The modern photographic emulsion is an extremely sensitive detector of light quanta. In fact, high speed black-and-white film (which consists mainly of microcrystalline grains of AgBr or AgBrI suspended in gelatin on an inert base) ranks with the photomultiplier tube as capable of detecting a very few individual light quanta. Exposure times can vary from minute fractions of a second to hours. Unlike the multiplier tube, the photographic emulsion also responds well to relatively high light flux, that is, film can have wide exposure latitude. In many respects such as sensitivity, latitude, resolution, etc., film sets the pace as new multiplier tubes and charge-coupled devices are developed.

Of course, the high sensitivity of film comes about because light absorbed within an emulsion grain causes a latent image to be formed (usually on the surface), which then renders the entire grain developable. The tabular grains of a modern high speed emulsion are largely triangular or hexagonal single-crystal platelets approximately 10 ü across and 0.1 ü thick. Such a grain will contain about 109 silver atoms, and it can be sensitized by a few molecules of gold sulfide or other additives adsorbed on its surface.

In the concentration speck theory of the latent image, light is absorbed throughout the grain, but this energy is transported by the motion of electrons to a sensitivity center usually on the surface where the latent image is formed.

The earliest refractories for containing melts were quarried from natural deposits of limestone and dolomite. Today these two carbonate rocks serve important roles in the production of metal contact refractories. Early refractories for glass melting on the other hand were manufactured from clays and claystones. These materials are also still used extensively for the batch melting of glasses that are hand formed or blown into art and tableware. Glass contact refractories for the continuous (tank) melting of glass are often fired, cast into large shapes, and arranged in a soldier course which constitutes the sidewalls of the glass tank.

In this brief exposition of refractories technology and allied research, the articles by B. Brezny, T.F. Vazza and T.A. Leitzel, and by T.S. Busby cover materials development, selection, and properties of the systems which have evolved for the efficient melting of steels and glasses. As such they relate to extremes of technological flux in the processes for the manufacture of steel and glass, respectively.

The continuous melting of large volumes of commercial glasses has been carried out in tanks equipped with reverberators for at least 70 years. The basic design of the overall system and of many of the glass fabrication machines for pressing, rolling, and blowing the glass has been constant since World War I. Only the introduction of the float glass method, the famous Pilkington process, for the production of flat glass, has interrupted the slow quiet progress in the technology of continuous glass making.

Since the discovery by Karl Alex Muller and Johannes Georg Bednorz (IBM Zurich) in January 1986 that an oxide of barium, lanthanum and copper might be superconducting at temperatures up to 35 K, the superconducting transition temperature has jumped to 125 K in only two years. A great tide of high Tc superconductor exploration has swept across the world. Like their counterparts in other countries, Chinese scientists also stood in the frontline of this revolution.

When Prof. Zhao Zhongxian obtained Bednorz and Müller's paper in September 1986, he thought their ideas were reasonable. A team at the Institute of Physics, Academia Sinica was then organized to search for high Tc materials. In December 1986, Zhao and his colleagues successfully obtained superconducting samples of Sr0.25La4.75Cu5Ox with onset temperature Tcon 48.6 K and Ba0.5La4.5Cu5Ox with Tcon = 46.3 K (Figure 1). This occurred only a few days after the announcement of the confirmation of superconductivity by diamag-netic observation from Japan. These were the highest records for superconducting transition temperature Tcon in the world at that time. Moreover, a sign of superconductivity with Tc0 (zero resistivity) around 70 K was also observed in some La-Ba-Cu-O samples. Because these samples were unstable, however, the Tc decreased after several days storage in air. Since then Zhao and his colleagues have searched for materials with higher Tc by using various compositions and substitutions, different sintering processes, and heat treatments.

Fracture mechanics technology is an effective tool for characterizing the rates of fatigue crack propagation. Generally, fatigue crack growth rate (da/dN) in each loading cycle can be presented as a function of stress intensity range (ΔK), where ΔK = Kmax — Kmin, Kmax and Kmin are the maximum and the minimum stress intensities, respectively. A typical fatigue crack growth rate curve of da/dN versus ΔK can be divided into three regimes, i.e., Stage I (near-threshold), Stage II (Paris), and Stage III (fast) crack growth regions, as shown in Figure 1.

