Abstract:
The invention is for an apparatus and method for removal of waste heat at high-flux from electronic, photonic, and other components. The apparatus of the present invention is a self-contained unit comprising a closed flow loop flowing liquid metal coolant pumped by an integral magneto-hydrodynamic (MHD) pump. Liquid metal coolant flow is arranged to impinge onto a thin member mounting a heat load. Impinging flow of liquid metal coolant offers a high heat transfer coefficient, which translates to comparably low thermal resistance between the heat load and the liquid metal coolant. As a result, the apparatus may remove heat from the heat load at very high flux. Waste heat acquired from the heat load may be transferred at reduced flux into a flowing secondary coolant, heat pipe, structure, or a radiation panel. Temperature of the heat load may be varied by varying the MHD pump drive current.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. provisional patent applications U.S. Ser. No. 61/686,134, filed on Mar. 30, 2012 and entitled “Thermal Management for Solid State High-Power Electronics,” the entire contents of which are hereby expressly incorporated by reference This patent application is a continuation-in-part patent application of: U.S. Ser. No. 12/290,195 filed on Oct. 28, 2008 and entitled HEAT TRANSFER DEVICE; U.S. Ser. No. 12/584,490 filed on Sep. 5, 2009 and entitled HEAT TRANSFER DEVICE; U.S. Ser. No. 12/932,585 filed on Feb. 28, 2011 and entitled THERMAL INTERFACE DEVICE; and U.S. Ser. No. 13/385,317 filed on Feb. 13, 2012 and entitled THERMAL MANAGEMENT FOR SOLID STATE HIGH-POWER ELECTRONICS the entire contents of all of which are hereby expressly incorporated by reference. 
     
    
     GOVERNMENT RIGHTS NOTICE 
       [0002]    This invention was made with Government support under Contract No. FA9453-10-C-0061 awarded by the U.S. Air Force. The Government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    This invention relates generally to a removal of heat from a heat-generating component and more specifically to a removal of heat at high flux. 
       BACKGROUND OF THE INVENTION 
       [0004]    The subject invention is an apparatus and a method for removal of waste heat from heat-generating components including analog solid-state electronics, digital solid-state electronics, semiconductor laser diodes, light emitting diodes, photovoltaic cells, vacuum electronics, and solid-state laser crystals. 
         [0005]    There are many devices generating waste heat as a byproduct of their normal operations. These include analog solid-state electronic components, digital solid-state electronic components, semiconductor laser diodes, light emitting diodes for solid-state lighting, solid-state laser components, laser crystals, vacuum electronic components, and photovoltaic cells. Waste heat must be efficiently removed from such components to prevent overheating and consequential loss of efficiency, malfunction, or even catastrophic failure. Methods for waste heat management may include conductive heat transfer, convective heat transfer, and radiative heat transfer, or various combinations thereof. For example, waste heat removed from heat generating components may be transferred to a heat sink by a flowing heat transfer fluid. Such a heat transfer fluid is also known as a coolant. 
         [0006]    Meeting the cooling requirements for the new generation of heat-generating components (HGC) is very challenging for the thermal management technologies of prior art. For example, an ongoing miniaturization of semiconductor digital and analog electronic devices requires removal of heat at ever increasing fluxes now approaching the order of several hundreds of watts per square centimeter. Traditional heat sinks and heat spreaders have a large thermal resistance, which contributes to elevated junction temperatures and thus reducing device reliability. As a result, removal of heat often becomes the limiting factor and a barrier to further performance enhancements. Improved thermal management is necessary to boost heat transfer rates, eliminate hot spots, and reduce volume, while allowing for higher electric current density. 
         [0007]    High-power electronic chips (such as transistors and diodes) may be used for construction of electronic inverters, which invert direct current (DC) into 3-phase alternating current (AC). Such inverters are critical components of some electric vehicles, hybrid electric vehicles, and solar power plants. 
         [0008]    High-brightness light emitting diodes (LED) being developed for solid-state lighting for general illumination in commercial and household applications also require improved thermal management. These new light sources are becoming of increased importance as they offer up to 75% savings in electric power consumption over conventional lighting systems. Waste heat must be effectively removed from the LED chip to reduce junction temperature, thereby prolonging LED life and making LED cost effective over traditional lighting sources. 
