Effect of Pressure on the Resistance of Pyrolytic Graphite G. A. Samara and H. G. Drickamer Citation: The Journal of Chemical Physics 37, 471 (1962); doi: 10.1063/1.1701359 View online: http://dx.doi.org/10.1063/1.1701359 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/37/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A pyrolytic graphite resistive heating element J. Vac. Sci. Technol. A 5, 383 (1987); 10.1116/1.574165 Binding of nickel on pyrolytic graphite J. Vac. Sci. Technol. 14, 475 (1977); 10.1116/1.569222 Pressure and temperature dependences of the elastic constants of compressionannealed pyrolytic graphite J. Appl. Phys. 45, 3309 (1974); 10.1063/1.1663777 Expansion of Annealed Pyrolytic Graphite J. Appl. Phys. 35, 1992 (1964); 10.1063/1.1713794 Elastic Moduli of Pyrolytic Graphite J. Acoust. Soc. Am. 35, 521 (1963); 10.1121/1.1918521 This article is copyrighted as indicated in the article. 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Downloaded to IP: 130.113.111.210 On: Sun, 21 Dec 2014 23:56:46 http://scitation.aip.org/content/aip/journal/jcp?ver=pdfcov http://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/www.aip.org/pt/adcenter/pdfcover_test/L-37/327320036/x01/AIP-PT/JCP_ArticleDL_101514/PT_SubscriptionAd_1640x440.jpg/47344656396c504a5a37344142416b75?x http://scitation.aip.org/search?value1=G.+A.+Samara&option1=author http://scitation.aip.org/search?value1=H.+G.+Drickamer&option1=author http://scitation.aip.org/content/aip/journal/jcp?ver=pdfcov http://dx.doi.org/10.1063/1.1701359 http://scitation.aip.org/content/aip/journal/jcp/37/3?ver=pdfcov http://scitation.aip.org/content/aip?ver=pdfcov http://scitation.aip.org/content/avs/journal/jvsta/5/3/10.1116/1.574165?ver=pdfcov http://scitation.aip.org/content/avs/journal/jvst/14/1/10.1116/1.569222?ver=pdfcov http://scitation.aip.org/content/aip/journal/jap/45/8/10.1063/1.1663777?ver=pdfcov http://scitation.aip.org/content/aip/journal/jap/45/8/10.1063/1.1663777?ver=pdfcov http://scitation.aip.org/content/aip/journal/jap/35/6/10.1063/1.1713794?ver=pdfcov http://scitation.aip.org/content/asa/journal/jasa/35/4/10.1121/1.1918521?ver=pdfcov THE JOURNAL OF CHEMICAL PHYSICS VOLUME 37, NUMBER 3 AUGUST 1,1962 Effect of Pressure on the Resistance of Pyrolytic Graphite* G. A. SAMARA AND H. G. DRICKAMER Department of Chemistry and Chemical Engineering, University of Illinois, Urbana, Illinois (Received April 10, 1962) The effect of pressure to several hundred kilo bars has been measured on samples of pyrolytic graphite of density 2.18 and 2.20. Measurements along the c axis show a relatively high resistance which decreases with increasing pressure and levels at high pressure, apparently due to strongly decreasing compressibility. Along the a axis the resistance is low, but increases at higher pressures apparently due to recrystallization. THE effect of pressure to several hundred kilobars has been measured on the resistance of pyrolytic graphite. The experimental procedures have been pre- viously described.t-a The samples were polished to size on OOOO-grade emery paper. Since it was impractical to establish the contact resistance, the relative resistances are presented without correction. Copper, amalga- mated copper, and gold leads were used, with no dif- ference in the measurements. Two pieces of pyrolytic graphite one with density= 2.18 glcc, the other with density=2.20 glcc were ob- tained from High Temperature Materials Incorpo- rated, Brighton, Massachusetts. The density of pure graphite is 2.25 glce. All runs but two were made on samples cut from the first piece. The resistance was measured along the a and c axes (see discussion below) and on fused powder samples. The results are pre- sented and discussed below. CONDUCTIVITY ALONG c AXIS Four runs were made measuring the conductivity along the c axis. A typical run is shown in Fig. 1. The resistance drops rapidly with pressure in the low- pressure region, reaching, by 150 kbar, a value about four times smaller than that at 10 kbar. Above 200 kbar there is a very small drop in resistance, the value at 300 kbar being only 5% - 6% lower than that at 