Patent Publication Number: US-2021167270-A1

Title: Thermoelectric conversion element, thermoelectric conversion module, optical sensor, method of producing thermoelectric conversion material, and method of producing thermoelectric conversion element

Description:
TECHNICAL FIELD 
     The present disclosure relates to a thermoelectric conversion element, a thermoelectric conversion module, an optical sensor, a method of producing the thermoelectric conversion material, and a method of producing the thermoelectric conversion element. The present application claims a priority based on Japanese Patent Application No. 2018-164419 filed on Sep. 3, 2018, the entire content of which is incorporated herein by reference. 
     BACKGROUND ART 
     In recent years, renewable energy has been drawing attention as clean energy to replace a fossil fuel such as petroleum. Such renewable energy include energy obtained through power generation using solar light, hydraulic power, and wind power, as well as energy obtained through power generation involving thermoelectric conversion using a temperature difference. In the thermoelectric conversion, heat is directly converted into electric power. Hence, an unnecessary waste is not discharged during the conversion. Because the thermoelectric conversion requires no driving unit such as a motor, the thermoelectric conversion has a characteristic to facilitate maintenance of devices. 
     Efficiency η in converting a temperature difference (heat energy) into electric energy using a material (thermoelectric conversion material) for thermoelectric conversion is given by the following formula (1): 
       η=Δ T/T   h ·( M− 1)/( M+T   c   /T   h )   (1)
 
     where η represents conversion efficiency, ΔT represents a difference between T h  and T c , T h  represents a temperature on the high temperature side, T c  represents a temperature on the low temperature side, M equals to (1+ZT) 1/2 , ZT equals to α 2 ST/ κ , ZT represents a dimensionless figure of merit, α represents a Seebeck coefficient, S represents an electric conductivity and κ represents a thermal conductivity. The conversion efficiency is a monotonously increasing function of ZT. It is important to increase ZT in developing a thermoelectric conversion material. 
     For a thermoelectric conversion material, a technique has been reported in which Au nano particles are formed in SiGe (silicon germanium) by heating a layered body obtained by layering Si, Ge, and Au (for example, NPL 1). For a thermoelectric conversion material, a technique has been reported in which Si/GeB is used (for example, NPL 2). 
     PTL 1 discloses a thermoelectric conversion material in which nano particles including a base material element and a type of element different from the base material element are included in a base material composed of a semiconductor material constituted of the base material element. 
     CITATION LIST 
     Non Patent Literature 
     NPL 1: Hiroaki Takiguchi et al., “Nano Structural and Thermoelectric Properties of SiGeAu Thin Films”, Japanese Journal of Applied Physics 50 (2011) 041301 
     NPL 2: Akinari Matoba et al., “Crystallinity and Thermoelectric Properties of Si/GeB Multilayers Prepared with Si Buffer Layer and SiO 2  Substrates”, Japanese Journal of Applied Physics 48 (2009) 061201 
     Patent Literature 
     PTL 1: WO 2014/196475 
     SUMMARY OF INVENTION 
     A thermoelectric conversion element according to the present disclosure includes: a thermoelectric conversion material portion composed of a material having a band gap; a first electrode disposed in contact with the thermoelectric conversion material portion; a second electrode disposed in contact with the thermoelectric conversion material portion and disposed to be separated from the first electrode; and a sealing portion that seals the thermoelectric conversion material portion. A partial pressure of oxygen in a region surrounding the thermoelectric conversion material portion is maintained by the sealing portion so as to be lower than a partial pressure of oxygen in an external air. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing a structure of a π type thermoelectric conversion element (power generation element) serving as a thermoelectric conversion element in a first embodiment. 
         FIG. 2  is a flowchart showing a representative production process of the thermoelectric conversion element. 
         FIG. 3  is a graph showing a relation between temperature and resistivity in a thermoelectric conversion material portion. 
         FIG. 4  shows an exemplary structure of a power generation module in a second embodiment. 
         FIG. 5  shows an exemplary structure of a power generation module in the second embodiment. 
         FIG. 6  shows an exemplary structure of a power generation module in a third embodiment. 
         FIG. 7  shows an exemplary structure of an optical sensor. 
     
    
    
     DETAILED DESCRIPTION 
     Problems to be Solved by the Present Disclosure 
     There has been required a thermoelectric conversion element employing a thermoelectric conversion material having a conversion efficiency higher than that of each of the thermoelectric conversion materials disclosed in NPL 1, NPL 2, and PTL 1. The efficiency of thermoelectric conversion of a thermoelectric conversion material included in a thermoelectric conversion element can be improved when the resistivity of the thermoelectric conversion material can be maintained to be low. The resistivity is a reciprocal of electric conductivity. 
     In view of the above, it is one of objects to provide: a thermoelectric conversion element having improved efficiency of thermoelectric conversion; a thermoelectric conversion module; an optical sensor; a method of producing the thermoelectric conversion material; and a method of producing the thermoelectric conversion element. 
     Advantageous Effect of the Present Disclosure 
     According to the thermoelectric conversion element of the present disclosure, the efficiency of thermoelectric conversion can be improved. 
     Description of Embodiments 
     First, embodiments of the present disclosure are listed and described. A thermoelectric conversion element according to the present disclosure includes: a thermoelectric conversion material portion composed of a material having a band gap; a first electrode disposed in contact with the thermoelectric conversion material portion; a second electrode disposed in contact with the thermoelectric conversion material portion and disposed to be separated from the first electrode; and a sealing portion that seals the thermoelectric conversion material portion. A partial pressure of oxygen in a region surrounding the thermoelectric conversion material portion is maintained by the sealing portion so as to be lower than a partial pressure of oxygen in an external air. 
