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Research Papers: Heat and Mass Transfer

Characterization of Phase Change Heat and Mass Transfers in Monoporous Silicon Wick Structures

[+] Author and Article Information
Steve Q. Cai

Teledyne Scientific and Image Company,
1049 Camino Dos Rios,
Thousand Oaks, CA 91360
e-mail: qingjun.cai@teledyne.com

Avijit Bhunia

Teledyne Scientific and Image Company,
1049 Camino Dos Rios,
Thousand Oaks, CA 91360

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received July 9, 2013; final manuscript received March 5, 2014; published online March 26, 2014. Assoc. Editor: Sujoy Kumar Saha.

J. Heat Transfer 136(7), 072001 (Mar 26, 2014) (8 pages) Paper No: HT-13-1344; doi: 10.1115/1.4027152 History: Received July 09, 2013; Revised March 05, 2014

Silicon is the primary material of integrated circuit (IC) manufacturing in microelectronic industry. It has high thermal conductivity and superior thermomechanical properties compatible to most semiconductors. These characteristics make it an ideal material for fabricating micro/mini heat pipes and their wick structures. In this article, silicon wick structures, composed of cylindrical pillars 320 μm in height and 30–100 μm in diameter, are developed for studies of phase change capability. Fabrication of the silicon wick structures utilizes the standard microelectromechanical systems (MEMS) approach, which allows the precise definition on the wick dimensions, as well as the heated wick area. On these bases, experimental characterizations of temperature variations versus input heat fluxes, associated with simultaneous visualization on the liquid transport and the dryout, are performed to investigate the wick dimensional effects on the maximum phase change capability. On the wick structure with the pillar diameter/pores of 100 μm and a heated wick area of 2 mm × 2 mm, the phase change reached a maximum heat flux of 1130 W/cm2. Despite of the liquid bottom-feed approach, interactions between liquid and vapor phases enables the heated wick structure absorb liquid from its surrounding wick area, including from its top side with a longer liquid transport path. In contrast, a wick structure with fine pillars (10 μm in diameter) inhibited the generation of nucleate boiling. Evaporation on the meniscus interface becomes the major phase change mechanism. A large heated wick area (4 mm × 4 mm) increases the viscous loss in transporting liquid to wet the entire wick, advancing the dryout at 135 W/cm2. Mass transfer analysis, as well as discussion of the experimental results, indicates that a dimensional ratio r/l (pillar diameter/characteristic length of the heated wick area) is a key parameter in determining the maximum phase change capability. A low r/l ratio enhances heat and mass transport capability, as well as heat transfer coefficient.

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Figures

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Fig. 1

Silicon wick structures A and B: (a) design dimensions of silicon wick structure A, (b) wick structure A made through a MEMS process (r:l = 1: 40 and 1:133 for wicks A and B)

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Fig. 2

Silicon wick structure C: (a) micropores in two different scales, (b) the wick C etched through a dual-step process (r:l = 1:400)

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Fig. 3 PRTs

(a) a square PRT/heater deposited on the backside of the silicon substrate (b) thicknesses of PRT, adhesive metal layer, SiO2 insulation layer and silicon substrate

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Fig. 4

Experimental studies of silicon wick structures: (a) system setup for visualization and heat transfer characterizations, (b) the sample cylinder that separates the saturated environment from the electrical measurement circuit, and (c) schematic cross section of the test chamber

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Fig. 5

Phase change images of the silicon wick structure A: (a) 180 W/cm2, (b) 300 W/cm2, (c) 450 W/cm2, (d) 650 W/cm2, (e) 1130 W/cm2, and (f) 1300 W/cm2

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Fig. 6

Zoom-in views of the phase change zone on the wick structure A: (a) liquid supply path and directions at 650 W/cm2 and (b) dryout in the center of the heated wick area at 1300 W/cm2

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Fig. 7

Input heat fluxes versus the wall temperatures of the silicon wick structure A

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Fig. 8

Input heat fluxes versus the wall temperatures of the silicon wick structure B

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Fig. 9

Phase change images of the silicon wick structure C: (a) 22.5 W/cm2, (b) 30 W/cm2, (c) 45 W/cm2, and (d) 135 W/cm2

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Fig. 10

Zoom-in view at the center of the heated wick area of the wick structure C: (a) liquid recedes into the large pores at 45 W/cm2 and (b) the small pores dry at 135 W/cm2

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Fig. 11

Input heat fluxes versus the wall temperatures of the silicon wick structure C

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Fig. 12

Heat transfer coefficients versus heat fluxes of the silicon wick structures A, B, and C

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