Thermoelectric conversion material, thermoelectric conversion element and thermoelectric conversion module

A thermoelectric conversion material includes: a base material that is a semiconductor; and an additive element that differs from an element constituting the base material. An additional band formed of the additive element is present within a forbidden band of the base material. A density of states of the additional band has a ratio of greater than or equal to 0.1 relative to a maximum value of a density of states of a valence band adjacent to the forbidden band of the base material.

TECHNICAL FIELD

The present invention relates to a thermoelectric conversion material, a thermoelectric conversion element, and a thermoelectric conversion module. This application claims priority to Japanese Patent Application No. 2016-169406 filed on Aug. 31, 2016. The entire disclosure of this application is incorporated herein by reference

Background Art

In recent years, as clean energy replacing fossil fuels such as petroleum, renewable energy has attracted attention. The renewable energy includes, in addition to electric power generation using solar light, water power, and wind power, electric power generation by thermoelectric conversion using temperature difference. In thermoelectric conversion, since heat is directly converted to electricity, no extra waste is discharged during the conversion. Also, since thermoelectric conversion does not need a drive unit such as a motor, it has a feature that device maintenance is easy. In addition, an optical sensor such as an infrared sensor using thermoelectric conversion is available.

The conversion efficiency η of temperature difference (thermal energy) to electric energy using a material for conducting thermoelectric conversion (thermoelectric conversion material) is given by Equation (1) below.
η=ΔT/Th·(M−1)/(M+Tc/Th)  (1)
where η is the conversion efficiency, ΔT=Th−Tc, This a temperature on the high-temperature side, Tcis a temperature on the low-temperature side, M=(1+ZT)1/2, ZT=α2ST/κ, ZT is a dimensionless figure of merit, α is a Seebeck coefficient, S is an electric conductivity, κ is a thermal conductivity. As above, the conversion efficiency is a monotonically increasing function of ZT. Increasing ZT is important in the development of thermoelectric conversion materials.

Many examinations have been made for the development of thermoelectric conversion materials, in which ZT is generally approximately 1. In contrast to this, a report has been made on a thermoelectric conversion material in which ZT has been increased using a quantum effect (e.g., see NPL 1 and NPL 2).

CITATION LIST

Non Patent Literature

NPL 1: L. D. Hicks et al., “Effect of quantum-well structures on the thermoelectric figure of merit,” PHYSICAL REVIEW B, The American Physical Society, VOLUME 47, NUMBER 19, 47(1993) 12727NPL 2: L. D. Hicks et al., “Experimental study of the effect of quantum-well structures on the thermoelectric figure of merit,” PHYSICAL REVIEW B, The American Physical Society, VOLUME 53, NUMBER 16, 53(1996) R10493

SUMMARY OF INVENTION

The thermoelectric conversion material of the present disclosure includes a base material that is a semiconductor and an additive element that differs from an element constituting the base material. An additional band formed of the additive element is present within a forbidden band of the base material. A density of states of the additional band has a ratio of greater than or equal to 0.1 relative to a maximum value of a density of states of a valence band adjacent to the forbidden band of the base material.

DETAILED DESCRIPTION

Problem to be Solved by the Present Disclosure

According to NPL 1 and NPL 2 described above, reduction in the dimension of carriers and increase in phonon scattering are obtained by the quantum effect, whereby the Seebeck coefficient and the thermal conductivity can be controlled.

However, the thermoelectric conversion materials reported in NPL 1 and NPL 2 above are meant to improve the density of states in the band by the quantum effect. Therefore, while the Seebeck coefficient rises, the electric conductivity does not rise so greatly. As a result, a problem has arisen that inductive noise becomes great.

In view of the above, an objective of the present disclosure is providing a thermoelectric conversion material capable of raising the electric conductivity thereby increasing the value of ZT.

Advantageous Effect of the Present Disclosure

According to the thermoelectric conversion material of the present disclosure, it is possible to provide a thermoelectric conversion material capable of raising the electric conductivity thereby increasing the value of ZT.

DESCRIPTION OF EMBODIMENTS

First, embodiments of the invention of this application will initially be listed and described. The thermoelectric conversion material in this application includes a base material that is a semiconductor and an additive element that differs from an element constituting the base material. An additional band formed of the additive element is present within the forbidden band of the base material. The density of states of the additional band has a ratio of greater than or equal to 0.1 relative to the maximum value of the density of states of a valence band adjacent to the forbidden band of the base material.

According to examinations by the present inventors, it is possible to widely raise the electric conductivity thereby increasing ZT, by adding an additive element to a base material that is a semiconductor so as to form, within the forbidden band of the base material, an additional band having a density of states great to some extent, specifically, a density of states having a ratio of greater than or equal to 0.1 relative to the maximum value of the density of states of the valence band adjacent to the forbidden band of the base material. In the thermoelectric conversion material in this application, there is present, in the forbidden band of the base material, an additional band that is formed of an additive element and has a density of states having a ratio of greater than or equal to 0.1 relative to the maximum value of the density of states of the valence band adjacent to the forbidden band of the base material. Therefore, according to the thermoelectric conversion material in this application, it is possible to provide a thermoelectric conversion material capable of raising the electric conductivity thereby increasing the value of ZT.

