bluuebunny/crossref_metadata_2025 huggingface dataset:
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.
Nonevaporable Getters: Properties and Applications
Authors: Mazza F. , Boffito C.
Published: August 2016

Many advanced technologies, such as surface science, semiconductor processing and high energy physics, call for vacuum levels of the order of 10−11 mbar and lower. These pressures can not be reached without a careful choice of materials, treatments, and evacuation means for the vacuum device involved. Non-evaporable getters (NEGs) are increasingly being recognized as an interesting and powerful solution for many vacuum problems. NEGs have been used extensively in sealed-off devices such as microwave tubes, traveling wave tubes, x-ray tubes, lamps, and infrared detector dewars, in which their main role is to assure the desired vacuum level throughout the life of the sealed device. The getter material can be considered as a chemical pump which removes the active gases in the residual atmosphere of the vacuum device by forming stable chemical compounds.

The choice of materials, treatments, and structures of nonevaporable getter materials is critical for the optimization of the sorption and diffusion processes which are the basis of the NEG pumping mechanism. The effectiveness of this pumping mechanism at very low pressures, and the cleanliness and simplicity of operation have made this pumping approach ideal, in combination with other pumping technologies, for reaching the extreme high vacuums today's advanced technologies require. This article will explain the mechanism of the gettering process, describing materials, treatments, and structures used in standard vacuum practice, and will review some of the most typical and interesting applications.
Electron Tunneling in High Tc Superconductors
Authors: Valles J.M. , Dynes R.C.
Published: August 2016

Electron tunneling measurements have proven enormously valuable in studies of conventional superconductors. Very early measurements confirmed, in an especially convincing way, the existence of the superconducting energy gap, and more detailed studies demonstrated the spectral form of the gap and its temperature dependence. These measurements were instrumental in confirming in some detail the predictions of the Bardeen, Cooper, Schrieffer (BCS) theory of superconductivity in simple metals. For example, it was shown very clearly that the ratio of the energy gap (2Δ) and critical temperature Tc was close to the BCS value (2Δ/kTc = 3.5). As the sophistication of the technique improved, deviations from this BCS weak coupling limit became apparent (2Δ/kTc was measured to be >4 in materials like Pb, for example), and subtle structure in the current-voltage characteristics of tunnel junctions unearthed a signature of the electron-phonon interaction—the microscopic mechanism responsible for superconductivity in these traditional materials. Through a quantitative analysis of this structure, people were able to extract a function α2(ω)F(ω), which is the phonon density of states F(ω) modulated by the electron-phonon coupling function α2(ω). This function gave a quantitative description of the electron-phonon interaction and confirmed beyond a doubt that the electron-phonon interaction was responsible for superconductivity.
The Nature of Superconductivity in Ba1-XKXBiO3
Authors: Hinks D.G.
Published: August 2016

Superconductivity in potassium-doped BaBiO3 was first observed by Mattheiss et al. in 1988 and the structure of the superconducting material was determined by Cava et al. The material crystallizes in a simple perovskite structure with potassium substituted on the barium site as shown in Figure 1. Unlike the layered copper-oxide materials, this compound is simple cubic and thus isotropic in the superconducting state and shows no magnetism. Band theory calculations show that the electronic structure is simple, with conduction bands composed of Bi(sp)O(p) hybridized orbitals and, unlike the copper-oxide materials, there are no d electrons at the Fermi surface or involved in the superconductivity. Photoemission and inverse-photoemission agree well with the theoretical calculations and show metallic behavior with a low density of states at the Fermi level consisting of Bi-O sp hybrids. This material is structurally and electronically simple compared to the copper-oxide compounds and, therefore, should be much easier to investigate experimentally and understand theoretically. This article discusses recent results concerning the structural properties of the material and describes several experiments that give information on the superconducting state.
Optical Storage Disk Technology
Authors: Gambino Richard J.
Published: August 2016

Optical storage of digital information has reached the consumer market in the form of the compact audio disk. In this technology, information is stored in the form of shallow pits embossed in a polymer surface. The surface is coated with a reflective thin metallic film, and the digital information, represented by the position and length of the pits, is read out optically with a focused, low-power (5 mW) laser beam. When used for information storage for a computer this device is called a CD-ROM, a Compact Digital-Read Only Memory. The user can only extract information (digital data) from the disk without changing or adding any data. That is, it is possible to “read” but not to “write” or “erase” information.

