- Email: [email protected]
Formation of highly oriented layers of graphite in glass-like carbon heat-treated under pressure
FORMATION OF HIGHLY ORIENTED LAYERS OF GRAPHITE IN GLASS-LIKE CARBON HEAT-TREATED UNDER PRESSURE K. KAMIYA Department of Industrial Chemistry, Faculty of Engineering, Mie University, Tsu, Mie, 514,Japan and
M. INAGAKI Toyohashi University of Technology, Materials Science Tempaku-cho, Toyohashi, 440, Japan (Received 6 March 1980) Abstract-A glass-like carbon of homogeneous structure cured up to ItXWC (GC-IO) was heat-treated under pressure of 5 kbar. The heat ~eatment was performed in two different cell a~angements; in one a~angement quasi-hydrostatic pressure was dominant and in the other there were many contact points between the carbon investigated and small angular grains of glass-like carbon heat-treated to 2080°C. In the first arrangement, the glass-like carbon did not graphitize even at 1900°C.In the second, graphitization was observed above 1750°C. Below 17WC, optically anisotropic areas were initiated at contact points with the angular $rains where the stress concentrations occur. These areas show turbostratic structure. At a little higher temperature, they transform to graphite. The graphitized parts in cross section look like bamboo leaves and grow into the bulk of the carbon at the expense of non~aphitized parts. The graphite layers were found to align ~~endicularly to the compressive stress, contrary to our previous report. The previously observed o~entation was probably due to oriented regions originally present in the starting material. The mechanism of the spreading of the graphitization from the stress concentration points at the surface is discussed.
of oriented carbon layers around pores,, which easily graphitize by heat-treatment at high temperature under normal pressure[l3]. In ~aphitization of non-~aphitizing carbons under pressure, the presence of these originally oriented parts of carbon layers make it difficult to decide whether the graphite layer planes are newly formed by the heat-treatment under pressure or not. Therefore, it became necessary to reinvestigate the aligment of graphite Iayers with respect to the direction of compressive stress. In order to avoid the difficulty mentioned above, in the present work, a block of glass-like carbon with homogeneous structure was used as a starting material. The carbon was heat-treated under 5 kbar, and the development of graphite structure was pursued by X-ray diffraction and optical microscopy using polarized light.
l.INTRODUCTION
The influence of pressure on graphitization of carbons was investigated by the present authors and the results obtained are summarized as follows; (1) The externally applied pressure above 3 kbar accelerates markedly the graphitization of carbons [ 1,2], with non-graphitizing carbons such as phenolformaldehyde resin char being drastically transformed into graphite at relatively low temperature of 15~16~C under a pressure of 5 kbar [3]. (2) High bulk density and various degrees of preferred orientation of crystallites are attained by selecting suitable starting materials of different origin, of different shape and size, and also different thermal histories [4,5]. (3) The graphitization is initiated at the contact points between carbon grains where the applied stress is concentrated and spreads into the remaining non~aphitized regions [6,7]. The aligment of graphite layers formed at the contact points between grains was concluded from the observations under polarized light to be parallel to the compressive stress [7]. However, graphite layer aligment ~rpendicular to the compressive stress was reported for a stress-graphitized pyrolytic carbon [83, for glass-like carbon grains heat-treated at 2650°C under a pressure of OS-O.9 kbar [9], and also for carbon fiber/glassy carbon composite, the compressive stress being probably induced by the large contraction of the glassy carbon matrix [IO-121. One of the present authors found that a phenoiformaidehyde resin char after carbonization shows regions
ZEXPERIMENTAL
2.1 Specimen A glass-like carbon rod (6 mm in diameter and 6 mm in length)t was cured up to ca. 1000°C. No heterogeneity, such as pores and/or crack, was observed in this specimen under an optical microscope. Neither multi-phase graphitization nor localized development of anisotropic areas were induced by heat-treatment even at cu. 3000°C under normal pressure, indicating again that the specimen had microscopically homogeneous texture and structure. 2.2 Heat-treatment under high pressure The high pressure apparatus used was a simple-pistoncylinder type made of a tool steel. The pressure cell arrangement is shown in Fig. 1. The graphite heater had
tKindly supplied by Tokai Carbon Mfg. Co. and commercially named as GC-IO. 45
K. KAMIYAand M. INACAKI
46
MO-1900°C was a diffuse band, without any indication of the presence of graphite.
