In a permeable MgO ceramic containing TiO2 and Al2O3 as matrix MgO.Al2O3-2MgO.TiO2 solid solutions were detected and no Al2O3.TiO2 was observed. Therefore, the solid state reaction among MgO, TiO2 and Al2O3 with excess MgO was studied at various firing temperatures. The formation mechanism of MgO.Al2O32MgO. TiO2 solid solutions was studied from the results of interface reactions between MgO-TiO2 and MgO-Al2O3. MgO.Al2O3-2MgO.TiO2 solid solutions were detected at 1573-1673 K and increased with the firing temperature. 2MgO.TiO2 Was formed in the system MgO-TiO2, but was not produced in the system MgO-TiO2-Al2O3, because the amount of Mg diffusion at the interface of aggregate and matrix additives was not enough due to the large aggregate size of MgO. MgO.Al2O3 and 2MgO.TiO2 formed at the interface between MgO-TiO2 and MgO-Al2O3 decreased with the firing time at 1873 K. On the other hand, MgO.Al2O3-2MgO.TiO2 solid solutions increased. Mg was recognized in the matrix without addition of MgO as a matrix. Al diffused in the layer of MgO-TiO2, in opposition Ti diffused in the layer of MgO-Al2O3 by EPMA analysis. From the above results, the formation mechanism of a MgO.Al2O3-2MgO.TiO2 solid solution was proposed; (1) Mg diffusion at the interface of MgO aggregate and matrix additives, (2) formation of MgO.TiO2 and MgO.Al2O3, (3) formation of 2MgO.TiO2 due to the reaction with MgO.TiO2 and MgO, (4) reaction of the solid solution with MgO.Al2O3 and 2MgO.TiO2. In the reactions between MgO-TiO2 and MgO-Al2O3 the product layer of MgO.Al2O3-2MgO. TiO2 solid solution increased as the firing time increased. The diffusion coefficient of Mg was about 4.8 x 10(-7)(cm2/s), in a good agreement with that of Mg which was reported.
