Millimeter-Wave Dielectric Properties Of Infrared Window Materials

The millimeter-wave dielectric properties of a series of IR window materials were determined over the temperature range 23-1000°C. Materials studied included Al203, ZnS, ZnSe, aluminum oxynitride (ALON), and magnesium-spinel (MgAl20 4). These materials all exhibited fairly high millimeter-wave dielectric constants (e' = 8-10), but with essen-tially negligible room-temperature losses for most applications. However, both the dielectric constant and loss tangent increase significantly with increasing temperatures. The increases in dielectric constant with temperature can be analyzed in terms of a macroscopic dielectric virial expansion model, and are primarily due to the effective increase in volume for each polarizable unit of the material. Consequently, a strategy to overcome this degradation would be to search for new materials or composite structures with low thermal expansion coefficients. The observed millimeter-wave loss properties are characteristic of contributions from intergranular impurities and show an onset of increased absorption at ca. 500°C. However, even at 1000°C, typical loss tangents are still below 0.05, and should be acceptable in most millimeter-wave window applications for reasonable thicknesses (/ < X).