Resorcinol formaldehyde xerogels for loop heat pipe applications

Abstract

Heat pumps are being targeted in research and development because they represent energy-efficient means of transporting heat. Of particular interest are loop heat pipes (LHPs), passive systems based on continuous evaporation and condensation of a fluid in a closed-loop fashion. One key component in a LHP is the evaporator where the liquid enters at least one tube-shaped porous body (wick) and leaves it as gas through the porous network. While typical wick structures are composed of sintered metal (stainless steel, copper), also polymer-based examples are known to be effective in LHPs. As wicks ideally possess high porosity and pores in the (sub-)micrometer range, xerogels might be efficient candidates for wick development. We report on the production of wick-shaped xerogels based on Fricke-type resorcinol-formaldehyde (RF) xerogels. Beside the synthesis of RF with varying solid content the shaping of monoliths into wick bodies using a sequence of machining processes is presented. Furthermore, the characterization of the material in terms of density, porosity, and bubble point analysis is presented. Studies towards the compatibility of the RF wick with various fluids typically employed in LHPs are demonstrated. Finally, preliminary studies on a generic wick demonstrator housed in stainless steel tubing are presented.