Annual Review of Materials Research - Volume 53, 2023

Hydrous transition metal oxides (TMOs) are redox-active materials that confine structural water within their bulk, organized in 1D, 2D, or 3D networks. In an electrochemical cell, hydrous TMOs can interact with electrolyte species not only via their outer surface but also via their hydrous inner surface, which can transport electrolyte species to the interior of the material. Many TMOs operating in an aqueous electrochemical environment transform to hydrous TMOs, which then serve as the electrochemically active phase. This review summarizes the physicochemical properties of hydrous TMOs and recent mechanistic insights into their behavior in electrochemical reactions of interest for energy storage, conversion, and environmental applications. Particular focus is placed on first-principles calculations and operando characterization to obtain an atomistic view of their electrochemical mechanisms. Hydrous TMOs represent an important class of energy and environmental materials in aqueous and nonaqueous environments. Further understanding of their interaction with electrolyte species is likely to yield advancements in electrochemical reactivity and kinetics for energy and environmental applications.

The energy-efficient manipulation of the properties of functional materials is of great interest from both a scientific and an applied perspective. The application of electric fields is one of the most widely used methods to induce significant changes in the properties of materials, such as their structural, transport, magnetic, and optical properties. This article presents an overview of recent research on the manipulation of the electronic and magnetic properties of various material systems via electrolyte-based ionic gating. Oxides, magnetic thin-film heterostructures, and van der Waals 2D layers are discussed as exemplary systems. The detailed mechanisms through which ionic gating can induce significant changes in material properties, including their crystal and electronic structure and their electrical, optical, and magnetic properties, are summarized. Current and potential future functional devices enabled by such ionic control mechanisms are also briefly summarized, especially with respect to the emerging field of neuromorphic computing. Finally, a brief outlook and some key challenges are presented.

We review the class of materials known as polar metals, in which polarity and metallicity coexist in the same phase. While the notion of polar metals was first invoked more than 50 years ago, their practical realization has proved challenging since the itinerant carriers required for metallicity tend to screen any polarization. Huge progress has been made in the last decade, with many mechanisms for combining polarity and metallicity proposed and the first examples, LiOsO3 and WTe2, identified experimentally. The availability of polar metallic samples has opened a new paradigm in polar metal research, with implications in the fields of topology, ferroelectricity, magnetoelectricity, spintronics, and superconductivity. Here, we review the principles and techniques that have been developed to design and engineer polar metals and describe some of their interesting properties, with a focus on the most promising directions for future work.

The increasing consumption of nonrenewable materials urgently calls for the design and fabrication of sustainable alternatives. New generations of materials should be derived from renewable sources, processed using environmentally friendly methods, and designed considering their full life cycle, especially their end-of-life fate. Here, we review recent advances in developing sustainable polymers from biological matter (biomatter), including progress in the extraction and utilization of bioderived monomers and polymers, as well as the emergence of polymers produced directly from unprocessed biomatter (entire cells or tissues). We also discuss applications of sustainable polymers in bioplastics, biocomposites, and cementitious biomaterials, with emphasis on relating their performance to underlying fundamental mechanisms. Finally, we provide a future outlook for sustainable material development, highlighting the need for more accurate and accessible tools for assessing life-cycle impacts and socioeconomic challenges as this field advances.

Scanning transmission electron microscopy (STEM) is one of the most powerful characterization tools in materials science research. Due to instrumentation developments such as highly coherent electron sources, aberration correctors, and direct electron detectors, STEM experiments can examine the structure and properties of materials at length scales of functional devices and materials down to single atoms. STEM encompasses a wide array of flexible operating modes, including imaging, diffraction, spectroscopy, and 3D tomography experiments. This review outlines many common STEM experimental methods with a focus on quantitative data analysis and simulation methods, especially those enabled by open source software. The hope is to introduce both classic and new experimental methods to materials scientists and summarize recent progress in STEM characterization. The review also discusses the strengths and weaknesses of the various STEM methodologies and briefly considers promising future directions for quantitative STEM research.

