Surface Modifications of Bulk Micromachined Titanium Pillar Arrays: A Wick Material for Thin Flat Heat Pipes

Wicking materials with tunable wettability are of great importance for both fundamental research and practical applications such as heat pipes. In this work, we adopt recently developed titanium bulk micromachining[1] techniques to fabricate pillar arrays. Then we modify the micromachined pillars to form micro- & nano-textured (bitextured) titania structures (BTS). Further, we investigated how to plate gold on the modified surfaces to tune the wettability. A wicking material for heat pipe requires super wetting by common fluids such as water. We show theoretical studies and experimental work to investigate the wetting behavior of two different designs/samples. For heat pipe applications the BTS and plating gold not only increases the capillary pressure which enhances liquid pumping from condenser to evaporator, but also increases the heat transfer performance by extended surface and smaller pore sizes[2]. Testing results show that water can completely wet the micromachined Ti pillars (Design A: 5μm in diameter/5μm gap). The BTS helps increase the wetting speed by over 100% for this design. A second design with much larger diameter and gap (Design B: 100μm in diameter/50μm in gap) is also tested to compare with design A for wetting speed. Results show that Design B gives a wetting speed twice of Design A. Plating method is used to decrease pillar gap (from 50μm to 5μm) by growing gold on surfaces. This will help increase thermal conductivity of wicking material which is preferred for the evaporator and condenser regions of heat pipes. Wetting experiment is done on Sample B after plating with gold. Wetting results after Au plating show that wetting velocity decreases but is still significantly large.