Fig. 1. Pressure cell arrangement.(a) metal disk, (b) graphite plate (c) pyrophyllite, (d) graphite plate (e) graphite heater, (f) specimen (g) boron nitride, (h) artificial graphite sleeve (I) or glass-like carbon powder (II)
an outer dia. of 10 mm and an inner dia. of 8 mm. The
other parts of the cell were the same as those described elsewhere [41. The space between the glass-like carbon specimen and the graphite heater h, was filled by an artificial graphite sleeve of 8 mm in o.d. and 6 mm in i.d. (the cell arrangement I), or by glass-like carbon powder of cu. 70 pm particle size (the ceil arrangement II). The an~Iar glass-like carbon particles previously heattreated to 2000°Cunder normal pressure (GC-20) were as hard as the GC-10 rod. In the cell arrangement I the specimen was subjected to a quasi-hydrostatic pressure, because graphite and pyrophyllite surrounding the specimen are good pressure transmitting mediums. In the arrangement II numerous contact points between the particles and the specimen rod are present, where the applied stress is concentrated. The heat-treatments were performed at several temperatures ranging from 1600”to 1900°Cfor 1 hr under an external pressure of 5 kbar.
3.2 Head-~reatmeRtin contact wifh angular grains (cell arrangement II) In Fig. 2, the changes in the 002 diffraction line profile of the W-10 rod occurring with increase in heat-treatment temperature are shown. The original GC-10 shows a very diffuse band. For the specimen heat-treated at 175O”C,however, a sharp diffraction line develops at the diffraction angle 28 of 26.5” (corresponding to graphite) overlapping the diffuse band. This indicates occurrence of localized graphitization in the specimen. A more remarkable development of graphite structure is observed above 1800°C. Prior to the appearance of graphite structure, that is, below 17WC, some sharpening of the diffuse line is observed. Optically anisotropic areas were observed for speimens heat-treated above 1700°C. A polarized light micrograph of the specimen heat-treated at 1700°C is shown in Fig, 3(a). The upper half of the micrograph corresponds to the GC-10 rod and the lower half to the GC-20 grains surrounding the rod. Anisotropic areas are seen at the surface of the GC-10 rod where there are many contact points with the GC-20 grains (Fig. 3a), but not at all in the bulk of the rod. No anisotropic areas are observed at the same heat-treatment temperature in the ceil a~angement I (see 3.1 and Fig. 2). In Fig. 3(b), the alignment of carbon layers in the area indicated by an arrow in Fig. 3(a) is schematically drawn on basis of microscopic observations. The carbon layers are aligned along the interface between the GC-10 rod and the GC-20
2.3 X-Ray di~roc~io~and mjcroscupjc obse~a~io~ After heat-treatment under pressure the glass-like carbon rod was cut along the cylinder axis. One half of the specimen was crushed into powder of -200 mesh size and the 002 diffraction line profile was measured, using Ni-filtered Cul(a radiation. The cut surface of the other half of the specimen was polished using Sic (- 8OO-- 1000 mesh) and finished with A&O3 powders (- 2000 mesh), and then looked at under an optical microscope in polarized light. By inserting a gypsum test plate the aligment of carbon layers in the optically anisotropic areas was judged from the change of additive and subtractive regions of retardation on rotation of the microscope stage, keeping in mind that carbon and graphite are optically uniaxial negative. 3. RESULTS
3.1 Heoi-trea~rne~~ in 4uasi-~ydro~~u~ice~#ironmen~ (cell arrangement I) The rod of glass-like carbon was not cracked into I pieces by heat-treatment under pressure in the cell 23 24 25 26 27 28 ; 21 22 arrangement I, this indicating the dominance of the 2 B (degree) quasi-hydrostatic pressure in this cell arrangement. No anisotropic areas were observed in the specimen rod Fig. 2. 002 diffraction line profile for the W-10 rod heat-treated heat-treated up to lWC, under the optical microscope. under 5 kbar for 1 hr at various ~mperatures. The cell arrangement II. The 002 diffraction line for the specimens heat treated at
:ated under pressure
5
1 I
ib.l700'C,60 ml”
GC lo
I L
(GC-particle ) 100 pm
I
50 5 kb
ji
m
, 17OO’C
5kb
, 60 min 04
Fig. 3. Polarized light micro~ph of the GC-10 rod heat-treated at 1700°Cunder 5 kb in contact with the GC-20 grains. In the upper part is the GC-10 rod and in the lower are the packed GC-20 grains. (b) Schematic representation of the carbon layer alignment in the region indicated by an arrow in Fig. 3(a)
grains. The same feature in the alignment of carbon layers in Fig. 3(b) are observed at other contact points. At contact points between angular GC-20 grains in the lower part of Fig. 3(a), a number of anisotropic areas are seen. This agrees with the observations on various nongraphitizing carbons reported previously [5-71. In the 1750”Ctreated rod the anisotropic areas are observed to grow into the bulk of the rod with sharp edges, as shown in Fig. 4(a). They look like bamboo leaves or flower petals. These bamboo leaves are distinguished from areas which remained isotropic by their optical a~sotropy under polarized Lightand also by their grayish colour under normal light.%e powder sampled from these bamboo leaves in the rod gave a sharp 002 diffraction line profile at the angle of graphite structure. The schematic alignment of carbon layers is given in Fii. 4(b). The layers are seen to be aliied along the periphery of the leaves, that is, the boundary between isotropic matrix and anisotropic graphitized parts. No anisotropic areas were observed in the bulk of GC-IO. The leaf-shaped graphitized areas are found to grow more extensively at the expense of the isotropic matrix at 18WC, as seen in Fig. 5(a). The leaves reach cu. 1mm
,175O
"C
,
60min (b)
Fig. 4. (a) Normal light micrograph of graphitized region of the GC-IO rod heat-treated at 1750°Cunder 5 kbar in contact with the GC-20 grains. (b) Schematic representation of the graphite layer alignment in the region shown in Fig. 4(a).
