Common experience with the thermomechanical properties and behavior of matter is usually at the continuum level, i.e., observations concern such phenomena as elastic and inelastic deformation of solids, flow of fluids, and conduction of heat. Continuum descriptions of these phenomena are expressed in terms of partial differential equations representing the principles of balance of mass, momentum, and energy. Since the basic principles apply to all materials, it is apparent that they alone will not suffice for solving specific problems. The peculiarities of individual materials are expressed in terms of constitutive equations, of which Hooke's law of elasticity and Newton's law of viscosity are examples. For the most part, “continuum modeling” refers to the process of devising constitutive equations. This work requires exercise of physical insight at both the macroscopic and microscopic levels, consideration of experimental observations, and application of formal mathematical principles.

Much of materials science is devoted to development and application of materials that have been selected for, or designed to have, useful properties different from those of existing materials. Many materials are of interest precisely because they have unusual properties. For example, paints and other coatings are often designed to flow easily when spread, but resist running afterward. Much work is devoted to analyzing manufacturing processes, whether they be processing of foodstuffs, forging metals, drawing films and fibers, or curing polymers. Properties and physical states of materials that are important during manufacture are often very different from those desired under service conditions.

The growth of thin films has been instrumental in the study of many areas of material science, physics, metallurgy, and chemistry and is an important ingredient in the development of many devices. Although experimental studies have been extensively pursued for many years, theoretical studies have only been performed using model calculations which rely on a number of unknown parameters a priori. Only recently have attempts been made to understand thin film growth using realtime numerical simulation. The main reason for the recent increase of such studies is the development of computers capable of tackling a problem of the magnitude required to understand thin film growth. The phenomena present in thin film growth occur for systems containing many particles (e.g., columnar growth) and long relaxation times, which strain the capabilities presently available in modern supercomputers. Further increases in computational power might bring a number of important problems within reach and improve our understanding of thin film growth at the microscopic level.

I will present a number of epitaxial growth studies we have performed using molecular dynamics (MD) techniques. I will show that a number of properties predicted by these calculations are in good agreement with experimental observations. These include the microcrystalline and epitaxial growth of metal films, the growth of amorphous films in mixtures of metals, and the vapor phase growth of silicon. Finally, I will outline several important studies yet to be implemented.

The generally accepted definition for thermal analysis is that it covers “a group of techniques in which a physical property for a substance and/or its reaction product(s) is measured as a function of temperature while the substance is subjected to a controlled temperature program.” From this broad scope, it is evident that at some point in their careers nearly all experimental materials scientists have been practicing thermal analysts. It is also obvious that the techniques of thermal analysis are applicable to the entire range of materials. Consequently, it is difficult, it not impossible, to properly review such a topic in several pages, but I hope that a carefully selected handful of examples will acquaint the readers with the standard techniques and the enormous versatility inherent in thermal methods. One aspect which simplifies the review is that the methods are conceptionally very simple and do not require extensive descriptions.

Table I describes some of the more common techniques for which examples will be presented. Obviously, many more properties have been studied; however, those listed will suffice as an introduction to the subject.

Because all information derived from thermal methods frequently has only an indirect correlation with the chemical concentration or process of interest, it is important to use more than one technique where possible.

The present trend of developing electronic devices with increasingly fine dimensions borders on a number of fundamental scientific questions about the very nature of how materials at ultrafine dimensions behave. This article addresses some of these questions. The fabrication of discrete metallic phases in porous and nonporous glassy matrices presents a number of exciting device possibilities. Methods of fabricating ultrafine metallic phases in silica via the sol-gel route are presented.

In attempting to fabricate materials with ultrafine physical dimensions for a wide variety of applications, several fundamental questions arise about the nature of materials behavior. For example, how many metal atoms are necessary to form a cluster exhibiting “metallic“ properties? Moreover, does the number of atoms necessary depend upon which metallic property is examined? This question has been partly addressed by D.C. Johnson and co-workers with regard to magnetism in osmium clusters. Their results show a threefold increase in magnetic susceptibility between clusters containing 3–10 osmium atoms.

Another important question, especially when considering device applications, is how the relative contributions of surface and bulk thermodynamics affect such properties as phase transformations. In addition, ultrafine phase dimensions interact with the fundamental unit lengths of a wide range of processes, including the wavelength of visible light, the mean free path lengths of conduction processes, the wavelengths of phonon vibrations, etc. How do these interactions affect optical, thermal, and electronic properties?

Fabrication of ultrafine metallic particles in porous and nonporous matrices may lead to many possible device applications including heterogeneous catalysts, nonlinear optic devices, highvoltage switching devices based on interparticle tunneling, and perhaps even new types of charge storage devices (capacitors).

