Annual Review of Materials Research - Volume 51, 2021

Polymers that simultaneously transport electrons and ions are paramount to drive the technological advances necessary for next-generation electrochemical devices, including energy storage devices and bioelectronics. However, efforts to describe the motion of ions or electrons separately within polymeric systems become inaccurate when both species are present. Herein, we highlight the basic transport equations necessary to rationalize mixed transport and the multiscale material properties that influence their transport coefficients. Potential figures of merit that enable a suitable performance benchmark in mixed conducting systems independent of end application are discussed. Practical design and implementation of mixed conducting polymers require an understanding of the evolving nature of structure and transport with ionic and electronic carrier density to capture the dynamic disorder inherent in polymeric materials.

This review discusses selective and fast transport of ionic species (ions and their associates) through systems as diverse as ion-conducting transmembrane proteins and ion exchange membranes (IEMs) in aqueous environments, with special emphasis on the role of electrostatics, specific chemical interactions, and morphology (steric effects). Contrary to the current doctrine, we suggest that properly balanced ion-coordinating interactions are more important than steric effects for selective ion transport in biological systems. Steric effects are more relevant to the selectivity of ionic transport through IEMs. As a general rule, decreased hydration leads to higher selectivity but also to lower transport rate. Near-perfect selectivity is achieved by ion-conducting channels in which unhydrated ions transfer through extremely short hydrophobic passages separating aqueous environments. In IEMs, ionic species practically keep their hydration shell and their transport is sterically constrained by the width of aqueous pathways. We discuss the trade-off between selectivity and transport rates and make suggestions for choosing, optimizing, or developing membranes for technological applications such as vanadium-redox-flow batteries.

Neuromorphic computing is becoming increasingly prominent as artificial intelligence (AI) facilitates progressively seamless interaction between humans and machines. The conventional von Neumann architecture and complementary metal-oxide-semiconductor transistor scaling are unable to meet the highly demanding computational density and energy efficiency requirements of AI. Neuromorphic computing aims to address these challenges by using brain-like computing architectures and novel synaptic memories that coallocate information storage and computation, thereby enabling low latency at high energy efficiency and high memory density. Though various emerging memory devices have been extensively studied to emulate the functionality of biological synapses, there is currently no material/device system that encompasses both the needed metrics for high-performance neuromorphic computing and the required biocompatibility for potential body-computer integration. In this review, we aim to equip the reader with general design principles and materials requirements for realizing high-performance organic neuromorphic devices. We use instructive examples from recent literature to discuss each requirement, illustrating the challenges as well as future research opportunities. Though organic devices still face many challenges to become major players in neuromorphic computing, mostly due to their lack of compliance with back-end-of-line processes required for integration with digital logic, we propose that their biocompatibility and mechanical conformability give them an advantage for creating adaptive biointerfaces, brain-machine interfaces, and biology-inspired prosthetics.

Polymeric mixed ionic-electronic conductors (MIECs) combine aspects of conjugated polymers, polymer electrolytes, and polyelectrolytes to simultaneously transport and couple ionic and electronic charges, opening exciting new applications in energy storage and conversion, bioelectronics, and display technologies. The many applications of polymeric MIECs lead to a wide range of transport conditions. Ionic and electronic transport are directly coupled through electrochemical doping, while the mechanisms of ionic and electronic transport depend on distinctly different chemical functionality, (macro)molecular structure, and morphology. Despite this, ionic and electronic transport are surprisingly tunable, independent of one another. We review the various types of polymeric MIECs, the mechanisms of ionic and electronic charge transport across conditions, and the interrelations between the two, with special emphasis on the unique aspects of polymeric MIEC transport phenomena.

Shock loading takes materials from ambient conditions to extreme conditions of temperature and nonhydrostatic stress on picosecond timescales. In molecular materials the fast loading results in temporary nonequilibrium conditions with overheated low-frequency modes and relatively cold, high-frequency, intramolecular modes; coupling the shock front with the material's microstructure and defects results in energy localization in hot spots. These processes can conspire to lead to a material response not observed under quasi-static loads. This review focuses on chemical reactions induced by dynamical loading, the understanding of which requires bringing together materials science, shock physics, and condensed matter chemistry. Recent progress in experiments and simulations holds the key to the answer of long-standing grand challenges with implications for the initiation of detonation and life on Earth.