     The thermoelectric conversion element includes the sealing portion that seals the thermoelectric conversion material portion composed of a material having a band gap. The partial pressure of oxygen in the region surrounding the thermoelectric conversion material portion is maintained by the sealing portion so as to be lower than the partial pressure of oxygen in the external air. Therefore, formation of an oxide film on a surface of the material of the thermoelectric conversion material portion can be suppressed as compared with that in the external air. As a result, the resistivity of the thermoelectric conversion material portion can be suppressed from being increased, thereby maintaining ZT to be high. According to such a thermoelectric conversion element, improvement in efficiency of thermoelectric conversion can be attained. 
     In the thermoelectric conversion element, the thermoelectric conversion material portion may be composed of a semiconductor material containing an amorphous solid. Since the band gap of the semiconductor material is larger than that of an electrically conductive material, the Seebeck coefficient can be made large. With the semiconductor material including the amorphous solid, thermal conductivity can be made low. Therefore, dimensionless figure of merit ZT can be made large in the thermoelectric conversion material portion included in the thermoelectric conversion element. As a result, improvement in efficiency of thermoelectric conversion can be attained. 
     In the thermoelectric conversion element, the sealing portion may surround the thermoelectric conversion material portion with the region being interposed between the sealing portion and the thermoelectric conversion material portion. The partial pressure of oxygen in the region may be less than or equal to 2×10 5  Pa. In this way, formation of an oxide film on a surface of the material of the thermoelectric conversion material portion can be suppressed efficiently. Therefore, high efficiency of thermoelectric conversion can be secured by maintaining the state in which the resistivity of the thermoelectric conversion material portion is low. 
     In the thermoelectric conversion element, a pressure in the region may be less than or equal to 1×10 −2  Pa. In this way, formation of an oxide film on a surface of the material of the thermoelectric conversion material portion can be suppressed by decreasing the amount of oxygen in the above-described region. Therefore, high efficiency of thermoelectric conversion can be secured by maintaining the state in which the resistivity of the thermoelectric conversion material portion is low. It should be noted that the pressure in the region is preferably more than or equal to 1×10 −7  Pa. 
     In the thermoelectric conversion element, an inert gas may be sealed in the region. In this way, formation of an oxide film on a surface of the material of the thermoelectric conversion material portion can be suppressed more securely. Therefore, high efficiency of thermoelectric conversion can be secured by maintaining the state in which the resistivity of the thermoelectric conversion material portion is low. Examples of the inert gas include argon gas and nitrogen gas. 
     In the thermoelectric conversion element, the sealing portion may be composed of a sealing material provided in the region. With the sealing material, the thermoelectric conversion material portion can be suppressed from being brought into contact with oxygen molecules. As a result, formation of an oxide film on a surface of the material of the thermoelectric conversion material portion can be suppressed. Therefore, high efficiency of thermoelectric conversion can be secured by maintaining the state in which the resistivity of the thermoelectric conversion material portion is low. Examples of the material of the sealing material includes an epoxy resin. 
     In the thermoelectric conversion element, the semiconductor material may have n type conductivity and may include Si, Ge, Fe, and P. According to such a thermoelectric conversion element, improvement in thermoelectric efficiency can be attained by securely maintaining the state in which the resistivity of the thermoelectric conversion material portion is low. 
     A thermoelectric conversion module according to the present disclosure includes a plurality of the above-described thermoelectric conversion elements. According to the thermoelectric conversion module of the present disclosure, a thermoelectric conversion module having improved efficiency of thermoelectric conversion can be obtained by including the plurality of thermoelectric conversion elements of the present disclosure excellent in the efficiency of thermoelectric conversion. 
     An optical sensor according to the present disclosure includes: an absorber that absorbs optical energy; and the above-described thermoelectric conversion element of the present disclosure, wherein the first electrode is connected to the absorber. 
     In such an optical sensor, improvement in the efficiency of thermoelectric conversion is attained in the thermoelectric conversion element. Accordingly, an optical sensor having high sensitivity can be provided. 
     A method of producing a thermoelectric conversion material according to the present disclosure includes: obtaining powder of an amorphous semiconductor material by performing mechanical alloying in a reducing atmosphere; and forming a pressed powder body by pressing and solidifying the amorphous powder (the powder of the amorphous semiconductor material). 
     According to such a method of producing the thermoelectric conversion material, the mechanical alloying is performed in the reducing atmosphere. Hence, oxide films can be suppressed from being formed on surfaces of particles of the pressed powder body. By using the pressed powder body obtained in this way, a thermoelectric conversion material having improved efficiency of thermoelectric conversion can be provided. 
     In the method of producing the thermoelectric conversion material, the mechanical alloying may be performed in a mixed gas of hydrogen and nitrogen with a content ratio of hydrogen being less than or equal to 4 volume %. In this way, an oxide film can be suppressed from being formed in the obtaining of the powder. 
     A method of producing a thermoelectric conversion element according to the present disclosure includes: obtaining powder of an amorphous semiconductor material by performing mechanical alloying; forming a thermoelectric conversion material portion constituted of a pressed powder body by pressing and solidifying the amorphous powder; heating the thermoelectric conversion material portion while maintaining a state in which a partial pressure of oxygen in an atmosphere to which the thermoelectric conversion material portion is exposed is lower than a partial pressure of oxygen in an external air; and sealing the thermoelectric conversion material portion by a sealing portion such that a partial pressure of oxygen in a region surrounding the thermoelectric conversion material portion becomes lower than the partial pressure of oxygen in the external air. 
     According to the method of producing the thermoelectric conversion element in the present disclosure, the thermoelectric conversion material portion is heated while maintaining the state in which the partial pressure of oxygen in the atmosphere to which the thermoelectric conversion material portion is exposed is lower than the partial pressure of oxygen in the external air. Hence, oxygen atoms of oxide films on surfaces of particles of the pressed powder body can be diffused into the inside of the particles, thus resulting in thin oxide films on the surfaces of the particles. Accordingly, electric conductivity between the particles of the pressed powder body can be improved, thereby attaining a low resistivity of the thermoelectric conversion material portion. Further, the thermoelectric conversion material portion is sealed by the sealing portion such that the partial pressure of oxygen in the region surrounding the thermoelectric conversion material portion becomes lower than the partial pressure of oxygen in the external air. Hence, oxide films can be suppressed from being formed on the surfaces of the particles of the pressed powder body, thereby maintaining the state in which the resistivity of the thermoelectric conversion material portion is low. Therefore, according to such a method of producing the thermoelectric conversion element, a thermoelectric conversion element having improved efficiency of thermoelectric conversion can be produced. 
