The optimization of mechanical properties through microstructural design in metallurgical industries has been used successfully for many years. In comparison, the microstructural design in ceramics has been less successful, mainly due to the complexity and difficulties associated with low diffusivity (in their monolithic form) and to the ability to be sintered at elevated temperatures. For years, many structural ceramics could not be sintered to high densities (>95% of their theoretical density) without large quantities of sintering aids. A typical example is silicon carbide (SiC), which has a unique combination of properties, such as: high hardness and wear resistance, considerable strength at high temperatures, and excellent thermal conductivity, but, due to a high level of covalent bonding and consequently low diffusion rates, it cannot be sintered to high density (without the addition of sintering aids) even at temperatures approaching 3000°C.

Since the first successful sintering of SiC with small additions of boron and carbon in the early 1970s, remarkable progress has been made in developing a wide range of properties for the microstructure, including fracture toughness from 3–4 MPa m½ to 12–14 MPa m½ four-point-bending strength from 300 MPa to over 900 MPa, and thermal conductivity from 60 to 260 W/m K.4

This article presents the most recent developments in SiC ceramics, emphasizing optimization of mechanical properties through microstructure control. The discussion is limited to β-phase SiC pressureless sintering, using oxide additives.

When the first paper describing the formation of self-assembled monolayers (SAMs) of octadecyltrichlorosilane [CH3(CH2)17SCl3, or OTS] by adsorption on SiO2 was published, it could not have been predicted that this area of research would become so important in only one decade. Although Zisman was the first to discover that monolayers can be prepared by adsorption of a surfactant onto a clean metal surface, the real revolution in the field occurred when Nuzzo and Allara showed that SAMs of alkanethiolates on gold can be prepared by adsorption of di-n-alkyl disulfides from dilute solutions. A decrease in the use of moisture-sensitive alkyl trichlorosilanes and the increased use of crystalline gold surfaces were two important reasons for the success of these SAMs. Indeed, monolayers of alkanethiolates on gold are the most studied SAMs to date and thus deserve the most detailed discussion.

SAMs have been intensively studied in the past few years because of their relevance to science and technology. Due to their dense and stable structure, SAMs have potential applications in corrosion prevention, wear protection, and biosensing, for example. The ability to tailor both head and tail groups of the constituent molecules makes them ideal for gaining a more fundamental understanding of phenomena affected by competing intermolecular, molecular-substrate, and molecule-solvent interactions like ordering and growth, wetting adhesion, lubrication, and corrosion.

For the design of functionally gradient materials (FGMs), necessary material properties, such as thermal-expansion-coefficient and Young's modulus in the specific region, are optimized by controlling the distribution profiles of composition and microstructures, as well as micropores in the materials. For this purpose, our research team employs the inverse design procedure in which both the basic material combination and the optimum profile of the composition and microstructures are determined with respect to the objective structural shape and the thermomechanical boundary conditions. Figure 1 shows the inverse design procedure for FGM, in which the final structure to be developed, as well as the boundary conditions, are specified first. After the fabrication method and an allowable material combination are selected from the FGM database, the estimation rules for the material properties of the intermediate compositions are determined based upon the micro-structure. Then, the temperature distribution and the thermal-stress distribution are calculated with the assumed profiles of the distribution functions for the constituents. Other possible combinations and different profiles are also investigated until the optimum is obtained.

In an integrated circuit (IC), the global interconnects are used to run power and ground to the individual transistors as well as to send signals across the chip. The width of interconnects can vary, depending on the current that is carried by the interconnect. Figure 1 shows a cross section of a double-metal complementary metal oxide semiconductor (CMOS) circuit illustrating the major components of a multilevel metallization circuit. The global interconnect connects to the diffusion and polysilicon gates through the contacts. The intermetal dielectric electrically separates the different levels of interconnect. The connection between the global interconnects at adjacent levels is made through the vias. The choice of a global interconnect for a multilevel metallization forces one to consider how the interaction between the various components of this system can affect the performance of the interconnect. For example, the intermetal dielectric changes the mechanical stress in the interconnect; the presence of W plugs in vias can affect the electromigration resistance of interconnects. In this article, we will examine the problems that are encountered when using Al alloys as a global interconnect and illustrate how the material properties can be modified to solve these problems.

Interactions of proteins with cell membranes are of great interest in studies such as molecular recognition at cell surfaces. A monolayer lipid film at the airwater interface is useful in cell-surface modeling. Studies in binding behavior of proteins from a solution with a lipid monolayer have been reported by using various in situ techniques: surface-tension measurements, fluorescent-labeling techniques, radio-labeling techniques, fluorescence reflection methods, surface plasmon resonance, and surface-force measurements. These methods have potential applications in the observation of protein bindings. However, they require large, expensive equipment for in situ measurements and present difficulties for quantitatively obtaining the amount of protein adsorbed and the time frame for both binding and dissociation processes.

