bluuebunny/crossref_metadata_2025 huggingface dataset:
Hairpin Defects in Liquid-Crystalline Polymers
Authors: Williams David R.M. , Halperin Avi
Published: August 2016

Conventionally, the term “defects” in liquid-crystalline systems refers to microscopic faults in the orientational order, which are usually visible optically. These are discussed in other articles in this issue. Our use of the term defect is entirely different. The defects we shall be considering, “hairpins,” occur on the scale of several angstroms and are abrupt reversals in the trajectory of a single liquid-crystalline-polymer (LCP) chain (Figure 1). In comparison to conventional defects, the direct observation of hairpin defects is much more difficult, yet their presence has important effects. Among the affected properties are the dimensions and the elasticity of the chains, the elastic behavior of the bulk nematic, and its dielectric response. Their presence should also give rise to a family of interfacial phase transitions in solutions of LCPs in nematic solvents. In turn, these are of interest in the design of liquid-crystalline displays. Hairpins in LCPs are superficially reminiscent of similar configurations in proteins and in homopolymers that undergo fold crystallization. These similarities are misleading because hairpins in proteins are permanent structures due to the chemical bonds. The folds in crystalline polymers are also fixed structures. In marked contrast, hairpin defects are mobile topological excitations that are created and annihilated continuously. Our emphasis in this article is on main-chain, semiflexible, nematic LCPs, consisting of mesogenic monomers joined by flexible spacer chains (Figure la). These polymers combine the orientational order of monomeric nematics with the flexibility and randomness inherent in polymers. The appearance of an oriented, nematic phase can be controlled either by temperature or by concentration.
Defect-Mediated Rheology of Block Copolymers
Authors: Hudson Steven D. , Amundson Karl R. , Jeon Hong G. , Smith Steven D.
Published: August 2016

Block copolymer melts with a near-symmetric composition can microphase-separate to form a lamellar morphology where unfavorable monomeric interactions are reduced by an antiparallel layering of the polymer chains (see Figure 2, discussed later). The symmetry of such a block copolymer is the same as for small-molecule, smectic-A liquid crystals, which also exhibit (parallel or antiparallel) layering. Because of their shared symmetry, their quasistatic mechanical properties are of the same form. To lowest order, the energy of distortion of the lamellar pattern can be expressed as a sum of a compressional/dilational and a bend energy (see, for example, Reference 2):

where n is a unit vector normal to the layers, and ε and ∇ n are the layer dilational and bending strain, respectively. B and K are the corresponding moduli. These have been measured only recently for block copolymers. The ratio of these moduli yields a characteristic microscopic length:

which is calculated to be a fraction of the layer spacing d0.

Similarities are also found in the dynamic properties of polymeric and smallmolecule smectics. These similarities are found at low strain rate and amplitude where the rheological response is controlled by their similar layered structures and associated defects. During either steady or dynamic shear, certain defect and domain motions are induced such that a steady-state alignment of the director has been observed. In this report, we discuss defect structure and how the much more viscous nature of these polymeric layered materials influences the distribution and relative population of each defect type. We also discuss how these defects can control material stress-relaxation behavior.
Polymer Adsorption
Authors: Chakraborty Arup K. , Tirrell Matthew
Published: August 2016

Long polymer chains adsorbed onto solid surfaces are important in a wide range of applications and are relevant to many issues in biology and medicine. Adsorbed polymer layers used widely in the stabilization of colloidal suspensions, for example, are essential to the formulation of paints, coatings, printing inks, drilling needs, and ceramics processing. Adsorbed polymer layers also play a crucial role in many tribological applications such as boundary lubricants.

The interfacial behavior of biological macromolecules (such as proteins) plays an important role in biomedical applications as well as in the function of living organisms. The development of biocompatible solid materials that can be used safely and efficiently in vascular and joint prostheses, catheters, artificial-heart valves and whole hearts, cardiac-arrest devices, and hemodialysis cartridges is critically important. In some instances, the adsorption of biological macromolecules onto the artificial material can lead to deleterious effects. For example, many vascular grafts and catheters fail because of thrombotic occlusion initiated by protein adsorption. Protein adsorption is also the initial subprocess that leads to plaque formation in teeth and the fouling of contact lenses. Given the central role of protein adsorption in many physiological systems, and the great benefits that could be derived by designing materials that do not adsorb biological macromolecules, understanding the interfacial behavior of biological polymers is important.

