Many studies have been performed on the flat-plate heat pipes with sintered wick. It was found that during the evaporation process, the heat transfer characteristics of hydrophilic surface performed better than hydrophobic surface. This work investigated the heat transfer characteristics of flat-plate heat pipes in which the bottom surface was modified with various gradient contact angles by a sol-gel method. This method was applied to create a gradient surface on copper-plate surface. The coated nanoparticles were immobilized on the surface after the surface was heated in a furnace at a working temperature of 120°C. The thermal resistance results of flat plate heat pipes with either homogeneous superhydrophilic surface or a gradient wettability are reported in this study.

For the gradient wettability, the evaporation region was super-hydrophilic and the condense region was super-hydrophobic. The heat transfer ability was both increased in evaporation region and condense region. Furthermore, the reflux ability of the working fluid was performed better due to the unbalanced surface tension on the gradient surface and the impact of gravity force of inclination angle (α). By manipulating different surfaces with different contact angles (gradient surface, contact angle = 150 ° /110 ° /20 ° /10 ° and uniform surface, contact angle <10°) and different inclination angles (α = 0°, 10°), we managed to find the better combination to improve the thermal performance of flat-plate heat pipe.

The results indicated that the thermal performance of flat plate heat pipe with a gradient wettability is better than homogeneous superhydrophilic surface. The evaporation resistance of gradient wettability surface (gradient & α = 10°) has achieved to 0.098 °C /W, and reduced 30% than homogenous superhydrophilic surface (CA <10° & α = 0°). The gradient wettability surface in this work performed as well as the traditional sintered wick flat-plate heat pipe.

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