bluuebunny/crossref_metadata_2025 huggingface dataset:
Challenges in Materials Research for Sustainable Nuclear Energy
Authors: Raj Baldev , Vijayalakshmi M. , Rao P.R. Vasudeva , Rao K.B.S.
Published: February 2011

Global energy demand is expected to increase steeply, creating an urgent need to evolve a judicious global energy policy, exploiting the potential of all available energy resources, including nuclear energy. With increasing awareness of environmental issues, nuclear energy is expected to play an important role on the energy scenario in the coming decades. The immediate thrust in the science and technology of nuclear materials is to realize a robust reactor technology with associated fuel cycle and ensure the cost competitiveness of nuclear power and to extend the service life of reactors to 100 years. Accordingly, the present-generation materials need to be modified to meet the demands of prolonged exposure to irradiation and extended service life for the reactor. Emerging nuclear systems incorporate features to ensure environmental friendliness, effective waste management, enhanced safety, and proliferation resistance and require development of high-temperature materials and the associated technologies. Fusion, on a longer horizon of about fve decades, also requires the development of a new spectrum of materials. The development of next-generation materials technology is expected to occur in short times and is likely to be further accelerated by strong international collaborations.
Nuclear Fuel Cycle: Environmental Impact
Authors: Ewing Rodney C.
Published: February 2011

Every energy source has environmental impacts—positive and negative. Nuclear power is a carbon-free source of energy that can reduce CO2 emissions by displacing the use of fossil fuels. The present level of carbon displacement is approximately 0.5 gigatonnes of carbon per year (GtC/year), compared to the nearly 8 GtC/year emitted by the use of fossil fuels. However, there are three major negative environmental impacts of nuclear power: catastrophic accidents, nuclear weapons, and nuclear waste. The last two, weapons and waste, are directly tied to the type of nuclear fuel cycle (Figure 4 in the main nuclear article by Raj et al. in this issue). The different fuel cycles refect different strategies for the utilization of fssile nuclides, mainly 235U and 239Pu, and these different strategies have important implications for nuclear waste management and nuclear weapons proliferation.
Preventing Nuclear Weapon Proliferation as Nuclear Power Expands
Authors: Hecker Siegfried S.
Published: February 2011

Raj et al. describe the promise of nuclear energy as a sustainable, affordable, and carbon-free source available this century on a scale that can help meet the world's growing need for energy and help slow the pace of global climate change. However, the factor of millions gain in energy release from nuclear fssion compared to all conventional energy sources that tap the energy of electrons (Figure 1) has also been used to create explosives of unprecedented lethality and, hence, poses a serious challenge to the expansion of nuclear energy worldwide. Although the end of the cold war has eliminated the threat of annihilating humanity, the likelihood of a devastating nuclear attack has increased as more nations, subnational groups, and terrorists seek to acquire nuclear weapons.
Another Pathway to Large-Scale Power Generation: Concentrating Solar Power
Authors: Mehos Mark
Published: February 2011

Photovoltaics is not the only means of using sunlight to generate electricity. Another major solar technology is called “concentrating solar power” or CSP. CSP technologies use concentrating optics to generate high temperatures that are used to drive conventional steam or gas turbines. CSP is generally considered a central generation technology, rather than a source of distributed generation. That is, a large amount of power is generated in one location, with transmission and distribution to the various points of use, rather than generating small amounts of the power at numerous points of use. Because of this feature, CSP is predominantly a utility-scale source of power.
Thermoelectrics: Direct Solar Thermal Energy Conversion
Authors: Tritt Terry M. , Böttner Harald , Chen Lidong
Published: February 2011

The field of thermoelectricity began in the early 1800s with the discovery of the thermoelectric effect by Thomas Seebeck. Seebeck found that, when the junctions of two dissimilar materials are held at different temperatures (ΔT), a voltage (V) is generated that is proportional to ΔT. The proportionality constant is the Seebeck coeffcient or thermopower: α = −δV/ΔT. When the circuit is closed, this couple allows for direct conversion of thermal energy (heat) to electrical energy. The conversion effciency, ηTE, is related to a quantity called the fgure of merit, ZT, that is determined by three main material parameters: the thermopower α, the electrical resistivity ρ, and the thermal conductivity κ.
Cellulosic Ethanol: A Unique Sustainable Liquid Transportation Fuel
Authors: Wyman Charles E.
Published: February 2011

Although ethanol is now made from the sugars in the starch fraction of corn and other crops and from the sugar in sugarcane, a much greater impact for ethanol in terms of fuel use could be realized if the sugars from more recalcitrant cellulosic biomass could be converted to ethanol. Cellulosic biomass is the structural portion of plants and includes agricultural (e.g., corn stover, which is all of the above-ground portion of the corn plant, excluding the grain) and forestry (e.g., sawdust) residues, major fractions of municipal solid waste (e.g., waste paper and yard waste), and herbaceous (e.g., switchgrass) and woody (e.g., poplar) crops grown as energy resources. Although distinctive in outward appearance, these materials all comprise about 40–50% cellulose and 20–30% hemicellulose, with lesser amounts of lignin and other compounds such as sugars, oils, and minerals. Cellulose is a polymer of glucose sugar molecules that are physically linked together in a crystalline structure to provide structural support for plants. Hemicellulose is also made up of sugars covalently joined together in long chains, but it generally includes fve different sugars: arabinose, galactose, glucose, mannose, and xylose. In addition, hemicellulose is an amorphous, branched material. Lignin is a phenylpropene compound that can be viewed as a low-sulfur, immature coal.
Advancing Materials and Technologies for Water Purification
Authors: Shannon Mark A. , Semiat Raphael
Published: February 2011

