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Expanded pyrolytic graphite: Structural and transport properties
EXPANDED PYROLYTIC GRAPHITE: STRUCTURAL AND TRANSPORT PROPERTIES L. C. OLSEN,
S. E. SEEMAN
and H. W. SCOTT
McDonnell Douglas Astronautics Company, Richland, Wash. 99352 (Received 9 July 1969)
Abstract-A process has been developed for fabrication of a highly oriented expanded pyrolytic graphite (EPG). After forming a graphite bisulfate with pyrolytic graphite (PC), the material is heated to 500°C for several minutes to evolve acidic molecules and obtain expansion. X-ray analysis shows that the orientation density of two-fold and threefold EPG is approximately the same as that for the parent material-PC heat treated at 3100°C. Describing the orientation density by COS’%, where 6 is the polar angle of a
crystallite with respect to the deposition plane; m-values of 300-600 are obtained. Transport properties are discussed. The thermal conductivity (K), electrical conductivity a) and thermoelectric power (A) were measured in the ab- and c-directions, both as a func-
tion of expansion ratio and of temperature. Kab was found to decrease from the values characteristic of pyrolytic graphite inversely with expansion ratio. KC for two-fold and three-fold expanded material is -0.03 W/cm per “K. EPG transport properties are explained quite well by assuming a structural model consisting of a lamellar configuration involving two layers: one for which approximately 80% of the layer consists of PG and 20% void space; and the other composed of a low-density carbonaceous material which has literally been blown from the other type of layer. 1. INTRODUCTION
pounds. The thermoelectric studies were originally made by reacting selected additives with PG, but this procedure was awkward; PG compounds tended to delaminate. The use of EPG as a starting material eliminated this difficulty. Thermoelectric studies of compounds formed with EPG have been reported [4]. Because of the potential applications of EPG, an understanding of its structure and transport properties is desirable. Therefore, X-ray and transport studies were conducted. Resulting data can be interpreted quite well if EPG is considered to be a highly oriented lamellar structure with two kinds of layers: one consisting primarily of graphite crystallites (as in PC,), the other similar to a lowdensity carbon.
Graphite undergoes numerous reactions to form lamellar compounds in which the planar skeleton of carbon rings is preserved. The properties of some of these compounds have been reviewed by Ubbelohde [l] Rudorff [2] and Hennig[3]. In all cases, formation of these compounds involves intercalation of the additive atoms between the ab planes. Thus, as compound formation takes place, (or the graphite lattice tends to expand exfoliate) in the c direction. This phenomenon can be utilized to produce an expanded graphite material. The most useful approach consists of first forming a lamellar compound with pyrolytic graphite (PC), then heattreating the expanded graphitic matrix to evolve the additives and yield an expanded pyrolytic graphite (EPG). EPG was developed at the Donald W. Douglas Laboratories for various hightemperature applications and to study the thermoelectric properties of graphite com-
2. EXPANSION
PROCESS
The expansion process is similar to those described by McKay[5] and in a British Patent[6]. In principle, it is possible to use 85
86
L. C. OLSEN,
S. E. SEEMAN
several lamellar compounds to make EPG. However, it is most straightforward to use graphite bisulfate. The bisulfate is formed by subjecting graphite to an acidic mixture of H&SO, and HN03. Ubbelohde [l] indicates that the composition of graphite bisulfate is approximately C$,( HSO,)-a2 ( H2S04) and that the interlayer spacing is 7.98 A. As reported by McKay [5], the optimum mixture of sulfuric and nitric acids is three parts H$O., and one part HNOB. Water must be added to control the reaction rate. After chemical treatment, the sample is slowly heated to 500°C and kept at that temperature for several minutes. During this time, the acidic molecules are boiled off. In the heating process, the material not only expands, but carbonaceous planes rebond and integral expanded pyrolytic graphite if formed. Expansion ratios greater than 10 may be achieved; however, for applications requiring a high degree of anisotropy, ratios of less than 10 are most practical. In general, the potential shapes of EPG are limited by the configurations in which PG can be obtained. Figure 1 shows several EPG samples of different sizes and shapes. The arrow indicates the c or expansion direction of the material. All of the experimental studies reported here were made using cross-sections with rectanguiar samples measuring 4 X 4 in. or 1 X ) in. Lengths along the c-axes of these samples are related to the length of the starting PG material and the expansion ratio. All EPG samples used in these studies were heat-treated at 2000°C to evolve residual impurities. The PC was obtained from Super-Temp Corporation and had been heat-treated at 3100°C. The PG density was 2.22 g/cc. 3. X-RAY STUDIES* It is clear from Fig. 1 that the expanded material retains a high degree of orientation. *The authors wish to acknowledge the services who made all of the X-ray measurements.
of W. G. Jolley
and H. W. SCOTT
X-ray studies were performed to obtain a quantitative measure of this property. All EPG samples were found to have an X-ray diffraction pattern essentially identical to that of the parent material (PG heat-treated at 31OO”C), with respect to both location of lines and to line-intensities. To further determine the amount of preferred orientation retained in EPG, orientation density was determined for both PG and EPG. Orientation density is a measure of angular distribution of crystallite c-axes (ci) relative to the apparent c-axis of the sample. If the individual crystallites have angular coordinates, 6 and 4, with respect to the sample c-axis (as shown by the insert in Fig. 3), the number of crystallites with coordinates 6 and Cpwill, in general, depend on both of these variables. Because of the fabrication procedure used, PG samples usually have azimuthal symmetry with respect to the normal of the deposition plane (or sample c-axis). This also is the case for EPG. Thus, distribution of the crystallites depends only on 6. If the number of crystallites having c-axis lying in an interval, a%, about 6 is N(6)&, then the normalized orientation density, n(6), is given by n(S) = E.
