Abstract:
A high-strength, single-staged, composite thermoelectric cooler ( 18 ) for stabilizing the temperature of an uncooled, infrared detector ( 16 ) comprising a pair of ceramic plates ( 20, 22 ), a plurality of thermoelectric elements ( 24 ) sandwiched between the plates ( 20, 22 ) such that the thermoelectric elements ( 24 ) and the ceramic plates ( 20, 22 ) define a plurality of chambers ( 26 ), and a thermoelectric insulator ( 28 ) which substantially fills the chambers ( 26 ) inside the thermoelectric cooler ( 18 ) forming a high-strength composite structure with the thermoelectric elements ( 24 ) and the ceramic plates ( 20, 22 ).

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to a single-staged thermoelectric cooler for stabilizing the temperature of an uncooled, infrared detector and, in particular to, a high-strength, composite thermoelectric cooler that resists fracture due to compressive or tensile stresses and dampens shearing forces. 
     BACKGROUND OF THE INVENTION 
     Every object, whether cold or hot, emits electromagnetic radiation. The radiation spectrum, however, for hot objects differs from that of cold objects. For example, the sun emits much of its radiation as visible light (0.4 μm to 0.7 μm wavelength). While colder objects, such as people, trees and automobiles, emit most of their radiation in the lower energy, infrared (IR) part of the spectrum (3 μm to 12 μm wavelengths). Since the human eye cannot detect this low energy radiation, IR-sensitive detectors must be used to visually represent IR radiation. 
     Conventional infrared detectors, known as photon detectors, produce an electrical response directly as a result of absorbing IR radiation. These detectors are strongly dependent on temperature. It is necessary to cryogenically cool these detectors to temperatures of approximately 80 K (−193° C.) in order to maintain high sensitivity. 
     An alternate type of IR sensor uses a thermal detector. These detectors do not require cryogenic temperatures to operate. Significant advances, in both simplicity and performance, have been achieved in this uncooled infrared technology field over the past several years. Uncooled IR systems have many advantages over conventional cooled IR systems, including cost, weight, size and power consumption. In addition, uncooled IR detection technology has allowed development of IR systems for commercial and military applications where low-cost, light weight, high reliability and low power consumption are critical requirements. These applications include surveillance devices, man-portable weapon sights, driver&#39;s aids, and seekers for missiles and smart submunitions. 
     However, like their cooled IR systems counterparts, the uncooled IR systems are temperature sensitive. The uncooled IR systems use thermal detectors to absorb IR radiation. The IR radiation causes the thermal detectors to experience a temperature change which in turn creates an electrical response which can be displayed on a video monitor. For proper operation, these detectors must be thermally isolated from their immediate surroundings to maximize the temperature change that results from the absorption from a small amount of IR radiation. In order to stabilize the temperature of the IR detector, current systems employ a thermoelectric cooler along with a temperature sensor. With the use of the thermoelectric cooler, the IR detector can remain at the optimum detector operating temperature for peak performance over varying ambient temperatures. This optimum temperature for uncooled IR systems is approximately room temperature, or 295 K. 
     Thermoelectric coolers are well-known in the art. Typical thermoelectric coolers use arrays of thermocouples which operate using the Peltier or Seebeck effects. The thermocouples are formed from a P-type thermal element and an N-type thermal element which have long been known for producing heating or cooling. These thermocouples generally use a P-type semiconductor or thermal element connected to an N-type semiconductor or thermal element to form a thermoelectric element. Thus, depending on the direction of the current flowing across the N and P junctions, the device may produce heating or cooling at the junction. 
     Typical single-staged thermoelectric coolers have two ceramic plates, a cold plate and a hot plate, located on either end of the thermoelectric elements. Depending upon the direction of the current, heat will be pumped from one plate to the other. Typically the top surface, the cold plate, will be held at a constant temperature. A temperature sensor on the cold plate sends signals to a power supply to control the direction of current flow which in turn controls the direction of heat flow between the cold plate and the hot plate. 
     An important characteristic of thermoelectric coolers are their efficiency ratings, which are inversely related to the thermal conductivity of heat between the cold plate and the hot plate. Thermal conductivity, and therefore efficiency, are related to the size of the thermoelectric elements, the number of elements and the air gap between the two plates. For example, the larger the air gap between the two plates, the lower the thermal conductivity and the higher the efficiency rating. In typical uncooled infrared detector applications, optimum efficiency can be reached when the thermoelectric cooler is placed in a ceramic package and the air is evacuated from the system. 
