Ishimaru, Kengo

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Title Formation Mechanism of Microstructure in carbonized woodAuthor(s)Ishimaru, KengoCitationSustainable humanosphere : bulletin of Research Institute forSustainable Humanosphere Kyoto University (2008), 4: 31-32Issue Date2008-09-01URL http://hdl.handle.net/2433/182108RightType Departmental Bulletin PaperTextversionpublisherKyoto UniversityABSTRACTS (PH D FOR GRADUATE SCHOOL OF AGRICULTURE)Formation Mechanism of Microstructure in carbonized woodpmt olshimaruLaboratory oflnnovative Humano-habitability, RISH, Kyoto University(Present: Central Research LaboratoiTy, Daiwa House Indusn y Co., Ltd.)     Carbon materials are classified by graphite-related and diamond related carbon materials. The formeris composed of stacking hexagonal carbon layers based on sp2-bonded carbon atoms; the latter consists ofsp3-bonded carbon atoms involved in tetrahedral bonds. Microstmcture of graphite-related carbon materialssuch as a carbonized wood consists of mainly carbon crystallites, cross-linking between them, andoxygen-containing functional groups. The mechanical, electronic, and stmctural properties of thegraphite-related carbon materials depend mainly on the three-dimensional arrangement of the carboncrystallites forming the carbon skeleton. The recent developments in wood-based carbon materials providenew application fields for the new functions of carbonized wood [1, 2], while at the sarne timedemonstrating the necessity of finding a way to control its microstmcture. The further development ofvarious wood-based carbon materials requires that the carbonization process of wood and the fomiationmechanism of the microstructure of carbonized wood be sufficiently clarified, and that their microstructureon the nano scale be fu11y controlled. In the present study, the formation mechanism of the microstmctureof carbonized wood was investigated systematically, focusing on the carbonization behavior andmicrostructural change in the carbonization process between 500 and 1800eC of Japanese cedar byspectroscopic and microscopic analysis. As a result, new interesting information was found with respect tothe microstructure and its formation process in the wood carbonization. The details are summarized asfollows.     Firstly, the carbonization behavior of wood at the early stage of the carbonization process between500 and 1000eC is discussed in detail by comparison with that of cellulose and lignin [3]. A carboncrystallite composed ofpolyaromatic carbon planes in carbonized-wood, -cellulose, and -lignin was formeddue to the decomposition and volatilization of non-aromatic functional groups up to 600eC. The degree ofordering of carbon crystallites at each heat treatment temperature is described in the ascending order ofcarbonized-lignin, -wood, and -cellulose. This could be due to differences among carbonized-wood,-cellulose, and -lignin in the degree of cross-linking, such as ether bridges and disordered aromaticstmctures, in the range of 500-600"C. It appears that the degree and process of carbon crystallites'development of carbonized wood are closer to those of carbonized cellulose than to those of carbonizedlignin. This suggests that the degree of development of the carbon crystallites and the cross-linking forcarbonized-wood, -cellulose, and -lignin during carbonization are closely related to the OIC atomic ratiobefore carbonization. Byrne and Nagle have proposed a cellulose microfibri1 dominance model for theformation mechanism of carbonized wood microstmctures [4]. The present results prove that themicrostmcture in carbonized wood is strongly affected by the composition ratio ofcarbon to oxygen atomsin the raw wood.     Secondly, structural changes of sp2-bonded carbons as the main carbon skeleton of carbonized woodbetween 500 and 1800"C was studied by XPS, Raman, XRD, and TEM [5]. The results ofthe spectroscopicanalysis of carbonized wood showed that the microstmcture of carbonized wood changes drastically atapproximately 1400eC. The XPS and Raman results proved that the degree of disorder in carbon crystallitesdecreased significantly from 800 to 14000C with decomposing of the cross-linkings such asoxygen-containing functional groups and amorphous phase sp2-bonded carbon up to 1400eC. XRD resultsshowed that the plane size of the hexagonal carbon layer in a carbon crystallite increased mainly up to1400eC, and the number of the hexagonal carbon layers in a carbon crystallite increased significantly above1400"C. TEM observation of wood carbonized at 700 and 1800eC visually showed that its predominantstmcture was turbostratic, and carbon crystallites approximately 1 nm in size at 700"C grew up to about a 3to 1O nm size at 1800eC. The growth ofthe carbon crystallites was observed clearly above l400eC.     