Preparation And Investigation Of Mullite Lightweight Thermal Insulation Material By Microwave Sintering

At the era background of advocating a low-carbon economy and sustainable development,industrial furnaces as high energy-consuming equipment need to improve the energy efficiency.Lightweight thermal insulation materials are widely used in industrial furnaces in order to reduce the heat loss during the production process.Mullite lightweight thermal insulation material are widely used in many fields due to the excellent high temperature performance,low thermal conductivity and excellent thermal shock resistance.As an advanced material preparation technology,microwave sintering has become a new research hotspot in the field of material sintering due to its excellent characteristics,such as rapid heating rate,high energy utilization rate,safety and less pollution.However,the raw materials of preparing mullite such as Al₂O₃ and Si O₂could hardly couple with microwave to generate heat.Therefore,it is difficult to prepare the mullite lightweight thermal insulation material using the microwave sintering.In this paper,aluminum oxide（Al₂O₃）was used as raw material,ethyl orthosilicate was used as silicon source to prepare amorphous Si O₂.Silicon carbide（SiC）,aluminum fluoride（AlF₃）and PMMA microspheres were used as microwave hotspot,sintering additive and pore forming agents,respectively.The mullite lightweight thermal insulation materials was successfully prepared by microwave sintering in air atmosphere.This article mainly studied the effects of SiC content,AlF₃content,sintering temperature and PMMA microspheres content on the properties of mullite lightweight insulation materials.SiC particles could couple with microwave to generate heat at low temperature and stimulate the mullite generation reaction.The optimal amount of SiC need to be find not only to ensure the normal temperature rise and normal reaction of the sample,but also to take into account the final performance of the sample.Increasing the amount of SiC can increase the heating rate of microwave sintering and reduce the time required for sample preparation.However,excessive amount of SiC will cause SiC remain in the prepared samples,which will seriously affect the thermal insulation performance.The optimal amount of SiC should be controlled at 6 vol.%with an average heating rate of 39.5℃,the thermal conductivity of the sample between 30℃and 950℃was from 0.913 to 1.382 W/（m·k）.AlF₃ sintering additive could react with O₂,Si O₂ to generate Al OF and Si F₄ vapor.Vapor phase could be activited into plsama under the action of microwave electromagnetic field.The formation of microwave plasma provides much amount of energy for the formation of mullite.Subsequently,Al OF and Si F₄ reacted along the release direction to generate mullite whiskers.The optimal amount of AlF₃ was 3 wt.%,mullite whiskers were induced to form an interlocking structure and greatly enhance the thermal shock resistance of the sample.The critical thermal shock temperature difference could reach 1300℃.The pore size of most of the pores were less than 1μm,which reduces the thermal conductivity of the sample and the thermal conductivity between 30℃and 950℃was from 0.819 to 1.021 W/（m·k）.Microwave sintering could reduce the activation energy required for the reaction.Reducing the preparation temperature could reduce the time required for the reaction and save energy.The minimum temperature for microwave synthesis of mullite lightweight insulation material was 1400℃The material has good thermal shock resistance and low thermal conductivity.The critical thermal shock temperature difference was slightly less than 1300℃and the thermal conductivity between 30℃and 950℃was from 0.543 to 0.845 W/（m·k）.The PMMA microspheres pore forming agent could increase the porosity of the sample and greatly reduce the thermal conductivity.The optimal amount of PMMA microspheres was 30 wt.%.The porosity,density and flexural strength were 71.2%,0.99 g/cm³ and 3.8 MPa,respectively.The critical thermal shock temperature difference was slightly less than 1300℃and the thermal conductivity between 30℃and 950℃was from 0.201 to 0.313 W/（m·k）.Finally,mullite lightweight thermal insulation material with good thermal shock resistance and excellent thermal insulation properties were successfully prepared at 1400℃ by microwave sintering.