The structure and morphology of a thin film is, apart from composition, probably its most significant feature. Again and again we find that electronic, chemical, mechanical, magnetic, optical, and superconducting properties depend on the arrangement of atoms and on disorder in this arrangement. As most thin films are created using some form of deposition technique in which atoms are brought together at a surface, it is at the surface that the first opportunity occurs for formation of a particular atomic structure and morphology. As deposition proceeds, a moving surface, the “growth front,” continues to serve as the template for structure formation. Various kinetic processes, such as atomic transport and the adsorption or desorption of atoms from already-formed structures, determine the final morphology of the film. Subsequent treatment can, of course, modify the structure and morphology, but clearly the initial events occurring at the surface influence, to a large measure, the final outcome.

Techniques sensitive to surface structure and morphology are therefore an important component in the toolbox of methods for the quantitative analysis of the properties of thin films. Electron diffraction techniques have been, and continue to be, the most powerful and versatile of these. Even though electron diffraction is also useful for the structural analysis of the bulk, when one speaks of electron diffraction in the context of thin films, one almost invariably refers to surface-sensitive diffraction. Other methods, in particular scanning probe microscopy, have recently become popular. In many cases, such methods serve to re-emphasize the value of electron diffraction for quantitative structural analysis of thin films.

Concrete is the most widely used man-made product; in the United States we use about two tons per year per person. It has been estimated that the replacement value of concrete structures is over six trillion dollars, and that over one trillion dollars needs to be spent in the repair and rehabilitation of these structures over a 20-year period. By contrast, the resources devoted to the materials issues of the industry have been meager. This neglect may in part explain why the development of the science of cementitious materials had lagged behind that of other materials. The situation has been changing rapidly in recent years, however, and the articles in this issue have been selected to illustrate some of the advances that are being made.

One key to the resurgence in research in cement-based materials is the realization of the pressing need to upgrade our deteriorating infrastructure. Another is the realization that a materials science and engineering approach is needed to provide a solid knowledge base to underpin new technologies. Chemistry took us a long way in the 1950s and 1960s, but an appreciation of how chemistry and processing could combine to create desirable micro-structures that would achieve desirable bulk properties came much later. It is significant that, with the noticeable exception of the Pennsylvania State University, cement research in universities in the United States previously was funded through civil engineering departments. Similarly, in Europe and Japan, research was conducted primarily in engineering faculties, although there were programs in a few chemistry departments. Now, however, on both sides of the Atlantic Ocean, research groups are well established in materials science departments. It is noteworthy that on an NSF Science and Technology Center devoted to advanced cement-based materials is funded through the Division of Materials Research.

The recent rapid growth in the emerging areas of magnetic and magnet-related materials research and applications has led to worldwide recognition of the increased importance of research and technology using high magnetic fields. New high-field magnet facilities and major upgrades of existing facilities are being planned and implemented by a number of countries, among them Japan, Germany, France, the Netherlands, Belgium, Great Britain, Poland, Australia, and the United States. Over the next ten years, these developments will advance the state of the art in magnet-related materials science and technologies by a significant quantum jump. Support by many of the national agencies and a strong corporate commitment to stimulate rapid growth in the development of capabilities at higher magnetic fields and in related technologies results in part from an awareness of the impact these technologies will have in developing the new emerging industrial technologies of the 21st century.

The Francis Bitter National Magnet Laboratory (FBNML) at the Massachusetts Institute of Technology has been one of the pre-eminent facilities in developing and advancing science and technology in high magnetic fields. The new U.S. National High Magnetic Field Laboratory (NHMFL) at Florida State University, at the University of Florida, and at Los Alamos National Laboratory builds on the success of existing facilities. NHMFL will provide the necessary environment to develop the next generation of high magnetic fields: 30–50-tesla continuous fields, 60-tesla quasi-continuous fields, and pulsed fields from 60–1,000 tesla. The ability to develop broad user capabilities at these extreme fields is crucial for the advancement of the frontiers of science and of magnet-related industries.

Computer modeling and simulation of microwave processing systems provide significant advantages that are expected to play an important role in the development, scale-up, commercialization, and effective utilization of microwave heating technology. These advantages include the following:

• Understanding of the physical aspects of microwave interaction with materials. Microwave heating is a complex process, and computer modeling may help identify the role of materials properties, dimensions, input power, heating rates, etc. on the effective use of microwave heating procedures. Hybrid microwave heating often refers to the use of SiC rods, or lining the insulation surrounding the sample with thin SiC coatings, to help stimulate the microwave heating process. This is particularly important when microwaves are used to sinter low-loss materials that do not effectively absorb electromagnetic radiation. This complex hybrid heating process may lead to unpredictable results in routine microwave heating experiments; and many aspects of its procedure may be simulated, evaluated, and possibly optimized using computer modeling. For example, the number, dimensions, and locations of SiC rods in a typical picketfence arrangement and the thickness of SiC sheets that may be used to allow effective stimulation of the heating process, while still not shielding the sample from the microwave energy, may be better understood, and guidelines for experimentations may be developed using numerical simulation.

