Patent Publication Number: US-2020279988-A1

Title: Thermoelectric module

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage entry according to 35 U.S.C. 371 of International Application No. PCT/JP2018/039497, filed on Oct. 24, 2018, which claims priority to Japanese Patent Application No. 2017-206262, filed on Oct. 25, 2017, the contents of which are entirely incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a thermoelectric module, and more particularly, to a thermoelectric module used for temperature control in an automotive seat cooler or a fuel cell, in particular. 
     BACKGROUND 
     For example, a thermoelectric module undergoes a difference in temperature between one principal surface and the other principal surface with the supply of electric power to thermoelectric elements. Moreover, for example, a thermoelectric module produces electric power via thermoelectric elements upon a difference in temperature between one principal surface of the module and the other principal surface. Thermoelectric modules having such useful characteristics are used for temperature control purposes or thermoelectric power generation purposes, for example. 
     An example of such thermoelectric modules includes: a pair of support substrates including mutually opposed regions; wiring conductors disposed on opposed one principal surfaces of the pair of support substrate, respectively; a plurality of thermoelectric elements disposed between the one principal surfaces of the pair of support substrates; and a lead member joined to the wiring conductor located on one support substrate of the pair of support substrates. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Publication JP-A 2008-244239 
     SUMMARY 
     A thermoelectric module according to an aspect of the disclosure includes: a pair of support substrates including mutually opposed regions; wiring conductors disposed on one principal surface of one support substrate of the pair of support substrates and one principal surface of another support substrate of the pair of support substrates, respectively, the one principal surface of the one support substrate and the one principal surface of the other support substrate being opposed to each other; a plurality of thermoelectric elements disposed between the one principal surface of the one support substrate of the pair of support substrates and the one principal surface of the other support substrate of the pair of support substrates; a lead member joined to one wiring conductor of the wiring conductors, the one wiring conductor located on either the one support substrate or the other support substrate of the pair of support substrates; and an electrically conductive joining material which joins the one wiring conductor and the lead member together. The lead member includes a core, and a covering layer which covers a rear end-side part of the core, and which does not cover a front end-side part of the core. A bonding interface between the electrically conductive joining material and the one wiring conductor is smaller in width on a side of the bonding interface which is close to the thermoelectric elements than on a side of the bonding interface which is away from the thermoelectric elements, as viewed in a section in a direction perpendicular to an axial direction of the lead member. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic perspective view showing an example of the thermoelectric module; 
         FIG. 2  is a plan view of the thermoelectric module shown in  FIG. 1 ; 
         FIG. 3  is a partially transparent side view of the thermoelectric module shown in  FIG. 2 ; 
         FIG. 4  is a schematic sectional view of the thermoelectric module taken along the line IV-IV in  FIG. 2 ; 
         FIG. 5  is a sectional view of a main part of the thermoelectric module taken along the line V-V in  FIG. 4 ; 
         FIG. 6  is a sectional view of a main part of the thermoelectric module taken along the line VI-VI in  FIG. 4 ; 
         FIG. 7  is a schematic sectional view showing another example of the thermoelectric module; 
         FIG. 8  is a schematic sectional view showing still another example of the thermoelectric module; and 
         FIG. 9  is a schematic sectional view showing still another example of the thermoelectric module. 
     
    
    
