bluuebunny/crossref_metadata_2025_split huggingface dataset:
Tuned Tilt of Epitaxial Crystals
Authors: Flynn C.P. 
Published: August 2016

At first sight it may seem self-evident that epitaxial crystals must grow with their atomic planes parallel to the lattice planes of the substrate crystal. The simplified sketch in Figure 1a shows atoms raining onto a substrate from a molecular beam, and then diffusing to step-edges in a desirably controlled growth process termed step-edge (or ledge) flow. Drawn this way, the conclusion that the planes must lie parallel seems obvious. If, however, the process is heteroepitaxial, and the new lattice (solid circles) differs in spacing or structure from the substrate template (lines), the outcome is less straight-forward. Accommodating the difference of step heights at the interface then requires local tilts amounting to a fraction of one atomic spacing in, say, 25 atoms or perhaps half a degree, depending from one point to the next on details of the neighboring interfacial structure.In what follows I describe a qualitatively different and newly discovered behavior we term coherent tilt. The coherent tilt process causes the atomic planes of the epilayer to grow at an accurately predictable and reproducible angle with respect to the substrate planes. The tilt axis, tilt angle, and sense of rotation remain precisely fixed over an entire substrate, with the result that the process occurs coherently over the dimensions of a macroscopic sample. Large tilts over 7° have already been produced with the angle predictable from first principles to 0.1° The characteristics of coherent tilt are sketched in Figure 1b.
Solid Lubricants: Overview and Recent Developments
Authors: Sutor Paul 
Published: August 2016

A renaissance in research and development of solid lubricants has taken place during the past 10 years. This renaissance has been driven by the needs of advanced technological applications, for which liquid lubricants alone have become either ineffective or undesirable. Increasing the efficiency of jet and automobile engines, which entails engine operation at significantly higher temperatures, and increasing the longevity of space vehicles have been central technology drivers.In the past 10 years, many new solid lubricants have been developed to address these needs for higher temperature operation and longer life. Their development, for the most part, has taken advantage of what is known concerning their mechanism of action. Although this mechanistic knowledge is incomplete, particularly at high temperatures, significant advances in material performance have been made. In addition, new methods of applying solid lubricant coatings, namely plasma and ion-beam-assisted vacuum deposition methods, have recently come into their own and produced improved materials.This article will highlight developments in the 1980s which produced new solid lubricants and the advanced technologies which these materials will enable.Several reviews of solid lubricants survey the area in more detail than is possible here. For overview purposes, this article will briefly describe the classes of materials used as solid lubricants, their mechanism of action, and areas of application.
Theoretical and Experimental Studies of Ceramic:Metal Wetting
Authors: Russell Kenneth C. , Oh S-Y. , Figueredo A. 
Published: August 2016

Surface phenomena, including wetting, have surely fascinated humans foi millennia. The breakup of a falling stream of water and the behavior of froths, both driven by surface energy, certainly caught the interest of people long before records were kept. The Phoenicians are credited with inventing that remarkable wetting agent, soap, in about 600 B.C., and a 2000-year-old soap factory was unearthed in the ruins of Pompeii.It is hard to pinpoint the first scientific studies of surfaces and wetting, but it is a good guess that the earliest scientists, or natural philosophers, were interested. In 1805 Young noted that in a system consisting of a liquid and a solid phase, the former will wet the latter to a degree dictated by the surface energies of the system. If a liquid drop rests on a solid substrate, as shown in Figure 1, the wetting angle, θ, characterizes the wetting of solid by liquid. The fundamental relationship between wetting angle and surface energy in equilibrium was given by Young as:where γsv is the surface energy of solid, γsl is the solid-liquid interfacial energy, γlv is the surface energy of liquid, and θ is the wetting angle.
Geometrical Origins of Interfacial Strength
Authors: Eberhart M.E. , Clougherty D.P. , Louwen J.N. 
Published: August 2016

The advent of new materials processing and fabrication techniques has made it possible to control atomic-scale structure to an extent only dreamed of as recently as 20 years ago. This improved structure control takes us one step closer to synthesizing materials with a combination of properties tailored to the intended application. It is this ability to control structure that will accelerate the pace of technological advancement which is limited by the rate of materials development.Mechanical performance of materials restricts their use, even for nonstructural applications. Magnetic composites used in recording heads are an example. The mechanical and tribological properties of these materials are a major consideration when designing reliable components.Another example is electronic materials, where the strength of the interface between the polymer packaging and the semiconductor controls the lifetime of the semiconductor device. The semiconductor device undergoes thermal cycling as it is used, straining the polymer/semiconductor interface. Environmental impurities such as moisture can enter through cracks in the interface and degrade the semiconductor, causing device failure.
High Resolution Z-Contrast Imaging of Semiconductor Interfaces
Authors: Jesson D.E. , Pennycook S.J. 
Published: August 2016

