Advanced two-channel ac calorimeter for simultaneous measurements of complex heat capacity and complex thermal conductivity

Abstract

The advanced construction of a two-channel ac calorimeter for simultaneous measurements of frequency-dependent complex heat capacity C ( ω) and complex thermal conductivity λ ( ω) is presented. In the new calorimeter, the number of interfaces with thermal-wave reflections was reduced. Thus, the new construction can be easily calibrated with higher precision and is simpler in handling than the previous one. The new construction allows to measure thermal conductivity in steady-state mode, as well as frequency-dependent complex thermal properties in ac mode, in the same measuring cell. The capabilities of this technique were demonstrated, being applied for simultaneous measurements of complex effusivity, diffusivity, heat capacity, and thermal conductivity of glycerol in the glass transition region. The so-called ac and dc thermal conductivities of glycerol were measured as a function of temperature. It was shown that the double-channel ac calorimetry is a technique, which can be used for reliable distinguishing of relaxation processes related to relaxing thermal conductivity or relaxing heat capacity. In the region apart from phase transitions, the calorimeter provides the unique possibility of simultaneous measurements of the thermal contact properties together with the sample’s thermal parameters. The improvement of the accuracy gave us the possibility to observe the thermal contact resistance, leading to a step of 1 and 5% in the temperature-modulation amplitude at the cell/sample interface in the case of liquid samples such as Apiezon™-H grease and glycerol, respectively. A step of 25% was observed in the case of a dry thermal contact between the cell and an ethylene-1-octene copolymer sample. Thus, the thermal contact resistance must be taken into account in the temperature-modulated calorimetry, especially in the case of a dry cell/sample contact.