While it is an advantage to have permanently stored information in some cases — for example when listening to Beethoven's Ninth Symphony—in other situations the read-only feature is not appropriate. For most computer applications, it is essential that the user be able to store information on the disk and read it back at will. For example, in a word processing task—such as typing this article — it is often necessary to store the document on a disk. Optical data storage for this purpose is available in the form of a Write Once Read Many times (WORM) optical disk drive. The operating principle in a WORM drive is to use a focused laser beam (20 – 40 mW) to make a permanent mark on a thin film on a disk. The information is then read out as a change in the optical properties of the disk, e.g., reflectivity or absorbance. These changes can take various forms: “hole burning” is the removal of material (typically a thin film of tellurium) by evaporation, melting or spalling — sometime s referred to as laser ablation; bubble or pit formation involves deformation of the surface, usually of a polymer overcoat on a metal reflector.
Multilayered Thin-Film Materials for Phase-Change Erasable Storage
Authors: Libera Matthew , Chen Martin
Published: August 2016

Phase-change erasable optical recording uses a focused laser beam as a heat source to reversibly switch a micron-sized area in a thin film between the amorphous and crystalline states. A bit of information is stored as an amorphous spot in a crystalline background, and the state of the bit is determined by the differing optical properties of the amorphous and crystalline phases. This concept was first demonstrated in 1971 and then, after about a decade of exploratory work, the field accelerated throughout the 1980s at several research laboratories. Currently the subject of number of reviews, the field of phase-change materials promises to broaden and intensify in the 1990s.

The active layer, where the storage occurs, is typically a tellurium-based alloy with a variety of solute species. Early work studied the recording properties of single-layered films, but it has been clearly shown that multilayered films, where the active layer is sandwiched between two or more dielectric layers, have superior recording properties and resistance to irreversible damage caused by laser heating. The dielectric layers (typically SiO2, Si3N4, or ZnS) provide barriers to active-layer oxidation and contamination, help prevent the hole formation associated with ablative write-once storage methods, and act as crucibles and heat sinks which contain the molten spot and influence its cooling properties, respectively. A typical multilayer structure is shown in the cross-sectional transmission electron micrograph of Figure 1.
Magnetic Storage: Principles and Trends
Authors: Lemke James U.
Published: August 2016

Magnetic recording can store digital information at an areal density well beyond the fundamental density-limit of optical systems. The central problems in achieving high trans-optical information density with magnetic recording reside primarily in the materials area. In addition to improved magnetic properties, such as head materials with high saturation magnetization and recording media with low noise and high coercivity, tribological considerations impose constraints on the ultimate density that will be attained. The exponential loss of signal due to spacing between the head and recording medium is inherent in all magnetic recording, necessitating the development of durable quasi-contact interfaces.

Magnetic recording is the universal technology for electronic information mass storage. Its presence is ubiquitous as audio tapes, VCRs, floppy disks, computer hard disks, credit cards, etc. Sales of magnetic recording products exceed $50 billion annually, with strong growth projected into the foreseeable future. A steady progression in storage density and corresponding reduction in cost has characterized all phases of magnetic recording; the literature is replete with historical and projective curves showing cost and density numbers. Computer disk file memories have doubled in areal density every 2.5 years for about the past 30 years. Although many competing technologies have been proposed through the years, none has been able to displace or even significantly impact magnetic recording. The first challenge was thermoplastic recording, followed by at least three resurrections of magneto-optic recording, bubbles, semiconductors, and various nonmagnetic optical storage devices.
Particulate Recording Media
Authors: Sharrock M.P.
Published: August 2016

Magnetic recording is a very useful and versatile technology, and one that is continuously evolving to serve the increasing demand for information storage and to meet the challenge of competitors such as optical recording. The basic principles of magnetic recording are described in detail elsewhere and briefly below.