The ultimate goal of research in polymer adsorption is to facilitate the manipulation of the properties of adsorbed polymer layers (or polymer-solid interfaces) so that materials with required properties can be fabricated. To take steps toward this goal, understanding how the nature of the polymer, the substrate, and other prevailing conditions (such as the type of solvent) affect the macroscopic properties of the interface is crucial.
Polymer Surface Dynamics
Authors: Granick Steve
Published: August 2016

A major surge of activity is underway to understand the dynamics of polymer chains at interfaces. This stands in contrast to the situation a generation ago when much of polymer-materials research revolved around understanding dynamics in the bulk (isotropic) state. Building in part on earlier studies that had been somewhat neglected, striking new findings have been obtained. The new methods and equipment include surface-specific spectroscopies; advanced, in situ time-resolved methods to determine surface structure and composition; and the surface-forces apparatus for measuring adhesion and interfacial rheology. Also, older methods (such as contact angle) have been revitalizated and applied to new problems. Theoretical calculations and molecular-dynamics simulations are also emerging.

Appreciation is growing that scientific understanding is possible of these systems that are so complex and, often, so far from equilibrium. Polymer surfaces are becoming recognized as an area with many opportunities to do exciting and useful surface science, particularly regarding kinetics, diffusion, surface chemistry, and other rate-dependent processes.

The engineering significance is that while polymers and plastics-based applications are rooted in our economic life, too often the technologies and formulations are empirically derived. One tends to take plastics and their communication with adjoining materials for granted. A molecular understanding is needed so that better design can emerge by rational extension.

During the course of these new activities, the community of polymer science has rubbed shoulders with and has thereby become increasingly integrated with other disciplines-colloid science, surface science, biomedical science, and microelectronics, to cite a few examples. When dealing with interfaces, one's parochial materials interests quickly become generalized.
Ion-Beam Analysis of Polymer Surfaces and Interfaces
Authors: Kramer Edward J.
Published: August 2016

Ion-beam analysis of chemical composition as a function of depth is by now well-established for inorganic materials and is an important method of investigating growth of thin films. It has been applied to polymers much more recently, perhaps because fairly obvious problems with radiation damage discouraged workers in this field initially. Ion-beam analysis has developed, however, into a analytical tool that complements other methods, such as x-ray photoelectron spectroscopy and neutron reflection, very well. The purpose of this short article is to give the reader an introduction to its current uses in polymers.

The ion beams of ion-beam analysis are typically highly energetic (1–5 MeV) beams of 4He++. While other beams are used, for example, 3He and 15N, alpha particle beams are used in the vast majority of experiments reported in the literature. Two major categories of experiments are carried out with such beams. Rutherford backscattering (RBS) spectrometry to detect heavy elements in the polymer and forward recoil spectrometry (FRES) (also known as elastic recoil detection) to detect the isotopes hydrogen and deuterium. The basic principles for each method are similar.
Characterizing Polymer Surfaces and Interfaces
Authors: Russell T.R
Published: August 2016

The presence of a surface or interface can markedly alter the configuration and spatial distribution of polymer molecules. In the bulk, polymer molecules—comprised of numerous monomers covalently linked together—pervade 10s of nanometers spatially. However, packing such chains at an interface—under the constraint that a solid polymer is essentially incompressible—necessitates perturbations to the chain configurations near the interface. This may result in a collapse of coils at the surface or in a preferential orientation of the monomers with respect to the interface plane. For amorphous homopolymers, however, simulations indicate that, on a segmental level, surface effects are generally damped within several segment diameters from the surface. Interactions between the surface and the polymer chains place additional constraints on the molecular configurations. As the architecture of the polymer chain becomes more complex, as with diblock copolymers in which two chemically distinct polymer chains covalently bond together at one end, the packing of chains at the interface must take into account the relative interactions of the two portions of the chains with the interface. Due to the connectivity of the blocks, preferential interactions of the blocks with the interface can influence the spatial distribution of the chains far from the interface. As the number of components increases, as with a simple binary-polymer mixture, not only must packing constraints be satisfied, but also the interactions of the two chains with the surface and with each other must be taken into account. In the case of homogeneous mixtures, the preferential interaction of one chain with an interface can lead to a substantial excess of that chain at the interface which, depending upon the proximity to the demixing point, can lead to surface effects that propagate many molecular diameters into the sample. The key, however, is the connectivity of the monomers in the polymer chain, which can enhance surface effects, as in the case of block copolymers or polymer mixtures, or suppress surface effects, as in the case of homopolymers.
Effects of Stress on Step Energies and Surface Roughness
Authors: Roland Christopher
Published: August 2016

Strain relaxation in lattice-mismatched, heteroepitaxial systems is one of the classic problems in materials physics, which has gained new urgency with the increased applications of strained layers in microelectronic systems. In general both the structure and the integrity of the thin films are strongly influenced by strain. For instance it has long been known that under strain, the growth changes from an initial layer-by-layer growth mode to one with three-dimensional islanding. In the seminal works of van der Merwe, and Matthews and Blakeslee, this change in growth mode is explained in terms of the introduction of strain-relieving misfit dislocations, which appear when the film has reached some critical thickness. Recently it has become clear that this change in growth mode can take place even without the introduction of misfit dislocations. Such dislocation-free coherent islanding, or “roughening,” has been observed experimentally both in Ge/Si and in InGaAs/GaAs systems. Furthermore recent experiments show that in Ge/Si(100) systems, the thin films display a curious asymmetry with respect to the sign of the strain: Films under compression roughen by forming coherent islands while those under tension remain relatively smooth. A possible mechanism behind this strain-induced type of roughening is the subject of this article.
Substrate Engineering With Plastic Buffer Layers
Authors: Schowalter Leo J.
Published: August 2016

