THERMOELECTRIC CONVERSION ELEMENT AND THERMOELECTRIC CONVERSION DEVICE

A thermoelectric conversion device (20) includes a substrate (22) and a plurality of thermoelectric conversion elements (24, 25) on the substrate (22) . Each of the plurality of thermoelectric conversion elements (24, 25) has a rectangular parallelepiped shape and is made of an alloy including Fe3Sn2, an iron nitride (such as Fe16N2), or a rare-earth element and Co, the alloy exhibiting an anomalous Nernst effect. The thermoelectric conversion elements (24, 25) are arranged parallel to a direction (y direction) perpendicular to a longitudinal direction (x direction) to form a serpentine shape, and electrically connected in series.

TECHNICAL FIELD

The present invention relates to a thermoelectric conversion element and a thermoelectric conversion device including the thermoelectric conversion element.

BACKGROUND ART

The Seebeck effect is known as a thermoelectric mechanism that generates an electric voltage when a temperature gradient is applied to a material (see, for example, Patent Literature 1). Unfortunately, in the thermoelectric mechanism using the Seebeck effect, main materials usable at room temperature or higher include bismuth, tellurium, lead, or the like, and thus are not suitable for practical use due to high toxicity and are not durable due to mechanical fragility and vulnerability to vibration. The Seebeck effect generates the electric voltage in the same direction as the temperature gradient, which requires a complicated three-dimensional structure in which p-type modules and n-type modules are alternately arranged and stand in a direction perpendicular to a surface of a heat source. This leads to a rise in manufacturing cost. In addition, such a three-dimensional structure makes it difficult to achieve a large-area thermoelectric module.

The anomalous Nernst effect is known as another thermoelectric mechanism that generates an electric voltage due to a temperature gradient. The anomalous Nernst effect is a phenomenon observed when a heat current flowing through a magnetic material creates a temperature difference, which generates an electric voltage in a direction perpendicular to both a magnetization direction and the temperature gradient. Recent studies have shown that topological electronic structures contribute to increasing a Nernst coefficient much higher than a previously known value (0.1 µV/K).

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, although various magnetic materials exhibiting the anomalous Nernst effect have been developed so far, such metal materials are relatively expensive, which leads to a high cost.

The invention has been made in view of the foregoing, and an object of the invention is to provide a thermoelectric conversion element made of an inexpensive and non-toxic material and exhibiting an anomalous Nernst effect, and provide a thermoelectric conversion device.

Solution to Problem

A thermoelectric conversion element according to one embodiment of the invention is made of an alloy including a transition metal. The alloy is a compound having a crystal structure including a Kagome lattice plane constituted by the transition metal, and exhibits an anomalous Nernst effect.

A thermoelectric conversion device according to one embodiment of the invention includes a substrate and a plurality of thermoelectric conversion elements on the substrate. Each of the plurality of thermoelectric conversion elements is defined as the thermoelectric conversion element described above, and has a shape extending in one direction. The plurality of thermoelectric conversion elements are arranged in parallel in a direction perpendicular to the one direction and electrically connected in series.

A thermoelectric conversion device according to another embodiment of the invention includes a hollow member and the thermoelectric conversion element described above. The thermoelectric conversion element is a sheet-shaped element or a wire rod covering an outer surface of the hollow member.

Advantageous Effects of Invention

According to the invention, a thermoelectric conversion element made of an inexpensive and non-toxic material is capable of exhibiting an anomalous Nernst effect.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments of the invention will be described below with reference to the accompanying drawings. The drawings are schematic, and a relationship between a plane dimension and a thickness and a ratio of the thickness between members are different from actual ones.

First, a thermoelectric conversion element according to the embodiments of the invention and a thermoelectric mechanism thereof will be described with reference toFIG.1.

