Measuring the specific heat of solids using a CO2 laser calorimeter

Abstract

A method has been devised to measure the specific heat of solid materials, especially those used as optical components for the infrared part of the spectrum, using a CO2 laser vacuum calorimeter. The CO2 laser vacuum calorimeter is a device that can precisely determine the absorptivity, at 10.6 μm, of an optic that has been coated to reflect the laser beam. A sample to be tested is fabricated in the shape of a simple optical element, and a high-reflecting thin-film coating is applied. Witness samples of known specific heat are first tested to determine the percent absorption of the coating. The sample of unknown specific heat is then tested by irradiation by the CO2 laser in vacuum to find the specific heat of the substrate. The method was tested using samples fabricated of ZnSe, GaAs, Ge, and CdTe. Specific heat values calculated using this method were within 10% of commonly reported values. The accuracy of this method is ±15%. Because the mirror coating is highly reflective, substrate materials that are not normally capable of withstanding 10.6 μm radiation can also be tested. This test can be extended to include any solid material that can be formed into the proper geometrical shape.