manner along the 
fiber axis. 



73 



IV. FIBER SURFACE CHEMISTRY 



A. X-RAY PHOTOELECTRON SPECTROSCOPY (XPS) 

1. Background and Experimental 

Analysis of the LTC carbon fibers by X-ray photoelectron spectroscopy (XPS) was 
included to provide information on surface elemental composition and surface chem- 
istry. It was anticipated that the surface composition and chemistry might be a func- 
tion of the temperature and time at temperature during carbonization. Only unsized 
fibers were examined. The XPS analyses were performed on a VG ESCALAB Mkll 
surface analysis instrument. The analysis starts with photon-source irradiation of the 
sample (in this case with Mg Ka X rays at 300 W, hv = 1253.6 eV), which causes ejec- 
tion of photoelectrons from the surface. Only photoelectrons from the surface are 
detected because photoelectrons generated in the bulk are reabsorbed and do not 
escape the solid, giving rise to a surface sensitivity typically in the range of 50 to 100 
A deep. The kinetic energies (KE) of the ejected photoelectrons are analyzed, which 
gives binding energy (BE) information about electrons in the solid according to the 
simplified relationship, hv - KE + BE. A typical XPS spectrum shows electron 
intensity (count rate) as a function of BE. Each element is characterized by a unique 
set of electron binding energies. Quantitative surface compositions can be obtained 
from measurement of peak areas, and identification of chemical states can often be 
deduced from the exact positions and shapes of the peaks. 

Two fiber tows approximately 3 cm long were glued at one end with conductive, silver- 
filled epoxy to a standard 1-cm diameter flat-topped sample stub for analysis. Scan- 
ning electron beam imaging was used to choose an analysis area on the unglued end 
of the bundles where the fibers were overhanging the stub. Analysis chamber pres- 
sure during XPS data acquisition was < 5 x 10'^° torr to minimize sample contamina- 
tion. Comprehensive wide scans were run initially for each sample to determine 
which elements were present in the surface region. The XPS detection limit is about 
0.1 mol% for most elements (H and He cannot be detected). Binding energy and 
peak area measurements were made from high-resolution (HR) spectral scans 
(20-40 eV wide) of appropriate elemental regions. Data acquisition and analyses 
were done on a system-dedicated DEC PDP 11/73 computer. 



PRECEDING PAGE BLANK NOT FILMED 




75 .^NliUfiaa tiUM 



Surface concentrations are calculated as follows. Peak areas are measured from the 
HR elemental scans, scaled by elemental sensitivity factors, then normalized to report 
as mol%. It is important to note that these normalized concentrations do not reflect 
the presence of elements not detected (including hydrogen) or not measured. Also, 
elemental sensitivity factors are of limited accuracy for complex solid matrices. Con- 
sequently, "absolute" surface concentrations can be in error by as much as a factor of 
2 or more, but relative concentrations for a given element in comparable matrices 
should be accurate to about 10% for all but trace components. The elemental sensi- 
tivity factors used in this study for O relative to C and for Si relative to O give the 
expected stoichiometry within 10% for adsorbed CO and fused silica, respectively. 

The depth of analysis by XPS varies from about 50 to 100 A, and is most strongly 
dependent on the KE of the emitted photoelectrons. Electrons with low kinetic ener- 
gy have a shorter mean free path in the solid and, consequently, have a lower proba- 
bility of escaping the solid without energy loss. Differences in mean free path are in- 
cluded in calculations of relative elemental sensitivity factors, but significant errors 
still arise when surface composition is heterogeneous, e.g., in the presence of contam- 
ination overlayers. The apparent stoichiometry of a substrate can be substantially 
altered by an overlayer's different attenuation of electron signals with widely different 
KE. 

2. Results and Discussion 

XPS surface composition data for the eight unsized LTC carbon fibers is tabulated in 
T^ble 6. For comparison, data from two PAN-based fibers, T300 and T50, a pitch- 
based fiber, P55, and HOPG graphite are included in the table. The surfaces of all 
the LTC fibers were found to incorporate moderate concentrations of nitrogen and 
moderate to high concentrations of oxygen. Silicon was a moderate level contaminant 
on the surfaces of the Textron and Hercules fibers. The measured BE of the Si2p 
photoelectron peaks on these four fiber samples was 103-104 eV, indicating that the 
silicon is present as Si02. The XPS data do not provide any clues as to the source of 
this considerable silica contamination. Silicon was present at a low concentration, less 
than 1%, on the BASF and Amoco fiber samples. Trace concentrations of sulfur, 
sodium, chlorine, and calcium were detected on some of the LTC sample surfaces. 
Moderate sodium contamination was detected on the T300 fibers analyzed for 
comparison. 



76 



Table 6. XPS Surface Composition Data 











Surface mol% 








Unsized PAN Fibers 


C 





N 


Si 


S 


Na 


CI 


Ca 


Avcarb B-2 


74 


17 


5 


5 


< 0.1 


nd 


nd 


nd 


Avcarb G 


68 


18 


8 


5 


< 0.1 


nd 


nd 


nd 


Hercules R879-01 


57 


29 


4 


9 


0.3 


nd 


< 0.1 


nd 


Hercules R879-02 


53 


30 


6 


10 


0.4 


0.4 


nd 


nd 


DG Rayon 1 


82 


11 


4 


1 


0.5 


0.2 


< 0.1 


< 0.1 


DG Rayon 2 #1 

#2 


84 
86 


10 
9 


5 
4 


0.8 
0.6 


< 0.1 
0.1 


< 0.1 

< 0.1 


0.1 
< 0.1 


nd 
nd 


Amoco T350/23X #1 

#2 


87 
85 


8 
10 


5 
5 


0.2 
0.2 


< 0.1 

< 0.1 


nd 
nd 


nd 
nd 


nd 
nd 


Amoco T350/25X 
(shear treated) 


79 


15 


6 


< 0.1 


0.1 


nd 


< 0.1 


< 0.1 


Amoco T300 
(not shear treated) 


87 


8 


2 


nd 


0.1 


4 


tr 


nd 


Amoco T50 


96 


3 


0.2 


nd 


0.1 


0.2 


nd 


nd 


Amoco P55 (pitch) 


96 


4 


nd 


nd 


0.1 


nd 


tr 


nd 


HOPG graphite 


100 


0.4 


nd 


nd 


nd 


nd 


nd 


nd 


nd = Not detected. 



