(1)
The simplest way to obtain n(6) is to use cylindrical samples cut with the cylindrical axis parallel to the ab planes [7]. This approach is not feasible with EPG and therefore, a flat sample was used in conjunction with the experimental arrangement depicted in Fig. 2. If 1(6) is the integrated line intensity for the (002) reflection when the sample is rotated through an angle, 6, then the normalized orientation density is given by n(6’
f(6) sin 0 cos 8 = Z(0) sin (@-- 3) *
If cylindrical specimens are given directly by 1(6)/1(O).
used,
(2) n(6)
is
Fig. 1. EPG samples
of various shapes and PG specimens expanding process.
used for the
(Facingpage 86]
87
EXPANDED PYROL.YTIC GRAPHITE C-AXIS
Typical
C.
results
orientation
s
by
are
density
shown
~ lb ORIENTATIONVARIABLESOF INDIVIDUALCRYSTALLITE
INCIDENl
Fig. 2. Experimental
arrangement studies.
used for X-ray
2
8
b
4
INCLINATION
(3)
The PG used for this work is characterized by a value of m between 300 and 600. This result agrees well with Guentert and Klein’s data for PG heat-treated at 3OOO”C[8]. As shown in Fig. 3, typical samples of EPG exhibit the same high degree of orientation as the parent material. For comparison purposes, a curve for m = 20 is also shown; the corresponding orientation density is typical of as-deposited PG.
0 0
be described
PI n(6f = cos%.
6
in Fig. 3. The
can usually
ANGLE
161
Fig. 3. Normalized orientation density. of PG and EPG. (Solid curves are plots of 1209% for various vaiues of m.)
L. C. OLSEN, S. E. SEEMAN and H. W. SCOTT
88 4. THERMAL
CONDUCTIVITY
Results of thermal conductivity studies of expanded graphite materials are summarized in Figs. 4 and 5. Measurements of thermal conductivity parallel to the a6 planes, Kab, and along the c-axis, K,, were made in the conventional manner by determining the thermal gradient in the direction of interest for a given heat input. The main experimental problems in the two cases are the establishment of isothermal planes perpendicular to the a6 planes in the u-axis measurement, and the reduction of heat loss in the c-axis measurement. Consistent results were obtained for a6 studies by using a silicone heat-sink compound at the heater-sample and sample-sink junctions. The c-axis measurements were successfully made using carbon black for insulation. The a6 properties for various expansion ratios (the ratio of EPG length to PG length) as a function of temperature are shown in Fig. 4. Included are data taken for PG.
15.01
T
.
PG
a 0
EPG 68-30X=2.0 EPG 66-11 X=3.1
.s-. --. .
.
0
x=3.1
330
360
T?KI
390
420
Fig. 4. Thermal conductivity along a-axis for PC and samples of EPG.
PO
3-o
EXPANSION RATIO
Fig. 5. c-Axis thermal conductivity vs. expansion
ratio at 350°K.
Although the temperature range is small, it appears that Kab m l/T for all EPG samples. This temperature dependence is, of course, the same as observed for PG in this temperature range[9]. The dominant mechanism of thermal resistance for crystalline graphite and PG is that of phonon-phonon scattering. Thus, thermal conduction along the a6 planes in the expanded material seems to be characterized by the same kind of scattering processes as in the parent material, PG. In general, thermal conduction for the more defective graphites will be reduced in magnitude relative to PC and will exhibit a temperature dependence of Tn with -1 =S n < 2. Because of the observed temperature dependence of Kab, it appears that highly oriented graphite crystallites are responsible for a6 conduction in EPG. The reduction in magnitude is primarily a result of lower density. Figure 5 gives the dependence of K, on the expansion ratio; the solid curves are determined using an idealized theoretical model. Figure 6 shows this idealized model by which the thermal and electrical transport properties of EPG can be explained. Basically, the expansion process consists of two steps: (1) chemical treatment during which a PG
I
d+ 1 A
x=2.0
-1.0
EXPANDED
PYROLYTIC
PG
v CHEMICAL
TREATMENT
w
GRAPHITE BISIJLFATE
w
89
GRAPHITE
it is assumed that this low-density material similar to those of soft has properties carbon which has been heat-treated at = llOO”C, then the thermal and electrical properties of EPG can be understood. Soft carbon which has been heat-treated at 1100°C consists of crystallites with dimension in the ah direction of = 30 A; PG heat-treated at 3000°C is characterized by crystallites with ah dimensions of = 8000 A. To derive expressions for the thermal transport properties of this system, the following terms are defined:
HEATTREATMENT KJCT,)
=
(electrical)
conduc-
u-axis thermal tivity of PG;
(electrical)
conduc-
K,~((T,*)
=
K,((T,)
=
K,(&)
= bulk thermal (electrical) tivity of EPG parallel expansion axis;
EPG
Fig. 6. Model proposed for expansion process.
