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 than
Ceramics 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 whiskers 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 industrially 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 elimination of by-products. In this process, the diffusion of the chemical species in the gas phase and the surface 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 studies 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 substrate, chemical species must be first dissolved in the liquid phase for crystal growth. Next, they diffuse 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 nucleation 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
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 5535
136 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 supersaturation of the liquid phase is high enough. On the basis of the above presumption, we coated fine-grained SisN4 whiskers on Si3N4 whisker substrates.6 In the present study, we investigated the effect of liquid phase thickness on the dimensions of coated whiskers. 2 EXPERIMENTAL PROCEDURE The concept for the Si3N4 whisker coating is illustrated 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 prepared 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-AlY-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 ethanol 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 whiskers were calcined at 500°C for 1 h in air to convert the alkoxides to an oxide. A reported phase diagram 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 morphologies of the seed whiskers [Fig. 3(a)] and the resulting 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
Crystal growth of silicon nitride whiskers (a 1(d)国 黍 Fig 3. SEM micrographs of (a)seed whiskers, (b) fine-grained whiskers from solution B, (c) long whiskers from solution A and (d)short whiskers from fine-grained whisker-like products were formed on Regarding whisker growth through VLS the surface of the seed whiskers. The morphology mechanisms, the following mechanism is sugges of the growth was just like rose-twigs [Fig. 3(b)]. ted. Gas species are dissolved in liquid droplets on There is no significant variation in thickness of the a substrate, and then they are supplied to the sur- seeds before and after the whisker coating. This face of the substrate, where crystal growth starts indicates that a thin oxide layer was coated on the Accompanied by the crystal growth, the droplets seeds. X-ray diffraction indicated that the newly are lifted-up. If the components of the droplets are formed whiskers are the B-Si3N4 not all dissolved in the grown crystals, further In the case of solution A, the newly formed crystal growth is continued. If not, the droplets are whiskers on the seeds were much longer than those consumed and ultimately disappear obtained from the solution b, and the morphology Whisker growth in the present study would be of the growth was just like centipedes having long ascribed to the mechanisms shown in Fig. 4. With legs[Fig. 3(c)and (d) a thin oxide layer, the droplets on the tips of Clearly, the diameters of the seeds increase. This hikers are quickly consumed, accompanied by is probably because a thick oxide layer was coated whisker growth. Since the Si-Al-Y-O component although its thickness was determined. No droplets of the liquid phase is partly dissolved in the si3N4 were found on the tips of the fine-grained whiskers whiskers. It is widely known that SiO2-AlO3- from solution B [Fig. 3(b)] and relatively long Y2O3 oxides are conventional sintering aids for whiskers from solution A [Fig. 3(c)]. Droplets were liquid phase sintering of Si, N4 ceramics, when a found on the tips of relatively short whiskers from part of them is dissolved in Si3 N4: SiaION cera solution A[Fig 3(d)I mics are formed. 10 Therefore the whiskers are
Crystal growth of silicon nitride whiskers 137 Fig ;. 3. SEM micrographs of (a) seed whiskers, (b) fine-grained whiskers from solution B, (c) long whiskers from solution A al (d) short whiskers from solution A. nd fine-grained whisker-like products were formed on the surface of the seed whiskers. The morphology of the growth was just like rose-twigs [Fig. 3(b)]. There is no significant variation in thickness of the seeds before and after the whisker coating. This indicates that a thin oxide layer was coated on the seeds. X-ray diffraction indicated that the newly formed whiskers are the /?-Si3N4. In the case of solution A, the newly formed whiskers on the seeds were much longer than those obtained from the solution B, and the morphology of the growth was just like centipedes having long legs [Fig. 3(c) and (d)]. Clearly, the diameters of the seeds increase. This is probably because a thick oxide layer was coated although its thickness was determined. No droplets were found on the tips of the fine-grained whiskers from solution B [Fig. 3(b)] and relatively long whiskers from solution A [Fig. 3(c)]. Droplets were found on the tips of relatively short whiskers from solution A [Fig. 3(d)]. Regarding whisker growth through VLS mechanisms, the following mechanism is suggested.9 Gas species are dissolved in liquid droplets on a substrate, and then they are supplied to the surface of the substrate, where crystal growth starts. Accompanied by the crystal growth, the droplets are lifted-up. If the components of the droplets are not all dissolved in the grown crystals, further crystal growth is continued. If not, the droplets are consumed and ultimately disappear. Whisker growth in the present study would be ascribed to the mechanisms shown in Fig. 4. With a thin oxide layer, the droplets on the tips of whiskers are quickly consumed, accompanied by whisker growth. Since the Si-Al-Y-O component of the liquid phase is partly dissolved in the Si3N4 whiskers. It is widely known that Si02-Al@- Y20s oxides are conventional sintering aids for liquid phase sintering of S&N4 ceramics, when a part of them is dissolved in Si3N4; SiAlON ceramics are formed.‘O Therefore, the whiskers are