The advantage that epitaxy offers the electronics and optoelectronics industries is that it allows the possibility of producing precisely controlled layers of very high crystal quality. Heteroepitaxy of different materials offers the promise of tailoring device layers in clever ways that nature did not intend. However unlike fruit juices, nature has made it difficult to epitaxially combine different materials. As the preceding articles have clearly pointed out, it is very difficult to obtain smooth epitaxial layers that are free both of defects and strain when there is a lattice mismatch between the layers and their substrates.

As already discussed in this issue, a uniform network of dislocations at the interface between a flat, uniform epitaxial layer and its substrate can completely relieve strain in the majority of the epitaxial layer. This would be a satisfactory situation for many devices so long as the active region of the device could be kept away from the interface. The problem is how to introduce the dislocations in an appropriate way. When an epitaxial layer has a larger lattice parameter than the underlying substrate, a misfit dislocation running along the interface represents a plane of atoms that has been removed from the epitaxial layer. (One would insert a plane of atoms if the epitaxial lattice parameter was smaller. For simplicity however we will continue to assume that the epitaxial layer has a larger lattice parameter.) It is not possible for a whole half plane of atoms, bounded by the dislocation at the interface and the substrate edges along the two sides, to be removed at once. The boundary between where the extra plane of atoms has been removed and where the epitaxial layer has not relaxed yet will represent a threading dislocation. This threading dislocation would continue to move as the size of the misfit dislocation along the interface grows. Ideally it moves all the way out to the substrate edge and vanishes there while the misfit dislocation along the interface would end up extending from one side of the substrate to the other. However other dislocations and other kinds of defects can effectively pin the threading dislocation resulting in an epitaxial layer with many threading dislocations. Unfortunately these threading dislocations are generally detrimental to most kinds of devices. It is precisely this high density of threading dislocations that limits applications of many heteroepitaxial layers.
Applications of Intermetallic Compounds
Authors: Westbrook J.H.
Published: August 2016

Just what is an intermetallic compound? An intermetallic compound is a true compound of two or more metals that has a distinctive structure in which the metallic constituents are in relatively fixed abundance ratios and are usually ordered on two or more sublattices, each with its own distinct population of atoms. Often substantial or complete disorder may be obtained as a result of a low ordering energy or the intervention of some external agency—for example, extreme cooling rates or radiation. Deviations from precise stoichiometry are common on one or both sides of the nominal atomic ratio, necessitating some misordering and/or the introduction of vacancies. Usually metal-metalloid compounds such as silicides, arsenides, or tellurides are also included in the category since the phenomenology of most such compounds is similar to that of metal-metal compounds. More than 25,000 distinct intermetallics are now known.

To many, intermetallic compounds (IMCs) are classed as “exotics” or “advanced materials.” Such categorization should apply only to the recent interest in their potential use as monolithic high-temperature structural materials. Actually IMCs have been used for thousands of years in an astonishing range of applications exploiting not only their mechanical properties but their chemical, electrical, magnetic, optical, and semiconducting properties as well. Most readers will be surprised to learn that they carry within their own bodies (dental fillings) or on their persons (jewelry or pocket lighters) IMCs for which selection and processing are uniquely appropriate to those applications. The economic impact of IMCs is as unappreciated as the diversity of their applications. Some are regularly produced in quantities of 1,000s of tons per year while others, although not in tonnage production, are key factors in devices with billion-dollar markets.
Metallurgical and Chemical Applications of Intermetallics
Authors: Westbrook Jack H.
Published: August 2016

Practical metallurgical application of intermetallic compounds (IMCs) occurred more than 3,000 years before they were recognized as distinct entities in alloys. Pliny, The Elder (A.D. 23–79) recorded in his encyclopedia a then old practice: the use of mercury both to recover gold from sands and other dispersed sources, and to gild less-noble metal objects. In both cases, the key factor is the formation of a Au-Hg intermetallic compound (amalgam) stable at room temperature but readily decomposable on heating to produce solid gold. An illustration of the Au-recovery process, reproduced from Ercker (1574) appears in Figure 1. Use of the amalgam processes for silver apparently occurred later. Bronze mirrors were silver-coated with the amalgam process by the Chinese in the second century B.C., and silver was recovered from crushed sulfide ores using mercury in the famous Potosi process (1566, but probably known much earlier).