     In the method of producing the thermoelectric conversion element, the mechanical alloying may be performed in a reducing atmosphere. In this way, in the obtaining of the powder of the amorphous semiconductor material, oxide films can be suppressed from being formed on the surfaces of the particles of the pressed powder body. Therefore, a thermoelectric conversion element having a thermoelectric conversion material portion with a low resistivity can be produced. 
     In the method of producing the thermoelectric conversion element, the mechanical alloying may be performed in a mixed gas of hydrogen and nitrogen with a content ratio of hydrogen being less than or equal to 4 volume %. In this way, in the obtaining of the powder, oxide films can be suppressed from being formed. Therefore, a thermoelectric conversion element having improved thermoelectric efficiency can be produced. 
     Details of Embodiments of the Present Disclosure 
     Next, the following describes one embodiment of a thermoelectric conversion element of the present disclosure with reference to figures. It should be noted that in the below-described figures, the same or corresponding portions are denoted by the same reference characters and are not described repeatedly. 
     First Embodiment 
     The following describes a configuration of a thermoelectric conversion element according to a first embodiment of the present application.  FIG. 1  is a schematic view showing a structure of a π type thermoelectric conversion element (power generation element)  21  serving as the thermoelectric conversion element in the first embodiment. With reference to  FIG. 1 , π type thermoelectric conversion element  21  includes: thermoelectric conversion material portions  22 ,  23  each composed of a material having a band gap, specifically, thermoelectric conversion material portions  22 ,  23  each constituted of a pressed powder body of a semiconductor material containing an amorphous solid; a high temperature side electrode  24  serving as a first electrode disposed in contact with thermoelectric conversion material portions  22 ,  23 ; low temperature side electrodes  25 ,  26  serving as second electrodes disposed in contact with thermoelectric conversion material portions  22 ,  23  and disposed to be separated from high temperature side electrode  24 ; and a sealing portion  30  that seals thermoelectric conversion material portions  22 ,  23 . In  FIG. 1 , sealing portion  30  is schematically shown by a chain double-dashed line. Thermoelectric conversion material portion  22  is a p type thermoelectric conversion material portion  22 . Thermoelectric conversion material portion  23  is an n type thermoelectric conversion material portion  23 . 
     Thermoelectric conversion material portion  22  is composed of a thermoelectric conversion material having a component composition adjusted to have p type conductivity, for example. Thermoelectric conversion material portion  22  has p type conductivity because the thermoelectric conversion material of thermoelectric conversion material portion  22  is doped with a p type impurity for generating p type carriers (positive holes), which are majority carriers, for example. 
     Thermoelectric conversion material portion  23  is composed of the thermoelectric conversion material having a component composition adjusted to have n type conductivity, for example. Thermoelectric conversion material portion  23  has n type conductivity because the thermoelectric conversion material of thermoelectric conversion material portion  23  is doped with an n type impurity for generating n type carriers (electrons), which are majority carriers, for example. 
     Thermoelectric conversion material portion  22  and thermoelectric conversion material portion  23  are disposed side by side with a space being interposed therebetween. High temperature side electrode  24  is disposed to extend from one end portion  31  of thermoelectric conversion material portion  22  to one end portion  32  of thermoelectric conversion material portion  23 . High temperature side electrode  24  is disposed in contact with both one end portion  31  of thermoelectric conversion material portion  22  and one end portion  32  of thermoelectric conversion material portion  23 . High temperature side electrode  24  is disposed to connect between one end portion  31  of thermoelectric conversion material portion  22  and one end portion  32  of thermoelectric conversion material portion  23 . High temperature side electrode  24  is composed of an electrically conductive material such as a metal. High temperature side electrode  24  is in ohmic contact with thermoelectric conversion material portion  22  and thermoelectric conversion material portion  23 . 
     Low temperature side electrode  25  is disposed in contact with the other end portion  33  of thermoelectric conversion material portion  22 . Low temperature side electrode  25  is disposed to be separated from high temperature side electrode  24 . Low temperature side electrode  25  is composed of an electrically conductive material such as a metal. Low temperature side electrode  25  is in ohmic contact with thermoelectric conversion material portion  22 . 
     Low temperature side electrode  26  is disposed in contact with the other end portion  34  of thermoelectric conversion material portion  23 . Low temperature side electrode  26  is disposed to be separated from high temperature side electrode  24  and low temperature side electrode  25 . Low temperature side electrode  26  is composed of an electrically conductive material such as a metal. Low temperature side electrode  26  is in ohmic contact with thermoelectric conversion material portion  23 . 
     Wiring  27  is composed of an electrical conductor such as a metal. Wiring  27  electrically connects between low temperature side electrode  25  and low temperature side electrode  26 . 
     Sealing portion  30  is, for example, a box-shaped member and seals thermoelectric conversion material portions  22 ,  23 . Sealing portion  30  surrounds thermoelectric conversion material portions  22 ,  23  with a region  35  being interposed therebetween. A partial pressure of oxygen in region  35  surrounding thermoelectric conversion material portions  22 ,  23  is maintained by sealing portion  30  so as to be lower than a partial pressure of oxygen in an external air. In this embodiment, the pressure in region  35  is less than or equal to 1×10 −2  Pa. Specifically, the pressure in region  35  is maintained to be more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa. The inside of region  35  is substantially vacuum. As the material of sealing portion  30 , a silicon resin is selected, for example. 