         [0009]    Another class of electronic components requiring improved cooling are semiconductor-based high-power laser diodes used for direct material processing and pumping of solid-state lasers. Generation of optical output from laser diodes is accompanied by production of large amount of waste heat that must be removed at a flux on the order of several hundreds of watts per square centimeter. In addition, the temperature of high-power laser diodes must be controlled within a narrow range to avoid undesirable shifts in output wavelength. 
         [0010]    Photovoltaic cells (including solar electric cells and thermo-photovoltaic cells) are becoming increasingly important for generation of electricity. However, the cost per unit area of photovoltaic cells remains high. Happily, by using light concentrators, one may generate more electric power from smaller, more economical cells. For this approach to be successful, it is necessary to remove waste heat from the photovoltaic cells at increased flux. Similarly, high-performance crystals used in solid-state lasers generate waste heat that may require removal at fluxes in the neighborhood of thousand watts per square centimeter. 
         [0011]    Anodes in x-ray tubes are subjected to very high thermal loading. Rotating anodes are frequently used to spread the heat to avoid overheating. Such rotating anodes inside a vacuum enclosure are impractical for use in a new generation of x-ray tubes for use in compact and portable devices in medical and security applications. A compact and lightweight heat transfer component having no moving parts inside the vacuum is very desirable. 
         [0012]    Current approaches for removal of waste heat at high fluxes include 1) spreading of heat with elements having high thermal conductivity and/or 2) forced convection cooling using liquid coolants. However, even with heat spreading materials having extremely high thermal conductivity such as diamond films and certain graphite fibers, a significant thermal gradient is required to rapidly conduct large amounts of heat even over short distances. In addition, passive heat spreaders are not conducive to temperature control of the HGC. Forced convection methods for removal of waste heat at high fluxes may use microchannel heat exchangers or impingement jets. Literature indicates that conventional coolants such as water, alcohol, ethylene glycol, or fluorocarbons (e.g., Freon®) have a thermal conductivity less than 1 watt per meter-degree Kelvin. To obtain a desirably high heat transfer coefficient with conventional coolants may require flowing such coolants at very high velocity. However, this results in undesirably high coolant consumption and it requires a large pumping system. The latter is complex, costly to construct, and it requires significant amount of power to operate. High flow velocities also cause deleterious flow-induced vibrations, which are extremely undesirable in many precision systems, such as optical systems and lasers, especially on vibration-sensitive platforms such as spacecraft. 
         [0013]    Metals have a thermal conductivity several orders-of-magnitude greater than water and organic liquids. Liquid (molten) metals have a viscosity comparable to that of water. As a result, liquid metals are excellent candidates for advantageous cooling in many demanding applications, especially where heat must be removed at high heat flux. Initially, cooling by liquid metal (i.e., liquid metal cooling) was developed for thermal management of nuclear reactors on submarines in the 1950&#39;s. These large systems used eutectic alloy of sodium and potassium (also known as NaK) and in some cases, eutectic alloys of lead and bismuth. A large number of patents have been awarded in connection with these large-scale systems. 
         [0014]    One advantage of liquid metal coolants is that they can be readily pumped by the magneto-hydrodynamic (MHD) effect. In particular, MHD pumps do not have any moving components, which greatly simplifies their construction and improves their reliability. 
         [0015]    In addition, MHD pumps can be made very compact and lightweight. Liquid metal cooling for small commercial applications (e.g., electronics) is deemed to have been enabled by the discovery of a low melting point (−19° C.) eutectic alloy of gallium, indium, and tin, which is known as galinstan (see, for example, U.S. Pat. No. 5,800,060). Galinstan is non-toxic, stable in air, and it readily wets many materials. Other room-temperature-melting alloys of gallium have been reported. This opportunity was recognized in several recent disclosures, for example, U.S. Pat. Nos. 7,505,272, 7,697,291, 7,539,016, 7,764,499, 7,701,716, 7,672,129, 7,245,495, 7,861,769, and 7,131,286. No devices based on these disclosures are known to be currently on the market. 
