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Silicon Nitride
Silicon Nitride Vladimir V. Zakorzhevsky
Silicon nitride was originally synthesized in 1844 through silicon annealing in nitrogen atmosphere. Si3N4-based ceramics are used to manufacture units and parts operating under severe thermal and mechanical loading, e.g., cutting plates, ball-bearings, turbine blades, armor elements. There are two modifications of silicon nitride, α and β, and they differ in lattice structure. Low-temperature α-Si3N4 is used to make high-quality ceramics. It is a metastable compound and transitions to stable β-Si3N4 at T > 1450°C under specific conditions (treatment period, catalytic additives). Silicon nitride synthesis in the combustion mode is based on combustion of silicon powder in nitrogen with regulating additives such as silicon nitride powder as an inert diluent, salts (NH4Cl, NH4F, Na2SiF6, (NH4)2 SiF6), and so on, as illustrated here: 3Si + 2N2
combustion, Si3 N4
!
Si3 N4
3Si + Si3 N4 + NH4 Cl + 2N2 ! 2Si3 N4 + HCl + 0:5N2 + 1:5H2 The first papers on SHS of silicon nitride were published in the 1980s; such gasifying additives as ammonium chloride and fluoride were used to synthesize α-phase for the first time [1]. Solid nitriding components such as alkali metal azides were used as a source of nitrogen [2]. The synthesis is carried out by the following scheme: 14Si + 6NaN3 + ðNH4 Þ2 SiF6 + N2 ! 5Si3 N4 + 6NaF + 4H2 It is also possible to use NH4Cl, NH4F, Na2SiF6, and (NH4)2SiF6 as salt additives to the green mixture. When Si burns in nitrogen, Si3N4 is formed by the mechanism of a vapor-liquidcrystal process. The limiting stage of the process is silicon evaporation [3], which
Concise Encyclopedia of Self-Propagating High-Temperature Synthesis http://dx.doi.org/10.1016/B978-0-12-804173-4.00134-4
Copyright © 2017 Elsevier Inc. All rights reserved.
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Concise Encyclopedia of Self-Propagating High-Temperature Synthesis
means that the temperature in the synthesis zone must exceed the melting point of silicon (1420°C). The minimum temperature of silicon combustion in the Si + Si3N4 + N2 system is about 1750°C, because lower temperatures do not allow evaporation of silicon in an amount sufficient to maintain SHS [4]. Introduction of salt additives is necessary for controlling the synthesis temperature regime; it is especially important for the synthesis of α-Si3N4. Salt decomposition products react with silicon particles and provide silicon transition to the gas phase, which allows carrying out the low-temperature regime by means of an intermediate product formation. The α-phase of silicon nitride was obtained in the Si + Si3N4 + N2 system without gasifying additives, but with submicron silicon powder (d50 ¼ 200 nm, Ssp ¼ 12 m2/g). The temperature of the green mixture combustion was noticed to be 1346°C, which is lower than the Si melting point (1420°C). This result was explained by the existence of the nanofraction of the silicon powder [5]. The particle morphology of silicon nitride synthesized in the combustion mode depends on the synthesis conditions. At the low-temperature mode (T < 1650°C) with participation of gasifying additives, α-Si3N4 formed as whiskers (Fig. 1). Particles of high-temperature β-phase formed as hexagonal columnar crystals (Fig. 2).
6 µm
Fig. 1 SHS silicon nitride crystals obtained by low temperature burning.
Silicon Nitride
6 µm
Fig. 2 SHS silicon nitride crystals obtained by high temperature burning.
REFERENCES [1] Merzhanov AG, Borovinskaya IP, Martynenko VM. Production method of silicon nitride, Auth. Cert. No. 1533215, USSR; 1983. [2] Kosolapov VT, et al. Production method of carbonitrides, Auth. Cert. No 738242, USSR; 1978. [3] Mukasyan AS, Stepanov BV, Galchenko YuA, Borovinskaya IP. Fizika goreniya i vzryva 1990;1:45–52. [4] Grachev VV, Shatalov BN, Borovinskaya IP. All-Russian conference “combustion and explosion processes in physical chemistry and technology of inorganic materials,” Moscow, 2002, 89-93; 2002. [5] Zakorzhevsky VV, Borovinskaya IP. Powder Metall Metal Ceram 2009;48(7/8):375–80.
