Influence of SiO2 and CaO additions on the microstructure and magnetic properties of sintered Sr-hexaferrite

The effect of simultaneous addition of CaO and SiO2 on the microstructure and magnetic properties of sintered SrO-excess Sr-hexaferrites was studied. Both additives markedly affect the grain growth behavior and the magnetic properties. CaO-additions promote densification, which results in increased remanence, but due to simultaneuous grain growth the coercivity drops to <100 kA/m. SiO2 additions are known to suppress grain growth. Simultaneous addition of CaO and SiO2 is shown to be very beneficial in tailoring a dense microstructure with relatively small grains. The ratio of CaO/SiO2 was found to be optimum at about 1, and magnets with a remanence of 430 mT and a coercivity of 300 kA/m were obtained. Transmission electron microscopy (TEM) studies and investigations by energy-dispersive analysis of X-rays (EDX) in the scanning TEM (STEM) mode show that both CaO and SiO2 are concentrated at grain boundaries and grain junctions forming an amorphous secondary phase.