         [0016]    The above disclosures typically suggest a traditional layout for a thermal management system found already in the above mentioned nuclear systems: a heat exchanger (HEX) for receiving heat, HEX for rejecting heat, plumbing, and a pump. Such configurations may not self-contained and may be impractical for many applications because they may have a large size, are complex, have many seals, and are costly to produce. In addition, above disclosures do not address the challenges of handling and pumping liquid metal, namely:
       1) Galinstan has a specific gravity of about 6.4, which means that galinstan flow loop may require nearly 7-times more pumping power to operate than a comparable water flow loop having the same flow velocity.   2) Gallium alloys have a tendency to form amalgams with other metals, which may result in severe corrosion in commonly used engineering metals. In addition, the solid inter-metallic compounds produced by the corrosive action may form deposits inside the liquid metal flow channel, impeding the heat transfer, and possibly block the flow channels.   3) Pumping of liquid metal with an electromagnetic pump may be very simple in theory, but it may be challenging in practice due to possible complex magneto-hydro-dynamic (MHD) boundary layers and MHD instabilities.   4) Volumetric specific heat of liquid metal may be only about half of that of water. Hence, a liquid metal cooling loop may require higher flow throughput to carry away the same amount of heat as a comparable water loop having the same temperature rise in the coolant.       
 
         [0021]    The above indicates that for a superior performance, a liquid metal cooling hardware may not have an arbitrary configuration and/or arbitrary operating parameters. 
         [0022]    In summary, prior art does not teach a thermal management device capable of removing heat at very loads and high fluxes that is also compact, lightweight, self-contained, capable of accurate temperature control, has a low thermal resistance, is easy to fabricate, is robust to corrosion by liquid metal, and requires very little power to operate. It is against this background that the significant improvements and advancements of the present invention have taken place. 
       SUMMARY OF THE INVENTION 
       [0023]    The present invention provides a thermal management device (TMD) suitable for interfacing a heat generating component (HGC), which requires removal of waste heat at high-flux to a heat sink, which may have only low-heat flux capability. The TMD of the present invention is a self-contained device with an internal flow loop flowing liquid metal coolant, which is pumped by an MHD pump. The TMD of the present invention may be used to cool HGC requiring removal of waste heat at very high heat flux. In particular, the TMD removes waste heat at high flux from an HGC and transfers it to a heat sink or environment at a reduced heat flux. For example, TMD may transfer acquired heat to a structure, heat pipe, secondary liquid coolant, phase change material (PCM), gaseous coolant, or ambient air. The TMD of the present invention may be used to cool solid-state electronic chips, semiconductor laser diodes, light emitting diodes for solid-state lighting, solid-state laser components, laser crystals, optical components, vacuum electronic components, and photovoltaic cells. 
         [0024]    In one preferred embodiment of the present invention, the TMD comprises a body having a first surface, a second surface, a closed flow channel, and an MHD pump. The first surface is adapted for receiving heat from an HGC and the second surface is adapted for transferring (rejecting) heat to a heat sink. The flow channel forms a closed flow loop with two branches. Each of the flow loop branches is arranged to pass in the proximity of the first surface and in the proximity of the second surface. The flow channel is filled with a suitable liquid metal, which is flowed by the MHD pump. The flow loop is arranged to form impinging flow on a thin member separating the first surface from the flow channel. Heat transfer in the impinging flow is very high, which allows for removal of heat from HGC at high flux. Heat acquired by the liquid metal flow is carried by the flow and eventually transferred to an interface member separating the second surface and the flow channel. Heat is then rejected through the second surface from the TMD to a suitable heat sink or the environment. 
         [0025]    Accordingly, it is an object of the present invention to provide a thermal management device (TMD) for removing waste heat from HGC. The TMD of the present invention is simple, compact, lightweight, self-contained, easy to fabricate, can be made of materials with a coefficient of thermal expansion (CTE) matched to that of the HGC, requires relatively little power to operate, and it is suitable for large volume production. 
         [0026]    It is another object of the invention to provide means for cooling HGC. 
         [0027]    It is still another object of the invention to provide means for temperature control of HGC. 
         [0028]    It is yet another object of the invention to cool a semiconductor electronic components. 
