JP5511941B2 - Thermoelectric conversion element evaluation apparatus and evaluation method - Google Patents

Description

  The present invention relates to an apparatus for performing conversion efficiency and power evaluation of a thermoelectric conversion element in combination of power evaluation using a four-terminal method and penetration heat quantity evaluation using a flow calorimeter.

The thermoelectric material used for power generation loses about 2/3 of the consumed energy as waste heat, and is used to reuse this waste heat. This thermoelectric material generates an electromotive force by applying temperature to the upper and lower surfaces of the thermoelectric material, and takes out the electric power. And in the case of the power generation using this thermoelectric material, since it is possible to generate power by using only it, there is an advantage that it is maintenance-free and easy to miniaturize.
Thermoelectric materials are commercially available in the form of a multi-pair module composed of a plurality of thermoelectric conversion elements having different electrical characteristics. Evaluation of the conversion efficiency from thermal energy to electrical energy for this module is underway (for example, Patent Documents 1 and 2).
The measured value of the conversion efficiency of the module is almost always different from the conversion efficiency expected from the thermoelectric material alone constituting the module. This is because the temperature difference during evaluation of the thermoelectric material alone is smaller than the temperature difference during module evaluation. In addition, since the module is formed by bonding different types of materials such as p-type and n-type thermoelectric semiconductors and ceramics such as aluminum nitride and alumina, the influence of this bonding is evaluated on the thermoelectric material alone. It is known that the evaluation of thermoelectric conversion elements and modules is affected. Therefore, it is important to evaluate the conversion efficiency of the thermoelectric conversion elements before the multi-pairing.

  Usually, in the case of the conversion efficiency evaluation of a many-pair module composed of many pairs of elements, power load measurement and heat flow evaluation are easy. This is because the measurement in an environment where a heat flow is applied during measurement is because the electromotive force generated by the module is close to the volt order and the resistance of the module is close to the ohm order. On the other hand, also in the case of heat flow evaluation, the cross-sectional area of the heat channel is large, and a sufficient amount of through heat can be obtained. Using the heat flow evaluation method disclosed in Patent Document 1, it is possible to accurately measure the amount of through heat.

  However, the conversion efficiency evaluation using only the thermoelectric material or a pair of modules cannot use the conversion efficiency evaluation of the module. This is because the voltage generated from the thermoelectric material alone is on the order of millivolts, and the resistance of the module is on the order of milliohms. In addition, in the heat flow evaluation, since the cross-sectional area of the heat flow path is small and the amount of through heat is very small due to the structure of a single element or a pair, it is difficult to measure a minute heat flow with the heat flow evaluation method disclosed in Patent Document 1.

  Moreover, when evaluating using the apparatus disclosed by patent document 1, optimization is needed in order to perform cooling of a low temperature part, and temperature evaluation of a solvent simultaneously. However, in the case of a multi-pair module, since the cross-sectional area of the heat flow path is large, a large amount of heat of penetration flows, and if the cooling capacity of the low temperature part is increased, the circulation rate of the solvent becomes faster and the temperature evaluation of the solvent becomes difficult There is. Moreover, in order to easily evaluate the temperature of the solvent, there is a problem that if the circulation rate of the solvent is lowered, the cooling capacity is lowered. Therefore, it is difficult to optimize the cooling of the low temperature part and the temperature evaluation of the solvent in the multi-pair module, and the evaluation is made with a structure in which the heat flow evaluation part and the cooling part are separated.

JP 2004-296959 A JP 2005-302783 A

  Therefore, the present invention focuses on the fact that the heat passage has a small cross-sectional area in the case of a single thermoelectric material or only a pair of modules, so that the amount of through heat is small, eliminating the problems of many-pair modules and reducing the temperature. It is an object of the present invention to provide an evaluation method and apparatus capable of optimizing cooling of a part and temperature evaluation of a solvent.

