Study Of High-performance Silicon Nitride Ceramic Powders

Silicon nitride (Si3N4) is a kind of excellent high-temperature structure ceramic material, which has been hailed as a "all-around champion" in the world of ceramic materials. It can exhibit high strength, high temperature resistant, wear resistant, low gravitiy and large elastic modulus, as well as good chemical stability even in almost all inorganic acid except hydrofluoric acid. Therefore, Si3N4 ceramics are widely used in electronics, military industry, metallurgy, machinery, and chemical engineering. As the most important raw materials for Si3N4 ceramic, Si3N4 powder has attracted great attention, including the a phase, fine grain and narrow size distribution.The direct nitridation of silicon (Si) powder in nitrogen at high temperature is the main route for the synthesis of Si3N4 powder. However, the great heat accumulation occur during the reaction, which leads to the phase transformation from α to β. The great reaction heat may also result in the melted Si, which hinders the further nitridation. Based on this problem, this thesis propose some new idea to enhance the a phase, including the incorporation of nanosize Si, Si3N4 in the primary Si source, using porous Si powder as source, and two-step nitridation. In addition, we also investigate the ball-milling and acid pickling process to further improve the size distribution and purity.At first, we propose the nitridation process in rotating furnace. Comparing to the static furnace, the rotating furnace can improve the heat accumulation and hinder the agglomeration of Si3N4 powder, leading to the high a phase. Moreover, the dispersed Si3N4 powder can avoid the mechanical crushing process and improve the uniform dispersion of diluting agent.Then, we add the nanosize Si into the micrometer Si powder as the source for the nitridation. The nanosize Si can be nitridated at relatively low temperature to form dispersed Si3N4 powder in the micrometer Si powder, which can act as the diluting agent for the subsequent nitridation process. The existence of the diluting agent can buffer the heat accumulation during the reaction, which can lead to high a phase of resultant Si3N4 powder. Based on the similar idea, we add the Si3N4 powder into the primary micrometer Si source as the diluting agent, which can also enhance the a phase of resultant Si3N4 powder. In addition, we propose the two-step nitridation process. The Si3N4 powder formed in the first step can also act as the diluting agent to improve the ratio of a phase.The porous Si powder owns the high specific surface area and porous structure, which can improve the nitride reaction rate and buffer the heat accumulation. Therefore, the porous Si powder can facilitate to prepare the high a phase Si3N4 powder.The ball-milling of Si3N4 powder can reduce the particle size and improve the size distribution. The best ball mill process is adopting the unified 2 mm silicon nitride ceramic balls, the ball material ratio of 6:1, the ball milling speed of 200 r/min and ball milling for 15 h. The acid pickling is the common method for material purification. For Si3N4 powder, the compound of hydrofluoric acid and nitric acid can remove the isolated Si, silicon oxide and silicon nitride oxide. The use of the compound of hydrofluoric acid and nitric acid, hydrochloric acid, and ammonia water in turn can remove the metal efficiently.