Patent Description:
In recent years, there has been proposed thermoelectric conversion devices using the anomalous Nernst effect (see Patent Literature <NUM>, for example). The anomalous Nernst effect is a phenomenon observed when heat current flowing through a magnetic material creates a temperature difference, which generates an electric voltage in a direction perpendicular to both a direction of magnetization and a temperature gradient.

The Seebeck effect is well known as a thermoelectric mechanism that also generates an electric voltage due to a temperature gradient. The Seebeck effect generates the electric voltage in a direction of the temperature gradient, which causes a complicated three-dimensional structure of thermoelectric modules. This makes it difficult to achieve large-area thermoelectric modules and film-shaped thermoelectric modules. In addition, toxic and rare materials are used in the Seebeck effect, leading to fragile, vibration sensitive modules, and a high manufacturing cost. In contrast, the anomalous Nernst effect generates the electric voltage in the direction perpendicular to the temperature gradient, which enables a lateral configuration of thermoelectric modules to cover a heat source. Such a configuration is advantageous for achieving large-area thermoelectric modules and film-shaped thermoelectric modules. Further, inexpensive, low-toxicity, highly durable materials can be selected for the anomalous Nernst effect. Furthermore, Literature <NUM> discloses calculations on the pyrochlore iridates Pr<NUM>Ir<NUM>O<NUM> showing Weyl points in the band structure. Literature <NUM> deals with ZrTe<NUM> having Weyl points. Literature <NUM> discloses the effect of Weyl nodes on the Nernst effect in semimetals on the example of NbP. Literature <NUM> discloses advantages of thermoelectric power generation using anomalous Nernst effect.

Patent Literature <NUM>: <CIT> Literature <NUM> :<NPL> Literature <NUM>:<NPL> Literature <NUM>: <NPL> Literature <NUM>: <NPL>.

Although the anomalous Nernst effect has advantages over the Seebeck effect as described above, the current power generation capacity by the anomalous Nernst effect using typical magnetic materials is still insufficient for practical applications.

In view of the foregoing, an object of the present invention is to provide a thermoelectric conversion element that can exhibit a much larger anomalous Nernst effect than ever before, and to provide a thermoelectric conversion device including the thermoelectric conversion element.

A thermoelectric conversion element according to a first aspect of the present invention is made of a magnetic material with a band structure having Weyl points which exist within an range of ±<NUM> eV from Fermi energy. The thermoelectric conversion element has a thermoelectric mechanism for generating electromotive force by the anomalous Nernst effect.

A thermoelectric conversion device according to a second aspect of the present invention includes a substrate, and a power generator provided on the substrate and including a plurality of thermoelectric conversion elements. Each of the plurality of thermoelectric conversion elements has a shape extending in one direction, and is made of a magnetic material with a band structure having Weyl points which exist within an range of ±<NUM> eV from the Fermi energy. In the power generator, the plurality of thermoelectric conversion elements is arranged in parallel to one another in a direction perpendicular to the one direction and electrically connected in series to one another in a serpentine shape.

A thermoelectric conversion device according to a third aspect of the present invention includes: a thermoelectric conversion element made of a magnetic material with a band structure having Weyl points which exist within an range of ±<NUM> eV from the Fermi energy; and a hollow member. The thermoelectric conversion element is a sheet-shaped element covering an outer surface of the hollow member.

According to the present invention, it is possible to achieve a much larger anomalous Nernst effect than ever before by using a thermoelectric conversion element made of a magnetic material whose band structure has Weyl points which exist within an range of ±<NUM> eV from Fermi energy.

Exemplary embodiments of the present invention will be described below with reference to the accompanying drawings.

In recent years, it is theoretically known that topological electronic structures relate to a thermoelectric mechanism based on the anomalous Nernst effect. In particular, recent studies have indicated that the Berry curvature of Weyl points residing in the vicinity of Fermi energy EF can potentially enhance the anomalous Nernst effect, and it is therefore expected that search for materials containing the Weyl fermions and new material synthesis are effective in development of a thermoelectric conversion device using the anomalous Nernst effect.