Depending on the region of crack growth, fatigue crack growth behavior can be sensitive to microstructure, environment, and loading conditions [e.g., R (load) ratio = Kmin / Kmax]. In the nearthreshold region, fatigue crack growth rates are very slow, ranging from approximately 10−10 to 10−8 m/cycle. In this region, the fatigue crack growth rate curve eventually reaches a threshold stress intensity range, ΔKth, below which the crack would not grow or grow at an extremely slow rate. Typically, the value of ΔKth is operationally defined as the stress intensity range which gives a corresponding crack growth rate of 10−10 m/cycle. In the nearthreshold region, the influence of microstructure, environment, and load ratio on the rates of crack propagation is very significant.

The iron and steel industry is constantly undergoing technological changes in improving its processes to produce the highest quality world-class steels. As a consequence, the refractories industry has been challenged to keep in lock-step with the iron and steel industry to enhance, not detract from these processes.

Certain internal combinations have enabled these refractories changes. Three expositions can display the sophistication to which refractories have advanced, so this article will explore refractories for primary steelmaking, ladles, and slide-gate devices for continuous casting.

The start-up of the first top-blown oxygen vessel at the Linz Works in Austria 37 years ago was a historical milestone in the evolution of steelmaking. At that time the open-hearth process was the dominant steelmaking technology in the world. Ten to 12 hours were required to produce one batch of steel in the open-hearth furnace, but only one hour is needed to refine the same amount of steel in a basic oxygen furnace (BOF). In a top-blown BOF all oxygen required for refining is introduced into the vessel through the oxygen lance inserted through the top of the vessel.

Toward the end of the 1960s, bottom-blown processes, such as OBM and Q-BOP, became very important in oxygen steelmaking. In the bottom-blown process, oxygen and lime are blown into the steel batch through bottom tuyeres.

Recently the science and technology of fine particles have been greatly advanced to meet the urgent demands of modern industries for specific and sophisticated functions for various materials, including catalysts, sensors, electromagnetic devices, and photosensitive materials. Monodispersed colloidal Systems are invaluable for this purpose because the entire system's uniform physicochemical properties directly reflect the properties of each constituent particle. Procedures for modifying monodispersed particles have progressed remarkably in recent years so that specific characteristics can be achieved. This article focuses on the backgrounds of science and technology for controlling the properties of inorganic monodispersed particles and on new developments in this field.

Colloidal particles are normally formed through a sequential process of nucleation and growth of the nuclei. To obtain a monodisperse System, the two stages must be strictly separated and nucleation avoided during the growth period. Since the steady concentration of monomers in the growth stage is determined by the balance between the rates of generation of monomers and their consumption by particle growth, the generation or introduction rate of monomers must be controlled so that it is low enough to keep the balanced monomer concentration below the critical supersaturation after the nucleation period. Typically, the initial concentrations of metal sait, pH, and temperature are adjusted to meet this requirement for the formation of monodispersed metal hydrous oxide particles by forced hydrolysis of metal ions.

An artificial separation between nucleation and growth processes may be achieved by “seeding,” in which foreign particles are introduced into the solution of monomers below the critical supersaturation. One may also lower the pH in hydrolysis of metal ions, dilute with solvent, add chelating agents, or suddenly change the temperature just after limited nucleation. All these procedures could cause the monomer concentration above the critical supersaturation to plunge to a level below it.

Welding science is an interdisciplinary field involving metallurgy and materials science, heat, fluid, and mass transfer, and arc and plasma physics to name a few. Three areas of welding science have been the focus of much research the past few years as welding scientists attempt to understand the phenomena responsible for producing fusion welds with acceptable service properties. Broadly defined, these areas are:

1. physical and numerical modeling of the heat and fluid flow during fusion welding,

2. understanding the microstructural evolution during solidification and cooling of welds, and

3. welding of advanced materials.

This short review will focus on these fusion welding research themes to provide the reader with a flavor of the work in progress at several major government, industry, and university laboratories.

Much of both the experimental and theoretical studies that have advanced the understanding of fusion welding processes revolves around delineating the roles of mass, momentum, and energy transfer. The earliest models of fusion welding processes were analytical models of heat transfer by conduction. These models not only excluded much of the physics of the process, but assumed point or linear heat sources. Nevertheless, these models provided useful insight about the interplay between power and weld speed, particularly regarding weld pool shape. Also, although quantitative predictions of weld pool shape were generally inaccurate, predicted thermal contours and cooling rates away from the fusion zone were useful in understanding the extent of heat-affected zones.

