Thermal Characterization and Effect of Deposited \hbox {CO}_{2} on a Cryogenic Insulation System Based on a Spherical Powder

The objective of this work is to study the effect of deposited \(\hbox {CO}_{2}\) on the solid thermal conductivity of a cryogenic insulation system. Therefore, measurements were performed using a guarded hot plate apparatus at temperatures in the range from 80 K to 290 K in combination with a bellow acting as the sample containment. The unique experimental setup and sample preparation are described in detail. Furthermore, existing thermal models which are based on a superposition of thermal transfer due to radiation and solid thermal conductivity were modified to account for the thermal effects of deposited gases and the consequently increased solid thermal conductivity for a spherical powder. Measurements showed a significant increase of the solid thermal conductivity depending on the amount of \(\hbox {CO}_{2}\) that was provided for deposition–evacuation. 2.77 Vol-‰\(\hbox {CO}_{2}\) resulted in an increase of 5.5 % in the overall solid thermal conductivity. Twice this amount (5.54 Vol-‰\(\hbox {CO}_{2}\)) and four times this amount (11.1 Vol-‰\(\hbox {CO}_{2}\)) resulted in an increase of \(8.8\,\%\) and 14.1 % in the overall solid thermal conductivity, respectively. Due to additional temperature sensors, it was possible to measure the effective thermal conductivity in different layers of the insulation material. Thus, a significant change in the innermost layer of \(75\,\%\) was measured for the solid thermal conductivity comparing the evacuated sample with the \(\hbox {CO}_{2}\)-loaded (11.1 Vol-‰\(\hbox { CO}_{2})\) sample.