We determine the nonequilibrium grain size distribution (GSD) during the crystallization of a solid in d-dimensions under fixed thermodynamic conditions, for the random nucleation and growth model, and in the absence of grain coalescence. Two distinct generalizations of the theory established earlier are considered. A closed analytic expression of the GSD useful for experimental studies is derived for anisotropic growth rates. The main difference from the isotropic growth case is the appearance of a constant prefactor in the distribution. The second generalization considers a Gaussian source term: nuclei are stable when their volume is within a finite range determined by the thermodynamics of the crystallization process. The numerical results show that this generalization does not change the qualitative picture of our previous study. The generalization only affects quantitatively the early stage of crystallization when nucleation is dominant. The remarkable result of these major generalizations is that the nonequilibrium GSD is robust against anisotropic growth of grains and fluctuations of nuclei sizes.

The microstructure evolution and diffusion of silicon during heat-treatment and plastic deformation process were studied on the clad plates of Al–Mn/Al–Si aluminum composite fabricated by continuous casting. The results show that when the clad slab is homogenized and hot rolled, silicon diffuses across the interface from the Al–Si alloy (4004) side to the Al–Mn alloy (3003) side and dissolves into the 3003 matrix forming a solid solution. However, after deformation by cold-rolling, the increased driving force for precipitation of the solute elements in the core alloy side along with the abundant defects introduced by the severe deformation promotes the precipitation. Some Mg2Si particles precipitate from the solid solutions to form a transition region close to the interface of the two components. The presented transition area not only benefits the microstructure of the clad sheet but also improves the distribution of the microhardness across the interface, a tendency of gradient transition.

During the advance of the nuclear fission reaction, fission products accumulate and form pores (gas bubbles) that decrease the thermal conductivity of the nuclear fuel, potentially leading to overheating of the fuel element. To investigate this important phenomenon, a finite-element method is used to simulate the effect of 3-dimensional (3D) distributions of pores on the thermal transport in a nuclear fuel element consisting of uranium oxide (UO2) nuclear fuel pellet and Zircaloy cladding. Spherical pores ranging in size from 70 to 172 µm are introduced to create up to 30 vol% total porosity. The simulations demonstrate that the centerline temperature increases with the total porosity and the increase is nonlinear. The results also show that the centerline temperature, at fixed total porosity, weakly depends on the pore size distribution. This method can provide useful information regarding the effect of high porosity levels that may occur in off-normal operation conditions.

This article provides an overview of the key concepts and recent theoretical developments in computational modeling of complex metal hydrides with a focus on applications in hydrogen storage. Density functional theory based first-principles calculations have played an important role in understanding the structural and thermodynamic properties of these materials. Methods for predicting crystal structures and hydrogen positions in complex hydrides have been developed to complement experimental synthesis and characterization. Together with an efficient formalism for determining multinary phase diagrams under variable temperature and hydrogen pressure (the grand-canonical linear programming method), they constitute a complete first-principles framework for designing new hydrogen storage reactions. We also review the progress in modeling reaction kinetics in a prototypical complex hydride (i.e., a transition metal catalyzed sodium alanate [NaAlH4]). While many aspects of titanium-doped NaAlH4 remain hotly disputed, we discuss areas where satisfactory quantitative understanding has been achieved: diffusive metal mass transport, bulk substitution of Ti, and hydrogen dissociation.

The hydrogenation of metals often leads to changes in optical properties in the visible range. This allows for fundamental studies of the hydrogenation process, as well as the exploration of various applications using these optical effects. Here, we focus on recent developments in metal hydride-based optical fiber and plasmonic sensors and smart windows. Both applications benefit from the existence of a reflective metallic state, which is lost on hydrogenation and allows for large reversible optical changes. In this article, we review the status of both technologies and their prospects for applications.

Nano-sizing and scaffolding have emerged in the past decade as important strategies to control the kinetics, reversibility, and equilibrium pressure for hydrogen storage in light metal hydride systems. Reducing the size of metal hydrides to the nanometer range allows fast kinetics for both hydrogen release and subsequent uptake. Reversibility of the hydrogen release is impressively facilitated by nanoconfining the materials in a carbon or metal–organic framework scaffold, in particular for reactions involving multiple solid phases, such as the decomposition of LiBH4, NaBH4, and NaAlH4. More complex is the impact of nanoconfinement on phase equilibria. It is clear that equilibrium pressures, and even decomposition pathways, are changed. However, further experimental and computational studies are essential to understand the exact origins of these effects and to unravel the role of particle size, physical confinement, and interfaces. Nevertheless, it has become clear that nanoconfinement is a strong tool to change physicochemical properties of metal hydrides, which might not only be of relevance for hydrogen storage, but also for other applications such as rechargeable batteries.

Metal hydrides are a fascinating class of materials that can be utilized for a surprising variety of clean energy applications, including smart solar collectors, smart windows, sensors, thermal energy storage, and batteries, in addition to their traditional application for hydrogen storage. Over the past decade, research on metal hydrides for hydrogen storage increased due to global governmental incentives and an increased focus on hydrogen storage research for polymer electrolyte membrane fuel cell operation. Tremendous progress has been made in so-called complex metal hydrides for hydrogen storage applications with the discovery of many new hydrides containing covalently bound complex anions. Many of these materials have applications beyond hydrogen storage and are being investigated for lithium-ion battery separator and anode materials. In this issue of MRS Bulletin, we present the state of the art of key evolving metal-hydride-based clean energy technologies with an outlook toward future needs.