One of the qualities that distinguishes living systems from inanimate matter is the ability to adapt to changes in the environment. Smart materials have the ability to perform both sensing and actuating functions and are, therefore, capable of imitating this rudimentary aspect of life. Poled piezoelectric ceramics, for instance, are capable of acting as both sensor and actuator. External forces are detected through the direct piezoelectric effect, and a response is elicited through the converse piezoelectric effect, in which a voltage of suitable phase, frequency, and amplitude is applied to the same ceramic.

In this special issue, emphasis is placed on actuators, with articles on piezoelectric, electrostrictive, magnetostrictive, and shape memory materials. This is not to say that sensor materials are any less important; it is simply a matter of space. Optical fiber sensors, chemical sensors, thermistors, micromachined semiconductors, and other smart materials deserve special issues of their own.

Smart materials can be conveniently subdivided into passively smart materials that respond to external change without assistance, and actively smart materials that utilize a feedback loop enabling them to both recognize the change and initiate an appropriate response through an actuator circuit.

Zinc oxide varistors are passively smart materials capable of self-protection against high voltage breakdown. When struck by lightning, the ceramic varistor loses most of its electrical resistance, and the current is bypassed to ground. The resistance change is reversible, and acts as a standby protection phenomenon.

Advances in our understanding of the phenomena of adhesion, friction, and lubrication are facilitated by the recent development of new tools that allow the study of contacts in close-to-ideal conditions. These new tools are the surface force spparatus (SFA) and the atomic force microscope (AFM). The first was developed by Israelachvili in the 1970s. In this device, contact between two atomically flat surfaces of mica occurs over an area of several micrometers in diameter after the mica sheets, glued onto two perpendicular cylindrical lenses, are compressed. Force, area of contact, and separation distance can be controlled at the atomic scale. The second device, the AFM, was developed by Binnig et al. in 1986. The sharp tip of the AFM is a convenient idealization of a single asperity. In addition, the AFM can be used to image the surface in the weak repulsive or in the attractive modes so that minimum perturbation is introduced by the imaging process itself. These two devices have the necessary sensitivity to allow the application of forces weak enough not to dislodge atoms from their sites during contact. The order of magnitude of the force that can lead to the rupture of chemical bonds is a convenient figure to keep in mind in this context. A simple estimate of this force is obtained by considering a bond-length increase of 1 Å as leading to dissociation. For a bond energy of ≈1 eV, Fb ≈ 1 eV/1 Å ≈ 1 × 10−9 N.

Since the birth of integrated circuitry about thirty five years ago, microelectronics design and manufacturing technologies have evolved toward higher integration density with smaller design rules. As the semiconductor industry moves into ultra-large-scale integration (ULSI), device geometries continue to shrink into the sub-half-micron region while circuit densities increase to optimize reliability and improve performance. The resulting demands on interconnect technologies necessitate the exploitation of all development avenues: design, materials, and manufacturing.

Emerging sub-half-micron technologies require multilevel metallization (MLM) design schemes that reduce interconnection lengths and lead to lower signal transmission delays and enhanced device speeds. MLM schemes also permit increased device density, due to the ability to use the third (vertical) dimension, and easier signal routing because of higher flexibility in architectural design. These schemes, in turn, demand interconnect metals that can handle the higher current densities resulting from the decreasing size of device features, without the loss of electrical and structural integrity, and deliver the sheet resistance needed to meet performance demands. They also require reliable deposition techniques to successfully fabricate the increasingly complex architectures as lateral feature sizes are scaled down more rapidly than conductor or insulator thicknesses.

The vast majority of solar cells used in the field are based on single-crystal silicon. There are several reasons for this. First, by using this material, photovoltaic manufacturers can benefit from the economies of scale of the much larger microelectronics industry, where crystalline silicon also dominates. Since lower-quality silicon is acceptable for solar cells, cell manufacturers are able not only to benefit from large production volumes, but also to use off-grade material. The relatively high efficiencies that result from this material, its excellent reliability in the field, its almost complete lack of environmental problems, and recent laboratory progress strengthen the position of this technology.

