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In situ synthesis of high-purity α-Si3N4 whiskers from amorphous Si-N-C powders
November 1994
Materials Letters 21 (1994) 325-328
In situ synthesis of high-purity a-Si,N, whiskers from amorphous Si-N-C powders Ya-li Li, Yong Liang, Zhuang-qi Hu State Key Laborator?, ofRSA, Institute ofMetal Research, Academia Sinica, 72 Wenhua Road, Shenyang 110015, China Received 22 March 1994; in final form 18 July 1994; accepted 28 July 1994
Abstract a-S&N, whiskers with diameters of 0.1-0.5 pm and lengths in the millimeter range were synthesized from organosilane-derived nanometer-sized amorphous B-N-C powders at 1873 K in 1 atm of nitrogen. The whiskers nucleated on the a-S&N,grains crystallizedfrom the Si-N-C powdersand grewin situ by the vapor-solid (VS) mechanism. The absence of a catalyst and the in situ formation of the whiskers ensured that the whiskers were quite pure.
1. Introduction
Silicon nitride whiskers are promising for applications to enhance the mechanical properties of composites, ceramics and metals for elevated temperature use [I]. Studies of the synthesis of S&N, whiskers, as a promising reinforcing materiaf, have received extensive attention and various methods for preparation were developed. The synthesis routes for Si3N4 whiskers varied and can be classified into nitriding of metallic silicon [ 2,3 1, thermal reduction with carbon of a silica-containing compound [ 4-7 1, and gas phase reactions involving silicon halides [ 8 1. In silicon nitriding, a metallic baffle is needed for nucleation sites for S&N4 whisker growth and this method is liable to result in the introduction of metallic impurities into the whiskers [ 21. In the silicacarbon thermal reduction method, a homogeneous pre-mixing of the silica and carbon in the exact ratio is necessary to raise the solid-solid reaction efficiency between the two kinds of solids and thus avoid the retention of unreacted materials in the whiskers. However, the impurities come from the silica-con-
taining compound droplets, which are on the whisker tips, due to the vapor-liquid-solid (VLS) whisker formation mechanism [ 7 1. In addition, incomplete reaction of the raw materials [ 61 often degrades the whisker purity. In the silicon halide method, Fe or Co catalysis are needed for whisker growth. This paper reports a new method for synthesizing high-purity Si3N4 whiskers from nanometer-sized amorphous SGN-C powders. This method has the following advantages over the other methods mentioned above: ( 1) the formation of the Si3N4 whiskers takes place in situ, (2) the absence of droplets on the whisker tips required for the vapor-solid (VS) growth mechanism and use of catalysts, (3) the high reaction efficiency due to the atomic state of the Si, N, C elements in the amorphous Si/N/C powders, (4) the relatively lower cost of the Si/N/C powders compared with the monolithic nanometer Si3N4, SIC and high-purity Si powders.