Worldwide, 1.2 billion people lack access to sufficient amounts of clean water, and 2.6 billion lack adequate sanitation. Also, industry relies on large quantities of water during manufacturing, which is then returned to the environment. Having adequate water supplies, and removing pathogens, chemicals, and other contaminants with high throughput at a low cost is a growing challenge around the world. This issue of MRS Bulletin examines how materials research, through the development of membranes, catalysts, nanoparticles, and other materials, is addressing these needs.
Micropower Materials Development for Wireless Sensor Networks
Authors: Steingart Dan , Roundy Shad , Wright Paul K. , Evans James W.
Published: February 2011

Subcentimeter wireless computers capable of interfacing physically with their environment and communicating with each other have progressed from concept to commercial reality in the past decade. Wireless sensor nodes are an exciting technology, as they provide a backbone to measure almost any quantity in a spatially disperse way, allowing time-synchronized correlations over meters or miles. Before these devices can be deployed to monitor and protect environments (such as grid power distribution systems, buildings, factories, or even the human body) for long periods of time, they need a power source. Environmental generation looks to be a promising method.
Materials Challenges Facing Electrical Energy Storage
Authors: Whittingham M. Stanley
Published: February 2011

During the past two decades, the demand for the storage of electrical energy has mushroomed both for portable applications and for static applications. As storage and power demands have increased predominantly in the form of batteries, the system has evolved. However, the present electrochemical systems are too costly to penetrate major new markets, still higher performance is required, and environmentally acceptable materials are preferred. These limitations can be overcome only by major advances in new materials whose constituent elements must be available in large quantities in nature; nanomaterials appear to have a key role to play. New cathode materials with higher storage capacity are needed, as well as safer and lower cost anodes and stable electrolyte systems. Flywheels and pumped hydropower also have niche roles to play.
Environmental Performance Enters Construction Materials
Authors: Bonfield Peter
Published: February 2011

The environmental sustainability of materials used in construction applications is driving a requirement for the quanti-fcation of performance attributes of such materials. For example, the European Union (EU) Energy Performance in Buildings Directive will give commercial buildings an energy rating when rented or sold. The Code for Sustainable Homes launched by the U.K. Government's Department for Communities and Local Government (CLG) in January 2007 sets out the requirement for all new homes to be carbonneutral by 2016. In addition, homes in the United Kingdom will need to signifcantly reduce water consumption from today's average 160 liters (1) per person per day to less than 801 per person per day. Similarly stringent targets are required for waste, materials, and other factors. Such environmental and energy standards are complementing characteristics such as strength, stiffness, durability, impact, cost, and expected life with factors such as “environmental profle,” “ecopoints” (a single unit measurement of environmental impact arising from a product throughout its lifecycle that is used in the United Kingdom), “carbon footprint” (amount of CO2 produced for the lifecycle of the item), “recycled content,” and “chain of custody” (a legal term that refers to the ability to guarantee the identity and integrity of a specimen from collection through to reporting of test results).
Environmental Degradation of Materials in Advanced Reactors
Authors: Cabet C. , Jang J. , Konys J. , Tortorelli P. F.
Published: February 2011

Advanced fission-based reactors challenge our ability to fully understand environment–materials reactions in terms of fundamental stability and kinetics, including the influences of composition, microstructure, and system design, and to predict associated long-term performance. This article briefly describes corrosion reactions and the processes by which such are managed for several elevated-temperature environments associated with advanced reactor concepts: helium, molten Pb–Bi, fluorides, and supercritical water. For most of the subject environments, corrosion resistance critically depends on the ability to form and maintain protective surface layers. Effects of corrosion on mechanical behavior can be from thermally and chemically induced changes in microstructures or from environmental effects on cracking susceptibility. In most cases, the simultaneous effects of chemical reactivity and radiation have not been fully addressed, nor has much attention been paid to newly emerging alloy compositions or the effects of substantially increased operating temperatures.
Fuels for Advanced Nuclear Energy Systems
Authors: Petti D. , Crawford D. , Chauvin N.
Published: February 2011

