Study Of The Pore Control Mechanism Of Alumina Ceramics In The Initial Stage Of Freeze-casting And Solidification

Porous ceramic is a material with many pores.It has excellent performances such as good chemical stability,large specific surface area,high poriness,and high permeability.It is widely used in the fields of biological scaffolds,filter materials,fuel cells and catalyst carriers.Porous ceramics with a pore size of more than 100μm have important applications in some special fields.For example,biological scaffolds should have a main nutrient metabolism channel which is not less than 100μm,and fuel cells need a pore size of 50～150μm.Among the methods for preparing porous ceramics,the Freeze-casting method has the characteristics of strong pore structure design,large controllable range of pore size,and orderly arrangement of pores.It has attracted wide attention from scholars at home and abroad,but it is difficult to prepare large size porous ceramics which is over 100μm in the initial stage of solidification.In response to the above problems,this paper proposes a method for adjusting the pores in the initial stage of solidification:by changing the thickness and thermal diffusivity of the freezing plate to control the quasi-stable cooling curve at the output end of the freezing plate,thereby constant cooling rate which is in the initial stage of solidification can be obtained,then the problems of pore controlling can be consequently solved.The following studies have been carried out through mathematical modeling,finite element simulation,experimental analysis and verification:Firstly,according to Fourier’s law of heat transfer,a one-dimensional heat transfer equation about the thickness,thermal diffusivity and temperature changes of the freezing plate is established,and the equations are solved and analyzed,then the influence that the changes of the thickness and thermal diffusivity of the freezing have on the temperature at the output end of the freezing plate.A finite elment simulation experiment about ceramic slurry was carried out,and the effect of changing the thickness and thermal diffusivity of the freezing plate on the cooling rate of the solidification end of the ceramic slurry was analyzed.It is found that increasing the thickness can reduce the cooling rate of the freezing plate and the slurry,reducing its thermal diffusivity can obtain the same result.and the cooling rate changes parabolically,what’s more,reducing the freezing temperature can increase the coolingThen the experimental study clarified the rules of influence that the thickness of the freezing plate and the thermal diffusivity on the average pore width of the porous ceramics.On the condition that,at a distance of 1mm from the freezing plate,the freezing temperature is-28℃,the solid content is 25wt.%,and the thermal diffusivity is 3.94m2/s,when the thickness of the freezing plate is 50mm,the average pore widths of the transverse and longitudinal sections respectively are 98.58μm and 164.59μm.However,when the thickness is 175mm,the average pore widths of the transverse and longitudinal sections respectively are 340.32μm and 401.64μm,and the pore widths are significantly increased.When the thermal diffusivity of the freezing plate is 115.46m2/s and the thickness is 50mm,the average pore widths of the cross section respectively are 30.32μm and 95.64μm,and the pore widths are significantly reduced.Finally,the results of experimental and simulation are compared and analyzed,when the thickness of the freezing plate increases or the thermal diffusivity decreases,the slurry can maintain a constant cooling rate in the initial stage of solidification,and porous ceramics with uniform pore widths are prepared.It was found that when the freezing temperature is-28℃,the solid content is 25wt.%,the freezing plate is stainless steel 304,and the thickness is 175mm,simultaneously,when the moving distance of the solidification end is 1mm and 2mm,all the cooling rate is 0.35℃/min and all the hole widths of cross-sectional are 340.32μm,meanwhile,the pore widths of the longitudinal section respectively are 401.64μm and 400.68μm,which indicates that within 2mm of the initial freezing stage,the cooling rate of the slurry can be adjusted by increasing the thickness of the freezing plate and reducing the thermal diffusivity to keep the cooling rate constant,thereby preparing size-large,uniform and orderly porous ceramics.