     In π type thermoelectric conversion element  21 , for example, when a temperature difference is formed to attain a high temperature on the side of one end portion  31  of thermoelectric conversion material portion  22  and one end portion  32  of thermoelectric conversion material portion  23  and a low temperature on the side of the other end portion  33  of thermoelectric conversion material portion  22  and the other end portion  34  of thermoelectric conversion material portion  23 , p type carriers (positive holes) are moved from the one end portion  31  side toward the other end portion  33  side in thermoelectric conversion material portion  22 . On this occasion, in thermoelectric conversion material portion  23 , n type carriers (electrons) are moved from the one end portion  32  side toward the other end portion  34  side. As a result, current flows into wiring  27  in a direction of arrow I. In this way, power generation by thermoelectric conversion using the temperature difference is attained in π type thermoelectric conversion element  21 . That is, π type thermoelectric conversion element  21  is a power generation element. 
     Next, the following describes a configuration of thermoelectric conversion material portion  23 . Thermoelectric conversion material portion  23  is constituted of a pressed powder body of a semiconductor material containing an amorphous solid. In the present embodiment, the semiconductor material employs Si and Ge as base material elements. The semiconductor material includes a first additional element and a second additional element. The semiconductor material includes Fe as the first additional element. Fe forms a first addition level as a new level in a forbidden band (band gap) of SiGe. The semiconductor material includes P as the second additional element. P forms a second addition level between the first addition level and a conduction band in the forbidden band of SiGe. A Fermi level is adjusted by this second addition level. That is, in thermoelectric conversion material portion  23  in the present embodiment, a donor level can be formed by the second addition level. It should be noted that thermoelectric conversion material portion  22  is produced in the same manner as described above with Si, Ge, Au and B being used, for example. 
     Thermoelectric conversion element  21  including such thermoelectric conversion material portions  22 ,  23  is produced as follows.  FIG. 2  is a flowchart showing a representative production process of thermoelectric conversion element  21 . First, a method of producing n type thermoelectric conversion material portion  23  will be described. With reference to  FIG. 2 , measured amounts of Si, Ge, Fe, and P are introduced into a pot composed of stainless steel. In this case, the content ratios of the elements are adjusted as follows: Si 63 Ge 24 P 10 Fe 3 . Moreover, forming gas is introduced into the pot to provide a reducing atmosphere. The forming gas is a mixed gas of hydrogen and nitrogen with the content ratio of hydrogen being less than or equal to 4 volume %. Then, with mechanical alloying, amorphous powder in which Fe and P are added to SiGe is obtained. That is, the mechanical alloying is performed in the mixed gas of hydrogen and nitrogen with the content ratio of hydrogen being less than or equal to 4 volume %. In this way, the powder of amorphous semiconductor material is obtained by performing mechanical alloying in the reducing atmosphere (S 11 ). 
     Next, with a glove box having a nitrogen gas atmosphere therein, the obtained powder is introduced into a die to form a pressed powder body by a hot pressing method. A pressure on this occasion can be 400 MPa, and a temperature on this occasion can be 400° C. In this way, thermoelectric conversion material portion  23  constituted of the pressed powder body is formed by pressing and solidifying the amorphous powder (S 12 ). In this case, the amorphous powder is pressed and solidified while applying heat. It should be noted that the pressed powder body has become a sintered material on this occasion. These steps S 11  and S 12  are examples of representative steps in the method of producing the thermoelectric conversion material in the present application. That is, the method of producing the thermoelectric conversion material in the present application includes the steps of: obtaining powder of an amorphous semiconductor material by performing mechanical alloying in a reducing atmosphere; and pressing and solidifying the amorphous powder to form a pressed powder body. It should be noted that thermoelectric conversion material portion  22  is produced in the same manner as described above with Si, Ge, Au, and B being employed and the content ratios of the elements being adjusted. 
     Next, obtained thermoelectric conversion material portion  23  is heated while maintaining a state in which a partial pressure of oxygen in an atmosphere to which thermoelectric conversion material portion  23  is exposed is lower than a partial pressure of oxygen in an external air (S 13 ). In this case, thermoelectric conversion material portion  23  is disposed and heated in an atmosphere (vacuum) having a pressure of more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa. Then, thermoelectric conversion material portion  23  is cooled in an atmosphere having a pressure of more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa. The same applies to thermoelectric conversion material portion  22 . 
     Next, high temperature side electrode  24 , low temperature side electrodes  25 ,  26 , and wiring  27  are attached to thermoelectric conversion material portion  22  and thermoelectric conversion material portion  23 . Then, sealing is performed using sealing portion  30 . Here, thermoelectric conversion material portions  22 ,  23  is sealed by sealing portion  30  such that the partial pressure of oxygen in region  35  surrounding thermoelectric conversion material portions  22 ,  23  becomes lower than the partial pressure of oxygen in the external air (S 14 ). In this case, thermoelectric conversion material portion  22  and thermoelectric conversion material portion  23  having high temperature side electrode  24 , low temperature side electrodes  25 ,  26 , and wiring  27  attached thereto is sealed by sealing portion  30  in the atmosphere having a pressure of more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa. In this way, thermoelectric conversion element  21  including thermoelectric conversion material portions  22 ,  23  is obtained. 
       FIG. 3  is a graph showing a relation between temperature and resistivity in thermoelectric conversion material portion  23 . In  FIG. 3 , the vertical axis represents a resistivity (m Ωcm) and the horizontal axis represents a temperature (° C.). The resistivity is measured by a four-terminal method. 
     With reference to  FIG. 3 , it is understandable that the resistivity is decreased significantly by performing heating from a room temperature as indicated by an arrow  36   a  and circular marks while maintaining the state in which the pressure is more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa. Specifically, the resistivity, which is 220 m Ωcm at the room temperature, is decreased to be less than or equal to 10 m Ωcm at a temperature around and more than 700° C. Then, even when thermoelectric conversion material portion  23  is cooled to the room temperature as indicated by an arrow  36   b  and quadrangular marks while maintaining the state in which the pressure is more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa, the resistivity is maintained to be about 10 m Ωcm. It should be noted that the low resistivity is maintained also during the decrease of the temperature. Next, even when the temperature is increased again to 800° C. as indicated by an arrow  36   c  and triangular marks while maintaining the state in which the pressure is more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa, the resistivity is maintained to be less than or equal to 10 m Ωcm. On the other hand, when cooling to the room temperature is performed while maintaining the state in which the pressure is more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa as indicated by an X mark at the room temperature and then thermoelectric conversion material portion  23  is exposed to an external air, the resistivity is increased to about 110 m Ωcm. It should be noted that the resistivity is decreased in the following case: after exposing to the external air, the pressure is brought to be more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa again and the temperature is increased as indicated by an arrow  36   d  and X marks. 
