The structural and compositional integrity of interfaces between semiconductor multilayers can profoundly influence the optical and electronic properties of epitaxially grown heterostructures. Understanding the atomic-scale interfacial structure and chemistry is therefore essential to correctly relate electrical measurements to theoretical models and to correlate such effects with growth conditions. High-resolution electron microscopy (HREM) has played a pivotal role in this process, providing important information on interface commensurability and revealing the presence and nature of defects.

More recently, significant advances have been made in applying HREM to the difficult problem of chemical composition mapping in systems where no structural change occurs across the interfaces. The basis of such methods involves using the objective lens as a bandpass filter and tuning in on a specific range of spatial frequencies to form a chemically sensitive interference pattern. By using a suitable low-index zone axis and choosing an optimum range of specimen thickness, the patterns can indeed be extremely sensitive to the strength and periodicities of the projected potential.

This article briefly reviews the relationships between point defects and ion diffusion in nonstoichiometric ionic crystals, with special emphasis on cubic oxides. It focuses on crystalline materials with negligibly small concentrations of nonequilibrium defects such as dislocations and grain boundaries. First, the concepts used to analyze the point defect structure and the diffusion of ions in nonstoichiometric crystals will be discussed. Then, specific oxides will be considered as examples. These oxides are manganosite, Mn1−ΔO, and spinels of the type Me3−δO4 with Fe and Mn cations, respectively.

Since 1959 when P. Duwez showed at Caltech that Au-Si alloys could be quenched into a glassy state, there has been much interest in elucidating the nature of these amorphous materials. Certainly part of the motivation for studying amorphous alloys derives from their potential technological value: they are characterized by high hardness, corrosion and oxidation resistance, and high magnetic permeabilites and electrical resistivities but an equally strong motivation is simply that they are interesting materials. Scientific curiosity is stimulated by such fundamental questions as: What is their structure? Can defects be defined within this structure? What are the possible mechanisms of atomic transport? In addition, amorphous alloys provide a paradigm of a dense metastable structure, and how atomic transport can take place in such systems without transforming to more stable configurations is not well understood. Yet, as materials formed by nonequilibrium processing, e.g., nanocrystals, superlattices, rapidly solidified and ion-beam-modified materials, etc., find their way into technology, this question becomes increasingly germane. Ironically, much of the renewed interest in diffusion in amorphous alloys has been stimulated by the discovery, also at Caltech, of the solid-state amorphizing reaction, where multilayers of crystalline metal films transform to amorphous alloys by solid-state diffusion, rather than vice versa.

The U.S. Environmental Protection Agency (EPA) is one of several agencies that serve as members of the Federal Coordinating Council of Science, Engineering, and Technology. In this role, EPA chairs the Environmental, Safety, and Health Work Group of the Committee on Materials. One of the primary purposes of the Committee on Materials is to strengthen the government-industry:university partnership in materials research. Strengthening and building this partnership is also one of the goals of the Materials Research Society. You bring together an interdisciplinary group of scientists and engineers from industry, government, and academia to exchange ideas on the forefront of materials research.

A recent National Research Council report, Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials, identified six major science and engineering initiatives. One of these six initiatives was the environment. I believe that we will see, over the next few years, a major thrust in materials research to develop environmentally benign or “environmentally friendly” materials—materials that are less toxic and less polluting and do not deplete scarce natural resources, materials that have longer useful lives, and materials that can be recycled and require less energy to manufacture.

Numerous commercial products either exist as concentrated suspensions of small particles or involve the processing of concentrated suspensions during some stage of their manufacture. Examples include foods, adhesives and glues, ceramic dispersions, paints, and polymer dispersions such as polyvinyl chloride plastisols. As a result, it is important for engineers to understand the flow behavior of these systems and how the flow behavior affects the way these materials can be processed.

For mahy years, progress in understanding the flow behavior of concentrated suspensions was slow compared to progress on dilute systems, partly because of how the study of suspensions evolved. Building on Einstein's classical work for dilute suspensions of rigid spheres, many authors attempted to modify his equations to predict the flow behavior of more concentrated suspensions, but the extension of Einstein's work met with limited success, because nonhydrodynamic interactions cari be just as important as the hydrodynamic interactions considered by Einstein, and multiple particle interactions quickly complicate the problem as the particle concentration increases.

