Patent Publication Number: US-2022216387-A1

Title: Thermoelectric conversion device, method for controlling thermoelectric conversion device, method for cooling and/or heating object by using thermoelectric conversion device, and electronic device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a thermoelectric conversion device, a method for controlling the thermoelectric conversion device, a method for cooling and/or heating an object using the thermoelectric conversion device, and an electronic device including the thermoelectric conversion device. 
     BACKGROUND ART 
     PTL 1, PTL 2, and NPL 1 disclose periodic structures including a plurality of through holes. In these periodic structures, the through holes are regularly arranged in a thin film with a period of the order of nanometers (in the range of 1 nm to 1000 nm) in plan view. Each periodic structure is one type of phononic crystal structure. The phononic crystal structure of such a type generally has a unit cell that is a minimum unit forming the arrangement of the through holes. With this phononic crystal structure, the thermal conductivity of the thin film can be reduced. The thermal conductivity of a thin film can be reduced also by, for example, porosification. This is because the pores introduced into the thin film by the porosification reduce the thermal conductivity of the thin film. However, in the thin film having the phononic crystal structure, the thermal conductivity of the base material itself forming the thin film can be reduced. Therefore, it is expected to further reduce the thermal conductivity of such a thin film as compared with that achieved by simple porosification. 
     A thermoelectric conversion element including a thermoelectric converter containing a thermoelectric conversion material is a known art. The use of the thermoelectric conversion element allows a thermoelectric conversion device to be constructed. The thermoelectric conversion device can cool and/or heat an object by utilizing the Peltier effect. PTL 3 discloses a thermoelectric conversion element including a p-type thermoelectric conversion material and an n-type thermoelectric conversion material. 
     CITATION LIST 
     Patent Literature 
     PTL 1: U.S. Patent Application Publication No. 2017/0047499 
     PTL 2: U.S. Patent Application Publication No. 2017/0069818 
     PTL 3: International Publication No. WO2011/048634 
     Non Patent Literature 
     NPL 1: Nomura et al., “Impeded thermal transport in Si multiscale hierarchical architectures with phononic crystal nanostructures”, Physical Review B 91, 205422 (2015) 
     SUMMARY OF INVENTION 
     Technical Problem 
     The present disclosure provides a thermoelectric conversion device that can cool and/or heat an object with a high degree of flexibility and is, for example, suitable for maintaining variations in the temperature of the object within a prescribed range. 
     Solution to Problem 
     The present disclosure provides the following thermoelectric conversion device.
         A thermoelectric conversion device including:   a first thermoelectric conversion module;   a first insulating layer disposed on the first thermoelectric conversion module; and   a second thermoelectric conversion module disposed on the first insulating layer,   wherein the first thermoelectric conversion module includes one or two or more thermoelectric conversion elements, a first connection electrode, and a second connection electrode,   wherein the thermoelectric conversion elements of the first thermoelectric conversion module are electrically connected to the first connection electrode and the second connection electrode and located on an electric path connecting the first connection electrode and the second connection electrode,   wherein the second thermoelectric conversion module includes one or two or more thermoelectric conversion elements, a third connection electrode, and a fourth connection electrode,   wherein the thermoelectric conversion elements of the second thermoelectric conversion module are electrically connected to the third connection electrode and the fourth connection electrode and located on an electric path connecting the third connection electrode and the fourth connection electrode,   wherein each of the thermoelectric conversion elements includes a thermoelectric converter,   wherein the thermoelectric converter of at least one of the thermoelectric conversion elements includes a phononic crystal layer having a phononic crystal structure including a plurality of regularly arranged through holes, and   wherein a through direction of the plurality of through holes in the phononic crystal structure is substantially parallel to a stacking direction of the first thermoelectric conversion module, the first insulating layer, and the second thermoelectric conversion module.       

