Ceramics and ceramic composites are a rapidly evolving category of materials capable of being tailored to have unique combinations of electrical, optical, mechanical, and chemical properties that make them essential and irreplacable in many engineering applications. Conventional ceramics have both enabled and enhanced many important aspects of our modern technological society; the next generation of ceramics will play a further enabling role for still higher performance devices.

A briefing panel of the National Academies of Science and Engineering recently identified four areas into which promising research areas for advanced ceramics can be grouped:

1. 1. New thin films and layer structures with improved properties.

2. 2. Exploration of completely new, multicomponent ceramic crystal structures and composites.

3. 3. The mechanical behavior of ceramics and tough composites.

4. 4. Ceramic processing of large parts and assemblies.

Three mechanisms for achieving critical mass in ceramics research were recommended:

1. 1. University/industry programs.

2. 2. A summer institute.

3. 3. A large interdisciplinary center.

Recent advances in the property and performance requirements of ceramics have stimulated new approaches to ceramics processing. One approach being studied in numerous laboratories around the world involves the introduction of specialty chemicals to the sequential processing steps. Because many of these new approaches involve changes in equipment, raw materials, and processing variables, this paper focuses on an examination of the process economics. In some instances we can project the costs for these new technologies by considering the costs of similar commercial processes.

This paper gives the synthesis of several metallo-organic (MO) compounds. The MO compounds have been synthesized so that thermal decomposition produces fine metal oxide powders, or films ⋍0.1 to 0.3 μm thick without going through the powder stage. The Metallo-organic decomposition (MOD) process differs from the SOL-GEL process in that a gel is never formed, and the compound requirements are considerably different. Ferroelectric BaTiO3 and PbTiO3 powders were formed at 600°C and 450°C respectively, which are much lower temperatures than those required by conventional techniques. Defect free tetragonal PbTiO3 films have been formed 1 to 2 μm thick over an area of 1 cm × 1 cm at ⋍470°C, which is 20 to 30°C below its Curie Temperature.

Colloidal interactions in a heteroparticulate mixture of zirconia and alumina in water were studied for use in a transformation toughened alumina composite. The microelectrophoresis technique was used to measure the mobility of three zirconia powders and an alumina powder. The electro-phoretic mobility and particle size data were used to calculate total potential energy curves. The maximum height of the total potential energy barrier was used to predict the stability of a zirconia/alumina mixture. Theoretical predictions were compared to experimental results obtained from sedimentation and rheology measurements carried out as a function of pH of the dispersion. For a 5 v/o aqueous zirconia/alumina system stable dispersions were made at pH 3 and pH 5.

Laser synthesized silicon powders have been used to make reaction bonded silicon nitride samples. Maximum hardness (11.3 GNm−2), fracture toughness (3.6 MNm−3/2), pore size (Hg porosimetry 50–300Å radius) and strength (∼460 MNm−2) values reflect the superior microstructures that are observed. With anaerobic anhydrous processing, these powders nitride to completion in less than 7 hours at 1400°C.

Heterogeneities in a sintering compact constitute a source of stress and distortion. Procedures for calculating the stresses and the distortion are described. Specific trends in the stress with initial differentials in density or particle size and with temperature are then discussed. Some effects of non-sinterable heterogeneites (hard agglomerates or whiskers) on sintering retardation are also analyzed. Finally, some remarks concerning sintering damage are presented.

Clean-room powder processing, furnace firing, and CO2-laser heating techniques to produce high-purity Al2O3 ceramics were applied to a new source of high-purity Al2O3 ( 〉13 ppm total and 〉8 ppm cation detected impurities). The chemical analyses of the material after each stage of processing and firing indicate that the procedures used in this work give no detectable contamination of the material. The microstructure of a CO2-laser ultra high-purity Al2O3 is illustrated. Densification is incomplete for this material. Calcination and deagglomeration were not optimized. Procedures, such as these are required to control the trace impurity contents in a fired ceramic material

The effects of excess AI2O3 and TiO2 on microstructure, bending strength and thermal expansion of Al2TiO5 ceramics were studied by sintering various compositions of CVD Al2O3-TiO2 powders at 1310°C for 4 h. Excess Al2O3 resulted in much higher bending strength with slightly higher thermal expansion, while excess Ti02 resulted in slightly higher strength with lower thermal expansion. The excess AI2O3 and TiO2 were uniformly dispersed as particles on the Al2TiO5 grain boundaries, creating interlocked grains and dispersed microcracks. Excess TiO2 enhanced the material diffusion while AI2O3 did not, and appeared to develop more anisotropy of Al2TiO5 grains, resulting in lower expansion. It was proved that the overall properties of Al2TiO5 ceramics could be improved by dispersing excess Al2O3 or TiO2 uniformly onto the grain boundaries.

LiNbO3 tolerates large amounts of disorder in the form of Li2O- deficiency, oxygen-deficiency, and substitutional impurities. These should all result in the formation of a self-consistent set of lattice defects, and there will thus be interactions between the different types of disorder. It is shown that reduction will result in an increase of the Li2O activity in LiNbO3 when it is kinetically hindered from exchanging Li2O with its surroundings. This is in fact the situation for exposure to reducing atmospheres for several hours near 1000°C. Available data on compositions and defect concentrations at 1050°C indicate that reduction should result in separation of a Li20-rich second phase for oxygen partial pressures below 10−17 about 10 atm. Evidence for such phase separation is described.

