Conventional (VS mechanism) (a) Seed whisker Diffusion Substrate Crystal Growth (b) Oxide coating (b)Existence of liquid layer(VLS mechanism) (c) Nucleation in liquid phase Liquid Phase trate Crystal Growth (d)Crystal growth Nucleation Fig. 1. Schematic illustration of two types of CVD mechanisms hose through the VS mechanism, if the supersat uration of the liquid phase is high enough Fig. 2. Illustrations of a surface modification of ceramic On the basis of the above presumption, we hikers using a VLS mechanism coated fine-grained Si3 N4 whiskers on Si3N4 whis- ker substrates. 6 In the present study we investiga- oxide content. The oxides content are summarised ted the effect of liquid phase thickness on the in Table 1 dimensions of coated whiskers The seed whisk ers were ipped and were then drawn and dried. The treated whis- kers were calcined at 500C for 1 h in air to convert 2 EXPERIMENTAL PROCEDURE the alkoxides to an oxide. a reported phase dia- gram indicates that the resulting oxidc glass on the The concept for the Si3N4 whisker coating is illus- seed whiskers melts at about 1400 C.8 Finally, the trated in Fig. 2. An oxide layer, which melts and Si3 N4 whiskers coated with the SiOz-AhO3-Y2O3 forms a liquid phase at high temperature, is coated glass were heated with the compacted bodies of an on the Si3N4 whisker substrate. The oxide-coated amorphous Si3N4(produced by Mitsui-Toatsu Co whiskers are heated at temperatures above the Japan) and TiO2 (produced by Tayka Co Japan) melting point of the oxide in source gases. After powders at 1490C, 700 Torr for 2h in N the dissolution of the source gases in the resulting Obtained products were observed with scanning liquid phase, nucleation and crystal growth occur electron microscope(SEM), and were identified by on the surface of the substrate in the liquid phase X-ray diffraction from which whiskers grow diameters of about 0. 4 to 0. 7 um, which were pre- 3 RESULTS AND DISCUSSION pared by a reaction of amorphous Si3N4 with TiO2 powders, were used as seeds. Next, two kinds of Figure 3 shows the comparison of the morpholo ethanol solutions of metal alkoxides of thc Si-Al- gies of the secd whiskers [Fig 3 (a)] and the result Y-O system were prepared; tetraethoxysilane ing product. In the case of solution B, a number of TEOS, produced by Sasaki-Kagakuyakuhin C Japan) and alkoxides-derivatives, including Al and Table 1. the concentration of alkoxides prepared Y(Autoform MS-Al and MsY, produced by for。 xide coating Fuji-Kagakuyakuhin Co Japan), were mixed with Content of oxides(kg m-3 a composition of SiO2: A12 03: Y2 034: 2: 4(weight oxide ratio). As-prepared solution was diluted with etha- SiO2 5535 1384 (Solution B)to prepare solutions with different 20 3 nol one(Solution A: no dilution)or four times Alao 27.67 691 5535136 C. Kawai, A. Yamakawa (a) Conventional (VS mechanism) Interfacial layer Substrate (b) Existence of liquid layer (VLS mechanism) (Dissolutio& Liquid Phase Substrate Adsorption Migration d Crystal Growth NUClWtioll Fig. 1. Schematic illustration of two types of CVD mechanisms. those through the VS mechanism, if the supersat￾uration of the liquid phase is high enough. On the basis of the above presumption, we coated fine-grained SisN4 whiskers on Si3N4 whis￾ker substrates.6 In the present study, we investiga￾ted the effect of liquid phase thickness on the dimensions of coated whiskers. 2 EXPERIMENTAL PROCEDURE The concept for the Si3N4 whisker coating is illus￾trated in Fig. 2. An oxide layer, which melts and forms a liquid phase at high temperature, is coated on the SiJN4 whisker substrate. The oxide-coated whiskers are heated at temperatures above the melting point of the oxide in source gases. After the dissolution of the source gases in the resulting liquid phase, nucleation and crystal growth occur on the surface of the substrate in the liquid phase, from which whiskers grow. Alpha-SisN4 whiskers (single crystals) having diameters of about 0.4 to 0.7 pm, which were pre￾pared by a reaction of amorphous S&N4 with TiOz powders,7 were used as seeds. Next, two kinds of ethanol solutions of metal alkoxides of the Si-Al￾Y-O system were prepared; tetraethoxysilane (TEOS, produced by Sasaki-Kagakuyakuhin Co. Japan) and alkoxides-derivatives, including Al and Y (Hautoform MS-Al and MS-Y, produced by Fuji-Kagakuyakuhin Co. Japan), were mixed with a composition of Si02:A120s:Y203 = 4:2:4 (weight ratio). As-prepared solution was diluted with etha￾nol one (Solution A: no dilution) or four times (Solution B) to prepare solutions with different (a) Seed whisker b) Oxide coating (C) Nucleation in liquid phase (d) Crystal growth Fig. 2. Illustrations of a surface modification of ceramic whiskers using a VLS mechanism. oxide content. The oxides content are summarised in Table 1. The seed whiskers were dipped in the solutions, and were then drawn and dried. The treated whis￾kers were calcined at 500°C for 1 h in air to convert the alkoxides to an oxide. A reported phase dia￾gram indicates that the resulting oxide glass on the seed whiskers melts at about 14OO”C.* Finally, the Si3N4 whiskers coated with the SiOz-A120s-Y203 glass were heated with the compacted bodies of an amorphous SisN4 (produced by Mitsui-Toatsu Co. Japan) and Ti02 (produced by Tayka Co. Japan) powders at 149o”C, 700 Torr for 2h in NZ. Obtained products were observed with scanning electron microscope (SEM), and were identified by X-ray diffraction.- 3 RESULTS AND DISCUSSION Figure 3 shows the comparison of the morpholo￾gies of the seed whiskers [Fig. 3(a)] and the result￾ing product. In the case of solution B, a number of Table 1. The concentration of alkoxides prepared for oxide coating Oxide Content of oxides (kg m-3) Solution A Solution 6 SiOp 55.35 13.84 A1203 27.67 6.91 y203 55.35 13.84