Research Of The Valence Electron Theory Of Phase Transition And The Doped Phase Stability Of Zirconia

In this thesis, the valence electron structure of pure zirconia, zirconia doped with yttria and zirconia doped with magnesia are calculated on the basis of the Yu's "the empirical electron theory in solid and molecule (EET)" and an average ionic model advanced here. The results deducted from the valence electron structure combining "the C-Me segregating theory in solid alloys" advanced by Liu Zhilin and his colleagues are as follows.The calculation results show that the total numbers of the covalent bonds which form the strong bond framework of c-phase, t-phase and m-phase of pure zirconia are 3.19184, 3.45528 and 3.65935, respectively. According to the viewpoint of the C-Me segregating theory in solid alloys, it can be deduced that the phase transformation order of zirconia should be liquid phase→cubic phase→tetragonal phase→monoclinic phase. The deduction from valence electron structure is completely in accordance with the experiment results.The deduction from the results of the valence electron structure of the zirconia-yttria solid solution are as follows: when YO_1.5 content( mol%) is between 0 and 3mol%, m phase of zirconia-yttria solid solution is stable at room temperature. When it is between 5.5mol% and 11mol%, t phase is stable at room temperature. Only when it is greater than 13mol%, c phase is stable at room temperature. The deduction is completely in accordance with the experiment results.The deduction from the results of the valence electron structure of the zirconia-magnesia solid solution are as follows: when MgO content( mol%) is lower than 2%, m phase of zirconia-magnesia solid solution is the most stable phase structure. When it is between 2 mol% and 7mol%, t phase is stable at room temperature, but it is easy to decompose. Only when it is greater than 8%, c phase is stable at room temperature, and between 10% and 12% , the stability of c phase is the highest. When it is greater than 13%, the stability of c phase is decreased, and it is easy to decompose. The deduction is also in accordance with the experiment results. Zirconia phase transformation order and its doped content range of phase stability which are deduced from its valence electron structure are in accordance with the experiment results. It is indicated that the theory advanced in this paper can be applied in the search of the new zirconia phase stabilize, and the theory can also be extended to other ceramic phase transformation research.