JP2003347609A - Thermoelectric apparatus utilizing nernst effect and seebeck effect - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus structure which improves the figure of merit of an apparatus utilizing Nernst effect and Seebeck effect. <P>SOLUTION: The apparatus comprises a plurality of elements which are made of a thermoelectric material and each have at least a first and second elements at mutually opposite positions through central portions of the elements in their sectional view. To extract voltages, the first and second electrodes of the element are disposed at a high and low temperature sides, respectively. The first electrodes of the prestage element and the poststage element are mutually connected through a cable. The second electrodes of the prestage element and the poststage element are mutually connected through a cable. The prestage element and the poststage element have mutually different polarities. <P>COPYRIGHT: (C)2004,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermoelectric conversion element, and more particularly to a thermoelectric device utilizing the Nernst effect and the Seebeck effect.

[0002]

2. Description of the Related Art The Nernst effect means that when a magnetic field is applied to a conductor or semiconductor having a temperature gradient and heat is flowing in a direction perpendicular to the direction of heat flow, a magnetic field and a direction perpendicular to the temperature gradient are applied. This is a phenomenon that causes a potential difference.

The Seebeck effect is to generate an electric field parallel to the direction of the temperature gradient. The magnitude of the thermoelectromotive force is determined only by the temperature difference between the contacts if the material of the conductor is homogeneous. When a temperature difference ΔT is given to both ends of the conductor, heat is transferred from the high temperature side to the low temperature side by the flow of electrons and phonons,
The phonons interact with the electrons while moving, and drag the electrons. The thermoelectromotive force is an electromotive force S _d ΔT (S _d is an electron diffusion component, for example, S _d = (π / 2) (k /
_{e) (kT / E F)} ; k is the Boltzmann constant, e is the electron charge, T is the absolute temperature, _{E F} is the metal Fermi energy), given by the sum of the phonon drag component _{S g} [Delta] T. In the case of a semiconductor, the charge carrier (N
In the case of a type semiconductor, heat is transferred from the high temperature side to the low temperature side by the flow of electrons and phonons in the case of a P-type semiconductor such as electrons and Ge. In semiconductors, the concentration of charge carriers is
Lower than the metal of the conduction electrons (metal conduction electrons follow the Fermi-Dirac statistics), according Makusuferu-Boltzmann distribution, S _d is given by 3/2 (k / e).

Conventionally, Peltier devices using thermoelectric semiconductors are currently on the market. The Peltier element is a phenomenon in which when a current is passed through a contact between different conductors or semiconductors, heat is generated and absorbed in addition to Joule heat at the contact of the Peltier effect, which is a reverse process of the Seebeck effect. The same phenomenon is used for cooling and temperature control. That is, the Peltier element applies a voltage from outside the element and causes a current to flow through the element to generate a temperature difference, thereby performing cooling, heating, and temperature control. Then, applying a temperature difference to the thermoelectric semiconductor,
An element that generates power is called a “Seebeck element”. These two phenomena (the Peltier effect and the Seebeck effect) are physically the same process, but have been named independently by their discoverers in history.

[0005]

SUMMARY OF THE INVENTION The present invention has been made based on a study of a power generator using the Nernst effect and the Seebeck effect (thermoelectric power). Another object of the present invention is to provide a completely new device structure that improves the performance index of a device using the Seebeck effect.

[0006]

According to one aspect of the present invention, there is provided an apparatus for achieving the above object.
A plurality of thermoelectric elements in which a temperature difference is provided between both ends are cascaded in a plurality of stages, and a potential difference between both ends of a thermoelectric element having a relation of a former stage and a latter stage among a plurality of thermoelectric devices has a reverse polarity, and a plurality of thermoelectric devices are For each of the cables cascading the two ends per cable,
They are arranged on the same side of the high temperature side or the low temperature side.

In the present invention, a magnetic field is applied in a direction perpendicular to the cross section of the thermoelectric element, and a voltage based on the product of the magnetic field and the temperature difference between the high temperature side and the low temperature side is applied between the electrodes of one thermoelectric element. And a voltage based on the temperature difference between the high-temperature side and the low-temperature side.

