The development of a new nickel based superalloy for use in the high-pressure turbine disk of a modern gas turbine is one of the most expensive and protracted exercises undertaken by gasturbine manufacturers. A typical complete development cycle can take more than 10 years. This delay arises not only because of the extensive mechanical testing and Validation required to ensure the safety of the critical part, but also because of the difficulty of finding a material chemistry that provides the correct degree of balance between mechanical properties and long-term material stability. The conventional approach to this latter problem is to use a combination of experience, empirical wisdom, and a few basic calculation tools based on electron-valency theory (PHACOMP). There is thus an urgent need to develop a tool that reliably summarizes the known body of knowledge about the stability of Ni-based superalloys and provides a method that can quickly and reliably guide the alloy developer through the initial alloy-chemistry selection phase of development.

Calculation of phase equilibria using calculated phase-diagram (CALPHAD) techniques has now matured to the point where it is possible for the alloy developer to quickly and reliably calculate phase equilibrium in a matter of minutes without an in-depth knowledge of thermodynamics. There thus exists a tool that enables the definition of alloy chemistries so that specific assumptions can be tested and the final chemistry selected without having to carry out the traditional manufacturing, processing, testing, and examination of a large raft of experimental chemistries. This article aims to show how such a tool has been used and high lights the success of this approach in the field of Ni-based superalloy development.

Intergranular separation in polycrys-talline materials involves breaking metallic bonds along grain boundaries in response to stress. The surfaces created in this manner expose the grain facets on either side of the original boundary, as shown in Figure 1. This mode of fracture often occurs at much lower fracture stress and energy than cracking by ductile processes through the interior of grains. The exposure of specific materials to certain environments and stress can promote this low-energy, intergranular mode of separation, even when fracture of the same material in vacuum would occur along a ductile transgranu-lar path. Three types of environment-assisted intergranular cracking can occur in a wide variety of alloy/environment systems: intergranular stress-corrosion cracking (IGSCC), intergranular hydrogen embrittlement, and intergranular liquid-metal embrittlement.

Figure 1 shows an example of IGSCC. This type of cracking is a pervasive problem in many technological applications, leading to extensive repairs, loss of service function, and safety concerns. IGSCC occurs in the weld-heat-affected zones of stainless-steel pipes in high-purity primary coolant waters within nuclear power plants, and in nickel-based alloys utilized as heat-exchanger tubing when exposed to the high-purity primary as well as secondary coolant waters in power plants. It is also seen in Al-based alloys used for fuselage skins and structural components in military and commercial aircraft when exposed to humid atmospheric conditions. Ferrous alloys used in the oil and gas industry are also susceptible. For instance, IGSCC of mild steels used in buried gas-transmission pipelines is a widespread international problem, leading to explosions when leaking natural gas ignites.

The neutron is a spin 1/2 particle with a well-specified magnetic dipole moment which interacts with magnetic fields in solids. Because typical neutron wavelengths are comparable to interatomic distances in solids, the fields that can be probed range from the dipoles of individual electron spins to those associated with flux lines in superconductors. In this article, we give a brief introduction to the use of magnetic neutron scattering in materials science.

Progress in materials science typically involves new materials or the improved fabrication of previously known materials, followed by measuring quantities such as electrical resistivity, magnetic susceptibility, or specific heat, typically as a function of variables such as temperature, magnetic field, chemical composition, or pressure. A material is deemed interesting if a simply defined bulk characteristic either undergoes a spectacular evolution as a function of one of the variables—superconductivity comes to mind—or has a range of values that makes it uniquely suited for some predetermined application. Once the material is thought to be interesting, there are natural drives to understand the origin of its interesting properties as well as to improve them. At this point, a microscopic tool such as magnetic neutron scattering provides valuable insights that bulk measurements cannot yield.

Nanometer-scale multilayer materials exhibit a wealth of interesting structural and mechanical property behaviors. Physical-vapor-deposition technology allows almost unlimited freedom to choose among elements, alloys, and Compounds as layering constituents and to design and produce materials with compositional and structural periodicities approaching the atomic Scale. These materials have tremendous interface area density, approaching 106 mm/mm3, so that a Square centimeter area of a one-micron-thick multilayer film with a bilayer period of 2 nm has an interface area of roughly 1,000 cm2. Hence interfacial effects can dominate multilayer structure and properties leading to unusually large strains and frequently stabilization of metastable structures. The atomic-scale layering of different materials also leads to very high hardnesses and good wear resistance. These materials are a test-bed for examination of the fundamental aspects of phase stability and for exploring mechanical strengthening mechanisms. They are also becoming increasingly interesting for applications such as hard coatings, x-ray optical elements, in microelectromechanical Systems (MEMS), and in magnetic recording media and heads.

