Effect of Annealing on the Thermoelectricity Properties of the WRe26-In2O3 Thin Film Thermocouples

WRe26-In2O3 (WRe26 (tungsten-26% rhenium) and In2O3 thermoelectric materials) thin film thermocouples (TFTCs) have been fabricated based on magnetron sputtering technology, which can be used in temperature measurement. Many annealing processes were studied to promote the sensitivity of WRe26-In2O3 TFTCs. The optimal annealing process of the thermocouple under this kind of RF magnetron sputtering method was proposed after analyzing the properties of In2O3 films and the thermoelectric voltage of TFTCs at different annealing processes. The calibration results showed that the WRe26-In2O3 TFTCs achieved a thermoelectric voltage of 123.6 mV at a temperature difference of 612.9 K, with a sensitivity of up to 201.6 µV/K. Also, TFTC kept a stable thermoelectric voltage output at 973 K for 20 min and at 773 K for two hours. In general, the WRe26-In2O3 TFTCs developed in this work have great potential for practical applications. In future work, we will focus on the thermoelectric stability of TFTCs at higher temperatures.


Introduction
The accurate measurement of high temperature is particularly important in modern science. With the development of MEMS technology, TFTCs are widely used in many areas [1][2][3][4]. TFTCs have many advantages, such as fast response, high measurement accuracy, and easy integration [5][6][7]. Traditionally, for metal TFTCs, Such as type-K (Ni 10 Cr/Ni 5 Si) and type-S (Pt-10%Rh/Pt) TFTCs [8][9][10][11]. This kind of metal TFTCs have low sensitivity and low thermoelectric voltage output. To achieve high sensitivity and oxidation resistance, some silicide, carbides, and conductive oxides have been developed as alternative electrodes for high temperature measurement, such as the working temperature of CrSi 2 -TaC TFTCs in a vacuum or inert gas going up to 1080 • C while the thermoelectric output remains stable. When it was in an oxidizing atmosphere, it failed at 455 • C. Meanwhile, CrSi 2 can only work stably in an oxidizing environment at 670 • C; for more than 180 h, its sensitivity coefficient is 102 µV/ • C [12,13]. MoSi 2 -TiSi 2 carbide TFTCs was used to high temperature of 1200 • C. However, at high temperatures, SiO 2 is formed due to oxygen entering the film, which leads the stability of the TFTCs to become worse due to the composition of the thin-film changes [14]. Compared to carbide and silicide thin film thermocouples, oxide ceramic TFTCs have more potential for high temperature (1) where the S AB (T) is the Seebeck coefficient of TFTC, S A (T) is the Seebeck coefficient of material A; S B (T) is the Seebeck coefficient of material B, θ is the temperature of hot junction; θ 0 is temperature of cold junction. At the same time, the Seebeck coefficient of the conductive oxides are different from the metals. In 2 O 3 is an N-type non-degenerate semiconductor material. The Seebeck coefficient of In 2 O 3 is gave as: where S is Seebeck coefficient, K is the Boltzmann constant, h is the Planck constant, e respects electronic charges; N D is carrier concentration, m e is effective mass, A is a transport constant [27]. If additional oxygen enters the In 2 O 3 , it will affect the Seebeck coefficient of the In 2 O 3 . The conductive carriers of In 2 O 3 mainly comes from the electrons released by the oxygen vacancy, and one oxygen vacancy contributes two electrons (Equation (3)) [28]. V O are doubly charged oxygen vacancies. When additional oxygen occupied the oxygen vacancy of the In 2 O 3 film, it caused the carrier concentration in the In 2 O 3 film to decrease while increasing the Seebeck coefficient of the In 2 O 3 .
To verify whether the thermoelectric voltage output of WRe26-In 2 O 3 was better than the pure oxide combination (ITO-In 2 O 3 ), thermoelectricity simulation of TFTCs with different thermoelectric material combinations was required. The thermoelectric characteristics of the ITO-WRe26, WRe26-In 2 O 3 and ITO-In 2 O 3 TFTCs were studied by using commercial software COMSOL to ensure the results of model analysis. Figure 1 shows the model of the three combinations of TFTCs. The single size of the TFTC is 30 mm × 90 mm. The area of hot junctions is 4 mm × 10 mm. In this analysis, the temperature of hot junctions was increased from 300 K to 1300 K, and the cold junctions were set to 293 K. The Finite Element Analysis results of temperature gradient and thermoelectric voltage distribution are presented in Figure 2. The maximum temperature of the hot junctions are 1300 K. Figure 3 shows the thermoelectric voltage output of TFTCs. According to the simulation results, thermoelectric output of WRe26-In 2 O 3 is the biggest at 1300 K, which means the sensitivity coefficient of this combination is bigger than ITO-In 2 O 3 in theory.
carriers of In2O3 mainly comes from the electrons released by the oxygen vacancy, and one oxygen vacancy contributes two electrons (Equation (3)) [28]. VO are doubly charged oxygen vacancies. When additional oxygen occupied the oxygen vacancy of the In2O3 film, it caused the carrier concentration in the In2O3 film to decrease while increasing the Seebeck coefficient of the In2O3.