The Weyl fermions are massless fermions defined by the Dirac equation. As shown in <FIG>, the Weyl points are present at intersections of linear band dispersions and appear in pairs with opposite chirality (right handed and left handed). A pair of Weyl points can be regarded as positive and negative magnetic poles in a fictitious magnetic field (Berry curvature) in a momentum space and is considered to affect motion of electrons in a material, as with magnetic fields in a real space.

Recent first-principles calculations have shown that metals with composition Co<NUM>TX are potential Weyl metals in which Weyl points exist in the momentum space near the Fermi energy EF. Here, T is a transition metal, and X is one of Si, Ge, Sn, Al, and Ga. As an example of such metals, the following embodiments are directed to Co<NUM>MnGa, which is a full Heusler ferromagnet.

<FIG> schematically shows a crystal structure of Co<NUM>MnGa. Co<NUM>MnGa has a L2<NUM>-type cubic full Heusler structure as shown in <FIG>. A unit cell of the L2<NUM> structure consists of four face-centered cubic (fcc) sublattices with Co atoms at (<NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>) and (<NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>), Mn atom at (<NUM>, <NUM>, <NUM>) and Ga atom at (<NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>) in a lattice coordinate system. The crystal structure of Co<NUM>MnGa can be determined by a variety of diffraction methods, such as X-ray diffraction.

Next, a thermoelectric conversion element according to the embodiments of the present invention and a thermoelectric mechanism thereof will be described with reference to <FIG>. As shown in <FIG>, an assumption is made that a thermoelectric conversion element <NUM> according to the embodiments is made of Co<NUM>MnGa, has a box shape extending in one direction (direction y), has a thickness (length in a direction z) greater than or equal to <NUM>, and is magnetized in a direction +z. When heat current Q (~VT) flows through the thermoelectric conversion element <NUM> in a direction +x, a temperature difference is created in the direction +x. As a result, the anomalous Nernst effect generates electromotive force V (~M×VT) in the thermoelectric conversion element <NUM> in an outer product direction (direction y) perpendicular to both the direction of the heat current Q (direction +x) and the direction of magnetization M (direction +z).

Reference will now be made to a verification experiment of the anomalous Nernst effect of the thermoelectric conversion element <NUM>.

Single crystals of Co<NUM>MnGa were prepared by the Czochralski method after making polycrystalline samples by arc-melting Co, Mn, and Ga with an appropriate ratio. X-ray diffraction showed that the produced Co<NUM>MnGa had a lattice constant a=<NUM>(<NUM>) angstroms. In the experiment, three box-shaped samples with a size of <NUM>×<NUM>×<NUM><NUM> were produced as the thermoelectric conversion element <NUM>. The three samples are distinguished according to crystal orientations parallel to directions of magnetic fields B, and a B∥[<NUM>] sample, a B∥[<NUM>] sample, and a B∥[<NUM>] sample are denoted by #<NUM>, #<NUM>, and #<NUM>, respectively. In the embodiments, transport phenomena (the Nernst effect, the Seebeck effect, and the Hall effect) were measured for each sample by using a known method.

<FIG> show the results of the measurement of the Nernst effect, the Seebeck effect, and the Hall effect for each sample.

A graph a in <FIG> shows magnetic field dependence of a Nernst coefficient -Syx at room temperature (T=<NUM>) and shows observational results obtained when a magnetic field B parallel to [<NUM>], a magnetic field B parallel to [<NUM>], and a magnetic field B parallel to [<NUM>] are applied to the samples and the heat current Q parallel to [<NUM>] or [<NUM>-<NUM>] flows through the samples. A graph b in <FIG> shows temperature dependence of -Syx obtained when a magnetic field B=<NUM> T is applied to each of the samples. As is clear from the graph b in <FIG>, -Syx increases with elevating temperature, reaches |Syx|~<NUM>µV/K at room temperature, and even approaches |Syx|~<NUM>µV/K at <NUM>, which are one order of magnitude larger than typical observed values known for the anomalous Nernst effect.

