Effects of carbon nanotube arrays on nucleate pool boiling

Experiments were performed to assess the impact coating silicon and copper substrates with nanotubes (CNTs) have on pool boiling performance. Different CNT array densities and area coverages were tested on 1.27 mm2 samples in FC-72. The CNT preparation techniques used provided strong adherence of CNTs to both substrate materials. Very small contact angle enabled deep penetration of FC-72 liquid inside surface cavities of smooth uncoated silicon surfaces, requiring unusually high surface superheat to initiate boiling. Fully coating the substrate surface with CNTs was highly effective at reducing the incipience superheat and greatly enhancing both the nucleate boiling heat transfer coefficient and critical heat flux (CHF). Efforts to further improve boiling performance by manipulating CNT area coverage of the substrate surface proved ineffective; best results were consistently realized with full surface coverage. Greater enhancement was achieved on silicon than on copper since, compared to uncoated copper surfaces, the uncoated silicon surfaces were very smooth and void of any sizeable nucleation sites to start with. This study is concluded with detailed metrics to assess the enhancement potential of the different CNT array densities and area coverages tested

Metal Functionalization of Carbon Nanotubes for Enhanced Sintered Powder Wicks

Phase change cooling schemes involving passive heat spreading devices, such as heat pipes and vapor chambers, are widely adopted for thermal management of high heat-flux technologies. In this study, carbon nanotubes (CNTs) are fabricated on a 200 micrometer thick sintered copper powder wick layer using microwave plasma enhanced chemical vapor deposition technique. A physical vapor deposition process is used to coat the CNTs with a varying thickness of copper to promote surface wetting with the working fluid, water. Thermal performance of the bare sintered copper powder sample (without CNTs) and the copperfunctionalized CNT-coated sintered copper powder wick samples is compared using an experimental facility that simulates the capillary fluid feeding conditions of a vapor chamber. A notable reduction in the boiling incipience superheat is observed for the nanostructured samples. Additionally, nanostructured samples having a thicker copper coating provided a considerable increase in dryout heat flux, supporting heat fluxes up to 457 W/cm^2 from a 5 mm x 5 mm heat input area, while maintaining lower surface superheat temperatures compared to a bare sintered powder sample; this enhancement is attributed primarily to the improved surface wettability. Dynamic contact angle measurements are conducted to quantitatively compare the surface wetting trends for varying copper coating thicknesses and confirm the increase in hydrophilicity with increasing coating thickness