Carbon Alloys part 4 doc

Control of Interface and Microstructure in Carbon Alloys 89 ‘0 Fig. 6. Schematics of a layered structure of oxidized carbide at a low pressure of oxygen. An interesting result concerning the interface of carbon fibers is that residual carbons in a Si-Ti-C-0 fiber system were deposited on the surfaces as a homogeneous carbodgraphite layer and this layer acted as a sliding layer in the composites [29]. As a result, this layer changed the fracture behavior of the composites from brittle to pseudo-plastic. At low oxygen pressures, metal carbides were transformed into a layered structure consisting of graphite and metal oxides 1301. The schematics of this are illustrated in Fig. 6. In composites using Si-Ti-C-0-system fibers the same phenomena took place and a graphite layer was formed. Ion implantation techniques are used as surface treatments of carbon-related materials [31]. Metal ions were implanted in a thin layer of the surface and the layer became amorphous with a dose of 10l6 to lo1’ with 150 keV acceleration. Annealing treatment tended to enrich the ions on surfaces and the metal ions became stable. It was thought that amorphous carbon on the surfaces of the implanted carbons would improve their performance as electrodes. Therefore, they can be used as special electrodes [31]. 3 Microstructure Control As mentioned above, thermal conductivity of graphitic materials is strongly affected by their microstructure. A typical example is demonstrated in a C/C composite. Figure 7 shows thermal conductivity k of a UD-C/C composite for a thermosetting resin derived matrix. Thermal conductivity increased significantly with increasing HTT. This change coincides with increases in the crystallite size (110 diffraction band), which coincides with the mean free path length of phonons calculated from Eq. (3) as seen in Fig. 8 [32]. Generally, precursors with a large molecular weight gave smaller domain sizes in carbon products than those having a small molecular weight 1221. The former precursors are of high viscosity at low temperatures, with the latter showing low char yields and melting during carbonization at atmospheric pressure. To change the microstructure of carbon products, ratios of small to large molecules were controlled 1221 and additions of small amounts of fine graphite/carbon particles were found to be 90 Chapter 5 7 ""t Longitudinal direction E 9-&- 1 .o 0 .1- 1000 2000 30C I Heat-treatment Temperature / "C Fig. 7. Change of thermal conductivity of UD C/C composite as a function of heat treatment temperature 1321. effective [33]. However, it is quite difficult to control the viscosity of carbonizing systems. Therefore, it is preferable to use low molecular weight precursors and then polymerize them at low temperatures in order to produce a high char yield. Dehydration and/or cross-linking by air-oxidation and treatments with chlorine at about 200°C were found to be effective in stabilizing precursors during carboniza- tions. Some disadvantages of these treatments are that air-oxidation took a long time at the relatively high temperature of above 250°C and chlorine treatment caused localized reactions resulting in heterogeneous structures. Recently, it was found that iodine treatment at ca. 100°C for coal-tar-pitch, softening point 103"C, resulted in a decreased benzene-soluble fraction, an increasing pyridine-insoluble fraction in the pyrolyzed pitch, an increasing char yield to 75 wt% and a decreasing domain size in the resultant carbon [34]. Detailed studies [35,36] revealed that iodine formed a charge transfer complex with pitch molecules, and that the complex increased char yield and also viscosity during carbonization. Condensed aromatic rings consisting of molecules having ca. 14 benzene rings were suitable for complex formation. A structural model for the complex was produced. Iodine affected dehydration and polymerization of the pitch during its pyrolysis, with U I ohttice Spacing 4600 \ oCrystaIIite Size 300 8 2.45 5 2.44 100 2000 2400 2800 200 3 v) m V Heat-treatment Temperature / "C Fig. 8. Crystallite size of (110) plane in resin derived composite [32]. Control of Inteflace and Microstructure in Carbon Alloys 91 polymerizations taking place at a temperature 200°C lower than for the original pitch. This low temperature polymerization brought about the high char yield. An interesting phenomenon has been observed with a glassy carbon. On heat treatment, a surface layer, only 20 to 30 nm deep, is graphitized, the interior remaining more or less unchanged [37]. Surfaces themselves have an important role in the determination of crystal structure. New types of carbon alloys with interesting properties have appeared as a result of the project “Carbon Alloys”. For example, activated carbons with controlled pore size(s) have been produced