Study On Preparation And Mechanism Of AlN Transparent Ceramics Under High-temperature And High-pressure

Aluminum nitride（AlN）is a kind of ceramic materials with high hardness, highthermal conductivity, low dielectric loss and dielectric constant, coefficient of thermalexpansion that matches with silicon, insulation, and high transmittance. This unique setof properties, makes AlN a candidate for applications like heat dissipation materials，substrate materials of large scale integration, dome materials and infrared windowmaterials. However, AlN is difficult to sinter because of its strong covalent bond andsmall self-diffusion coefficient. In addition, the sintering temperature of transparentceramics is higher than normal ceramics, and it is easy for AlN to hydrolyze to importoxygen impurities. As a result of above, the reports of preparing AlN transparentceramics successfully are rather less.In this paper, AlN transparent ceramics were prepared by high-pressure sintering(HPS). HPS, which refers to sintering above1GPa, has definitely superiority inmaterials’ sintering. It not only can make materials achieve high-consistency swiftly,obtaining small grains, but also can lead the grain structure, even the atom and electronstate to produce changes at high pressure. There are wide application prospects for HPS.Commercially available AlN powder produced by TOYO ALUMINIUM, withsmall particle size, high purity and low content of impurity, is used as starting material.The CaC2used as sintering additive is chemical pure.Two kinds of transparent ceramics are prepared in this experiment, one is pure AlNtransparent ceramic，another is AlN transparent ceramic with sintering additive. Thepreparation technology of pure AlN transparent ceramic is5GPa/30min/1850~1900℃,of which infrared transmittance is20.04%. The preparation technology of AlNtransparent ceramic with CaC2of3wt%is5GPa/30min/1800℃, of which infraredtransmittance is56.35%.To pure AlN transparent ceramic, the increase of sintering pressure is beneficial forthe improvement of sintering properties and microstructure, and5GPa is a proper value.On the whole, the sintering pressure has little impact on the relative density and infraredtransmittance of the AlN ceramic; extension of sintering time is beneficial to growth ofcrystalline and improvement of relative density and infrared transmittance, and30min is a proper value; rise of temperature is beneficial for the improvement of relativedensity, infrared transmittance and crystalline growth, and AlN sintered at1900℃acquired best sintering properties.To AlN transparent ceramic with additive,3wt%is a proper content for CaC2, withwhich the sintered body has a higher relative density and infrared transmittance;extension of sintering time is beneficial to growth of crystalline and improvement ofrelative density and infrared transmittance, and30min is a proper value; rise oftemperature is beneficial for the improvement of relative density, infrared transmittanceand crystalline growth, and AlN sintered at1850℃acquired best sintering properties.To pure AlN transparent ceramic, full crystalline grains, regular crystalline shapeand purity of grain boundaries are all the reasons of being transparent. However，because of the existence of oxygen and pores, the transmittance is not enough high. ToAlN transparent ceramic with additive, full crystal grains with typical hexagonalstructure, thin grain boundaries, without impurity or pores all result in a hightransmittance.Compared with the pure AlN transparent ceramic, the one with additive has higherrelative density and infrared transmittance, which is the result of the adoption of CaC2.CaC2is an excellent sintering additive, which has important effects on two aspectsbelow: it can react with oxygen compounds in the AlN powder surface and generateliquid phase, which can volatilize and remove the impurity without pollution for thesamples, improving the transmittance further.It is found that HPS is an effective means to compress lattice space, promotesintering and improve densification for AlN ceramic. By analyzing the transmittancemechanism, the influence factors of the transmittance for the AlN ceramic are oxygenimpurity, pore phase, crystal grain situation and surface finish.This paper provides theoretical basis and experimental reference for preparation ofAlN transparent ceramic, and introduces a new idea for preparations of other transparentceramics.