Showing papers in "Mrs Bulletin in 2018"

NOMAD: The FAIR concept for big data-driven materials science

Abstract: Data are a crucial raw material of this century. The amount of data that have been created in materials science thus far and that continues to be created every day is immense. Without a proper infrastructure that allows for collecting and sharing data, the envisioned success of big data-driven materials science will be hampered. For the field of computational materials science, the NOMAD (Novel Materials Discovery) Center of Excellence (CoE) has changed the scientific culture toward comprehensive and findable, accessible, interoperable, and reusable (FAIR) data, opening new avenues for mining materials science big data. Novel data-analytics concepts and tools turn data into knowledge and help in the prediction of new materials and in the identification of new properties of already known materials.

Ferroelectric hafnium oxide for ferroelectric random-access memories and ferroelectric field-effect transistors

Abstract: Ferroelectrics are promising for nonvolatile memories. However, the difficulty of fabricating ferroelectric layers and integrating them into complementary metal oxide semiconductor (CMOS) devices has hindered rapid scaling. Hafnium oxide is a standard material available in CMOS processes. Ferroelectricity in Si-doped hafnia was first reported in 2011, and this has revived interest in using ferroelectric memories for various applications. Ferroelectric hafnia with matured atomic layer deposition techniques is compatible with three-dimensional capacitors and can solve the scaling limitations in 1-transistor-1-capacitor (1T-1C) ferroelectric random-access memories (FeRAMs). For ferroelectric field-effect-transistors (FeFETs), the low permittivity and high coercive field Ec of hafnia ferroelectrics are beneficial. The much higher Ec of ferroelectric hafnia, however, makes high endurance a challenge. This article summarizes the current status of ferroelectricity in hafnia and explains how major issues of 1T-1C FeRAMs and FeFETs can be solved using this material system.

Lead-free piezoceramics: Status and perspectives

Abstract: The field of lead-free piezoceramics, which aims to replace lead zirconate titanate (PZT) and related perovskite materials, has been vibrant for almost 15 years. Once the science in this field attained a certain stage of maturity, materials with properties better than PZT have appeared, and the first products are about to reach the marketplace. This article describes the three most promising lead-free piezoceramics currently under discussion to replace PZT. Each has a pronounced property profile geared for specific applications. Guidelines for directions for fundamental future research on as well as technology transfer to industry of lead-free piezoceramics are provided.

Lead-free piezoelectrics—The environmental and regulatory issues

Abstract: The search for lead-free alternatives to Pb(Zr,Ti)O3 (PZT) piezoelectric ceramics has become a major topic in functional materials research due to legislation in many countries that restricts the use of lead alloys and compounds in commercial products. This article examines both the necessity for regulation and the impacts those regulations have created in the context of piezoelectric materials. It reviews the toxicity of lead, describes current legislation to control the spread of lead in the environment, and attempts to define the risks associated with the manufacture, use, and disposal of lead-based piezoelectric materials. The consequences of current legislation, both intended and unintended, are examined.

Materials for energy harvesting: At the forefront of a new wave

Abstract: The rapid increase and dependency on mobile electronic devices and burgeoning importance of sensor network systems and Internet of Things (IoT) to sustain an aging society indicates the strong need to develop battery-less and mobile power sources. Materials for energy harvesting from environmental sources, including mechanical vibrations, magnetic field, heat, and light have become highly relevant for implementation of the IoT vision that requires self-powered wireless sensor networks for sustainable deployment. The articles in this issue cover piezoelectric materials, magnetoelectrics, and thermoelectrics and provide a summary of state-of-the-art energy-harvesting approaches, various material design strategies being targeted by the community, and fundamental challenges in finding an optimum solution and future roadmap. Flexibility of energy harvesters is also emphasized, given the huge potential for wearables. Photovoltaics are briefly covered with respect to wearables and textiles.

