Effects Of Additives On Thermal Shock Resistance Of Magnesia Ceramic

Magnesia,cubic system,sodium chloride crystal structure,theoretical density of 3.58g/cm3,melting point 800?,has excellent high temperature resistance and wear resistance,is widely used in many fields.Magnesia ceramic crucible has been widely used in smelting rare metals due to it has excellent chemical stability,oxidation resistance and resistance to molten metal corrosion,in recent years.However,the application of magnesia oxide ceramic crucible is greatly limited because its poor thermal shock resistance is led by the larger thermal expansion coefficient of periclase((14-15)× 10-6?,0-1 500?).The magnesia ceramic samples with high purity nanometer magnesia powder as raw material,four kinds of different rare earth oxides and three kinds of different crystal nanometer zircoma as additives,were prepared in order to improve the thermal shock resistance of magnesia ceramics after bitching,mixing,forming,sintering and holding two hours at 1 350 ?,1 450 ?,1 550 ?respectively in this essay.The main contents of the study include:1)Effect of four different rare earth oxides(yttrium oxide,lanthanum oxide,erbium oxide,cerium oxide)and three different crystal nanometer zirconia on thermal shock resistance of magnesia ceramics;2)Effect of adding amount and sintering temperature of yttrium oxide on thermal shock resistance of magnesia ceramics;3)Effect of the amount of monoclinic nanometer zirconia and sintering temperature on thermal shock resistance of magnesia ceramics;4)Effect of yttrium oxide and monoclinic nanometer zirconia composite proportion and sintering temperature on thermal shock resistance of magnesia ceramics.The results show that:1)The addition of three nanometer zirconia and four rare earth oxides can improve the density and thermal shock resistance of magnesia ceramics.The optimum additive is monoclinic nanometer zirconia and yttrium oxide.2)The yttrium oxide mainly exists at the grain boundary of periclase,which inhibits the growth rate of the grains of periclase,and increases the sintering density of magnesia ceramics in the sample of adding yttrium oxide.The thermal shock resistance of the sintered sample is improved by the second phase toughening mechanism and the micro crack toughening mechanism caused by yttrium oxide.Thermal shock times up to 11 times which sample with 2 wt%yttrium oxide sintered at 1450 Co.The microstructure uniformity is improved because zirconia disperses at the grain boundary of periclase in the sample of adding monoclinic nanometer zirconia.Sintering is promoted because periclase crystal lattice is activated due to solid solution formed by monoclinic nanometer zirconia and magnesia.The thermal shock resistance of the sintered sample is improved by the micro crack toughening mechanism and the zirconia phase transition toughening mechanism in the sample of adding monoclinic nanometer zirconia.Thermal shock times up to 10 times which sample with 12 wt%monoclinic nanometer zirconia sintered at 1450 C°.3)Sintering is promoted because tetragonal zirconia solid solution formed by monoclinic nanometer zirconia and magnesia,yttrium oxide and magnesia in samples with multiple additives.The density of sintered samples is improved because dispersed the tetragonal zirconia solid solution at the grain boundary of periclase inhibits the growth rate of the periclase grains and favor pore's discharge.The energy required for the extension of the main crack is increased because the phase-change of tetragonal zirconia solid solution produces compression stress on the matrix in the samples with multiple additives.The fracture surface energy of the main crack is consumed by second phase particle of zirconia solid solution.The thermal shock resistance of the sintered samples is improved because micro cracks in the sample decrease elastic modulus,dissipate the strain energy of the main crack tip and effectively inhibit the crack's extension.