Method for producing thermoelectric conversion apparatus and thermoelectric conversion apparatus

A thermoelectric conversion element includes: a first film including a perovskite structure; a second film and a third film, including a perovskite structure, disposed in such a manner that the first film is interposed between the second film and the third film; a fourth film, including a perovskite structure, disposed so as to interpose the second film with the first film; and a fifth film, including a perovskite structure, disposed so as to interpose the third film with the first film, wherein an offset in conduction band between the first film and the second film and an offset in conduction band between the first film and the third film is less than 0.25 eV, and an offset in conduction band between the second film and the fourth film and an offset in conduction band between the third film and the fifth film is more than 1 eV.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-202211, filed on Oct. 13, 2015, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a thermoelectric conversion element, a method for producing the thermoelectric conversion element, a thermoelectric conversion apparatus, and the like.

BACKGROUND

Attention has been focused on thermoelectric conversion elements, which are clean power generation systems, from the viewpoints of a reduction in carbon dioxide (CO2) emission and environmental protection.

Japanese Laid-open Patent Publication No. 2010-93009 and International Publication Pamphlet No. WO2004105144 disclose the related art.

SUMMARY

According to an aspect of the embodiments, a thermoelectric conversion element includes: a first film including a perovskite structure; a second film and a third film, including a perovskite structure, disposed in such a manner that the first film is interposed between the second film and the third film; a fourth film, including a perovskite structure, disposed so as to interpose the second film with the first film; and a fifth film, including a perovskite structure, disposed so as to interpose the third film with the first film, wherein an offset in conduction band at an interface between the first film and the second film and an offset in conduction band at an interface between the first film and the third film is less than 0.25 eV, and an offset in conduction band at an interface between the second film and the fourth film and an offset in conduction band at an interface between the third film and the fifth film is more than 1 eV.

DESCRIPTION OF EMBODIMENTS

Use of thermoelectric conversion elements enables thermal energy, which is used to be exhausted, to be converted into electric energy and reused.

There may be room for improving, for example, the thermoelectric efficiency of thermoelectric conversion elements.

FIGS. 1A and 1Billustrate an example structure of a thermoelectric conversion element.

A thermoelectric conversion element10includes a Sr0.95La0.05TiO3film11and SrTiO3films12and13between which the Sr0.95La0.05TiO3film11is interposed as illustrated inFIG. 1A. The thermoelectric conversion element10also includes SrZrO3films14and15between which the SrTiO3film12, the Sr0.95La0.05TiO3film11, and the SrTiO3film13are interposed. For example, the SrTiO3film12is interposed between the Sr0.95La0.05TiO3film11and the SrZrO3film14, and the SrTiO3film13is interposed between the Sr0.95La0.05TiO3film11and the SrZrO3film15. The offset in conduction band at the interface between the Sr0.95La0.05TiO3film11and the SrTiO3film12and the offset in conduction band at the interface between the Sr0.95La0.05TiO3film11and the SrTiO3film13may be 0.25 eV or less. The offset in conduction band at the interface between the SrTiO3film12and the SrZrO3film14and the offset in conduction band at the interface between the SrTiO3film13and the SrZrO3film15may be 1.0 eV or more.

The thickness of the Sr0.95La0.05TiO3film11may be set to 4 to 100 Å in order to achieve sufficient quantum confinement. The thicknesses of the SrTiO3films12and13may be set to 4 to 1,000 Å in order to reduce the influence of electrons migrating from the Sr0.95La0.05TiO3film11into the SrTiO3films12and13. The thicknesses of the SrZrO3films14and15may be set to 100 Å or more in order to ensure the insulating capabilities of the SrZrO3films14and15. However, the insulating capabilities of the SrZrO3films14and15is saturated when the thicknesses of the SrZrO3films14and15reach about 10,000 Å, and setting the thicknesses of the SrZrO3films14and15to more than 10,000 Å vainly increases the cost. Thus, the thicknesses of the SrZrO3films14and15may be set to 10,000 Å or less.

The offset in the conduction band ECbetween the Sr0.95La0.05TiO3film11and the SrTiO3film12and the offset in the conduction band ECbetween the Sr0.95La0.05TiO3film11and the SrTiO3film13are considerably small as illustrated inFIG. 1B. This causes charge to migrate from the Sr0.95La0.05TiO3film11into the SrTiO3films12and13, thereby a reduction in charge density occurring. As a result, the Seebeck coefficient may be improved.