It is seen in Tkble 6 that there is a correlation between the surface nitrogen and oxy- 
gen concentrations and the degree of graphitization. HOPG graphite has only a trace 
(tr) level of oxygen, T50 has 4% oxygen, T300 has 8% oxygen (about 2% of this oxy- 
gen can be assumed to be associated with the sodium oxide surface contamination), 
and the LTC fibers have 7 to 13% oxygen that is not associated with SiOa contamina- 
tion. The Amoco T350/25X sample is shear treated and has a slightly higher surface 
oxygen concentration (16%) than the nonshear-treated fibers. The presence of widely 
varied concentrations of surface silica and the apparent variability in surface oxygen 
between different samples of the same fiber type make it impossible to correlate the 
surface oxygen concentrations of the LTC fibers with the degree of graphitization 
determined by X-ray diffraction. 

The lower thermal conductivity of the LTC fibers is due to their retention of a large 
amount of nitrogen. At higher temperatures, most of this nitrogen is driven off, leav- 
ing a purer carbon fiber with higher thermal conductivity. Comparing the PAN-based 



77 



fibers, it can be seen that residual surface nitrogen from the precursor PAN correlates 
with the temperature and time at temperature during carbonization. The nitrogen 
concentration was measured to be 0.2% on T50, 2% on T300, and 4 to 8% on the 
LTC fibers. There was no consistent difference between the LTC carbon fibers. Het- 
erogeneity makes it difficult to correlate surface nitrogen levels with the extent of 
graphitization. The Nls photoelectron peak on the LTC samples is split into two 
peaks, one of which can be assigned to nitrides or CN at about 398.5 eV BE, and the 
other of which can be assigned to NH4 or NR4 at about 400.5 eV BE. On most of the 
samples, the ratio of the lower BE peak to the higher BE peak was about 2:3. Addi- 
tionally, on some of the Nls spectra it was necessary to add a third peak at about 
402.5 eV to obtain a smooth curve fit. This peak accounted for about 10% of the to- 
tal peak intensity and can be assigned to oxidized nitrogen. Shear treatment does not 
appear to decrease the amount of nitrogen on the LTC fiber surface. 

The Cls photoelectron peaks measured for the LTC carbon fibers have significant 
intensity at higher binding energy due to the presence of oxygen-containing function- 
alities bonded to the fiber surfaces. This is illustrated in Figure 55a, where the higher 
BE peaks are seen to give rise to an asymmetric tail on the dominant carbon peak 
arising from the carbonized surface. The Cls peaks of the LTC fibers have been 
curve fit to determine the relative contribution of different classes of oxygen- 
containing functionalities to the high BE tail. These data are shown in Table 7. A 
study of well-characterized polymers'^ has shown that each bond formed between a 
carbon atom and an oxygen atom leads to an electron withdrawing shift to higher BE 
on the carbon atom of about 1.5 eV. The LTC carbon fiber Cls data could be fit by 
assuming the presence of functionalities with one, two, and three bonds to oxygen 
(e.g., alcohol or ether, ketone or ketal, and acid or ester, respectively). The major 
limitation to this approach arises from the observation that a clean, essentially 
nonfunctionalized graphite surface has an asymmetric Cls photoelectron peak, as seen 
in Figure 55b for an HOPG graphite freshly cleaved surface. Although this surface 
was measured to have 0.4% surface oxygen, a standard Cls curve-fitting approach 
indicates about 15% of the surface carbon to have one or more bonds to oxygen, as 
seen in Table 7. It is not legitimate to do a simple subtraction of the clean graphite 
spectrum from a more oxidized fiber Cls spectrum because the line width changes 
with graphitization along with the line shape. Thus, the observations made on the 
basis of the Cls curve fits are relative and at best semiquantitative. 

The FWHM of the dominant carbon peak at 284 to 285 eV BE is also tabulated in 
Table 7 with the curve-fit information. It can be seen that the more graphitized fibers, 
T50 and P55, have a FWHM comparable to HOPG graphite at about 1 eV. T300 and 
the LTC fibers have significantly broader peak widths, in the range 1.6 to 1.8 eV. The 
carbon fiber surfaces all have significantly increased levels of Cls higher binding 



78 




o.oo 



(a) 



2K.H 



2BG.M »-M 29°'°* 

Binding Energy (eV) 



292.00 



294.00 




0.00 



(b) 



27B.00 



280.00 2*2.00 2«4.00 286.04 28B.00 

Binding Energy (eV) 



290.00 292.00 



294.00 



Figure 55. XPS Cls spectra, (a) Amoco T350/23X fiber, 
(b) HOPG cleaved surface. 