     Advantageous Effects of Invention 
     The present disclosure can provide a thermoelectric conversion device that can cool and/or heat an object with a high degree of flexibility and is, for example, suitable for maintaining variations in the temperature of the object within a prescribed range. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view schematically showing an example of the thermoelectric conversion device of the present disclosure. 
         FIG. 2  is a cross-sectional view schematically showing an example of a thermoelectric converter in a thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 3  is a cross-sectional view schematically showing another example of the thermoelectric converter in the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 4  is a plan view when the thermoelectric converter in  FIG. 3  is viewed from a first phononic crystal layer side. 
         FIG. 5  is a plan view when the thermoelectric converter in  FIG. 3  is viewed from a second phononic crystal layer side. 
         FIG. 6A  is a schematic illustration showing an example of a unit cell of a phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 6B  is a schematic illustration showing another example of the unit cell of the phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 6C  is a schematic illustration showing yet another example of the unit cell of the phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 6D  is a schematic illustration showing still another example of the unit cell of the phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 7  is a plan view schematically showing an example of the phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 8A  is a schematic illustration showing a unit cell of a first domain included in the phononic crystal structure in  FIG. 7  and the orientation of the unit cell. 
         FIG. 8B  is a schematic illustration showing a unit cell of a second domain included in the phononic crystal structure in  FIG. 7  and the orientation of the unit cell. 
         FIG. 9  is an enlarged view of region R 1  in the phononic crystal structure in  FIG. 7 . 
         FIG. 10  is a plan view schematically showing another example of the phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 11  is enlarged view of region R 2  in the phononic crystal structure in  FIG. 10 . 
         FIG. 12  is a plan view schematically showing yet another example of the phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 13  is an enlarged view of region R 3  in the phononic crystal structure in  FIG. 12 . 
         FIG. 14  is a plan view schematically showing still another example of the phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 15  is a plan view schematically showing even another example of the phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 16  is a plan view schematically showing even another example of the phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 17A  is a schematic illustration showing an example of the unit cell of the phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 17B  is a schematic illustration showing another example of the unit cell of the phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 18  is a plan view schematically showing even another example of the phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 19  is a plan view schematically showing even another example of the phononic crystal structure that the thermoelectric conversion element can have. 
         FIG. 20A  is a plan view schematically showing an example of the phononic crystal layer that the thermoelectric conversion element can have. 
         FIG. 20B  is a cross-sectional view showing a cross section  20 B- 20 B of the phononic crystal layer in  FIG. 20A . 
         FIG. 21  is a cross-sectional view schematically showing another example of the thermoelectric converter in the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 22A  is a plan view schematically showing another example of the phononic crystal layer that the thermoelectric conversion element can have. 
         FIG. 22B  is a cross-sectional view showing a cross section  22 B- 22 B of the phononic crystal layer in  FIG. 22A . 
         FIG. 23  is a cross-sectional view schematically showing another example of the thermoelectric converter in the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24A  is a schematic cross-sectional view illustrating an example of a method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24B  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24C  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24D  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24E  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24F  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24G  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24H  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24I  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24J  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24K  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24L  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24M  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24N  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 24O  is a schematic cross-sectional view illustrating the example of the method for producing the thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have. 
         FIG. 25  is a cross-sectional view schematically showing another example of the thermoelectric conversion device of the present disclosure. 
         FIG. 26  is a cross-sectional view schematically showing yet another example of the thermoelectric conversion device of the present disclosure. 
         FIG. 27  is a flowchart showing an example of a control method in the present disclosure. 
         FIG. 28  is a graph showing voltage application patterns in an example of the control method in the present disclosure. 
         FIG. 29  is a graph showing voltage application patterns in an example of the control method in the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     (Findings Underlying the Present Disclosure) 
     The thermoelectric conversion device of the present disclosure includes a plurality of thermoelectric conversion modules stacked together. The thermoelectric conversion modules can be controlled independently through connection electrodes disposed in the respective thermoelectric conversion modules. For example, a thermoelectric conversion module close to an object is controlled differently from a thermoelectric conversion module away from the object. In this manner, the degree of flexibility in controlling cooling and/or heating of the object can be increased. 
     Moreover, the thermoelectric conversion device of the present disclosure includes a thermoelectric conversion element including a thermoelectric converter having a phononic crystal structure. This can enhance the thermal insulation performance of the thermoelectric conversion modules each including the above element, typically the thermal insulation performance of the plurality of thermoelectric conversion modules in their stacking direction. The enhanced thermal insulation performance improves the thermoelectric conversion efficiency of the thermoelectric conversion modules. The enhanced thermal insulation performance also improves the degree of flexibility in control patterns for thermoelectric conversion modules adjacent to each other when these modules are controlled independently. These contribute synergistically to the improvement in the degree of flexibility in controlling the cooling and/or heating of the object. 
     (Embodiments of the Present Disclosure) 
     Embodiments of the present disclosure will be described with reference to the drawings. The embodiments described below show general or specific examples. Numerical values, shapes, materials, components, arrangements and connections of the components, process conditions, steps, the order of the steps, etc. shown in the following embodiments are merely examples and are not intended to limit the present disclosure. 
     Among the components in the following embodiments, components not described in an independent claim representing the broadest concept will be described as optional components. The drawings are schematic drawings and are not necessarily strictly accurate illustrations. 
     [Thermoelectric Conversion Device] 
     FIRST EMBODIMENT 
       FIG. 1  shows a thermoelectric conversion device in a first embodiment. The thermoelectric conversion device  1  in  FIG. 1  includes a first thermoelectric conversion module  2 , a first insulating layer  3 , and a second thermoelectric conversion module  4 . The first insulating layer  3  is disposed on the first thermoelectric conversion module  2 . The second thermoelectric conversion module  4  is disposed on the first insulating layer  3 . The first thermoelectric conversion module  2 , the first insulating layer  3 , and the second thermoelectric conversion module  4  each have a laminar shape and are stacked in this order to form a layered structure  5 . 
     The first thermoelectric conversion module  2  includes two or more thermoelectric conversion elements  21 ( 21   a ), a first connection electrode  11 , and a second connection electrode  12 . The thermoelectric conversion elements  21   a  of the first thermoelectric conversion module  2  are electrically connected to the first connection electrode  11  and the second connection electrode  12 . The electrical connection of each thermoelectric conversion element  21   a  to the first connection electrode  11  or the second connection electrode  12  is a direct connection or an indirect connection through another thermoelectric conversion element  21   a.  The thermoelectric conversion elements  21   a  are located on an electric path connecting the first connection electrode  11  and the second connection electrode  12 . In the example in  FIG. 1 , the two or more thermoelectric conversion elements  21   a  are electrically connected to each other in series between the first connection electrode  11  and the second connection electrode  12 . However, the electrical connection form of the thermoelectric conversion elements  21   a  between the first connection electrode  11  and the second connection electrode  12  is not limited to the above example. For example, a combination of series and parallel connections may be used. By applying a voltage through the first connection electrode  11  and the second connection electrode  12 , the thermoelectric conversion elements  21   a  and the first thermoelectric conversion module  2  operate as Peltier elements and a Peltier module, respectively. The Peltier module is, for example, a Peltier-type cooling module, a Peltier-type cooling/heating module, or a Peltier type heating module. 
     The second thermoelectric conversion module  4  includes two or more thermoelectric conversion elements  21 ( 21   b ), a third connection electrode  13 , and a fourth connection electrode  14 . The thermoelectric conversion elements  21   b  of the second thermoelectric conversion module  4  are electrically connected to the third connection electrode  13  and the fourth connection electrode  14 . Each of the thermoelectric conversion elements  21   b  is electrically connected to the third connection electrode  13  or the fourth connection electrode  14  directly or indirectly through another thermoelectric conversion element  21   b.  The thermoelectric conversion elements  21   b  are located on an electric path connecting the third connection electrode  13  and the fourth connection electrode  14 . In the example in  FIG. 1 , the two or more thermoelectric conversion elements  21   b  are electrically connected to each other in series between the third connection electrode  13  and the fourth connection electrode  14 . However, the electrical connection form of the thermoelectric conversion elements  21   b  between the third connection electrode  13  and the fourth connection electrode  14  is not limited to the above example. For example, a combination of series and parallel connections may be used. By applying a voltage through the third connection electrode  13  and the fourth connection electrode  14 , the thermoelectric conversion elements  21   b  and the second thermoelectric conversion module  4  operate as Peltier elements and a Peltier module, respectively. 
     In each of the first thermoelectric conversion module  2  and the second thermoelectric conversion module  4 , the two or more thermoelectric conversion elements  21  are typically arranged in an array. The first thermoelectric conversion module  2  and/or the second thermoelectric conversion module  4  may include one thermoelectric conversion element  21 . 
     The electrical connection form of the thermoelectric conversion elements  21  in the first thermoelectric conversion module  2  may be the same as or different from the electrical connection form of the thermoelectric conversion elements  21  in the second thermoelectric conversion module  4 . 
     In the example in  FIG. 1 , the number of first connection electrodes  11 , the number of second connection electrodes  12 , the number of third connection electrodes  13 , and the number of fourth connection electrodes  14  are each  1 . However, the number of connection electrodes may be two or more. 
     The voltage applied between the first connection electrode  11  and the second connection electrode  12  and the voltage applied between the third connection electrode  13  and the fourth connection electrode  14  can be controlled independently. This allows the first thermoelectric conversion module  2  and the second thermoelectric conversion module  4  to be controlled independently. For example, a first voltage may be applied to the first thermoelectric conversion module  2 , and a second voltage with an application pattern different from that of the first voltage may be applied to the second thermoelectric conversion module  4 . 
     The thermoelectric conversion device  1  in  FIG. 1  further includes a substrate (base layer)  6 , a second insulating layer  7 , and a protective layer  8 . The layered structure  5  is disposed on the substrate  6  with the second insulating layer  7  therebetween. The protective layer  8  is disposed on the second thermoelectric conversion module  4 . The protective layer  8  is disposed as an outermost layer of the thermoelectric conversion device  1  (the outermost layer on the side opposite to the substrate  6 ). The thermoelectric conversion device  1  has a structure including the substrate  6 , the second insulating layer  7 , the first thermoelectric conversion module  2 , the first insulating layer  3 , the second thermoelectric conversion module  4 , and the protective layer  8  that have been stacked in this order. The first connection electrode  11 , the second connection electrode  12 , the third connection electrode  13 , and the fourth connection electrode  14  are via lines embedded in through holes passing through the layer(s) and extending in the stacking direction of the layered structure  5 , reach the upper surface of the protective layer  8 , and are exposed at the upper surface. The exposed end of each of the connection electrodes can be, for example, a connection point for a controller and/or a control module that controls the voltage applied to the first thermoelectric conversion module  2  or the second thermoelectric conversion module  4 . 
     Each thermoelectric conversion element  21  includes a p-type thermoelectric converter  22  and an n-type thermoelectric converter  23  that serve as thermoelectric converters and further includes a first electrode  24 , a second electrode  25 , and a third electrode  26 . A first end of the p-type thermoelectric converter  22  and a first end of the n-type thermoelectric converter  23  are electrically connected through the first electrode  24 . A second end of the p-type thermoelectric converter  22  is electrically connected to the second electrode  25 . A second end of the n-type thermoelectric converter  23  is electrically connected to the third electrode  26 . One selected from the second electrode  25  and the third electrode  26  is disposed on the electric path connecting the corresponding connection electrodes and located on the upstream side in the path. The other selected from the second electrode  25  and the third electrode  26  is disposed on the electric path connecting the corresponding connection electrodes and located on the downstream side in the path. 
     In other words, a voltage can be applied to the thermoelectric conversion element  21  through the second electrode  25  and the third electrode  26 . The second electrode  25  of each of the thermoelectric conversion elements  21   a  is electrically connected to the third electrode of an adjacent one of the thermoelectric conversion elements  21   a.  The upstream and downstream sides in the electric path may be determined, for example, based on the direction of the electric current flowing through the path when a typical voltage is applied to the thermoelectric conversion module. In each thermoelectric conversion element  21 , the direction in which a pair of electrodes sandwiching the thermoelectric converter therebetween are connected is generally the stacking direction of the layered structure  5 . In other words, the direction in which the flow of heat is controlled in the thermoelectric conversion elements  21  and the thermoelectric conversion device  1  is generally the stacking direction of the layered structure  5 . In each of the thermoelectric conversion elements  21  in  FIG. 1 , an insulating portion  27  is disposed between the p-type thermoelectric converter  22  and the n-type thermoelectric converter  23 , and this configuration allows electric insulation between the thermoelectric converters  22  and  23  to be maintained. 
     The p-type thermoelectric converter  22  and/or the n-type thermoelectric converter  23  in each thermoelectric conversion element  21 , typically each of the p-type thermoelectric converter  22  and the n-type thermoelectric converter  23 , includes a phononic crystal layer. The phononic crystal layer has a plurality of regularly arranged through holes. The through direction of the plurality of through holes in the phononic crystal structure is substantially parallel to the stacking direction of the layered structure  5 . The phononic crystal layer includes, for example, a first phononic crystal layer and a second phononic crystal layer described later. The through holes include, for example, first through holes and second through holes described later. In the first embodiment, all the thermoelectric conversion elements  21  include the respective phononic crystal layers. 
     However, not all the thermoelectric conversion element  21  may include the phononic crystal layers. The term “substantially parallel” as used herein means that, even when the relation between two directions deviates from a parallel relation by, for example, 5 degrees or less, preferably 3 degrees of less, and more preferably 1 degree or less, these directions are regarded as parallel to each other. 
     In insulators and semiconductors, heat is transferred mainly by lattice vibrations called phonons. The thermal conductivity of a material composed of an insulator or a semiconductor is determined by the dispersion relation of phonons in the material. The dispersion relation of phonons means the relation between their frequency and wavenumber or the band structure of phonons. In insulators and semiconductors, phonons that transfer heat are present in a wide frequency band of from 100 GHz to 10 THz. This frequency band is a thermal band. The thermal conductivity of a material is determined by the dispersion relation of phonons in the thermal band. In the phononic crystal structures, the dispersion relation of phonons in the material can be controlled by the periodic structure formed from the through holes. In other words, in a thermoelectric converter having a phononic crystal structure, the thermal conductivity itself of the material of the thermoelectric converter such as its base material can be controlled. In particular, the formation of a phononic band gap (PBG) by the phononic crystal structure can significantly reduce the thermal conductivity of the material. No phonons are allowed to be present in the PBG. Therefore, the PBG located in the thermal band can serve as a gap for thermal conduction. Moreover, in frequency bands other than the PBG, the gradients of the phonon dispersion curves are reduced by the PBG. The reduction in the gradients reduces the group velocity of phonons, causing a reduction in the speed of heat conduction. These characteristics significantly contribute to a reduction in the thermal conductivity itself of the material. 
     &lt;Phononic Crystal Structure&gt; 
     A description will be given of a phononic crystal structure that the thermoelectric converter of each thermoelectric conversion element  21  can have. The p-type thermoelectric converter  22  is exemplified as the thermoelectric converter. The n-type thermoelectric converter  23  can also have the phononic crystal structure described below. 
       FIG. 2  shows an example of the p-type thermoelectric converter  22 . The p-type thermoelectric converter  22  in  FIG. 2  includes a first phononic crystal layer  44  having a first phononic crystal structure including a plurality of first through holes  43  arranged regularly. The p-type thermoelectric converter  22  in  FIG. 2  is a monolayer structural body including the first phononic crystal layer  44 . The through direction of the plurality of first through holes  43  in the first phononic crystal structure and in the first phononic crystal layer  44  is a direction connecting a first end  41  of the p-type thermoelectric converter  22  and its second end  42 . The first electrode  24  is disposed on the first end  41 . The second electrode  25  is disposed on the second end  42 . The above direction is substantially perpendicular to the connection surface between the p-type thermoelectric converter  22  and the first electrode  24  and the connection surface between the p-type thermoelectric converter  22  and the second electrode  25 . The term “substantially perpendicular” as used herein means that, even when the relation between two directions deviates from a perpendicular relation by, for example, 5 degrees or less, preferably 3 degrees or less, and more preferably 1 degree or less, these directions are regarded as perpendicular to each other. 
     Another example of the p-type thermoelectric converter  22  is shown in  FIG. 3 . The p-type thermoelectric converter  22  shown in  FIG. 3  further includes, in addition to the first phononic crystal layer  44 , a second phononic crystal layer  46  having a second phononic crystal structure including a plurality of second through holes  45  arranged regularly. The first phononic crystal layer  44  and the second phononic crystal layer  46  are stacked in a direction connecting the first end  41  and the second end  42  of the p-type thermoelectric converter  22 . The first phononic crystal layer  44  and the second phononic crystal layer  46  are stacked in the stacking direction of the layered structure  5 . The through direction of the plurality of first through holes  43  in the first phononic crystal structure and in the first phononic crystal layer  44  is substantially parallel to the through direction of the plurality of second through holes  45  in the second phononic crystal structure and in the second phononic crystal layer  46 . The p-type thermoelectric converter  22  in  FIG. 3  is a multilayer structural body including the first phononic crystal layer  44  and the second phononic crystal layer  46 . The first phononic crystal layer  44  and the second phononic crystal layer  46  are in contact with each other. 
     The PBG is distributed three-dimensionally, and it is expected that a heat flow in each phononic crystal layer can be controlled not only in its in-plane directions but also in its thickness direction and that the thermal conductivity can be reduced by controlling the heat flow. The phrase “the thickness direction of a phononic crystal layer” means the through direction of a plurality of regularly arranged through holes in  FIGS. 2 and 3 . In the p-type thermoelectric converter  22  in  FIG. 3 , at least two phononic crystal layers are stacked in the thickness direction. It is expected that the stack with an increased thickness will allow the heat flow in the p-type thermoelectric converter  22  in the thickness direction to be controlled more reliably. 
     The thickness of the first phononic crystal layer  44  and the thickness of the second phononic crystal layer  46  are, for example, equal to or more than 10 nm and equal to or less than 500 nm. When the p-type thermoelectric converter  22  includes two or more phononic crystal layers, the thicknesses of these phononic crystal layers may be the same as or different from each other. 
     No limitation is imposed on the number of phononic crystal layers included in the p-type thermoelectric converter  22 . When the p-type thermoelectric converter  22  includes two or more phononic crystal layers, the phononic crystal layers may be stacked in contact with each other or may be stacked with another member interposed therebetween. The other member is, for example, an oxide film such as a SiO 2  film or a buffer layer described later. 
       FIG. 4  is a plan view showing the p-type thermoelectric converter  22  in  FIG. 3  when it is viewed from the first phononic crystal layer  44  side.  FIG. 5  is a plan view showing the p-type thermoelectric converter  22  in  FIG. 3  when it is viewed from the second phononic crystal layer  46  side. In the p-type thermoelectric converter  22  in  FIGS. 3, 4, and 5 , the first phononic crystal structure that the first phononic crystal layer  44  has structurally differs from the second phononic crystal structure that the second phononic crystal layer  46  has. Specifically, the period P of the arrangement of the first through holes  43  differs from the period P of the arrangement of the second through holes  45 . When the first phononic crystal structure structurally differs from the second phononic crystal structure, at least part of the second through holes  45  are generally not in communication with the first through holes  43 . In a p-type thermoelectric converter  22  including two or more phononic crystal layers, the phononic crystal layers may be structurally the same as each other. 
     The thickness of the phononic crystal layers  44  and  46  that corresponds to the length of the through holes  43  and  45  may be equal to or larger than twice the diameter of the through holes. In this case, the distance between the upper and lower surfaces of each of the phononic crystal layers  44  and  46  can be increased. This allows the temperature difference between the upper and lower surfaces of each of the phononic crystal layers  44  and  46  to be increased, so that the thermoelectric conversion efficiency can be improved. 
     As used herein, the term “the upper surface” and “the lower surface” of a phononic crystal layer mean, respectively, one principal surface of the phononic crystal layer and the other principal surface opposite to the one principal surface when the phononic crystal layer is viewed in the through direction of the through holes. The term “the principal surface” means a surface having the largest area. The upper limit of the thickness of each of the phononic crystal layers  44  and  46  is, for example, equal to or less than 100 times the diameter of the through holes and may be equal to or less than 80 times, equal to or less than 60 times, and equal to or less than 50 times the diameter of the through holes. 
     The ratio of the total volume of the through holes  43  or  45  included in each of the phononic crystal layers  44  and  46  to the volume of the each of the phononic crystal layers  44  and  46 , i.e., the porosity of the phononic crystal layer, may be equal to or more than 10%. In this case, the volumes of the phononic crystal layers  44  and  46  excluding the through holes  43  and  45  can be reduced, so that the effect of the PBG can be increased. Therefore, the thermal conductivity of each of the phononic crystal layers  44  and  46  can be further reduced, and the thermoelectric conversion efficiency can be increased. The upper limit of the porosity of each of the phononic crystal layers  44  and  46  is, for example, equal to or lower than 90% and may be equal or lower than 70%, equal to or lower than 50%, and equal to or lower than 40%. 
     Examples of the case where the first phononic crystal structure structurally differs from the second phononic crystal structure include the following cases. A plurality of cases may be used in combination.
         The period P of the arrangement of the first through holes  43  differs from the period P of the arrangement of the second through holes  45 .   The diameter D of the first through holes  43  differs from the diameter D of the second through holes  45 .   The type of unit cell  91  including first through holes  43  differs from the type of unit cell  91  including second through holes  45 .       