[0008] In another aspect (side) of the device according to the present invention, the device is made of a thermoelectric material, and the first and second electrodes are provided on the side of the element at positions opposed to each other via the center of the cross section of the element. In order to take out a voltage, at least a plurality of elements provided are provided, and the first electrode of the element is disposed on a high-temperature side, the second electrode of the element is disposed on a low-temperature side, and the former and the latter elements are provided. Are connected to each other by a cable, and the second electrodes of the former and subsequent elements are connected by a cable.

In the apparatus according to the present invention, a magnetic field is applied in a direction perpendicular to the cross section, and a product of the magnetic field and a temperature difference between the high temperature side and the low temperature side is applied between the first and second electrodes. And a voltage based on the temperature difference between the high-temperature side and the low-temperature side, a combined voltage is obtained.

In the present invention, the polarities of the elements at the front and rear stages are different from each other.

In the device according to the present invention, a cross-sectional shape of the element is rectangular, and the first and second electrodes are provided on side surfaces of the element near first and second vertices defining a diagonal line of the rectangle. Each of them may be provided.

In the device according to the present invention, the elements at the front and rear stages are P-type and N-type semiconductor elements.

[0013]

Embodiments of the present invention will be described. According to the present invention, thermoelectric elements having a temperature difference between both ends are cascaded in a plurality of stages (for example, the element 10 in FIG. 8).
A, 10B, 10A,...), A potential difference between both ends of a thermoelectric element in a relation of a former stage and a latter stage among a plurality of thermoelectric elements has opposite polarity, and a cable for cascade-connecting a plurality of thermoelectric elements (13 in FIG. 8). ), Both ends of one cable are arranged on the same side of the high temperature side or the low temperature side with respect to the temperature. Function.

Temperature gradient direction (vector grad T)
Defines one of the X-axis and the Y-axis orthogonal to each other in one section of the thermoelectric element, and applies a magnetic field (B) to the section in a normal direction to the one section. Near one point on the outer circumference of the cross section corresponding to one end of
The electrode (1) is located near the one point and near another point on the outer circumference of the cross section located diagonally with respect to the cross section.
1, 12), and a voltage VN based on the product of the magnetic field B and the temperature difference gradT between the high temperature side and the low temperature side, and a temperature difference between the high temperature side and the low temperature side, between the electrodes (11, 12). Thus, a combined voltage (= VN + Vα) based on the voltage Vα is obtained.

In the following, the Nernst-Seebeck device according to the present invention and the element structure constituting the device will be described.
An explanation will be given based on the principle and experimental results.

FIG. 1 is a diagram for explaining a unit element configuration of a Nernst-Seebeck element in a thermoelectric device (thermoelectric generator) according to the present invention. Sample (element) 1
0 has a rectangular shape (the size is a in the horizontal direction and b in the vertical direction), and the upper part thereof has a high temperature side Th (high temperature sid).
e) The lower part is the low temperature side Tl (low temperature side). The magnetic flux B is applied in the vertical direction in FIG. gr
ad T is the temperature gradient.

Electrodes (electrodes) 11 and 12 are attached to the sides of the periphery (CONTOUR) of the element 10 near the vertices connected by diagonal lines of the rectangular cross section. Electrodes (electrodes) 11 and 12 are elements 1
0 extends in the thickness direction of the side surface. In FIG. 1, the rectangular or square rectangular cross section is merely an example, and in the present invention, the cross sectional shape may be another polygon or the like.

The voltage Vα generated by the thermoelectromotive force (referred to as “Seebeck voltage”) is generated in the vertical direction of the element 10 in FIG. 1, and the voltage (Nernst voltage) VN generated by the Nernst effect is calculated in the horizontal direction in FIG. Occurs.

The corresponding Ohm's law, Seebeck voltage and Nernst voltage are expressed by equations (1), (2) and (3).

[0020] E = α · gradT + NB · gradT (1) Vα = α · ΔT (2) VN = NB · ΔT · (a / b) (3)

Where α is the Seebeck coefficient, N is the Nernst coefficient, T is the temperature, B is the magnetic field (vector), ΔT is the temperature difference, and a and b are the samples (10 in FIG. 1). The length of each side. gradT is a gradient (vector) of the temperature T, and B × gradT is a vector product of the vector B and the vector gradT.

