JP2024023735A - Thermoelectric conversion system and thermoelectric conversion method - Google Patents

Abstract

PROBLEM TO BE SOLVED: 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

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to a thermoelectric conversion system and a thermoelectric conversion method that generate electricity using an applied temperature gradient.

For example, 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 having different Seebeck coefficients and electrodes connected to these layers. It is difficult to supply power to such thermoelectric conversion elements from the outside, and they can be used in various environments remote from the operator. As an example of this, for example, Patent Document 1 describes the use of such a thermoelectric conversion element as a power source for temperature management of a cask that contains spent nuclear fuel.

In this case, the cask is used as a heat source, and the thermoelectric conversion element spontaneously generates electricity using the decay heat of the nuclear fuel within the cask. With this configuration, it is not necessary to supply power from outside to a location where a cask with a high radiation dose is located, and the frequency of entry by workers can be reduced.

JP2018-72167A

However, in conventional Seebeck elements, this radiation causes crystal defects in the semiconductor material used. These crystal defects deteriorated the characteristics of the Seebeck element, and its output gradually decreased. Therefore, when the cask was used as a heat source as described above, the radiation emitted from the cask caused such deterioration. Particularly, radiation that causes crystal defects in this manner includes gamma rays.

A thick heavy metal layer is required to shield gamma rays. When a cask is used as a 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, the output also decreased due to a decrease in the amount of heat flowing into the thermoelectric conversion element side.

For this reason, a thermoelectric conversion system or thermoelectric conversion method that is less degraded by radiation has been desired.

The present invention has been made in view of these problems, and it is an object of the present invention to provide an invention that solves the above problems.

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 due to a temperature gradient applied from the outside, and is formed on a substrate and has a magnetic field in an in-plane direction. It is composed of a ferromagnetic layer and a metal material that exhibits a reverse spin Hall effect, and is formed on the ferromagnetic layer, and the first electrode and the second electrode intersect in the in-plane direction and the magnetic field. a metal layer with a thickness of 50 nm or less formed at locations spaced apart in the direction, the heat source for imparting the temperature gradient is a cask containing a radioactive substance therein, and the substrate is made of gallium gadolinium garnet. (GGG) and coated with boron (B) or cadmium (Cd), and the temperature gradient is applied in the thickness direction of the ferromagnetic layer .
In the thermoelectric conversion system of the present invention, a magnetic field application layer that applies the magnetic field to the ferromagnetic layer and the metal layer is connected to the end portions of the ferromagnetic layer and the metal layer in the in-plane direction. Features.
In the thermoelectric conversion system of the present invention, the ferromagnetic layer is made of yttrium-iron-garnet (YIG) or YIG added with gallium (Ga).
In the thermoelectric conversion system of the present invention, the metal layer includes platinum (Pt), gold (Au), niobium (Nb), nickel (Ni), or stainless steel.
In the thermoelectric conversion method of the present invention, a plurality of the thermoelectric conversion systems are arranged so that the magnetic field in each of them is in a common direction, and the first electrode and the second electrode in each are on a common side. It is characterized by being placed on the surface of the heat source to be applied and connected in series.
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 the outer peripheral surface of the substantially cylindrical shape.

According to the present invention, it is possible to obtain a thermoelectric conversion system or a thermoelectric conversion method that is less degraded by radiation.

1 is a diagram showing the structure of a thermoelectric conversion system of the present invention. It is the result of measuring the magnetic field dependence of the output voltage in an example of the present invention. It is the temperature difference dependence of the output voltage in the example before γ-ray irradiation (a) and after γ-ray irradiation in a high temperature and dry atmosphere (b). The temperature difference dependence of the output voltage for each gamma ray exposure dose (a) and the exposure dose dependence of the temperature coefficient (b) in gamma ray irradiation at high temperature and in steam are shown, respectively. 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.

