Effect of Heat Treatment on Crystallite Size and Grain Morphology Of

Note Journal of the Ceramic Society of Japan 107 [3] 285-289 (1999)

Effect of Heat Treatment on Crystallite Size and Grain Morphology of Silicon Carbide Synthesized from Carbon-Silica Hybrid Precursors Masaki NARISAWA, Yoshio OKABE, Kiyohito OKAMURA and Yasuo KURACHI* DepartmentofMetallurgy and Materials Science, College ofEngineering, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai-shi 599-8531 *Konica Corporation , 1, Sakura-machi,Hino-shi, Tokyo 191-0063

炭素-シリカ複合体から合成される炭化ケイ素の結晶成長と粒の形状に対する熱処理の効果成澤雅紀・岡部義生・岡村清人・倉地育夫* 大阪府立大学工学部材料工学科, 599-8531大阪府堺市学園町1-1 *コニカ (株)メディカル&グラフィック事業本部, 191-0063日野市さくら町1

Carbon-silica hybrid precursors for SiC with different carbon contents were prepared by firing hybrid gels consisting of crosslinked phenol resin and silica gel. The SiC precipitation processes in these precursors at 1873K were traced in terms of X-ray diffraction and scanning electron microscope (SEM) observation. Ex cess carbon in the system reduces the size of the formed SiC crystallite during the heat treatment. The precursor with high carbon content yielded aggregated grains consisting of small SiC crystallite, while the precursor with low carbon content yielded coarse isolated grains of 100-200nm after a long-term of heat treatment. [Received October 12,1998; Accepted January 20, 1999]

Key-words: Silicon carbide particle, Aggregation, Carbothermic reduction, Ethyl silicate, Phenol resin, Precursor method

1. Introduction (Sumiliteresin PR-50781, Sumitomo Durez Co., Ltd.) Precursor methods are unique technique for synthesizing without any additional solvents is essential to prepare the inorganic materials, which are difficult to be obtained with starting gels.12),13)Firing of the gels up to 1273K yielded traditional powder sintering processes. In particular, syn carbon-silica hybrid precursors, and the obtained precur thesis of silicon carbide with precursor methods is impor sors were converted to silicon carbide powders by sudden tant, because the silicon carbide is covalent bond materials heat treatments at 1873K in a graphite furnace with an Ar with high mechanical and high chemical resistant proper gas flow. ties in a wide temperature range, but is not easily sintered. An automatic balance was attached to the graphite fur Synthesis of silicon carbide fibers by pyrolysis of fibrous nace, and mass loss during the SiC formation was polycarbosilane is available in industrial scale at present.1) The SiC materials obtained from similar carbosilane or silane backbone precursors are intrinsically amorphous con taining considerable amount of dangling bond.2)-4) Recent ly, the carbothermic reduction process of carbon-silica hybrid precursors prepared with sot-gel methods have been widely investigated, because such precursors can be prepared in relatively low prices, and yield crystalline silicon carbide materials with heat treatments beyond 1773K.5)-8) Control of the silicon carbide precipitation pro cess in the prepared precursor at high temperature range is not easy, One of the major problems is possibly originated from the variety in microstructure of such gels prepared with crosslinked polymers and silica gels.9)-11) We have investigated the simultaneous condensation reaction in liquid mixtures of ethyl silicate and water-solu ble liquid phenol resin and succeeded in preparing transparent gels at 35-55mass% silica contents after a catalyst addition. Firing of these transparent gels at 1273K yields the carbon-silica hybrid precursors, which are sufficiently reliable for synthesizing nano-crystalline SiC with stable conversion rate constants.12),13) We selected the water-soluble liquid resin from various commercialized phenol resins in order to improve the compatibility between the hydrolyzed ethyl silicate and the resin. In this article, we describe about the SiC crystallite size and the grain mor phology in the precursors with different carbon contents at various heat treatment periods.

2. Experimental The outline of the SiC powder preparation methods is Fig. 1. Flowchart of silicon carbide powder synthesis from the shown in Fig. 1. The use of water-soluble liquid resin carbon-silica hybrid precursors.