The energy of the conduction bands of the SrZrO3film14, the SrTiO3film12, the Sr0.95La0.05TiO3film11, the SrTiO3film13, and the SrZrO3film15form a quantum well as illustrated inFIG. 1B. Therefore, due to the quantum confinement, charge migration into the SrZrO3films14and15may be reduced. For example, an excessive reduction in charge density may be reduced. Accordingly, a figure of merit (ZT), which is one of measures of thermoelectric efficiency, may be improved.

FIGS. 2A and 2Billustrate an example structure of a thermoelectric conversion element.

A thermoelectric conversion element20includes a Sr0.90La0.10TiO3film21instead of the Sr0.95La0.05TiO3film11as illustrated inFIG. 2A. The thermoelectric conversion element20also includes the SrTiO3films12and13and the SrZrO3films14and15as in the thermoelectric conversion element10illustrated inFIG. 1A. The offset in conduction band at the interface between the Sr0.90La0.10TiO3film21and the SrTiO3film12and the offset in conduction band at the interface between the Sr0.90La0.10TiO3film21and the SrTiO3film13may be 0.25 eV or less. The offset in conduction band at the interface between the SrTiO3film12and the SrZrO3film14and the offset in conduction band at the interface between the SrTiO3film13and the SrZrO3film15may be 1.0 eV or more. The thickness of the Sr0.90La0.10TiO3film21may be, for example, 4 to 100 Å.

The offset in the conduction band ECbetween the Sr0.90La0.10TiO3film21and the SrTiO3film12and the offset in the conduction band ECbetween the Sr0.90La0.10TiO3film21and the SrTiO3film13are considerably small as illustrated inFIG. 2B. This causes charge to migrate from the Sr0.90La0.10TiO3film21into the SrTiO3films12and13, thereby a reduction in charge density occurring. As a result, the Seebeck coefficient may be improved.

The energy of the conduction bands of the SrZrO3film14, the SrTiO3film12, the Sr0.90La0.10TiO3film21, the SrTiO3film13, and the SrZrO3film15form a quantum well as illustrated inFIG. 2B. Therefore, due to the quantum confinement, charge migration into the SrZrO3films14and15may be reduced. For example, an excessive reduction in charge density may be reduced. Accordingly, a figure of merit (ZT), which is one of the measures of thermoelectric efficiency, may be improved.

FIGS. 3A and 3Billustrate an example structure of a thermoelectric conversion element.

A thermoelectric conversion element30includes a Sr0.95Nb0.05TiO3film31instead of the Sr0.95La0.05TiO3film11as illustrated inFIG. 3A. The thermoelectric conversion element30also includes the SrTiO3films12and13and the SrZrO3films14and15as in the thermoelectric conversion element10illustrated inFIG. 1A. The offset in conduction band at the interface between the Sr0.95Nb0.05TiO3film31and the SrTiO3film12and the offset in conduction band at the interface between the Sr0.95Nb0.05TiO3film31and the SrTiO3film13may be 0.25 eV or less. The offset in conduction band at the interface between the SrTiO3film12and the SrZrO3film14and the offset in conduction band at the interface between the SrTiO3film13and the SrZrO3film15may be 1.0 eV or more. The thickness of the Sr0.95Nb0.05TiO3film31may be, for example, 4 to 100 Å.

The offset in the conduction band ECbetween the Sr0.95Nb0.05TiO3film31and the SrTiO3film12and the offset in the conduction band ECbetween the Sr0.95Nb0.05TiO3film31and the SrTiO3film13are considerably small as illustrated inFIG. 3B. This causes charge to migrate from the Sr0.95Nb0.05TiO3film31into the SrTiO3films12and13, thereby a reduction in charge density occuring. As a result, the Seebeck coefficient may be improved.

The energy of the conduction bands of the SrZrO3film14, the SrTiO3film12, the Sr0.95Nb0.05TiO3film31, the SrTiO3film13, and the SrZrO3film15form a quantum well as illustrated inFIG. 3B. Therefore, due to the quantum confinement, charge migrating into the SrZrO3films14and15may be reduced. For example, an excessive reduction in charge density may be reduced. Accordingly, a figure of merit (ZT), which is one of the measures of thermoelectric efficiency, may be improved.