Fuels for advanced nuclear reactors differ from conventional light water reactor fuels and also vary widely because of the specific architectures and intended missions of the reactor systems proposed to deploy them. Functional requirements of all fuel designs for advanced nuclear energy systems include (1) retention of fission products and fuel nuclides, (2) dimensional stability, and (3) maintenance of a geometry that can be cooled. In all cases, anticipated fuel performance is the limiting factor in reactor system design, and cumulative effects of increased utilization and increased exposure to inservice environments degrade fuel performance. In this article, the current status of each fuel system is reviewed, and technical challenges confronting the implementation of each fuel in the context of the entire advanced reactor fuel cycle (fabrication, reactor performance, recycle) are discussed.
Nanopolymeric Therapeutics
Authors: Tong Rong , Christian David A. , Tang Li , Cabral Horacio , Baker James R. , Kataoka Kazunori , Discher Dennis E. , Cheng Jianjun
Published: February 2011

Polymers have been extensively utilized in the design of nanometer-sized delivery vehicles of chemotherapeutics for clinical cancer therapy. Polymeric nanoparticulate delivery vehicles, with chemotherapeutics being either conjugated or encapsulated, have been developed into a variety of different architectures, including polymer-drug conjugates with linear or branched polymers, micelles, and polymersomes. This review describes the progress that has been made in the field of polymeric nanomedicine that brings the science closer to clinical realization of nanopolymeric therapeutics for its application in cancer treatment.
Magnetic Nanoparticles for Early Detection of Cancer by Magnetic Resonance Imaging
Authors: Lin Wenbin , Hyeon Taeghwan , Lanza Gregory M. , Zhang Miqin , Meade Thomas J.
Published: February 2011

This article provides a brief overview of recent progress in the synthesis and functionalization of magnetic nanoparticles and their applications in the early detection of malignant tumors by magnetic resonance imaging (MRI). The intrinsic low sensitivity of MRI necessitates the use of large quantities of exogenous contrast agents in many imaging studies. Magnetic nanoparticles have recently emerged as highly efficient MRI contrast agents because these nanometer-scale materials can carry high payloads while maintaining the ability to move through physiological systems. Superparamagnetic ferrite nanoparticles (such as iron oxide) provide excellent negative contrast enhancement. Recent refinement of synthetic methodologies has led to ferrite nanoparticles with narrow size distributions and high crystallinity. Target-specific tumor imaging becomes possible through functionalization of ferrite nanoparticles with targeting agents to allow for site-specific accumulation. Nanoparticulate contrast agents capable of positive contrast enhancement have recently been developed in order to overcome the drawbacks of negative contrast enhancement afforded by ferrite nanoparticles. These newly developed magnetic nanoparticles have the potential to enable physicians to diagnose cancer at the earliest stage possible and thus can have an enormous impact on more effective cancer treatment.
Cantilever Sensors: Nanomechanical Tools for Diagnostics
Authors: Datar Ram , Kim Seonghwan , Jeon Sangmin , Hesketh Peter , Manalis Scott , Boisen Anja , Thundat Thomas
Published: February 2011

Cantilever sensors have attracted considerable attention over the last decade because of their potential as a highly sensitive sensor platform for high throughput and multiplexed detection of proteins and nucleic acids. A micromachined cantilever platform integrates nanoscale science and microfabrication technology for the label-free detection of biological molecules, allowing miniaturization. Molecular adsorption, when restricted to a single side of a deformable cantilever beam, results in measurable bending of the cantilever. This nanoscale deflection is caused by a variation in the cantilever surface stress due to biomolecular interactions and can be measured by optical or electrical means, thereby reporting on the presence of biomolecules. Biological specificity in detection is typically achieved by immobilizing selective receptors or probe molecules on one side of the cantilever using surface functionalization processes. When target molecules are injected into the fluid bathing the cantilever, the cantilever bends as a function of the number of molecules bound to the probe molecules on its surface. Mass-produced, miniature silicon and silicon nitride microcantilever arrays offer a clear path to the development of miniature sensors with unprecedented sensitivity for biodetection applications, such as toxin detection, DNA hybridization, and selective detection of pathogens through immunological techniques. This article discusses applications of cantilever sensors in cancer diagnosis.
Nanoscience and Nanotechnology: The Key to New Studies in Areas of Science Outside of Nanoscience and Nanotechnology
Authors: Gleiter Herbert
Published: February 2011

In recent years, a new branch of nanoscience/nanotechnology seems to be emerging. This branch is characterized by the application of preparation methods and/or the diagnostic tools developed in nanoscience/nanotechnology in order to perform either new, decisive experiments or to open the way to novel applications in areas of science that were originally not related to nanoscience/nanotechnology, such as cancer research or quantum physics. In order to highlight the diversity of this new branch, we shall discuss the following four areas in which methods of nanoscience/nanotechnology are applied to other areas of science: (1) cancer therapy, (2) cellular labeling, (3) the synthesis of solid materials with tunable atomic structures, and (4) the new opportunities provided by nanoscience/nanotechnology to probe the limits of quantum physics, one of the classical problems of physics.
Surface Defects and Passivation of Ge and III–V Interfaces
Authors: Houssa Michel , Chagarov Evgueni , Kummel Andrew
Published: February 2011

