Preparation Of Mullite Whiskers And Research On Enhancement Of Mullite Ceramics

Mullite whisker is single crystal with a length greater than 10μm,aspect ratio greater than 20,which is mullite crystals grow along the c-axis after nucleation.Mullite Whiskers is an attractive potential engineering ceramic because it has mechanical strength and high creep resistance at low and high temperatures,excellent thermal shock,a low thermal expansion coefficient,and good chemical and thermal stability.Mullite Whiskers has attracted attention as a possible reinforcement for ceramic matrix,metal matrix,and polymer matrix composites.Mullite whiskers were prepared by sol-foaming method with different raw materials,and applied it to the mullite ceramics toughening.Firstly,the mullite whiskers were prepared by foaming method by using alumina,aluminum hydroxide and fused silica as the raw materials respectively,and the aluminum fluoride as the addition agent.The influences of technological conditions such as content of addition agent,sintered temperature and raw material granularity on the mullitization and grain growth behavior were discussed.Second,the morphology and microstructure of mullite whiskers were observed.Growth mechanism of the mullite whiskers was discussed.Thirdly,mullite whiskers enhancement the mullite ceramic matrix composites was prepared by the external addition,in-situ growth method and gas phase infiltration,and discussed the enhancing mechanism.The main achievements of the dissertation are as following:1.Preparation of the mullite whiskersThe mullite whiskers were prepared by sol-gel method,and the factors that influence the growth of mullite whiskers are discussed.Alumina and aluminum hydroxide all could be used as the raw materials to prepared the mullite whiskers.（1）Whenα-Al2O3 was used as the aluminum source,α-Al2O3 and SiO2 were mullitization at about 1200℃.The optimum conditions for whisker growth were as follows:8%¹0%of AlF₃·3H₂O as additive,firing temperature of 1600℃,and soaking time of 6 h.When Al（OH）₃ was used as the aluminum source,Al（OH）₃ and SiO₂ were mullitization at about 1100℃.The optimum conditions for whisker growth were as follows:7.5¹0%of AlF₃·3H₂O as additive,firing temperature of 1400℃,and soaking time of 6 h.（2）The suitable amount of AlF₃·3H₂O makes the anisotropic growth rate of mullite crystals vary greatly,and the mullite whiskers have a long length and a large aspect ratio.Continue to increase the amount of aluminum fluoride,the growth ratio at thickness could be faster and the mullite whiskers from the needle-like transition to the rods.The temperature could change the anisotropic grain growth rates.However,if the temperature is too high,the difference between the grain growth rate will decrease,the whisker diameter will increase,and the aspect ratio will decrease.（3）At the top of the prepared mullite whisker,the crystal surface is clearly variable,the surface of the whisker is smooth,and no liquid phase appears during crystal growth,which means that the whisker grows according to the VS growth mechanism.The reaction mechanism is characterized by the existence of gas phase at high temperature.After the nucleation of the nucleus,the reaction component passes through the gas phase and gas-solid reaction occurs on the surface of the nucleus.The crystal grows along the c-axis to form mullite whiskers.2 Mullite whiskers enhanced compositesThe addition of whiskers has a certain reinforcing effect on the ceramic material.When the amount of mullite whisker added is 12%,the reinforcing effect is the best,and the high temperature strength is increased by 5¹0%;the effect of in-situ whisker reinforced ceramic material is not obviously,the main reason is that AlF₃·3H₂O exists as a gas phase at high temperature.Excess AlF₃·3H₂O addition increases the internal porosity of the ceramic material.The whiskers are scattered in the matrix of the ceramic material and hinder the shrinkage and densification of the ceramic pores.The gas phase infiltration enhances the effect of the mullite ceramics,and the AlF₃·3H₂O is decomposed into a gas phase medium to penetrate into the ceramic material at a high temperature so as to avoid an increase in the porosity due to the sintering of the auxiliary AlF₃·3H₂O inside the ceramic material.The formation of mullite whisker in the ceramic matrix enhances the mechanical strength of the ceramic material,and the reinforcing effect is obvious,and the high temperature bending strength can be increased by 13¹6%.