Until the 1980s, most people thought of ceramics as artware and artifacts. The recent development of advanced ceramics for high performance thermal insulation (space shuttle tiles), high temperature structures (heat engines), and electronics (superconductors) has dramatically changed perceptions about the utility of ceramics.

High technology ceramics are related to “traditional” ceramics only to the extent that they are inorganic, nonmetallic materials. Traditional ceramics are derived from minerals. For example, dinnerware and bricks consist mostly of clay, while sand is the major ingredient in flat glass and containers. Abundant raw materials, simple processing, adequate performance at low cost, and technological evolution have kept these industries viable for several thousand years.

But, for demanding electronic or structural applications, synthesizing ceramics from minerals is often unacceptable. The chemical variability of mineral deposits, the difficulty of obtaining a homogeneous mixture of powders, and the problems of consolidating the powders into a uniform ceramic with desirable grain morphology, chemistry and grain boundary phases have stimulated the development of chemically derived ceramic precursors.

Ceramic precursors are the chemical constituents of ceramic materials in a form in which they can be manipulated and shaped prior to processing to form a dense body. Generally these precursors are finely ground crystalline materials such as oxides or clays. Recently, however, much effort has focused on preparing precursors by chemical rather than mechanical means. The goal is to prepare purer, more homogeneous, and more reactive precursors than mixed crystalline powders. A wide variety of chemical processes have been devised to grow ceramic precursors, including powders formed by precipitation reactions, gels formed by hydrolysis and condensation reactions, and gas phase pyrolysis, to name just a few. Due to the random nature of the growth processes, these precursors often have structures wholly unlike their mechanically derived counterparts on both an atomic and a “macromolecular” scale. Random growth and structure and the effect of these structures on the subsequent processing of ceramics is an area of growing interest.

Random structures are inherently more difficult to describe and study than are the ordered, periodic structures of crystals. With the exception of a few special cases, such as random walk polymers, little of a quantitative nature could be said about them until the development of fractal geometry and the accompanying realization that the structures which result from many random growth processes are, in fact, fractal.

In the early 1980s Prof. Bowen at Massachusetts Institute of Technology advanced the concept that sintered ceramic bodies could be improved by decreasing the defects in green bodies. In addition, his group advanced the idea that ideal green bodies should be composed of monodisperse particles packed into an ordered array. They used the hydrolysis of metal alkoxides to produce narrow size distribution sol-gel powders (i.e., amorphous). These powders were allowed to settle under the force of gravity into an ordered array which was dried and sintered. Sintering took place at much lower temperatures and at faster rates than traditional ceramic processing (e.g., broad size distributions of crystalline powders). Improvements in strength and toughness of the sintered body were never demonstrated by Bowen's group for these novel ceramics.

Ordered particles have packing faults which lead to ordered domains, as shown in Figure 1, similar to grains in polycrystalline materials. When sintered, the ordered domains shrink separately and pull away from each other as shown in Figure 2. This leads to defects in the sintered body of sizes similar to those of the ordered domains which can encompass as many as 10,000 particles. These ordered domains lead to weakness in the sintered body according to Griffith's theory.

Electronic ceramic materials research is one of the fastest growing, most highly publicized areas of materials science. Subjects receiving considerable attention include high temperature superconductors, multilayer ceramic composites for high density microelectronics packaging, and ferroelectric electro-optic thin films. A complete review of all aspects of electronic ceramics research is beyond the scope of this article, which will focus on two general topics whose development is representative of recent contributions to the field. These two areas are synthesis and characterization of electronic ceramic films,1 and controlled use of low level dopants (1,000 ppm or less) in bulk polycrystalline ceramics, thin films, and single crystals to achieve desired properties. Perspective of the progress in ceramic film development is given by a review of single-crystal synthesis and properties.

Several examples of the impact that low level dopants and thin film synthesis have on electronic ceramics development are presented. Dopant concentrations of 1,000 ppm or less can have a dramatic effect on microstructural, optical, and electrical properties. For example, a decrease in aluminum content of 150 ppm resulted in an increase in grain size from 1 to 25 microns in otherwise identical ZnO varistors. Background aluminum concentrations for these varistors were less than 10 ppm. In another example, the photorefractive effect, the change in refractive index with optical light intensity, has been shown to be altered by orders of magnitude with ppm doping levels in ferroelectric electro-optic materials.

Several electronic ceramic devices have recently been developed due to improvements in ceramic film processing. Examples of these devices include: 1. multilayer PZT transformers, which allow fabrication of complex monolithic passive multicom-ponent networks, 2. liquid cooled multilayer ceramic substrates, with 400×800 micron liquid transfer capillaries integrated into the multilayer structure via ceramic processing techniques for high density VLSI packaging, and 3. ferroelectric electrooptic thin films that are compatible with silicon or III-V technology. For all the above applications, synthesis of electronic ceramic materials into high purity films is essential.

The growth of integrated circuit applications has been a strong influence in the expansion of markets for ferroelectric ceramics. Ferroelectrics perform three major functions in circuits:

1. They store energy with a high volume efficiency,

2. They have very useful large changes in impedance with frequency, and

3. They transduce between various energy forms and electrical signals.

For many years commercial ferroelectric ceramics have been dominated by the barium titanate and lead zirconate titanate (PZT) systems. A tremendous research effort has been dedicated to these systems with very interesting studies still progressing on basic understanding, reproducibility, and modifications to utilize inexpensive electrodes. Processing studies are also seeking to reduce the size of devices and develop new transducing and sensing applications. The need to reduce cost and to fulfill specific applications is creating demands for new materials. Much of this effort has centered on lead-based systems referred to as relaxor ferroelectrics.

