The extreme variations in the mechanical as well as acoustical properties of wood, not only within a given species, but even within the same tree, have plagued violin researchers for nearly 100 years and violin makers for more than 300 years. The lore of violin making tells us that a spruce tree growing on the north side of a mountain should be cut in the spring when the water is moving up in the wood cells, and that the first six or eight feet of the trunk should be discarded because the twisting and the weight of the big trunk cause increased density in the wood with a less even grain structure. The wood should then be cut into lengths, free of knots and sap cracks, and split into so-called flitches that can be used for various violin-family instruments, depending on size. When a flitch is properly cut and joined for a violin top, the annual-ring spacing, or grain, ideally should be narrow in the center of the violin top, gradually becoming wider toward the edges. Also, the grain should be vertical to the bottom surface of the top plate. The more researchers study the desirable parameters of spruce for the proper tuning of violin top and back free plates and the effects they have on the finished instrument, the more the seemingly unsupported lore of violin making makes very good sense.

Fast ions create linear trails of intense atomic disorder in many solids. The particle tracks are in themselves scientifically interesting because they consist of unique, localized radiation damage. They also are noteworthy for their diverse practical uses, which range from improved high field superconductors to mineral exploration and bird altimetry. The two areas—what tracks are and what they do practically—are the subjects of this introduction and the following three articles. Although the mechanism for producing tracks in insulators is semi-quantitatively well-established, there is a distinct mystery as to the formation mechanism in superconductors, inter-metallics, and metals. This mystery is the subject of the next two articles written by discoverers of tracks in these materials.

We will not discuss in detail the multitude of scientific uses for these tracks as particle-track detectors. Uses range from nuclear, elementary-particle, and cosmic-ray physics to geochronology, geochemistry, and geophysics; chemistry; and radiobiology. The interested reader can learn more on the subject through a book, part of which surveys scientific applications of particle tracks in solids. The key to these uses—and most of the practical uses—is that, in materials where tracks can be observed, either directly or by a widely applicable trick to be described, each detector sample is a nuclear particle-track chamber—the solid-state equivalent of the well-known gaseous and liquid detectors (i.e., cloud chambers and bubble chambers). The major distinction is that tracks in solids are long-lasting rather than transient features.

Over the past 30 years or so, some fascinating new developments have taken place in a field that is now called “nonlinear science.” These developments have been of such magnitude that historian Alan Beyerchen concluded “There is every reason to believe that the westernized world is in the early stages of an intellectual transformation of major proportions, perhaps as significant as the emergence of the modern worldview in the fifteenth through seventeenth centuries.”

These new developments are rooted in work conducted by mathematician Jules Henri Poincaré around the turn of the century. He carried out some calculations regarding planetary orbits and demonstrated the possibility of erratic or “chaotic” dynamical behavior. What is particularly interesting about such behavior is that it is exhibited by deterministic systems, that is, systems that have no stochastic (noisy) character of any kind. Little was done with Poincaré's findings until the 1960s when new work in nonlinear science, based on numerical models, led to the discovery of amazingly complex behavior that can be exhibited even by very simple deterministic systems. Since that time, the discipline has grown enormously and has been applied in practically every area of science and engineering, as well as in other diverse areas such as the financial market, political science, economics, and even Adlerian psychology.

