The following series of articles focuses attention on five important groups of glassy materials: silica, oxynitride, phosphate, chalcogenide, and halide glasses. Each author has made a concerted effort to indicate the nature of the scientific questions and technological applications which make his individual area of materials research both exciting and timely. Nevertheless, these five articles can cover only a few of the identifiable areas of glass science and technology which are active and find new importance today. A broader picture can be obtained by perusal of the general texts listed in References 1-13.

Glasses are usually defined as a sub-class of amorphous materials which exhibit a glass transition and are formable by quenching from the melt. They include numerous oxides, chalcogenides, halides, polymers, organic glasses, and amorphous metals. We have chosen to discuss only the first three of these groupings, which largely comprise the traditional inorganic glasses. The oxide glasses are only partly covered by our first three articles; completeness would require descriptions of activities involving alkalisilicate glasses, alumino-silicates, boro-silicates, boro-phosphates, and more.

Amorphous materials which are not viewed as glasses include tetrahedral amorphous semiconductors like a-Ge and a-Si, either pure or containing alloy levels of H, F, Cl, B, P, or other elements.

Fifty years ago, who could have imagined that silicon dioxide—the material of ordinary beach sand—would become one of the most important materials of present-day optics and electronics? Yet SiO2 is arguably the most crucial material component in current-generation fiber optics and metal-oxide-semiconductor (MOS) device technology. In MOS field-effect transistors (MOSFETs), SiO2 serves not only as the gate insulator, but also as the “field oxide” (which isolates various components of an integrated circuit) and as the packaging material which seals the device from outside contamination. In these roles silica acts as a “perfect dielectric,” being characterized by an essentially infinite resistivity (actually ~1016 Ohm · m at 300 K). The ability to form such a high quality dielectric film with a near-perfect lattice match on single-crystal silicon continues to favor silicon-based MOS technology over technologies founded on electrically superior GaAs.

In the rapidly developing fiber optic arena, fused silica is still “king” due to a combination of properties, including extremely high transparency over a range of usable wavelengths (Figure 1), low material dispersion (~0 at 1.3/üm), high tensile strength (~ 150 kpsi), and high chemical durability. In addition, bulk forms of silica continue to find application in lenses, prisms, windows, and low-coefficient-of-thermal-expansion reflective optics; thin silica films are common components of the highly reflective and anti-reflective surface coatings which are laid down on reflective and transmissive optics, respectively.

Research on oxynitride glasses has expanded considerably since the first systematic compositional investigations were begun some ten years ago. This review will briefly introduce that work with an emphasis on more recent developments. Also included is some discussion of the applications of oxynitride glasses in both bulk and thin film form. The literature review is selective, however, and the recently developed phosphorus oxynitride glasses are only briefly mentioned. For more detailed information on earlier work on silicon oxynitride glasses see the reviews by Jack and Loehman. For discussions of phosphorus oxynitride glasses see Marchand, Peng and Day, and Rajaram and Day.

The development of bulk oxynitride glasses has been closely associated with research on Si3N4 and other nitrogen-containing ceramics. The initial discovery that large amounts of nitrogen can be incorporated in silicate glasses originated with analyses of glassy grain boundary phases in Si3N4 that was hot-pressed or sintered with different additives to promote liquid phase formation at the processing temperature. (Some applications of oxynitride glasses take advantage of that close connection to Si3N4. See Applications of Oxynitride Glasses below.) Later, systematic study of the bulk oxynitride glasses showed that nitrogen incorporation increases density, hardness, glass transition temperature and fracture toughness, and, to a certain extent, decreases the thermal expansion coefficient. Nitrogen additions also tend to lower glass solubility in aqueous solutions.

Until recently, phosphate glasses were regarded as somewhat of a scientific curiosity—materials limited in technological application but nevertheless exhibiting interesting fundamental properties. Although the basic properties of many phosphate glasses have been studied on a laboratory scale during the past 50 years, very few of these materials proved to be of commercial interest. This lack of extensive practical application was primarily related to their poor chemical durability and a tendency to crystallize during processing.

The pace of modern technological development continues to result in an increased demand for new materials with specific characteristics, and this demand has led to renewed interest in a number of previously neglected materials—including phosphate glasses. The combined physical and chemical characteristics of phosphate glasses make them relatively unique among the families of inorganic glasses. These characteristics include: large thermal expansion coefficients, low preparation and softening temperatures, low melt viscosities, chemical compatibility with living bone, and a polymeric structure similar to that in organic polymers. Most recent research on phosphate glasses has emphasized one or more of these characteristics.

The major scientific questions pertinent to technological applications of phosphate glasses are related to utilizing the desirable characteristics of these materials while maximizing their corrosion resistance and thermal stability. In order to tailor a phosphate glass for a particular application, the corrosion mechanism of the glass must often be understood in detail, along with the relationship between the microscopic structure of the glass and its macroscopic properties.

