squares using a diamond tipped scribe. The scribe is used in place of the dicing saw to keep the silicon surface free from contaminants because the CNT growth ...
Proceedings of IMECE: International Mechanical Engineering Congress and Exposition November 5-11, 2005, Orlando, Florida IMECE2005-80065
ENHANCED POOL BOILING USING CARBON NANOTUBE ARRAYS ON A SILICON SURFACE Sebastine O. Ujereh Jr, Issam Mudawar, Placidus B. Amama, Timothy S. Fisher, Weilin Qu School of Mechanical Engineering Purdue University West Lafayette, Indiana ABSTRACT Progress in integrated circuit technology has caused device density and power dissipation to increase, resulting in significant cooling challenges. Pool boiling is an attractive cooling option because of its unique combination of passive fluid circulation and high heat flux capability. Having no mechanical pumps, pool boiling hardware is less complex, easier to seal, and free of pump-induced fluid pulsations that are present with many alternative approaches. One of the main obstacles for improvements in pool boiling technology is the limiting factor of critical heat flux (CHF), which limits cooling capacity. The present experimental work considers the introduction of carbon nanotube (CNT) arrays on the chip surface to delay CHF and to enhance boiling heat transfer. Pool boiling curves for a smooth silicon surface and a silicon surface coated with CNTs were obtained. Tests were conducted in which power was input in 1 W increments to the respective silicon surfaces immersed in FC-72 fluid. These experiments reveal significant boiling enhancement. Testing reveals a measured CHF of approximately 15 W/cm2 for a CNT-coated silicon wafer and a CHF of approximately 10 W/cm2 for bare silicon wafers. Further, superheat at fully developed boiling is reduced on CNT-coated surfaces by up to 60%, and effective heat transfer coefficients are enhanced by approximately 400% by the presence of CNTs. NOMENCLATURE ∆T= temperature difference, oC qflux = heat flux, W/cm2 A = chip cross sectional area, cm2 h = heat transfer coefficient, W/m2*K Ts= temperature of silicon surface, K Tsat = saturation temperature of the fluid, K CHF = critical heat flux, W/cm2 INTRODUCTION Since the invention of computers, improvements and innovations in chip technology have been made at extraordinarily rapid rates. The advancement of integrated circuit technology is the driving force of this trend. According to Moore’s Law [1], the number of transistors on a chip doubles
every 18 months. Given this steady increase in chip density, the ability to remove large amounts of heat from small areas has become a pressing concern in the electronics industry. The increasing necessity of computer technology, along with the market-driven desire for small, quiet, inexpensive, and reliable devices has further increased the complexity of the heat removal task. The potential of direct liquid immersion cooling, pool boiling, as a suitable alternative to air cooling and indirect liquid cooling technologies has been suggested [2], but improvements in critical pool boiling metrics are needed to optimize thermal performance. The effects of surface enhancements on pool boiling have been studied extensively. On the larger-scale, Anderson and Mudawar [3] demonstrated the effects of fin and microstud augmentation to enhance pool boiling. In these experiments, microstud enhanced surfaces were shown to be the most effective. Their experiments revealed that the increased surface area with only a mild increase in fin volume which microstuds provide effectively delays CHF [3]. The effects of small-scale surface augmentation have also been observed. Augmentation techniques include surface roughening and microporous coatings. O’Connor et al. [4] have shown CHF enhancement of up to 100% on diamond painted surfaces. They also observed low superheats associated with boiling incipience and the commensurate improvements in heat transfer coefficients. The unique surface structure appears to enhance vapor entrapment, resulting in an increased nucleation site density. Rainey and You [5] developed a microporous chip coating that provided significant heat transfer enhancement (approximately 330%) and CHF was increased (approximately 100%). Carbon nanotubes (CNTs) are relatively new materials whose unique structure and superior mechanical, electrical and thermal properties make them ideal candidates for numerous engineering applications. CNTs exist in either multi-walled (MWNT) or single-walled (SWNT) form. The typical diameter of a multi-walled tube is 10-100 nm while the typical length is approximately between 1 and 50 µm [6]. In this experiment the unique shape and the thermal properties of MWNTs are of particular interest. The thermal conductivity of individual MWNTs has been measured to exceed of 3000 W/mK [7]. Vertically oriented MWNTs have been grown using a plasma
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enhanced chemical vapor deposition (PECVD) system [8], which is used in this work. Enhancing pool boiling using CNT arrays has not been studied to date, and this work’s aim is to characterize pool boiling heat transfer from CNT arrays on a silicon surface and to compare the results to those of a bare, polished silicon surface using a common two-phase fluid (FC72, 3M).