sample is delaminated into a large number of slabs consisting of PG and regions of graphite bisulfate; and (2) heat treatment during which acidic molecules are evolved. Figure 6 illustrates the partial bisulfate reaction that is expected to exist in one of the slabs after chemical treatment. Because acidic molecules probably enter the graphite crystallites along the edges, it is reasonable to assume that pockets of bisulfate are formed near crystallite boundaries as shown. During heat treatment, it is assumed that these pockets essentially explode and leave a void region. It is further postulated that the acidic molecules evolve and leave a low-density, isotropic, carbonaceous material between the planar arrays of PG crystallites and the voids. The carbonaceous material should consist of crystallites much smaller than those characterizing PG and therefore could be expected to have properties resembling those of lower-grade carbons. In fact, if
c-axis thermal tivity of PG;
thermal (electrical) of the ‘unknown’ material;
conductivity carbonaceous
Kab(&,) = bulk thermal (electrical) tivity perpendicular expansion axis;
conducto the conducthe
t.0
&Ii= ab dimensions of crystallite in EPG;
ith
graphite
l,i = C dimension of crystallite in EPG.
ith
graphite
(Note that the terms ‘c-axis’ and ‘expansion axis’ are used interchangeably when discussing EPG properties.) Consider an EPG sample with a rectangular cross-section and let the dimensions perpendicular to the c-axis be L, and L,, where the x- and y-axes have been chosen parallel to the sides of this rectangular cross-section. The following parameters may then be defined:
L. C. OLSEN, S. E. SEEMAN and H. W. SCOTT
90
EC=
@
along c-axis.
( > L
L, is the length of the sample in the c-direction. Thus, the effective cross-sectional area for thermal conduction via graphite crystallites along the x-axis is l&,c&&. Along the y-axis, the effective area is E,~~,L,L,. For thermal conduction in the x- or y-directions (layers involving crystallites and voids and layers of the ‘unknown’ material) will be in parallel. If Kabis defined in the usual manner, Xab =
(~&ab)~ab+
(1
-~ch
(4)
In the case of conduction processes parallel to the c-axis, the two types of layers are in series
If the expansion
ration is defined as
and K, = 0.065 (at T = 350”K), and for various choices of the ratio, K,/K,. For calculations of thermoelectric power in the next section, K,/K, will be set = 2.8. Thus it appears that K, = 0.02 W/cm per “K. This is not an unreasonable value for a porous carbon material comprised of small particles (230 8, in size) [lo] as the ‘unknown’ material is expected to be. 5. THERMOELECTRIC PROPERTIES The electrical conductivity and thermoelectric power of EPG have been extensively studied, with particular emphasis on the c-axis properties. As in the case of thermal measurements, the experimental techniques are fairly straightforward and will not be discussed in detail. The most difficult measurement is that of determining the a-axis electrical conductivity. Rather limited u-axis conductivity studies were accomplished by brazing copper plates across the sample ends (perpendicular to the ab planes) for current electrodes, and by using razor blades for potential probes. Figures 7 through 10 show the experimental results for electrical conductivity
X = L&0, where L,, is the original PG c-axis length, then cc = l/X. Thus, EPG thermal conductivities should be given by
Kc= KU+
X(%zb)2Wc
(x-
1) (E,b)2Kc’
0 A
EPG 67-90 X=2.0 EPG 67-67 X-2.8
(6)
At this point, two quantities, cab and K~, are unknown. It is reasonable to assume that In fact, the approximation, &b = KU 4 K&. e&&X, fits the experimental data fairly well if cob is set = 0.9 and the experimental curve in Fig. 4 for K&(T) is used. The solid lines in Fig. 4 are determined in this way. The solid-line curves of Fig. 5 show the theoretical prediction for Kc when ab = 0.9.
O_ PO Fig.
0
2.0 EXPANDED
RATIO
X
7. Electrical conductivity along a-axis EPG as a function of expansion ratio.
for
EXPANDED 25.0 ,
I
I
I
PYROLYTI(:
I
(:RAPHITE
91
parallel to the ab planes (&J and parallel to the c-direction (22,); and the thermoelectric power in the same directions (ArLband A,.) for samples of EPG of different expansion ratios (X). In all cases, the solid lines are theoretically predicted as discussed below. Using the model described in the last the thermoelectric properties of section, EPG are given by
I
OlEPG 67-89 X=1.8 VEPG 67-84 X=2.7 AEPG 67.lWX=2.8
I
ot 300
I
I
I
500
400
600
700
800
TEMPERATURE l°Kl Fig.
8.
c-Axis electrical conductivity vs. temperature for various EPG samples.
=%bGb%b+
A
ab -30.0 / s
/\/
-20.0
AN /
A./A
A, =
y-0
"2 1
(x--l)~tL%d (x- 1Ia,
K,&+ (Eabj2 (x-l)a, Kc z+
(X-
I) (c,,,)’
(7) ’
A/ -10.0
a PG 0 EPG 68-30X-2.0 A EPG 67-69X=2.8
OP
@'
Yoo
400
500
600
700
Tl'Ki
Fig.
%zb cab+
9. Thermoelectric power parallel to u-axis vs. temperature for PG and EPG.
the
TEMPERATURE IOK)
Fig. 10. c-Axis thermoelectric power vs. temperature for PG and EPG.