For example, the thermoelectric conversion element illustrated inFIGS. 1A and 1Bis prepared, and measurement of offset in conduction band is made. The thicknesses of the SrZrO3films14and15may be, for example, 180 Å. The thicknesses of the SrTiO3films12and13may be, for example, 18 Å. The thickness of the Sr0.95La0.05TiO3film11may be as large as the thickness of one atomic layer. The SrZrO3film14, the SrTiO3film12, the Sr0.95La0.05TiO3film11, the SrTiO3film13, and the SrZrO3film15are formed on a (LaAlO3)0.3—(SrAl0.5Ta0.5O3)0.7(LSAT) substrate by pulse laser deposition (PLD). As a result of the measurement, the offset in conduction band at the interface between the Sr0.95La0.05TiO3film11and the SrTiO3film12and the offset in conduction band at the interface between the Sr0.95La0.05TiO3film11and the SrTiO3film13were 0.25 eV or less. The offset in conduction band at the interface between the SrTiO3film12and the SrZrO3film14and the offset in conduction band at the interface between the SrTiO3film13and the SrZrO3film15were 1.9 eV.

For example, the thermoelectric conversion element illustrated inFIGS. 2A and 2Bis prepared, and measurement of offset in conduction band is made. As a result the measurement, the offset in conduction band at the interface between the Sr0.90La0.10TiO3film21and the SrTiO3film12and the offset in conduction band at the interface between the Sr0.90La0.10TiO3film21and the SrTiO3film13were 0.25 eV or less. The offset in conduction band at the interface between the SrTiO3film12and the SrZrO3film14and the offset in conduction band at the interface between the SrTiO3film13and the SrZrO3film15were 1.9 eV.

For example, the thermoelectric conversion element illustrated inFIGS. 3A and 3Bis prepared, and measurement of offset in conduction band is made. As a result of the measurement, the offset in conduction band at the interface between the Sr0.95Nb0.05TiO3film31and the SrTiO3film12and the offset in conduction band at the interface between the Sr0.95Nb0.05TiO3film31and the SrTiO3film13were 0.25 eV or less. The offset in conduction band at the interface between the SrTiO3film12and the SrZrO3film14and the offset in conduction band at the interface between the SrTiO3film13and the SrZrO3film15were 1.9 eV.

The Sr0.95La0.05TiO3film11, the SrTiO3films12and13, and the SrZrO3films14and15may be formed by RF magnetron sputtering or the like. The Sr0.90La0.10TiO3film21and the Sr0.95Nb0.05TiO3film31may be formed by RF magnetron sputtering or the like.

FIG. 4is a cross-sectional view of an example structure of a thermoelectric conversion apparatus.

For example, a thermoelectric conversion apparatus40illustrated inFIG. 4includes three thermoelectric conversion elements10. The thermoelectric conversion apparatus40also includes p-type films46which are disposed so as to interpose the corresponding one of the SrZrO3films with the corresponding one of the SrTiO3films12. The thermoelectric conversion apparatus40also includes a plurality of conductive films47and a plurality of conductive films48with which Sr0.95La0.05TiO3films11and the p-type films46are electrically coupled to each other in series. The conductive films47couple the respective Sr0.95La0.05TiO3films11to the respective p-type films46disposed on a side of the SrZrO3film14in each thermoelectric conversion element10. The conductive films48couple the respective Sr0.95La0.05TiO3films11to the respective p-type films46disposed on a side of the SrZrO3film15in each thermoelectric conversion element10. The conductive films47and the conductive films48are disposed opposite to each other across the thermoelectric conversion elements10. InFIG. 4, the conductive films47are disposed in the upper portions of the thermoelectric conversion elements10, and the conductive films48are disposed in the lower portions of the thermoelectric conversion elements10. The thermoelectric conversion apparatus40further includes a heat sink42disposed on a side of the conductive films47are disposed and a heat sink44disposed on a side of the conductive films48are disposed. A protective film41is interposed between the heat sink42and the conductive films47. A protective film43is interposed between the heat sink44and the conductive films48.

The thermoelectric conversion apparatus40enables a figure of merit (ZT), which is one of the measures of thermoelectric efficiency, to be improved similarly to the thermoelectric conversion element illustrated inFIGS. 1A and 1B. Thus, the thermoelectric conversion apparatus40may have a high thermoelectric efficiency. The thermoelectric conversion apparatus40may include the thermoelectric conversion element20or30instead of the thermoelectric conversion element10.

The composition of the material of the first film is not limited to Sr0.95La0.05TiO3, Sr0.90La0.10TiO3, or Sr0.95Nb0.05TiO3and, for example, A1-aMaB1-bNbO3, where 0.01≤a≤0.9 and 0.01≤b≤0.9 may be used. The composition of the material of the second and third films is not limited to SrTiO3and, for example, ACBdO3may be used. In order to form a perovskite phase, 0.5<c/d<1.5 is preferably satisfied. The composition of the material of the fourth and fifth films is not limited to SrZrO3and, for example, AZr1-eBeO3may be used.