Study On Preparation Method Of Porous Ceramics By Centrifugal Freeze Drying

Porous ceramic not only has the inherent characteristics of ceramic Matrix itself,such as high temperature resistance,high hardness,stable physical and chemical properties,but also obtains significant characteristics such as large specific surface area and high porosity due to its rich pore structure.It has dual application value of structure and function.Then it is involved in the fields of biomedicine,catalytic reaction,energy absorption and shock absorption.The preparation of porous ceramics by freeze casting method has the advantages of environmental protection and simple operation.Foreign and domestic Scholars focus on affecting the pore size distribution and structure by controlling the growth of ice crystals,but they can not change the macro distribution of ceramic particles.In view of the above problems,this paper studies a method of preparing porous ceramics with dense outside and sparse inside particles based on centrifugal freeze casting technology: by changing the viscosity and rotation speed of the slurry,the centrifugal movement of ceramic particles is controlled to change the particle distribution,and a controllable porous ceramic with gradient structure is prepared,which has a certain application prospect in the field of artificial bone.Through the methods of theoretical analysis,simulation and experimental verification,a detailed study on the influence of binder,solid content,rotating speed and other factors on the internal and external porosity and pore structure of alumina porous ceramics was carried out,so as to solve the problem of regulating its pore structure and particle distribution.Firstly,the experimental process of centrifugal freeze casting technology is determined.On the basis of freeze casting technology,porous ceramics with dense outside and sparse inside can be prepared by combining with centrifugal technology.Secondly,according to the centrifugal velocity formula of ceramic particles,the effects of rotating speed,density difference and viscosity on the centrifugal effect of particles are analyzed,and the finite element simulation of rotating fluid discrete phase（DPM）is established.In order to simulate the effect of freezing die on slurry temperature in the freezing process,slurry heat transfer simulation was established.The results show that increasing the rotating speed or reducing the fluid viscosity will make the centrifugal movement effect of particles more obvious.The temperature gradient of the slurry can be increased by using the freezing die with high thermal conductivity.Finally,the experimental results show that the centrifugal effect of particles is directly proportional to the rotating speed and inversely proportional to the ceramic solid content and binder content.The use of copper freezing die will reduce the pore structure and affect the centrifugal effect of particles.Under the condition of solid content of 40wt% and 1wt% CMC Na,when the rotating speed increases from 800 rmp to 1500 rmp,the difference between internal and external porosity also increases from 8.61% to 19.23%.With the increase of rotating speed,the pore structure of porous ceramics gradually changes from lamellar to cellular,which indicates that rotation will inhibit the anisotropy of water-based solution.Based on the porous ceramics prepared by centrifugal freeze casting,the pore size changes from small to large along the radius from the circumferential position to the center of the circle,and the change law of porosity is on the contrary.On the whole,the porosity and pore size change in a gradient.