Study On The Design,Preparation And Performance Of Foamed Ceramics With High Closed Porosity

Foamed ceramics are lightweight structural materials with the advantages of heat insulation,heat preservation,sound insulation and corrosion prevention,which are widely used in the fields of aviation,aerospace,chemical industry,metallurgy,biology,medicine,construction,etc.Most of the foamed ceramics prepared in the present research are open-pore type,and there is lack of research on closed-pore type foamed ceramics.In recent years,with the development of science and technology,more and more special fields put forward functional requirements for traditional structural materials.Foamed ceramics,which are important lightweight structural materials,are usually required to have more performance,such as waterproof,moisture-proof and special electromagnetic performance.Compared with open-pore foamed ceramics,closed-pore foamed ceramics have gained much attention due to their superior heat insulation,heat preservation and sound insulation effect,as well as excellent performance of waterproof and moisture-proof.The closed-pore foamed ceramics prepared by the existing technology generally have the problems of high open porosity,low closed porosity and high sintering temperature.In order to improve the comprehensive performance of closed-pore foamed ceramics,reduce the manufacturing cost,satisfy the requirement of mass production,this study takes silicon nitride and calcium carbonate as the foaming agent,explore to prepare foamed ceramics with high closed porosity using different material system at different foaming temperature.In response to the current research hotspot,closed-pore foamed ceramics with good microwave absorption performance are prepared by changing the composition of the raw materials and improving the sintering process.(1)In our previous research,quartz-based closed-pore foamed ceramics with a closed porosity of 85% and compressive strength of 6.9 MPa were prepared using silica nitride as the foaming agent and using calcium carbonate and talcum powder as the performance improver by a high temperature foaming method at 1200 ?C,and the foaming mechanism of the closed-pore foamed ceramics was revealed.Based on this research,the comprehensive performance of the closed-pore foamed ceramics was improved by adding a small amount of sodium tetraborate in raw materials and coordinating the sintering temperature and sintering time.The changes of the sodium tetraborate content in raw materials,sintering temperature and sintering time have effects on the generation of foaming gas and the viscosity of the matrix which is in molten state,and then impact on the foaming performance of the foamed ceramics.The closed-pore foamed ceramics prepared by sintering the green bodies with 1~2wt% sodium tetraborate at 1250 ?C for 1 h have closed porosity as high as 88%,low density of 0.33~0.34 g/cm3 and compressive strength of 4.8~4.9 MPa.The closed-pore foamed ceramics prepared by sintering the green bodies with 3 wt%sodium tetraborate at 1200 ? C for 2 h have closed porosity as high as 88%,low density of 0.31 g/cm3 and compressive strength of 6.0 MPa.The closed-pore foamed ceramics prepared by sintering the green bodies with 5~6 wt% sodium tetraborate at1150 ?C for 3 h have closed porosity as high as 89%,low density of 0.32 g/cm3 and compressive strength of 6.2 MPa.(2)The effects of the Si C content in raw materials on the foaming performance and microwave absorption performance of the foamed ceramics were studied by adding Si C in raw materials.During sintering at high temperature,the oxidation of Si C in the green bodies which is from the outside in will form a continuous gradual distribution of Si C in the foamed ceramics.The foamed ceramics prepared from the green bodies with 3 wt% Si C have the best comprehensive performance: low density of 0.48 g/cm3,high compressive strength of 9.6 MPa,high total porosity of 82%,high closed porosity of 80%.The foamed ceramics also have excellent microwave absorption performance with a mean microwave reflectivity of-11.9 d B in the frequency range of 8.0~18.0 GHz and a minimum microwave reflectivity of-14.6 d B at the frequency of 12.7 GHz.More importantly,the foamed ceramics also have excellent stability at high temperature.After a 1000 ?C heat treatment for 20 h,the mean microwave reflectivity of the foamed ceramics can still reach-11.3 d B in the frequency range of 8.0~18.0 GHz.(3)Foamed ceramics with high closed porosity were successfully prepared by sintering the green bodies with calcium carbonate as the foaming agent at temperatures below 900 °C.The aggregation of the decomposition–derived CO2 from calcium carbonate by the molten green bodies with appropriate viscosity and good continuity during sintering is the most likely foaming mechanism of foamed ceramics.The composition of raw materials,heating rate and sintering temperature are three key factors affecting the foaming performance of foamed ceramics,and rapid heating is the necessary condition for preparing foamed ceramics.In the rapid heating process,the temperature at the surface of the green bodies rises much faster than that inside the green bodies,and the green bodies turn into molten state from the outside in with the increase of time.After the surface of the green bodies has melted into a whole with appropriate viscosity and good continuity,the decomposition–derived CO2 from the calcium carbonate inside the green bodies is sealed by the molten surface.As the temperature inside the green bodies rises,an increasing of the decomposition–derived CO2 is better sealed by the more newly–formed molten matrix.With the extension of sintering time,the aggregation of the decomposition–derived CO2 leads to large volume expansion of the molten green bodies like slowly blowing up a balloon.With the method of rapid heating,foamed ceramics with good foaming performance could be prepared by coordinating the composition of raw materials and sintering temperature.Take the maximum volume expansion as the standard,the foamed ceramics sintered from the green bodies with 10~20 wt% of calcium carbonate and 40wt% of sodium tetraborate at 750~800 °C show the best foaming performance with high total and closed porosity of 80~82% and 76~78% respectively and compressive strength of 2 MPa.From above results,foamed ceramics with high closed porosity can be prepared by using silicon nitride and calcium carbonate as the foaming agent from appropriate raw materials and reasonable sintering process.The sintering temperature of foamed ceramics must be higher than the foaming temperature of the foaming agent.When using silica nitride as the foaming agent,the sintering temperature of foamed ceramics must be high due to the limitation of the foaming temperature of silica nitride,and the best foaming temperature is 1150~1250 °C When using calcium carbonate as the foaming agent,the sintering temperature of foamed ceramics can be low as 750 °C because of the low foaming temperature of calcium carbonate.In addition,by coordinating the composition of raw materials and sintering process,foamed ceramics could be given more performance on the condition that the original performance of foamed ceramics are not or little reduced.This study provides many useful ideas for the design,preparation and application of high closed-pore foamed ceramics.