A ratio of the observed value -Syx to the Seebeck coefficient Sxx indicated by a graph b in <FIG> (namely, the Nernst angle θN≈tanθN=Syx/Sxx) is also an unprecedented large value. In fact, |Syx/Sxx| is greater than <NUM>, as shown by a right vertical axis of the graph a in <FIG>. The graphs a and b in <FIG> further indicate that there is almost no anisotropy in Syx.

As shown in graphs c and d of <FIG>, the Hall resistivity ρyx reaches <NUM>µΩcm at room temperature and reaches a maximum <NUM>µΩcm around <NUM>. The Hall angle θH≈tanθH=ρyx/ρxx (a right vertical axis of the graph c in <FIG>) is also large and exceeds <NUM> at room temperature. Here, ρxx is longitudinal resistivity. A graph a in <FIG> shows temperature dependence of ρxx at zero magnetic field.

Graphs e and f in <FIG> show magnetic field dependence of the magnetization M at room temperature and temperature dependence of the magnetization M at magnetic field B=<NUM> T, respectively. The graphs a, c, and e in <FIG> show that the Hall effect and the Nernst effect have nearly the same magnetic field dependence as that of the magnetization curve. This indicates that the anomalous contribution (∝M) to the Hall effect and the Nernst effect is dominant and the normal contribution (∝B) is negligibly small at T=<NUM>. As indicated by the graphs e and f in <FIG>, saturated magnetization Ms reaches <NUM>µB at T=<NUM>, gradually grows on cooling, and reaches about <NUM>µB at T=<NUM>, which is consistent with the predicted value based on the Slater-Pauling rule. The graphs e and f in <FIG> indicate that the anisotropy for the magnetization is negligibly small at T=<NUM>, which is fully consistent with the cubic structure.

The observed Hall resistivity |ρyx|~<NUM>µΩcm is one of the largest known for the anomalous Hall effect. Similarly, the Hall conductivity is also exceptionally large. A graph a in <FIG> shows temperature dependence of the Hall conductivity σyx at B=<NUM> T. Here, σyx=-ρyx/ (ρxx<NUM>+ρyx<NUM>) is satisfied. -σyx monotonically increases on cooling and reaches -σyx~<NUM>Ω-<NUM>cm-<NUM> at absolute zero. This value is of the same order of magnitude as the one known for the layered quantum Hall effect.

The Nernst coefficient Syx can be defined by the Peltier coefficient αyx. In general, electric current is generated by both electric field ε and temperature gradient VT and expressed by J=σ·ε-α·∇T. Here, J, σ and α are an electric current density tensor, an electric conductivity tensor, and a thermoelectric conductivity tensor, respectively. Assuming that the direction of the magnetic field B is parallel to the direction z and the temperature gradient VT is parallel to the direction x, and setting J=<NUM>, the following is obtained: Jy=σyxSxx+σxxSyx-αyx=<NUM>. Here, a cubic symmetry provides σxx=σyy. That is, the Peltier coefficient, which is a transverse thermoelectric coefficient, is given by the following Expression (<NUM>): <MAT>.

According to Expression (<NUM>), the Peltier coefficient determines the magnitude of the Nernst coefficient, and it is effective to evaluate the Peltier coefficient for determination of the anomalous Nernst effect.

A graph b in <FIG> shows the results of calculation of temperature dependence of -αyx using Expression (<NUM>) and the values obtained from <FIG> and <FIG> and the graph a in <FIG>. As indicated by the graph b in <FIG>, -αyx increases almost linearly with T up to T~<NUM>, reaches a maximum around T~<NUM>, and then gradually decreases with a rise in T. Notice that the curve of the temperature dependence of -αyx closely resembles - TlogT behavior. In more detail, by plotting data of -αyx/T (right vertical axis of <FIG>) against logT, a crossover is found between two distinct behaviors: -αyx~T at low temperatures and -αyx~-TlogT at high temperatures.