The elements of a magnetic recording system are a magnetizable surface layer carried on a flexible tape or on a rotating disk, and a transducer that can both write information to and read information from this surface. The tape or disk, often called a recording medium, and the transducer, called a head, move with respect to each other. The information to be stored is originally contained in an electrical signal, either by direct analog representation or via frequency, phase, amplitude, or pulse-code modulation. In response to this signal, the head in the writing mode generates an intense, localized magnetic field that is capable of changing the direction and degree of the magnetization in the surface material. Each time the input signal changes sign, the writing field changes direction and a transition between regions of opposite magnetization is created. As the head moves along the surface, a series of these transitions is created along a track. The resulting magnetization pattern of the tape or disk becomes itself the source of a spatially varying magnetic field.
Chemical Reactions with Single Microparticles
Authors: Davis E. James , Buehler Mark F.
Published: August 2016

Fine particles can be produced via aerosol processes either by means of vapor phase reactions that produce solid or liquid particles or by reactions between a preexisting solid or liquid particle and a reactive gas. This article examines the latter processes because a strong interest has developed in the production of materials via aerosol processing. Although fine particles are frequently produced using flow systems, such as in the laminar flow aerosol reactor of McRae and his co-workers, fundamental studies of the chemical kinetics are more readily done using single microparticles or microdroplets. Design of an aerosol reactor requires knowledge of the reaction rates, for there must be a sufficient residence time of the reacting species in the reactor to complete the desired reaction.

Matijević reviewed early work on preparing well-defined and very pure metal oxides by hydrolysis of alkoxide aerosol particles, and Ingebrethsen and co-workers studied the hydrolysis rates of aerosol droplets of aluminum and titanium alkoxides and mixtures of the two alkoxides. Following Matijevic and his colleagues, Okuyama et al. used the thermal decomposition of metal alkoxide vapors to produce ultrafine particles of the oxides of titanium, silicon, and aluminum. The preparation of polymeric aerosols has been studied by Partch et al. and by Ward et al. The latter investigators used single-particle techniques (the electrodynamic balance) to obtain polymerization rate data for the photochemical polymerization of acrylamide monomer microparticles.
Processing of Fine Ceramic Powders
Authors: Ring Terry A.
Published: August 2016

This article discusses the fundamentals required to produce narrow size distribution fine ceramic powders, make suspensions of fine ceramic powders, and make green bodies with a uniform packing of these particles. In all cases, the interface that fine ceramic powders present to their environment is a very important parameter in controlling the properties of the powders during processing.

There are two major classifications for ceramics: structural and functional. The former includes high and low temperature applications. High temperature ceramics are needed for kiln furniture, ladles, catalyst substrates, and insulations. Low temperature uses are represented by the traditional white ware, as well as hardness applications, such as coatings, armor, and cutting tools. Electrical functions include superconductivity, dielectrics, piezoelectrics, and varistors; magnetic functions are represented by ferrite magnets and SQUIDs (Superconducting Quantum Interference Devices); and optical functions include optical and infrared windows, as well as radar windows. Each class of ceramics has different processing problems and, therefore, different research and development directions. The major areas of research advances for structural and functional ceramics are described below.
Preparing Narrow Size Distribution Particles from Amphiphilic Association Structures
Authors: Ward Anthony J.I. , Friberg Stig E.
Published: August 2016

Small inorganic particles with a narrow size distribution have been prepared from aqueous and organic solutions under varied temperatures, pressures, and concentrations. This article reviews the preparation of such particles from microemulsions and liquid crystals and shows how these amphiphilic association structures can have a pronounced influence on the size, shape, and even chemical composition of the precipitated particles.

Surfactants associate in the presence of water to form micelles and lyotropic mesophases. This is a consequence of the molecular geometrical packing requirements, superposed on the dissolution behavior of these substances. Associated structures that have shapes with the lowest free energy are preferentially formed. A balance is attained between the interactions of the hydrophobic portions of the surfactant, reducing the unfavorable contact with aqueous environs and the interactions between the headgroups. The composition regions of these different phases have been widely studied. Micelles exist as normal and reversed structures (Figure 1), while the liquid crystalline phases show a variety of structures which have been studied in great detail.

