Design and Test of Carbon Nanotube Biwick Structure for High-Heat-Flux Phase Change Heat Transfer

Abstract

With the increase in power consumption in compact electronic devices, passive heat transfer cooling technologies with high-heat-flux characteristics are highly desired in microelectronic industries. Carbon nanotube (CNT) clusters have high thermal conductivity, nanopore size, and large porosity and can be used as wick structure in a heat pipe heatspreader to provide high capillary force for high-heat-flux thermal management. This paper reports investigations of high-heat-flux cooling of the CNT biwick structure, associated with the development of a reliable thermometer and high performance heater. The thermometer/heater is a 100-nm-thick and 600 μm wide Z-shaped platinum wire resistor, fabricated on a thermally oxidized silicon substrate of a CNT sample to heat a 2×2 mm2 wick area. As a heater, it provides a direct heating effect without a thermal interface and is capable of high-temperature operation over 800°C. As a thermometer, reliable temperature measurement is achieved by calibrating the resistance variation versus temperature after the annealing process is applied. The thermally oxidized layer on the silicon substrate is around 1-μm-thick and pinhole-free, which ensures the platinum thermometer/heater from the severe CNT growth environments without any electrical leakage. For high-heat-flux cooling, the CNT biwick structure is composed of 250 μm tall and 100 μm wide stripelike CNT clusters with 50 μm stripe-spacers. Using 1×1 cm2 CNT biwick samples, experiments are completed in both open and saturated environments. Experimental results demonstrate 600 W/cm2 heat transfer capacity and good thermal and mass transport characteristics in the nanolevel porous media.

Recent Advances in Vapor Chamber Transport Characterization for High-Heat-Flux Applications

“…A 2 mm × 2 mm platinum heater was fabricated on the back side of the silicon growth substrate, and the capillary-fed evaporation/boiling performance was evaluated in open and saturated vapor environments. A maximum heat flux of ~600 W/cm 2 was measured at surface superheats of only 35-45°C [45].…”

“…Further, while the hydrophilicity of NWs and CNTs with water has been reported in the literature [205,206], aligned arrays of nanotubes have also been shown to behave as superhydrophobic surfaces [207]. Hence, surfactants may be used for liquid conveying applications [208], or nanostructures may be functionalized for heat transfer applications via metallization [51], hydrochloric acid treatment [209], or ultraviolet excitation [210]. Nanostructures have a high number of pores per unit substrate area, and thereby may also offer an increase in the thin-film area for evaporation.…”

“…Cai et al [45] investigated a CNT 'biwick' structure composed of a uniform 250 μm-thick CNT array with parallel interspersed microgrooves. It was postulated that the nanoscale pores of the CNT array would provide a large increase in the area for thin-film evaporation and boiling heat transfer, while the groove spacing would provide area for bulk liquid supply and vapor removal.…”

“…The wick structure was fabricated by using lithography processes to define the catalyst deposition area and resulting CNT growth pattern. An acid-treatment process was used to make the CNTs hydrophilic [45]. The structures tested had 100 μm-wide CNT strips with 50 μm-wide microgrooves.…”

Metal functionalization of carbon nanotubes for enhanced sintered powder wicks

“…Several modifications to traditional monoporous wicks have been proposed in prior work [12][13][14][15][16] to improve the dryout heat flux and to decrease the thermal resistance of the wick structure to meet the cooling requirements for ultra-high heat flux applications. Semenic and Catton [12] compared the critical heat flux (CHF) of monoporous and biporous wicks (wicks with two different pore sizes) using water as the working fluid, under sub-atmospheric saturation pressure (< 13 kPa) conditions and reported high CHF values of 520 W/cm 2 and 990 W/cm 2 at superheats of 50 °C and 147 °C for 800 μm and 3000 μm thick biporous wicks, respectively.…”