79 



Table 7. XPS Curve-Fit Data for Cls Photoelectron Peaks 



CIS Curve Fit Data 
(Normalized) 


Unsized Fibers 


C-C 
C-H 


C-OR 
(R = C,H) 


RO-C-OR 
orC = 



// 
C-OR 


Jt-*7l* 


FWHM 
of C-C 


Avcarb B-2 


79 


12 


5 


2 


2 


1.8 


Avcarb G 


74 


18 


5 


2 


1 


1.8 


Hercules R879-01 


77 


15 


5 


3 


1 


1.8 


Hercules R879-02 


73 


16 


6 


3 


1 


1.8 


DG Rayon 1 


74 


17 


6 


3 


1 


1.7 


DG Rayon 2 #1 

#2 


71 
71 


20 

18 


5 
6 


2 

3 


2 

2 


1.8 
1.8 


Amoco T350/23X #1 

#2 


75 
70 


16 
18 


5 
5 


2 
3 


2 

3 


1.7 
1.6 


Amoco T350/25X 
(shear treated) 


72 


15 


7 


4 


1 


1.7 


Amoco T300 


74 


14 


4 


3 


3 


1.7 


Amoco T50 


77 


14 


3 


3 


3 


1,1 


Amoco P55 


76 


14 


3 


2 


3 


1.1 


HOPG graphite 


81 


13 


1 


1 


3 


1.0 



intensity compared to the HOPG graphite. Most of the LTC fiber surfaces have an 
additional increase in the Cls higher binding energy intensity compared to T50 and 
P-55. This additional increase falls primarily into the curve-fit peaks associated with 
one and two bonds between carbon and oxygen. The shear-treated T350/25X has an 
additional increase in the surface acid/ester functionality compared to the nonshear- 
treated LTC fibers. The n^-u* transition peak indicated in Table 7 is a Cls peak 
feature at 291 eV BE associated with delocalized bonding in the carbon network, and 
is expected to be strongest for the most highly graphitized materials. 

3. Summary 

XPS analysis finds that there are correlations between the HTT and the surface com- 
position and chemistry for carbon fiber samples. The concentration of surface oxygen 
and nitrogen (PAN-based fibers only) is shown to decrease with increasing HTT. The 
details of the Cls photoelectron peak, i.e., the FWHM of the dominant carbon peak 



80 



and the intensity of the higher binding energy tail, also indicate major changes. The 
line width of the dominant carbon peak decreases, and the percentage of the higher 
binding energy intensity decreases with increasing graphitization. None of the above- 
mentioned indicators measured from the XPS data was found to have adequate reso- 
lution to distinguish differences in the extent of graphitization of the set of LTC 
carbon fibers analyzed in this study. The comparative analyses were hindered by 
significant variability in surface contamination levels, particularly of silica, and by vari- 
ability in surface composition between different samples taken from the same batch of 
fibers. 

B. ACID/BASE CHARACTER 

In order to gain some insight into the acid/base character of the fiber surfaces, the pH 
of water in which unsized fibers were immersed for 4 hr at room temperature was 
measured. These data are shown in Table 8, along with the pH of the water before 
fiber addition. With the exception of T300 fibers, the addition of fibers to water 
results in very little pH change. The addition of the T300 fibers significantly raises the 
pH, indicating the presence of basic material on the surface. This is most likely due 
to the sodium contamination found on these fibers by XPS. Shear treatment does not 
appear to effect the acid/base character of the fiber surfaces. 

Table 8. Acid/Base Character of Unsized Fiber Surfaces 







pH of Distilled Water 




Fiber Designation 


Before Fiber Addition 


After Fiber Addition 


Hercules R879-01 


6.6 






6.4 


Hercules R879-02 


6.6 






6.7 


Textron Avcarb G 


5.8 






5.9 


Textron Avcarb B-2 


5.8 






6.3 


Amoco T350/25X (shear treated) 


6.1 






6.4 


Amoco T350/23X 


5.8 






6.3 


BASF DG Rayon 1 


5.8 






6.2 


BASF DG Rayon 2 


5.8 






5.9 


Amoco T300 (shear treated) 


6.1 






9.1 


Amoco T300 (not shear treated) 


6.1 






8.7 



81 



V. FIBER SIZING LEVELS 



A. BACKGROUND 

Fiber size is designed to protect the fibers from damage during weaving and handling. 
Each fiber manufacturer was given the option of sizing the fibers with a phenohc- 
compatible size of their choice. Each also chose the preferred sizing level for each 
candidate fiber. Applied-sizing levels were measured both at Lockheed Research and 
Development Division and at the Weaver, Katema Textile Product Division. Lock- 
heed used a burnoff technique, and Katema used an acetone solvent extraction tech- 
nique. Results of the two techniques differed, and Aerospace was requested by NASA 
to make an independent evaluation. 

B. EXPERIMENTAL 

We used the acetone extraction technique, but we dried the fibers in a vacuum oven 
(at 30 ± 5°C and 250 torr for 16 hr) before solvent extraction. This was not done at 
Katema. The fibers were dried, weighed before extraction, refluxed in acetone, rinsed 
with acetone, and then dried and reweighed. The entire weight loss was assumed to 
be due to sizing removal. 

C. RESULTS 

The results are shown in Table 9, which also includes the fiber sizing data obtained at 
Lockheed (burnoff technique) and Katema (acetone extraction technique). We found 
that the fibers must be vacuum dried before extraction in order to obtain consistent 

Table 9. Fiber Sizing Levels 



Fiber Designation 


% Sizing, wt/wt 
(Lockheed) 


% Sizing, 
(Aerospc 


wt/wt 
ice) 


% Sizing, wt/wt 
(Katema) 


Hercules R879-01 


0.32 


0.26 


3.4 


Hercules R879-02 


0.43 


0.76 




5.1 


Textron Avcarb G 


9.22 


0.57 




6.1 


Textron Avcarb B-2 


2.88 


2.64 




3.5 


Amoco T350/25X 


1.56 


1.63 




2.9 


Amoco T350/23X 


1.3 


1.48 




4.6 


BASF DG Rayon 1 


0.28 


0.8 




0.8 


BASF DG Rayon 2 


0.56 


1.16 




1.0 



83 



f^ 



PRECEDirSKS PAGE BLAf^K NOT FILWED *****-L=— i^iiNliUNAia OLfiiM 



results. We suspect that the fiber sizing levels observed by Katema are too high 
because the fibers were vacuum dried after extraction, but not before. The weight 
loss reported by Katema is probably the sum of the weights of absorbed water and 
sizing. 

