Mechanism of Seebeck coefficient variation at the output of NiCr/NiSi thin film thermocouple with different wires

In this paper, by using magnetron sputtering to prepare NiCr/NiSi thin film thermocouples, the static calibration method is used for NiCr/NiSi thermocouples with rapid temperature calibration experiments. Different temperature calibration curves are obtained. The Seebeck coefficient of NiCr/NiSi thin film thermocouples connected to NiCr/NiSi wires is significantly higher (41.39 μV/°C) than that of NiCr/NiSi wires (0 μV/°C). The Seebeck coefficient (41.39 μV/°C) of the NiCr/NiSi thin-film thermocouple connected to copper wire is significantly higher than that of the thermocouple connected to copper wire (0 μV/°C). The problem of the Seebeck coefficient of the K-type thermocouple is analyzed by experimental data, which provides the relevant parameter basis for the use of the K-type thermocouple. The method has the advantages of simple equipment, convenient operation, accurate and reliable data, and provides a basis for the sensor to measure the temperature measurement.


Introduction
Current research on thin-film thermocouples mainly focuses on improving the preparation methods and expanding the application areas.However, regardless of the preparation method and the application of thin-film thermocouples, their temperature measurement principle is based on the Seebeck effect [1][2][3][4][5].Therefore, the use of thermocouples to accurately measure transient surface temperatures presupposes the ability to accurately calibrate the Seebeck coefficient of the thin film thermocouple.The static calibration process for ordinary filament thermocouples involves placing them in a metering furnace with a standard platinum resistor, fixing the temperature of the cold end and keeping the measuring end constant at different temperatures, and establishing the thermocouple's thermopotential-temperature relationship by measuring the calibration temperature from the standard filament thermocouple [6][7].When this method is used to calibrate the tiny-size thin-film thermocouple, the entire sensor is placed in the homogenized temperature field of the metering furnace.The temperature gradient is mainly concentrated in the compensation wire, which is inconsistent with the temperature distribution of thinfilm thermocouple applications, and it is not possible to realize effective calibration, which affects the temperature measurement accuracy of thin-film thermocouples in practical applications [8][9].As the NiCr/NiSi thermocouple connected to a different line Seebeck is also different, the thermoelectric properties of the electric potential difference are still very large.The calibration in this paper analyzes the above problems well.Only by analyzing the static calibration method of thin-film thermocouple in detail can we accurately evaluate its static characteristics so as to further improve the accuracy of temperature measurement.

Thin film thermocouple structure design
When designing the NiCr/NiSi thin film thermocouple temperature, first of all, the structure of the thin film thermocouple temperature sensor is simple, and the preparation process is easy to realize, which can meet the needs of the sensor and calibration.The NiCr/NiSi thin film thermocouple temperature has a sensitivity and fast response speed, which can meet the requirements of temperature testing [10][11][12][13][14].The connection between the thin film thermocouple temperature thermal electrode material and the compensation wire is stable and reliable, will not fall off easily, and can stably transmit the temperature signal.Based on the above principles, the NiCr/NiSi thin-film thermocouple temperature design uses a glass substrate with a rectangular shape, as shown in Figure 1.For the shape of a thin-film thermocouple, thermal electrodes are prepared in this paper using a mechanical mask.

Preparation of NiCr/NiSi thin film thermocouple
NiCr/NiSi thin-film thermocouple was prepared as follows: the polished mask, fixture, and glass substrate were cleaned with acetone, ethanol, and deionized water for 20 minutes, respectively, and blown dry with nitrogen.The mask is positioned and mounted on the fixture and fixed on the sample tray with high-temperature tape.The mask should be mounted carefully to tightly bond the mask to the substrate to avoid diffraction.When the vacuum degree of the vacuum coating chamber reaches 1.0×10 - 3 , the vacuum chamber of the vacuum coating equipment is closed.When the vacuum degree of the vacuum coating chamber reaches 6.0×10 -3 , the air pressure of the vacuum coating chamber is adjusted to 0.6 Pa.After the pressure in the vacuum chamber is balanced, the power switch is activated, and at the same time, the baffle is turned on for pre-sputtering for 5 minutes in order to eliminate the impurities on the target material and to ensure the stability of the sputtering process.After pre-sputtering, the target baffle was closed, and the sputtering of NiCr functional films was formally initiated.After the sputtering of the NiCr functional film is finished, it is necessary to gradually close the gas valve molecular pump, the insert valve, and the mechanical pump, and then wait for the completion of the venting and cooling, and finally remove the sample.The mask and target material are replaced to sputter NiSi and SiO2 in turn.The sputtering parameters are shown in Table 1 [18], and the operation procedure is the same as that for NiCr.Sensor performance testing is an important part of the sensor production and commissioning process [15].In order to ensure that the developed sensors can be used in industry, scientific research, and other places to obtain accurate, reliable, stable, and rapid measurement results, in sensor manufacturing, packaging must be tested after the performance parameters.Static characteristics are important performance indicators of thin film thermocouple temperature sensors [16].The NiCr/NiSi thin-film thermocouple temperature sensor developed in this paper is a non-standard sensor.In this paper, the developed NiCr/NiSi thin film thermocouple temperature sensor is calibrated using static calibration of laboratory equipment.As can be seen from Figure 3, the equation between the thermopotential E sensor and the temperature ș is: E = 0.04139ș + 0.04139.By analyzing the results of the static calibration experiments, it can be concluded that the developed NiCr/NiSi temperature sensor is connected to the NiCr/NiSi wire with a Seebeck coefficient of 41.39 ȝV / ႏ.