Hereinafter, a thermoelectric conversion system according to an embodiment of the present invention will be described. This thermoelectric conversion system is similar to conventional thermoelectric conversion systems in that it generates electricity using applied heat (temperature gradient), but its structure and power generation mechanism are different from conventional thermoelectric conversion systems.

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

On the other hand, the thermoelectric conversion system of the present invention utilizes the spin Seebeck effect described in, for example, Japanese Patent Application Laid-Open Nos. 2009-130070 and 2015-179746. In thermoelectric conversion elements that use the spin Seebeck effect, when a magnetic field is applied to the ferromagnetic layer or when the ferromagnetic layer is magnetized, electrons flow parallel to the magnetic field (up-spin electrons). , a difference is created in the flow of electrons with antiparallel spins (down-spin electrons), and this difference is output as a voltage (electromotive force) due to the reverse spin Hall effect in the paramagnetic metal layer that is in contact with the ferromagnetic layer. .

In a thermoelectric conversion element that utilizes the Seebeck effect in a semiconductor as described above, the flow of holes and electrons that generate electromotive force is greatly affected by crystal defects in the semiconductor. Since these crystal defects are generated particularly by radiation irradiation, they increase over time in an environment with a high radiation dose, and the output voltage decreases accordingly.

On the other hand, as will be described later, the spin current in a ferromagnetic material is less affected by radiation irradiation than the flow of holes and electrons in a semiconductor. Therefore, a thermoelectric conversion system using such a spin Seebeck effect is particularly suitable for use in an environment with a high radiation dose.

FIG. 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 sequentially formed on a substrate 10 along the film thickness direction (z direction).

Further, a heat source 40 that provides a temperature gradient to this thermoelectric conversion system 1 is connected to the lower side of the substrate 10. Furthermore, magnetic field application layers 51 and 52 are connected to both sides of this structure in the x direction, respectively. If the metal layer 30 were made thick enough to have negligible sheet resistance, the metal layer 30 would have the same potential in the in-plane direction, but as will be described later, since the metal layer 30 is formed sufficiently thin, the metal layer 30 A potential difference occurs in the in-plane direction of the layer 30. Since a 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, and the first electrode 31 and the second electrode Electromotive force is extracted between the electrodes 32.

As the substrate 10, a material having sufficient mechanical strength and on which a high-quality ferromagnetic insulating layer 20 can be formed is used. For example, a 1 mm thick Gd ₃ Ga ₅ O ₁₂ (gadolinium gallium garnet) :GGG) single crystal is used. As the ferromagnetic insulating layer (ferromagnetic layer) 20, a ferrite magnetic material such as Y ₃ Fe ₅ O ₁₂ (yttrium-iron-garnet: YIG), which is magnetic garnet, or YIG added with gallium (Ga) is used. It has a thickness of about 100 nm and is formed by a coating printing method, a sputtering method, 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 also be used, such as magneto-optical materials such as MnSb and MnBi. Materials and rare earth compounds such as SmCo can also be used. 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. Therefore, it is preferable to use the ferromagnetic insulating layer 20.

As the metal layer 30, metal materials such as platinum (Pt), gold (Au), niobium (Nb), nickel (Ni), stainless steel, etc., which have a large spin-orbit interaction and strongly exhibit a reverse spin Hall effect, are preferably used. . As described above, the thickness of the metal layer 30 is sufficiently thin, 50 nm or less, for example, about 10 nm, so that the sheet resistance is sufficiently high even when a conductive metal material is used. The metal layer 30 is formed by, for example, a sputtering method. The electromotive force generated in this thermoelectric conversion system 1 is extracted from the first electrode 31 and the second electrode 32 as described above.

The magnetic field applying layers 51 and 52 are each made of a hard magnetic material having spontaneous magnetization in the illustrated direction, and are insulating or provided on both sides in the x direction so as to be insulated from the metal layer 30 etc. There is. The magnetic field applying layers 51 and 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.

