As the dimensions of ultralarge-scale-integration devices scale to smaller feature sizes and larger die dimensions, the resistance-capacitance (RC) delay of the metal interconnect will increasingly limit the performance of high-speed logic chips. This is because the transistor capacitance and resistance both scale to lower values as the feature size is reduced, while both the line-to-line capacitance and resistance of the metal-interconnect lines increase as their dimensions decrease. For interconnects 5-mm long, the crossover feature size at which the interconnect delay dominates the transistor delay is approximately 0.5 μm. Since this interconnect RC delay increases roughly quadratically with decreasing feature size versus the historical quadratic reduction in transistor delay, device designers currently face difficult barriers to continued performance increases with scaling. Figure 1 presents the components of the RC delay for a single-transistor/single-interconnect combination with 0.35-μm feature sizes. The total delay can be broken into four additive components: the transistor delay R 0 C 0, the interconnect delay rLcL, and the two transistor/interconnect cross terms where R 0 and C 0 are the transistor resistance and capacitance, r and c are the specific resistance and capacitance, and L is the interconnect length. As can be seen for interconnect lengths less than about 100-μm long, the intrinsic transistor delay dominates. However for interconnect lengths between approximately 100 μm and 10 mm, the resistance of the transistor coupled with the capacitance of the interconnect dominates the combined delay, resulting in a linear increase in delay with increasing line length.

The control of surface chemistry and topography has great technological relevance for numerous applications of polymers in textiles, adhesives, coatings, packaging, membranes, and biomedical implants. Conventionally, chemical modification of polymer surfaces has been achieved through kinetically governed practices that allow little control over the final surface composition or morphology. These chemically generated surfaces are also prone to reconstruction. Hence the development of inexpensive, scaleable routes to impart stable and more complex chemical functionality to polymer surfaces continues to be an active area of research. Apart from surface chemistry, the topography of a polymer surface often plays a determinant role in the adhesive, optical, and wetting characteristics of the surface. Consequently methods to produce surfaces of controlled texture are also of interest. Toward these goals, new, statistical, mechanics-based theoretical approaches, coupled with increased computing power, can now facilitate the first-principles design of polymer surfaces that are chemically and structurally “tailored” for a given application. In this article, we review some of the recent, significant developments in this area.

Scientists have studied the mechanical properties of quasicrystalline materials for quite some time. However the difficulty in obtaining material of reasonable quality hampered systematic investigations. The progress in materials preparation in recent years has triggered new activity in this field. Furthermore the new ternary and multicomponent alloys have demonstrated great promise for use as coatings with good wear resistance and low coefficients of sliding friction. However the physical reasons for these properties and their correlation with the particular structure of quasicrystals are still not understood. As in conventional alloys, experiments under well-defined conditions are required that can serve as a basis for understanding the intrinsic mechanical properties of quasicrystals. Such studies are now increasingly possible after the development of techniques to grow larger single quasicrystals up to a few centimeters in size directly from the melt.

Since the mechanical behavior of quasicrystalline alloys is to a great extent determined by a brittle-to-ductile transition at about 70% of the absolute melting temperature, it is useful to discuss the mechanical properties with reference to appropriately defined low-temperature and high-temperature regions.

Nanoscale materials show a variety of interesting physical and chemical properties. Since the properties are mostly determined by mesoscopic- and atomicscale areas of the materials, characterization of nanoscale materials at the atomic level is crucial for understanding the origin of their properties as well as for tailoring them to industrial needs. High-resolution electron microscopy (HREM) is an effective method to observe atomic structures in nanophase materials. The “structure-imaging technique,” originated by Ueda et al., and Cowley and Iijima, contributed much to the development of the method. Innovations are anticipated in (1) three-dimensional analysis of nanophase materials in real space, (2) observation of dynamic phenomena, and (3) extraction of compositional and physical information from nanometer-sized areas. Using the structure-imaging technique, one can observe directly nanometer-scale particles and defect structures such as voids and line and plane defects in crystals, including surfaces and interfaces. There were previously some limitations on the study of the dynamic process on the atomic level since images were captured on films. The recent development of high-sensitivity silicon-integrated-target-television (SIT-TV) cameras has initiated a new stage of HREM.