Although there had been earlier work with what might be termed “cast multicrystalline silicon,” the first efficient crystalline silicon cells were reported in 1954. These cells displayed energy conversion efficiencies of about 6%, a substantial improvement over what had been previously demonstrated. Performance improved rapidly through the 1950s, with efficiencies up to about 15% demonstrated in the early 1960s. By this time, their use on spacecraft had become the major commercial purpose of the cells, with reliability a more important attribute than efficiency. The technology then stabilized for a decade. In the early 1970s, another burst of activity pushed the performance of laboratory cells to about 17%. At about this time, a simple processing sequence was developed, based on screen printing the metal contacts to the cell. Anisotropic chemical etching of the silicon to produce surface texture to reduce reflection loss was also implemented, to give the cell structure shown in Figure 1.

One of the requirements for an intelligent system is to construct a virtual reality in the computer. For materials development, the “reality” is a laboratory or a production line used to improve some aspect of a material by changing its composition, processing parameters, service conditions, etc. In the case of information in libraries, the reality is a search that uses a thesaurus and bibliographic (fact) databases. The greatest barrier faced by users of materials information is that of accessing necessary information through both a librarian's view and a scientific/technological expert's view. One of the objectives of a virtual production line is to reduce this barrier through user-friendly interfaces.

In designing materials, two typical approaches are combined to solve a given problem. The first is a top-down approach, in which a number of requirements are resolved to a set of possible and practical solutions for satisfying these requirements to a certain level. Almost all alloy development has followed such an approach. The second approach is a bottom-up approach, where different materials properties are described on the basis of underlying theories, preferably using first principles and fundamental data. But the vast number of possible materials makes the bottom-up approach unrealistic on its own. Instead, semi-empirical approaches are needed to bridge the gap between ad hoc data sets for practical applications and results produced by the bottom-up approach using fundamental data and first-principle calculations.

As applications for polymeric materials continue to expand, cost-effective and environmentally responsible processing methods become vitally important. It is also becoming increasingly apparent that the success of some new materials is tightly coupled to their processing technologies because the development of their properties depends directly on the processing. In thermoplastic-toughened thermoset resins, for example, morphology development during processing is critical to the mechanical performance of the final material.

Microwave processing has enjoyed growing interest primarily because of the potential of uniformly heating relatively large volumes of materials very rapidly; this overcomes limitations due to the poor thermal conductivity of polymeric materials. Microwave radiation has a very large penetration depth in polymeric materials, and absorption follows a simple Beer-Lambert Law. Hence, depending on the material, very uniform heating profiles throughout the thickness of a material can be obtained up to 0.5 m. This bulk absorption of energy allows for substantially higher heating rates than can be obtained using conventional or infrared methods and for the retention of an acceptable heating profile. Other advantages of this technology include (1) more efficient heating and thus efficient use of energy (only the part is heated, not the walls and surrounding air); (2) shorter processing time for batch operations; (3) shorter process lines for continuous operations; (4) environmental superiority; (5) production of properties or morphologies not attainable using other processing techniques; and (6) the possibility of selective heating—the selective absorption of energy by one phase or part of a material—leading to a custom temperature profile within a part.

Many of the useful properties of Portland cement concrete stem from chemical reactions between cement and water, which cause the slow transformation of the initial fluid suspension into a rigid solid. Because this transformation does not begin immediately, concrete may be processed—mixed, transported, consolidated into a mold, and given a smooth surface — before it becomes rigid (i.e., sets), which typically occurs a few hours after initial mixing. As a result of the need to maintain a workable suspension during processing, the flow properties of cementitious systems have seen considerable attention. However, control of workability remains poorly understood, in part because the relationships between flow and process-ability are not clear. Recent advances have been made in understanding the flow of nonreactive suspensions, and the objective of this article is to explore how these advances provide new insights concerning the behavior of cementitious materials.

There are two motivations driving investigations of cement rheology. The first is that understanding flow properties is the key to better processing. The second is that paste rheology can be used to gain a fundamental understanding of the initial cement microstructure and of the variables influencing early cement hydration reactions.

Rutherford backscattering spectrometry (RBS) is probably one of the most frequently used techniques for quantitative analysis of composition, thickness, and depth profiles of thin solid films or solid samples near the surface region. It has evolved in the past few years from an obscure nuclear technique into a major materials characterization technique. This is primarily due to its simplicity, versatility, and the amount of information it can produce in a short time. Because of its quantitative feature, RBS analysis often serves as a standard for other techniques. The purpose of this article is to introduce the topic to a broad community of materials researchers. Readers who want a more rigorous treatment of the topic for utility purposes can refer to a monograph.