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EXPERIMENTAL PROCEDURE Substrate Preparation. Float zone silicon wafers were used for all testing. The double-side-polished silicon wafers have a 100 mm diameter with a thickness of 400µm and bulk resistivity greater than 15000 Ω-cm. The wafer was diced into 12.7 mm x 12.7 mm squares using a Tempress 602 dicing saw. In the experiments, heat is transferred to the chip via a solid copper block. The top surface dimensions of the block are 12.7 mm x 12.7 mm. A thick film heater with an electrical resistance of 100 ohms is soldered onto the bottom surface of the copper block to facilitate heat input to the silicon. A single K-type thermocouple is embedded in the center of the base of the copper block to monitor temperature, and two heater leads are soldered to opposite edges of the thick film heater. Hightemperature solder (melting point ~ 300 oC) is used to solder the heater and the copper leads to the bottom side of the copper block. The square chip is soldered to the top surface of the copper block (see Figs. 1-3) using indium foil (25 µm, Honeywell) in a Lindberg Blue industrial oven at a temperature of 175 oC. Care must be taken to ensure the silicon surface remains scratch-free throughout the adhesion process.
Fig. 1. Copper block and material components
Fig 2. Photograph of copper block with CNT-coated silicon surface attached. Two (black) heater leads are soldered to the bottom of the heater. A single thermocouple is embedded in the center of the block.
Fig. 3. Dimensions of the copper block CNT Surface Preparation. An identical four-inch double side polished silicon wafer was diced into 12.7 mm x 12.7 mm squares using a diamond tipped scribe. The scribe is used in place of the dicing saw to keep the silicon surface free from contaminants because the CNT growth catalyst is sensitive to impurities. A 30 nm titanium layer was deposited on the top layer of the chip using the Varian electron beam evaporator. The purpose of the layer is to aid with anchoring of CNTs on the pure silicon surface [8]. Anchoring of the CNTs is a major concern, as they must withstand the vigorous nature of the boiling process. Catalyst Preparation. A fourth-generation (G4) PAMAM (polyamidoamine) dendrimer with amine peripheral groups (0.42 g) purchased from Dendritic Nano Technologies was dissolved in 20 mL of water, and 0.5 g of FeCl3.6H2O (Aldrich) was dissolved in 20 mL of water separately. The two solutions were mixed and stirred vigorously for 2 hours. The cleaned wafer was dipped in the catalyst solution for 24 hours. The dendrimer was then removed by mild calcination of the immobilized catalyst at 250°C for 30 min to avoid the passivation of the catalyst by the dendrimer. Work by Crooks et al. [9] details the synthesis and application of the dendrimer catalyst. The calcination temperature is kept low to decrease the aggregation of the metallic clusters. The calcined catalyst is used for the growth of carbon nanotubes in the PECVD system under optimal conditions for Fe catalyst at 900oC with a methane flowrate of 10 sccm (standard cubic centimeter per minute) and a hydrogen flowrate of 50 sccm. The PECVD was run for 20 minutes to maximize CNT coverage of the substrate. The chip was then adhered to
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the copper block as previously discussed. Much care must be taken to prevent damage to the CNTs during this process. A photograph of the CNT-coated surface is shown in Figure 4, and scanning electron microscope (SEM) images of the sample are shown in Fig. 5. The dimensions of the CNT samples used in this experiment are approximately 30 nm in diameter and 20 – 30 µm in length as estimated from SEM images.
Boiling Test Cell Setup. Once the silicon surface was adhered to the copper block, the block was mounted in the test section, and the test section was installed in the pool boiling facility shown in Fig 6. The cartridge heater located at the bottom of the chamber was used to bring the FC-72 fluid to its saturation temperature and to maintain saturated conditions. The test section is protected from leakage by sealing the top surface edges with high-temperature silicone RTV. Thermocouple
Thermocouple
vent Coil condenser View port
G-10 fiberglass chamber Lexan view window
Fig 4. CNT-coated silicon wafer in test section of boiling facility. High temperature RTV was used to seal edges and to protect test section from leakage. heater module
Test section Cartridge heaters
Fig 6. Schematic of boiling test cell.