200 kbar. A fifth run was made on the denser of the two samples (density=2.20 glee), and the results are also shown in Fig. 1. The initial drop in the resistance was smaller than that for the first sample, the resistance at 150 kbar being only about one-half of the value at 10 kbar. Above 80 kbar, the resistance curve is identical with that of the lower density sample. CONDUCTIVITY ALONG a AXIS Six runs were made. The resistance drops by about 20%-30% by 50 kbar. The results were nearly identi- cal for all runs up to 60 -70 kbar. Between 80 and 200 ⢠This work was supported in part by the United States Atomic Energy Commission. 1 A. S. BaJchan and H. G. Drickamer, Rev. Sci. Instr. 32, 308 (1961). 2 S. Minomura and H. G. Drickamer, J. Phys. Chern. Solids 23, 451 (1962). 3 G. A. Samara and H. G. Drickamer, J. Phys. Chern. Solids 23,457 (1962). kbar the changes in resistance differed considerably from run to run, usually, the features becoming more drastic the thicker the sample. Figure 2 shows the results on two runs. In the figure, the resistances are normalized to the same sample thickness in order to show the reproducibility of the data in the low-pressure region. In five of the runs, a sharp rise in resistance oc- curred at 80-100 kbar. The resistance decreased again with increasing pressure above 120-130 kbar. In the case of run No.4, a second sharp increase in resistance occurred at 160 kbar followed a decrease above 225 kbar. Above 250 kbar the resistance changed in a regu- lar manner, decreasing, smoothly, by only about 4% between 250 and 500 kbar. The resistance is reversible in the low-pressure region (below 60 kbar). The re- versibility of the high-pressure region is shown by the dashed curves in Fig. 2. A plausible explanation for these irregularities is given in the discussion below. CONDUCTIVITY OF FUSED POWDER One run was made on fused powder, and the results are shown in Fig. 3. The shape of the resistance curve is quite similar to that in (a), the main features being a large drop in resistance followed by a leveling-off region above 200 kbar. The resistance drops by about a factor of 2 between 100 and 150 kbar and by only 7% - 8% between 200 and 400 kbar. DISCUSSION Many of the physical and electrical properties of graphite can be predicted from its known crystal structure. In the graphite crystal, the carbon atoms are arranged in infinite sheets of regular hexagons, the carbon-carbon distance being 1.42 A. The closeness of this value to that for most aromatic hydrocarbons sug- gests a similarity in the type of bonding involved. The distance between layers is 3.35 A, a value large enough to preclude covalent bonding between them. Layers are held together by long-range van der Waals forces. The stacking of the layers is such that alternate layers are laterally displaced from each other, producing two types of lattice sites. In one case, each atom has atoms directly above and below it in neighboring planes, while in the other, each atom is located above and below the centers of open hexagons in neighboring planes. This stacking pattern determines the degree 471 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Sun, 21 Dec 2014 23:56:46 472 G. A. SAMARA AND H. G. DRICKAMER ~ 0.11 J: 0 tl 0.10 z ~ !Q0.09 RESISTANCE OF PYROLYTIC GRAPHITE 473 is such that it is highest just above and just below each layer. The various calculations on the energy band in- dicate that there exists a large energy gap in the (j band system, while the two halves of the 11" band touch (i.e., zero energy gap) at the corners of the hexagonal Brillouin zone. The touching energy lies within the (J- band gap.4 The complete 11" band can accommodate two electrons per atom; however, since each carbon contributes only one 11" electron, the band is only half- filled. This band is nearly symmetrical about its mid- point, with the density of states N(E) falling to zero at this point. At OOK the lower half of the band is full. the upper half empty. At room temperature enough electrons are excited to be upper half (leaving an equal number of holes in the lower half) to give graphite its high conductivity. A