Tailor-made additives (TMAs) have found a role in crystal morphology engineering and control by specific binding to crystal surfaces through stereo-chemical recognition. The utility of TMAs, however, has been largely limited to crystal growth from solutions. In this review, we illustrate examples where TMAs have been used to influence the growth of crystals during cooling of their melts. In solution, the crystal growth driving force is governed by solute supersaturation, which corresponds to the deviation from equilibrium. In growth from melts, however, undercooling is the important thermodynamic parameter responsible for crystallization outcomes, a key difference that can influence the manner in which TMAs affect growth kinetics, crystal morphology, nucleation, enantioselective surface recognition, and the determination of the absolute sense of polar axes. When the crystallization driving force in a melt is small and diffusion is comparatively high, TMAs can exert their influence on well-faceted single crystals with the stereochemical richness observed in solution growth. Under high supercooling, where the driving force is large, ensembles of crystals can grow radially, masking stereochemical information and requiring new optical tools for understanding the influence of TMAs on emerging crystals.

Piezoelectric materials possess the capability to interchangeably convert electrical energy into a mechanical response. While current piezoelectric materials exhibit strong properties, known limitations have inhibited further development. This review describes the ability to combine different piezoelectric materials into a composite to create well-rounded properties. The different types of connectivity classes are described as well as important design considerations and theoretical models. The contributions from the active and passive phases are outlined, focusing primarily on ferroelectric ceramics and polymer-based composites. The key advantage of piezoelectric composites is their ability to combine the flexibility of polymers with the high electromechanical coupling and piezoelectric coefficients of ferroelectric ceramics or single crystals appropriate for a variety of applications. Composites are prominent in medical ultrasound imaging and therapy, underwater acoustic sensing, industrial structural health monitoring, energy harvesting, and numerous other emerging applications.

Natural wood has been used for construction, fuel, and furniture for thousands of years because of its versatility, renewability, and aesthetic appeal. However, new opportunities for wood are arising as researchers have developed ways to tune the material's optical, thermal, mechanical, and ionic transport properties by chemically and physically modifying wood's naturally porous structure and chemical composition. Such modifications can be used to produce sustainable, functional materials for various emerging applications such as automobiles, construction, energy storage, and environmental remediation. In this review, we highlight recent advancements in engineered wood for sustainable technologies, including thermal and light management, environmental remediation, nanofluidics, batteries, and structural materials with high strength-to-weight ratios. Additionally, the current challenges, opportunities, and future of wood research are discussed, providing a guideline for the further development of next-generation, sustainable wood-based materials.

Soft robotics aims to close the performance gap between built and biological machines through materials design. Soft robots are constructed from soft, actuatable materials to be physically intelligent, or to have traits that living organisms possess such as passive adaptability and morphological computation through their compliant, deformable bodies. However, materials selection for physical intelligence often involves low-performance and/or energy-inefficient, stimuli-responsive materials for actuation. Additional challenges in soft robot sensorization and control further limit the practical utility of these machines. Recognizing that electrically controllable materials are crucial for the development of soft machines that are both physically and computationally intelligent, we review progress in the development of electroprogrammable materials for soft robotic actuation. We focus on thermomechanical, electrostatic, and electrochemical actuation strategies that are directly controlled by electric currents and fields. We conclude with an outlook on the design and fabrication of next-generation robotic materials that will facilitate true bioinspired autonomy.

Noncentrosymmetric (NCS) materials feature an exciting array of functionalities such as nonlinear optical (NLO) responses and topological spin textures (skyrmions). While NLO materials and magnetic skyrmions display two different sets of physical properties, their design strategies are deeply connected in terms of atomic-scale precision, structural customization, and electronic tunability. Despite impressive progress in studying these systems separately, a joint road map for navigating the chemical principles for NCS materials remains elusive. This review unites two subtopics of NCS systems, NLO materials and magnetic skyrmions, offering a multifaceted narrative of how to translate the often-abstract fundamentals to the targeted functionalities while inviting innovative approaches from the community. We outline the design principles central to the desired properties by exemplifying relevant examples in the field. We supplement materials chemistry with pertinent electronic structures to demonstrate the power of the fundamentals to create systems integration relevant to foreseeable societal impacts in frequency-doubling instrumentation and spin-based electronics.