In this article, we review some of the interesting structures and mechanical properties that have been observed in nanometer-scale artificial multilayer structures.

Superlattice thin films are readily deposited by vapor-phase techniques such as sputter deposition, evaporation, and chemical vapor deposition, as well as by electrochemical deposition. Superlattice deposition Systems are similar to conventional film deposition Systems, except for the provision to modulate the fluxes and thereby produce alternating super-lattice layers.

The development of biomaterials to treat, repair, or reconstruct the human body is an increasingly important component of materials research. Collaboration between materials researchers and their industrial and clinical partners is essential for the development of this complex field. To demonstrate the importance of these interactions, two articles in this issue focus on advances in biomaterials relating to the use of colloidal systems for transport, drug delivery, and other medical applications. These articles were coordinated by Dominique Muster (Université Louis Pasteur, Strasbourg) and Franz Burny (Hôpital Erasme, Brussels). The following is the first of these two articles.

A large variety of colloidal Systems involving highly fluorinated components have been prepared and investigated in recent years. These fluorinated Systems comprise diverse ty pes of emulsions (e.g., direct, reverse, and multiple emulsions; microemulsions; gel emulsions; waterless emulsions) with a fluorocarbon phase (and often a fluorinated Surfactant), and a ränge of self-assemblies (vesicles, tubules, helices, ribbons, etc.) made from fluorinated amphiphiles. Fluorinated Langmuir films and fluorinated black lipid membranes (BLMs) also have been investigated.

Research in this area was driven by the potential applications of such materials in medicine and biology. Fluorocarbon-based products are being developed as injectable oxygen carriers (“blood Substitutes”), media for liquid Ventilation, drug delivery Systems, and contrast agents for ultrasound imaging. One such agent has recently been approved for use in Europe and the United States. Several more products are in an advanced stage of clinical evaluation, and others are in various stages of preclinical development. From a more fundamental Standpoint, these materials are being investigated for assessing and understanding the impact that fluorinated components have on the formation, stability, structure, and properties of colloida l Systems in comparison with their hydrocarbon counterparts. The attention given to fluorinated colloids prompted the synthesis of numerous new families of fluorinated amphiphiles, which were to become components of such colloids.

Semiconductors doped with rare-earth (RE) elements have attracted a lot of attention as alternative materials for producing electrically pumpe d semiconductor lasers whose emission wavelength is very weakly dependent on temperature. This prospect is especially attractive in the case of indirect-gap Silicon, whose photonic applications as the material for light emitters still remain more of a hope than a reality. In view of a desirable emission wavelength at 1.5 μm, a lot of research has concentrated on Si:Er (see Coffa et al. for a recent review). It is generally recognized that doping with Er ions presents one of the most promising approaches to Silicon photonics. However, despiteintensive investigations, stimulated emission has not been conclusively demonstrated for Si.Er or for any other RE-doped semiconductor. This is in striking contrast to optical amplifiers and lasers based on various erbium-doped glasses. In this article, which builds on recent articles in MRS Bulletin on Silicon photonics, we will address the issues relevant to efficient light generation by semiconductors doped with RE elements in general, and specifically by Si:Er-based structures.

The intraimpurity electronic structure of RE ions is dominate d by electron-electron and spin-orbit interactions within the 4f shell. In the case of Er3+, they produce separated J-multiplets with 4I15/2 and 4I13/2 as the ground and the lowest-lying excited states, respectively. Due to the effective Screening of 4f electrons by the outer electron Shells, the host has a very limited influence and changes only slightly the relative positions of the levels. Depending on a particular site symmetry, the even terms of the crystal field split the free-ion J-multiplets into the Stark components typically by several meV for the ground State. The energy-level diagram of an Er3+ ion in a cubic crystal field is shown in Figure 1, where the energy transfer paths relevant for Si:Er are also schematically indicated. The odd terms of the crystal field potential admix the states of opposite parity to the 4f11 configuration of the Er3+ ion, thereby introducing a certain degree of electric-dipole strength into the otherwise forbidden intra-4f-shell transitions. This effect enhance s slightly the magnetic-dipole strength of the 4I15/2 ↔ 4I13/2 transition and is host- and site-dependent. There-fore, Er-related center s of different microstructure can be fairly easily identified.