Structural materials have lagged behind other classes in the use of combinatorial and high-throughput (CHT) methods for rapid screening and alloy development. The dual complexities of composition and microstructure are responsible for this, along with the need to produce bulk-like, defect-free materials libraries. This review evaluates recent progress in CHT evaluations for structural materials. High-throughput computations can augment or replace experiments and accelerate data analysis. New synthesis methods, including additive manufacturing, can rapidly produce composition gradients or arrays of discrete alloys-on-demand in bulk form, and new experimental methods have been validated for nearly all essential structural materials properties. The remaining gaps are CHT measurement of bulk tensile strength, ductility, and melting temperature and production of microstructural libraries. A search strategy designed forstructural materials gains efficiency by performing two layers of evaluations before addressing microstructure, and this review closes with a future vision of the autonomous, closed-loop CHT exploration of structural materials.

Herein, we critically evaluate computational and experimental studies in the emerging field of high-entropy ultra-high-temperature ceramics. High-entropy ultra-high-temperature ceramics are candidates for use in extreme environments that include temperatures over 2,000°C, heat fluxes of hundreds of watts per square centimeter, or irradiation from neutrons with energies of several megaelectron volts. Computational studies have been used to predict the ability to synthesize stable high-entropy materials as well as the resulting properties but face challenges such as the number and complexity of unique bonding environments that are possible for these compositionally complex compounds. Experimental studies have synthesized and densified a large number of different high-entropy borides and carbides, but no systematic studies of composition-structure-property relationships have been completed. Overall, this emerging field presents a number of exciting research challenges and numerous opportunities for future studies.

Co base superalloys strengthened by coherent L12 ordered γ′ precipitate in a disordered face-centered cubic γ matrix represent a new opportunity for high-temperature alloy development. The emergence of alloys with low density and high specific yield strength at elevated temperatures has further energized the research and development efforts in the last 5 years. Initially stabilized by the addition of small amounts of Nb and Ta, these new generations of alloys with multiple alloying additions to form basic quaternary and ternary alloys have steadily expanded the property envelopes to raise hope for a modern class of superalloys with higher-temperature capabilities. This article reviews the work of a vibrant set of researchers across the globe whose findings are constantly unlocking the potential of these alloys. These developments have achieved high-temperature strength (at 870°C) >0.6 GPa, γ′ solvus temperature exceeding 1,100°C, and densities between 7.8 and 8.6 g/cm³.

The mechanical properties of superalloys are strongly governed by the resistance to shearing of ordered precipitates by dislocations. In the operating environments of superalloys, the stresses and temperatures present during thermomechanical loading influence the dislocation shearing dynamics, which involve diffusion and segregation processes that result in a diverse array of planar defects in the ordered L12 γ′ precipitate phase. This review discusses the current understanding of high-temperature deformation mechanisms of γ′ precipitates in two-phase Ni-, Co-, and CoNi-base superalloys. The sensitivity of planar fault energies to chemical composition results in a variety of unique deformation mechanisms, and methods to determine fault energies are therefore reviewed. The degree of chemical segregation in the vicinity of planar defects reveals an apparent phase transformation within the parent γ′ phase. The kinetics of segregation to linear and planar defects play a significant role in high-temperature properties. Understanding and controlling fault energies and the associated dislocation dynamics provide a new pathway for the design of superalloys with exceptional properties.

Processing science for nanocrystalline metals has largely focused on far-from-equilibrium methods that can generate many grain boundaries with excess defect energy. Conversely, the science of stabilizing nanocrystalline alloys has largely focused on the lowering of that excess defect energy through grain boundary segregation, bringing nanocrystalline structures closer to equilibrium. With increasing technological adoption of stabilized nanocrystalline alloys, there is a substantial need for research at the intersection of these two fields. This review lays out the basic thermodynamic issues of the two subfields and surveys the literature on the most common processing methods, including severe plastic deformation, ball milling, physical vapor deposition, and electrodeposition. We provide an overview of studies that have examined grain boundary segregation through each of these methods and identify general themes. We conclude that there is substantial scope for more systematic work at the intersection of these fields to understand how nonequilibrium processing affects grain boundary segregation.