-αyx~T behavior at low temperatures is consistent with the Mott formula, which defines the relation between αyx at low temperatures (kBT<<EF) and the energy derivative of the Hall conductivity σyx at T=<NUM> (αyx~-(π<NUM>kB<NUM>T/3e)(∂σyx/∂EyF)). Here, kB is the Boltzmann constant. On the other hand, -αyx~-TlogT behavior at high temperatures (between T~<NUM> and T~<NUM>) indicates violation of the Mott formula. The -TlogT behavior of the thermoelectric coefficient can be understood in terms of Weyl fermions, as will be described below.

To provide evidence for the existence of Weyl points, the focus is first put on a Fermi surface closest to the Fermi energy EF of Co<NUM>MnGa. <FIG> shows the band structure of Co<NUM>MnGa obtained from the first-principles calculations. Here, the magnetization M is <NUM>µB, and the magnetization direction is along [<NUM>]. In <FIG>, a band that forms the largest Fermi surface, which is located near the Brillouin zone boundary and closest to the Fermi energy EF (=<NUM> eV), is drawn by a thick line. <FIG> show that the Weyl points (+-) are located around E<NUM>≈<NUM> meV in the vicinity of the Fermi energy EF, and this Fermi surface has a large Berry curvature |Ωz| in the vicinity of the Weyl points (+-) (<FIG>).

<FIG> and <FIG> show that the Weyl points (+-) are located on the Brillouin zone boundary along U-Z-U line at ±k<NUM>=±(2π/a)×<NUM> in a ka=kb plane spanned by the momentum kUZ along U-Z and kc. In <FIG>, the distribution of z component of the Berry curvature |Ωz| in the ka=kb plane is expressed by grayscale, and the Weyl points (+-) appear at locations where the Berry curvature |Ωz| is relatively large (dense portion). The Weyl points can be searched in the Brillouin zone by a method of <NPL>)).

<FIG> and <FIG> show that the band that forms the largest Fermi surface and another band intersect with each other to provide linear dispersion in the vicinity of the Fermi energy EF, and density of states (DOS) increases because the dispersions of the two bands are almost flat. <FIG> shows a spin-decomposed density of states of Co<NUM>MnGa obtained from the first-principles calculations, and <FIG> shows the density of states near the Fermi energy EF. As shown in <FIG>, the density of states has peaks around the Fermi energy EF and around <NUM> meV. That is, the density of states of Co<NUM>MnGa has a local maximum value in the vicinity of Fermi energy EF.

The right handed (+) and left handed (-) Weyl fermions are described by low-energy Hamiltonians, as given by Expression (<NUM>). <MAT> <MAT> σx, σy, σz: Pauli matrix Here, v<NUM>, v<NUM>, and v⊥ are three independent velocity parameters, and h is the Planck constant. According to the first-principles calculations of Co<NUM>MnGa described above, the Weyl fermions are located at ±k<NUM>~(2π/a)×<NUM> in the vicinity of E<NUM>≈<NUM> meV, and a tilt parameter v<NUM>/v<NUM>=<NUM> and v<NUM>≈<NUM><NUM> m/s are obtained. The tilt parameter v<NUM>/v<NUM>=<NUM> corresponds to a quantum critical point, v<NUM>/v<NUM><<NUM> corresponds to type-I Weyl fermions, and v<NUM>/v<NUM>><NUM> corresponds to type-II Weyl fermions. In the type-I Weyl fermions (v<NUM>/v<NUM><<NUM>), the density of states at the Weyl points is zero, whereas in the type-II Weyl fermions (v<NUM>/v<NUM>><NUM>), the density of states at the Weyl points is finite so that electron and hole pockets touch.

At the quantum critical point (v<NUM>/v<NUM>=<NUM>), the energy derivative of the Hall conductivity ∂σyx/∂E displays log divergent behavior at low energy. The low energy theory suggests that αyx(T, µ) indicating temperature and chemical potential dependence of the Peltier coefficient in the vicinity of the quantum critical point can be expressed in terms of a dimensionless scaling function G over a wide range of temperatures (see <FIG>). Here, µ is the chemical potential. <FIG> shows that the scaling functions obtained from experiment (T<NUM>=<NUM>) and density functional theory (DFT) calculations (T<NUM>=<NUM>) match with the results from the low energy theory over a decade of temperatures (solid line in <FIG> when (µ-E<NUM>) /kBT<NUM>=-<NUM>).

