Читать книгу Transparent Ceramics - Adrian Goldstein - Страница 15

The fundamental useful characteristic of a transparent ceramic is obviously its electromagnetic transmission capability, especially in the optical region and its general optical properties [K55, K58]. The spectral transmission window width and/or its level are the most important features. Transmission level is not exactly identical with the transparency concept. The transparency (see Section 1.4) is assessed by the fraction between the total forward transmission (TFT) and the so called real in-line transmission (RIT). For most applications, transparency is the important feature, but in some cases, like streetlamp envelopes, the TFT is the relevant parameter. However it has to be emphasized that transparency alone is rarely sufficient to make a transparent ceramic-based product interesting. The optical transmission property has to be accompanied by other favorable functional properties. The nature of these properties depends on the specific application. For instance, hardness and stiffness are critical properties for use as armored windows; ferroelectric properties count the most for antiglare goggles; a high refractive index coupled with a low optical dispersion are important for lenses. For all transparent ceramics, high resistance to ambient chemical aggressive agents is a fundamental requirement. A transparent ceramic suitability to a destined application is expressed using figures of merit, which take into account a weighted assessment of all relevant properties.

In many cases besides performance, the practical value of a transparent ceramic is strongly affected by the availability of suitable geometric dimensions and/or shapes. For instance, for armor applications, fabrication technology is fully developed for plates, 30–100 cm wide (sometimes exhibiting some curvature). For missile IR sensors, dome or cone-shaped protective noses are needed. Certain laser applications require a thin chip, or fiber geometry.