Selecting materials to contain high-level nuclear waste presents an unparalleled challenge to materials scientists and engineers because of the very long times required for containing the waste and the high degree of certainty required when predicting a container's long-term performance. Material endurance without severe degradation is the primary property sought in container materials. There are, of course, additional factors related to mechanical properties, fabricability and weldability, and economics, but these factors are usually secondary to the chemical properties of the material. Metals and alloys have been given prime consideration as container materials because metal materials are ductile and tough, they can be easily fabricated in mass quantities and large container sizes, and they can be readily joined to create intact gas-tight seals between parts.

In selecting a suitable container material, the container's environment is the most important consideration. Several geological formations and resulting geochemical environments are under consideration as disposal sites in different countries. This short review will discuss the principles governing corrosion of the primary container materials being considered in various national nuclear waste disposal programs and will show how this background can guide materials selection. Important candidate materials are then considered along with examples of how these materials will be expected to perform under repository conditions existing in a range of national programs.

Electron holography was originally proposed as a means of improving the resolution of the transmission electron microscope (TEM). And within the last decade dramatic improvements in coherent field-emission electron sources, coupled with TEM instrumentation, have allowed the promise of holography to come to fruition. These improvements in TEM resolution notwithstanding, it has been the application of holography techniques and coherent-beam imaging to the investigation of magnetic and electric fields that has sparked the interest of materials scientists. Electron holography is a unique tool for probing phenomena in the microscopic world because all the information about the object is contained in the hologram (both the electron wave's amplitude and phase), whereas conventional microscopy records only the intensity. It is in this sensitivity to the phase that materials scientists can observe the true potential of electron holography.

Electromagnetic phenomena in a wide range of materials systems have been observed via electron holography; domain structures in magnetic particles and electric fields in p-n junctions are just two examples. Furthermore, fundamental physical properties, such as the existence of the vector potential predicted by Aharonov-Bohm (the AB effect), have been conclusively demonstrated by electron holography. For more information, interested readers are referred to several reviews of electron holography.

The concept for the disposal of high-level radioactive waste (HLW) in Switzerland is illustrated in Figure 1: the waste, solidified in a borosilicate glass matrix, is encapsulated in a 25-cm-thick steel canister which has a minimum life expectancy of 1,000 years. After this time, the heat produced by 90Sr and 137Cs will have dissipated and the repository will have adjusted to the host rock ambient temperature of around 60°C.

The steel canisters will be placed in horizontal tunnels, 3.7 m in diameter, at a depth of around 1,000 m in the granite of northern Switzerland, and the tunnels then backfilled. The backfill material, an important link in the chain of engineered and natural safety barriers, satisfies both physical/mechanical and chemical requirements. The most important of these are:

• Low hydraulic conductivity compared to the host rock;

• Good swelling properties to seal construction-caused joints and rock fractures;

• Plasticity to absorb rock movements and to distribute pressure homogeneously;

• Good retention of radionuclides; and

• Stability over a period of at least 106 years.

Economic aspects and availability should also be considered: each canister requires 88 tons of backfill material, or of the order of 250,000 metric tons for the planned repository.

Thermal-barrier coatings are finding increasing use in engineering applications, particularly in gas turbines. Such coatings, consisting of ceramic insulating layers bonded to the superalloy substrate by oxidation-resistant alloy coatings, are deposited onto components to reduce heat flow through the cooled substrate and to limit operating temperature. They have been used effectively on static components such as combustor cans, flare heads, hot gas seal segments, fuel evaporators, and deflector plates, giving considerable improvements in component life. They have been used successfully on vane platforms. In recent years, the emphasis has shifted toward the development of coatings for high-risk areas, such as turbine blades.

A ceramic thermal-barrier coating needs to be refractory and chemically inert, and to have low thermal conductivity. However, it also needs to possess a high thermal expansion coefficient of ~11 × 10−6 K−1, to match the nickel-base superalloy substrate. The latter specification has focused attention on ZrO2. However, ZrO2 is polymorphic and undergoes two phase changes, cubic to tetragonal at 2350°C and tetragonal to monoclinic at 1170°C. The latter transformation is accompanied by a 5% volume increase which means that ZrO2 has to be alloyed to stabilize one of the high-temperature phases. Early systems in the 1970s consisted of ZrO2 stabilized with MgO, but this has been shown to be a metastable system. Present-day commercial thermal-barrier coatings consist of a plasma-sprayed yttria- or magnesiastabilized zirconia layer on top of an M-Cr-Al-Y bond coat. The latter plays a very important role by helping to key the ceramic to the alloy substrate and to accommodate the mechanical strains arising because of differences in thermal expansion coefficients and elastic moduli between the ceramic and the substrate.