To verify whether the thermoelectric voltage output of WRe26-In2O3 was better than the pure oxide combination (ITO-In2O3), thermoelectricity simulation of TFTCs with different thermoelectric material combinations was required. The thermoelectric characteristics of the ITO-WRe26, WRe26-In2O3 and ITO-In2O3 TFTCs were studied by using commercial software COMSOL to ensure the results of model analysis. Figure 1 shows the model of the three combinations of TFTCs. The single size of the TFTC is 30 mm × 90 mm. The area of hot junctions is 4 mm × 10 mm. In this analysis, the temperature of hot junctions was increased from 300 K to 1300 K, and the cold junctions were set to 293 K. The Finite Element Analysis results of temperature gradient and thermoelectric voltage distribution are presented in Figure 2. The maximum temperature of the hot junctions are 1300 K. Figure 3 shows the thermoelectric voltage output of TFTCs. According to the simulation results, thermoelectric output of WRe26-In2O3 is the biggest at 1300 K, which means the sensitivity coefficient of this combination is bigger than ITO-In2O3 in theory.

Experiment
In order to study the effect of different annealing on the thermoelectric voltage of TFTCs, In2O3 film samples and WRe26-In2O3 TFTCs were prepared by RF magnetron sputtering. RF Magnetron sputtering technology is widely used because of the good adhesion of the films on the substrate, good thickness uniformity and high film density [29][30][31]. High purity WRe26 and In2O3 Target (purity 99.999 wt.%, diameter: 101.6 mm, and thickness: 3 mm) were been used while the distance between target and substrate was 80 mm. In Figure 4, WRe26 and In2O3 films were deposited on the Si3N4 substrate. The mass size of Si3N4 substrate is 30 mm × 90 mm × 3 mm, and the TFTC is 8 mm × 70 mm ×2 um.

Experiment
In order to study the effect of different annealing on the thermoelectric voltage of TFTCs, In2O3 film samples and WRe26-In2O3 TFTCs were prepared by RF magnetron sputtering. RF Magnetron sputtering technology is widely used because of the good adhesion of the films on the substrate, good thickness uniformity and high film density [29][30][31]. High purity WRe26 and In2O3 Target (purity 99.999 wt.%, diameter: 101.6 mm, and thickness: 3 mm) were been used while the distance between target and substrate was 80 mm. In Figure 4, WRe26 and In2O3 films were deposited on the Si3N4 substrate. The mass size of Si3N4 substrate is 30 mm × 90 mm × 3 mm, and the TFTC is 8 mm × 70 mm ×2 um.