The need for high-κ gate dielectrics and metal gates in advanced integrated circuits has reopened the door to Ge and III–V compounds as potential replacements for silicon channels, offering the possibility to further increase the performances of complementary metal oxide semiconductor (CMOS) circuits, as well as adding new functionalities. Yet, a fundamental issue related to high-mobility channels in CMOS circuits is the electrical passivation of their interfaces (i.e., achieving a low density of interface defects) approaching state-of-the-art Si-based devices. Here we discuss promising approaches for the passivation of Ge and III–V compounds and highlight insights obtained by combining experimental characterization techniques with first-principles simulations.
Source and Drain Contacts for Germanium and III–V FETs for Digital Logic
Authors: Dimoulas Athanasios , Toriumi Akira , Mohney Suzanne E.
Published: February 2011

The scaling of transistors to smaller dimensions and the exploration of devices with III–V and Ge channels for digital logic places serious demands on the ohmic contacts used in these devices. Contacts with extremely low specific contact resistances are required to take full advantage of the performance promised by alternative semiconductor materials. In addition, device processes and contact morphologies must be compatible with the geometry and feature sizes of the transistors. In this article, we begin by reviewing what is known about contacts to Ge, InGaAs, InAs, and InSb, including the role of Fermi level pinning on the Schottky barrier that is often formed at the metal/semiconductor interface and common strategies for forming ohmic contacts. Then we turn our attention to the additional challenges faced when preparing ohmic contacts for the many types of field-effect transistors now under development for Ge and III–V complementary field-effect transistor technology.
On the Genesis of Nuclei and Phase Separation on an Atomic Scale
Authors: Seidman David N.
Published: February 2011

Turnbull performed much experimental and theoretical research on nucleation and on fundamental aspects of phase transformations in condensed matter. Nucleation of precipitates followed in parallel or sequentially by their growth and coarsening is a complex scientific subject and technologically highly relevant because of the many critical roles played by structural metallic alloys and their relevance to the energy problem. The focus herein is on nucleation, growth, and coarsening of precipitates employing atom-probe tomography, lattice kinetic Monte Carlo simulations, and diffusion theory, which constitute a unique approach for studying phase separation in concentrated multicomponent alloys.
Hard Materials with Tunable Porosity
Authors: Erlebacher Jonah , Seshadri Ram
Published: February 2011

Porous metals and ceramic materials are of critical importance in catalysis, sensing, and adsorption technologies and exhibit unusual mechanical, magnetic, electrical, and optical properties compared to nonporous bulk materials. Materials with nanoscale porosity often are formed through molecular self-assembly processes that lock in a particular length scale; consider, for instance, the assembly of crystalline mesoporous zeolites with a pore size of 2–50 nm or the evolution of structural domains in block copolymers. Of recent interest has been the identification of general kinetic pattern-forming principles that underlie the formation of mesoporous materials without a locked- in length scale. When materials are kinetically locked out of thermodynamic equilibrium, temperature or chemistry can be used as a “knob” to tune their microstructure and properties. In this issue of the MRS Bulletin, we explore new porous metal and ceramic materials, which we collectively refer to as “hard” materials, formed by pattern-forming instabilities, either in the bulk or at interfaces, and discuss how such nonequilibrium processing can be used to tune porosity and properties. The focus on hard materials here involves thermal, chemical, and electrochemical processing usually not compatible with soft (for example, polymeric) porous materials and generally adds to the rich variety of routes to fabricate porous materials.
Materials at 200 mph: Making NASCAR Faster and Safer
Authors: Leslie-Pelecky Diandra
Published: February 2011

Speed is the ultimate goal of racing, and materials are an increasingly important area of research for making race cars faster. The splitter, which produces front downforce, is made from Tegris, a polypropylene composite offering comparable stiffness and improved impact properties at significantly lower cost than alternative materials. Engine blocks must be cast iron, but careful control of microstructure using precision manufacturing methods produces a lighter engine block that generates more horsepower.

Speed and excitement must be balanced with safety, and materials are key players here, as well. Energy-dissipating foams in the car and the barriers surrounding the tracks allow drivers to walk away uninjured from accidents. Fire-resistant polymers protect drivers from high-temperature fuel fires, and technology transfer from the National Aeronautics and Space Administration (NASA) to the National Association for Stock Car Auto Racing (NASCAR) in the form of a low-temperature carbon monoxide catalyst filters the drivers' air.

Sports are an outstanding way of showing the public how materials science and engineering are relevant to their lives and interests. Materials science and engineering is just that much more exciting when it's traveling at two hundred miles an hour.
Piezoelectric Thin Films for Sensors, Actuators, and Energy Harvesting
Authors: Muralt P. , Polcawich R. G. , Trolier-McKinstry S.
Published: February 2011