The structural and compositional integrity of interfaces between semiconductor multilayers can profoundly influence the optical and electronic properties of epitaxially grown heterostructures. Understanding the atomic-scale interfacial structure and chemistry is therefore essential to correctly relate electrical measurements to theoretical models and to correlate such effects with growth conditions. High-resolution electron microscopy (HREM) has played a pivotal role in this process, providing important information on interface commensurability and revealing the presence and nature of defects.More recently, significant advances have been made in applying HREM to the difficult problem of chemical composition mapping in systems where no structural change occurs across the interfaces. The basis of such methods involves using the objective lens as a bandpass filter and tuning in on a specific range of spatial frequencies to form a chemically sensitive interference pattern. By using a suitable low-index zone axis and choosing an optimum range of specimen thickness, the patterns can indeed be extremely sensitive to the strength and periodicities of the projected potential.
Pattern Formation During the Growth of Liquid Crystal Phases
Authors: Oswald Patrick , Bechhoefer John , Melo Francisco 
Published: August 2016

Liquid crystals, discovered just a century ago, have wide application to electrooptic displays and thermography. Their physical properties have also made them fascinating materials for more fundamental research.The name “liquid crystals” is actually a misnomer for what are more properly termed “mesophases,” that is, phases having symmetries intermediate between ordinary solids and liquids. There are three major classes of liquid crystals: nematics, smectics, and columnar mesophases. In nematics, although there is no correlation between positions of the rodlike molecules, the molecules tend to lie parallel along a common axis, labeled by a unit vector (or director) n. Smectics are more ordered. The molecules are also rodlike and are in layers. Different subtypes of smectics (labeled, for historical reasons, smectic A, smectic B,…) have layers that are more or less organized. In the smectic A phase, the layers are fluid and can glide easily over each other. In the smectic B phase, the layers have hexagonal ordering and strong interlayer corrélations. Indeed, the smectic B phase is more a highly anisotropic plastic crystal than it is a liquid crystal. Finally, columnar mesophases are obtained with disklike molecules. These molecules can stack up in columns which are themselves organized in a two-dimensional array. There is no positional correlation between molecules in one column and molecules in the other columns.
Neutron Scattering in Materials Sciences
Authors: Moss Simon C. , Shapiro Stephen M. 
Published: August 2016

At the 1989 MRS Fall Meeting the present guest editors and J.D. Jorgensen chaired a symposium on “Neutron Scattering for Materials Science.” Approximately 80 papers were presented, covering a great variety of topics which joined members of the materials science and neutron scattering communities. Because of that symposium's success, it was decided to bring the topic of Neutron Scattering to the wider attention of the materials community through this special issue of the MRS BULLETIN.Our purpose is twofold. First, neutrons have increasingly come to play a crucial role, both here and especially in Europe, in our understanding of the structure and properties of materials. Through the manipulation of materials (radiation-induced effects, transmutation doping of semiconductors), nondestructive materials testing (residual stress measurements on industrial-sized objects, depth profiling of ion-implanted semiconductors) and structural and dynamical studies, academic, government, and industrial scientists and engineers are coming to recognize the broad utility of neutron methods. We would like to highlight some of the advances in this field for MRS BULLETIN readers.Second, neutron scattering presents an excellent example of the contribution of our large research facilities to the solution of both basic and applied problems in materials science. Without the major neutron scattering centers we would be severely limited in the scope of our materials activities (no knowledge of magnetic structures and only primitive insight into polymer structures, for example).
Neutron Powder Diffraction
Authors: Jorgensen James D. , Newsam John M. 
Published: August 2016

For many classes of materials, neutron diffraction is the best way to obtain detailed atomic-level structural information. Diffraction experiments on single crystals provide the most precise data, but sufficiently large specimens (>0.1–0.5 mm3) are often not available. Steady development of instrumentation and data analysis techniques, however, has now made it possible to obtain comparably precise structural information from neutron diffraction experiments on powder samples. Such studies have played a prominent role in solid state physics, chemistry, and materials science in recent years. The special capabilities that have contributed to the success of this technique include atomic cross sections that are often favorable for a particular structural problem, high neutron penetrating power, the excellent resolution achieved with state-of-the-art diffractometers, and steadily advancing analysis techniques that facilitate obtaining structural information from a diverse range of polycrystalline materials.As Axe, Pynn, and Hayter note in their introductory article in this issue of the MRS BULLETIN, atomic scattering cross sections for neutrons are not simply a function of atomic number, as is the case for x-rays. The scattering is predominantly from the nuclei (thus avoiding the form factor diminution observed for x-ray scattering), and coherent neutron scattering cross sections can, generally, be as large for light atoms as for heavy atoms. Light atoms, such as hydrogen (deuterium), oxygen, nitrogen, carbon, or lithium, can therefore be located in the presence of heavier atoms. This advantage has led to the widespread use of neutron powder diffraction for studing metal hydrides and, more recently, oxide superconductors.
Neutron Scattering Studies of Surfaces and Interfaces
Authors: Majkrzak Charles F. , Felcher Gian P. 
Published: August 2016

