Microstructure And Properties Of High Porosity Calcium Hexaaluminate Foams By Thermo-foaming Method

The high porosity calcium hexaaluminate foams have the characteristics of large specific surface area,low thermal expansion coefficient,good stability under alkaline and reducing atmosphere,low thermal conductivity at high temperature,can be widely used in thermal insulation,high temperature filtration and catalyst carriers materials.However,it is difficult for the calcium hexaaluminate crystals to grow alongside the c-axis direction,leading to the flaky-like microstructure of calcium hexaluminate crystal grains,and the difficult sintering of calcium hexaaluminate foams.Further,this will result in low strength of foams,which makes it hard to prepare high porosity calcium hexaaluminate foams.In view of this problems,a new method for preparing high porosity foams--thermo-foaming method is selected in this paper,and high-porosity calcium hexaaluminate foams are successfully prepared.In this paper,the raw materials for the synthesis of calcium hexaaluminate and the dispersion used in the thermo-foaming method are designed and selected.On the basis of this,the effects of foaming temperature,sintering temperature and the amount of barium carbonate added to the calcium hexaluminate foams on pore structure and properties are investigated.Through the above research work,the main conclusions are as follows:?1?Calcium chloride,calcium hydroxide,and calcium gluconate cannot be used as a calcium source to produce a green.Both calcium sulfate and calcium carbonate can be used as calcium sources to prepare calcium hexaaluminate foams.Finally,calcium carbonate is selected as the calcium source for preparing calcium hexaaluminate foams by thermo-foaming method.?2??-Al₂O₃ micropowder and calcium carbonate micropowder are selected as raw materials for synthesic calcium hexaaluminate.The parperaed calcium hexaaluminate foams have the characteristics of uniform pore distribution and high spherical degree,and the through-hole structure is formed between the adjacent pores through the cell windows.?3?Calcium hexaluminate foams with porosity of 91.7 to 97.0%can be synthesized with sucrose and anhydrous glucose as a dispersion medium.After heat treatment at high tempurature,the structure of calcium hexaluminate foams are cell-window porous structure,and the pores are divided into two categories:one is spherical large pores?cells?,another one is circular window-like opening pores on the wall of the large spherical pores?cell windows?,and the cells are connected through the windows.?4?The dispersion is a pseudoplastic fluid,and the viscosity decreases with the increase of shear rate.The viscosity determines the foaming performance of the whole dispersion system,and the larger viscosity is favorable for the uniformity and stability of the green pores.The performance of sucrose-based calcium hexaaluminate foams are better than the anhydrous glucose-based calcium hexaaluminate foams.?5?As the foaming temperature increases,the foaming height of the dispersion increases,the porosity of the sample increases,the apparent density decreases,and the compressive strength first increases and then decreases.The change of foaming temperature is inversely proportional to the viscosity of dispersion and the thermal conductivity at room temperature.The higher foaming temperature and lower foaming temperature is not conducive to the stability of the pores and the growth of pores,respectively.In addition,when the foaming temperature is too high,the pressure difference between the inside and outside of the cells is large,the cells are easily broken,and the number of cell windows is significantly increased.Compared with the foaming temperature of 130?,the pore size of calcium hexaaluminate foams is larger,and the number of cell windows increases significantly when the foaming temperature is 145?.?6?As the firing temperature is raised from 1450?to 1600?,the apparent density of the samples gradually increase from 0.13 g/cm³ to 0.25 g/cm³,and the linear shrinkage rate increases from 19.3%to 22.6%,the compressive strength increases from 0.36 MPa to 0.79 MPa,the thermal conductivity increases from 0.072 W·?m·K?^-1 to 0.087W·?m·K?^-1,the average pore diameter decreases,and the pore size distribution obeys a single peak distribution.?7?The addition of barium carbonate can change the growth of calcium hexaaluminate grains in the vertical c-axis direction,and make the calcium hexaluminate grain thicker,which will also change the rheological properties of the dispersion and reduce the viscosity of the dispersion.This is not conducive to the stability of pores during foaming process of the dispersion.As the addition of barium carbonate increases from 1wt%to 4 wt%,the apparent density and the shrinkage rate of the samples after sintering are gradually reduced,and the compressive strength of the samples after sintering is first increased and then decreased.Better performance can be obtained by adding 2 wt%of barium carbonate,the apparent density is 0.17 g/cm³,and the compressive strength is 0.59MPa.