Thermoelectric conversion system and thermoelectric conversion method

Abstract

To obtain a thermoelectric conversion system or a thermoelectric conversion method with less deterioration due to radiation.SOLUTION: In a thermoelectric conversion system 1, a ferromagnetic insulating layer 20 and a metal layer 30 are sequentially formed on a substrate 10 along a film thickness direction (z direction). A heat source 40 that imparts temperature gradient to the thermoelectric conversion system 1 is connected to a lower side of the substrate 10. Since a potential difference due to the spin Seebeck effect occurs in a y-axis direction in the figure, a first electrode 31 and a second electrode 32 are connected to both ends of the metal layer 30 in the y-axis direction, respectively. Since the thermoelectric conversion system 1 has high γ-ray resistance, the thermoelectric conversion system does not require a heavy metal layer for shielding γ-rays.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a thermoelectric conversion system and a thermoelectric conversion method that generate electricity by using an applied temperature gradient. [Background technology]

[0002] A Seebeck element is known as a thermoelectric conversion element that generates electricity using heat (temperature gradient). The Seebeck element is composed of two types of semiconductor layers with different Seebeck coefficients and electrodes connected to them. It is difficult to supply power to such a thermoelectric conversion element from the outside, and it can be used in various environments remote from the operator. As an example, Patent Document 1 describes the use of such a thermoelectric conversion element as a power source for temperature control of a cask that holds spent nuclear fuel inside.

[0003] In this case, the cask is used as a heat source, and the thermoelectric conversion element generates electricity spontaneously using the decay heat of the nuclear fuel inside the cask. With this configuration, it is no longer necessary to supply power from outside to the area where the cask is located, which has a high level of radiation, and the frequency with which workers must enter the area can be reduced. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2018-72167 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the semiconductor materials used in the conventional Seebeck elements, this radiation causes crystal defects. These crystal defects deteriorate the characteristics of the Seebeck element, and its output gradually decreases. For this reason, when a cask is used as a heat source as described above, this deterioration occurs due to the radiation emitted from the cask. Gamma rays are particularly a type of radiation that causes crystal defects.

[0006] A thick heavy metal layer is necessary to block gamma rays. When a cask is used as the heat source as described above, it is necessary to provide a heavy metal layer between the thermoelectric conversion element and the cask in order to suppress such deterioration. However, in this case, the amount of heat flowing into the thermoelectric conversion element is reduced, and the output is also reduced.

[0007] For this reason, a thermoelectric conversion system or method that is less susceptible to degradation due to radiation is desired.

[0008] The present invention has been made in consideration of the above problems, and an object of the present invention is to provide an invention that solves the above problems. [Means for solving the problem]

[0009] In order to solve the above problems, the present invention has the following configuration. The thermoelectric conversion system of the present invention is a thermoelectric conversion system that generates an electromotive force between a first electrode and a second electrode by an externally applied temperature gradient, and includes a ferromagnetic layer formed on a substrate and having a magnetic field in an in-plane direction, and a metal material that exhibits an inverse spin Hall effect, and a thickness of 1000 nm formed on the ferromagnetic layer at a location where the first electrode and the second electrode are separated in the in-plane direction and in a direction intersecting the magnetic field. is 10 nm or more, a metal layer of 50 nm or less; a heat source for applying the temperature gradient is a cask containing a radioactive material therein; the substrate is made of gallium gadolinium garnet (GGG) and is coated with boron (B) or cadmium (Cd); and the temperature gradient is applied in the thickness direction of the ferromagnetic layer. The thermoelectric conversion system of the present invention is characterized in that a magnetic field application layer that applies the magnetic field to the ferromagnetic layer and the metal layer is connected to ends of the ferromagnetic layer and the metal layer in the in-plane direction. In the thermoelectric conversion system of the present invention, the ferromagnetic layer is made of yttrium iron garnet (YIG) or YIG doped with gallium (Ga). In the thermoelectric conversion system of the present invention, the metal layer is platinum (Pt), gold (Au), niobium (Nb), nickel (Ni), or stainless steel. Consists of It is characterized by: The thermoelectric conversion method of the present invention is characterized in that a plurality of the thermoelectric conversion systems are arranged on the surface of a heat source that imparts the temperature gradient and connected in series so that the magnetic fields in each system are oriented in a common direction and the first electrode and the second electrode in each system are on a common side. The thermoelectric conversion method of the present invention is characterized in that the heat source has a substantially cylindrical shape, and a plurality of the thermoelectric conversion systems are arranged along the circumferential direction of an outer circumferential surface of the substantially cylindrical shape. Effect of the Invention