Piezoelectric microelectromechanical systems (MEMS) offer the opportunity for high-sensitivity sensors and large displacement, low-voltage actuators. In particular, recent advances in the deposition of perovskite thin films point to a generation of MEMS devices capable of large displacements at complementary metal oxide semiconductor-compatible voltage levels. Moreover, if the devices are mounted in mechanically noisy environments, they also can be used for energy harvesting. Key to all of these applications is the ability to obtain high piezoelectric coefficients and retain these coefficients throughout the microfabrication process. This article will review the impact of composition, orientation, and microstructure on the piezoelectric properties of perovskite thin films such as PbZr1−xTixO3(PZT). Superior piezoelectric coefficients (e31, fof −18 C/m2) are achieved in {001}-oriented PbZr0.52Ti0.48O3films with improved compositional homogeneity on Si substrates. The advent of such high piezoelectric responses in films opens up a wide variety of possible applications. A few examples of these, including low-voltage radio frequency MEMS switches and resonators, actuators for millimeter-scale robotics, droplet ejectors, energy scavengers for unattended sensors, and medical imaging transducers, will be discussed.
Molecular, Supramolecular, and Macromolecular Motors and Artificial Muscles
Authors: Li Dongbo , Paxton Walter F. , Baughman Ray H. , Huang Tony Jun , Stoddart J. Fraser , Weiss Paul S.
Published: February 2011

Recent developments in chemical synthesis, nanoscale assembly, and molecular-scale measurements enable the extension of the concept of macroscopic machines to the molecular and supramolecular levels. Molecular machines are capable of performing mechanical movements in response to external stimuli. They offer the potential to couple electrical or other forms of energy to mechanical action at the nano- and molecular scales. Working hierarchically and in concert, they can form actuators referred to as artificial muscles, in analogy to biological systems. We describe the principles behind driven motion and assembly at the molecular scale and recent advances in the field of molecular-level electromechanical machines, molecular motors, and artificial muscles. We discuss the challenges and successes in making these assemblies work cooperatively to function at larger scales.
Reticular Chemistry and Metal-Organic Frameworks for Clean Energy
Authors: Yaghi Omar M. , Li Qiaowei
Published: February 2011

Reticular chemistry concerns the linking of molecular building blocks into predetermined structures using strong bonds. We have been working on creating and developing the conceptual and practical basis of this new area of research. As a result, new classes of crystalline porous materials have been designed and synthesized: metal-organic frameworks, zeolitic imidazolate frameworks, and covalent organic frameworks. Crystals of this type have exceptional surface areas (2,000−6,000 m2/g) and take up voluminous amounts of hydrogen (7.5 wt% at 77 K and 3−4 × 106 Pa), methane (50 wt% at 298 K and 2.5 × 106 Pa), and carbon dioxide (140 wt% at 298 K and 3 × 106 Pa). We have driven the basic science all the way to applications without losing sight of our quest for understanding the underlying molecular aspects of this chemistry. The presentation was focused on the design concepts, synthesis, and structure of these materials, with emphasis on their applications to onboard energy storage.
An Atom-Probe Tomography Primer
Authors: Seidman David N. , Stiller Krystyna
Published: February 2011

Atom-probe tomography (APT) is in the midst of a dynamic renaissance as a result of the development of well-engineered commercial instruments that are both robust and ergonomic and capable of collecting large data sets, hundreds of millions of atoms, in short time periods compared to their predecessor instruments. An APT setup involves a field-ion microscope coupled directly to a special time-of-flight (TOF) mass spectrometer that permits one to determine the mass-to-charge states of individual field-evaporated ions plus theirx-,y-, andz-coordinates in a specimen in direct space with subnanoscale resolution. The three-dimensional (3D) data sets acquired are analyzed using increasingly sophisticated software programs that utilize high-end workstations, which permit one to handle continuously increasing large data sets. APT has the unique ability to dissect a lattice, with subnanometer-scale spatial resolution, using either voltage or laser pulses, on an atom-by-atom and atomic plane-by-plane basis and to reconstruct it in 3D with the chemical identity of each detected atom identified by TOF mass spectrometry. Employing pico- or femtosecond laser pulses using visible (green or blue light) to ultraviolet light makes the analysis of metallic, semiconducting, ceramic, and organic materials practical to different degrees of success. The utilization of dual-beam focused ion-beam microscopy for the preparation of microtip specimens from multilayer and surface films, semiconductor devices, and for producing site-specific specimens greatly extends the capabilities of APT to a wider range of scientific and engineering problems than could previously be studied for a wide range of materials: metals, semiconductors, ceramics, biominerals, and organic materials.
Solid-State Ionics in the 21st Century: Current Status and Future Prospects
Authors: Kim Sangtae , Yamaguchi Shu , Elliott James A.
Published: February 2011

The phenomenon of ion migration in solids forms the basis for a wide variety of electrochemical applications, ranging from power generators and chemical sensors to ionic switches. Solid-state ionics (SSI) is the field of research concerning ionic motions in solids and the materials properties associated with them. Owing to the ever-growing technological importance of electrochemical devices, together with the discoveries of various solids displaying superior ionic conductivity at relatively low temperatures, research activities in this field have grown rapidly since the 1960s, culminating in “nanoionics”: the area of SSI concerned with nanometer-scale systems. This theme issue introduces key research issues that we believe are, and will remain, the main research topics in nanoionics and SSI during the 21st century. These include the application of cutting-edge experimental techniques, such as secondary ion mass spectroscopy and nuclear magnetic resonance, to investigate ionic diffusion in both bulk solids and at interfaces, as well as the use of atomic-scale modeling as a virtual probe of ionic conduction mechanisms and defect interactions. We highlight the effects of protonic conduction at the nanometer scale and how better control of interfaces can be employed to make secondary lithium batteries based on nanoionics principles. Finally, in addition to power generation and storage, the emergence of atomic switches based on cation diffusion shows great promise in developing next-generation transistors using SSI.
Ion Dynamics at Interfaces: Nuclear Magnetic Resonance Studies
Authors: Heitjans Paul , Wilkening Martin
Published: February 2011