The development of high-temperature processes and tighter environmental regulations requires utilization of efficient gas-separation processes that will provide high fluxes, high selectivity of separation, and the ability to operate at elevated temperatures. Dense inorganic membranes and membrane reactors are especially well suited for high-temperature reactions and separations, due in part to their thermal stability and high separation selectivity (in theory, infinite). Furthermore, membrane reactors offer an inherent advantage of combining reaction, product concentration, and separation in a single-unit operation for the improvement of process economics and waste minimization.

The classification of membrane reactors can either be by membrane material and geometry or by the configuration of the reactor. Porous and dense membranes in both tubular and disk forms have been used for membrane reactors. The membrane can either be catalytically active (catalytic membrane reactor [CMR]) or simply act as a separation medium. In the latter case, the catalyst is packed in the reactor, whose walls are formed by the membrane (packed-bed membrane reactor [PBMR]). In addition, if the membrane is also catalytically active, the reactor is called a packed-bed catalytic membrane reactor (PBCMR).

The principal materials from which porous inorganic (ceramic) membranes are made are alumina, zirconia, and glass. Alumina and zirconia membranes are usually asymmetric and composite, with a porous support (0.5–2.0 mm thick) for mechanical strength and one or more thin layers for carrying out separations.

On the other hand, glass membranes, such as Vycor and microporous glass, have symmetric pores. Materials commonly used as the porous support are alumina, granular carbon, sintered metal, and silicon carbide.

The development of biomaterials to treat, repair, or reconstruct the human body is an increasingly important component of materials research. Collaboration between materials researchers and their industrial and clinical partners is essential for the development of this complex field. To demonstrate the importance of these interactions, two articles in this issue focus on advances in biomaterials relating to the use of colloidal systems for transport, drug delivery, and other medical applications. These articles were coordinated by Dominique Muster (Université Louis Pasteur, Strasbourg) and Franz Burny (Hôpital Erasme, Brussels). The following is the second of these two articles.

There are two important objectives in drug delivery research. The first is to maximize the effectiveness of drugs by increasing the amount of drug reaching the target tissue while sparing other tissues the deleterious effects of the drug. The second is to control the release of a drug, so that the period of optimal drug concentration in the target tissue is maximized. A numbe r of different Systems have been investigated to achieve these objectives, including soluble polymeric delivery Systems and a range of colloidal drug delivery forms such as liposomes, emulsions, micelles, microcapsules, microparticles, and nanoparticles. This article focuses on polymeric materials for the production of micro- or nanoparticle Systems for dru g delivery by injection, and their characterization and Performance in vivo.

Colloidal particles have a number of advantages as drug delivery Systems; they are easy to prepare, have the potential for high drug loading, and release of the drug can be controlled. However, without surface modification, colloidal particles are difficult to target because they are directed largely to the liver and spieen after intravenous injection. The reasons for this can be found in the context of the body's defenses. In order to protect against disease, the body has a complex System to ensure that invading microorganisms are identified and neutralized at the earliest possible opportunity. Most parasitic or invading organisms which pose a threat are particulate in form, and thus any colloidal drug delivery System will have to evade detection by these mechanisms in order to reach its target.

The appeal of nanolayered materials from a mechanical viewpoint is that, in principle, plastic deformation can be confined to small volumes of material by Controlling both the frequency and magnitude of obstacles to dislocation motion. As we shall see, the spacing of obstacles can be used to impart large plastic anisotropy and work hardening. However, how strong can such materials be made as layer thickness (and therefore obstacle spacing) is decreased to the nanoscale level? In perspective, large, micron-scale, polycrystalline materials generally display improved yield strength (and fracture toughness) as grain size is decreased. This behavior at the micron scale can be explained via modeis that are built on two assumptions: (1) the strength of obstacles to crystal slip is sufficiently large to require pileups of numerous dislocations in order to slip past them; and (2) the strength of such obstacles does not change, even if their spacing is decreased. The modeling presented here shows that these assumptions may break down at the nanometer scale. The result is that there is a critical layer thickness in the nanometer range, below which improvement in strength does not occur.

Our discussion to follow briefly outlines a more macroscopic, micron-scale approach to determine yield strength, and then contrasts that with a sequence of events leading up to yield in nanolayered materials. We also address whether nanoscale materials are expected to exhibit more uniform or coarse slip than micron-scale materials. Finally, a semi-quantitative model of yield strength is developed which requires, as input, the strength of an interface to crystal slip transmission across it. We discuss several contributions to the interfacial strength and apply the theory to demonstrate a peak in strength for a 50 vol% Cu-50 vol% Ni multilayered sample.