     As shown in a phononic crystal structure A described later, the arrangement of the first through holes  43  in the first phononic crystal structure and the arrangement of the second through holes  45  in the second phononic crystal structure are not always constant over the entire phononic crystal layers. In consideration of the above, when the first phononic crystal structure structurally differs from the second phononic crystal structure, the p-type thermoelectric converter  22  can have configurations described below. The p-type thermoelectric converter  22  may have a configuration obtained by combining any of the configurations described below. 
     Configuration A: The first phononic crystal structure includes a domain A that is a phononic crystal region. The second phononic crystal structure includes a domain B that is a phononic crystal region. The domain A and the domain B overlap with each other when viewed in the through direction of the first through holes  43  and the second through holes  45 . The period P of the arrangement of the first through holes  43  in the domain A differs from the period of the arrangement of the second through holes  45  in the domain B. 
     Configuration B: The first phononic crystal structure includes a domain A that is a phononic crystal region. The second phononic crystal structure includes a domain B that is a phononic crystal region. The domain A and the domain B overlap with each other when viewed in the through direction of the first through holes  43  and the second through holes  45 . The diameter of the first through holes  43  in the domain A differs from the diameter of the second through holes  45  in the domain B. 
     Configuration C: The first phononic crystal structure includes a domain A that is a phononic crystal region. The second phononic crystal structure includes a domain B that is a phononic crystal region. The domain A and the domain B overlap with each other when viewed in the through direction of first through holes  43  and the second through holes  45 . 
     The type of unit cell including first through holes  43  in the domain A differs from the type of unit cell including second through holes  45  in the domain B. 
     Each of the domains, which are phononic crystal regions, is a region having an area of, for example, equal to or more than 25 P 2  in plan view, where P is the period of the arrangement of the through holes  43  or  45 . To control the dispersion relation of phonons using the phononic crystal structure, the domain may have an area of at least equal to or more than 25 P 2 . When the length of the sides of a square domain in plan view is equal to or more than 5&gt;P, the area of the domain can be equal to or more than 25 P 2 . 
     No limitation is imposed on the shape of each domain in plan view. The shape of each domain in plan view is, for example, a polygonal shape such as a triangular, square, or rectangular shape, a circular shape, an elliptical shape, or a combination thereof. Each domain may have an irregular shape in plan view. No limitation is imposed on the number of domains included in each phononic crystal structure. No limitation is imposed on the size of each domain included in the phononic crystal structure. One domain may be spread over the entire phononic crystal layer. The term “in plan view” as used herein means that the phononic crystal layer is viewed in the through direction of the through holes. 
     The period P of the arrangement of the through holes  43  or  45  is, for example, equal to or more than 1 nm and equal to or less than 300 nm. This is because the wavelength of phonons carrying heat ranges mainly from 1 nm to 300 nm. The period P is determined by the center-to-center distance between adjacent through holes  43  or  45  in plan view. 
     The diameter D of the through holes  43  or  45  satisfies, for example, D/P≥0.5, where D/P is the ratio of the diameter D to the period P. If the ratio D/P&lt;0.5, the porosity of the phononic crystal structure is excessively small, so that the heat flow may not be controlled sufficiently, e.g., the thermal conductivity may not be sufficiently reduced. The upper limit of the ratio D/P is, for example, less than 0.9 in order to prevent contact between adjacent through holes  43  or  45 . The diameter D of the through holes  43  or  45  is the diameter of their openings. When the openings of the through holes  43  or  45  have a circular shape in plan view, the diameter D is the diameter of the circular shape. The openings of the through holes  43  or  45  may have a non-circular shape in plan view. In this case, the diameter D is defined as the diameter of a virtual circle having the same area as the area of the openings. 
     Examples of the type of unit cell  91  including a plurality of regularly arranged through holes  43  or  45  include a square lattice ( FIG. 6A ), a hexagonal lattice ( FIG. 6B ), a rectangular lattice ( FIG. 6C ), and a centered rectangular lattice ( FIG. 6D ). However, the type of unit cell  91  is not limited to these examples. 
     The material M forming the p-type thermoelectric converter  22 , the n-type thermoelectric converter  23 , and the phononic crystal layers that the p-type thermoelectric converter  22  and the n-type thermoelectric converter  23  can have is typically a semiconductor material doped with an impurity element such that the material is of an appropriate semiconductor type such as the p or n type. The semiconductor material is, for example, silicon (Si), Ge, SiGe, SiC, ZnSe, CdSn, ZnO, GaAs, InP, or GaN. The material M may be a material other than the semiconductor materials, and such a material is, for example, TiN, SiN, or VO 2 . However, the material M is not limited to the above examples. 
     Among semiconductor materials, a Si-based semiconductor material generally has a relatively high thermal conductivity. Therefore, in a conventional thermoelectric conversion element including thermoelectric converters formed of a Si-based semiconductor material, it is difficult to obtain high thermoelectric conversion efficiency. However, in the thermoelectric conversion element  21 , the thermoelectric converters each have a phononic crystal layer. Therefore, in the thermoelectric conversion elements  21  and the thermoelectric conversion device  1  including the thermoelectric conversion elements  21 , high thermoelectric conversion efficiency can be obtained even when the thermoelectric converters are formed of a Si-based semiconductor material. 
     The following advantages, for example, are obtained when the thermoelectric converters can be formed of a Si-based semiconductor material. The substrate  6  may be used as a base substrate.
         The thermoelectric conversion elements and a thermoelectric conversion device including these elements can be formed on a base substrate formed of a Si-based semiconductor material such as a Si wafer.   The thermoelectric conversion elements and the thermoelectric conversion device can be embedded in a base substrate formed of a Si-based semiconductor material. In this case, for example, an integrated circuit such as a CPU or GPU can be formed on the base substrate in which the thermoelectric conversion elements or the thermoelectric conversion device is embedded. This means, for example, that an electronic device such as an integrated circuit device in which a Peltier-type cooling device is embedded can be produced. The integrated circuit device may be a semiconductor element including a thermoelectric conversion device and an integrated circuit integrated together and housed in one package.       

