Rare-earth based alloys for hard-magnetic applications: Temperature and pressure dependent phase stabilities

硬磁材料在当前欧洲发展可持续能源概念以保护气候的努力中发挥着核心作用。大多数包含电动发动机（如电动或混合动力汽车）或发电机（如风力涡轮机）的设备的功能至关重要地取决于永磁体的性能。目前的趋势是采用高磁能密度的系统，以提高效率，减少设备的尺寸、重量和生产成本。为此，Nd2Fe14B是目前的首选系统。然而，全世界都在努力取代这种材料体系的主要原因是，所涉及的稀土元素（如Dy或Tb）以及Nd的价格高昂。与它们相反，Ce是一种相当丰富的稀土元素，当用于化合物时，可以产生所谓的稀土平衡磁铁。然而，铈基硬磁材料很难生产，因为在产生的微观结构中，磁性有趣的三元相经常与二元相（如CeFe2）竞争。因此，当前研究项目的一个主要目标是实现对RE基合金热力学稳定性背后的物理原因的基本理解，RE=Ce， Pr和Nd，并从那里开发合成含Ce的金属间化合物的有效途径。使用各种理论和实验方法，我们将推导出这些材料的相图。三个自由度将是研究的重点：(i)温度，T，是生产过程中的关键控制参数，也是许多应用；（ii）压力P可以对元稳定性和磁性有很强的影响，（iii）化学成分X可以改变，使得相稳定性的物理趋势变得更加明显，并且内在和外在磁性得到进一步优化。这些问题将在三个合作机构的共同努力下得到解决：马克斯-普朗克研究所（Max-Planck-Institut fr Eisenforschung）将使用基于密度泛函数理论的技术，从第一原理推导出全参数空间（P-T-X）的自由能和所有相关熵贡献。强电子相关性的理论挑战将由Ecole Polytechnique解决，特别关注使用动态平均场理论（DMFT）的化学趋势。先进的合成和广泛的H-， P-和t -范围的综合表征将在Technische Universität达姆施塔特进行，获得的实验数据将用作理论基准。合成也将受益于理论输入，以开发P-T-X图中的热机械合成路线，从而稳定所需的磁相。

Hard-magnetic materials play a central role in current European efforts to develop sustainable energy concepts that allow climate protection. The functionality of most devices that contain electric engines (such as electrical or hybrid cars) or generators (such as wind turbines) depends crucially on the performance of the permanent magnets. The current trend goes towards systems with a high magnetic energy density to increase efficiency and to reduce size, weight and production costs of devices. For this Nd2Fe14B is currently the system of choice. The main reason for intensive world-wide efforts to replace this material system is, however, the high price for the involved rare-earth elements such as Dy or Tb, but also Nd. In contrast to them, Ce is a rather abundant rare-earth element, which when used in compounds could lead to so-called rare earth balance magnets. Ce-based hard-magnetic materials are, however, difficult to produce, since the magnetically interesting ternary phases are often competing with binary phases such as CeFe2 in the resulting microstructures.A main goal of the current research project is therefore to achieve a fundamental understanding for the physical reasons behind the thermodynamic stability of RE-based alloys, with RE=Ce, Pr, and Nd and to develop from there efficient routes to synthesise Ce-containing intermetallics. Using a variety of theoretical and experimental methods, we will derive phase diagrams for these materials. Three degrees of freedom will be in the focus of the investigations: (i) the temperature, T, is a key control parameter in the production process, but also for many applications; (ii) the pressure, P, can have a strong impact on meta-stabilities as well as magnetic properties, and (iii) the chemical composition, X, can be varied such that physical trends for phase stabilities become more apparent and that intrinsic and extrinsic magnetic properties are further optimized. These questions will be addressed in a joint effort of three partner institutions: Density-functional theory-based techniques will be used at Max-Planck-Institut für Eisenforschung to derive free energies and all relevant entropy contributions for the full parameter space, P-T-X, from first principles. The theoretical challenge of strong electronic correlations will be addressed by Ecole Polytechnique with a particular focus of chemical trends using dynamic mean-field theory (DMFT). Advanced synthesis and comprehensive characterization in wide H-, P- and T-ranges will be performed at the Technische Universität Darmstadt and the obtained experimental data will be used as benchmark for theory. Synthesis will also benefit from the theoretical input to develop a thermomechanical synthesis route in the P-T-X diagram that allows stabilizing the desired magnetic phases.