The United States is at an important crossroads in its management of high-level radioactive waste (HLW). It is presently in the process of changing over from a strategy that was developed during World War II, which involves storing HLW in a relatively mobile liquid form, to a strategy of the 1990s, which involves removing and isolating potentially harmful radionuclides and immobilizing them into solid and inert forms, more specifically, borosilicate glass. The resulting waste glass products will then be permanently disposed of by deep burial, within stable geologic formations, where they become one element of a multibarrier waste-isolation system. This barrier system is designed to retain radionuclides so they can be permanently isolated from the public and from the accessible environment. Important contributing factors to the success of this strategy include the excellent stability and technical performance of waste glass forms and the ability of the glass, as well as waste package materials, to retain radionuclides even when exposed to potential leachants within a repository environment.

Ceramic joining has become an area of widespread and increasing engineering importance as the aerospace, automotive, chemicals, metals, and utilities industries seek improved performance at higher temperature and in more corrosive service environments. The brittle nature of ceramics, the difficulty of fabricating near net-shape components, the expense of ceramic machining (requiring diamondtipped cutting tools), and the lack of reliable in-process nondestructive evaluation (NDE) methods have driven the development of a variety of joining techniques. This article discusses recent advances in ceramic joining using microwave energy as a fast, efficient means to obtain joints indistinguishable from as received material.

Ceramic materials of engineering interest include oxides such as Al2O3 and Al6Si2O13 (mullite), Si3N4, and SiC. Joining applications include attachment of components such as piston liners, combustor cans and turbine rotors in high-efficiency engines; fabrication of tube assemblies for high-temperature, high-pressure heat exchangers and temperature and corrosion resistant radiant burners; and installation of hermetic seals for vacuum system components. The most common joints are mechanical attachments which take advantage of the coefficient of thermal expansion (CTE) difference between ceramics and metals. Ceramic-ceramic and ceramicmetal joining methods using metallic, glassy, and glass-ceramic interlayers and diffusion bonding have also been successfully demonstrated in recent years.

The ability to provide rapid and volumetric heating of the parts to be joined makes microwave joining attractive. The feasibility and simplicity of this approach was first demonstrated at Los Alamos National Laboratory in 1985 by Meek and Blake, who used a home microwave oven to join Al2O3 to itself and to metal with a borosilicate glass interlayer.

Continuing advances in the fields of very-large-scale integration (VLSI), ultralarge-scale integration (ULSI), and gigascale integration (GSI), leading to the continuing development of smaller and smaller devices, have continually challenged the fields of materials, processes, and circuit designs. The existing metallization schemes for ohmic contacts, gate metal, and interconnections are inadequate for the ULSI and GSI era. An added concern is the reliability of aluminum and its alloys as the current carrier. Also, the higher resistivity of Al and its use in two-dimensional networks have been considered inadequate, since they lead to unacceptably high values of the so-called interconnection delay or RC delay, especially in microprocessors and application-specific integrated circuits (ICs). Here, R refers to the resistance of the interconnection and C to the total capacitance associated with the interlayer dielectric. For the fastest devices currently available and faster ones of the future, the RC delay must be reduced to such a level that the contribution of RC to switching delays (access time) becomes a small fraction of the total, which is a sum of the inherent device delay associated with the semiconductor, the device geometry and type, and the RC delay.

Cement-based materials are random composite materials over many length scales. In common with any random material, the two main problems that must be solved to obtain quantitative theoretical understanding of microstructure-property relationships are: (1) what actually is the microstructure, and (2) given enough knowledge of a micro-structure, how can properties be calculated?

In the past few decades, physicists and chemists have studied microstructure/property relationships for atomically disordered materials like semiconductor and metallic glasses, while mechanical engineers have done the same for continuum problems of macroscopic inclusions of one material distributed in a matrix of a second material. With the large memories and fast computational speeds of late 20th-century computers, it is now possible to attack problems that lie between these two extremes, namely random grain problems like those of cement-based and ceramic materials. For these kinds of materials, the important randomness for most problems is at the grain or particle level, not the atomic level, and is complicated by the fact that their microstructure changes markedly on time scales of hours and days, either due to sintering (ceramics), or hydration reactions (cement-based materials). The following sections of this paper describe how microstructures are simulated and properties like electrical conductivity and elastic moduli are computed-mostly for cement-based materials, but with some mention of parallel work on ceramic materials.

