The materials science of radioactive waste forms and containment materials has long been a subject of interest to the Materials Research Society. One of the earliest (and continuing) MRS symposia, the Scientific Basis for Nuclear Waste Management, has been held 18 times since 1978. This symposium rotates abroad every third year: Berlin in 1982, Stockholm in 1985, Berlin in 1988, Strasbourg in 1991, and Kyoto this past October. Nearly 170 papers were presented at the Kyoto meeting.

Materials science issues for nuclear waste disposal are unique in their scale and consequences. The wastes include an extremely wide variety of materials: spent nuclear fuel from commercial and research reactors; high-level liquid waste produced at West Valley, New York, during the reprocessing of commercial spent nuclear fuel; high-level waste (HLW) generated by the nuclear weapons program; nearly pure plutonium from the dismantling of nuclear weapons; highly enriched uranium from weapons; low-level, medium-level, and mixed waste from laboratories and medical facilities; and, finally, mill tailings from uranium mines and the residues from chemical processing, such as the radium-bearing filtrate presently in storage at Fernald, Ohio, and Niagara Falls, New York. Some material can be simply stabilized and monitored in situ, as is done for most uranium mill tailings and residues, but other materials require retrieval, processing, immobilization, and permanent disposal. The volumes of material that will require handling, immobilization, and disposal are enormous. In the United States, much of the weapons program waste is stored in tanks at Hanford, Washington and Savannah River, South Carolina.

The atom probe field ion microscope is the most powerful and direct method for the analysis of materials at the atomic level. Since analyses are performed by collecting atoms one at a time from a small volume, it is possible to conduct fundamental characterization of materials at this level. The atom probe technique is applicable to a wide range of materials since its only restriction is that the material under analysis must possess at least some limited electrical conductance. Therefore, since its introduction in 1968, the atom probe field ion microscope has been used in many diverse applications in most branches of materials science. Many of the applications have exploited its high spatial resolution capabilities to perform microstructural characterizations of features such as grain boundaries and other interfaces and ultrafine scale precipitation that are not possible with other microanaly tical techniques. This article briefly outlines some of the capabilities and applications of the atom probe. The details of the atom probe technique are described elsewhere.

The power of the atom probe may be demonstrated by its ability to see and identify a single atom, which is particularly useful in characterizing solute segregation to grain boundaries or other interfaces. An example of a brightly-imaging solute atom at a grain boundary in a nickel aluminide is shown in Figure 1. In order to conclusively determine its identity, its image is aligned with the probe aperture in the center of the imaging screen and then the selected atom is carefully removed by field evaporation and analyzed in the time-of-flight mass spectrometer. This and many other bright spots in this material were shown to be boron atoms. This example also illustrates the light element analytical capability of the atom probe. In fact, the atom probe may to used to analyze all elements in the periodic table and has had applications ranging from characterizing the distribution of implanted hydrogen to phase transformations in uranium alloys.

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.

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

2. 2. Chemical reaction at interfaces;

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

4. 4. Complex coupled phenomena (heat transfer, diffusion, chemical reaction, phase transformation like solidification, free surface, electromagnetic force, and bubble flow).

In the microelectronics industry, integrated circuit (IC) device performance is continually increasing while the critical feature sizes are rapidly decreasing. Since this trend is expected to continue for future generations of ICs, areal density constraints often require that circuit designs utilize multilevel structures with vertical interconnects. It was recently demonstrated that the resistivity of the metal interconnects may limit device performance in multilevel thin-film structures. Although Al metallurgy (Al/2 wt.% Cu alloy) is extensively used for IC metallization today, lower resistivity metals, such as gold, copper, and silver may be necessary for designs requiring feature sizes of 0.25 μm or less. Chemical vapor deposition (CVD) is an attractive technique for the conformal filling of submicron vertical interconnects. For CVD to be generally applicable to IC fabrication, volatile precursors with adequate stability must be designed and optimized. Lastly, IC metallization typically requires that both uniformity and conformality be achieved simultaneously in a single process step.

In 1831, Faraday reported to the Royal Society of London that granular material inside a container, when vibrated, would spontaneously begin to exhibit convection rolls, similar to what is observed in normal fluids when heated from below. This observation indicated that not only can a granular material act like a fluid, but also that vibrations can affect the properties of these materials in important ways. Such phenomena are of immediate practical importance because granular materials exist all around us. We use sand and gravel to build the roads we drive on; we process grain to provide our food supply; we mine ore to provide coal, minerals, and precious commodities; we take powders and pills to cure what ails us. Many of the phenomena observed in granular media are prototypical examples of complex, nonequilibrium behavior that is also found in an increasing number of other systems. As a result, sandpiles have served as a macroscopic and visually appealing metaphor for thinking about a number of microscopic systems that are not directly accessible to our senses. Despite the common occurrence of these materials, their properties are not at all well understood and most of our knowledge centers on the subset of static, equilibrium properties of granular matter. Only over the last few years have physicists and engineers begun to unravel some of the exceptional time-dependent, nonequilibrium properties that these seemingly simple materials exhibit. This review focuses on recent developments in the newly emerging field of granular dynamics and, in particular, addresses the role of vibration in determining the phenomena observed in such media.

