The Preparation And Microstructure Analysis Of Nanoscale Silicon Nitride Podwer

In the world,the energy and environment has become the two serious problems.The motor vehicle,having the advantages of high efficiency,low noise,less pollution,easy operation and so on,is considered to be more and more important.In agriculture,the complex application environment and abominable working conditions assert the demands of high power,high voltage and high current for IGBT power module in the motor vehicle.However the high power and the large difference in temperature of IGBT module may lead the IGBT chip burned or produce thermal shock crack,which cause the IGBT power module can not serve.Therefore,the heat dissipation substrate materials of IGBT power module must have the high thermal conductivity,excellent mechanical performance and thermal shock resistance,simultaneously.Having the high hardness,high strength,excellent thermal shock resistance and corrosion resistance and so on,the Si3N4 ceramics has been widely studied and applied as a kind of structural ceramics.In recent years,the study found that the Si₃N₄ ceramics has thermal expansion coefficient matching the chip and theoretical high thermal conductivity,which has been widely concerned as electrical functional ceramics and is considered to be the most potential electronic heat dissipation material at present.But for the Si₃N₄ prepared by existing methods,whose thermal conductivity is still far below the theoretical level.The mainly reason is that the Si3N4 powder can not meet the requirements of obtaining Si₃N₄ ceramics with high thermal conductivity in phase composition and morphologies.The research shows that the Si3N4 powder with the nanoscale,high?-Si3N4 content is beneficial to obtain the Si3N4 ceramics with high density,high thermal conductivity.Therefore,searching the ways for preparing nanoscale and high?-Si₃N₄ in raw materials has important theoretical significance and practical significance.This paper mainly studies the influence of preparation methods and experimental conditions on the phase composition and microstructure of products and analyses the growth mechanism of various morphologies of products,and has prepared the Si3N4 powder with nanoscale and high?-Si₃N₄ content.It was divided into the three parts:1)Using Si powder as and Si₃N₄ as diluent,preparing Si₃N₄ products by direct nitriding,the effects of heating temperature,heating time and the ratio of raw materials to diluent on the phase content and morphology of the products were studied.The experimental results show that with the increase of temperature,the content of?-Si₃N₄ and?-Si₃N₄ in the product increases synchronously,but when the temperature is above 1450°C,because of the transformation of???,the content of?-Si₃N₄ decreases.Similarly,as the heating time increases,the content of?-Si₃N₄ decreases,adding the Si₃N₄ diluent in raw materials is beneficial to the formation of?-Si₃N₄.The morphologies of the product is divided into two layers:the upper layer is fibrous Si₃N₄;the lower layer is blocky Si₃N₄.With the increase of temperature and time,the diameter of Si₃N₄ nanowires increases,the content of Si₃N₄ diluent has no effect on the morphology of the products in low layer.2)Adding Si₃N₄ diluent and molten salt NaCl into Si raw materials,preparing Si₃N₄products by molten salt nitriding,the effects of heating temperature,NaCl content on the phase content and morphology of the products were studied and the growth mechanism of morphologies were analysed.The results show that,increasing temperature,the content of?-Si₃N₄ increased at 1250?-1450?,but when the temperature is too high,the content of?-Si₃N₄ decreased.And with the increase of NaCl content,the content of?-Si₃N₄ decreased.The morphology of the product is divided into two layers:the upper layer contain Si₃N₄nanowires and nanobelts,with the increase of temperature and NaCl content,the radial size of Si₃N₄ nanowires and nanobelts are gradually increased;the lower layer contain blocky and short needle-like Si₃N₄.With the increase of heating temperature,the short needle-like products increase,with the increase of NaCl content,the short needle-like products reduced.It is analyzed that the morphologies of the products are related to the state of the raw materials'aggregation,different aggregates state form different morphologies of the products.3)Adding NaCl and NH₄Cl in Si powders,preparing Si₃N₄ by NH₄Cl assisted nitriding,we studied the influence of temperature,NaCl content and NH₄Cl content on the phase content and morphology of the products and analysed the growth mechanism of morphology.The results show that,with the increase of temperauture,NaCl content and NH₄Cl content,the?-Si₃N₄ content in products appeared the tendency that first increase and then decrease.And when the temperature was 1450°C,the NaCl content was 30 wt%,the NH₄Cl content was 3wt%,the maximum of?-Si₃N₄ content was 96 wt%.The products showed two layers:the upper layer consisted of a thin layer of nanowires,in which there are Si₃N₄ nanowires,nanobranches and so on;the lower layer is mainly a blocky,and short needle-like product.The process of Si nitridation was devided to three stages,the introduction of NaCl and NH₄Cl facilitated the evaporation of Si powders,caused the decomposition of Al₂O₃ from porcelain boat and furnace tube.So there were a trace of O and Al elements in reaction atmosphere,which promote the formation of products with different morphologies.And the growth mechanisms of nanowires located on the upper have the VS,VLS and the double-stage VLS-base and VS-tip growth mechanisms and so on.