The present invention relates generally to heat transfer apparatus. More particularly, the present invention relates to an apparatus using electrowetting (EW) to transfer heat.
According to one aspect, a heat transfer device is provided that includes a filler fluid positioned adjacent an area to be cooled, a plurality of droplets, and a plurality of electrodes selectively receiving voltage to control the position of the plurality of droplets. The electrodes direct at least one droplet to a hot spot on the area to be cooled to receive heat from the hot spot and directs the droplet away from the hot spot into the filler fluid to be cooled by the filler fluid.
According to another aspect, a heat transfer device is provided that includes a filler fluid positioned adjacent an area to be cooled, a plurality of droplets, and means for directing the plurality of droplets to portions of the area with elevated temperature and directing the plurality of droplets between the portions with elevated temperature to portions of the area with lowered temperature so that heat is transported from the portions with elevated temperature to the areas with lowered temperature.
According to another aspect, a method for transferring heat is provided including the steps of providing a filler fluid positioned adjacent an area to be cooled and a plurality of droplets, the area having at least one hot spot, moving at least one of the droplets to the hot spot with electrowetting, transferring heat from the hot spot to the at least one droplet, moving the at least one droplet away from the hot spot to a portion of the area spaced apart from the hot spot and surrounded by filler fluid with electrowetting, and transferring a majority of the heat received from the hot spot to the filler fluid.
Additional features of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of the presently perceived best mode of carrying out the invention.
The detailed description of the drawings particularly refers to the accompanying figures in which:
The embodiments disclosed below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may utilize their teachings.
As shown in
For the case when the filler fluid temperature is less than the droplet temperature, droplet 14 picks up heat from hot spot 12 and transfers part of it to the colder surrounding filler fluid 20 during each oscillatory cycle. When a droplet 14 above a particular hot spot 12 becomes too hot to maintain sufficient heat transfer, it is moved to a cooler region within filler fluid 20 (such as one of the corners of the chip shown in
For the case of the filler fluid temperature being greater than the droplet temperature, droplet 14 would pick up heat from filler fluid 20 in addition to hot spot 12. In this case, droplet 14 is moved away from hot spot 12 when it reaches a threshold temperature, and is replaced by a new cold droplet 14. The hot droplet 14 is pumped away to a fluid reservoir for external cooling, prior to recirculation.
Droplet motion throughout the entire droplet life cycle is controlled by electrowetting based pumping action, which has very low power consumption. This heat spreading mechanism based on electrowetting induced droplet motion can be used in place of conventional heat spreaders as well as for hot spot thermal management of microelectronic chips, resulting in lower temperature gradients in the chip, reduced thermal stresses, lower overall chip temperatures and increased reliability.
This technology can be used to design heat spreading devices for various heat transfer applications. In particular this technology can be used for hot spot thermal management of microelectronic chips which have specified maximum temperature limits to ensure long term reliability. This technology can be used to control hot spot temperatures by extracting and spreading away heat from hot spot 12. The concept of electrowetting induced pumping can also be used to design electrowetting-based microchannel heat sinks, wherein discrete droplets 14 moving in microchannels are used to remove heat from the hot substrate.
Heat transfer device 10 provides localized convective heat extraction with higher heat transfer coefficients, when compared to high thermal conductivity solid materials, ensuring higher heat dissipation capacities than conduction-based spreaders. Heat transfer device 10 affords greater hot spot temperature control possibilities resulting from enhanced control of droplet motion by electrowetting. Heat transfer device 10 provides site-specificity (i.e., dynamic selection of cooling location). Heat transfer device 10 also lends itself easily to integration with microelectronic chips and is totally noiseless. Heat transfer device 10 also has very low power consumption as compared to other heat removing technologies.
Heat transfer device 10 uses localized convective heat removal instead of conduction-based heat spreaders, although such heat spreaders may be used in conjunction with heat transfer device 10. Heat transfer device 10 utilizes a controller, which can be the chip being cooled or another controller that uses feedback from surface thermal sensors 15 to develop dynamic cooling solutions for time varying heat flux situations. Thermal sensors 15 can be used to detect hot spots 12 and to detect when a droplet 14 is above a particular temperature and needs to be replaced by another droplet 14 to maintain sufficient heat transfer as described above. The controller monitors the temperature of droplets 14 using input from heat sensor 15 and directs them to and from hot spots 12 using electrowetting based pumping. Microfabricated thermocouples can be used as thermal sensors 15 which is known to those skilled in the art.
The position of droplets 14 are tracked by the controller. In one tracking method, the controller assumes the respective droplets 14 follow the path specified by the controller. The control can verify that the respective droplet 14 is following the specified path by tracking the thermal signature of respective droplet using thermal sensors 15. If a droplet 14 is “lost” by the controller, the controller selects another droplet 14 to be a replacement.
In many chips, the location of hot spots 12 is predictable based on the layout of heat generating circuitry within the chip. In such circumstances, the location of hot spots 12 can be programmed into the memory of the controller of heat transfer device 10 without any reliance on thermal sensors 15. Movement of droplets 14 to and from hot spots 12 can be controlled based on predetermined timing.