Piezotronics and piezo-phototronics with third-generation semiconductors

Abstract: When uniform strain is applied to noncentrosymmetric semiconductor crystals, which are piezoelectric, static polarization charges are induced at the surface. If the applied strain is not uniform, these charges can even be created inside the crystal. The applied strain affects electronic transport and also photonic processes, and thus can be used to tune the material properties statically or dynamically. As a result, two new fields have emerged, namely piezotronics and piezo-phototronics. This article reviews the history of the two fields and gives a perspective on their applications. The articles in this issue of MRS Bulletin highlight progress in these two fields, and this article places this progress into perspective.

Nano-microstructural control of phonon engineering for thermoelectric energy harvesting

Abstract: Manipulating the thermal conductivity of solids is important for practical applications. Due to the fact that phonons in thermoelectric materials have longer mean free paths (MFPs) than electrons, strengthening phonon scattering to reduce lattice thermal conductivity (κlat) becomes the most straightforward and effective approach to enhance the thermoelectric figure of merit, ZT, which determines the maximum device efficiency. Phonons have a wide range of MFPs in semiconductors, and different dimensions of lattice defects can be targeted to scatter particular phonons with distinct relaxation times. Designing hierarchical nano-microstructures, spanning from point defects to volume defects, would be beneficial to achieve low κlat via a full spectrum of phonon scattering. Herein, we review the formation and underlying mechanisms for lattice defects and highlight the role of all-scale hierarchical nano-microstructure on phonon engineering. Existing challenges in simulations are also discussed.

Relaxor-ferroelectric transitions: Sodium bismuth titanate derivatives

Abstract: Sodium bismuth titanate (NBT) and its solid solutions with other ABO3 perovskites are of great interest for lead-free ferroelectric and piezoelectric applications. In this article, we provide an introduction to the complex structure of NBT, including atomic displacements and nanoscale defects. We also review poling effects and properties as well as NBT-ABO3 phase equilibria. The interesting relaxor properties, frequency dispersion in dielectric permittivity, and field-induced structural phase transitions of these systems are discussed. Finally, we describe other functional, mechanical, and electrical properties of NBT.

Nanoporous metal by dealloying for electrochemical energy conversion and storage

Abstract: Metallic materials are key for electrochemical energy conversion and storage when they are tailored into electrodes designed for rapid reaction kinetics, high electrical conductivities, and high stability. Nanoporous metals formed by dealloying could meet all of these requirements, as the dealloyed products beckon energy researchers with their fascinating structures and outstanding performance. In this article, we discuss the characteristics of dealloyed materials related to their functions in energy devices. We then review nanoporous metal electrodes for applications in fuel cells, supercapacitors, and batteries to provide insights into selection and design criteria for meeting the diverse needs of energy conversion and storage.

Dealloyed nanoporous materials with interface-controlled behavior

Abstract: Dealloying, the selective dissolution of less noble elements from an alloy, enables the preparation of monolithic macroscale bodies, which at the nanostructure level exhibit a network of “ligaments” with a well-defined characteristic size that can be tuned to between a few nanometers and several microns. These porous solids can be made with macroscale dimensions, and, prior to dealloying, can be shaped to form engineered components. Their surface-to-volume ratio is extremely large and their bicontinuous structure provides transport pathways to tune the surface state under control of an electric or chemical potential. These materials present new opportunities for exploring the impact of surfaces on material behaviors and for exploiting surface effects in novel materials design strategies. New experimental approaches unraveling surface effects involving small-scale plasticity and elasticity have been demonstrated. Approaches to new functional materials include electrochemical potential switching of strength, stiffness, fracture resistance, fluid sorption, actuation, and quasi-piezoelectric strain sensing.

Future of dynamic random-access memory as main memory

Abstract: Dynamic random-access memory (DRAM) is the main memory in most current computers. The excellent scalability of DRAM has significantly contributed to the development of modern computers. However, DRAM technology now faces critical challenges associated with further scaling toward the ∼10-nm technology node. This scaling will likely end soon because of the inherent limitations of charge-based memory. Much effort has been dedicated to delaying this. Novel cell architectures have been designed to reduce the cell area, and new materials and process technologies have been extensively investigated, especially for dielectrics and electrodes related to charge storage. In this article, the current issues, recent progress in and the future of DRAM materials, and fabrication technologies are discussed.