         [0029]    It is yet further object of the invention to cool semiconductor laser diodes. 
         [0030]    It is a further object of the invention to cool LED for solid-state lighting. 
         [0031]    It is still further object of the invention to cool computer chips. 
         [0032]    It is an additional object of the invention to cool photovoltaic cells. 
         [0033]    These and other objects of the present invention will become apparent upon a reading of the following specification and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  is an isometric view of the TMD in accordance with one preferred embodiment of the present invention. 
           [0035]      FIG. 2  cross-sectional view  2 - 2  of the TMD of  FIG. 1 . 
           [0036]      FIG. 3  is a cross-sectional view  3 - 3  of the TMD of  FIG. 1 . 
           [0037]      FIG. 4  is a cross-sectional view  4 - 4  of the TMD of  FIG. 1 . 
           [0038]      FIG. 5  is an isometric view of the TMD body including a partial cross-section to expose the small opening. 
           [0039]      FIG. 6  shows an enlarged cross-sectional view of a portion  6  of  FIG. 2 . 
           [0040]      FIG. 7  is an isometric view of the MHD pump assembly. 
           [0041]      FIG. 8  is a cross-sectional view  8 - 8  of the MHD pump assembly of  FIG. 7 . 
           [0042]      FIG. 9  is a cross-sectional view  9 - 9  of the MHD pump assembly of  FIG. 7 . 
           [0043]      FIG. 10  is an isometric view of the magnet core assembly. 
           [0044]      FIG. 11  is an isometric view of the magnet core assembly of  FIG. 10  flipped over. 
           [0045]      FIG. 12  is cross-sectional view  12 - 12  of the magnet core assembly of  FIG. 10 . 
           [0046]      FIG. 13  is cross-sectional view  13 - 13  of the magnet core assembly of  FIG. 10 . 
           [0047]      FIG. 14  is an isometric view of the fill plug. 
           [0048]      FIG. 15  is an isometric view of a prismatic flow separator. 
           [0049]      FIG. 16  is an isometric view of a conical flow separator. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0050]    Selected embodiments of the present invention will now be explained with reference to drawings. In the drawings, identical components are provided with identical reference symbols in one or more of the figures. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are merely exemplary in nature and are in no way intended to limit the invention, its application, or uses. 
         [0051]    Referring now to  FIG. 1 , there is shown a thermal management device (TMD)  100  in accordance with one preferred embodiment of the present invention generally comprising a body  102 , MHD pump assembly  170 , and manifolds  199 .  FIGS. 2 ,  3 , and  4  show principal cross-sectional views of the TMD  100  exposing additional elements including the fill plug  172  ( FIGS. 2 and 4 ). As seen in  FIG. 2 , the body  102  together with the MHD pump assembly  170  and with the fill plug  172  in an assembled condition form an internal cavity defined as a flow channel comprising flow channel portions  104   a ,  104   b , and  104   c . The flow channel is substantially filled with liquid metal coolant  116 . 
         [0052]    The body  102  (shown as a stand alone component in  FIG. 5 ) further comprises a large opening  184 , a small opening  182 , a thin member  196 , interface members  198   a  and  198   b , and a flow separator  148  ( FIG. 6 ). The large opening  184  of the body  102  is suitable for precision fitting of the MHD pump assembly  170 . Referring now again to  FIG. 2 , the small opening  182  of the body  102  is suitable for receiving the fill plug  172 . In addition, the body  102  comprises a heat receiving surface  106  ( FIG. 6 ) forming the exterior surface of the thin member  196 . Furthermore, the body  102  comprises heat rejection surfaces  108   a  and  108   b  forming the exterior surfaces of the respective interface members  198   a  and  198   b . The heat receiving surface  106  is adapted for receiving heat from a heat generating component (HGC), and the heat rejection surfaces  108   a  and  108   b  are adapted for rejecting heat to a heat sink or environment. In particular, the heat rejection surfaces  108   a  and  108   b  are formed into a multitude of channels  167  ( FIGS. 2 ,  3 , and  5 ) suitable for flowing a secondary coolant. As seen in  FIG. 2 , the channels  167  fluidly couple to internal distribution channels inside the manifolds  199  to facilitate supply and drainage of secondary coolant. 