     DETAILED DESCRIPTION 
     In a conventional thermoelectric module with a lead member soldered to a wiring conductor located on a low-temperature-side support substrate of the pair of support substrates which has a relatively low temperature, the lead member is positioned in parallel with the principal surface of the support substrate. 
     In the thermoelectric module so constructed, transmission of heat from the lead member to the wiring conductor and the support substrate leads to poor cooling performance, and, the lead member may become detached from the wiring conductor under external forces, for example. 
     The disclosure addresses the problems discussed above, and aims to provide a thermoelectric module that achieves reduction in cooling performance degradation, and reduction in separation of a lead member from a wiring conductor. 
     A thermoelectric module in accordance with an embodiment of the invention will now be described with reference to drawings. 
     A thermoelectric module  10  shown in  FIGS. 1 to 4  includes: a pair of support substrates  11  and  12  including mutually opposed regions; wiring conductors  21  and  22  disposed on opposed one principal surfaces of the support substrates  11  and  12 , respectively; a plurality of thermoelectric elements  31  and  32  disposed between the one principal surfaces of the support substrates  11  and  12 ; a lead member  4  joined to the wiring conductor  21  located on one support substrate  11  of the pair of support substrates; and an electrically conductive joining material  5  for joining the wiring conductor  21  and the lead member  4  together. 
     The pair of support substrates  11  and  12  constituting the thermoelectric module  10  include mutually opposed regions of, for example, a rectangular shape, for holding and supporting the plurality of thermoelectric elements  3  therebetween in sandwich style. For example, dimensions of each of the mutually opposed rectangular regions can be set to 40 to 80 mm in longitudinal length, 20 to 40 mm in transverse length, and 0.25 to 0.35 mm in thickness in plan configuration. 
     An upper surface of the support substrate  11  is placed so as to serve as one principal surface facing the support substrate  12 , and, a lower surface of the support substrate  12  is placed so as to serve as one principal surface facing the support substrate  11 . For example, the support substrate  11  serves as a low-temperature-side support substrate which has a relatively low temperature, whereas the support substrate  12  serves as a high-temperature-side support substrate which has a relatively high temperature. 
     The support substrate  11  bears the wiring conductor  21  on its upper surface serving as one principal surface facing the support substrate  12 , and, the support substrate  12  bears the wiring conductor  22  on its lower surface serving as one principal surface facing the support substrate  11 . Thus, the upper-surface side of the support substrate  11  and the lower-surface side of the support substrate  12  are each made of an insulating material. For example, the pair of support substrates  11  and  12  are each constructed of a 50 to 200 μm-thick substrate body made of alumina filler-added epoxy resin, with a 50 to 500 μm-thick copper sheet bonded to the outward principal surface of the substrate body. Alternatively, the pair of support substrates  11  and  12  may each be constructed of a substrate body made of a ceramic material such as alumina or aluminum nitride, with a metal sheet such as a copper sheet bonded to the outward principal surface of the substrate body. In another alternative, each support substrate may be constructed of a substrate body made of a conductive material such as copper, silver, or a silver-palladium material, with an insulating layer made of, for example, epoxy resin, polyimide resin, alumina, or aluminum nitride formed on the inward principal surface of the substrate body. 
     The wiring conductors  21  and  22  are disposed on the opposed inward one principal surfaces of the support substrates  11  and  12 , respectively. For example, the wiring conductors  21  and  22  are obtained by laminating a copper sheet to each of the opposed inward principal surfaces of the support substrates  11  and  12 , with a mask placed on each of a part of the copper sheet which constitutes the wiring conductor  21  and a part of the copper sheet which constitutes the wiring conductor  22 , and removing mask-free areas of each copper sheet by etching. Alternatively, it is possible to use copper sheets die-cut in the form of the wiring conductors  21  and  22 . The material of construction of the wiring conductors  21  and  22  is not limited to copper. For example, silver or a silver-palladium material may be used instead. 
     Between the opposed inward one principal surfaces of the support substrates  11  and  12 , there are provided the plurality of thermoelectric elements  3  electrically connected to one another via the wiring conductors  21  and  22 . The plurality of thermoelectric elements  3  include p-type thermoelectric elements  31  and n-type thermoelectric elements  32 . The thermoelectric elements  3  are members for temperature control utilizing the Peltier effect, or members for power generation utilizing the Seebeck effect. For example, the plurality of thermoelectric elements  3  are arranged in a matrix of rows and columns with spacing which equals 0.5 to 2 times the diameter of each thermoelectric element  3 . The thermoelectric elements  3  are soldered to the wiring conductors  21  and  22 . More specifically, the p-type thermoelectric elements  31  and the n-type thermoelectric elements  32  are alternately disposed adjacent each other, while being electrically connected in series via the wiring conductors  21  and  22  and solder. That is, all the thermoelectric elements  3  are connected in series. 
