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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

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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

fn. dj^^i^^^-^

P. M. Adams

Materials Evaluation and Survivability
Department

Ca**f^'^..

inger (/

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 ^^

B. Preferred Orientation

1 S

1. Background

2. Experimental

3. Results ^^

C. Small-Angle X-Ray Scattering 19

in
1 Background

2. Experimental

3. Results ^^

a. Qualitative Observations ^^

O 1

b. Ouantiative Measurements -'^

D. Summary ^^

III. TRANSMISSION ELECTRON MICROSCOPY 37

A. Background

B. Experimental

C. Results and Discussion

L Macroscopic Texture

a. Longitudinal

b. Transverse

PR^CE0^^K5 PAGP BLANK NOT FILMED

V ^ ^ JuMiumuai=«wJi..»»^

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 ^^

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

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

I 10

O Thermal Conductivity
• Tensile Strength

-1 700

- 600

Rayon Thermal
Conductivity

Rayon Tensile Strength

500

400

300

200

So
w

- 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

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

, FOCUS

PRIMARY BEAM

(c)

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

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

1.5

0.5

40 60

Two Theta (degrees)

(100)

40 60

Two Theta (degrees)

1000
800 U
600
400
200

(100)

(101)

''^"***'*'***>l»**«*

»»l< H *'»»»<l

(a)

100

(110)

(b)

ILi.

100

(110)

(112)

(C)

60 80

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

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

J L

4 6 8 10 12

TW THETA (d»9r««t)

2.5 ^

002

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

? "

1.5 _

1 _

0.5

002

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.

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

TWOTHETA («

0.1 _
0.05

yli'\

DG1,DG2

"^^"

70 79 80 89

1W0 THETA (dagrMt)

0.35

0.3

45 SO

TWO THETA (dagrM*)

J 0.25

i 0.2
u

* 0.15

0.1

0.05

M^ai*'''*^^^

23,25,Lri,IJ-2

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

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

(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

Heat treatment temperature,

3000 3500 4000

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

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).

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°.

=- 40

•T50

T300, Celion

•

LF2. 25X DG1

• • 23X Ro

DG2 , S2 ^

LF1

•

Preferred Orientation (FWHM) (Deg)

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

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)

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.

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

= f s2l(s) ds . (9)

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)

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.

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.

ORIGINAL P,

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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

ORIGINAL PAGE
BLACK AND V;H1TE PHOTOGRAHh

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

■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.

"R,s^

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

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

Characteristic Curve for DEF Film

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

3.5

2.5

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 _

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

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.

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

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.

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.

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

external
suiface

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

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

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

BASAL
PLANES

0.1 \im

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

BASAL
PLANES

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

0.5 nm

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

Figure 28. Sketch showing the macrotexture.

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

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

CORE

OUTER
REGION

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

CORE

OUTER
REGION

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

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

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.

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

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).

((

(a)

,.^

'( ^ )) (b)

"^ —

o s » X

(c)

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

(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.

BLACK AND WHiTE PiiOTOGRAhh

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

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

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

8LACK Ar^D V.'H;rr P''— v^^o

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

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

(a) Tilting 1 '

(b) Tilting 2°

(d) Tilting 4'"

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

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

BLACK

ORIGINAL
AND WH.TE

PAGC

PHOTOGRAPH

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

'-;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).

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).

lU.'-i^i. ■■

£ FHCTQGRAPH

• > -*'*' 'iM'&k-miu^mw^''

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

BLACK AiSO WHITE FHOTOGRAFh

100 A , k-|

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

Dl .'

■ '^rt^

HGTOGRAPH

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

BLACK AND WHiT£ f'iiOTOGRAPh

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

BLACK IK

■^ ^'^0]OGf^;APH

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

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

BLACK mu VVhlTt PHOTOGRAPH

(a) BF Image

(b) SAD Pattern

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

ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH

Figure 54. DF image of Amoco PKX) fiber.

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.

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.