The electric conductivity of the above thermoelectric conversion material may be greater than or equal to 50 kS/m and less than or equal to 1.5 MS/m. With the electric conductivity of greater than or equal to 50 kS/m, high ZT can be easily achieved. With the electric conductivity of less than or equal to 1.5 MS/m, reduction in Seebeck coefficient and rise in thermal conductivity can be prevented or reduced.

In the above thermoelectric conversion material, the half bandwidth of the additional band may be less than or equal to 50 meV. With this, higher electric conductivity can be achieved and thus ZT can be easily raised widely.

In the above thermoelectric conversion material, the additive element may have an unoccupied orbital in d orbital or f orbital located inside the outermost shell. With this, it becomes easy to form an addition band small in energy width.

In the above thermoelectric conversion material, the additive element may be a transition metal. With this, it becomes easy to form an addition band small in energy width.

In the above thermoelectric conversion material, the additional band may lie in a region within 100 meV from the valence band or a conduction band of the base material. With this, it becomes easy to obtain high ZT when the temperature has risen.

In the above thermoelectric conversion material, the base material may be a silicon germanium (SiGe) based material. The SiGe-based material is suitable as the base material of the thermoelectric conversion material in this application. The SiGe-based material refers to a material of SixGey(0≤x, 0≤y, and 0<x+y) and a material in which part of at least one of Si and Ge of SixGeyhas been replaced with carbon (C), tin (Sn), etc.

In the above thermoelectric conversion material, the additive element may be one kind or more selected from the group consisting of gold (Au), copper (Cu), nickel (Ni), and iron (Fe). All of Au, Cu, Ni, and Fe are suitable as the additive element when the base material is an SiGe-based material.

In the above thermoelectric conversion material, the base material may be a manganese silicon (MnSi) based material. The MnSi-based material is suitable as the base material of the thermoelectric conversion material in this application. The MnSi-based material refers to a material of MnxSiy(0.90≤x≤1.10 and 0.75≤y≤5.70) and a material in which part of at least one of Mn and Si of MnxSiyhas been replaced with aluminum (Al), tungsten (W), etc.

In the above thermoelectric conversion material, the additive element may be one kind or more selected from the group consisting of tantalum (Ta), tungsten (W), and rhenium (Re). Ta, W, and Re are suitable as the additive element when the base material is an MnSi-based material.

In the above thermoelectric conversion material, the base material may be an aluminum manganese silicon (AlMnSi) based material in which part of at least one of Mn and Si of the MnSi-based material has been replaced with Al. The AlMnSi-based material is suitable as the base material of the thermoelectric conversion material in this application. The AlMnSi-based material refers to a material of AlxMnySiz(0.00<x≤3.67, 0.90≤y≤1.10, 1.50≤x+z≤5.70, and z≥0.43x) and a material in which part of at least one of Al, Mn and Si of AlxMnySizhas been replaced with W, etc.

In the above thermoelectric conversion material, the additive element may be one kind or more selected from the group consisting of ruthenium (Ru), Ta, W, and Re. Ru, Ta, W, and Re are suitable as the additive element when the base material is an AlMnSi-based material.

In the above thermoelectric conversion material, the base material may be a tin selenium (SnSe) based material. The SnSe-based material is suitable as the base material of the thermoelectric conversion material in this application. The SnSe-based material refers to a material of SnxSey(0<x, 0<y, and ⅔≤y/x≤3/2) and a material in which part of at least one of Sn and Se of SnxSeyhas been replaced with scandium (Sc), titanium (Ti), zirconium (Zr), etc.

In the above thermoelectric conversion material, the additive element may be one kind or more selected from the group consisting of Sc, Ti, and Zr. Sc, Ti, and Zr are suitable as the additive element when the base material is an SnSe-based material.

In the above thermoelectric conversion material, the base material may be a copper selenium (CuSe) based material. The CuSe-based material is suitable as the base material of the thermoelectric conversion material in this application. The CuSe-based material refers to a material of CuxSey(0<x, 0<y, and ¼≤y/x≤1) and a material in which part of at least one of Cu and Se of CuxSeyhas been replaced with Sc, Ti, vanadium (V), etc.

In the above thermoelectric conversion material, the additive element may be one kind or more selected from the group consisting of Sc, Ti, and V. Sc, Ti, and V are suitable as the additive element when the base material is a CuSe-based material.

The thermoelectric conversion element in this application includes: a thermoelectric conversion material section; a first electrode placed in contact with the thermoelectric conversion material section; and a second electrode placed in contact with the thermoelectric conversion material section but apart from the first electrode. The thermoelectric conversion material section is made of the thermoelectric conversion material in this application of which the ingredient composition has been adjusted to have p-type or n-type conductivity.

The thermoelectric conversion element in this application has the thermoelectric conversion material section made of the thermoelectric conversion material, excellent in thermoelectric conversion characteristics, of which the ingredient composition has been adjusted to have p-type or n-type conductivity. Therefore, according to the thermoelectric conversion element in this application, a thermoelectric conversion element excellent in conversion efficiency can be provided.