The thermoelectric power of the ‘unknown’ material is denoted as CY~,while CY, and (Y& denote the thermoelectric power in the c and ab directions for PG. Properties of various carbons differ so extensively that it is difficult to make an estimate of CY, and cru, and then test the theoretical model against the experimental results. Instead, the data is analyzed by first assuming that KJK, = 2-8 and E,,, = 0.90 and then determining if these data can be fit with the assumption of reasonable values for (Y, and CT,. The theoretical model allows for a good description of EC and &,, if it is assumed that cc e cr, < flab. Results for A, and &I, are also described quite well if CY, is assumed to be independent of X and behaves with temperature as shown in Fig. IO. To calculate EC and Eat,, the values used for V, and (Tabare, of course, those appropriate for PG as shown in Figs. 7 and 8. The results for PG seem to be in good agreement with results recorded by previous workers [I I]. Assumed values for (Y, are in reasonable
L. C. OLSEN,
92
agreement carbons[l2].
with
results
obtained
S. E. SEEMAN
for
soft
6. SUMMARY AND CONCLUSIONS An approach for fabricating expanded graphitic materials that retain a high degree of anisotropy has been discussed. The appropriate structural model for EPG seems to be that described in Fig. 6, namely, a lamellar configuration consisting of two kinds of layers: one for which approximately 80 per cent of the layer consists of PG and 20 per cent void space (implied by l& = O-g), the other composed of a low-density carbonaceous material which has literally been blown from the other type of layer. Although the properties of the low-density material undoubtedly vary with density (and thus with expansion ratio X), it appears that the thermal and electrical properties are of the order of uu = 0.02 W/cm per “K (Y, = - ~O/.LV/“K 5R-’
cm-l 4 uU e 2 X 104fi-’
cm-‘.
These values resemble those obtained for low-grade carbons [lo, 121 which are composed of particles on the order of 3OA. Thus, results of X-ray studies, the study of thermal and electrical properties, and theoretical interpretation of transport properties of EPG tend to indicate that during the expansion process: (1) regions of graphite bisulfate are formed around the boundaries of and (2) these regions then crystallites, essentially explode during heat treatment, resulting in a highly orientated lamellar structure. The proposed analytical model seems to be quite adequate. There are certainly other possible models to be considered for explaining the transport data for EPG. The most obvious one would involve ‘path considerations-that is, contortuosity’ sideration of the effects of misorientated crystallites on transport properties. In
and H. W. SCOTT
particular, contributions to c-axis properties by transport along ab planes must be considered. It is generally accepted that c-axis measurements on highly oriented pyrolytic graphite materials reflect true c-axis properties of graphite crystallites. X-ray analyses indicate that EPG contains a large amount of PG, and that the PG portion of the system is characterized by the same orientation density as the parent material. Thus, there is apparently no significant change in the degree of misorientation of graphite crystallites between PG and EPG. In addition to the X-ray analysis, the c-axis electrical conductivity data also provides regarding the ‘path negative evidence model. The ab electrical contortuosity’ ductivity of the PG is essentially constant from 300” to 800°K. Thus, if ab crystallite conduction accounted for a significant portion of the c-axis conduction in EPG, 2, would show less temperature dependence than crc. Instead, Z, increases with temperature at a greater rate than uc. It therefore appears that explanation of EPG transport properties requires the postulation of a third material component. The simplest model involving such an assumption is the one used here. This model is also quite compatible with the expansion process. Some comments here regarding possible impurity content are appropriate. A relatively small concentration of foreign atoms has a large effect on the thermoelectric power of graphite materials. Thermoelectric power was measured for EPG samples after heat treatment at progressively higher temperatures. Very small changes in the thermoelectric power were found after the heat treating temperature was raised above 1400°C. Thus, it is felt that materials heat treated at insignificant 2000°C have an impurity content. In addition, it was found that f&b agrees very well with ff&; if the impurity content was significant, such agreement would not be expected. In general, EPG should be useful for appli-
EXPANDED
PYKOLYTIC
cations requiring extremely anistropic thermal or electrical properties and hightemperature stability. EPG may also be useful for high-temperature thermal insulation, particularly where some degree of structural strength or an unusual configuration is required. EPG was developed primarily for investigating the thermoelectric properties of graphite compounds; the expanded material has served very well for this purpose. Graphite compounds can be formed by reacting EPG with the desired additive without delamination occurring which often happens when PG is used for a starting material. 1. Ubbelohde
REFERENCES A. R. and Lewis F. A., Gru@ite
2. 3. 4.
5. 6. 7. 8. 9. 10. 11. 12.
GKAPHITE
93
and its Crystal Compounds, Chap. 17. Oxford University Press (1960). Rudorff W., Advan. Znorg. Chem. 1, 223 (1959). Hennig G. R., Prog. Znorg. Gem. 1, 125 (1959). Olsen, L. C., Bull. Am. Phys. Sot., Series II, 13, 1644 (1968). Seeman S. E. and Olsen L. C., Bull. Am. Phys. SOL., Series 11, 13, 1644 (1968). McKay S. F., J. Appl. Phys. 35, 1992 (1964). British Patent 991, 581 (Cl.Colb), May 12, 1965. Guentert 0. J., J. Chem. Phys. 3’7, 884 (1962). Guentert 0. J. and Klein C. A., A#. Phys. Letters 2, 125 (1963). Hooker C. N., et al., Proc. Roy. Sot. A284, 17 (1964). Jamieson C. P. and Mrozowski S., Proc. 2nd Carbon ConJ, Univ. of Buffalo, p. 155 (1956). Klein C. A., Kev. Mod. Phys. 34,56 (1962). Pinnick H. T., Proc. 1st and 2n,d Carbon Conj., Univ. of Buffalo, p. 3 (1954).