Some twenty years ago, it appeared that the crystallographic database had grown so large that it could no longer be described by conventional methods (see, for example, Structure Reports). Today we have available both computerized databases (CRYSTDAT, JCPDS diffraction data, etc.) and excellent bibliographies which enable us to search much larger databases rapidly and far more systematically, resulting in qualitative changes in methods and results. Following are five examples illustrating this point.

The spin-polarized phases of the quantum fluids and solids, liquid 3He, solid 3He, and spin-aligned hydrogen have generated considerable excitement over the past fifteen years. The introduction of high magnetic fields (B ∼ 10–30 T) in conjunction with low temperatures (T ≲ 100 mK) has given rise to opportunities for exploring some of the new phases predicted for these materials. There is a broad range of physical phenomena that can be accessed in this regime of parameter space—unconventional superfluidity, unusual magnetic ordering, Bose-Einstein condensation and Kosterlitz-Thouless transitions, to name a few. This is most surprising since this plethora of complicated states of matter are present in some of the most uncomplicated materials. The rich variety of phases found in these materials are all examples of collective phenomena of quantum many-body systems, and they serve as prototypes for developing an understanding of magnetism and order/disorder processes in other systems, and for the design and characterization of new materials.

CuInSe2 and related chalcopyrite semiconductors are among the compound semiconductors that have been considered for thin solar cells for about the past 20 years. Recently, high efficiencies close to 17% have been achieved. This result could be the starting point for a new category of solar cells—high-performance thin-film cells—that would combine the high performance of single-crystal cells with possible low-cost thin-film processing.

The development of CuInSe2 cells started in 1974, when single-crystal cells with an efficiency of 12% were reported by a group at Bell Laboratories. Soon after, thin-film solar cells were demonstrated by Kazmerski et al. CuInSe2 thin films have been deposited by evaporating the CuInSe2 source material to completion and adding Se from a separate source. It was found that straight-forward evaporation of the compound does not generally lead to films with stoichiometric composition. By coevaporation of the elements, films with any desired composition can be obtained, provided there is appropriate process control. A “bilayer” recipe developed by Boeing, namely combining Cu-rich films and In-rich films, solved the problem of combining larger grains with suitable electronic properties. By this method, the first CuInSe2 thin-film solar cells with an efficiency exceeding 10% conversion efficiency were fabricated.

Alloying CuInSe2 with CuGaSe2 and CuInS2 considerably increases the potential for the innovative development of solar cells from these materials. The energy gaps covered by these alloys range from about 1 eV to about 2.4 eV The possibility of increasing the energy gap and achieving absorber layers with graded bandgaps has many advantages for the application of these materials in thin-film solar cell modules.

In this article, we examine materials behavior in the magnetic field region from about 40 T to 500 T using pulsed magnet technology. Examples of materials science using two different pulsed magnet technologies are described in this article.

Semicontinuous Magnets

Since the late 1960s, the University of Amsterdam has operated a semicontinuous magnetic field installation that produces magnetic fields up to 40 T with typical time constants of about one second. The magnet coil is constructed from hard-drawn copper wire with a reinforcement cylinder of maraging steel positioned at roughly one third of the outer diameter. Before operation, the coil is cooled to 30 K by cold neon gas. The power for this installation is taken directly from a 10 kV connection to the public electricity grid. By means of a thyristor-based power control system, highly flexible field-time profiles can be realized: step-wise pulses can be generated with field levels constant within 10−4 during 100 ms; linearly increasing and decreasing fields as well as exponentially ripple-free decreasing fields are other examples of standard field-time profiles. Among the measuring techniques frequently used are magnetization, magneto-transport, quantum oscillations, relaxation phenomena, etc. Temperatures at which experiments can be performed range from 400 mK to room temperature. In the Netherlands, the Amsterdam High Field Facility has recently been combined with the High Magnetic Field Laboratory in Nijmegen, where static magnetic fields up to 30 T are produced in hybrid magnet systems, to form the Amsterdam-Nijmegen Magnet Laboratory (ANML). The high field research of ANML comprises semiconductors, magnetism in transition-metal compounds, heavy-fermion physics, superconductors, organic conductors, and magnetic separation. We present here a few selected topics.