         [0053]    The body  102  may be formed as a monolithic structure or as an assembly made from discrete components. In either case, the thin member  196  and the interface members  198   a  and  198   b  of the body  102  are each preferably made of material having very high thermal conductivity. Suitable materials for construction of the thin member  196  and the interface members  198   a  and  198   b  of may include copper, copper tungsten alloy, tungsten, molybdenum, aluminum, silicon, berylia, silicon carbide, and aluminum nitride. The thin member  196  and the interface members  198   a  and  198   b  do not have to be made from the same materials. Thin member  196  is preferably made between 0.25 and 1.00-milimeter thick to facilitate efficient flow of heat therethrough while maintaining structural integrity. Referring now to  FIG. 6 , the thin member  196  may include a flow separator  148  positioned to be in the center of the flow channel portion  104   c . The surface of the thin member  196  facing the flow channel may also include ridges or extensions to increase the contact area with the liquid metal coolant  116 . 
         [0054]    A heat generating component (HGC)  114  ( FIGS. 1 ,  2 ,  4 , and  6 ) may be also attached to the heat receiving surface  106  of the body  102  and arranged to be in a good thermal communication therewith. Preferably, the HGC  114  is centered on the flow separator  148  ( FIG. 6 ). The HGC  114  may be, but it is not limited to a solid-state electronic chip, semiconductor laser diode, light emitting diodes (LED), solid-state laser crystal, optical component, x-ray tube anode, or a photovoltaic cell. If desired, the thin member  196  of the body  102  may be made from material having a coefficient of thermal expansion (CTE) matched to the CTE of the HGC  114 . The HGC  114  may be attached and thermally coupled to the first surface  106  with a suitable joining material having acceptably good thermal conductivity. Suitable joining materials may include solder, epoxy, and adhesive. Alternatively, HGC  114  may be diffusion bonded onto surface  106 . As another alternative, the HGC  114  may be maintained in a mechanical contact with the heat receiving surface  106  and suitable enhancement to thermal contact therebetween may be provided by thermally conductive paste or fusible alloy or liquid metal. If desired, a suitable electrical insulating member having high thermal conductivity (not shown) may be installed between the HGC  114  and the surface  106 . Such an electrical insulating member may be fabricated from aluminum nitrate, berylia, or alike. 
         [0055]    Referring now to  FIG. 7 , there is shown an isometric view of the MHD pump assembly  170  comprising two magnet core assemblies  180   a  and  180   b  and electrodes  130   a  and  130   b . The two magnet core assemblies  180   a  and  180   b  are relatively positioned as shown in  FIG. 7  to form the flow channel portion  104   c  (see also  FIGS. 8 and 9 ). The two magnet core assemblies  180   a  and  180   b  are of identical structural construction except that the orientation of the magnetization vector of magnets  128  ( FIG. 10 ) is arranged so that when the magnet core assemblies are configured into the MHD pump assembly  170  as shown in  FIG. 7 , the magnetization vectors are aligned with the arrow  181 . Another words, the magnetization vectors of the magnets  128  in magnet core assemblies  180   a  and  180   b  should be arranged so that their magnetization vectors are substantially pointing in the same direction when the MHD pump assembly  170  ( FIG. 7 ) is formed. Because of the magnetization vector alignment, two magnet core assemblies  180   a  and  180   b  attract each other. As a result, the MHD pump assembly  170  may be formed without any fasteners, thus allowing for simple construction and assembly. 
         [0056]    Isometric views of the magnet core assembly  180   a  are shown in  FIGS. 10 and 11 .  FIG. 12  is a cross-sectional view  12 - 12  of the magnet core assembly of  FIG. 10  showing a core structure  186  equipped with a magnet  128 , electrically insulating filler material  192 , and an electrically insulating film  194 . The core structure  186  is formed from a suitable ferromagnetic material capable of carrying magnetic flux at high density such as iron, steel, low carbon steel, core iron (e.g., Consumet® by Cartpenter Steel), pure iron, nickel-iron alloys such as Hiperco®, or alike. The electrically insulating filler material  192  may be epoxy, or plastic (e.g., Ultem), ceramic, or other suitable material having good electrical insulating properties. The core structure  186  has a grove  188  designed to form a portion of the flow channel when the MHD pump assembly  170  is formed and installed into the body  102 . The grove  188  is substantially circumferential having a width “W” and a height “H”. Preferably, the width “W” is much larger than the height “H”. For example, the width “W” may have a dimension in the range of 3 millimeters to 20 millimeters and the height “H” may have a dimension in the range of 0.1 millimeters to 2 millimeters. The width W″ and the height “H” may not have to be constant around circumference of the structure  186 . 