Beta and β″-alumina are remarkable solid electrolytes whose study has been a principal theme in the field of solid state ionics ever since Yao and Kummer first reported the exceptionally high conductivity of sodium β-alumina in 1967. The unusual properties of sodium β-alumina stimulated widespread interest in both the science and technology of high conductivity solid electrolytes. Since Yao and Kummer's seminal work, many solid electrolytes have been explored, but interest in the β and β″-aluminas has endured, principally because of their virtually unique ability to undergo ion exchange in which the sodium ions are replaced by a wide variety of monovalent, divalent, and trivalent cations as well as various protonic species. The β and β″-aluminas thus are not single compounds, but a family of solid electrolytes with diverse properties and potential technological applications.

Kummer first pointed out the ion exchange properties of sodium β-alumina by showing that the sodium ions in the structure can be replaced by different monovalent cations, such as Li+, K+, as well as H(H2O)x+ and NH4+. More recent studies have shown that the ion exchange chemistry of sodium β″-alumina, is far richer than that of sodium β″-alumina. In fact, the sodium ion content of β″-alumina can be exchanged for virtually any +1, +2, or +3 cation in the periodic chart.

Diamond synthesis has attracted attention ever since it was established in 1797 that diamond is a crystalline form of carbon. Initially, synthesis was attempted at high pressures because diamond is the densest carbon phase. As understanding of chemical thermodynamics developed through the 19th and 20th centuries, the pressure-temperature range of diamond stability was explored. These efforts culminated in the announcement in 1955 of a process for diamond synthesis with a molten transition metal solvent-catalyst at pressures where diamond is thermo-dynamically stable. Worldwide sales of synthetic diamond now approach 330 million carats (73 tons) with a market price of between $500 million to $1 billion.

Over the past 40 years a parallel effort has been directed toward the growth of diamond at low pressures, where it is metastable. Although diamond was successfully produced, low-pressure synthesis was plagued by extremely low growth rates. Recent developments have led to much higher growth rates, creating great interest in the field. Polycrystalline diamond films can now be produced on a variety of substrates at linear growth rates of tens to hundreds of micrometers per hour. In addition, the recognition of an entirely new class of solids, the so-called “diamondlike” carbons and hydrocarbons has arisen from this work. This article will discuss both crystalline diamond grown at low pressure and the diamondlike phases.

The interest in diamond is driven by its extreme properties, summarized in Table I. Diamond stands alone as the densest (number density), strongest (elastic modulus), and hardest known material.

A material's susceptibility to cracking may be significantly affected by its chemical environment. Stress corrosion cracking (SCC), liquid metal embrittle-ment (LME), hydrogen embrittlement (HE), and corrosion fatigue are examples of environmental effects which cause ductility or endurance losses through environment-assisted cracking (EAC). Under certain conditions, virtually all commercially important materials are susceptible to one or more of the above embrittlement processes. Cracking may occur intergranularly, transgranularly, or in a mixed mode, depending on conditions. Much is known about the metallurgical and environmental conditions which promote environment-assisted cracking, and prudent control of these is often successful in mitigating or preventing cracking. However, in spite of our understanding of the factors controlling SCC, LME, and HE, the responsible mechanisms remain elusive.

This article will (1) review some of the important variables affecting these phenomena, such as stress, stress intensity, material microstructure, strain rate, electrochemical potential and pH, and (2) attempt to relate phenomeno-logical characteristics of environment-induced embrittlement to several mechanisms proposed for environment-assisted cracking, as they are understood today.

The problem of stress corrosion cracking is unquestionably the most costly of environmental cracking phenomena, with losses occurring in a wide variety of service environments. Liquid metal embrittlement is of concern in nuclear power and other industries. Hydrogen embrittlement, first recognized as an embrittler of iron in 1873, causes cracking problems in applications ranging from welding to oil drilling. In all, the list of situations in which environment-assisted cracking occurs is long and is likely to grow as materials are increasingly challenged by the severity of their service conditions.

Modeling methods are now established as a major technique in defect physics and chemistry. Indeed, much of the contemporary understanding of complex defect processes derives from applying these techniques in conjunction with experimental methods.

The silver halides possess unusual defect properties and pose unique challenges to simulation studies. This article aims to outline the general features of defect simulations, and to review the status of their application to the defect properties of silver halides.

Technical aspects of the field have been detailed elsewhere, so this account is brief. An important feature of recent studies of the silver halides concerns developments in interatomic potentials, which will be considered, followed by a presentation and discussion of the calculated defect parameters.

The basis of the simulation techniques is the specification of an interatomic potential model for the system, i.e., an analytical or possibly a numerical description of the energy of the system as a function of atomic coordinates. For polar materials, the model must include the following terms: Coulomb and short-range energies, and ionic polarization.

