Annual Review of Materials Research - Volume 27, 1997

The layered perovskite cuprate materials are a unique class of superconductors with unusual normal-state and superconducting properties. The common physics to all these materials is that of the underlying CuO2 planes. This review provides a survey of and guide to their physical properties as it relates to the superconductivity of this interesting group of conducting oxides.

Scintillators are the primary radiation sensor in many applications such as medical diagnostics, medical radiographs, and industrial component inspection. Some of the limitations in the properties of single-crystal scintillators are discussed for imaging applications, and the advantages of a new class of polycrystalline ceramic scintillators are described in detail. After the important scintillator properties of transparency, X-ray stopping power, light output, primary speed, luminescent afterglow, and radiation damage are described, the processing and performance of ceramic scintillators (Y,Gd)2O3:Eu,Pr; Gd2O2S:Pr,Ce,F; and Gd3Ga5O12:Cr,Ce are discussed. Ceramic scintillator uses and trends are presented in light of issues related to their uses in advanced medical and industrial X-ray detectors for CT imaging applications. Finally, some of the challenges are given for successfully developing a polycrystalline ceramic scintillator for use in photon-counting applications.

The materials properties and physical phenomena exhibited by layered silicate clays and clay intercalation compounds, a subgroup of the general class of layered solids, are reviewed. The importance of layer rigidity is emphasized. Clays are compared and contrasted with the more familiar layered solids such as graphite and dichalcogenides. Some of the unusual structural features of clays including interstratification, swelling, and the lack of staging are discussed and explained qualitatively and quantitatively. Novel magnetic phenomena such as that associated with a disordered two-dimensional kagomé antiferromagnet formed in synthetic clays and the effect of co-intercalated water on the crystal field–induced magnetic ordering in natural clays are described and analyzed. The vibrational excitations in clays are addressed in terms of lattice dynamical models for the phonon dispersion curves. The theoretical models are compared with experimental measurements including neutron scattering and Raman spectroscopy.

Cholesteric liquid crystals possess a helical structure and exhibit two stable states at zero field: the planar texture and the focal conic texture. In the planar texture, they reflect circularly polarized light, whereas in the focal conic texture, they scatter light in forward directions. They can be switched from the planar texture to the focal conic texture by a low-voltage pulse and switched from the focal conic texture to the planar texture by a high-voltage pulse. The wavelength of the reflected light is easily adjusted by varying the pitch of cholesteric liquid crystals. They can be used to make reflective displays that do not need a back light and have a good readability under room-light conditions. We first review the optical properties of bistable cholesteric reflective displays and discuss the techniques used to achieve high contrast and large viewing angle. We then discuss the transitions among the cholesteric textures and the drive schemes used to address bistable cholesteric reflective displays.

The physico-chemical processes that occur during binder thermolysis of porous ceramic bodies are reviewed. The discussion initially focuses on intrinsic and extrinsic mechanisms of polymer degradation, with emphasis on the formation of volatile degradation products and involatile carbonaceous residue. The transport of volatile species (e.g. residual solvents, plasticizers, etc) and degradation products through both empty and binder-filled pores is then considered. The particular case of debinding highly loaded ceramic bodies is addressed in detail. Recent developments in modeling the onset of defect formation and the effects of capillary redistribution of thermoplastic binders on removal kinetics are outlined. On the basis of this fundamental knowledge, the criteria for optimizing binder removal processes is established.

Applications of state-of-the-art atomic force microscopy methods to the elucidation of the surface and near-surface structure of polymeric solids are described. Contact, tapping, force modulation, frictional force, and other modes of atomic force microscopy are described, and recent results are summarized. Conformational and chain order, crystalline order, polymer crystals, lamellar structures, lamellar surfaces, fold surfaces, and fibers and films with highly oriented molecules all yield important information. Controlled deformation of polymer surfaces, both reversible and irreversible, with the atomic force microscope, provides a wealth of information about mechanical properties on a nanometer scale. The observation of phase-separated regions and of polymer crystals lying below a smooth surface shows that not only topography but also elastic inhomogeneity can be observed in great detail with the atomic force microscope. This is a rapidly developing field, and some indications of future developments are presented.