Applications of lead-free piezoelectric materials

Abstract: The piezoelectric properties of lead-free ferroelectric materials have been dramatically improved over the past two decades. For some limited applications, their properties have reached the same levels or have even surpassed the properties of the benchmark lead-based material Pb(Zr,Ti)O3 (PZT). Initial commercial lead-free products, including powders, ceramic components, films, and devices (e.g., ultrasonic cleaner, knocking sensor), are now available on the market. Several prototype devices, such as inkjet printheads, ultrasonic motors, angular sensors, and energy harvesters, have been developed. Their overall performance is still inferior to that of PZT-based devices; however, these prototypes and products point the way for future applications. Here, we provide an overview of recent industrial developments in the field and discuss the main advantages and disadvantages of lead-free piezoceramics for individual applications.

Multicaloric materials and effects

Abstract: Multicaloric materials show thermal changes that can be driven simultaneously or sequentially by more than one type of external field. The use of more than one driving field can induce larger thermal changes, with smaller field magnitudes, over wider ranges of operating temperature, and can also eliminate hysteresis in one control parameter by transferring it to another. The thermodynamics behind multicaloric effects is well established, but only a small number of multicaloric materials have been experimentally studied to date. Here, we describe the fundamentals of multicaloric effects and discuss the performance of representative multicaloric materials. Exploiting multicaloric effects could aid the future development of cooling devices, where key challenges include energy efficiency and the span of the operating temperature.

Mechanical response of nanoporous metals: A story of size, surface stress, and severed struts

Abstract: Nanoporous metals made by dealloying are macroscopic network architectures that can contain ∼1015 nanoscale struts or ligaments per sample. Their mechanical performance is critical to their applications as functional or lightweight high-strength materials. Testing nanoporous metals at the macroscopic scale offers opportunities for unraveling the properties of nanoscale solids in general. The central questions in this area include whether the macroscopic strength and elastic modulus of nanoporous metals can be correlated with the properties of nanoscale ligaments by the classical Gibson–Ashby equations, whether the dealloying-made network structure differs from the conventional foam metals, how network connectivity influences mechanical response, and how ligament size and surface properties affect the elastic and plastic response of nanoscale solids and that of nanoporous metals, particularly the tension–compression asymmetry in strength. This article reviews the fundamental observations of the mechanical response of nanoporous metals with a focus on gold and the emerging understanding of the aforementioned issues.

On the topological, morphological, and microstructural characterization of nanoporous metals

Abstract: The structural characterization of dealloyed nanoporous metals is a fundamental and active area of research, needed for the optimization of these structures for catalytic, electrosensing, biomedical, and mechanical functions. The prediction of properties requires identifying and quantifying salient structural characteristics, while insights into the relevant mechanisms of dealloying and coarsening can be achieved through in situ observations of structural evolution. Three-dimensional structural characterization techniques have advanced such that nanoscale quantification of topology, morphology, and crystallographic parameters are achievable, yet the field is new enough that the assessment and comparison of such parameters of different nanoporous metals are just beginning. Here, we explore the state of the art in structural characterization, focusing on nanoporous gold to exemplify the challenges, the achievements, and the potential associated with establishing an appropriate set of structural parameters for this unique class of materials.

Architectural design and fabrication approaches for solid-state batteries

Abstract: Solid-state batteries are promising candidates for energy storage due to their potential advantages in safety, working temperature range, and energy density compared to traditional liquid-electrolyte-based batteries. Rational battery architecture design and a scalable fabrication approach are critical to realize solid-state batteries. In this article, we present the architecture, fabrication procedure, and related challenges of sulfide and oxide electrolyte-based solid-state batteries. Approaches toward intimate solid−solid contact, thin solid-electrolyte fabrication, and scale-up production are discussed. Finally, we discuss the future research directions of solid-state batteries.

