For more than 50 years, scientists have studied the “magic dust” of high-temperature oxidation—certain oxygen active or “reactive” elements which, when added to alloys in small quantities, effect profound improvements in their oxidation resistance. In general, high-temperature oxidation resistance is achieved by the oxidation of one or more alloy components to form a dense, stable, slow-growing, external oxide layer, or ’scale” such as α-Cr2O3, α-Al2O3, or SiO2. When added properly, reactive elements have a beneficial effect on the formation and growth of both α-Cr2O3 and α-Al2O3 scales. A standard list of reactive element (RE) effects would include: (1) an improvement in scale adhesion or resistance to spallation, (2) a change in the scale growth mechanism, (3) a reduction in the oxidation rate, related to the change in mechanism, (4) a modification in the scale microstructure, and (5) in the case of alloys that form Cr2O3 scales, an improvement in selective oxidation, meaning that a lower Cr concentration in the alloy is required to form and maintain an external Cr2O3 scale.

Since the discovery of superconductivity in layered cuprates there has been a strong desire to grow thin films of these materials in a so-called layer-by-layer manner, where each layer represents one atomic monolayer. There has been worldwide interest in this problem and numerous groups have invested time and equipment in an attempt, first, to grow the known materials using a sequential deposition technique so that the atomic monolayers are stacked artificially on top of one another in a sequence defined by the structure of the given unit cell, and second, to search for new stacking sequences of such atomic monolayers, thus making metastable compounds.

Understanding the growth of thin films using any deposition technique requires information about two fundamental processes, the nucleation of a crystal and its growth. Both are strongly affected by the three thermodynamic parameters, temperature, pressure, and chemical potential (composition), at the growth front. The third parameter, the surface composition from which the desired compound can nucleate and propagate, probably provides the greatest degree of freedom in the growth process. The compositional phase diagrams of most cuprates show that several phases can coexist with the superconducting phase at a certain temperature and pressure. This is also the main reason why a widely changing surface composition, such as that present during layer-by-layer or block-by-block deposition, still produces high-quality thin films.

This article describes the application of transmission electron microscopy (TEM) to real-time, in situ dynamic observations of dislocations in strained epitaxial semiconductor heterostructures. Such experiments allow us to directly observe the formation, motion, and interaction of mis-fit dislocations. Preliminary extension of this work to the in situ measurement of the electrical properties of misfit dislocations will also be described.

The Fundamental Scientific Issue

It is well established that it is possible to grow a thin, coherent epitaxial layer on a substrate with a slightly different lattice parameter, as illustrated in Figure la. This concept is known as strained layer epitaxy. In the fields of semiconductor physics and device design, strained layer epitaxy offers many exciting new opportunities (see Reference 1 for a review). A coherently strained structure, however, will store an enormous elastic strain energy density in the epitaxial layer, due to the distortion of interatomic bonds. Therefore, as the epitaxial layer increases in thickness during growth, it will become increasingly energetically favorable to relax this strain energy. A number of relaxation routes exist: (1) roughening of the epitaxial layer surface (see, for example, Reference 2); (2) interdiffusion of the layers (this will generally only be significant at temperatures which are a large fraction of the layer melting temperatures (e.g., Reference 3)); and (3) introduction of a dislocation network into the substrate/epilayer interface, which as shown schematically in Figure lb, will allow the epitaxial layer to relax toward its bulk lattice parameter. This dislocation mechanism is the most prevalent strain relaxation mechanism at typical crystal growth and processing temperatures, and we concentrate on this mechanism in our experimental studies.

Nanostructured materials have recently attracted the attention of some materials scientists. Because of their unique properties occurring in low-dimensional structures, nanostructured materials are sought for their possible industrial applications. This article introduces a specific nanostructured material, the carbon nanorube—an extremely thin filaments of graphite considered to be a quasi one-dimensional structure, with a simple well-understood atomic structure. Because of these qualities, the carbon nanorube has elicited great interest from diverse fields of basic and technological research. My discovery of carbon nanotubes was inspired by the discovery of C60 and its family and their mass production. The carbon nanotubes were serendipitously found during the examination of fullerene materials by a high-resolution transmission electron microscope (HRTEM). Since introducing this technique in 1971, I have been employing HRTEM to characterize the microscopic structural details of a variety of materials, including carbonaceous materials. So far, only nanotubes have been revealed with HRTEM.

Interest in the carbon nanorube is multifold. Academically the nanotube is an ideal model structure for a quasi one-dimensional structure since its known atomic structure makes computer simulations more reliable. It is worthwhile to study both rare structures of graphite—cylindrical forms with a helical arrangement of carbon atom hexagons and flexible graphitic sheets containing topological surface defects. These materials may find practical uses as tough graphite fibers, molecular wires, catalyst supports, molecular adsorbers, and so on.