The sizing values obtained by the Lockheed fiber burnoff technique are in substantial 
agreement with the extraction values obtained by us, with the exception of Avcarb G. 
The two most likely possibilities for this discrepancy are: (1) the burnoff technique 
drives off volatile fiber fragments. The Avcarb G fibers have the lowest carbon con- 
tent and may be easier to oxidize than the other fibers, resulting in high burnoff val- 
ues; or (2) the values obtained by each method are correct, but the sizing content 
from spool to spool is nonuniform. 



84 



VI. CONCLUSIONS 

Based on wide-angle X-ray diffraction, the LTC PAN fibers all appear to have a simi- 
lar turbostratic structure with large 002 d-spacings, small crystallite sizes, and mod- 
erate preferred orientation (Tables 3 and 4). The Textron Avcarb fibers show the 
least-ordered structure, whereas the Hercules fibers exhibit the most order. All the 
LTC fibers are slightly less ordered than T300 fibers, which are much less ordered 
than T50 fibers. This orientation is consistent with relative heat-treatment tempera- 
tures (HTTs) of the fibers. 

Limited small-angle X-ray scattering (SAXS) results indicate that, with the exception 
of the BASF fibers, the LTC fibers do not have well-developed pores (Table 5). This 
suggests that they have not been subjected to a high enough HTT to drive off all of 
their volatile constituents (mainly nitrogen). The BASF fibers produce SAXS patterns 
intermediate between T300 and the other LTC fibers (Figures 21 and 22). 

Transmission electron microscopy shows that the texture of the LTC PAN-based fibers 
studied (Amoco T350/23X and /25X) consists of multiple sets of parallel, wavy, bent 
layers that interweave with each other forming a complex, three-dimensional network 
oriented randomly around the fiber axis. In the cases of higher HTT fibers (T300 and 
T50), the texture consists of two distinct regions with the core's texture being similar 
to that of the LTC fibers, whereas the outer region (Figure 32) is better ordered and 
oriented. In LTC fibers (T350/23X and /25X), the two-phase structure was not 
observed. The results of our study are in good agreement with the proposed interlink- 
ing ribbon model ^''-^-'"^^ (Figure 23). With increasing HTT, the microstructure of these 
fibers evolves from the formation of isolated, short strip-like domains to a long, inter- 
connected ribbon structure. Crystallite size and the extent of graphitization, the latter 
based on latticed spacing, and perfection of basal planes, both increased with in- 
creased HTT. These results are consistent with those from X-ray diffraction. 

X-ray photoelectron spectroscopy (XPS) analysis finds that there are correlations 
between the HTT and the surface composition and chemistry for carbon fiber samples 
(Table 6). The concentration of surface oxygen and nitrogen for PAN-based fibers 
decreases with increasing HTT, which is consistent with increased volatilization of 
nitrogen and greater extent of graphitization. The details of the Cls photoelectron 
peak, i.e., the full-width at half-maximum (FWHM) of the dominant carbon peak and 
the intensity of the higher binding energy tail, indicate an increasing extent of graphi- 
tization with increasing HTT (Table 7). The line width of the dominant carbon peak 
decreases, and the percentage of higher binding energy intensity decreases with 
increasing graphitization. None of the above-mentioned indicators measured from 
the XPS data was found to have adequate resolution to distinguish differences in the 



85 



extent of graphitization of the LTC carbon fibers analyzed in this study. The compar- 
ative analyses were also hindered by significant variability in surface contamination 
levels, particularly of silica, and by variability in surface composition between different 
samples taken from the same batch of fibers. 



86 



REFERENCES 

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6. D. P Anderson, "Carbon Fiber Morphology: Wide Angle X-ray Studies of Pitch 
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12. R. J. Diefendorf, "Carbon/Graphite Fibers," in Engineering Materials Handbook, 
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87 



13. D. Johnson and C. N. Tyson, "The Fine Structure of Graphitized Fibres,"/ Phys. 
D 2, 787-795 (1969). 

14. D. J. Johnson and C. N. Tyson, "Low Angle X-ray Diffraction and Physical Proper- 
ties of Carbon Fibers," / Phys. D 3, 526-534 (1970). 

15. R. Perret and W. Ruland, "X-ray Small-angle Scattering of Non-graphitizing Car- 
bons,"/ Appl. Cryst. 1, 308-313 (1968). 

16. R. Perret and W. Ruland, "Single and Multiple X-ray Scattering of Carbon Fibres," 
/ Appl. Cryst. 2, 209-218 (1969). 

17. R. Perret and W. Ruland, "The Microstructure of PAN-based Carbon Fibres,"/ 
Appl. Cryst. 3, 525-532 (1970). 

18. R. Perret and W. Ruland, "X-ray Small-Angle Scattering of Glassy Carbon," / 
Appl. Cryst. 5, 183-187 (1972). 

19. W. Ruland, "Small Angle Scattering of Two Phase Systems: Determination and 
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(1971). 

20. M. Shioya and A Takaku. "Characterization of Microvoids in Carbon Fibers by 
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4074-4079 (1985). 

21. A. Takaku and M. Shioya, "Characterization of Microvoids in Polyacrylonitrile- 
based Carbon Fibres,"/ Mater. Sci. 2\, 4443-4450 (1986). 

22. M.Tang, G. G. Rice, J. F. Fellers, and J. S. Lin, "X-ray Scattering Studies of 
Graphite Fibers,"/ Appl. Phys. 60, 803-808 (1986). 

23. L. Alexander, "X-ray Diffraction Methods in Polymer Science," John Wiley, New 
York, NY, 280-386 (1968) . 

24 P. Debye and A. M. Bueche, "Scattering by an Inhomogeneous Solid," / Appl. 
Phys. 20, 518-525 (1949). 

25. P. Debye, H. R. Anderson, Jr., and H. Brumberger, "Scattering by an Inhomoge- 
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26. G. Porod, "Die Rontgenkleinwinkelstreuung von Dichtgepacken Kolloiden Syste- 
men." Kolloid-Z \2A, 83-114 (1951). 



