A liquid metal seebeck coefficient measurement system and method based on combination potential method

Abstract

A liquid metal Seebeck coefficient measuring device and method based on combination potential method, using a T-shaped welded thermocouple to directly contact with liquid metal, forming good electrical contact, applying temperature difference through bottom cooling and upper heating, the cooling and heating mode is water bath temperature control, the liquid metal measuring section is placed in a constant temperature cover to control the temperature during the whole measuring process, using the method of common negative electrode to measure the thermoelectric potential and Seebeck potential of all thermocouples, the potential difference between the copper wire and the constantan wire of a thermocouple is the thermoelectric potential, the potential difference between the same materials of any two thermocouples is the Seebeck potential, without obtaining temperature difference, only measuring potential, a physical quantity, can calculate the Seebeck coefficient of the material.

Description

Technical Field

[0001] This invention relates to the field of liquid metal thermophysical property measurement technology, and in particular to a device and method for measuring the Seebeck coefficient of liquid metal based on the combined potential method, for measuring the Seebeck coefficient of any liquid metal under unknown temperature conditions. Background Technology

[0002] The Seebeck effect is a thermoelectric phenomenon in which the voltage difference between two materials is caused by the temperature difference between two different electrical conductors or semiconductors. The Seebeck coefficient is a physical quantity that measures the intensity of the Seebeck effect, defined as the ratio of the temperature to the potential difference between two points. The thermoelectric effect has many applications, such as waste heat recovery from engines, remote power generation, integrated circuit refrigeration, and solid-state refrigeration. Meanwhile, in the field of liquid metals, liquid metals have important applications in fusion devices due to their excellent electrical and thermal conductivity and fluidity. Faced with the extreme conditions of strong magnetic fields and high temperature gradients in fusion devices, the Seebeck effect generated inside liquid metals cannot be ignored. The Seebeck potential caused by the Seebeck effect can seriously affect the measurement of the velocity potential inside liquid metals, as demonstrated in the research of Weissenfluh and Davoust et al. To decouple the velocity potential and Seebeck potential of liquid metals, it is first necessary to accurately measure the Seebeck coefficient of liquid metals.

[0003] Existing Seebeck coefficient measurement devices all measure solid metal samples and calculate the material's Seebeck coefficient by measuring only the potential difference between a single electrode. This requires measuring the temperature at the two electrode locations and using the ratio of potential difference to temperature difference for calculation. Currently mature commercial measurement systems, such as... Figure 1 As shown; the entire testing system is equipped with an infrared furnace to change the temperature of the sample environment, ensuring that the entire sample is in a uniform temperature environment and allowing for coefficient measurements under different temperature conditions; the Seebeck coefficient of the sample is calculated using the traditional differential principle, and thus the Seebeck coefficient can be expressed as... The accuracy of the coefficient measurement at room temperature is within ±5%. The test sample is made into a strip and clamped between two platinum electrodes. A temperature difference is applied by heating the bottom. The temperature difference and potential difference are measured using two standard Pt-Pt+10%Rh thermocouples. The Seebeck coefficient of the sample can be expressed by the ratio of the potential difference to the temperature difference. Good thermoelectric contact is ensured between the thermocouples and the sample. The potential difference measured in this device is only the potential difference between the two platinum electrodes. The entire measurement process is carried out in a helium environment. The Seebeck coefficient is calculated using the single-point method and the slope method.

[0004] Similar measuring devices are only suitable for measuring the Seebeck coefficient of solid metal samples. On the one hand, due to the good fluidity of liquid metal, it is impossible to make strip-shaped samples and clamp them between fixed blocks. On the other hand, applying a temperature difference inside liquid metal by heating from the bottom and cooling from the top will create thermal convection, resulting in a very small actual temperature difference. Only by heating from the top and cooling from the bottom can the liquid metal form a stratified flow, thus creating a larger temperature difference. In addition, current Seebeck measurement systems require the measurement of two physical quantities, temperature difference and potential difference, when calculating the Seebeck coefficient. The measurement of these two quantities will introduce errors, leading to deviations in the calculated coefficient results. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the present invention aims to provide a device and method for measuring the Seebeck coefficient of liquid metal based on the combined potential method. This method uses T-type welded thermocouples in direct contact with the liquid metal to form a good electrical contact. A temperature difference is applied by cooling the bottom and heating the top. The thermoelectric potential and Seebeck potential of all thermocouples are measured using a common negative electrode method. The potential difference between the copper and constantan wires of a thermocouple is the thermoelectric potential, and the potential difference between any two thermocouples made of the same material is the Seebeck potential. Without needing to obtain the temperature difference, the Seebeck coefficient of the material can be calculated simply by measuring the potential.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A liquid metal Seebeck coefficient measurement system based on the combined potential method includes multiple T-shaped thermocouples arranged on the wall of an plexiglass cavity 1. Liquid metal 12 is contained inside the plexiglass cavity 1. The solder joints of copper wire 9 and constantan wire 10 in the thermocouples are in contact with the liquid metal 12.