The evolution of silicon nitride ceramics over the last two decades has brought about the advancement of materials which were first fabricated by the application of mechanical pressure and temperature (i.e., hot pressing) resulting in high flexure strengths (e.g., 700–800 MPa) but rather poor resistance to creep at temperatures of ~1200°C. At the same time, these ceramics remained quite brittle with fracture-toughness values of 4–5 MPa m½, such that strengths were very sensitive to flaw or crack sizes. As a result, measured strengths exhibited considerable scatter, as reflected by a low Weibull modulus. In the ensuing years, approaches were sought to develop more economical methods of fabricating silicon nitride components by densifying to near-net shape. Methods were also sought for increasing the elevated-temperature reliability by minimizing the additives employed to promote densification and by utilizing additives that produced more stable and refractory grain boundary phases. The application of gas-pressure sintering methods, utilizing gaseous environments of 10–100 atmospheres, led to the ability to produce dense near-net shaped components with very high fracture strengths (e.g., ≥1000 MPa). At the same time, advances in processing and additive chemistry, sometimes combined with additional fabrication methods (e.g., hot isostatic pressing), have resulted in ceramics with excellent creep resistances at temperatures in excess of 1300°C. Some of these silicon nitride ceramics exceed the elevated-temperature capability of superalloys by 200°C. The initial desire for light-weight ceramic components that could sustain tensile loads for high-temperature applications is, indeed, beginning to bear fruit. One of the most impressive examples of the development of a complexly shaped lightweight component is the silicon nitride turbocharger rotor used in a number of Japanese automobiles, which is currently manufactured at a cost approaching that of the opposing superalloy rotor and provides exceptionally high mechanical reliability and production yields. Currently, there are also earnest efforts to incorporate silicon nitride valves for engines, as well as in a variety of other components (e.g., combustion swirl chambers, valve-lifter pads, etc.). The acceptance and use of this class of brittle materials, which were once considered prohibitively expensive for fabrication into complex shapes and not suited for such applications, is a remarkable testimony of the progress that has been made.

The properties of imperfections (or defects) of the atomic or molecular order in condensed matter can be conveniently described under two headings: (1) Topological properties—Defects break a specific symmetry of the ordered system at a local scale, that is, along a point defect, a line defect (a dislocation or a disclination), or a surface defect (a wall). (2) Elastic properties—Defects are sources of two types of distortions of the order: long-range distortions, which depend crucially on the broken symmetry but also on the material constants, and short-range distortions in the “core” region of the defect where the order parameter of the ordered phase is broken. These distortions are irreversible in the sense that defects appear during plastic deformation (in solids) or rheological flow (in liquid crystals).

To illustrate this classification, let us recall the example of dislocation lines in solids. These defects break translational symmetries (henceforth a dislocation is defined topologically by the translation b it breaks, the so-called Burgers vector). They are at the origin of rather weak, long-range, internal distortions and stresses that depend on the elastic constants (in the region of the good crystal) and rather strong, short-range distortions and stresses in the “core” region, implying a complete rearrangement of the molecular order. These stresses are different in the static and dynamic states, and the shape of the dislocation line, as well as its size, etc., depend on the history of the sample.

In this article, we will focus on defects in liquid-crystalline polymers. A synthetic polymer that displays mesomorphic order (intermediate between crystalline and liquid) is usually made of units that are themselves mesogenic and that align coherently when in contact.

The application of functionally graded materials (FGM) principles is widespread in Europe, despite the lack of national or European programs coordinating the efforts of the individual institutions and researchers. In the following article, the status of research in Europe will be outlined by examples of research projects from various application fields. More detailed information on European FGM research is available from the proceedings of the FGM '94 symposium held in October 1994 in Lausanne, Switzerland. The symposium was organized by B. Ilschner through the Materials Department of the Swiss Federal Institute of Technology of Lausanne.

The following is an edited transcript of the plenary lecture presented by Robert Langer at the 1995 MRS Spring Meeting on April 17, 1995 in San Francisco.

Over the last 21 years I have been involved in examining and studying, and in some cases synthesizing new polymers and biomaterials, looking at how they might be able to solve two problems. One set of problems involves drug delivery; a second involves creating new tissues.

Controlled drug release is a relatively young field that has involved a fundamental change in the way drugs are delivered. Transdermal drug delivery, for example, uses five layers of polymers, yet the whole system is 0.2 mm thick. One such polymer contains nitroglycerine, which is slowly released for one day from a patch through the skin into the blood circulation. This process provided a novel way to treat angina for patients with chest pains. First introduced in 1982, such drug-delivery systems saw a usage rate of over 500 million last year.

Norplant systems, which are silicone capsules placed under the forearm, slowly release a birth-control drug for over 2,000 days, or five years, from what are actually six small implants about the size of match sticks. These two examples illustrate that drugs can be made to slowly diffuse through polymers over a long period of time.