         [0057]    The electrically insulating film  194  may be a suitable firm formed from plastic (e.g., Mylar® or Kapton®), epoxy, or other material having good electrical insulating properties. The electrically insulating filler material  192 , and the electrically insulating film  198  are applied in a suitable manner that prevents electrical contact between each of the electrodes  130   a  and  130   b  and each the core structure  186 , between each of the electrodes  130   a  and  130   b  and the magnets  128  of either magnet core assembly  180   a  and  180   b.    
         [0058]    The magnet  128  ( FIGS. 10 ,  12 , and  13 ) is a suitable permanent magnet magnetized through its large faces in a direction parallel to the arrow  181 . The magnet  128  is preferably a rare earth permanent magnet formed from samarium-cobalt (SmCo) or from neodymium-iron-boron (NdFeB) materials. 
         [0059]    The electrodes  130   a  and  130   b  are preferably made of tungsten, tantalum, or other suitable material having high electrical conductivity as well as robustness to erosion by electrical arc. Alternatively, the electrode may be made of copper or copper alloy and it may be plated with a suitable refractory metal such as, but not limited to molybdenum, tungsten, tantalum, ruthenium, osmium, and iridium. The edge  152  of the electrodes facing the flow channel  104   c  may be curved (as shown in  FIG. 4 ) or it may be straight. Curved edge is deemed to make the electrode less susceptible to electrical arcing. 
         [0060]    The body  102  and the MHD pump assembly  170  are arranged so that during installation, the MHD pump assembly  170  slides precisely into the large opening  184  of the body  102 . Once the MHD pump assembly  170  is installed into the body  102 , a suitable adhesive or sealant (e.g., epoxy) may be applied along the joints to hold the MHD pump assembly  170  in the body  102  and to seal the flow channel to prevent potential leakage of the liquid metal  116  from within. Adhesive or sealant may be also applied along the joints between magnet core assemblies  180   a  and  180   b , and near the electrodes. 
         [0061]    With the MHD pump assembly  170  installed into the body  102 , a flow channel is formed generally by the gap between the MHD pump assembly  170  and the body  102 . The flow channel forms a closed flow loop having a general shape of figure “8” having a main flow channel common  104   c  and two branches, one formed by the branch flow channel portion  104   a  and the other by the branch flow channel portion  104   b . The flow channel portions  104   a ,  104   b , and  104   c  contain a suitable liquid metal coolant  116 . Preferably, the flow channel is not entirely filled with the liquid metal and at least some small void space free of liquid metal is provided inside the flow channel to allow for thermal expansion (and/or phase change expansion) of the liquid metal coolant. Such void space may be filled with suitable elastomeric material. 
         [0062]    Preferably, the liquid metal coolant  116  has a good thermal conductivity, low viscosity, and low freezing point. For the purposes of this disclosure, the term “liquid metal” shall mean suitable metals (and their suitable alloys) that are in a liquid (molten) state at their operating temperature. Examples of suitable liquid metals include nontoxic room temperature melting alloys comprising of gallium, indium, and tin (GaInSn). Ordinary or eutectic liquid metal alloys may be used. Examples of suitable gallium-based liquid eutectic metal alloys include Indalloy 51 and Indalloy 60 (manufactured by Indium Corporation in Utica, N.Y.), galinstan (obtainable from Geratherm Medical AG in Geschwenda, Germany). In particular, galinstan is an eutectic alloy reported to contain 68.5% by weight of gallium, 21.5% by weight of indium and 10% by weight of tin, and having a melting point around minus 19 degrees Centigrade. Examples of suitable gallium-based liquid metal alloys may be also found in the U.S. Pat. No. 5,800,060 issued to G. Speckbrock et al., on Sep. 1, 1998. A new class of liquid metal alloys recently disclosed by Brandeburg et al. in the U.S. Pat. No. 7,726,972 and having reportedly extended useful temperature range down to minus 36 degrees Centigrade may be also usable with the subject invention. The Brandeburg&#39;s alloy differs from the commercially available GaInSn alloy in that it additionally includes 2%-10% of zinc (Zn). 