GC
5 kb, WO’C, 60mr
04
10
K. KAMNAand M. INAGAKI
48
GC
particle
0.5
5ib.‘k3COTC.60min
mm
(4
5 kb , 1800'C , 60 min (b)
Fig. 5. (a) Normal light micrograph of the graphitized region grown at the surface of the W-10 rod at 1800°Cunder 5 kbarin contact with the GC-20 grains. (b) Schematic representation of the graphite layer alignment in the leaf-shaped parts shown by arrows in Fig. S(a).
in length. The row of leaves can be seen even by naked eye. With the inserted gypsum test-plate under polarized light the yellowish and bluish stripes are observed arranged side by side. This shows that the graphitized part consists of highly oriented graphite layer packets. The alignment of graphite layers in the corresponding regions is given schematically in Fig. 5(b). In addition to the leaf-shaped graphitized areas adjacent to the surface, discrete graphitized areas further away from the surface appeared in the non-graphitized matrix like inbedded particles of which section look similar to a flower such as a morning-glory. A section across such a particle is seen in Fig. 6(a) and the graphite layer alignment in it is parallel to the periphery as schematically shown in Fig. 6(b). 4
DISCUSSION
Graphitization, especially so-called heterogeneous graphitization[7], does not occur in the glass-like carbon CC-10 even at 1900°Cunder pressure of 5 kbar in the cell arrangement I in which a quasi-hydrostatic pressure is dominant. This heat-treatment temperature is sulhciently high for the angular CC-10 powder particles packed in the graphite heater to become graphitized under such externally applied pressure of * 5 kbar as previously reported [l-7]. This fact ascertains that a quasi-hydrostatic pressure much higher than 5 kbar would be required for graphitization of this carbon to occur at relatively lower temperatures such as 1500-1600°C without presence of local stress accumulation. Using the cell arrangement II, the graphitization of the CC-10 rod was forced to occur at the contact points with the angular grains of CC-20 in heat-treatment under an externally applied pressure of 5 kbar. As was shown above, even though small parts having optically anisotropic structure were observed in the 1700°Cheat-treated specimen, no sharp line of graphite structure was detected yet in its 002 diffraction profile. de Fonton et al. [14] also observed, by using high resolution electron micros-
GC
IO
Skb , 18OOT,
60 min
(b) Fig. 6. (a) Micrograph of a discrete graphitized region formed inside of the W-10 rod heat-treated at 1800°Cunder 5 kbar. (b) Schematic graphite layer alignment in the graphitized region shown in Fig. 6(a).
copy, theat in non-graphitizing carbons turbostratic particles develop before appearance of graphite structure in heat-treatment under pressure of 5 kbar. Thus it can be surmized that in heat-treatment of CC-10 above 17OO”C, the not yet graphitized but optically anisotropic areas have a turbostratic structure, which is initiated below 1750°C at the contact points with CC-20 grains. These anisotropic areas are then transformed into graphite at a somewhat higher temperature. Above 1750°C the graphitized bamboo-leaf looking areas grow inward at the expense of the non-graphitized isotropic part. As the heat-treatment temperature is raised, the leaf-shaped graphitized areas grow mostly in longitudinal direction, with their width changing very little. After 1700°C heat-treatment, some CC-20 grains are seen to have broken into the CC-10 rod. This thrusting is probaly due to localized deformation of the CC-10 rod occurring as a result of the transformation of the randomly oriented structure into the well-oriented less bulky carbon layer packet at the contact area relaxing the high strain due to stress concentration. In the area of contact with a CC-20 grain, the compressive stress contour is thought to be parallel to the grain/rod interface, in other words, pressure gradient is normal to the interface. Since the carbon layers are observed to be aligned parallel to
Glass-like carbon heat-treated under pressL:re
the interface in the contact areas, one concludes that the carbon layer alignment induced is perpendicular to the compressive stress. This conclusion disagree with that reported in our previous work [7] on stress graphitization of phenolformaldehyde resin char. The starting grains of the material were found later to be heterogeneous in structure having graphitizable oriented carbon layer packets at the outside surface and around internal pores [13]. Thus the highly oriented carbon layer packets observed in the previous work at the contact points between grains are believed now to have been present all along and not newly formed in heat-treatment under pressure. On the contrary, there is no doubt that the carbon layer packets in our W-10 rod at the contact points with angular carbon CC-20 grains are formed only