Abstract:
An actively cooled system includes a heat generating device and at least one heat transfer device. The heat transfer device includes a refrigerant loop including a compressor for providing a superheated vapor state from a vapor stream, a condenser comprising a membrane coupled to an actuator, the condenser including a condensing surface for condensing the superheated vapor into a plurality of droplets, and an evaporator for receiving the droplets. An expansion structure is interposed between the condenser and the evaporator, wherein the membrane ejects the plurality of droplets toward the evaporator during refrigerant cycle intervals when the expansion structure is open. At least a portion of the heat generating device is in thermal contact with the evaporator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to U.S. patent application Ser. No. 09/874,656, entitled THERMAL MANAGEMENT DEVICE which was filed on Jun. 4, 2001 now U.S. Pat. No. 6,598,409, which itself claims priority to U.S. Provisional Patent Application No. 60/209,335, filed Jun. 2, 2000, both of which are incorporated herein in their entirety. 

   FIELD OF THE INVENTION 
   The present invention generally concerns a vapor compression refrigeration cycle thermal management devices and more particularly a modularized, high energy transfer rate, and gravity insensitive heat transfer device. 
   BACKGROUND OF THE INVENTION 
   More efficient and scalable thermal management systems are required in many applications ranging from electronics cooling to medical practice where localized cooling is needed as differentiated from macrocooling of a large environment. 
   For example, the drive for increased performance has led to smaller, faster transistors and consequently, integrated circuits with larger transistor density, higher Input/Output count, and faster clock frequency. The larger transistor density at nearly constant supply voltage and ever increasing clock frequencies has resulted in increased dynamic power dissipation. This increasing power must be dissipated by the thermal management scheme employed in the package. These trends are evidenced by the exponential increase of power density over time for state-of-the-art integrated circuits. In the latest projections of the Semiconductor Industry Association (1999), the total power dissipation is expected to push the present state-of-knowledge for thermal management. The challenge for the identification of a future thermal management technology arises from the requirement of the package to provide a robust mechanical support, a low-distortion electrical conduit for the incoming and outgoing signals, environmental protection, and thermal dissipation at low cost and high reliability. 
   Currently, several approaches to thermal management are used in production chip packages. For example, buoyancy-driven convective heat transfer from the heat sink to the ambient is employed for portable integrated circuit (IC) applications, while forced convection is used for high-performance IC applications. In the past, mainframe computers and supercomputers have employed complex and expensive closed-loop cooling systems using liquids. Most microprocessor and microelectronic systems have avoided closed-loop thermal management approaches due to their high cost, high power, high acoustic noise, and low reliability. These macro-scale techniques employing bulky refrigeration units are not compatible with many future microelectronic applications in high performance markets. 
   Efficient two-phase boiling and condensing systems capable of transferring more energy across a smaller temperature gradient can significantly help meet performance requirements for high power density and minaturized physical dimensions. Even though dropwise condensation offers heat transfer coefficients at least an order of magnitude higher than filmwise condensation, conventionally, filmwise condensation has been used in industrial condensers but not in miniaturized applications. 
   With the rapid advances in the area of micro-electro-mechanical systems (MEMS) in recent years, miniaturized devices are achieving higher energy effectiveness. Membranes are of particular interests in MEMS for their use as valves, pumps, and compressors in micro-fluidic devices. Membranes can use electrostatic, piezoelectric or thermal actuation to pressurize a fluid in a cavity. More recently, design concepts of miniaturized cooling systems have been proposed based on the refrigeration vapor-compression cycle (Shannon et al., 1999, and Ashraf et al., 1999). In particular, Shannon et al. (1999) have used an electrostatic diaphragm with valves to perform compression, whereas Ashraf et al. (1999) have used a centrifugal compressor. However both used conventional heat exchanger condenser and evaporator. The herein cyclic thermal management system is also based on the refrigeration vapor compression cycle, however possesses original components. In the herein system an actuated-membrane is adopted as the condensing surface as well as the ejecting device. Therefore the droplets ejected serve the dual purpose for maintaining dropwise condensation and creating a spray for highly efficient cooling. 
   Thus, there is a strong need for a compact, highly energy efficient device. Such a device could be connected with other similar devices to form arrays and could be incorporated in many useful devices. 
   SUMMARY OF THE INVENTION 
   These and other needs are met or exceeded by the present vapor compression cycle heat transfer device with a dropwise condenser. High efficiency cooling available in conventional large mechanical compressor vapor compression heat transfer devices is produced by the present invention in a substantially different physical embodiment similar to integrated circuit packagings, and which may be constructed using traditional and microfabrication techniques. Heating is also available from the device of the invention, since a portion of the device will expel heat into an adjacent atmosphere, fluid or object while another portion of the device will absorb heat from an adjacent atmosphere, fluid or object. Individual, self-contained devices of the invention draw little electrical power and may be interconnected with like devices to satisfy localized cooling or heating over a desired area of atmosphere, fluid or object. 
   A device of the invention includes a housing having integrated compressor, condenser, expansion, and evaporator structures, with the evaporator structure removing heat from an adjacent atmosphere, fluid or object and the condenser structure expelling heat into an adjacent atmosphere, fluid or object. The compressor structure includes a compressor body defining a compressor cavity and a flexible compressor diaphragm mounted in the compressor cavity that compresses refrigerant within the cavity and promotes circulation of the refrigerant through a closed path defined through the compressor, condenser, expansion, and evaporator structures. 
   The condenser structure is in fluid communication with the compressor structure and includes a flexible condenser diaphragm that promotes growth of a plurality of droplets to form upon a cooled condenser surface and propels the droplets from the condenser surface of the condenser diaphragm into the expansion structure. The expansion structure includes an expansion chamber in fluid communication with the condenser structure and which is in expansive receipt of the droplets propelled from the condenser diaphragm. Finally, the evaporator structure includes an evaporator chamber which is proximate a top end of the expansion chamber and which is in fluid communication with the expansion chamber and the compressor structure. 
   The device is modularized, energy efficient and gravity insensitive. It provides high cooling rates for electronic instruments, and offers a novel means for thermal management. It can also be scaled to accommodate different types of applications. 