from a mixture of a standard resin with an ion-exchanged resin containing different cations, as well as hard carbons with nitrogen alloying, and semiconductor carbons [38]. The concept of Carbon Alloys can be extended to ceramic materials containing carbons [39]. For example, the synthesis of fine carbide powders through a sol-gel route employed a mixture of resin precursors and metal alkoxides. In this process, homogeneous structures with small quantities of doping additives in resultant carbons can be easily achieved. Ultra-fine particles of ceramics were also synthesized at low temperatures. For PAN fibers mixed with fine particles of Fe or Ni dispersed in the carbon, reaction rates were accelerated at 1250 K including the evolution of nitrogen gas resulting in low nitrogen contents and high carbon yields from the PAN fibers [40]. An interesting effect of alloying on the oxidation of resin-derived carbon was found. Additions of small amounts, <1 wt% of Ta or Ti, into a resin precursor enhanced the oxidation resistance of the resultant carbons [41]: in a typical case, oxidation rates decreased by a factor of ten. Detailed mechanisms still await clarifica- tion. The number of the active sites in the carbon decreases and oxidation rates were lower than the desorption rates of CO and CO, [42]. For amorphous films of hydrogenated carbon nitride formed by a CVD process, the hardness of the films depended upon amounts of hydrogen that terminated the (b) Fig. 9. Cluster models for amorphous hydrogenated carbon nitride. Each hexagon indicates a cluster consisting of carbon and nitrogen network. Hydrogen atoms terminate the networks. (a) Relatively small amount of hydrogen and the boundary is mechanically strong, and (b) relatively large amount of hydrogen and the boundary is weak [43]. 92 Chapter 5 .8. Oneat -7 - ATi02 .6 - .5 - 1000 1500 2000 2500 3000 0 I I , '0 - Heat-treatment Temperature 1 "C Fig. 10. Density change of inorganic powder contained composites [4]. clusters. The model in Fig. 9 indicates hardness increases with hydrogen content [43]. The hexagons indicate clusters of amorphous hydrogenated carbon nitride with carbon and nitrogen networks being terminated by hydrogen. As the shear strength of the graphene layer planes of HOPG is as low as 3 MPa, then the high orientation of graphite crystals in the matrix has to be restrained. To make high shear strength materials, a fine microstructure is required, achieved when fine particles were added to the precursor. In general terms, density, grain size, and orientation of both the crystallite in the grains and the grains themselves mainly control the mechanical properties of ceramics. The density of the composites mixed with fine particles is plotted against €TIT, as in Fig. 10. The densities of the composite, except for Sic-doped composites, increased with increasing HTI'. However, the density of Sic-doped composites decreased dramatically above 2400°C. The phase diagram of the silicon-carbon system indicates the incongruent melting of Sic at about 255OoC, the vapor pressure of the molten silicon being well above this temperature [44]. The (002) diffraction profiles of a furan-resin derived C/C composite were asymmetric, the profile being composed of three peaks, from the fiber, from the glass-like matrix carbon and from the partially graphitized matrix. Peak separation was carried out by computer using a Pierson-VI1 type function. Composites with added B,C had the smallest interlayer d,, spacings reaching the value of single crystal graphite of 0.336 nm at 2400°C. Doping with TiO, accelerated the graphitization at above 2600°C of composites [44]. Shear strengths of selected composites, measured by the DNC method, are shown in Fig. 11. Although some additives affected strength, additions of tantalum, effective as an anti-oxidant, had little effect on shear strength even for composites with high HTT. Shear strengths of composites without additives have much higher values than reported values, until now. The shear strength of a UD C/C composite made using a short curing period was about 30 MPa (H'IT 1000°C) [14], this being about 40% lower than that of a fully cured composite [45]. In resin-derived matrix composites the curing process is quite important in determining the properties of composites, Control of Integace and Microstnccture in Carbon Alloys 93 1 I I I O*o.k 0.A 0.h 0.42 0.54 do02 "m Fig. 11. Shear strength as a function of interlayer spacing of graphite component in the matrix (Neat, Sic, B,C, TalO,, TiOJ [45]. 4 Conclusion Interactions or reactions between carbodgraphite and metals are observed directly by TEM at temperatures from 1200 to 1500°C [38]. 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Akatsu and Y. Tanabe, Proc. 2nd Int. Meeting of Pacific Rim Ce- 45. Y. Tanabe, Y. Hotta, T. Akatsu, S. Yamada and E. Yasuda, J. Mater. Sci., 16: 557-559, 239-244,2000. ramic Societies (PacRim 2), 1996. 