In order to solve the above-described problems, the present inventors have found the following solution as a result of intensive studies.
That is, the first solving means of the thermal conversion element evaluation apparatus of the present application includes an upper block and a lower block made of a high thermal conductive material arranged so as to sandwich the thermoelectric conversion element to be evaluated, The upper block includes a temperature raising means and a temperature raising control means, a cooling means constituted by a liquid circulation path is connected to the lower block, and liquid is introduced to the lower block inlet side and the outlet side of the liquid circulation path. Temperature measuring means is provided, and electrodes for measuring voltage and supplying current are provided on the side of the upper block and the lower block in contact with the thermoelectric conversion element.
The second solving means is characterized in that, in the first solving means, the cross-sectional areas of the upper block and the lower block are reduced toward the thermoelectric conversion element side.
The third solving means is characterized in that, in the above-mentioned solving means, heat insulating plates are arranged adjacent to each other at a predetermined interval along the longitudinal direction of the upper block on both sides of the upper block.
The fourth solving means is characterized in that, in the above-mentioned solving means, the liquid circulation path includes flow rate control means for controlling the flow rate of the liquid .

  According to the present invention, the conversion efficiency of the thermoelectric conversion element is measured by measuring the amount of heat passing through the thermoelectric conversion element via the liquid temperature of the liquid circulation path. It is possible to evaluate a single thermoelectric conversion element.

Explanatory drawing of the apparatus of one embodiment of this invention Explanatory drawing of the measurement parameter in the evaluation method of one embodiment of this invention Explanatory drawing of the measurement parameter in the evaluation method of other embodiment of this invention The graph which shows the evaluation result of an Example The graph which shows the evaluation result of an Example The graph which shows the evaluation result of an Example

As the thermoelectric conversion element to be evaluated in the present invention, for example, a BiTe-based or FeSi-based thermoelectric material having a rectangular parallelepiped shape with a side of 2 to 4 mm is used.
On the upper and lower surfaces of the thermoelectric conversion element, electrodes made of a material having high conductivity ( ¹ MS · m ⁻¹ or more) and low Seebeck coefficient (± 20 μV · K ⁻¹ or less) such as copper or gold are provided. Is configured as a measurement sample.

The measurement sample 1 is attached to the evaluation device 2 shown in FIG. 1 for evaluation.
The evaluation apparatus 2 shown in the figure has a temperature riser for controlling the temperature of the temperature raising means in the chamber 4 to which the vacuum pump 3 is connected by controlling the temperature raising means such as a heater and the energization amount to the temperature raising means. An upper block 5 provided with a temperature control means (not shown), and a lower block 8 provided with a cooling means 6 to which a liquid circulation path is connected and liquid temperature measuring means 7a and 7b are provided. Outside the chamber 3 of the liquid circulation path 6, a pump 9 for controlling the flow rate of the liquid per unit time is provided. The liquid temperature measuring means 7a and 7b are constituted by a thermocouple or a resistance thermometer.
The upper block 5 is made of a material having good thermal conductivity such as aluminum nitride, copper, and aluminum, and the sample 1 is arranged to give a sufficient heat flow to the thermoelectric conversion element (sample) 1 which is a small sample piece. The cross-sectional area is reduced toward the side. A temperature measuring means (not shown) for the upper block 5 such as a thermocouple or a resistance thermometer and an electrode 10 are provided on the surface where the upper block 5 and the sample 1 are in contact. Further, on both sides in the longitudinal direction of the upper block 5, heat shield plates 11 are arranged adjacent to each other at intervals in order to prevent heat propagation due to radiation from the lower block 8.
The lower block 8 is made of a material having good thermal conductivity, such as aluminum nitride, copper, and aluminum, in the same manner as the upper block 4 in order to measure the amount of through heat conducted through the upper block 5 and the sample 1. And like the upper block 5, it is comprised so that a cross-sectional area may become small toward the side by which the sample 1 is arrange | positioned. Further, a temperature measuring means (not shown) for the lower block 5 such as a thermocouple or a resistance thermometer and an electrode 10 are provided on the side in contact with the sample 1. Further, the lower block 8 is provided on a heat insulating structure such as foamed polystyrene, and is configured to block heat from other than the liquid circulation path 6.
The sample 1 is disposed between the electrodes 10 provided in the upper block 5 and the lower block 8, and an AC or DC current input and voltage for measuring resistance by a four-terminal method are applied to these electrodes 10. Measurement is performed.

  In the apparatus 2, a part of the heat from the upper block 5 is converted into electric power by the measurement sample 1, and the remaining heat, that is, the heat passing through the measurement sample 1 is conducted to the lower block 8. The penetration heat quantity is measured by the liquid temperature measuring means 7a and 7b provided in the liquid circulation path 6 which is a cooling means connected to the lower block 8. According to this configuration, the flow rate of the liquid in the liquid circulation path 6 can be adjusted by the pump 9 to obtain an optimum measurement condition, and the efficiency of the thermoelectric element itself, which is smaller than that of many pairs of thermoelectric elements, can be evaluated. it can.

