Abstract:
The present invention is a thermally controlled switch with high thermal or electrical conductivity. Microsystems Technology manufacturing methods are fundamental for the switch that comprises a sealed cavity formed within a stack of bonded wafers, wherein the upper wafer comprises a membrane assembly adapted to be arranged with a gap to a receiving structure. A thermal actuator material, which preferably is a phase change material, e.g. paraffin, adapted to change volume with temperature, fills a portion of the cavity. A conductor material, providing a high conductivity transfer structure between the lower wafer and the rigid part of the membrane assembly, fills another portion of the cavity. Upon a temperature change, the membrane assembly is displaced and bridges the gap, providing a high conductivity contact from the lower wafer to the receiving structure.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to a structure for thermal or electrical control, particularly for thermal control in space applications. 
       BACKGROUND OF THE INVENTION 
       [0002]    In many devices, wherein a substantial amount of heat is generated, there is a need for an active thermal control, in order to maintain the desired operational temperature for the device. A common solution is to use the air in the atmosphere for transport of the excessive heat by use of electromechanical fans or ventilators. This is an effective but sometimes noisy solution, wherefore conduction of the heat through passive or active heat conductors to a thermal radiator in many times is a preferred solution. In particular, in space applications, operating in vacuum, this is the only solution if direct radiation of the heat into space is impossible. 
         [0003]    For example, in the development of small but very efficient spacecraft with high internal power density thermal control becomes a growing area of concern. The low thermal mass of a small spacecraft makes it necessary to radiate excessive heat when active, but on the other hand the internal part of the spacecraft must be thermally isolated from external radiator surfaces when passive in order to keep the internal temperature at an acceptable level. If the active and passive modes are synchronized with entering or leaving eclipse (earth shadow) the problem becomes even worse. To solve the problem an active thermal control system with a heat flux modulation capability must be used. 
         [0004]    Such a heat flux modulation can be based on a number of design principles. A liquid can be pumped around in the system carrying the heat from the source to the radiator. Passive heat pipes (extremely good thermal conductors) or active heat pipes, in which a liquid in vapor phase is used in a tube to transport the heat. The heat transport capability in such a heat-pipe is normally directly related to the temperature on the hot side. In some variable active heat pipers, the heat transport capability can be controlled by controlling the boil rate of the liquid. Another alternative is mechanical systems, where mechanical switches are used together with very good thermal conductors, i.e. passive heat pipes. The mechanical switch creates a gap with very low thermal conductivity in the off-mode. 
         [0005]    The heat flux modulation is a key parameter for all thermal control systems. Particular on the small spacecraft with a modern distributed functionality the mechanical system is most likely to prefer due to the simplicity, given that the heat switches have high modulation capability, are compact and have low mass. 
         [0006]    A switch designed for high thermal conductivity may naturally be particularly useful as an electrical conductor as well. When optimized for high electrical conductivity such a switch may be used as a high current electrical switch. 
         [0007]    However, in general, mechanical switches according to prior art have rather low heat flux modulation capability or current switching capability, especially in relation to their physical size. In particular, since the trend is that other components of spacecraft or other systems are miniaturized using for example Microsystems Technology (MST) or Microelectromechanical Systems (MEMS), conventional mechanical switches become too large and inefficient, or cannot readily be implemented in such a miniaturized system. 
       SUMMARY OF THE INVENTION 
       [0008]    Obviously the prior art has drawbacks with regards to being able to provide thermally controlled high conductivity switches with high switching capability compared to the physical size of the switch. 
         [0009]    The object of the present invention is to overcome the drawbacks of the prior art. This is achieved by the device as defined in claim  1 . 
         [0010]    The high conductivity switch according to the invention comprises a sealed cavity with a first wall and a second wall, wherein at least the second wall is a membrane assembly. The second wall is adapted to be arranged with a gap to a receiving structure. A thermal actuator material that is adapted to change volume with temperature fills a portion of the cavity. A conductor material fills another portion of the cavity. The conductor material provides a high conductivity transfer structure between the first wall and the second wall. The thermal actuator material is arranged to upon a temperature induced volume change, displace the second wall, so that the gap to the receiving structure can be bridged, providing a high conductivity contact from the first wall to the receiving structure. 
         [0011]    The cavity may be formed within bonded wafers, preferably silicon wafers, but metal sheets, ceramic, polymer or glass are examples of other wafer materials. 
         [0012]    The temperature induced volume change may at least partly be caused by a phase change of the actuator material, typically from liquid to solid state, occurring at a predefined temperature or temperature interval. Paraffin is a preferred actuator material with such properties. 
         [0013]    To provide a flexible heat transfer structure the conductor material may be in liquid phase at least at the phase change temperature of the actuator material. Metal or metal alloys may be used and are kept in a central position within the cavity by using coatings with particular wetting properties and/or enclosure posts protruding from at least on wafer. 