         [0063]    It is important that all surfaces of TMD  100  that may come into contact with the liquid metal coolant  116  be made of compatible materials. In particular, it is well know that liquid gallium and its alloys severely corrode many metals. Literature indicates that certain refractory metals such as tantalum, tungsten, and ruthenium may be stable in gallium and its alloys. See, for example, “Effects of Gallium on Materials at Elevated Temperatures,” by W. D. Wilkinson, Argonne National Laboratory Report ANL-5027, published by the U.S. Atomic Energy Commission (August 1953). To protect against corrosion, vulnerable surfaces that may come into contact with the liquid metal coolant (for example portions of the body  102 ) may be coated with suitable protective film. Suitable protective coatings and films for copper parts (e.g., the body  102 ) may include sulfamate (electroless) nickel, electroplated ruthenium, titanium nitride (TiN), and diamond-like coating (DLC). Diamond-like coating may be obtained from Richter Precision in East Petersburg, Pa. The Applicant has determined that core structure  186  made of substantially pure iron or core iron (e.g., Consumet® by Cartpenter Steel) may not require a protective coating. Reduced need for protective coatings simplifies fabrication and reduces cost. 
         [0064]    Referring now again to  FIG. 2 , the TMD  100  may be filled with liquid metal coolant  116  by removing the fill plug  172  and orienting the small opening  182  of the body  102  in upward direction. Liquid metal coolant may be poured into the small opening  182 . When the fill is complete, the fill plug  172  may be pushed into the small opening  182  to close it. Proper orientation of the fill plug  172  with respect to the body  102  is ensured with alignment pins  175 . Any excess liquid metal coolant may be released through the vent ports  173 . After removing the excess liquid metal coolant from the vent port, the vent port  173  may be sealed using a suitable sealant  177 . Secondary coolant manifolds  199  may be attached with bolts  110  and sealed with o-rings  112  ( FIG. 3 ). 
         [0065]    In operation, direct current electric potential is applied to the electrodes  130   a  and  130   b  of the TMD  100  ( FIG. 1 ). The liquid metal coolant  116  inside the flow channel portion  104   c  makes an electrical contact with the electrodes ( FIG. 3 ) and allows an electric current to flow through the liquid metal coolant from one electrode to electrode. The direction of the electric current (as defined by the polarity of the electric current source) drawn though the liquid metal coolant is coordinated with the direction of the magnetic field generated by the magnets  128  in the MHD pump assembly  170 , so that the resulting magneto-hydrodynamic (MHD) effect causes the liquid metal coolant  116  to flow inside the main flow channel portion  104   c  in the direction indicated by the arrow  124   c  in  FIG. 2 . Referring now to  FIG. 6 , a stream of liquid metal coolant identified by the arrow  124   c  impinges onto the flow separator  148 . The flow separator  148  has a sharp edge. As a result, the liquid metal coolant stream identified by the arrow  124   c  becomes separated (split) into two streams respectively identified by the arrows  124   a  and  124   b . The two streams respectively identified by the arrows  124   a  and  124   b  are about the same size. Heat transfer from the thin member  196  to the liquid metal coolant is greatly enhanced because of the sharp changes in the direction of liquid metal coolant flow in the vicinity of the flow separator  148 . Referring now again to  FIG. 2 , the liquid metal coolant stream identified by the arrow  124   a  flows through the branch flow channel portion  104   a  toward the fill plug  172 . The liquid metal coolant stream identified by the arrow  124   b  flows through the branch flow channel portion  104   b  toward the fill plug  172 . In vicinity of the fill plug  172  the liquid metal coolant stream identified by the arrow  124   a  and the liquid metal coolant stream identified by the arrow  124   b  are combined and fed into the flow channel portion  104   c  in the MHD pump assembly  170 . As a result, liquid metal coolant completes a round trip through the closed flow loop. 