88 



GA 



27. D. J. Johnson, "Structure-Property Relationships in Carbon Fibres,"/ of Phys. D 
Appl. Phys. 30, 286 (1987). 

28. M. A. A. Jarro, W. R. Ladner, and T. D. Rantell, "Characteristics of Coal-Based 
Carbonised Fibres," Carbon 14, 219 (1976). 

29. M. Guigon, A. Oberlin, and G. Desarmot, "Microtexture and Structure of Some 
High Tensile Strength, PAN-Based Carbon Fibres," Fibre Sci. and Technol 20, 55 
(1984). 

30. D. J. Johnson, "Direct Lattice Resolution of Layer Planes in Polyacrylonitrile 
Based Carbon Fibres," Nature 226, 750 (1970). 

31. M. Guigon and A. Oberlin, "Preliminary Studies of Mesophase-Pitch-Carbon 
Fibres: Structure and Microtexture," Composite Sci. and Technol. 25, 231 (1986). 

32. B. J. Wicks, "Direct Observations of the Internal Structure of Carbon Fibers," /. 
Mater Sci. 23, 173 (1971). 

33. W. Watt and W. Johnson, "The Effect of Length Changes During the Oxidation of 
Polyacrylonitrile Fibers on the Young's Modulus of Carbon Fibers " Appl. Polymer 
Symp. 9, 215 (1969). 

34. D. V. Badami, J. C. Joiner, and G. A. Jones, "Microstructure of High Strength, 
High Modulus Carbon Fibres," Nature 215, 386 (1967). 

35. W T Brydges, D. V. Badami, and J. C. Joiner, "The Structure and Elasltic Proper- 
ties of Carbon Fibres," App. Polymer Symp. 9, 255 (1969). 

36. D. J. Johnson and C. N. Tyson, "The Fine Structure of Graphitized Fibres," Br J. 
Appl. Phys. D2, 787 (1969). 

37. D. J. Johnson, D. Crawford, and C. Gates, "The Fine Structure of a Range of 
PAN-Based Carbon Fibers," 10th Bienn. Conf., Carbon, Lehigh Univ., Bethlehem, 
PA, 29 (1971). 

38. M. Guigon, J. Ayache, and A. Oberlin, "Structure and Microstructure of Some 
Glassy Polymers-Carbon Fibers Composites," 15th Bienn. Conf., Carbon, Philadel- 
phia, PA, 288-289 (1981). 

39. F. R. Barnet and M. K. Noor, "Carbon Fiber Etching in an Oxygen Plasma," Car- 
bon 11, 281 (1973). 

40. S. C. Bennett and D. J. Johnson, "Electron-Microscope Studies of Structural Het- 
erogeneity in PAN-Based Carbon Fibres," Carbon 17, 25 (1979). 



89 



41. B. J. Wicks and R. A. Coyle, "Microstructural Inhomogeneity in Carbon Fibres,"/. 
Mater. Sci. 11, 376 (1976). 

42. R. J. Diefendorf and E. Tokarsky, "High-Performance Carbon Fibers," Polymer 
Eng. Sci. 15, 150 (1975). 

43. D. D. Edie, N. K. Fox, B. C. Barnett, and C. C. Fain, "Melt-Spun Non-Circular 
Carbon Fibers," Carbon 24, 477 (1986). 

44. M. Endo, "Structure of Mesophase Pitch-Based Carbon Fibers,"/ Mater Sci 23 
598 (1988). 

45. S. C. Bennett, D. J. Johnson, and R. Murray, "Structural Characterization of a 
High-Modulus Carbon Fibre by High Resolution Electron Microscopy and Elec- 
tron Diffraction," Carbon 14, 117 (1976). 

46. G. R. Millward and D. A. Jefferson, "Lattice Resolution of Carbons by Electron 
Microscopy," in Chem. and Phys. of Carbon, Vol. 14, R L. Walker and P A. 
Thrower, eds.. Marcel Dekker, NY, 1-82 (1978). 

47. A. Oberlin, "Application of Dark-Field Electron Microscopy to Carbon Study," 
Carbon 17, 7 (1979). 

48. D. T. Clark and H. R. Thomas, "Applications of ESCA to Polymer Chemistry, 
XVII. Systematic Investigation of the Core Levels of Simple Homopolymers," / 
of Polymer Sci.: Polymer Chem. Edition, 16, 791-820 (1978). 



90 



APPENDIX A 



WIDE-ANGLE X-RAY DIFFRACTION SCANS 



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APPENDIX B 
PREFERRED-ORIENTATION JCANS 



PRECEDING PAGE BLANK NOT FILMED , ^ / 



107 ^^^KdX^mtHJmMiX tflAM 



Plot of fibe 
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Angle Beta [deg] 



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Angle Beta [deg] 



108 



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Angle Beta [deg] 



109 



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Angle Beta [deg] 



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Plot of fiber orientation angle (P) vs. intensity of {002} reflection for 
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AVCARB B2 








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111 



APPENDIX C 



SMALLANGLE X-RAY SCATTERING PLOTS 



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AilSN3a "IVOLLdO i3N 



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118 



INTERNAL DISTRIBUTION LIST 



REPORT TITLE CHARACTERIZATION OF LOW THERMAL CONDUCTIVITY 
PAN-BASED CARBON FIBERS 



REPORT NO. 