When the electrodes 11 and 12 are mounted in a diagonal direction of the device 10, the Nernst voltage VN and the Seebeck voltage Vα
Can be detected.

By the way, when the Seebeck element and the Peltier element described above are manufactured, they have considerably different structures.

The reason is that the Seebeck element and the Peltier element flow greatly different currents. In other words, in the case of the Peltier element, an actually necessary temperature difference cannot be obtained unless a relatively large current flows. On the other hand, the power that can be extracted under the condition that the same temperature difference is applied to the element is the product of the voltage generated by the element and the current.
As compared with the former, it is 1/5 or less, so that the electrode may be small. This will be described below.

In the following, a conventionally used bismuth tellurium-based thermoelectric material will be considered. In particular,
The parameter of the transport coefficient (thermal conductivity) is determined as follows based on commercially available materials.

Α = 205 μV / K η = 1.11 × 10 ⁻⁵ Ωm κ = 1.35 W / m / K

The size of the bismuth tellurium-based thermoelectric material is S = 10 mm × 10 mm (cross-sectional area) d = 2 mm (thickness).

Assuming that the transport coefficient does not change over the temperature range used, the heat flow equation is:
It is expressed by the following equations (4) and (5).

[0029] _{q c = αT c I-0.5r} e I 2 -KΔT ... (4) q h = αT h I + 0.5r e I 2 -KΔT ... (5)

Here, q _c is the amount of heat absorbed on the low temperature side, q _h is the amount of heat radiation on the high temperature side, _re is the resistance of the element, I is the current, K is the thermal conductance, and α is the Seebeck coefficient.

Further, the voltage applied to both ends of the element at this time is _{V = r e I + αT h} ... (6).

As an example of using a Peltier element, let us assume that a maximum temperature difference is provided. In this case, assuming that the high temperature side in FIG. 1 is constant, the current that gives the minimum _Tc is obtained by differentiating the above equation (4) with the current I and setting it to zero, and is given by the following equation (7).

I = αΔT / r _e (7)

Then, the relationship between the low temperature side temperature _Tc and the current I is represented as shown in FIG.

The calculation _conditions, is set to T h = 300K.

As shown in FIG. 2, the lowest temperature is 227K, and the current value at that time is 210A. Therefore, when used as a Peltier element, the electrodes and their wiring
It is necessary to deal with 10A, which means that the size becomes large.

When the thickness of the thermoelectric semiconductor is reduced by half,
The optimum current value is doubled, but the lowest temperature does not change.

Next, description will be made on a case where the device is used as a power generating element (Seebeck element). In this case, the temperature difference is
Assuming that 80K (= 300K−220K), the generated voltage is only the second term on the right side of the above equation (6).
Obtain 6.4 mV.

On the other hand, since the electrical resistivity is known at the above size, the electrical resistance is determined, and is 0.222 mΩ.

In the Seebeck effect, the thermoelectric element is a battery, and the maximum power that can be taken out is obtained when the external resistance is equal to the internal resistance. The current flowing at this time is
36.9A.

Accordingly, the value is 17.6% of the current for obtaining the maximum temperature difference. That is, even if the same thermoelectric material is used as a Peltier element or as a Seebeck element, the electrode structure greatly changes, and it can be seen that the current capacity may be low with a Seebeck element.

Further, when the thickness of the element is doubled, the flowing current is halved. This is because the voltage generated by the Seebeck element is determined only by the temperature difference between the high-temperature side and the low-temperature side, but the element thickness (the element thickness corresponds to the direction perpendicular to the plane of FIG. 1) is 2
This is because, when the current is doubled, the resistance value is doubled, and the flowing current is halved. However, in this case, the conversion efficiency does not change. Conversely, the reciprocal 1 / d of the thickness d of the element 10
In addition, it can be seen that the power that can be extracted per unit area is proportional.