Static calibration of thin-film thermocouples with copper leads.
In the same way, the developed NiCr/NiSi thin-film sensor is placed in the hot end.The compensation wire connected to the copper wire is placed in a freezing-point constant thermostat.0 ႏ constant is set.The static calibration temperature range is set as 50 ~ 400 ႏ.The dry metering furnace temperature is maintained at every 10 ႏ rise 5 min constant temperature for calibration to obtain data.The expression obtained by fitting the data is shown in Figure 4: As can be seen from Figure 4, the thin film thermocouple thermoelectric potential E and the temperature ș of the equation between E = 0.By analyzing the results of the static calibration experiments, it can be concluded that the developed NiCr/NiSi thin-film sensor is connected to the copper wire and has a Seebeck coefficient of 0 ȝV / ႏ.

Experiments and results
Through the previous study, this paper found that the Seebeck of NiCr/NiSi thin film thermocouple connected to NiCr/NiSi wire (41.39 ȝV/°C) is significantly higher than that of NiCr/NiSi thin film thermocouple connected to copper wire (0 ȝV/°C).Detailed comparison data are shown in Figure 5.This is because no electric potential is generated between the copper compensation wires.Therefore, the connection to the electric potential of the copper wires is provided only by the film thermocouple and the hot and cold ends of the film thermocouple are in the same temperature field, so the Seebeck coefficient is 0 ȝV/°C.
Through the NiCr/NiSi thin film thermocouple connected to the wire is not the same as analyzing the Seebeck coefficient value.It can be reliably applied to temperature measurement, smart tools, smart manufacturing, and other fields.

Conclusion
In this paper, a NiCr/NiSi thermocouple is connected to different wires with different Seebeck.Using rapid temperature calibration experiments, different temperature calibration curves are obtained.The study of NiCr/NiSi thin film thermocouple connected to the NiCr/NiSi wire Seebeck coefficient (41.39 ȝV / ႏ) is significantly higher than that of NiCr/NiSi film thermocouple connected to copper wire (0 ȝV/°C).The problem of the Seebeck coefficient of the K-type thermocouple is analyzed by experimental data, which provides the relevant parameter basis for the use of the K-type thermocouple.The method has the advantages of simple equipment, convenient operation, accurate and reliable data, and provides a basis for the sensor to measure the temperature measurement.Using the comparative method of NiCr/NiSi thermocouple connected to a different line, Seebeck was analyzed in detail for accurate measurement of temperature.The study of tool wear, improved machining accuracy, machining efficiency, and tool durability is of great significance.

2. 3 . 1
Calibration system design.Temperature sensor static calibration refers to the temperature sensor in the temperature field that reaches the static standard conditions and determines the linear relationship between the sensor input and output[17].The aim is to obtain the developed sensor's Seebeck coefficient, linearity, repeatability, and other indicators.The temperature sensor used in this paper mainly consists of FLUKE-9144 dry metering furnace, PLANCK-6190A type icemaker, and DMM7510 digital multimeter.The thin film thermocouple temperature static calibration schematic system is shown in Figure2[19][20].

Figure 3 .
Figure 3. Static calibration curve of thin film sensor.

Figure 4 .
Figure 4. Static calibration curves for thin film thermocouples.

Figure 5
Figure 5 Static calibration curves for wo outlets of a thin-film Sensor.