summary of the results of this work is shown in Fig. 4, where the resistances have been normalized to the same sample thickness. The larger drop in resist- ance along the c axis, as compared with that along the a axis, in the low-pressure region is associated with the higher compressibility along this direction. In all cases, the drop in resistance is a result of the increased elec- tronic interaction caused by bringing the atoms and layers closer together. We believe this increased inter- action causes the two halves of the 11" band to overlap, which leads to higher conductivity. It is believed that the sharp decrease in slope of the resistance-pressure curve can be explained in terms of greatly reduced compressibility. Bridgman's5 com- pressibility data to 100 kbar can be extrapolated smoothly on a log-log plot. The results are shown in Table 1. The compressibility decreases rapidly at high TABLE I. Fractional volume change of graphite vs pressure (extrapolated from Bridgman's data). P (khar) AV/Vo 20 0.0458 40 0.0715 60 0.0907 80 0.1057 100 0.1169 120 0.1253 140 0.1317 160 0.1366 180 0.1401 200 0.1427 220 0.1445 240 0.1458 260 0.1467 280 0.1474 300 0.1478 6 P. W. Bridgman, Proc. Am. Acad. Arts Sci. 76, 55 (1948). 2.5 41 ~2.0 ~ V) Hi a:: 9 \.5 1.0 0.10 100 Kb 200Kb 300Kb 0.12 (viVo) 0.14 FIG. 5. Rel'>istance vs fractional change in volume. pressure because of the low polarizability of the carbon atoms. Figure 5 shows a plot of resistance vs density over the region from 100-300 kbar. Since the density extrapolation is probably not exact, the details of the curve probably are not significant, but the smooth curvature indicates that nothing drastic has occurred to the sturcture at 200 kbar. Similar results have been observed for fused-ring aromatic compounds.6 These are in marked contrast to results obtained for metals, 1 some zinc-blende-type compounds2,3 and the thallous halides,7 where considerable change in resistance was observed above 200 kbar. The irregularities in the results above 60 kbar along the a axis can be explained as follows. The weak binding between the layers in graphite permits these layers to slide and slip past each other. The absence of a perfectly hydrostatic medium, as is the case in this work, enhances this slip. We attribute the rise in re- sistance at 80-100 kbar to this effect, the decrease in conductivity being due to the increased scattering caused by lattice disorder and the random rearrange- ment of the layers. One would expect this effect to be more drastic the thicker the sample and the less hy- drostatic the medium.s The results bear this out. Run No.4 in Fig. 2 illustrates this point. In this case, the second rise in resistance at 160 kbar is also attributed to the same effect. From the shape of the reversibility 6 G. A. Samara and H. G. Drickamer, J. Chern. Phys. (to be published) . 7 G. A. Samara and H. G. Drickamer, J. Phys. Chern. Solids (to be published). 8 Our apparatus is such that the pressures are somewhat less hydrostatic for thick samples. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Sun, 21 Dec 2014 23:56:46 474 G. A. SAMARA AND H. G. DRICKAMER curve, it is believed that the slip effect causes a reori- entation of the lattice, going from a-axis to c-axis ori- entation along the direction of compression. To check this, the pressure was increased and decreased on one sample three times. The results are summarized by curves (3-a, -b, -c) in Fig. 4. The shape of the resist- ance curve after the second compression was quite similar to that along the c axis. The resistance of the fused powder was in between those along the a and c axes. The pressure dependence THE JOURNAL OF CHEMICAL PHYSICS was similar to that along the c axis, although there is a smaller decrease in the resistance at low pressure. The sharp drop in resistance below 20 kbar is caused by the fusing of the sample. In all three cases, the small decreases in resistance above 250 kbar were of about the same magnitude. ACKNOWLEDGMENT The authors wish to thank D. Shiff of High Tem- perature Materials for the graphite samples. VOLUME 37, NUMBER 3 AUGUST 1,1962 