The thermoelectric conversion module in this application includes a plurality of such thermoelectric conversion elements. According to the thermoelectric conversion module in this application, by having a plurality of thermoelectric conversion elements excellent in thermoelectric conversion efficiency, a thermoelectric conversion module excellent in thermoelectric conversion efficiency can be obtained.

DETAILS OF EMBODIMENTS

Next, an embodiment of the thermoelectric conversion material according to the present invention will be described hereinafter with reference to the accompanying drawings.

FIGS. 1 and 2are views showing the band structure of silicon (Si) that is a semiconductor. InFIGS. 1 and 2, the horizontal axis represents the energy. The vertical axis represents the density of states inFIG. 1and the spectral conductivity inFIG. 2. The spectral conductivity σ (ε, T) is expressed by Equation (2) below.
σ(ε,T)=ν2(ε)τ(ε)N(ε)  (2)
where ν is the heat speed of a carrier, τ is the relaxation time of the carrier, and N is the density of states. That is, the spectral conductivity is obtained by multiplying the density of states by the relaxation time of the carrier, etc., and is therefore proportional to the density of states. Referring toFIGS. 1 and 2, Si as a semiconductor has a forbidden band3between a valence band1and a conduction band2.

FIGS. 3, 5, and 7, corresponding toFIG. 2described above, are views showing the band structure of the thermoelectric conversion material of this embodiment. Referring toFIGS. 3, 5, and 7, the thermoelectric conversion material of this embodiment includes a base material that is a semiconductor and an additive element that differs from an element constituting the base material. An additional band4formed of the additive element is present within forbidden band3lying between valence band1and conduction band2of the base material. The density of states of additional band4has a ratio of greater than or equal to 0.1 relative to the maximum value of the density of states of valence band1adjacent to forbidden band3of the base material.

FIGS. 4, 6, and 8are views showing the energy-ZT relationships calculated based on the band structures inFIGS. 3, 5, and 7, respectively. InFIGS. 4, 6, and 8, the horizontal axis represents the energy and the vertical axis represents ZT. ZT has been calculated assuming that additional band4follows the Gaussian distribution. Also, ZT has been calculated for a plurality of cases different in the formation position of additional band4in forbidden band3. The plurality of curves inFIGS. 4, 6, and 8correspond to the different formation positions of additional band4.

Referring toFIG. 3, when the half bandwidth of additional band4is 20 meV and the strength of the spectral conductivity is 0.2×106S/m, ZT is of the order of 0.4 as shown inFIG. 4. This value of ZT corresponds to the order of eight times as large as that when additional band4is not present.

Referring toFIG. 5, when the strength of the spectral conductivity is changed to 3.0×106S/m, ZT becomes of the order of 3 as shown inFIG. 6. This value of ZT corresponds to the order of 60 times as large as that when additional band4is not present. Note that, as the lattice thermal conductivity, 0.5 W/mK is used. This value corresponds to a thermoelectric conversion material low in lattice thermal conductivity among usable thermoelectric conversion materials.

Referring toFIG. 7, when the half bandwidth of additional band4is changed to 10 meV, ZT becomes of the order of 8 as shown inFIG. 8. This value of ZT corresponds to the order of 180 times as large as that when additional band4is not present. At this time, compared with the case where additional band4is not present, the electric conductivity is 500 times, the Seebeck coefficient is 0.6 times, and the electron thermal conductivity is 16 times. That is, the rise of the electric conductivity greatly contributes to the rise of ZT in the thermoelectric conversion material of this embodiment.

Additional band4may just be present in forbidden band3, but preferably lies in a region within 100 meV from valence band1or conduction band2. With this, it becomes easy to obtain high ZT when the temperature rises. Further, it is especially preferable for additional band4to lie at the end of valence band1or conduction band2closer to forbidden band3, that is, at either end of forbidden band3. With this, ZT can be improved in high-temperature operation in a temperature range of approximately greater than or equal to 600 K.

The half bandwidth of additional band4is preferably less than or equal to 50 meV. Also, the density of states of additional band4preferably has a ratio of greater than or equal to 0.3 relative to the maximum value of the density of states of valence band1adjacent to forbidden band3of the base material. With this, it becomes easy to achieve higher electric conductivity thereby widely raising ZT.

The presence of additional band4can be confirmed by angle-resolved photoelectron spectroscopy.FIG. 9is a view showing an example of measurement results by the angle-resolved photoelectron spectroscopy. InFIG. 9, the horizontal axis represents the wavenumber, and the vertical axis represents the binding energy.FIG. 10is a view showing the relationship between the binding energy and the intensity (arbitrary unit) corresponding to the density of states, obtained from the measurement results ofFIG. 9. InFIG. 10, the horizontal axis represents the binding energy, and the vertical axis represents the intensity in arbitrary units corresponding to (proportional to) the density of states.