Microwave heating equations are shown to have a complex functional dependence on material and process parameters such as composition, density, temperature, frequency and geometry. Power flow and its deposition in the presence of simple material shapes are used to illustrate relevant electromagnetic boundary conditions. Various types of applicator design using standing wave and travelling wave structures are discussed with respect to the results on power density, process efficiency and controllability. The problem of measuring and controlling the temperature inside a microwave system is addressed with emphasis on the use of thermocouples and infrared thermometry. Some of the problems peculiar to high temperature processing are considered. A review of the dielectric and magnetic property variation with temperature and frequency is given in order to understand the complexity and difficulty of uniformity of heating even with microwaves. The use of combination processing, for example, microwaves and infrared heating, is discussed. A short bibliography is presented listing some important publications in the field.

Sintering behavior of gel-derived alumina containing 2.8 mol percent cerium oxide (Ce2 O3) was studied at 1400°C. The growth morphology of alpha alumina in cerium oxide-containing aluminas was found to be significantly different form that of pure gel-derived alumina. Alumina gel containing 2.8 mol % Ce2 O3 was found to densify to 81% of the theoretical density as compared to 65% for pure alumina gels. The powdered cerium oxide-containing gels, pressed into a pellet, could be sintered to 97% of the theoretical density at 1400°C as compared to 87% found for pure alumina gel powder processed in an identical manner.

A review is given of the controlled use of compositional variation in the sintering of ceramics. The difficulties in determining the mechanisms of operation of sintering additives are outlined and the mechanistic approaches for studying the subject are described. Important remaining issues, new developments and opportunities for developing unique ceramic microstructures through the controlled use of compositional variation are described. Attention is focused on lattice defects and grain growth inhibition, particle pinning, multiple doping, anion effects, anisotropy effects, scavenging, surface diffusion and the formation of wavy grain boundaries.

Rheology and the component interactions which affect rheology were studied for a tape casting composition similar to commercial systems. Viscosity measurements at different shear rates were compared to measured tape properties to determine if high or low shear rate rheological behavior controls tape characteristics. Relative viscosity was measured to assess the contribution of each component to the stability of the dispersion.

The major hindrance in using structural ceramics in well defined engineering applications is their lack of reliability caused by uncontrolled flaw populations introduced during fabrication. Mechanical reliability is thus a matter of fabrication reliability. The strengthening that can be achieved by either eliminating or reducing the size of flaw populations through changing either processing or microstructure will be reviewed.

Constant compressive stress creep experiments in the temperature and stress ranges of 1730K - 2100K and 16 MN/m2 - 70 MN/m2 on HIPed NbC0.74 have revealed stress exponents of 2.0 under stress levels of 16–54 MN/m2 at all temperatures investigated and 3.2 under stress levels of 54–70 MN/m2 at 1830K. The activation energy of steady state creep is approximately 230 kJ/mol in the temperature range of 1730K - 1930K under 48–54 MN/m and 470 kJ/mol in the temperature range of 1900K - 2100K under 64 MN/m2. TEM of the annealed but uncrept material reveals grown-in dislocation subboundaries. At 1730K and under 34–54 MN/m2, these subboundaries become single dislocations and dipoles. At 1830K and under 54–70 MN/m the subboundaries evolve into simple tilt boundaries which are occasionally knitted, indicating more glide activity at higher stresses. At 1930K and under 34–54 MN/m , hexagonal subboundaries form, but are not as well defined as in the annealed material. At 2100K and under 16–30 MN/m2, the subboundaries are well-defined hexagonal networks which become polygonized under higher stresses on 64 MN/m2. The experimental and TEM results indicate that at low temperatures (below 0.5 Tm = 2073K) and at all stresses, creep occurs by dislocation glide which is accompanied by subgrain and high angle boundary interaction. At high temperature (above 0.5 Tm), strain occurs by glide and subboundary movement; recovery occurs by climb in the subboundary.

We show how the entropies of formation and migration of point defects may be calculated accurately. The approximations inherent within static lattice calculations are assessed, in particular the Vineyard reaction rate theory.

We present results of EXAFS and computer simulation studies of the defect structure of yttria-stabilised zirconia. The results are analysed in terms of dopant-vacancy association models for this phase.

Electronic current flow over grain boundary potential barriers for low resistance grains is first reviewed. It is then shown that if the resistance of the grains increases and/or grain size is reduced, the grains may be totally depleted of mobile charge. The grain boundary barrier is thus reduced to (D/2W)2 of its original large grain value, D being grain width and W the full space charge width. For high resistivity grains satisfying the relation D ≪ 2W, the conduction band becomes essentially flat. Phenomena formerly caused by grain boundary potential barriers (varistor and PTC effects seen in semiconducting ceramic) will be greatly reduced, or eliminated.

Commercial COG and X7R MLC capacitors exhibit a transition from super-ohmic to ohmic behavior at high voltages, paralleling the behavior of the varistor. Two possible mechanisms that could account for this are varistor-like grain boundary behavior, or space charge limited diffusion current.

In MgO irradiated at high dose rates and high temperatures with 1.8 MeV electrons, a suppression of the Fe3+ optical absorption band at 290 nm is observed. This suppression, a function of both dose rate and temperature, is consistent with a reduction process induced by oxygen displacement damage. Both thermal and radiation enhanced diffusion are involved and lead to the formation of iron containing precipitates. Similar results have been obtained for Ni2+.

The present understanding of the structure of grain boundaries and phase boundaries in ceramic materials is briefly reviewed with a view to extracting general conclusions on the structure of such interfaces. Examples of experimental studies of grain boundaries in alpha-alumina, spinel and germanium are discussed together with illustrations of the wustite/spinel and sesquioxide/spinel interface. In the oxide systems discussed here, both the grain boundaries and phase boundaries can facet even in the case where the anion sublattice is effectively undisturbed, this observation emphasises the influence of the cations on the selection of the boundary plane.