     The body of each of the plurality of thermoelectric elements  3  is formed of a thermoelectric material made of A 2 B 3  crystal (A refers to Bi and/or Sb, and B refers to Te and/or Se), or preferably formed of a Bi (bismuth) and Te (tellurium)-based thermoelectric material. More specifically, the p-type thermoelectric element  31  is formed of, for example, a thermoelectric material made of a solid solution of Bi 2 Te 3  (bismuth telluride) and Sb 2 Te 3  (antimony telluride). On the other hand, the n-type thermoelectric element  32  is formed of, for example, a thermoelectric material made of a solid solution of Bi 2 Te 3  (bismuth telluride) and Bi 2 Se 3  (bismuth selenide). 
     For example, the thermoelectric element  3  may be shaped in a circular cylinder or a polygonal prism such as a quadrangular prism. The thermoelectric element  3  of circular cylinder shape, in particular, is less influenced by thermal stress caused therein under heat cycles during use. For example, dimensions of the circular cylinder-shaped thermoelectric element  3  are set to 0.5 to 3 mm in diameter and 0.3 to 5 mm in height. 
     For example, the thermoelectric material constituting the p-type thermoelectric element  31  is formed as a rod-like body having a circular sectional profile of 0.5 to 3 mm in diameter from a p-type thermoelectric material made of Bi, Sb, and Te, which has undergone one melting-and-solidification process, through unidirectional solidification using Bridgman method. Moreover, the thermoelectric material constituting the n-type thermoelectric element  32  is formed as a rod-like body having a circular sectional profile of 0.5 to 3 mm in diameter from an n-type thermoelectric material made of Bi, Te, and Se, which has undergone one melting-and-solidification process, through unidirectional solidification using Bridgman method. 
     After being coated on its side surface with a resist to prevent adhesion of plating as required, each thermoelectric material is cut in a length (thickness) of, for example, 0.3 to 5 mm with a wire saw. Subsequently, on an as needed basis, a Ni layer is formed only on the cut surface of the material by electrolytic plating, for example, and then a Sn layer is formed on the Ni layer. Thus, the p-type thermoelectric elements  31  and the n-type thermoelectric elements  32  are obtained. 
     A sealing material made of, for example, resin such as silicone resin or epoxy resin may be provided as required around the plurality of thermoelectric elements  3  disposed between the support substrate  11  and the support substrate  12 . Although the outer periphery of the construction becomes deformed greatly due to a difference in temperature between the pair of support substrates  11  and  12 , the sealing material for filling the gaps among a plurality of outer periphery-side thermoelectric elements  3  disposed between the one principal surface of the support substrate  11  and the one principal surface of the support substrate  12  serves as a reinforcing material, thereby restraining the thermoelectric elements  3  from separating from the wiring conductors  21  and  22 . 
     One support substrate  11  of the pair of support substrates  11  and  12  is provided with an extended portion  111  as required. The extended portion  111  is a part of the support substrate  11  which lies outside the part thereof opposed to the support substrate  12  as seen in a plan view, or equivalently, a part of the support substrate  11  which lies to the left of the chain double-dashed line in  FIG. 3 . 
     For example, extending amount (extending length) of the extended portion  111  are set to 1 to 5 mm, and a width along the entire length of a short side of the support substrate  11  is set to 5 to 40 mm. 
     The wiring conductor  21  disposed on the one principal surface of the support substrate  11  lies also on the extended portion  111 , and an end of the lead member  4  is joined, with the electrically conductive joining material  5  such as solder, to the wiring conductor  21  disposed on one principal surface of the extended portion  111 . The wiring conductor  21  and the lead member  4  may be joined together by laser beam welding rather than soldering. 
     The lead member  4  is intended for electrical connection between the thermoelectric module  10  and an external circuit, and provides electric power to the thermoelectric element  3  or extracts electric power produced by the thermoelectric element  3 . The lead member  4  includes a core  41  and a covering layer  42 . The front end of the lead member  4 , which is joined to the wiring conductor  21 , is made as a bared portion of the core  41 . Moreover, the lead member  4  includes the covering layer  42  with which the core  41  is covered on the side located close to the rear end of the lead member  4  rather than on the side located close to the front end thereof. Expressed differently, the covering layer  42  is disposed about the periphery of the core  41 , except at least for the front end of the core  41  which is electrically connected to the wiring conductor  21 . 