As shown inFIG.1, a thermoelectric conversion element 1 according to the embodiments has a rectangular parallelepiped shape extending in one direction (y direction), has a predetermined thickness (length in a z direction), and is magnetized in a +z direction. When a heat current Q (∝ -VT) flows through the thermoelectric conversion element 1 in a +x direction, a temperature difference is created in the +x direction. As a result, in the thermoelectric conversion element 1, the anomalous Nernst effect generates electromotive force V (∝ M × (-∇T)) in an outer product direction (y direction) orthogonal to both the direction of the heat current Q (+x direction) and the direction of magnetization M (+z direction) .

Among materials exhibiting the anomalous Nernst effect, Co2MnGa has a Nernst coefficient that reaches a record high value of about 6 µV/K at room temperature and about 8 µV/K at 400 K, reported by the inventor of the present application (see Nature Physics 14, 1119-1124 (2018) and International Publication No. WO 2019/009308) .

The inventor of the present application was able to demonstrate that a Nernst coefficient of an alloy of Fe (iron) and Sn (tin), which are inexpensive and non-toxic materials, approaches the record high value.

FIG.2AandFIG.2Bshow a crystal structure of Fe2Sn2capable of exhibiting the anomalous Nernst effect. The crystal structure of Fe3Sn2is composed of a Fe-Sn Kagome bilayer and a Sn layer stacking alternately along a c-axis direction. In the Fe-Sn Kagome bilayer, the Fe atoms form two Kagome lattice layers. The Sn layer has a honeycomb structure. As shown inFIG.2A, two Fe-Sn Kagome bilayers are separated by the Sn layer. As shown inFIG.2B, each of the Fe-Sn Kagome bilayers has two kinds of equilateral triangles with different Fe-Fe lengths (solid line, broken line) , and the Sn atoms within the bilayer occupy centers of hexagons of the Kagome lattice.

Such an Fe2Sn2ferromagnetic material can be prepared by a known method such as a self-flux method. Alternatively, a sputtering method, a molecular beam epitaxy (MBE) method, a chemical vapor deposition (CVD) method, or other such method may be employed to produce a thin film of Fe3Sn2. Fe3Sn2may be a single crystal or a polycrystal. Because Fe3Sn2is a very stable material, the crystal can be pulverized into powder to produce ink.

FIG.3Ashows temperature dependence of magnetization M of Fe3Sn2in zero-field cooling (ZFC) and field cooling (FC) when a magnetic field B of 100 Oe is applied parallel to an ab plane.FIG.3Bshows a temperature derivative of the magnetization shown inFIG.3A(dM/dT) .FIG.3Cshows magnetic field dependence of the magnetization M of Fe2Sn2at T = 5 K, 200 K, and 300 K when the magnetic field B is applied parallel to the ab plane.

FIG.3Aindicates that the magnetization in FC is slightly larger than that in ZFC at a low temperature because domains are aligned in FC while the domains are not aligned in ZFC.FIGS.3A and3Bindicate that a transition temperature (Curie temperature) Tc of Fe3Sn2is 656 K.FIG.3Cindicates that at T = 5 K, saturation magnetization of Fe2Sn2reaches 6.03 µB/F.U., and the saturation magnetization per Fe atom is 2.01 µB.

FIG.4Ashows the temperature dependence of the magnetization M of Fe2Sn2in ZFC and FC when the magnetic field B of 100 Oe is applied parallel to the c-axis.FIG.4Bshows the magnetic field dependence of the magnetization M of Fe2Sn2at T = 5 K, 200 K, and 300 K when the magnetic field B is applied parallel to the c-axis.FIG.4Asuggests that even at a low temperature, almost the same magnetization is observed in FC and ZFC.FIG.4Bsuggests that almost the same saturation magnetization as that inFIG.3Cis observed.