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Fig 5. SEM images of CNT samples. a) 300nm scale, b) 4µm scale, c) 10µm scale, and d) 4µm scale at a different location on the surface from (b).
The heater leads are connected to a Variac power source, which supplies power to the silicon surface in the boiling chamber. Total power was quantified using a Yokogama power meter, which monitors current and voltage to the heater. Two K-type thermocouples were immersed in the fluid to measure its temperature. Cooling water was supplied to condense the vapor generated within the test cell. All temperatures of interest were collected using a Labview data acquisition system connected to a computer. The primary data acquisition instrument was a NI Daqpad 4350 interfaced with a NI TC 2190 thermocouple port. The test facility was charged with two gallons of FC-72 at atmospheric pressure, and the test fluid was brought to its boiling point (56.6oC) by setting the cartridge heater power source to 90 W. The FC-72 boiled vigorously for 45 minutes for deaeration. At this point, power input to the fluid was reduced to 50 W to maintain saturated conditions. The power was then reduced to zero and increased in approximately 0.5 W/cm2 increments. The temperatures of the FC-72 and the heated surface were monitored and recorded continually, and the power input, current, and voltage were recorded manually. The power was increased incrementally until CHF was observed as indicated by a rapid increase in the chip temperature at a constant input power. Upon reaching CHF, the
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heat flux was reduced and subsequently increased in smaller increments to obtain a more accurate CHF value. For all reported data points, steady state conditions were ensured by applying wait periods of 2 minutes for lower heat fluxes and 5 minutes for higher heat fluxes during which changes in all temperatures remained constant (within the uncertainty of ±0.4°C). RESULTS AND DISCUSSION Effect of CNTs on CHF. The smooth bare silicon surface was tested first as a control. Two separate experimental trials yielded CHF at substrate temperatures of 103.6oC and 103.7oC, occurring at heat fluxes of 11.37 W/cm2 and 11.40 W/cm2 respectively. The curve for the latter experiment is shown in Fig. 7.
Heat Flux, q" (W/cm2)
12 CHF
10 8 6
Fully developed
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indicate an approximately 35% increase in CHF for the CNTcoated silicon surface based on averaging the two tests from each case. Effect of CNT coating on boiling development. The results of the boiling curves suggest that the surface structure of CNTs promotes much lower superheats for boiling incipience, perhaps by enhancing the number of vapor entrapment locations. This effect facilitates added bubble nucleation sites. For the present purposes, fully developed pool boiling condition is defined to be the superheat temperature at which bubbles nucleation occurs over the entire surface of the substrate. Based on visual observation, fully developed boiling for the bare silicon surface occurred at approximately 29.5oC, and the corresponding condition for the CNT surface occurred o at only 10.2 C as shown in Figs. 7 and 8. This result represents a 65% reduction in fully developed superheat temperature produced by a CNT-coated silicon surface. Comparison of the photographic images in Figures 9 and 10 below shows that bubble nucleation begins at a much lower superheat (and heat flux level) for the CNT-coated surface than for the bare silicon surface. Boiling from the CNT-coated surface occurs at very low heat fluxes, with bubble departure beginning at a heat flux of 0.6 W/cm2 and a superheat of 7.2oC. Bubble departure begins with significant coverage on the bare silicon surface at a heat flux of 2.61 W/cm2 and a superheat of 23.6oC.
0 0
10
20
30
40
50
60
Superheat, Ts-Tsat [K]
Fig 7. Boiling curve for a smooth bare Si chip 16
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Heat Flux, q"(W/cm )
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CHF
12
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10 8 6 4
Fully developed
2 0 0
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Superheat, T s-T sat [K]
Fig 8. Boiling curve for CNT-coated Si chip Separate experiments conducted on CNT-coated silicon o o surfaces yielded CHF temperatures of 70.1 C and 72.3 C at heat 2 2 fluxes of 15.81 W/cm and 14.88 W/cm . The experimental boiling curve for the latter case is shown in Fig. 8. The results
Fig 9. Photographs boiling from bare silicon surface at 2 2 2 various heat fluxes a) 1.3 W/cm b) 5.1 W/cm c) 8.5 W/cm 2 d) 10.5 W/cm (the heat flux prior to CHF).
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Finally, we note two further observations. First, the area coverage of CNTs on silicon was not complete, and we believe that increased coverage would improve results further. Secondly, we note that we did not observe any degradation of the CNT surface, and this result suggests that the CNTs were well bonded to the substrate.