Aqueous-solution/solid interfaces are ubiquitous in modern manufacturing environments as well as in our living environment, and studies of such interfaces are an active area of science and engineering research. An important area is the study of liquid/solid interfaces under active electrochemical control, which has many immediate technological implications, for example, corrosion/passivation of metals and energy storage in batteries and ultracapacitors. The central phenomenon of electrochemistry is the charge transfer at the interface, and the region of interest is usually wider than a single atomic layer, ranging from a monolayer to thousands of angstroms, extending into both phases.

Despite the technological and environmental importance of liquid/solid interfaces, the atomic level understanding of such interfaces had been very much hampered by the absence of nondestructive, in situ experimental techniques. The situation has changed somewhat in recent decades with the development of the largely ex situ ultrahigh vacuum (UHV) surface science, modern spectroscopic techniques, and modern surface microscopy.

However in situ experiments of electrochemical interfaces are difficult, stemming from the special nature of these interfaces. These are so-called buried interfaces in which the solid electrode surface is covered by a relatively thick liquid layer. For this reason, the probe we use in the structural investigation must satisfy simultaneously two conditions: (1) the technique must be surface/interface sensitive, and (2) absorption of the probe in the liquid phase must be sufficiently small for penetration to and from the interface of interest without significant intensity loss.

Most microfabrication techniques employ masks to transfer the desired microstructure onto a wafer using ultraviolet light, x-rays, electrons, or ions for the projection of the structures. Generally, photoresist processing and etching follow to form the final structures. In all cases, the facilities necessary to perform these processes grow increasingly more complex as the feature size of the structures diminishes, and these processes face their practical or economic limits at dimensions of about 50 nm. Thus alternative approaches are under investigation, including different self-assembly techniques. They require no costly facilities and no masks with nanometer structures, and they promise high throughput, since the patterning is directly achieved by a physical or chemical process. Self-assembled monolayers of long-chain organic molecules are the most widely studied examples, where chemisorption and spontaneous self-ordering of the molecules are observed on appropriate substrates. Another interesting example is island-ordering, laterally or in a vertical direction, during epitaxial growth. The lattice-mismatched islands tend to nucleate preferentially on top of each other when separated by a thin spacer layer, due to the associated strain field. Another approach is the use of specific stressor layers on the surface to obtain alignment of buried precipitates along the stressor lines. However, the main challenges of all self-assembly techniques are precise control of the dimensions of the structures and reproducibility.

As discussed in the accompanying articles in this issue of MRS Bulletin, the optical properties of rare-earth (RE) elements have led to many important photonic applications, including solid-state lasers, components for telecommunications (optical-fiber amplifiers, fiber lasers), optical storage devices, and displays. In most of these applications, the host materials for the RE elements are various forms of oxide and nonoxide glasses. The emission can occur at visible or infrared (IR) wavelengths, depending on the electronic transitions of the selected RE element and the excitation mechanism. Until recently, the study of semiconductors doped with RE elements such as Pr and Er has concentrated primarily on the lowest excited state as an optically active transition. The presence of transitions at IR wavelengths (1.3 and 1.54 μm) that are coincident with minima in the optical dispersion and the loss of silica-based glass fibers utilized in telecommunications, combined with the prospect of integration with semiconductor device technology, has sparked considerable interest.

The status and prospects of obtaining stimulated emission in Si:Er are reviewed by Gregorkiewicz and Langer in this issue and by Coffa et al. in a previous MRS Bulletin issue. While great progress is being made in enhancing the emission intensity of Er-doped Si, it still experiences significant loss in luminescence efficiency at room temperature, as compared with low temperatures. This thermal quenching was shown by Favennec et al. to de crease with the bandgap energy of the semiconductor. Hence wide-bandgap semiconductors (WBGSs) are attractive candidates for investigation as hosts for RE doping.

Despite a variety of shapes and sizes, the generic mechanical structure of cells is remarkably similar from one cell type to the next. All cells are bounded by a plasma membrane, a fluid sheet that controls the passage of materials into and out of the cell. Plant cells and bacteria reinforce this membrane with a cell wall, permitting the cell to operate at an elevated osmotic pressure. Simple cells, such as the bacterium shown in Figure 1a, possess a fairly homogeneous interior containing the cell's genetic blueprint and protein workhorses, but no mechanical elements. In contrast, as can be seen in Figure 1b, plant and animal cells contain internal compartments and a filamentous cytoskeleton—a network of biological ropes, cables, and poles that helps maintain the cell's shape and organize its contents.