Advanced experimental and numerical approaches are being developed to capture the localization of plasticity at the nanometer scale as a function of the multiscale and heterogeneous microstructure present in metallic materials. These innovative approaches promise new avenues to understand microstructural effects on mechanical properties, accelerate alloy design, and enable more accurate mechanical property prediction. This article provides an overview of emerging approaches with a focus on the localization of plasticity by crystallographic slip. New insights into the mechanisms and mechanics of strain localization are addressed. The consequences of the localization of plasticity by deformation slip for mechanical properties of metallic materials are also detailed.

If variety is the spice of life, then abnormal grain growth (AGG) may be the materials processing equivalent of sriracha sauce. Abnormally growing grains can be prismatic, dendritic, or practically any shape in between. When they grow at least an order of magnitude larger than their neighbors in the matrix—a state we call extreme AGG—we can examine the abnormal/matrix interface for clues to the underlying mechanism. Simulating AGG for various formulations of the grain boundary (GB) equation of motion, we show that anisotropies in GB mobility and energy leave a characteristic fingerprint in the abnormal/matrix boundary. Except in the case of prismatic growth, the morphological signature of most reported instances of AGG is consistent with a certain degree of GB mobility variability. Open questions remain, however, concerning the mechanism by which the corresponding growth advantage is established and maintained as the GBs of abnormal grains advance through the matrix.

Grain boundaries in polycrystalline materials migrate to reduce the total excess energy. It has recently been found that the factors governing migration rates of boundaries in bicrystals are insufficient to explain boundary migration in polycrystals. We first review our current understanding of the atomistic mechanisms of grain boundary migration based on simulations and high-resolution transmission electron microscopy observations. We then review our current understanding at the continuum scale based on simulations and observations using high-energy diffraction microscopy. We conclude that detailed comparisons of experimental observations with atomistic simulations of migration in polycrystals (rather than bicrystals) are required to better understand the mechanisms of grain boundary migration, that the driving force for grain boundary migration in polycrystals must include factors other than curvature, and that current simulations of grain growth are insufficient for reproducing experimental observations, possibly because of an inadequate representation of the driving force.

Owing to its unique structure, morphology, and crystal quality, low-dimensional (L-D) ice has attracted increasing attention in recent years. With a size (at least in one dimension) between that of a single water molecule and a snowflake, L-D ice does not only appear as an intermediate state during the dimensional change but can also manifest extraordinary characteristics, from its molecular structures to its physical properties, which offer exciting opportunities for a better understanding and utilization of ice. In this article, we start with a brief introduction to the crystal growth, structure, and typical characterization techniques of ice and then review recent progress in the study of crystal growth, molecular structures, phase morphologies, and physical properties of zero-, one-, and two-dimensional (0-, 1-, and 2D) ice. Extraordinary behaviors of ice in low dimensions and extreme conditions are highlighted. Finally, the future outlook for the physical study and technological applications of L-D ice is briefly discussed.

High-throughput data generation methods and machine learning (ML) algorithms have given rise to a new era of computational materials science by learning the relations between composition, structure, and properties and by exploiting such relations for design. However, to build these connections, materials data must be translated into a numerical form, called a representation, that can be processed by an ML model. Data sets in materials science vary in format (ranging from images to spectra), size, and fidelity. Predictive models vary in scope and properties of interest. Here, we review context-dependent strategies for constructing representations that enable the use of materials as inputs or outputs for ML models. Furthermore, we discuss how modern ML techniques can learn representations from data and transfer chemical and physical information between tasks. Finally, we outline high-impact questions that have not been fully resolved and thus require further investigation.

Most heterogeneous catalytic processes occur between combinations of gases, liquids, and solids at elevated temperatures. They play a critical role for society in energy production, health care, a cleaner environment, industrial products, food, fuel cells, battery technologies, and photocatalysis. Dynamic gas–solid catalyst reactions take place at the atomic level, with active catalyst structures forming, and often also progressively and competitively deactivating, under reaction conditions. There is increasing evidence that single atoms and small clusters of atoms can act as primary active sites in catalytic reactions. Understanding and directing the reactions at the atomic level under controlled operating conditions are crucial for the development of improved materials and processes. We review advances in dynamic in situ microscopy for directly probing heterogeneous catalysis at the atomic level in live action and real time. Benefits include new knowledge and improved management of process fundamentals for greater efficiency and sustainability.