Various heat sources 40 are used depending on the manner in which this thermoelectric conversion system 1 is used. However, as will be described later, since this thermoelectric conversion system 1 has high radiation resistance, a cask that is used in an environment with a high radiation dose, such as the cask described in Patent Document 1, can be particularly preferably used.

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 increasing power generation efficiency. For example, when the heat source 40 emits neutrons like a cask, and the substrate 10 is made of GGG containing Gd, which has a large neutron capture cross section, the substrate 10 itself also functions as a heat source. In this case, power generation efficiency can be further improved by coating the substrate 10 with boron (B) or cadmium (Cd), which have a large reaction cross section with neutrons, for example. Even in this case, the temperature gradient formed is as shown in FIG.

In the above structure, the spin Seebeck effect in the ferromagnetic insulating layer 20 and the reverse spin Hall effect in the metal layer 30 are similar to the structures described in JP 2009-130070A and JP 2015-179746A. Accordingly, the electromotive force due to the temperature gradient and the magnetic field can be taken out from the metal layer 30. On the other hand, when the metal layer 30 is a ferromagnetic material, the abnormal Nernst effect as described in JP-A-2016-103535, for example, occurs in the metal layer 30 regardless of the presence or absence of the ferromagnetic insulating layer 20. do. The direction of the electromotive force (electric field) generated in the metal layer 30 due to this anomalous Nernst effect is the same as that due to the spin Seebeck effect. Therefore, in reality, the electromotive force output from the metal layer 30 in the structure of FIG. 1 is the sum of a component due to the spin Seebeck effect and a component due to the anomalous Nernst effect.

In FIG. 1, the heat source 40 is connected to the lower side of the substrate 10 (negative side in the z direction), but conversely, the heat source 40 may be connected to the upper side of the metal layer 30 (positive side in the z direction). In this case, since the direction of the temperature gradient is opposite to that in FIG. 1, if the direction of the applied magnetic field is the same as in FIG. The direction of power is reversed. Further, in this case, the first electrode 31 and the second electrode 32 are provided between the metal layer 30 and the heat source 40.

As mentioned 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. Magnetic field application layers 51 and 52 are used to generate H. 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 when the externally applied magnetic field becomes zero can be increased, 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. be.

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

Here, γ-rays, which have the greatest effect as radiation, are irradiated in the z direction in Fig. 1 as described above, and the ΔT dependence of the output voltage (saturation value) as described above is expressed as the γ-ray dose (exposure amount). Measured every time. ⁶⁰ Co was used as the γ-ray source, and the maximum exposure dose was about 1 MGy.

Furthermore, 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. FIG. 3(a) shows the ΔT dependence of the output voltage V0 before irradiation with γ-rays (a), and the same characteristics (b) after 0.86 MGy irradiation under the environment of (2), respectively. There is no significant difference between these, and γ-ray irradiation of about 1 MGy under the environment (2) does not cause the deterioration of the characteristics of the nuclear battery 1 described above. Similar results were obtained under the environment (1). Further, the output voltage in this case has high linearity with respect to ΔT.

On the other hand, in case (3), the output voltage slightly deteriorated as the exposure amount increased. Figure 4(a) shows the ΔT dependence of the output voltage for each exposure dose in this case, and Figure 4(b) shows the exposure dose dependence of the temperature coefficient (proportionality coefficient (V/K) in Figure 4(a)). Each shows its gender. Compared to (1) and (2) above, characteristic deterioration is observed due to γ-ray irradiation, but the deterioration is to the extent that the output voltage is halved at about 1 MGy. Therefore, the thermoelectric conversion system 1 described above has high gamma ray resistance and does not require a heavy metal layer for shielding gamma rays.

As shown in FIG. 2 and the like, the output voltage of this thermoelectric conversion system 1 is about 10 ⁻⁶ V, which is smaller than other commonly used batteries. However, in this thermoelectric conversion system 1, the output voltage is taken out in the in-plane direction due to the temperature difference in the thickness direction as shown in FIG. By providing a difference, a large output voltage can be obtained.