Progress in the synthesis and engineering of advanced porous materials demands better pore structure characterization. The analysis of pore structure is complicated by (1) the wide range in pore sizes observed, from molecular (<1 nm) to macroscopic (>1 mm) dimensions, (2) complex pore shapes and connectivities, (3) chemical and physical heterogeneities, and (4) pore structure changes that can occur during characterization.

The required pore structure information varies with application. Bulk density and the pore-size distribution are needed for thermal insulation. In this case, the dimension of interest is the so-called hydraulic radius since, for small pores, the gas-phase conductivity is proportional to the mean hydraulic radius to the mean free path. A few large but isolated pores will significantly affect conductivity but will go undetected in typical gas-absorption methods. In contrast, for separations, bottlenecks control performance. For transport, such as migration through geologic formations, both the pore-size distribution and pore connectivity are important. For adsorption, surface area and pore size are the relevant factors. Finally, the conventional concepts of pore structure lose meaning as the pore size approaches molecular dimensions, typical of adsorbents and gas-separation membranes.

Printing and writing paper represents 30% by weight of all domestically made paper products. Today, however, less than 50% of that paper is recycled. The problem with waste paper from homes and offices is that it contains dyes, inks, and chemicals. If these additives are not removed properly—with no fiber degradation—the recycled paper will be of an inferior quality for writing and printing.

Recycling, however, is the future. In 1990, 28.9 million tons of paper were collected for recycling, representing a collection rate of 33.5%. By the year 1995, the collection rate goal is 40%, with favorable economics supporting increased utilization of recovered paper. For example, a ton of paper made from 100% waste paper saves 17 trees, 4,100 kWh of energy (approximately 6 months of power used by the average home), 7,000 gallons of water, 60 pounds of air-polluting effluents, and 3 cubic yards of landfill.

Two of the issues facing recycled fiber utilization are the cost and visual quality of the final sheet. Paper brightness, opacity, and printability are the industry's biggest concerns when comparing the quality of recycled fibers to that of virgin fibers. The common approach to increasing brightness is to add a white filler pigment such as precipitated calcium carbonate (PCC), ground limestone, clay, and/or titanium dioxide. Chemical optical brighteners have also been used, but their use will decline as consumers ask what impact the brightening chemicals have on the environment.

Investment in high-performance and large-scale integration (LSI) has been increasing and low-cost fabrication equipment, simple processing, and short turnaround are big concerns. The propagation delay time in a device is reduced with a reduction in device size, and wiring delay becomes longer than device delays. Wiring then has become as important as the device itself. Future multilevel wiring will require five to seven levels. The multilayer wiring itself will not be easy to achieve, because of increased wiring cost, process complexity, and longer turnaround. We must overcome these problems.

The materials required for wiring so that ULSI performance and reliability in RISC (reduced instruction set computer), and CISC (complex instruction set computer) type processors and DSPs (digital signal processors) can be increased must have a lower resistivity and higher resistance to electromigration (EM) and stress migration (SM) than do aluminum alloys. Gold, copper, and silver are candidate wiring materials. Copper is especially attractive, but we must overcome several obstacles to take advantage of its potential.

This article will discuss wring and via resistance delay, Cu CVD technology, the surface reaction mechanism, copper patterning technology, copper oxidation and diffusion prevention, and copper wiring reliability.

Several emerging technologies depend on the development of porous materials with pore dimensions in the nanometer range (1 nm = 10 Å). Based on the canonical classification scheme, such materials are defined as “mesoporous materials.” Specialty uses, such as separation applications, require porosities engineered to meet stringent pore-size requirements, particularly for separating high-boiling-temperature or-ganics. In addition, commodity products, such as high-performance thermal insulation, await credible manufacturing methods to produce fine-scale pores. Finally, information discovered during the development of mesoporous materials could be helpful in finding new strategies to generate void-free ceramic materials by reverse engineering.

Although several methods exist to generate nanometer porosity, our ability to tailor porosity for specific applications is still primitive. Because we lack adequate structural data and models for pore formation, the relationship between synthetic protocol and pore structure remains enigmatic.

Since the early work of Ilan Blech and Gene Meieran more than 20 years ago, electromigration (mass transport produced in a metallic conductor by current flow) has been recognized as an important reliability hazard for solid-state electronic devices. The continuing shrinkage of dimensions in VLSI devices has resulted in a continuing increase in the current density in the interconnection lines and a corresponding increase in the potential electromigration hazard. Although there have been improvements in the materials and structures used for interconnections, notably the addition of copper to aluminum and the use of layered structures, the provision of an adequate margin of safety has become increasingly difficult and is an important constraint on both device design and process development.