   
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the will become apparent upon reading the following detailed description, while referring to the attached drawings, in which: 
       FIG. 1  is a schematic representation of the present invention. 
       FIG. 2A  is a schematic diagram of the vapor-compression refrigeration thermodynamic cycle followed by the present invention. 
       FIG. 2B  is a schematic temperature-entropy diagram of the ideal vapor-compression refrigeration thermodynamic cycle followed by the present invention. 
       FIG. 2C  is a schematic pressure-enthalpy diagram of the ideal vapor-compression refrigeration thermodynamic cycle followed by the present invention. 
       FIG. 3  is a schematic illustration of operation of a first embodiment of the present invention while the expansion valve is open. 
       FIG. 4  is a schematic illustration of operation of a second embodiment of the present invention showing a first and a second expansion valve. 
       FIG. 5  is a schematic illustration of operation of a third embodiment of the present invention. 
       FIG. 6  shows a detail view of an exemplified electrostatically actuated compressor diaphragm. 
       FIG. 7  shows a detail view of an exemplified electrostatically actuated compressor valve. 
       FIG. 8  shows a detail view of an exemplified piezoelectrically actuated compressor diaphragm. 
       FIG. 9  shows a detail view of an exemplified first and a second electrostatically actuated expansion valves. 
       FIG. 10  shows a system comprising an integrated circuit disposed on the evaporator of membrane actuated condenser/evaporator micro-cooling device, according to an embodiment of the invention. 
       FIG. 11  shows a personal cooling system in the form of a fabric surface having a plurality of micro-coolers attached thereto, according to another embodiment of the invention. 
       FIG. 12  shows a microcooler having a condenser positioned at the bottom and integrated with a gaseous fluid based heat sink, according to another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is more particularly described in the following examples that are intended to be illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. As used in the specification and in the claims, the singular form “a,” “an,” and “the” may include plural referents unless the context clearly dictates otherwise. 
   Referring now is specific detail to the drawings in which like reference numerals designate like or equivalent elements throughout the several views, and initially to  FIG. 1 , the present invention is directed to a compact, integrated and self-contained vapor compression cycle heat transfer device  2 . The device  2  of the invention includes a housing  6  having integrated compressor  10 , condenser  20 , expansion  30 , and evaporator  40  structures, with the evaporator  40  structure removing heat from an adjacent atmosphere, fluid or object and the condenser  20  structure expelling heat into an adjacent atmosphere, fluid or object. 
   Now referring to  FIG. 3 , generally, the compressor  10  structure includes a compressor body  12  defining a compressor cavity  14  and a flexible compressor diaphragm (or membrane)  16  mounted in the compressor cavity  14  that compresses refrigerant within a compressor portion  21  of the cavity  14  and promotes circulation of the refrigerant through a closed path defined through the compressor  10 , condenser  20 , expansion  30 , and evaporator  40  structures. The condenser  20  structure includes a flexible condenser diaphragm (or membrane)  22  in fluid communication with the compressor portion  21  of the compressor  10  structure. The condenser diaphragm  22  includes a cooled condenser surface  24  that promotes growth of a plurality of refrigerant droplets thereon and propels the droplets from the condenser surface  24  of the condenser diaphragm  22  into the expansion  30  structure. The expansion  30  structure includes an expansion chamber  32  in fluid communication with the condenser  20  structure and which is in expansive receipt of the droplets propelled from the condenser diaphragm  22 . Finally, the evaporator  40  structure includes an evaporator chamber  42  which is proximate a top end  34  of the expansion chamber  32  and which is in fluid communication with the expansion chamber  32  and the compressor portion  21  of the compressor  10  structure. As one will appreciate, the localized cooling and heating effect of the device  2  may be expanded via interconnection with like devices  2 . 
   The heat transfer device  2  of the present invention follows a vapor-compression refrigeration cycle. As illustrated in  FIGS. 2A-2C , the ideal vapor-compression refrigeration cycle includes four processes:
         1-2: Isentropic compression in a compressor;   2-3: Constant pressure heat rejection in a condenser;   3-4: Throttling in an expansion device; and   4-1: Constant pressure heat absorption in an evaporator.
 
The coefficient of performance denoted COP is defined as the ratio of the cooling load (Q L ) to the work output (W in ):
   COP=Q L /W in .
 
The preferred embodiment is designed to produce 2-6 Watts cooling capacity while operating between 20 deg. C. and 50 deg. C. At those conditions its actual coefficient of performance (COP) will be equal to the product of the ideal COP, the isentropic efficiency of the actual compressor, and any irreversibilities due to heat transfer, which is not precisely determined but is expected to be approximately 0.8. Thus, final COP of the device  2  is predicted to be in the approximate range of 4 to 7 COP. The robustness of the device  2  will permit operation over a wide range of conditions. As an example, its efficiency should be comparable at both 10 deg. C. and 40 deg. C., while pressure and flow rates would be correspondingly lower at the lower temperature. Power consumption should be in the range of about 0.4 to 0.9 W, while weight of an individual mass produced unit should be about 20-100 grams. Obviously, this opens a broad range of applications for the device  2  of the invention due to the small size, efficient cooling, and small power demand of the unit.
       

   The small charge required by an individual device  2  also permits refrigerants which might not otherwise be considered in conventional units from being utilized since there are fewer toxicity and flammability concerns when used in a small individual device  2 . Since the refrigerant charge of each device  2  is individual and self-contained, this concern also does not arise when many individual device  2   s  of the invention are operationally combined in an array. FC-72 is a preferred refrigerant pursuant to experiments conducted to date, but others are suitable. FC-72 is highly dielectric and provides an excellent insulating fluid for interfacing with electrical device  2   s . Generally, preferred refrigerants will require low pressure lifts in the compressor, while exhibiting good thermodynamic properties. Example potential candidates include R12, R13, R13B1, R14, R21, R23, R115, R123a, R124, R134a, R141b, R142b, R143, R152a, R218, RC270, RC318, R227ea, R236ea, R245cb, R600, pentane [n-pentane], 2-methyl butane [iso-pentane], R744, RE134, RE245, RE245ca, R236fa, R1270, R116, RE1170. However, high volumetric flow refrigerants, such as water, are also suitable refrigerants. 
