Analysis and measurement of thermal conductivity of polypropylene laminated paper impregnated with subcooled liquid nitrogen

•The thermal conductivity of PPLP impregnated with subcooled LN2 was measured.•We obtained a value of 0.23–0.26 W/m K for a bath temperature of 65–75 K.•The obtained value is five times that measured in a vacuum.

We measured the thermal conductivity of polypropylene laminated paper (PPLP) impregnated with subcooled liquid nitrogen. PPLP is widely used for the electrical insulation of high-Tc superconducting (HTS) power transmission cables. Although the thermal conductivity of PPLP is an important factor in the design of HTS cables, there has been very limited work on its measurement in subcooled liquid nitrogen. We prepared PPLP samples and symmetrically stacked them on both sides of a heater. The stacked samples were immersed in liquid nitrogen in an open cryostat. A cryocooler mounted on the cryostat was used to maintain the subcooled temperature of the liquid nitrogen. The thermal conductivity of the stacked PPLPs was measured by the steady state method at a bath temperature of 65–75 K and was found to be 0.23–0.26 W/m K, which is about five times that measured in a vacuum as presented in available literature. We also discuss possible mechanisms for boosting the thermal conductivity of PPLP by liquid nitrogen impregnation.