Several microwave absorbers have been characterized in terms of transmittance and reflectance at frequencies between 35 GHz and 3 THz. The materials studied were a series of iron-loaded cast epoxy absorbers known as Eccosorb. Measurements show that reflectance and absorption coefficient increase with the iron density. A dramatic decrease, by as much as a factor of 2, in absorption coefficient was observed when the samples were cooled from ambient to cryogenic temperatures. A blackbody calibrator to be operated at liquid helium temperature was constructed using the measured optical constants for these absorbers. The measured absorption coefficient for cold Eccosorb CR-110 is within 20% of that reported recently by Peterson and Richards.
Hamid Hemmati, John C. Mather, and William L. Eichhorn, "Submillimeter and millimeter wave characterization of absorbing materials," Appl. Opt. 24, 4489-4492 (1985)