Characterization and nanostructured enhancement of boiling incipience in capillary-fed, ultra-thin sintered powder wicks

Next-generation thermal management applications will require passive heat spreading at a lower thermal resistance, higher dryout tolerance, and with thinner profile devices than current vapor chambers. Such performance improvements may be achieved by augmenting evaporation and boiling heat transfer via patterning the internal wick or nanostructuring the wick surface in the region of heat input. Test samples composed of 200 μm thick sintered copper powder layers are investigated because they can be integrated into vapor chambers with an overall thickness of 1 mm. Carbon nanotubes (CNTs) are grown onto patterned and monolithic samples by a microwave plasma chemical vapor deposition synthesis technique, and are functionalized to ensure high wettability with the test fluid, water. Performance of the test samples is evaluated in an experimental facility which replicates the heat input and capillary fluid-feeding mechanisms at the evaporator section of a vapor chamber. High-speed visualizations are performed to identify the vapor formation regimes. Monolithic samples are shown to dissipate heat fluxes greater than 400 W/cm² over 0.25 cm² prior to dryout. A noteworthy heat transfer enhancement mechanism observed is reduction of the required superheat for boiling incipience by addition of a CNT coating. Predictable transition from the evaporation to boiling regimes at a lowered superheat is critical due to the lower thermal resistance associated with boiling. Multiple repeated tests on identically prepared samples reveal that the CNT coating reduces the average incipience substrate superheat by 5.6 °C compared to uncoated samples.