Although there are some significant exceptions, most important glass-forming systems contain elements from the sixth, or chalcogenide, column of the periodic table (oxygen, sulfur, selenium, or tellurium). The glasses which contain oxygen are typically insulators, while those which contain the heavier chalcogen elements are usually semiconductors. Even though oxygen is technically a chalcogen element, the term “chalcogenide glass” is commonly used to denote those largely covalent, semiconducting glasses which contain sulfur, selenium, or tellurium as one of the constituents.

The chalcogenide glasses are called semiconducting glasses because of their electrical properties. The electrical conductivity in these glasses depends exponentially on the temperature with an activation energy which is approximately one half of the optical gap. In this sense these glasses exhibit electrical properties similar to those in intrinsic crystalline semiconductors. The analogy is by no means perfect. The mobilities for the charge carriers in these glasses are very low (< 10 cm2/V-s) compared to crystalline semiconductors, and there are even discrepancies in determining the sign of the charge carriers from measurements of the Hall effect and the Seebeck effect.

The first detailed studies of the chalcogenide glasses were performed about 30 years ago. For many years the prototype compositions have been selenium (Se), arsenic triselenide (As2Se3) or arsenic trisulfide (As2S3), and germanium diselenide (GeSe2) or germanium disulfide (GeS2).

The term “halide glass” refers to glasses in which the anions are from elements in Group VIIA of the periodic table, namely, F, Cl, Br and I, as opposed, for example, to “oxide glasses,” such as silicates, borates, phosphates, etc. Two known single component halide melts are glassforming, BeF2 and ZnCl2, but the majority of halide glasses are multicomponent. Practical interest in halide glasses has been generated almost entirely by their optical properties, which cannot be duplicated in a more conventional oxide glass. Barriers to the practical deployment of halide glasses have their origin in materials properties in which they can be markedly inferior to oxide glasses, e.g., mechanical strength, resistance of the melt to crystallization, chemical durability, etc.

In the past decade there has been considerable and accelerating research activity in the area of halide glass science and engineering. Halide glass research up to 1980 has been reviewed by Baldwin et al. and oxide and halide glasses for laser applications have been compared by Weber. Four international symposia on halide glass science and engineering have been held in the period 1982–1987, the proceedings of the last two of which have been or will be shortly published. The proceedings of a 1986 NATO-sponsored meeting on halide glasses have also been published in book form.

Two recent DOE information exchanges exemplifying the breadth and depth of materials research supported through BES/DMS were held at Los Alamos National Laboratory, New Mexico, on June 15-16 and at Sandia National Laboratory, Albuquerque, New Mexico on June 17-18, 1987. The Division of Materials Sciences (DMS), Office of Basic Energy Sciences (BES), U.S. Department of Energy, holds annual information exchanges at each of the national laboratories. Representatives of all DOE national laboratories as well as representatives of other government, academic, and industrial laboratories are invited to attend the information exchanges.

The LANL meeting was opened by laboratory director Sig Hecker, who set the stage for a day and a half of technical presentations concerning BES-supported research at Los Alamos. Organizing the meeting and chairing the first day's session was James L. Smith, chairman of LANL's Center for Materials Science. A wide variety of topics were covered. Leading off was Zachary Fisk, who discussed correlated electrons and high Tc superconductivity. He described work on both heavy fermion systems and the new ceramic oxide superconductors.

Subsequent presentations dealt with localized vibrational modes (soliton-like) which can be sustained in solids showing both dispersion and nonlinear effects. Thermal physics and quantum fluids were described and a novel acoustic dilution refrigerator concept was demonstrated. Studies designed to elucidate the structure of molecular solids were also discussed.

At the Savannah River Plant (SRP), construction of what will be the world's largest solidification facility for nuclear waste has been under way since 1983. Beginning in 1990, the nearly 100 million liters of liquid high-level nuclear waste now stored on the site will be made into a durable borosilicate glass in this Defense Waste Processing Facility (DWPF).

In developing a slurry-fed melting process for the DWPF, we made advances in understanding both glass processing and glass durability. This article focuses on what we learned and what further advances are likely to be made.

Generally speaking, the goal of any glass technologist is to make a good glass and to make it well. In the glass industry a good product is whatever people will buy. To make it well means, above all, to make the product as economically as possible. Thus, the commercial glass technologist will control the composition of the melter feed material very closely to ensure that only the components necessary for glass performance are included, and in the least expensive form possible. The commercial glass technologist may also tolerate low yields or specify several stages of post-melt processing if it is necessary to produce a product to demanding specifications.

To the nuclear waste glass technologist, however, a good product is one which will be stable in geologic environments for millions of years.