at heat-treatment under pressure. So, the relation of carbon layer alignment to the direction of stress deduced in the previous work [7] has to be retracted. The carbon layer alignment as found in our case (graphitization under pressure) agrees with other results reported [g-12]. The motive force for the structural change in the first stages of the graphitization of the CC-10 rod under pressure may be the relaxation of stress concentrated at the contact points with W-20 grains. The stress due to anisotropic thermal expansion has been proposed as a motive force for graphitizing carbons]151 and non-graphitizing carbons [ 161 under normal pressure. Once highly strained regions at the contact points have changed in texture to a less bulky oriented state, the locally concentrated stress will relax. A pressure of 5 kbar is not high enough to force the progress of graphitization of the remaining GC-10 rod if the specimen is subjected to a quasihydrostatic pressure, as understood from the results obtained with the cell arrangement I. The propagation of graphitization into the GC-10 rod, therefore, requires a travel of the stress-concentration region. This can be explained as follows; The graphitized region formed at the surface of the rod is much softer than the remaining non-graphitic part, and acts as a flaw around which the externally applied stress becomes concentrated. The stresses are mostly concentrated at the tip of the graphitized flaw, forcing this region to undergo graphitization. The compressive stress contour is parallel to the top surface of the grown flaw, which results in the alignment of carbon layer parallel to the periphery of the graphitized region. Newly formed graphitized region at the tip of the flaw shifts the stress accumulation point. As a result, the graphitized gregion of flaw grows into the GC-10 rod at the expense of nongraphitized material, keeping a sharp edge at the tip and forming the leaf-shaped cross section. Pressure or compressive stress does not seem to cause plastic flow of carbon on our case, the heat-treatment temperature being too low for that. However, ruptures of
CAR Vol. 19. No. I-D
49
cross-linking bonds in the highly strained areas and flattening of the micropores with walls consisting of tiny layer packets perpendicular to the compressive stress may occur and such ruptures and deformations may explain the formation of highly oriented regions of carbon or graphite layers in heat-treatment under pressure, as shown by high resolution electron microscopy [14]. The heat-treatment temperature needed to induce the graphitization of the GC-10 rod in contact with the W-20 grains is 175O”C,that is 3-C higher than that for the packed angular grains of a phenolformaldehyde resin char. This lowering of graphitization temperature is possibly due at least partly to the microheterogeneity of the phenolformaldehyde resin char. In the course of heat-treatment of angular grains of a phenolformaldehyde resin char with strong structural heterogeneity, internal stresses generated by the difference in thermal expansion between oriented and isotropic regions may add to the externally applied pressure in promoting graphitization as suggested by Fischbach [17], thus lowering the graphitization temperature of the char. Acknowledgements-The authors wish to thank Dr. T. Noda, Prof. Emeritus of Nagoya University and Mie University, and Prof. S. Sakka of Mie University for their valuable discussion and encouragement.
REFERENCES
:: 3. 4. 5. 6. 7.
T. Noda and H. Kato, Carf~on3 289 (1965). T. Noda, K. Kamiya and M. Inagaki, Bull. Chem. Sot. Japan 41 445 (1968). K. Kamiya, M. Inagaki, M. Mizutani and T. Noda, Bull. Chem. Sot. Japan 412169 (1%8). K. Kamiya, T. Noda, M. Inagaki and H. Saito, J. Mat Sci. 7 1244(1972). M.1naaakiandS.Naka.J. Mat. Sci. 10814(1975):M.lnagaki.K Horii and S. Naka, &bon 13 97 (1975). K. Kamiya, M. Inagaki and T. Noda, Carbon 9 287 (1971). K. Kamiya, M. Inagaki and T. Noda, High Temp.-High Press. 5 331 (1973).
8. D. B. Fischbach, J. Chim. Phys. (special issue), p. I21 (1969). 9. W. C. Chard, R. D. Reiswig, L. S. Levinson and T. D. Baker, Carbon 4 950 (1%8).
IO. R. D. Reiswig, L. S. Levinson and J. K. O’Rourke, Carbon 6 124(1968). Il. Y. Hishiyama, M. Inagaki, S. Kimura and S. Yamada, Curbon 12 249 (1974). 12. S. Kimura, E. Yasuda, H. Tanaka and S. Yamada, J. Ceram. Assoc. Japan 83 122 (1975).
13. K. Kamiya and K. Suzuki, Carbon 13 317 (1975). 14. M. Inagaki, A. Oberlin and S. de Fonton, High Tern.-High Press. 9 453 (1977); S. de Fonton, A. Oberlinand M. Inagaki, J. Mol. Sci. 15 909 (1980). IS. S. Mrozowski, Proc. 1st and 2nd Carbon Con]. Buffalo, p. 31. Waverley Press, Baltimore (1956). 16. R. E. Franklin, Proc. Roy. Sot. (London) Au)9 1% (1951). 17. D. B. Fischbach, Chemistry and Physics of Carbon (Edited by P. L. Walker), pp. 77-83. Marcel Dekker, New York (1971).