That is, based on the scaling function of the low energy theory, the logarithmic divergence of ∂σyx/∂E at the quantum critical point can lead to αyx~Tlog(|EF-E<NUM>|/(hv<NUM>k<NUM>/2π)) behavior at low temperatures. On the other hand, this logarithmic divergence can lead to αyx~Tlog(kBT/(hv<NUM>k<NUM>/2π)) behavior at high temperatures kBT>|EF-E<NUM>|, which does not follow the Mott formula (αyx~T). Thus, the temperature dependence of αyx can be understood in terms of a scaling function of the low energy theory in the vicinity of the quantum critical point between the type I and the type II over a decade of temperatures.

When the chemical potential µ is tuned to the Weyl points (µ=E<NUM>), the scaling function does not follow the Mott formula even at any low temperature (broken line in <FIG>).

A graph a in <FIG> shows energy dependence of the number of Weyl points for the magnetization along [<NUM>], a graph b in <FIG> shows energy dependence of -σyx at T=<NUM>, and a graph c in <FIG> shows energy dependence of -αyx/T at T=<NUM>. In the graph a of <FIG>, the chirality (right handed and left handed) of Weyl points is denoted by +<NUM> and -<NUM>. According to the graphs a, b and c, -αyx/T has a sharp peak and the number of Weyl points is large in the vicinity of E<NUM>~<NUM> eV. Further, in the vicinity of E~-<NUM> eV, -αyx/T has an extreme value and the number of Weyl points is much larger. Thus, the Weyl points can exist within the range of ±<NUM> eV from the Fermi energy EF.

<FIG> and <FIG> each schematically shows a relation between the band dispersion and the Nernst coefficient. As shown in <FIG>, with respect to the type-I Weyl fermions separated from the quantum critical point (v<NUM>=v<NUM>), two energy bands are in point contact with each other, and the density of states at the Weyl points is zero. The Nernst coefficient in this case is about <NUM>µV/K. The Nernst coefficient increases with approaching the quantum critical point. At the quantum critical point (<FIG>), the two energy bands provide flat dispersion, and the density of states at the Weyl points increases. At this point, the Nernst coefficient has a local maximum value, reaching about <NUM>µV/K. Thus, the flat dispersion increases the Nernst coefficient by one order of magnitude.

As described above, the first-principles calculations of Co<NUM>MnGa show the existence of the type-I Weyl fermions, which are located at ±k<NUM>~(2π/a)×<NUM> in the vicinity of E<NUM>≈<NUM> meV and have the tilt parameter of v<NUM>/v<NUM>=<NUM>, thereby obtaining the Nernst coefficient in the vicinity of the quantum critical point.

To provide further evidence for the existence of Weyl fermions in Co<NUM>MnGa, measurements of magnetic field dependence of the longitudinal conductivity σxx with different electric current directions and measurements of angle dependence of magneto-conductivity σxx(B)-σxx(<NUM>) with different electric current directions were performed on the thermoelectric conversion element <NUM>.

<FIG> is a graph illustrating the results of the measurements of the magnetic field dependence of the longitudinal conductivity σxx at T=<NUM> and T=<NUM> when the magnetic field B and the electric current I are parallel to each other (I∥B) and when the magnetic field B and the electric current I are perpendicular to each other (I⊥B). <FIG> is a graph illustrating the results of the measurements of the angle dependence (an angle θ between the magnetic field B and the electric current I) of the magneto-conductivity at T=<NUM> and |B|=<NUM> T for I∥[<NUM>], I∥[<NUM>], and I∥[<NUM>]. θ=<NUM>°, <NUM>° and <NUM>° correspond to I∥B, and θ=<NUM>° and <NUM>° correspond to I⊥B. <FIG> indicates that the magneto-conductivity displays cos<NUM>θ behavior. As is clear from <FIG>, the electric current I is allowed to flow easily in high magnetic fields (e.g., above |B|~<NUM> T) when the magnetic field B is parallel to the electric current I. This implies occurrence of chiral anomaly that appears in a material containing Weyl fermions.