         [0066]    Concurrently, secondary coolant streams  179   a  and  179   b  of fresh secondary coolant are injected into the manifolds  199  as shown in  FIG. 1 . Suitable secondary coolant may be a liquid (such as water, ethylene glycol, alcohol, or Freon®) or gas such as air. The HGC  114  is installed onto the thin member  196  and operated. Waste heat from the HGC  114  is transferred through the heat receiving surface  106  into the thin member  196  of the body  102  ( FIG. 6 ). The liquid metal coolant streams identified by the arrows  124   a  and  124   b  sweep by the thin member  196 , receive the waste heat from the thin member  196 , and carry the waste heat to the interface members  198   a  and  198   b  respectively. The waste heat is then transferred from the liquid metal coolant to the interface members  198   a  and  198   b , conducted through them, and respectively transferred to the secondary coolant streams  179   a  and  179   b  flowing through the channels  167  ( FIG. 2 ). Expended (warmer) secondary coolant exits the manifolds  199  as secondary coolant streams  179   a ′ and  179   b ′. Note that the heat flux in the interface members  198   a  and  198   b  can be much lower than in the thin member  196 . 
         [0067]    Temperature of the HGC  114  may be controlled by controlling the flow velocity of the coolant  116  inside the closed flow loop. This can be accomplished by controlling the current drawn through the coolant  116  via electrodes  130   a  and  130   b . For example, by drawing more current through the coolant  116 , the coolant flow velocity may be increased, and the HGC waste heat may be removed at a lower temperature differential between the HGC and the secondary coolant streams  179   a  and  179   b . Conversely, by drawing less current through the liquid metal coolant  116 , the liquid metal coolant velocity may be decreased, and the HGC waste heat may be removed at a higher temperature differential between the HGC and the secondary coolant streams  179   a  and  179   b . Thus, by drawing more current through the coolant  116 , the temperature of the HGC  114  may decreased, and by drawing less current through the coolant  116 , the temperature of the HGC  114  may be increased. An automatic closed-loop temperature control of the HGC  114  can be realized by sensing HGC temperature (for example, with a thermocouple or infrared sensor) and using this information to appropriately control the current drawn through the coolant  116 . In particular, if the HGC  114  is an LED, its temperature may be inferred from the output light spectrum. A means for sensing the LED light spectrum may be provided for this purpose. If the HGC  114  is a semiconductor laser diode, its temperature may be inferred from the output light center wavelength. A means for sensing the semiconductor laser diode output light center wavelength may be provided for this purpose. If the HGC  114  has electric currents flowing therethrough, HGC temperature may be determined from certain current and/or voltages supplied to or flowing through in the HGC. If the coolant used in the TMD  100  is susceptible to freezing (solidifying) due to ambient conditions during inactivity, the TMD may be equipped with an electric heater to warm the coolant up to at least its melting point. Alternatively, the HGC  114  may be operated to warm up the TMD. 
         [0068]    The invention may be also practiced with alternative heat rejection surfaces  180   a  and  108   b . For example, the heat rejection surfaces  180   a  and  108   b  may be formed for interfacing a solid heat sink, a heat pipe, a radiation panel, or a structure. The invention may be also practiced with flow separators of various shapes. For example,  FIG. 15  shows a flow separator  148  having a general prismatic shape. As another example,  FIG. 16  shows a flow separator  148  having a general conical shape. 
         [0069]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” and “includes” and/or “including” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0070]    The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies. 
         [0071]    The term “suitable,” as used herein, means having characteristics that are sufficient to produce a desired result. Suitability for the intended purpose can be determined by one of ordinary skill in the art using only routine experimentation. 
         [0072]    Moreover, terms that are expressed as “means-plus function” in the claims should include any structure that can be utilized to carry out the function of that part of the present invention. In addition, the term “configured” as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function. 
         [0073]    Different aspects of the invention may be combined in any suitable way. 
         [0074]    While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the present invention as defined in the appended claims. Furthermore, the foregoing description of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the present invention as defined by the appended claims and their equivalents. Thus, the scope of the present invention is not limited to the disclosed embodiments.