Electrical characterization methods for the analysis of alternating current thin-film electroluminescent (ACTFEL) devices are reviewed. Particular emphasis is devoted to electrical characterization techniques because ACTFEL devices are electro-optic display devices whose performance is to a large extent determined by their electrical properties. A systematic procedure for ACTFEL electrical assessment is described. The utility of transient charge, voltage, current, and phosphor field analysis is explained. Steady-state electrical characterization methods discussed in this review include charge-voltage (Q-V), capacitance-voltage (C-V), internal charge-phosphor field (Q-Fp), and maximum charge-maximum applied voltage (Qmax-Vmax) analysis. These electrical characterization methods are illustrated by reviewing relevant results obtained from the analysis of evaporated ZnS:Mn and atomic layer epitaxy (ALE) SrS:Ce ACTFEL devices.

Recent progress in the field of mechanical behavior of layered ceramics is reviewed. Several processing techniques are described with reference to the fabrication of ceramic laminates. These include tape-casting, centrifugal casting, slip-casting, electrophoretic deposition (EPD), and the production of fibrous monolith materials. The review discusses various laminar design approaches for achieving improvements in strength, toughness, work of fracture, R-Curve behavior, and contact damage resistance. Examples of effective strategies include manipulation of residual stresses, the incorporation of weak interlayers to induce crack deflection, and promotion of synergistic effects between layer materials that exhibit intrinsically different responses.

New thin-film electroluminescent (EL) materials with blue and broadband emissions recently have been developed that meet the phosphor requirements for commercial multicolor and full-color EL display products. Improvements have been achieved in phosphor luminance, luminous efficiency, emission color, and aging characteristics. This paper reviews the latest advancements in red, green, and blue thin-film phosphors for patterned phosphor EL display applications, and broadband emission phosphors for the filtered white or color-by-white EL display approach. Focus is placed on the new EL materials and the composition modifications that have improved the performance of well-known EL materials.

Since the early 1970s, three major prerequisites have brought the success of the liquid crystal display (LCD) technology to its key role of today. Namely, the discovery of electro-optical field-effects on which the displays are based, the successful search for liquid crystals (LCs) with material properties that meet the complex requirements of electro-optical effects and render the effects applicable in displays, and last but not least, the development of the technological tools required for manufacturing displays.

Virtually all of today's commercial LCDs are based on the twisted nematic (TN) or on various supertwisted nematic (STN) effects whose extensive development and improvement over the past 25 years is still rapidly progressing. Those liquid crystal material properties and electro-optical effects that essentially determine the performance of nematic displays are reviewed. Correlations between molecular functional structural groups, LC material properties, and their electro-optical relevance for TN and STN displays are outlined. Included are dual-frequency addressing phenomena in liquid crystal materials, in situ dielectric heating of displays, and conductivity phenomena that are related and that may either hamper or improve the performance of high-information content LCDs. Moreover, we review some recent developments made in our laboratories on novel electro-optical devices and device-specific functional organic materials, e.g. optical alignment of monomeric and polymeric liquid crystals by linearly polarized light; the generation of photo-patterned multidomain twisted nematic displays with broad field of view; the operation of displays with circularly polarized light, as well as compact and bright cholesteric LCD projection optics whose polarizers, filters, and modulators are all based on liquid crystal elements.

Atomic force microscopy is an imaging tool used widely in fundamental research, although it has, like other scanned probe microscopies, provided only limited information about the chemical nature of systems studied. Modification of force microscope probe tips by covalent linking of organic monolayers that terminate in well-defined functional groups enables direct probing of molecular interactions and imaging with chemical sensitivity. This new chemical force microscopy technique has been used to probe adhesion and frictional forces between distinct chemical groups in organic and aqueous solvents. Contact mechanics provide a framework to model the adhesive forces and to estimate the number of interacting molecular groups. In general, measured adhesive and frictional forces follow trends expected from the strengths of the molecular interactions, although solvation also plays an important role. Knowledge of these forces provides a basis for rationally interpretable mapping of a variety of chemical functionalities and processes such as protonation and ionization.