Numerous metallic alloys are now known to exhibit a shape memory effect through which an article deformed at a lower temperature will regain its original undeformed shape when heated to a higher temperature. This behavior is basically a consequence of a martensitic phase transformation. When compared, the various shape memory materials are found to have common characteristics such as atomic ordering, a thermoelastic martensitic transformation that is crystallographically reversible, and a martensite phase that forms in a self-accommodating manner. The explanation of the shape memory phenomenon is now universal and well in hand. In addition to the familiar “one-way” memory, shape memory alloys also exhibit a “two-way” memory as well and a “mechanical” shape memory resulting from the formation and reversal of stressinduced martensite.

Fundamental to the shape memory effect (SME) is the occurrence of a martensitic phase transformation and its subsequent reversal Basically, a shape memory alloy (SMA) is deformed in the martensitic condition (martensite), and the shape recovery occurs during heating when the specimen undergoes a reverse transformation of the martensite to its parent phase. This is the essence of the shape memory effect. Materials that exhibit shape memory behavior also show a two-way shape memory, as well as a phenomenon called superelasticity. These are also discussed.

The shape memory response after deformation and thermal stimulation constitutes “smart” behavior, i.e., Stimulated Martensite-Austenite Reverse Transformation.

Microwave heating offers the unique potential for selectively heating a strongly microwave-absorbing material within a nonabsorbing matrix, providing the mixture is homogeneous and the absorbing phase is diluted enough to retain a penetration depth sufficient for volumetric heating. Mainly for this reason, microwave processing opens up a way to produce entirely new materials and microstructures that cannot be processed by conventional methods. By proper selection of raw materials and microwave heating conditions, processes can be tailored to produce desired materials and structures, such as powders, compacts, or functional gradient materials, as described in this article.

Synthetic powders with controlled morphology, agglomerate structure, and composition are necessary to improve the reliability of ceramic materials. A novel approach to synthesizing such powders is based on microwave heating of metalorganic precursor compounds. In contrast to conventional precursor-based methods for powder syntheses, which use diluted solutions of precursor compounds, the main objective of microwave processing is to develop a method for direct pyrolysis of the precursor compound into a ceramic powder, produced by simultaneous decomposition of the precursor within the whole volume of the reaction mixture. Besides a simple pyrolysis of a single precursor compound, decomposition of precursor mixtures is also possible, eventually followed by further reaction of the components. Furthermore, an inert or reactive powdery matrix-material can be impregnated with the precursor and then converted to an “alloyed” ceramic powder.

A new method for microstructural modeling of ceramic materials combines precursor compounds with heating by microwave radiation to introduce a second phase and to control grain growth.

Hydrodynamics and elastohydrodynamics have been successful in describing lubrication by micron-thick films. However, these continuum theories begin to break down as film thicknesses become comparable to molecular dimensions. An increasing number of applications require an understanding of lubricants in such severely confined geometries. Examples include lubrication of nanoscale bearings in micromachinery or high-density magnetic disk drives, as well as asperity interactions in macroscopic bearings that operate in the mixed lubrication regime.

Development of new experimental and theoretical techniques for studying thin lubricant films has paralleled the growing interest in their properties. The surface force apparatus (SFA) allows normal and shear forces to be measured between atomically flat solid surfaces while their separation is determined to within 0.1 nm using interferometry or capacitance. The contact area in the SFA is typically 100 μm across, much larger than the separation between solid walls. The atomic force microscope (AFM) can be used to explore friction in lubricated contacts whose diameter is comparable to the separation (5 nm). This allows spatial resolution of the frictional force on a molecular scale. Quartz-crystal oscillators have been used to determine the frictional forces between a surface and an adsorbed film of one or more monolayers. Theoretical advances have been aided by the advent of supercomputers that allow thin films to be simulated at the molecular level using molecular dynamics. These new experimental and theoretical techniques reveal a sequence of dramatic changes in the static and dynamic properties of fluid films as their thickness approaches molecular scales.