Advanced materials can provide specialized properties, or combinations of properties, that cannot be obtained in conventional materials. However, advanced materials generally require unusual processing operations in order to achieve their unique microstructures and the resulting greatly improved properties. These materials also tend to be expensive because of the high value added by unusual processing operations that may be labor intensive. Because the relationships among the processing parameters, microstructure, and resulting material properties and performance are not fully understood, and because the microstructure is difficult to control, reproducibility in these materials is often unsatisfactory. A very promising direction toward overcoming these difficulties involves intelligent processing of materials, a computer-based approach to automatically controlling the evolution of microstructure during processing.

In a conventional automated materials processing system, automation involves utilizing sensors to monitor process variables such as temperature and pressure. Data from these sensors are compared with preset values automatically in order to maintain these values with control devices. Nevertheless, the microstructure and properties often experience significant variations. The variations are detected after the fact either by destructive analysis in a quality control laboratory or by nondestructive evaluation (NDE) at the end of the manufacturing process.

In contrast, an intelligent processing system utilizes a new class of NDE sensors to characterize the microstructure of the material in real time. Moreover, the real-time data plus data from conventional process variable sensors are transmitted to a computerized decision maker.

Optical technologies have advanced dramatically in recent years. In just two decades the transparency of optical fibers has improved by four orders of magnitude. Semiconductor lasers have evolved from a new invention to highly reliable, high performance commercial devices for wide bandwidth optical communications. New approaches to higher frequency modulation, wider bandwidth transmission, more sensitive detection and optical amplification are constantly being developed. Fundamental limitations are sufficiently far removed from current capabilities that considerable further progress can be anticipated. These advances have provided the stimulus for a much broader investigation of the potential of optics in future information technologies in which optics and electronics play complementary roles. This rapidly developing field is referred to as “photonics.” Increasing attention is now being paid to applying optics to wide bandwidth switching systems and to exploring the potential of optics for image processing and computation.

Past progress in optical communication can be traced largely to the dramatic progress in optical fiber and compound semiconductor materials technologies. Likewise, future opportunities in photonic switching and information processing will depend critically on the development of improved photonic materials. The future role of optics in these conventionally electronic technologies, and the extent of that role, depends on whether materials can be designed and fabricated with the required characteristics.

Imaging is the cornerstone of materials characterization. Until the middle of the present century, visible light imaging provided much of the information about materials. Though visible light imaging still plays an extremely important role in characterization, relatively low spatial resolution and lack of chemical sensitivity and specificity limit its usefulness.

The discovery of x-rays and electrons led to a major advance in imaging technology. X-ray diffraction and electron microscopy allowed us to characterize the atomic structure of materials. Many materials vital to our high technology economy and defense owe their existence to the understanding of materials structure brought about with these high-resolution methods.

Electron microscopy is an essential tool for materials characterization. Unfortunately, electron imaging is always destructive due to the sample preparation that must be done prior to imaging. Furthermore, electron microscopy only provides information about the surface of a sample. Three dimensional information, of great interest in characterizing many new materials, can be obtained only by time consuming sectioning of an object.

The development of intense synchrotron light sources in addition to the improvements in solid state imaging technology is revolutionizing materials characterization. High resolution x-ray imaging is a potentially valuable tool for materials characterization. The large depth of x-ray penetration, as well as the sensitivity of absorption crosssections to atomic chemistry, allows x-ray imaging to characterize the chemistry of internal structures in macroscopic objects with little sample preparation. X-ray imaging complements other imaging modalities, such as electron microscopy, in that it can be performed nondestructively on metals and insulators alike.

Research in magnetism has a long, redoubtable history, and one fundamental reason for its continuing productivity is the fact that advances are very often coupled intimately with progress in materials science. New materials, preparation methods, and characterization technique s continu e to provide opportunities for exploring magnetic phenomena in a progressively broader array of systems with a growing arsenal of analytical tools. Challenges to theory are often presented by discoveries on the experimental side, engendering constructive interaction which enhances our basic understanding of the materials and the physics controlling their properties. Technology invariably benefits from that ongoing process. The following series of five articles in this issue of the MRS BULLETIN aims at conveying the excitement and vitality of current work in magnetism and magnetic materials. Each author or set of author s has endeavored to provide a succinct overview of a specific area, and in combination the papers offer an informative (although certainly not exhaustive!) survey of this very active field.

Crystal growth is a vital and fundamental part of materials science and engineering, since crystals of suitable size and perfection are required for fundamental data acquisition and for practical devices such as integrated circuits.

The word “crystal” comes from the Greek and means “congealed by cold.” The term was originally applied to ice crystals and to crystalline quartz found in such locations as the Alps and thought, at one time, to be some permanently frozen form of water. As a crystallization process, crystal growth extends throughout recorded history—the crystallization of salt from sea water may have preceded pottery-making as the oldest method for controlled phase transformation of materials.

The crystallization of salts such as sodium carbonate was known to the Egyptians, and the process of purification by recrystallization from aqueous solution was described by Greek and Roman writers before the Christian era. The “father” of crystal growth, however, is generally considered to be the English scientist, Robert Boyle, who in 1672 described experiments relating the form of crystals to impurities and changes in growth rate. Boyle believed in the healing powers of crystals, part of the mysticism which has had a strong revival in recent times. We understand that crystals are particularly valued for enhancing the ability of the third eye to perceive auras, but are unable to offer warranted samples.