FIG. 5(a) shows a first example of such a configuration. In this case, n thermoelectric conversion systems 1 are arranged horizontally in the figure on a support substrate (support base) 100, and in each thermoelectric conversion system 1, the magnetic field H due to the magnetic field application layers 51 and 52 is directed downward in the figure. The temperature gradient ΔT caused by the heat source 40 is applied in a direction perpendicular to the plane of the paper. At this time, since the direction of the magnetic field H is common to all thermoelectric conversion systems 1, the strength of the magnetic field H is maintained uniformly high within the plane. Furthermore, since the temperature gradient ΔT is uniformly applied throughout the plane, each thermoelectric conversion system 1 can obtain a stable output. 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 allows a high output voltage to be obtained. Similarly, the output voltage can be further increased by arranging such thermoelectric conversion systems 1 in parallel also in the vertical direction in FIG. 5(a) and connecting the outputs of the two-dimensional array in series. In such a case, the substrate 10 may be shared by adjacent or all thermoelectric conversion systems 1 in FIG. 1 .

Further, as shown as a second example in FIG. 5(b), the thermoelectric conversion systems 1 can be arranged on the circumference on the outer circumferential surface of the cylindrical heat source 40 in the same manner as in FIG. 5(a). Here, two rows of arrays are formed in the vertical direction as described above, but the number of arrays in the vertical direction can also 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 cases, the thermoelectric conversion system 1 with high radiation (gamma ray) resistance is particularly effective.

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 (direction in which the output voltage is extracted) are assumed to be orthogonal to each other. They do not need to be strictly orthogonal, and the relationship between them may be shifted from orthogonal depending on the element configuration. That is, these directions may intersect with each other as long as a sufficient output voltage can be obtained.

Further, the materials constituting the substrate, the ferromagnetic layer, and the metal layer can be appropriately set other than the above-mentioned examples.

1 Thermoelectric conversion system 10 Substrate 20 Ferromagnetic insulating layer (ferromagnetic layer)
30 Metal layer 31 First electrode 32 Second electrode 40 Heat sources 51, 52 Magnetic field application layer

Claims (6)

A thermoelectric conversion system that generates an electromotive force between a first electrode and a second electrode due to a temperature gradient applied from the outside,
a ferromagnetic layer formed on a substrate and having a magnetic field in an in-plane direction;
It is made of a metal material that exhibits a reverse spin Hall effect, is formed on the ferromagnetic layer, and is located at a location where the first electrode and the second electrode are spaced apart in the in-plane direction and in the direction intersecting the magnetic field. A formed metal layer with a thickness of 50 nm or less,
Equipped with
The heat source imparting the temperature gradient is a cask containing a radioactive substance therein,
The substrate is made of gallium gadolinium garnet (GGG) and coated with boron (B) or cadmium (Cd),
A thermoelectric conversion system, wherein the temperature gradient is applied in the thickness direction of the ferromagnetic layer. 2. A magnetic field applying layer that applies the magnetic field to the ferromagnetic layer and the metal layer is connected to an end of the ferromagnetic layer and the metal layer in the in-plane direction. thermoelectric conversion system. 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. The metal layer according to claim 1 , wherein the metal layer contains platinum (Pt), gold (Au), niobium (Nb), nickel (Ni), or stainless steel. Thermoelectric conversion system. The thermoelectric conversion system according to any one of claims 1 to 4 is arranged such that the magnetic field in each is in a common direction, and the first electrode and the second electrode in each are on a common side. A thermoelectric conversion method characterized in that a plurality of heat sources are arranged on the surface of a heat source that imparts the temperature gradient and connected in series. The heat source has a substantially cylindrical shape,
6. The thermoelectric conversion method according to claim 5 , wherein a plurality of said thermoelectric conversion systems are arranged along the circumferential direction of said substantially cylindrical outer peripheral surface.

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