Preparation And Properties Of Porous Alumina With Inter-Locked Platelets Structure

Porous alumina ceramics always possesses,low thermal conductivity,high high temperature strength and high chemical stability.It has a good application prospect in the fields of purification and separation,sound absorption and shock absorption,heat insulation and so on.High porosity and high specific surface area are the goals pursued in the preparation of porous materials.However,high porosity results in a rapid decline of strength.In addition,most of the ceramic pores prepared by the traditional methods are surrounded by equiaxed grains.Material tends to fracture at the grains’contact position when applied with external force due to the small contact area between grains,which is not conducive to improve the mechanical properties.So the combination of high porosity and high strength has always been a difficulty in preparing porous materials.In this experiment,porous alumina ceramics with alumina platelets was prepared by vapor-solid reaction sintering with the reaction of HF and Al₂O₃（1300℃～1600℃,1 h,in an airtight container）.The effect of sintering temperatures on the grain morphology,size,bonding and mechanical properties of porous alumina ceramics were investigated.The formation mechanism of porous alumina with inter-locked platelets structure was explored.At the same time,the inter-locked platelets structure was optimized by adding SiO₂.The effects of SiO₂content on the composition,morphology,strength and inter-locked structure of ceramics were investigated.The conclusions are as follows:（1）Porous alumina ceramics with alumina platelets was prepared by vapor-solid reaction sintering with the reaction of HF and Al₂O₃.As the sintering temperature increases from 1300℃to 1600℃,the linear shrinkage rate of the sample increases from 2.7%to 10.2%,and the porosity decreases from 61.6%to 48.4%.（2）When the sintering temperature increases from 1300℃to 1600℃,the average alumina platelets diameter and the average thickness increased.With the increase of sintering temperature,the sintered neck becomes more obvious,the bonding strength of alumina platelets improved.At 1600℃,the porosity of porous alumina ceramics with inter-locked platelets structure is 48.4%and the maximum compressive strength is 29.8 MPa.（3）The formation process of inter-locked platelets structure can be divided into two stages.Firstly,the precursor powder sintered and grew into alumina platelets,the grain grew up by the transport of materials in the vapor phase reaction.Secondly,sintered neck grew through solid phase diffusion.With the increase of the sintering temperature,the platelets bonded to each other by sintered necks.The sintering mechanisms of alumina platelets includes vapor deposition and solid diffusion.（4）The shrinkage rate of the samples with SiO₂ content between 15%～25%did not change significantly,and the phase composition of the samples were allα-Al₂O₃.When the content of SiO₂ is 30%,the ceramic shrinkage rate increases to 15.9%,and the phase composition of the sample isα-Al₂O₃ and mullite.（5）With the increase of SiO₂ content from 15%to 30%,the average particle size of ceramic samples decreases first and then increases.The average particle size is 10.8μm at 20%SiO₂ content.The porosity and compressive strength of porous alumina ceramics are 54.3%and 45.6 MPa when the content of SiO₂ is 20%.The improvement of compressive strength is attributed to the decrease of grain size and the deepening of inter-locked structure.