Measuring device and method for measuring the thermoelectric properties of a sample

Abstract

Measuring instrument for measuring thermoelectric properties of a sample, comprising a first electrode arm (14) and a second electrode arm (15) designed to hold the sample (16) between them and introduce an electric current into the sample, and comprising a first sensor (19) and a second sensor (20) for contacting the surface of the sample (16), characterized in that a thermal choke (22) is arranged between the first electrode arm (14) and the sample (16).

Description

[0001] The invention relates to a measuring device for measuring the thermoelectric properties of a sample. The measuring device comprises a first electrode arm and a second electrode arm. The electrode arms are designed to hold the sample between them and introduce an electrical current into the sample. The measuring device comprises a first measuring sensor and a second measuring sensor, which are designed for contact with the surface of the sample.

[0002] Measuring devices of this type are known in which both electrode arms rest directly on the sample via large contact surfaces. An intensive heat exchange takes place between the sample and the electrode arms via these contact surfaces. This is desirable for determining certain thermoelectric quantities (e.g., the Seebeck coefficient), where a fixed temperature gradient must be maintained across the sample.

[0003] Such a measurement setup does not allow the direct determination of the dimensionless figure of merit ZT, which is a measure of the efficiency with which a material can convert heat into electrical energy. This would require adiabatic conditions, which are not present when a significant amount of heat flows from the sample into the first electrode arm.

[0004] The invention is based on the object of presenting a measuring device and an associated method with an expanded range of applications. Based on the cited prior art, this object is achieved with the features of the independent claims. Advantageous embodiments are specified in the subclaims.

[0005] In the measuring device according to the invention, a heat choke is arranged between the first electrode arm and the sample piece.

[0006] The heat choke according to the invention minimizes heat transfer between the sample and the first electrode arm. This makes it possible to perform thermoelectric measurements without the measurement result being distorted by heat transfer between the sample and the first electrode arm. This opens up a new area of ​​application for the measuring device according to the invention. In particular, it makes it possible to directly measure the dimensionless figure of merit (ZT).

[0007] The dimensionless figure of merit ZT is a measure of the efficiency with which a material can convert heat into electrical energy. The dimensionless figure of merit ZT is proportional to the square of the Seebeck coefficient S, the electrical conductivity σ, and the temperature T, and inversely proportional to the thermal conductivity κ of the material.

[0008] The electric current between the sample and the first electrode arm flows through the heat choke. The heat choke should therefore have good electrical conductivity. There are several options for the design of the heat choke. For example, it is possible to place a needle-shaped element between the first electrode arm and the sample. Such a needle-shaped element is suitable for introducing an electric current into the sample. On the other hand, the cross-sectional area is so small that no significant heat transfer occurs.

[0009] A needle-shaped element has the additional property of being able to transmit compressive forces along its length. The measuring device can therefore be designed so that the needle-shaped element is also used to hold the sample in position. The electrode arm can thus exert a force on the sample via the needle-shaped element to hold it in position. In particular, the electrode arm can be designed so that the needle-shaped element rests on the sample in a spring-loaded manner.

[0010] In an alternative embodiment, the heat choke is designed as a wire. The wire is flexible and therefore not suitable for transmitting forces in the longitudinal direction. Furthermore, due to its small cross-section, the wire is suitable for introducing electrical currents into the sample in a similar way to the needle-shaped element, while keeping heat transfer low.

[0011] The electrical contact resistance between the heat choke and the sample piece should be low in order to allow the electrical current to pass as freely as possible from the electrode arm into the sample piece. For this purpose, a transition piece made of an electrically conductive material can be arranged between the heat choke and the sample piece. The transition piece preferably lies on the sample piece over a large area. This means that the contact surface between the transition piece and the sample piece is significantly larger than the cross-sectional area of ​​the heat choke, for example, at least by a factor of 5, preferably at least by a factor of 10, more preferably at least by a factor of 20 larger than the cross-sectional area of ​​the heat choke. The cross-sectional area of ​​the heat choke can, for example, be between 0.0002 mm 2 and 0.02 mm 2The transition part can be permanently connected to the heat choke or be present as a separate part that is positioned between the heat choke and the sample piece when the first electrode arm is brought towards the sample piece. In both cases, it is advantageous if the transition part is pressed against the sample piece to reduce the contact resistance. The transition part can be a copper sheet, for example. An electrically conductive thermal paste can be arranged between the transition part and the sample piece. In an alternative embodiment, a direct connection is made between the heat choke and the sample piece, for example by connecting the heat choke to the sample piece by welding or soldering.

