404 Chapter 24 polyimide molecules, PMDAPPD is the most flat, as shown in Fig. 4. This molecule does not contain ether oxygen but its film is very brittle. A PMDA/PPD film, named PPT, was prepared at the Research LaboratoIy of Toho Rayon Co. Ltd. with a small amount of additive to keep the film flexible (Fig. 4). Rectangular specimens, approximately 26 mm long and 10 mm wide, were cut from a 45 pm thick PPT film, sandwiched between polished artificial graphite plates and heated by infrared radiation to 900°C at a rate of 2°C mind in a flow of nitrogen for 1 h. These carbonized films were then stepwise heat-treated, 1800-3200°C in flowing argon. The residence times at final temperatures were 30 min between 1800 and 3000°C, and 10 min at 3200°C. Figure 5 shows changes in the X-ray 006 diffraction profiles (using Cu Ka radiation) of the specimen films with increasing HTI'. For each film specimen, in the /Ayc 18WC I I I I I I I I 82 83 84 85 86 87 88 89 J 2500'C J I I t L 85 86 87 88 89 2W) Fig. 5. '006' Diffraction profiles of PFT-derived carbon films heat-treated at various temperatures (reprinted with the permission of Elsevier Science from Ref. [lo]). Magnetoresistance 0.343Or I 1 I 1 I I II 405 - 0.341 0 - 0.3400 - 5 0.3390 - 4 0.3380 - 0.3370 0.3360 - 0.3350 - o-mm*18&3 - 2dOO 2200 2400 2600 2800 3000 Hl-r ("C) Fig. 6. d,,,,, for PPT-derived carbon films heat-treated at temperatures between 1800 and 3000°C (reprinted with the permission of Elsevier Science from Ref. [lo]). ' 1800 2dOO 22bO 24bO BOO 28100 3d0( HTT("C) Fig. 7. Mosaic spread for PPT-derived carbon films heat-treated at temperatures between 1800 and 3000°C (reprinted with the permission of Elsevier Science from Ref. [lo]). 406 Chapter 24 -2.0 s 2000 2200 2400 2800 2800 3000 1800 2000 2200 2400 HTT ("C ) HTT ("C ) Fig. 8. (A~/P)~~~ measured in a field of 1 T for PPT-derived carbon films heat-treated at temperatures between 1800 and 3000°C (reprinted with the permission of Elsevier Science from Ref. [lo]). The values of (AP/P"),,,~~ for the 2200°C specimen, at 77 K, are positive at all magnetic fields up to 1 T, but are slightly negative for (A~/P~)~,,,~~ and (Ap/po)rLmin, -0.01% and -0.009%, respectively. This dependence of magnetic field orientation at 1 T results from the superposition of two magnetoresistance components, one compo- nent with positive magnetoresistance and the other with negative magnetoresistance. Because the negative magnetoresistance relates to the turbostratic structures and the positive to graphite, the 2200°C specimen consists, electrically, of two phases. But no direct evidence for such a composite structure has been found in X-ray diffraction studies. The negative magnetoresistance decreases its absolute value more rapidly with increasing temperature of measurement than the positive magnetoresistance [12]. Hence, the value of (A~/P~),,,~ of the 2200°C specimen, when measured at room temperature, should be larger than that at 77 K. Results in a field of 1 T confirm this as being 1.03% at room temperature and 0.400% at 77 K, as indicated by open circles in Fig. 8a. Also, the 2250°C specimen gives nearly the same values, 1.33 and 1.37%, respectively. Further increases of HTT up to 2300 and 2400°C cause reversal of the relative magnitudes as indicated by changes to arrows from solid to open circles in Fig. 8a. With a decreasing contribution of the negative component, the net magneto- resistance is reduced with increasing temperature. This is the case for the specimen of HTT 2400°C (Fig. 8a), and less markedly for the specimens of HTT 2250 and 2300°C. Magnetoresistance 407 I I I I 1 I 1800 2000 2200 2400 2600 2800 3000 HTT ("C ) Fig. 9. r for PPT-derived carbon films heat-treated at