Multiple enhancement on the magnetic and electric properties of SrM hexaferrites by phase-change perovskite additives

Abstract

M−type strontium (SrM) hexaferrites, which possess a high saturation magnetization, coercivity, and maximum magnetic energy product, have been widely used as permanent magnets in electric machines. However, emerging high-speed and high-power electric machines put forward higher requirements on the loss characteristics of ferrites at high frequencies and high temperatures. Here, the effects of two phase-change perovskite additives, barium titanate (BTO) and BTO-based ceramic with a positive temperature coefficient (PTCC), on the magnetic properties, electrical properties, and their temperature dependence of SrM hexaferrite are reported. The spatial concentration distribution and diffusion kinetics of the perovskite additives are investigated, showing their accumulation at the grain boundaries and gradual diffusion into the crystallites. Due to the high melting points, the addition of BTO and PTCC at the grain boundaries hinder the grain growth and reduce the grain size. Due to the strong pinning effect of the impurities near the grain boundaries and the reduction of grain size, enhancement in coercivity up to 10 % as well as an increase of 225 % in resistivity are observed. More interestingly, a positive temperature dependence of the resistivity is achieved by adding various amounts of PTCC. These results demonstrate the capability of phase-change perovskite additives to achieve simultaneous enhancement in multiple electromagnetic and temperature properties of SrM hexaferrite.