Figure 1 gives a simplified example of the structural conditions in different parts of a phase diagram of the three components water/surfactant/amphiphilic substance (e.g., medium chain length alcohol). The interaction between water and the surfactant is sufficiently pronounced to permit the formation of reversed micelles. The association structures of different regions shown in Figure 1 include normal (L1) and reversed micelles (L2). The hexagonal phase (E) consists of a hexagonal array of cylinders with the hydrocarbon chains situated in the center of the cylinder, while the reverse hexagonal phase (F) is built of cylinders with aqueous centers. The lamellar phase (Lα) is made up of infinite sheets of bimolecularly arranged surfactant molecules separated by aqueous layers. The physical properties and reasons for the formation of such lyotropic phases have been discussed extensively.
Up Close: Materials Sciences at Lockheed Palo Alto Research Laboratory
Authors: Littauer Ernest L.
Published: August 2016

Lockheed's Research Laboratory was established in 1956 to support the recently founded Lockheed Missiles and Space Division. The earliest products of the division were a new generation of submarine-launched ballistic missile—Polaris and the Agena spacecraft. It was immediately recognized that both required advances in materials technology and the Materials Sciences Directorate was established in the research laboratory to address those needs. Today, materials researchers are conducting investigations in a wide range of topics in metallurgy, chemistry, structures, and nondestructive test/evaluation.

The laboratory has a particularly strong capability in performance analysis and problem identification/resolution for solid rocket propulsion systems. Current work on the Trident II continues the laboratory's contributions to the Polaris, Poseidon, and Trident I missiles. Efforts are also focused on materials for other aerospace applications, such as advanced aircraft, the NASA Space Station (Freedom), and the Strategie Defense Initiative.

Examples of materials sciences achievements include the development of the space shuttle's heat shield tiles, the first use of lithium as an anode in batteries, a real-time x-ray inspection system for large rocket motors, and coated and uncoated refractory metal hot gas manifolds for ballistic missile propulsion. The organization has a staff of about 170 professionals.
A First-Hand View of Shape-Selective Catalysis in Zeolites: New Science and Technology
Authors: Weisz Paul B.
Published: August 2016

The discovery and demonstration of molecular shape-selective catalysis in zeolitic silicate structures has led to a large segment of new catalytic science as well as to chemical process technologies with broad impacts on industry and society. Manmade catalysts are beginning to acquire skills of molecular structure selectivity and activity seen in the world of enzymes.

All our activities, including those of life itself, are dominated by the presence and action of ever transforming molecules. Heat alone transforms them but, basically, decomposes them indiscriminately. Catalysts, on the other hand, can direct transformations with meticulous selectivity as if to preserve a continuing purpose and order, and life itself. However, manmade catalysts, although the major tool of the chemical industry, have been highly simplistic and crude in their discriminatory capabilities. For example, we have solid acidic catalysts called cracking catalysts that will break carbon-carbon bonds. They have been employed for decades in the petroleum fuels industries and represent the largest volume of any manufactured catalyst. They will rather indiscriminately break C—C bonds in nearly any aliphatic organic structure, with only some structural preference for tertiary over secondary over primary carbon links.

In another class of manmade catalysts, molecules with unsaturated bonds are hydrogenated by a variety of metal-based catalysts. This addition of hydrogen proceeds nearly regardless of where the bond occurs in the structure. Or, take a synthesis reaction, such as that of xylenes from the addition of methanol to toluene. The usual acidic catalysts will add the new methyl group in any and all of the three possible isomeric positions to produce ortho-, meta-, and para-xylenes.
Solid State Ionics
Authors: Huggins Robert A.
Published: August 2016

This issue of theMRS BULLETINcontains three articles relating to the general field that has come to be known as Solid State Ionics. The central feature of this area of science and emerging technology is the rapid transport of atomic or ionic species within solids, and the various phenomena, of both scientific and technological interest, that are related to it.

Attention to this area has grown greatly in recent years because of the rapidly increasing recognition of the possibility of a wide range of interesting technological applications. One example already widespread is the use of an oxygen-conducting solid electrolyte as the critical element in the oxygen sensors installed in the exhaust systems of almost all current automobiles to reduce deleterious emissions and improve the efficiency of the combustion process.

Work is under way in a number of other directions, including static and dynamic chemical sensors, solid state electrochemical reactors, low impedance selective atomic filters, new concepts for the direct conversion of heat to electricity by the use of sodium- or hydrogen-transporting cycles, a novel method for the low cost electrolysis of water at intermediate temperatures, batteries that can store greatly increased amounts of energy, ion exchange materials, solid state laser hosts, high efficiency fuel cells, electrochromic materials and configurations for both optical displays and “smart windows,” advanced catalysts, atomic reservoirs and pumps, high temperature superconductors, and possibly solid state fusion hosts.