Four principal types of filaments are found in the cytoskeleton: spectrin, actin, microtubules, and a family of intermediate filaments. Not all filaments are present in all cells. The chemical composition of the filaments shows only limited variation from one cell to another, even in organisms as diverse as humans and yeasts. Membranes have a more variable composition, consisting of a bi-layer of dual-chain lipid molecules in which are embedded various proteins and frequently a moderate concentration of cholesterol. The similarity of the cell's mechanical elements in chemical composition and physical characteristics encourages us to search for universal strategies that have developed in nature for the engineering specifications of the cell. In this article, we concentrate on the cytoskeleton and its filaments.

The exponential increase in computing power realized over the past three decades has required devices of ever-decreasing dimensions. With these reductions in feature size, each generation of circuit technology creates a new set of materials-science challenges, as exemplified by the use of silicide-based diffusion barriers.

The use of transition-metal silicides in the semiconductor industry began in the early 1980s with (in retrospect) fairly thick films, simple compositions, and minimal microstructural requirements. The implementation of thin-film binary silicides in integrated circuit (IC) applications required the development of appropriate modeling techniques to select the deposited metal, its gaseous precursor, and the Silicon precursor. These vectors were used to define experimental conditions to yield desired films. Chemical-vapor-deposition (CVD) experiments were simulated and CVD phase diagrams were used to describe the changing film properties with different thermodynamic conditions.

In the early 1990s, research began to focus on CVD of ternary Systems (Ta-Si-N, Ti-Si-N). Modeling these complex Systems required optimization of the thermodynamic data and careful evaluation of the ternary phase diagrams.

Current-generation materials are deposited in extremely thin layers (ULSI, or ultralarge-scale integration), composed of multiple elements from a variety of gas sources, an d have tailored micro-structures. As described in this article, early thermodynamic modeis helped develop deposition techniques for early-generation silicides. As the technological requirements increased, the modeling method s evolved in parallel, yielding continued insight into the relevant processes. Current work on CVD modeling couples thermodynamic calculations with heat and mass transfer. Incorporating both kinetic and thermodynamic effects, these methods provide a more realistic description of the CVD reactor.

Pervaporation is a membrane process used to separate liquid mixtures. Separation is achieved by a combination of evaporation and membrane permeation. As a result, the process offers the possibility of removing dissolved volatile organic compounds (VOCs) from water, dehydrating organic solvents, and separating mixtures of components with close boiling points or azeotropes that are difficult to separate by distillation or other means.

A schematic diagram of the pervaporation process is shown in Figure 1. In the example shown, the feed liquid is a solution of toluene in water which contacts one side of a membrane that is selectively permeable to toluene. The permeate, enriched in toluene, is removed as a vapor from the other side of the membrane. The driving force for the process is the difference in the partial vapor pressures of each component in the feed liquid and the permeate gas. This driving force can be increased by raising the temperature of the feed liquid to increase its vapor pressure or by decreasing the permeate gas pressure. The permeate gas pressure can be adjusted by using a vacuum pump, but industrially the most economical method is to cool and condense the vapor. Condensation spontaneously generates a vacuum. The permeate vapor pressure is then determined by the temperature of the permeate condenser and the composition of the permeate liquid generated by cooling and condensing the permeate vapor.

Pervaporation membranes are made by coating a thin layer of selective polymer material onto a microporous support.

Neutron scattering had its origin in 1932, the year that marked the discovery of the neutron by Chadwick, and the first nuclear reactors were successfully operated in Chicago and Oak Ridge, Tenn., in the early 1940s. During its initial stages, neutron scattering was used mainly for the study of “hard” crystalline materials. For example, Shull and Wollan's pioneering research, which led to the 1994 Nobel Prize in physics, began with studies of iron, chromium, and iridium, and was followed by the development of polarization analysis to determine the structure of magnetic materials. Such studies continue to yield important structural information (see the articles on magnetism by Aeppli and Hayden and on crystallography by Radaelli and Jorgensen in this issue of MRS Bulletin), although during the last two decades, the technique has been increasingly used by scientists from other disciplines (chemistry, biology, polymer science), and many of these newer applications have involved “soft” matter such as polymers, colloids, and gels. By definition, these substances are “plastic” or “squishy,” and easy to mold into different shapes; because of this flexibility, they have become some of the most practical and widely used materials today.