Experiment
In order to study the effect of different annealing on the thermoelectric voltage of TFTCs, In 2 O 3 film samples and WRe26-In 2 O 3 TFTCs were prepared by RF magnetron sputtering. RF Magnetron sputtering technology is widely used because of the good adhesion of the films on the substrate, good thickness uniformity and high film density [29][30][31]. High purity WRe26 and In 2 O 3 Target (purity 99.999 wt.%, diameter: 101.6 mm, and thickness: 3 mm) were been used while the distance between target and substrate was 80 mm. In Figure 4, WRe26 and In 2 O 3 films were deposited on the Si 3 N 4 substrate. The mass size of Si 3 N 4 substrate is 30 mm × 90 mm × 3 mm, and the TFTC is 8 mm × 70 mm × 2 um.   Table 1 shows the detail sputtering parameters of the TFTCs preparation. The order position of the two legs of the TFTCs were especially important. The leg of WRe26-In2O3 TFTC ttern was transferred by using photolithography. In2O3 films deposited by magnetron sputterin r 4 h. Then, In2O3 films were soaked in different annealing processes. After the TFTCs were cleane , the WRe26 films were sputtered for 90 min with a high power of 400 w. Finally, the Al2O otective layer was covered on the sensitive layer again. The In2O3 films samples at different annealing processes were presented in Figure 5a. The Al2O bstrate was 14 mm × 20 mm × 1 mm. The color of film samples obviously changed under differe nealing processes. The crystal structure of In2O3 at different annealing conditions was analyzed b ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) was used to characterize i emical composition. Scanning electron microscopy (SEM) was used to observe the micr  Table 1 shows the detail sputtering parameters of the TFTCs preparation. The order of deposition of the two legs of the TFTCs were especially important. The leg of WRe26-In 2 O 3 TFTCs pattern was transferred by using photolithography. In 2 O 3 films deposited by magnetron sputtering for 4 h. Then, In 2 O 3 films were soaked in different annealing processes. After the TFTCs were cleaned up, the WRe26 films were sputtered for 90 min with a high power of 400 w. Finally, the Al 2 O 3 protective layer was covered on the sensitive layer again. The In 2 O 3 films samples at different annealing processes were presented in Figure 5a. The Al 2 O 3 substrate was 14 mm × 20 mm × 1 mm. The color of film samples obviously changed under different annealing processes. The crystal structure of In 2 O 3 at different annealing conditions was analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) was used to characterize its chemical composition. Scanning electron microscopy (SEM) was used to observe the micro-morphology of In 2 O 3 at different annealing conditions, and the WRe26-In 2 O 3 TFTCs were prepared to find the best annealing process by thermoelectric voltage testing ( Figure 5b). Fabricated WRe26-In2O3 TFTCs were static tested in muffle furnace (LHT0820, Nabertherm, Lilienthal, Germany). As shown in Figure 6, one K-type thermocouples and WRe26-In2O3 TFTCs were placed in the muffle furnace to get the temperature of hot junctions. Another K-type was used to monitor the cold junctions. Cold junctions of the TFTCs were cooled by circulating cold water to maintain a big temperature gradient. Then thermoelectric voltage of K-type thermocouples and the WRe26-In2O3 TFTCs were recorded with a data collector (Hioki, LR8410-30, Nagano, Japan).  Fabricated WRe26-In 2 O 3 TFTCs were static tested in muffle furnace (LHT0820, Nabertherm, Lilienthal, Germany). As shown in Figure 6, one K-type thermocouples and WRe26-In 2 O 3 TFTCs were placed in the muffle furnace to get the temperature of hot junctions. Another K-type was used to monitor the cold junctions. Cold junctions of the TFTCs were cooled by circulating cold water to maintain a big temperature gradient. Then thermoelectric voltage of K-type thermocouples and the WRe26-In 2 O 3 TFTCs were recorded with a data collector (Hioki, LR8410-30, Nagano, Japan). Fabricated WRe26-In2O3 TFTCs were static tested in muffle furnace (LHT0820, Naberther ilienthal, Germany). As shown in Figure 6, one K-type thermocouples and WRe26-In2O3 TFTCs we laced in the muffle furnace to get the temperature of hot junctions. Another K-type was used onitor the cold junctions. Cold junctions of the TFTCs were cooled by circulating cold water aintain a big temperature gradient. Then thermoelectric voltage of K-type thermocouples and t Re26-In2O3 TFTCs were recorded with a data collector (Hioki, LR8410-30, Nagano, Japan).

Result and Discussion
The X-ray diffraction (XRD) patterns of In 2 O 3 film samples at different annealing process were presented in Figure 7. As shown in Figure 7a, the (222) peak of In 2 O 3 is very small at no annealing. With increasing of air annealing temperature, the (222) and (400) peaks of In 2 O 3 were promoted a great deal, especially the (222) peak increases in the air annealing at 1000 • C. This indicates that the preferred growth of the crystal plane are (222) and (400) crystal planes. In Figure 7b, it was obvious that each peak of In 2 O 3 in XRD was nearly unchanged at the anaerobic annealing processes.
The X-ray diffraction (XRD) patterns of In2O3 film samples at different annealing process were presented in Figure 7. As shown in Figure 7a, the (222) peak of In2O3 is very small at no annealing. With increasing of air annealing temperature, the (222) and (400) peaks of In2O3 were promoted a great deal, especially the (222) peak increases in the air annealing at 1000 °C. This indicates that the preferred growth of the crystal plane are (222) and (400) crystal planes. In Figure 7b, it was obvious that each peak of In2O3 in XRD was nearly unchanged at the anaerobic annealing processes.