FIGS.5A to5Cshow results of observation of the anomalous Nernst effect and an anomalous Hall effect of Fe2Sn2. Specifically,FIG.5Ashows magnetic field dependence of a Nernst coefficient Syxof Fe2Sn2at T = 200 K, 300 K, and 390 K when the magnetic field B is applied parallel to [0001] and a heat current Q flows parallel to [2-1-10].FIG.5Bshows magnetic field dependence of Hall resistivity ρyxof Fe2Sn2at T = 200 K and 300 K when the magnetic field B is applied parallel to [0001] and a current I flows parallel to [2-1-10].FIG.5Cshows temperature dependence of the Nernst coefficient Syxof Fe2Sn2when the magnetic field B = 2T is applied parallel to [0001] and the heat current Q flows parallel to [2-1-10]. Here, four integers in a square bracket represent Miller indices.

FIGS.5A and5Csuggest that the Nernst coefficient Syxof Fe3Sn2has large temperature dependence, and its absolute value | Syx| is about 2.4 µV/K at T = 300 K and reaches about 5.1 µV/K at T = 390 K. In addition, | Syx| at T = 150 K or lower is found to be almost zero, whereas | Syx| at a temperature exceeding T = 150 K is found to rapidly increase with increase in temperature. Further,FIG.5Bsuggests that Fe2Sn2exhibits a large anomalous Hall effect at room temperature.

As described above, a large anomalous Nernst effect can be exhibited in Fe2Sn2.

Iron Nitride

The inventor of the present application has further found that the anomalous Nernst effect is also exhibited in an iron nitride.

FIG.6shows a crystal structure of Fe16N2capable of exhibiting the anomalous Nernst effect. In a unit cell of Fe16N2, N (nitrogen) atoms have a body-centered cubic (bcc) structure, and Fe atoms occupy three crystallographically different sites (4e, 4d and 8h) . Fe16N2can be prepared by a known method. For example, Fe16N2can be prepared by sputtering iron in a nitrogen atmosphere.

FIG.7Ashows magnetic field dependence of the Nernst coefficient Syxof a first sample made of Fe16N2at T = 300 K.FIG.7Bshows magnetic field dependence of the Nernst coefficient Syxof a second sample made of Fe16N2at T = 300 K. Although the first sample and the second sample are both prepared on substrates made of MgO, a contact state between the first sample and a terminal is different from a contact state between the second sample and a terminal. As shown inFIGS.7A and7B, | Syx| of Fe16N2is about 0.6 µV/K and about 0.8 µV/K, suggesting that the anomalous Nernst effect is exhibited.

Even in an iron nitride other than Fe16N2, the anomalous Nernst effect is expected to be observed. For example, Fe4N has a crystal structure belonging to a space group Pm-3m, and a Curie temperature Tc is as high as 760 K. By high-throughput calculation, a maximum value of a transverse thermoelectric conductivity a [A/Km] of Fe4N at Fermi energy is estimated to be 2.4 A/Km at T = 500 K or lower.

R-Co Based Alloy

Further, the inventor of the present application has found that the anomalous Nernst effect is also exhibited in an R—Co based alloy as typified by RCo5. Here, R is a rare-earth element, and examples of such a rare-earth element include Y (yttrium), La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium) , Sm (samarium), Gd (gadolinium) , Tb (terbium), Dy (dysprosium), Ho (holmium), and Er (erbium).

FIG.8shows a crystal structure of RCo5. RCo5has a hexagonal structure with a space group P6/mmm in which an a-axis and a b-axis share the same length, an angle a between the b-axis and a c-axis and an angle β between the c-axis and the a-axis are a = β = 90°, and an angleYbetween the a-axis and the b-axis isY= 120°. In RCo5, R atoms occupy 1a sites, and Co atoms occupy 2c sites and 3g sites constituting a Kagome lattice plane. Related substances of RCo5include R2Co7and R2CO17.

FIG.9Ashows an X-ray diffraction pattern of a YCo5polycrystal prepared by arc melting. For comparison, the X-ray diffraction pattern obtained from simulation is also shown. These X-ray diffraction patterns can confirm that the YCo5polycrystal sample has a single phase.