     The first phononic crystal structure and the second phononic crystal structure may have the following configuration. Each phononic crystal structure includes a first domain and a second domain that are phononic crystal regions. The first domain has a plurality of through holes regularly arranged in a first direction in a cross section perpendicular to the through direction of the through holes. The second domain has a plurality of through holes regularly arranged in a second direction different from the first direction in a cross section perpendicular to the through direction of the through holes. Such a phononic crystal structure having a plurality of domains distinguished by their arrangement orientation is hereinafter referred to as a phononic crystal structure A. The arrangement orientation can be determined by the orientation of the unit cell. 
     According to studies by the present inventors, the degree of reduction in thermal conductivity obtained by a phononic crystal structure depends on the angle between the direction of heat transfer and the orientation of the unit cell of the phononic crystal structure. This may be because factors relating to heat conduction such as the number of PBGs, the band width of each PBG, the average group velocity of phonons depend on the above angle. As for heat transfer, phonons flow in a direction from a high temperature side to a low temperature side in a macroscopic sense. When attention is focused on micro-regions of the order of nanometers, the flow of phonons has no directivity. Specifically, phonons do not flow in a uniform direction in a microscopic sense. 
     The above-described Patent Literature and Non Patent Literature disclose members each having a plurality of phononic crystal regions with the same unit cell orientation. In these members, their interaction with phonons flowing in a specific direction is maximized in a microscopic sense, but the interaction with phonons flowing in the other directions is weakened. The phononic crystal structure A includes two or more phononic crystal regions with different unit cell directions. Therefore, the interaction with phonons flowing in a plurality of directions can be enhanced in a microscopic sense. This feature allows the degree of flexibility in controlling the heat flow to be further improved. 
     The following description relates to the phononic crystal structure A that at least one phononic crystal layer selected from the first phononic crystal layer  44  and the second phononic crystal layer  46  can have. When a plurality of phononic crystal layers have their respective phononic crystal structures A, these phononic crystal structures A may be structurally the same as or different from each other. 
     An example of the phononic crystal structure A is shown in  FIG. 7 .  FIG. 7  shows a plan view of part of a phononic crystal layer  56 . The phononic crystal layer  56  may be at least one phononic crystal layer selected from the first phononic crystal layer  44  and the second phononic crystal layer  46 . The phononic crystal layer  56  is a thin film having a thickness of, for example, equal to or larger than 10 nm and equal to or less than 500 nm. The phononic crystal layer  56  is rectangular in plan view. A plurality of through holes  50  extending in the thickness direction of the phononic crystal layer  56  are provided in the phononic crystal layer  56 . The phononic crystal structure A that the phononic crystal layer  56  has is a two-dimensional phononic crystal structure in which the plurality of through holes  50  are regularly arranged in in-plane directions. 
     The phononic crystal structure A includes the first domain  51 A and the second domain  51 B that are phononic crystal regions. The first domain  51 A has a phononic single crystal structure including a plurality of through holes  50  arranged regularly in a first direction in plan view. The second domain  51 B has a phononic single crystal structure including a plurality of through holes  50  arranged regularly in a second direction different from the first direction in plan view. In each of the single crystal structures, the plurality of through holes  50  have the same diameter and arranged with the same period. In each of the single crystal structures, the orientations of unit cells  91 A or  91 B of the plurality of regularly arranged through holes  50  are the same as each other. The first domain  51 A and the second domain  51 B each have a rectangular shape in plan view. The shape of the first domain  51 A and the shape of the second domain  51 B are the same in plan view. The phononic crystal structure A is also a phononic polycrystal structure  52  that is a complex body including a plurality of phononic single crystal structures. 
     As shown in  FIGS. 8A and 8B , in the phononic crystal structure A, the orientation  53 A of each unit cell  91 A in the first domain  51 A differs from the orientation  53 B of each unit cell  91 B in the second domain  51 B in plan view. The angle between the orientation  53 A and the orientation  53 B in plan view is, for example, equal to or more than 10 degrees. When the unit cell  91 A and the unit cell  91 B are identical and have an n-fold rotational symmetry, the upper limit of the angle between the orientation  53 A and the orientation  53 B is less than 360/n degrees. When each unit cell has n-fold symmetries for a plurality of n&#39;s, the largest one of the n&#39;s is used to determine the upper limit of the angle. For example, a hexagonal lattice has a 2-fold rotational symmetry, a 3-fold rotational symmetry, and a 6-fold rotational symmetry. In this case, “6” is used for the n defining the upper limit of the angle. Specifically, when the unit cells  91 A and  91 B are each a hexagonal lattice, the angle between the orientation  53 A and the orientation  53 B is less than 60 degrees. The phononic crystal structure A includes at least two phononic crystal regions having different unit cell orientations. The phononic crystal structure A may further include any other phononic crystal regions and/or regions having no phononic crystal structure so long as the above condition is met. 
     The orientation of a unit cell can be determined based on any rule. However, it is necessary that the same rule be applied to different domains to determine the orientations of their unit cells. The orientation of a unit cell is, for example, the extending direction of a straight line bisecting the angle between two non-parallel sides included in the unit cell. However, it is necessary to use the same rule for different domains to define their two sides. 
       FIG. 9  shows an enlarged view of region R 1  in the phononic crystal structure A in  FIG. 7 . The orientations  53 A and  53 B of the unit cells  91 A and  91 B change at the interface  55  between the first domain  51 A and the second domain  51 B adjacent to each other. The interface  55  at which the orientations of the unit cells change has a large interface resistance to heat macroscopically flowing through the phononic crystal structure A. The interface resistance is based on a mismatch between the group velocity of phonons in the first domain  51 A and the group velocity of phonons in the second domain  51 B. The interface resistance contributes to a reduction in the thermal conductivity of the phononic crystal layer  56  having the phononic crystal structure A. In  FIG. 9 , the interface  55  extends linearly in plan view. The interface  55  extends in the width direction of the rectangular phononic crystal layer  56  in plan view. The width direction may be a direction perpendicular to the extending direction of the centerline of the phononic crystal layer  56  that is determined by the direction of macroscopic heat transfer. The interface  55  divides the phononic crystal structure A in a direction substantially perpendicular to the direction of macroscopic heat transfer in plan view. 
     In the phononic crystal structure A in  FIG. 7 , the period P of the arrangement of the plurality of through holes  50  in the first domain  51 A is the same as the period P of the arrangement of the plurality of through holes  50  in the second domain  51 B. 
     In the phononic crystal structure A in  FIG. 7 , the diameter of the plurality of through holes  50  regularly arranged in the first domain  51 A is the same as the diameter of the plurality of through holes  50  regularly arranged in the second domain  51 B. 
     In the phononic crystal structure A in  FIG. 7 , the type of unit cell  91 A in the first domain  51 A is the same as the type of unit cell  91 B in the second domain  51 B. The unit cell  91 A and the unit cell  91 B in  FIG. 7  are each a hexagonal lattice. 
     No limitation is imposed on the number of domains included in the phononic crystal structure A. The larger the number of domains included in the phononic crystal structure A is, the larger the effect of the interface resistance at the interfaces between domains is. 
     Other examples of the phononic crystal structure A will be shown. 
     In a polycrystal structure  52  that is a phononic crystal structure A in  FIGS. 10 and 11 , the interface  55  between a first domain  51 A and a second domain  51 B adjacent to each other extends in the direction of the long sides of the rectangular phononic crystal layer  56  in plan view. The phononic crystal structure A in  FIGS. 10 and 11  is structurally the same as the phononic crystal structure A in  FIG. 7  except for the above feature.  FIG. 11  is an enlarged view of region R 2  in  FIG. 10 . 
     In the phononic crystal structures A in  FIGS. 7 and 10 , the size of the first domain  51 A is the same as the size of the second domain  51 B in plan view. However, the sizes of the first and second domains  51 A and  51 B included in a phononic structure A may differ from each other in plan view. 
     In a polycrystal structure  52  that is a phononic crystal structure A in  FIGS. 12 and 13 , a first domain  51 B is surrounded by a second domain  51 A in plan view. The first domain  51 A has a rectangular outer shape in plan view. The second domain  51 B has a rectangular shape in plan view. The size of the first domain  51 A differs from the size of the second domain  51 B in plan view. In plan view, the interface  55  between the second domain  51 B and the first domain  51 A surrounding the second domain  51 B forms the outer edge of the second domain  51 B. The phononic crystal structure A in  FIGS. 12 and 13  is structurally the same as the phononic crystal structure A in  FIG. 7  except for the above feature.  FIG. 13  is an enlarged view of region R 3  in  FIG. 12 . 
     In the phononic crystal structure A in  FIGS. 12 and 13 , the interface  55  has bent portions. 
     Moreover, the phononic crystal structure A in  FIGS. 12 and 13  includes the second domain  51 B that is not in contact with the sides of the phononic crystal layer  56 . 
     In a polycrystal structure  52  that is a phononic crystal structure A in  FIG. 14 , a first domain  51 A and a second domain  51 B are disposed so as to be spaced apart from each other in plan view. More specifically, in plan view, a region  201  having no through holes  50  is disposed between the first domain  51 A and the second domain  51 B so as to extend in the long side direction of the phononic crystal layer  56 . The phononic crystal structure A in  FIG. 14  is structurally the same as the phononic crystal structure A in  FIG. 7  except for the above feature. 
     In a polycrystal structure  52  that is a phononic crystal structure A in  FIG. 15 , a first domain  51 A and a second domain  51 B are disposed so as to be spaced apart from each other in plan view. More specifically, in plan view, a region  202  having randomly arranged through holes  50  is disposed between the first domain  51 A and the second domain  51 B so as to extend in the long side direction of the phononic crystal layer  56 . In the region  202 , the through holes  50  are not arranged regularly in plan view. Alternatively, in the region  202 , the area of a regular arrangement region is, for example, less than 25 P 2  in plan view. Here, P is the period of the arrangement of the through holes  50 . The phononic crystal structure A in  FIG. 15  is structurally the same as the phononic crystal structure A in  FIG. 7  except for the above feature. 
     A polycrystal structure  52  that is a phononic crystal structure A in  FIG. 16  includes a plurality of domains  51 A,  51 B,  51 C,  51 D,  51 E,  51 F, and  51 G having different shapes in plan view. In each of the domains, the period of the arrangement of a plurality of through holes  50  and the unit cell orientation are constant. However, the unit cell orientations of the domains differ from each other. In plan view, the sizes and shapes of the domains differ from each other. In this configuration, the number of unit cell orientations in the phononic crystal structure A as a whole is larger than that in the configurations exemplified above. Therefore, the effect of reducing the thermal conductivity that is based on the difference in unit cell orientation is more significant. In this configuration, interfaces  55  between the domains extend in a plurality of random directions in plan view. Therefore, the effect of reducing the thermal conductivity based on the interface resistance is more significant. 
     In the phononic crystal structure A in  FIG. 16 , the interface  55  between the first domain  51 A and the second domain  51 B adjacent to each other extends in a direction inclined with respect to the width direction of the phononic crystal layer  56  in plan view. The interfaces  55  also have bent portions in plan view. 
     A polycrystal structure  52  that is a phononic crystal structure A may include a first domain  51 A and a second domain  51 B that differ in the period P of the arrangement of through holes  50  and/or in the diameter D of the through holes  50 . The diameter D of through holes  50  in a first domain  51 A shown in  FIG. 17A  differs from the diameter D of through holes  50  in a second domain  51 B shown in  FIG. 17B . The period P of the arrangement of the through holes  50  in the first domain  51 A shown in  FIG. 17A  is the same as the period P of the arrangement of the through holes  50  in the second domain  51 B shown in  FIG. 17B . 
     A phononic crystal structure A shown in  FIG. 18  has a first domain  51 A in which a plurality of through holes  50  having a smaller diameter D are regularly arranged with a smaller period P and a second domain  51 B in which a plurality of through holes  50  having a larger diameter D are regularly arranged with a larger period P. The phononic crystal structure A shown in  FIG. 18  includes a region  92  including a plurality of through holes  50  with a smaller period P and a smaller diameter D and a region  93  including a plurality of through holes  50  with a larger period P and a larger diameter D. The region  92  is adjacent to the region  93 . The region  92  and the region  93  each include a plurality of domains having different shapes and different unit cell orientations in plan view, as in the example shown in  FIG. 16 . The region  92  and the region  93  divide the phononic crystal structure A in a direction substantially parallel to the direction of macroscopic heat transfer. In this configuration, the frequency band of a PBG formed in the first domain  51 A differs from the frequency band of a PBG formed in the second domain  51 B, and therefore, the effect of reducing the thermal conductivity is particularly significant. 
     A phononic crystal structure A shown in  FIG. 19  includes a first domain  51 A in which a plurality of through holes  50  having a smaller diameter D are regularly arranged with a smaller period P and a second domain  51 B in which a plurality of through holes  50  having a larger diameter D are regularly arranged with a larger period P. The phononic crystal structure A in  FIG. 19  includes a plurality of domains having different shapes in plan view and different unit cell orientations. In this configuration, the frequency band of a PBG formed in the first domain  51 A differs from the frequency band of a PBG formed in the second domain  51 B, and therefore the effect of reducing the thermal conductivity is particularly significant. 
     The phononic crystal layer  56  has, for example, a polygonal shape such as a triangular, square, or rectangular shape, a circular shape, an elliptical shape, or a combination thereof in plan view. However, the shape of the phononic crystal layer  56  is not limited to the above examples. 
     The thermoelectric converter has, for example, a polygonal shape such as a triangular, square, or rectangular shape, a circular shape, an elliptical shape, or a combination thereof in plan view. However, the shape of the thermoelectric converter is not limited to the above examples. The thermoelectric converter may have a rectangular parallelepipedic or cubic shape. 
     The thermoelectric converter may include two or more first phononic crystal layers  44  and/or two or more second phononic crystal layers  46 . The thermoelectric converter may further include a phononic crystal layer having a phononic crystal structure having a configuration different from those of the first phononic crystal structure and the second phononic crystal structure. 
     Another example of the phononic crystal layer  56  is shown in  FIGS. 20A and 20B .  FIG. 20B  shows a cross section  20 B- 20 B of the phononic crystal layer  56  in  FIG. 20A . The phononic crystal layer  56  shown in  FIGS. 20A and 20B  further includes a plurality of pillars  61 . The pillars  61  are columnar members extending linearly. Each of the pillars  61  is filled into a corresponding one of the through holes  50  in the phononic crystal layer  56 . The circumferential surface of each of the pillars  61  is covered with an oxide film  62 . In this configuration, the through holes  50  that are vacant holes are filled with the respective pillars  61 . Therefore, for example, the degree of flexibility in controlling the characteristics of the phononic crystal layer  56  in the through direction of the through holes  50  can be increased. More specifically, for example, in a thermoelectric converter that is a stacked structural body including two or more phononic crystal layers  56 , the electron conductivity between a first end  41  and a second end  42  can be improved while the low thermal conductivity based on the phononic crystal structures is maintained. 
     When the material the phononic crystal layer  56  into which the pillars  61  have been filled is the same as the material of the pillars  61 , the circumferential surface of each of the pillars  61  is covered with the oxide film  62 . When the material the phononic crystal layer  56  into which the pillars  61  have been filled is different from the material of the pillars  61 , the oxide film  62  is not always necessary. 
     The phononic crystal layer  56  further including the pillars  61  is, for example, the first phononic crystal layer  44  and/or the second phononic crystal layer  46 . The pillars  61  may be filled into the first through holes  43  and also into the second through holes  45 . 
     Typically, the pillars  61  are formed of a semiconductor material. The material forming the pillars  61  is, for example, Si, SiGe, SiC, TiN, SiN, or VO 2 . However, the material forming the pillars  61  is not limited to the above examples. 
     The oxide film  62  is, for example, a SiO 2  film. However, the oxide film  62  is not limited to the above example. 
       FIG. 21  shows an example of the p-type thermoelectric converter  22  including the first phononic crystal layer  44  and the second phononic crystal layer  46  with the pillars  61  filled thereinto. The p-type thermoelectric converter  22  in  FIG. 21  includes the phononic crystal layers  56  shown in  FIGS. 20A and 20B  as the first phononic crystal layer  44  and the second phononic crystal layer  46 . The p-type thermoelectric converter  22  in  FIG. 21  is a two-layer structural body including two phononic crystal layers  56 . A buffer layer  63  is disposed between the first phononic crystal layer  44  and the second phononic crystal layer  46 . The material forming the pillars  61  (excluding the oxide film  62 ) in the first phononic crystal layer  44  is the same as the material forming the buffer layer  63 . The material forming the buffer layer  63  is the same as the material forming the second phononic crystal layer  46  (excluding the pillars  61  and the oxide film  62 ). 
     Another example of the phononic crystal layer  56  is shown in  FIGS. 22A and 22B .  FIG. 22B  shows a cross section  22 B- 22 B of the phononic crystal layer  56  in  FIG. 22A . The phononic crystal layer  56  shown in  FIGS. 22A and 22B  further includes a plurality of pillars  61 . Each of the pillars  61  is filled into a corresponding one of the through holes  50  in the phononic crystal layer  56 . The material forming the pillars  61  differs from the material forming the phononic crystal layer  56 . 
       FIG. 23  shows an example of the p-type thermoelectric converter  22  including the first phononic crystal layer  44  and the second phononic crystal layer  46  with pillars  61  filled thereinto. The p-type thermoelectric converter  22  in  FIG. 23  is a three-layer structural body which includes three phononic crystal layers  56  and in which a first phononic crystal layer  44 , a second phononic crystal layer  46 , and a first phononic crystal layer  44  are disposed in this order. A first buffer layer  63 A is disposed between the lowermost first phononic crystal layer  44  and the second phononic crystal layer  46 . A second buffer layer  63 B is disposed between the second phononic crystal layer  46  and the uppermost first phononic crystal layer  44 . The material forming the pillars  61  in the first phononic crystal layer  44  is the same as the material forming the second buffer layer  63 B. 