M. INAGAKI Toyohashi University of Technology, Materials Science Tempaku-cho, Toyohashi, 440, Japan (Received 6 March 1980) Abstract-A glass-like carbon of homogeneous structure cured up to ItXWC (GC-IO) was heat-treated under pressure of 5 kbar. The heat ~eatment was performed in two different cell a~angements; in one a~angement quasi-hydrostatic pressure was dominant and in the other there were many contact points between the carbon investigated and small angular grains of glass-like carbon heat-treated to 2080°C. In the first arrangement, the glass-like carbon did not graphitize even at 1900°C.In the second, graphitization was observed above 1750°C. Below 17WC, optically anisotropic areas were initiated at contact points with the angular $rains where the stress concentrations occur. These areas show turbostratic structure. At a little higher temperature, they transform to graphite. The graphitized parts in cross section look like bamboo leaves and grow into the bulk of the carbon at the expense of non~aphitized parts. The graphite layers were found to align ~~endicularly to the compressive stress, contrary to our previous report. The previously observed o~entation was probably due to oriented regions originally present in the starting material. The mechanism of the spreading of the graphitization from the stress concentration points at the surface is discussed.
of oriented carbon layers around pores,, which easily graphitize by heat-treatment at high temperature under normal pressure[l3]. In ~aphitization of non-~aphitizing carbons under pressure, the presence of these originally oriented parts of carbon layers make it difficult to decide whether the graphite layer planes are newly formed by the heat-treatment under pressure or not. Therefore, it became necessary to reinvestigate the aligment of graphite Iayers with respect to the direction of compressive stress. In order to avoid the difficulty mentioned above, in the present work, a block of glass-like carbon with homogeneous structure was used as a starting material. The carbon was heat-treated under 5 kbar, and the development of graphite structure was pursued by X-ray diffraction and optical microscopy using polarized light.
l.INTRODUCTION
The influence of pressure on graphitization of carbons was investigated by the present authors and the results obtained are summarized as follows; (1) The externally applied pressure above 3 kbar accelerates markedly the graphitization of carbons [ 1,2], with non-graphitizing carbons such as phenolformaldehyde resin char being drastically transformed into graphite at relatively low temperature of 15~16~C under a pressure of 5 kbar [3]. (2) High bulk density and various degrees of preferred orientation of crystallites are attained by selecting suitable starting materials of different origin, of different shape and size, and also different thermal histories [4,5]. (3) The graphitization is initiated at the contact points between carbon grains where the applied stress is concentrated and spreads into the remaining non~aphitized regions [6,7]. The aligment of graphite layers formed at the contact points between grains was concluded from the observations under polarized light to be parallel to the compressive stress [7]. However, graphite layer aligment ~rpendicular to the compressive stress was reported for a stress-graphitized pyrolytic carbon [83, for glass-like carbon grains heat-treated at 2650°C under a pressure of OS-O.9 kbar [9], and also for carbon fiber/glassy carbon composite, the compressive stress being probably induced by the large contraction of the glassy carbon matrix [IO-121. One of the present authors found that a phenoiformaidehyde resin char after carbonization shows regions
ZEXPERIMENTAL
2.1 Specimen A glass-like carbon rod (6 mm in diameter and 6 mm in length)t was cured up to ca. 1000°C. No heterogeneity, such as pores and/or crack, was observed in this specimen under an optical microscope. Neither multi-phase graphitization nor localized development of anisotropic areas were induced by heat-treatment even at cu. 3000°C under normal pressure, indicating again that the specimen had microscopically homogeneous texture and structure. 2.2 Heat-treatment under high pressure The high pressure apparatus used was a simple-pistoncylinder type made of a tool steel. The pressure cell arrangement is shown in Fig. 1. The graphite heater had
tKindly supplied by Tokai Carbon Mfg. Co. and commercially named as GC-IO. 45
K. KAMIYAand M. INACAKI
46
MO-1900°C was a diffuse band, without any indication of the presence of graphite.
Fig. 1. Pressure cell arrangement.(a) metal disk, (b) graphite plate (c) pyrophyllite, (d) graphite plate (e) graphite heater, (f) specimen (g) boron nitride, (h) artificial graphite sleeve (I) or glass-like carbon powder (II)
an outer dia. of 10 mm and an inner dia. of 8 mm. The
other parts of the cell were the same as those described elsewhere [41. The space between the glass-like carbon specimen and the graphite heater h, was filled by an artificial graphite sleeve of 8 mm in o.d. and 6 mm in i.d. (the cell arrangement I), or by glass-like carbon powder of cu. 70 pm particle size (the ceil arrangement II). The an~Iar glass-like carbon particles previously heattreated to 2000°Cunder normal pressure (GC-20) were as hard as the GC-10 rod. In the cell arrangement I the specimen was subjected to a quasi-hydrostatic pressure, because graphite and pyrophyllite surrounding the specimen are good pressure transmitting mediums. In the arrangement II numerous contact points between the particles and the specimen rod are present, where the applied stress is concentrated. The heat-treatments were performed at several temperatures ranging from 1600”to 1900°Cfor 1 hr under an external pressure of 5 kbar.