0167-577x/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved S’SDIO167-577x(94)00182-0
326
Y. Li et al. /Materials Letters 21 (I 994) 325-328
2. Experimental
3. Results
2.1. Raw materials
White, 3 mm long, wool-like whiskers formed on the surface of the original Si-N-C compact after 1 h at 1873 K and 1 atm NZ. Whiskers weighing 2 g were obtained after separating from the discs. The weight loss of the original compact was 40%. XRD revealed that the whiskers were a-Si3N4 and no other crystallized phases present, whereas the material remaining on the discs after removal of the whiskers, consisted of a-Si3N4, a-Sic, P-Sic and graphite (Fig. 2 ) . It was found that no whiskers formed at temperatures lower than 1873 K. The Si-N-C powders began to crystallize to o-S&N4 at 1773 K and to form aSiJN4, o-Sic and B-Sic at 1973 K (Fig. 2). We also found that no whiskers formed when the starting SiN-C powders were replaced with nanometer-sized S&N4 powders and an atmosphere of Ar gas at 1873 K. Only Sic particles were formed. When the whiskers were used for STEM examinations, we found that the whiskers were insulators sug-
The raw material, nanometer-sized amorphous SiN-C powders, was synthesized from hexamethyldisilazane and ammonia by laser-induced vapor-phase reactions. The synthesis method and the powder characteristics were reported elsewhere [ 9 1. The powders used for this work had a mean particle diameter of 80 nm measured by TEM, and were amorphous as determined by XRD. The composition of the powder was: Si 50 wt%, N 28 wt%, C 15 wt% and 0 5 wt%. The oxygen was due to surface adsorption of water and oxygen in air due to the extraordinary high specific area of the powers [ 9 1, 2.2. Whisker synthesis The schematic diagram of the whisker synthesis apparatus is shown in Fig. 1. The Si-N-C powders were first slightly cold-pressed to discs in order to avoid the loss of powders during evacuating the system. Each disc weighed 1 g and had dimensions of 10 mm in diameter and 5 mm thick. The discs were placed in a BN crucible and positioned in the graphite resistance furnace. The system was evacuated to 1.3 Pa followed by the introduction of 1 atm of nitrogen into the furnace. Then, the sample was heated at a heating rate of 20 K/min to a defined temperature, kept constant for 1 h, then cooled to room temperature by terminating the power to the furnace. Several runs were done following this procedure for synthesis between 1673 and 1973 K in 100 K intervals. The above experiments were repeated using 1 atm of Ar instead of nitrogen, and nanometer-sized amorphous Si3N4 powders instead of Si-N-C, so as to understand the whisker formation mechanism. c rauiet.ance
Row material
BN crucible
Fig. 1. Schematic diagram of the whisker synthesis apparatus.
DEGREE (28) Fig. 2. XRD patterns of the as-formed whiskers and the crystallized products of S-N-C powders under various conditions: (a) remaining amorphous material after heating at 1673 K under 1 atm Nz, (b) a-S&N4 particulates at 1773 K under 1 atm N2, (c) a-SisN., whiskers formed at 1873 K under 1 atm N,, (d) crystallized particulates beneath the whiskers at 1873 K under 1 atm N2, (e) at 1873 K and 1 atm Ar, and (f) at 1973 K and 1 atm NZ.
Y. Li et al. /Materials
Letters 21 II 9941325-328
327
were in the range 0.1-0.5 pm and the length in millimeters as estimated from the STEM measurements. A typical TEM morphology of the whiskers is shown in Fig. 4.
4. Discussion
Fig. 3. STEM morphologies of the S&N4 whiskers: (a) whisker growth on the surface of the Si-N-C powder compact and (b) general morphology of the whiskers.
No frozen spherical droplets were observed by TEM on the whisker tips, as shown in Fig. 5a, indicating the absence of the VLS growth mechanism for whisker growth [lo]. Whisker growth on the grains can be seen in Fig. 5b, from which we know that the whiskers nucleated on the pre-crystallized a-Si-,N, on the powder discs. The main components in the gas phase for whisker formation are SiO, CO and N2. Their partial pressures influence solid phase formation and their supersaturation ratios determine the formation of whiskers [ 6 ]. In the Si-N-C system, SisN, and Sic are stable phases between 1773 and 1973 K under 1 atm N2 and SIC is the only stable phase when 1 atm of Ar was used [ 111. This explains the coexistence of Si3N4and Sic at 1873 and 1973 K and the formation
Fig. 4. Typical TEM morphology of the as-produced a&N, whiskers.
gesting that little C existed in the whiskers. The STEM observations were then carried out after a layer of amo~hous C was sprayed onto the whiskers. The STEM results are shown in Fig. 3. Fig. 3a shows the whisker growth morphology on the powder compact surface, and Fig. 3b shows the typical morphology of the whiskers formed. Most of the whiskers arc straight while some are bent. The diameters of the whiskers
Fig. 5. TEM morphologies of the a-Si,N, whisker tips: (a) the smooth whisker tip and (b) the whisker tip bonded to a small a&N, gram.
