Investigations on ThMn12-type and Mn-Al compounds as permanent magnet candidates

Permanent magnets (PM) are key components on the emerging and growing technologies related to renewable energies and electromobility, besides the vast use in data storage, sensors, robotics and automatization and many other consumer technologies. The combination of this scenario highlights the observed increase in the high-performance magnets demand that will grow in the following years. However, even though these key components are considered the main solution of such developments, they are also in the center of concerns and uncertainty. The rare earth (RE) elements used to obtain the high magnetic properties are very sensitive to market and price fluctuations, besides the risks regarding supply because of geopolitical or sustainable development issues. In this context, finding suitable compounds that show promising magnetic properties, while overcoming these issues, is of a great importance. In this dissertation, two types of prospective materials for permanent magnetic applications were investigated: the RE-lean ThMn12-type and the RE-free Mn-Al compounds. A correlation between microstructure and extrinsic magnetic properties was investigated to understand the discrepancy in relation to the intrinsic properties. With this aim, multi scale characterization techniques, allied with different types of simulations, were used to give a better understanding of the overall aspects of phase formation, phase stability and to build the knowledge on the relation structure-processing-microstructure-magnetic properties. Through this evaluation and identification of microstructural weak links, insights about the coercivity mechanism could be drawn, which can be used to further create strategies to develop the permanent magnet candidates investigated in this work. The unfolding of such study is important to improve and create alternative magnets that can possibly reduce the dependency on critical raw materials, enabling possibilities for a more sustainable development of different technologies and applications

Mechanochemical Synthesis and Nitrogenation of the Nd1.1Fe10CoTi Alloy for Permanent Magnet

In this work, the mechanochemical synthesis method was used for the first time to produce powders of the nanocrystalline Nd1.1Fe10CoTi compound from Nd2O3, Fe2O3, Co and TiO2. High-energy-milled powders were heat treated at 1000 °C for 10 min to obtain the ThMn12-type structure. Volume fraction of the 1:12 phase was found to be as high as 95.7% with 4.3% of a bcc phase also present. The nitrogenation process of the sample was carried out at 350 °C during 3, 6, 9 and 12 h using a static pressure of 80 kPa of N2. The magnetic properties Mr, µ0Hc, and (BH)max were enhanced after nitrogenation, despite finding some residual nitrogen-free 1:12 phase. The magnetic values of a nitrogenated sample after 3 h were Mr = 75 Am2 kg–1, µ0Hc = 0.500 T and (BH)max = 58 kJ·m–3. Samples were aligned under an applied field of 2 T after washing and were measured in a direction parallel to the applied field. The best value of (BH)max ~ 114 kJ·m–3 was obtained for 3 h and the highest µ0Hc = 0.518 T for 6 h nitrogenation. SEM characterization revealed that the particles have a mean particle size around 360 nm and a rounded shape