Development of new high anisotropy phases for permanent magnet applications

Abstract

The development of last generation permanent magnets is strongly linked to the use of critical and / or strategic raw materials such as rare earths (RE), and especially heavy rare earths. In the case of Nd2Fe14B, magnets, the addition of Dy, one of the scarcest REs, is necessary to increase its maximum energy product and working temperature. However, the use of Dy raises the production costs of the new magnets and compromises their supply. In this thesis we have used new techniques to improve the magnet performance, suppressing the content of Dy. We also study new magnetic phases; with tetragonal structure type-ThMn12 in Fe rich alloys with reduced rare earths and other with nil content of them. For the phases of NdFeB, the control of the crystallisation process by adding traces of other elements as Cu and Nb has been studied, followed by grain boundary diffusion processes (GBDP) in nanocrystalline samples, through infiltration of a eutectic alloy, to enhance the coercivity. For the ThMn12 phases, the elements that can stabilize the phase have been studied first. Subsequently, the improvement of their magnetic properties by nitrogenation and the optimum microstructure of the samples has been studied, through thermal treatments and mechanical grinding. For the RE-free alloys, the Fe-Co-Ta system has also been screened looking for new phases that present uniaxial anisotropy at room temperature.