328
Y. Li et al. /lateral
of Sic when Ar is used. It is known that Si-N-C powders are thermodynamically metastable and are liable to give off gases such as Co, Nz and SiO during synthesis at temperatures up to 1673 K under 1 atm Nz [ 121. According to thermodynamic analysis of S&N4formation associated with these gases, the most favorable reaction [ 61 is 3SiO(g)S3CO(g)+2N,(g)4i3N4(s)+3C02(g), logk(1873K)
=-11.1
a
The small ~uilib~um constant at 1873 K means that the reaction is controlled by kinetic factors such as the release rate of the gases from the Si-N-C powders. Since the concentration of these gases was higher in the gas phase and attained the proper concentration, the whiskers formed only on the powder surface and the nucleation sites may be the first crystallized a-Si,N4 grains. Because none or small amounts of SiO and CO were released from the pure S&N4 powders, no whiskers form when S&N4 powders are used. As mentioned previously, the use of the Si-N-C amorphous powders as raw materials for whisker growth has several advantages over other methods. One shortcoming in the present synthesis method is the separation of the whiskers from the annealed powder compact. Actually, because there was no other impurity or elements in the raw materials except a little graphite, the presence of Si3N4 or Sic particulates in the whiskers does not influence the application of the whiskers as reinforcing materials. In most circumstances, the mixing of particulates with the whisker is necessary to produce high-density sintered bodies. Also, the proper design of the Si-N-C powder com~sition may eliminate the retention of graphite. It is known that the Si-N-C powders with a wider composition range can be synthesized by laser-induced gas-phase reaction [ 9 1. A study of the influence of powder composition on the formation of whiskers is now under way.
Letters 21(1994) 325-328
5. Conclusions a-Si3N4 whiskers with diameters of 0.1-0.5 pm and lengths in the millimeter range can be synthesized in situ from amorphous nanometer-sized Si-N-C powders at 1873 K in 1 atm N2 in a graphite resistance furnace. There were no impurity frozen droplets on the whisker tips, suggesting that the VLS (vapor-liquid-solid) growth mechanism was absent. The absence of any other catalysts and the in situ formation of the whiskers ensured that the whiskers were quite pure. No whiskers formed at 1773 and 1973 K, or with the use of Ar instead of N2 or using nanometer S&N4 powders as raw material. The whiskers were formed by the VS (vapor-solid) mechanism involving a gas-phase reaction among SiO, CO and N2 which were released from the Si-N-C powders during synthesis.
References [ I] W. Wada, in: Processing of ceramic and metal matrix composites, ed. H. Mostaghaci (Pergamon Press, London, 1989) p. 3. [2] A.L. Cunning and L.G. Davis, SAMPE 15 (1969) 120. [ 31 Y. Inomata and T. Yamane, J. Crystal Growth 2 1 ( 1974) 317. [ 4 ] R.C. Johnson, J.K. Alley and W.H. Warwick, U.S. Patent 3, 244,486 (1966). [ 51 S. Zhang and W.R. Cannon, J. Am. Ceram. Sot. 67 ( 1984) 691. [6] M.J. Wang and H. Wada, J. Mater. Sci. 25 (1990) 1690. [7] V.N. Gribkov, V.A. Silaev, B.V. Schetanov, E.L. Umantsev and A.S. Isaikin, Soviet Phys. Crystall. I6 ( 1972) 852. [8] K. Kijima, N. Setaka and H. Tanaka, J. Crystal Growth 24,25 (1974) 183. /9] Y.L. Li, Y. Liang, F. Zheng and Z.Q. Hu, Mater. Sci. Eng. 174 (1994) L23. [lo] W.B. Campbell, in: Whisker technology, ed. A.P. Levitt (Wiley, New York, 1970) p. 37. [ 111 R.V. Krishnarao and M.M. Godkhindi, J. Mater. Sci. 27 (1992) 2726. [ 121 G.W. Rice, J. Am. Ceram. Sot. 69 (1986) C-183.