In a commercially available Peltier device, electrodes are attached to the entire surface on the side opposite to the high temperature side and the surface facing the low temperature side. From the above examination, when the electrode is used as a Seebeck device, it is no longer necessary. Also, a small electrode and wiring configuration may be used.

Referring to FIG. 1 again, in such an element structure, by selecting the positions of the electrodes, the members, and the like so that the sum of the Seebeck voltage Vα and the Nernst voltage VN can be obtained.
Substantially corresponds to the increase in Seebeck coefficient.

That is, the figure of merit Z (Fi
gure of merit), which is expected to improve conversion efficiency.

In particular, in the case of the Nernst effect, the heat flow flows from the low temperature side to the high temperature side, and is in a direction opposite to the Seebeck effect. Therefore, even if the thermoelectric conversion elements have the same figure of merit, a large conversion efficiency can be obtained. This will be specifically described below.

The material used as the element (sample) 10 in FIG. 1 is polycrystalline bismuth. Size is 12mm
At the corners (a = 12 mm, b = 12 mm in FIG. 1), the thickness is
1.8 mm.

A copper block (not shown) is electrically insulated from the element 10 and is configured to maintain a constant temperature on the low temperature side and the high temperature side. A heater (not shown) was attached to the copper block on the high temperature side. Thermocouples (not shown) were directly attached to bismuth at two locations. Thus, the temperature of the device (sample) 10 was measured.

FIG. 3 shows the electrodes 11, 1 arranged diagonally.
2 shows the relationship between the voltage and the magnetic field between the two. The voltage observed increases or decreases depending on the direction of the magnetic field. This means that the sign of the Nernst voltage is inverted,
This is because the Seebeck voltage itself changes.

In this embodiment, the maximum value is obtained near the magnetic field of -2.5 T, and the value gradually decreases as the magnetic field increases in the negative direction. On the other hand, when a magnetic field in the positive direction is applied, the induced voltage decreases, becomes zero near + 2.0T, and the sign is reversed when the magnetic field is further increased. Until + 5.0T, the induced voltage did not tend to be saturated.

On the other hand, at this time, the electric resistance and the heat conduction between the electrodes 11 and 12 on the diagonal line of the element (sample) 10 are changed by the magnetic field. FIG. 4 shows diagonal electrodes 11, 1
The relationship between the electric resistance and the magnetic field between the two is shown, and FIG. 5 shows the relationship between the heat conduction and the magnetic field.

In general, the electric resistance increases with the application of a magnetic field, and the heat conduction decreases. As shown in FIG. 4 and FIG. 5, the same result was obtained in the present measurement.

At this time, the figure of merit Z is given by the following equation (8).

Z = α ² / (κ · η) (8)

Here, η is a transport parameter, R is a resistance value, and κ is a thermal conductivity.

The figure of merit Z may increase or decrease depending on the application of a magnetic field. The magnetic field dependence of the figure of merit is a complicated phenomenon, and it is difficult to simply estimate. In fact, several papers and the like have been published on the magnetic field dependence of thermoelectric semiconductors, and these have obtained mutually contradictory results.

However, using the above data,
It is possible to estimate how the figure of merit changes with the magnetic field. That is, the following equation (9) is estimated in order to evaluate the effective figure of merit Z _eff .

Z _eff = (V _eff / ΔT) ² / (κ · R) (9)

Where V _eff is the observed voltage, ΔT
Is the temperature difference of the sample, R is the resistance value, and κ is the thermal conductivity.

By evaluating the above equation (9), a figure of merit when the electrodes are arranged on a diagonal line in a magnetic field can be effectively estimated. FIG. 6 shows the results.

In FIG. 6, the horizontal axis is the magnetic field, and the vertical axis is the relative value of the figure of merit. The figure of merit when the magnetic field is zero is 1.
Assuming that the value is 0, when the magnetic field is -2.5T, the value is approximately 2.2 times. Then, when a magnetic field in the direction in which the signs of the Seebeck voltage and the Nernst voltage are reversed is applied, the figure of merit decreases due to the increase in the magnetic field, and becomes almost zero.
Increase. This increase is because the sign of the Seebeck coefficient itself is inverted by the application of the magnetic field. Therefore, it can be seen that the figure of merit increases with the application of the magnetic field.