Interface engineering and the study of diffusion and transport processes through and along interfacial regions play important roles in materials science and energy research. For the latter, nanostructured materials are increasingly considered to act as powerful electrodes and solid electrolytes in sustainable energy systems, such as Li ion batteries. This is due to reduced diffusion lengths achieved when going to the nanometer scale and the fact that nanocrystalline materials with an average particle size of less than about 50 nm often show an enhanced diffusivity of their charge carriers. In this article, we show examples of how solid-state nuclear magnetic resonance (NMR) spectroscopy can be used to study the diffusion parameters of Li cations located in the interfacial regions separately from those in the interior of the grains. This article will demonstrate the future challenges and perspectives of Li NMR as a powerful tool of probing dynamic properties in functional materials.
Li Ion Migration at the Interfaces
Authors: Jamnik Janko , Gaberscek Miran
Published: February 2011

During the past decade, the electrochemical properties (energy density, power capability, and cycling stability) of practical lithium (Li) batteries have been enormously improved. Surprisingly, although the knowledge exists of how to prepare excellent batteries, a detailed understanding of how they actually work is still lacking. In particular, the impact of interfaces in electrode composites is poorly understood. Here, we collect the most advanced mechanistic studies performed in our laboratory or published in recent literature and try to embed this knowledge into the well-established concepts used in solid-state ionics for many decades. In particular, we focus on the so-called perpendicular and parallel interfacial effects. We show that much of the old wisdom can be applied to batteries, although there are several important differences. We discuss, in some detail, the effects of charge incorporation, electronic interphase contacting, electrode porosity, and heterogeneous doping in selected advanced electrode materials and emphasize the future perspectives.
The Chemistry of Single-Walled Nanotubes
Authors: Strano Michael S. , Boghossian Ardemis A. , Kim Woo-Jae , Barone Paul W. , Jeng Esther S. , Heller Daniel A. , Nair Nitish , Jin Hong , Sharma Richa , Lee Chang Young
Published: February 2011

The unique structural, electronic, and mechanical properties of single-walled carbon nanotubes (SWNTs) have opened the doors to developments that push the limits of science. These advancements not only further scientific discovery, but also result in the development of everyday practical applications. These applications vary from singlemolecule sensors to nano-scaled transistors to multi-modal biosensors. This article focuses on three distinct developments made as a result of recent advances in spectroscopy of SWNTs. The first system examines the use of SWNTs for molecular detection using near-infrared light to produce tunable fluorescent sensors that are highly photostable. The second system examines the use of a 4-hydroxybenzene diazonium reagent to sort SWNTs based on electronic structure to create on-chip modifications of nano-electronic devices. The third system characterizes nanotube networks for such applications as flexible electronics, exploring the irreversible binding of adsorbates onto nanotube networks using electrical transport and Raman spectroscopy.
Atomistic Simulations of Mechanics of Nanostructures
Authors: Huang Hanchen , Van Swygenhoven Helena
Published: February 2011

Nanostructures can be in the form of nanoparticles or nanograins, nanowires or nanotubes, and nanoplates or multilayers. These nanostructures may be used individually or embedded in a bulk material. In both cases, they share two common features. First, the small dimensions minimize or even eliminate the presence of defects. Second, nanostructures entail large surface or interface areas. The absence of defects makes nanostructure materials stronger than their bulk counterparts, leading to the eventual realization of ideal strength. The presence of surfaces and interfaces may either reduce or increase the strength. Atomistic simulations can provide insight into the deformation mechanism at the atomic and electronic level, something that is very difficult to obtain from experiments. This article describes generic features of nanostructures and summarizes the five areas presented in the articles in this issue.
Mechanics of Ultra-Strength Materials
Authors: Zhu Ting , Li Ju , Ogata Shigenobu , Yip Sidney
Published: February 2011

Recent experiments on nanoscale materials, including nanowires, nanopillars, nanoparticles, nanolayers, and nanocrystals, have revealed a host of “ultra-strength” phenomena, defined by stresses in the material generally rising up to a significant fraction of the ideal strength—the highest achievable strength of a defect-free crystal. This article presents an overview of the strength-controlling deformation mechanisms and related mechanics models in ultra-strength nanoscale materials. The critical role of the activation volume is highlighted in understanding the deformation mechanisms, as well as the size, temperature, and strain rate dependence of ultra strength.
What is the Limit of Nanoparticle Strengthening?
Authors: Chrzan D.C. , Morris J.W. , Osetsky Y.N. , Stoller R.E. , Zinkle S.J.
Published: February 2011

