Explorations of Carbon Nanotube Wick Structure for High Heat Flux Cooling

Multiwall carbon nanotube (MCNT) has high thermal conductivity, nano size pores and high capillary pressure. All these physical properties make it an ideal candidate as a wick structure in a micro sized heat pipe/spreader. In this paper, experimental investigations evaluate heat transfer performance of the carbon nanotube (CNT) wick and demonstrate its ability to handle high heat flux cooling. The CNT wick structure used for high heat flux experiments employs the bi-wick structure design to overcome high flow resistance in CNT clusters. The wick fabrication technique integrates both microelectromechanical systems (MEMS) patterning and thermal chemical vapor deposition (CVD) CNT growth processes. In high heat flux experiments, the CNT cluster functions as the first order wick structure and provides a large capillary force. The spacing among CNT clusters acts as the second order wick structure thus setting up low resistance liquid supply channels and vapor ventilation paths. Preliminary experiments are conducted in an open chamber system with vertical CNT bi-wick sample setup. Heat flux, as high as 400W/cm2, is demonstrated over 0.16mm2 heating area. Dryout was not observed, whereas the heater soft-bonding material fails at the higher testing heat flux. The experimental results indicate that the CNT bi-wick structure is capable of high heat flux cooling and promises to be the heat transfer element in new generation microelectronics cooling systems.

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Investigations of High Heat Flux Cooling Using Carbon Nanotube Bi-Wick Structure and Integrated Platinum Thermometer/Heater

Volume 2: Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Computational Heat Transfer ◽

10.1115/ht2009-88077 ◽

2009 ◽

Cited By ~ 1

Author(s):

Qingjun Cai ◽

Yuan Zhao ◽

Chialun Tsai ◽

Chung-lung Chen

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Carbon Nanotube ◽

High Performance ◽

Empty Space ◽

High Heat ◽

High Heat Flux ◽

Resistance Variation ◽

Wick Structure ◽

Reliable Temperature

With the increase of power consumption in compact electronic devices, passive heat transfer cooling technologies with high heat flux characteristics are highly desired in microelectronics industries. Carbon nanotube (CNT) cluster/forest has high effective thermal conductivity, nano pore size and large porosity, which can be used as wick structure in a heat pipe heatspreader and provides high capillary force for high heat flux thermal management. In this research, investigations of high heat flux cooling of the CNT bi-wick structure are associated with the development of a reliable thermometer and high performance/interface free heater. A 100nm thick and 600μm wide Z-shaped platinum wire resistor is fabricated on the backside of a CNT sample substrate to heat a 2×2mm2 wick area. As a heater, it provides direct heating effect without thermal interface and is capable of over 800°C high temperature operation. As a thermometer, reliable temperature measurement is achieved by calibrating the resistance variation with temperature after the annealing process is applied. The CNT sample substrate is silicon. The backside of the silicon substrate is thermally oxidized to create a 2μm thick and pinhole-free SiO2 layer so that the platinum heater and thermometer can survive from the server CNT growth environments and without any electrical leakage. For high heat flux cooling, the CNT bi-wick structure is composed of 250μm tall, 100μm wide stripe-like CNT clusters and 50μm empty space. Using 1×1cm2 CNT bi-wick samples, experiments are completed in both the open and saturated environments. Testing results of CNT bi-wick structure demonstrate 600W/cm2 heat transfer capacity and good thermal & mass transport characteristics in the nano level porous media.

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High Heat Flux Phase Change on Silicon Wick Structures

Volume 7: Fluids and Heat Transfer, Parts A, B, C, and D ◽

10.1115/imece2012-88302 ◽

2012 ◽

Cited By ~ 1

Author(s):

Qingjun Cai ◽

Avijit Bhunia ◽

Yuan Zhao

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Phase Change ◽

Test Chamber ◽

Nucleate Boiling ◽

High Heat ◽

High Heat Flux ◽

Maximum Heat ◽

Wick Structure ◽

Silicon Pillars

Silicon is the major material in IC manufacture. It has high thermal conductivity and is compatible with precision micro-fabrication. It also has decent thermal expansion coefficient to most semiconductor materials. These characteristics make it an ideally underlying material for fabricating micro/mini heat pipes and their wick structures. In this paper, we focus our research investigations on high heat flux phase change capacity of the silicon wick structures. The experimental wick sample is composed of silicon pillars 320μm in height and 30 ∼ 100μm in diameter. In a stainless steel test chamber, synchronized visualizations and measurements are performed to crosscheck experimental phenomena and data. Using the mono-wick structure with large silicon pillar of 100μm in diameter, the phase change on the silicon wick structure reaches its maximum heat flux at 1,130W/cm2 over a 2mm×2mm heating area. The wick structure can fully utilize the wick pump capability to supply liquid from all 360° directions to the center heating area. In contrast, the large heating area and fine silicon pillars 10μm in diameter significantly reduces liquid transport capability and suppresses generation of nucleate boiling. As a result, phase change completely relies on evaporation, and the CHF of the wick structure is reduced to 180W/cm2. An analytical model based on high heat flux phase change of mono-porous wick structures indicates that heat transfer capability is subjected to the ratio between the wick particle radius and the heater dimensions, as well as vapor occupation ratio of the porous volume. In contrast, phase change heat transfer coefficients of the wick structures essentially reflect material properties of wick structure and mechanism of two-phase interactions within wick structures.

