Abstract:
A thermoelectric module includes: thermoelectric semiconductor elements; printed metal conductors for interconnecting the semiconductor elements; and at least one base support for the printed conductors, the base support including a metal matrix composite.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to thermoelectric modules, in particular for use as thermoelectric generators. 
         [0003]    2. Description of the Related Art 
         [0004]    When integrated into an appropriate generator, thermoelectric modules make it possible to generate current by utilizing a temperature gradient in a system. 
         [0005]    The basic structure of a known thermoelectric module (TEM) is outlined in  FIG. 1 . Printed conductors  102  of metal are situated on a ceramic base support  100 . They are used for the circuit routing of a plurality of thermoelectric semiconductors of n-type  104  or p-type  106 . Printed conductors  102  may, for example, be made of copper. Semiconductor elements or thermocouples  104  may be surrounded on both sides by printed conductors  102  and base plates  100 . 
         [0006]    A known integration of a thermoelectric module as a thermoelectric generator  200  in a system having a hot side  202  and a cold side  203  is outlined in  FIG. 2 . A heat exchanger  204  to which an insulator  206  is attached is located facing hot side  202 . A conductor or a printed conductor  208  which may, for example, correspond to one of printed conductors  102  from  FIG. 1  is located on the heat exchanger. A thermocouple of n-type  212  and a thermocouple of p-type  213  (each, for example, corresponding to one of thermocouples  104  and  106  from  FIG. 1 ) are connected to printed conductor  208  via metallic areas  210 . Metallic areas  214 , conductors or printed conductors  216 , insulator  218  and heat exchanger  220  correspond to components  210 ,  208 ,  206  and  204 , as described above. Heat exchanger  220  faces cold side  203  of the system. 
         [0007]    Normally, metals such as aluminum, stainless steel, titanium or copper are used as the material of heat exchangers  204  and  220 . Heat exchangers  204  and  220  may be joined to the usually ceramic base plate or base support  206  or  218  (corresponding to support  100  from  FIG. 1 ), for example, by welding, hard or soft soldering, cementing or friction-locked joining processes. 
         [0008]    A robust use of thermoelectric generator  200 , i.e., its reliable and long-lived operation, requires minimization of thermomechanical stresses occurring within generator  200 . This problem arises in particular on the side of generator  200  facing hot side  202  and the metal-ceramic composite of layers  204  and  206  in that area. Stresses may result from the different thermal expansions of the used materials. Previous systems having the materials used heretofore also display very high thermal resistances. No systems have been available up to now that would make possible a simple adaptation of a general thermoelectric module or generator to different systems or processes or process environments. Conversely, a specific module has only been designed for a specific environment or a specific process and it cannot be used on or adapted to other process environments in a simple way without this adversely impacting its reliability and long life. 
         [0009]    Due to high costs for the development and adaptation to specific process environments and/or correspondingly limited efficiency, thermoelectric generators have primarily been used only in the environment of aerospace engineering up to now. However, the provision of thermoelectric modules that could be more easily adapted to specific process environments would also open up, for example, vehicle construction as a field of application where such modules or generators would make possible more efficient utilization of waste heat from internal combustion engines or electric motors. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The present invention provides a thermoelectric module having the following components: Thermoelectric semiconductor elements, printed conductors made of metal for interconnecting the semiconductor elements, and at least one base support for the printed conductors. The base support includes a metal matrix composite. 
         [0011]    In one specific embodiment of the thermoelectric module, a metal content of the metal matrix composite has a gradient between a side of the base support facing the printed conductors and a side of the base support facing away from the printed conductors. 
         [0012]    For example, the metal matrix composite on the side of the base support facing the printed conductors may have 0 volume percent (vol %) metal. If the composite has, for example, a (porous) ceramic base substance, the porosity of which on the side of the base support facing the printed conductors is zero, an insulating ceramic layer is present there which assumes the insulator function. The metal-ceramic composite on the side of the base support facing away from the printed conductors has a metal content of 0 to 100%. 