ATR-91(6819)-1 



1. FOR OFF-SITE PERSONNEL, SHOW LOCATION SYMBOL, 
e.g., JOHN 0. PUBLIC/VAFB 
NAME (Include Initials) 



P. M. Adams 
W. T Barry 
H. M. Blaes 
M. Buechler 
0. Denault 
K. Drake 
D. J. Evans 
R. W. Fillers 
C. A. Gaulin 
T. W. Giants 
W. Goodman 
G. F. Hawkins 
J. R. Hallett 
C. S. Hemminger 
W James 
W. H. Kao 
H. A. Katzman 
R. W. Kobayashi 
E. Landsbaum 
T. D. Le 
S. R. Lin 
J. J. Mallon 
N. Patel 
G. A. Paulikas 
G. S. Rellick 
E. Y. Robinson 
H. Rugge 
R M. Sheaffer 



(3) 



MAIL CODE * 



M2/250 

M2/248 

M2/242 

M5/582 

Ml/292 

M4/919 

M2/264 

M2/247 

M2/250 

M2/250 

M4/920 

M2/248 

M2/278 

M2/250 

M5/712 

M2/242 

M2/248 

M2/248 

M4/969 

M2/248 

M4/920 

M2/248 

M4/920 

Ml/009 

M2/248 

M2/248 

M2/264 

M2/248 



APPROVED BY 



^^^■!U=^^^^^-^ 



PUBLICATION DATE 

15 March 1992 



SECURITY CLASSIFICATION 

Unclassified 



2 IF LIST IS ALTERED, INITIAL CHANGE(S) AND SHOW AFFILIATION 
* FOR SECRET REPORTS SHOW BLDG AND ROOM, NOT MAIL STATION 

MAIL CODE 



N AME (Include Initials) 

A. J. Schiewe 
J. J. Shaffer 
G. L. Steckel 
W. K. Stuckey 
D. G. Sutton 
G. A. To 
J. C. Uht 
S. L. Zacharius 
R. J. Zaldivar 
Library (2) 
Technical Counsel 



DATE 



,F LIST COMPRISES TWO OR MORE SHEETS, COMPLETE ABOVE BLOCK ON LAST SHEET ONLY 



Ml/002 

M2/279 

M2/321 

M2/250 

M2/248 

M2/250 

M2/250 

M2/250 

M2/248 

Ml/ 199 

Ml/040 




<2^ 



SHEET 



AEROSPACE FORM 2394 REV 3-85 



EXTERNAL DISTRIBUTION LIST 



REPORT Tl 



^^CHARACTERIZATION OF LOW THERMAL CONDUCTIVITY 
PAN-BASED CARBON FIBERS 



REPORT NO. 



ATR-91(6819)-1 



MILITARY AND GOVERNMENT OFFICES 



PUBLICATION DATE 

15 March 1992 



SECURITY CLASSIFICATION 

Unclassified 



ASSOCIATE CONTRACTORS AND OTHERS 



1. SHOW FULL MAILING AODHESS: INCLUDE ZIP CODE, 

2. IF LIST IS ALTERED, INITIAL CHANGE(S) AND SHOW 



AF BMO/MYET 

Norton Air Force Base, CA 92409-6468 

Attn: Lt. R. Einhorn 

Phillips Laboratory 
Edwards AFB, CA 93533 
Attn: J. Wanchek 

L. Tepe PL/RKS 

J. Buch 

B. Wells PL/RK 

Defense Technical Information Center 
Cameron Station (12 unc) 
Alexandria, VA 22314 
Attn: DTIC-TC 

Institute for Defense Analyses 
1801 N. Beauregard St. 
Alexandria, VA 22311-1772 
Attn: Library 
J. Hovey 

AF/SSD 

Attn: Col. Randma 

Jet Propulsion Laboratory 
4800 Oak Grove Drive 
Pasadena, CA 91103 
Attn: W. Phillips 
Library 

Naval Surface Warfare Center 
10901 New Hampshire Ave. 
Silver Spring, MD 20903-5000 
Attn: A. L. Bertram R32 



MILITARY OFFICE SYMBOL, AND ■'ATTENTION" LINE 
AFFILIATION 



Acurex Corp. 
PO Box 7040 

Mountain View, CA 94039 
Attn: B. Laub 

Aerojet Strategic Propulsion Co. 
PO Box 15699C 
Sacramento, CA 95813 
Attn: S. C. Brown 

KETEMA 

3611 S. Harbor Blvd. Suite 225 
Santa Ana, CA 92704-6928 
Attn: J. Garibotti 
T Lascarus 

AMOCO Performance Products, Inc. 
4500 McGinnis Ferry Road 
Alpharetta, GA 30202 
Attn: R. Bacon 

M. Towne 

C. McGuire 

AMOCO Performance Products, Inc. 
375 Northridge Road Suite 600 
Atlanta, GA 30350-3297 
Attn: L. Matthews 

Atlantic Research Corporation 
5390 Cherokee Avenue 
Alexandria, VA 22312 
Attn: J. Baetz 



DISTRIBUTION LIMITATIONS MARKED ON THE COVER/ TITLE PAGE ARE AUTHORIZED BY SIGNATURE BELOW 



APPROVED BY . 
(AEROSPACE) 

APPROVED BY . 
(AF OFFICE) 



DATE 



DATE 



INOT REQUIRED FOR ATR CATEGORY) 



IF LIST COMPRISES TWO OR MORE SHEETS, COMPLETE ABOVE BLOCK ON LAST SHEET ONLY 



SHEET . 



kEROSPACE FORM 2380 REV 11-85 



EXTERNAL DISTRIBUTION LIST 



REPORT TITLE CHARACTERIZATION OF LOW THERMAL CONDUCTIVITY 
PAN-BASED CARBON FIBERS 



REPORT NO 

A1R-91(6819)-1 


PUBLICATION DATE 

15 March 1992 


SECURITY CLASSIFICATION 

Unclassified 


MILITARY AND GOVERNMENT OFFICES 


ASSOCIATE CONTRACTORS AND OTHERS 



r SHOW FULL MAILING ADDRESS: INCLUDE ZIP CODE, MILITARY OFFICE SYMBOL, AND ■■ATTENTION" LINE 
2, IF LIST IS ALTERED, INITIAL CHANGE(S) AND SHOW AFFILIATION 