   Referring now to  FIG. 3 , a first embodiment of a heat transfer device  2  in accordance with the present invention is shown. The device  2  includes a generally layered structure, including a condenser layer  20 ′ (including a flexible condenser diaphragm  22 ), a compressor layer  10 ′ (including a body  12  and a flexible compressor diaphragm  16 ), an expansion layer  30 ′ (including an expansion chamber  32 ), and an evaporator layer  40 ′(including an evaporator chamber  42 ). As noted above, operation of the device  2  is through a general vapor compression cycle with the compressor diaphragm  16  being electrically stimulated to compress refrigerant and drive refrigerant through the closed path defined within the device  2  which also includes an inlet conduit  50  and an outlet conduit  60 . The condenser diaphragm  22  is electrically stimulated to propel refrigerant that has condensed on a condenser surface  24  of the condenser diaphragm  22  into the expansion chamber  32 . Refrigerant circulates, heat is dispelled into the atmosphere by the condenser  20  on a portion of the device  2 , and is absorbed from the atmosphere by the evaporator  40  on a top side  8  of the housing  6  of the device  2 . The novel structure of the invention provides a compact, integrated, self-contained and generally modular device  2 . 
   As shown in  FIGS. 3 and 6 , an operational cycle of the device  2  starts with the refrigerant vapor being compressed by the compressor  10 . The compression causes an increase in temperature of refrigerant fluid within the compressor cavity  14 . The compressor has a body  12  that defines the compressor cavity  14 . The body  12  may be typically formed from two connected compressor members  12   a ,  12   b . The body  12  also mounts an electrically grounded compressor diaphragm  16  such that the diaphragm  16  is capable of movement within the compressor cavity  14 . A voltage applied between a pair of opposing capacitive electrical contacts  11  disposed on opposing surfaces  13 ′,  13 ″ of the compressor cavity  14  creates a capacitive force between the conductive planes defined by the compressor cavity  14  and the flexible compressor diaphragm  16 . In the illustrated embodiments, both surfaces of the compressor diaphragm  16  are conductive, while opposing portions of the outer surface  13 ′,  13 ″ of the compressor cavity  14  are conductive. Alternatives include having only one electrical contact on the outer surface  13  of the compressor cavity  14  and/or having only one conductive surface on the compressor diaphragm  16 . 
   In the preferred embodiment, the opposing capacitive compressor electrical contacts II include an upper compressor electrode  15  and a lower compressor electrode  17 . The compressor diaphragm  16  is adapted to selectively deflect toward the upper and lower compressor electrodes  15 ,  17 . The upper and lower compressor electrodes  15 ,  17  are generally integral with opposing portions of the body  12  to form an upper electrode surface  18  and a lower electrode surface  19  in the compressor cavity  14 . The compressor diaphragm  16  conforms to the respective electrode surfaces  18 ,  19  when it is electrostatically driven to one or the other surface through application of a voltage to the particular electrode  15 ,  17  via a voltage source for the upper compressor electrode  15  and a voltage source for the lower compressor electrode  17 . The compressor diaphragm  16  and the upper and lower electrode surfaces  18 ,  19  may be coated with thin dielectric layers (not shown) for electrical insulation and protection. 
   In operation, the compressor diaphragm  16  is movable from a first relaxed position, to a second intake position, and to a third compressed position. In the first position, the upper and lower compressor electrodes  15 ,  17  are deactivated and the compressor diaphragm  16  is in its original, relaxed, position. In the second position, the upper and lower electrodes  15 ,  17  are activated with the appropriate polarization to move the compressor diaphragm  16  toward the upper compressor electrode  15  (toward the upper electrode surface  18 ) to maximize the available volume within the compressor portion  21  of the compressor cavity  14 . Finally, in the third position, the upper and lower compressor electrodes  15 ,  17  are activated to move the compressor diaphragm  16  toward the lower compressor electrode  17  (toward the lower electrode surface  19 ) to minimize the volume within the compressor portion  21  of the compressor cavity  14 . The movement of the compressor diaphragm  16  from the second position to the third position compresses and transforms the refrigerant into a superheated vapor. It is contemplated that the condenser diaphragm  16  will be actuated once per refrigeration cycle. 
   In the present invention, the compressor diaphragm  16  may be stretched or tensile loaded, however, it is preferred that the compressor diaphragm  16  be formed in a prebuckled shape, so that, in the first position, when the compressor diaphragm  16  is in the interim position between the upper and lower compressor electrodes  15 ,  17 , the buckles compress and the shape of the compressor diaphragm  16  is somewhat irregular. Upon movement toward the upper or lower electrode surface  18 ,  19 , the buckled diaphragm  16  straightens out to form a smooth, uniform surface that may fully engage the respective electrode surface  18 ,  19 . Buckled diaphragms have a larger volume per stroke that can be obtained with reduced actuation force when compared to stretched or tensile loaded diaphragms. Additionally, buckled diaphragms are almost stress free in both the second and third positions which results in a system that is less sensitive to temperature variations. 
   The body  12  may be constructed of, for example, connected layers of silicon or by molding a high temperature plastic such as ULTEM®, (registered trademark of General Electric Company, Pittsfield, Mass.), CELAZOLE®, (registered trademark of Hoechst-Celanese Corporation, Summit, N.J.), or KETRON®, (registered trademark of Polymer Corporation, Reading, Pa.). The upper and lower electrodes  15 ,  17  themselves can be formed via common manufacturing methodologies such as, for example, printing, plating, sputtering, or EB deposition of metal followed by patterning by using dry film resist, as is known in the art. Low temperature organic and inorganic dielectric may also be used as an insulator between the actuating electrodes  15 ,  17 . 
   The compressor diaphragm  16  may be made from metal coated polymers such as, for example, KAPTON® (registered trademark of E. I. du Pont de Nemours &amp; Co., Wilmington, Del.), KALADEX®. (registered trademark of ICI Films, Wilmington, Del.) and MYLAR® (registered trademark of E. I. du Pont de Nemours &amp; Co., Wilmington, Del.), metal, or a conductive flexibly elastic polymer that permits it to conform its surface area to the curved surfaces. Both metal and elastic polymer diaphragms can be flat or buckled. Typically, the polymeric material have elastomeric properties sufficient to permit movement between said curved surfaces. For example, fabrication of the diaphragm  16  is based upon technology developed for keyboard and flexible circuits that are produced in huge quantities making the fabrication process well optimized. Preferred diaphragms  16  are made from polymer films such as KAPTON® or MYLAR® (registered trademark of E. I. du Pont de Nemours &amp; Co., Wilmington, Del.), or different polyesters that are commercially available. 