During the past decade, scientific and technological interest in the properties of surfaces and interfaces has grown at an astounding rate. On first thought, one might not consider a neutron or even an x-ray photon to be a particularly sensitive surface probe given their relatively weak interactions with matter compared to that of a low-energy electron or atom. Indeed, low-energy electron diffraction and atomic beam scattering techniques have contributed significantly to our understanding of surface phenomena. Nonetheless, the very fact that electrons and atoms are so strongly interacting makes quantitative analysis of their scattering data difficult. The interaction of neutrons or x-rays with matter, on the other hand, is weak enough that the potential can be characterized by a relatively simple scattering amplitude. Presently attainable neutron intensities, though not yet comparable to those of x-ray synchrotron sources, are still of sufficient strength to permit a variety of surface or near surface reflectivity and grazing angle diffraction experiments. Because neutrons can distinguish between different isotopes of the same element, most notably hydrogen and deuterium, as well as couple to atomic magnetic moments via a dipolar interaction, they can be indispensable and complementary probes.More conventional neutron diffraction techniques can also be applied to the study of interfacial phenomena and the effects of reduced dimensionality and compositional modulation in super-lattice structures grown by a variety of thin film deposition methods. In this article we will differentiate between reflectivity and diffraction measurements as follows: if the scattering occurs at a wavevector transfer low enough that the scattering medium appears as a continuum, so that amorphous and crystalline states are indistinguishable, then it will be considered to be in the reflectivity regime whereas diffraction will be taken to correspond to higher wavevector transfer where the precise arrangement of atoms is discernible.
Interfaces Part II: Mechanical and High-Temperature Behavior
Authors: Wolf Dieter , Yip Sidney 
Published: August 2016

This is the second of two issues of the MRS BULLETIN on interface materials and is entirely devoted to their mechanical and high-temperature behavior. Mechanical properties provide a rich area for investigating the effect of the local inhomogeneities near the interfaces, and their effect on the interrelation between the structure and chemistry on one hand, and the elastic and fracture behavior on the other.Based on much experimental work on grain-boundary fracture it seems that, with the exception of “beneficial” segregants, the embrittlement potential of most impurities is governed by their propensity for segregation to the grain boundaries, which in turn is strongly influenced by the energies of the pure boundaries. To investigate the role of the grain-boundary structure in its fracture behavior, one must therefore consider the correlations between (1) the structure (i.e., the five macroscopic degrees of freedom) and the energy of pure grain boundaries, (2) impurity segregation and the grain boundary energy, (3) structure, impurity segregation and elastic response at the interface, and finally (4) the correlation between embrittlement and segregation. In addition, the mobility of dislocations near a crack tip also plays an important role. Unfortunately, relatively little knowledge has been accumulated on most of these complex interrelations even though their unraveling is widely recognized as the ultimate goal.
Nanophase Materials Assembled from Atomic Clusters
Authors: Siegel Richard W. 
Published: August 2016

The synthesis of nanometer-sized atomic clusters of metals and ceramics by means of the gas-condensation method, followed by their in situ consolidation under high-vacuum conditions, has resulted in a new class of ultrafine-grained interface materials. These nanophase materials, with average grain sizes presently ranging from about 5 to 25 nm, exhibit properties that are often rather different and improved relative to those of conventional materials. In addition, their processing characteristics appear in some cases to be greatly improved over their conventional coarser grained counterparts. The synthesis of nanophase materials by means of cluster assembly under controlled conditions should enable the design of materials, heretofore unavailable, with improved or unique properties. As such, it is likely that the combination of new capabilities to synthesize, characterize, anc) engineer the properties of materials based on the assembly of atomic clusters will significantly impact materials science and engineering in the coming years.The assembly of matter by the consolidation of gas-condensed atomic clusters is probably as old as the universe itself, since this is thought to be the way condensed matter formed during the cooling period that followed the “big bang,” as evidenced by the structure of the earliest meteorites. The modern synthesis of ultrafine-grained materials by the in situ consolidation of nanometer-sized gas-condensed ultrafine particles or atomic clusters, however, was first suggested by Gleiter.
Materials in the Undergraduate Chemistry Curriculum
Authors: Whittingham M. Stanley 
Published: August 2016