The areas of application of ferroelectrics are narrowed in this review by eliminating the interfacial (grain-grain boundary) devices and electro-optic applications discussed in “Electronic Ceramic Thin Films” by Bruce Tuttle in this issue. Also, this article can only cover a small fraction of the information indicated by the title.

Condensation from plasmas has been shown to yield very fine, high purity powders from even the most refractory materials. Elementary particles in these powders are often single crystals, as fine as 50 nm diameter. These particles vary widely in agglomeration state; in some systems they are essentially independent, while in others hard agglomerates as large as several microns are observed. In this article the geometric aspects of reactor design which are thought to influence these powder structures are reviewed, along with three case studies in powder microstructure. Finally, the structure of the “typical” plasma powder is compared with expectations from alternative preparation routes.

The development of new, energy-efficient processing strategies for ceramic fabrication has become a rapid growth field over the last ten years. Most of these strategies rely on minimizing diffusion requirements during densification by improving both the chemical homogeneity and the initial density of the powder compact to be consolidated; both requirements dictate unusual care in the synthesis of the starting powders. In the oxide systems most extensively studied to date, the required powders can often be produced by precipitation from aqueous solutions and/or emulsions. In the carbide and nitride systems widely envisaged for the next generation of structural applications, these techniques are difficult and gas-phase synthesis routes appear more attractive. In fact, even the leading practitioners of the precipitation arts write enthusiastically of aerosol synthesis methods for the oxides.

This article will provide information about chemical processes which rely on the heat evolved during reaction to synthesize and, in some cases, to simultaneously density single-phase or composite ceramic materials. Although the basic concept underlying these processes is simple, the high temperature reactions are complex and require careful study with individual systems before their potential as fabrication processes can be fully realized.

Many reactions between solids involving elements and/or compounds, or between solids and gases are highly exothermic. Listed in Table I is a selected group of typical chemical reactions accompanied by their calculated adiabatic temperatures. As a general rule any reaction with an adiabatic temperature ~2000°C or over can be reacted under combustion conditions. For example, suppose that a cold-pressed cylindrical compact of titanium and boron powder (Eq. 1) is ignited at the top surface with a convenient source of heat such as a laser. From the igniting surface a combustion wave rapidly self-propagates down the compact, transforming the reactants into the TiB2 product. Figure 1 shows a Ti and C powder compact where the combustion wave has progressed about halfway down the compact.

Whisker-reinforced ceramic matrix composites have recently received a great deal of attention for applications as high temperature structural materials in, for example, advanced heat engines and high temperature energy conversion systems. For applications requiring mechanical reliability, the improvements that can be realized in fracture strength and fracture toughness are of great interest. Of particular importance for optimizing the mechanical reliability of these composites is the effect of the whisker/matrix interfacial characteristics on the strengthening and toughening mechanisms. Whisker reinforcements are primarily utilized to prevent catastrophic brittle failure by providing processes that dissipate energy during crack propagation. The degree of energy dissipation depends on the nature of the whisker/matrix interface, which can be controlled largely by the matrix chemistry, the whisker surface chemistry, and the processing parameters.

It is generally believed that a strong interfacial bond results in a composite exhibiting brittle behavior. These composites usually have good fracture strengths but low fracture toughnesses. If the interfacial bond is weak, the composite will not fail in a catastrophic manner due to the activation of various energy dissipation processes. These latter composites tend to have high fracture toughnesses and low fracture strengths. Generally, the interface should be strong enough to transfer the load from the matrix to the whiskers, but weak enough to fail preferentially prior to failure. Thus, local damage occurs without catastrophic failure. It is therefore necessary to control the interfacial chemistry and bonding in order to optimize the overall mechanical performance of the composites.

Ceramics have long been known for their refractoriness, or ability to bear loads at elevated temperatures. However, until the 1960s, the predominant refractory ceramics were oxide-based materials such as silica, zirconia, alumina, mullite, magnesia, and their combinations, including silicates. These ceramics were, and still are, used for firebrick furnace linings, crucibles and liquid metal carrier liners, regenerators, and recuperators.

However, these materials all possess some characteristic which precludes their use for very high stress, high temperature applications. Typically, the silicates form viscous liquids which allow creep, while zirconia and alumina suffer from poor thermal shock resistance, and magnesia possesses a large thermal expansion coefficient. Consequently, for heat engine applications which involve high temperatures, high stresses, sudden temperature changes (e.g., startup), and may involve the maintenance of tight operating tolerances, a new family of materials is required. The requisite properties for heat engine applications may be found in certain non-oxide materials, namely silicon nitride and silicon carbide. They possess high strength even at high temperatures, low thermal expansion coefficient, and excellent thermal shock resistance. These materials are not thermodynamically stable in air at elevated temperatures and will eventually react to form oxides. Nonetheless, they possess excellent oxidation resistance by virtue of protective silica-based glass oxidation layers.

The magic of multilayer structures — comprised of a regular repeating pattern of alternating layers that vary in thickness from less than 1 nm to more than several micrometers — is present today in nearly every facet of materials science.

A technological gamut of issues has been approached through the use of multilayer structures. Material systems have been routinely investigated, ranging from simple binary through complex quaternary and including nearly every element of the periodic table. Applications include thin-film semiconductor devices in the form of strained-layer superlattices; protective coatings for improved surface hardness and wear resistance of conventional alloys (tribology); and reflective and transmissive x-ray optical devices. Multilayer structures have also provided an experimental means to investigate the interdiffusion and stability of alloy phases; the relationship between the presence, lack, or variations of crystallinity with magnetism (and most recently superconductivity); and the role of interfaces in adhesion and elastic/plastic behavior.