In the early 1960s, several of the low-energy atomic and nuclear physics laboratories started to use their accelerators to do research in solid-state physics. This activity opened up a fertile interdisciplinary area called ion-solid interaction, which impacts materials research significantly.

The 20th century has seen considerable technological achievement that has had far-reaching consequences on the way people live and do business. The first half of the century saw remarkable developments in transportation with the automobile and the airplane and in communications with radio and television. These developments would not have been possible without associated advances in materials technology. The second half of the century has been characterized by the development of the computer (see the cover of this issue).

Nineteenth century futurists equated progress with scale. The bigger the better. One can only speculate on what these savants would say if introduced to what we currently call progress. Progress is now measured by how small you can make a device, how narrow you can make a conductor, and how many circuits you can fit onto the point of a pin (see Figure 1).

Worldwide environmental problems such as the greenhouse effect and acid rain have been caused by the human race's continuous reliance on the combustion of petroleum for fuel.

Solar energy, which is clean and practically unlimited, is expected to be a desirable alternate energy source to conventional power supplies, and demand for the photovoltaic system has increased throughout the world, especially in Europe and the United States.

Photovoltaic cells are probably the most effective method for capturing solar energy, since they are easy to use and are the most effective means of directly generating electricity.

Many kinds of solar cells have been developed in past years, especially since the first oil crisis in 1973. Among them, solar cells from cast multicrystalline silicon (also refereed to as (cast) polycrystalline silicon or semicrystalline silicon) are considered to be one of the most promising types, capable of achieving both high efficiency and low cost.

In 1975, Wacker proposed a new manufacturing method for silicon substrates, using the casting method. Since then, many organizations have been involved in the research and development of multicrystalline ingots and solar cells using multicrystalline silicon substrates.

Multicrystalline silicon substrates contain many kinds of defects compared to single-crystal silicon substrates, so the efficiency of multicrystalline silicon solar cells has been inferior to that of single-crystal cells. Recent research on multicrystalline silicon solar cells has resulted in substantial improvements and in the demonstration of high-efficiency cells.

The field of electronic ceramics includes components as diverse as Mn-Zn ferrites, lead zirconate titanate transducers, ZnO varistors, and Al2O3 or cordierite-based packages for integrated circuits. An overview of electroceramics is given in Figure 1, which depicts schematically the types of electronic and ionic phenomena of interest. Many of the materials pictured utilize some bulk property of the ceramic, such as the presence of ferroelectric of ferrimagnetic dipoles, electronic or ionic conductivity, or a phase transition to provide a useful function. In other materials, extraordinary responses can be engineered by suitably tailoring the thickness and properties of a grain boundary phase. Varistors, barrier layer capacitors, and PTC thermistors, for example, all rely on the preparation of semiconducting grains and insulating grain boundaries to create properties that cannot be found in single crystals. In a similar way, some of the porous ceramics utilized in humidity sensors rely on surface properties rather than on a bulk response to detect changes in the ambient humidity (Kulwicki, 1992). Finally, in materials for insulators or substrates, all of the otherwise interesting phenomena must be eliminated. With the exception of the last category, all of the phenomena depicted in Figure 1 can be utilized in ceramic sensors. While sensing and actuating materials are currently a fraction of the overall multibillion-dollar electroceramics market, the percentage is expected to grow as more and more devices are made “smart.”

As discussed in the introduction to this issue, smart materials are capable of sensing a change in the environment and responding in a useful way.

Interconnection technology is a key factor in the continual advancement of integrated systems. The rapid increase in device density and circuit complexity through scaling demands a similar increase in the interconnection density. Traditionally, this is achieved by reducing the metal pitch as well as gradually increasing the number of interconnection levels. As the width and spacing of interconnections are scaled down to submicron dimensions at the chip level and micron dimensions at the board level, signal delay, crosstalk, electromigration, and stress-induced migration become important concerns.

Cu holds promise as an alternative metallization material to Al alloy due to its low resistivity and ability to reliably carry high-current densities. Cu has a bulk resistivity of 1.68 μΩ-cm, whereas Al has a bulk resistivity of 2.65 μΩ-cm. The only metal with a resistivity lower than Cu is Ag. Since Cu has a melting point and atomic weight both higher than Al, it is expected to have better resistance to electromigration, although properties such as grain structure and resistance to corrosion at high temperatures may also affect electromigration characteristics.