In the above, the results of experiments actually performed are cited. Is it possible to make an energy conversion element that can be put into practical use by using such experimental results? Unfortunately, the answer is no ("No"). The reason is that if an element is manufactured using such a material, a plurality of elements are electrically connected in series. This is the same as a normal Peltier element or Seebeck element.

FIG. 7 is a diagram schematically showing a configuration in which electrodes 11 and 12 of a plurality of elements 10 (see FIG. 1) are connected in series. As shown in FIG. 7, the wiring cable 13 to be electrically connected has a structure connecting the high temperature side and the low temperature side.
Then, since the cable 13 uses a good conductor such as copper, heat flows from the high-temperature side to the low-temperature side through the cable 13 to generate a large loss. Therefore, such an element structure is impossible in principle (cannot operate)
It is.

Therefore, further studies were made on the possibility of a Nernst-Seebeck element using a magnetic field.

The sign of the Seebeck coefficient is determined by the polarity of the thermoelectric semiconductor device. That is, depending on the P type and N type,
The sign becomes positive or negative.

On the other hand, it is said that the Nernst coefficient is determined by the scattering process of carriers of the thermoelectric element. Therefore, if the carrier scattering process is the same irrespective of the P type and the N type, the same sign is assigned.

[0067]

FIG. 8 is a diagram for explaining the structure of one embodiment. 8, the configuration of a single element is the same as the configuration shown in FIG.

In the example shown in FIG. 7, bismuth as a material constituting the element 10 is N-type, so that the high temperature side of the element 10 always has the potential on the low temperature side (l).
owtemperature side). For this reason, as shown in FIG. 7, a cascade configuration is adopted in which the high-temperature side electrode 11 of one element and the low-temperature side electrode 12 of the adjacent element are connected by wiring 13.

On the other hand, since the magnetic field B is generated by a superconducting magnet or the like, the sign cannot be rapidly reversed at a small distance.

Therefore, the direction of the Nernst electric field is P
It is spatially unidirectional, regardless of the polarity of the type and N-type.

Therefore, in this embodiment, the cable 13 connected to the electrode of the element 10 is connected to one element and the high temperature side (high temperature sid) of the adjacent element (the former and the latter).
e) between the electrodes 11 or between one element and an adjacent element (the former and the latter elements)
Since the electrodes 12 on the side) are connected to each other, substantially no temperature difference occurs between both ends of the cable 13. In other words, a plurality of thermoelectric elements having different polarities (Nernst-Seebeck elements in FIG. 1) arranged opposite to each other on the high temperature side and the low temperature side, respectively, and the cable 13 for cascading the thermoelectric elements has two ends. , On either the high temperature side or the low temperature side. This thermoelectric element has an electrode 1
Element 1 is a P-type element (10A) on the high temperature side, N-type (10B) is next to P-type elements, and P-type elements are next to each other.

The electrodes 11, 12 of the element 10A at the left end in FIG.
The difference voltage between the electrodes (with reference to the electrode 12) is -ΔVA, and the element 10B
The difference voltage between the electrodes 11 and 12 (with reference to the electrode 12) is ΔV
B, assuming that the difference voltage between the electrodes 11 and 12 of the rightmost element 10A in FIG. 8 (with reference to the electrode 12) is −ΔVA, the electrode 11 of the leftmost element 10A in FIG. 8 and the rightmost element 10A in FIG. A voltage equivalent to 2 × ΔVA + ΔVB can be obtained between the electrodes.

Then, a structure is obtained in which the performance index of the material improved by the application of the magnetic field described with reference to FIG. 6 can be used as it is as the performance index of the element. The Nernst coefficient has the same sign depending on the polarity of the material.

A material having a high Seebeck coefficient and a high Nernst coefficient at the same time is appropriately used. Bismuth is
Among the thermoelectric elements, both the Seebeck coefficient and the Nernst coefficient are high.