Hybrid halide perovskite semiconductors exhibit complex, dynamical disorder while also harboring properties ideal for optoelectronic applications that include photovoltaics. However, these materials are structurally and compositionally distinct from traditional compound semiconductors composed of tetrahedrally coordinated elements with an average valence electron count of silicon. The additional dynamic degrees of freedom of hybrid halide perovskites underlie many of their potentially transformative physical properties. Neutron scattering and spectroscopy studies of the atomic dynamics of these materials have yielded significant insights into their functional properties. Specifically, inelastic neutron scattering has been used to elucidate the phonon band structure, and quasi-elastic neutron scattering has revealed the nature of the uncorrelated dynamics pertaining to molecular reorientations. Understanding the dynamics of these complex semiconductors has elucidated the temperature-dependent phase stability and origins of defect-tolerant electronic transport from the highly polarizable dielectric response. Furthermore, the dynamic degrees of freedom of the hybrid perovskites provide additional opportunities for application engineering and innovation.

The next generation of nuclear reactors will expose materials to conditions that, in some cases, are even more extreme than those in current fission reactors, inevitably leading to new materials science challenges. Radiation-induced damage and corrosion are two key phenomena that must be understood both independently and synergistically, but their interactions are often convoluted. In the light water reactor community, a tremendous amount of work has been done to illuminate irradiation-corrosion effects, and similar efforts are under way for heavy liquid metal and molten salt environments. While certain effects, such as radiolysis and irradiation-assisted stress corrosion cracking, are reasonably well established, the basic science of how irradiation-induced defects in the base material and the corrosion layer influence the corrosion process still presents many unanswered questions. In this review, we summarize the work that has been done to understand these coupled extremes, highlight the complex nature of this problem, and identify key knowledge gaps.

Perovskite ABO3 oxides that have Bi and Pb at the A site and transition metals at the B site, when stabilized by high-pressure synthesis at several gigapascals, provide a rich parameter space of fascinating properties. Stereochemical 6s² lone pairs of Bi³⁺ and Pb²⁺ induce polar or antipolar distortions. 6s² and 6s⁰ (Bi⁵⁺ and Pb⁴⁺) charge degree of freedom enable intermetallic charge transfer transitions. The structural distortion and the charge degree of freedom are coupled with magnetism of transition metals, resulting in various functionalities. In particular, we highlight magnetization reversal by electric field and polarization rotation in BiFe1−xCoxO3, negative thermal expansion in modified BiNiO3 and PbVO3, and systematic charge distribution changes in PbMO3 (M = 3d transition metal).

Successful strategies for the design of crystalline materials with useful function are frequently based on the systematic tuning of chemical composition within a given structural family. Perovskites with the formula ABX3, perhaps the best-known example of such a family, have a vast range of elements on A, B, and X sites, which are associated with a similarly vast range of functionality. Layered double perovskites (LDPs), a subset of this family, are obtained by suitable slicing and restacking of the perovskite structure, with the additional design feature of ordered cations and/or anions. In addition to inorganic LDPs, we also discuss hybrid (organic-inorganic) LDPs here, where the A-site cation is a protonated organic amine. Several examples of inorganic LDPs are presented with a discussion of their ferroic, magnetic, and optical properties. The emerging area of hybrid LDPs is particularly rich and is leading to exciting discoveries of new compounds with unique structures and fascinating optoelectronic properties. We provide context for what is important to consider when designing new materials and conclude with a discussion of future opportunities in the broad LDP area.

