Processamento e propriedades dielectricas de ceramicos de titanato de magnesio

MgTiO_3-based ceramics are used as components in microwave circuits for communications systems, namely, as dielectric resonators. An important factor that affects the characteristics of these ceramics is powder processing. In this work, an alternative method (chemical route) was used to produce MgTiO_3 powders instead of the conventional method (mixed-oxide route). With this chemically prepared powder it was possible to obtain dense, single-phase and more homogeneous MgTiO_3 ceramics at a much lower sintering temperature than conventionally used. Hence, ceramics with much lower dielectric loss were obtained. Aiming to a better understanding of the factors that affect microwave dielectric loss, the effect of some additives (CaTiO_3, La, Sr, Nb, Cr, Fe, Co and Ni) were studied. Two different situations could be pictured. One, in which second phases were formed and the other, where a solid solution took place between the additives and the MgTiO_3. The presence of second phases have always increased dielectric loss of MgTiO_3 ceramics, since themselves have higher losses at microwave frequencies. When additives were present in solid solution with MgTiO_3 by cation substitution, dielectric loss increased for most of the cases but in a lesser degree. However, when with its introduction some lattice defects are eliminated, this effect superimposes on the one caused by cation substitution and dielectric loss might even decrease (Nb case). In the case of the additives that formed a solid solution, dielectric loss is discussed in terms of their position in the MgTiO_3 lattice. The role of MgTiO_3 non-stoichiometry on its characteristics was for the first time studied. Although there was a tendency for the decrease of dielectric loss in the compositions with magnesium excess (in solid solution), the alterations were small when compared, for instance, with the ones caused by the additives. The effect of cooling rate after sintering upon the characteristics of pure and doped MgTiO_3 was also studied. In both cases the defects generated in the MgTiO_3 lattice were responsible for the increase in dielectric loss at microwave frequencies. At last, an assessment of microwave..Available from Fundacao para a Ciencia e a Tecnologia, Servico de Informacao e Documentacao, Av. D. Carlos I, 126, 1200 Lisboa / FCT - Fundação para o Ciência e a TecnologiaSIGLEPTPortuga