temperatures between 1800 and 3000°C (reprinted with the permission of Elsevier Science from Ref. [lo]). Figure 8b shows the dependence of (A~/P~),,,~~ on H'IT between 2250 and 3000°C. (Ap/p,Jrnax exceeds 900% for the specimen of HlT 3000°C and reaches 1206% for the specimen of H'IT 3200°C (held for 10 min). It must be noted that values of (AP/~~),,,~~, observed in regular quality HOPG, are in the range of 1100-1300% [SI. The textures of the carbon films are similar to that of ideal graphite but are less perfect. The values of rT and rTL of each heat-treated film are close to zero, but are not identical with each other, because of a small inhomogeneity (heterogeneity) in texture. Therefore, the mean value of rT and rTL, r = (rT + rTL)/2, was used as a parameter for the degree of orientation. In Fig. 9, r is plotted as a function of HTT. The trend in Fig. 9 can be interpreted as follows. The orientation of the specimen of HTT 1800°C is turbostratic and with a relatively high degree of orientation of the graphene layers along the film surface. The degree of orientation was improved by successive heat treatment up to 2000°C even though the transformation from turbostratic structure to graphite was initiated. A further increase in HTT caused slight deformation of the film, probably due to the growth of the graphite crystallites. At higher values of Hl'T, the crystallites continue to grow and, as a consequence, r increased gradually. It seems that at a certain HlT an annealing effect of lattice imperfections takes place in addition to the crystal growth. A critical HTT appears at -2600°C when the maximum value of r was obtained. However, the annealing effect might granulate graphite crystallites, so that r begins to level off changing little on heating to 2900°C. The r value at 3200°C is 0.0333 (not shown in Fig. 9). The resistivity ratio pRT/p4.2K of this carbon specimen is 3.45, pRT and p4,2K being the resistivities at room and liquid helium temperatures with the crystallinity corresponding to that of an HOPG. Kaburagi et al. [ll] obtained highly graphitic films from PPT-derived carbon films by heat treatment at 3200°C. Rectangular strips of film, 3 mm wide and 20 mm in length, were cut from the PPT film and carbonized by infrared heating. The 408 Chapter 24 1400 1200 1000 n 800 ae v x a E n 600 a Q W 400 200 I1 I I I I 0123456 (T) Fig. 10. (Ap/pJmar at liquid helium temperature for PPT3200-2, together with those KG4.65, KG11.9, HOPG3600 and PPT3200-1 (reprinted with the permission of Material Research Society from Ref. [ll]). carbonized film-strips were then heat treated using two different procedures. Each of the carbonized films was sandwiched between two polished artificial graphite plates. One, called PPT3200-1, was heated up 3200°C in a graphite resistance furnace in a flow of argon (soak time 10 min). The second strip, called PPT3200-2, was heated to 3100°C (soak time 40 min) and then at 3200°C (soak time 23 min). The d,,, values for these specimens were identical, i.e. 0.3354 nm. The resistivity ratio pRT/p4,2Kwas 3.45 for PPT3200-1 and 4.90 for PPT3200-2, while (AP/~,),,,~ in a field of 1 T at 77 K was 1206% for PPT3200-1 and 1621 % for PPT3200-2, indicating higher crystallinity of PP3200-2. This is supported by a measurement of the Shubnikov-de Haas oscillation observed of (AP/~~),,,~~ at 4.2 K for PPT3200-2, as shown in Fig. 10. Here, the results for KG4.65, KG11.9, HOPG3600 and PPT3200-1 are shown for comparison (KG is kish graphite) followed by its resistivity ratio pRT/p4,2K, the number after HOPG is the annealing temperature. The r values for PPT3200-1 and PPT-3200-2 are 0.0170 and 0.0051, and that of HOPG3600 is 0.0051. These results indicate the extensive