As shown inFIG. 9, an image corresponding to the density of states (a white image inFIG. 9) is obtained by the angle-resolved photoelectron spectroscopy. By analyzing the measurement results, the relationship between the binding energy and the intensity (arbitrary unit) corresponding to the density of states is obtained as shown inFIG. 10. When additional band4is present, an image corresponding to additional band4is obtained along a straight line drawn along the location corresponding to 0.0 of the vertical axis inFIG. 9. By analyzing this, a peak corresponding to additional band4shown inFIG. 10is obtained. In this way, the presence of additional band4can be confirmed by the angle-resolved photoelectron spectroscopy. Also, by performing such analysis, the ratio of the density of states of additional band4relative to the maximum value of the density of states of valence band1of the base material can be calculated.

The electric conductivity of the thermoelectric conversion material of this embodiment is preferably greater than or equal to 50 kS/m and less than or equal to 1.5 MS/m. With the electric conductivity of greater than or equal to 50 kS/m, high ZT can be easily achieved. With the electric conductivity of less than or equal to 1.5 MS/m, reduction in Seebeck coefficient and rise in thermal conductivity can be prevented or reduced.

The additive element of the thermoelectric conversion material of this embodiment is preferably an element having an unoccupied orbital in d orbital or f orbital located inside the outermost shell, e.g., a transition metal. With this, it becomes easy to form an addition band small in energy width.

The thermoelectric conversion material of this embodiment can be produced by following the procedure of forming a thin film by molecular beam epitaxy (MBE) and then performing thermal treatment. Specifically, a thin film including a base material and an additive element, for example, is formed and thermal treatment is performed to produce a material including a mother phase and an additive element. By this thermal treatment, a crystalline body precipitated out of the mother phase and an aggregate of the additive element can be obtained. The crystalline body of the mother phase and the aggregate of the additive element have grain sizes of 0 to 50 nm. In particular, grain sizes of 3 to 15 nm are preferred because the phonon scattering becomes significant, decreasing the thermal conductivity to as small as less than or equal to 3 W/mK, and improving the thermoelectric characteristics ZT. The grain size of either one of the crystalline body and the aggregate is preferably less than or equal to 10 nm, more preferably less than or equal to 5 nm. Such a grain size is preferred because the thermal conductivity can be widely reduced.

(Thermoelectric Conversion Material Having SiGe-Based Material as Base Material)

In this embodiment, an SiGe-based material can be used as the base material. The SiGe-based material as used herein refers to a material of SixGey(0≤x, 0≤y, and 0<x+y) and a material in which part of at least one of Si and Ge of SixGeyhas been replaced with C, Sn, etc. In this case, as the additive element, it is possible to use one kind or more selected from the group consisting of Au, Cu, Ni, and Fe. Specifically, the thermoelectric conversion material of this embodiment includes a mother phase constituted by an SiGe-based material and an additive element of one kind or more selected from the group consisting of Au, Cu, Ni, and Fe, for example. With this, a sharp band small in half bandwidth formed of the additive element can be formed within the forbidden band of the base material.

As for Si or Ge forming the SiGe-based material,FIG. 11shows an example of the band structure in the case where an additional band of Au, Cu, or Ni as the additive element is present within the forbidden band of Si band,FIG. 12shows an example of the band structure in the case where an additional band of Au, Cu, or Ni as the additive element is present within the forbidden band of Ge band, andFIG. 13shows an example of the band structure in the case where an additional band of Fe as the additive element is present within the forbidden band of Si band. As shown inFIGS. 11 to 13, Au, Cu, Ni, and Fe as the additive elements can form sharp bands small in half bandwidth within the forbidden bands of Si and Ge. Also, by using a mother phase constituted by an SiGe-based material, addition bands narrow in half bandwidth can be formed at ends of the forbidden bands of the SiGe-based material.

Note that, inFIGS. 11 and 12, the Si and Ge bands are calculated by the full-potential linearized augmented plane wave (FLAPW) method, and the exchange interaction thereof is handled within the frame of the generalized gradient approximation (GGA) method. The bands of Si36H36and Ge36H36are cluster-calculated (i.e., calculated using a cluster model, or calculated by DV-Xα method, which also applies to similar cases). The bands of the 5d orbital of Au, the 3d orbital of Cu, and the 3d orbital of Ni are cluster-calculated. InFIG. 13, the bands of Si and Fe in Si143Fe1are shown by the total density of states (tdos). The “×3” in the notes for the 3d orbital of Cu and the 5d orbital of Au and “×4” at the peak of the Si band inFIG. 11indicate that the represented signals have been “tripled” and “quadrupled,” respectively.

(Thermoelectric Conversion Material Having MnSi-Based Material as Base Material)

In this embodiment, an MnSi-based material can be used as the base material. The MnSi-based material as used herein refers to a material of MnxSiy(0.90≤x≤1.10 and 0.75≤y≤5.70) and a material in which part of at least one of Mn and Si of MnxSiyhas been replaced with Al, W, etc. In this case, as the additive element, it is possible to use one kind or more selected from the group consisting of Ta, W, and Re. Specifically, the thermoelectric conversion material of this embodiment includes a mother phase constituted by an MnSi-based material and an additive element of one kind or more selected from the group consisting of Ta, W, and Re, for example. With this, a sharp band small in half bandwidth formed of the additive element can be formed within the forbidden band of the base material.