Pattern formation during electrochemical and liquid metal dealloying

Abstract: Dealloying has evolved from a problematic corrosion process to a versatile tool for scalable fabrication of nanostructured metals. While the original, and majority of, work in the area has focused on electrochemical dealloying, a powerful variation of dealloying—liquid metal dealloying—has recently gained popularity. This process relies on a melt to carry out selective dissolution, replacing the traditional electrolyte solution. While electrolytes and molten metals are both suitable dealloying media, they can lead to very different morphologies. In this article, we compare and contrast what is known about the microscale physics and chemistry controlling microstructural evolution in electrochemical and liquid metal dealloying. We conclude that the core phenomenology of porosity evolution—a competition between dissolution and interface diffusion—is similar in both dealloying processes, but that the relative magnitudes of these two processes control interfacial pattern formation.

Shifting the phase boundary: Potassium sodium niobate derivates

Abstract: The focus on piezoelectric ceramics based on the potassium sodium niobate system began in 2004. After years of dedicated research, these materials can be considered one of the most promising lead-free piezoceramics with comprehensive performance. While their structure–property relationships are still not completely understood, the thermal stability issue is partly resolved, which leaves further room for phase-boundary engineering. Technological advancement has recently focused on using base metals as inner electrodes for multilayer actuators, which provides cost benefits as compared to lead zirconate titanate devices. The remaining challenges, however, such as poor sinterability and weak reproducibility of functional properties, still hinder extensive applications of these materials.

Nanoporous metals by alloy corrosion: Bioanalytical and biomedical applications.

Abstract: Nanoporous metals obtained by dealloying have attracted significant attention for their unusual catalytic properties, and as model materials for fundamental studies of structure–property relationships in a variety of research areas. There has been a recent surge in the use of these metals for biomedical and bioanalytical applications, where many exciting opportunities exist. The goal of this article is to provide a review of recent progress in using nanoporous metals for biological applications, including as biosensors for detecting biomarkers of disease and multifunctional neural interfaces for monitoring and modulating the activity of neural tissue. The article emphasizes the unique properties of nanoporous gold and concludes by discussing its utility in addressing important challenges in biomedical devices.

Theory of piezotronics and piezo-phototronics

Abstract: Piezotronic and piezo-phototronic devices exhibit high performance and have potential applications especially in next-generation self-powered, flexible electronics and wearable systems. In these devices, a strain-induced piezoelectric field at a junction, contact, or interface can significantly modulate the carrier generation, recombination, and transport properties. This mechanism has been studied based on the theory of piezotronics and piezo-phototronics. Simulation-driven materials design and device improvements have been greatly propelled by the finite element method, density functional theory, and molecular dynamics for achieving high-performance devices. Dynamical piezoelectric fields can also control new quantum states in quantum materials, such as in topological insulators, which pave a new path for enhancing performance and for investigating the fundamental physics of quantum piezotronics and piezo-phototronics.

Caloric effects in ferroic materials

Abstract: The fundamentals and applications of ferroic materials—ferromagnetic, ferroelectric, and ferroelastic—are common subjects discussed in just about every graduate course related to functional materials. Looking beyond today’s traditional uses, such as in permanent magnets, capacitors, and shape-memory alloys, there are worthwhile and interesting questions common to the caloric properties of these ferroic materials. Can ferroic materials be used in a cooling cycle? Why are these materials susceptible to external fields? Which combination of properties is required to make some of them suitable for efficient cooling and heat pumping? We address these questions in this introduction to ferroic cooling, which comprises magnetocaloric, electrocaloric, elastocaloric and barocaloric approaches and combinations thereof (i.e., multicalorics). These are addressed in greater detail in the articles in this issue.