The characterization of fracture behavior is a continuing challenge to the cement and concrete community. The performance of a material can be evaluated by its stress-strain response. For an ideally brittle material, elastic response is terminated when stress suddenly drops to zero, as shown in Figure 1a. However, cement-based materials are considered quasi-brittle because they respond nonlinearly prior to peak stress, and their stress gradually decreases after reaching a peak, as indicated in Figure 1b.

To make cement-based materials stronger and tougher, one needs to understand the fracture mechanisms associated with nonlinear stress-strain behavior and to characterize material fracture properties based on these fracture mechanisms. Three novel techniques are being used at the Center for Advanced Cement-Based Materials (ACBM) to detect the quasi-brittle nature of cement-based materials. These three techniques are laser holographic interferometry, acoustic emission, and microscopic surface analysis. This article summarizes both the fracture mechanisms in cement-based materials and the application of the three techniques to characterize and measure fracture properties.

Polymers, the main ingredient of plastics, constitute the fastest-growing sector of materials—having been introduced only in this century. Their application, however, is plagued by inappropriate use and resulting premature failure. Conversely, polymers are also being attacked as indestructible contributors to the waste stream. Although objective discussion of their merits is complicated by the enormous financial interests involved, it is clear that their rapid use with a short-term experience base has led to many problems. On the positive side, they are amenable to good theoretical and experiential understanding because of the excellent organic and physical chemistry background underlying the field. Hence, we can expect to see even more widespread use of polymers and hopefully fewer problems, as our knowledge base grows, is disseminated, and is applied. This article will address the current field of polymer environmental stability and summarize a few select areas where major technical efforts are being directed.

Recent interest in copper-based metallization for ultra fast logic devices has stimulated extensive studies on thermal stability issues, as well as the search for novel deposition and etching processes. In copper wiring technology, thermal stability problems include: (1) rapid diffusion of copper into dielectric substrates, (2) low-temperature reaction with most near-noble metals and their silicides, (3) the susceptibility of copper to oxidation, (4) poor adhesion characteristics of copper on most dielectric materials, and (5) thermal-stress-induced void formation and cracking. The basic issues have been summarized in a recent review.

When copper is in contact with a silicon substrate, copper can diffuse into active regions of devices where it becomes a recombination-generation center. In addition, copper forms the silicide Cu3Si by reacting with the substrate at temperatures less than 200°C. After the formation of the Cu3Si phase, the underlying silicon in the Cu3Si/Si structure is readily oxidized even at room temperature. In the case of copper in contact with silicon dioxide under bias thermal stress (BTS), copper is found at the SiO2/Si interface under a positive electric field. Copper diffusion in doped glass, e.g., phosphosilicate glass and silicon nitride films, is a factor at temperatures below 400°C.

The development of microelectronic miniaturization has taken an almost revolutionary course. For more than 20 years, important performance features have doubled every one to one and a half years on a per-chip level and, despite some recent economic worries, most people believe that we will see equally rapid progress for at least another 10 years. CMOS technology is making the fastest advances among the different microelectronic technologies and is pushing forward most powerfully. The acronym CMOS stands for Complementary Metal Oxide Semiconductor (chapter 8 in Reference 1) and means that two complementary types of transistor devices carry a current for complementary bias conditions. A device of this kind often used for switching purposes is drawn schematically in Figure 1. It uses a control electrode, called the gate, to switch a current flow from the terminal source to the terminal drain. The channel can be made conducting depending on the potential applied to the gate which is separated from the semiconductor by an insulating dielectric layer, namely amorphous SiO2. Control is via a field effect; for example, in one of the two complementary types of devices, a positive gate voltage can draw negatively charged electrons toward the interface, which causes conduction from the source to the drain through the otherwise blocked source and drain contacts.