     Thus, the low resistivity can be maintained as long as the pressure is maintained to be more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa. On the other hand, the resistivity becomes large when exposed to the external air. This is presumably due to the following reason: when exposed to the external air, oxide films are formed on surfaces of particles of the pressed powder body. It should be noted that the pressure of 1×10 −7  Pa results from use of a turbo-molecular pump. The same result was obtained also when an experiment was performed at a pressure of more than or equal to 1×10 −11  Pa by using an ion pump or a cryopump to decrease the pressure. 
     According to the method of producing thermoelectric conversion element  21  in the present application, each of thermoelectric conversion material portions  22 ,  23  is heated while maintaining the state in which the partial pressure of oxygen in the atmosphere to which each of thermoelectric conversion material portions  22 ,  23  is exposed is lower than the partial pressure of oxygen in the external air. Hence, oxygen atoms of oxide films on surfaces of particles of the pressed powder body can be diffused into the inside of the particles, thus resulting in thin oxide films on the surfaces of the particles. Accordingly, electric conductivity between the particles of the pressed powder body can be improved, thereby attaining a low resistivity of each of thermoelectric conversion material portions  22 ,  23 . Further, each of thermoelectric conversion material portions  22 ,  23  is sealed by sealing portion  30  such that the partial pressure of oxygen in the region surrounding thermoelectric conversion material portions  22 ,  23  becomes lower than the partial pressure of oxygen in the external air. Hence, oxide films can be suppressed from being formed on the surfaces of the particles of the pressed powder body, thereby maintaining the state in which the resistivity of each of thermoelectric conversion material portions  22 ,  23  is low. Therefore, according to such a method of producing thermoelectric conversion element  21 , a thermoelectric conversion element  21  having improved efficiency of thermoelectric conversion can be produced. 
     Moreover, according to the method of producing the thermoelectric conversion material in the present application, the mechanical alloying is performed in the reducing atmosphere. Hence, oxide films can be suppressed from being formed on the surfaces of the particles of the pressed powder body. By using the pressed powder body obtained in this way, a thermoelectric conversion material having improved efficiency of thermoelectric conversion can be provided. 
     In the present embodiment, the mechanical alloying is performed in the reducing atmosphere. Hence, formation of oxide films can be decreased on the surfaces of the particles of the pressed powder body. By using the pressed powder body obtained in this way, a thermoelectric conversion element  21  having improved efficiency of thermoelectric conversion can be produced. 
     In the present embodiment, the mechanical alloying is performed in a mixed gas of hydrogen and nitrogen with the content ratio of hydrogen being less than or equal to 4 volume %. Therefore, in the step of obtaining the powder, oxide films can be suppressed from being formed. 
     It should be noted that the method of producing thermoelectric conversion element  21  may include the steps of: obtaining powder of an amorphous semiconductor material by performing mechanical alloying; forming a thermoelectric conversion material portion  23  constituted of a pressed powder body by pressing and solidifying the amorphous powder; heating thermoelectric conversion material portion  23  while maintaining a state in which a partial pressure of oxygen in an atmosphere to which thermoelectric conversion material portion  23  is exposed is lower than a partial pressure of oxygen in an external air; and sealing thermoelectric conversion material portion  23  by sealing portion  30  such that a partial pressure of oxygen in a region surrounding thermoelectric conversion material portion  23  becomes lower than the partial pressure of oxygen in the external air. 
     According to such a method of producing thermoelectric conversion element  21 , thermoelectric conversion material portion  23  is heated while maintaining a state in which a partial pressure of oxygen in an atmosphere to which thermoelectric conversion material portion  23  is exposed is lower than a partial pressure of oxygen in an external air. Hence, oxygen atoms of oxide films on surfaces of particles of the pressed powder body can be diffused into the inside of the particles, thus resulting in thin oxide films on the surfaces of the particles. Accordingly, electric conductivity between the particles of the pressed powder body can be improved, thereby attaining a low resistivity of the thermoelectric conversion material portion. Moreover, thermoelectric conversion material portion  23  is sealed by sealing portion  30  such that the partial pressure of oxygen in the region surrounding thermoelectric conversion material portion  23  becomes lower than the partial pressure of oxygen in the external air. Hence, oxide films can be suppressed from being formed on the surfaces of the particles of the pressed powder body, thereby maintaining the state in which the resistivity of thermoelectric conversion material portion  23  is low. Therefore, according to such a method of producing thermoelectric conversion element  21 , a thermoelectric conversion element  21  having improved efficiency of thermoelectric conversion can be produced. 
     In the present embodiment, thermoelectric conversion element  21  includes sealing portion  30  that seals thermoelectric conversion material portions  22 ,  23 . The partial pressure of oxygen in the region surrounding each of thermoelectric conversion material portions  22 ,  23  is maintained by sealing portion  30  so as to be lower than a partial pressure of oxygen in an external air. Therefore, formation of oxide films on the surfaces of the particles of the pressed powder body can be suppressed as compared with that in the external air. As a result, the resistivity of each of thermoelectric conversion material portions  22 ,  23  can be suppressed from being increased, thereby maintaining high ZT. According to such a thermoelectric conversion element  21 , improvement in the efficiency of thermoelectric conversion can be attained. 