As for Mn1Si1.73as the MnSi-based material,FIG. 14shows an example of the band structure in the case where an additional band of Ta as the additive element is present within the forbidden band of Mn band,FIG. 15shows an example of the band structure in the case where an additional band of W as the additive element is present within the forbidden band of Mn band, andFIG. 16shows an example of the band structure in the case where an additional band of Re as the additive element is present within the forbidden band of Mn band. As shown inFIGS. 14 to 16, Ta, W, and Re as the additive elements can form sharp bands small in half bandwidth within the forbidden band of Mn band. Also, by using a mother phase constituted by an MnSi-based material, addition bands narrow in half bandwidth can be formed at ends of the forbidden bands of the MnSi-based material.

Note that, inFIGS. 14 to 16, the bands of the 3d orbital of Mn, the 5d orbital of Ta, the 5d orbital of W, and the 5d orbital of Re are cluster-calculated.

(Thermoelectric Conversion Material Having AlMnSi-Based Material as Base Material)

In this embodiment, an AlMnSi-based material in which part of at least one of Mn and Si of the MnSi-based material described above has been replaced with Al can be used as the base material. The AlMnSi-based material as used herein refers to a material of AlxMnySiz(0.00<x≤3.67, 0.90≤y≤1.10, 1.50≤x+z≤5.70, and z≥0.43x) and a material in which part of at least one of Al, Mn and Si of AlxMnySizhas been replaced with W, etc. In this case, as the additive element, it is possible to use one kind or more selected from the group consisting of Ru, Ta, W, and Re. Specifically, the thermoelectric conversion material of this embodiment includes a mother phase constituted by an AlMnSi-based material and an additive element of one kind or more selected from the group consisting of Ru, Ta, W, and Re, for example. With this, a sharp band small in half bandwidth formed of the additive element can be formed within the forbidden band of the base material.

As for Al1Mn1Si1as the AlMnSi-based material,FIG. 17shows an example of the band structure in the case where an additional band of Ru as the additive element is present within the forbidden band of Mn band,FIG. 18shows an example of the band structure in the case where an additional band of Ta as the additive element is present within the forbidden band of Mn band,FIG. 19shows an example of the band structure in the case where an additional band of W as the additive element is present within the forbidden band of Mn band, andFIG. 20shows an example of the band structure in the case where an additional band of Re as the additive element is present within the forbidden band of Mn band. As shown inFIGS. 17 to 20, Ru Ta, W, and Re as the additive elements can form sharp bands small in half bandwidth within the forbidden band of Mn band. Also, by using a mother phase constituted by an AlMnSi-based material, addition bands narrow in half bandwidth can be formed at ends of the forbidden bands of the AlMnSi-based material.

Note that, inFIGS. 17 to 20, the band of the 3d orbital of Mn is calculated by the FLAPW method and the GGA method described above. The bands of the 4d orbital of Ru, the 5d orbital of Ta, the 5d orbital of W, and the 5d orbital of Re are cluster-calculated.

(Thermoelectric Conversion Material Having SnSe-Based Material as Base Material)

In this embodiment, an SnSe-based material can be used as the base material. The SnSe-based material as used herein refers to a material of SnxSey(0<x, 0<y, and ⅔≤y/x≤3/2) and a material in which part of at least one of Sn and Se of SnxSeyhas been replaced with Sc, Ti, Zr, etc. In this case, as the additive element, it is possible to use one kind or more selected from the group consisting of Sc, Ti, and Zr.

Specifically, the thermoelectric conversion material of this embodiment includes a mother phase constituted by an SnSe-based material and an additive element of one kind or more selected from the group consisting of Sc, Ti, and Zr, for example. With this, a sharp band small in half bandwidth formed of the additive element can be formed within the forbidden band of the base material.

As for Sn1Se1as the SnSe-based material,FIG. 21shows an example of the band structure in the case where an additional band of Sc as the additive element is present within the forbidden band of Sn1Se1band, andFIG. 22shows an example of the band structure in the case where an additional band of Ti or Zr as the additive element is present within the forbidden band of Sn1Se1band. As shown inFIGS. 21 and 22, Sc, Ti, and Zr as the additive elements can form sharp bands small in half bandwidth within the forbidden band of Sn1Se1band. Also, by using a mother phase constituted by an SnSe-based material, an addition band narrow in half bandwidth can be formed at an end of the forbidden band of the SnSe-based material.

Note that, inFIGS. 21 to 22, the Sn1Se1band is calculated by the FLAPW method and the GGA method described above. The bands of the 3d orbital of Sc, the 3d orbital of Ti, and the 4d orbital of Zr are cluster-calculated.

