Study of Producing α-Si3N4 Powder by Low-temperature Vapor-phase Reaction Method

Abstract

Most silicon nitride powders, industrially used in various applications under extreme conditions, are produced by the diimide process. The synthesis of diimide is carried out in the range -50–0 °C using liquid-phase reactants with organic solvent. This process, however, consumes a considerable amount of energy. One promising method for the synthesis of silicon nitride powder is the vapor-phase reaction of SiCl4 with NH3, and this synthetic process is also energy efficient. In this study, the processing parameters of the vapor-phase reaction for the synthesis of silicon nitride were investigated for producing diimide. The vapor-phase reaction completed at room temperature with a down-top flow, and solid products were obtained at the bottom of the reactor. Si(NH)2 decomposed at temperatures >150 °C according to the result of the thermogravimetric analysis, limiting the reaction temperature. The reaction temperature increased with increasing flow rates of the reactants and decreased with increasing flow rate of the carrier gas. The reaction yield decreased with increasing flow rate of the carrier gas. Under the optimized reaction conditions, 87% yield was obtained. Amorphous silicon nitride powder prepared by low-temperature vapor-phase reaction was heated at various temperatures for different periods of time to examine the crystallization behavior. The effects of the heat-treatment temperature and duration on the degree of crystallization were investigated along with the effect of the heat-up rate on the particle size, and the distribution of the crystallized α-phase silicon nitride powder. A phase transition from amorphous to α-phase occurred at temperatures >1,400 °C. The crystallization process was completed after heating at 1,500 °C for 3 h or 1,550 °C for 1 h. The crystallization process starts at the surface of the amorphous particle, whereas the outer regions of the particle become crystalline and the inner part remains amorphous. The re-arrangement of the Si and N atoms on the surface of the amorphous particle leads to the formation of hexagonal crystals that are separated from the host amorphous particle. The particle size and size distribution were controlled by varying the heat-treatment profile (namely, the heat-treatment temperature, heating rate, and heating duration at the specified temperature) and can be used to control the relative extent of the nucleation and growth. The completion of most of the nucleation process by lowering the heat-up rate was used to achieve a singlet particle size distribution. A bimodal particle size distribution was achieved by fast heating during the crystallization process. In this study, we developed a novel synthesis method for the preparation of Si3N4 nanowires from the amorphous silicon nitride (a-Si3N4) powder synthesized by low-temperature vapor-phase reaction method. Highly crystallized α-Si3N4 nanowires were synthesized by heating Si3N4 powder under ammonia atmosphere. The surface of the nanowires was smooth and clean without any attached particles. The thickness of the nanowires was in the range 200–300 nm with lengths of tens of micrometers. The nucleation of nanowires from the reaction between SiO and N2 occurs on the surface of a-Si3N4 powder, covered by a thin layer of SiO2, and the nanowires grow from the re-arrangement of Si and N atoms of the a-Si3N4 powder. The reduction of SiO2 to SiO by ammonia was promoted by the presence of Ni catalyst, and thus the growth was observed at lower temperature when Ni was added to the a-Si3N4 powder than when Fe was added. The growth of α-Si3N4 nanowires occurs along the [100] and [101] directions and follows Vapor–Solid–Solid mechanism. Silicon nitride is an alternative material widely used for silica crucibles for directional solidification of multi-Si ingots, and its main advantages is the reusability in successive castings and elimination for a source for oxygen contamination of the ingot. In this study, multi-Si ingot was cast in the synthesized Si3N4 coated-crucibles and compared to the reference ingots cast in commercial crucibles. The quality assessment of the coating layer was investigated. The thickness of the coating layer was 257 μm with a uniformity of 27.15%. The inside crack and inclusion were not observed in the 25 bricks. The average MCLT and resistivity of the slugs were 3.9 μs and 4.4 Ω·s, respectively. The contents of O, C, and metal impurities were 4.4, 1.1, and 7.3 ppm, respectively. These results indicate that the performance of the synthesized Si3N4 coated-crucible was almost similar to the commercial one. For application in the structural ceramics, pressureless sintering of the synthesized powder was carried out. Density and microstructure of the sintered body were measured, and the results were compared to the sintered body from Ube E10 powder. The pressureless sintering of the synthesized powder showed 99% sintering density at 1,650 °C, whereas Ube E10 powder showed 96% sintering density. The density of the synthesized powder was higher than that of Ube E10 powder. The Si3N4 powder synthesized by low-temperature vapor-phase reaction would be a good candidate for coating and the structural ceramic materials.