In this article, we discuss a new, easy in situ technique to detect interactions of adsorbed proteins with a phospholipids monolayer through the use of a Langmuir film balance on which a quartz-crystal microbalance (QCM) is horizontally attached to the lipid monolayer from the air phase (see Figure 1). QCMs are known to be very sensitive mass-measuring devices because their resonance frequency decreases with an increase of a given mass on a QCM at the nanogram level. Adsorption and penetration behavior of proteins can be observed quantitatively from the frequency changes (ΔF) of the QCM on the monolayer and the surface-pressure changes (Δπ) of the monolayer responding to the addition of proteins. The amount of adsorbed proteins (Δm) wasalso obtained from the frequency changes after lifting and drying the sample in air.

Recently, QCMs have become popular tools for detection of bioactive compounds such as odorous and bitter substances, and for measurement of protein adsorption, immuno-assays, DNA hybridization, enzyme reactions, and cell growth.

We all know that the properties of a material can change with time, but is the inverse possible? How can materials have an effect on time? In the strictest sense, the idea is absurd. However, our ability to keep track of time is clearly dependent on the materials we use to construct our clocks. The performance of many early mechanical clocks depended on the materials of fabrication, and today's high-technology clocks depend no less critically on materials.

My objective in this brief article is to divulge a few materials problems involved in clock technology. Since I do not attempt a comprehensive report, I will undoubtedly leave out some issues that colleagues in the field would consider important.

Navigation has been a key driving force for the development of accurate timekeeping, at least since the 17th century. While many other important applications require accurate timekeeping (e.g., synchronization for telecommunications and electrical power distribution), I focus on navigation because of its historical significance and its particularly graphic illustration of why clocks have attained such prominence.

The advent of electron accelerators dedicated to the production of high-intensity x-rays has revitalized experimental techniques based on x-ray absorption. A recent variant is to use circularly polarized x-rays generated either by use of out-of-plane radiation from a bending magnet or from a specially designed “insertion device.” This new field of x-ray magnetic circular dichroism (XMCD) shows considerable promise in spectroscopy and microscopy of magnetic materials. In this article, we describe the nature of XMCD, offer a few examples of recent progress, and review the prospects for future development using the newly constructed Advanced Light Source (ALS).

Defects play a significant role in determining the physical and thermodynamic behaviors of a material. This precept can be especially helpful when one attempts to interpret the engineering properties of a biological material in the context of its complex, hierarchical microstructure. An excellent example of an impressive engineering material used by nature is silk, which has a composite microstructure assembled from one or more protein polymers. Spiders, aided by several million years of evolution, have succeeded in producing a polymer fiber [major ampullate silk (MAS)] with a strength-to-weight ratio exceeding that of nearly all synthetic materials. Even the exceptions—specialty fibers such as carbon, silica, and some variants of Kevlar—pale in comparison when toughness (energy required to bring about failure) is taken into account; they are relatively brittle and lack the viscoelastic extensibility of MAS. When one considers that spiders produce this naturally biodegradable material under ambient conditions and with an aqueous solvent, the feat becomes even more impressive. It is clear that the spider could be an excellent instructor in both polymer processing and microstructural design.

where the α-carbon is bonded to a functional “R” group that distinguishes the monomer as being one of the 20 commonly occurring natural a-amino acids.

As recently as 1993, the prevailing presumption among the semiconductor technical community was that then-current development efforts associated with aluminum lines and tungsten damascene vias needed to shift rapidly to copper multilevel interconnect schemes. This is exemplified by the June 1993 issue of the MRS Bulletin, which featured copper metallization as its theme. In the intervening years, however, that same technical community revised the Semiconductor Industry Association (SIA) roadmap and placed renewed emphasis on the use of an all-aluminum interconnect scheme. This was done largely in deference to the costs associated with converting existing semiconductor lines to copper-compatible facilities. In addition to tooling costs, there is a learning curve for copper systems that remains to be established for device reliability, field failures, yield learning, and process maturation. On the other hand, existing fabs are already compatible with aluminum metallurgies, and there is a rich history of reliability and yield data.

This change in direction creates two immediate needs: (1) the need to fill small-diameter vertical interconnects (vias) with void-free aluminum and (2) the need to remove the top surface aluminum resulting from its blanket deposition (overburden) following the metal fill. In addition, for high circuit-density applications, it may be desirable, if not necessary, to form the metal lines using the same damascene fill method as is used for the vias. This process strategy replaces metal etching and insulator gap fill with insulator (usually silicon oxide) etching and metal gap fill.