(Thermoelectric Conversion Material Having CuSe-Based Material as Base Material)

In this embodiment, a CuSe-based material can be used as the base material. The CuSe-based material as used herein refers to a material of CuxSey(0<x, 0<y, and ¼≤y/x≤1) and a material in which part of at least one of Cu and Se of CuxSeyhas been replaced with Sc, Ti, vanadium (V), etc. In this case, as the additive element, it is possible to use one kind or more selected from the group consisting of Sc, Ti, and V. Specifically, the thermoelectric conversion material of this embodiment includes a mother phase constituted by a CuSe-based material and an additive element of one kind or more selected from the group consisting of Sc, Ti, and V, for example. With this, a sharp band small in half bandwidth formed of the additive element can be formed within the forbidden band of the base material.

As for Cu2Se1as the CuSe-based material,FIG. 23shows an example of the band structure in the case where an additional band of Sc, Ti, or V as the additive element is present within the forbidden band of Cu2Se1band. As shown inFIG. 23, Sc, Ti, and V as the additive elements can form sharp bands small in half bandwidth within the forbidden band of Cu2Se1band. Also, by using a mother phase constituted by a CuSe-based material, an addition band narrow in half bandwidth can be formed at an end of the forbidden band of the CuSe-based material.

Note that, inFIG. 23, the Cu2Se1band is calculated by the FLAPW method and the GGA method described above. The bands of the 3d orbital of Sc, the 3d orbital of Ti, and the 3d orbital of V are cluster-calculated. The “×8” in the note for the Cu2Se1band inFIG. 23indicates that the represented signal has been “octupled.”

Next, as an embodiment of the thermoelectric conversion element and thermoelectric conversion module using the thermoelectric conversion material according to the present invention, a power generation element and a power generation module will be described.

FIG. 24is a schematic view showing a structure of a π-shaped thermoelectric conversion element (power generation element)10as the thermoelectric conversion element of this embodiment. Referring toFIG. 24, π-shaped thermoelectric conversion element10includes a p-type thermoelectric conversion material section11as a first thermoelectric conversion material section, an n-type thermoelectric conversion material section12as a second thermoelectric conversion material section, a high-temperature side electrode21, a first low-temperature side electrode22, a second low-temperature side electrode23, and an interconnection31.

P-type thermoelectric conversion material section11is made of the thermoelectric conversion material of Embodiment 1 of which the ingredient composition has been adjusted to have p-type conductivity. P-type thermoelectric conversion material section11is made to have p-type conductivity by doping the thermoelectric conversion material of Embodiment 1 constituting p-type thermoelectric conversion material section11with p-type impurities that cause generation of p-type carriers (holes) as majority carriers, for example.

N-type thermoelectric conversion material section12is made of the thermoelectric conversion material of Embodiment 1 of which the ingredient composition has been adjusted to have n-type conductivity. N-type thermoelectric conversion material section12is made to have n-type conductivity by doping the thermoelectric conversion material of Embodiment 1 constituting n-type thermoelectric conversion material section12with n-type impurities that cause generation of n-type carriers (electrons) as majority carriers, for example.

P-type thermoelectric conversion material section11and n-type thermoelectric conversion material section12are placed side by side with a spacing therebetween. High-temperature side electrode21is placed to extend from one end11A of p-type thermoelectric conversion material section11to one end12A of n-type thermoelectric conversion material section12. High-temperature side electrode21is placed to be in contact with both end11A of p-type thermoelectric conversion material section11and end12A of n-type thermoelectric conversion material section12. High-temperature side electrode21is placed to connect end11A of p-type thermoelectric conversion material section11and end12A of n-type thermoelectric conversion material section12. High-temperature side electrode21is made of a conductive material, e.g., a metal. High-temperature side electrode21is in ohmic contact with p-type thermoelectric conversion material section11and n-type thermoelectric conversion material section12.

First low-temperature side electrode22is placed to be in contact with the other end11B of p-type thermoelectric conversion material section11. First low-temperature side electrode22is placed apart from high-temperature side electrode21. First low-temperature side electrode22is made of a conductive material, e.g., a metal. First low-temperature side electrode22is in ohmic contact with p-type thermoelectric conversion material section11.

Second low-temperature side electrode23is placed to be in contact with the other end12B of n-type thermoelectric conversion material section12. Second low-temperature side electrode23is placed apart from high-temperature side electrode21and first low-temperature side electrode22. Second low-temperature side electrode23is made of a conductive material, e.g., a metal. Second low-temperature side electrode23is in ohmic contact with n-type thermoelectric conversion material section12.

Interconnection31is made of a conductor such as a metal. Interconnection31electrically connects first low-temperature side electrode22and second low-temperature side electrode23.

In π-shaped thermoelectric conversion element10, when a temperature difference is formed so that the temperature is high on the end11A side of p-type thermoelectric conversion material section11and the end12A side of n-type thermoelectric conversion material section12and is low on the end11B side of p-type thermoelectric conversion material section11and the end12B side of n-type thermoelectric conversion material section12, for example, p-type carriers (holes) migrate from the end11A side toward the end11B side in p-type thermoelectric conversion material section11. At this time, in n-type thermoelectric conversion material section12, n-type carriers (electrons) migrate from the end12A side toward the end12B side. As a result, in interconnection31, a current flows in the direction of arrow α. In this way, in π-shaped thermoelectric conversion element10, electric power generation by thermoelectric conversion using the temperature difference is achieved. That is, π-shaped thermoelectric conversion element10is a power generation element.