285 286 Effect of Heat Treatment on Crystallite Size and Grain Morphology of Silicon Carbide Synthesized from Carbon-Silica Hybrid Precursors

measured. Two precursors with different carbon contents Such mass losses should be originated from the carbother were prepared from the gels with the different resin con mic reduction process. If the cabothermic reduction pro tents. Carbon burning tests of the prepared precursors and ceeds completely in the precursor with limiting SiO evolu the powders were performed with TG-8110 (Rigaku) in air tion, the whole reaction scheme in the SiO2-3.8C precursor flow with a heating rate of 5K/min. The X-ray diffraction can be sketched as follows:14)-16) (XRD) patterns of the resulting powders were measured SiO2+3.8CSiC+2CO+0.8C (1) by RINT-1100 (Cu Kƒ¿, Rigaku). The morphology of the The mass loss in terms of Eq. (1) is 53mass%, and the formed powders is observed with FE-SEM, S-4500 theoretical carbon content in a resulting powders is 19 (Hitachi) after Pd-Pt coatings. mass%. Carbon burning tests of the resulting powders after the 3.6ks heat treatment period indicated the substantial 3. Results and discussion carbon contents of 20mass%. Figure 2 shows the TG curves of the finely ground car In the case of the SiO2-2.1C precursor, there is not bon-silica precursors in air. Carbon in the precursor began sufficient carbon for the complete carbothermic reduction. to burn at 800K and completely burned out at 1150 and Thus the reaction scheme can be sketched as follows: 1250K, respectively. The molar ratios of carbon to silica in SiO2+2.1C0.55SiC+0.45SiO+1.55CO (2) the precursors calculated from the mass losses are 3.8 and Equation (2) was derived from the combination of the 2.1. reaction schemes, SiO2+CSiO+CO and SiO2+3C Figure 3 shows the isothermal mass loss curves of these SiC+2CO, with assuming the minimum SiO evolution. precursors at 1873K. The mass losses of the precursors Since the precursor is exposed to an Ar gas flow without a finish at about 1.5ks into 57 and 65mass%, respectively. cover, the evolved SiO is considered to get out the heated

Fig. 2. TG curves of the finely ground carbon-silica precursors Fig. 3. Isothermal TG curves of the precursors at 1873K in an with a heating rate of 5K/min in air. (a) SiO2-3.8C precursor, Ar atmosphere. (a) SiO2-3.8C precursor, (b) SiO2-2.1C precur (b) SiO2-2.1C precursor. sor.

Fig. 4. XRD patterns of the powders synthesized from the SiO2-3.8C precursor at various heating periods at 1873K. Masaki NARISAWA et al. Journal of the Ceramic Society of Japan 107 [3] 1999 287 precursor to react with the graphite parts equipped with the The theoretical estimations are quite consistent with the furnace. The mass loss in terms of Eq. (2) is 74mass%, substantial measurements in particular in the SiO2-3.8C and a resulting powder should contain no carbon. Influence precursor. The reported step by step mechanism, including of the evolved SiO reaction with the graphite crucible must SiO gas evolution process at the silica-carbon interface and be considered. The carbon burning tests indicated little the SiC whisker precipitation on the carbon rich surface by residual carbon below 0.5mass% in the resulting powder. the SiO gas reaction, should produce the loss of the evolved SiO gas beyond the theoretical estimation, in particular at early stage of the reaction.14)-16) Such loss in SiO would give rise to the large mass loss with the high residual carbon in the powder preparation. Thus the SiC formation in the prepared precursors possibly obeys one step carbothermic reduction during the heat treatments. Figure 4 shows the XRD patterns of the SiO2-3.8C precursor after standing the precursor in the furnace for the selected heat treatment periods. The precursor is initially amorphous, and SiC crystallites precipitate in the precursor during the heat treatments. The apparent SiC crystallite sizes in the precursors are estimated from half height line width of (111) line of SiC according to the Scherrer equa tion, and plotted against the heat treatment periods (Fig. 5). The crystallites grow rapidly at initial stage of heat treatments below 1.0ks, and the increasing rates are reduced at the later stage. Such SiC crystal growth is consis tent with the idea proposed by Avrami, Erofe'ev and Chris tian, which assumes the nuclei growth model with reducing the growth rate at a large crystallite size.12),17) The crystallite size in the SiO2-3.8C precursor continues to in crease moderately at a long heat treatment period. That in the SiO2-2.1C precursor, however, increases again beyond

