Infrared-optical characteristics of ceramics at elevated temperatures

Abstract In this work the infrared-optical properties of ceramic samples at ambient and elevated temperatures are presented. The aim of this work is the determination and interpretation of the spectral transmittance, reflectance and emittance of three oxide ceramic samples (MgO, Al 2 O 3 /ZrO 2 , TiO 2 /Y 2 O 3 ) and one carbide ceramic sample (SiC) as a function of the temperature from 300 K up to 1100 K. The semi-transparency of the ceramics in the near infrared region has to be considered when performing the measurements and evaluating the data. The experimentally derived spectra are compared with the values derived from theoretical calculations. The complex dielectric function of a ceramic can be deduced from the Lorentz oscillator model. The complex refractive index is correlated with the complex dielectric function by the Maxwell relation. Scattering effects can be described by radiative transfer models. Finally the infrared-optical properties transmittance, reflectance and emittance depend on the complex refractive index and the morphology of the ceramic sample. Of special interest is the Christiansen wavelength as well as the location of the free running frequencies of the longitudinal and transversal optical oscillations. Usually the ceramic is highly reflecting between the free running frequencies of the longitudinal and transversal optical oscillations whereas at the Christiansen wavelength the reflectance vanishes which leads to an emittance of one for an optically thick sample at the Christiansen wavelength. Due to the known emittance it is possible to use the Christiansen wavelength for determining the temperature of the investigated sample. Finally the obtained infrared-optical spectra are presented and their temperature dependencies are discussed which are mainly due to phonon excitations. Additionally it has been shown that for example the location of the Christiansen wavelength can be influenced by varying the composition of the ceramic.