Materials Letters 21 (1994) 325-328
In situ synthesis of high-purity a-Si,N, whiskers from amorphous Si-N-C powders Ya-li Li, Yong Liang, Zhuang-qi Hu State Key Laborator?, ofRSA, Institute ofMetal Research, Academia Sinica, 72 Wenhua Road, Shenyang 110015, China Received 22 March 1994; in final form 18 July 1994; accepted 28 July 1994
Abstract a-S&N, whiskers with diameters of 0.1-0.5 pm and lengths in the millimeter range were synthesized from organosilane-derived nanometer-sized amorphous B-N-C powders at 1873 K in 1 atm of nitrogen. The whiskers nucleated on the a-S&N,grains crystallizedfrom the Si-N-C powdersand grewin situ by the vapor-solid (VS) mechanism. The absence of a catalyst and the in situ formation of the whiskers ensured that the whiskers were quite pure.
1. Introduction
Silicon nitride whiskers are promising for applications to enhance the mechanical properties of composites, ceramics and metals for elevated temperature use [I]. Studies of the synthesis of S&N, whiskers, as a promising reinforcing materiaf, have received extensive attention and various methods for preparation were developed. The synthesis routes for Si3N4 whiskers varied and can be classified into nitriding of metallic silicon [ 2,3 1, thermal reduction with carbon of a silica-containing compound [ 4-7 1, and gas phase reactions involving silicon halides [ 8 1. In silicon nitriding, a metallic baffle is needed for nucleation sites for S&N4 whisker growth and this method is liable to result in the introduction of metallic impurities into the whiskers [ 21. In the silicacarbon thermal reduction method, a homogeneous pre-mixing of the silica and carbon in the exact ratio is necessary to raise the solid-solid reaction efficiency between the two kinds of solids and thus avoid the retention of unreacted materials in the whiskers. However, the impurities come from the silica-con-
taining compound droplets, which are on the whisker tips, due to the vapor-liquid-solid (VLS) whisker formation mechanism [ 7 1. In addition, incomplete reaction of the raw materials [ 61 often degrades the whisker purity. In the silicon halide method, Fe or Co catalysis are needed for whisker growth. This paper reports a new method for synthesizing high-purity Si3N4 whiskers from nanometer-sized amorphous SGN-C powders. This method has the following advantages over the other methods mentioned above: ( 1) the formation of the Si3N4 whiskers takes place in situ, (2) the absence of droplets on the whisker tips required for the vapor-solid (VS) growth mechanism and use of catalysts, (3) the high reaction efficiency due to the atomic state of the Si, N, C elements in the amorphous Si/N/C powders, (4) the relatively lower cost of the Si/N/C powders compared with the monolithic nanometer Si3N4, SIC and high-purity Si powders.