     In the present embodiment, each of thermoelectric conversion material portions  22 ,  23  is composed of the semiconductor material containing the amorphous solid. Specifically, each of thermoelectric conversion material portions  22 ,  23  is constituted of the pressed powder body of the semiconductor material containing the amorphous solid. Since the band gap of the semiconductor material is larger than the conductive material, the Seebeck coefficient can be made large. With the semiconductor material containing the amorphous solid, thermal conductivity can be made low. Therefore, dimensionless figure of merit ZT can be made large in each of thermoelectric conversion material portions  22 ,  23  included in thermoelectric conversion element  21 . As a result, improvement in efficiency of thermoelectric conversion can be attained. 
     In the present embodiment, the pressure in region  35  is more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa. Therefore, by decreasing the amount of oxygen in region  35 , oxide films can be suppressed from being formed on the surfaces of the particles of the pressed powder body. As a result, high efficiency of thermoelectric conversion can be secured by maintaining the state in which the resistivity of each of thermoelectric conversion material portions  22 ,  23  is low. 
     The semiconductor material of thermoelectric conversion material portion  23  has n type conductivity and includes Si, Ge, Fe, and P. According to such a thermoelectric conversion element  21 , improvement in thermoelectric efficiency can be attained by securely maintaining the state in which the resistivity of thermoelectric conversion material portion  23  is low. 
     It should be noted that in the above-described embodiment, the semiconductor material employs Si and Ge as the base material elements but is not limited thereto. A SiGe-based material may be employed. The SiGe-based material refers to SiGe and a material in which other element(s) such as C and/or Sn substitute for part of at least one of Si and Ge in SiGe. Moreover, the semiconductor material may be a MnSi-based material. The MnSi-based material refers to MnSi and a material in which other element(s) such as Al and/or W substitute for part of at least one of Mn and Si in MnSi. Further, the semiconductor material may be a SnSe-based material. The SnSe-based material refers to SnSe and a material in which other element(s) substitute for part of at least one of Sn and Se in SnSe. Further, the semiconductor material may be a Cu 2 Se-based material. The Cu 2 Se-based material refers to Cu 2 Se and a material in which other element(s) substitute for part of at least one of Cu and Se in Cu 2 Se. Moreover, the first additional element may not be included. The second additional element may not be included. 
     Moreover, in the embodiment, each of thermoelectric conversion material portions  22 ,  23  is composed of the semiconductor material, specifically, each of thermoelectric conversion material portions  22 ,  23  is constituted of the pressed powder body of the semiconductor material containing the amorphous solid; however, it is not limited thereto. Each of thermoelectric conversion material portions  22 ,  23  may be composed of, for example, an electrically conductive material as long as the material has a band gap. A semiconductor material containing an amorphous solid is more suitable. Also in this way, formation of an oxide film on a surface of the material of each of thermoelectric conversion material portions  22 ,  23  can be suppressed as compared with that in the external air. As a result, the resistivity of each of thermoelectric conversion material portions  22 ,  23  can be suppressed from being increased, thereby maintaining ZT to be high. According to such a thermoelectric conversion element  21 , improvement in the efficiency of thermoelectric conversion can be attained. 
     In the above-described embodiment, after heating (S 13 ) obtained thermoelectric conversion material portions  22 ,  23  while maintaining the state in which the partial pressure of oxygen in the atmosphere to which each of thermoelectric conversion material portions  22 ,  23  is exposed is lower than the partial pressure of oxygen in the external air, each of thermoelectric conversion material portions  22 ,  23  is sealed (S 14 ) by sealing portion  30  such that the partial pressure of oxygen in region  35  surrounding each of thermoelectric conversion material portions  22 ,  23  becomes lower than the partial pressure of oxygen in the external air. However, steps S 13  and S 14  may be performed in a reversed order. That is, after sealing (S 14 ) each of thermoelectric conversion material portions  22 ,  23  by sealing portion  30  such that the partial pressure of oxygen in region  35  surrounding each of thermoelectric conversion material portions  22 ,  23  becomes lower than the partial pressure of oxygen in the external air, each of obtained thermoelectric conversion material portions  22 ,  23  may be heated (S 13 ) while maintaining the state in which the partial pressure of oxygen in the atmosphere to which thermoelectric conversion material portions  22 ,  23  is exposed is lower than the partial pressure of oxygen in the external air. In this case, a member having durability even when the temperature is high due to heating is employed for sealing portion  30 . Alternatively, while heating (S 13 ) obtained thermoelectric conversion material portions  22 ,  23  while maintaining the state in which the partial pressure of oxygen in the atmosphere to which each of thermoelectric conversion material portions  22 ,  23  is exposed is lower than the partial pressure of oxygen in the external air, each of thermoelectric conversion material portions  22 ,  23  may be sealed (S 14 ) by sealing portion  30  such that the partial pressure of oxygen in region  35  surrounding each of thermoelectric conversion material portions  22 ,  23  becomes lower than the partial pressure of oxygen in the external air. That is, steps S 13  and S 14  may be performed in parallel. 
     In the embodiment, the pressure in region  35  is more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa; however, it is not limited thereto. The partial pressure of oxygen in the atmosphere in region  35  may be less than or equal to 2×10 5  Pa. In this way, formation of oxide films on surfaces of the particles of the pressed powder body can be suppressed efficiently. Therefore, high efficiency of thermoelectric conversion can be secured by maintaining the state in which the resistivity of each of thermoelectric conversion material portions  22 ,  23  is low. 
     In the above-described embodiment, an inert gas may be sealed in region  35 . In this way, formation of oxide films on surfaces of the particles of the pressed powder body can be suppressed more securely. Therefore, high efficiency of thermoelectric conversion can be secured by maintaining the state in which the resistivity of each of thermoelectric conversion material portions  22 ,  23  is low. Examples of the inert gas include argon gas and nitrogen gas. 
     In the above-described embodiment, the π type thermoelectric conversion element has been described as an exemplary thermoelectric conversion element of the present application; however, the thermoelectric conversion element of the present application is not limited thereto. The thermoelectric conversion element of the present application may be a thermoelectric conversion element having a different structure, such as an I type (uni-leg type) thermoelectric conversion element. 