     The front end of the lead member  4  refers to an end of the lead member  4  which is joined to the wiring conductor  21  located on the support substrate  11 . The front end of the lead member  4  in the form of the bared portion of the core  41  refers to a part of the core  41  which extends beyond an edge of the covering layer  42  located close to the front end for electrical connection of the lead member  4  to the wiring conductor  21 . For connection between the lead member  4  and an external circuit, the rear end of the lead member  4  may also be made as a bared portion of the core  41 , or the rear end of the lead member  4  may be provided with a connector. 
     For example, the core  41  is formed of a bundle of a plurality of metallic wires such as copper wires, for example, a bundle of 15 to 30 copper wires, each having a diameter of 0.15 to 0.30 mm. For example, the covering layer  42  is formed of a 0.2 to 0.4 mm-thick sheet made of polyvinyl chloride or polyethylene. 
     As shown in  FIGS. 5 and 6 , the bonding interface between the electrically conductive joining material  5  and the wiring conductor  21  is smaller in width on the side of the bonding interface which is close to the thermoelectric element  3  than on the side of the bonding interface which is away from the thermoelectric element  3 , as viewed in a section in a direction perpendicular to an axial direction of the lead member  4 . This reduces the contact area on the side close to the thermoelectric element  3  where wide temperature variations are encountered. Thus, for the support substrate  11  serving as a low-temperature-side support substrate which has a relatively low temperature, transmission of heat from the lead member  4  to the wiring conductor  21  is reduced, and consequently the thermoelectric module  10  delivers a higher level of cooling performance. Moreover, the contact area is large on the side away from the thermoelectric element  3  enough to restrain the lead member  4  from separating from the wiring conductor  21 , and consequently the thermoelectric module  10  becomes more durable against external force. 
     Let it be assumed that a junction between the electrically conductive joining material  5  and the wiring conductor  21  is divided lengthwise into two portions, namely a portion located farther away from the thermoelectric element  3  than the lengthwise midpoint of the junction, and a portion located nearer to the thermoelectric element  3  than the lengthwise midpoint of the junction. When viewed in a section in a width direction perpendicular to the length direction of the lead member  4 , given that the width of the junction on the side close to the thermoelectric element  3  is 0.5 to 1.0 mm, then the junction on the side away from the thermoelectric element  3  has a width two to three times the width of the junction on the side close to the thermoelectric element  3 . 
     As shown in  FIGS. 3 and 4 , in the thermoelectric module  10 , the lead member  4  may be inclined relative to a direction parallel to the one principal surface, as viewed in a section in a direction perpendicular to the one principal surface of the support substrate  11 , as well as along the axial direction of the lead member  4 . This reduces clearance between the support substrate  11  and the lead member  4 , and thus can reduce ingress of moisture into the wiring conductor  21 . Moreover, for the support substrate  11  serving as a high-temperature-side support substrate which has a relatively high temperature, on the occurrence of downwardly-curved convex warpage in the module which is in operation as a product due to the thermal expansion of the support substrate  11  and the thermal shrinkage of the support substrate  12 , the lead member  4  takes a nearly horizontal position within a housing case accommodating the thermoelectric module  10 . Thus, the module becomes more durable during the passage of current therethrough. 
     For example, the angle of inclination of the lead member  4  with respect to the one principal surface of the support substrate  11  falls in the range of 1 degree to 30 degrees. 
     Moreover, as shown in  FIG. 7 , a resin material  6  may be provided to cover the electrically conductive joining material  5  and at least part of the core  41 . This achieves greater mechanical strength with which the support substrate  11  becomes resistant to deformation under the application of external force to the lead member  4 . Thus, the lead member  4  can be restrained from separating from the wiring conductor  21 , and consequently the thermoelectric module  10  becomes more durable against external force. 
     Moreover, as shown in  FIG. 8 , a void  7  may be provided in a part of the boundary of the electrically conductive joining material  5  and the resin material  6 . This reduces distortion resulting from a difference in thermal expansion between the electrically conductive joining material  5  and the resin material  6 , and thus can restrain the resin material  6  from separating from the electrically conductive joining material  5 . 