         [0014]    The conducting properties of the high conductivity switch can be optimized for thermal or electrical control by choosing a conductor material with high electrical or thermal conductivity. A switch according to the present invention with high electrical conductivity may be provided with electrical feed-through integrated in the wafers. 
         [0015]    Thanks to the invention it is possible to provide miniaturized mechanical switches with improved on/off modulation with respect to high thermal and electrical conductivity. 
         [0016]    One advantage of the switch according to the invention is that the switch can be arranged to be automatically and reversibly activated by the heat generated by the heat source. 
         [0017]    Embodiments of the invention are defined in the dependent claims. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0018]    Preferred embodiments of the invention will now be described with reference to the accompanying drawings, wherein 
           [0019]      FIG. 1  is a schematic illustration of a general mechanical thermal control system, 
           [0020]      FIG. 2  is a cross-sectional view of a switch according to the present invention, 
           [0021]      FIG. 3  is a cross-sectional view of a switch according to the invention that comprises enclosure posts, 
           [0022]      FIG. 4  is a top view of the switch in  FIG. 3  illustrating the enclosure of the heat transfer structure in the switch, 
           [0023]      FIG. 5   a  is a cross-sectional view of a switch in the low temperature off mode, 
           [0024]      FIG. 5   b  is a cross-sectional view of a switch at the moment of thermal contact, 
           [0025]      FIG. 5   c  is a cross-sectional view of a switch in the over temperature mode, 
           [0026]      FIG. 6  is cross-sectional view of an implementation of the present invention in a freestanding, normally off, thermal switch between two heat conductors, 
           [0027]      FIG. 7   a  is a cross-sectional view of a electrical high power switch with multiple through plated via holes, and 
           [0028]      FIG. 7   b  is a cross-sectional view of an electrical high power switch with a solid metal plug with screw attachment. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0029]    A high conductivity switch according to the present invention opens new possibilities for thermal and electrical control and for the implementation of different miniaturized systems, particularly in space applications. 
         [0030]    An active thermal control system is schematically illustrated in  FIG. 1 . If an excessive amount of heat is generated in an arbitrary device  100 , i.e. the heat source, it might be necessary to conduct some heat away from the device  100  in order to avoid overheating. This is accomplished through one or two heat conductors  103  to a thermal heat sink  104 , which can be a radiator or a latent heat storage device. The two heat conductors  103  are separated by an air gap  102  in sequence with a thermal switch  101 . At a certain predetermined temperature the switch  101  closes the air gap  102  permitting a high heat flux to flow from the heat source  100  to the heat sink  104 . A desired feature of the thermal switch  101  is to have as high temperature modulation as possible, i.e. the ratio between heat conductivity in off state and on state shall be as high as possible. 
         [0031]    The high conductivity switch according to the present invention, which is based on MEMS/MST, is primary intended for applications where small size and mass are desirable features and provides unsurpassed high thermal conductivity in the on state. The total thickness of the switch  101  can be less than 1 mm with a cross-section area matching the size of the heat conductors  103 , i.e. a few mm 2  up to several cm 2 . 
         [0032]    One embodiment of the present invention comprises at least two horizontal wafers  201 ,  202  bonded together, as illustrated in  FIG. 2 . A sealed cavity  213  is formed between the two wafers  201 , 202 , wherein the lower wafer  201  provides a lower first wall  203  and the upper wafer  202  provides an upper second wall  204  of the cavity  213 . The cavity  213  is filled with both a thermal actuator material  215  and a heat transfer structure  216  comprising a conductor material making a central connection between the lower wall  203  and the upper wall  204  that is formed as a membrane assembly  205  comprising a thin (and corrugated) membrane  207  and a rigid central part  206  above the cavity  213 . The purpose of the heat transfer structure  216  is to ensure a very good thermal contact between the central part  206  of the membrane  205  in wafer  202  and the wall  204  of wafer  201  where the main part of the input heat flux  220  is entering the system. There is also a lateral heat flux  222 , but as the thin (and corrugated) membrane  207  is a poor heat conductor, the most of the heat flux will go down into wafer  201  and further into the heat transfer structure  216 . The heat transfer structure  216  must be flexible as the distance between the central membrane  206  and the lower wall  203  changes when the actuator material  215  is activated. Preferably an actuator material  215  that goes through a phase change, e.g. a transition from solid to liquid state, at a given temperature or at a temperature interval is utilized. As more and more of the actuator material  215  goes through the phase change, the central part  206  of the flexible membrane  205  will move upwards until the gap  209  is closed and a good thermal contact with the heat conductor in the receiving structure  210  or pickup structure is established, permitting the heat flux  220  to flow towards the heat sink  104 . When the temperature is going down, the actuator material  215  solidifies with decreasing volume as a consequence and the thermal contact to the heat sink  104  is broken. 