Gallium is a metal that literally melts in your hand. It has low toxicity, near-zero vapor pressure, and a viscosity similar to water. Despite possessing a surface tension larger than any other liquid (near room temperature), gallium can form nonspherical shapes due to the thin, solid native oxide skin that forms rapidly in oxygen. These properties enable new ways to pattern metals (e.g., injection and printing) to create stretchable and soft devices with an unmatched combination of mechanical and electrical properties. The oxide skin can be transferred to other substrates and manipulated electrochemically to lower the interfacial tension to near zero. The reactivity of gallium can drive a wide range of reactions. The liquid state of gallium makes it easy to break into particles for making colloids and soft composites that have unusual properties due to the deformable nature of the filler. This review summarizes the truly unique and exciting properties of gallium liquid metals.

In recent decades, research on persistent luminescence has led to new phosphors and promising performances. Efforts to improve the quality of phosphors’ afterglow have paved the way toward innovative solutions for many disciplines. However, there are few examples of the implementation of luminescent materials. In addition to providing a general background on persistent luminescence, the techniques used for its analysis, and its multidisciplinary potential in energy and environmental science, this article aims to explain the existing gap between the physical-chemical approach and the effective implementation of luminescent materials in larger-scale applications. It investigates engineering solutions in terms of the possible benefits of luminescence in lighting energy savings and passive cooling of urban surfaces. Finally, this article aims to reduce the abovementioned gap by suggesting what is most needed for the successful application of luminescent materials in the built environment.

Cultural heritage materials, ranging from archaeological objects and sites to fine arts collections, are often characterized through their life cycle. In this review, the fundamentals and tools of materials science are used to explore such life cycles—first, via the origins of the materials and methods used to produce objects of function and artistry, and in some cases, examples of exceptional durability. The findings provide a window on our cultural heritage. Further, they inspire the design of sustainable materials for future generations. Also explored in this review are alteration phenomena over intervals as long as millennia or as brief as decades. Understanding the chemical processes that give rise to corrosion, passivation, or other degradation in chemical and physical properties can provide the foundation for conservation treatments. Finally, examples of characterization techniques that have been invented or enhanced to afford studies of cultural heritage materials, often nondestructively, are highlighted.

Oxides with mixed protonic and p-type electronic conductivity (and typically containing also mobile oxygen vacancies) are important functional materials, e.g., for oxygen electrodes in protonic ceramic electrochemical cells or for permeation membranes. Owing to the presence of three carriers, their defect chemical behavior is complex. Deviations from ideal behavior (defect interactions) have to be taken into account, which are related to the partially covalent character of the transition metal–oxygen bonds. Compared to acceptor-doped Ba(Zr,Ce)O3−z electrolytes, perovskites with redox-active transition-metal cations typically show smaller degrees of hydration. Trends in the proton uptake of (Ba,Sr,La)(Fe,Co,Y,Zn)O3−δ perovskites are analyzed and correlated to structural features (local lattice distortions) and electronic properties (the position of oxygen states on an absolute energy scale). The proton mobility in such mixed-conducting perovskites is estimated. Specific aspects of the application of protonic and electronic mixed-conducting oxides in protonic ceramic electrochemical cells are discussed, and an overview of recent materials and device developments is given.

Quantum spin liquids are unique quantum states of matter predicted to arise in low-dimensional, frustrated, and quantum magnetic systems. Compared with conventional ferromagnetic and antiferromagnetic states, quantum spin liquids are expected to display a variety of novel and exotic properties, making their realization in materials a highly appealing prospect. While an unambiguous realization of this long-sought-after state remains elusive, a growing number of materials candidates show promise in revealing the properties of quantum spin liquids. In this review, we present some of the key challenges and current opportunities in the synthesis, characterization, and understanding of quantum spin liquids from the perspective of the broad and interdisciplinary field of materials research.

A renaissance of interest in crystallographic shear structures and our recent work in this remarkable class of materials inspired this review. We first summarize the geometrical aspects of shear plane formation and possible transformations in ReO3, rutile, and perovskite-based structures. Then we provide a mechanistic overview of crystallographic shear formation, plane ordering, and propagation. Next we describe the energetics of planar defect formation and interaction, equilibria between point and extended defect structures, and thermodynamic stability of shear compounds. Finally, we emphasize the remaining challenges and propose future directions in this exciting area.