[0012] Direct thermal contact can be established between the second (other) electrode arm and the sample piece via a large contact area, ensuring good heat transfer between the second electrode arm and the sample piece, as well as good heat transfer within the second electrode arm. Furthermore, the second electrode arm can be designed as a support on which the sample piece rests during the measurement. The invention also encompasses embodiments in which the second electrode arm is also equipped with a heat choke.

[0013] The measuring device according to the invention preferably comprises a device for bringing the second electrode arm to a temperature different from the ambient temperature. In particular, a heating device can be provided for the second electrode arm, so that the second electrode arm can be brought to a temperature higher than the ambient temperature. This can be used to heat an end of the sample piece adjacent to the electrode arm. This makes it possible to generate a temperature gradient across the sample piece.

[0014] The sample piece can comprise two parallel surfaces to which the electrode arms engage. For example, the sample piece can be cylindrical, with the electrode arms engaging the end surfaces of the cylinder. Prism-shaped sample pieces with two parallel surfaces are also possible. Preferably, the measuring device comprises a drive motor to move the two electrode arms toward each other so that they hold the sample piece between them. A manually operated mechanism is also possible, with which the sample arms are moved relative to each other.

[0015] A lateral surface extends between the two parallel surfaces of the sample piece. The measuring sensors can be designed to pick up the measured values ​​via the lateral surface of the sample piece. The measuring sensors are preferably designed in accordance with the heat choke so that they cannot dissipate large amounts of heat. The measuring sensors therefore also preferably have a small cross-sectional area. The connection between the measuring sensor and the sample piece can be made, for example, by welding or soldering. It is also possible for the contact between the measuring sensor and the sample piece to be established simply by pressing them together. The measuring sensors are preferably configured so that they can record temperature and / or electrical voltage measurements.

[0016] The measuring device according to the invention can comprise a measuring chamber in which the measurement is carried out. The measuring device can comprise a heating device and / or cooling device for the measuring chamber. The measuring device can, for example, be designed to enable measurements in the temperature range from -100°C to 1500°C. The temperature in the measuring chamber forms the ambient temperature for the measurement. In addition, a susceptor is mounted above the two electrode arms and the sample piece to minimize heat exchange by radiation. If a temperature gradient across the sample piece is required for the measurement, a different temperature is applied to one end of the sample piece via the second electrode arm. The second electrode arm is preferably designed to enable a temperature difference between 10°C and 100°C, in particular of at least 50°C, compared to the temperature in the measuring chamber.

[0017] The measuring device can also be designed so that the measurement can be performed at a pressure other than atmospheric pressure. Preferably, the measurement is performed at a pressure lower than atmospheric pressure, i.e., under vacuum. For this purpose, the measuring device preferably includes a vacuum pump. The absolute pressure in the measuring chamber during the measurement can, for example, be between 10 -3 mbar and 10 -5 mbar.

[0018] A measuring device configured in this way enables direct measurement of the dimensionless figure of merit ZT. To also enable direct measurement of other thermoelectric quantities (e.g., Seebeck coefficient, electrical conductivity), the measuring device can be designed as a measuring system comprising a plurality of electrode inserts that can be arranged between the first electrode arm and the sample piece. Preferably, the measuring system comprises a first electrode insert with a heat choke and a second electrode insert without a heat choke. Without a heat choke means that the heat can be transferred from the sample piece to the electrode insert essentially unhindered and that the heat is dissipated essentially unhindered within the electrode insert. The first electrode arm can therefore act as a heat source or heat sink for the sample piece.Depending on the measurement to be performed, either the first electrode insert or the second electrode insert can be used.

[0019] The measuring device according to the invention preferably comprises a control unit configured to enable the measuring device to automatically perform one or more of the steps required for the measurement. In particular, the control unit can be configured to output a control command according to which the first electrode arm and / or the second electrode arm are brought toward the sample piece, thereby establishing electrical contact between the electrodes and the sample piece. The control unit can be configured to output a control command according to which the first measuring sensor and / or the second measuring sensor are brought toward the sample piece and brought into contact with the sample piece.

