o 1997 Elsevier Science Limited and Techna S.r.L. Printed in Great Britain. All rights reserved PII:S0272-8842(97)00042 0272884297S17.00+00 Crystal Growth of Silicon Nitride Whiskers Through a VLs Mechanism Using SiO2- Al2O3Y2O3 Oxides as liquid Phase Chihiro Kawai Akira Yamakawa Itami Research Laboratories, Sumitomo Electric Industries, Ltd, I-I-1, Koya-Kita, Itami, Hyogo, 664 Japan (Received 4 February 1997: accepted I July 1997) Abstract: Si3 N4 whiskers were coated on Si,N4 whisker substrates through a VLs mechanism using SiO2-Al2O3-Y2O3 oxides as liquid phases, Alpha-Si3N4 whis kers having diameters of 0. 4 to 0. 7 um were used as seeds. After the seed whiskers e coated with SiOz Al2O3-Y2O3 oxides, the oxide coated whiskers were heated at 1490 C N2 including gas species generated by reacting amorphous Si3N4 and TiO2 powders. Resultantly, whisker-like products were newly formed on the seeds. The morphologies of the resulting whisker growth on the seeds resembled rose-twigs and centipedes with long legs. X-ray diffraction analysis indicated that the newly formed whiskers were B-Si3N4. Droplets were observed on the tips of some whiskers. This suggested that the whisker growth proceeded id-liquid (VLS)mechanism. C Is and Techn S.r. l 1 INTRODUCTION their fast diffusion rate in the gas phase and fast surface migration rate on the substrate Gas phase process such as chemical vapour Crystal growth via liquid phase by the Vls deposition(CVD) has been uscd as onc of indust- mcchanism has been uscd to prepare ceramic rially important coating techniques due to high whiskers such as Si3 N4 and Sic. 1-5 uniformity of the coating. Figure 1 shows two To our knowledge, however, there are few stud ypes of it illustrated schematically. Usual CVd ies focusing on the use of the vls mechanism as a process proceeds through the vapour-solid (S) coating technique on substrates. As shown in mechanism [Fig. I (a). Chemical species diffuse Fig. I(b), if a liquid-phase layer exist on a sub- toward a substrate through an interfacial gas layer, strate, chemical species must be first dissolved then are adsorbed on the surface of the substrate the liquid phase for crystal growth. Next, they dif- and move on it. Subsequently, nucleation and fuse in it and are adsorbed on the substrate crystal growth occur, accompanied by the elimina Finally, crystal growth occurs via nucleation in the ion of by-products. In this process, the diffusion of liquid phase the chemical species in the gas phase and the sur In the crystal growth from liquid phase, the face migration of them on the substrate are fast, higher supersaturation leads to the higher nuclea compared with the vapour-liquid solid (vls) tion density if the growth rate of nucleus is small mechanism[ Fig. 1(b)] via the liquid phase. In the enough. As shown in Fig. 1(b), the diffusion rates VS mechanism, therefore, high deposition rate will of the chemical specics in the liquid phase are be obtained if a reaction between the chemical probably much smaller than those in the gas phase species is fast enough. This often results in the [Fig. 1(a). Therefore, the rate of the supplement of deposit of coarse-grained crystals, however, chemical species to nucleus for crystal growth is because the chemical species are easy to consume very small. This probably results in high nucleation or crystal growth rather than for nucleation due to density and the formation of the finer crystals thanCeramics International 24 (1998) 135-138 Q 1997 Elsevier Science Limited and Techna S.r.1. Printed in Great Britain. All rights reserved PII: SO272-8842(97)00042-4 0272-8842/97 $17.00+ .OO Crystal Growth of Silicon Nitride Whiskers Through a VLS Mechanism Using SiOT A1203_Y203 Oxides as Liquid Phase Chihiro Kawai & Akira Yamakawa Itami Research Laboratories, Sumitomo Electric Industries, Ltd, l-l-1, Koya-Kita, Itami, Hyogo, 664 Japan (Received 4 February 1997; accepted 1 July 1997) Abstract: Si3N, whiskers were coated on SisN4 whisker substrates through a VLS mechanism using SiOz-Al@-Y203 oxides as liquid phases. Alpha-S&N4 whis￾kers having diameters of 0.4 to 0.7 pm were used as seeds. After the seed whiskers were coated with Si02-A1203-Y203 oxides, the oxide-coated whiskers were heated at 1490°C N2 including gas species generated by reacting amorphous Si3N4 and TiOz powders. Resultantly, whisker-like products were newly formed on the seeds. The morphologies of the resulting whisker growth on the seeds resembled rose-twigs and centipedes with long legs. X-ray diffraction analysis indicated that the newly formed whiskers were p-Si3N4. Droplets were observed on the tips of some whiskers. This suggested that the whisker growth proceeded through a vapour-solid-liquid (VLS) mechanism. 0 1997 Elsevier Science Ltd and Techna S.r.1. 1 INTRODUCTION Gas phase process such as chemical vapour deposition (CVD) has been used as one of indust￾rially important coating techniques due to high uniformity of the coating. Figure 1 shows two types of it illustrated schematically. Usual CVD process proceeds through the vapour-solid (VS) mechanism [Fig. l(a)]. Chemical species diffuse toward a substrate through an interfacial gas layer, then are adsorbed on the surface of the substrate and move on it. Subsequently, nucleation and crystal growth occur, accompanied by the elimina￾tion of by-products. In this process, the diffusion of the chemical species in the gas phase and the sur￾face migration of them on the substrate are fast, compared with the vapour-liquid-solid (VLS) mechanism [Fig. l(b)] via the liquid phase. In the VS mechanism, therefore, high deposition rate will be obtained if a reaction between the chemical species is fast enough. This often results in the deposit of coarse-grained crystals, however, because the chemical species are easy to consume for crystal growth rather than for nucleation due to their fast diffusion rate in the gas phase and fast surface migration rate on the substrate. Crystal growth via liquid phase by the VLS mechanism has been used to prepare ceramic whiskers such as Si3N4 and SIC. Id5 To our knowledge, however, there are few stud￾ies focusing on the use of the VLS mechanism as a coating technique on substrates. As shown in Fig. l(b), if a liquid-phase layer exist on a sub￾strate, chemical species must be first dissolved in the liquid phase for crystal growth. Next, they dif￾fuse in it and are adsorbed on the substrate. Finally, crystal growth occurs via nucleation in the liquid phase. In the crystal growth from liquid phase, the higher supersaturation leads to the higher nuclea￾tion density if the growth rate of nucleus is small enough. As shown in Fig. l(b), the diffusion rates of the chemical species in the liquid phase are probably much smaller than those in the gas phase [Fig. l(a)]. Therefore, the rate of the supplement of chemical species to nucleus for crystal growth is very small. This probably results in high nucleation density and the formation of the finer crystals than 135