     The material forming the pillars  61  in the second phononic crystal layer  46  is the same as the material forming the first buffer layer  63 A. The material forming the first phononic crystal layers  44  (excluding the pillars  61 ) is the same as the material forming the first buffer layer  63 A. The material forming the second phononic crystal layer  46  (excluding the pillars  61 ) is the same as the material forming the second buffer layer  63 B. The p-type thermoelectric converter  22  in  FIG. 23  is formed from two types of materials. The two types of materials may be semiconductor materials. 
     The first connection electrode  11 , the second connection electrode  12 , the third connection electrode  13 , the fourth connection electrode  14 , the first electrodes  24 , the second electrodes  25 , and the third electrodes  26  are each formed of a conductive material. The conductive material is typically a metal. The metal is, for example, chromium (Cr), aluminum (Al), gold (Au), silver (Ag), or copper (Cu). However, the conductive material is not limited to the above examples. At least one selected from the first connection electrode  11 , the second connection electrode  12 , the third connection electrode  13 , the fourth connection electrode  14 , the first electrodes  24 , the second electrodes  25 , and the third electrodes  26  may include a phononic crystal layer. The through direction of the plurality of through holes in the phononic crystal layer may be substantially parallel to the stacking direction of the layered structure  5 . 
     The substrate (base layer)  6  is typically formed of a semiconductor material. The semiconductor material is, for example, Si. The substrate  6  may be a Si wafer. An oxide film may be formed on the upper surface of the substrate  6  formed of Si. The oxide film is, for example, a SiO 2  film. The oxide film may be the second insulating layer  7 . The structure of the substrate  6  is not limited to the above example. For example, an integrated circuit may be embedded in the substrate  6 . The substrate  6  may have a multilayer structure including a plurality of stacked layers. At least part of the substrate  6  may include a phononic crystal layer. The through direction of the plurality of through holes in the phononic crystal layer may be substantially parallel to the stacking direction of the layered structure  5 . 
     The first insulating layer  3  may function as a layer for maintaining electrical insulation between the first thermoelectric conversion module  2  and the second thermoelectric conversion module  4 . The second insulating layer  7  may function as a layer for maintaining electrical insulation between the substrate  6  and the first thermoelectric conversion module  2 . The first insulating layer  3 , the second insulating layer  7 , and the insulating portions  27  are typically formed of an insulating material. The insulating material is, for example, any of oxides, nitrides, and oxynitrides of metals including Si. The insulating material may be SiO 2 . However, the insulating material is not limited to the above examples. At least one selected from the first insulating layer  3 , the second insulating layer  7 , and the insulating portions  27  may include a phononic crystal layer. The through direction of the plurality of through holes in the phononic crystal layer may be substantially parallel to the stacking direction of the layered structure  5 . 
     The protective layer  8  may function as a layer that protects the thermoelectric conversion device  1 . The protective layer  8  is formed of, for example, an insulating material. Examples of the insulating material are as described above. The protective layer  8  may include a phononic crystal layer. The through direction of the plurality of through holes in the phononic crystal layer may be substantially parallel to the stacking direction of the layered structure  5 . 
     When a member of each thermoelectric conversion element  21  other than the thermoelectric converters includes a phononic crystal layer, the thermal conductivity of the thermoelectric conversion device  1  in in-plane directions can be reduced. This reduction allows the thermoelectric conversion efficiency of the thermoelectric conversion device  1  to be further improved. Moreover, the reduction can inhibit the diffusion of heat in the in-plane directions, so that the degree of flexibility in the construction of an electronic device including the thermoelectric conversion device  1  can be increased. 
     The thermoelectric conversion device  1  may further include a temperature detection module. In this case, for example, the first thermoelectric conversion module  2  and/or the second thermoelectric conversion module  4  can be controlled based on the information about temperature acquired by the temperature detection module. The information about the temperature is, for example, the value of the temperature, the rate of change in the temperature, or the history of the temperature. However, the information about the temperature is not limited to the above examples. The thermoelectric conversion device  1  in  FIG. 1  includes a temperature detection module  28  disposed inside the first insulating layer  3 . The temperature detection module  28  is located at the center of the layered structure  5  when it is viewed in the stacking direction. However, the location of the temperature detection module  28  is not limited to the above example. The temperature detection module  28  includes, for example, at least one selected from a thermocouple element, a resistance thermometer bulb, and a thermistor. 
     The thermoelectric conversion device  1  may further include a control module for controlling the voltage applied to the first thermoelectric conversion module  2  and/or the second thermoelectric conversion module  4 . The control module may be composed of, for example, an integrated circuit. The control module may include a power source that applies the voltage to the first thermoelectric conversion module  2  and/or the second thermoelectric conversion module  4  or may include a signal transmitter that transmits a control signal to a power source disposed separately from the control module. The control module may be connected to the temperature detection module  28 . 
     The thermoelectric conversion device  1  may further include, for example, an optional member and/or an optional module other than the components described above. 
     The thermoelectric conversion device  1  can be used as a Peltier-type cooling and/or heating device. An object to be heated and/or cooled by the thermoelectric conversion device  1  is, for example, a heat source. The heat source is, for example, an integrated circuit such as a CPU or a GPU or an integrated circuit device including the integrated circuit. However, the object is not limited to the above examples. The amount of heat generated by an integrated circuit varies irregularly depending on the load thereon. 
     Therefore, although it is desirable that the temperature of the integrated circuit is constant, it is inevitable that the temperature of the integrated circuit varies irregularly. With the thermoelectric conversion device  1 , for example, the irregular variations described above can be reduced, and the variations in the temperature of the integrated circuit are maintained within a prescribed range. In other words, the thermoelectric conversion device  1  is particularly advantageous when the object is an integrated circuit and/or an integrated circuit device. 
     The thermoelectric conversion device  1  may be used as a Seebeck-type power generator. 
     &lt;Production Method&gt; 
     The thermoelectric conversion device of the present disclosure can be produced using a combination of any of various thin film forming methods such as chemical vapor deposition (CVD), sputtering, and vapor deposition and any of various micromachining methods and pattern forming methods such as electron beam lithography, photolithography, block copolymer lithography, selective etching, and chemo-mechanical polishing (CMP). The block copolymer lithography is suitable for the formation of the phononic crystal structures. 
     An example of a method for producing a thermoelectric conversion element  21  including a phononic crystal layer will be described with reference to  FIGS. 24A to 24O . However, the method for producing the thermoelectric conversion element that the thermoelectric conversion device  1  can include is not limited to the following example. 
       FIG. 24A : A substrate  71  is prepared. An oxide film  72  has been provided on the upper surface of the substrate  71 . The oxide film  72  is, for example, a SiO 2  film. 
       FIG. 24B : A metal layer  73  is formed on the oxide film  72 . The metal layer  73  later becomes the first electrode  24 . The metal layer  73  is, for example, a Cr layer. The metal layer  73  is formed, for example, by sputtering. The thickness of the metal layer  73  is, for example, 50 nm. 
       FIG. 24C : A semiconductor layer  74  is formed on the metal layer  73 . The semiconductor layer  74  is, for example, a polycrystalline Si layer. The semiconductor layer  74  is formed, for example, by CVD. The thickness of the semiconductor layer  74  is, for example, 200 nm. 
       FIG. 24D : A hard mask  75  is formed on the semiconductor layer  74 . The hard mask  75  is, for example, a SiO 2  layer. The hard mask  75  is formed, for example, by CVD. The thickness of the hard mask  75  is, for example, 30 nm. The hard mask  75  is used to form a phononic crystal structure in the semiconductor layer  74 . 
       FIG. 24E : A self-assembled film  76  of a block copolymer is formed on the hard mask  75 . The self-assembled film  76  is used for block copolymer lithography for forming a phononic crystal structure. 
       FIG. 24F : A plurality of regularly arranged through holes  77  are formed in the hard mask  75  by block copolymer lithography. 
       FIG. 24G : A plurality of regularly arranged through holes  50  are formed in the semiconductor layer  74  by selective etching using the hard mask  75  as a resist at positions corresponding to the plurality of through holes  77  in plan view. The plurality of through holes  50  form a phononic crystal structure. The semiconductor layer  74  later becomes the phononic crystal layer  56 . 
       FIG. 24H : The hard mask  75  and the self-assembled film  76  are removed. 
       FIG. 24I : An oxide film  62  is formed on the inner circumferential surface of each of the through holes  50  in the phononic crystal layer  56 . The oxide film  62  is, for example, a SiO 2  film. The oxide film  62  is formed, for example, by thermal oxidation. The thickness of the oxide film  62  is, for example, 1 nm. 
       FIG. 24J : The through holes  50  in the phononic crystal layer  56  are filled with a semiconductor to form pillars  61  having the oxide film  62  on their circumferential surface. The pillars  61  are formed of, for example, polycrystalline Si. The pillars  61  are formed, for example, by CVD. In this case, a layer  78  formed of the semiconductor material forming the pillars  61  is formed on the phononic crystal layer  56 . 
       FIG. 24K : The layer  78  is removed by a method such as CMP. In this manner, the phononic crystal layer  56  further including the pillars  61  is formed. 
       FIG. 24L : Impurity ions are implanted into a partial region of the phononic crystal layer  56  using a method such as photolithography to dope the partial region with the impurity ions, and a p-type thermoelectric converter  22  is thereby formed. The impurity ions are, for example, boron ions. 
       FIG. 24M : Impurity ions are implanted into a region of the phononic crystal layer  56  that differs from the p-type thermoelectric converter  22  using a method such as photolithography to dope the region with the impurity ions, and an n-type thermoelectric converter  23  is thereby formed. The impurity ions are, for example, phosphorus ions. The p-type thermoelectric converter  22  is spaced apart from the n-type thermoelectric converter  23 . 
       FIG. 24N : The entire product is subjected to heat treatment (annealing) to activate the dopant impurity ions. 
       FIG. 24O : A second electrode  25  is formed on the p-type thermoelectric converter  22 . A third electrode  26  is formed on the n-type thermoelectric converter  23 . The second electrode  25  and the third electrode  26  are formed of, for example, Al. A thermoelectric conversion element  21  is thereby formed. A region of the phononic crystal layer  56  that remains present between the p-type thermoelectric converter  22  and the n-type thermoelectric converter  23  serves as an insulating portion  27 . The insulating portion  27  has a phononic crystal structure including a plurality of regularly arranged through holes  50 . In this configuration, the in-plane thermal conductivity of a portion of the element  21  that is located between the p-type thermoelectric converter  22  and the n-type thermoelectric converter  23  can be reduced. The reduction in the in-plane thermal conductivity allows the thermoelectric conversion efficiency of the thermoelectric conversion element  21  and the thermoelectric conversion device  1  to be further improved. 
     SECOND EMBODIMENT 
     A thermoelectric conversion device in the second embodiment is shown in  FIG. 25 . The thermoelectric conversion device  1  in the second embodiment has the same structure as that of the thermoelectric conversion device  1  in the first embodiment except that the thermoelectric conversion device  1  further includes a third insulating layer  10  disposed on the second thermoelectric conversion module  4  and a third thermoelectric conversion module  9  disposed on the third insulating layer  10  and that the protective layer  8  is located on the third thermoelectric conversion module  9 . The thermoelectric conversion device  1  in the second embodiment has a structure including three thermoelectric conversion modules  2 ,  4 , and  9  stacked together. The thermoelectric conversion device of the present disclosure may include an additional thermoelectric conversion module in addition to the first thermoelectric conversion module  2  and the second thermoelectric conversion module  4  so long as the thermoelectric conversion device has a layered structure  5 . No limitation is imposed on the number of thermoelectric conversion modules included in the thermoelectric conversion device of the present disclosure. 
     The third thermoelectric conversion module  9  includes two or more thermoelectric conversion elements  21 ( 21   c ), a fifth connection electrode  15 , and a sixth connection electrode  16 . In the third thermoelectric conversion module  9 , the thermoelectric conversion elements  21   c  are electrically connected to the fifth connection electrode  15  and the sixth connection electrode  16 . Each thermoelectric conversion element  21   c  is located on an electric path connecting the connection electrodes  15  and  16  included in the third thermoelectric conversion module  9 . By applying a voltage through the connection electrodes  15  and  16 , the thermoelectric conversion elements  21   c  and the third thermoelectric conversion module  9  operate as Peltier elements and Peltier modules, respectively. The third thermoelectric conversion module  9  may have the same structure as the structure of the first thermoelectric conversion module  2  and/or the second thermoelectric conversion module  4  except for the features described above. In the thermoelectric conversion device  1  in the second embodiment, the thermoelectric conversion modules  2 ,  4 , and  9  can be controlled independently. By increasing the number of thermoelectric conversion modules that can be controlled independently, the degree of flexibility in controlling the cooling and/or heating of the object can be further improved. 
     In the thermoelectric conversion device  1  in the second embodiment, a layered body including the first thermoelectric conversion module  2  and the second thermoelectric conversion module  4  with the first insulating layer  3  interposed therebetween may be interpreted as a layered structure  5 ( 5   a ), and a layered body including the second thermoelectric conversion module  4  and the third thermoelectric conversion module  9  with the third insulating layer  10  interposed therebetween may be interpreted as a layered structure  5 ( 5   b ). The thermoelectric conversion modules in the layered structure  5   b  can be controlled independently in the same manner as those of the layered structure  5   a.  The control of the layered structure  5   b  may be the same as the control of the layered structure  5   a.    
     THIRD EMBODIMENT 
     A thermoelectric conversion device in a third embodiment is shown in  FIG. 26 . Each of the thermoelectric conversion elements  21  in the first embodiment and the second embodiment includes a p-type thermoelectric converter  22  and an n-type thermoelectric converter  23  and is referred to as a  7   c -type element by those skilled in the art. The thermoelectric conversion element that the thermoelectric conversion device of the present disclosure can have is not limited to the  7   c -type element. The thermoelectric conversion device  1  in the third embodiment includes thermoelectric conversion elements  31  different from the  7   c -type elements. The thermoelectric conversion device  1  in the third embodiment has the same structure as that of the thermoelectric conversion device  1  in the first embodiment except that the thermoelectric conversion elements  31  are provided instead of the thermoelectric conversion elements  21 . 
     Each of the thermoelectric conversion elements  31  includes two thermoelectric converters  32  and  33  adjacent to each other. The thermoelectric converters  32  and  33  have the same conductivity type. In other words, each thermoelectric conversion element  31  has two p-type or n-type thermoelectric converters adjacent to each other. Each thermoelectric conversion element  31  includes a fourth electrode  34 , a fifth electrode  35 , and a sixth electrode  36 . A first end of the thermoelectric converter  32  and a first end of the thermoelectric converter  33  are electrically connected to each other through the fourth electrode  34 . The fourth electrode  34  electrically connects the lower surface of the thermoelectric converter  32  to the upper surface of the thermoelectric converter  33 . The fourth electrode  34  includes a via line  37 ( 37   a ) extending in the stacking direction of the layered structure  5 . A second end of the thermoelectric converter  32  is electrically connected to the fifth electrode  35 . A second end of the thermoelectric converter  33  is electrically connected to the sixth electrode  36 . One selected from the fifth electrode  35  and the sixth electrode  36  is disposed on an electric path connecting the corresponding connection electrodes and located on the upstream side in the path. The other selected from the fifth electrode  35  and the sixth electrode  36  is disposed on the electric path connecting the corresponding connection electrodes and located on the downstream side in the path. In other words, a voltage can be applied to the thermoelectric conversion element  31  through the fifth electrode  35  and the sixth electrode  36 . In the thermoelectric conversion element  31 , a direction connecting a pair of electrodes holding one of the thermoelectric converters therebetween is generally the stacking direction of the layered structure  5 . When an electric current is caused to flow through the electric path, the directions of the electric current flowing through the two adjacent thermoelectric converters  32  and  33  are the same (see arrows in  FIG. 26 ). The thermoelectric conversion element  31  is known as a uni-leg type element to those skilled in the art. 
     Each of the thermoelectric conversion modules  2  and  4  in  FIG. 26  includes two or more thermoelectric conversion elements  31 . In two elements  31  adjacent to each other, the fifth electrode  35  of one element  31  and the sixth electrode  36  of the other element  31  are electrically connected through a via line  37 ( 37   b ) extending in the stacking direction of the layered structure  5 . 
     Each thermoelectric conversion element  31  can have any known uni-leg type structure so long as the thermoelectric converters each have a phononic crystal layer. 
     [Method for Controlling Thermoelectric Conversion Device] 
     An example of a method for controlling the thermoelectric conversion device  1  is shown in  FIG. 27 . The control method in  FIG. 27  includes a step of applying a first voltage to a first thermoelectric conversion module  2  and applying a second voltage to a second thermoelectric conversion module  4 . The application pattern of the first voltage differs from the application pattern of the second voltage. The control method in  FIG. 27  is a method for independently controlling the thermoelectric conversion modules included in the thermoelectric conversion device  1 . 
     Examples of the form of the application pattern are as follows. However, the form of the application pattern is not limited to the following examples.
         At least one selected from the effective voltage value, the maximum voltage value, and the minimum voltage value is different.   At least one selected from the width of pulses, their period, their waveform, and the duty cycle during the application of the pulses is different.       