S. E. SEEMAN
and H. W. SCOTT
McDonnell Douglas Astronautics Company, Richland, Wash. 99352 (Received 9 July 1969)
Abstract-A process has been developed for fabrication of a highly oriented expanded pyrolytic graphite (EPG). After forming a graphite bisulfate with pyrolytic graphite (PC), the material is heated to 500°C for several minutes to evolve acidic molecules and obtain expansion. X-ray analysis shows that the orientation density of two-fold and threefold EPG is approximately the same as that for the parent material-PC heat treated at 3100°C. Describing the orientation density by COS’%, where 6 is the polar angle of a
crystallite with respect to the deposition plane; m-values of 300-600 are obtained. Transport properties are discussed. The thermal conductivity (K), electrical conductivity a) and thermoelectric power (A) were measured in the ab- and c-directions, both as a func-
tion of expansion ratio and of temperature. Kab was found to decrease from the values characteristic of pyrolytic graphite inversely with expansion ratio. KC for two-fold and three-fold expanded material is -0.03 W/cm per “K. EPG transport properties are explained quite well by assuming a structural model consisting of a lamellar configuration involving two layers: one for which approximately 80% of the layer consists of PG and 20% void space; and the other composed of a low-density carbonaceous material which has literally been blown from the other type of layer. 1. INTRODUCTION
pounds. The thermoelectric studies were originally made by reacting selected additives with PG, but this procedure was awkward; PG compounds tended to delaminate. The use of EPG as a starting material eliminated this difficulty. Thermoelectric studies of compounds formed with EPG have been reported [4]. Because of the potential applications of EPG, an understanding of its structure and transport properties is desirable. Therefore, X-ray and transport studies were conducted. Resulting data can be interpreted quite well if EPG is considered to be a highly oriented lamellar structure with two kinds of layers: one consisting primarily of graphite crystallites (as in PC,), the other similar to a lowdensity carbon.
Graphite undergoes numerous reactions to form lamellar compounds in which the planar skeleton of carbon rings is preserved. The properties of some of these compounds have been reviewed by Ubbelohde [l] Rudorff [2] and Hennig[3]. In all cases, formation of these compounds involves intercalation of the additive atoms between the ab planes. Thus, as compound formation takes place, (or the graphite lattice tends to expand exfoliate) in the c direction. This phenomenon can be utilized to produce an expanded graphite material. The most useful approach consists of first forming a lamellar compound with pyrolytic graphite (PC), then heattreating the expanded graphitic matrix to evolve the additives and yield an expanded pyrolytic graphite (EPG). EPG was developed at the Donald W. Douglas Laboratories for various hightemperature applications and to study the thermoelectric properties of graphite com-
2. EXPANSION
PROCESS
The expansion process is similar to those described by McKay[5] and in a British Patent[6]. In principle, it is possible to use 85
86
L. C. OLSEN,
S. E. SEEMAN
several lamellar compounds to make EPG. However, it is most straightforward to use graphite bisulfate. The bisulfate is formed by subjecting graphite to an acidic mixture of H&SO, and HN03. Ubbelohde [l] indicates that the composition of graphite bisulfate is approximately C$,( HSO,)-a2 ( H2S04) and that the interlayer spacing is 7.98 A. As reported by McKay [5], the optimum mixture of sulfuric and nitric acids is three parts H$O., and one part HNOB. Water must be added to control the reaction rate. After chemical treatment, the sample is slowly heated to 500°C and kept at that temperature for several minutes. During this time, the acidic molecules are boiled off. In the heating process, the material not only expands, but carbonaceous planes rebond and integral expanded pyrolytic graphite if formed. Expansion ratios greater than 10 may be achieved; however, for applications requiring a high degree of anisotropy, ratios of less than 10 are most practical. In general, the potential shapes of EPG are limited by the configurations in which PG can be obtained. Figure 1 shows several EPG samples of different sizes and shapes. The arrow indicates the c or expansion direction of the material. All of the experimental studies reported here were made using cross-sections with rectanguiar samples measuring 4 X 4 in. or 1 X ) in. Lengths along the c-axes of these samples are related to the length of the starting PG material and the expansion ratio. All EPG samples used in these studies were heat-treated at 2000°C to evolve residual impurities. The PC was obtained from Super-Temp Corporation and had been heat-treated at 3100°C. The PG density was 2.22 g/cc. 3. X-RAY STUDIES* It is clear from Fig. 1 that the expanded material retains a high degree of orientation. *The authors wish to acknowledge the services who made all of the X-ray measurements.
of W. G. Jolley
and H. W. SCOTT
X-ray studies were performed to obtain a quantitative measure of this property. All EPG samples were found to have an X-ray diffraction pattern essentially identical to that of the parent material (PG heat-treated at 31OO”C), with respect to both location of lines and to line-intensities. To further determine the amount of preferred orientation retained in EPG, orientation density was determined for both PG and EPG. Orientation density is a measure of angular distribution of crystallite c-axes (ci) relative to the apparent c-axis of the sample. If the individual crystallites have angular coordinates, 6 and 4, with respect to the sample c-axis (as shown by the insert in Fig. 3), the number of crystallites with coordinates 6 and Cpwill, in general, depend on both of these variables. Because of the fabrication procedure used, PG samples usually have azimuthal symmetry with respect to the normal of the deposition plane (or sample c-axis). This also is the case for EPG. Thus, distribution of the crystallites depends only on 6. If the number of crystallites having c-axis lying in an interval, a%, about 6 is N(6)&, then the normalized orientation density, n(6), is given by n(S) = E.