The key properties of intermetallics that make possible their diverse applications in chemical and metallurgical processes are their high melting points relative to one or all of their constituent elements, their often sharply defined composition, their brittleness, and their controllable reactivity/stability–that is, systems can be chosen such that a stable intermetallic forms easily at room or low temperatures that is nonetheless readily decomposable at a higher temperature. Once the intermetallic forms however, a useful physical property (e.g., hardness or conductivity) or chemical property (oxidation, sulfidation, corrosion resistance, nonsticking quality, etc.) may be that which is ultimately exploited in use.

We will review these two classes of applications using representative examples from both process metallurgy and chemistry.
From Gibbsian Thermodynamics to Electronic Structure: Nonempirical Studies of Alloy Phase Equilibria
Authors: de Fontaine Didier
Published: August 2016

The following is an edited version of the David Turnbull Lectureship address, given by recipient Didier R. de Fontaine at the 1995 MRS Fall Meeting, De Fontaine received the lectureship “in recognition of fundamental contributions and insights in the fields of order/disorder phenomena in materials and computational techniques for phase diagrams.” De Fontaine is a professor in the Department of Materials Science and Mineral Engineering at the University of California—Berkeley and holds a joint appointment with the Lawrence Berkeley National Laboratory.
Use of Plasma Processing in Making Integrated Circuits and Flat-Panel Displays
Authors: Gottscho Richard A. , Barone Maria E. , Cook Joel M.
Published: August 2016

The ever-shrinking dimensions of microelectronic devices has mandated the use of plasma processing in integrated circuit (IC) factories worldwide. Today the plasma-processing industry has grown to over $3 billion in revenues per year, well in excess of predictions made only a few years ago. Plasma etching and deposition systems are also found throughout flat-panel-display (FPD) factories despite the much larger dimensions of thin-film transistors (TFTs) that are used to switch picture elements (pixels) on and off. Besides the use of plasma in etching and depositing thin films, other processes include the following: removal of photoresist remnants after development (descumming), stripping developed photoresist after pattern transfer (ashing), and passivating defects in polycrystalline material. Why are plasma processes so prevalent?

In etching, plasmas are used for high-fidelity transfer of the photolithographically defined pattern that defines the device or circuit. More generally, plasma provides the means to taper sidewalls. In Si processing, the sidewalls must be nearly vertical to obtain high density integration and faster performance. However in making FPDs, sidewalls are tapered to obtain uniform step coverage and reduce shorting. In deposition, plasmas are used to enable processing at low temperature. For both etching and deposition, only plasma processing provides an economically viable means for processing large area substrates: 300 mm for Si and more than 550 × 650 mm for FPDs. It is the ability to scale uniform reactant generation to larger areas that sets plasma apart from beam-based processes that might otherwise offer the desired materials modifications. The nonequilibrium characteristics of plasma further distinguish this processing method. Energetic electrons break apart reactant precursors while ions bombard the surface anisotropically.
Materials in Active-Matrix Liquid-Crystal Displays
Authors: Hanna Jun-ichi , Shimizu Isamu
Published: August 2016

In today's world of increasing office automation and computer-aided personal-communications systems, display devices play a very important role as person-machine interfaces. Above all, high-definition, full-color flat-panel displays will be key devices in the near future when processing huge amounts of information—including pictorial images via computer networks and telecommunication systems that transcend the present limitations of time and place—will be possible.

Passive-matrix liquid-crystal displays (LCDs) represent the most widely used choice for portable display devices. Figure 1 illustrates the essential components and operating principle of a typical LCD. Each pixel is addressed by the top- and bottom-line electrodes of the cell based on information signals, producing a light image. By installing a color filter of red, green, or blue for each pixel, full-color images can be displayed. However, the essential problems of crosstalk among pixels and low response speed become serious with an increase in the number of pixels, resulting in a low contrast ratio and failure of the display to keep up with the signals.
Materials and Manufacturing Issues for Color Plasma Displays
Authors: Weber Larry F. , Birk Jane D.
Published: August 2016

Recent demonstrations of color-plasma displays with exceptional image quality and performance that meet and in some cases exceed that of cathode-ray tubes (CRTs) have resulted in a renewed interest in this 25-year-old technology. The tremendous market potential for large-area plasma displays for both home-entertainment and computer applications now appears to be an attainable goal. This new generation of plasma displays features full color capability, high brightness and contrast ratio, wide viewing angles, and rapid refresh rate. The success of today's plasma displays is the result of the integration of advancements in driver electronics and the materials and processes used in the fabrication of the plasma-display structures.