[0010] According to the present invention, it is possible to obtain a thermoelectric conversion system or a thermoelectric conversion method that is less susceptible to deterioration due to radiation. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing a structure of a thermoelectric conversion system according to the present invention. [Diagram 2] 13 shows the results of measuring the magnetic field dependency of the output voltage in an embodiment of the present invention. [Diagram 3] 1 shows the temperature difference dependence of the output voltage in an embodiment before gamma ray irradiation (a) and after exposure to gamma rays in a high-temperature, dry atmosphere (b). [Figure 4] The graphs show (a) the temperature difference dependence of the output voltage for each dose of gamma ray exposure when irradiated with gamma rays at high temperatures in steam, and (b) the dose dependence of the temperature coefficient. [Diagram 5]FIG. 1 is a diagram showing an example of a configuration in which a plurality of thermoelectric conversion systems according to an embodiment of the present invention are combined. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] A thermoelectric conversion system according to an embodiment of the present invention will be described below. This thermoelectric conversion system is similar to a conventional thermoelectric conversion system in that it generates electricity by applying heat (temperature gradient), but its structure and power generation mechanism are different from those of the conventional thermoelectric conversion system.

[0013] In a conventional Seebeck element used for thermoelectric conversion, for example, p-type and n-type semiconductor layers are used, and an electromotive force is generated due to the flow of holes in the p-type semiconductor and electrons in the n-type semiconductor when a temperature gradient exists.

[0014] On the other hand, the thermoelectric conversion system of the present invention utilizes the spin Seebeck effect described in, for example, JP 2009-130070 A and JP 2015-179746 A. In a thermoelectric conversion element using the spin Seebeck effect, when a magnetic field is applied to a ferromagnetic layer or when the ferromagnetic layer is magnetized, a difference occurs between the flow of electrons parallel to the magnetic field (up spin electrons) and the flow of electrons with antiparallel spin (down spin electrons), and this difference is output as a voltage (electromotive force) due to the inverse spin Hall effect in a paramagnetic metal layer in contact with the ferromagnetic layer.

[0015] In the thermoelectric conversion element using the Seebeck effect in the semiconductor as described above, the flow of holes and electrons that generate electromotive force is significantly affected by crystal defects in the semiconductor. These crystal defects are generated by irradiation of radiation in particular, and therefore increase over time in an environment with a high radiation dose, causing a decrease in the output voltage.

[0016] In contrast, as described below, the spin current in a ferromagnetic material is less susceptible to the adverse effects of radiation compared to the flow of holes and electrons in a semiconductor, making such a thermoelectric conversion system using the spin Seebeck effect particularly suitable for use in environments with high levels of radiation.

[0017] 1 is a cross-sectional perspective view showing the structure of a thermoelectric conversion system 1 according to an embodiment of the present invention. In this thermoelectric conversion system 1, a ferromagnetic insulating layer 20 and a metal layer 30 are successively formed on a substrate 10 along the film thickness direction (z direction).

[0018] A heat source 40 that applies a temperature gradient to the thermoelectric conversion system 1 is connected to the underside of the substrate 10. Magnetic field application layers 51 and 52 are connected to both sides of this structure in the x direction. If the metal layer 30 is made thick enough that its sheet resistance can be ignored, the metal layer 30 will have the same potential in the in-plane direction. However, as described below, the metal layer 30 is formed to be sufficiently thin, so that a potential difference occurs in the in-plane direction of the metal layer 30. Since the potential difference due to the spin Seebeck effect occurs in the y-axis direction in the figure, a first electrode 31 and a second electrode 32 are connected to both ends of the metal layer 30 in the y-axis direction, respectively, and an electromotive force is taken out between the first electrode 31 and the second electrode 32.