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A Mathematical Modeling Method for Capillary Limit of Micro Heat Pipe with Sintered Wick

Solid State Phenomena ◽

10.4028/www.scientific.net/ssp.175.335 ◽

2011 ◽

Vol 175 ◽

pp. 335-341

Author(s):

Xi Bing Li ◽

Chang Long Yang ◽

Gong Di Xu ◽

Wen Yuan ◽

Shi Gang Wang

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Heat Pipe ◽

Heat Dissipation ◽

High Heat ◽

Experimental Investigations ◽

High Heat Flux ◽

Micro Heat Pipe ◽

Copper Powders ◽

Capillary Limit

With heat flux increasing and cooling space decreasing in microelectronic and chemical products, micro heat pipe has become an ideal heat dissipation device in high heat-flux products. Through the analysis of its working principle, the factors that affect its heat transfer limits and the patterns in which copper powders are arrayed in circular cavity, this paper first established a mathematical model for the crucial factors in affecting heat transfer limits in a circular micro heat pipe with a sintered wick, i.e. a theoretical model for capillary limit, and then verified its validity through experimental investigations. The study lays a powerful theoretical foundation for designing and manufacturing circular micro heat pipes with sintered wicks.

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

Journal of Heat Transfer ◽

10.1115/1.4000469 ◽

2010 ◽

Vol 132 (5) ◽

Cited By ~ 50

Author(s):

Qingjun Cai ◽

Chung-Lung Chen

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Carbon Nanotube ◽

Silicon Substrate ◽

High Performance ◽

High Heat ◽

High Heat Flux ◽

Resistance Variation ◽

Variation Versus ◽

Reliable Temperature

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.

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A Mathematical Modeling Method on Micro Heat Pipe with a Trapezium-Grooved Wick Structure

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.29-32.1686 ◽

2010 ◽

Vol 29-32 ◽

pp. 1686-1694

Author(s):

Xi Bing Li ◽

Shi Gang Wang ◽

Jian Hua Guo ◽

Dong Sheng Li

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Heat Pipe ◽

Chemical Engineering ◽

High Heat ◽

High Heat Flux ◽

Micro Heat Pipe ◽

Flow Limit ◽

Wick Structure ◽

Design And Manufacture

With heat flux increasing and cooling space decreasing in the products in microelectronics and chemical engineering, micro heat pipe has become an ideal heat radiator for products with high heat flux. Through analyzing the factors influencing the structure, strength and heat transfer limits of circular micro heat pipe with trapezium-grooved wick structure, the heat transfer models are established in this paper, including the models of viscous limit, sonic limit, entrainment limit, capillary limit, condensing limit, boiling limit, continuous flow limit and frozen startup limit. The study lays a powerful theoretical foundation for the design and manufacture of circular micro heat pipe with a trapezium-grooved wick structure.

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Dryout Studies of Carbon Nanotube Bi-Porous Structure

ASME/JSME 2011 8th Thermal Engineering Joint Conference ◽

10.1115/ajtec2011-44168 ◽

2011 ◽

Author(s):

Qingjun Cai ◽

Ya-Chi Chen ◽

Chung-lung Chen

Keyword(s):

Heat Transfer ◽

Mass Transfer ◽

Heat Flux ◽

Carbon Nanotube ◽

Heat Pipe ◽

Maximum Temperature ◽

High Heat Flux ◽

Evaporator Temperature ◽

Maximum Heat ◽

Wick Structure

Dryout occurring in a heat pipe evaporator section is caused by insufficient coolant supply of wick structure, and generally considered as a failure mode of heat pipe operation. However, traditional dryout theory does not fit the bi-porous (bi-wick) wick structure research on the new mass transfer mechanism, such as liquid splash at high heat flux. The reported maximum heat flux on the similar wick structure may show a large deviation. Accordingly, dryout studies of bi-wick structures become critical and necessary to understand the limitation of the heat and mass transfer. In this article, carbon nanotube (CNT) clusters are used to investigate dryout of bi-wick structures. Within a closed system, evaporation and boiling phase change on CNT bi-wick structures is visualized to provide direct views on the occurrence and expansion of dryout zone. At the same time, the evaporator temperature variations versus heat flux are measured to characterize the temperature responses upon the bi-wick dryout. Investigations based on both visualization and measurement results show that dryout of CNT bi-wick structures are caused by insufficient liquid supply to create temperature elevation and in-plane heat transfer increase of the evaporator substrate. On the curvatures of heat flux versus the evaporator temperature, dryout can be defined as the appearance of the inflexion point on the heating section, and associated by the existence of large hysteresis of heat transfer performance. Numerical modeling of the temperature distribution on dried wick structure further indicates that traditional temperature measurement approaches are hardly to detect the occurrence of dryout and to provide the maximum temperature. High temperature hotspot on dried wick structure can be more destructive than temperature sensor measured.

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