Using color-plasma technology, two-inch-thick direct-view displays can be made that are up to 60 inches in diagonal. During the past several months, numerous articles have appeared in the media announcing major investments in facilities and technology for largescale manufacturing of these large-area plasma displays. Table I lists the companies committed to plasma-display research and development. Additional advancements in materials, processes, and electronics will be required to achieve high-volume, low-cost manufacturing.
Magnetic Recording Head Materials
Authors: Brug James A. , Anthony Thomas C. , Nickel Janice H.
Published: August 2016

The materials used in magnetic recording heads have recently received a tremendous amount of attention. This has been the result of a fortunate set of circumstances. Ever-increasing demands for information storage, especially for graphics-intensive applications, have necessitated unprecedented increases in disk-drive areal densities. Combined with this are recent discoveries in the area of magnetoresistive materials, enabling the design and fabrication of much more sensitive recording heads. The end result is a flurry of activity that has come to dominate the field of magnetics. This article will explore choices for magnetoresistive read head materials, with an emphasis on the materials challenges.

The recording heads that are used in high-performance disk drives typically consist of separate magnetoresistive read and inductive write heads (see Figure 1) where previously a single inductive head performed both functions. Separation of the two heads allows each to be optimized for their individual function, an essential factor in enabling disk drives to contain gigabytes of storage. The write head is the simpler of the two, consisting of a U-shaped ferromagnet surrounding a set of coils. The ends of the ferromagnet are the magnetic poles defining the write gap. When current passes through the coils, a field bridges the gap, setting the orientation of the magnetization in the media. Information is stored by changing the polarity of the current in order to write a pattern of magnetic domains in the media. The materials used in write poles will be reviewed in the section, Write Head Materials.
Flow in Porous Media: Permeability and Displacement Patterns
Authors: Wong Po-zen
Published: August 2016

Porous media represent a broad class of mesoscopically disordered materials that are important in many industrial processes, mostly involving the transport of fluids. For examples, the flow of oil and water in sedimentary rock determines the producibility of oil reservoirs; diffusion and dispersion of molecules in soil govern the spreading of both fertilizers and contaminants; chemical reactors use porous catalysts to enhance the mixing of reagents; the containment of chemical and nuclear wastes depends on the low permeability of concrete barriers. These are just a few problems familiar to our everyday lives. The study of porous media has traditionally been under the disciplines of chemical, mechanical, and petroleum engineering. Only in recent years has the field begun to attract the interest of physicists. The displacement of one fluid by another in a porous medium is now recognized as one of the archetypal systems for studying the physics of pattern formation and interface growth. Fluids transport is known to involve effects due to the microgeometry of the pores, the connectivity of the pore network, and the wetting properties of the pore surfaces. These are all fundamental scientific issues that require the understanding of the physics and chemistry associated with the porous materials. Tackling these problems requires the collaboration of scientists from many different disciplines. Although much work has been done and a great deal has been learned, many interesting and important issues remain. In an article written five years ago, I gave a pedagogical overview of some of the problems in porous media that captivated my own interest.
Evolution of Ge Precipitate Morphology in Al
Authors: Dahmen U.
Published: August 2016

Static electron microscopy provides views of microstructures frozen in time. For many problems, such snapshots give a clear picture of the material's characteristics after a particular processing step, but for other problems it is the evolution of the microstructure itself, its mechanisms and kinetics, that are important to understand. For this class of problem, in situ electron microscopy is an indispensable tool because it can provide real-time dynamic observations of processes, showing, for example, where dislocations nucleate, how a particle grows, by what mechanism and how fast an interface migrates. The major limitation to such experiments is that foils must be extremely thin to be electron transparent. The proximity of the free surfaces can have a strong effect on the dynamic equilibrium. For that reason, high-voltage electron microscopes are particularly useful for in situ TEM experiments. The greater penetration depth of high energy electrons makes it possible to observe processes in foils that are thick enough to avoid the dominant influence of the surfaces.

This article will describe some experiments in which the dynamic behavior of precipitates in a simple alloy system was examined during in situ temperature cycling in order to understand the effect of bicrystal anisotropy on the characteristics of interface motion.
Organic Thin Films: An Overview
Authors: Whitten David G. , Kajiyama Tisato , Kunitake Toyoki
Published: August 2016

The six articles comprising this issue of the MRS Bulletin deal with ultrathin films formed from organic molecules by a variety of techniques. In each case the component molecule forming the film is a relatively simple, single molecule which may or may not have important self-organizing properties that facilitate the formation of a film or related ordered molecular assembly. Taken together, the series of articles offer a concise look at the remarkable diversity and complexity of molecular thin films in terms of preparation and their properties.
II-VI Blue-Green Laser Diodes: A Frontier of Materials Research
Authors: Gunshor Robert L. , Nurmikko Arto V.
Published: August 2016

The current interest in the wide bandgap II-VI semiconductor compounds can be traced back to the initial developments in semiconductor optoelectronic device physics that occurred in the early 1960s. The II-VI semiconductors were the object of intense research in both industrial and university laboratories for many years. The motivation for their exploration was the expectation that, possessing direct bandgaps from infrared to ultraviolet, the wide bandgap II-VI compound semiconductors could be the basis for a variety of efficient light-emitting devices spanning the entire range of the visible spectrum.