     When the thermoelectric conversion device includes the first temperature detection module  28 , the application pattern of the first voltage and/or the application pattern of the second voltage may be controlled based on the information about the temperature acquired by the temperature detection module  28 . 
     The object to be cooled and/or heated by the thermoelectric conversion device  1  may be disposed near the thermoelectric conversion device  1 . The object is, for example, a heat source. Examples of the heat source are as described above. The object is disposed, for example, at a position opposite to the substrate  6  of the thermoelectric conversion device  1 . The object may be in contact with the thermoelectric conversion device  1 . The object may be in contact with the protective layer  8 , the insulating layer, or one of the thermoelectric conversion modules of the thermoelectric conversion device  1 . In this case, at least one selected from the following control A, control B, and control C may be performed. 
     Control A: The object includes a second temperature detection module, or the second temperature detection module is disposed between the object and the thermoelectric conversion device  1 . Based on the information about temperature acquired by the second temperature detection module, the application pattern of the first voltage and/or the application pattern of the second voltage is controlled. In this manner, the degree of flexibility in controlling the cooling and/or heating of the object can be further improved. 
     Control B: The application pattern of the first voltage and/or the application pattern of the second voltage is controlled such that the voltage applied to a thermoelectric conversion module that is selected from a group of thermoelectric conversion modules included in the thermoelectric conversion device  1  and is closer to the object is changed more frequently than the voltage applied to a thermoelectric conversion module farther from the object. The group of thermoelectric conversion modules in each of first and third embodiments 1 and 3 includes the first thermoelectric conversion module  2  and the second thermoelectric conversion module  4 . The group of thermoelectric conversion modules in the second embodiment includes the first thermoelectric conversion module  2 , the second thermoelectric conversion module  4 , and the third thermoelectric conversion module  9 . 
     Among a thermoelectric conversion module group including three or more thermoelectric conversion modules, any two thermoelectric conversion modules adjacent to each other with an insulating layer interposed therebetween may be selected as the first thermoelectric conversion module  2  and the second thermoelectric conversion module  4  to which the first voltage and the second voltage, respectively, are to be applied. 
     Control C: The application pattern of the first voltage and/or the application pattern of the second voltage is controlled such that variations in the temperature of the object are within a prescribed range. 
     A more specific example of the control B is shown in  FIGS. 28 and 29 . A control method in  FIGS. 28 and 29  is a method for controlling a thermoelectric conversion device  1  including three thermoelectric conversion modules. In the control method in  FIGS. 28 and 29 , the voltage application patterns are controlled such that the voltage applied to a thermoelectric conversion module A closest to the object is changed more frequently than the voltages applied to thermoelectric conversion modules B and C farther from the object. Moreover, the voltage application patterns are controlled such that the voltage applied to the thermoelectric conversion module B is changed more frequently than the voltage applied to the thermoelectric conversion module C farther from the object than the thermoelectric conversion module B. In the control in  FIG. 29 , voltages are applied irregularly to the thermoelectric conversion modules A and B. 
     The above control method is also a method for cooling and/or heating the object using the thermoelectric conversion device  1 . In other words, in another aspect different from the above aspect, the present disclosure provides a method for cooling and/or heating an object using a thermoelectric conversion device. In this method, the thermoelectric conversion device is the thermoelectric conversion device of the present disclosure. The method includes a step of applying a first voltage to a first thermoelectric conversion module of the thermoelectric conversion device and applying a second voltage to a second thermoelectric conversion module in an application pattern different from that for the first voltage. In this method, one or two or more types of control described above can be performed. 
     Electronic Device] 
     In another aspect, the present disclosure provides an electronic device including an integrated circuit and a thermoelectric conversion device that cools and/or heats the integrated circuit. The thermoelectric conversion device is the thermoelectric conversion device of the present disclosure. Examples of the electronic device are as described above. 
     INDUSTRIAL APPLICABILITY 
     The thermoelectric conversion device of the present disclosure can be used as, for example, a Peltier-type cooling device and/or a Peltier-type heating device. 
     Examples of the invention derived from the above-disclosed contents are enumerated below. 
     (Item 1) 
     A thermoelectric conversion device including:
         a first thermoelectric conversion module;   a first insulating layer disposed on the first thermoelectric conversion module; and   a second thermoelectric conversion module disposed on the first insulating layer,   wherein the first thermoelectric conversion module includes at least one thermoelectric conversion element, a first connection electrode, and a second connection electrode,   wherein the at least one thermoelectric conversion element of the first thermoelectric conversion module is electrically connected to the first connection electrode and the second connection electrode and located on an electric path connecting the first connection electrode and the second connection electrode,   wherein the second thermoelectric conversion module includes at least one thermoelectric conversion element, a third connection electrode, and a fourth connection electrode,   wherein the at least one thermoelectric conversion element of the second thermoelectric conversion module is electrically connected to the third connection electrode and the fourth connection electrode and located on an electric path connecting the third connection electrode and the fourth connection electrode,   wherein each of the at least one thermoelectric conversion element of the first thermoelectric conversion module and the at least one thermoelectric conversion element of the second thermoelectric conversion module includes a thermoelectric converter,   wherein the thermoelectric converter includes a phononic crystal layer having a phononic crystal structure including a plurality of regularly arranged through holes, and   wherein a through direction of the plurality of through holes is substantially parallel to a stacking direction of the first thermoelectric conversion module, the first insulating layer, and the second thermoelectric conversion module.       