         [0033]    The wafer  201 , 202  material will most likely be silicon as silicon is the most common material in the MST/MEMS field. However it can also be e.g. metal sheets, micromachinable glass, polymer or a ceramic material. For the application as an electrical switch, in which good electrical isolation is a major concern, the insulator materials are of particular interest. The electrical switch embodiment is presented later in this description. Suitable methods for shaping the wafers are, but is not limited to, etching, injection molding, electro discharge machining (EDM), rolling, laser ablation, punching etc. The wafers are bonded together. Bonded should here be interpreted in a general way meaning joining the wafers in a manner that is suitable for the materials used. Bonding include, but is not limited to fusion bonding, anodic bonding, using adhesives, welding, soldering, clamping. 
         [0034]    As mentioned, the thermal actuator material  215  may be a phase change material, due the attractive properties of such materials. In particular paraffin or paraffin-like material can be used if the switch shall be activated at a certain over temperature. Paraffin materials expand with as much as 10 to 20% in the transition from solid to liquid and the melting point temperature can be chosen from minus several tens of C.° to plus several hundreds C.°. Melting occurs over a very limited or a broader temperature interval depending on the composition of the paraffin and the lengths of the hydrocarbon chains in the paraffin. On the other hand, if the switch shall be activated when temperature is going down, a material with opposite properties can be used. Water is a good example as it expands around 10% in the transition from liquid to solid (water to ice). The main drawback with paraffin as an actuator material and a thin flexible membrane is the rather poor heat conductivity through the paraffin and also, although not necessarily, through the thin membrane. By the inclusion of a thermal bridge, i.e. the heat transfer structure, of liquid conductor material the conductivity is dramatically improved. This results in a much higher heat conductivity modulation. An alternative to the phase change materials is to use the thermal expansion of materials within the same phase, wherein the switch is designed so that the expansion of the thermal actuator material makes the flexible membrane bridge the gap at a certain temperature. 
         [0035]    The conductor material in the heat transfer structure  216  may be a low melting point metal or metal alloy. The melting point temperature for the metal or metal alloy is lower than the phase change temperature for the actuator material  215 . Either the conductor material in the heat transfer structure  216  is solid in the off-state and then melts in the on state or the conductor material  216  is liquid all the time. 
         [0036]    Another embodiment of the present invention is shown in  FIG. 3 . Two micromachined silicon wafers  201 , 202  are bonded together forming a sealed cavity  213  with a flexible membrane  205 , which comprises a rigid central part  206  and a concentric thin and corrugated part  207 , in the upper wafer  202 . A number of enclosure posts  208  protruding from the central part  206  of the flexible membrane  205  form a more or less open cage surrounding the low melting point metal or metal alloy  216 . The liquid metal  216  is kept in place due to two factors. First, the wafer  201 ,  202  surfaces inside the posts  208  are coated with a coating  209 , e.g. a metal or metal alloy, with good wetting properties against the liquid metal  216 . Second, as the liquid metal  216  does not mix with the actuator material  215  or wets against the non-coated wafer material it will not pass the surrounding posts  208 . A picture of a cross-section A-A through wafer  201  is given in  FIG. 4  showing eight posts  208  arranged to keep the liquid metal  216  inside the posts  208  that are enclosed by the actuator material  215  within the cylindrical cavity  213 . The interface between the actuator material  215  and the liquid metal  216  is located in between the posts  208 , and when the actuator material  215  expands, increasing the pressure in the cavity  213 , the interface border  217  is pushed towards the centre. The number of post  208  as well as the internal diameter  223  and the external diameter  224  can be optimized for each design case. For small switches, it is possible that the posts  208  can be totally omitted. 
         [0037]    The switch according to the invention is arranged to be automatically and reversibly activated by the heat generated by the device  100 . In one embodiment an electrical heater (not shown) inside or in thermal contact with the actuator material  215  can be used to heat and activate the actuator material  215  if electrical control of the switch function is preferred before the thermal actuation. 
         [0038]    In another embodiment of the present invention the single central heat transfer structure  216  is replaced by distributed heat transfer structures, i.e. several columns of heat transfer structure material with smaller diameter, each surrounded by actuator material  215 . Consequently the cross-section area becomes smaller, but the heat distribution to the actuator material  215  is different, since a larger portion of the actuator material  215  is in close contact to the heat transfer material  216 . 
         [0039]    In one embodiment of the present invention comprising two bonded micromachined silicon wafers  201 , 202 , the heat transfer structure  216  does not have complete contact with the membrane  205 . A thin layer of the enclosing actuator material  215  is present between the membrane  205  and the heat transfer structure  216 . Enclosure posts  208  protruding from the lower wafer  201  and a coating  209  on the wafer  201  in an area defined by the posts  208  keeps the conductor material  216  in place. 