[0019] The substrate 10 is one on which a good quality ferromagnetic insulating layer 20 can be formed and has sufficient mechanical strength. For example, a 1 mm thick Gd3Ga5O 12 Gadolinium gallium garnet (GGG) single crystal is used. The ferromagnetic insulating layer (ferromagnetic layer) 20 is made of magnetic garnet Y3Fe5O 12A ferrite-based magnetic material such as yttrium-iron-garnet (YIG) or YIG doped with gallium (Ga) is formed to a thickness of about 100 nm by coating printing, sputtering, or the like. Since the spin Seebeck effect can be obtained even when a conductive (metal) ferromagnetic layer is used instead of the ferromagnetic insulating layer 20, other ferromagnetic materials can be used, for example, optical magneto-optical materials such as MnSb and MnBi, and rare earth compounds such as SmCo, etc. However, when a thick conductive ferromagnetic layer is used, it is difficult to obtain an electromotive force between the first electrode 31 and the second electrode 32, as in the case where the metal layer 30 is thick as described above, so it is preferable to use the ferromagnetic insulating layer 20.

[0020] As the metal layer 30, platinum (Pt), gold (Au), niobium (Nb), nickel (Ni), stainless steel, or the like, which are metal materials having large spin-orbit interactions and a strong inverse spin Hall effect, are preferably used. As described above, even when a conductive metal material is used, the film thickness of the metal layer 30 is set to be sufficiently thin, 50 nm or less, for example, about 10 nm, so that the sheet resistance is sufficiently high. The metal layer 30 is formed, for example, by a sputtering method. The electromotive force generated in this thermoelectric conversion system 1 is taken out from the first electrode 31 and the second electrode 32, as described above.

[0021] The magnetic field application layers 51, 52 are made of hard magnetic materials each having spontaneous magnetization in the direction shown in the figure, and are insulating or are provided on both sides in the x direction so as to be insulated from the metal layer 30, etc. The magnetic field application layers 51, 52 apply a magnetic field to the ferromagnetic insulating layer 20 in the in-plane direction from the negative side to the positive side in the x direction in FIG.

[0022] Various types of heat source 40 are used depending on the mode of use of this thermoelectric conversion system 1. However, as described later, since this thermoelectric conversion system 1 has high radiation resistance, it is particularly preferable to use one that is in an environment with a high radiation dose, such as the cask described in Patent Document 1.

[0023] Furthermore, when the heat source itself emits radiation, such as a cask, the radiation can be absorbed by the substrate 10, and the substrate 10 itself can be used as a heat source, further improving the power generation efficiency. For example, when the heat source 40 emits neutrons, such as a cask, if the substrate 10 is made of GGG, which contains Gd, which has a large neutron capture cross section, the substrate 10 itself functions as a heat source in this way. In this case, the power generation efficiency can be further improved by coating the substrate 10 with, for example, boron (B) or cadmium (Cd), which has a large reaction cross section with neutrons. In this case, the temperature gradient formed is also as shown in FIG. 1.

[0024] In the above structure, similar to the structures described in JP 2009-130070 A and JP 2015-179746 A, the spin Seebeck effect in the ferromagnetic insulating layer 20 and the inverse spin Hall effect in the metal layer 30 allow the electromotive force due to the temperature gradient and the magnetic field to be extracted from the metal layer 30. On the other hand, when the metal layer 30 is a ferromagnetic material, the anomalous Nernst effect as described in JP 2016-103535 A, for example, appears in the metal layer 30 regardless of the presence or absence of the ferromagnetic insulating layer 20. The direction of the electromotive force (electric field) generated in the metal layer 30 by this anomalous Nernst effect is the same as that by the spin Seebeck effect. Therefore, the electromotive force actually output from the metal layer 30 in the structure of FIG. 1 is the sum of the component caused by the spin Seebeck effect and the component caused by the anomalous Nernst effect.

[0025] 1, the heat source 40 is connected to the lower side (negative side in the z direction) of the substrate 10, but conversely, the heat source 40 may be connected to the upper side (positive side in the z direction) of the metal layer 30. In this case, the direction of the temperature gradient is opposite to that in FIG. 1, and therefore, when the direction of the applied magnetic field is the same as that in FIG. 1, the direction of the electromotive force generated between the first electrode 31 and the second electrode 32 is opposite. In this case, the first electrode 31 and the second electrode 32 are provided between the metal layer 30 and the heat source 40.