     Moreover, as shown in  FIG. 9 , the core  41  may be configured to extend through and beyond the electrically conductive joining material  5 , so that the front end of the core  41  is bare of the electrically conductive joining material  5 . This increases the surface area of the joined portion of the lead member  4 , and thus achieves greater heat-dissipating capability with which Joule heat generated in the joined portion can be dissipated efficiently. Dissipation of heat from the lead member  4  can minimize cooling performance degradation. 
     EXAMPLES 
     The following describes examples. 
     As the first step, p-type and n-type thermoelectric materials made of Bi, Sb, Te, and Se were melted and solidified by Bridgman method to prepare rod-like materials each having a circular sectional profile of 1.5 mm in diameter. More specifically, the p-type thermoelectric material was formed from a solid solution of Bi 2 Te 3  (bismuth telluride) and Sb 2 Te 3  (antimony telluride), and the n-type thermoelectric material was formed from a solid solution of Bi 2 Te 3  (bismuth telluride) and Bi 2 Se 3  (bismuth selenide). The surfaces of the p-type thermoelectric material and the n-type thermoelectric material each in rod-like form were roughened by etching using nitric acid. 
     Next, the rod-like p-type thermoelectric material and the rod-like n-type thermoelectric material were cut into 1.6 mm in height, or 1.6 mm in thickness with a wire saw to obtain a p-type thermoelectric element and an n-type thermoelectric element. A nickel layer was formed on each of the cut surfaces of the obtained p-type thermoelectric element and n-type thermoelectric element by electrolytic plating. 
     Next, a substrate clad on both principal surfaces with copper, which is prepared by bonding a 105 μm-thick copper sheet to both sides of alumina filler-added epoxy resin under pressure, was printed with a solder paste by screen printing. 
     On the solder paste, 127 p-type thermoelectric elements and 127 n-type thermoelectric elements were arranged electrically in series with a mounter. The arrangement of the p-type thermoelectric elements and the n-type thermoelectric elements was sandwiched between a pair of support substrates. The construction so obtained was heated in a reflow furnace, with its upper and lower surfaces subjected to pressure, and, the thermoelectric elements were soldered to corresponding wiring conductors. 
     Next, a silicone-made sealing material was applied to between the pair of support substrates with an air dispenser. 
     To permit the passage of electric current through the obtained thermoelectric module, two lead members were joined to the construction with a solder-made conductive joining material. At this time, samples in which, by carrying out adjustments of the amount of solder supply and the angle at which the lead member was joined, the area of contact between the conductive joining material and the wiring conductor (a width of a bonding interface between the conductive joining material and the wiring conductor) was adjusted, as shown in  FIGS. 4 to 6 , were prepared (Sample No. 1 and Sample No. 2). Table 1 shows a list of bonding interface widths as viewed in a section in the width direction of the lead member. 
     Moreover, samples in which, by applying thermosetting epoxy resin so as to cover the joined portion of the lead member (an electrically conductive joining material), with an air dispenser, and thereafter curing the epoxy resin under heat in a dryer, the electrically conductive joining material and at least part of the core were covered with the resin material, as shown in  FIG. 7 , were prepared (Sample No. 3 to Sample No. 5). At this time, samples in which a void was provided in the boundary of solder and epoxy resin, as shown in  FIG. 8 , were prepared (Sample No. 4 and Sample No. 5). 
     In addition, sample in which a core length of a lead member was changed and the core of the lead member extended beyond the surface of the joined portion (the electrically conductive joining material) to provide a bared core portion, as shown in  FIG. 9 , was prepared (Sample No. 5). 
     Each sample was manufactured by 20 pieces (n=20), and measurement results described later were an average of values of 20 pieces. 
     For each of the samples thus prepared, a horizontal force was applied to the lead member using a tensile strength tester, and the strength (before the endurance test) when the lead member was separated was measured. Table 1 shows the results. 
     Next, a thermal conductive grease was applied to a surface of the pair of support substrates of the obtained thermoelectric module, the thermoelectric module was set on a heat sink whose temperature was controlled at 75° C., and 60 W of power was supplied to the thermoelectric module to generate a temperature difference. A temperature difference at the maximum voltage was defined as the cooling performance. Thereafter, an endurance test in which the energization direction was reversed every 30 seconds was carried out for 10000 cycles. 
     Then, for the samples after the endurance test, a horizontal force was applied to the lead member using the tensile strength tester, the strength when the lead member was separated was measured, and a change rate of the lead member tensile strength before and after the endurance test was calculated. Table 1 shows the results. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Length of electrically 
                   