[0008] All thermocouples are divided into two groups, and the distance between the two groups of thermocouples is half the height of the plexiglass cavity 1;

[0009] A single copper wire 8 is used as the common negative terminal of all thermocouples, with one end in contact with the liquid metal 12 and the other end directly connected to the negative terminal of the data acquisition system 13.

[0010] All the copper wires 9 and constantan wires 10 of the thermocouples are connected to the positive terminal of the data acquisition system 13 after passing through the zero-degree constant temperature bath 14.

[0011] The data acquisition system 13 is a voltage acquisition instrument with a precision of six and a half digits;

[0012] The organic glass cavity 1 is provided with a heating top plate 2 and a cooling bottom plate 3 on the outer side of the upper and lower walls, and the whole is placed in a water bath insulation cover 7.

[0013] The plexiglass cavity 1 is disposed on the rectangular hollow tube 5 in the copper basin 4.

[0014] The copper basin 4 is mounted on the leveling plate 6.

[0015] The organic glass cavity 1 is wrapped with multiple layers of thermal insulation cotton.

[0016] The heating top plate 2 and cooling bottom plate 3 are made of copper.

[0017] A method for measuring the Seebeck coefficient of liquid metal based on the combined potential method includes the following steps:

[0018] Two thermocouples from two groups are randomly selected and brought into electrical contact with liquid metal. The temperatures at the solder joints are T1 and T2, respectively, and T0 is the reference temperature of the thermocouples. The potential difference ΔV between the copper wires of the two thermocouples is obtained using the common negative terminal method. Cu for:

[0019] ΔV Cu =S Cu (T0-T2)+S L (T2-T1)+S Cu (T1-T0)=(S Cu -S L (T1-T2) (1)

[0020] At the same time, it is also possible to obtain the potential difference ΔV between the two thermocouple constantan wires. CuNi ,for

[0021] ΔV CuNi (=S CuNi (T0-T2)+S L (T2-T1)+S CuNi (T1-T0)=(S CuNi -S L (T1-T2) (2)

[0022] Combining equations (1) and (2), by eliminating the temperature difference (T1-T2) through the ratio of potential differences, the relationship between the ratio of Seebeck coefficients and the potential difference is obtained:

[0023]

[0024] Since the Seebeck coefficients of copper and constantan are known, the Seebeck coefficient S of the liquid metal can be calculated simply by measuring the potential difference between the copper wires and the constantan wires. L for:

[0025]

[0026] Since the data acquisition system 13 introduces measurement errors during the actual measurement process, and these errors in this method originate from voltage measurement errors, the coefficient expression for the combined potential method, which considers measurement errors, is as follows:

[0027]

[0028] Wherein, δ(ΔV Cu ) represents the error when measuring the potential difference between copper wires, δ(ΔV) CuNi The error is the measurement error when measuring the potential difference between constantan wires.

[0029] Compared with the prior art, the advantages of the present invention are:

[0030] 1. The system of the present invention is bottom-cooled and top-heated. This temperature difference application method is different from the existing commercial measurement system and overcomes the heat convection phenomenon caused by bottom heating and top cooling or horizontal application of temperature difference.

[0031] 2. The system of this invention uses a common negative electrode for measurement, which can obtain the potential difference between any two single wires of the same material and two single wires of different materials. This also allows for the acquisition of raw data from the single-point method and the slope method, making it applicable to existing methods. Furthermore, the combined potential method proposed in this invention is a novel and highly accurate calculation method.