graphitization of PPT3200-2. Magnetoresistance 409 5 Negative Magnetoresistance in Boron-doped Graphites Boron substitutes carbon atoms in graphene layers, the maximum solid solubility being 2.35 at% at 2350°C [13]. Hishiyama et al. [14-16] and Sugihara et al. [17] found a weak negative transverse magnetoresistance for three kinds of boron-doped graph- ites with the field perpendicular to the specimen surface, and characterized by a field dependence proportional to B1" at temperatures below 4.2 K. The boron-doped natural graphite compacts, boron-doped Grafoil materials and boron-doped graphite films were studied, the boron-doped graphite films being Kapton-derived highly graphitic graphite films (HOGF). All of the boron-doped carbons have interlayer spacing, doOz, lower than for single crystal graphite, 0.3345 nm, and electrical resistivities which exhibit weak temperature dependence but which, at substantially lower temperatures, increase with decreasing temperature expressed as TI". This type of negative magnetoresistance is not due to increases in carrier density, as for turbo- stratic carbons [4], nor to two-dimensional weak localization which was used by Bayot et al. to explain the negative magnetoresistance of turbostratic carbons, because of the different type of field dependence of the negative magnetoresistance and the different type of dependence of the resistivity on temperature [6]. It is probably explained by a three-dimensional weak localization in graphites as proposed by Hishiyama et al. [16,17]. Hishiyama et al. [16] related the negative magnetoresistance of boron-doped graphite to a disordered structure created by the substitution by boron atoms. An X-ray study of the boron-doped HOGFs (B-HOGFs) was carried out as well as a cross-sectional study by scanning electron microscopy (SEM) and Raman scattering. Hishiyama et al. [16] used HOGF, about 11 pm in thickness, for boron doping. The high level of orientation of the graphene layers is slightly disturbed by boron doping. The disturbance is seen in cross-sectional SEM micrographs of HOGF and B-HOGF. The lattice constant c,, was determined from the 002, 004 and 006 diffraction lines measured in the reflection mode with a,) being measured from the 100 and 110 diffraction lines in the transmission mode. The atomic fraction of dissolved boron x, for B-HOGFs was estimated from X-ray diffraction measurements from relationships of the lattice constants a,, and c,) vsx, given by Lowel [13]. The values of a,, and c,,,x, and the full width at half maximum of the peak intensity of the 002 diffraction (PI!? for the samples are listed in Table 1. The X-ray diffraction results for the B-HOGF indicated well-crystallized materials, but appear to be rather disordered as indicated by the Raman spectra. Figure 11 shows the first and second order Raman spectra of HOPG, HOGF, B-HOGFs and glass-like carbon heat-treated at 1600°C (GC-1600). The Raman spectra of HOPG and HOGF are those of well-crystallized graphite materials and show a G band at 1585 cm-I, an overtone band at 2441 cm-', a doublet at 2680 cm-' (GI' band) and 2725 cm-' (Gi band) and an overtone band at 3249 cm-I. The spectra of B-HOGF-0.4 and B-HOGF-2.2 are similar to those of carbons with small crystallites [18,19]. The spectra show the relatively strongD band (1367 cm-I), the D'band (1623 410 Chapter 24 Table 1 The lattice constants a. and co, atomic fraction of boron dissolved into latticexB, the full width at the half-maximum of the peak intensity recording of the 002 diffraction I$,,2, room temperature resistivity p300K, resistivity ratio p300K/p3K and transverse magnetoresistance measured at 3 Kin a field of 1 T (Ap/po)3K,IT for HOGF and B-HOGFs [16] Sample code a. co