NASA 

Ames Research Center 
Moffett Field. CA 94035 
Attn: Library 

NASA 

Marshal Space Flight Center 
Huntsville, AL 35812 
Attn: R. G. Clinton, Code EH 34 
Library 

A. Puckett, Code EH 34 
R. Nichols, Code ER 41 
M. Stacker. Code ER 41 

B. Neighbors 

NASA Scientific & Technical Information 

Facility (unc) 

PO Box 8757 
Baltimore- Washington International Airport 

MD 21240 
Attn: SAK/DL 

Office of Naval Technology 
800 N. Quincy St. 
Arlington, VA 22217-5000 
Attn: W. Messick 



BF Goodrich Research & Development Center 
9921 Brecksville Road 
Brecksville, OH 44141 
Attn: E. R. Stover 

BF Goodrich 
11120 Norwalk Blvd. 
Santa Fe Springs, CA 90670 
Attn: W H. Pfeifer 

G. B. Engle - 

W Terasen 

General Electric 
3198 Chestnut St. 
Philadelphia, PA 19101 
Attn: R. Randolph 

Hercules Aerospace Co. 
PO Box 98 

Magna, UT 84044-0098 
Attn: J. P George 

S Lewis 

C. M. Heyborne, M/S B2 

K. H. Hill. M/S B2 

J. Larson, M/S B2 

F P Magin, M/S XllTl 

H. M. Pressley, Jr., M/S B2 

G. M. Wendel, M/S NlEQl 

Lockheed Palo Alto Research Laboratory 
3251 Hanover St. 
Palo Alto, CA 94304-1191 
Attn: P C. Pinoli. Org 93-30, Bldg 204 
F C. Weiler, Org 93-30, Bldg 251 

Materials Sciences Corporation 
930 Harvest Drive, Suite 300 
Union Meeting Corporate Center 
Blue Bell, PA 19477 
Attn: J. J. Kibler 
B. J. Sullivan 





1 






DISTRIBUTION LIMITATIONS MARKED ON THE COVER/TITLE PAGE ARE AUTHORIZED BY SIGNATURE BELOW 




APPROVED BY 


DATE 




(AEROSPACE) 
APPROVED BY 


DATE 


(AF OFFICE) 


(NOT REQUIRED FOR ATR CATEGORY) 



IF LIST COMPRISES TWO OR MORE SHEETS. COMPLETE ABOVE BLOCK ON LAST SHEET ONLY 



SHEET . 



AEROSPACE FORM 2380 REV 11-85 



EXTERNAL DISTRIBUTION LIST 



REPORT TITLE 



CHARACTERIZATION OF LOW THERMAL CONDUCTIVITY 
PAN-BASED CARBON FIBERS 



REPORT NO, 



ATR-91(6819)-1 



MILITARY AND GOVERNMENT OFFICES 



PUBLICATION DATE 

15 March 1992 



SECURITY CLASSIFICATION 

Unclassified 



ASSOCIATE CONTRACTORS AND OTHERS 



1. SHOW FULL MAILING ADDRESS: INCLUDE ZIP CODE, 

2. IF LIST IS ALTERED, INITIAL CHANGE(S) AND SHOW 

Office of Director of Defense for Research & 

Engineering 
The Pentagon 
Washington, DC 20301 
Attn: J. Persh 

Staff Speciahst for Materials & Structures 
Room 3D 1089 

Office of Naval Research 
800 N. Quincy St. 
Arlington, VA 22217-5000 
Attn: S. Fishman, Code 1131 

SSD/AFWAL 
Attn: P Propp 

U.S. Army Materials Technology Laboratory 
Watertown, MA 02172 
Attn: J. Dignam 
S. Wentworth 

Wright Laboratory 
Wright-Patterson AFB 
Dayton, OH 45433 
Attn: D. Schmidt 

AFIT 

Wright-Patterson AFB, OH 45433 

Attn: Technical Library 

U.S. Army Ballistic Missile Agency 
Technical Documents Library 
Redstone Arsenal, AL 35809 
Attn: Commander 

AFSC (DLF) 

Andrews AFB, DC 20331 



MILITARY OFFICE SYMBOL, AND "ATTENTION" LINE 
AFFILIATION 

Prototype Development Assoc, Inc. 
2975 Redhill Ave. 
Costa Mesa, CA 92626 
Attn: J. G. Crose 

Southern Research Institute 
PO Box 55305 

Birmingham, AL 35255-5305 
Attn: J. Koenig 

C. Pears 

W. Lundblad 

Science Applications Intl. Corp. 
18872 Bardeen Ave. 
Irvine, CA 92715 
Attn: W. C. Loomis 
E I. Clayton 

TRW/EDC 
PO Box 1310 

San Bernardino, CA 92402 
Attn: B. Balachandra 
T Serafini 

Union Carbide Corporation 
Parma Technical Center 
PO Box 6116 
Cleveland, OH 44101 
Attn: S. Strong 

United Technologies Corporation 
Chemical Systems Division 
PO Box 49028 
San Jose, CA 95161-9028 
Attn: E. R. Mills 



APPROVED BY . 
(AEROSPACE) 

APPROVED BY . 
(AF OFFICE) 



DISTRIBUTION LIMITATIONS MARKED ON THE COVER/TITLE PAGE ARE AUTHORIZED BY SIGNATU RE BELOW 

DATE 



DATE 



(NOT REQUIRED FOR ATR CATEGORY) 



IF LIST COMPRISES TWO OR MORE SHEETS, COMPLETE ABOVE BLOCK ON LAST SHEET ONLY 



SHEET . 



.Of. 