The first stainless steels, mainly low carbon chromium-iron alloys, have been known since the beginning of this century. These steels show good resistance against wet corrosion and high-temperature corrosion. This article focuses on high-temperature corrosion, with emphasis on gaseous sulfidizing and oxidizing environments. The discussion is limited to these two gases since corrosion involving halogen-and/or carbon-containing gases involves other specific processes. The behavior of binary and ternary alloys will be successively examined, then the role of minor elements will be considered.

Fundamental Mechanisms of High-Temperature Corrosion of Stainless Steel

Usually, a dry corrosion process results in the formation of corrosion products, giving a simple or complex oxide or sulfide scale on a metallic substrate, separating it from the aggressive gaseous environment and, consequently, acting as a protective barrier. Scale growth is controlled by the conductivity of the reaction products which are solid electrolytes. Generally, the mechanism of scale growth is governed by outward cation or inward anion diffusion processes. This is the basis of the model originally put forward by Wagner for a single metal and subsequently developed for alloys, and particularly, for stainless steels. This one-way point-defect diffusion process is responsible for the observed parabolic scaling kinetics characterized by a parabolic rate constant kp. This model is well described in the literature.

In the case of stainless steels, formation of a protective scale is required; this is possible if the oxide or sulfide products have a low diffusivity to cations or anions due to a low density of point defects in the crystal lattice. The protective characteristics of the corrosion products may be experimentally determined by measurement of their electrical conductivity, although the scales should also be effective against short-circuit transport of ions, atoms, or molecules. The best barriers consist of oxides, such as Al2O3, SiO2, and Cr2O3.

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.

Borosilicate glass is the principal solid matrix for immobilizing 99% of the highly radioactive, heat-generating nuclides extracted by reprocessing spent nuclear fuel. Production of the glass has begun in several countries, yet no final disposal site is available anywhere in the world. This is due partly to political issues and partly to the difficulties of credibly demonstrating that nuclear waste can be safely isolated in deep underground repositories for hundreds of generations. The release of hazardous quantities of radionuclides from a repository is prevented by a multiple barrier containment system, including a central engineered system consisting of the canistered glass, an overpack, and backfill materials. If the glass could retain all radionuclides upon contact with groundwater, it would not be necessary to demonstrate that geological isolation is safe. However, the glass corrodes slowly in water and humid air, and inevitably, certain quantities of radionuclides are mobilized. The glass is not inherently corrosion-resistant, but rather depends on the waste package and on surrounding geochemical and hydrological constraints. The difficulty is predicting the release/retention of radionuclides for long time periods while considering the interactions with other engineered and natural barriers.

The photorefractive (PR) effect has been studied for more than 25 years and many applications for optical signal processing such as correlation, real-time holography, dynamic interconnections, and optical memories have been developed. The main focus of study for the PR effect has been oxides (ferroelectrics and sillenites) in which the useful spectral range lies in the visible. Applications for telecommunication systems and eye-safe devices have required extending the spectral range into the near infrared (1.0 to 1.5 μm), and so the exploration of different materials. It has been shown that the bulk semi-insulating III-V semiconductors GaAs and InP, and more recently the II-VI compound CdTe, were efficient materials for this spectral range. III-V materials offer the advantage of availability as bulk semiinsulating materials of high crystalline perfection and homogeneity regarding their electrical properties due to their importance as substrate materials in micro and optoelectronic technology. However, these materials have not been optimized for PR applications, so quantitative analyses of PR experiments related to the specific material defect properties are necessary for further developments. It has equally been shown that the PR effect can be used as an efficient tool for materials characterization.

Besides the numerical tools which have been developed for solving the continuity equations in materials processing, the prediction of microstructures and defects in such processes is becoming an important step in assessing the quality, and ultimately the mechanical properties, of the final products. Because the typical length scales associated with the process and with the microstructure differ widely (typically a factor of 104), special techniques have to be used when coupling the macroscopic and microscopic levels. This contribution will briefly present some of the modeling tools recently developed for solidification. The trend is now to replace deterministic models used over the last 20 years by stochastic approaches which directly generate computed micrographs. Among those, Monte Carlo techniques originally developed for the grain growth in solids were adapted to the solidification of alloys, and cellular automata models specifically take into account the dendrite growth mechanisms.