     The use of such uncooled IR detection systems, however, is limited due to the brittle nature of the thermoelectric elements which may result in fracture or breakage under rough handling or use in hostile environments. Such failures within the thermoelectric elements are typically caused by small shifts in the ceramic plates resulting in shearing forces within the thermoelectric elements. Also of concern, but less common, is failure due to compressive or tensile stresses within the thermoelectric elements. 
     In order to increase the strength of the thermoelectric coolers, conventional devices have increased the size of the thermoelectric elements. However, with an increase in size of the thermoelectric elements comes a decrease in thermal efficiency of the thermoelectric cooler. Other devices have potted the cooler with an epoxy resin which greatly increases the strength of the thermoelectric cooler, however, this approach also results in a decrease in efficiency of the thermoelectric cooler. A need has therefore arisen for a thermoelectric cooler for stabilizing the temperature of an uncooled infrared detector having high strength without having a loss in efficiency. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein comprises a high-strength, single-staged, composite thermoelectric cooler for stabilizing the temperature of an uncooled, infrared detector. The composite structure of the thermoelectric cooler resists compressive and tensile stresses and dampening shearing forces. Complementing the high-strength feature of the composite structure, this thermoelectric cooler operates at a high level of efficiency. 
     In accordance with one aspect of the present invention, the high-strength, composite thermoelectric cooler comprises a pair of parallel ceramic plates, a cold plate and a hot plate. A plurality of thermoelectric elements is thermally coupled between the cold plate and the hot plate. The thermoelectric elements are made of N and P type semiconductor material such as bismuth telluride (Bi 2 Te 3 ). The unoccupied volume between the cold plate and the hot plate defines a plurality of chambers. These chambers are substantially filled with a thermoelectric insulator creating a composite structure which resists compressive and tensile stresses and dampens shearing forces. 
     To maintain a suitable efficiency rating, the thermoelectric insulator material has a very low density and a very low thermal conductivity similar to that of air. This low thermal conductivity allows the thermoelectric cooler to operate at an efficiency rating substantially the same as that for a thermoelectric cooler operating with an air gap between the ceramic plates, noting that operating in a vacuum the thermal conductivity of air will increase. The thermoelectric insulator can be selected from a group consisting of an aerogel, a xerogel or other similar porous material having a low thermal conductivity. 
     In accordance with another aspect of the current invention, a wet precursor gel is potted into the chambers between the ceramic plates in the thermoelectric cooler. At a point above the critical point of the gel, pore fluid is extracted from the gel forming the aerogel within the thermoelectric cooler thereby creating a high strength composite structure. In accordance with another aspect of the present invention, liquid CO 2  is used to replace pore fluid in the wet precursor gel. At a point above the critical point of CO 2 , the CO 2  is extracted from the gel forming the aerogel between the two ceramic plates thereby creating a high strength composite structure within the thermoelectric cooler. 
     In yet another aspect of the present invention the wet precursor gel is placed in a mixture with a surface modifying compound and a solvent. This mixture is washed and potted into the chambers between the ceramic plates in the thermoelectric cooler. The mixture is dried in the chambers under ambient conditions forming an xerogel in a composite structure inside the thermoelectric cooler. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, including its features and advantages, reference is now made to the following detailed description, taken in conjunction with the accompanying drawing in which: 
     FIG. 1 is a perspective view illustrating a typical infrared detector assembly; 
     FIG. 2 is an exploded view of the infrared detector assembly of FIG. 1; and 
     FIG. 3 is a sectional view taken along line  3 — 3  of FIG. 2 illustrating the composite structure of a thermoelectric cooler. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention comprises a high-strength thermoelectric cooler and a method for strengthening a thermoelectric cooler. The apparatus and method comprise filling the chambers between the ceramic plates of a thermoelectric cooler with a thermoelectric insulator that has a very low density and a very low thermal conductivity to provide high strength without loss of thermal efficiency. 
     Referring initially to FIG. 1, a drawing representing a typical uncooled infrared detector assembly is depicted. The infrared detector assembly is generally designated as  10 . The housing of detector assembly  10  is a 40-pin ceramic package  12 . Infrared radiation pass through germanium window  14  into detector assembly  10 . Detector assembly  10  has military applications such as weapon sights and seeker for missiles and smart submunitions as well as commercial applications including surveillance devices for law enforcement, stationary security, and vision enhancement for trucks and automobiles. 
     Referring next to FIG. 2, an exploded view of detector assembly  10  is depicted. Between ceramic package  12  and germanium window  14  is infrared detector  16  and thermoelectric cooler  18 . Infrared detector  16  is a thermal detector which experiences temperature changes when IR radiation is absorbed. These temperature changes are converted to electrical responses which are typically transformed into a video display. For peak performance infrared detector  16  operated at or near room temperature. Thermoelectric cooler  18  and a temperature sensor (not pictured) are used to stabilized the temperature of infrared detector  16  to maintain peak performance. 
