Effects of Heat-Treatment Temperature on the Microstructure, Electrical and Dielectric Properties of M-Type Hexaferrites
M-type hexaferrite BaCrxGaxFe12−2xO19 (x = 0.2) powders have been synthesized by use of a sol–gel autocombustion method. The powder samples were pressed into 12-mm-diameter pellets by cold isostatic pressing at 2000 bar then heat treated at 700°C, 800°C, 900°C, and 1000°C. X-ray diffraction patterns of the powder sample heat treated at 1000°C confirmed formation of the pure M-type hexaferrite phase. The electrical resistivity at room temperature was significantly enhanced by increasing the temperature of heat treatment and approached 5.84 × 109 Ω cm for the sample heat treated at 1000°C. Dielectric constant and dielectric loss tangent decreased whereas conductivity increased with increasing applied field frequency in the range 1 MHz–3 GHz. The dielectric properties and ac conductivity were explained on the basis of space charge polarization in accordance with the Maxwell–Wagner two-layer model and Koop’s phenomenological theory. The single-phase synthesized materials may be useful for high-frequency applications, for example reduction of eddy current losses and radar absorbing waves.