High-Performance Hexaferrite Ceramic Magnets Made from Nanoplatelets of Ferrihydrite by High-Temperature Calcination for Permanent Magnet Applications

Abstract

Highly aligned ceramic hexaferrite magnets with high-energy products (BH) max and a density exceeding 90% of theoretical density have been fabricated. The precursors were an antiferromagnetic powder, a six-line ferrihydrite mixed with SrCO 3 , and a grain growth inhibitor SiO 2 . Conventional cold compaction of the precursor powders was employed prior to calcination at temperatures of 1050, 1150, and 1250 °C. The influence of calcination temperature and magnetic properties has been systematically studied in the produced ceramic magnets. Conventional cold compaction is a favorable route for industrial production when compared with other compaction techniques like spark plasma sintering, hot compaction, or electroforging. A high (BH) max of 25.2 kJ/m 3 was obtained for the best magnet along with an appreciable coercivity, H c , of 187 kA m −1 , a high squareness ratio, M r /M s , of 0.84, and a saturation magnetization, M s , of 73 A m 2 /kg. Texture and crystallite size analysis were extracted from 2D synchrotron transmission powder diffraction measurements. We have demonstrated that high-performance bulk magnets for permanent magnet applications can be produced from nonmagnetic interacting crystallites mixed with a grain growth inhibitor without applying a magnetic field for alignment.

Permanent magnets based on hard ferrite ceramics

“…Recently, Vijayan et al reported the use of SPS not only for densification of ferrite powders but for the direct synthesis of aligned SrM magnets [75][76][77]. In this study, SrM is synthesized directly during the SPS process using a precursor powder of antiferromagnetic six-line ferrihydrite (FeOOH) platelets mixed with SrCO 3 .…”