Next, a method of evaluating using the apparatus 2 having the above configuration will be described with reference to FIG.
First, the inside of the chamber 4 is set to a vacuum atmosphere of 10 Pa or less.
The temperature T _a [K] on the upper surface of the measurement sample 1 and the temperature T _b [K] on the lower surface are measured by the temperature measuring means provided in each block. Next, the temperature of the upper block 5 is controlled to T _u [K] by the temperature increase control means to increase the temperature, and the flow rate is adjusted to the lower block 8 by the pump 9 to adjust the liquid at the speed v [ml · s ⁻¹ ]. The liquid (water) is allowed to flow through the circulation path 6 and the difference between the temperature T _in [K] at the inlet 7a and the temperature T _out [K] at the outlet 7b into the lower block 8 of the liquid (water) is determined as the liquid temperature measuring means 7a, 7b. Measure by T _u [K] is not particularly limited, but is preferably 100 ° C. or higher. Further, in the case of the measurement sample 1 having the shape described above, the flow rate is preferably 1/10 to 10 [ml · s ⁻¹ ], and the upper block temperature is 150 ° C. or less in the temperature range of 1/6 to 1/2. [ml · s ⁻¹ ] is preferred.

The conversion efficiency η of the measurement sample 1 into electric power is obtained by the following formula based on the result of the penetration heat quantity Q _s [W] of the measurement sample 1 and the generated power Q _e [W]. The penetration heat quantity Q _s [W] is the quantity of heat conducted from the upper block 5 to the lower block 8, and the temperature _Tu [K] of the upper block 5 and the temperature change amount of the liquid passing through the lower block 8 ( T _out [K] −T _in [K]), and a specific method for calculating the amount of heat will be described later.
η = Q _e / (Q _s + Q _e ) (Formula 1)
Further, the maximum value η _max of the conversion efficiency of the measurement sample 1 into power is calculated using the maximum power [W] when the through heat quantity Q _s [W] takes the maximum value.
Note that the penetration heat quantity Q _s [W] is measured when the upper block 5 is heated and controlled at a constant temperature and no current for measurement is supplied.

The amount of heat Q _w [W] of the lower block 8 is calculated based on the following formula 2.
T _out -T _in ∝Q _w / C ・ 1 / ν ・ ・ ・ (Formula 2)
In the above formula 2, C [J · m ⁻³ · K ⁻¹ ] is the volume specific heat capacity of the liquid circulating in the liquid circulation path 6.

The power evaluation (Q _e = V · II ² · R) is performed by changing the input current I [A] to maximize the electric power, and the voltage V [V] provided to each electrode 10 and the input Assume that the current I [A] is substituted into the equation (Q _e = V · II ² · R).
In the case of direct current measurement, the electric resistance R is obtained from the current dependency of the voltage under a constant current closed circuit. However, the voltage is corrected by the upper surface temperature of the sample 1, the lower surface temperature of the sample 1, and the Seebeck coefficient. In the case of AC measurement, it is obtained from the voltage under a closed circuit in which the current is periodically changed. If there is frequency dependence, correction is also performed. Further, the Seebeck coefficient is evaluated by the upper surface temperature of sample 1 and the lower surface temperature and voltage of sample 1 when no current is supplied.

As described above, the heat consumption of the thermoelectric conversion element is obtained through the through heat quantity Q _s [W] that is consumed by conduction from the upper block 5 to the thermoelectric conversion element, and the electromotive force and electric resistance for this are obtained. Thus, the conversion efficiency of the thermoelectric conversion element can be measured.

Further, when the penetration heat quantity Q _s [W] of the sample 1 whose physical properties are unknown is measured based on the temperature difference measured by the liquid temperature measuring means 7a, 7b, or evaluated by the following method. .