[0075]

As described above, according to the present invention,
The cable connected to the electrode of the element is connected in series by connecting one element and the electrode on the high temperature side of the adjacent element, and connecting one element and the electrode on the low temperature side of the adjacent element. Therefore, there is no temperature difference, and the figure of merit of the material improved by the application of the magnetic field can be used as it is as the figure of merit of the element, so that a desired voltage can be efficiently extracted.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a Nernst-Seebeck element.

FIG. 2 is a diagram illustrating a relationship between a low-temperature side temperature and a current value.

FIG. 3 is a diagram illustrating a relationship between a diagonal voltage and a magnetic field.

FIG. 4 is a diagram showing the relationship between the resistance between diagonal electrodes and the magnetic field.

FIG. 5 is a diagram showing the magnetic field dependence of heat conduction.

FIG. 6 is a diagram showing the magnetic field dependence of the effective figure of merit.

FIG. 7 is a diagram illustrating an example of a wiring structure of a Nernst-Seebeck element.

FIG. 8 is a diagram showing an example of a wiring structure of a Nernst-Seebeck element using P-type and N-type semiconductors in one embodiment of the present invention.

[Explanation of symbols]

10 elements (sample) 11 electrodes 12 electrodes 13 Cable

Claims (9)

[Claims] A thermoelectric element having a temperature difference between both ends is cascaded in a plurality of stages, and a potential difference between both ends of a thermoelectric element having a relation of a former stage and a latter stage among the plurality of thermoelectric devices has an opposite polarity. The thermoelectric device is characterized in that, for each of the cables that cascade-connect the plurality of thermoelectric elements, both ends per cable are arranged on the same side of the high temperature side or the low temperature side with respect to the temperature. . 2. A temperature difference direction defines one of an X axis and a Y axis orthogonal to each other in one cross section of the thermoelectric element, and applies a magnetic field along a normal direction to the cross section. In the vicinity of one point of the outer periphery of the cross section corresponding to one end side of the one axis, and near other points of the outer periphery of the cross section located diagonally with respect to the one point and the cross section, The thermoelectric device according to claim 1, further comprising an electrode, wherein a combined voltage is obtained between the electrodes by a voltage based on a product of the magnetic field and the temperature difference and a voltage based on the temperature difference. apparatus. 3. A plurality of devices comprising a thermoelectric material and comprising at least first and second electrodes at positions opposing each other with a central portion of the device cross section on the side surface of the device, and extracting voltage. In this case, the first electrode of the element is disposed on a high-temperature side, the second electrode of the element is disposed on a low-temperature side, and the first electrodes of the former and subsequent elements are connected by a cable. The second electrodes of the elements at the front and the rear are connected by a cable, and for each of the cables connecting the electrodes between the elements, both ends of the cable are on either the high-temperature side or the low-temperature side. A thermoelectric device, wherein the temperature of both ends per one cable is the same. 4. A magnetic field is applied in a direction perpendicular to the cross section, and a voltage based on a product of the magnetic field and a temperature difference between the high temperature side and the low temperature side is applied between the first and second electrodes. A composite voltage is obtained by the voltage based on the temperature difference between the low side and the low side,
The thermoelectric device according to claim 3, wherein: 5. The device according to claim 3, wherein said first and second elements have polarities different from each other.
A thermoelectric device as described. 6. The element has a rectangular cross section, and the first and second electrodes have first and second electrodes defining a diagonal line of the rectangle.
The thermoelectric device according to claim 3, wherein the thermoelectric device is disposed on each of the element side surfaces near the second vertex. 7. The thermoelectric device according to claim 3, wherein the polarities of the plurality of elements are alternately different. 8. The thermoelectric device according to claim 3, wherein said first and second elements are P-type and N-type semiconductor elements. 9. A flat thermoelectric material having a predetermined thickness,
An element including at least first and second electrodes is provided on a side surface of the flat plate at a position opposed to each other via a center portion of the cross section of the flat plate. , Second
Electrodes are arranged on the high temperature side and the low temperature side, and a voltage based on the product of the magnetic field and the temperature difference between the high temperature side and the low temperature side, between the first and second electrodes; A thermoelectric device, wherein a combined voltage is extracted from the voltage based on the temperature difference of the thermoelectric device.