[0026] As described above, YIG, which is the material used for the ferromagnetic insulating layer 20, is a soft magnetic material, and its coercive force and residual magnetization are very small, so the magnetic field application layers 51 and 52 are used to generate a magnetic field H along the x-direction in the ferromagnetic insulating layer 20. On the other hand, the spin Seebeck effect as described above occurs even if the ferromagnetic insulating layer 20 is made of a hard magnetic material. In this case, the residual magnetization can be increased when the magnetic field applied from the outside becomes zero, and the strength of the magnetic field in FIG. 1 can be maintained by the residual magnetization. Therefore, when the ferromagnetic insulating layer 20 is made of a hard magnetic material, the magnetic field application layers 51 and 52 are not necessary if a magnetic field as shown in FIG. 1 is applied in advance in the thermoelectric conversion system 1.

[0027] In order to investigate the characteristics of this thermoelectric conversion system 1, a laminated structure consisting of only the substrate 10, the ferromagnetic insulating layer 20, and the metal layer 30 was formed, excluding the magnetic field application layers 51 and 52 in FIG. 1, and a temperature gradient (temperature difference ΔT) in the z direction and a magnetic field H in the x direction were applied from the outside to measure the potential difference (output voltage V0) on both sides of the y direction in the metal layer 30. Here, GGG with a thickness of 1 mm was used as the substrate 10, YIG with a thickness of 200 nm was used as the ferromagnetic insulating layer 20, and Pt with a thickness of 10 nm was used as the metal layer 30, and ΔT=8K was used. FIG. 2 shows the results of measuring the dependency of the output voltage V0 on the magnetic field H. Here, the magnetic field H was scanned on both the positive and negative sides, and when the magnetic field H is near zero, a certain residual magnetization in the ferromagnetic insulating layer 20 becomes the magnetic field in the x direction in FIG. 1, so that hysteresis occurs according to the direction of the residual magnetization. The output voltage V0 of the thermoelectric conversion system 1 having this configuration becomes a saturated value on the positive side and the negative side of the magnetic field H in FIG. 2, and is about 20 μV.

[0028] Here, gamma rays, which have the greatest influence as radiation, were irradiated in the z direction in FIG. 1, and the ΔT dependence of the output voltage (saturation value) was measured for each dose (exposure amount) of gamma rays, as described above. 60 Co was used, and the maximum exposure dose was approximately 1MGy.

[0029] Three types of irradiation environments were selected: (1) room temperature and dry atmosphere, (2) high temperature (150°C) and dry atmosphere, and (3) high temperature (150°C) and steam atmosphere. Figure 3(a) shows the ΔT dependence of the output voltage V0 before gamma ray irradiation (a) and the same characteristics after 0.86 MGy irradiation in the environment of (2) (b). No significant difference was observed between these, and gamma ray irradiation of about 1 MGy in the environment of (2) did not cause any deterioration in the characteristics of the above-mentioned nuclear battery 1. Similar results were obtained in the environment of (1). In addition, the output voltage in this case has a high linearity with respect to ΔT.

[0030] On the other hand, in the case of (3), the output voltage slightly deteriorated with the amount of radiation exposure. Figure 4(a) shows the ΔT dependence of the output voltage for each amount of radiation exposure in this case, and Figure 4(b) shows the dependence of the temperature coefficient (proportionality coefficient (V / K) in Figure 4(a)) on the amount of radiation exposure. Compared to (1) and (2) above, some degradation in characteristics due to gamma ray irradiation was observed, but the degradation was only to the extent that the output voltage was halved at about 1 MGy. For this reason, the above thermoelectric conversion system 1 has high gamma ray resistance and does not require a heavy metal layer to shield against gamma rays.

[0031] As shown in FIG. 2 and other figures, the output voltage of the thermoelectric conversion system 1 is 10 -6 V, which is smaller than other batteries that are commonly used. However, since this thermoelectric conversion system 1 extracts output voltage in the in-plane direction due to the temperature difference in the thickness direction as shown in Figure 1, a large output voltage can be obtained by connecting multiple thermoelectric conversion systems 1 in series and applying a temperature difference in the thickness direction of each system.