         [0013]    In other specific embodiments of the thermoelectric module according to the present invention, a bilateral gradient is present in the metal content which falls from maximum values on the side of the base support facing the printed conductors and the side of the base support facing away from the printed conductors to an intermediate metal content minimum. In this specific embodiment, at least one part of the printed conductors is implemented by the areas of the base support having maximum metal content values. The intermediate minimum should preferably be at a metal content of 0 vol %. The maximum metal content values on both sides of the base support may vary from one another. If the printed conductors are formed by the metallization of the metal matrix composite, a maximum value allowing an adequate current conduction and accordingly an interconnection of the thermocouples must be targeted. The maximum value on the side of the base support facing away from the printed conductors must be selected in such a way that thermal expansion and connection to the cold or hot side of the system is optimized. 
         [0014]    In certain specific embodiments of the thermoelectric module according to the present invention, the base support includes an additional material having 0 vol % metal on the side of the metal matrix composite facing the printed conductors. This may in particular be 100 vol % of a ceramic material. For example, a ceramic having 100% volume filling may be applied, for example, by a sinter bonding method. In addition or alternatively, the base support may include an area having 100 vol % metal on the side of the metal matrix composite facing away from the printed conductors. A corresponding metal layer may be applied, for example, by recasting onto the metal matrix composite. 
         [0015]    The present invention also provides a thermoelectric generator, which is designed in particular for a drive system in the transportation industry, for example, for an internal combustion engine or an electric motor of a vehicle. This generator has a thermoelectric module as described above. 
         [0016]    Furthermore, a method for manufacturing a thermoelectric module as described above has the following steps: provision of a ceramic preform as a base support of the thermoelectric module; and infiltration of the ceramic preform with metal. 
         [0017]    If an appropriate preform design having a porosity gradient is present, a gradient may be produced in the metal content of the later base support during infiltration. 
         [0018]    Certain specific embodiments of the method include another upstream or downstream step of applying another material having 0 vol % metal. This may be in particular a ceramic material. This material is applied to one side of the preform. The material may be applied, for example, by a sinter bonding method before or after the metal infiltration. 
         [0019]    In addition or alternatively, another upstream or downstream step of applying an area having 100 vol % metal to one (other) side of the preform may be provided. For example, a metal layer may be applied to the side of the preform facing away from the later printed conductors using a recasting method. This makes it possible to implement a heat exchanger having an environment of the later generator or possibly a connection to such a heat exchanger. 
         [0020]    According to the present invention, the use of a metal matrix composite for thermoelectric modules or thermoelectric generators is also described. 
         [0021]    The use of a metal matrix composite in a base support of a thermoelectric module makes it possible to adapt the coefficients of thermal expansion within the module in the presence of a simultaneously high thermal conductivity as a function of the specific process environment. The provision of a gradient in the metal content of the metal matrix composite makes it possible to implement the two functionalities of (1) insulating the printed conductors and (2) optimizing the connection to the hot or cold side of a system using the composite of the base support. 
         [0022]    For example, the metal matrix composite on the side of the base support facing the printed conductors may have 0 vol % metal. If the composite has, for example, a (porous) ceramic base substance, the porosity of which on the side of the base support facing the printed conductors is zero, an insulating ceramic layer which assumes the insulator function is present there. On the side of the base support facing away from the printed conductors, the metal matrix composite has a metal content of 0 to 100%. 
         [0023]    The suitable selection of the metal content allows a high thermal conductivity and simultaneously an optimized connection to the system in which the module or the generator is used while simultaneously ensuring the stability of the base support. 
         [0024]    The described method for manufacturing a thermoelectric module makes it possible to use metal matrix composites as base plates or as base supports of thermoelectric modules, it being possible to ensure an integral connection between the insulator on the one side and the heat exchanger on the other side in a simple manner including optimal heat transfer and optimized coefficients of thermal expansion within the module and therefore the corresponding effects on the robustness of the module or generator. 
