Design of Integrated Nanostructured Wicks for High-Performance Vapor Chambers

The performance of passive phase-change cooling devices, such as vapor chambers or heat pipes, may be significantly enhanced by exploiting the superior thermal properties of carbon nanotube (CNT) arrays. The potential for large reductions in overall package resistance with the use of high-conductivity wick materials enhanced with CNT nanostructures is investigated. While such nanostructured wicks feature very small pore sizes that support high capillary pressures, it is shown that the high fluid flow resistance through these dense arrays prevents their use as the lone fluid transport mechanism. It is proposed that evaporator surfaces comprised of nanostructured wicks fed by interspersed conventional wick materials (such as sintered powders) can provide the required permeability for fluid flow while simultaneously decreasing the effective evaporator thermal resistance. Optimization of wicks with integrated sintered and nanostructured areas requires a study of the trade-offs between the greater permeability of the sintered materials and the greater capillary pressure and thin-film evaporation area offered by the nanostructures. A numerical model is developed to estimate the thermal resistance of the evaporator region compared to that of a homogeneous sintered powder wick. The inputs needed for this model include the permeability and the capillary pressure in the two regions. A parametric study is conducted as a function of the ratio of conduction and evaporative resistances for the nanostructured and sintered regions. For a given heat input, the optimal liquid-feeding geometry that minimizes thermal resistance is obtained. In the best cases, the thermal resistance is reduced by a factor of thirteen through the use of the integrated nanostructured wicks compared to the resistance of a homogeneous sintered powder wick.