During the last few years, liquid crystalline elastomers (LCEs) have been systematically produced by cross-linking liquid crystalline side-chain polymers. In these networks, a liquid crystalline molecule is fixed at each monomeric unit. LCEs exhibit a novel combination of properties. Due to liquid crystalline groups, they show anisotropic liquid crystalline properties similar to conventional liquid crystals (LCs); but due to the three-dimensional network-structure of the polymer chains, they show typical elastomer properties, such as rubber elasticity or shape stability. One exceptional property of this combination is demonstrated when a mechanical deformation to the LCE causes macroscopically uniform orientation of the long molecular axis of the LC units (the so-called “director”).

This response of the LC-phase structure to an applied mechanical field is similar to the effect of electric or magnetic fields on low molecular weight liquid crystals (LMLC), as illustrated in Figure 1. Figure la shows an undeformed LCE. Because of the non-uniform orientation of the director, the sample scatters light strongly so the elastomer is translucent like frosted glass. On the other hand, applying a mechanical field the director becomes uniformly aligned and the sample is transparent (Figure 1b). Such a macroscopically ordered rubber exhibits optical properties very similar to single crystals. These propertie s of LCEs offer new prospects for technical application, e.g., in nonlinear and integrated optics.

Characterization of microstructural defects in high critical temperature oxide superconductors has been very valuable in gaining an understanding of their phase transformations and insight into the relationship between defects and some superconducting properties such as critical current densities Jc. For this purpose, transmission electron microscopy (TEM) techniques have been proved powerful. In this article we will briefly summarize typical defects in the perovskite-based layered oxides, i.e., [La(1−x)(Ba,Sr)x]2CuO4, YBa2Cu3O7−δ, and (Bi,Pb)2Sr2Can−1Cun−O4+2n+δ. Then we will discuss the nature of defects in YBa2Cu3O7−δ in detail since this oxide has been most extensively studied. Among the many varieties of microstructural defects in YBa2Cu3O7−δ, we will emphasize only those we consider most important. An account of the field before 1990 is given in an excellent review by Beyers and Shaw and a more recent review of the structure of the grain boundaries in YBa2Cu3O7−δ by Babcock.

Since the types and the nature of the defects depend greatly on the crystal structure of the materials, the basic structures of the three oxides are shown in Figures la to 1c: (a) [La1−x(Sr,Ba)x]2CuO4, (2:1:4), (b) YBa2Cu3O7 (1:2:3:), and (c) Bi2Sr2CaCu2O8 (2:2:1:2). (The structure of Tl cuprates are very similar to the Bicuprate and will not be discussed here.)

Friction is often a nuisance, but it can be useful too. Brakes, clutches, and tires rely on it, of course, though the inevitable fractional heat remains a problem. Other applications use frictional heat: friction cutting and welding, skiing, skating, and curling. The damage to magnetic disks caused by head-disk contact and the striking of matches are also examples.

This article illustrates a framework where the thermal aspects of friction can be analyzed in an informative way. It uses a unified approach to the calculation of flash and bulk heating, and a helpful diagram—the frictional temperature map—to display the results. The method is approximate, but the approximations have been carefully chosen and calibrated to give precision adequate to most tasks, and the gain in simplicity is great.

The symbols used in this article are defined in Table I.

When two contacting solids 1 and 2, pressed together by a normal force F, slide at a relative velocity ν and with coefficient of friction ü, heat is generated at the surface where they meet. The heat generated, q, per unit of nominal contact area, An, per second is

The heat flows into the two solids, partitioned between them in a way that depends on their geometry and thermal properties. Figure 1 shows one geometry commonly used for laboratory tests: the pin-on-disk configuration. The pin is identified by the subscript 1, the disk by subscript 2. Solid 1 can have properties which differ from those of solid 2.

Among the many point defects in crystalline solids, antisite defects play a key role in the stability of intermetallics. Such defects are either thermal equilibrium defects or are introduced by some external forcing.

There are, indeed, many examples where intermetallics or compound semiconductors are “driven,” i.e., sustained in nonequilibrium configurations by external forcing. Good examples include the following:

∎ Intermetallics in alloys under irradiation, like FeZr2 in Zircalloy used as a cladding material in pressurized water nuclear reactors, or any of the compounds produced by ion implantation or ion beam mixing (atoms are continuously forced to change lattice sites because of replacement collsions). See, for example, Reference 1.