(1)
The simplest way to obtain n(6) is to use cylindrical samples cut with the cylindrical axis parallel to the ab planes [7]. This approach is not feasible with EPG and therefore, a flat sample was used in conjunction with the experimental arrangement depicted in Fig. 2. If 1(6) is the integrated line intensity for the (002) reflection when the sample is rotated through an angle, 6, then the normalized orientation density is given by n(6’
f(6) sin 0 cos 8 = Z(0) sin (@-- 3) *
If cylindrical specimens are given directly by 1(6)/1(O).
used,
(2) n(6)
is
Fig. 1. EPG samples
of various shapes and PG specimens expanding process.
used for the
(Facingpage 86]
87
EXPANDED PYROL.YTIC GRAPHITE C-AXIS
Typical
C.
results
orientation
s
by
are
density
shown
~ lb ORIENTATIONVARIABLESOF INDIVIDUALCRYSTALLITE
INCIDENl
Fig. 2. Experimental
arrangement studies.
used for X-ray
2
8
b
4
INCLINATION
(3)
The PG used for this work is characterized by a value of m between 300 and 600. This result agrees well with Guentert and Klein’s data for PG heat-treated at 3OOO”C[8]. As shown in Fig. 3, typical samples of EPG exhibit the same high degree of orientation as the parent material. For comparison purposes, a curve for m = 20 is also shown; the corresponding orientation density is typical of as-deposited PG.
0 0
be described
PI n(6f = cos%.
6
in Fig. 3. The
can usually
ANGLE
161
Fig. 3. Normalized orientation density. of PG and EPG. (Solid curves are plots of 1209% for various vaiues of m.)
L. C. OLSEN, S. E. SEEMAN and H. W. SCOTT
88 4. THERMAL
CONDUCTIVITY
Results of thermal conductivity studies of expanded graphite materials are summarized in Figs. 4 and 5. Measurements of thermal conductivity parallel to the a6 planes, Kab, and along the c-axis, K,, were made in the conventional manner by determining the thermal gradient in the direction of interest for a given heat input. The main experimental problems in the two cases are the establishment of isothermal planes perpendicular to the a6 planes in the u-axis measurement, and the reduction of heat loss in the c-axis measurement. Consistent results were obtained for a6 studies by using a silicone heat-sink compound at the heater-sample and sample-sink junctions. The c-axis measurements were successfully made using carbon black for insulation. The a6 properties for various expansion ratios (the ratio of EPG length to PG length) as a function of temperature are shown in Fig. 4. Included are data taken for PG.
15.01
T
.
PG
a 0
EPG 68-30X=2.0 EPG 66-11 X=3.1
.s-. --. .
.
0
x=3.1
330
360
T?KI
390
420
Fig. 4. Thermal conductivity along a-axis for PC and samples of EPG.
PO
3-o
EXPANSION RATIO
Fig. 5. c-Axis thermal conductivity vs. expansion
ratio at 350°K.
Although the temperature range is small, it appears that Kab m l/T for all EPG samples. This temperature dependence is, of course, the same as observed for PG in this temperature range[9]. The dominant mechanism of thermal resistance for crystalline graphite and PG is that of phonon-phonon scattering. Thus, thermal conduction along the a6 planes in the expanded material seems to be characterized by the same kind of scattering processes as in the parent material, PG. In general, thermal conduction for the more defective graphites will be reduced in magnitude relative to PC and will exhibit a temperature dependence of Tn with -1 =S n < 2. Because of the observed temperature dependence of Kab, it appears that highly oriented graphite crystallites are responsible for a6 conduction in EPG. The reduction in magnitude is primarily a result of lower density. Figure 5 gives the dependence of K, on the expansion ratio; the solid curves are determined using an idealized theoretical model. Figure 6 shows this idealized model by which the thermal and electrical transport properties of EPG can be explained. Basically, the expansion process consists of two steps: (1) chemical treatment during which a PG
I
d+ 1 A
x=2.0
-1.0
EXPANDED
PYROLYTIC
PG
v CHEMICAL
TREATMENT
w
GRAPHITE BISIJLFATE
w
89
GRAPHITE
it is assumed that this low-density material similar to those of soft has properties carbon which has been heat-treated at = llOO”C, then the thermal and electrical properties of EPG can be understood. Soft carbon which has been heat-treated at 1100°C consists of crystallites with dimension in the ah direction of = 30 A; PG heat-treated at 3000°C is characterized by crystallites with ah dimensions of = 8000 A. To derive expressions for the thermal transport properties of this system, the following terms are defined:
HEATTREATMENT KJCT,)
=
(electrical)
conduc-
u-axis thermal tivity of PG;
(electrical)
conduc-
K,~((T,*)
=
K,((T,)
=
K,(&)
= bulk thermal (electrical) tivity of EPG parallel expansion axis;
EPG
Fig. 6. Model proposed for expansion process.
