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ABSTRACT
This article first deals with analysis techniques adapted to the characterization of the main defects (pores, inclusions, impurities, etc.) that can impair the optical transparency of ceramics. The identification and quantification of defects is not easy in these materials because of their very low concentration, which requires the implementation of appropriate methods. These methods are mainly based on optical or electronic microscopy and on spectroscopy. In a second part, the properties of the most common transparent ceramics are described in close relation to their major fields of scientific and industrial application.
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Read the articleAUTHORS
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Rémy BOULESTEIX: Senior Lecturer - Ceramics Research Institute, IRCER UMR CNRS 7315, University of Limoges, France
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Alexandre MAÎTRE: Professor - Ceramics Research Institute, IRCER UMR CNRS 7315, University of Limoges, France
INTRODUCTION
By their very nature and the manufacturing processes used, transparent ceramics must be able to combine the properties of single crystals (compactness, purity, homogeneity) with the characteristics of ceramics (ease of shaping, large parts, flexibility of composition). They also offer enhanced thermo-mechanical performance (mechanical strength, toughness, resistance to thermal shock), making them particularly well-suited to applications involving high thermal and/or mechanical stress.
Several compositions are now being developed in the form of transparent polycrystalline ceramics, such as alumina, YAG Y 3 Al 5 O 12 , spinel MgAl 2 O 4 , calcium fluoride CaF 2 or zinc selenide. The variety of materials available makes it possible to cover a wide range of applications, such as lighting, watchmaking, shielding or power lasers. Nevertheless, one of the main limitations of transparent polycrystalline ceramics obtained by powder sintering remains the presence of residual defects, in particular porosity, which alter their optical transparency through light scattering. The characterization of these defects, which remains difficult due to their very low proportion in transparent ceramics (< 0.1% vol.), requires the use of adapted techniques.
The first section of this article aims to present characterization methods that provide access to the nature and, possibly, quantity of residual defects in transparent ceramics with a view to optimizing their manufacture. A second section details the physico-chemical properties of the most common transparent ceramics, along with examples of industrial and scientific applications.
The general properties and manufacturing processes of transparent ceramics are described in detail in
At the end of the article, readers will find a glossary and a table of the notations used.
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KEYWORDS
spectroscopy | microscopy | transparent ceramics | optical lenses | ballistic armor | solid-state lasers
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Identification and quantification of defects in transparent ceramics
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Among all transparent crystalline materials (monocrystalline or polycrystalline), YAG, AlON, spinel and sapphire currently account for the bulk of the market, each with an equivalent share of close to 20% (figure 1 ).
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