Epitaxial thin-film growth of high-critical-temperature (Tc) superconductors has been intensively studied not only because it is one of the key technologies for electronic application but also because it provides suitable specimens for elucidating the superconducting mechanism. For simply making thick (>100 nm) epitaxial films, various deposition techniques such as sputtering, pulsed laser deposition (PLD), evaporation, including molecular beam epitaxy (MBE), and chemical vapor deposition (CVD) have been verified as applicable. For instance, high-quality YBa2Cu3O7–δ (YBCO) films, in terms of superconducting properties (Tc and critical current Jc), can be made by adjusting the cationic composition and choosing the right deposition conditions, e.g., oxygen pressure and temperature close to the decomposition line in the phase diagram. The knowledge and techniques accumulated in the high Tc field have been successfully transferred for the film growth of such oxides as dielectric, ferroelectric, magnetic, and optically functional materials. Pulsed laser deposition, especially, is now widely used for those materials and was addressed in a previous issue of the MRS Bulletin. However, as the demand for film quality increases, allowing films to be used in complex heterostructures like Josephson tunnel junctions and in well-designed physics studies, the meaning of the term “highquality film” has been changing.

Processing has always been a key component in the development of new materials. Basic scientific understanding of the reactions and transformations that occur has obvious importance in guiding progress. Invaluable insight can be provided by observing the changes during processing, especially at high magnification by in situ microscopy. Now that this can be achieved at the atomic level by using high-resolution electron microscopy (HREM), atomic behavior can be seen directly. Accordingly, many deductions concerning reactions in materials at the atomic scale are possible.

The purpose of this article is to illustrate the level reached by in situ HREM. The essential procedure is to form a high-resolution image of a standard transmission electron microscope (TEM) sample and then to alter the structure by some means in a controlled manner, such as by heating. Continual recording on videotape allows subsequent detailed analysis of the behavior, even on a frame-by-frame (1/30 second) basis. The most obvious advantage is to follow the atomic rearrangements directly in real time. However, in addition, by continuous recording no stages in a reaction are missed, which can often occur in a series of conventional ex situ annealed samples because of the limited number of samples that can realistically be examined by HREM. One can be sure that the same reaction, in the same area, is being studied. Furthermore, by changing the temperature systematically, extremely precise kinetic measurements can be made (e.g., for activation energies and kinetic laws) and the whole extent of a material transformation can be investigated in one sample, something that would take months of work if studied conventionally. The information provided by in situ HREM is often unique and so it can become an important technique for fundamental materials investigations.

An atomic-level understanding of surface phenomena is becoming increasingly important as materials scientists and engineers begin to fabricate new materials by controlling their growth at the nanometer or subnanometer scale. Recent advances in molecular beam epitaxy and chemical vapor deposition make it possible to assemble a crystalline solid or epitaxial overlayer literally one atomic layer at a time. The need to characterize the structure and composition of these complex materials in finer and finer detail has forced the traditional analytical tools (e.g., electron microscopy) to strive for better and better spatial resolution. It has also generated a virtual explosion in the proliferation of scanning probe microscopies inherently capable of viewing surface structure at the atomic level. This same need has recently rekindled an interest in the technique that first allowed scientists to view a solid surface in atomic detail: the field ion microscope (FIM). The unique attributes of this instrument and its successor, the atom probe mass spectrometer, make it possible to observe individual atoms on a solid surface, to remove atoms from the surface one atomic layer at a time, and to determine the chemical identity of the atoms as they are removed. The close match between these capabilities and the requirements of modern-day materials analysis have stimulated renewed efforts to use the FIM to gain fundamental insight into materials problems. This article discusses a few selected applications of the FIM, individually and combined with the atom probe, to phenomena occurring at the surface of solid materials.

Aqueous foams are macroscopically homogeneous complex fluids composed of co-existing gas and liquid phases. A hierarchy of structure and self-organization at progressively smaller length scales is ultimately responsible for the unique material properties of foams, and is shown schematically in Figure 1. A key feature of this microstructure is the continuous random network of liquid which typically occupies 1–10 vol% and separates the tightly packed gas bubbles. As shown by the photographs in Figure 2, the average bubble diameter can vary widely from about 10 μm to 1 cm, while the average bubble shape can vary from nearly spherical to nearly polyhedral, depending on the liquid volume fraction. At the smallest structural scale, surface active molecules are preferentially adsorbed at the gas/liquid interfaces and give rise to several physical-chemical effects that deter the coalescence of neighboring bubbles and thereby lend stability to the foam. However, the delicate structure formed by gas bubbles packed in a continuous liquid phase is not truly stable with either time or applied forces. This article reviews recent progress in understanding the structural response of aqueous foams to temporal and mechanical perturbations.