To evaluate these terms, charges must be assigned to all atoms. In most studies reported to date, the fully ionic model has been used, i.e., integral charges have been assigned. The procedure has been followed in modeling the silver halides. Care must be taken in summing r-1 terms. Simple truncation procedures give unreliable results.

The successful development of fusion power critically depends on structural materials which can maintain their integrity and dimensional stability in an extremely hostile service environment involving high temperatures and heat fluxes, corrosive media, high stresses, and intense fluxes of neutrons. Neutron irradiation results in a variety of mechanical, physical, and chemical property changes that are collectively referred to as radiation effects. The primary sources of damage are neutron-induced atomic displacements and transmutation products.

High-energy neutrons produce primary recoil atoms (PKAs) with energies ranging from less than 1 keV to more than 100 keV. The energetic PKAs create a cascade of additional displacements. About 10% of the displaced atoms survive cascade recombination as isolated vacancies and self-interstitials or small clusters of these defects. The mobile defects migrate to pre-existing sinks such as grain boundaries and dislocations, or cluster to form a variety of extended defects such as voids (vacancies) or faulted dislocation loops (interstitials).

If you are an economist in this day and age, and especially if you are an economist at MIT, you often get asked “Is economics really a science?”

I used to give a very complicated answer to that question, but recently I discovered a much simpler one: When the Discovery went up, the Space Administration announced that the success of the Discovery was entirely due to the scientific achievement of economists because in shifting from the Challenger to the Discovery the Space Aclrninistration had to do a lot of experiments, and they had to replace rats with economists in all these experiments. They announced three reasons for replacing rats with economists: First, there were now more economists and they were cheaper; second, you sometimes emotionally get attached to the rats; and third, there are some things that rats just won't do.

The serious answer to that question, however, is that economics is much like geology. Geologists understand earthquakes and volcanoes perfectly—it's plate tectonics. But if you ask a geologist, “Tell me the precise timing and the precise magnitude of the next earthquake in Los Angeles,” you are in trouble.

The problem is that people like to ask economists that question: “Give me the precise timing and the precise magnitude of some event.” And economists have a character weakness. They tend to answer those kinds of questions. And those kinds of questions simply can't be answered.

Petrographic analysis is not new to the study of archaeological ceramics, but it is receiving more attention as it is applied to interesting and diverse archaeological problems. Petrography was developed by geologists to identify minerals in rocks. Archaeologists use it to study the minerals that occur in prehistoric clay matrices transformed by heat to a rocklike state.

Petrographic analysis requires a polarizing microscope and a piece of ceramic, called a thin section, 30 microns (0.03 mm) thick. As light from this specialized microscope passes through a mineral, it produces optical properties unique to that mineral. Birefringence and pleochroism are two such properties used to identify minerals, while physical properties such as cleavage and relief are also important discriminators. Several references fully explain these properties and relate them to specific minerals and rocks.

Using petrographic techniques, as many as five materials can be studied in a ceramic—the clay itself, natural inclusions in the clay, purposefully added inclusions, and glazes or slips on the clay surface. Under certain conditions the clay or inclusions may melt and a fifth material, glass, may form. Depending on the research problem to be addressed and the samples available, the petrdgrapher studies one or more of these materials.

Twelve compounds with the formula RBa2Cu3O7-δ where δ≍0.1 and R=Y or a lanthanide element except Ce, Pm, or Tb, crystallize in the same layered orthorhombic perovskite-like structure (see the article by I.K. Schuller and J.D. Jorgensen in this issue). All such compounds are superconducting with superconducting transition temperatures Tc≍92-94 K, except for R=Pr. The compound PrBa2Cu3O7-δ appears to be a special case, since it does not exhibit metallic behavior and is not superconducting. Historically, the prototype YBa2Cu3O7-δ compound is singularly important since it was the first superconducting material with a Tc greater than 77 K, the boiling point of liquid nitrogen. In the meantime, yet higher Tc's have been obtained in YBa2Cu3O7-δ at very high pressure (Tc˜107 K at 150 kbar)5 and in new layered compounds in the Bi-Sr-Ca-Cu-O (maximum Tc≍110 K)6 and Tl-Ba-Ca-Cu-O (maximum Tc≍125 K)7 systems (see the article by A.W. Sleight, M.A. Subramanian, and C.C. Torardi in this issue).