As the material constituting p-type thermoelectric conversion material section11and n-type thermoelectric conversion material section12, used is the thermoelectric conversion material of Embodiment 1 with a value of ZT having increased by raising the electric conductivity. As a result, π-shaped thermoelectric conversion element10is a highly efficient power generation element.

Moreover, by electrically connecting a plurality of π-shaped thermoelectric conversion elements10with each other, a power generation module as the thermoelectric conversion module can be obtained. A power generation module50as the thermoelectric conversion module of this embodiment has a structure of a plurality of π-shaped thermoelectric conversion elements10connected in series.

Referring toFIG. 25, power generation module50of this embodiment includes p-type thermoelectric conversion material sections11, n-type thermoelectric conversion material sections12, low-temperature side electrodes22,23corresponding to first low-temperature side electrodes22and second low-temperature side electrodes23, high-temperature side electrodes21, a low-temperature side insulator substrate33, and a high-temperature side insulator substrate34. Low-temperature side insulator substrate33and high-temperature side insulator substrate34are made of a ceramic such as alumina. P-type thermoelectric conversion material sections11and n-type thermoelectric conversion material sections12are arranged alternately. Low-temperature side electrodes22,23are placed in contact with p-type thermoelectric conversion material sections11and n-type thermoelectric conversion material sections12, as in π-shaped thermoelectric conversion element10described above. High-temperature side electrodes21are placed in contact with p-type thermoelectric conversion material sections11and n-type thermoelectric conversion material sections12, as in π-shaped thermoelectric conversion element10described above. Each of p-type thermoelectric conversion material sections11is connected to n-type thermoelectric conversion material section12adjacent on one side via common high-temperature side electrode21. Also, this p-type thermoelectric conversion material section11is connected to n-type thermoelectric conversion material section12adjacent on the other side via common low-temperature side electrode22,23. In this way, all p-type thermoelectric conversion material sections11and n-type thermoelectric conversion material sections12are connected in series.

Low-temperature side insulator substrate33is placed on the principal plane side of plate-shaped low-temperature side electrodes22,23opposite to the side in contact with p-type thermoelectric conversion material sections11and n-type thermoelectric conversion material sections12. One low-temperature side insulator substrate33is placed for the plurality of (all) low-temperature side electrodes22,23. High-temperature side insulator substrate34is placed on the side of plate-shaped high-temperature side electrodes21opposite to the side in contact with p-type thermoelectric conversion material sections11and n-type thermoelectric conversion material sections12. One high-temperature side insulator substrate34is placed for the plurality of (all) high-temperature side electrodes21.

Interconnections72and73are connected to high-temperature side electrodes21or low-temperature side electrodes22,23that are in contact with p-type thermoelectric conversion material sections11or n-type thermoelectric conversion material sections12located at both ends of serially-connected p-type thermoelectric conversion material sections11and n-type thermoelectric conversion material sections12. When a temperature difference is formed so that the temperature is high on the high-temperature side insulator substrate34side and is low on the low-temperature side insulator substrate33side, a current in the direction of arrow α flows by serially-connected p-type thermoelectric conversion material sections11and n-type thermoelectric conversion material sections12, as in the case of π-shaped thermoelectric conversion element10. In this way, in power generation module50, power generation by thermoelectric conversion using the temperature difference is achieved.

Next, as another embodiment of the thermoelectric conversion element using the thermoelectric conversion material according to the present invention, an infrared sensor that is an optical sensor will be described.

FIG. 26is a schematic cross-sectional view showing the structure of an infrared sensor40that is the thermoelectric conversion element of this embodiment. Referring toFIG. 26, infrared sensor40includes a p-type thermoelectric conversion section41and an n-type thermoelectric conversion section42placed adjacent to each other. P-type thermoelectric conversion section41and n-type thermoelectric conversion section42are formed on a substrate80.

Infrared sensor40includes a substrate80, an etching stop layer82, an n-type thermoelectric conversion material layer83, an n+-type ohmic contact layer84, an insulator layer85, a p-type thermoelectric conversion material layer86, n-side ohmic contact electrodes87, p-side ohmic contact electrodes88, a heat absorption pad89, an absorber90, and a protection film91.

Substrate80is made of an insulator such as silicon dioxide. A concave portion81is formed on substrate80. Etching stop layer82is formed to cover the surface of substrate80. Etching stop layer82is made of an insulator such as silicon nitride, for example. A gap is formed between etching stop layer82and concave portion81of substrate80.

N-type thermoelectric conversion material layer83is formed on the principal surface of etching stop layer82opposite to the surface on which substrate80is formed. N-type thermoelectric conversion material layer83is made of the thermoelectric conversion material of Embodiment 1 of which the ingredient composition has been adjusted to have n-type conductivity, for example. The thermoelectric conversion material of Embodiment 1 constituting n-type thermoelectric conversion material layer83is doped with n-type impurities that cause generation of n-type carriers (electrons) as majority carriers, for example, whereby n-type thermoelectric conversion material layer83has n-type conductivity. N+-type ohmic contact layer84is formed on the principal surface of n-type thermoelectric conversion material layer83opposite to the surface on which etching stop layer82is formed. N+-type ohmic contact layer84is doped with n-type impurities that cause generation of n-type carriers (electrons) as majority carriers, for example, at a higher concentration than n-type thermoelectric conversion material layer83, whereby n+-type ohmic contact layer84has n-type conductivity.