Our time-resolved-high-resolution-electron-microscopy (TRHREM) system is composed of a 200-kV HREM (JEOL: JEM-2010) equipped with a high-sensitivity SIT-TV camera, various types of dedicated sample holders, an ultrahigh-vacuum (UHV) sample-preparation chamber, and a digital videotape recorder, as partly illustrated in Figure 5. The HREM images at 0.8–1.0-million magnification are recorded on the videotape recorder. In the U.S.-Japan TV system, one “frame image” of 1/30-s time resolution is composed of two interlaced “field images” of 1/60-s time resolution. The digital videotape recorder enables us to output the two “field images” separately from two frame memories.

The compaction process involves stress transmission via rigid or flexible (die) walls and the propagation of stresses within a powder mass. The particles that comprise the powder distribute the stress by a variety of kinematic processes that involve sliding, rotation, particle deformation, and rupture. In practice the “particles” are often agglomerates of finer particles that have a range of properties. All of these factors must be considered in developing a comprehensive predictive model for compaction.

The modeling of powder-compaction processes has a significant history that has been greatly advanced by the relatively recent general availability of powerful computers and their peripherals as well as by appropriate softwares. Compaction modeling may attempt to provide a basis for machine-loading specifications, or it may provide guidelines to help minimize “capping” defects where failure cracks form at the top of the green compact. It may also provide “green-body heterogeneity” through predicted stress and density distributions within a compact. Likewise compaction models may be combined with binder burnout and sintering models to predict internal microstructural features such as grain size and porosity, and the external shape of the sintered product. This article will deal only with the modeling of the compaction process; important elements such as powder flow for die filling and subsequent processing steps such as sintering and net shape predictions are not directly addressed.

A long deoxyribose-nucleic-acid (DNA) molecule can be seen directly under an optical microscope if it is suitably decorated by fluorescent dyes. This makes it possible to see how a long chain responds to external forces f. The conformation of a uniformly stretched chain has been discussed by Pincus. Under a force f, acting on the two free ends, the chain configuration is modified only above a scale length ξ = kT/f where T is temperature and k is Boltzmann's constant. The chain (Figure 1a), containing N monomers, can be pictured as a string of blobs of size ξ containing g monomers with ξ = gva, where a is the monomer size (v = 1/2 in a θ solvent: the chain is ideal, v = 3/5 in a good solvent). The end-to-end distance is L = (N/g)ξ, that is,

For DNA, which is a semirigid chain, ideal behavior has been observed for chain lengths up to 20 μm when f is small. At higher f ( ≅ 100 pN), Cluzel et al. found a structural phase transition, corresponding to a plateau in the curve f versus elongation. Going even further, one reaches the break point.

Another method to stretch DNA is based on optical tweezers. Here one end of the DNA is attached to a bead of glass of high polarizability, which is attracted by a region of large (optical) electric field. This was used by the Stanford group: In one series of experiments, they observed the conformations of the chain, pulled at one end by a constant force f. The friction on monomers are cumulative, and the tension along the chain is not uniform—it increases from the free end to the tethered end.

Quasicrystals may have important applications as new technological materials. In particular, work in our laboratory has shown that some quasicrystals may be useful as hydrogen-storage materials.

Some transition metals have a capacity to store hydrogen to a density exceeding that of liquid hydrogen. Such systems allow for basic investigations of solid-state phenomena such as phase transitions, atomic diffusion, and electronic structure. They may also be critical materials for the future energy economy. The depletion of the world's petroleum reserves and the increased environmental impact of conventional combustion-engine powered automobiles are leading to renewed interest in hydrogen. TiFe hydrides have already been used as storage tanks for stationary nonpolluting hydrogen internal-combustion engines. Nickel metal-hydride batteries are commonly used in a wide range of applications, most notably as power sources for portable electronic devices—particularly computers. The light weight and low cost of titanium-transition-metal alloys offer significant advantages for such applications. Unfortunately they tend to form stable hydrides, which prevents the ready desorption of the stored hydrogen for the intended use.

Some titanium/zirconium quasicrystals have a larger capacity for reversible hydrogen storage than do competing crystalline materials. Hydrogen can be loaded from the gas phase at temperatures as low as room temperature and from an electrolytic solution. The hydrogen goes into solution in the quasicrystal structure, often avoiding completely the formation of undesirable crystalline hydride phases. The proven ability to reversibly store variable quantities of hydrogen in a quasicrystal not only points to important areas of application but also opens the door to previously inaccessible information about the structure and dynamics of this novel phase. Selected results illustrating these points appear briefly here.