Fig. 5. SiC crystallite size estimated from half height line width 2.2ks. This is possibly due to the consumption of the (111 line) of the XRD patterns: (•œ) SiO2-3.8C precursor, residual carbon in the system. The difference in the (•›) SiO2-2.1C precursor. crystallite sizes of the individual precursors is widened as

(a) (b)

(c) (d) Fig. 6. SEM micrographs of the powders synthesized from the SiO2-3.8C precursor at 1873K at various heat treatment periods. (a) 0.3ks (reaction rate: 40%), (b) 0.7ks (reaction rate: 80%), (c) 1.5ks (reaction rate: 100%), (d) 3.0ks (reaction rate: 100%). 288 Effect of Heat Treatment on Crystallite Size and Grain Morphology of Silicon Carbide Synthesized from Carbon-Silica Hybrid Precursors the heat treatment period increases. The excess carbon ed SiO and CO from the resulting powders.19)-21) The swept out from nano-crystalline SiC particles during the car residual carbon was also reduced to a minimum. bothermic reduction may block SiC crystal growth at the Such gel precursor is economical as compared with the later stage of the heat treatment process. foregoing organosilicon precursors which yield silicon car The SEM micrographs of the powders synthesized from bide after heat treatments, because the resin and ethyl the SiO2-3.8C precursor and the SiO2-2.1C precursor are silicate are widely commercialized. The controlled shown in Figs. 6 and 7, respectively. Before the heat treat crystallite size and grain morphology in the formed ment, the precursor surface is completely plain, and no powders are attractive in the production of sintering SiC structure is observed with SEM in the magnification ratios powders. of 10000-100000. The change in the surface morphology is observed at the 0.3ks heat treatment period, where the 4. Conclusions XRD patterns indicate the precipitation of 8nm SiC formation process at 1873K in the prepared carbon crystallites. The individual crystallite can not be, however, silica hybrid precursors is consistent with the complete car distinguished with SEM at the 0.3ks heat treatment bothermic reduction mechanism. The sizes of SiC period. In the SiO2-3.8C precursor, formation of partially crystallites in the precursors rapidly increases up to 15 nm aggregated grains is observed at the 0.7ks heat treatment in the 1.0ks period, but growing rates are slowed down at period. As the heat treatment period increases, aggregation 1.0-2.2ks. At the heat treatment period of 2.2ks, the of the grains become remarkable.16),18) The observed grain crystallite sizes in the precursors reach to 20nm. The should consist of the several SiC crystallites, because the ap crystallite size in the SiO2-3.8C precursor continues to in parent size of the grain is always larger than the crystallite crease moderately, while the crystallite size in the SiO2 size estimated from the XRD pattern at the same heat treat -2.1C precursor jumps up again beyond 2.2ks. Excess car ment conditions. In the SiO2-2.1C precursor, the apparent bon in the system reduces the crystallite size, and promotes grain sizes are roughly consistent with the sizes estimated the aggregation of the small SiC crystallites. Even below from the XRD patterns at the 0.7 and 1.5ks heat treatment 2.2ks, the carbon content in the precursor has influenced periods. At the 3.0ks heat treatment period, the grain size the formed particle morphology. jumps up to 100-200nm. This size is beyond efficiency of the half height line width technique. The individual grain Acknowledgment The establishment of the synthesis process may consist of single SiC crystallite. SiC whisker, which are of carbon-silica hybrid possessing stable rate constants for SiC con usually formed in the carbothermic reduction of silica in an version was supported by Nippon Sheet Glass Foundation for Ar gas flow, was never observed in the powders from the Materials Science and Engineering. Subsequent study of the car bon content effects on the SiC crystallite growth was partially sup precursors. This should be due to the minimum SiO evolu ported by the Ministry of Education, Science, Sports and Culture, tion from the precursors and the rapid diffusion of the evolv

(a) (b)

(c) (d) Fig. 7. SEM micrographs of the powders synthesized from the SiO2-2.1C precursor at 1873K at various heat treatment periods. (a) 0.3ks (reaction rate: 40%), (b) 0.7ks (reaction rate: 80%), (c) 1.5ks (reaction rate: 100%), (d) 3.0ks (reaction rate: 100%). Masaki NARISAWA et al. Journal of the Ceramic Society of Japan 107 [3] 1999 289

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