sample is delaminated into a large number of slabs consisting of PG and regions of graphite bisulfate; and (2) heat treatment during which acidic molecules are evolved. Figure 6 illustrates the partial bisulfate reaction that is expected to exist in one of the slabs after chemical treatment. Because acidic molecules probably enter the graphite crystallites along the edges, it is reasonable to assume that pockets of bisulfate are formed near crystallite boundaries as shown. During heat treatment, it is assumed that these pockets essentially explode and leave a void region. It is further postulated that the acidic molecules evolve and leave a low-density, isotropic, carbonaceous material between the planar arrays of PG crystallites and the voids. The carbonaceous material should consist of crystallites much smaller than those characterizing PG and therefore could be expected to have properties resembling those of lower-grade carbons. In fact, if
c-axis thermal tivity of PG;
thermal (electrical) of the ‘unknown’ material;
conductivity carbonaceous
Kab(&,) = bulk thermal (electrical) tivity perpendicular expansion axis;
conducto the conducthe
t.0
&Ii= ab dimensions of crystallite in EPG;
ith
graphite
l,i = C dimension of crystallite in EPG.
ith
graphite
(Note that the terms ‘c-axis’ and ‘expansion axis’ are used interchangeably when discussing EPG properties.) Consider an EPG sample with a rectangular cross-section and let the dimensions perpendicular to the c-axis be L, and L,, where the x- and y-axes have been chosen parallel to the sides of this rectangular cross-section. The following parameters may then be defined:
L. C. OLSEN, S. E. SEEMAN and H. W. SCOTT
90
EC=
@
along c-axis.
( > L
L, is the length of the sample in the c-direction. Thus, the effective cross-sectional area for thermal conduction via graphite crystallites along the x-axis is l&,c&&. Along the y-axis, the effective area is E,~~,L,L,. For thermal conduction in the x- or y-directions (layers involving crystallites and voids and layers of the ‘unknown’ material) will be in parallel. If Kabis defined in the usual manner, Xab =
(~&ab)~ab+
(1
-~ch
(4)
In the case of conduction processes parallel to the c-axis, the two types of layers are in series
If the expansion
ration is defined as
and K, = 0.065 (at T = 350”K), and for various choices of the ratio, K,/K,. For calculations of thermoelectric power in the next section, K,/K, will be set = 2.8. Thus it appears that K, = 0.02 W/cm per “K. This is not an unreasonable value for a porous carbon material comprised of small particles (230 8, in size) [lo] as the ‘unknown’ material is expected to be. 5. THERMOELECTRIC PROPERTIES The electrical conductivity and thermoelectric power of EPG have been extensively studied, with particular emphasis on the c-axis properties. As in the case of thermal measurements, the experimental techniques are fairly straightforward and will not be discussed in detail. The most difficult measurement is that of determining the a-axis electrical conductivity. Rather limited u-axis conductivity studies were accomplished by brazing copper plates across the sample ends (perpendicular to the ab planes) for current electrodes, and by using razor blades for potential probes. Figures 7 through 10 show the experimental results for electrical conductivity
X = L&0, where L,, is the original PG c-axis length, then cc = l/X. Thus, EPG thermal conductivities should be given by
Kc= KU+
X(%zb)2Wc
(x-
1) (E,b)2Kc’
0 A
EPG 67-90 X=2.0 EPG 67-67 X-2.8
(6)
At this point, two quantities, cab and K~, are unknown. It is reasonable to assume that In fact, the approximation, &b = KU 4 K&. e&&X, fits the experimental data fairly well if cob is set = 0.9 and the experimental curve in Fig. 4 for K&(T) is used. The solid lines in Fig. 4 are determined in this way. The solid-line curves of Fig. 5 show the theoretical prediction for Kc when ab = 0.9.
O_ PO Fig.
0
2.0 EXPANDED
RATIO
X
7. Electrical conductivity along a-axis EPG as a function of expansion ratio.
for
EXPANDED 25.0 ,
I
I
I
PYROLYTI(:
I
(:RAPHITE
91
parallel to the ab planes (&J and parallel to the c-direction (22,); and the thermoelectric power in the same directions (ArLband A,.) for samples of EPG of different expansion ratios (X). In all cases, the solid lines are theoretically predicted as discussed below. Using the model described in the last the thermoelectric properties of section, EPG are given by
I
OlEPG 67-89 X=1.8 VEPG 67-84 X=2.7 AEPG 67.lWX=2.8
I
ot 300
I
I
I
500
400
600
700
800
TEMPERATURE l°Kl Fig.
8.
c-Axis electrical conductivity vs. temperature for various EPG samples.
=%bGb%b+
A
ab -30.0 / s
/\/
-20.0
AN /
A./A
A, =
y-0
"2 1
(x--l)~tL%d (x- 1Ia,
K,&+ (Eabj2 (x-l)a, Kc z+
(X-
I) (c,,,)’
(7) ’
A/ -10.0
a PG 0 EPG 68-30X-2.0 A EPG 67-69X=2.8
OP
@'
Yoo
400
500
600
700
Tl'Ki
Fig.
%zb cab+
9. Thermoelectric power parallel to u-axis vs. temperature for PG and EPG.
the
TEMPERATURE IOK)
Fig. 10. c-Axis thermoelectric power vs. temperature for PG and EPG.