         [0025]    A gradient in the metal content of the later base support ensures optimal connection to the environment of the module or generator on the one hand and the insulation of the printed conductors on the other hand. The method makes it possible to adapt the modules manufactured in this manner to specific process environments in a simple way. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  schematically shows the general structure of a thermoelectric module of the related art. 
           [0027]      FIG. 2  schematically shows the general structure of a thermoelectric generator of the related art. 
           [0028]      FIG. 3  schematically shows a first exemplary embodiment of a thermoelectric module/generator according to the present invention. 
           [0029]      FIG. 4  shows a second exemplary embodiment of a thermoelectric module/generator according to the present invention. 
           [0030]      FIG. 5  shows a third exemplary embodiment of a thermoelectric module/generator according to the present invention. 
           [0031]      FIG. 6  shows an exemplary embodiment of a method according to the present invention for manufacturing a thermoelectric module. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]      FIG. 3  shows an exemplary embodiment of a thermoelectric module  300  according to the present invention. Module  300  includes a plurality of p-type thermoelectric semiconductors  302  and n-type thermoelectric semiconductors  304 , each of which is interconnected to one another in a suitable manner across a connecting layer  306  using printed conductors  308 . Printed conductors  308  may, for example, be made of copper. Printed conductors  308  are located on a base support  310  which includes a graduated metal matrix composite (MMC). The graduation of the MMC is identified by an arrow  311  pointing in the direction of increasing metal content. In the example from  FIG. 3 , the graduation is discrete, i.e., base support  310  includes a total of five different layers denoted by reference numerals  312 - 320 . The layers vary in the porosity of the preform from which base support  310  was manufactured or they vary correspondingly with respect to metal content. However, the graduation may also be designed to be continuous. 
         [0033]    Specifically, layer  312 , which lies on side  322  of base support  310  facing printed conductors  308  and is used as a support and insulator for printed conductors  308 , may, for example, be made of up to 100% of a ceramic material, i.e., the original preform had 0% porosity in the area of layer  312 . On the other hand, when module  300  is used as a generator, layer  320  lying on side  324  of base support  310  facing away from printed conductors  308  is itself used as a back plate, heat exchanger and/or for connecting to a heat exchanger which represents a hot side of a system, and is therefore made of 100% metal. Layers  314 ,  316  and  318  located between insulator layer  312  and heat exchanger or connection layer  320  have a graduated metal content of, for example, 25%, 50%, 75% metal content, the preform having by analogy, for example, a porosity of 25%, 50%, 75%. 
         [0034]    Another exemplary embodiment of a thermoelectric module  400  according to the present invention is shown in  FIG. 4 . Thermocouples of n-type  402  or p-type  404  are applied to conductor structures  406 , which in this example are designed integrally with the base plate or base support  408 . The integral design of base support  408  having conductor structures  406  simplifies in particular the manufacture of module  400 . In contrast to exemplary embodiment  300  of  FIG. 3  in which gradient  311  passes unilaterally, a bilateral gradient  410  is present in the case of base support  408  including conductor structures  406 . This gradient has a minimum metal content in an area  412  on a side  413  facing printed conductors  406 , while both an area  414  on side  418  facing away from conductor structures  406  and an area of printed conductors  406  each have a maximum metal content. 
         [0035]    The metal content in areas  416  or  414  must be suitable for making ( 416 ) an interconnection of thermocouples  402 ,  404  or a thermal connection to the system in which module  400  is to be used ( 414 ). The metal content in areas  416  and/or  414  may thus vary from 100%. 