Electrowetting provides reliable and enhanced control of droplet motion. Additional description of electrowetting is provided in U.S. Pat. No. 6,911,132, to Pamula et al.; entitled “Apparatus for manipulating droplets by electrowetting-based techniques;” U.S. Pat. No. 6,565,727 to Shenderov, entitled “Actuators for microfluidics without moving parts;” U.S. Pat. No. 6,629,826, to Yoon et al., entitled “Micropump driven by movement of liquid drop induced by continuous electrowetting;” and U.S. Pat. No. 6,773,566, to Shenderov, entitled, “Electrostatic actuators for microfluidics and methods for using same;” the disclosures of which are expressly incorporated by reference herein. Additional description of hot spot cooling strategies is provided in U.S. Patent Application Publication No. 2005/0212124, to Wang, entitled “Device for cooling hot spot in micro system; “U.S. Patent Application Publication No. 2005/0183844, to Tilton et al., entitled “Hotspot spray cooling; “and U.S. Patent Application Publication No. 2003/0229662, to Luick, entitled “Method and apparatus to eliminate processor core hot spots,” the disclosures of which are expressly incorporated by reference herein.
The working of the electrowetting based heat spreading device 10 is illustrated in
Water droplets 14 are surrounded by non-evaporating and thermally conducting filler fluid 20, like silicone oil. Each droplet 14 picks up heat from a respective hot spot 12 and the surrounding filler fluid 20, thereby preventing any further increase in the hot spot temperature. As a respective droplet 14 heats up, its heat dissipation capacity decreases. When the respective droplet 14 reaches a threshold temperature, beyond which the heat dissipation capacity is less than required, the oscillatory motion of the respective droplet 14 is stopped. If a region of filler fluid 20 is less than a predetermined temperature, the “hot” droplet 14 is directed by the controller to that cooler region (such as the corners of chip 22 as shown in
If filler fluid 20 is above a predetermined temperature, the respective droplet 14 is then withdrawn to a hot fluid reservoir 26 for external cooling by an external heat transfer device 27, such as a heat exchanger before recirculation. This droplet 14 is replaced by a new cold droplet 14 which is created from a cold fluid reservoir 28 and moved over to the respective hot spot 12. In the exemplary embodiment, the motion of the respective droplet 14 throughout its entire life cycle is controlled entirely by the controller and electrodes 24 using electrowetting. According to other embodiments, portions of the motion may be provided by other means than electrowetting. If filler fluid 20 is sufficiently cooled, external cooling is not provided using reservoirs 26, 28 and external heat transfer device 27. Thus, in some embodiments, external reservoirs 26, 28 and external heat transfer device 27 are not provided.
A cross section of an EW-based spreading device 10 is shown in
When a voltage is applied to the right electrode 24 on top plate 18 (shown in
The EW induced actuation force on a droplet 14 is modeled by using the principle of energy minimization. EW actuation results from a reduction in the dielectric-liquid interfacial energy in the presence of an applied voltage. The total droplet surface energy is estimated as a function of the transition position of the droplet. The energy gradient gives the actuation force on a droplet 14 at that position. This actuation force model is combined with semi-empirical models which predict the forces opposing droplet motion to yield a model which predicts EW induced droplet motion. The significant opposing forces consist of the shear force from the top and bottom plates 18, 30, the viscous force offered by filler fluid 20, and the contact-line friction force. Equation (1) represents the model to predict transient EW induced droplet motion of a rectangular droplet.
The hot spot cooling capacity resulting from droplet motion can be estimated by a transient thermal analysis of a droplet 14 oscillating around a hot spot 12. The cooling capacity is measured in terms of the heat flux dissipation at a specified hot spot temperature (which prevents any further temperature rise). A droplet 14 picks up heat from a hot spot 12 as well as surrounding filler fluid 20, which is assumed to be at a fixed lower temperature than the respective hot spot 12. A 1 mm square hot spot 12 is analyzed and a rectangular droplet 14 is oscillated around the respective hot spot 12. The droplet dimensions and the oscillation magnitude is chosen to ensure that the respective hot spot 12 is completely covered by droplet 14 during all stages of the oscillation. The droplet temperature is allowed to reach a maximum permissible value, after which droplet 14 is moved away from hot spot 12 by EW based pumping and replaced by a new droplet 14 as discussed above.
The equation representing the transient thermal behavior of the droplet is:
where T is the droplet temperature,
Top plate 18 is assumed to be adiabatic in the above analysis. Key dimensions and parameters used in the foregoing analysis are detailed in Table 1.
The maximum and average (averaged over the entire cooling duration) heat dissipation capacities were estimated for varying hot spot temperatures. The filler fluid temperature was assumed to be 10° C. less than the hot spot temperature and the maximum permissible droplet temperature was fixed to be 20° C. less than the hot spot temperature.
Heat spreading device 10 has very low power consumption. The average power consumption over a droplet oscillation cycle is 4.1 μW. The high heat flux dissipation capacity, low power consumption, noiselessness and ease of integration with the heated surface are features which make electrowetting based heat spreading device 10 attractive for hot spot thermal management.
Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the spirit and scope of the invention as described and defined in the following claims.
The present application claims priority to U.S. Provisional Patent Application Ser. No. 60/747,980, entitled “ELECTROWETTING BASED HEAT SPREADER,” filed May 23, 2006, to Bahadur et al., the entire disclosure of which is expressly incorporated by reference herein.
Number | Date | Country | |
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60747980 | May 2006 | US |