Preparation,performance And Mechanism Of Mullite-Reinforced Flexible Aluminum Titanate Ceramics

Aluminum titanate（Al₂TiO₅）ceramics have been widely used in high temperature refractory fields due to its excellent properties such as low thermal expansion coefficient,low thermal conductivity,high melting point and good thermal shock resistance.Because of the high thermal expansion anisotropy,a large number of microcracks tend to be generated in the sintering and cooling process.Consequently,this property allows the production of flexible aluminum titanate ceramics with a structure similar to“itacolumite”.However,a large number of grain boundary microcracks lead to low mechanical strength of aluminum titanate ceramic,which limits its wide applications.To solve this issue,mullite as an enhanced phase was used to reinforce flexible aluminum titanate ceramic due to its desirable high-temperature performance,high mechanical strength and favorable chemical compatibility with aluminum titanate in this thesis.Mullite was introduced for improving the mechanical strength while maintaining the flexibility of the aluminum titanate ceramics.The preparation and properties of mullite reinforced flexible aluminum titanate ceramics were investigated in detail,and the related mechanism was discussed.The main content of this thesis is separated into two parts:Firstly,flexible aluminum titanate ceramics were fabricated by solid reaction sintering of Al₂O₃ and Ti O₂ with additives of Fe₂O₃ and Mg O at 1500°C for 2 h.The influences of heating rates and cooling rates on the microstructures and mechanical properties of flexible aluminum titanate ceramics were investigated in detail.Furthermore,the effects of microcrack structure（microcrack width and microcrack density）on the mechanical properties of flexible aluminum titanate ceramics were quantitatively studied.Meanwhile,the damage mechanism of flexible aluminum titanate ceramics with various microcrack structures under stressed was also studied.Results show that both the microcrack width and microcrack density would rise with increase of cooling rate.In addition,the increase of microcrack width can lead to a suitable increase of flexibility but a slight decrease of mechanical strength.When the heating rate is 5°C/min and cooling rate is air cooling（≈400°C/min）（with the median microcrack width of 0.533μm and microcrack density of 6831.88 N/mm²）,the aluminum titanate ceramic possesses the best flexibility,with the bending strain of 1.10%.According to the above research,it is found that the mechanical strength of flexible aluminum titanate ceramics cannot be fundamentally improved by regulating the cooling rates.Based on the above,Mullite was introduced to reinforce the flexible aluminum titanate ceramics.Because the particle sizes and crystal types of the Al source have a great impact on the microstructures and mechanical properties of the synthesized mullite ceramics,furtherly affecting the mullite reinforced flexible aluminum titanate ceramics.Therefore,the influences of particle sizes and crystal types of Al source on the microstructures and mechanical properties of mullite ceramics were firstly studied.Results show that the mullite ceramics prepared with coarser Al source exhibit the coarser porous skeletons,leading to the higher bulk density and greater mechanical strength at the same pre-sintering temperature.Since the mullite ceramic prepared by PS500-C（Al source obtained by pre-sintering coarse Al（OH）₃ at 500°C）was consistent with the solid reaction sintering process of the flexible aluminum titanate ceramic,PS500-C mullite was introduced into the flexible aluminum titanate ceramic.In this thesis,mullite reinforced flexible aluminum titanate ceramics were prepared by two preparation processes,named as two steps sintering and one step sintering.The effect of mullite contents on the microstructures and mechanical properties of flexible aluminum titanate ceramics was studied in detail.The differences of microstructures and mechanical properties of samples prepared by two steps sintering and one step sintering were comparatively analyzed.Results show that the introduction of mullite can refine the Al₂Ti O₅ grains and reduce the microcrack width of aluminum titanate.The grain boundary microcracks will be filled with glass phase if the content of mullite is in a high level.With the increase of mullite content,the strength of the samples increases and the flexibility decreases.However,samples prepared by one step sintering possess greater strength and better flexibility than two steps sintering.When the mullite content is 5 wt.%,samples prepared by one step sintering exhibits the best comprehensive mechanical properties,whose mechanical strength can be increased by 97.28%,from 6.24 MPa to 12.31 MPa,compared with virgin sample.The bending strain is only reduced by 7.84%,from 1.02%to 0.94%.It is shown that the introduction of mullite can maintain the flexibility while tremendously increase the strength of flexible aluminum titanate.