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CONCLUSIONS Carbon nanotubes (CNTs) were deposited on the surface of a bare silicon wafer. Boiling performance on this enhanced surface was analyzed in comparison to a bare silicon surface. Controlled testing yielded moderate CHF augmentation, considerable reduction in boiling incipience superheat, and a significant increase in heat transfer coefficients. Pool boiling experiments showed a 35% increase in CHF for the CNTcoated silicon surface and a 60% reduction superheat at the point of fully developed boiling. The net effect of these changes is an increase of greater than 400% in the effective heat transfer coefficient. These results suggest that CNTs may be effective in promoting efficient pool boiling and that they can remain well adhered to boiling surfaces.
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ACKNOWLEDGMENTS We gratefully acknowledge the assistance of Jun Xu in CNT fabrication, 3M for supplying the working fluid, and the GEM Fellowship Program for financial support of the lead author. (c)
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REFERENCES
Fig 7. Photographs of CNT-coated silicon surface at various 2 2 2 heat fluxes a) 1.3 W/cm b) 4.9 W/cm c) 9.2 W/cm d) 14.5 2 W/cm (heat flux prior to CHF).
Effect of CNTs on heat transfer coefficient. An effective boiling heat transfer coefficient can be calculated as the ratio of heat flux to surface-to-fluid temperature rise as
h=
q" Ts − Tsat
(1)
Heat flux levels recorded just before CHF are used for these calculations. For the bare silicon surfaces, the heat transfer coefficients are 0.289 W/cm2K and 0.286 W/cm2K. For the CNT-coated surface, the heat transfer coefficients are 1.17 W/cm2K and 1.07 W/cm2K, representing an increase of more than 400% in effective heat transfer coefficient. Both the CHF enhancement and heat transfer coefficient enhancement can be attributed primarily to the unique surface structure of the CNT coating. As shown in Fig. 8, boiling incipience occurred early in the heating cycle (i.e., at low heat flux) in the case of CNT-coated samples whereas incipience on bare silicon surfaces occurred much later. This result is important because the boiling mechanism keeps the surface at a moderate temperature. The superheat prior to CHF associated with the CNT-coated surface (13 K) is much smaller than that of the bare silicon surface (47 K). The high thermal conductivity of CNTs is believed to also add to this heat transfer enhancement. Future work must be conducted to quantify such effects.
1. G. E. Moore, “No exponential is forever: But “forever” can be delayed!,” IEEE International Solid-State Circuits Conference, 2003. 2. Mudawar, I, and Anderson, T.M., 1989 “Microelectronic Cooling by Enhanced Pool Boiling of a Dielectric Flourocarbon Liquid.” Transactions of the ASME. Vol 111. pp. 752-759. 3. Mudawar, I, and Anderson, T.M., 1993, “Optimization of Enhanced Surfaces for High Flux Chip Cooling by Pool Boiling.” ASME Journal of Electronic Packaging. Vol 115. pp. 89-99. 4. O’Connor, J.P., You, S.M., and Price, D.C., 1995, “A Dielectric Surface Coating Technique to Enhance Boiling Heat Transfer from High Power Microelectronics.” IEEE Transactions on Components, Packaging, and Manufacturing Technology. Part A.Vol 18. No 3. pp. 656-653. 5. Rainey, K.N., and You, S.M., 2000, “Pool Boiling Heat Transfer from Plain and Microporous, Square Pin-Finned Surfaces in Saturated FC-72.” ASME Journal of Heat Transfer. Vol 122. pp. 509-516. 6. Terrones, M., 2003, “Science and Technology of the 21st Century: Synthesis, Properties and Applications of Carbon Nanotubes.” Annual Reviews of Materials Research. pp. 30-61. 7. Kim, P., Shi, L., Majumdar, A., and McEuen, P. L., 2001, “Thermal Transport Measurements of Individual Multiwalled Carbon Nanotubes,” Physics Review Letters. Vol 87. pp. 215502:1-4. 8. Jun Xu, T.S. Fisher, “Enhancement of Thermal Interface Materials with Carbon Nanotube Arrays,” in review, International Journal of Heat and Mass Transfer. 2005.
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9. Crooks et al., 2001, “Dendrimer Encapsulated Nanoparticles: Synthesis, Characterization, and Applications to Catalyst.” Accounts of Chemical Research. Vol 34 num 3. pp. 181-190.
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