Study of Thermal Diffusivity of Dielectric-Metal Sandwich Structures at Low Temperatures

The objectives of the thesis is to study the AC measurement method and use it to obtain the thermal diffusivity for sandwich structures that consist of different materials in series. Two sandwich structures were investigated, varying in the amount and the material of the interfaces. It was relevant to detect which of these structures can transport heat loads the fastest to the mixing chamber of a dilution refrigerator in order to decide which sandwich structure is most suitable for AEgIS. Based on the measurements performed in the study, many characteristics of the AC method were detected. Two values, obtained with the measurement of a sinusoidal heat signal, can be analyzed to determine the thermal diffusivity - the phase shift and the amplitude attenuation. It has been proven that the phase shift method is not suitable for measurements on the sandwich in a dilution refrigerator, since an AC resistance bridge is necessary for these studies. The resistance bridge causes an additional phase shift. More- over, the accuracy of the method is not high enough to measure the small phase shifts expected for the samples. The frequency dependence of the method is clearly visible for all samples and a correct measurement is only possible in a small frequency range where the thermal diffusivity is constant. Ideas where the frequency dependence originates from and how the constant "plateau" could be extended to lower frequencies were discussed. The thermal diffusivity for both sandwich structures was measured between 3 K and 30 K and it was evident that the thermal diffusivity is the same for both sandwich mock-ups at about 3 K. Because of different sensor distances for S1 and S2 the same thermal diffusivity means a higher thermal boundary resistance of about factor 2 for sandwich S2 caused by the additional metallic boundaries