Polymer and composite electrolytes

Abstract: Solid inorganic and polymeric electrolytes have the potential to enable rechargeable batteries with higher energy densities, compared to current lithium-ion technology, which uses liquid electrolyte. Inorganic materials such as ceramics and glasses conduct lithium ions well, but they are brittle, which makes incorporation into a battery difficult. Polymers have the flexibility for facile use in a battery, but their transport properties tend to be inferior to inorganics. Thus, there is growing interest in composite electrolytes with inorganic and organic phases in intimate contact. This article begins with a discussion of ion transport in single-phase electrolytes. A dimensionless number (the Newman number) is presented for quantifying the efficacy of electrolytes. An effective medium framework for predicting transport properties of composite electrolytes containing only one conducting phase is then presented. The opportunities and challenges presented by composite electrolytes containing two conducting phases are addressed. Finally, the importance and status of reaction kinetics at the interfaces between solid electrolytes and electrodes are covered, using a lithium-metal electrode as an example.

Predictive modeling and design rules for solid electrolytes

Abstract: All-solid-state batteries utilizing a ceramic instead of an organic liquid as an electrolyte have the potential to be safer and more energy dense than traditional rechargeable lithium-ion batteries. This emergent energy-storage technology, however, is still critically limited by the performance of the solid electrolyte and its interface with electrodes. Here, we present a review of recent efforts in predictive modeling and materials design for lithium and sodium solid electrolytes using advanced computational approaches. These approaches have enabled the efficient design and discovery of new functional materials with desired properties, such as high alkali ionic conductivity, good phase and electrochemical stability, and low cost, accelerating the development of all-solid-state alkali batteries.

Overcoming fatigue through compression for advanced elastocaloric cooling

Abstract: Elastocaloric materials exhibit extraordinary cooling potential, but the repetition of cyclic mechanical loadings during long-term operation of cooling systems requires the refrigerant material to have long fatigue life. This article reviews the fundamental cause of fatigue from aspects of initiation and propagation of fatigue cracks in shape-memory alloys (SMAs) that are used as elastocaloric materials, and highlights recent advances in using compression to overcome fatigue by curtailing the generation of surfaces associated with crack propagation. Compression is identified as a key means to extend fatigue lifetime in engineering design of elastocaloric cooling drive mechanisms. We summarize the state-of-the-art performance of different SMAs as elastocaloric materials and discuss the influence of low cyclic strains and high resistance to transformation. We present integration of compression-based material assemblies into a cooling system prototype and optimization of the system efficiency using work recovery and related measures.

Magnetoelectric magnetic field sensors

Abstract: Highly sensitive magnetic field sensors using magnetoelectric (ME) bulk and thin-film composites consisting of magnetostrictive and piezoelectric phases are discussed. Examples include PZT (Pb(ZrxTi1–x)O3) fibers and AlN as the piezoelectric component and amorphous magnetostrictive material, respectively, or their multilayers. Additionally, self-organized ME composites are discussed. These ME sensors offer a passive (consuming little to no power) nature, high sensitivities, large effect enhancements at mechanical resonances, and large linear dynamic ranges. At mechanical resonance, limits of detection in the fT/Hz1/2 range can be achieved. Below the mechanical resonance frequency, the sensitivity can be enhanced through frequency conversion using alternating current magnetic or electric fields or by using magnetic field-induced changes of the elastic properties, the delta-E effect, where E represents Young’s modulus. Noise floors of about 1–100 pT/Hz1/2 at a frequency of f = 1 Hz can be obtained depending on the sensor size and the operational mode. For applications in unshielded environments, approaches to suppress acoustic and vibrational cross-sensitivities are presented.

Magnetocaloric materials for refrigeration near room temperature

Abstract: This article overviews the current status of magnetocaloric materials for room-temperature refrigeration. We discuss the underlying mechanism of the magnetocaloric effect and illustrate differences between first- and second-order type materials starting with gadolinium as a reference system. Beyond the key functional properties of magnetocaloric materials, the adiabatic temperature, and entropy change, we briefly address the criticality of the most promising materials in terms of their supply risk. Looking at practical applications, suitable geometries and processing routes for magnetocaloric heat exchangers for device implementation are introduced.

