UC Irvine

This study reports experimental measurements of the thermal resistances of a thermal diode structure, consisting of microscale-thickness layers of distinct physical properties. The concept was developed based on the cathode structure of PEM fuel cell, which promotes water and heat removal in one direction by heat pipe effects in porous media. The diode layers have an overall thickness of 100-1,000 micrometers, similar to the overall thickness of the cathode in a PEM fuel cell. The unique thermal property is enabled by the heat pipe effects, directed by the arrangement of the sublayers through the capillary action. Experimental measurement is carried out on the simplest structure that consists of two sublayers with one being hydrophilic and the other hydrophobic. Various materials of distinct wettability properties are used and tested, including printing paper, carbon paper, cellulose paper, and polypropylene paper. The testing was also conducted under various conditions such as temperature, water contents, and sublayer properties, which may greatly influence the unique thermal property of this type of thermal diodes. Two water contents, 40% and 60% (volume percent) and temperature range of 30 ˚C-90 ˚C were examined. Results show that under 90 ˚C and 60% water contents. we achieve resistance 0.280 K/W in the conductive direction, over three times smaller than that in the opposite direction (0.874 K/W) for cellulose paper and polypropylene paper materials. For the sample of printing paper with carbon paper, and the sample of cellulose paper with carbon paper, the thermal resistances also have about two times difference. The thermal diodicity increases with temperature and water content in the most cases. Future work is to fabricate the diodicity materials for larger than 10 thermal diodicity.