Cadmium telluride is currently the most promising material for high efficiency, low-cost thin-film solar cells. Cadmium telluride is a compound semiconductor with an ideal 1.45 eV bandgap for direct light-to-electricity conversion. The light absorption coefficient of CdTe is high enough to make a one-micrometer-thick layer of material absorb over 99% of the visible light. Processing homogenous polycrystalline thin films seems to be less critical for CdTe than for many other compound semiconductors. The best small-area CdTe thin-film cells manufactured show more than 15% conversion efficiency. Large-area modules with aperture efficiencies in excess of 10% have also been demonstrated. The long-term stability of CdTe solar cell structures is not known in detail or in the necessary time span. Indication of good stability has been demonstrated. One of the concerns about CdTe solar cells is the presence of cadmium which is an environmentally hazardous material.

Microwave processing technology has taken on increasing significance in manufacturing. Numerous equipment configurations that take advantage of heating by coupling microwave radiation to materials have been developed for curing, bonding, and sintering materials. Such industrial microwave systems generally operate at centimeter wavelength frequencies of 0.915 or 2.45 GHz. Although they offer new methods of industrial processing, their application is often limited by the degree of coupling of the microwave energy to the materials to be processed.

Recently, millimeter wavelength generators have become available. They have enabled scientists and engineers to devise new methods for processing materials. Radiation in these superhigh frequency (SHF) domains interacts differently with materials than that at centimeter wavelengths. For many materials, the absorption of microwave energy at millimeter wavelengths is significantly greater than at 0.915 or 2.45 GHz. This in turn offers new opportunities for thermally processing materials in significantly shorter processing times, or for modifying their properties.

The recent collapse of the Soviet Union has opened previously closed laboratories and industrial facilities. Discoveries and equipment that were once closely guarded secrets are now being shown and shared. Institutes, which only a few years ago would have been suspicious of Western contacts now welcome potential investors, licensers, or co-workers. Some of these institutes have made significant advances in microwave instrumentation and its application to industrial processing. The Institute of Applied Physics in Nizhny Novhorod (Gorky), Russia, has developed high-power gyrotrons that operate at millimeter wavelengths.

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.)

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).

Reliable long wavelength laser diodes emitting in the 1.30−1.55 μm regime with an expected operating life greater than 25 years for optical fiber communication applications are now fabricated using a combined heavy screening and accelerated aging process. For a given laser structure, the reliability of the devices depends intricately on both the crystalline perfection of the complex buried-heteroepitaxial semiconductor structures as well as the qualities of structures external to the semiconductor, such as electrical contact, dielectric coating, bonding, and packaging structures external to the semiconductor. The former (crystalline perfection) determines the intrinsic property of the laser, while the latter qualities (electrical contact, etc.) determine the contact resistance, parasitic capacitance, heating, and mechanical and thermal stresses to which a packaged laser device is subjected during operation. Since device heating and external stresses both degrade laser performance and accelerate permanent damage—through processes such as crystalline defect formation, current leakage path development, and doping profile redistribution—a reliable laser device, therefore, requires both a perfect semiconductor structure and also a high-quality external structure. Realistically, however, this may not be easily achievable, especially when a development program is limited in resource and time. Proper choice of the critical material issues in a development process becomes crucial to the success of the program.

The infrastructures of the United States and other industrialized nations are, to a large degree, based upon concrete. With such a pervasive application, the importance of accurately analyzing concrete structure failures and addressing durability problems is obvious, as is the timely recognition of premature structure deterioration, so that an appropriate recommendation for remediation may be made. Although concrete is one of the most important engineering materials in use today, it is perhaps also the least understood, primarily because it is chemically reacting and changing throughout its service life. The chemical and physical changes that occur in this material can have degenerative effects on its mechanical properties and, ultimately, on its durability. Understanding how these chemical reactions occur within the concrete and affect its physical characteristics is often not straightforward. For example, the reactions can be affected by internal components (cement, fine and coarse aggregate), mixing procedures, curing cycle parameters, and/or the choice of additives. External influences on these reactions include weather exposure, chemically aggressive environments, and climate-mitigating surface treatments. Furthermore, there are some deleterious internal reactions that tend to occur sequentially and others that may occur concurrently. The overlap of simultaneous reactions can make it difficult to isolate the initial occurrence and to identify the deleterious component of deterioration. To resolve such questions, it is necessary to understand the interrelationship among the concrete components and their processing, and to examine the material with a variety of complementary techniques and sample configurations.

There have been many achievements in the last decade in the development of materials science, chemistry, and physics (experimental and theoretical). However, the most difficult problem—calculating the intrinsic properties of multicomponent compounds starting from the knowledge of their constituent components' properties—still remains unsolved. Calculations or predictions based on only the properties of constituent components (or simply, properties) are called a priori calculations or predictions. These difficulties are due to the solution of mathematical problems arising in the quantum mechanical calculations of multi-electronic systems. As a result, scientists make use of many empirical prediction methods that use existing regularities from a variety of property data. Some of the empirical criteria for the formation of compounds with predefined properties use the rules of Hume-Rothery, Laves, Mathias, Goldschmidt, Villars, and Darken-Gurry.

