Dense strontium hexaferrite-based permanent magnet composites assisted by cold sintering process

TL;DR

This study explores a novel cold sintering process combined with post-annealing to produce rare-earth-free strontium ferrite permanent magnets with improved magnetic properties and density, offering an environmentally friendlier alternative to traditional methods.

Contribution

It introduces a new sintering approach using cold sintering and post-thermal treatment to enhance the magnetic properties of strontium ferrite composites without rare-earth elements.

Findings

Achieved 92% relative density in sintered magnets.

Obtained coercivity up to 3.0 kOe in the final magnets.

Demonstrated control of magnetic properties via composition and grain size.

Abstract

The use of rare-earth-based permanent magnets is one of the critical points for the development of the current technology. On the one hand, industry of the rare-earths is highly polluting due to the negative environmental impact of their extraction and, on the other hand, the sector is potentially dependent on China. Therefore, investigation is required both in the development of rare-earth-free permanent magnets and in sintering processes that enable their greener fabrication with attractive magnetic properties at a more competitive price. This work presents the use of a cold sintering process (CSP) followed by a post-annealing at 1100 {\deg}C as a new way to sinter composite permanent magnets based on strontium ferrite (SFO). Composites that incorporate a percentage < 10% of an additional magnetic phase have been prepared and the morphological, structural and magnetic properties have been evaluated after each stage of the process. CSP induces a phase transformation of SFO in the composites, which is partially recovered by the post-thermal treatment improving the relative density to 92% and the magnetic response of the final magnets with a coercivity of up to 3.0 kOe. Control of the magnetic properties is possible through the composition and the grain size in the sintered magnets. These attractive results show the potential of the sintering approach as an alternative to develop modern rare-earth-free composite permanent magnets.