[0032] FIG. 5(a) shows a first example of such a configuration. In this case, n thermoelectric conversion systems 1 are arranged in the horizontal direction in the figure on a support substrate (support base) 100, and in each thermoelectric conversion system 1, the magnetic field H by the magnetic field application layers 51 and 52 is formed so as to be applied downward in the figure, and the temperature gradient ΔT by the heat source 40 is applied in the direction perpendicular to the paper surface. At this time, since the direction of the magnetic field H is common to all the thermoelectric conversion systems 1, the strength of the magnetic field H is maintained uniformly high in the plane. In addition, since the temperature gradient ΔT is also uniformly applied to the entire surface, a stable output can be obtained in each thermoelectric conversion system 1. In this case, if each thermoelectric conversion system 1 is connected in series, an output voltage of n×V0 can be obtained, assuming that the output voltage of a single thermoelectric conversion system 1 is V0. This makes it possible to obtain a high output voltage. Similarly, an array of such thermoelectric conversion systems 1 can be arranged in parallel in the vertical direction in FIG. 5(a) and the output of the two-dimensional array can be connected in series to further increase the output voltage. In such a case, the substrate 10 may be common to adjacent or all of the thermoelectric conversion systems 1 in FIG.

[0033] As shown in Fig. 5(b) as a second example, the thermoelectric conversion systems 1 can be arranged circumferentially on the outer peripheral surface of a cylindrical heat source 40, similar to Fig. 5(a). Here, two rows are arranged vertically as described above, but the number of rows arranged vertically can be increased. In this case, a cylindrical cask can be preferably used as the heat source 40, as described in Patent Document 1. In such a case, a thermoelectric conversion system 1 having high resistance to radiation (gamma rays) is particularly effective.

[0034] In the above example, the direction of the magnetic field H, the direction of the temperature gradient ΔT, and the direction in which the first and second electrodes are formed (the direction in which the output voltage is taken out) are mutually orthogonal, but these do not need to be strictly orthogonal, and the relationship between these may be shifted from orthogonal depending on the element configuration. In other words, as long as a sufficient output voltage is obtained, these directions may intersect each other.

[0035] Moreover, the materials constituting the substrate, ferromagnetic layer, and metal layer can be appropriately set to materials other than those mentioned above. [Explanation of symbols]

[0036] 1 Thermoelectric conversion system 10 Substrate 20 Ferromagnetic insulating layer (ferromagnetic layer) 30 metal layer 31 1st electrode 32 2nd electrode 40 Heat source 51, 52 Magnetic field application layer

Claims

1. A thermoelectric conversion system in which an electromotive force is generated between a first electrode and a second electrode by an externally applied temperature gradient, a ferromagnetic layer formed on a substrate and having a magnetic field in an in-plane direction; a metal layer having a thickness of 10 nm or more and 50 nm or less, the metal layer being made of a metal material that exhibits an inverse spin Hall effect, the metal layer being formed on the ferromagnetic layer, the metal layer being formed at a location where the first electrode and the second electrode are spaced apart in the in-plane direction and in a direction intersecting the magnetic field; Equipped with the heat source that imparts the temperature gradient is a cask containing radioactive material therein; the substrate is composed of gallium gadolinium garnet (GGG) and is coated with boron (B) or cadmium (Cd); The thermoelectric conversion system according to claim 1, wherein the temperature gradient is applied in a thickness direction of the ferromagnetic layer.

2. 2. The thermoelectric conversion system according to claim 1, wherein a magnetic field application layer that applies the magnetic field to the ferromagnetic layer and the metal layer is connected to ends of the ferromagnetic layer and the metal layer in the in-plane direction.

3. 3. The thermoelectric conversion system according to claim 2, wherein the ferromagnetic layer is made of yttrium iron garnet (YIG) or YIG doped with gallium (Ga).

4. 4. The thermoelectric conversion system according to claim 1, wherein the metal layer is made of platinum (Pt), gold (Au), niobium (Nb), nickel (Ni), or stainless steel.

5. 5. A thermoelectric conversion method comprising: arranging a plurality of thermoelectric conversion systems according to claim 1 in series on a surface of a heat source that imparts the temperature gradient, so that the magnetic fields in each system are oriented in a common direction and the first electrode and the second electrode in each system are on a common side.

6. The heat source is generally cylindrical in shape, The thermoelectric conversion method according to claim 5 , wherein a plurality of the thermoelectric conversion systems are arranged in a circumferential direction of an outer circumferential surface of the substantially cylindrical shape.

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