The stress required to deform a perfect crystal to its elastic limit while maintaining perfect periodicity, the so-called ideal strength, sets the gold standard for the strength of a given material. Materials this strong would be of obvious engineering importance, potentially enabling more efficient turbines for energy production, lighter materials for transportation applications, and more reliable materials for nuclear reactor applications. In practice, the strength of engineering materials is often more than two orders of magnitude less than the ideal strength due to easily activated deformation processes involving dislocations. For many materials, precipitate strengthening is a promising approach to impede dislocation motion and thereby improves strength and creep resistance. This observation begs the question: What are the limits of nanoparticle strengthening? Can the ideal strength of a matrix material be reached? To answer these questions, we need a detailed, atomic scale understanding of the interactions between dislocations and obstacles. Fortunately, simulations are beginning to explore this interaction.
Mechanics of Crystalline Nanowires
Authors: Park Harold S. , Cai Wei , Espinosa Horacio D. , Huang Hanchen
Published: February 2011

Nanowires are among the most exciting one-dimensional nanomaterials because of their unique properties, which result primarily from their chemical composition and large surface area to volume ratio. These properties make them ideal building blocks for the development of next generation electronics, opto-electronics, and sensor systems. In this article, we focus on the unique mechanical properties of nanowires, which emerge from surface atoms having different electron densities and fewer bonding neighbors than atoms lying within the nanowire bulk. In this respect, atomistic simulations have revealed a plethora of novel surface-driven mechanical behavior and properties, including both increases and decreases in elastic stiffness, phase transformations, shape memory, and pseudoelastic effects. This article reviews such atomistic simulations, as well as experimental data of these phenomena, while assessing future challenges and directions.
Nanomechanical Architectures—Mechanics-Driven Fabrication Based on Crystalline Membranes
Authors: Liu Feng , Lagally Max G. , Zang Ji
Published: February 2011

Bending of thin sheets or ribbons is a ubiquitous phenomenon that impacts our daily lives, from the household thermostat to sensors in airbags. At nanometer-scale thicknesses, the mechanics responsible for bending and other distortions in sheets can be employed to create a nanofabrication approach leading to novel nanostructures. The process and resulting structures have been aptly referred to as “nanomechanical architecture.” In this article, we review recent progress in atomistic simulations that not only have helped to reveal the physical mechanisms underlying this nanofabrication approach, but also have made predictions of new nanostructures that can be created. The simulations demonstrate the importance of the atomic structure of the crystalline membrane and of the intrinsic surface stress in governing membrane bending behavior at the nanoscale and making the behavior fundamentally distinct from that at the macroscale. Molecular dynamics simulations of the bending of patterned graphene (a single-atomic layer film) suggest a new method for synthesizing carbon nanotubes with unprecedented control over their size and chirality.
Crystal Growth from the Melt under External Force Fields
Authors: Rudolph P. , Kakimoto K.
Published: February 2011

The present and future demands of industrial bulk crystal growth from the melt are concentrated on improved crystal quality, increased yield, and reduced costs. To meet these challenges, the size of the melt volume must be markedly increased. As a result, violent convective perturbations appear within the melts due to turbulent heat and mass flows. They disturb the single crystal growth and give rise to compositional inhomogeneities. The application of external force fields is an effective method to dampen and control these flows. After introducing different stabilizing variants, such as constant and accelerated melt rotation, mechanical vibrations, and electric current, this article focuses on the use of magnetic fields. Nonsteady fields became very popular because, in this case, the needed strength of the magnetic induction is much lower than for steady fields. A new low-energy low-cost technology that combines heat and magnetic field generation in one module placed close to the melt crucible is introduced.
Advances in Bulk Crystal Growth of AlN and GaN
Authors: Ehrentraut Dirk , Sitar Zlatko
Published: February 2011

Aluminum nitride (AlN) and gallium nitride (GaN) play an essential role in modern electronics, particularly in optoelectronics. Highly efficient light-emitting devices covering the ultraviolet to green spectral region are fabricated from these materials. Despite all efforts, the growth of large-size and high-quality AlN and GaN crystals for substrates, which are thermally and lattice-matched to the AlGaN-based device structures, is still in its infancy. This is due to the high equilibrium vapor pressure of nitrogen above these compounds, which requires growth techniques employing either the vapor phase or liquid solutions. The best commercially available GaN substrates show a high dislocation density of >105 per cm2 and strong bowing with a radius of curvature smaller than 10 m. This article reviews current growth techniques that look promising and may become commercially viable.
Growth of Optical Crystals by the Micro-Pulling-Down Method
Authors: Yoshikawa Akira , Chani Valery
Published: February 2011

The micro-pulling-down technique is a crystal growth method that has been mostly developed since 1992. The general scheme of the growth system is relatively simple: the melt (oxide, fluoride, metal) residing in a crucible is transported in downward through microcapillary channel(s) made in the bottom of the crucible. Two driving forces (capillary action and gravity) support the delivery of the melt to the liquid/solid growth interface formed under the crucible due to a properly established temperature gradient. Appropriate configuration of the crucible bottom allows for controlling of the crystal shape (fibers, rods, tubes, plates) and the dimensions of the crystals' cross sections that range approximately from 0.1 to 10 mm. A great number of scientifically and industrially important optical crystal fibers have been successfully produced using this method.
Development of Bulk GaN Crystals and Nonpolar/Semipolar Substrates by HVPE
Authors: Fujito Kenji , Kubo Shuichi , Fujimura Isao
Published: February 2011