In the apparatus configuration shown in FIG. 3, the auxiliary plate 13 made of a high thermal conductivity material such as aluminum nitride / sapphire on the bottom surface of the upper block 5 and the auxiliary plate 14 made of the same material on the upper surface of the lower block 8 The auxiliary plate 13 of the upper block 5 is provided with a common electrode 16 for the measurement sample 1 and the reference sample 15, and the upper surface of the lower block 8 is referred to the electrode 17 for the measurement sample 1 with a space therebetween. An electrode 18 for the sample 15 is provided. In addition, a reference material having a known physical property value for comparison with the measurement sample is formed in the same shape as the thermoelectric conversion element, and the electrode and the thermal conductivity are 10 W · m ⁻¹ · K ⁻¹ or more. An auxiliary plate having a thickness of 1 mm or less is provided to constitute a reference sample. The auxiliary plate 13 is provided on the upper block 5 and the lower block 8 in order to make the conditions of the portion not covered with the electrode and the portion covered with the electrode the same.
Then, by comparing the measurement result of the calorific value of the lower block 8 using the standard sample with the measurement result of the measurement sample 1, the penetration heat quantity Q _s [W] of the measurement sample 1 is evaluated. Specifically, as shown in ASTM E 1530, heat flow is measured for a standard sample, and a calibration equation is derived from the thermal resistance (thickness / thermal conductivity) and heat flux (through heat / cross-sectional area) of the standard sample. Evaluate unknown samples. Using this, not only the thermal conductivity of an unknown sample is obtained, but also a one-dimensional heat transfer model (Qs / A = λ (Tb-Ta) / d A: cross section, λ: thermal conductivity, d: thickness The true heat of penetration can also be evaluated based on

Next, a measurement example using the above-described apparatus will be described.
Example 1
As a measurement sample, n-Bi ₂ Te ₃ (4 × 4 × 4 mm) was used.
(Example 2)
As a measurement sample, p-Bi _0.3 Sb _1.7 Te ₃ (4 × 4 × 4 mm) was used.

In FIG. 4, the horizontal axis is the flow velocity of the liquid in the liquid circulation path 6, and the vertical axis is the temperature T _in [K] of the inlet 7a of the liquid (water) to the lower block 8 measured by the liquid temperature measuring means 7a, 7b. And the difference between the temperature T _out [K] at the outlet 7b is plotted ((a) Example 1 (b) Example 2).
FIG. 5 plots the horizontal axis as the direct current applied to the measurement sample 1 and the vertical axis as the voltage measured from the measurement sample 1 ((a) Example 1 (b) Example 2).
In FIG. 6, the horizontal axis is plotted as the current input to the measurement sample 1, and the vertical axis is plotted as the conversion efficiency measured from the measurement sample 1 ((a) Example 1 (b) Example 2).

Tables 1 and 2 below show the maximum conversion efficiency (η _max ), Seebeck coefficient (S / μV · K ⁻¹ ), and electrical resistivity (ρ / μΩ · m) and thermal conductivity (λ / Wm ⁻¹ · K ⁻¹ ).

  As described above, in the present example, it was found that a highly accurate measurement of power conversion efficiency was possible even for a thermoelectric material of 4 mm square.

DESCRIPTION OF SYMBOLS 1 Measurement sample 2 Evaluation apparatus 3 Vacuum pump 4 Chamber 5 Upper block 6 Cooling means (liquid circulation path)
7a, 7b Liquid temperature measuring means 8 Lower block 9 Pump 10 Electrode 11 Heat shield plate 13, 14 Auxiliary plate 15 Reference sample 16, 17, 18 Electrode

Claims (4)

An upper block and a lower block made of a high thermal conductivity material arranged so as to sandwich a thermoelectric conversion element to be evaluated are provided, and the upper block includes a temperature raising means and a temperature raising control means, and the lower block Is connected to cooling means constituted by a liquid circulation path, and includes liquid temperature measurement means on the inlet side and outlet side of the lower block of the liquid circulation path, and the thermoelectric conversion elements of the upper block and the lower block A device for evaluating a thermoelectric conversion element, comprising an electrode for measuring voltage and supplying current on a side in contact with the thermoelectric conversion element.   2. The thermoelectric conversion element evaluation apparatus according to claim 1, wherein cross-sectional areas of the upper block and the lower block are configured to decrease toward the thermoelectric conversion element.   The thermoelectric conversion element evaluation apparatus according to claim 1 or 2, wherein heat insulating plates are arranged adjacent to each other on both sides of the upper block at a predetermined interval along the longitudinal direction of the upper block. Said liquid circulation path is evaluated equipment of the thermoelectric conversion element according to claim 1, characterized in that it comprises a flow rate control means for controlling the flow rate of the liquid.

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