[0020] The control unit can be configured to send a control command to the second electrode arm, according to which the heating device of the second electrode arm is activated. The control unit can be configured to output a control command, according to which an electric current is introduced into the sample piece via the electrodes. This is preferably an electric current with a constant current intensity. The control unit can be configured to output a control command, according to which the measuring sensors record a temperature measurement value, in particular a measurement value of the temperature difference between the two measuring sensors. The control unit can be configured to output a control command, according to which the measuring sensors record a voltage measurement value, preferably a measurement value of the voltage applied between the two measuring sensors.The control unit can be configured to issue a control command according to which the measuring chamber is brought to a predetermined temperature and / or pressure. These functions can be implemented individually or in any combination in the control unit.

[0021] The measuring device according to the invention can comprise an evaluation unit designed to determine a thermoelectric characteristic from measured values ​​recorded with the measuring device. In particular, the evaluation unit can be designed to determine the dimensionless figure of merit ZT and / or the Seebeck coefficient. Additionally or alternatively, the evaluation unit can be designed to determine the electrical conductivity and / or the thermal conductivity. The measuring device can comprise a display and / or an interface via which the thermoelectric characteristic is output.

[0022] The invention further relates to a method for measuring thermoelectric properties of a sample piece. In the method, a first electrode arm and a second electrode arm are brought close to a sample piece. A first measuring sensor and a second measuring sensor are brought close to the sample piece. According to the invention, a heat choke is arranged between the first electrode arm and the sample piece. The method can be further developed with further features that are described in connection with the measuring device according to the invention. The measuring device can be further developed with further features that are described in connection with the method according to the invention.

[0023] The invention is described below by way of example with reference to advantageous embodiments in the accompanying drawings. They show: Fig. 1: a schematic representation of a measuring device according to the invention; Fig. 2: a block diagram of a measuring device according to the invention; and Fig. 3: the view according to Fig. 1 in a different state of the measuring device.

[0024] A measuring device according to the invention in Fig. 1 comprises a first electrode arm 14 and a second electrode arm 15, between whose front ends a sample piece 16 is arranged. The sample piece 16 is cylindrical and is in electrical contact with electrodes 17, 18 of the electrode arms 14, 15 via its end faces. The electrical contact between the upper end face of the sample piece 16 and the electrode 17 of the first electrode arm 14 is established via a copper plate 21 and a heat choke in the form of a needle-shaped element 22.

[0025] The lower end face of the sample piece 16 rests flat against the electrode 18 of the second electrode arm 15. The second electrode arm 15 is provided with a heating device 25, with which heat can be introduced into the sample piece 16 via the electrode 18. The two electrode arms 14, 15 exert pressure on the end faces of the sample piece 16, so that the sample piece 16 is held in position by the two electrode arms 14, 15.

[0026] Two measuring sensors 19, 20 are welded to the outer surface of the sample piece 16. The measuring sensors 19, 20 are designed to record temperature and electrical voltage measurements on the sample piece 16.

[0027] The electrode arms 14, 15 and the sample piece 16 are arranged in a measuring chamber that is hermetically sealed from the environment during the measurement. Fig. Figure 1 shows a section of the housing 23 surrounding the measuring chamber. The housing 23 is equipped with a heating device 24 for heating the measuring chamber. The housing 24 also includes a vacuum pump 28 for creating a vacuum in the measuring chamber.

[0028] According to the block diagram in Fig. 2, the measuring device also comprises a control unit 26 and an evaluation unit 27. The control unit 26 is designed to send control commands to components of the measuring device so that the measurement can be carried out largely automatically.

[0029] When the electrode arms 14, 15 with the sample piece 16 are arranged in the measuring chamber and the housing 23 is closed, the control unit 26 sends a control command to the heating device 24, according to which the heating device 24 begins to heat the measuring chamber. At the same time, a control command is sent to the vacuum pump 28, according to which the vacuum pump 28 generates a negative pressure in the measuring chamber.

[0030] In the next step, an electric current of constant intensity is passed through the sample piece 16 via the electrodes 17, 18. Due to the Peltier effect, one end of the sample piece 16 is heated, while the other end of the sample piece 16 cools. The measuring sensors 18, 19 receive the control command to measure the voltage and the temperature difference between the two points on the surface of the sample piece 16 where the measuring sensors 18, 19 are applied. The measured values ​​are fed to the evaluation unit 27, which determines the dimensionless figure of merit ZT from the measured values. The result is shown on a display 29 of the measuring device.