1997. Part 3 Typical Carbon Alloys and Processing [...]... Yb, Cu and Ag Surface Sci., 45 4 -45 6: 43 744 1,2000 41 N Akuzawa, K Kaneko, N Miyata, Y Soneda and Y Takahashi, Doping of IBr into graphite and carbon materials with different graphitization degree Tanso, 229-232,1999 42 N Akuzawa, T Tajima, M Watanabe, Y Soneda and Y Takahashi, Sorption of molecules by alkali metal -carbon alloys Proc Eurocarbon 2000, Vol 2, pp 827-828,2000 43 N Akuzawa, M Murakami, M... Liq Cryst., 340 : 59- 64, 2000 44 H Sato, O.E Andersson, T Enoki, I.S Suzuki and M Suzuki, Dimensional crossover and angular dependent magnetoresistance of magnetic graphite intercalation compounds; MC1,GIC's (M=Cu and Co) J Phys SOC Jpn., 69 (4) : 1136-1 144 ,2OOO 45 M Inoue and M Kimura, Alkyl derivatives of boehmite having the second stage structure Mol Cryst Liq Cryst., 341 : 43 143 6,2000 46 M Inoue, M... fullerenes 142 1 Later, Ebbesen et al found the optimum arc-evaporation conditions for the production of carbon nanotubes in bulk quantities [43 ] Apart from the arc-discharge method, several other methods have been proposed, e.g catalytic pyrolysis of hydrocarbons [44 -46 ] and condensation of a laser-vaporized carbonetalyst mixture [47 l In the Scientific Research on Priority Areas, Carbon Alloys , the... [33, 34] By adding fluorine atoms singly to the CZ4H12 cluster, fluorine atoms first terminated at the edge sites of the cluster, and then the interior carbon atoms were doped by the fluorine atoms by breaking the n-bonds between a carbon atom and its neighboring carbon atom Control of properties of carbons by fluorination was extensively investigated [35-391 Carbon nanotubes prepared by pyrolytic carbon. .. Res., 4: 1560-1568, 1989 2 T Zheng and J.R Dahn, Applications of carbon in lithium-ion batteries, In: T.D Burchell (Ed.), Carbon Materials for Advanced Technologies,p 341 Pergamon, Amsterdam, 1999 3 M Endo, C Kim, T Karaki, T Kasai, M.J Matthews, S.D.M Brown, M.S Dresselhaus, T Tamaki and Y Nishimura, Structural characterization of milled mesophase pitch-based carbon fibers Carbon, 3 6 1633-1 641 ,1998 4. .. Electrochimica Acta, 4 4 1713-1722, 1999 8 T Nakajima and M Koh, Synthesisof high crystalline carbon- nitrogen layered compounds by CVD using nickel and cobalt catalysts Carbon, 35: 203-208,1997 9 M Koh, T Nakajima and R N Singh, Synthesis and electrochemicalbehavior of carbon alloy CJV Mol Cryst Liq Cryst., 310 341 - 346 ,1998 10 T Nakajima, M Koh and M Takashima, Electrochemical behavior of carbon alloy C$... of single-walledcarbon nanotubes with lithium Chem Phys Lett., 307: 153-157,1999 23 M Inaba, M Fujikawa, T Abe and Z Ogumi, Calorimetric study on the hysteresis in the charge-discharge profiles of mesocarbon microbeads heat-treated at low temperatures J Electrochem SOC., 147 : 40 08 -40 12,2000 24 T Zhen, W.R McKinnon and J.R Dahn, Hysteresis during lithium insertion in hydrogen-containing carbons J Electrochem... pitch-based carbon fibers Carbon, 37: 561-568,1999 5 M Endo, C Kim, Y Nishimura, T Fujino and K Miyashita, Recent development of carbon materials for Li ion batteries Carbon, 38: 183-197,2OoO 6 M Koh and T Nakajima, Synthesis of well crystallized boron -carbon filament by chemical vapor deposition using a nickel catalyst Carbon, 36: 913-920,1998 7 M Koh and T Nakajima, Electrochemical behavior of carbon. .. [2] Several carbons ranging from non-graphitizable carbon to graphite were examined Further improvements are required and the results of this Carbon Alloys project are summarized in Part 5 This section focuses on the Li-intercalation (and de-intercalation) properties of these carbon materials 101 Intercalation Compounds host materials acceptor compounds I MCIz - GICs (M= Cu Cof graphite carbon fibers... Yoshizawa et al 1 141 have pointed out 2.2.2 Carbonization of mesoporous aerogel Mesoporous carbon can also be prepared by carbonizing mesoporous organic aerogels prepared by a sol-gel reaction The preparation of organic aerogels and their carbonizations were originally carried out by Pekala et al [15], who found that the resultant carbon aerogels had high porosities (> 80%) and high surface areas (40 0-900 m2 . Yb, Cu and Ag. Surface Sci., 45 4 -45 6: 43 744 1,2000. 41 . N. Akuzawa, K. Kaneko, N. Miyata, Y. Soneda and Y. Takahashi, Doping of IBr into graphite and carbon materials with different. SOC. Jpn., 69 (4) : 1136-1 144 ,2OOO. 45 . M. Inoue and M. Kimura, Alkyl derivatives of boehmite having the second stage structure. Mol. Cryst. Liq. Cryst., 341 : 43 143 6,2000. 46 . M. Inoue,. Technol., 40 . T. Watanabe, Y. Ohtsuka and Y. Nishiyama, Carbon, 32 329-3 34, 19 94. 41 . E. Yasuda, S. Park, T. Akatsu and Y. Tanabe, J. Mater Sci. Lett., 13: 378-380, 19 94. 42 . Y. Tanabe,