     Second Embodiment 
     By electrically connecting a plurality of π type thermoelectric conversion elements  21 , a power generation module serving as a thermoelectric conversion module can be obtained. A power generation module  41 , which is a thermoelectric conversion module of the present embodiment, has a structure in which a plurality of π type thermoelectric conversion elements  21  are connected in series. 
     Each of  FIG. 4  and  FIG. 5  shows an exemplary structure of a power generation module  41  in a second embodiment. In order to facilitate understanding, in  FIG. 4 , parts of configurations are not shown and shapes thereof are indicated by chain double-dashed lines. 
     With reference to  FIG. 4 , power generation module  41  of the present embodiment includes p type thermoelectric conversion material portions  22 , n type thermoelectric conversion material portions  23 , low temperature side electrodes  25 ,  26 , high temperature side electrodes  24 , a low temperature side insulator substrate  29 , a high temperature side insulator substrate  28 , and a frame body  37 . Each of low temperature side insulator substrate  29  and high temperature side insulator substrate  28  is composed of a ceramic such as alumina. Thermoelectric conversion material portions  22  and thermoelectric conversion material portions  23  are alternately disposed side by side. As with π type thermoelectric conversion element  21  described above, each of low temperature side electrodes  25 ,  26  is disposed in contact with thermoelectric conversion material portion  22  and thermoelectric conversion material portion  23 . As with π type thermoelectric conversion element  21  described above, each of high temperature side electrodes  24  is disposed in contact with thermoelectric conversion material portion  22  and thermoelectric conversion material portion  23 . Each thermoelectric conversion material portion  22  is connected to an adjacent thermoelectric conversion material portion  23  on one side by a common high temperature side electrode  24 . Moreover, each thermoelectric conversion material portion  22  is connected to an adjacent thermoelectric conversion material portion  23  on a side different from the one side by common low temperature side electrodes  25 ,  26 . In this way, all the thermoelectric conversion material portions  22  and thermoelectric conversion material portions  23  are connected in series. 
     Low temperature side insulator substrate  29  is disposed on the side of each of low temperature side electrodes  25 ,  26  that are each in the form of a plate, the side being opposite to the side thereof in contact with thermoelectric conversion material portion  22  and thermoelectric conversion material portion  23 . One low temperature side insulator substrate  29  is disposed for the plurality of (all the) low temperature side electrodes  25 ,  26 . High temperature side insulator substrate  28  is disposed on the side of each of high temperature side electrodes  24  that are each in the form of a plate, the side being opposite to the side thereof in contact with thermoelectric conversion material portion  22  and thermoelectric conversion material portion  23 . One high temperature side insulator substrate  28  is disposed for the plurality of (all the) high temperature side electrodes  24 . 
     Frame body  37  is constituted of a frame-shaped member having a certain thickness along the external shape of low temperature side insulator substrate  29 . Frame body  37  has a quadrangular tubular shape. Frame body  37  is composed of a material such as a silicon resin. Frame body  37  is attached and interposed between low temperature side insulator substrate  29  and high temperature side insulator substrate  28 . In this case, frame body  37  is joined to each of low temperature side insulator substrate  29  and high temperature side insulator substrate  28 . Thermoelectric conversion material portions  22 ,  23 , low temperature side electrodes  25 ,  26 , and high temperature side electrodes  24  are disposed in region  35  surrounded by low temperature side insulator substrate  29 , high temperature side insulator substrate  28 , and frame body  37 . Sealing portion  30  included in thermoelectric conversion element  21  in the present embodiment is constituted of low temperature side insulator substrate  29 , high temperature side insulator substrate  28 , and frame body  37 . The pressure in region  35  is more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa (vacuum), for example. 
     Wirings  42 ,  43  are connected to high temperature side electrode  24  or low temperature side electrodes  25 ,  26  in contact with thermoelectric conversion material portion  22  or thermoelectric conversion material portion  23  located at respective ends among thermoelectric conversion material portions  22  and thermoelectric conversion material portions  23  connected in series. When a temperature difference is formed to attain a high temperature on the high temperature side insulator substrate  28  side and a low temperature on the low temperature side insulator substrate  29  side, current flows in a direction of arrow I by thermoelectric conversion material portions  22  and thermoelectric conversion material portions  23  connected in series, as with π type thermoelectric conversion element  21 . In this way, power generation by thermoelectric conversion using the temperature difference is attained in power generation module  41 . 
     Power generation module  41  includes a plurality of thermoelectric conversion elements  21  described above. Here, since power generation module  41  includes the plurality of thermoelectric conversion elements  21  of the present application excellent in the efficiency of thermoelectric conversion, there can be obtained a power generation module serving as a thermoelectric conversion module and having improved efficiency of thermoelectric conversion. It should be noted that an inert gas may be sealed in region  35  shown in  FIG. 4 . 
     Third Embodiment 
     Sealing portion  30  in each of the above-described embodiments surrounds thermoelectric conversion material portions  22 ,  23  with region  35  being interposed therebetween; however, it is not limited thereto. The following configuration may be employed. A power generation module  41  in a third embodiment has basically the same configuration and exhibits basically the same effect as those in the second embodiment. However, power generation module  41  of the third embodiment is different from that in the second embodiment in that sealing portion  30  is constituted of a sealing material  38  provided in region  35 . 