During the past thirty years or so, development of the narrower gap III-V compound semiconductors, such as gallium arsenide and related III-V alloys, has progressed quite rapidly. A striking example of the current maturity reached by the III-V semiconductor materials is the infrared semiconductor laser that provides the optical source for fiber communication links and compact-disk players. Despite the fact that the direct bandgap II-VI semiconductors offered the most promise for realizing diode lasers and efficient light-emitting-diode (LED) displays over the green and blue portions of the visible spectrum, major obstacles soon emerged with these materials, broadly defined in terms of the structural and electronic quality of the material. As a result of these persistent problems, by the late 1970s the II-VI semiconductors were largely relegated to academic research among a small community of workers, primarily in university research laboratories.
Organic Conductors and Superconductors: New Directions in the Solid State
Authors: Brooks J.S.
Published: August 2016

In single-crystal organic salts, we find a keen competition between superconducting, magnetic, insulating, and metallic states. The physics of these materials is further enriched by the sensitivity of these states to pressure, temperature, chemical formulation, and magnetic field. A growing international community of scientists have turned their attention to these materials, and are applying the techniques and theories of metal and semiconductor physics to probe these new systems. In this article we will explore these materials. We will discover that these materials have given us many new things: a renaissance in fermiology, new high-magnetic-field states of matter, a bulk quantum Hall effect, new challenges in the calculation of energy bands on a small energy scale, and elusive behavior which seems one step away from our present understanding of physics in low dimensions. Electron correlations probably play an important role in determining the phenomena, and should be considered in more microscopic theoretical treatments of these systems.
Conductors from Superconductors: Conventional Low-Temperature and New High-Temperature Superconducting Conductors
Authors: Larbalestier David C. , Maley Martin P.
Published: August 2016

A useful superconducting conductor must have several properties. Some of the key properties among these are illustrated by the cross section of a Nb-47wt%Ti/Cu composite (Figure 1) which was manufactured for the dipole magnets of the Superconducting Super Collider (SSC). It represents the state of the art for conventional conductor fabrication and is thus an excellent place to start in considering what is needed for any new conductor. First among the essential properties is a high critical current density (Jc); the lower limit of useful Jc is ~104 A/cm2, but really useful values lie between 105 and 106 A/cm2. The SSC conductor achieves this at fields up to 9 T at 4.2 K, the normal temperature used for magnets cooled by liquid helium.

A critical second requirement is that the superconductor be paralleled by an intimately connected good normal conductor, in this case high-conductivity Cu. One function of the Cu is to stabilize the superconductor against small temperature disturbances that lead to flux jumps that could result in local quenching of superconductivity. This requirement forces the subdivision of a given cross section of the superconductor into many filaments having a maximum diameter of no more than about 50 μm, since bigger filaments store more electromagnetic energy than can safely be deposited in the filament without locally heating it above its critical temperature (Tc). One advantage of high-temperature superconducting (HTS) materials is that they can operate at temperatures above ~10 K. Since the specific heat is a strongly increasing function at low temperatures, this permits the safe filament size to greatly increase too. The need to minimize hysteresis losses, however, often provides a separate drive to minimize the filament diameter, as in the conductor of Figure 1, where there are some 7,000 filaments which are only 6 μm in diameter. The overall Cu:Nb-Ti ratio is about 1.5:1. This represents a compromise between the need to minimize the dilution of the supercurrent density by Cu and the need to provide sufficient high-conductivity normal metal to pass the current when the magnet makes the transition from the superconducting to the normal state (a quench).
Environmental Stability of High Tc Superconducting Ceramics
Authors: Barkatt Aaron , Hojaji Hamid , Amarakoon Vasantha R.W. , Fagan James G.
Published: August 2016

The discovery of high Tc superconductivity in layered cuprate ceramics has so far led to the identification of about 35 distinct superconducting cuprate systems, the latest of which is the 133 K superconducting system Hg-Ba-Ca-Cu-O. For most of their proposed applications, high Tc ceramics have to be resistant to environmental degradation both with respect to atmospheric water vapor, e.g., in storage, and to liquid water (produced by condensation on warm-up from cryogenic conditions). The presence of CO2 is an important factor in both environments. Increasing environmental stability involves improving the processing methods to eliminate pores, cracks, and other macroscopic defects (e.g., highly leachable impurity phases) which are prevalent in materials prepared by solid-state sintering. Furthermore, protective coatings and hermetic seals are necessary in many applications involving films because of small film thickness. (Wires are usually drawn inside metal tubes, which provide protection.)
Single-Crystal Silicon: Photovoltaic Applications
Authors: Green Martin A.
Published: August 2016

The vast majority of solar cells used in the field are based on single-crystal silicon. There are several reasons for this. First, by using this material, photovoltaic manufacturers can benefit from the economies of scale of the much larger microelectronics industry, where crystalline silicon also dominates. Since lower-quality silicon is acceptable for solar cells, cell manufacturers are able not only to benefit from large production volumes, but also to use off-grade material. The relatively high efficiencies that result from this material, its excellent reliability in the field, its almost complete lack of environmental problems, and recent laboratory progress strengthen the position of this technology.