<FIG> shows comparison of magnitude of the Peltier coefficient |αyx| for various ferromagnets and antiferromagnet Mn<NUM>Sn. As is clear from <FIG>, the magnitude of the Peltier coefficient of Co<NUM>MnGa is significantly greater than those of the other ferromagnets and the antiferromagnet Mn<NUM>Sn.

Next, reference will be made to a thermoelectric conversion device including the thermoelectric conversion element according to the embodiments in the form of modules.

<FIG> shows an exterior configuration of a thermoelectric conversion device <NUM> according to the embodiments. The thermoelectric conversion device <NUM> includes a substrate <NUM> and a power generator <NUM> placed on the substrate <NUM>. In the thermoelectric conversion device <NUM>, when the heat current Q flows from the substrate <NUM> side toward the power generator <NUM>, a temperature difference is created in the power generator <NUM> in the direction of the heat current, and an electric voltage V is generated in the power generator <NUM> by the anomalous Nernst effect.

The substrate <NUM> has a first surface 22a on which the power generator <NUM> is placed, and a second surface 22b opposite to the first surface 22a. Heat from a heat source (not shown) is applied onto the second surface 22b.

The power generator <NUM> includes a plurality of thermoelectric conversion elements <NUM> and a plurality of thermoelectric conversion elements <NUM>, each having a three-dimensional L shape and being made of a material identical to that of the thermoelectric conversion element <NUM> shown in <FIG>. As shown in <FIG>, the plurality of thermoelectric conversion elements <NUM> and the plurality of thermoelectric conversion elements <NUM> are alternately arranged in parallel to one another on the substrate <NUM> in a direction (direction y) perpendicular to a longitudinal direction (direction x) of the thermoelectric conversion elements. The number of the thermoelectric conversion elements <NUM> and <NUM> that constitute the power generator <NUM> is not limited to a specific number.

The plurality of thermoelectric conversion elements <NUM> and the plurality of thermoelectric conversion elements <NUM> are arranged such that magnetization M1 of the thermoelectric conversion elements <NUM> is oriented opposite to a direction of magnetization M2 of the thermoelectric conversion elements <NUM>. Further, the plurality of thermoelectric conversion elements <NUM> has the Nernst coefficient with the same sign as that of the Nernst coefficient of the plurality of thermoelectric conversion elements <NUM>.

Each of the thermoelectric conversion elements <NUM> has a first end face 24a and a second end face 24b, both of which are parallel to the longitudinal direction (direction x) of each of the thermoelectric conversion elements <NUM>. Each of the thermoelectric conversion elements <NUM> has a first end face 25a and a second end face 25b, both of which are parallel to the longitudinal direction (direction x) of each of the thermoelectric conversion elements <NUM>. The first end face 25a of the thermoelectric conversion element <NUM> is connected to the second end face 24b of one thermoelectric conversion element <NUM> adjacent thereto on one side thereof, and the second end face 25b of the thermoelectric conversion element <NUM> is connected to the first end face 24a of another thermoelectric conversion element <NUM> adjacent thereto on the opposite side thereof. With this structure, the plurality of thermoelectric conversion elements <NUM> and the plurality of thermoelectric conversion elements <NUM> are electrically connected in series to one another. That is, the power generator <NUM> is provided on the first surface 22a of the substrate <NUM> in a serpentine shape.

When heat is applied from the heat source onto the second surface 22b of the substrate <NUM>, the heat current Q flows in the direction +z toward the power generator <NUM>. When the heat current Q creates a temperature difference, the anomalous Nernst effect causes each of the thermoelectric conversion elements <NUM> to generate electromotive force E1 in the direction (direction -x) perpendicular to both the direction of the magnetization M1 (direction -y) and the direction of the heat current Q (direction +z). The anomalous Nernst effect causes each of the thermoelectric conversion elements <NUM> to generate electromotive force E2 in the direction (direction +x) perpendicular to both the direction of the magnetization M2 (direction +y) and the direction of the heat current Q (direction +z).