Despite this recent attention, however, it is worth noting that interest in solids in which ionic species can move with unusual rapidity is actually not new at all. As early as 1839, Michael Faraday reported measurements on several materials that showed an unusual increase in electrical conductivity at elevated temperatures, contrary to that found in normal metals.
Fundamental Studies of Irradiation Effects in Fusion Materials
Authors: Guinan M. W.
Published: August 2016

Fusion is the energy production process which drives the universe. Controlled utilization of this process for the benefit of man has been the illusive goal of the world's fusion power research since the 1950s. The most easily utilized fusion reaction is the fusion of deuterium and tritium. Every major fusion power research program is directed toward utilization of the energy released from this reaction.

When the hydrogen isotopes deuterium (D) and tritium (T) are fused, 80% of the energy released is carried by a single neutron. This neutron moves at 5 × 107 m/s and a kinetic energy of 14 MeV, so the designer of a magnetically confined reactor is faced with the reality that 80% of the power produced will impinge on the structure facing the burning plasma as a “current” of 14 MeV neutrons. The D-T reaction is illustrated in Figure 1.

Neutrons are not charged and do not interact with electrons in material through which they move. They collide with nuclei. The result of these collisions is always some combination of the in-situ creation of one or more energetic ions, alteration of chemistry through transmutation, and the introduction of radioactivity. Each changes material properties.

Nearly all the present experimental data base of neutron irradiation effects has come from fission reactor irradiations.
Reduced Activation Alloy Development for Fusion
Authors: Doran D.G. , Klueh R.L.
Published: August 2016

Developing materials that resist degradation when exposed to neutrons is a key element in international programs to demonstrate the engineering and economic viability of fusion power. An additional requirement is that these materials exhibit low-induced long-lived radioactivity in order to enhance fusion power's environmental (and economic) attractiveness. This article briefly overviews current efforts to develop reduced activation structural alloys for fusion reactor applications.

A specific objective of the U.S. Fusion Materials Program is that all reactor components will meet the requirements for near-surface burial. Presently, this is interpreted to mean that all radioactive waste should satisfy Part 61 of Title 10 of the U.S. Code of Federal Regulations (10CFR61), prepared by the Nuclear Regulatory Commission (NRC). Specific activity limits are placed on particular long-lived radionuclides, which, when combined with activation calculations for conceptual fusion devices, place constraints on material compositions. Regulation 10CFR61 was prepared to cover low-level waste produced by the fission reactor industry; it has been necessary to extend it, in principle, to include additional radionuclides. Unlike anticipated waste from a fusion industry, only a small fraction of the fission waste stream is activated metals; hence, the applicability of the regulation is questionable. Further analysis of the probable disposal method for fusion wastes is needed, along with consideration of maintenance and decommissioning scenarios.
Structural, Electronic, and Magnetic Properties of Thin Films and Superlattices
Authors: Freeman A.J. , Continenza A. , Li Chun
Published: August 2016

The importance of interfaces in determining the physical properties of technologically important materials — addressed in the Guest Editors' introduction to the articles in this issue — has stimulated theoretical efforts to determine from first principles a detailed understanding of their chemical, electronic, and mechanical properties. Fortunately, this has now been made possible as a result of the dramatic advances in condensed matter theory made in the last decade, driven in large part by new and sophisticated experiments on high-purity materials that have been well and carefully characterized. Particularly in electronic structure, these advances may be attributable directly to the close collaboration of theoretical and experimental researchers. Indeed, the new found ability to apply fundamental theoretical concepts to real materials (rather than to simple model systems) made possible by using the continued rapid development of computer power, has served to fill the increasingly urgent demand of experimentalists for theoretical interpretation of their data. Also, in some cases, these computational efforts can be used to provide data that would be currently impossible or impractical to obtain experimentally. This development has been an essential element in the phenomenal growth in this area of materials science.

More specifically, the advent of accurate self-consistent (local) spin density functional (LSDF) calculations for surfaces, interfaces, and multilayers means that theory is no longer limited to simple, parameter-dependent models. These complex systems are of growing interest because the reduced symmetry, lower coordination number, and availability and role of highly localized surface and interface states offers the possibility of inducing new and exotic phenomena and promotes the possibility of new device applications.
Solute-Atom Segregation at Internal Interfaces
Authors: Foiles S.M. , Seidman D.N.
Published: August 2016

In an alloy, the equilibrium composition in the vicinity of an inhomogeneity, such as a surface or a grain boundary, will generally differ from the composition in the bulk. This was first recognized by Gibbs. This change in local composition can potentially affect the structure and mechanical properties of the boundary. As an example, experiments by Sass and co-workers have shown that the Burgers vector of the dislocations present in Fe-Au twist boundaries depends on the Au concentration at the boundary. This demonstrates that the structure does, in fact, depend on the local composition of the alloy at the interface.