The remarkable progress in nonpolar and semipolar devices based on gallium nitride (GaN) in recent years has been driven by not only advancements in the epitaxial growth technique but also improvements in the quality of bulk nonpolar and semipolar GaN substrates. At present, high-quality nonpolar/semipolar substrates are only made by slicing thick bulk GaN crystals grown by hydride vapor-phase epitaxy (HVPE). Although HVPE is currently the most successful method for obtaining high-quality bulk GaN crystals, it is still difficult to obtain uniform crystals with large diameters and thicknesses. The size of the nonpolar/semipolar substrates has been limited by the growth thickness along the c-axis of bulk GaN crystals. Here we review the growth of bulk GaN crystals by HVPE to achieve high-quality and large-sized nonpolar and semipolar substrates.
GaN-Based Light-Emitting Diodes on Selectively Grown Semipolar Crystal Facets
Authors: Scholz Ferdinand , Wunderer Thomas , Neubert Barbara , Feneberg Martin , Thonke Klaus
Published: February 2011

In this article, we briefly review a particular approach to fabricate light-emitting diode (LED) structures on the semipolar side facets of triangular GaN stripes grown by selective area epitaxy. This approach enables a significant reduction of the internal piezoelectric fields in the LED's active area, while still maintaining the well-established c-direction as the main epitaxial growth direction for GaN-based devices on large area substrates. For the latter, these internal fields are responsible for the lower efficiency of GaN-based LEDs in the longer (green) wavelength range. The reduced internal fields of such semipolar LEDs can be directly determined by photoluminescence (PL) investigations on pre-biased LED structures and further confirmed by time-resolved PL studies. The epitaxial growth behavior is strongly facet-dependent, leading to different surface flatnesses on different semipolar facets formed by this procedure and different – indium incorporation efficiencies. An increased indium uptake on semipolar {1101} facets as compared to conventional c-plane layers can help to shift the LED emission to longer wavelengths near 500 nm, despite the significantly reduced field-dependent Stark shift, which helps to reach the green wavelength range in polar LEDs.
Polytype Replication in Heteroepitaxial Growth of Nonpolar AlN on SiC
Authors: Suda Jun , Horita Masahiro
Published: February 2011

Zinc-blende and wurtzite are the most common structures for binary compound semiconductors. Aluminum nitrides (AIN), one of the most promising materials for deep ultraviolet light-emitting diodes, have a wurtzite structure as an equilibrium phase due to its strong ionicity. Silicon carbide (SiC) is widely used as a substrate for heteroepitaxial growth of AlN, since SiC has a hexagonal structure whose lattice constant is close to that of AIN. Different from other compound semiconductors, SiC can have many different crystalline structures, called polytypism. Among various polytypes of SiC, large-size high-quality wafers are available for 4H and 6H structures. When AlN is grown on a 4H- or 6H-SiC basal plane (0001), normal, wurtzite-structured AIN is obtained. On the other hand, when AlN is grown on a nonbasal SiC plane, such as nonpolar (1100) or (1120), what is expected? If ideal growth is realized, AIN will follow the crystalline structure of SiC (i.e., the polytype of the SiC substrate will be replicated to the AIN epitaxial layer). Nonpolar nitride growth has attracted much attention to eliminate undesirable internal electric fields due to the polarization in nitride heterostructures. In addition, nonpolar nitride growth on SiC also allows an opportunity to obtain nitrides with new crystalline structures. In this article, the polytype replication growth of AIN on nonpolar SiC substrates is reviewed.
Degradable, Multifunctional Cardiovascular Implants: Challenges and Hurdles
Authors: Jung Friedrich , Wischke Christian , Lendlein Andreas
Published: February 2011

Polymer-coated and polymer-based cardiovascular implants are essential constituents of modern medicine and will proceed to gain importance with the demographic changes toward a society of increasing age-related morbidity. Based on the experiences with implants such as coronary or peripheral stents, which are presently widely used in clinical medicine, several properties of the next generation of cardiovascular implants have been envisioned that could be fulfilled by multifunctional polymers. The challenge is to combine tailored mechanical properties and rapid endothelialization with controlled drug release in order to modulate environmental cells and tissue. Additionally, degradability and sensitivity to external stimuli are useful in several applications. A critical function in terms of clinical complications is the hemocompatibility. The design of devices with improved hemocompatibility requires advancedin vitrotest setups as discussed in depth in this article. Finally, degradable, multifunctional shape-memory polymers are introduced as a promising family of functional polymers that fulfill several requirements of modern implants and are of high relevance for cardiovascular application (e.g., stent technology). Such multifunctional polymers are a technology platform for future cardiovascular implants enabling induced autoregeneration in regenerative therapies.