   As noted above, the closed loop formed within the device  2  also includes an inlet conduit  50  and an outlet conduit  60 . Both the inlet and outlet conduits  50 ,  60  are microchannels that are in fluid communication with the compressor portion  21  of the compressor cavity  14 . The inlet conduit  50  is also in fluid communication with the evaporator chamber  42 . Further, the outlet conduit  60  is in fluid communication with the condenser diaphragm  22  of the condenser  20 . Preferably, to enhance the efficiency of the compression stroke of the compressor  10 , each conduit  50 ,  60  also contains a compressor valve  70  for controlling the flow of refrigerant into and out of the compressor portion  21  of the compressor cavity  14 . The compressor valves  70  may be pressure actuated flapper valves, as known in the art, that open and close automatically due to the pumping action of the compressor  10 . It is preferred however, to electrostatically actuate the compressor valves  70  to reduce leakage or back-pressure losses. 
   As shown in  FIGS. 3 and 7 , the inlet conduit  50  has an inlet wall surface  52  and the device  2  has a first compressor valve  70 ′ disposed within the inlet conduit  50 . Preferably, the first compressor valve  70 ′ is a hinged tab  72 ′ that has a scaling edge portion  74 ′ that is complementarily shaped to a first portion  54  of the inlet wall surface  52 . The tab  72  is hinged proximate a second portion  56  of the inlet wall surface  52 . In similar fashion, the outlet conduit  60  has an outlet wall surface  62  and the device  2  has a second compressor valve  70 ″ disposed within the outlet conduit  60 . The second compressor valve  70 ″ is a hinged tab  72 ″ which has a sealing edge portion  74 ″ that is complementarily shaped to a first portion  64  of the outlet wall surface  62 . The tab  72 ″ is hinged proximate a second portion  66  of the outlet wall surface  62 . Thus, in operation, the first compressor valve  70 ′ and the inlet wall surface  52  and the second compressor valve  70 ″ and the outlet wall surface  62  form sealable contacts when the respective sealing edge portions  74 ′,  74 ″ of the compressor valves  70 ′,  70 ″ are placed into contact with the respective first portions  54 ,  64  of the inlet and outlet wall surfaces  52 ,  62  to prevent refrigerant flow through the inlet conduit  50  and/or the outlet conduit  60 . 
   More particularly, the first compressor valve  70 ′ is movable from a closed position to an open position. In the closed position, the sealing edge  74 ′ of the first compressor valve  70 ′ is sealed to the first portion  54  of the inlet wall surface  52  to prevent flow of fluid from the evaporator chamber  42  via the inlet conduit  50  into the compressor portion  21  of the compressor cavity  14 . In the open position, the sealing edge  74 ′ of the first compressor valve  70 ′ is drawn away from the first portion  54  of the inlet wall surface  52  toward the second portion  56  of the inlet wall surface  52  (which, as noted above, is proximate the hinge of the first compressor valve  70 ′). As one will appreciate, as the sealing edge  74 ′ of the first compressor valve  70 ′ is drawn away from the first portion  54  of the inlet wall surface towards the second portion of the inlet wall surface, the inlet conduit  50  is opened which allows flow of refrigerant through the inlet conduit  50  and into the compressor portion  21  of the compressor cavity  14 . 
   Similarly, the second compressor valve  70 ″ is movable from a closed position to an open position. In the closed position, the sealing edge  74 ″ of the second compressor valve  70 ″ is sealed to the first portion  64  of the outlet wall surface  62  to prevent flow of fluid through the outlet conduit  60  and into the condenser  20 . In the open position, the sealing edge  74 ″ of the second compressor valve  70 ″ is drawn away from the first portion  64  of the outlet wall surface  62  toward the second portion  66  of the outlet wall surface  62  (which, as noted above, is proximate the hinge of the second compressor valve  70 ″). As the sealing edge  74 ″ of the second compressor valve  70 ″ is drawn away from the first portion  64  of the outlet wall surface  62  towards the second portion  66  of the outlet wall surface  62 , the outlet conduit  60  is opened which allows flow of refrigerant from the compressor portion  21  of the compressor cavity  14 , through the outlet conduit  60  and into the condenser  20 . 
   As noted above, the first and second compressor valves  70 ′,  70 ″ may comprise a hinged tab  72 ′,  72 ″ movable toward and away from the respective first and second portions  54 ,  64 ,  56 ,  66 , of the respective inlet and outlet wall surfaces  52 ,  62 . The tab  72 ′,  72 ″ is fixed at one end (i.e., hinged) in cantilever fashion with respect to the respective inlet and outlet wall surfaces  52 ,  62 . The tab  72 ′,  72 ″ may be substantially rigid or, preferably, may be flexible. If the tab  72 ′,  72 ″ is flexible, it is preferred that the tab  72 ′,  72 ″ be formed from a polymeric material that has elastomeric properties. The movable tab  72 ′,  72 ″ may be formed through techniques known in the art, such as selective etching of a silicon member and/or the selective bonding of a polymer flap. 
   The first and second compressor valves  70 ′,  70 ″ are preferably electrostatically controlled. Electrostatically controlled valves are well known in the art. Referring generally to  FIG. 8 , in this electrostatically controlled embodiment, each compressor valve  70 ′,  70 ″ further includes two opposing capacitive compressor valve electrical contacts  75 ,  76  on the tab  72 ′,  72 ″ and proximate the second portions  56 ,  66  of the respective inlet and outlet wall surfaces  52 ,  62 . To utilize the capacitive action for the compressor valves  70 ′,  70 ″, the area about the second portions  56 ,  66  of the inlet and outlet wall surfaces  52 ,  62  must be made conductive, with a dielectric above the compressor valve electrical contact  76  within the inlet and outlet wall surfaces  52 ,  62 . Similarly, the tab  72 ′,  72 ″ should have a conductive plane to mate with the conductive portion of the inlet and outlet wall surfaces  52 ,  62 . 
