Study On The Catalytic Nitridation Of Silicon Powder By Cr₂O₃ And TiO₂

As a traditional structural ceramic material,silicon nitride（Si₃N₄）ceramic material has many excellent mechanical,physical and chemical properties and is widely used in automotive,metallurgy,aerospace and other fields.At the same time,Si₃N₄ ceramic has high theoretical thermal conductivity and is considered as one of the most promising alternative materials for electronic packaging ceramic substrates.The preparation of high quality Si₃N₄ ceramic powder is the basis for the preparation of high performance Si₃N₄ ceramics.The major problem in the preparation of Si₃N₄ceramic powder is how to obtain Si₃N₄ ceramic powder at low temperature.In this paper,the effects of raw material,graphite paper substrate,nitridation gas,nitridation temperature,nitridation time and catalyst on nitridation process and phase composition and microstructure of the product were studied.The catalytic mechanism of metal oxide was also studied.The results are as follows:The optimum nitridation experimental parameters were:micron Si as raw material,nitrogen-hydrogen mixed gas as nitridation gas,and graphite paper as substrate.When the catalyst content and the nitridation time were constant,the degree of nitridation increases as the temperature increased.When the catalyst content and the nitriding temperature were constant,the degree of nitridation increased as the holding time increases.Nitriding experiments were carried out using Cr₂O₃ as a catalyst,and the degree of nitridation increased with the increase of Cr₂O₃ content.The Si powder with 2.5-8wt%Cr₂O₃ was completely nitrided at 1350℃for 2h.The main crystal phase of the sample wasα-Si₃N₄,the secondary phases wereβ-Si₃N₄ and Si₂N₂O.The Si powder with 2.5-8wt%Cr₂O₃ was nitrided at 1400℃for 4h to obtain pure Si₃N₄ powder with the main crystal phaseα-Si₃N₄ and a small amount ofβ-Si₃N₄.Cr₂O₃ promoted the nitridation of Si powder by mutual conversion with Cr₂N,and mainly promoted the conversion of Si toα-Si₃N₄.TiO₂ was used as the catalyst for nitridation experiments.At 1300℃,the addition of TiO₂ inhibited the nitridation of Si.At 1350℃,with the increase of TiO₂ content,the degree of nitridation increases and theβ-Si₃N₄ phase increased.The samples with 2.5-8 wt%TiO₂ achieved complete nitridation at 1400℃ for 2h and the main crystal phase wasβ-Si₃N₄,the second phase wasα-Si₃N₄,and the impurity phase was Si₂N₂O.Unfortunately Si₂N₂O could not be removed with increase of the nitridation time.TiO₂ promoted the nitridation of Si powder by mutual conversion with TiN,and mainly promoted the formation ofβ-Si₃N₄ phase.The catalytic nitridation mechanisms of the two catalysts were similar,but the specific catalytic temperature,the temperature of complete nitridation and the main crystalline phase of the final product were different.This was related to the stability of the metal oxide,the more stable the metal oxide was,the more difficultly the metal oxide was reduced.