Pure carbon materials, graphite and diamond, possess a wide array of interesting physical properties, and so attract a large spectra of interests and applications. Carbon microparticles (carbon black) and carbon fibers are widely used in practical applications including common materials (paints, inks, polymers, etc.) and high-performance composite materials.

Carbon displays a remarkably rich and complex chemical behavior (three different possible hybridizations: sp1, sp2, and sp3). In particular, the covalent carboncarbon bond is one of the strongest in nature, and induces a high melting temperature (> 4000°C). The phase changes associated with unusually high temperatures and pressures as revealed in the carbon phase diagram, and the fact that the solid sublimates at low pressures before melting, lead to many experimental difficulties in the study of high-temperature properties of carbon materials. Experiments must therefore rely on transient melting, for example, laser vaporization or arc-discharge heating. This explains why fullerenes and related graphitic structures have only recently been discovered.

From a fundamental point of view, the discovery of fullerenes has introduced new ideas about how carbon atoms bond. The curvature and closure of graphitic surfaces has become a standard concept in carbon chemistry, and recently a wide range of structures formed by curved graphitic networks has been observed. A surprising aspect of fullerene research is that these novel graphitic structures were found in well-known experiments, and that they had been overlooked for so many years.

This article will describe recent progress in the generation and physical characterization of graphitic nanoparticles, or multishell fullerenes. The lack of an efficient method for producing, as well as a method for purifying these particles makes it difficult to characterize them and to develop possible applications.

The versatility of wood as a raw material is emphasized by the realization that the mass of wood consumed annually in the United States is nearly that of the combined total U.S. consumption of aluminum, plastics, cement, and steel (Figure 1). Partly as a result of the enormous quantities consumed, many wood and paper products also make up significant fractions of the materials disposed in landfills, despite accelerated recycling efforts, notably those of the paper industry. With a target recycle rate of 40% by 1995, the paper industry will make further progress in alleviating some of the disposal problems, but additional efforts to recycle wood and wood-fiber-based materials into other types of products will also be needed. Many of these opportunities have been described. A common denominator in these utilization schemes is to consider how the morphology of secondary wood-based materials may limit their use, and how the morphological characteristics of recycled fiber and wood may influence the properties of the materials produced from them. These considerations suggest, at least partly, a materials science approach to the utilization of recycled fiber and wood, particularly for the fabrication of wood-based composites.

Continued dimensional scaling of the elements of integrated circuits places significant restrictions on the width, density, and current carrying capability of metallic interconnects. It is expected that, by the year 2000, the transistor channel length will be at 0.18 μm, while microprocessors will pack more than 15 million transistors over an area ~700 mm. To conserve area, interconnects will continue to be stacked at an increasing number of levels (six by the year 2000, versus four in today's leading microprocessors), and the minimum spacing and width within an interconnect layer will shrink to 0.3 μm. In addition, it is expected that future interconnects will need to sustain increasingly higher current densities without electromigration failures.

Aluminum alloys are the conductors of choice in present-day interconnects, and much effort is focused on means to extend the usefulness of aluminum through improvements in reliability, either by new alloy formulations or by the development of complicated multimetal stacks. A more radical approach, which is gaining increased attention, is the replacement of aluminum altogether by copper. The bulk resistivity of copper is significantly lower than that of aluminum (1.7 μΩ cm for Cu versus 3.0 μΩ cm for Al-Cu), which is expected to translate to interconnects of higher performance because of reduction in signal propagation delay. In addition, the significantly higher melting temperature of copper (~1100°C versus ~600°C for Al-Cu alloys) and its higher atomic weight are expected to translate to improved resistance to electromigration.

Bandgap engineering of thin semiconductor layers and defect engineering combine to form photorefractive (PR) quantum well structures. PR quantum wells are semi-insulating thin films useful for dynamic holography and other coherent and incoherent optical applications. As materials for thin-film dynamic holography, they have high nonlinear-optical sensitivity and high speed.

The PR effect translates a spatially varying irradiance, from the interference of two or more coherent light beams, into a refractive index grating. The multiple-step PR process begins with photoexcitation of charge carriers, followed by transport and trapping of charge at deep defects. The trapped space-charge generates electric fields that alter the refractive index of the material through the electrooptic effect. The same laser beams that generate the gratings diffract from the gratings, leading to a rich variety of multiple-beam effects, such as two-wave and four-wave mixing.

Because the PR process involves several distinct physical parameters, such as carrier mobility and electrooptic coefficients, optimized performance requires a coincidence of favorable properties in a single material. Rather than relying on coincidence, bandgap engineering of multiple layers of semiconductors provides a way to individually tune the separate material pa rameters. Likewise, defect engineering in semiconductors provides flexibility in the choice of defects, their concentrations, and degree of compensation. Bandgap and defect engineering combined make custom designed PR materials possible.