XPS was used to analyze the oxygen element in In2O3 films at different annealing conditions. O 1s core energy spectrum of In2O3 films are shown in Figure 8. The O1s spectrum of In2O3 films has two peaks. The binding energy of 529 eV corresponds to the O element peak and binding energy of 531 eV corresponds to the O 2− element peak in In2O3 films. The area ratio under peak of O1s (I) and O1s (II) increased after 600 °C air annealing for 2 h. It is mainly because a large amount of oxygen in the air will not enter the film at a low temperature. Instead, oxygen escaped from the film to produce more oxygen vacancies and the O 2− element was increased. Then, In2O3 films recrystallized after annealing at 1000 °C for 2 h. More oxygen entered the In2O3 film, and oxygen vacancy defects were reduced, causing the carrier concentration of In2O3 to be reduced.  XPS was used to analyze the oxygen element in In 2 O 3 films at different annealing conditions. O 1s core energy spectrum of In 2 O 3 films are shown in Figure 8. The O1s spectrum of In 2 O 3 films has two peaks. The binding energy of 529 eV corresponds to the O element peak and binding energy of 531 eV corresponds to the O 2− element peak in In 2 O 3 films. The area ratio under peak of O1s (I) and O1s (II) increased after 600 • C air annealing for 2 h. It is mainly because a large amount of oxygen in the air will not enter the film at a low temperature. Instead, oxygen escaped from the film to produce more oxygen vacancies and the O 2− element was increased. Then, In 2 O 3 films recrystallized after annealing at 1000 • C for 2 h. More oxygen entered the In 2 O 3 film, and oxygen vacancy defects were reduced, causing the carrier concentration of In 2 O 3 to be reduced.

Result and Discussion
The X-ray diffraction (XRD) patterns of In2O3 film samples at different annealing process were presented in Figure 7. As shown in Figure 7a, the (222) peak of In2O3 is very small at no annealing. With increasing of air annealing temperature, the (222) and (400) peaks of In2O3 were promoted a great deal, especially the (222) peak increases in the air annealing at 1000 °C. This indicates that the preferred growth of the crystal plane are (222) and (400) crystal planes. In Figure 7b, it was obvious that each peak of In2O3 in XRD was nearly unchanged at the anaerobic annealing processes. XPS was used to analyze the oxygen element in In2O3 films at different annealing conditions. O 1s core energy spectrum of In2O3 films are shown in Figure 8. The O1s spectrum of In2O3 films has two peaks. The binding energy of 529 eV corresponds to the O element peak and binding energy of 531 eV corresponds to the O 2− element peak in In2O3 films. The area ratio under peak of O1s (I) and O1s (II) increased after 600 °C air annealing for 2 h. It is mainly because a large amount of oxygen in the air will not enter the film at a low temperature. Instead, oxygen escaped from the film to produce more oxygen vacancies and the O 2− element was increased. Then, In2O3 films recrystallized after annealing at 1000 °C for 2 h. More oxygen entered the In2O3 film, and oxygen vacancy defects were reduced, causing the carrier concentration of In2O3 to be reduced.    Figure 9 exhibits the SEM of the In2O3 films under different annealing conditions. Compared to anaerobic and air annealing, as the temperature increased, the microstructures of In2O3 just became denser at anaerobic annealing. But the microstructures of In2O3 were changed significantly under air annealing. The organization grains of In2O3 became denser and larger, and the cellular crystals were formed at 1000 °C, implying that the oxygen entered the thin film structure at 1000 °C air annealing, and the oxygen occupied the oxygen vacancy of the In2O3 films, making the conductive electrons in the In2O3 film decrease rapidly according to the Equation (3). As a result, the Seebeck coefficient of In2O3 increased. To verify this phenomenon, Figure 10 shows the result of a static test of TFTCs from room temperature to 673 K at the different annealing process. It is obvious that the thermoelectric voltage was the smallest at no annealing. There were slight changes in the microstructures of In2O3 in 600 °C and 1000 °C anaerobic annealing. The thermoelectric voltage was significantly smaller than air annealing treatment. Thermoelectric voltage at 1000 °C air annealing was much bigger than 600 °C air annealing, which means the Seebeck coefficient of In2O3 films can be improved under air annealing processes, making the performance of WRe26-In2O3 TFTCs better.  To verify this phenomenon, Figure 10 shows the result of a static test of TFTCs from room temperature to 673 K at the different annealing process. It is obvious that the thermoelectric voltage was the smallest at no annealing. There were slight changes in the microstructures of In 2 O 3 in 600 • C and 1000 • C anaerobic annealing. The thermoelectric voltage was significantly smaller than air annealing treatment. Thermoelectric voltage at 1000 • C air annealing was much bigger than 600 • C air annealing, which means the Seebeck coefficient of In 2 O 3 films can be improved under air annealing processes, making the performance of WRe26-In 2 O 3 TFTCs better.