Friction, or the resistance to motion of two bodies in contact, and the related phenomenon of wear are two of the more costly problems facing industry today. Despite their importance, a fundamental understanding of friction and wear, especially at the atomic scale, has remained elusive. This is rapidly changing, however, as new scientific instrumentation has been developed that allows, for the first time, the study of friction at the atomic scale. These pioneering efforts have led to the emergence of a rapidly growing field called nanotribology, the subject of this issue of the MRS Bulletin. Some of the contributing techniques include the surface force apparatus, which has been used to study the rheology of molecularly thin liquid layers, a quartz-crystal microbalance, which has been used to measure the sliding friction of molecularly thin adsorbed films, and the atomic force microscope (AFM), which has been used to measure the frictional force between a sharp tip (possibly a single asperity) and a flat surface during sliding. In addition to providing a vast amount of information related to friction on the atomic scale, these innovative experiments have provided the necessary data to test the validity of older theoretical models and have stimulated new theoretical work. For instance, atomic-scale friction has been investigated theoretically using analytic models, first principles calculations, and molecular dynamics simulations.

The precision machining of metal surfaces and the ductile-regime grinding of ceramic surfaces are examples of fundamental cutting processes used in fabricating high-tolerance parts. Components with dimensional tolerances of a few tens of nanometers are currently being produced by direct machining with single-point diamond tools. Despite the ability to fabricate these parts, little is understood of the basic deformation mechanisms that determine how material is removed and deformed, how a tool-tip interacts with a workpiece, how induced surface and subsurface damage occurs, and how cutting tools wear.

The key to solving these problems is a fundamental understanding of basic tribological processes such as surface indentation and scraping. Indentation experiments measure the mechanical response of a surface, the onset of plastic deformation, and material hardness. Macroscopic hardness measurements have been shown to correlate well with observed tensile yield strengths. Microscopic indentation studies, where the indentation size is smaller than the material grain size, show new and interesting phenomena. In the pioneering work of Gane and Bowden, no permanent penetration occurred until a critical load was achieved. They related this critical yielding to the theoretical shear strength in the metal, the strength required to create dislocations. Yielding of this sort has since been observed by many investigators.

The environmental degradation of materials poses a serious limitation in the utility of engineering materials. The corrosion of metals, for example, has been estimated to represent a 4–5% decrease in the Gross National Product each year. To this, add losses involved in the replacement or restoration of ceramic structures such as buildings and transportation systems, i.e., the “infrastructure,” and the result is a significant sacrifice of economic strength.

Most of us are familiar with the consequences of exposing materials to environments in which the materials are chemically unstable and convert into substances that are unable to perform the function for which the original material was selected. The corrosion of metals into soluble or insoluble oxidation reaction products, chain scission or molecular mutation in polymers, even hydrolysis and leaching of silicious ceramic compounds represent behavior which ultimately limits the service applications of most engineering materials. For example, aluminum and its alloys are unsuitable for use in environments where oxide formation rates are high enough to represent a problem with respect to useful service life.

While there are only two group IV semiconductors, Si and Ge, the III-V material system offers a nearly infinite number of single-crystal semiconducting compounds to choose from. For example, there are nine binary compounds resulting from combining the group III elements Al, Ga, or In with the group V elements P, As, or Sb. From this group, high-performance photovoltaic cells have been made from GaAs, GaSb, and InP. Using ternary III-V compounds, high-performance photovoltaic devices have been made with AlGaAs, GalnAs, GaAsP, and InGaP2. All of the above-cited photovoltaic cells have respectable energy conversion efficiencies of over 15%.

A primary strength associated with the III-V semiconductor material system lies therefore in the diversity of materials to choose from. Thus, a single-crystal III-V compound can be found with almost any bandgap ranging from 0.16 eV for InSb to 2.24 eV for GaP (or 3.5 eV for GaN). But not only can one choose a material with a particular bandgap, one can, more importantly, as a result of tetrahedral covalent bonding, also be assured that that material can be grown as a single crystal with either n- and p-type doping, and that junctions can be formed with nearly ideal rectifying characteristics. The implication this holds for solar cells of ideal diode characteristics is the achievement of theoretical limit open-circuit voltages and fill factors. An additional important feature associated with many of the III-V compounds is the direct-gap photon absorption which translates to a short optical absorption distance, with the result that excited-state carriers in a solar cell are created near the junction.

Portland cement and Ca(OH)2, “slaked lime,” are representative of a family of materials that have conceptual and practical advantages as matrices for the immobilization of wastes. These are relatively proven construction materials with an extensive history of use in various ground-water regimes, and in various climates. The raw materials are widely available and they exhibit a reasonable amount of resistance to physical damage and attrition. They are also durable. Although modern Portland cements are only about 150 years old, numerous examples of Roman concrete made from slaked lime and volcanic pozzolanic ash survive; the Pantheon in Rome (circa A.D. 300) continues in regular use.

Satisfactory immobilization matrices can also be formed by incorporating other artificial waste materials, e.g. coal combustion fly ash, blast furnace slag, and natural materials such as calcined kaolin or siliceous volcanic ashes containing glass and zeolites. These materials react with Ca(OH)2 and cement to become an integral part of a cement matrix. Thus one waste (ash, slag) can be used to help immobilize another, and may actually improve the retentive capacity of a cementitious matrix.

Since cements require water to harden, they are tolerant of wet wastes and are especially useful for many effluents. In addition, they are low-cost, nonflammable materials with the potential to provide chemical and physical immobilization of wastes, but their set characteristics can be affected by certain material additions. Table I balances some advantages against disadvantages of the use of cement systems for waste immobilization.