3.2 Head-~reatmeRtin contact wifh angular grains (cell arrangement II) In Fig. 2, the changes in the 002 diffraction line profile of the W-10 rod occurring with increase in heat-treatment temperature are shown. The original GC-10 shows a very diffuse band. For the specimen heat-treated at 175O”C,however, a sharp diffraction line develops at the diffraction angle 28 of 26.5” (corresponding to graphite) overlapping the diffuse band. This indicates occurrence of localized graphitization in the specimen. A more remarkable development of graphite structure is observed above 1800°C. Prior to the appearance of graphite structure, that is, below 17WC, some sharpening of the diffuse line is observed. Optically anisotropic areas were observed for speimens heat-treated above 1700°C. A polarized light micrograph of the specimen heat-treated at 1700°C is shown in Fig, 3(a). The upper half of the micrograph corresponds to the GC-10 rod and the lower half to the GC-20 grains surrounding the rod. Anisotropic areas are seen at the surface of the GC-10 rod where there are many contact points with the GC-20 grains (Fig. 3a), but not at all in the bulk of the rod. No anisotropic areas are observed at the same heat-treatment temperature in the ceil a~angement I (see 3.1 and Fig. 2). In Fig. 3(b), the alignment of carbon layers in the area indicated by an arrow in Fig. 3(a) is schematically drawn on basis of microscopic observations. The carbon layers are aligned along the interface between the GC-10 rod and the GC-20
2.3 X-Ray di~roc~io~and mjcroscupjc obse~a~io~ After heat-treatment under pressure the glass-like carbon rod was cut along the cylinder axis. One half of the specimen was crushed into powder of -200 mesh size and the 002 diffraction line profile was measured, using Ni-filtered Cul(a radiation. The cut surface of the other half of the specimen was polished using Sic (- 8OO-- 1000 mesh) and finished with A&O3 powders (- 2000 mesh), and then looked at under an optical microscope in polarized light. By inserting a gypsum test plate the aligment of carbon layers in the optically anisotropic areas was judged from the change of additive and subtractive regions of retardation on rotation of the microscope stage, keeping in mind that carbon and graphite are optically uniaxial negative. 3. RESULTS
3.1 Heoi-trea~rne~~ in 4uasi-~ydro~~u~ice~#ironmen~ (cell arrangement I) The rod of glass-like carbon was not cracked into I pieces by heat-treatment under pressure in the cell 23 24 25 26 27 28 ; 21 22 arrangement I, this indicating the dominance of the 2 B (degree) quasi-hydrostatic pressure in this cell arrangement. No anisotropic areas were observed in the specimen rod Fig. 2. 002 diffraction line profile for the W-10 rod heat-treated heat-treated up to lWC, under the optical microscope. under 5 kbar for 1 hr at various ~mperatures. The cell arrangement II. The 002 diffraction line for the specimens heat treated at
:ated under pressure
5
1 I
ib.l700'C,60 ml”
GC lo
I L
(GC-particle ) 100 pm
I
50 5 kb
ji
m
, 17OO’C
5kb
, 60 min 04
Fig. 3. Polarized light micro~ph of the GC-10 rod heat-treated at 1700°Cunder 5 kb in contact with the GC-20 grains. In the upper part is the GC-10 rod and in the lower are the packed GC-20 grains. (b) Schematic representation of the carbon layer alignment in the region indicated by an arrow in Fig. 3(a)
grains. The same feature in the alignment of carbon layers in Fig. 3(b) are observed at other contact points. At contact points between angular GC-20 grains in the lower part of Fig. 3(a), a number of anisotropic areas are seen. This agrees with the observations on various nongraphitizing carbons reported previously [5-71. In the 1750”Ctreated rod the anisotropic areas are observed to grow into the bulk of the rod with sharp edges, as shown in Fig. 4(a). They look like bamboo leaves or flower petals. These bamboo leaves are distinguished from areas which remained isotropic by their optical a~sotropy under polarized Lightand also by their grayish colour under normal light.%e powder sampled from these bamboo leaves in the rod gave a sharp 002 diffraction line profile at the angle of graphite structure. The schematic alignment of carbon layers is given in Fii. 4(b). The layers are seen to be aliied along the periphery of the leaves, that is, the boundary between isotropic matrix and anisotropic graphitized parts. No anisotropic areas were observed in the bulk of GC-IO. The leaf-shaped graphitized areas are found to grow more extensively at the expense of the isotropic matrix at 18WC, as seen in Fig. 5(a). The leaves reach cu. 1mm
,175O
"C
,
60min (b)
Fig. 4. (a) Normal light micrograph of graphitized region of the GC-IO rod heat-treated at 1750°Cunder 5 kbar in contact with the GC-20 grains. (b) Schematic representation of the graphite layer alignment in the region shown in Fig. 4(a).
