Isotropic nanocrystalline MnAl(C) permanent magnet powder

Resumen

Permanent magnet composites are characteristic materials that can create their own persistent magnetic field without the need of any other external magnetic field. This unusual property has been taken advantage in the developing of new technological applications, facilitating the production of faster and smaller devices. The development of new and better permanent magnet compounds during the last century has been possible thanks to the research of new structures and/or through the creation of novel compounds. This tireless research has allowed overcoming some socio-political problems in the continuous technological development and the resource shortage of some valuable elements (rare earths) needed to produce permanent magnets and required in many technological applications. Nowadays, the challenge of finding new permanent magnet materials without Neodymium and Dysprosium (main rare earth elements used in the strongest permanent magnets aimed for room temperature applications) remains. The goal is achieving a diversification in the permanent magnet market sector with the development of rare earth-free alternatives able of filling the gap between ferrites, Alnico and Nd,Dy-based magnets. This thesis has been focused on the study of one of the most promising alternatives to substitute the bonded Nd-based permanent magnets based on its outstanding intrinsic magnetic properties and abundance of its constituent elements: MnAl. In this research, it is presented an in deep study of the MnAl system by the modification of its micro- and nano-structure and the control of the MnAl-phases present in the material. This research has been performed through the combined use of the gas atomization and the ball-milling techniques, and importantly, exploring the short milling time regime by optimization of the milling process. This approach has made possible to modify its nanostructure, phase transformation process and, in consequence, its permanent magnet properties (with a strong effort focused on coercivity development). The influence that the choice of parameters in the fast-ball milling process has on the nanostructuring and phase transformation of the MnAl system has been studied in detail. The control of the milling time and the impact energy exerted on the MnAl particles during the process, and the subsequent annealing treatment needed for development of the permanent magnet properties, are demonstrated to be determining factors to tune magnetization and coercivity in a controlled manner. The possibility of applying temperature simultaneously during recording of XRD patterns has allowed the configuration of Temperature-XRD maps, thus allowing to observe the gradual phase evolution in MnAl with temperature and, accordingly, the crystallization of the ferromagnetic -MnAl phase. This information, in combination with microstructural results, has been decisive to understand the mechanisms behind the development of permanent magnet properties in the MnAl system. The second part of the research presented in this thesis has focused on establishing the correlation between composition and structure of the starting MnAlC powder with its magnetic properties after the nanostructuring process. The results show a strong dependence of the initial composition in the effectiveness of the nanostructuring process, demonstrating that the Mn:Al ratio in the starting composite influences strongly the final magnetic properties of the alloy. It has also been studied the effect of the initial size of the gas atomized MnAl particles on the ball-milling process, proving that there is a threshold size above which the ultrafast ball milling technique is not efficient in improving the permanent magnet properties. The novel route here used consisting of combining gas-atomization and ultrafast-milling provides a new method for coercivity development in isotropic nanocrystalline MnAl powders for permanent magnet applications, with potential for industrial implementation based on its short processing time, low-energy consumption, foreseen cost-efficiency, and easy scalability.