Study Of Preparation For Medium Temperature Thermal Storage Ceramics By Using Low-grade Raw Materials

In the solar thermal power generation,industrial waste heat recovery system and other heat utilization systems,the heat storage devices must be used to ensure the continuous operation.Ceramic is a kind of potential thermal storage material because of its high temperature stability,chemical resistance and other advantages.Shale is rich in the crust,and the emissions of fly ash and red mud are huge.These three low-quality raw materials contain high contents of SiO₂ and Al₂O₃,which can be used for the preparation of mullite ceramics to promote its resource utilization.In this paper,a kind of low-cost,higher thermal conductivity and heat storage density thermal storage ceramic for medium temperature was prepared by using low-quality raw materials such as shale,fly ash and red mud.First,the chemical composition,phase composition and microstructure of the clay shale were analyzed,and then the possibility of preparing thermal storage ceramics with shale is explored.In order to improve the thermal shock resistance of ceramics,the thermal storage ceramics were prepared by using both fly ash and shale.The mechanism of adding fly ash to improve the thermal shock resistance of ceramic was studied.In order to enhance the thermal conductivity and heat storage density of the ceramic,a certain amount of red mud was added on the basis of the fly ash-based ceramic,and the effect of the red mud on the ceramic properties was studied.The relationships between the chemical composition,preparation process,structure and properties have been investigated by TG-DSC,XRD,SEM,EPMA,etc.The main research results are listed as follows:（1）The chemical composition,phase composition and microstructure of the shale were analyzed.Then the thermal storage ceramic was prepared from clay shale as the main raw material at a low temperature.The result showed that the content of Fe₂O₃ in shale was 7.22%,which indicated the shale was a kind of low-quality clay.The sample（75%shale,10%kaolin,10%potash feldspar and 5%soda feldspar）fired at 1080°C showed the best comprehensive properties,in which the water absorption,porosity,bulk density,thermal conductivity at 500°C and theoretical heat storage density（room temperature-500°C）were 0.11%,0.29%,2.60g/cm³,2.33W/（m·K）,1.51KJ/cm³,respectively.But the thermal shock resistance of the sample was poor and the bending strength of samples plummeted after 30 cycle times.（2）The ceramic samples were prepared by using fly ash and shale and the effects of composition and firing temperature on the physical properties were discussed.The result indicated that the sample（40%fly ash,15%shale,20%soda feldspar and 25%quartz）fired at 1180°C showed the best comprehensive properties,in which the water absorption,open porosity,bulk density,bending strength,thermal conductivity at500°C and theoretical heat storage density（room temperature-500°C）were 0.25%,0.61%,2.43g/cm³,75.32MPa,1.94W/（m·K）,1.19KJ/cm³,respectively.The bending strength increased by 16.21%after 30 thermal shock cycles.In the process of firing,the synthesis of mullite started from the surface of the glass beads in the fly ash.The internal pressure of the glass bead would increase rapidly until rupture when the temperature increased and the volume of the glass bead reduced.Meanwhile,the rod-like mullite intertwined into a nested structure with the broken glass bead as templates.（3）A certain amount of red mud was added on the basis of the fly ash based ceramics and the influence of the addition of red mud on the physical performance was discussed.The results showed that the iron oxides introduced by red mud promoted the formation of liquid phase.A large amount of liquid phase filled the pores in the body by the viscous flow under the action of surface tension,thereby improving the heat storage performance of samples.The sample（40%fly ash,15%shale,20%soda feldspar,25%quartz and 15%red mud）fired at 1140°C showed the best comprehensive properties,in which the water absorption,open porosity,bulk density,bending strength,thermal conductivity at 500°C and theoretical heat storage density（room temperature-500°C）were 0.14%,0.37%,2.54g/cm³,89.81MPa,3.62W/（m·K）,1.56KJ/cm³,respectively.Especially,thermal conductivity and heat storage density were increased by 87%and 30%compared to the sample without the red mud adding.Besides,the fired samples had a high thermal shock resistance.Although the bending strength of the sample fired at 1140°C decreased by 20.29%after 30 thermal shock cycles,it still maintained a bending strength of 73.78 MPa.