GC
5 kb, WO’C, 60mr
04
10
K. KAMNAand M. INAGAKI
48
GC
particle
0.5
5ib.‘k3COTC.60min
mm
(4
5 kb , 1800'C , 60 min (b)
Fig. 5. (a) Normal light micrograph of the graphitized region grown at the surface of the W-10 rod at 1800°Cunder 5 kbarin contact with the GC-20 grains. (b) Schematic representation of the graphite layer alignment in the leaf-shaped parts shown by arrows in Fig. S(a).
in length. The row of leaves can be seen even by naked eye. With the inserted gypsum test-plate under polarized light the yellowish and bluish stripes are observed arranged side by side. This shows that the graphitized part consists of highly oriented graphite layer packets. The alignment of graphite layers in the corresponding regions is given schematically in Fig. 5(b). In addition to the leaf-shaped graphitized areas adjacent to the surface, discrete graphitized areas further away from the surface appeared in the non-graphitized matrix like inbedded particles of which section look similar to a flower such as a morning-glory. A section across such a particle is seen in Fig. 6(a) and the graphite layer alignment in it is parallel to the periphery as schematically shown in Fig. 6(b). 4
DISCUSSION
Graphitization, especially so-called heterogeneous graphitization[7], does not occur in the glass-like carbon CC-10 even at 1900°Cunder pressure of 5 kbar in the cell arrangement I in which a quasi-hydrostatic pressure is dominant. This heat-treatment temperature is sulhciently high for the angular CC-10 powder particles packed in the graphite heater to become graphitized under such externally applied pressure of * 5 kbar as previously reported [l-7]. This fact ascertains that a quasi-hydrostatic pressure much higher than 5 kbar would be required for graphitization of this carbon to occur at relatively lower temperatures such as 1500-1600°C without presence of local stress accumulation. Using the cell arrangement II, the graphitization of the CC-10 rod was forced to occur at the contact points with the angular grains of CC-20 in heat-treatment under an externally applied pressure of 5 kbar. As was shown above, even though small parts having optically anisotropic structure were observed in the 1700°Cheat-treated specimen, no sharp line of graphite structure was detected yet in its 002 diffraction profile. de Fonton et al. [14] also observed, by using high resolution electron micros-
GC
IO
Skb , 18OOT,
60 min
(b) Fig. 6. (a) Micrograph of a discrete graphitized region formed inside of the W-10 rod heat-treated at 1800°Cunder 5 kbar. (b) Schematic graphite layer alignment in the graphitized region shown in Fig. 6(a).
copy, theat in non-graphitizing carbons turbostratic particles develop before appearance of graphite structure in heat-treatment under pressure of 5 kbar. Thus it can be surmized that in heat-treatment of CC-10 above 17OO”C, the not yet graphitized but optically anisotropic areas have a turbostratic structure, which is initiated below 1750°C at the contact points with CC-20 grains. These anisotropic areas are then transformed into graphite at a somewhat higher temperature. Above 1750°C the graphitized bamboo-leaf looking areas grow inward at the expense of the non-graphitized isotropic part. As the heat-treatment temperature is raised, the leaf-shaped graphitized areas grow mostly in longitudinal direction, with their width changing very little. After 1700°C heat-treatment, some CC-20 grains are seen to have broken into the CC-10 rod. This thrusting is probaly due to localized deformation of the CC-10 rod occurring as a result of the transformation of the randomly oriented structure into the well-oriented less bulky carbon layer packet at the contact area relaxing the high strain due to stress concentration. In the area of contact with a CC-20 grain, the compressive stress contour is thought to be parallel to the grain/rod interface, in other words, pressure gradient is normal to the interface. Since the carbon layers are observed to be aligned parallel to
Glass-like carbon heat-treated under pressL:re
the interface in the contact areas, one concludes that the carbon layer alignment induced is perpendicular to the compressive stress. This conclusion disagree with that reported in our previous work [7] on stress graphitization of phenolformaldehyde resin char. The starting grains of the material were found later to be heterogeneous in structure having graphitizable oriented carbon layer packets at the outside surface and around internal pores [13]. Thus the highly oriented carbon layer packets observed in the previous work at the contact points between grains are believed now to have been present all along and not newly formed in heat-treatment under pressure. On the contrary, there is no doubt that the carbon layer packets in our W-10 rod at the contact points with angular carbon CC-20 grains are formed only