341
Silicon nitride was originally synthesized in 1844 through silicon annealing in nitrogen atmosphere. Si3N4-based ceramics are used to manufacture units and parts operating under severe thermal and mechanical loading, e.g., cutting plates, ball-bearings, turbine blades, armor elements. There are two modifications of silicon nitride, α and β, and they differ in lattice structure. Low-temperature α-Si3N4 is used to make high-quality ceramics. It is a metastable compound and transitions to stable β-Si3N4 at T > 1450°C under specific conditions (treatment period, catalytic additives). Silicon nitride synthesis in the combustion mode is based on combustion of silicon powder in nitrogen with regulating additives such as silicon nitride powder as an inert diluent, salts (NH4Cl, NH4F, Na2SiF6, (NH4)2 SiF6), and so on, as illustrated here: 3Si + 2N2
combustion, Si3 N4
!
Si3 N4
3Si + Si3 N4 + NH4 Cl + 2N2 ! 2Si3 N4 + HCl + 0:5N2 + 1:5H2 The first papers on SHS of silicon nitride were published in the 1980s; such gasifying additives as ammonium chloride and fluoride were used to synthesize α-phase for the first time [1]. Solid nitriding components such as alkali metal azides were used as a source of nitrogen [2]. The synthesis is carried out by the following scheme: 14Si + 6NaN3 + ðNH4 Þ2 SiF6 + N2 ! 5Si3 N4 + 6NaF + 4H2 It is also possible to use NH4Cl, NH4F, Na2SiF6, and (NH4)2SiF6 as salt additives to the green mixture. When Si burns in nitrogen, Si3N4 is formed by the mechanism of a vapor-liquidcrystal process. The limiting stage of the process is silicon evaporation [3], which
Concise Encyclopedia of Self-Propagating High-Temperature Synthesis http://dx.doi.org/10.1016/B978-0-12-804173-4.00134-4
Copyright © 2017 Elsevier Inc. All rights reserved.
339
340
Concise Encyclopedia of Self-Propagating High-Temperature Synthesis
means that the temperature in the synthesis zone must exceed the melting point of silicon (1420°C). The minimum temperature of silicon combustion in the Si + Si3N4 + N2 system is about 1750°C, because lower temperatures do not allow evaporation of silicon in an amount sufficient to maintain SHS [4]. Introduction of salt additives is necessary for controlling the synthesis temperature regime; it is especially important for the synthesis of α-Si3N4. Salt decomposition products react with silicon particles and provide silicon transition to the gas phase, which allows carrying out the low-temperature regime by means of an intermediate product formation. The α-phase of silicon nitride was obtained in the Si + Si3N4 + N2 system without gasifying additives, but with submicron silicon powder (d50 ¼ 200 nm, Ssp ¼ 12 m2/g). The temperature of the green mixture combustion was noticed to be 1346°C, which is lower than the Si melting point (1420°C). This result was explained by the existence of the nanofraction of the silicon powder [5]. The particle morphology of silicon nitride synthesized in the combustion mode depends on the synthesis conditions. At the low-temperature mode (T < 1650°C) with participation of gasifying additives, α-Si3N4 formed as whiskers (Fig. 1). Particles of high-temperature β-phase formed as hexagonal columnar crystals (Fig. 2).
6 µm
Fig. 1 SHS silicon nitride crystals obtained by low temperature burning.
Silicon Nitride
6 µm
Fig. 2 SHS silicon nitride crystals obtained by high temperature burning.
REFERENCES [1] Merzhanov AG, Borovinskaya IP, Martynenko VM. Production method of silicon nitride, Auth. Cert. No. 1533215, USSR; 1983. [2] Kosolapov VT, et al. Production method of carbonitrides, Auth. Cert. No 738242, USSR; 1978. [3] Mukasyan AS, Stepanov BV, Galchenko YuA, Borovinskaya IP. Fizika goreniya i vzryva 1990;1:45–52. [4] Grachev VV, Shatalov BN, Borovinskaya IP. All-Russian conference “combustion and explosion processes in physical chemistry and technology of inorganic materials,” Moscow, 2002, 89-93; 2002. [5] Zakorzhevsky VV, Borovinskaya IP. Powder Metall Metal Ceram 2009;48(7/8):375–80.
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