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.

An evacuable guarded hot plate for thermal conductivity measurements between À200 and 800 C was used to investigate the heat transfer in barley straw. The different thermal transfer mechanisms (solid, gaseous conduction and infrared-radiative heat transfer) as well as coupling effects were separated. The measured thermal conductivities ( ¼ 0.041 W m À1 K À1 ) are similar to those of conventional insulation materials such as foams, glass or mineral fibres which are widely used as building insulation materials. Straw from barley or wheat, which is a low-cost, renewable material readily available world-wide, is therefore an interesting alternative to conventional insulation materials.

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