XB QlIZ P3WK lo7 P3lMK/P3K APIP0)3K,1T (nm) (nm) (at%) ("1 (W (%I HOGF 0.24612 0.67076 0 4.04 6.33 1.13 901 B-HOGF-0.4 0.24623 0.67054 0.4 4.62 13.0 1.0391 0.137 B-HOGF-0.5 - 0.67046 0.5 - 14.2 0.9911 0.003 B-HOGF-0.9 - 0.67022 0.9 - 19.5 0.9735 -0.183 B-HOGF-1.4 - 0.66992 1.4 - 24.8 0.9645 -0.179 B-HOGF-2.2 0.24682 0.66944 2.2 10.23 26.4 0.9785 -0.169 1000 1500 2000 2500 3000 3500 Raman sh i f t (cm-'1 Fig. 11. Raman spectra of HOPG, HOGF, B-HOGF-0.4, B-HOGF-2.2, and GC-1600 with 514.5-nm excitation [16]. Magnetoresistance 411 cm-') at the high frequency side of the G band (1584 cm-'), the weak 2441 cm-' band, and the single unsymmetrical G' band (2720 cm-') which is a merged band of G' and G bands observed in HOPG and HOGF, the D" band (2962 cm-I), and the band at 3246 cm-I. The occurrence of D, D' and D" bands is a characteristic of a disordered graphite structure and is related to the substituted boron atoms in the graphene layers. With increasingx,, the relative intensities of the D andD' bands to the G band increase and that of the G' band to the G band decreases. The Raman spectrum of GC-1600 is similar to that of B-HOGF-2.2, showing a higher disorder. The room temperature electrical resistivity, p300K, the resistivity ratio p300K/p3K, where p3K is the resistivity at 3.0 K and transverse magnetoresistance at 3.0 K in a magnetic field of 1 T (AP/&~,'~ for HOGF and B-HOGFs are listed in Table 1. 0. 5 1.01 1 + t 3 0 rn 1.00 Q Q t-r 0 OC 'A 0 2 4 6 8 10 12 14 16 T1/2 (K1/2) Fig. 12. Relative resistivity p7/p300K for B-HOGFs plotted as a function of TI'' [16]. 412 Chapter 24 0.2 0. 1 0 14 0 -0.1 -0. 2 -0.3 -0.4 -0.5 0 0. 5 1.0 1.5 2.0 B1/2 (T1/2) Fig. 13. Transverse magnetoresistance Ap/pn measured at 3.0 K for B-HOGFs, plotted as a function of B'" 1161. HOGF is characterized by a large value of (Ap/p0)3K,IT, while each of the B-HOGFs is differentiated by a small positive or a small negative value of The dependence of the resistivity pT on temperature for HOGF is weakened at the lowest x, value, while keeping the weak dependence for all of the B-HOGF carbons. This fact indicates that scattering from the substituted boron atoms dominates over the lattice scattering [20]. Values of p7/p30nK for B-HOGF carbons are plotted as a function of T112 in Fig. 12 where the number represents thevalue ofx,. With increasing TI", starting from the lowest temperature, the resistivity decreases gradually and linearly with to about 20 K, then decreases a little gradually to pass through a shallow minimum and then finally increases. The temperature of minimum resistivity increases with increasing xB, exceeding 300 K for B-HOGF-1.4 and -2.2. Because the temperature dependence of the resistivity for B-HOGF-1.4 and -2.2 is very weak, the linear TIi2 dependence at low temperature is related to an additional resistivity superimposed onto the Boltzmann contribution. The additional resistivity could be attributed to the quantum correction of the resistivity due to the 3-D weak localiza- tion 6p which is obtained by extending the Kawabata's theory to the SWMcC band [ 16,171. Figure 13 shows the transverse magnetoresistance Ap/pn measured at 3.0 K for B-HOGF-1.4 and -2.2 plotted as a function ofB112 and indicating that Ap/pO is negative Magnetoresistance 413 and decreases with increasing B’”. For B-HOGF-0.9, Ap/po is negative in low fields, decreases linearly with B’” in fields below 0.41 T, passes through a minimum and then increases with a change in sign with a further increase of B. Similarities among the curves of the field dependence of Ap/po for B-HOGF-0.4,0.5, and 0.9, and the linear B’” dependence of Aplp,, in low fields for these samples is qualitatively explained by Hishiyama et al. [16] and Sugihara et al. [17]. For B-HOGF-1.4 and -2.2, Ap/p,, decreases linearly with B’” in fields above about 0.8 T. References 1. Y. Hishiyama, Y. Kaburagi and M. Inagaki, Characterization of structure and microtexture of carbon materials by magnetoresistance technique. In: P.A. Thrower (Ed.), Chemistry and Physics of Carbon, Vol. 23, pp. 1-68. Marcel Dekker, New York, 1991. 