Table 6. XPS Surface Composition Data

Surface mol%

Unsized PAN Fibers

Avcarb B-2

< 0.1

Avcarb G

< 0.1

Hercules R879-01

0.3

< 0.1

Hercules R879-02

0.4

DG Rayon 1

0.5

0.2

< 0.1

DG Rayon 2 #1

84
86

10
9

5
4

0.8
0.6

< 0.1
0.1

< 0.1

0.1
< 0.1

nd
nd

Amoco T350/23X #1

87
85

8
10

5
5

0.2
0.2

< 0.1

nd
nd

Amoco T350/25X
(shear treated)

< 0.1

0.1

< 0.1

Amoco T300
(not shear treated)

0.1

Amoco T50

0.2

0.1

0.2

Amoco P55 (pitch)

0.1

HOPG graphite

100

0.4

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

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

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.

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

1.8

Avcarb G

1.8

Hercules R879-01

1.8

Hercules R879-02

1.8

DG Rayon 1

1.7

DG Rayon 2 #1

71
71

5
6

1.8
1.8

Amoco T350/23X #1

75
70

16
18

5
5

2
3

1.7
1.6

Amoco T350/25X
(shear treated)

1.7

Amoco T300

1.7

Amoco T50

1,1

Amoco P55

1.1

HOPG graphite

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

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

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

BASF DG Rayon 2

0.56

1.16

1.0

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.

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

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.

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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).

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).

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
H

erculL'^T.nTL72%%7rs^.'''''' '^' ^^ ' ^"^^^^^^^ °^ ^^O^) reflect!

on for

Sample: R879-01-18

Angle Beta [deg]

♦0 50 io 70 BO SO

Angle Beta [deg]

108

Plot of fiber orientation angle (P) vs. intensity of {002} reflection for
BASF DGl and DG2 fibers.

Sample: RAYON- 1

-1 — ■ — r*-" — r

-1 — ' — I — ' — r-

-I — ' — I — ■ — 1 — ' — r-

FWHM-42*
GAU5S-Fit

-BO -80 -70 -60 -50 -40 -30 -20 -10 10 20 30 40 50 60 70 80 90

Angle Beta [deg]

Sample:

RAYON -2

1.0

— , — J...., r ' I '

D.9

FWHU-39*
CAUSS-nt

O.S

>»

0.7

O.S

13
0)

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0.5
0.4
0.3
0.2
0.1
0.0

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. . t ...J 1 . 1 . L_

. ,.1.1.1

-80 -70 -60 -50 -40 -30 -20 -10 10 20 30 40 M 60 70 60 SO

Angle Beta [deg]

109

Plot of fiber orientation angle (p) vs. intensity of {002} reflection for
Amoco 23 and 25 fibers.

-eo -70 -«0 -50 -40 -30 -20 -ID

Angle Beta [deg]

Sample: T350/25X

(.0

' 1 ' 1 ' 1 - -1 — ■ — 1 — . — 1 — . — 1 — . — , — . — 1 — , — 1 — ,

0.1

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./■ \

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D.S

■a

O.J

FWHU-38*
GAUSS-Flt

-80 -70 -M -M -40 -30 -20

10 ao 30 40 30 to 70 00 10

Angle Beta [deg]

110

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

Somple:

AVCARB B2

1.0

■ 1 • ! • 1 ■

FWHU-4B'

0.9

GAUSS-fll

fl.B

"(0

0.7

a.a

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0.4
0.3
0.2
0.1
0.0

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__. — 1 — . — 1 — ■ — 1 — i — 1 —

J I . 1 . — I i L

-90 -80 -70 -»0 -50 -40 -JO -20 -10

10 20 30 40 50 eo 70 80 90

Angle Beta [deg]

Sample; AVCARB G

-I — ' — t — • — I — ' — 1 — ■ — r-

-1 — ■ — I — ' — 1 — ' — r-

-1 — ■ — r— ' — I — ' — I — ' — 1 — ' — r-

FWMU-5J"
GAUSS-Flt

-«0 -70 -M -50 -40 -30 -20 -10 10 20 30 40 SO <0 70 SO 90

Angle Beta [deg]

111

APPENDIX C

SMALLANGLE X-RAY SCATTERING PLOTS

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118

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BF Goodrich
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General Electric
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Prototype Development Assoc, Inc.
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Fiberite
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Huntsville, AL 35806

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ICI Fiberite
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Textron Specialty Materials
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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

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

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