FIG.9Bshows magnetic field dependence of the magnetization M of the YCo5polycrystal at 10 K and 300 K.FIG.9Cshows temperature dependence of the magnetization M of the YCo5polycrystal when a magnetic field B = 1T is applied.FIG.9BandFIG.9Ccan confirm that a ferromagnetic state is realized.

FIG.9Dshows magnetic field dependence of the Nernst coefficient Syxof the YCo5polycrystal at 100 K, 200 K, and 300 K.FIG.9Dindicates that | Syx| takes a relatively large value of 1.7 µV/K at 300 K (room temperature).

In an RCo5single crystal, a larger Nernst coefficient can be obtained. Table 1 shows a composition formula, the space group, the Curie temperature Tc, the transverse thermoelectric conductivity a, and the Nernst coefficient Syxof the RCo5single crystal. The transverse thermoelectric conductivity a and the Nernst coefficient Syxshown in Table 1 are values at room temperature. Here, R = Y, Dy, Ho, Tb, Er, Gd, and Sm. As shown in Table 1, Syxof the YCo5single crystal at room temperature is 4.33 µV/K, which is a much larger value.

RCo5has a high Curie temperature of 900 K or higher, and shows strong uniaxial magnetic anisotropy. For example, when R is La, Ce, Pr, Sm, or Y, RCo5has an easy axis in the c-axis direction at room temperature. In addition, by pulverizing particles of RCo5to particles with a single magnetic domain size, it is possible to produce a permanent magnet with large coercivity, exhibiting the Nernst effect in a zero magnetic field.

The anomalous Nernst effect can also be exhibited in a compound with a composition of RCo4M. Here, M is B (boron) or Ga (gallium).

FIG.10shows a crystal structure of RCo4M. In RCo4M, the R atoms occupy 1a and 1b sites, and the Co atoms occupy 2c sites and 6i sites constituting the Kagome lattice plane.

FIG.11shows magnetic field dependence of the Nernst coefficient Syxof DyCo4Ga at 300 K and 350 K.FIG.11indicates that |Syx| takes a relatively large value of 1.5 µV/K at 350 K.

In addition to RCo5and RCo4M described above, a large Nernst coefficient value can also be obtained for R2Co7, R2Co17, RCo3, RCo4-xBx, and RCo4-xGax.

Next, reference will be made to a thermoelectric conversion device in which the thermoelectric conversion element of the embodiments is modularized.

FIG.12shows an external configuration of a thermoelectric conversion device20according to Example 1 of the embodiments. The thermoelectric conversion device20includes a substrate22and a power generator23placed on the substrate22. In the thermoelectric conversion device20, when a heat current Q flows from the substrate22side toward the power generator23, a temperature difference is created in a heat current direction in the power generator23, and the anomalous Nernst effect generates an electric voltage V in the power generator23.

The substrate22has a first surface22aon which the power generator23is placed, and a second surface22bopposite to the first surface22a. Heat from a heat source (not shown) is applied to the second surface22b. Examples of a material of the substrate22include, but are not limited to, MgO, Si, and Al2O3.

The power generator23includes a plurality of thermoelectric conversion elements24and a plurality of thermoelectric conversion elements25, and each of the thermoelectric conversion elements has an L-shaped three-dimensional shape and is made of Fe3Sn2, the iron nitride (Fe16N2or Fe4N) , or the R—Co based alloy described above. As shown inFIG.12, the plurality of thermoelectric conversion elements24and the plurality of thermoelectric conversion elements25are alternately arranged in parallel on the substrate22in a direction (y direction) perpendicular to a longitudinal direction (x direction) of each of the thermoelectric conversion elements. The number of the thermoelectric conversion elements24and the thermoelectric conversion elements25constituting the power generator23are not limited.