   Preferably, the opposing capacitive compressor valve electrical contacts  75 ,  76  include a movable first compressor valve electrode  77  capsulated within the tab  72 ′,  72 ″ and a fixed second compressor valve electrode  78  integral to and proximate the dielectric second portions  56 ,  66  of the respective inlet and outlet wall surfaces  52 ,  62 . As one will appreciate, the basic operation of the tab  72 ′,  72 ″ is simple; a voltage applied between the two compressor valve electrodes  77 ,  78  establishes an electrical attraction/repulsion. Operationally, the first and second compressor valve electrodes  77 ,  78  are selectively energized so that the tab  72 ′,  72 ″ is electrostatically positioned in the open or closed position. Normally, each tab  72 ′,  72 ″(and thus each compressor valve  70 ′,  70 ″) is in the closed position. Power is supplied to the respective opposing compressor valve electrical contacts  75 ,  76  to provide potentials of opposite polarity in the first and second compressor valve electrodes  77 ,  78 . This tends to draw the first and second compressor valve electrodes  77 ,  78  toward one another, eventually moving the tab  72 ′,  72 ″ into a complete open state. When power is supplied to the respective opposing compressor valve electrical contacts  77 ,  78  to provide potentials of identical polarity in the first and second compressor valve electrodes  77 ,  78 , the compressor valve electrodes  77 ,  78  are forced away from one another, thus forcing the tab  72 ′,  72 ″ into the closure position. 
   In the preferred flexible embodiment, the tab  72 ′,  72 ″ returns to the closed position under an internal, elastic force upon application of equal potential to the respective compressor valve electrodes  77 ,  78  or the shorting of the compressor valve electrical contacts  75 ,  76 . Thus, upon removal of the applied voltage, the inherent stress within the flexible tab  72 ′,  72 ″ curls the tab  72 ′,  72 ″ back into its original, closed, position. 
   Techniques for fabricating such an electrostatically driven tab  72 ′,  72 ″ are known in the art. In one example, the technique uses process and material used in the fabrication of VLSI integrated circuits. In this example five photolithographic steps are used to form the electrostatically actuated tab  72 ′,  72 ″, which, in this example, is flexible. Beginning with a silicon substrate with a polyimide insulating film, a Cr/Au/Cr metal film is deposited and pattered to form the second compressor valve electrode  78 . A polyimide film is then deposited, to insulate the second compressor valve electrode  78  from the environment. A release film of PECVD oxide is then deposited and patterned. This film is wet etched away at the end of the process to free the flexible films from the substrate. Another polyimide film is deposited to protect the bottom of the first flexible compressor valve electrode  77  from the environment and to prevent charges from being transferred from the first compressor valve electrode  77  to the second compressor valve electrode  77  in the second portion  56 ,  66  of the wall surface  52 ,  62  of the respective inlet and outlet conduit  50 ,  60 . This film is patterned to form vias between the flexible first compressor valve electrode  77  and the second compressor valve electrode  78  for ease of wiring the device  2 . Then a second Cr/Au/Cr metal film is deposited and patterned to form the first compressor valve electrode  77 . A final polyimide film is deposited and patterned to define the size and shape of the tab  72 ′,  72 ″ as well as to protect the top surface of the first compressor valve electrode  77  from the environment. This top film may be thicker than the bottom dielectric film in order to create stress in the tab  72 ′,  72 ″ which will cause the tab  72 ′,  72 ″ to reflexively curl away to the closed position in the respective inlet and outlet conduits  50 ,  60  when voltage is removed from the compressor valve electrodes  77 ,  78 . The final step is to etch away the PECVD oxide with HF, which releases the flexible tab  72 ′,  72 ″ from the substrate. The fabrication steps used in this exemplified construction can be done with conventional, prior generation VLSI equipment, including contact photolithography. Further, the substrate may be, for example, silicon, metal, plastic, glass, or like materials. 
   In operation, when the compression process is complete, i.e., the compressor diaphragm  16  is in the third position, the second compressor valve  70 ″ is selectively opened to let the superheated refrigerant vapor to flow through the outlet conduit  60  to the condenser  20 . The first compressor valve  70 ′ remains closed to reduce back-pressure losses. When all of the compressed superheated refrigerant has escaped to the condenser  20 , the second compressor valve  70 ″ is selected closed and will remain closed through out the remainder of the refrigeration cycle. The first compressor valve  70 ′ is selected open to allow vaporized fluid from the evaporator chamber  42  to be drawn into the compressor  10 . 
   Referring now to  FIGS. 3 and 8 , the drop-wise condenser  20  has a flexible condenser diaphragm  22  which is in fluid communication with compressed superheated refrigerant escaping the outlet conduit  60 . The flexible condenser diaphragm  22  has a condenser surface  24  which may be covered with a thin film of hydrophobic material (not shown) to promote dropwise condensation thereon the condenser surface  24 . The temperature of the condenser surface  24  is maintained at a generally constant temperature lower than the temperature of the superheated refrigerant vapor introduced via the outlet conduit  60 . The temperature difference may, for example, be approximately 1° C. to 7° C., and preferably, may be approximately 2° C. to 5° C. For example, the temperature of the saturated vapor exiting the compressor  10  may be approximately 50° C. and the temperature of the condenser surface  24  may be approximately 53° C., for a temperature difference of approximately 3° C. Once the droplets have grown to a desired size, the condenser diaphragm  22  is actuated to eject or propel the condensed droplets away from the condenser surface  24  of the condenser diaphragm  22 . As one will appreciate, the condenser diaphragm  22  will be actuated consistent with the rate of condensation of the selected refrigerant. In addition, consistent with the compressor diaphragm  16 , it is contemplated that the condenser diaphragm  22  will be actuated once per refrigeration cycle. Alternatively, it is contemplated that the condenser diaphragm  22  may be actuated at a predetermined frequency throughout the refrigeration cycle. 
   The condenser diaphragm  22  is connected to an electromechanical actuator  25 . A broad range of electromechanical actuators  25  which may be used with the present invention will be apparent to those skilled in the art and may utilize, for example, electrostatic, electromagnetic, piezoelectric, or magnetostrictive principles. However, preferably, the electromechanical actuator  25  is a piezoelectric actuator  25  which is operated by an electrical signal to its conductive condenser electrical contact  26 . 
