A new energy paradigm, consisting of greater reliance on renewable energy sources and increased concern for energy effi ciency in the total energy lifecycle, has accelerated research into energy-related technologies. Due to their ubiquity, magnetic materials play an important role in improving the effi ciency and performance of devices in electric power generation, conditioning, conversion, transportation, and other energy-use sectors of the economy. This review focuses on the state-of-the-art hard and soft magnets and magnetocaloric materials, with an emphasis on their optimization for energy applications. Specifi cally, the impact of hard magnets on electric motor and transportation technologies, of soft magnetic materials on electricity generation and conversion technologies, and of magnetocaloric materials for refrigeration technologies, are discussed. The synthesis, characterization, and property evaluation of the materials, with an emphasis on structure‐property relationships, are discussed in the context of their respective markets, as well as their potential impact on energy effi ciency. Finally, considering future bottlenecks in raw materials, options for the recycling of rare-earth intermetallics for hard magnets will be discussed.

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Magnetic materials and devices for the 21st century: Stronger, lighter, and more energy efficient

The provision of efficient electron and ion transport is a critical issue in an exciting new group of materials based on lithium metal phosphates that are important as cathodes for lithium-ion batteries. Much interest centres on olivine-type LiFePO(4), the most prominent member of this family. Whereas the one-dimensional lithium-ion mobility in this framework is high, the electronically insulating phosphate groups that benefit the voltage also isolate the redox centres within the lattice. The pristine compound is a very poor conductor (sigma approximately 10(-9) S cm(-1)), thus limiting its electrochemical response. One approach to overcome this is to include conductive phases, increasing its capacity to near-theoretical values. There have also been attempts to alter the inherent conductivity of the lattice by doping it with a supervalent ion. Compositions were reported to be black p-type semiconductors with conductivities of approximately 10(-2) S cm(-1) arising from minority Fe(3+) hole carriers. Our results for doped (and undoped) LiMPO(4) (M = Fe, Ni) show that a percolating nano-network of metal-rich phosphides are responsible for the enhanced conductivity. We believe our demonstration of non-carbonaceous-network grain-boundary conduction to be the first in these materials, and that it holds promise for other insulating phosphates.

Nano-network electronic conduction in iron and nickel olivine phosphates.

Modern society relies on readily available refrigeration. Magnetic refrigeration has three prominent advantages compared with compressor-based refrigeration. First, there are no harmful gases involved; second, it may be built more compactly as the working material is a solid; and third, magnetic refrigerators generate much less noise. Recently a new class of magnetic refrigerant-materials for room-temperature applications was discovered. These new materials have important advantages over existing magnetic coolants: they exhibit a large magnetocaloric effect (MCE) in conjunction with a magnetic phase-transition of first order. This MCE is larger than that of Gd metal, which is used in the demonstration refrigerators built to explore the potential of this evolving technology. In the present review we compare the different materials considering both scientific aspects and industrial applicability. Because fundamental aspects of MCE are not so widely discussed, we also give some theoretical considerations.

https://iopscience.iop.org/article/10.1088/0022-3727/38/23/R01/pdf

Developments in magnetocaloric refrigeration

The electronic structures of metal-rich binary phosphides M2P and M3P (M = Cr−Ni) have been examined by means of X-ray photoelectron and absorption spectroscopy and compared with those of the monophosphides MP. The P 2p3/2 binding and P K-edge absorption energies decrease with greater ionic character of the M−P bonding and indicate the presence of anionic phosphorus, contrary to previous theoretical assertions but consistent with our previous results. Interatomic effects play a more important role in affecting the energy shifts in these metal-rich phosphides than in the monophosphides, becoming more pronounced with higher metal concentration. Although the M 2p3/2 XPS spectra show no discernible shifts in binding energies, they reveal satellite features whose intensity can be related to metal charge in the Co- and Ni-containing phosphides. In these cases, the metal charge becomes less positive with higher metal concentration. For Ni2P and Ni3P, this trend was confirmed from an analysis of the Ni L-edge and...

X-ray Photoelectron and Absorption Spectroscopy of Metal-Rich Phosphides M2P and M3P (M = Cr−Ni)

Hysteresis is more than just an interesting oddity that occurs in materials with a first-order transition. It is a real obstacle on the path from existing laboratory-scale prototypes of magnetic refrigerators towards commercialization of this potentially disruptive cooling technology. Indeed, the reversibility of the magnetocaloric effect, being essential for magnetic heat pumps, strongly depends on the width of the thermal hysteresis and, therefore, it is necessary to understand the mechanisms causing hysteresis and to find solutions to minimize losses associated with thermal hysteresis in order to maximize the efficiency of magnetic cooling devices. In this work, we discuss the fundamental aspects that can contribute to thermal hysteresis and the strategies that we are developing to at least partially overcome the hysteresis problem in some selected classes of magnetocaloric materials with large application potential. In doing so, we refer to the most relevant classes of magnetic refrigerants La–Fe–Si-, Heusler- and Fe 2 P-type compounds. This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.2015.0308

Mastering hysteresis in magnetocaloric materials.

The magnetic properties of hexagonal Fe 2 P single crystal are studied by the measurements of magnetization, susceptibility, thermal expansion and electrical resistivity along the principal crystallographic axes. It is found that the saturation magnetization per molecule and the Curie temperature are 2.94 µ B and 208.6 K, respectively, and the anisotropy constant K 1 is 2.32×10 7 erg/cm 3 at 4.2 K, which is considerably larger than the values in 3d-transition compounds. The thermal expansion and the electrical resistivity show discontinuous changes at the Curie temperature indicating that the ferromagnetic to paramagnetic transition of Fe 2 P is accompanied by the first-order transition due to magnetoelastic effects. The exchange striction obtained from the measurement of thermal expansion is discussed on the basis of the theory of Bean and Rodbell.

Hōkabe Tsuneo

Papers

Magnetic Properties of Fe 2 P Single Crystal