[0032] 3. In the existing commercial measurement system, the temperature difference between two thermocouples and the potential difference between a single electrode are required to calculate the Seebeck coefficient of the sample. The combined potential method measurement system of the present invention can calculate the Seebeck coefficient of liquid metal without accurately measuring the temperature difference between two thermocouples, and its measurement accuracy is the same as that of the traditional single-point method and slope method.

[0033] 4. This invention establishes a novel coefficient measurement theory: the combined potential method, which uses multiple pairs of thermocouples in two groups at different positions to measure the coefficient, thus eliminating the influence of thermocouple defects on coefficient measurement. Attached Figure Description

[0034] Figure 1 It is an existing Seebeck coefficient measurement device for solid metal samples.

[0035] Figure 2 This invention relates to a Seebeck coefficient measurement system for liquid metals.

[0036] Figure 3 This is a schematic diagram illustrating the measurement principle of the combined potential method. Detailed Implementation

[0037] The present invention will now be described in detail with reference to the accompanying drawings.

[0038] Reference Figure 2 A Seebeck coefficient measurement system for liquid metal based on the combined potential method includes eight T-type thermocouples T1-T8 arranged on the wall 11 of an plexiglass cavity 1 containing liquid metal 12. Each thermocouple has a diameter of 1.0 mm. The solder joints of the copper wire 9 and constantan wire 10 in the thermocouples are in contact with the liquid metal 12. The formation of the solder joints facilitates good thermal and electrical contact with the liquid metal. Every four T-type thermocouples are grouped together, and the distance between two groups of thermocouples is 63 mm, which is half the height of the entire cavity, to ensure sufficient contact between the two thermocouples. Large temperature difference; a single copper wire 8 is used as the common negative terminal of all thermocouples, extending from the central hole at the top of the cavity to contact the liquid metal 14, to measure the potential difference of each individual thermocouple wire relative to this common negative terminal, thereby obtaining the temperature difference potential and Seebeck potential between any two individual wires; all the copper wires 9 and constantan wires 10 of the thermocouples are connected to the positive terminal of the data acquisition system 13 after passing through the zero-degree constant temperature bath 14, while the other end of the single copper wire serving as the common negative terminal is directly connected to the negative terminal of the data acquisition system 13 without passing through the zero-degree constant temperature bath 14.

[0039] The upper and lower outer walls of the acrylic glass cavity 1 are provided with a heating top plate 2 and a cooling bottom plate 3 to ensure that the acrylic glass cavity 1 has a constant temperature difference. The whole is placed in a water bath insulation cover 7, which reduces the heat loss of liquid metal in the acrylic glass cavity 1.

[0040] The plexiglass cavity 1 is mounted on the rectangular hollow tube 5 in the copper basin 4. The copper basin 4 and the rectangular hollow tube 5 are both designed to prevent heat from being conducted downwards, thus preventing the contact material of the cooling base plate 3 from carrying away too much heat.

[0041] The copper basin 4 is mounted on the leveling plate 6, which can adjust the overall levelness.

[0042] The heating top plate 2 and cooling bottom plate 3 are made of copper to ensure good temperature uniformity. A constant temperature water bath is used to control the temperature of the two plates. Through a 3-hour program adjustment, the upper and lower plates reach the set temperature.

[0043] To prevent heat loss, multiple layers of insulation cotton are first wrapped around the outside of the plexiglass cavity 1, and then it is placed in a copper water bath insulation cover. The insulation cover maintains the ambient temperature of the entire cavity at a uniform temperature between the upper and lower plates by regulating the temperature of the constant temperature water bath.

[0044] The working principle of the device of the present invention is as follows:

[0045] like Figure 2As shown, the temperatures of the heating top plate 2 and cooling bottom plate 3 are controlled by a program-regulated water chiller. First, a set temperature is given to the water chiller. High-precision thermistors are installed on the heating top plate 2 and cooling bottom plate 3 to detect the temperatures of the top and bottom plates. The measured temperature is compared with the set temperature, and the water chiller temperature is adjusted again. Through this adjustment, the heating top plate 2 and cooling bottom plate 3 reach the preset temperature, thus ensuring precise temperature difference application. Eight thermocouples are arranged on the walls of the plexiglass cavity. The thermocouple solder joints are in direct contact with the liquid metal 12 to form good electrical contact. The wall openings are sealed with silicone rubber to prevent liquid metal leakage. All thermocouples have a total of 16 single wires connected to 16 data acquisition points via a common negative terminal. Before being connected to the data acquisition system, all single wires first pass through a zero-degree constant temperature bath 14, which ensures that the measured thermocouple potential is the standard thermocouple potential. By subtracting the data from two designated channels, the potential difference between any two single wires can be obtained. All thermocouples are divided into two groups, with a distance of 63mm between the two groups to ensure that the temperature difference between each pair of thermocouples is large enough. Two thermocouples at the same position in the two groups form a pair of thermocouples, forming a total of 4 pairs of thermocouples. Each pair of thermocouples can obtain the raw data for calculating the Seebeck coefficient of liquid metal. By comparing the calculation results of the 4 pairs of thermocouples, the measurement error caused by the defects of the thermocouples themselves is eliminated.

[0046] A method for measuring the Seebeck coefficient of liquid metal based on the combined potential method includes the following steps:

[0047] Two thermocouples from two groups are selected, and they have good electrical contact with the liquid metal. The temperatures at the solder joints are T1 and T2, respectively, and T0 is the reference temperature of the thermocouples. The potential difference ΔV between the copper wires of the two thermocouples is obtained by using the common negative terminal method. Cu for:

[0048] ΔV Cu =S Cu (T0-T2)+S L (T2-T1)+S Cu (T1-T0)=(S Cu -S L (T1-T2) (1)

[0049] It can also obtain the potential difference ΔV between the two thermocouple constantan wires. CuNi ,for

[0050] ΔV CuNi =S CuNi (T0-T2)+S L (T2-T1)+S CuNi (T1-T0)=(S CuNi -S L(T1-T2) (2)

[0051] Combining equations (1) and (2), the temperature difference (T1-T2) can be eliminated by the ratio of potential differences, thus obtaining the relationship between the ratio of Seebeck coefficients and the potential difference:

[0052]

[0053] The Seebeck coefficient of liquid metal can be directly expressed by equation (3). Since the Seebeck coefficients of copper and constantan are known, the Seebeck coefficient of liquid metal can be calculated simply by measuring the potential difference between the copper wires and the constantan wires. It is:

[0054]

[0055] In actual measurement processes, due to voltage bias in the acquisition instrument itself or defects in the thermocouple, unavoidable errors will exist in the measured values. The expression for the coefficients of the combined potential method that include the measurement error is as follows:

[0056]

[0057] Wherein, δ(ΔV C u) and δ(ΔV CuNi These represent the errors when measuring the potential difference between copper wires and the potential difference between constantan wires, respectively.

[0058] In this invention, the Seebeck coefficient of liquid metal is calculated based on the combined potential method. Compared to the traditional single-point method and slope method, its advantage lies in that it eliminates the need to measure the temperature difference between the two thermocouples; the coefficient can be calculated simply by measuring the potential difference between the copper wires and the constantan wires of the two thermocouples. Figure 3 The theory of the combined potential method is established by two thermocouples. Taking a pair of thermocouples as an example, the theoretical principle of the combined potential method is explained, and the theoretical principles of the single-point method and the slope method are compared.

[0059] Equations (1)-(5) are the theoretical derivation of the Seebeck coefficient measurement method for liquid metal based on the combined potential method.

[0060] Compared to the combined potential method, the theoretical derivation processes of the existing single-point method and slope method are as follows:

[0061] In the process of using the single-point method and the slope method, it is necessary to measure the temperature difference (T1-T2) between the two thermocouples. Therefore, the thermocouples need to be calibrated first. The temperature at the location of each thermocouple is calculated from the calibration results. This temperature is obtained by converting the measured thermoelectric potential, that is, the potential difference between the copper wire and constantan wire of each thermocouple, into: From equation (1):

[0062]

[0063] The Seebeck coefficient S of the liquid metal is calculated using equation (6). L The single-point method and the slope method share the same theoretical basis; the difference lies in that the single-point method can calculate an S using a single data acquisition result. L The slope method, on the other hand, obtains the slope of a straight line by fitting multiple data points, and uses this slope as the Seebeck coefficient S of the liquid metal. L .

