Low temperature sintering and magnetic properties of garnet microwave magnetic materials

This study investigated the low temperature sintering for microwave magnetic materials [Bi-incorporated calcium vanadium garnets (Bi-CVG), (Bi_0.75Ca_1.2Y_1.05)(V_0.6Fe_4.4)O₁₂]. Sintering behavior and magnetic properties of Bi-CVG materials prepared from ultrafine-sized powders were compared with those of materials prepared from micron-sized powders. The activation energy for densification (E_d), derived from temperature and time dependence of materials' density, is markedly reduced by using ultrafine-sized powders as starting materials, i.e., (E_d)_ultra = 7.04 kJ mole^-1 and (E_d)_micron = 39.5 kJ mole^-1 for ultrafine and micron powder sintered samples, respectively. The activation energy for densification is further reduced to 4.59 kJ mole^-1 by using microwave sintering process with ultrafine powders. Remnant magnetization (B_r) of the Bi-CVG materials is similar for samples possessing the same high density, regardless of the microstructures. Conversely, the coercive field (H_c) of the materials is larger for samples containing fine grains and vice versa. While the low frequency magnetic properties of the Bi-CVG materials are microstructure-dependent, the microwave magnetic properties of the materials are not.