The production and use of aluminum alloys involves a wide range of processing and fabrication methods. For a number of reasons discussed in this article, process modeling has an increasingly important role in all areas of production. This article reviews some process modeling activities in aluminum processing. The aims of the review are to (1) describe the current state of research in two major processing areas, (2) establish the principal motivations in the aluminum industry for undertaking process modeling, and (3) to identify generic research issues most in need of attention. This review is not intended to be exhaustive, but considers only selected processes, based on work in Norwegian laboratories with which the authors are most familiar. There is considerable parallel modeling activity worldwide in industry and academia which will not be discussed here. The case studies selected are sufficient, however, to illustrate all the main points of the review.

A recent review of materials process modeling identifies a number of recurring themes which indicate the commercial reasons why process modeling is undertaken in various industries. They may be summarized as modeling to (1) develop new or scaled-up processes, (2) reduce the number of trials, (3) optimize an existing process, and (4) improve quality by reducing product variability.

So the overriding concerns in industry are improving productivity and quality, and reducing costs. Modeling is also important from the point of view of model based real-time control of processes, but developments in this area are at present limited to relatively slow processing operations. Models are also important tools for both industry and academia to improve scientific understanding of materials processing. Within a company, modeling serves as a means to assemble know-how about the company's processes, raising the level of training of the workforce and making it possible for people to learn a process quickly.

Porous metals are engineered materials; they are designed for special properties. Technological progress necessitates expanding the choices of such materials, making the development of new porous metals a relevant challenge for materials scientists.

Although a wealth of information has already been accumulated on these materials, new results are published every year, extending the engineer's capability to manufacture porous metals and revealing their unknown and often unusual properties. This survey describes the state of the art and some recent accomplishments in the field. This article discusses manufacturing practices, structure, properties, and applications of porous metals. Promising new research issues are also highlighted. Materials whose pores were not formed in situ, like honeycomb structures made by high-energy beams etc., are not covered.

The geologic disposal of spent nuclear fuel is currently under consideration in many countries. Most of this fuel is in the form of assemblies of zirconium-alloy-clad rods containing enriched (1–4% 235U) or natural (0.71% 235U) uranium oxide pellets. Approximately 135,000 Mg are presently in temporary storage facilities throughout the world in nations with commercial nuclear power stations.

Safe geologic disposal of nuclear waste could be achieved using a combination of a natural barrier (the host rock of the repository) and engineered barriers, which would include a low-solubility waste form, long-lived containers, and clay- and cement-based barriers surrounding the waste containers and sealing the excavations.

A requirement in evaluating the safety of disposal of nuclear waste is a knowledge of the kinetics and mechanism of dissolution of the waste form in groundwater and the solubility of the waste form constituents. In the case of spent nuclear fuel, this means developing an understanding of fuel microstructure, its impact on release of contained fission products, and the dissolution behavior of spent fuel and of UO2, the principal constituent of the fuel.

Over the past few years, recycling within the pulp and paper industry has been brought sharply into focus. The primary motivating factors for this have been the crisis in the landfills, of which paper is the largest component, the ensuing legislation, greater public awareness of recycling as an issue, and a desire to recycle more.

In reality, recycling has always been a significant part of papermaking. On a worldwide basis, about one third of the fiber supply is recycled fiber (Table I). A further significant portion of the fiber supply comes from the use of residues from sawmills and other woodworking operations; thus, in the strictest sense, the recycled content is even higher.

The amount of recycled paper that is used varies from 45–50% for Japan and Continental Europe, which have limited forest resources, to about 10% for Canada and Scandinavia, which are rich in forest resources.

In the United States, the recycle rate is about 26% and increasing. Estimates of future usage vary, but the industry as a whole is moving toward a recycle rate close to about 40% or higher by the end of the decade.

It is important to understand the effect that recycling has on fiber properties and also the effect that adding recycled pulp can have on the final sheet properties. To arrive at this, it is worthwhile to review the main aspects of the recycling process and how the process affects pulp properties.

The discovery that C60 Buckminsterfullerene may be created spontaneously in high yield when carbon vapor condenses indicates that graphene sheet curvature and closure is a common occurrence during carbon nucleation to form extended networks. As a consequence, a net microscopic perspective on graphitelike carbonaceous materials has evolved. This perspective is summarized here because the net observations relate to various types of nonplanar graphitic structures that promise to be useful as viable nanoscale engineering materials.