[0064] The measurement errors in this method mainly come from voltage measurement errors and temperature difference measurement errors. Therefore, the coefficient expressions for the single-point method and slope method, which include measurement errors, are as follows:

[0065]

[0066] Where δ(ΔT) is the error when measuring the temperature difference.

[0067] Equations (1)-(5) are the derivation process of the liquid metal Seebeck coefficient measurement method based on the combined potential method, and equations (6)-(7) are the derivation process of the liquid metal Seebeck coefficient measurement method based on the single-point method and the slope method. Comparing equations (4) and (6), the essential difference between the combined potential method and the single-point method and the slope method can be clearly seen. The combined potential method only needs to measure the potential difference between the same materials, and the temperature difference measurement error has no effect on the method. However, the single-point method and the slope method not only need to measure the potential difference, but also need to measure the temperature difference between the two thermocouples, resulting in two kinds of measurement errors. The combined potential method, the single-point method and the slope method include the coefficient expressions of the error as shown in equations (5) and (7). The measurement error introduced in the process of using the single-point method and the slope method is more.

Claims

1. A measurement method of a liquid metal Seebeck coefficient measurement system based on a combination potential method, the measurement system based on comprising a plurality of T-shaped thermocouples arranged on the wall surface of a plexiglass cavity (1), the plexiglass cavity (1) being filled with a liquid metal (12), the welding points of copper wires (9) and constantan wires (10) in the thermocouples being in contact with the liquid metal 12; all the thermocouples are divided into two groups, the distance between the two groups of thermocouples being half the height of the plexiglass cavity 1; A single copper wire (8) is used as the common negative electrode of all the thermocouples, one end of which is in contact with the liquid metal (12) and the other end is directly connected to the negative electrode of the data acquisition system (13); The copper wires (9) and constantan wires (10) of all the thermocouples are connected to the positive electrode of the data acquisition system (13) after passing through a zero-degree constant-temperature tank (14); The data acquisition system (13) is a voltage acquisition instrument with a precision of six and a half bits; The outer sides of the upper and lower walls of the plexiglass cavity (1) are provided with a heating top plate (2) and a cooling bottom plate (3), and the whole is placed in a water bath heat preservation cover (7); characterized in that The measurement method comprises the following steps: Select any two thermocouples in the two groups and liquid metal exists good electrical contact, the temperature of the welding point is T1 and T2, T0 is the reference temperature of the thermocouple, by the method of common negative electrode, the potential difference between the two thermocouple copper wires is obtained ΔV Cu is: ΔV Cu = S Cu (T0-T2) + S L (T2-T1) + S Cu (T1-T0) = (S Cu - S L )(T1-T2) (1) At the same time, the potential difference AV between the two thermocouple wires can also be obtained CuNi is: ΔV CuNi = S CuNi (T0-T2) + S L (T2-T1) + S CuNi (T1-T0) = (S CuNi - S L )(T1-T2) (2) Combining formula (1) with formula (2), the temperature difference (T1-T2) is eliminated by the ratio of the potential difference, and the relationship between the ratio of the Seebeck coefficient and the potential difference is obtained: Since the Seebeck coefficient of copper and constantan is a known quantity, it is only necessary to measure the potential difference between the copper wires and the potential difference between the constantan wires to calculate the Seebeck coefficient S of the liquid metal L is:

2. The measurement method according to claim 1, characterized in that, Since the data acquisition system (13) will introduce measurement errors in the actual measurement process, the measurement error under this method is due to the measurement error of the voltage, so the combination potential method considers the coefficient expression of the measurement error: where δ(ΔV Cu ) is the error in measuring the potential difference between copper wires, and δ(ΔV CuNi ) is the error in measuring the potential difference between constantan wires.

3. The measurement method according to claim 1, characterized in that, The plexiglass cavity (1) is arranged on a rectangular hollow tube (5) in a copper basin (4).

4. The measurement method according to claim 1, characterized by, The copper basin (4) is arranged on a horizontal adjusting plate (6).

5. The method of claim 1, wherein, The plexiglass cavity (1) is wrapped with multiple layers of thermal insulation cotton.

6. The measurement method of claim 1, wherein, The heating top plate (2) and the cooling bottom plate (3) use red copper material.

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