High Pressure Consolidation for advanced Ferrite Permanent Magnets

We present a study on the consolidation of advanced ferrite magnets by employing High Pressure technique. Ferrite magnets currently represent the most widely used permanent magnets in the world, covering 80% of the PM market even if their energy product and magnetization saturation are much smaller than rare-earth permanent magnets. Recently, a strong effort is being performed to improve their magnetic properties with the scope of substituting partially rare-earth compounds in magnets. Most of the strategies involve the nanostructuration of the ferrites to get high coercive field or the development of hybrid compounds with higher magnetization than commercial ferrites [1]. A bottleneck in the development of these solutions is the magnet production without modifying the properties of initial materials. In fact, the conventional preparation of magnets, i.e. the sintering process, that requires high temperatures (above 1200°C) and the oxidizing atmosphere, modifies the nanostructure thus inducing several changes in the final properties. Alternatively, novel strategies consider out-of-equilibrium methods or cold sintering processes [2] to overcome the problem. In this presentation we will show some results obtained by applying high pressures to induce the consolidation of ferrite powders so as to get dense magnets. The consolidation was performed using a multi-anvil (MA) press applying a quasi-isotropic pressure in the range up to 6 GPa or a piston-cylinder (PC) press able to apply a uniaxial pressure up to 1 GPa. In both cases, in situ annealing can be performed until 2000°C. We have investigated the consolidation of single phase Sr-hexaferrites (SrFe12O19) micrometric powders with moderate coercive fields (0.25 T) and nanometric, prepared by solid-state reaction and with high coercive field (0.6 T). Also, we have investigated the consolidation of composites mixture of micrometric ferrites with CoFe metallic powders that exhibit higher magnetization [3]. We observed that PC press produces fragment of dense magnets while ferrite and composite dense magnets are obtained with the MA press with excellent magnetic properties. The influence of the pressing parameters, pressure and temperature, in the final magnets characteristics (structure, morphology and magnetic properties) will be discussed. Notably, we observed that the particle’s morphology and size play a role to get dense and iso- or anisotropic magnets.