         [0036]    Another exemplary embodiment of a thermoelectric module  500  according to the present invention is shown in  FIG. 5 . In this example, thermocouples of n-type  502  and p-type  504  are applied to conductor structures  506  which were introduced into recesses of a base plate  508 . The introduction may be accomplished, for example using die casting, squeeze casting or gas pressure infiltration of metal. In the example of  FIG. 5 , a gradient  510  in the metal content of support  508  is unilateral and passes, for example, from 0% porosity of a ceramic preform of base plate  508  in an area  512  on side  513  of base support  508  facing printed conductors  506  to a maximum in the metal content in an area  514  on side  515  facing away from printed conductors  506  for connection to a system. 
         [0037]    Based on the flow chart shown in  FIG. 6 , a method for manufacturing a thermoelectric module ( 602 ) is described. In step  604 , a ceramic preform having a porosity gradient is provided as a base support of the later thermoelectric module. In step  606 , the ceramic preform is infiltrated with metal. In this step, a gradient is accordingly produced in the metal content of the later base support. In step  608 , another material having 0 volume percent metal is applied to one side of the preform. Alternatively, this material may already be represented using step  604 . In step  610 , an area having 100 volume percent metal is applied to another side of the preform. Alternatively, the area having 100 volume percent metal may be produced on the side facing away from the base support in step  606 . The manufacturing process ends in step  612 . 
         [0038]    While gradient  311  in  FIG. 3  is a graduated gradient, a bilateral gradient ( 410 ) or a unilateral gradient ( 510 ) may also pass without graduations, i.e., continuously from a minimum value to a maximum value of the metal content (or a porosity of a ceramic preform). 
         [0039]    The metal matrix composite of support  310 ,  408  or  508  may be manufactured from porous ceramic preforms via metal infiltration, for example, using pressure support, for example, die casting, squeeze casting or gas pressure infiltration (step  606 ). This makes it possible to adapt the coefficient of thermal expansion (CTE) within the module to the system requirements, simultaneously ensuring high thermal conductivity. The ceramic preform may have a porosity gradient of, for example, 0 vol % in areas  312 ,  412 ,  512  to, for example, a maximum of 50 vol %, 75 vol %, in particular approximately 65 vol % in areas  318 ,  414 ,  514 , sufficient mechanical stability still being ensured. 
         [0040]    Areas  312  or  512  having 100 vol % ceramic or 0 vol % porosity may also be applied to the preform or the base support (step  608 ) using a sinter bonding method, optionally before or after a metal infiltration. Areas  320 ,  414  or  514  in which the porosity reaches 100 vol % or the metal content reaches 100 vol % may be applied to the base support, for example, by recasting of metal during the metal infiltration (step  610 ). 
         [0041]    The present invention thus makes it possible to form an integral connection between insulator layer  312 ,  412  or  512  and the heat exchanger or connection side to system  320 ,  414  or  514 . This ensures optimal thermal transfer with simultaneously minimal thermomechanical stresses within the module or generator. 
         [0042]    Base support  310 ,  408  or  508  made from a metal matrix composite continues to offer an insulating base for circuit routing  308 ,  406  or  506  on the insulator or ceramic side, while a boundary surface having its coefficient of thermal expansion (CTE) adapted is available on the side having a high metal content  318 / 320 ,  414  and  514  for the metals of heat exchangers of the generator or system and/or the corresponding hot or cold side of the system. 
         [0043]    Since the CTE in the module may be optimally adapted to the system requirement, the module designed according to the present invention offers significantly higher reliability with regard the thermomechanical loads compared to conventional TEMs. Simultaneously, the flexibility is increased with regard to the usable design and connection techniques and with regard to the installation space (required volume and required shaping) within the system when used as a thermoelectric generator. This is of significance for applications, for example, in the exhaust branch of an internal combustion engine. 
         [0044]    According to the present invention, thermoelectric modules or generators made of graduated preform MMCs may be used economically at comparably low costs and increased energy efficiency for the efficient utilization of the waste heat of, for example, internal combustion engines or electric motors in the transportation industry (vehicle construction).