at heat-treatment under pressure. So, the relation of carbon layer alignment to the direction of stress deduced in the previous work [7] has to be retracted. The carbon layer alignment as found in our case (graphitization under pressure) agrees with other results reported [g-12]. The motive force for the structural change in the first stages of the graphitization of the CC-10 rod under pressure may be the relaxation of stress concentrated at the contact points with W-20 grains. The stress due to anisotropic thermal expansion has been proposed as a motive force for graphitizing carbons]151 and non-graphitizing carbons [ 161 under normal pressure. Once highly strained regions at the contact points have changed in texture to a less bulky oriented state, the locally concentrated stress will relax. A pressure of 5 kbar is not high enough to force the progress of graphitization of the remaining GC-10 rod if the specimen is subjected to a quasihydrostatic pressure, as understood from the results obtained with the cell arrangement I. The propagation of graphitization into the GC-10 rod, therefore, requires a travel of the stress-concentration region. This can be explained as follows; The graphitized region formed at the surface of the rod is much softer than the remaining non-graphitic part, and acts as a flaw around which the externally applied stress becomes concentrated. The stresses are mostly concentrated at the tip of the graphitized flaw, forcing this region to undergo graphitization. The compressive stress contour is parallel to the top surface of the grown flaw, which results in the alignment of carbon layer parallel to the periphery of the graphitized region. Newly formed graphitized region at the tip of the flaw shifts the stress accumulation point. As a result, the graphitized gregion of flaw grows into the GC-10 rod at the expense of nongraphitized material, keeping a sharp edge at the tip and forming the leaf-shaped cross section. Pressure or compressive stress does not seem to cause plastic flow of carbon on our case, the heat-treatment temperature being too low for that. However, ruptures of
CAR Vol. 19. No. I-D
49
cross-linking bonds in the highly strained areas and flattening of the micropores with walls consisting of tiny layer packets perpendicular to the compressive stress may occur and such ruptures and deformations may explain the formation of highly oriented regions of carbon or graphite layers in heat-treatment under pressure, as shown by high resolution electron microscopy [14]. The heat-treatment temperature needed to induce the graphitization of the GC-10 rod in contact with the W-20 grains is 175O”C,that is 3-C higher than that for the packed angular grains of a phenolformaldehyde resin char. This lowering of graphitization temperature is possibly due at least partly to the microheterogeneity of the phenolformaldehyde resin char. In the course of heat-treatment of angular grains of a phenolformaldehyde resin char with strong structural heterogeneity, internal stresses generated by the difference in thermal expansion between oriented and isotropic regions may add to the externally applied pressure in promoting graphitization as suggested by Fischbach [17], thus lowering the graphitization temperature of the char. Acknowledgements-The authors wish to thank Dr. T. Noda, Prof. Emeritus of Nagoya University and Mie University, and Prof. S. Sakka of Mie University for their valuable discussion and encouragement.
REFERENCES
:: 3. 4. 5. 6. 7.
T. Noda and H. Kato, Carf~on3 289 (1965). T. Noda, K. Kamiya and M. Inagaki, Bull. Chem. Sot. Japan 41 445 (1968). K. Kamiya, M. Inagaki, M. Mizutani and T. Noda, Bull. Chem. Sot. Japan 412169 (1%8). K. Kamiya, T. Noda, M. Inagaki and H. Saito, J. Mat Sci. 7 1244(1972). M.1naaakiandS.Naka.J. Mat. Sci. 10814(1975):M.lnagaki.K Horii and S. Naka, &bon 13 97 (1975). K. Kamiya, M. Inagaki and T. Noda, Carbon 9 287 (1971). K. Kamiya, M. Inagaki and T. Noda, High Temp.-High Press. 5 331 (1973).
8. D. B. Fischbach, J. Chim. Phys. (special issue), p. I21 (1969). 9. W. C. Chard, R. D. Reiswig, L. S. Levinson and T. D. Baker, Carbon 4 950 (1%8).
IO. R. D. Reiswig, L. S. Levinson and J. K. O’Rourke, Carbon 6 124(1968). Il. Y. Hishiyama, M. Inagaki, S. Kimura and S. Yamada, Curbon 12 249 (1974). 12. S. Kimura, E. Yasuda, H. Tanaka and S. Yamada, J. Ceram. Assoc. Japan 83 122 (1975).
13. K. Kamiya and K. Suzuki, Carbon 13 317 (1975). 14. M. Inagaki, A. Oberlin and S. de Fonton, High Tern.-High Press. 9 453 (1977); S. de Fonton, A. Oberlinand M. Inagaki, J. Mol. Sci. 15 909 (1980). IS. S. Mrozowski, Proc. 1st and 2nd Carbon Con]. Buffalo, p. 31. Waverley Press, Baltimore (1956). 16. R. E. Franklin, Proc. Roy. Sot. (London) Au)9 1% (1951). 17. D. B. Fischbach, Chemistry and Physics of Carbon (Edited by P. L. Walker), pp. 77-83. Marcel Dekker, New York (1971).