Effect of Pressure on the Resistance of Fused-Ring Aromatic Compounds* G. A. SAMARA AND H. G. DRICKAMER Department oj Chemistry and Chemical Engineering, University oj Illinois, Urbana, Illinois (Received April 10, 1962) The effect of pressure to several hundred kilo bars has been measured on the resistance of seven-fused ring aromatic compounds, including two polyacenes and five quinones. In general, there is a rapid drop in resistance at lower pressures, followed by a marked leveling at above 200-250 kbars. The leveling is ap- parently associated with the rapid decrease in compressibility at high pressure observed by Bridgman. The resistance at high pressure varied by several orders of magnitude among the compounds. It seems to be closely associated with the amount of overlap between adjacent molecules in the unit cell. The temperature coefficient of resistance at high pressure was obtained for three compounds. They all remained semicon- ductors at the highest pressure studied, but the activation energy was about one-sixth of the atmospheric pressure value. THE effect of pressure to several hundred kilobars has been measured on seven organic crystals in- cluding two polyacenes, tetracene and pentacene, and five quinones: violanthrone, isoviolanthrone, pyran- throne, dibenzo-(a,h)pyrene-7, 14-dione and dibenzo- (cd ,jk) pyrene-6, 12-dione.1 The structural formulas of these compounds are given in Fig. 1. In each case, the electrical resistance was measured as a function of pressure, at room temperature (23°-25°C). For three of the compounds, pentacene, violanthrone, and iso- violanthrone, the temperature coefficient of the re- sistance at high pressure was approximately deter- mined. The experimental techniques have been de- scribed elsewhere.2- 4 The samples were fused powder 0.001 in. thick, loaded as discussed.3.4 Relative resist- ances are reported as measured, as it was not possible to correct, for contact resistance. These problems are discussed in reference 4. By measuring both current and potential it could be established that the contacts were at least roughly Ohmic. Leads of copper, amal- * This work was supported in part by the United States Atomic Energy Commission. 1 For the sake of brevity we henceforth refer to the last two of these compounds as dibenzo-7, 14-, and dibenzo, -6, 12-, respec- tively. 2 A. S. Balchan and H. G. Drickamer, Rev. Sci. lnstr. 32, 308 (1961). as. Minomura and H. G Drickamer, J. Phys. Chern. Solids 23,451 (1962). 4 G. A. Samara and H. G. Drickamer, J. Phys. Chern. Solids 23,457 (1962). gamated copper, gold, and platinum were used with no differences in the results. In the usual apparatus it was not possible to measure resistances above 107 12 because of bomb resistance, so data are not shown until the resistance fell below that level. A special cell with Bakelite liner and oven-dried pyrophyllite was used for tetracene. With this cell resistance as high as 1010 12 could be measured, but pressures were limited to 250 kbar and there was considerable breakage. The results are qualitatively and, in some cases, quantita- tively similar, the main features being a large drop in resistance (linear, on a log resistance vs pressure plot) in the low-pressure region followed by a leveling-off region above 200 kbar. The atmospheric resistivities are high, ranging from 1010 12-cm for isoviolanthrone to 1015 12-cm for tetracene. Since, as mentioned earlier, the high-pressure cell could not be insulated to better than 107 12, it was not possible to measure the changes in resistance in the low-pressure region. In all cases, the resistances of the samples dropped to within range of the apparatus by 80-150 kbar. A detailed presentation of the results follows. TETRACENE This material was obtained from Eastman Kodak Company and was sublimed once under vacuum. The resistance never dropped below 106 12, and, for this reason, it was necessary to use the high-resistance cell with this compound. Three runs, up to 250 kbar, were This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Sun, 21 Dec 2014 23:56:46
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