The effect of Cr and La on MgTiO₃ and MgTiO₃–CaTiO₃ microwave dielectric ceramics

Magnesium titanate and MgTiO₃–CaTiO₃ ceramics were prepared by a chemical (Pechini) route and classical mixed oxide route. Selected specimens were doped with Cr or La. Specimens were sintered at 1350 °C and 1400 °C. Microstructures were examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy. Dielectric properties were determined at 8 GHz by the Hakki and Coleman method. The highest Q values were obtained for undoped, chemically prepared MgTiO₃ (20800); any dopants caused the Q value to be degraded. Additions of small amounts of Cr (≤1 mol %) to mixed oxide magnesium titanate increased the density to 97.1% theoretical, and increased the Q value (from 7000) to 13,000. Additions of La led to the formation of La₂Ti₂O₇ second phase and reduction in the Q value for both materials. Both Cr and La acted as effective sintering aids, increasing density (to a maximum of 99% theoretical for 1% mol La in chemically prepared samples) and relative permittivity (to 18.1 for the same specimens). The relative permittivity of MgTiO₃–CaTiO₃ ceramics increased with calcium content, but the corresponding Q values decreased (∈_r = 19.9, Q = 8500 for Mg:Ca = 94: 6). Small additions of La to Mg–Ca titanates enhanced the dielectric Q values but decreased the relative permittivities.