Preparation And Properties Of CA₆ Porous Materials

Calcium hexaaluminate（CA₆）has the characteristics of high refractoriness,excellent chemical stability,good thermal shock resistance,metal and slag erosion resistance,etc.,which can be made into porous materials with both CA₆and porous materials.In this paper,CA₆porous materials were prepared by traditional sintering method,molten salt pore-making method and molten salt-carbon black composite pore-making method.The effects of aluminum calcium raw materials,molten salt and molten salt-carbon black composite addition on phase composition,microstructure,conventional properties and thermal conductivity of porous materials were studied.The results are as follows:（1）The energy band and electronic density of states of the CA₆were calculated by first-principles,and the mechanical properties including its intrinsic elastic modulus were predicted.The calculation of electronic properties of CA₆show that its chemical properties are stable,with stable semiconductor or insulator properties.The calculation of mechanical properties show that CA₆crystal has good mechanical stability,resistance to volume change and deformation,and has certain tensile elasticity.（2）The properties and microstructures of CA₆porous materials are closely related to the types of aluminum-calcium raw materials.The effects of different aluminum-calcium materials on CA₆porous materials were studied.The results showed that the main phase of specimens with industrial Ca CO₃as the calcium source and theα-Al₂O₃micropowder（CA）and industrial Al₂O₃fines（CGA）as aluminum source was CA₆phase after firing at 1400°C,while the main crystal phase of the sample withγ-Al OOH（CAO）as aluminum source was CA₂phase and corundum phase.At 1500℃,the main phase of samples was CA₆phase.After firing at 1500℃,the morphology of CA₆in CA and CGA samples was sheet structure,whereas the size of CA₆sheet structure in CAO sample was small and CA₆particles were present.The linear change of CGA and CAO samples was shrunken and CA sample was expanded after firing.The CGA sample had the highest apparent porosity and the lowest compressive strength,while CA sample had the lowest porosity and the highest strength after burning.Compared with Ca CO₃as calcium source,the changes of phase composition,porosity and strength of the samples with Ca（OH）₂as calcium source were basically same.At 1500℃,the apparent porosity and compressive strength of CA sample were higher than that of CHA sample,the apparent porosity and compressive strength of CGA sample were lower than that of CHGA sample,the apparent porosity of CHAO sample was higher than that of CAO sample,and the compressive strength was lower than that of CAO sample.（3）CA₆porous materials were prepared by molten salt（KCl）pore-forming method using industrial Ca CO₃and Ca（OH）₂as calcium sources,α-Al₂O₃micropowder,industrial Al₂O₃andγ-Al OOH powder as aluminum sources.The results show that the contact area of Ca²⁺/Al₂O₃and increased the diffusion rate of Ca²⁺into Al₂O₃increased due to the liquid environment of KCl at high temperature.And the microstructure,pore size distribution,physical properties and thermal conductivity of CA₆porous materials could be controlled.When the sintering temperature was 1500℃and the mass ratio of aluminum-calcium to KCl was 4:2,the porous CA₆material had the better properties(the apparent porosity of the CAK-2sample was 67.80%,the strength was 21.65 MPa,and the thermal conductivity was0.784 W·m^-1·K^-1).At the same time,fractal dimension was introduced to analyze the pores,and the relationship among fractal dimension,apparent porosity and compressive strength was fitted,such as CAK series Y=-386.223+4.70917 X₁+363.252 X₂-4.58187 X₁×X₂（R²=0.997）（X₁:apparent porosity,X₂:fractal dimension;,Y:compressive strength）.（4）Based on the aluminum-calcium raw material:KCl=4:1,CA₆porous materials were prepared by molten salt-carbon black pore-forming method.The results present that the microstructure and pore size distribution,physical properties and thermal conductivity could be controlled by the addition of carbon black.When the sintering temperature was 1500℃and KCl:C was 4:2,the porous CA₆material had better properties(the apparent porosity of the CHAKC-2 sample was 64.84%,the strength was 18.51 MPa,and the thermal conductivity was 0.610 W·m^-1·K^-1at1000℃).The pores were analyzed by fractal dimension,and the relationship between fractal dimension,apparent porosity and compressive strength was obtained by fitting.Such as CAKC series Y=-393.78+31.9696 X₁-864.519 X₂-27.8451 X₁×X₂+0.0739 X₁²+901.703 X₂²（R²=0.985）.