This article presents computer simulation studies of equilibrium segregation in binary alloys at grain boundaries, and two experimental studies of interfaces using atom-probe field-ion-microscopy (APFIM). These studies address two fundamental questions: What is the composition of the boundary region as a function of the bulk composition and temperature? What is the spatial distribution of the two species in the boundary? Note, that the comparison of experiment and calculations, where possible, is crucial. Such comparisons can provide guidance in the experimental analysis, and the comparison also provides a stringent test of the reliability of the simulation methods.

This article first describes simulation and atom-probe field-ion microscope techniques and next discusses the segregation in Ni-Cu alloys to isolated edge dislocations. This is followed by a study of segregation at twist boundaries in Pt-Au alloys. Then, the segregation at an asymmetric mixed tilt-twist boundary in a Pt-Ni alloy is computed, and the results are compared with experimental atom-probe held-ion-microscopy results. Finally, the results of an atom-probe study of two-dimensional Re segregation at a twist boundary with a small tilt component in a W-Re alloy are presented.
Chemical Interfaces: Structure, Properties, and Relaxation
Authors: Ourmazd A.
Published: August 2016

Any finite system is delimited by interfaces. In this trivial sense interfaces are ubiquitous. However, modern epitaxial techniques seek to modify the properties of materials by the creation of interfaces. “Band gap engineering,” the attempt to tailor the electronic properties of semiconductors by interleaving many dissimilar layers, is an example of this approach. Interfaces between lattice-matched, isostructural, crystalline systems, (differing only in composition) are technologically the most advanced and thus most widely used. These “chemical” interfaces are formed by the introduction of dopant impurities, or by the stacking of dissimilar materials. Chemical interfaces formed by the epitaxial growth of dissimilar materials are the subject of this article.

A fundamental tenet of modern epitaxy is the tailoring of properties through control of structure. We will therefore outline how the “structure” of a chemical interface may be defined and experimentally determined. Determining the interfacial structure is a major experimental challenge, inextricably bound with our understanding of how structure affects other properties. The article will thus briefly discuss this vital link. We will conclude with an outline of the way chemical interfaces, in reality systems far from equilibrium, can relax, and how their relaxation mechanisms shed light on the fundamental properties of solids. Because the GaAs/AlGaAs system is the most technologically advanced, and to be concrete, we will illustrate the discussion by reference primarily to this system. Many of the concepts and experimental results, however, are more generally valid.
Materials Engineering and Fuel Cell Development
Authors: Steele B.C.H.
Published: August 2016

The direct conversion of chemical energy in a fuel cell has been one of the most challenging technological problems since a simple hydrogen/oxygen fuel cell was first demonstrated 150 years ago by Groves at the London Institution. Aqueous electrolyte fuel cells have been successfully used in space to power electrical equipment in the Apollo spacecraft and in the space shuttle, but terrestrial applications have been principally limited to a few specialized military and telecommunication applications.

In the original concept of a fuel cell, a primary fuel (e.g., hydrogen, hydrocarbon) is reacted with oxygen in an electrochemical cell incorporating an acid electrolyte, namely:

so as to convert the free energy of oxidation (ΔG°) of the fuel to carbon dioxide and water directly into electrical energy.

Fuel cells are attractive in that theoretical thermodynamic energy/fuel conversion ratios given by the appropriate value of ΔG°/ΔH° (free energy change/enthalpy change) can be very high (e.g., ~95% for H2/O2 reaction at 298 K) compared to the limitations imposed on heat engines by the Carnot cycle efficiency, (T2−T1)/T2. However, in practice it has proved difficult to realize the high electrical/fuel conversion efficiencies promised by fuel cell systems. This is caused by problems associated with the electrochemical kinetics and/or the requirement that in the Faradaic reaction both mass and electron transfer must occur in a spatially restricted region where an electrode, an electrolyte, and one of the reacting phases (usually a gas) are in intimate contact. This region is often referred to as the three-phase boundary.