Alternating cycles of isothermal magnetization and adiabatic demagnetization applied to a magnetocaloric material can drive refrigeration in very much the same manner as cycles of gas compression and expansion. The material property of interest in finding candidate magnetocaloric materials is their gravimetric entropy change upon application of a magnetic field under isothermal conditions. There is, however, no general method of screening materials for such an entropy change without actually carrying out the relevant, time-and effort-intensive magnetic measurements. Here we propose a simple computational proxy based on carrying out non-magnetic and magnetic density functional theory calculations on magnetic materials. This proxy, which we refer to as the magnetic deformation Σ M , is a measure of how much the unit cell deforms when comparing the relaxed structures with and without the inclusion of spin polarization. Σ M appears to correlate very well with experimentally measured magnetic entropy change values. The proxy has been tested against 33 known ferromagnetic materials, including nine materials newly measured for this study. It has then been used to screen 134 ferromagnetic materials for which the magnetic entropyhas not yet been reported, identifying 30 compounds as being promising for further study. As a demonstration of the effectiveness of our approach, we have prepared one of these compounds and measured its isothermal entropy change. MnCoP, with T C = 575 K, shows a maximum ∆S M = −6.0 J kg −1 K −1 for an applied field of H = 5 T.2

Solid-state lighting using laser diodes is an exciting new development that requires new phosphor geometries to handle the greater light fluxes involved. The greater flux from the source results in more conversion and therefore more conversion loss in the phosphor, which generates self-heating, surpassing the stability of current encapsulation strategies used for light-emitting diodes, usually based on silicones. Here, we present a rapid method using spark plasma sintering (SPS) for preparing ceramic phosphor composites of the canonical yellow-emitting phosphor Ce-doped yttrium aluminum garnet (Ce:YAG) combined with a chemically compatible and thermally stable oxide, α-AlO. SPS allows for compositional modulation, and phase fraction, microstructure, and luminescent properties of ceramic composites with varying compositions are studied here in detail. The relationship between density, thermal conductivity, and temperature rise during laser-driven phosphor conversion is elucidated, showing that only modest densities are required to mitigate thermal quenching in phosphor composites. Additionally, the scattering nature of the ceramic composites makes them ideal candidates for laser-driven white lighting in reflection mode, where Lambertian scattering of blue light offers great color uniformity, and a luminous flux >1000 lm is generated using a single commercial laser diode coupled to a single phosphor element.

We investigate the average structure, local structure, and magnetic behavior of Heusler alloys of the composition Mn 1−x Fe x Ru 2 Sn, between antiferromagnetic (AFM) MnRu 2 Sn and ferromagnetic (FM) FeRu 2 Sn (often written Ru 2 MnSn and Ru 2 FeSn). Using a combination of neutron total scattering, electron microscopy, and 57 Fe Mössbauer spectroscopy, we conclude that true solid solutions are formed across the compositional space investigated, with Fe substituting for Mn on the Heusler lattice, with little or no antisite disorder. Despite the lack of chemical phase separation, magnetic phase separation is present in compositions near x = 0.50, where the coexistence of AFM and FM domains is confirmed by 15 K neutron diffraction. At these intermediate compositions a large increase in magnetic coercivity is observed, in excess of 1 kOe, attributed to local exchange interactions.

Assisted microwave heating involves the use of a susceptor to initially heat up reactants in a microwave reaction. Once hot, the reactants themselves become directly susceptible to microwave heating, and interdiffuse to form products. Assisted-microwave methods are appealing for a wide variety of high-temperature solid-state reactions, reaching reaction temperatures of 1500 • C and more. Among the many advantages are that the direct volumetric heating associated with microwaves allows for rapid reaction times while employing significantly less energy than conventional furnace-based preparation. Shorter reaction times and selective heating permit volatile reactants to be incorporated stoichiometrically in the product. Undesirable reactions with containers or enclosures are also minimized. The morphology of powders obtained through microwave reactions are also more uniform and comprise smaller particles than obtained conventionally. This Methods/Protocols article is presented as a user manual for carrying out assisted-microwave preparation of bulk complex oxides in air or reducing atmospheres, sol-gel based processing of complex oxides, air sensitive intermetallics, and transition metal chalcogenides.

Materials with strongly coupled magnetic and structural transitions can display a giant magnetocaloric effect, which is of interest in the design of energy-efficient and environmentally-friendly refrigerators, heat pumps, and thermomagnetic generators. There also exist however, a class of materials with no known magnetostructural transition that nevertheless show remarkable magnetocaloric effects. MnB has been recently suggested as such a compound, displaying a large magnetocaloric effect at its Curie temperature (570 K) showing promise in recovering low-grade waste heat using thermomagnetic generation. In contrast, we show that isostructural FeB displays very similar magnetic ordering characteristics, but is not an effective magnetocaloric. Temperature-and field-dependent diffraction studies reveal dramatic magnetoelastic coupling in MnB, which exists without a magnetostructural transition. No such behavior is seen in FeB. Furthermore, the magnetic transition in MnB is shown to be subtly first-order, albeit with distinct behavior from that displayed by other magnetocalorics with first-order transitions. Density functional theory-based electronic structure calculations point to the magnetoelastic behavior in MnB as arising from a competition between Mn moment formation and B-B bonding.