   In the exemplified embodiment, the condenser surface  24  of the condenser diaphragm  22  forms a portion of a substantially flat bottom end  36  of the expansion chamber  32 . In a recess  38  defined within the bottom end  36 , a thin film  27  of a piezoelectric material, forming the piezoelectric actuator  25 , is seated therein and is in contact with the condenser electrical contact  26 . A base surface  23  of the condenser diaphragm  22 , which opposes the condenser surface  24  of the condenser diaphragm  22 , is connected to the piezoelectric actuator  25 . The condenser diaphragm  22  is connected to the edge area of the recess  38  so that the condenser surface  24  of the condenser diaphragm  22  is substantially planar to the bottom end  36  of the expansion chamber  32  when the condenser diaphragm  22  is in a first, unenergized, position. In this first position, the substantially planar condition of the condenser diaphragm  22  allows for the condensation of droplets on the condenser surface  24 . As one will appreciate, upon application of a pulse voltage to the condenser electrical contact  26 , the piezoelectric material  25  is actuated which forces the condenser diaphragm  22  to bow outward relative to the base surface  23  to a second position with sufficient force so that the condensed droplets of refrigerant are propelled from the condenser surface  24  toward the top end  34  of the expansion chamber  32 . 
   The condenser further includes a heat exchanger means for cooling the condenser surface  24  of the condenser diaphragm  22 . The heat exchanger means may comprise a heat-rejecting heat exchanger  90  that proximally bounds the condenser surface  24 . More particularly, the heat exchanger means may include a heat exchanger  90 , a fluid microchannel  92 , and a fluid pump  94 . The heat exchanger  90  may, for example, include a finned heat exchanger, such as known in the art, that is disposed on an exterior surface  9  of the housing  6  to reject heat to the surrounding atmosphere. The fluid microchannel  92  defines at least one closed flow path between the heat exchanger  90  and proximate the base surface  23  of the condenser  20 . The fluid pump  94  is disposed in the fluid channel  92  so that fluid, such as a refrigerant, is circulated therethrough the fluid channel  92 . The fluid pump  94  allows fluid that has been conductively heated by the condenser surface  24  to be drawn through the heat exchanger  90  where excess heat from the circulating fluid is rejected to the atmosphere to cool the fluid. The cooled fluid is drawn back through the microchannel  92  proximate the condenser surface  24  to cool and maintain the condenser surface  24  at the generally constant temperature. A broad range of fluid pumps which may be used with the present invention will be apparent to those skilled in the art and may utilize, for example, electrostatic, electromagnetic, piezoelectric, or magnetostrictive principles. However, preferably, the fluid pump  94  is a piezoelectric fluid pump such as, for example, the micropump disclosed in U.S. Pat. No. 5,876,187 to Forster et al., which in incorporated herein in its entirety. Alternatively, the fluid pump  94  may be an electrostatically driven diaphragm pump as described above in respect to the compressor  10 . 
   As one would appreciate, the expansion chamber  32  is in fluid communication with the condenser. The expansion chamber  32  has a wall surface  33  extending between the top end  34  and the bottom end  36  of the expansion chamber  32 . The wall surface  33  defines a first orifice  35  proximate the bottom end  36  of the expansion chamber  32  that serves as the outlet for the outlet conduit  60 . Further, the wall surface  33  defines a second orifice  37  proximate the top end  34  of the expansion chamber  32  that serves as the inlet for the inlet conduit  50 . As the refrigerant passes through the expansion chamber  32 , the temperature of the refrigerant undergoes a sudden drop. For example, the temperature may drop approximately 15° C. to 50° C., and, more preferably, approximately 20° C. to 40° C. Referring to  FIG. 3 , at least a portion of the wall surface  33  proximate the top end  34  of the expansion chamber  32  extends outwardly away from a longitudinal axis L of the expansion chamber  32 . The top end  34  of the expansion chamber  32  has a first width that is greater than a second width of width of the expansion chamber  32  taken proximate the bottom end  36 . Thus, the cross-sectional area of the expansion chamber  32  increases as the droplets pass from the bottom end  36  to the top end  34  of the expansion chamber  32  and into the evaporator chamber  42 . 
   The refrigeration cycle then completes when the cooled refrigerant absorbs heat from the atmosphere or object proximate the device  2  in the evaporator  40 . The evaporator chamber  42  has a conductive member  44  that may be placed into contact with a heat generating object for which cooling is desired. In the preferred embodiment the conductive member  44  forms at least a portion of the top side  8  of the housing  6  of the device  2 . The evaporator chamber  42  is proximate the top end  34  of the expansion chamber  32  and is fluid communication with the expansion chamber  32 . The conductive member  44  has an evaporation surface  46  upon which the cooled droplets impinge after passing though the top end  34  of the expansion chamber  32 . The evaporation surface  46  may be coated with a thin film of metal (not shown) to insure that the refrigerant wets the evaporation surface  46  to provide a large heat transfer rate. 
   As noted, the impinged droplets provide cooling by evaporation. As the refrigerant evaporates it is returned to the compressor cavity  14  via the inlet conduit  50 . As noted above, the first compressor valve  70 ′ opens (while the second compressor valve  70 ″ remains closed) to allow the vaporized refrigerant to pass into the compressor portion  21  of the compressor  10 . 
   Referring now to  FIGS. 4 and 9 , a second embodiment of the device  2  is shown. The construction of the second embodiment of the device  2  is similar to the first embodiment of the device  2  and, accordingly, the figures use the same reference numbers for similar components. Furthermore, the components in  FIGS. 1-4  and  5 - 9  that use the same reference numbers are substantially equivalent and the description thereof is omitted for the second embodiment. In this embodiment, at least one expansion valve  100  is connected to the wall surface  33  of the expansion chamber  32  intermediate the top end  34  and the bottom end  36  of the expansion chamber  32 . The expansion valve  100  is moveable from a closed position, in which a cavity  10  bounding the condenser diaphragm  22  is defined by a portion of the wall surface  33  of the expansion chamber  32 , the expansion valve  100 , and the bottom end  36  of the expansion chamber  32  (which includes the condenser diaphragm  22 ), to an open position, in coordinated response to the activation of the condenser diaphragm  22 , to allow droplets propelled from the condenser diaphragm  22  to pass though the expansion chamber  32  and into the evaporator chamber  42 . After the droplets have passed the expansion valve  100 , the expansion valve  100  returns to the closed position. In this embodiment, the cross-sectional area of the expansion chamber  32  may increase, or preferably, may be substantially constant from the bottom end  36  through the top end  34  of the expansion chamber  32 . 