2. D.E. Soule, Magnetic field dependence of the Hall effect and magnetoresistance in graph- ite single crystals. Phys. Rev., 112 698-707, 1958. 3. Y. Hishiyama, Negative magnetoresistance in soft carbons and graphite. Carbon, 8: 4. P. Delhaes, P. de Kepper and M. Uhlich, A study of the negative magnetoresistancc in 5. A.A. Bright, Negative magnetoresistance of pregraphitic carbons. Phys. Rev., B20: 6. V. Bayot, L. Piraux, J P. Michenaud, J P. Issi. M. Lelaurain and A. Moore, Two- 259-259,1970. pyrolytic carbons. Phil. Mag., 29: 1301-1330, 1974. 5142-5149, 1979. dimensional weak localization in partially graphiticcarbons. Phys. Rev., B41: 11770-1 1779, 1989. 7. M. Inagaki, Microtextures of carbon materials. Tanso (No. 122): 114-121,1985. 8. M. Inagaki, New Carbons. Elsevier Science, Oxford, 2000. 9. Y. Hishiyama, Y. Kaburagi, M. Inagaki, T. Imamura and H. Honda, Graphitization of ori- ented coke made from coal tar pitch in magnetic field. Carbon, 13: 54&542,1975. 10. Y. Hishiyama, M. Nakamura, Y. Nagata and M. Inagaki, Graphitization behavior of carbon film prepared from high modulus polyimide film: synthesis of high-quality graphite film. Carbon, 32 645-650,1994. 11. Y. Kaburagi, A. Yoshida and Y. Hishiyama, Microtexture of highly crystallized graphite as studied by galvano-magnetic properties and electron channeling contrast effect. J. Mater. Res., 11: 769-778,1996. 12. Y. Kaburagi, R.H. Bragg and Y. Hishiyama, Electrical resistivity, transverse magneto- resistance and Hall coefficient in pyrolytic carbon: correlation with interlayer spacing d,”,> Phil. Mag. B., 63: 417-436, 1991. 13. C.E. Lowell, Solid solution of boron in graphite. Am. Ceram. SOC., 50: 142-144, 1967. 14. Y. Hishiyama, Y. Kaburagi, K. Kobayashi and M. Inagaki, Structure and properties of boronated graphite. Molec. Cryst. Liquid Cryst., 310 279-284, 1998. 15. Y. Hishiyama, Y. Kaburagi and K. Sugihara, Negative magnetoresistance and of magnetic susceptibility of boronated graphite. Molec. Cryst. Liquid Cryst., 340: 337-342,2000. 16. Y. Hishiyama, H. Irumano, Y. Kaburagi and Y. Soneda, Structure, Rarnan scattering and transport properties of boron-doped graphite. Phys. Rev. B, 63: 245406-1-24506-11,2001. 17. K. Sugihara, Y. Hishiyama and Y. Kaburagi, Electronic and transport properties of boronated graphite: 3D-weak localization effect. Molec. Cryst. Liquid Cryst., 340: 367-371, 2000. [...]... electrode surface Carbon surfaces are modified easily by various chemical and physical treatments One of the objectives of the Carbon Alloys project is to modify carbon surfaces to derive new functionalities Thus, studies of electrochemical functions of carbon electrodes are of major interest to carbon alloys The objective of this chapter is to understand the electrochemicalcharacteristics of carbon materials... preparation history (pyrolysis and carbonization) [22] Carbons are classified into two types, i.e., graphitizable (soft carbons) and non-graphitizable (hard carbons) according to the parent material and heat treatment temperature ( H n ) Graphitizable carbons have an HTT > 2300°C Non-graphitizable carbons do not graphitize even at temperature >2800°C Nongraphitizable carbons are mostly prepared by solid-phase... capacity (mAh/g) ( ) Graphitizable carbon (HTT=3000%) 1 (21 Graphitizable carbon (HTT=2000%) (' Non-graphitizablecarbon (HTT=700%) 