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35SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS OF a-Si3N WITH PARTICIPATION OF SODIUM HALOGENIDES
V. V. Zakorzhevsky*", N. I. Mukhina", and I. D. Kovalev"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of
Sciences, Chernogolovka, Moscow, 142432 Russia
*e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10201
Ceramic materials from silicon nitride are widely used for the production of structural and functional ceramics operating under extreme conditions of high temperatures and mechanical loads. The strength properties of ceramics depend on the characteristics of the initial powders of silicon nitride. One of the important parameters affecting the properties of ceramics is the size and shape of particles. To obtain ceramics by hot pressing and slip casting, it is preferable to use powders with equiaxial particles.
Generally, silicon nitride powder is obtained by furnace method [1-3] and by synthesis and decomposition of silicon diimide [4]. Grinding and classification operations are performed to obtain equiaxed particles.
During the synthesis of the alpha phase of silicon nitride in the combustion mode, silicon nitride particles are formed in the form of filamentous crystals. Such a microstructure is formed due to realization of gas-phase mechanism of structure formation [5, 6]. Since the elongated silicon nitride particles are formed as a result of gas-phase reactions, it is necessary to create synthesis conditions that prevent the realization of the gas-phase mechanism of structure formation in order to obtain equiaxed particles. The paper presents the results of studies on the effect of sodium halide additives on the microstructure and phase composition of silicon nitride.
In the study of the effect of sodium fluoride on the microstructure and synthesis parameters of Si3N4, it was found that with the introduction of 24 wt % Si and 1 wt % NaF into the reaction mixture, the synthesis product is formed in the form of conglomerates which consist of particles of equiaxed shape of 0.3-3|m in size (Fig. 1). Combustion temperature was 1713°C. The alpha phase content increased from 97 to 98 wt %. By increasing the additive NaF to 3 wt %, the combustion temperature was 1773°C. The alpha phase content in the synthesis product was significantly decreased from 97 to 91 wt %. The synthesis product consisted of conglomerates formed by mainly equiaxial particles 0.5-3 |m in size with a predominance of particles larger than 1 |im in size (Fig. 2). The analysis carried out by the method of energy-dispersive scattering showed the presence of fluorine and sodium atoms in samples of Si3N4 in an amount of 0.7-1.3 wt %. The calculation of the equilibrium composition with sodium fluoride for the synthesis conditions showed the formation of Si3N4, Si-liquid, Na-gas, SiF, Na2SiO3-liquid. Sodium silicate is formed as a film on the surface of silicon particles when NaF interacts with a silicon oxide film by the reaction Si + SiO2 + NaF = Na2SiO3 + SiF. The film NaF, Na2SiO3 apparently prevents the gasification of silicon and, accordingly, the implementation of the gas-phase mechanism of structure formation. Nitriding occurs in the diffusion mode. Sodium silicate is also as a sintering additive.
When using a green mixture with 1 and 3 wt % sodium chloride, combustion temperature varied from 1615 to 1785°C for 24 and 26 wt % Si respectively. The microstructure of both samples is represented by particles of equiaxial and elongated shape (Figs. 3, 4). The content of the alpha phase increased by 2 wt %. Compared with the initial mixture, X-ray phase analysis showed that the NaCl phase is present only in the surface layer of the cake.
XV International Symposium on Self-Propagating High-Temperature Synthesis
Mag = 12.03 KX WD » 10min Signal A = SEI Data :28 Aug 2017 EHT=15.00ICV Time :11:00:15
Fig. 1. Microstructure of Si3N4. 24 wt % Si, 1 wt % NaF. Zcomb = 1713°C.
Fig. 3. Microstructure of Si3N4. 24 wt % Si, 1 wt % NaF. Zcomb = 1615°C.
Fig. 2. Microstructure of Si3N4. 26 wt % Si, 3 wt % NaF. Zcomb = 1773°C.
Fig. 4. Microstructure of Si3N4. 26 wt % Si, 3 wt % NaF. Zcomb = 1785°C.
The formation of filamentary crystals of Si3N4 indicates the realization of the gas-phase mechanism of structure formation. The calculation of the equilibrium composition with sodium chloride for the synthesis conditions showed the presence of Na-gas, SiCl, SiO, Si-liquid, which promote to the partial realization of the gas-phase mechanism of structure formation, by reaction SiCl + Na-gas + N2 = Si3N4 + NaCl. It was found that with the use of 3 wt % NaCl additives, the alpha phase content does not decrease at Zcomb = 1785°C, as compared to NaF. This can be explained by the fact that when using NaCl, volatile silicon suboxide (SiO) is formed in the combustion wave, which evaporates from the reaction zone, thereby not participating in the phase transition, unlike NaF, which forms the liquid phase of sodium silicate.
Samples of silicon nitride obtained with the participation of sodium fluoride were crushed with ceramic balls in water for 1 h to destroy the conglomerates and remove sodium fluoride. Silicon nitride was separated from the NaF solution using a vacuum filter and washed with distilled water. After drying, the silicon nitride was deagglomerated in a jet mill. The final product is a powder with equiaxed particle shape, polydisperse distribution. The average particle diameter J50 was 1.2-1.5 |im. The specific surface area was 5-6 m2/g.
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