   While one expansion valve  100  may be used, it is preferred that a first expansion valve  100 ′ and an opposing second expansion valve  100 ″ be provided. Each of the first and second expansion valves  100 ′,  100 ″ generally is a tab  102 ′,  102 ″, having a distal end  104 ′,  104 ″, that is moveable toward and away from the wall surface  33  of the expansion chamber  32 . More particularly, the first and second expansion valves  100 ′,  100 ″ are moveable from a closed position, in which the distal ends  104 ′,  104 ″ of the first and second expansion valves  100 ′,  100 ″ are sealed to one another to define the cavity  110  bounding the condenser diaphragm  22 , to an open position, in which the distal ends  104 ′,  104 ″ of the first and second expansion valves  100 ′,  100 ″ are drawn toward opposing portions  33 ′,  33 ″ of the wall surface  33  of the expansion chamber  32  so that the condensed droplets may flow through the expansion chamber  32 . 
   A wide variety of applications for the invention are possible because of the improved performance provided by devices according to the invention as compared to available cooling devices. Devices according to the invention can provide ultra-high heat flux (10 3 -10 5  W/Cm 2 ) and extremely low evaporator temperature (e.g. −40 C) because of the refrigeration cycle utilized. The heat flux level provided by the invention is believed to be approaching the maximum theoretical thermodynamic limit of heat transfer. The low evaporator temperature will mitigate the stringent upper temperature limits of device component materials. 
   In the embodiment shown in  FIG. 10 , system  1000  includes an integrated circuit  1010 , such as a microprocessor, disposed on the evaporation surface  1020  upon which the cooled droplets of membrane actuated condenser/evaporator micro-cooling device  1050  impinge. In the embodiment shown, integrated circuit  1010  and micro-cooler  1050  are separate components. The respective components of micro-cooling device  1050  can be analogous to heat transfer device  2  shown in FIG.  3 . Although a package encapsulating system  1000  is not shown, in many applications system  1000  comprising integrated circuit  1010  disposed on micro-cooler  1050  will be disposed within a package, such as a plastic package. 
   The high cooling level provided by micro-cooling device  1050  allows replacement of conventional heat sinks which generally comprise a solid metal slab with the integrated circuit placed on one surface of the slab and fins on the other surface of the slab to increase heat transfer. Moreover, the high heat flux provided by invention can permit integrated circuit  1010  to run at higher operating currents than are otherwise possible to achieve higher speeds, without impermissibly raising the junction temperature of the integrated circuit  1010 . 
   System  1000  can be fully integrated where the micro-cooler  1050  is fabricated on the same substrate (e.g. Si) as integrated circuit  1010 . In this embodiment, integrated circuit  1010  also utilizes evaporation surface  1020  of micro-cooler  1050  as a portion thereof. For the fully integrated embodiment of system  1000 , refrigerant for micro-cooler  1050  is generally filled prior to sealing the device. 
   The invention can be used for cooling high heat flux applications (10 2 -10 3  W/cm 2 ), such as generated by electronic components and related systems. For example, the invention can be used to provide high heat flux localized cooling for on board avionics, supercomputers, desktops, laptops, digital assistants, and cell phones. 
   The invention can also be used for local cooling of hot spots in a variety of macro devices and systems. For example, particle accelerators, turbine blades, laser weapons, radar systems, and rocket nozzles can utilize the invention. In macro applications, an array comprising a plurality of micro-coolers is generally utilized. This arrangement offers flexibility and increased reliability as the failure of a single micro-cooler in the array will not significantly diminish the overall cooling provided by array. 
   As shown in  FIG. 11 , the array concept is shown with reference to a personal cooling system  1100  comprising a plurality of micro-coolers  1110  attached to a fabric surface  1120 , according to another embodiment of the invention. In this embodiment, the evaporator surface (not shown) of micro-coolers  1110  are disposed on fabric surface  1120 , such as secured with a thermally conductive glue or tape. The fabric should be thin and strong to minimize the heat transfer resistance. It also should be selected to provide a reasonably high thermal conductivity, at least in regions where micro-coolers  1110  are disposed. Fabric can include a plurality of intermixed thermally conductive particles, such as where micro-coolers are located, to increase thermal conductivity thereof. Personal cooling system can be used for soldiers and for other outdoor activities. In an analogous arrangement, the invention can also be used with refrigeration bags. 
   The system can be used as a heating unit by attaching one or more microheaters to micro-scale heating bags or heating blankets for biomedical fluids and organs by reversing the refrigerant flow as in a conventional heat pump. In this mode, compressed refrigerant vapor is directed first to the surface to be heated. 
   The invention can be used as device which provides both heating and cooling. To permit cycling of the device between a refrigeration cycle to a heating cycle, analogous to a conventional heat pump, a controller and a reversing valve can be added to reverse the direction of the cycle when directed to by the controller, such as based on predetermined temperature limits. 
     FIG. 12  shows a micro-cooler system  1200  comprising a micro-cooler  1250  and a gaseous working fluid  1218  based heat sink  1225 , according to another embodiment of the invention. Gaseous working fluid based heat sink  1225  helps extend the life of micro-cooler  1250  as compared to liquid based heat sinks, since the gaseous working fluid  1218  minimizes induced stress and wear on the condenser diaphragm  1210  (e.g. a piezoelectric) when vibrating. The gaseous working fluid can comprise air, or more preferably gases having higher thermal conductivities, such as N 2  or H 2 . The gaseous working fluid can be held at a vacuum level to optimize heat transfer. 