3 Fig 8 Plots of voltage vs reversible capacity for (a) the second discharge and (b) charge cycle of representative carbon and graphite samples; (1) graphitizable carbons H T I 3000"C, (2) graphitizable carbon H'IT 2000"C, (3) non-graphitizablecarbon H'IT 700°C materials [25]... ordered pyrolytic graphite (HOPG), glassy carbon (GC), carbon fiber (CF), and activated carbons, etc Carbon surfaces, such as pore size, electronic states, and presence of functional groups, can be modified and controlled by various chemical and physical treatments Therefore, development of carbon electrodes with superior functions is the subject of the Carbon Alloys project Electrode materials in new... properties of conventional and novel types of carbon materials for Li-ion batteries Anode carbons include graphitizable carbons such as milled mesophase pitch-based carbon fibers, non-graphitizable carbon such as polyparaphenylene-based carbon heat-treated at low temperatures Market demand and the trends in lithium-ion secondary batteries are commented upon Kevw0rd.c Carbon, Microstructure, Anode material,... Sensor 1 Features of Carbon Materials as Electrodes Carbon materials, consisting of sp2 carbon atoms, show high electrical conductivity and several other prominent characteristics such as high chemical and thermal stability, catalytic properties, light weight, etc Furthermore, carbon materials with widely different properties and shapes are available at reasonable prices Therefore, carbon has been used... characteristics of carbons show them to be superior to typical metals, such as Pt and Hg as electrode materials [l-31 Firstly, a variety of carbons with different properties are obtained by controlling sizes and orientations of crystallites consisting of hexagonal planes Examples include highly oriented pyrolytic graphite (HOPG), glassy carbon (GC), carbon fibers (CF), and activated carbon fibers (ACF)... shifting with excitation wavelength for carbons and graphites Solid State Commun., 39: 341-344,1981 20 D.E Soule, The effect of boron on the electronic properties of graphite In: Proceedingsof the Fifth Conferenceon Carbon, Vol 1, pp 13- 21 Pergamon Press, Elmsford,N.Y., 1962 Part 5 Function Developments and Application Potentials 417 Chapter 25 Applications of Advanced Carbon Materials to the Lithium Ion... chapter is to understand the electrochemicalcharacteristics of carbon materials in relation to carbon alloys Applications of carbon electrodes, including future uses such as lithium rechargeable batteries, capacitors and sensors are described 2 Electrochemical Reactions on Carbon HOPG, a synthetic, highly graphitic carbon, is a semi-metal,with a small overlap (0.04 eV) between the conduction and valence... battery performance [20-221 Two types of carbon material have been used, that is highly ordered graphites heat-treated to 3000°C and non-graphitizable carbon heat-treated only to 1100°C Precursor materials include cokes, polymers and fibers The insertion behavior and mechanism of lithium ions into carbon and graphite hosts have been extensively studied [23-281 In particular, lithium insertion and resultant . Moore, Two- 259-259,1970. pyrolytic carbons. Phil. Mag., 29: 130 1 -133 0, 1974. 5142-5149, 1979. dimensional weak localization in partially graphiticcarbons. Phys. Rev., B41: 11770-1 1779,. and novel types of carbon materials for Li-ion batteries. Anode carbons include graphitizable carbons such as milled mesophase pitch-based carbon fibers, non-graphitizable carbon such as polyparaphenylene-based. Conference on Carbon, Vol. 1, pp. 13- 21. Pergamon Press, Elmsford, N.Y., 1962. Part 5 Function Developments and Application Potentials 417 Chapter 25 Applications of Advanced Carbon