AEROSPACE FORM 2380 REV I1-B5 



EXTERNAL DISTRIBUTION LIST 



REPORT TITLE 



"CHARACTERIZATION OF LOW THERMAL CONDUCTIVITY 
PAN-BASED CARBON FIBERS 



REPORT NO, 



ATR-91(6819)-1 



MILITARY AND GOVERNMENT OFFICES 



PUBLICATION DATE 

15 March 1992 



SECURITY CLASSIFICATION 

Unclassified 



ASSOCIATE CONTRACTORS AND OTHERS 



1, SHOW FULL MAILING ADDRESS. INCLUDE ZIP CODE, MILITARY OFFICE SYMBOL. AND '■AnENTION" LINE 

2. IF LIST IS ALTERED. INITIAL CHANGE(S) AND SHOW AFFILIATION 

BASF Structural Materials, Inc. 
Celion Carbon Fibers 
11501 Steele Creek Rd. 
PO Box 7687 
Charlotte, NC 28241 
Attn: K. Bloomer 

Aerojet Corp 
1 NASA Drive 
luka, MS 38852-8998 
Attn: W. Armour ■ 

US Polymeric 
700 East Dyer Rd. 
Santa Ana, CA 92702 
Attn: D. Beckley 

Thiokol Corp. 

MSFC, AL 35812 

Attn: R. Bunker, Bldg 4708, Room 254 

Thiokol Corp. Wasatch Division 

Advanced Technology 

PO Box 707 

Brigham City, UT 84302-0707 

Attn: A. Canfield 

Kaiser Aerotech 
880 Doolittle Drive 
San Leandro, CA 94577 
Attn: H. O. Davis 

Thiokol Corp. 

6767 Old Madison Pike, Suite 490 

Huntsville, AL 35806 

Attn: T Day 

ICI Fiberite 

4500 South 575 East, Suite B-lOO 

Murray, UT 84107 

Attn: N. DiMeo 



niSTRIBUTION LIMITATIONS MARKED ON THE COVER/TITLE PAGE ARE AUTHORIZED BY SIGNATURE BELOW 



APPROVED BY . 
(AEROSPACE) 

APPROVED BY . 
(AF OFFICE) 



(NOT REQUIRED FOR ATR CATEGORY) 



DATE 



DATE 



r 



LIST COMPRISES TWO OR MORE SHEETS, COMPLETE ABOVE BLOCK ON LAST SHEET ONLY 



SHEET . 



kEROSPACE FORM 2380 REV 11-85 



EXTERNAL DISTRIBUTION LIST 



"^^^ CHARACTERIZATION OF LOW THERMAL CONDUCTIVITY 
PAN-BASED CARBON FIBERS 



REPORT NO, 



ATR-91(6819)-1 



MILITARY AND GOVERNMENT OFFICES 



PUBLICATION DATE 

15 March 1992 



SECURITY CLASSIFICATION 

Unclassified 



ASSOCIATE CONTRACTORS AND OTHERS 



1, SHOW FULL MAILING ADDRESS: INCLUDE ZIP CODE. MILITARY OFFICE SYMBOL. AND "ATTENTION'' LINE 

2, IF LIST IS ALTERED, INITIAL CHANGE(S) AND SHOW AFFILIATION 

Hitco 

13722 Harvard PI 
Gardena, CA 90249 
Attn: P. A. Fordham 

Mississippi State University 
Chemical Engineering 
PO Drawer CN 
Mississippi State, MS 39762 
Attn: W.Hall 

Fiberite 
501 W 3rd St. 
Winona, MN 55987 
Attn: E. Hemmelman 

Aerotherm Corp. 

1500 Perimeter Park Way, Suite 225 

Huntsville, AL 35806 

Attn: F. Strobel 

ICI Fiberite 
600 Blvd, South 104 
Huntsville, AL 35802 
Attn: J. Thomas 

Textron Specialty Materials 
2 Industrial Ave. 
Lowell, MA 01851 
Attn: S. J. Whicher 

Johns Hopkins University/CPIA 
c/o Applied Physics Laboratory 
7302 John Hopkins Road 
Laurel, MD 20723 
Attn: T Wilson 









^ 










DISTRIBUTION 


LIMITATIONS MARKED ON THE COVER/TITLE PAGE ARE AUTHORIZED BY SlUNAIUHt btLuw 

DATE . 








APPROVED BY 
(AEROSPACE) 




DATE . 








APPROVFnRY 
(AF OFFICE) 




(NOT REQUIRED FOR ATR CATEGORY) 










IF LIST 


COMPRISES TWO OR MORE SHEETS, C 


OMPLETE ABOVE BLOCK ON LAST SHEET ONLY 

,SHFFT 


5 


_0F _ 


6 



AEROSPACE FORM 2360 REV 1185 



EXTERNAL DISTRIBUTION LIST 



REPORT T,TLE CHARACTERIZATION OF LOW THERMAL CONDUCTIVITY 
PAN-BASED CARBON FIBERS 



REPORT NO 

ArR-91(6819)-l 


PUBLICATJON OATC 

15 March 1992 


SECURITY CLASSIFICATION 

Unclassified 


MILITARY AND GOVERNMENT OFFICES 


ASSOCIATE CONTRACTORS AND OTHERS 



r SHOW FULL MAILING ADDRESS: INCLUDE ZIP CODE, MILITARY OFFICE SYMBOL, AND "AnENTION" LINE 
2 IF LIST IS ALTERED. INITIAL CHANGE(S) AND SHOW AFFILIATION 



Reinhold Industries 
12827 E. Imperial Highway 
Santa Fe Springs, CA 90670 
Attn: R. Pegg 



DISTRIBUTION LIMITATIONS MARKED ON THE COVER/ TITLE PAGE ARE AUTHORIZED BY SIGNATURE BELOW 



APPROVED BY . 
(AEROSPACE) 

APPROVED BY . 
(AF OFFICE) 



rx^ ^yji 



DATE 



DATE 



^,^^,^i 



(NOT REQUIRED FOR ATR CATEGORY> 



IF LIST COMPRISES TWO OR MORE SHEETS, COMPLETE ABOVE BLOCK ON LAST SHEET ONLY 



SHEET . 



AEROSPACE FORM 23BO REV 11-85