   Micro-cooler  1250  is built from a silicon or other similar substrate  1235  and includes compressor  1221  and evaporator  1230 . Condenser diaphragm  1210  of micro-cooler  1250  is positioned at the bottom of FIG.  12  and is and integrated with a gold layer  1220  of heat sink  1225 . Device  1250  includes thermally insulating coating layer  1260 . Heat sink  1225  includes membranes  1228  and  1229 . Membranes  1228  and  1229  vibrate up and down and preferably operate 180 degrees out of phase to create an oscillatory motion of the gaseous working fluid  118  as shown in FIG.  12 . 
   Heat sink  1225  provides convective cooling by oscillatory gaseous motion of the working fluid  1218  combined with micro fins  1215  and conventional fins  1238  which surround the volume encapsulating working fluid  1218 . Micro fins  1215  provide a ripple interface  1241  that induces microcirculation of the working fluid  1218  to enhance heat transfer. This arrangement provides an enhanced rate of cooling for the condenser  1210  and micro-cooler  1250 . 
   In operation of system  1200 , vapor within the micro-cooler  1250  refrigeration loop condenses at condenser diaphragm  1210  having a temperature T Vapor  and pressure P Vapor . Gaseous working fluid  1218  in heat sink  1225  is at P Coolant  and T Coolant , where T Coolant  is significantly below T Vapor , such as 30 C Heat is transferred from condenser diaphragm  1210  to gold film  1220  and then to working fluid  1218  and out to an ambient surrounding fins  1215  and  1238 . 
   It is preferred that the each expansion valve  100 ′,  100 ″ be an electrostatically drive valve similar in operation and construction to the electrostatically driven first and second compressor valves  70 ′,  70 ″ discussed above. In this preferred embodiment, each expansion valve  100 ′,  100 ″ includes opposing capacitive expansion valve electrical contacts  105 ,  106  on the tab  102 ′,  102 ″ and a portion of the wall surface  33 ′,  33 ″ of the expansion chamber  32  which are adapted to selectively move one of the respective first and second expansion valves  100 ′,  100 ″. As one will appreciate, the opposing capacitive expansion valve electrical contacts  105 ,  106  for each of the first and second expansion valves  100 ′,  100 ″ comprise a first expansion valve electrode  107  encapsulated within the tab  102 ′,  102 ″ and a second expansion valve electrode  108  proximate the wall surface  33  of the expansion chamber  32 . Preferably, the second expansion valve electrode  108  is integral with the body  12 . Upon selective application of a voltage of desired polarity, the first and second expansion valve electrodes  107 ,  108  can be selectively energized so that the respective tabs  102 ′,  102 ″ are electrostatically positioned in the open or closed position. The tab  102 ′,  102 ″ may be substantially rigid, however, it is preferred that the tab  102 ′,  102 ″ is formed from a polymeric material having elastomeric properties. 
   Referring to  FIG. 5 , a third embodiment of the device  2  is shown. The construction of the third embodiment of the device  2  is similar to the first and second embodiments of the device  2  and, accordingly, the figures use the same reference numbers for similar components. Furthermore, the components in  FIGS. 1-9  that use the same reference numbers are substantially equivalent and the description thereof is omitted for the third embodiment. One skilled in the art will appreciate that the general structures of the compressor  10 , the condenser  20 , the expansion chamber  32 , and the evaporator  40  are similar to the first and second embodiments described above. However, in this exemplified embodiment, the compressor  10  and the condenser  20  are formed within the same layer. This illustrates that many permutations of the layered approach to constructing the device  2  of the invention are possible and are contemplated. 
   Electrical vias provide electrical connections with the electromechanical actuator  25  of the condenser  20 , and leads connect vias to the first and second compressor valves  70 ′,  70 ″ in the preferred electrostatically clamped compressor valve embodiment. Additional leads provide electrical connection to the compressor electrodes  15 ,  17  in the compressor cavity  14 , preferably formed as multiple separate electrodes to encourage a zip action in the compressor  10  as it compresses. Still further leads connect vias and provide electrical connection to the expansion valve  100  in the electrostatically clamped expansion valve embodiment. Solder bumps in one layer oppose vias in another layer to provide electrical connections between layers in a manner commonly used to connect printed circuit board (PCB) layers. Outside control circuitry may be used to control compressor  10 , condenser  20 , compressor valve  70 , and expansion valve  100  actions, or an on board chip may be included. 
   Efficient operation of the device  2  requires thermal isolation between hot and cool areas of the device  2 . Isolation between the condenser and the compressor may be provided by the insertion of an insulator  120  between the relative hot and cold portions of the device  2 . The insulator  120  can also serve as a portion of the electrical connection to outside power sources through electrical connection network. 
   The device  2  of the invention is fabricated according to a combination of macro and microfabrication techniques. Low end dimensions in the device  2  of the invention are realizable through microfabrication techniques, while higher dimension features may be achieved via low pressure injection molding techniques. Two construction approaches, however, are preferred (microfabrication, injection molding). The application (cross sectional area, refrigerant choice, operation pressure) may drive the final choice of fabrication methods. The preferred method of fabricating the invention employs a layered approach, or an approach similar to laminate manufacturing, in order to provide a robust method for high volume production. Individual components of the device  2  are partially or wholly fabricated in layers, and then are assembled and bonded together. Components are aligned to communicate electrically and to communicate refrigerant fluid with other components. 
   For use of the invention over long time periods, refrigerants under pressure may eventually be lost to the surroundings due to the permeability of the material and the subsequent diffusion of the high pressure gases through the polymer walls. The rate of loss varies greatly between different polymers and refrigerants. Small molecule refrigerants tend to diffuse more rapidly through solid polymers than those comprised of larger molecules. Different polymers are also more or less permeable to molecules of various chemistries. Long term loss is exacerbated since the invention employs relatively large surface areas, compared to the total amount of refrigerant charge used. A diffusion or vapor barrier comprised of a thin film of metal may be added between the layers and/or on exterior surface of the housing to reduce the potential for diffusion. If the metal vapor layer is on the surface, a thin polymer coating can be placed over it to protect it from wear. 
   Although the illustrative embodiments of the present disclosure have been described herein with reference to the accompanying drawings, it is to be understood that the disclosure is not limited to those precise embodiment, and the various other changes and modifications may be affected therein by one skilled in the art without departing from the scope of spirt of the disclosure. All such changes and modifications are intended to be included within the scope of the disclosure as defined by the appended claims.