Titanium nitride (TiN) has been recognized as an excellent barrier material in various metallization structures of advanced microelectronic devices. TiN serves as a nucleation/glue layer as well as a barrier against WF6 attack in W plug filling. It serves as a diffusion barrier during or after high-temperature Al reflow processing for contact and via filling. TiN is considered as a diffusion-barrier material for Cu metallization as well. In addition, it is utilized as an antireflection coating layer, especially on top of Al, an application that will not be discussed in this article.

TiN films must conform to the extreme topographies used in devices in order to guarantee void-free plug formation as well as Jow junction leakage. This should be achieved with the thinnest films possible in order to reduce interconnect stack thickness and to lower contact or via resistance. (The TiN resistivity is higher than that of the other components of the metallization—Ti, Al, or W.) In addition, the good barrier properties must be retained following various thermal cycles used in multilevel metallization. Finally, the metallization must be manufacturing-worthy, namely, it should be reliable and reproducible, it should have a very low particle content, and it should have a low cost of ownership.

At present, TiN is mainly deposited by physical vapor deposition (PVD) via reactive sputtering. However, the poor conformality of sputtered TiN films over extreme topography limits the use of this deposition technique for deep sub-0.5 μm applications, especially those with features that have high aspect ratios.

Ellipsometry has been used in microelectronics since the early 1960s when it became necessary to measure the thickness of transparent films on reflective substrates. Ellipsometry is an old technique but with the increasing technological importance of thin films, particularly in microelectronics, and with the availability of faster computers for calculations, ellipsometry is in a renaissance. In principle it is a simple technique (don't confuse simple with easy!) that relies primarily on the physical optics of stratified media. Excellent treatments of ellipsometry exist (e.g., a book, recent reviews), and it is not the intent here to try to improve upon them. Rather, this article is intended to briefly discuss the essential ideas and then to show a few examples of in situ ellipsometry applied to modern materials-research problems. The emphasis here is on in situ ellipsometry, because it is one of the few techniques that can accurately monitor thin-film processes while they occur, yielding both fundamental properties and process information about the system under measurement. In addition it is now possible to use feedback to affect control of thickness, composition, and temperature, for example.

We first consider some basic questions: What is ellipsometry, what does it measure, and how well? Next, we discuss hardware for in situ ellipsometry, and finally we show a few examples.

The two-dimensional packing of organic molecules (small and large) produces ultimately thin organic films. Several possibilities exist for achieving this result from linear polymers, taking advantage of surfaces and interfaces as spatial templates. Monomolecular polymer layers can be prepared on the surface of water by polymerization of monolayers. They can be transferred onto solid supports by the Langmuir-Blodgett-Kuhn technique. Polyion complexation at the air-water interface provides a second method of two-dimensional arrangements of linear polymers. More recent approaches include polymerization-induced epitaxy (PIE) and alternate polyion adsorption. In the case of PIE, growing polymer chains in solution are adsorbed onto immersed graphite plates, most probably as single polymer layers (Figure la). Chain alignment is commensurate with the graphite lattice, as confirmed by scanning tunneling microscopy (STM). This epitaxial adsorption is independent of polymerization mechanisms. On the other hand, the alternate adsorption consists of sequential dipping of charged solid supports in aqueous solutions of oppositely charged linear polyions (Figure 1b). The regularity of the layer thickness is remarkable and, under appropriate conditions, the deposition is repeatable without limit. This method is applicable not only to pairs of linear polyions but also to combinations of linear polyions with water-soluble proteins or inorganic nanoparticles.