The structural dependence of the optical and dielectric physical properties of liquid crystal (LC) materials is investigated with the aid of molecular modeling. A material characterization of LC materials is made using fundamental physical measurement values, e.g. optical, dielectric, elastic, and viscous parameters. A novel mixture design based on selected LC materials leads to significant improvements that fulfill the requirements for active matrix LC displays, for instance, fast switching times, lower power consumption, and wide operation temperature ranges. The development of LC displays from the material supplier's point of view is briefly reviewed.

This review is intended to provide the ceramic engineer with information about the history and current use of ceramics in dentistry, contemporary research topics, and potential research agenda. Background material includes intra-oral design considerations, descriptions of ceramic dental components, and the origin, composition, and microstructure of current dental ceramics. Attention is paid to efforts involving net-shape processing, machining as a forming method, and the analysis of clinical failure. A rationale is presented for the further development of all-ceramic restorative systems. Current research topics receiving attention include microstructure/processing/property relationships, clinical failure mechanisms and in vitro testing, wear damage and wear testing, surface treatments, and microstructural modifications. The status of the field is critically reviewed with an eye toward future work. Significant improvements seem possible in the clinical use of ceramics based on engineering solutions derived from the study of clinically failed restorations, on the incorporation of higher levels of “biomimicry” in new systems, and on the synergistic developments in dental cements and adhesive dentin bonding.

This review focuses on the characterization of interfaces, specifically on the optical methods of characterization that utilize some form of spatial localization to circumvent the special problems accruing to interfaces. Experiments utilizing radiation in only the infrared through the ultraviolet regions of the electromagnetic spectrum are considered. We specifically exclude the vast and important array of experimental techniques that utilize the vacuum ultraviolet and X-ray spectral regions. In addition, the interfaces considered are those composed of solids with liquids, thin films, and other solids, thus largely ignoring the literature of ultrahigh vacuum surface science.

Optimal design of the fiber-matrix interface in ceramic-matrix composites is the key to achieving desired composite performance. In this paper the interface-controlling parameters are described. Techniques for measuring interfacial properties are reported. Examples of interface design of both oxide and non-oxide types are illustrated.

Growth of thin films from atoms deposited from the gas phase is intrinsically a non-equilibrium phenomenon dictated by a competition between kinetics and thermodynamics. Precise control of the growth becomes possible only after achieving an understanding of this competition. In this review, we present an atomistic view of the various kinetic aspects in a model system, the epitaxy of Si on Si(001), as revealed by scanning tunneling microscopy and total-energy calculations. Fundamentally important issues investigated include adsorption dynamics and energetics, adatom diffusion, nucleation, sticking, and detachment. We also briefly discuss the inverse process of growth, removal by sputtering or etching. We aim our discussions to an understanding at a quantitative level whenever possible.

Passive matrix, supertwisted nematic (STN) displays are well established as small- and medium-sized matrix displays and are presently being used in half of the notebook-sized computers. STN screens are being considered as desk-top CRT monitor replacements because their simpler construction allows them to be manufactured in larger sizes and at lower cost than active matrix LCDs. This chapter describes the fundamental physics behind STN operation, reviews the many device and materials developments that have improved STN performance, and points out likely future trends.

Rapid advances in the field of photorefractive polymers and composites have in the brief time since their inception in 1991 led to the development of high-performance materials with refractive index modulations approaching 0.01, dif-fraction efficiencies close to 100%, and net two-beam coupling gain coefficients exceeding 200 cm⁻¹ in samples typically 100 μm thick. This paper reviews the current state of research, from the most successful synthetic strategies to produce polymeric photorefractive materials, to their emerging uses in applications. Two-beam coupling and four-wave mixing measurement techniques are presented and their importance in the characterization of the photorefractive properties of new materials is explained. The physics of the photorefractive effect in polymers is discussed with emphasis placed on the differences compared with the traditional inorganic photorefractive crystals. In particular, the orientational enhancement mechanism, which is believed to be responsible for the high performance of most of the low-glass-transition-temperature systems, is discussed in detail.