                   
                   
                   
               
               
                   
                 conductive joining 
               
               
                   
                 material-wiring 
                   
                   
                   
                   
               
               
                   
                 conductor interface 
               
            
           
           
               
               
               
               
            
               
                   
                 Length 
                 Length 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 on side 
                 on side 
                   
                   
                   
                 Average 
                 Average of lead 
                   
               
               
                   
                 close to 
                 away from 
                   
                   
                   
                 of 
                 member tensile 
               
               
                   
                 thermo- 
                 thermo- 
                   
                   
                   
                 cooling 
                 strength (N) 
                 Average 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 electric 
                 electric 
                   
                   
                 Bared 
                 performance 
                 Before 
                 After 
                 of 
               
               
                 Sample 
                 elements 
                 elements 
                 Resin 
                   
                 core 
                 level 
                 endurance 
                 endurance 
                 change 
               
               
                 No. 
                 (mm) 
                 (mm) 
                 material 
                 Void 
                 portion 
                 (° C.) 
                 test 
                 test 
                 rate 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 3.2 
                 3.4 
                 Not 
                 Not 
                 Not 
                 63.6 
                 114 
                 93 
                 18.4% 
               
               
                   
                   
                   
                 provided 
                 formed 
                 provided 
               
               
                 2 
                 1.5 
                 3.6 
                 Not 
                 Not 
                 Not 
                 66.4 
                 117 
                 100 
                 14.5% 
               
               
                   
                   
                   
                 provided 
                 formed 
                 provided 
               
               
                 3 
                 1.6 
                 3.8 
                 Provided 
                 Not 
                 Not 
                 66.3 
                 121 
                 108 
                 10.7% 
               
               
                   
                   
                   
                   
                 formed 
                 provided 
               
               
                 4 
                 1.6 
                 3.7 
                 Provided 
                 Formed 
                 Not 
                 66.5 
                 118 
                 111 
                 5.9% 
               
               
                   
                   
                   
                   
                   
                 provided 
               
               
                 5 
                 1.5 
                 3.5 
                 Provided 
                 Formed 
                 Provided 
                 66.7 
                 114 
                 112 
                 1.8% 
               
               
                   
               
            
           
         
       
     
     As seen from Table 1, Sample No. 2 in which the area of contact between the electrically conductive joining material and the wiring conductor (the width of the bonding interface) is smaller on the side close to the thermoelectric elements than on the side away from the thermoelectric elements, is higher in cooling performance level and smaller in the change rate in lead member tensile strength than Sample No. 1 in which the area of contact on the side close to the thermoelectric elements is substantially equal to the area of contact on the side away from the thermoelectric elements. 
     Sample No. 3 in which the joined portion of the lead member is coated with the resin material is smaller in the change rate in lead member tensile strength and is thus more satisfactory than Sample No. 2. 
     Sample No. 4 in which the void is provided in the boundary of solder and epoxy resin is smaller in the change rate in lead member tensile strength and is thus more satisfactory than Sample No. 3. 
     Sample No. 5 in which the bared core portion extends beyond the surface of the joined portion (the electrically conductive joining material) is smaller in the change rate in lead member tensile strength and is thus more satisfactory than Sample No. 4. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10 : Thermoelectric module 
               11 ,  12 : Support substrate 
               111 : Extended portion 
               21 ,  22 : Wiring conductor 
               3 : Thermoelectric element 
               31 : p-type thermoelectric element 
               32 : n-type thermoelectric element 
               4 : Lead member 
               41 : Core 
               42 : Covering layer 
               5 : Electrically conductive joining material 
               6 : Resin material 
               7 : Void