One of n-side ohmic contact electrodes87is placed so as to be in contact with a center portion of the principal surface of n+-type ohmic contact layer84opposite to the surface on which n-type thermoelectric conversion material layer83is formed. N-side ohmic contact electrode87is made of a material capable of ohmic contacting with n+-type ohmic contact layer84, e.g., a metal. Insulator layer85made of an insulator such as silicon dioxide, for example, is placed on the principal surface of n+-type ohmic contact layer84opposite to the surface on which n-type thermoelectric conversion material layer83is formed. Insulator layer85is placed on a portion of the principal surface of n+-type ohmic contact layer84on the p-type thermoelectric conversion section41side with respect to n-side ohmic contact electrode87.

Protection film91is further placed on the principal surface of n+-type ohmic contact layer84opposite to the surface on which n-type thermoelectric conversion material layer83is formed. Protection film91is placed on a portion of the principal surface of n+-type ohmic contact layer84on the opposite side from p-type thermoelectric conversion section41with respect to n-side ohmic contact electrode87. The other n-side ohmic contact electrode87is placed on a portion of the principal surface of n+-type ohmic contact layer84opposite to the surface on which n-type thermoelectric conversion material layer83is formed so as to oppose the former n-side ohmic contact electrode87with protection film91interposed therebetween.

P-type thermoelectric conversion material layer86is placed on the principal surface of insulator layer85opposite to the surface on which n+-type ohmic contact layer84is formed. P-type thermoelectric conversion material layer86is made of the thermoelectric conversion material of Embodiment 1 of which the ingredient composition has been adjusted to have p-type conductivity, for example. The thermoelectric conversion material of Embodiment 1 constituting p-type thermoelectric conversion material layer86is doped with p-type impurities that cause generation of p-type carriers (holes) as majority carriers, for example, whereby p-type thermoelectric conversion material layer86has p-type conductivity.

Protection film91is also placed on a center portion of the principal surface of p-type thermoelectric conversion material layer86opposite to the surface on which insulator layer85is formed. A pair of p-side ohmic contact electrodes88are placed on the principal surface of p-type thermoelectric conversion material layer86opposite to the surface on which insulator layer85is formed with protection film91interposed therebetween. P-side ohmic contact electrodes88are made of a material capable of ohmic contacting with p-type thermoelectric conversion material layer86, e.g., a metal. One of paired p-side ohmic contact electrodes88closer to n-type thermoelectric conversion section42is connected to n-side ohmic contact electrode87.

Absorber90is placed to cover the principal surfaces of p-side ohmic contact electrode88and n-side ohmic contact electrode87connected to each other opposite to the surfaces where n+-type ohmic contact layer84is formed. Absorber90is made of titanium, for example. Heat absorption pad89is placed to be in contact with the top surface of other p-side ohmic contact electrode88that is not connected to n-side ohmic contact electrode87. Heat absorption pad89is also placed to be in contact with the top surface of other n-side ohmic contact electrode87that is not connected to p-side ohmic contact electrode88. Gold (Au)/titanium (Ti), for example, is used as the material of heat absorption pad89.

When infrared sensor40is irradiated with infrared light, absorber90absorbs energy of the infrared light. As a result, while the temperature of absorber90rises, temperature rise of heat absorption pads89is prevented. This causes a temperature difference between absorber90and heat absorption pads89. With this, in p-type thermoelectric conversion material layer86, p-type carriers (holes) migrate from the absorber90side toward the heat absorption pad89side. Likewise, in n-type thermoelectric conversion material layer83, n-type carriers (electrons) migrate from the absorber90side toward the heat absorption pad89sides. By taking out a current flowing as a result of the migration of carriers from n-side ohmic contact electrodes87and p-side ohmic contact electrodes88, infrared light is detected.

In infrared sensor40of this embodiment, as the material constituting p-type thermoelectric conversion material layer86and n-type thermoelectric conversion material layer83, used is the thermoelectric conversion material of Embodiment 1 with a value of ZT having increased by raising the electric conductivity. As a result, infrared sensor40is a highly sensitive infrared sensor.

It should be understood that the embodiments disclosed herein are illustrative in all aspects and are not restrictive by any means. It is intended that the scope of the present invention shall be defined, not by the above description, but by the appended claims, and cover all changes falling within the spirit and scope equivalent to the appended claims.

INDUSTRIAL APPLICABILITY

The thermoelectric conversion material, the thermoelectric conversion element, and the thermoelectric conversion module in this application are especially favorably applicable to a thermoelectric conversion material, a thermoelectric conversion element, and a thermoelectric conversion module that are required to improve conversion efficiency.

REFERENCE SIGNS LIST