C60 Buckminsterfullerene was discovered in 1985 among the products of chemical nucleation of carbon in the gas phase. The faint whisper, embedded in the helium wind that blew a laser-initiated carbon plasma into a mass spectrometer, was interpreted as the possible signature of the elegant soccer-ball-shaped molecule. That noticed signal, which indicated that a pure 60-carbon-atom molecule might be very stable, was quite unexpected and thus a truly serendipitous discovery. The original aim of the experiments was to simulate the conditions in a red giant carbon star and particularly to probe an earlier proposal that long carbon-chain molecules might have originated in such celestial objects and might have been subsequently ejected into interstellar space. The carbon chains themselves had been discovered to be abundant in certain regions of the interstellar medium by a radioastronomy search program in the late 1970s. The molecular rotational frequencies, which formed the basis for the radio search, resulted from a spectroscopic study of cyanopolyynes that had been synthesized in the laboratory.

The intermetallic compound, TiAl, and materials based on it have been receiving considerable attention because of their high specific strengths at elevated temperatures. Their mechanical properties are being extensively studied, with a view to application in aircraft or automobile engines. Before using these materials at elevated temperatures, their interactions with the environment should be investigated and suitable measures taken to overcome any degradation. It has, however, been reported that protective Al2O3 scales do not form when TiAl is exposed to an oxidizing atmosphere in spite of its high aluminum content. The formation of an A12O3 scale, or at least a scale containing a continuous A12O3 layer, is a prerequisite for providing sufficient protection. Therefore, much effort has been expended in choosing suitable alloying additions. However, relatively less effort has been devoted to surface coatings on such materials. The purpose of the alloying addition is two-fold: to improve mechanical properties, particularly ductility, and to improve oxidation resistance. However, compatible alloying elements are rare. For instance, some elements which improve the ductility, e.g., vanadium, chromium, and manganese, lower the oxidation resistance. Accordingly, a reasonable concept for the designer is to add effective alloying elements to improve the mechanical properties and to apply a coating to provide protection against the environment. For improved reliability, elements that improve the oxidation resistance can also be added to increase the protection unless they reduce the required mechanical properties.

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.

“Optical computing” is a term which has been used to describe many types of optical systems. This article concentrates on one optical computer design which appears to be particularly promising and competitive with digital electronic systems and which can use photorefractive (PR) crystals. We begin by discussing the motivation for constructing optical and optoelectronic computing systems and go on to describe two approaches to constructing optical computers. The first does not use PR crystals as yet but serves to indicate other approaches. The second approach involves the operation of one type of optical neural network. We discuss in detail the recording of holographic memories in a PR crystal for inclusion in the optical neural network described here.

The crystalline-to-amorphous (c-a) phase transformation can be induced by a variet of solid-state processes ranging from energetic particle irradiation, interface inter diffusion reactions, hydrogen charging and mechanical deformation to the application of high pressures. During the past decade, such transformations have become the focus of considerable research not only because of their potential technological applications, but also because of strong scientific interest in the relationship between the c-a transition and the melting process.

A common feature underlies all solid state amorphization processes: The atomic disorder created in the crystalline lattice in the form of static atomic displacement can induce volume change and elastic softening of the lattice. A particularly striking example of the softening effect is shown in Figure 1 for the case of radiation-induced amorphization of the intermetallic compound Zr3Al. The compound, which has the Ll2 (Cu3Au)-type superlattice structure, was irradiated with energetic ion at room temperature in a high-voltage electron microscope interfaced to a tandem ion accelerator. The rapid decrease in the intensities of both fundamental and superlattice reflections show that irradiation introduces antisite defects (chemical disorder) as well as static atomic displacements. The disordering of the long-range ordered structure, which occurs prior to the onset of amorphization, is accompanied by a volume expansion of about 2.5% and a ~25% decrease in the average velocity of sound. This decrease in sound velocity corresponds to a ~50% decrease in the average shear modulus, which is comparable to that observed for many metals during heating to melting. The volume dependence of this disorder-induced elastic softening is also similar to that associated with heating. In both cases, the shear modulus is a linearly decreasing function of volume expansion. However, for a given amount of expansion, the softening associated with static atomic displacements is nearly twice as large as that associated with increasing anharmonic lattice vibrations.

Synroc, a ceramic made from a reactive mixture of Al, Ba, Ca, Ti, and Zr oxides, is proving to be a suitable and effective medium for immobilizing nuclear wastes.

