Preparation And Properties Of Mullite Transparent Ceramics Sintered Under Vacuum

Infrared window and radome are the eyes of aircraft.The new generation of window materials not only need to bear sand and rain erosion,but also need to bear the huge load and high temperature caused by the high speed of aircraft.At present,the window materials generally have two fatal problems severe decline of strength in high temperature and poor thermal shock resistance.Mullite ceramics have excellent creep resistance,thermal shock resistance and corrosion resistance.What’s more,there is almost no decrease in mechanical properties at high temperature.They are potential infrared transparent materials for the new generation of aircraft.However,there are two main problems in the preparation of transparent mullite ceramics:the raw material synthesis is relatively difficult because the process is complex and difficult to control and the mechanism is not very clear.From the perspective of sintering methods,special sintering methods such as hot pressing and SPS（Spark Plasma Sintering）are adopted.It restricts the development and application of transparent mullite ceramics because it is difficult to prepare transparent mullite ceramics with complex shape and large size.Based on the above problems,this work fabricated transparent mullite ceramics by solid-state reactive sintering under vacuum from oxides for the first time.It provides a way to prepare transparent mullite ceramic with large size and complex shape.In addition,this paper prepared mullite transparent ceramics by vacuum sintering using powders synthesized by sol-gel method.the main research contents are as follows:（1）Mullite ceramics prepared by two-step method and reactive sinteringFor the solid-state reactive sintered sample from Al2O3 and amorphous Si O2,the shrinkage process almost completed at 1400℃.,the amorphous Si O₂ transformed into cristobalite at 1400℃and reacted completely at 1600℃,while the Al₂O₃ reacted completely at 1650℃.However,the Al₂O₃ precipitated at 1750℃and the amount of Al2O3 increased with the increasing of sintering temperature.When the temperature is1780℃,the transmittance of the solid-state reactive sintered mullite ceramics（thickness of 0.75mm）reached 65%at 4μm.The bending strength,compressive strength,Vickers hardness and fracture toughness of the samples were 155.6±9.8MPa,592.2±162MPa,12.8±2GPa and 1.16±0.04(MPa·m^1/2)respectively.This is the first report of solid-state reactive sintered transparent mullite ceramics.（2）Effect of MgO on reactive sintered mulliteFrom the XRD results,mullite began to formed at 1300℃when 0.3wt%and 0.5wt%MgO were added.while the mullite began to formed at 1400℃when 0.05wt%and0.2wt%MgO were added.All the samples reacted completely at 1600℃.With the increase of the amount of MgO,the transmittance of the sample at 4μm increased firstly and then decreased.When the amount of MgO was 0.2wt%,the transmittance of the sample（sintered at 1780℃with a thickness of 0.75mm）at 4μm was about 70%which was 5%higher than that of the samples without MgO.MgO inhibited Al2O3precipitation by reacting with Al2O3 precipitated at high temperature to form Al2O3-rich magnesium aluminum spinel phase at grain boundary.The average grain size of the sintered sample at 1780℃with the addition of 0.2wt%MgO is 13.8μm,which was smaller than that of the samples without MgO additives（19μm）.（3）Preparation of mullite transparent ceramics from mullite powder by sol-gel methodThe XRD results showed that the precursors transformed into aluminosilicate spinel phase at 1000℃and mullite phase at 1200℃.TG-DSC results showed that the adsorbed water in the precursor evaporated at 154.1℃,NO₃^-decomposed at 194.1℃,spinel phase formed at 987.5℃,and the weight almost had no changes after 600℃.The results of XRF showed that the purity of mullite powder was above 99.5%.Chemical composition of mullite ceramics had a great influence on its sintering process,microstructure,mechanical and optical properties.Al2O3 firstly precipitated and then dissolved in the samples with the content of 60mol%～66mol%Al2O3,while the samples with the content of 70mol%Al2O3 always had unreacted Al2O3.With the increase of Al2O3 content,the average grain size of the sample firstly increased and then decreased and the number of columnar grains decreased while the number of equiaxed grains increased.The flexural strength,compression strength,fracture toughness,Vickers hardness all decreased firstly and then increased.While the infrared transmittance increased firstly and then decreased.The flexural strength,compression strength and fracture toughness reached the minimum value when the Al2O3 content was 66mol%.At this content,the transmittance at 4μm（thickness of 0.75mm）reached its maximum（about 72%）when sintered at 1780℃.The transmittance of mullite ceramics prepared by sol-gel method is greatly affected by the addition of MgO.With the increase of the addition of MgO,the transmittance of the sample decreased firstly and then increases.When the amount of MgO is 0.2wt%,the transmittance of the sample（thickness of0.5mm）sintered at 1750℃reached its maximum（about 77%）at 4μm.