In the early 1970s in Japan, the United States and France it was found that additions of nitrogen into aluminum oxide resulted in new spinel-like phases. At about the same time there was much increased interest in oxynitrides, stimulated by Professor K. Jack in the UK and Y. Oyama in Japan. Following these activities a major research program in this area was initiated at the Army Materials and Mechanics Research Center in Watertown, Massachusetts in 1974. These efforts resulted in the first complete Al2O3–AlN phase equilibrium diagram and a process to reactively sinter to nearly full density, translucent aluminum oxynitride spinel ceramic, which was named AlON. Subsequently, the Raytheon Company further developed AlON into a highly transparent material (ALON™) with many applications including transparent armor and EM domes and windows, among others—the technology was recently transferred to the Surmet Corporation. This paper will review the early history, phase equilibrium, crystal chemistry, and properties of this material, along with more recent work in our laboratory on transient liquid phase sintering and new data on lattice parameter measurements. In addition, recent results of collaborative work on AlON's dynamic mechanical properties using plate impact, Kolsky bar and edge-on impact (EoI) experimental techniques, including preliminary modeling at the microstructural scale of AlON in the EoI test, will be presented.

AlON: A brief history of its emergence and evolution

Silicon nitride ceramics is a generic term for a variety of alloys of Si3N4 with additional compounds necessary for a complete densification of the Si3N4 starting powder. They are heterogeneous, multicomponent materials characterised by the inherent properties of the crystalline modifications α and β of Si3N4 and the significant influence of the densification additives. With a view to ability of the α and β modification to form solid solutions α-Si3N4 (αss) and β-Si3N4 (βss) solid solutions can be distinguished. Each group contains engineered materials with interesting properties for special applications. Phase relations and micro-structures determine the properties decisively. Composition of the phases, the distribution of the grains, their aspect ratio and the grain boundary phase are pronounced microstructural features. The formation of the microstructure strongly depends on the one hand on the quality of the Si3N4 starting powders, which closely is related to the chemistry of the production process, and on the other on the liquid phase sintering as the most important step in the densification route. The interrelation between pure Si3N4, the densification of the powder including the role of sintering additives, microstructural engineering, physicochemical properties of the sintered Si3N4 ceramics (SSN, GPSN, HPSN, HIP-SSN, HIP-SN) are described in more detail and compared to reaction bonded Si3N4 ceramics (RBSN), which are produced by nitridation of silicon powders.

Silicon Nitride Ceramics

Transparent ceramics have various potential applications such as infrared (IR) windows/domes, lamp envelopes, opto-electric components/devices, composite armors, and screens for smartphones and they can be used as host materials for solid-state lasers. Transparent ceramics were initially developed to replace single crystals because of their simple processing route, variability in composition, high yield productivity, and shape control, among other factors. Optical transparency is one of the most important properties of transparent ceramics. In order to achieve transparency, ceramics must have highly symmetric crystal structures; therefore, the majority of the transparent ceramics have cubic structures, while tetragonal and hexagonal structures have also been reported in the open literature. Moreover, the optical transparency of ceramics is determined by their purity and density; the production of high-purity ceramics requires high-purity starting materials, and the production of high-density ceramics requires sophisticated sintering techniques and optimized sintering aids. Furthermore, specific mechanical properties are required for some applications, such as window materials and composite armor. This review aims to summarize recent progress in the fabrication and application of various transparent ceramics.

Materials development and potential applications of transparent ceramics: A review

Transparent ceramic materials with optical qualities comparable to single crystals of similar compositions have been developed in recent years, as a result of the improved understanding of powder-processing-fabrication- sintering-property inter-relationships. These high-temperature materials with a range of thermal and mechanical properties are candidate envelopes for focused-beam, short-arc lamps containing various fills operating at temperatures higher than quartz. This paper reviews the composition, structure and properties of transparent ceramic lamp envelope materials including sapphire, small-grained polycrystalline alumina, aluminium oxynitride, yttrium aluminate garnet, magnesium aluminate spinel and yttria–lanthana. A satisfactory thermal shock resistance is required for the ceramic tube to withstand the rapid heating and cooling cycles encountered in lamps. Thermophysical properties, along with the geometry, size and thickness of a transparent ceramic tube, are important parameters in the assessment of its resistance to fracture arising from thermal stresses in lamps during service. The corrosive nature of lamp-fill liquid and vapour at high temperatures requires that all lamp components be carefully chosen to meet the target life. The wide range of new transparent ceramics represents flexibility in pushing the limit of envelope materials for improved beamer lamps.

http://iopscience.iop.org/article/10.1088/0022-3727/38/17/S07/pdf

Transparent ceramic lamp envelope materials

The dependence of the lattice parameter of γ-aluminium oxynitride (Alon) on its composition was investigated. It was found that the width of the homogeneity of Alon varies with temperature: at 1850°C Alon is the stable phase between 66 mol% Al2O3 (lattice parameter 0·7953 nm) and 81 mol% Al2O3 (lattice parameter 0·7932 nm). This region becomes smaller with lower temperatures. Below 1640 ± 10°C Alon is not stable.

https://pure.tue.nl/ws/files/2245646/621497.pdf

Thermodynamics of Alon II : phase relations

The formation and stability of Alon under various circumstances are described using Ellingham and activity diagrams. With the help of these diagrams the connection between seemingly uncorrelated experimental observations on the stability of Alon from the literature can be understood. Thermodynamic data sets for Alon are critically reviewed. It is shown that Alon is stable only within a small region of oxygen and nitrogen pressures, and that it is not stable below 1640 ± 10° C.

https://pure.tue.nl/ws/files/2314188/621498.pdf

R. Metselaar

Papers

Thermodynamics of Alon II : phase relations

Thermodynamics of Alon I: Stability at Lower Temperatures