Mullite (3AI2O3. 2Si02) is becoming increasingly important in electronic, optical, and high-temperature structural applications. This paper reviews the current state of mullite-related research at a fundamental level, within the framework of phase equilibria, crystal structure, synthesis, processing, and properties. Phase equilibria are discussed in terms of the problems associated with the nucleation kinetics of mullite and the large variations observed in the solid-solution range. The incongruent melting behavior of mullite is now widely accepted. Large variations in the solid solubility from 58 to 76 mot% alumina are related to the ordering/disordering of oxygen vacancies and are strongly coupled with the method of synthesis used to form mullite. Similarly, reaction sequences which lead to the formation of mullite upon heating depend on the spatial scale at which the components are mixed. Mixing at the atomic level is useful for lowtemperature (<lOOO"C) synthesis of mullite but not for low-temperature sintering. In contrast, precursors that are segregated are better suited for lowtemperature (1250" to 1500°C) densification through viscous deformation. Flexural strength and creep resistS M Wiederhorn-contributing editor

Mullite for Structural, Electronic, and Optical Applications

Inspection of the scientific literature reveals that intermetallic compounds have, in recent years, attracted considerable interest as a result of their unique elevated temperature characteristics. Among the wide range of intermetallic compounds that are actively being studied, MoSi2 has been singled out as a result of its unique combination of properties, which include an excellent oxidation resistance, a high modulus of elasticity, and an elevated melting point (2030°C). In view of this interest, the present work was undertaken with the objective of providing the reader with a comprehensive review of the mechanical and oxidation behaviour of MoSi2, paying particular attention to the synergism between processing and microstructure. Accordingly, synthesis techniques, including powder metallurgy, self-propagating hightemperature synthesis, spray processing, solid-state displacement reactions, and exothermic dispersion, are critically reviewed and discussed. In addition, recent efforts aimed at using MoSi2 as a matrix material in metal-matrix composites are also critically reviewed and discussed.

Processing of molybdenum disilicide

Because of its outstanding mechanical properties, zirconia-based ceramics are considered as some of the best potential materials within the engineering ceramics field that might be widely used to substitute various metallic parts and specific alloys. Taking into account the transformation toughening mechanisms that operates in their microstructure, important properties can be obtained. Phase transitions as well as transformation toughening in ZrO2 are reviewed briefly with the purpose to understand its effects in some composites and glass systems. Zirconia ceramics as high toughness materials for cutting tool, metal forming applications, mechanically superior ceramics called partially stabilised zirconia (PSZ), solid electrolytes, have been fabricated using the martensitic nature of the tetragonal to monoclinic phase transition.

Phase transitions in zirconium dioxide and related materials for high performance engineering ceramics

Basic research needs on high temperature ceramics for energy applications

The reaction bonding of silicon carbide (SiC) typifies liquid-solid reaction processes for the synthesis of refractory ceramic composites. These processes have particular advantages over conventional sintering and hot-pressing techniques in their lower processing temperatures, shorter times and near-net shape fabrication capabilities. Two particular modifications that we have employed in order to improve the mechanical properties and the ultimate use temperature of reaction-bonded SiC are the use of microporous carbon pre-forms derived from polyfurfural alcohol for refinement of microstructure, and the use of alloyed melts in order to replace detrimental residual silicon with a refractory silicide. The control of reaction rate is always a key issue in reaction processing. We have studied the kinetics and mechanisms of the liquid SiC reaction. Experiments on carbon fibers and plates show that the principle mechanism is one of solution-reprecipitation. There is an increased solubility of carbon at very fine SiC particles formed by the spallation of the misfitting carbide from the carbon interface, leading to reprecipitation of SiC at defective seed crystals. Molybdenum and boron at low concentrations (3.2 mol.%) have little effect on reaction kinetics, whereas aluminum is able to impede the reaction through the formation of an interfacial carbide layer.

Reaction-formed silicon carbide

Mechanical Properties of Sintered, In Situ-Reacted Mullite-Zirconia Composites

Mechanical Properties of Sintered and Hot‐Pressed Si3N4‐ZrO2 Composites

Mechanical properties of sintered, in situ-reacted mullite-zirconia composites

A novel method of sintering Si3 N4 shaped bodies which method comprises grinding Si3 N4 with the use of a grinding material down to a surface area magnitude of 10.5 to 35 m2 /g and an average particle size from 0.2 to 0.005 μm, shaping the powder obtained, and sintering the compact under inert gas or nitrogen at a temperature from 1700° to 1900° C.

Method of sintering articles of silicon nitride

Aligned eutectic composites of unstabilized ZrO/sub 2/ and Ta can be prepared crack-free even though the matrix has been completely transformed into the monoclinic lattice, i.e. the transformation-induced volume change does not result in the excessive cracking which is usually observed. Composites with Y/sub 2/O/sub 3/-stabilized ZrO/sub 2/, however, crack extensively and exhibit inhomogeneous eutectic structures. Similar results are obtained with directionally solidified, stabilized ZrO/sub 2/--W and stabilized HfO/sub 2/--W. The fact that ZrO/sub 2/ and HfO/sub 2/ have modifications with identical crystal structures suggested that compositions of unstabilized HfO/sub 2/ with Ta would also exhibit an improved solidification behavior compared with mixtures containing Y/sub 2/O/sub 3/-stabilized HfO/sub 2/. Eutectic areas of HfO/sub 2/--Ta sintered powder rods exhibited almost no cracking. There are two possible explanations for the fact that the composites with unstabilized ZrO/sub 2/ and HfO/sub 2/ survive the tetragonal..-->..monoclinic transformation without cracking: (1) the matrix is prestressed by the Ta fibers when expanding at the lattice tranformation; or (2) since the composites are sintered in vacuum, ZrO/sub 2/ or HfO/sub 2/ are substoichiometric. This condition, together with Ta in solid solution, may cause the monoclinic lattice cell to exhibit a density similar to that of the tetragonalmore » form. Both hypotheses, however, must be carefully investigated. (JRD)« less

Jürgen Jahn

Papers

Mechanical Properties of Sintered, In Situ-Reacted Mullite-Zirconia Composites

Mechanical Properties of Sintered and Hot‐Pressed Si3N4‐ZrO2 Composites

Mechanical properties of sintered, in situ-reacted mullite-zirconia composites

Method of sintering articles of silicon nitride

Directional Solidification of Unstabilized HfO2‐Ta