Sp3-bonded carbon of the sub-microstmctural component in wood carbonized from 700 to 1800"Cwas qualified by Raman and TEM analysis [5]. The TEM results showed the existence of an onion-like andnano-diamond structure based on sp3-bonded carbons in wood carbonized at 700"C, which suggested thatthe carbon stmcture of wood at the early stage of carbonization is a complex of various allotropic carbonstructures [6, 7]. Nano-diamond based on sp3-bonded carbon would be fomied by the reaction between31ABSTRACTS (PH D FOR GRADUATE SCHOOL OF AGRICULTURE)sp3-bonded carbon and volatilized oxygen atoms in carbonized wood at a low temperature of 700eC. TheRaman spectra of carbonized wood proved that part of the microstmcture in carbonized wood from 700 to1soO"C was made of the combination of sp2- and sp3-bonded carbon. Since sp3-bonded carbon is the rigidcross-linking of carbon crystallites, the microstructure of carbonized wood could be highly disordered. Itwas found that the sp3-bonded carbons were transformed from an amorphous to nanocrystalline stmcturewith the growth of carbon crystallites in the temperature range above 1400eC. The structural developmentof rigid cross-linking of sp3-bonded carbon would disturb the well-ordering of carbon crystallites in thehigh heat temperature range above 2000"C.     Finally, the microstructure in the cell walls ofwood carbonized from 700 to 1800eC was clarified onthe basis of the results of TEM observation and pinpoint Raman analysis [8]. The discrete layers in the cellwalls of the raw wood were no longer observed at 700eC. SEM- and TEM-observations confirmed that themicrostructure in a cell wall for carbonized wood was turbostratic in nature without any heterogeneityoriginating from the constitutional heterogeneity of wood. However, well-ordered carbon crystallites wereobserved on the inner surface ofa cell wall for a sample carbonized at 1800eC. This surface layer could beformed by deposition of vaporized carbon gasses on the surface of cell walls during heat treatment, andmay play an important role in the formation of the preferred orientation of carbon crystallites andmulti-phase graphitization ofcarbonized wood at high temperatures [9].     On the base of the concerns mentjoned above, the mjcrostructural controJ jn carbonjzed wood couJdbe applied to the development of various wood-based carbon materials. As one possibility, themicrostmcture on the nano scale of carbonized wood would be controlled by the adjustment in quantity ofsp3-bonded carbon as cross-linkings of carbon crystallites. For the development of wood catalyticgraphitization, carbonized wood, containing less amount of sp3-bonded carbon, should be used as a carbonprecursor because sp3-bonded carbon disturb the well alignment of carbon crystallites as cross-linkingduring graphitization. On the other hand, for the addition of more stiffhess to SiC and SiCIC composites,carbonized wood, ineluding a large amount of sp3-bonded carbon, should be utilized as a raw material inorder to increase the number of the edge of carbon crystallites where silicon atoms react to carbon atoms.As another possibility, the microstmcture between the inner surface and inside of the cell walls ofcarbonized wood would be controlled by the change of the permeability of volatized gases into cell 1umen.By the development of well alignment carbon layers on the inner surface of cell walls of carbonized wood,it has heterogeneous microstructure between the inner surface and inside of cell wall. Such carbonmaterials are available to be as solid cathode batteries such as an electric double layer capacitor and lithiumion battery. Thus, the microstmctural control in carbonized wood could be applied to the development ofvarious wood-based materials, and be broadened the utilization ofthem as carbon materials.REFERENCES[1] Fujisawa, M., Hata, T., Bronsveld, P., Castro, V., Tanaka, E, Kikuchi, H. and Imamura, Y., J. Eur. Ceram.Soc., 2005, 25, 2735-2738[2] Hata, T., Ishimaru, K., Fujisawa, M., Bronsveld, P., Vystavel, T., Hosson, JD., Kikuchi, H., Nishizawa, T.and Imamura, Y., Fullerene Nanotube Carbon Nanostr., 2005, l3, 435-445[3] Ishimaru, K., Hata, T., Bronsveld, P. and Imamura, Y., J. Mater. Sci., 2007, 42, 122-129[4] Byrne, CE. and Nagle, DC., Carbon, 1997, 35, 267-273[5] Ishimaru, K., Hata, T., Bronsveld, P., Nishizawa, T. and Imamura, Y., J. Wood Sci., 2007, 53, 442-448[6] Hata, T., Imamura, Y., Kobayashi, E., Yamane, T. and Kikuchi, K., J. Wood Sci., 2000, 46, 89-92[7] Ishimaru, K., Vystavel, T., Bronsveld, P., Hata, T., Imamura, Y. and Hosson, JD., J. Wood Sci., 2001, 47,414-416[8] Ishimaru, K., Hata, T., Bronsveld, P. and Imamura, Y., J. Mater. Sci., 2007, 42, 2662-2668[9] Otani, S. and Oya, A., Tanso, 1971, 64, 10-13 (in Japanese)32

Formation Mechanism of Microstructure in carbonized wood

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Formation Mechanism of Microstructure in carbonized wood

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