Development and Characterization of Low-Emitting Ceramics

The infrared-optical properties of ceramics are correlated with the complex index of refraction of the material and the structure of the ceramic. By changing these parameters, the infrared-optical properties can be changed over a relatively wide range. The correlation of the structural properties (like the porosity or the pore sizes) and the material properties (such as the complex index of refraction on the one hand and the infrared-optical properties such as emittance on the other) are described by a solution of the equation of radiative transfer and the Mie-theory. Within this work, low-emitting ceramics, which have significantly lower emittances than conventional ceramics, were prepared by optimizing their composition and structure. The spectral emittance of these ceramics was measured, and a total emittance dependent on temperature was calculated from the spectral emittance. As a result, one obtains ceramics which have a total emittance of 0.2 at a temperature of 1,100 K. In comparison to conventional ceramics with a typical total emittance of 0.8 at 1,100 K, the use of such low-e ceramics leads to a reduction in heat transfer of about 70% via thermal radiation. The results of our calculations were compared with experimental data to validate the theory.