       FIG. 6  shows an exemplary structure of the power generation module in the third embodiment. With reference to  FIG. 6 , the power generation module includes: thermoelectric conversion material portions  22 ,  23  each composed of a material having a band gap; high temperature side electrodes  24  serving as first electrodes disposed in contact with thermoelectric conversion material portions  22 ,  23 ; low temperature side electrodes serving as second electrodes disposed in contact with thermoelectric conversion material portions  22 ,  23  and disposed to be separated from high temperature side electrodes  24 ; and sealing portion  30  that seals thermoelectric conversion material portions  22 ,  23 . Sealing portion  30  is composed of sealing material  38  provided in region  35 . As sealing material  38 , an epoxy resin is selected, for example. Sealing material  38  is provided as follows, for example. That is, frame body  37  is joined onto low temperature side insulator substrate  29  to place low temperature side electrodes  25 ,  26 , thermoelectric conversion material portions  22 ,  23 , and high temperature side electrodes  24  on low temperature side insulator substrate  29 . Then, an uncured epoxy resin or the like is provided into gaps among low temperature side electrodes  25 ,  26 , thermoelectric conversion material portions  22 ,  23 , and high temperature side electrodes  24  in region  35 . Then, the epoxy resin is cured. In this way, sealing material  38  composed of the cured epoxy resin is provided. It should be noted that in this case, there is no gas around thermoelectric conversion material portions  22 ,  23  and no oxygen molecules exist. Hence, the partial pressure of oxygen in the region surrounding thermoelectric conversion material portions  22 ,  23  is 0. That is, in thermoelectric conversion element  21  according to the present application, the substance disposed in region  35  surrounding thermoelectric conversion material portions  22 ,  23  may be in the form of a gas or in the form of a solid such as sealing material  38 . With such a sealing material  38 , each of thermoelectric conversion material portions  22 ,  23  can be suppressed from being brought into contact with oxygen molecules. As a result, oxide films can be suppressed from being formed on the surfaces of the particles of the pressed powder body. Therefore, high efficiency of thermoelectric conversion can be secured by maintaining the state in which the resistivity of each of thermoelectric conversion material portions  22 ,  23  is low. 
     Fourth Embodiment 
     Next, the following describes an optical sensor as another embodiment employing power generation module  41  according to the second embodiment.  FIG. 7  shows an exemplary structure of an optical sensor. With reference to  FIG. 7 , optical sensor  44  includes: power generation module  41  described above; a cap  45  that covers power generation module  41 ; and a base  46  having a circular plate shape. Power generation module  41  is attached on one surface  48  of base  46 . Cap  45  includes a cylindrical portion  47   a  and a cover portion  47   b  that covers an opening of cylindrical portion  47   a  on one side. Cap  45  is attached on one surface  48  of base  46  to cover power generation module  41 . Cover portion  47   b  is provided with a transparent glass  49  to receive light. Wirings  42 ,  43  from power generation module  41  extends through base  46  in a plate thickness direction to enable an output to outside. In the present embodiment, power generation module  41  includes an absorber  50  that absorbs optical energy. Absorber  50  is disposed in contact with high temperature side insulator substrate  28  on the surface side facing glass  49 . In  FIG. 7 , absorber  50  is illustrated schematically. High temperature side insulator substrate  28  included in power generation module  41  is connected to absorber  50 . That is, high temperature side electrode  24 , which is the first electrode included in power generation module  41 , is connected to absorber  50  via high temperature side insulator substrate  28 . 
     Light enters cylindrical portion  47   a  through glass  49  in a direction indicated by an arrow in  FIG. 7  and absorber  50  is irradiated with the light in power generation module  41 . Here, when absorber  50  of power generation module  41  absorbs optical energy as a result of the irradiation of light, a temperature difference occurs between high temperature side insulator substrate  28  in contact with absorber  50  and low temperature side insulator substrate  29 . Due to thermoelectric conversion material portion  22  and thermoelectric conversion material portion  23  connected in series, current flows in the same direction as that in the case of π type thermoelectric conversion element  21 . 
     In optical sensor  44  of the present embodiment, improvement in the efficiency of thermoelectric conversion is attained in thermoelectric conversion element  21 . Accordingly, an optical sensor having high sensitivity can be provided. 
     It should be noted that in the present embodiment, optical sensor  44  employs power generation module  41  shown in  FIG. 4 ; however, it is not limited thereto. The following configuration may be employed. That is, for example, a power generation module  41  including no frame body  37  and having thermoelectric conversion material portions  22 ,  23  not sealed is placed as shown in  FIG. 7 . This is sealed using cap  45  and base  46  in a state in which a pressure is more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa (vacuum), for example. In this case, cap  45  and base  46  constitute sealing portion  30 . That is, thermoelectric conversion element  21  includes cap  45  and base  46  as sealing portion  30 . Also in this way, formation of an oxide film on a surface of the material of each of thermoelectric conversion material portions  22 ,  23  is suppressed as compared with that in the external air, thereby obtaining an optical sensor  44  having high sensitivity. 
     Moreover, instead of power generation module  41  shown in  FIG. 7 , optical sensor  44  may employ thermoelectric conversion element  21  shown in  FIG. 1  and including the pair of p type and n type thermoelectric conversion material portions  22 ,  23 . In this case, high temperature side electrode  24  is provided with absorber  50  that absorbs optical energy. Then, sealing is performed using cap  45  and base  46  in the state in which the pressure is more than or equal to 1×10 −7  Pa and less than or equal to 1×10 −2  Pa (vacuum). That is, also in this case, thermoelectric conversion element  21  includes cap  45  and base  46  as sealing portion  30 . Also in this way, formation of an oxide film on a surface of the material of each of thermoelectric conversion material portions  22 ,  23  is suppressed as compared with that in the external air, thereby obtaining an optical sensor  44  having high sensitivity. 
     The embodiments disclosed herein are illustrative and non-restrictive in any respect. The scope of the present invention is defined by the terms of the claims, rather than the embodiments described above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     REFERENCE SIGNS LIST 
       21 : π type thermoelectric conversion element;  22 ,  23 : thermoelectric conversion material portion;  24 : high temperature side electrode;  25 ,  26 : low temperature side electrode;  27 ,  42 ,  43 : wiring;  28 : high temperature side insulator substrate;  29 : low temperature side insulator substrate;  30 : sealing portion;  31 ,  32 ,  33 ,  34 : end portion;  35 : region;  36   a ,  36   b ,  36   c ,  36   d : arrow;  37 : frame body;  38 : sealing material;  41 : thermoelectric conversion module;  44 : optical sensor;  45 : cap;  46 : base;  47   a : cylindrical portion;  47   b : cover portion;  48 : surface;  49 : glass;  50 : absorber.