The thermoelectric power of the ‘unknown’ material is denoted as CY~,while CY, and (Y& denote the thermoelectric power in the c and ab directions for PG. Properties of various carbons differ so extensively that it is difficult to make an estimate of CY, and cru, and then test the theoretical model against the experimental results. Instead, the data is analyzed by first assuming that KJK, = 2-8 and E,,, = 0.90 and then determining if these data can be fit with the assumption of reasonable values for (Y, and CT,. The theoretical model allows for a good description of EC and &,, if it is assumed that cc e cr, < flab. Results for A, and &I, are also described quite well if CY, is assumed to be independent of X and behaves with temperature as shown in Fig. IO. To calculate EC and Eat,, the values used for V, and (Tabare, of course, those appropriate for PG as shown in Figs. 7 and 8. The results for PG seem to be in good agreement with results recorded by previous workers [I I]. Assumed values for (Y, are in reasonable
L. C. OLSEN,
92
agreement carbons[l2].
with
results
obtained
S. E. SEEMAN
for
soft
6. SUMMARY AND CONCLUSIONS An approach for fabricating expanded graphitic materials that retain a high degree of anisotropy has been discussed. The appropriate structural model for EPG seems to be that described in Fig. 6, namely, a lamellar configuration consisting of two kinds of layers: one for which approximately 80 per cent of the layer consists of PG and 20 per cent void space (implied by l& = O-g), the other composed of a low-density carbonaceous material which has literally been blown from the other type of layer. Although the properties of the low-density material undoubtedly vary with density (and thus with expansion ratio X), it appears that the thermal and electrical properties are of the order of uu = 0.02 W/cm per “K (Y, = - ~O/.LV/“K 5R-’
cm-l 4 uU e 2 X 104fi-’
cm-‘.
These values resemble those obtained for low-grade carbons [lo, 121 which are composed of particles on the order of 3OA. Thus, results of X-ray studies, the study of thermal and electrical properties, and theoretical interpretation of transport properties of EPG tend to indicate that during the expansion process: (1) regions of graphite bisulfate are formed around the boundaries of and (2) these regions then crystallites, essentially explode during heat treatment, resulting in a highly orientated lamellar structure. The proposed analytical model seems to be quite adequate. There are certainly other possible models to be considered for explaining the transport data for EPG. The most obvious one would involve ‘path considerations-that is, contortuosity’ sideration of the effects of misorientated crystallites on transport properties. In
and H. W. SCOTT
particular, contributions to c-axis properties by transport along ab planes must be considered. It is generally accepted that c-axis measurements on highly oriented pyrolytic graphite materials reflect true c-axis properties of graphite crystallites. X-ray analyses indicate that EPG contains a large amount of PG, and that the PG portion of the system is characterized by the same orientation density as the parent material. Thus, there is apparently no significant change in the degree of misorientation of graphite crystallites between PG and EPG. In addition to the X-ray analysis, the c-axis electrical conductivity data also provides regarding the ‘path negative evidence model. The ab electrical contortuosity’ ductivity of the PG is essentially constant from 300” to 800°K. Thus, if ab crystallite conduction accounted for a significant portion of the c-axis conduction in EPG, 2, would show less temperature dependence than crc. Instead, Z, increases with temperature at a greater rate than uc. It therefore appears that explanation of EPG transport properties requires the postulation of a third material component. The simplest model involving such an assumption is the one used here. This model is also quite compatible with the expansion process. Some comments here regarding possible impurity content are appropriate. A relatively small concentration of foreign atoms has a large effect on the thermoelectric power of graphite materials. Thermoelectric power was measured for EPG samples after heat treatment at progressively higher temperatures. Very small changes in the thermoelectric power were found after the heat treating temperature was raised above 1400°C. Thus, it is felt that materials heat treated at insignificant 2000°C have an impurity content. In addition, it was found that f&b agrees very well with ff&; if the impurity content was significant, such agreement would not be expected. In general, EPG should be useful for appli-
EXPANDED
PYKOLYTIC
cations requiring extremely anistropic thermal or electrical properties and hightemperature stability. EPG may also be useful for high-temperature thermal insulation, particularly where some degree of structural strength or an unusual configuration is required. EPG was developed primarily for investigating the thermoelectric properties of graphite compounds; the expanded material has served very well for this purpose. Graphite compounds can be formed by reacting EPG with the desired additive without delamination occurring which often happens when PG is used for a starting material. 1. Ubbelohde
REFERENCES A. R. and Lewis F. A., Gru@ite
2. 3. 4.
5. 6. 7. 8. 9. 10. 11. 12.
GKAPHITE
93
and its Crystal Compounds, Chap. 17. Oxford University Press (1960). Rudorff W., Advan. Znorg. Chem. 1, 223 (1959). Hennig G. R., Prog. Znorg. Gem. 1, 125 (1959). Olsen, L. C., Bull. Am. Phys. Sot., Series II, 13, 1644 (1968). Seeman S. E. and Olsen L. C., Bull. Am. Phys. SOL., Series 11, 13, 1644 (1968). McKay S. F., J. Appl. Phys. 35, 1992 (1964). British Patent 991, 581 (Cl.Colb), May 12, 1965. Guentert 0. J., J. Chem. Phys. 3’7, 884 (1962). Guentert 0. J. and Klein C. A., A#. Phys. Letters 2, 125 (1963). Hooker C. N., et al., Proc. Roy. Sot. A284, 17 (1964). Jamieson C. P. and Mrozowski S., Proc. 2nd Carbon ConJ, Univ. of Buffalo, p. 155 (1956). Klein C. A., Kev. Mod. Phys. 34,56 (1962). Pinnick H. T., Proc. 1st and 2n,d Carbon Conj., Univ. of Buffalo, p. 3 (1954).