Synroc-C, a titanate-based ceramic variant, was initially developed in 1978 by Ringwood et al. for immobilizing high-level nuclear waste (HLW) from nuclear power reactor fuel reprocessing. HLW is essentially a solution of radioactive fission products, actinides, and process contaminants in ~3 mol/L nitric acid. The developers of Synroc-C aimed to immobilize radioactive waste ions by incorporating them in a ceramic. They accomplished this by mixing the HLW solution (liquid waste) with a ceramic precursor, then forming the ceramic by drying, calcining, and hot-pressing the mixture in a metal container for two hours at 1200°C/20 MPa. The result, Synroc-C, is composed of hol-landite, zirconolite, perovskite, and rutile, together with a few percent of minor phases and metal alloys. The Synroc-C precursor has the following composition (wt%): Al203(5.4); BaO(5.6); CaO(11); TiO2(71.4); and ZrO2(6.6). Since 1984, it has been prepared by hydrolyzing a mixture of Al, Ti, and Zr alkoxides with an aqueous slurry of Ba and Ca hydroxide. The abundances of the phases, and the radionuclides contained in them in dilute solid solution, are identified in Table I.

Oxides exhibit most of the interesting phenomena known to occur in solid-state systems. As a class of materials they may be richer in phenomenology than any other comparable class. Oxides can be insulators, semiconductors, or metals. The copperoxide-based compounds we have studied are superconductors with the highest critical temperatures. In some oxides, electrons manifest simple single-particle transport properties, with a high mobility; in others, they show strongly correlated behavior resulting in a Mott-Hubbard transition, localization, and charge- or spin-density waves. In some oxides, electron-phonon coupling leads to polaronic transport. Others show collective states such as magnetism; in some there are large local magnetic moments that can couple to form ferromagnetic or antiferromagnetic phases that exist up to high temperatures. Yet others have large nonlinear dielectric and optical properties. In fact, it would seem there is very little that some such oxide couldn't do for or to the experimenter.

Porous polymeric foams are well-known materials and have been produced by a wide variety of techniques ranging from leaching soluble fillers through gas-blowing to phase separation. Although these materials are widely used, their structure is often irregular and difficult to control. However, a novel method for producing porous materials with a more regular structure has been developed based on high internal phase emulsions (HIPE). These foams are called emulsion-derived foams and are commercially known as Polyhipe® (Unilever) polymers.

If one takes a liquid oil, introduces water and a suitable emulsifier with agitation, a dilute emulsion is produced with small water droplets dispersed within the oil phase, about the consistency of milk. If the amount of the water (i.e., the internal or pore forming) phase is increased, a concentrated emulsion will be formed and the consistency of the mixture changes into a more viscous fluid. As further water is introduced, a limit is reached where the droplets are so prevalent that they approximate a close-packed structure (see cover of this issue); beyond this point the droplets must distort to accommodate any additional internal phase. For a collection of monodispersed spheres this packing limit corresponds to 74% by volume and is the accepted definition of a high internal phase emulsion, i.e., an emulsion with an internal phase greater than 74%.4 The structure of the emulsion is now analogous to soap bubbles, with thin films surrounding and separating the drops. Note also that, compared to “standard” emulsion polymerization, this is the exact reverse or “inverse” of that process. In the standard route, the dispersed oil phase is polymerized to yield a collection of polymeric spheres; with inverse emulsion, the continuous phase is polymerized, yielding a rigid matrix.

Welding is utilized in 50% of the industrial, commercial, and consumer products that make up the U.S. gross national product. In the construction of buildings, bridges, ships, and submarines, and in the aerospace, automotive, and electronic industries, welding is an essential activity. In the last few decades, welding has evolved from an empirical art to a more scientifically based activity requiring synthesis of knowledge from various disciplines. Defects in welds, or poor performance of welds, can lead to catastrophic failures with costly consequences, including loss of property and life.

Figure 1 is a schematic diagram of the welding process showing the interaction between the heat source and the base metal. During the interaction of the heat source with the material, several critical events occur: melting, vaporization, solidification, and solid-state transformations. The weldment is divided into three distinct regions: the fusion zone (FZ), which undergoes melting and solidification; the heat-affected zone (HAZ) adjacent to the FZ, that may experience solid-state phase changes but no melting; and the unaffected base metal (BM).

Creating the extensive experimental data base required to adequately characterize the highly complex fusion welding process is expensive and time consuming, if not impractical. One recourse is to simulate welding processes either mathematically or physically in order to develop a phenomenological understanding of the process. In mathematical modeling, a set of algebraic or differential equations are solved to obtain detailed insight of the process. In physical modeling, understanding of a component of the welding process is achieved through experiments designed to avoid complexities that are unrelated to the component investigated.

In recent years, process modeling has grown to be a powerful tool for understanding the welding process. Using computational modeling, significant progress has been made in evaluating how the physical processes in the weld pool influence the development of the weld pool and the macrostructures and microstructures of the weld.