Since the thermoelectric conversion elements <NUM> and the thermoelectric conversion elements <NUM>, which are arranged in parallel to one another, are electrically connected in series to one another as described above, the electromotive force E1 generated in one thermoelectric conversion element <NUM> can be applied to the adjacent thermoelectric conversion element <NUM>. Since the direction of the electromotive force E1 generated in the one thermoelectric conversion element <NUM> is opposite to the direction of the electromotive force E2 generated in the adjacent thermoelectric conversion element <NUM>, the electromotive force in the thermoelectric conversion element <NUM> and the electromotive force in the adjacent thermoelectric conversion element <NUM> are added up, thereby increasing an output voltage V.

As a modification of the thermoelectric conversion device <NUM> in <FIG>, another configuration may be employed in which the plurality of thermoelectric conversion elements <NUM> and the plurality of thermoelectric conversion elements <NUM> are arranged such that the thermoelectric conversion element <NUM> has the Nernst coefficient which is opposite in sign to the Nernst coefficient of the thermoelectric conversion element <NUM>, and the thermoelectric conversion element <NUM> and the thermoelectric conversion element <NUM> share the same direction of magnetization (that is, the magnetization M1 and the magnetization M2 have the same direction).

Aspects of the thermoelectric conversion device according to the embodiments should not be limited to the above-described embodiment shown in <FIG>. The anomalous Nernst effect allows temperature gradient, a direction of magnetization, and a direction of an electric voltage to be perpendicular to one another, which makes it possible to produce a thin sheet-shaped thermoelectric conversion element.

<FIG> shows an exterior configuration of a thermoelectric conversion device <NUM> including a sheet-shaped thermoelectric conversion element <NUM>. Specifically, the thermoelectric conversion device <NUM> includes a hollow member <NUM> and an elongated sheet-shaped thermoelectric conversion element <NUM> covering (winding around) an outer surface of the hollow member <NUM>. The thermoelectric conversion element <NUM> is made of a material identical to that of the thermoelectric conversion element <NUM> shown in <FIG>. A direction of magnetization of the thermoelectric conversion element <NUM> is parallel to a longitudinal direction (direction x) of the hollow member <NUM>. When heat current flows in a direction perpendicular to the longitudinal direction (direction x) of the hollow member <NUM> and a temperature gradient is created in a direction from inside to outside the hollow member <NUM>, the anomalous Nernst effect generates an electric voltage V along a longitudinal direction of the elongated thermoelectric conversion element <NUM> (i.e., a direction perpendicular to the direction of magnetization and the direction of heat current).

In <FIG> and <FIG>, let a 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 greater the generated voltage. Hence, the anomalous Nernst effect is expected to be enhanced by employing the power generator <NUM> (<FIG>) including the plurality of thermoelectric conversion elements <NUM> and the plurality of thermoelectric conversion elements <NUM> which are electrically connected in series to one another in a serpentine shape or employing the elongated sheet-shaped thermoelectric conversion element <NUM> (<FIG>).

The thermoelectric conversion device <NUM> and the thermoelectric conversion device <NUM> can be used for a variety of apparatuses. For example, a heat flux sensor provided with the thermoelectric conversion device enables evaluation of heat insulation performance of buildings. Further, an exhaust system of a motorcycle or other vehicles provided with the thermoelectric conversion device allows utilization of heat of exhaust gas (waste heat) for power generation. It is therefore possible to make effective use of the thermoelectric conversion device as an auxiliary power supply.

The embodiments have focused on the electric voltage generated by the anomalous Nernst effect. Instead, the output voltage can be increased by virtue of synergy among the electric voltage generated by the Seebeck effect resulting from a temperature difference, 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.

Claim 1:
A thermoelectric conversion element (<NUM>, <NUM>, <NUM>, <NUM>), characterized by being made of a magnetic material with a band structure having Weyl points which exist within a range of ±<NUM> eV from Fermi energy,
wherein the thermoelectric conversion element (<NUM>, <NUM>, <NUM>, <NUM>) has a thermoelectric mechanism for generating electromotive force by anomalous Nernst effect.