     Now referring to FIG. 3, a drawing representing a cross-section of thermoelectric cooler  18  is depicted. Thermoelectric cooler  18  has a cold ceramic plate  20  which is typically held at a constant temperature to stabilize the temperature of infrared detector  16  (see FIG. 2) Thermoelectric cooler  18  also has a hot ceramic plate  22  that is parallel with cold ceramic plate  20  and spaced a distance apart from cold ceramic plate  20 . Thermoelectric cooler  18  further comprises a plurality of thermoelectric elements  24  arranged in an array between cold ceramic plate  20  and hot ceramic plate  22 . 
     Thermoelectric elements  24  are thermocouples which operate using the Peltier or Seebeck effects. Thermoelectric elements  24  comprise a P-type thermal element and an N-type thermal element which produce heating or cooling in response to an electrical current. Thermoelectric elements  24  use a P-type semiconductor or thermal element connected to an N-type semiconductor or thermal element such that, depending on the direction of the current flowing across the N and P junctions, the device may produce heating or cooling at the junction. Heat flow may be either from cold ceramic plate  20  to hot ceramic plate  22  or from hot ceramic plate  22  to cold ceramic plate  20  depending on the direction of current flow as required to maintain a constant temperature for infrared detector  16 . 
     Still referring to FIG. 3, chambers  26  are defined by the unoccupied space between cold ceramic plate  20 , hot ceramic plate  22 , and thermoelectric elements  24 . Thermoelectric insulator  28  is potted within chambers  26  substantially filling chambers  26  forming a high strength composite structure with thermoelectric elements  24  that resists compressive and tensile stresses and dampens shearing forces. 
     In a preferred embodiment, thermoelectric insulator  28  is a material which has a very low density and a very low thermal conductivity such that thermoelectric insulator  28  has very little effect on the efficiency rating of thermoelectric cooler  18 . In a preferred embodiment, thermoelectric insulator  28  is selected from a group consisting of an aerogel, an xerogel, and other similar materials having high porosity and low thermal conductivity. 
     The data below is found in U.S. patent application Ser. No. 08/055,069 filed on Apr. 28, 1993 which is hereby incorporated by reference. In a preferred embodiment, thermoelectric insulator  28  is an aerogel having a porosity between about 0.85 and 0.98, a density as low as 0.003 g/cm 3 , and a thermal conductivity of about 0.02 W/mK. In another preferred embodiment, the aerogel has substantially the same thermal conductivity as air so as to minimize any loss of thermal efficiency within thermoelectric cooler  18 . 
     Aerogel is formed by drying a wet gel selected from a group consisting of inorganic metal oxide gels, composite inorganic-organic gels, and organic gels. For example, aerogels are made from inorganic metal oxides such as silica (SiO 2 ) or alumina (Al 2 O 3 ). In a preferred embodiment the wet gel is potted in chambers  26  of thermoelectric cooler  18  after thermoelectric cooler  18  has been mounted into ceramic package  12 . Ceramic package  12  is placed in an autoclave (not pictured) or similar device and raised above the critical point of the gel (Tc=243° C., Pc=63 bars for ethanol) forming a supercritical fluid. Pore fluid from the wet gel is extracted forming the aerogel in place, thereby creating a high strength composite structure within thermoelectric cooler  18 . 
     In another embodiment of the invention, the pore fluid of the wet gel is first replaced by liquid CO 2 . After potting chambers  26  with the wet gel, the liquid CO 2  is extracted from the gel at a temperature and pressure above the critical point of CO 2  (Tc=31° C., Pc=73 bars). This process occurs at a much lower temperature than the non CO 2  replacement process. 
     In one embodiment of the invention, thermoelectric insulator  28  is an xerogel. To form the xerogel, the wet gel is placed in a mixture with a surface modifying compound (having the general form of R x M y  where R is an organic group such as CH 3  or C 2 H 5  and X is a halogen such as Cl) and a solvent such as benzene or toluene. The wet gel is potted into chambers  26  of thermoelectric cooler  18  which has been mounted in ceramic package  12 . The wet gel is dried at a sub-critical pressure forming a xerogel within chambers  26  creating a high strength composite structure within thermoelectric cooler  18 . 
     In summary, an advantageous apparatus and method have been disclosed that feature a high strength composite thermoelectric cooler for stabilizing the temperature of an uncooled infrared detector comprising a thermoelectric insulator having a low thermal conductivity which has little effect on the efficiency rating of the thermoelectric cooler. 
     While preferred embodiments of the invention and their advantages have been disclosed in the above detailed description, the invention is not limited thereto but only by the spirit and scope of the appended claims.