C. Kawai A yamakawa 4 CONCLUSION (a) Thin liquid phase Gas species Si3 N4 whiskers were coated on a-Si3N4 whisker 幸￥YL1 seeds, on which Sioz-Al2O3 Y2O3 oxides were coated, through a VLS mechanism using gas spe Substrate Li Whiskers cies generated by reacting amorphous Si3N4 and TiO2 as source powders. With a thin oxide layer, (b) Thick liquid phase the coated whiskers were fine-grained and had no Gras species droplets on the tips of the whiskers. with a thick oxide layer, the coated whiskers consisted of short whiskers with droplets and long whiskers with no Whiskers droplets. These results indicated that the whisker Nucleation Crystal growt growth proceeded through a vls mechanism. the (Initial stage) (Final stage differences in morphology were considered to be caused by difference in thickness of the resulting Fig. 4. The comparison of whisker growth mechanisms; (a) liquid layer on the seeds hin oxide layer,(b) thick oxide layer. REFERENCES short and have no droplets, and are in the final stage of whisker growth [Fig 4(a) 1. WANG, M. J.& wADA, H, Mater. Sci. Forum, 47 With a thick oxide layer, however, the droplet (1989)267-281 2. WANG, M.J.& WADA, H.,J. Mater. Sci. 25(1990) are easy not to consume, since large droplets are 1690-1698 probably formed on the tips of whiskers due to the 3. HAYASHI, T. KAWABE, S. SArto,H, Yogyo. thick oxide layer. Therefore, whisker growth is Kyokai-shi, 94 (1986)29-30 continued for a long time and is finished when the 4. SAItO. H. hAYaSHI, t.& MiurA, K.,J. Che Soc.Jpn,9(1981)1371-1377 droplets are entirely consumed This results in the 5. BL, Y, YU, Z& ZHANG, P, Science of sintering growth of long whiskers having no droplets. On (1991)334 the other hand, relatively short whiskers having 6. KAWAL, C& YAMAKAWA, A,, J. Am. Ceram. Soc. 79(196)57-58 droplets are also seen with the long whiskers. They KAWAL C.& YAMAKAWA,J Mater. Sci. Lett. 14 probably start to grow after a considerably long (1995)192-193 duration; the liquid laycr remains on the substrate 8. LEVIN. E.M., ROBBINS. C. R& McMURDIE, H. F. Phase Diagram for Ceramists 1969 Supplement. Am. even though most of it is consumed in the growth Ceram Soc., Columbus, oH, 1969, p. 165 The short whisk 9. KATOH, N, Kessyo-Kohgaku-Ilandbook M. Yama an initial stage of whisker growth and have possl- moto kyoritsu-Shuppan, Tokyo, 1911,p.218 OMO. M.. In Chikkakeiso-Ceramics, ed. s. Soh- bility of further elongation with the increase in miya, M. Yoshimura M. Mitomo Uchida-Rokakuho, duration [Fig. 4(b)] Tokyo, 1987, pp. 1-8
138 C. Kawai, A. Yamakawa 4 CONCLUSION (a) Thin liquid phase (b) Thick liquid phase Gas species Finished Nucleation whiskers In giowing Crystal growth (Initial stage) (Final stage) Fig. 4. The comparison of whisker growth mechanisms; (a) thin oxide layer, (b) thick oxide layer. short and have no droplets, and are in the final stage of whisker growth [Fig. 4(a)]. With a thick oxide layer, however, the droplets are easy not to consume, since large droplets are probably formed on the tips of whiskers due to the thick oxide layer. Therefore, whisker growth is continued for a long time and is finished when the droplets are entirely consumed. This results in the growth of long whiskers having no droplets. On the other hand, relatively short whiskers having droplets are also seen with the long whiskers. They probably start to grow after a considerably long duration; the liquid layer remains on the substrate even though most of it is consumed in the growth of many long whiskers. The short whiskers are in an initial stage of whisker growth and have possibility of further elongation with the increase in duration [Fig. 4(b)]. S&N4 whiskers were coated on a-S&N4 whisker seeds, on which SiO2--Al#-Y203 oxides were coated, through a VLS mechanism using gas species generated by reacting amorphous S&N4 and Ti02 as source powders. With a thin oxide layer, the coated whiskers were fine-grained and had no droplets on the tips of the whiskers. With a thick oxide layer, the coated whiskers consisted of short whiskers with droplets and long whiskers with no droplets. These results indicated that the whisker growth proceeded through a VLS mechanism. The differences in molphology were considered to be caused by difference in thickness of the resulting liquid layer on the seeds. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. WANG, M. J. & WADA, H., Mater. Sci. Forum, 41 (1989) 267-281. WANG, M. J. & WADA, H., J. Mater. Sci., 25 (1990) 1690-1698. HAYASHI, T., KAWABE, S. & SAITO, H., YogyoKyokai-shi, 94 (1986) 29-30. SAITO, H., HAYASHI, T. & MIURA, K., J. Chem. Sot. Jpn, 9 (1981) 1371-1377. BI, Y., YU, Z. & ZHANG, P., Science of Sintering, 23 (1991) 3340. KAWAI, C. & YAMAKAWA, A., J. Am. Ceram. Sot., 79 (1996) 57-58. KAWAI, C. & YAMAKAWA, J. Mater. Sci. Lett., 14 (1995) 192-193. LEVIN, E. M., ROBBINS, C. R. & McMURDIE, H. F., Phase Diagram for Ceramists 1969 Supplement. Am. Ceram. Sot., Columbus, OH, 1969, p. 165. KATOH, N., Kessyo-Kohgaku-Handbook. M. Yamamoto Kyoritsu-Shuppan, Tokyo, 1971, p. 218. MITOMO, M., In Chikkakeiso-Ceramics, ed. S. Sohmiya, M. Yoshimura & M. Mitomo. Uchida-Rokakuho, Tokyo, 1987, pp. l-8