Sintering, microstructure and dielectric properties of MgO doped alumina ceramics co-doped with Gd₃O₂, and Pr₆O₂

There is a need for low dielectric, low loss ceramic substrates for microwave electronics. Earlier works report rare earth dopants are very much useful to control the dielectric properties of sintered ceramics. This paper describes the effect of Gd₂O₃ and Pr₂O₆ rare earth co-dopants on the dielectric properties of MgO-doped alumina ceramics. Sintering, microstructures and the dielectric properties are investigated and reported. First of all, ultrafine alumina particles were obtained by ball milling which were converted to sintered alumina ceramics at temperatures 1400, 1450, 1500 and 1550°C. Sintered α -Al₂O₃ ceramics showed nearly 70% theoretical sintered density at 1450 °C which was increased beyond 97% at 1550 °C. The SEM micro-structure images reveal the presence of elongated, spherical like Al₂O₃ grains in the sintered samples. The results on dielectric constant (ε _r), dielectric loss and a.c. resistivity indicate that sintered MgO doped alumina ceramics have low dielectric constant in the range ε _r <6 in presence of Gd₂O₃ and Pr₂O₆ co dopants which is one order less than the phase pure Al₂O₃ ceramics. The dielectric constant decreases with the increase in MgO. The resistivity of sintered alumina increases with increase in dopant concentration and decreases with increase in temperature. High vacancy concentration at the interfaces of GPMDA and Al₂O₃ phases is possibly responsible for the resistivity of G-P co doped Al₂O₃ ceramics. The fine grain microstructure of sintered alumina creates more interfaces which in turn responsible for the dielectric constant 4.9 to 8.7 for doped alumina at the frequency of 1 MHz.