Study On Solid State Synthesis And Performance Control Of MgAl₂O₄ And MgAl₂O₄-CaAl₁₂O₁₉ Ceramic Materials

Magnesium aluminum spinel(Mg Al₂O₄,MgO·Al₂O₃or MA)has superior high temperature mechanical properties,excellent peeling resistance and corrosion resistance.It is the most typical high temperature ceramic in the Al₂O₃-MgO system.The preferential growth of calcium hexaaluminate(CaAl₁₂O₁₉,CaO·6Al₂O₃or CA₆)crystal grains along the basal plane makes it grow into platelet or needle morphology,which can greatly enhance the toughness of the material.Calcium dialuminate(CaAl₄O₇,CaO·2Al₂O₃or CA₂)has a low coefficient of thermal expansion.When CA₂is compounded with other materials with high melting point and high coefficient of expansion,it can well resist damage caused by thermal shock.Therefore,MA-CA₆composites have received extensive attention as a new type of high temperature ceramic material in the high-temperature industry because of its comprehensive properties of CA₆and MA.In this paper,MA ceramic,MA-CA₂-CA₆ceramic composites and MA-CA₆ceramic composites were prepared by high temperature solid-phase sintering,and the influence of mineralizers on the properties of these ceramic materials were studied.The strengthening mechanism of mineralizers on the performance of ceramics were discussed,and the following research results were obtained:1)The results showed that the bulk density and flexural strength of MA ceramic materials increased gradually with the increase of sintering temperature.After sintering at 1600?for 2 h,the sintering performance of MA ceramic was poor,with a bulk density of 3.17 g/cm³and a flexural strength value of 133.31MPa.With the increase of the mineralizer Fe₂O₃,the bulk density of MA ceramic materials increased gradually,and the flexural strength first increased and then decreased.When the addition amount was 3 wt%,the flexural strength reached the maximum 209.3 MPa.2)The performance and phase composition of MA-CA₆ceramic are related to the particle size of Ca CO₃and?-Al₂O₃raw materials,the purity of?-Al₂O₃,the temperature of synthesis and the holding time.Using small particle size Ca CO₃and high purity?-Al₂O₃as raw materials,after sintering at 1600?and holding for 2 h,the synthesized MA-CA₆ceramic has great flexural strength.The particle size of Ca CO₃plays an important role in the formation of CA₆phase and the growth and development of crystal grains in MA-CA6 ceramic materials.At high temperature,the impurity Si in?-Al₂O₃will form a transient liquid phase,which makes the morphology of CA₆grains evolve from platelet to equiaxed.3)The effect of mineralizers ZnO and Mg(BO₂)₂on the properties of MA-CA₆composites and the strengthening mechanism were investigated.It is found that(Mg_1-xZn_x)Al₂O₄solid solution and boron-containing liquid phase formed by the mineralizers ZnO and Mg(BO₂)₂make the grain size of MA smaller and the content of MA increased.These dense phases are coated with microcrystalline MA particles to form regional dispersed dense bodies,which leads to the transformation of CA₆grains into equiaxed grains,thus promoting the densification of MA-CA₆ceramic materials and improving its flexural strength.4)By using analytically pure Al₂O₃instead of?-Al₂O₃,MA-CA₂-CA₆ceramic composites were synthesized from analytically pure raw materials.The effects of mineralizers SnO₂and H₃BO₃on the physical and mechanical properties,microstructure and phase composition of the composites were studied.The results show that solid solution and boron-containing transient liquid phase appear in the ceramic material after adding mineralizers SnO₂and H₃BO₃respectively,it makes the CA₂phase change to CA₆phase and accelerates the formation of MA and CA₆,thus improving the sintering activity of the ceramic material.The dense phase formed by the excess Ca makes the bond between MA and CA₆grains tight,which improves the mechanical properties of ceramic materials.