Although there had been earlier work with what might be termed “cast multicrystalline silicon,” the first efficient crystalline silicon cells were reported in 1954. These cells displayed energy conversion efficiencies of about 6%, a substantial improvement over what had been previously demonstrated. Performance improved rapidly through the 1950s, with efficiencies up to about 15% demonstrated in the early 1960s. By this time, their use on spacecraft had become the major commercial purpose of the cells, with reliability a more important attribute than efficiency. The technology then stabilized for a decade. In the early 1970s, another burst of activity pushed the performance of laboratory cells to about 17%. At about this time, a simple processing sequence was developed, based on screen printing the metal contacts to the cell. Anisotropic chemical etching of the silicon to produce surface texture to reduce reflection loss was also implemented, to give the cell structure shown in Figure 1.
Status of Computational Fluid Dynamics and Its Application to Materials Manufacturing
Authors: Sawada Ikuo , Tanaka Hiroyuki , Tanaka Masahiro
Published: August 2016

Computational fluid dynamics was born principally in the aerospace field as a method for fluid flow and heat transfer research methods following experimental and analytical approaches. Along with progress in the cost performance of computers, computational fluid dynamics is now establishing itself as a tool to improve production processes and product quality in the steel, nonferrous metals, glass, plastics, and composite materials industries.

Materials manufacturers use computational fluid dynamics for diverse purposes:

1. Reduction in experimental conditions and costs;

2. Detailed analysis of mechanisms with multifaceted information unobtainable through experimentation;

3. Universal tool for scale-up; and

4. Evaluation of novel processes.

It can be readily imagined that accuracy, flexibility, and other requirements of computational fluid dynamics should vary with specific applications.

Fluids generally observed in materials manufacturing processes are molten materials such as metal, glass, and plastics, and gases for stirring and refining. In the flow of such fluids, materials quality and process characteristics are governed by the following:

1. Transport phenomena in the bulk region (where fluid flow is normally turbulent);

2. Chemical reaction at interfaces;

3. Transport phenomena in boundary layers near the interfaces; and

4. Complex coupled phenomena (heat transfer, diffusion, chemical reaction, phase transformation like solidification, free surface, electromagnetic force, and bubble flow).
Stress-Induced Void Formation in Metal Lines
Authors: Flinn Paul A. , Mack Anne Sauter , Besser Paul R. , Marieb Thomas N.
Published: August 2016

The problem of stress-induced voiding in the metal lines used for VLSI interconnections appeared about 10 years ago and was first reported in the literature in 1984. Some typical voids are shown in Figure 1. It is not entirely coincidental that the problem is of about the same age as the personal computer. The continuing advances in process technology that make possible the ever lower costs and improved performance of VLSI devices often have unexpected and troublesome side effects. Continuous attention to potential effects of process changes on the behavior of the materials comprising a VLSI chip is essential to avoid passing on problems to customers. The history of stress-induced voiding provides a good illustration.
Reliability of Solder Joints
Authors: Frear D.R. , Yost F.G.
Published: August 2016

In early electronic technologies, circuit components were attached to circuit boards by mechanical means. The electrical leads were either twisted together or mechanically interlocked to a board prior to soldering. The possibility of an unreliable solder joint causing any kind of circuit failure was remote. Interconnections were made intrinsic to the board by applying solder to increase electrical and thermal conductance. Technological advances and the need for high-density electronics have since eliminated the luxury of mechanical interlocks. Soldering in advanced applications, like surface mount technology (SMT), provides electrical, thermal, and mechanical interconnections between the board and its electrical components. In SMT, solder joints are the only mechanical features on the board and must hold components in place in a wide range of environments. The solder joints themselves are decreasing in size as increased chip functionality and clock frequencies become available. The failure of a single solder joint can render a device, or an entire electrical system, inoperable. Therefore, as insignificant and innocuous as they may seem, solder joints have become a critical aspect of electronic circuit reliability.
Emerging Technologies for Reuse of Lignocellulosic Materials: Recycling Our Disposed Wood and Paper
Authors: Laufenberg Theodore L.
Published: August 2016

“Lignocellulosic materials” refer to the organic matter produced by plants on the Earth in the form of grasses, shrubs, agricultural crops, and trees. Worldwide, this plant growth (also called biomass) serves two sustaining functions for our planet: it degrades carbon dioxide to oxygen and it has been the origin or storage media of all non-atmospheric carbon. Leaving aside the related critical issues of fossil fuel depletion and global warming/cooling, lignocellulosic materials growing on the surface also serve many commercial purposes: food production, fuel sources, structural materials for housing and industry, paper products, and chemicals.