Although solids are one of the three states of matter, and the solid state is pervasive throughout science and our lives, students would not know it from the standard chemistry curriculum, which still emphasizes small molecules. Despite this education, a significant proportion (more than 30%) of all chemists end up as practitioners of materials chemistry, either in inorganic solids or in polymers, and they must therefore obtain on-the-job education. Not only should this need be reflected in the curriculum, but it should be possible through modern areas of chemistry such as materials to bring some of the excitement of the practicing chemist to the undergraduate student's first chemistry course, perhaps turning around the flight from science, and from chemistry and physics in particular. The American Chemical Society is encouraging this approach through the proposal of a certified BS degree in chemistry with emphasis in materials. To place the present position in perspective, one only needs to look at the recent figures tabulated by the National Science Foundation; there is a tremendous attrition of students planning to major in science and engineering during the freshman year (See Table I).Potential science majors are indeed there, but they are being lost due to their first experiences, which are usually in general chemistry and calculus, and a lesser number in biology and physics. It is therefore imperative that these courses encourage students rather than kill their enthusiasm.
Review of Wall Conditioning and Wall Materials for Fusion Research Devices
Authors: Langley Robert A. 
Published: August 2016

Wall materials for fusion research must meet stringent requirements. The purity of a fusion plasma is critical to obtaining meaningful results during fusion plasma experiments. The concentration of impurity ions in the plasma must be kept low because impurities result in radiative losses that at best decrease the efficiency of the plasma “burn” and at worst prevent ignition completely. Radiative losses scale with the effective charge, Z, of the plasma. Radiative losses due to bremsstrahlung are proportional to Z2; those due to recombination are proportional to Z4. Figure 1 shows the impurity concentration above which ignition cannot be achieved as a function of impurity atomic number for an experimental power reactor. The advantage of using a low-Z material as the first wall of a fusion machine (i.e., as the plasma-facing material) is evident.The major plasma impurities observed in most experiments are oxygen, carbon, and first-wall material. These impurities are introduced into the plasma by many mechanisms, including outgassing, desorption, chemical interaction, sputtering, and evaporation. Chemical and physical interactions between the first wall and the energetic hydrogenic ions produced in the plasma are important factors in impurity production. (Here “hydrogen” refers to hydrogen and its isotopes, deuterium and tritium.)The energy contained in the plasma can be quite large (many megajoules) and can interact with the first wall on a short rime-scale (e.g., a millisecond). It is possible that no material will prove suitable for all operational scenarios. Therefore, the burden of reducing the plasma/first-wall interaction to an acceptable level rests on both plasma and materials scientists.
Helium Pumping Strategies for D-T Fusion Devices
Authors: Krauss A.R. , Auciello O. , Brooks J.N. , Mattas R. , McGrath R. , Nygren R. , Smith D. L. 
Published: August 2016

In order for the new generation of long pulse D-T (deuterium-tritium) burning tokamaks to be successful, it is necessary to minimize the helium “ash” that accumulates during the thermonuclear burn. Since tritium is radioactive as well as relatively expensive, the tritium inventory stored at any given facility must also be minimized. It is therefore necessary either to preferentially remove the helium from the plasma, or to recycle all of the gas, extracting the deuterium and tritium for reuse as fuel. This latter approach requires incorporating a high pumping speed and correspondingly large pumping ducts into the fusion device design, as well as an external facility for separating helium from the D-T fuel. Since the He/H ratio in the plasma should not exceed ~5%, the pumping speed must be 20 times higher than required for a system which removes the helium only. The resulting complexity of the pumping system and the need to handle large quantities of tritium imposes a cost penalty of ~100M$ for a typical commercial fusion reactor design. It is therefore desirable to design a pumping system which removes helium only, leaving the D-T fuel in the fusion device. Recall, however, that existing pumping technologies generally provide better pumping of hydrogen, and many pump designs are totally ineffective at pumping helium.Despite the low solubility of helium in many metals, permanent trapping of helium atoms occurs at defect sites, culminating ultimately in the formation of helium bubbles. Retention of trapped inert gas is determined by diffusional processes and is consequently temperature-dependent. Hydrogen has not only a much higher solubility, but also a much higher diffusion coefficient than helium in most metals. Consequently, it is to be expected that for some metals, there is a temperature range in which implanted hydrogen is released much more quickly than implanted helium. Brooks and Mattas proposed using this “self-pumping” effect as the basis for a pump which preferentially removes helium in the presence of hydrogen.