[0031] A direct determination of the figure of merit ZT is only possible under adiabatic conditions. Therefore, a large amount of heat must not be transferred to the first electrode arm 14 via the upper end face of the sample piece 16. This is achieved according to the invention by the needle-shaped element 22, which is arranged between the upper end face of the sample piece 16 and the electrode 17. The needle-shaped element 22 is dimensioned such that it transfers only a very small amount of heat to the electrode 17 and thus acts as a heat choke within the meaning of the invention.

[0032] Due to the copper plate 21, which rests over a large area on the end face of the sample piece 16, the contact resistance between the sample piece 16 and the copper plate 21 is low. The needle-shaped element 22, like the copper plate 21, is an electrical conductor, so that the electrical current can flow essentially unhindered to the electrode 17.

[0033] To determine the Seebeck coefficient, the measuring device is used in a different configuration. Fig. 1, the electrode 17, the needle-shaped element 22 and the copper plate 21 form a replaceable electrode insert 30. This is in Fig. 2 is replaced by an alternative electrode insert 31, in which the electrode 17 is connected to the first electrode arm 14 in such a way that good heat transfer takes place from the sample piece 16 to the first electrode arm 14. The electrode insert 31 can thus act as a heat sink for the sample piece 16.

[0034] After changing the electrode insert, the electrode arms 14, 15 are retracted into the measuring chamber with the sample piece 16. The measuring chamber is closed, the heating device 24 heats the measuring chamber, and the vacuum pump 28 creates a vacuum in the measuring chamber.

[0035] A control command is sent to the heating device 25 of the second electrode arm 15, which activates the heating device 25. The lower end of the sample piece 16 is thereby brought to a temperature higher than the temperature in the measuring chamber. Thus, a temperature gradient exists across the sample piece 16. By using the opposite first electrode arm 14 as a heat sink, the temperature gradient can be kept essentially constant.

[0036] The two sensors 19 and 20 record temperature and voltage measurements. These are fed to the evaluation unit 27, which then calculates the Seebeck coefficient.

Claims

[1] A measuring device for measuring thermoelectric properties of a sample piece, comprising a first electrode arm (14) and a second electrode arm (15) designed to hold the sample piece (16) between them and to introduce an electric current into the sample piece, and comprising a first measuring sensor (19) and a second measuring sensor (20) for contacting the surface of the sample piece (16), characterized by that a heat choke (22) is arranged between the first electrode arm (14) and the sample piece (16). [2] Measuring device according to claim 1, characterized by that the heat choke (22) is designed as a needle-shaped element. [3] Measuring device according to claim 2, characterized by that the first electrode arm (14) is designed to exert a force on the sample piece (1) via the needle-shaped element (22) in order to hold the sample piece in position. [4] Measuring device according to one of claims 1 to 3, characterized bythat a transition part (21) is arranged between the heat choke (22) and the sample piece (16), wherein the contact area between the transition part (21) and the sample piece (16) is larger than the cross-sectional area of ​​the heat choke (22). [5] Measuring device according to one of claims 1 to 4, characterized by that the heat choke (22) is connected to the test piece (16) by welding or soldering. [6] Measuring device according to one of claims 1 to 5, characterized by that the second electrode arm (15) is provided with a heating device (25) for heating an end of the sample piece (16) adjacent to the electrode arm (15). [7] Measuring device according to one of claims 1 to 6, characterized by that the measuring sensors (19, 20) are connected to the sample piece (16) by welding or soldering. [8] Measuring device according to one of claims 1 to 7, characterized bya measuring chamber (23) which is provided with a heating device (24), a cooling device and / or a vacuum pump (28). [9] Measuring device according to one of claims 1 to 8, characterized by a set of replaceable electrode inserts (30, 31), wherein a first electrode insert (30) has a heat choke (22) and wherein a second electrode insert (31) has no heat choke. [10] Measuring device according to one of claims 1 to 9, characterized by an evaluation unit (27) which is designed to determine the dimensionless figure of merit ZT of the sample piece (16) and / or the Seebeck coefficient of the sample piece (16) from measured values ​​recorded with the measuring sensors (19, 20). [11] A method for measuring thermoelectric properties of a sample piece (16), in which a first electrode arm (14) and a second electrode arm (15) are brought to the sample piece in order to hold the sample piece (16) between them and to introduce an electric current into the sample piece (16), and in which a first measuring sensor (19) and a second measuring sensor (20) are brought to the sample piece (16), characterized by that a heat choke (22) is arranged between the first electrode arm (14) and the sample piece (16).

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