Silicon nitride is a highly covalent bonded compound which decomposes at 1877°C. Therefore, it is impossible to densify Si3N4 without sintering additives. Densification is achieved by liquid-phase sintering usually using metal oxides such as MgO, Y2O3, A12O3, and most of the rare-earth oxides as sintering additives. The oxides react with SiO2—always present at the surface of Si3N4 particles—to form an oxide melt and, with increasing temperature, an oxynitride melt by dissolution of Si3N4. The resulting microstructure consists of elongated Si3N4 needles embedded in a matrix of smaller equiaxed Si3N4 grains and a grain boundary phase, as shown in Figure 1. The amount and chemistry of the sintering aids determine the volume fraction of the grain boundary phase. The content required for complete densification depends on the sintering techniques: 2–5 vol% additives are sufficient if densification is supported by a high external pressure (hot pressing [HP] or hot isostatic pressing [HIP]); pressureless-sintered and gas-pressure-sintered (10-MPa nitrogen pressure) materials have additive contents of up to 15 vol%. Today, silicon nitride ceramics are regarded as a class of material comparable to steel. Different qualities depend on the size and shape of the silicon nitride grains and the amount and chemistry of the grain boundary phase. Materials with a high room-temperature strength exhibit a finegrained, elongated microstructure, while materials with a high fracture toughness are more coarse-grained. In both cases, a weak interface is required to induce transgranular fracture. (See Becher et al. in this issue.) Since all Si3N4 grains are completely wetted by the grain boundary phase, the interface strength is determined by the additive composition. Nevertheless, a contradiction arises between the development of high-strength and high-toughness Si3N4 ceramics and high-temperature resistant materials because the grain boundary phase is responsible for the excellent properties at low temperatures, but limits the properties at temperatures above its softening point.

Strings for musical instruments seem to be among the simplest of devices. Textbooks on acoustics commonly use them as the most elementary example of a vibrating system. However, the assumptions often made to simplify mathematics are far from the conditions that exist in real strings made of even the most suitable materials available.

Consider what a musician wants in any instrument: as wide a frequency range as practical; considerable acoustic power with a significant difference between loud and soft notes; and, if possible, the ability to play several notes at once. The last requirement dictates the presence of multiple oscillators. Long, thin strings in closely-spaced groups fit the bill, but they are inherently deficient in meeting the other criteria without help from additional devices.

Direct acoustic output from a vibrating string is negligible because it alone cannot move enough air. This deficiency has been corrected in hundreds of ingenious ways over thousands of years by mechanical coupling to a light, stiff structure large enough to radiate the lowest pitch being produced, often incorporating air-cavity resonators tuned to enhance low-frequency output. In this electronic era, unlimited power can be obtained using transducers to detect string motion as electrical voltages that can be amplified and directed to loud-speakers.

Transition metal oxides belong to the class of iono-covalent materials in which electrons are, to first order, localized in tight metal-oxygen bonds. They appear to be quite different from classical metals in which electrons are assumed to be delocalized in a free electron gas. Nevertheless, due to exchange interactions and hybridization of atomic orbitals, oxides exhibit many interesting physical properties like magnetic ordering and various electron transport properties that extend from wide gap insulators to narrow band conductors and high-temperature superconducting (HTS) superconductors. This class of materials appears well-suited to the study of the damage induced by heavy ion irradiation and its consequence on physical properties like electrical resistivity.

In this article, we will describe the results of heavy ion irradiations in magnetic oxides, exhibiting a wide palette of electron transport properties from insulators to metals through hopping semiconductors and in HTS copper oxides that are narrow-band metals above Tc. We will show that heavy ion irradiation, in addition to disordering matter, can induce changes in some physical properties like magnetic ordering and orientation of magnetic fields, and can produce large increases of the critical current. To account for these results, the thermal spike model will be considered as a possible way to understand the damage creation in iono-covalent compounds.

The following is an edited transcript of the presentation given by A. Paul Alivisatos, recipient of the Outstanding Young Investigator Award, at the 1995 MRS Spring Meeting in San Francisco.