The plurality of thermoelectric conversion elements24and the plurality of thermoelectric conversion elements25are arranged such that a direction of magnetization M1 of the thermoelectric conversion elements24is opposite to a direction of magnetization M2 of the thermoelectric conversion elements25. The Nernst coefficient of the plurality of thermoelectric conversion elements24has the same sign as that of the plurality of thermoelectric conversion elements25.

The thermoelectric conversion element24has a first side surface (+y side) and a second side surface (-y side) in the longitudinal direction (x direction), and a first end (+x side) on the first side surface (+y side) is defined as a first end face24a, and a second end (-x side) on the second side surface (-y side) is defined as a second end face24b. The thermoelectric conversion element25has a first side surface (+y side) and a second side surface (-y side) in the longitudinal direction (x direction), and a second end (-x side) on the first side surface (+y side) is defined as a first end face25a, and a first end (+x side) on the second side surface (-y side) is defined as a second end face25b.

The first end face25aof the thermoelectric conversion element25is connected to the second end face24bof the adjacent thermoelectric conversion element24on the +y side, and the second end face25bof the thermoelectric conversion element25is connected to the first end face24aof the adjacent thermoelectric conversion element24on the opposite side (-y side). With this structure, the plurality of thermoelectric conversion elements24and the plurality of thermoelectric conversion elements25are electrically connected in series. That is, the power generator23is provided in a serpentine shape on the first surface22aof the substrate22. The thermoelectric conversion elements24and the thermoelectric conversion elements25are insulated from each other except for the connection points.

When heat from the heat source is applied to the second surface22bof the substrate22, the heat current Q flows in the +z direction toward the power generator23. When the heat current Q creates the temperature difference, the anomalous Nernst effect generates an electromotive force E1 in a direction (-x direction) orthogonal to both the direction of the magnetization M1 (-y direction) and the direction of the heat current Q (+z direction) in the thermoelectric conversion elements24. In the thermoelectric conversion elements25, the anomalous Nernst effect generates an electromotive force E2 in a direction (+x direction) orthogonal to both the direction of the magnetization M2 (+y direction) and the direction of the heat current Q (+z direction).

As described above, since the thermoelectric conversion elements24and the thermoelectric conversion elements25, which are arranged in parallel, are electrically connected in series, the electromotive force E1 generated in one thermoelectric conversion element24can be applied to the adjacent thermoelectric conversion elements25. In addition, since the direction of the electromotive force E1 generated in one thermoelectric conversion element24is opposite to that of the electromotive force E2 generated in the adjacent thermoelectric conversion elements25, the electromotive force in the thermoelectric conversion elements24and the electromotive force in the adjacent thermoelectric conversion elements25are added up, thereby increasing an output voltage V.

A modified configuration of the thermoelectric conversion device20ofFIG.12may be employed such that the Nernst coefficient of the thermoelectric conversion element24is opposite in sign to that of the adjacent thermoelectric conversion element25, and the plurality of thermoelectric conversion elements24and the plurality of thermoelectric conversion elements25share the same magnetization direction (that is, the magnetization M1 and the magnetization M2 have the same direction).

FIG.13is a plan view of a thermoelectric conversion device30according to Example 2 of the embodiments. The thermoelectric conversion device30includes, as a power generator34, a plurality of rectangular parallelepiped-shaped thermoelectric conversion elements 1A having Nernst coefficients with the same sign. Each of the thermoelectric conversion elements 1A is made of Fe3Sn2, the iron nitride, or the R—Co based alloy described above.

The plurality of thermoelectric conversion elements 1A are arranged in parallel on a substrate32in a direction (y direction) perpendicular to the longitudinal direction (x direction) such that they share the same direction of the magnetization M (y direction). Each thermoelectric conversion element 1A has a first end (+x side) and a second end (-x side), and the first end of one thermoelectric conversion element 1A is connected to the second end of the adjacent thermoelectric conversion element 1A on the -y side via a copper wiring36, and thus the plurality of thermoelectric conversion elements 1A are electrically connected in series. Examples of a material of the substrate32include, but are not limited to, MgO, Si, and A12O3.