0167-577x/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved S’SDIO167-577x(94)00182-0
326
Y. Li et al. /Materials Letters 21 (I 994) 325-328
2. Experimental
3. Results
2.1. Raw materials
White, 3 mm long, wool-like whiskers formed on the surface of the original Si-N-C compact after 1 h at 1873 K and 1 atm NZ. Whiskers weighing 2 g were obtained after separating from the discs. The weight loss of the original compact was 40%. XRD revealed that the whiskers were a-Si3N4 and no other crystallized phases present, whereas the material remaining on the discs after removal of the whiskers, consisted of a-Si3N4, a-Sic, P-Sic and graphite (Fig. 2 ) . It was found that no whiskers formed at temperatures lower than 1873 K. The Si-N-C powders began to crystallize to o-S&N4 at 1773 K and to form aSiJN4, o-Sic and B-Sic at 1973 K (Fig. 2). We also found that no whiskers formed when the starting SiN-C powders were replaced with nanometer-sized S&N4 powders and an atmosphere of Ar gas at 1873 K. Only Sic particles were formed. When the whiskers were used for STEM examinations, we found that the whiskers were insulators sug-
The raw material, nanometer-sized amorphous SiN-C powders, was synthesized from hexamethyldisilazane and ammonia by laser-induced vapor-phase reactions. The synthesis method and the powder characteristics were reported elsewhere [ 9 1. The powders used for this work had a mean particle diameter of 80 nm measured by TEM, and were amorphous as determined by XRD. The composition of the powder was: Si 50 wt%, N 28 wt%, C 15 wt% and 0 5 wt%. The oxygen was due to surface adsorption of water and oxygen in air due to the extraordinary high specific area of the powers [ 9 1, 2.2. Whisker synthesis The schematic diagram of the whisker synthesis apparatus is shown in Fig. 1. The Si-N-C powders were first slightly cold-pressed to discs in order to avoid the loss of powders during evacuating the system. Each disc weighed 1 g and had dimensions of 10 mm in diameter and 5 mm thick. The discs were placed in a BN crucible and positioned in the graphite resistance furnace. The system was evacuated to 1.3 Pa followed by the introduction of 1 atm of nitrogen into the furnace. Then, the sample was heated at a heating rate of 20 K/min to a defined temperature, kept constant for 1 h, then cooled to room temperature by terminating the power to the furnace. Several runs were done following this procedure for synthesis between 1673 and 1973 K in 100 K intervals. The above experiments were repeated using 1 atm of Ar instead of nitrogen, and nanometer-sized amorphous Si3N4 powders instead of Si-N-C, so as to understand the whisker formation mechanism. c rauiet.ance
Row material
BN crucible
Fig. 1. Schematic diagram of the whisker synthesis apparatus.
DEGREE (28) Fig. 2. XRD patterns of the as-formed whiskers and the crystallized products of S-N-C powders under various conditions: (a) remaining amorphous material after heating at 1673 K under 1 atm Nz, (b) a-S&N4 particulates at 1773 K under 1 atm N2, (c) a-SisN., whiskers formed at 1873 K under 1 atm N,, (d) crystallized particulates beneath the whiskers at 1873 K under 1 atm N2, (e) at 1873 K and 1 atm Ar, and (f) at 1973 K and 1 atm NZ.
Y. Li et al. /Materials
Letters 21 II 9941325-328
327
were in the range 0.1-0.5 pm and the length in millimeters as estimated from the STEM measurements. A typical TEM morphology of the whiskers is shown in Fig. 4.
4. Discussion
Fig. 3. STEM morphologies of the S&N4 whiskers: (a) whisker growth on the surface of the Si-N-C powder compact and (b) general morphology of the whiskers.
No frozen spherical droplets were observed by TEM on the whisker tips, as shown in Fig. 5a, indicating the absence of the VLS growth mechanism for whisker growth [lo]. Whisker growth on the grains can be seen in Fig. 5b, from which we know that the whiskers nucleated on the pre-crystallized a-Si-,N, on the powder discs. The main components in the gas phase for whisker formation are SiO, CO and N2. Their partial pressures influence solid phase formation and their supersaturation ratios determine the formation of whiskers [ 6 ]. In the Si-N-C system, SisN, and Sic are stable phases between 1773 and 1973 K under 1 atm N2 and SIC is the only stable phase when 1 atm of Ar was used [ 111. This explains the coexistence of Si3N4and Sic at 1873 and 1973 K and the formation
Fig. 4. Typical TEM morphology of the as-produced a&N, whiskers.
gesting that little C existed in the whiskers. The STEM observations were then carried out after a layer of amo~hous C was sprayed onto the whiskers. The STEM results are shown in Fig. 3. Fig. 3a shows the whisker growth morphology on the powder compact surface, and Fig. 3b shows the typical morphology of the whiskers formed. Most of the whiskers arc straight while some are bent. The diameters of the whiskers
Fig. 5. TEM morphologies of the a-Si,N, whisker tips: (a) the smooth whisker tip and (b) the whisker tip bonded to a small a&N, gram.