The origin and nature of the high Tc superconductivity of the RBa2Cu3O7-δ compounds, and the other layered copper oxide compounds as well, are presently not understood and constitute a formidable challenge to experimentalists and theorists alike. One of the most intriguing possibilities is that a magnetic mechanism, rather than the electronphonon interaction, is responsible for the formation of the superconducting electron pairs in the high Tc copper oxides. The primary evidence for a magnetic pairing mechanism is the proximity of antiferromagnetism and superconductivity as the concentration of holes in the conducting CuO2 planes is varied,8 as discussed below for the RBa2Cu3O7-δ compounds.

The first report of a superconductor in this family came from the laboratory of B. Raveau. The system was Bi/Sr/Cu/O. The Tc's were in the range 10-20 K, and this system therefore did not attract much attention when there was intense effort on 90 K superconductors. Neither the structure nor the composition of the superconductor in the Bi/Sr/Cu/O system was known at the time of the first publication, a common feature with all discoveries of new high Tc superconductors in the past couple of years. We now know that the first discovered member of this new family of superconductors can be represented as Bi2Sr2CuO6. However, the composition and structure of this phase are still not precisely established.

Subsequent to the French work, the Bi/Sr/Ca/Cu/O system was studied in Germany, and higher Tc's were obtained. Although patents were filed; this work was not publicly announced. In January 1988, Maeda announced high Tc in the Bi/Sr/Ca/Cu/O system and Hermann announced high Tc in the Tl/Ba/Ca/Cu/O system.

It is now known that there exists a very large family of compounds of the type (AO)mM2Can-1CunO2n+2. The A cation can be Tl, Pb, Bi or mixtures of these elements. The value of m may be one or two but is only two when A is Bi. The M cation is Ba or Sr, and substitution of Ca by Sr is frequently observed. The number of consecutively stacked CuO2 layers is indicated by n. The highest temperature at which zero resistivity has been obtained in this family is 122 K, and this has been obtained for several distinctly different compositions (Table I).

Chaired by G. Slade Cargill, III (IBM Watson Research Center), D. Wayne Goodman (Texas A & M University), and J. Francis Young (University of Illinois), the 1988 MRS Fall Meeting was the largest on record. A strong and varied program of 24 technical symposia attracted over 3,700 participants from Monday, November 28 to Friday, December 4. Rounding out the meeting program were the lunchtime tutorial lectures of Symposium X, Frontiers of Materials Research; a forum on the Materials Science and Engineering Study, which previewed the forthcoming National Research Council report; the Plenary Address by Prof. Lester C. Thurow (Dean of the Sloan School of Management, Massachusetts Institute of Technology) on “Maintaining Technological Leadership in a World Economy”; and the presentation of the 1988 Von Hippel Award to Prof. Jacques Friedet (Professor of Solid State Physics, University of Paris).

The lectures by Thurow and Friedel were both excellent presentations, although quite different in style. Thurow confounded the scientists' cynical view of economists by giving a lucid and cogently argued presentation of the societal problems underlying the loss of United States dominance in technological developments. Readers will be able to peruse the text in an upcoming issue of the MRS BULLETIN but will miss the flavor of the oral presentation. Prof. Friedel gave a delightful personal account of his scientific career, which illustrated so well the unity of materials research despite the apparent diversity of approach. His account is published elsewhere in this issue.

Silver chloride and silver bromide, like sodium chloride and sodium bromide, are ionic crystals with the rock-salt structure, but despite the similarity in the crystal radii of Ag+ (1.29 Å) and Na+ (1.16 Å) the properties of these silver halides are very different from those of the corresponding sodium salts. Much of this difference is due to the filled 4d shell in the electron configuration of Ag+, which is responsible for the higher polarizability of Ag+ (2.37 − 2.44 Å3) than Na+ (0.41 Å3 and for the tendency of Ag+ to undergo quadrupolar deformation.

Associated with the high polarizability of Ag+ are the large van der Waals cation-cation and cation-anion interactions in AgCl and AgBr and the consequent high cohesive energies of these silver salts (9.26 eV and 9.12 eV, respectively). Bear in mind, however, that the large van der Waals coefficients that result from empirical fitting of two-body model crystal potentials to crystal properties may be partly due to the model attempting to simulate features of the real crystal which the model does not really include, such as many-body interactions. That such many-body interactions are important in AgCl and AgBr is evident from the large violations of the Cauchy relation between the elastic constants, C12 = C44, which ought to hold when each ion is at a center of cubic symmetry and the inter-ionic forces are derivable from two-body potentials.