Materials for heat-assisted magnetic recording

Abstract: Increasing the density of data storage is crucial to the future of inexpensive digital technology. The large majority of storage in the “Cloud” consists of magnetic hard-disk drives. The continued evolution of this USD$30 billion industry depends on the commercial introduction of heat-assisted magnetic recording (HAMR). This technology uses heat from a laser beam confined well below the diffraction limit, <50-nm wide, to write to media near 450°C with high magnetic anisotropy that would normally be unwriteable under available magnetic fields. This high anisotropy guarantees thermal stability even for grain sizes around 5-nm diameter, which are necessary for major increases in storage density. In this article and in the articles in this issue, we introduce HAMR requirements and discuss its numerous interdisciplinary materials challenges, including high-temperature/efficient plasmonic materials, low-loss optical materials, highly ordered/thermally anisotropic nanoscale magnetic grains, block copolymers for directed assembly below 10 nm, high-temperature nanometer-thick coatings/lubricants, materials/interfaces to control heat flow at nanometer length scales, and advanced spintronic materials.

Quantum materials for thermoelectricity

Abstract: Research in thermoelectric (TE) quantum structures was greatly propelled by the prediction in the early 1990s of a significant boost in TE efficiency by quantum size effects. Recently, research interest has shifted from quantum size effects in conventional semiconductors toward new types of quantum materials (e.g., topological insulators [TIs], Weyl and Dirac semimetals) characterized by their nontrivial electronic topology. Bi2Te3, Sb2Te3, and Bi2Se3, established TE materials, are also TIs exhibiting a bulk bandgap and highly conductive and robust gapless surface states. The signature of the nontrivial electronic band structure on TE transport properties can be best verified in transport experiments using nanowires and thin films. However, even in nanograined bulk, the typical peculiarities in the transport properties of TIs can be seen. Finally, the remarkable transport properties of Dirac and Weyl semimetals are discussed.

Wearable and flexible thermoelectrics for energy harvesting

Abstract: Conjugated polymers have emerged as potential candidates for thermal-energy harvesting. Their flexible and lightweight nature, as well as scalable processing, make them geometrically versatile for a large variety of applications, including powering wearable electronics that are not available for traditional inorganic materials. However, the long-range structural disorder greatly hinders their electrical conduction, and this far outweighs the induced low thermal conductivity; therefore, the thermoelectric performance needs to be significantly improved to fulfill the requirements of efficient devices. Composites and hybrid thermoelectric materials have been developed to capitalize on the individual strengths of conducting polymers and other components, including carbon nanotubes, graphene, and inorganic nanomaterials. In this article, we present recent advances in conjugated polymers, the associated hybrid thermoelectric composites, and the latest breakthroughs in the development of inorganic–organic hybrid superlattices.

Dual-stimulus magnetoelectric energy harvesting

Abstract: Harvesting energy from otherwise wasted resources has been intensively investigated as a promising technology especially for enabling the deployment of autonomous wireless-sensor networks for the Internet of Things. Multi-stimulus energy harvesting, simultaneously from different energy sources, provides an attractive opportunity to amplify the power density of harvesters, thereby extending their potential for self-powered devices. In this article, we review recent and ongoing research efforts aimed at enhancing the energy-harvesting performance of magnetoelectric (ME) composite harvesters employing dual stimuli, mechanical vibrations, and magnetic fields. After a brief introduction to vibration, magnetic field, and dual-mode energy harvesting, we survey the key materials utilized for ME energy harvesting. We then focus on progress in this area and discuss relevant ideas to realize electromechanical and magnetoelectric coupling for harvesting energy from mechanical vibrations and magnetic fields simultaneously. We provide perspectives and future directions as well.