     (Item 2) 
     The thermoelectric conversion device according to Item 1, wherein the at least one thermoelectric conversion element of the first thermoelectric conversion module includes two or more thermoelectric conversion elements. 
     (Item 3) 
     The thermoelectric conversion device according to Item 1, wherein the at least one thermoelectric conversion element of the second thermoelectric conversion module includes two or more thermoelectric conversion elements. 
     (Item 4) 
     The thermoelectric conversion device according to Item 2, wherein the two or more thermoelectric conversion elements are electrically connected in series between the first connection electrode and the second connection electrode. 
     (Item 5) 
     The thermoelectric conversion device according to Item 3, wherein the two or more thermoelectric conversion elements are electrically connected in series between the third connection electrode and the fourth connection electrode. 
     (Item 6) 
     The thermoelectric conversion device according to any one of Items 1 to 5, wherein the at least one thermoelectric conversion element of at least one thermoelectric conversion module selected from the group consisting of the first thermoelectric conversion module and the second thermoelectric conversion module includes:
         a p-type thermoelectric converter;   an n-type thermoelectric converter;   a first electrode;   a second electrode; and   a third electrode,   wherein the thermoelectric converter includes the p-type thermoelectric converter and the n-type thermoelectric converter,   wherein a first end of the p-type thermoelectric converter and a first end of the n-type thermoelectric converter are electrically connected to each other through the first electrode,   wherein a second end of the p-type thermoelectric converter is electrically connected to the second electrode,   wherein a second end of the n-type thermoelectric converter is electrically connected to the third electrode,   wherein one selected from the second electrode and the third electrode is disposed on the electric path and located on an upstream side therein, and   wherein the other selected from the second electrode and the third electrode is disposed on the electric path and located on a downstream side therein.       