The heat current flows from the substrate32side toward the power generator34(in a +z direction) . Since the thermoelectric conversion device30has a configuration in which the adjacent thermoelectric conversion elements 1A are connected to each other via the copper wiring36, the thermoelectric conversion device30can be manufactured more easily than the thermoelectric conversion device20of Example 1 shown inFIG.12.

The thermoelectric mechanism using the anomalous Nernst effect allows the temperature gradient, the magnetization direction, and the direction of the electric voltage to be orthogonal to one another, which makes it possible to produce a thin sheet-shaped thermoelectric conversion element.

FIG.14shows an external configuration of a thermoelectric conversion device40according to Example 3 including a sheet-shaped thermoelectric conversion element44. Specifically, the thermoelectric conversion device40includes a hollow member42and the elongated sheet-shaped (tape-shaped) thermoelectric conversion element44winding around the hollow member42to cover an outer surface of the hollow member42. The thermoelectric conversion element44is made of Fe3Sn2, the iron nitride, or the R—Co based alloy described above.

The magnetization of the thermoelectric conversion element44is parallel to a longitudinal direction (x direction) of the hollow member42. When the heat current flows from the inside toward the outside of the hollow member42and creates the temperature gradient, the anomalous Nernst effect generates an electric voltage V along a longitudinal direction of the elongated thermoelectric conversion element44(i.e., along a direction perpendicular to both the magnetization direction and the direction of the heat current) .

Instead of the elongated sheet-shaped thermoelectric conversion element44of the thermoelectric conversion device40ofFIG.14, a wire-rod thermoelectric conversion element may be wound around the hollow member42.

Here, inFIGS.12to14, let the longitudinal length of the thermoelectric conversion element (s) be denoted by L and a thickness (height) of the thermoelectric conversion element(s) be denoted by H. The electric voltage generated by the anomalous Nernst effect is proportional to L/H. That is, the longer and thinner the thermoelectric conversion element(s), the larger the generated electric voltage. Therefore, the anomalous Nernst effect is expected to be enhanced by employing the power generator having the thermoelectric conversion elements which are electrically connected in series, or employing the wire-rod thermoelectric conversion element or the elongated sheet-shaped thermoelectric conversion element.

The thermoelectric conversion devices described in Examples 1 to 3 are applicable to various fields. Specific examples of such applications include stand-alone power supplies for Internet of Things (IoT) sensors or heat flux sensors, in a temperature range between room temperature and several hundred degrees Celsius.

For example, by applying the thermoelectric conversion device of the embodiments to a heat flux sensor, it is possible to determine whether thermal insulation performance of a building is good or bad. By providing the thermoelectric conversion device in an exhaust device of a motor vehicle, it is possible to convert heat from exhaust gas (waste heat) into electricity, and to effectively use the thermoelectric conversion device as an auxiliary power supply. Further, by arranging heat flux sensors in a mesh-like pattern on a wall surface of a certain space, it is possible to perform space recognition of heat current or heat sources. This technique is expected to be applicable to, for example, a high-precision temperature control of high-density crop cultivation or livestock growth, or to a driver detection system for automatic driving. The heat flux sensors can also be used in room air conditioning management, or core body temperature management in medical treatment. In addition, the thermoelectric conversion element of the embodiments can be formed into powder or paste, thus offering promising applications in a wide range of fields.

The embodiments have focused on the electric voltage generated by the anomalous Nernst effect. In practice, the output voltage can be increased by synergy among the electric voltage generated by the Seebeck effect resulting from the temperature gradient, the Hall effect that occurs based on the electric voltage generated by the Seebeck effect, and the electric voltage generated by the anomalous Nernst effect.

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