328
Y. Li et al. /lateral
of Sic when Ar is used. It is known that Si-N-C powders are thermodynamically metastable and are liable to give off gases such as Co, Nz and SiO during synthesis at temperatures up to 1673 K under 1 atm Nz [ 121. According to thermodynamic analysis of S&N4formation associated with these gases, the most favorable reaction [ 61 is 3SiO(g)S3CO(g)+2N,(g)4i3N4(s)+3C02(g), logk(1873K)
=-11.1
a
The small ~uilib~um constant at 1873 K means that the reaction is controlled by kinetic factors such as the release rate of the gases from the Si-N-C powders. Since the concentration of these gases was higher in the gas phase and attained the proper concentration, the whiskers formed only on the powder surface and the nucleation sites may be the first crystallized a-Si,N4 grains. Because none or small amounts of SiO and CO were released from the pure S&N4 powders, no whiskers form when S&N4 powders are used. As mentioned previously, the use of the Si-N-C amorphous powders as raw materials for whisker growth has several advantages over other methods. One shortcoming in the present synthesis method is the separation of the whiskers from the annealed powder compact. Actually, because there was no other impurity or elements in the raw materials except a little graphite, the presence of Si3N4 or Sic particulates in the whiskers does not influence the application of the whiskers as reinforcing materials. In most circumstances, the mixing of particulates with the whisker is necessary to produce high-density sintered bodies. Also, the proper design of the Si-N-C powder com~sition may eliminate the retention of graphite. It is known that the Si-N-C powders with a wider composition range can be synthesized by laser-induced gas-phase reaction [ 9 1. A study of the influence of powder composition on the formation of whiskers is now under way.
Letters 21(1994) 325-328
5. Conclusions a-Si3N4 whiskers with diameters of 0.1-0.5 pm and lengths in the millimeter range can be synthesized in situ from amorphous nanometer-sized Si-N-C powders at 1873 K in 1 atm N2 in a graphite resistance furnace. There were no impurity frozen droplets on the whisker tips, suggesting that the VLS (vapor-liquid-solid) growth mechanism was absent. The absence of any other catalysts and the in situ formation of the whiskers ensured that the whiskers were quite pure. No whiskers formed at 1773 and 1973 K, or with the use of Ar instead of N2 or using nanometer S&N4 powders as raw material. The whiskers were formed by the VS (vapor-solid) mechanism involving a gas-phase reaction among SiO, CO and N2 which were released from the Si-N-C powders during synthesis.
References [ I] W. Wada, in: Processing of ceramic and metal matrix composites, ed. H. Mostaghaci (Pergamon Press, London, 1989) p. 3. [2] A.L. Cunning and L.G. Davis, SAMPE 15 (1969) 120. [ 31 Y. Inomata and T. Yamane, J. Crystal Growth 2 1 ( 1974) 317. [ 4 ] R.C. Johnson, J.K. Alley and W.H. Warwick, U.S. Patent 3, 244,486 (1966). [ 51 S. Zhang and W.R. Cannon, J. Am. Ceram. Sot. 67 ( 1984) 691. [6] M.J. Wang and H. Wada, J. Mater. Sci. 25 (1990) 1690. [7] V.N. Gribkov, V.A. Silaev, B.V. Schetanov, E.L. Umantsev and A.S. Isaikin, Soviet Phys. Crystall. I6 ( 1972) 852. [8] K. Kijima, N. Setaka and H. Tanaka, J. Crystal Growth 24,25 (1974) 183. /9] Y.L. Li, Y. Liang, F. Zheng and Z.Q. Hu, Mater. Sci. Eng. 174 (1994) L23. [lo] W.B. Campbell, in: Whisker technology, ed. A.P. Levitt (Wiley, New York, 1970) p. 37. [ 111 R.V. Krishnarao and M.M. Godkhindi, J. Mater. Sci. 27 (1992) 2726. [ 121 G.W. Rice, J. Am. Ceram. Sot. 69 (1986) C-183.