         [0040]      FIGS. 5   a ,  5   b  and  5   c  illustrate the conditions inside the switch for three operational modes: low temperature mode in  FIG. 5   a , thermal contact moment in  FIG. 5   b  and over temperature mode in  FIG. 5   c . At low temperature, the membrane  205  is approximately flat, see  FIG. 5   a , and the gap  102  between the receiving structure  210  and the membrane central part  206  is at its maximum. The heat transfer structure  216  is solid, bulging with a slight convex contour of the interface surface  217 . The actuator material  215  is also in the solid phase. 
         [0041]    When a heat flux is flowing into the device into the first wall  203 , the following will occur, see  FIG. 5   b . First, when the temperature is increased, at a certain temperature or within a limited temperature interval, the heat transfer material  216   b  melts. Second, at a higher temperature, the actuator material  215  phase change starts whereby the heat transfer structure  216   b  is squeezed together, the membrane  205  is lifted, and gap  102  is decreased. In the moment of thermal contact the insulating gap  102  is closed and a thermal contact  212  is formed between the rigid part  206  of the membrane and a receiving structure  210 . At this moment the solidification front  218  in the solid actuator material  215  and the liquid actuator material  215   b  has almost reached the membrane  205  and only a portion of the solid actuator material  215  remains. The membrane  205  is slightly deflected. 
         [0042]    When the temperature continues to increase, the switch is going into over-temperature mode, see  FIG. 5   c . Finally all actuator material  215   b  has melted. The liquid heat transfer structure  216   b  still has approximately the same shape as in  FIG. 5   b , as the receiving structure  210  above the thermal contact prevents the central part  206  of the membrane  205  to move further upwards. The additional volume caused by the phase change of the remaining part of the actuator material  215  in  FIG. 5   b  generates an increased deflection of the thin part of membrane  207 . 
         [0043]    The design of the switch according to the present invention is made to facilitate a reversible and stable operation of the switch. This is simplified by using a symmetrical structure where the heat flow is more or less symmetrical laterally, and by the fact that the membrane provides a spring force acting to return the membrane to the original position. The latter, in combination with a reduced pressure in the cavity upon solidification of the phase change material and surface forces in the interface between actuator material and conductor material, with a proper design, preserve the conditions described in  FIG. 5   a - c.    
         [0044]    In one embodiment the switch can be designed to be normally closed, i.e. with the second wall  204  in contact with the receiving structure  210  in analogy with the low temperature mode described above. When the actuator material  215  expand upon a temperature change, e.g. paraffin changes phase due to a temperature increase, the second wall  204  looses contact with the receiving structure  210  and the high conductivity contact is broken and width of the gap  102  with low conductivity increases. 
         [0045]    The switch device  101  can be an integrated part of a larger microsystem or be used as a freestanding device as in another embodiment of the present invention, which is illustrated in  FIG. 6 . The switch  101  is embedded in a support structure  106 . The heat conductors  103  are also fixed in the support structure  106 . A small gap  102  is left between one of the heat conductors  103  and the membrane  205  of the heat switch  101 . When the switch  101  is activated the gap  102  is closed and heat flux or an electrical current can flow from the input  220  to the output  221 . If the thermal switch  101  shall be used as an electrical switch  101  two conditions must be fulfilled. The support structure  106  or a part of it must provide electrical insulation between the input conductor  103  and the output conductor  103 . Inside the switch  101  an electrical feed-through contact from the outside to the metallic heat transfer structure inside the cavity must be provided. 
         [0046]    An electrical switch of this design has a several advantages compared to conventional electromagnetic relays. The large cross-section area of the transfer structure and the hydraulic motion and high contact pressure gives very high current capability versus size for the switch. High voltages can also be switched on or off if the volume  107  surrounding the switch is filled with isolating fluid such as transformer oil. 
         [0047]    For the electrical switch function a leak-tight electrical contact from the outside to the heat transfer structure is needed. It can be solved in a number of ways, whereof two possibilities are presented in  FIGS. 7   a  and  b . Multiple through plated holes  301  between an external metal layer  304  and an internal metal layer  303  are used in  FIG. 7   a . The internal layer  303  has a solder interface  302  to the heat transfer structure  216 . 
         [0048]      FIG. 7   b  illustrates a more straightforward method of making the contact. A solid metal plug  305  is inserted in the lower wafer  201 . A high temperature solder  306  is used to seal the plug  305 . Moreover a low temperature solder  302  is used between the plug  305  and the heat transfer structure  216 . The plug  305  can have any interface  307  to the external electrical conductor, such as screw, solder, welding, etc., and any suitable shape and surface coating to provide a good electrical contact on the surface exposed to the gap. 
         [0049]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, on the contrary, is intended to cover various modifications and equivalent arrangements within the appended claims.