The work I will describe on semiconductor nanocrystals started with the realization that it is possible to precipitate a semiconductor out of an organic liquid. We can precipitate out a semiconductor as a colloid—a very small-sized semiconductor with reduced dimensionality—that will show large, quantum size effects. A dream at that time was to make an electronic material by such a process in a liquid beaker, by starting with an organic fluid and somehow injecting something into the fluid to make very small particles, which we could use in electronics. The materials we use in electronics today have perfect crystalline order. We are able to put in dopants very specifically, or control precisely their arrangements in space in enormously complicated ways. The level of purity of electronic materials is so high that making an electronic material in a wet chemistry approach seems almost impossible. If, in addition, we specify that the size must be controlled precisely, we recognize the project is a problem for basic research, yet not one ready for applications. Many fundamental problems arise if we try to make semiconductor particles, in a liquid, of such high quality that they can be used as electronic materials.

Several authors have conducted research to characterize the woods used for musical instruments. The principle of such studies is that acoustical qualities of musical instruments are linked to modal properties of the resonator, especially to the soundboard. Therefore, by knowing the mechanical parameters of materials, it is easy to deduce the frequencies of the eigen modes of a vibrating structure. Equation 1 requires only the geometrical dimension of a plate, the density, and Young's modulus to define a material:

where l is length, b is width, d is thickness, E is Young's modulus, I (= bd3/12) is inertial momentum, ρ is density, n is partial number, and fn, is frequency mode(n).

But, how do we choose the values of each parameter, since woods are variable materials with a large distribution of values-for samples coming from the same tree?

Just as in ordinary metals, defects in intermetallic compounds fall into three basic categories: point defects (vacancies, substitutional and interstitial atoms), line defects (dislocations), and planar defects (stacking faults, interfaces, grain boundaries). Also like ordinary metals, many important physical and chemical properties of intermetallic compounds are governed by the presence of these defects and the effects on them from temperature, composition, chemical environment, elastic stress state, and so on.

What ultimately distinguishes an intermetallic compound from ordinary metals is its superlattice crystal structure. A two-dimensional analogue of the actual three-dimensional superlattice structure is shown in Figure 1a where the superlattice (unit cell marked by full lines) is made up of two identical sublat-tices (unit cells marked by dotted lines). A property of the sublattice is that it is exclusively occupied by one atom species, and accordingly sublattices are named after the atoms that occupy them, for example, the A and B sublattices in Figure 1a. In three dimensions, a super-lattice may consist of several sublattices. For example, the L12 superlattice of Ni3Al consists of four interpenetrating cubic sublattices, one occupied by Al atoms (Al sublattice), the other three by Ni atoms (Ni sublattices). When the sub-lattices are occupied exclusively by their designated atoms, the crystal is said to be fully ordered. The crystal will be partially ordered if a certain fraction of the sublattice sites is taken up by atoms that would otherwise sit at other sublattices; this fraction is used to describe the degree of long-range order.

The growth of Si3N4 grains in the usual sintering process occurs in an oxynitride liquid formed by reactions between sintering additives, impurity SiO2, and Si3N4. The shape of an Si3N4 grain in the liquid matrix is a hexagonal rod, although the aspect ratio (c/a) varies considerably, depending on the processing conditions and final crystalline forms of α or β. Figure 1 shows two types of microstructures observed in sintered Si3N4-based materials. The microstructure shown in Figure la is normal with unimodal grain-size distribution and that of Figure 1b is abnormal with a microstructure of exceptionally large elongated grains within fine matrix grains. When normal grain growth occurs, the microstructure varies little with sintering time, and the development may be described by a simple law. But, when abnormal grain growth occurs, a duplex grain structure with a bimodal grainsize distribution results; then no simple kinetic law can describe the microstructure development.

The grain growth in the microstructures shown in Figure 1 exemplifies the growth of faceted grains in a liquid matrix. The grain growth in a matrix occurs via growth of larger grains and dissolution of smaller ones by material transport through the liquid phase. The driving force of the material transport for an individual grain is determined by the difference between its size and the critical grain size, which is invariant at the moment of observation. Since the driving force is usually low, the volume change of each grain is relatively slow and analogous to the crystal growth or dissolution in a liquid matrix under low super- or undersaturation. Therefore, knowledge of the growth behavior of faceted crystals under low supersaturation in a liquid may provide fundamental understanding of the grain growth in Si3N4-based materials.