     (Item 7) 
     The thermoelectric conversion device according to any one of Items 1 to 5, wherein the at least one thermoelectric conversion element of at least one thermoelectric conversion module selected from the group consisting of the first thermoelectric conversion module and the second thermoelectric conversion module includes:
         two p-type thermoelectric converters adjacent to each other;   a fourth electrode;   a fifth electrode; and   a sixth electrode,   wherein the thermoelectric converter includes the two p-type thermoelectric converters,   wherein a first end of a first one of the thermoelectric converters and a first end of a second one of the thermoelectric converters are electrically connected to each other through the fourth electrode,   wherein a second end of the first one of the thermoelectric converters is electrically connected to the fifth electrode,   wherein a second end of the second one of the thermoelectric converters is electrically connected to the sixth electrode,   wherein one selected from the fifth electrode and the sixth electrode is disposed on the electric path and located on an upstream side therein,   wherein the other selected from the fifth electrode and the sixth electrode is disposed on the electric path and located on a downstream side therein, and   wherein, when an electric current is caused to flow through the electric path, the directions of the electrode current flowing through the two adjacent thermoelectric converters are the same.       

     (Item 8) 
     The thermoelectric conversion device according to any one of Items 1 to 5, wherein the at least one thermoelectric conversion element of at least one thermoelectric conversion module selected from the group consisting of the first thermoelectric conversion module and the second thermoelectric conversion module includes:
         two n-type thermoelectric converters adjacent to each other;   a fourth electrode;   a fifth electrode; and   a sixth electrode,   wherein the thermoelectric converter includes the two n-type thermoelectric converters,   wherein a first end of a first one of the thermoelectric converters and a first end of a second one of the thermoelectric converters are electrically connected to each other through the fourth electrode,   wherein a second end of the first one of the thermoelectric converters is electrically connected to the fifth electrode,   wherein a second end of the second one of the thermoelectric converters is electrically connected to the sixth electrode,   wherein one selected from the fifth electrode and the sixth electrode is disposed on the electric path and located on an upstream side therein,   wherein the other selected from the fifth electrode and the sixth electrode is disposed on the electric path and located on a downstream side therein, and   wherein, when an electric current is caused to flow through the electric path, the directions of the electrode current flowing through the two adjacent thermoelectric converters are the same.       

     (Item 9) 
     The thermoelectric conversion device according to any one of Items 1 to 8, wherein the phononic crystal layer includes a first phononic crystal layer and a second phononic crystal layer,
         wherein the first phononic crystal layer has a first phononic crystal structure including a plurality of regularly arranged first through holes that are part of the through holes,   wherein the second phononic crystal layer has a second phononic crystal structure including a plurality of regularly arranged second through holes that are part of the through holes, and   wherein the first phononic crystal layer and the second phononic crystal layer are stacked in the stacking direction.       

     (Item 10) 
     The thermoelectric conversion device according to Item 9, wherein the first phononic crystal layer and the second phononic crystal layer are in contact with each other. 
     (Item 11) 
     The thermoelectric conversion device according to Item 9 or 10, wherein at least part of the second through holes are not in communication with the first through holes. 
     (Item 12) 
     The thermoelectric conversion device according to any one of Items 1 to 11,wherein the phononic crystal structure has a first domain and a second domain that are phononic crystal regions,
         wherein the plurality of through holes in the first domain are arranged regularly in a cross section perpendicular to the through direction of the through holes, and   wherein the plurality of through holes in the second domain are arranged regularly in a second direction different from the first direction in the cross section perpendicular to the through direction of the through holes.       

     (Item 13) 
     The thermoelectric conversion device according to any one of Items 1 to 12, wherein the phononic crystal layer includes a plurality of pillars,
         wherein the pillars are columnar bodies extending linearly, and   wherein each of the pillars is filled into a corresponding one of the through holes in the phononic crystal layer.       

     (Item 14) 
     The thermoelectric conversion device according to Item 13, wherein the phononic crystal layer with the pillars filled thereinto and the pillars are formed of the same material as each other, and
         wherein a circumferential surface of each of the pillars is covered with an oxide film.       

     (Item 15) 
     The thermoelectric conversion device according to any one of Items 1 to 14, further including a temperature detection module. 
     (Item 16) 
     The thermoelectric conversion device according to any one of Items 1 to 15, further including a control module for controlling a voltage applied to at least one thermoelectric conversion module selected from the group consisting of the first thermoelectric conversion module and the second thermoelectric conversion module. 
     (Item 17) 
     A method for controlling a thermoelectric conversion device, the method including a step of applying a first voltage and a second voltage to the first thermoelectric conversion module and the second thermoelectric conversion module, respectively, of the thermoelectric conversion device according to any one of Items 1 to 16,
         wherein an application pattern of the first voltage differs from an application pattern of the second voltage.       

     (Item 18) 
     The method for controlling according to Item 17, wherein the thermoelectric conversion device includes a first temperature detection module, 
     wherein the application pattern of at least one voltage selected from the group consisting of the first voltage and the second voltage is controlled based on information about temperature acquired by the first temperature detection module. 
     (Item 19) 
     The method for controlling according to Item 17 or 18, wherein an object to be cooled and/or heated by the thermoelectric conversion device is disposed near the thermoelectric conversion device. 
     (Item 20) 
     The method for controlling according to Item 19, wherein the object includes a second temperature detection module, or the second temperature detection module is disposed between the object and the thermoelectric conversion device, and
         wherein the application pattern of at least one voltage selected from the group consisting of the first voltage and the second voltage is controlled based on information about temperature acquired by the second temperature detection module.       

     (Item 21) 
     The method for controlling according to Item 19 or 20, wherein the application pattern of at least one voltage selected from the group consisting of the first voltage and the second voltage is controlled such that a voltage applied to a thermoelectric conversion module that is selected from the first thermoelectric conversion module and the second thermoelectric conversion module and is located closer to the object is changed more frequently than a voltage applied to a thermoelectric conversion module that is selected from the first thermoelectric conversion module and the second thermoelectric conversion module and is located farther from the object. 
     (Item 22) 
     The method for controlling according to any one of Items 17 to 21, wherein the application pattern of at least one voltage selected from the group consisting of the first voltage and the second voltage is controlled such that variations in temperature of the object are within a prescribed range. 
     (Item 23) 
     The method for controlling according to any one of Items 17 to 22, wherein the object is a heat source. 
     (Item 24) 
     A method for cooling and/or heating an object using a thermoelectric conversion device, wherein the thermoelectric conversion device is the thermoelectric conversion device according to any one of Items 1 to 16,
         wherein the method includes a step of applying a first voltage and a second voltage to the first thermoelectric conversion module and the second thermoelectric conversion module, respectively, of the thermoelectric conversion device, and   wherein an application pattern of the first voltage differs from an application pattern of the second voltage.       

     (Item 25) 
     An electronic device including:
         an integrated circuit; and   a thermoelectric conversion device that cools and/or heats the integrated circuit,   wherein the thermoelectric conversion device is the thermoelectric conversion device according to any one of Items 1 to 16.       

     REFERENCE SIGNS LIST 
     
         
           1  thermoelectric conversion device 
           2  first thermoelectric conversion module 
           3  first insulating layer 
           4  second thermoelectric conversion module 
           5  layered structure 
           6  substrate 
           7  second insulating layer 
           8  protective layer 
           9  third thermoelectric conversion module 
           10  third insulating layer 
           11  first connection electrode 
           12  second connection electrode 
           13  third connection electrode 
           14  fourth connection electrode 
           21  thermoelectric conversion element (π type) 
           22  p-type thermoelectric converter 
           23  n-type thermoelectric converter 
           24  first electrode 
           25  second electrode 
           26  third electrode 
           27  insulator 
           28  temperature detection module 
           31  thermoelectric conversion element (uni-leg type) 
           32 ,  33  thermoelectric converter 
           34  fourth electrode 
           35  fifth electrode 
           36  sixth electrode 
           43  first through hole 
           44  first phononic crystal layer 
           45  second through hole 
           46  second phononic crystal layer 
           50  through hole 
           51 A first domain 
           51 B second domain 
           52  phononic polycrystal structure 
           53 A,  53 B orientation 
           55  interface 
           56  phononic crystal layer 
           61  pillar 
           62  oxide film 
           91 ,  91 A,  91 B unit cell