Recognition as the “founder” of the theory of crystal growth is normally given to the American J. Willard Gibbs, who in 1878 published a seminal work on phase transformations, including a theory of the energy to generate a crystal nucleus and of the equilibrium form of crystals.

In electronic processors, heat dissipation and interconnection delay are serious design and performance limiting factors. Why then consider photonic components where both the energy and size of the photon are large (˜1 eV and ˜1 μm, respectively) and the required nonlinear interactions between electric or magnetic fields and photons for switching or modulation are small? There are several answers to this question. First, the wide bandwidth of optical communications systems is taxing the current capabilities of electronic switching technologies. Even a slow optical switch can switch a very wide bandwidth optical signal from one fiber to another. External optical modulators will likely be required in ultrawide bandwidth communications because of basic limitations on direct modulation of lasers. Because of the weak electromagnetic interaction and low dispersion, optical interconnection of electronic circuits offers considerable advantages in high speed computer architectures. Some of these applications would appear to be relatively near term since they build on current capabilities of optical communication.

Longer term and more speculative are applications of photonics to computation and image processing — areas where electronics technology is already mature. Current research can be divided into two groups — ultrafast processing and parallel processing. The first group concentrates on processing with ultra-fast optical pulses. Optical pulses as short as 6 fs — orders of magnitude shorter than any electronic pulses — have been generated in the research laboratory. High processing rates are achievable by serial processing of high repetition rate ultrashort pulses. This approach requires ultrafast switches, which in turn requires materials with ultrafast nonlinear optical response time. Indeed, the shortest electrical signals are now measured by optical sampling techniques.

Molecular Beam Epitaxy (MBE) is a thin film deposition process in which thermal beams of atoms or molecules react on the clean surface of a single-crystalline substrate, held at high temperatures under ultrahigh vacuum conditions, to form an epitaxial film. Thus, contrary to the CVD processes described in the other articles, the MBE process is a physical method of thin film deposition.

The vacuum requirements for the MBE process are typically better than 10−10 torr. This makes it possible to grow epitaxial films with high purity and excellent crystal quality at relatively low substrate temperatures. Additionally, the ultrahigh vacuum environment allows the study of surface, interface, and bulk properties of the growing film in real time, by employing a variety of structural and analytical probes.

Although the MBE deposition process was first proposed by Günther in 1958, its implementation had to wait for the development of the ultrahigh vacuum technology. In 1968 Davey and Pankey successfully grew epitaxial GaAs films by the MBE process. At the same time Arthur's work on the kinetics of GaAs growth laid the groundwork for the growth of high quality MBE films of GaAs and other III-V compounds by Arthur and LePore and Cho.

The preservation of U.S. aeronautical leadership is an economic and military necessity, but it is by no means assured. The rise of Airbus, Ariane, and Embraer has been lightning fast; tomorrow could see the development of Japan's FSC or Israel's Lavi. Our competitors are well organized and often enjoy the support of their governments. Our capabilities are no longer unique; thus our future work is clearly defined for us.

The key to continued U.S. preeminence in aerospace is to be found in the further research, development, and application of a group of revolutionary technologies in the areas of propulsion, numerical and symbolic computation, laminar flow modeling, and advanced materials and structures. Exploitation of the emerging technologies in these areas by industry, government, and universities will significantly impact the performance and cost of future aerospace vehicles and systems. Materials science and engineering, particularly the discipline of nondestructive evaluation, will play a major role in making such continued aerospace leadership a reality.

From the use of plastic and glass radomes in the first jet engine demonstrators to the composite parts of today's most advanced aircraft, the need to ensure reliable materials has always been critical. Advanced materials and structural concepts offer the opportunity for significant airframe improvements on all types of aircraft. Indeed tomorrow's aerospace structures, such as the National Aerospace Plane, the Space Station, as well as the ATF and SDI-related items will employ a myriad of exotic materials that must be extremely reliable and highly producible.

Science often advances through the introdction of new ideas which simplify the understanding of complex problems. Materials science is no exception to this rule. Concepts such as nucleation in crystal growth and spinodal decomposition, for example, have played essential roles in the modern understanding of the structure of materials. More recently, fractal geometry has emerged as an essential idea for understanding the kinetic growth of disordered materials. This review will introduce the concept of fractal geometry and demonstrate its application to the understanding of the structure of materials.

Fractal geometry is a natural concept used to describe random or disordered objects ranging from branched polymers to the earth's surface. Disordered materials seldom display translational or rotational symmetry so conventional crystallographic classification is of no value. These materials, however, often display “dilation symmetry,” which means they look geometrically self-similar under transformation of scale such as changing the magnification of a microscope. Surprisingly, most kinetic growth processes produce objects with self-similar fractal properties. It is now becoming clear that the origin of dilation symmetry is found in disorderly kinetic growth processes present in the formation of these materials.