Although lignocellulosic materials represent a sustainable resource base, the expanding use of paper and wood products has caused the developed countries to reevaluate the use and recovery of these natural biopolymerics. This issue of the MRS Bulletin explores some of the intricacies surrounding the recovery and reuse of these biomaterials. The applications for materials research are by no means exhaustively covered in these articles. Instead we illuminate the critical issues facing the paper recycling community and the technologies being applied to this resource to create marketable materials.
Paper Recycling: Definitions and Developments
Authors: Mulligan Daniel B.
Published: August 2016

With the northeastern United States leading the way, tipping fees paid to landfill owners for solid waste disposal have increased as much as 533% since 1980. Fueled by the rapidly diminishing number of available landfills, these rates are expected to go only higher, adding to the public's frustration. Local, state, and federal officials are seeking solutions to mitigate the problem and the public outcry.

In an address at the May 14,1990 Waste-paper I Conference, James B. Malloy, president and chief operating officer of Jefferson Smurfit Corporation and Container Corporation of America, stated: “The bottom line is that our industry, not only in the U.S., but also around the world, must continue to strive for sensible waste reduction at the source as well as total integration of waste management options. In cooperation with, not in conflict with, the public sector we can continue to be positive, constructive participants in the search for workable solutions to the municipal solid waste challenge.”

Recycling is part of the solution. Paper, which contributes up to 40% of solid waste, offers an obvious solution (see Figure 1). The focus on recycling must remain high, encouraging the public to collect and sort waste paper as well as purchase recycled material. What follows is a description of some attempts to inject order into this otherwise complex issue.
Engineered Porous Materials
Authors: Schaefer Dale W.
Published: August 2016

Rustum Roy recently observed that, operating from a finite number of elements, materials science faces an over-supply problem, too many scientists, and too few elements. Like all Malthusian dilemmas, relief is unlikely within the assumptions. Materials scientists, however, enjoy opportunities to develop new materials through morphological engineering of traditional substances. Carbon, after all, provided a century of progress for polymer chemists and a revolution in the manufacturing world. The trend from atomic- to molecular- to chain-level engineering can obviously be extended to mesostructure engineering and beyond. The tailoring of porosity is an example of such an extension.

This issue of the MRS Bulletin examines the concept of engineered porosity from three perspectives. Smith, Hua, and Earl discuss characterization based on classic gas adsorption and more recent NMR and scattering methods. Two articles, one by Harold and Lee, and the other by Fain, look at the pore structure requirements for important applications, reactive separations and gas separation. Finally, the materials science of engineered porosity is discussed in four articles. Even describes emulsion-derived foams in which surface-active species impose micron-scale structure that is retained during polymerization. Shapovalov broadly reviews methods to introduce porosity in metals. Durian discusses recent work on relaxation phenomena in aqueous foams. Finally, I review the factors that control structure in mesoporous aerogels.
Catalysis with Inorganic Membranes
Authors: Harold M.P. , Lee C. , Burggraaf A.J. , Keizer K. , Zaspalis V.T. , de Lange R.S.A.
Published: August 2016

Catalytic inorganic membranes are among the most challenging and intriguing porous materials. Consisting of a thin film of mesoporous or microporous inorganic material deposited on a macroporous material, catalytic membranes are multifunctional materials that must be engineered for both chemical and physical properties. New approaches to carrying out chemical reactions are possible by tailoring the membrane catalytic activity and selectivity, permselectivity, and other thin film properties. Readers are referred to several recent reviews of inorganic membranes, in particular, Zaspalis and Burggraaf, Armor, Gellings and Bouwmeister, Hsieh, Stoukides, and Tsotsis et al.

Inorganic membranes are most conveniently classified according to pore size (see introductory article). Of particular importance is the ratio of the pore size to the molecular mean free path (MFP). Decreasing pore dimensions lead to increased selectivity with corresponding loss of permeability.Macroporousmembranes have a pore size much larger than the MFP, leading to molecular (bulk) diffusion or viscous flow. Knudsen diffusion dominates in themesoporousregime, where the pore size is comparable to the MFP. In addition, surface diffusion of the molecules along the pore walls may contribute, leading to an enhanced flux of the adsorbed species along the walls. Themicroporousregime is encountered when the pore size is comparable to the molecules. This regime makes possible much higher permselectivities, which depend on both molecular size and specific interactions with the solid. Finally, in dense membranes, molecular transport occurs through a solution-diffusion mechanism, which also involves specific interactions between the solute and membrane.