∎ Intermetallics in superalloys under cyclic fatigue (γ′ precipitates in persistent slip bands undergo sustained shearing, and sometimes redissolves).

∎ Intermetallics during high-energy ball milling, a promising technique to stabilize nonequilibrium phases.

∎ Ordered compounds when formed by vapor phase deposition.

In such compounds, atoms are forced to leave their optimum local surroundings by nuclear collisions, shearing, fracturing, and welding respectively, or land on a surface at a random position, while thermal jumps tend to restore some degree of local atomic order. For simplicity, we call such compounds “driven systems” or “driven intermetallics.” Indeed, such systems are driven away from the usual thermodynamic equilibrium by a permanent dynamical forcing (nuclear collisions, plastic shear, etc.).

On May 28, 1976, President Gerald R. Ford signed into law the Medical Device Amendments to the Federal Food, Drug and Cosmetic Act. The purpose of the Amendments is to ensure that medical devices are safe and effective, and properly labelled for their intended use. In order to accomplish this mandate, the Amendments provide the Food and Drug Administration (FDA) with authority to regulate devices during most phases of their development, testing, and use. The FDA's approach to this regulation focuses heavily on both the pre- and post-market phases of a device's lifetime. During the pre-market phase, the FDA concentrates on providing a reasonable assurance that new devices are adequately evaluated for safety and effectiveness. Implicit in this assessment is the concept of balancing risk to benefit. If the benefits significantly outweigh the risks for the intended use, the device will most likely be approved for marketing in the United States. Since risk/benefit assessments for new technology involve considerable clinical judgment, the FDA utilizes expert panels composed of clinicians, engineers, toxicologists, and other experts familiar with the devices. These panels are often directed to review the data provided by the manufacturer to support claims for device safety and effectiveness. If the advisory panel believes the data support the manufacturer's claims, a recommendation for approval for marketing is made to the FDA. The Agency must ultimately decide whether the device is to receive approval for marketing.

Surface phenomena, including wetting, have surely fascinated humans foi millennia. The breakup of a falling stream of water and the behavior of froths, both driven by surface energy, certainly caught the interest of people long before records were kept. The Phoenicians are credited with inventing that remarkable wetting agent, soap, in about 600 B.C., and a 2000-year-old soap factory was unearthed in the ruins of Pompeii.

It is hard to pinpoint the first scientific studies of surfaces and wetting, but it is a good guess that the earliest scientists, or natural philosophers, were interested. In 1805 Young noted that in a system consisting of a liquid and a solid phase, the former will wet the latter to a degree dictated by the surface energies of the system. If a liquid drop rests on a solid substrate, as shown in Figure 1, the wetting angle, θ, characterizes the wetting of solid by liquid. The fundamental relationship between wetting angle and surface energy in equilibrium was given by Young as:

where γsv is the surface energy of solid, γsl is the solid-liquid interfacial energy, γlv is the surface energy of liquid, and θ is the wetting angle.

Scanning electron microscopes offer several unique advantages and they have evolved into complex integrated instruments that often incorporate several important accessories. Their principle advantage stems from the method of constructing an image from a highly focused electron beam that scans across the surface of a specimen. The beam generates backscattered electrons and excites secondary electrons and x-rays in a highly localized “spot.” These signals can be detected, and the results of the analysis are displayed as a specific intensity on a screen at a position that represents the position of the electron spot. As with a television image, after a given period, information about the entire field of view is displayed on the screen, resulting in a complete image. If the specimen is thin, the same type of information can be gathered from the transmitted electrons, and a scanning transmission image is thus constructed.

The scanning electron microscope is highly versatile and widely used. The quality of the image is related to its resolution and contrast, which, in turn, depend on the diameter of the focused beam as well as its energy and current. Because electron lenses have inherently high aberrations, the usable aperture angles are much smaller than in a light microscope and, therefore, the electron beam remains focused over a relatively large distance, giving these instruments a very large depth of focus.

Scanning electron microscopes are versatile in their ability to detect and analyze a lot of information. As a result, modern versions of these instruments are equipped with a number of detectors. Developments are sometimes related to placing the detectors in a geometrically attractive position close to the specimen.