Micromachines 2020, 11, x 8 of 13 Figure 9 exhibits the SEM of the In2O3 films under different annealing conditions. Compared to anaerobic and air annealing, as the temperature increased, the microstructures of In2O3 just became denser at anaerobic annealing. But the microstructures of In2O3 were changed significantly under air annealing. The organization grains of In2O3 became denser and larger, and the cellular crystals were formed at 1000 °C, implying that the oxygen entered the thin film structure at 1000 °C air annealing, and the oxygen occupied the oxygen vacancy of the In2O3 films, making the conductive electrons in the In2O3 film decrease rapidly according to the Equation (3). As a result, the Seebeck coefficient of In2O3 increased. To verify this phenomenon, Figure 10 shows the result of a static test of TFTCs from room temperature to 673 K at the different annealing process. It is obvious that the thermoelectric voltage was the smallest at no annealing. There were slight changes in the microstructures of In2O3 in 600 °C and 1000 °C anaerobic annealing. The thermoelectric voltage was significantly smaller than air annealing treatment. Thermoelectric voltage at 1000 °C air annealing was much bigger than 600 °C air annealing, which means the Seebeck coefficient of In2O3 films can be improved under air annealing processes, making the performance of WRe26-In2O3 TFTCs better.  In order to find the optimal annealing processes, the In 2 O 3 films were annealed at 1000 • C for a longer time. As shown in Figure 11, with longer time in high temperature annealing, structure grains of In 2 O 3 continued to grow and became more uniform, especially at 10 h. But from the test results of of WRe26-In 2 O 3 TFTCs in Figure 12, it is observed that the thermoelectric voltage output is best at air annealing for 8 h. Thermoelectric voltage output for 10 h is smaller than that for 4 h. The reason was that voids appeared in the In 2 O 3 films (Figure 11d), the grain boundaries of the In 2 O 3 films structure become discontinuous, and the conductivity of the In 2 O 3 films became poor during long duration annealing processes, although oxygen promoted the growth of tissue grains, leading to poor conductivity of the In 2 O 3 film.
Micromachines 2020, 11, x 9 of 13 In order to find the optimal annealing processes, the In2O3 films were annealed at 1000 °C for a longer time. As shown in Figure 11, with longer time in high temperature annealing, structure grains of In2O3 continued to grow and became more uniform, especially at 10 h. But from the test results of of WRe26-In2O3 TFTCs in Figure 12, it is observed that the thermoelectric voltage output is best at air annealing for 8 h. Thermoelectric voltage output for 10 h is smaller than that for 4 h. The reason was that voids appeared in the In2O3 films (Figure 11d), the grain boundaries of the In2O3 films structure become discontinuous, and the conductivity of the In2O3 films became poor during long duration annealing processes, although oxygen promoted the growth of tissue grains, leading to poor conductivity of the In2O3 film.  The measured thermoelectric voltage depends on the difference between hot junction (Th) and cold junction (Tc) and the Seebeck coefficient of the metal materials. The sensitivity coefficient (S) of thermocouples is given as: In order to find the optimal annealing processes, the In2O3 films were annealed at 1000 °C for a longer time. As shown in Figure 11, with longer time in high temperature annealing, structure grains of In2O3 continued to grow and became more uniform, especially at 10 h. But from the test results of of WRe26-In2O3 TFTCs in Figure 12, it is observed that the thermoelectric voltage output is best at air annealing for 8 h. Thermoelectric voltage output for 10 h is smaller than that for 4 h. The reason was that voids appeared in the In2O3 films (Figure 11d), the grain boundaries of the In2O3 films structure become discontinuous, and the conductivity of the In2O3 films became poor during long duration annealing processes, although oxygen promoted the growth of tissue grains, leading to poor conductivity of the In2O3 film.  The measured thermoelectric voltage depends on the difference between hot junction (Th) and cold junction (Tc) and the Seebeck coefficient of the metal materials. The sensitivity coefficient (S) of thermocouples is given as: The measured thermoelectric voltage depends on the difference between hot junction (T h ) and cold junction (T c ) and the Seebeck coefficient of the metal materials. The sensitivity coefficient (S) of thermocouples is given as: where the ∆V is the thermoelectric voltage difference between the WRe26 and In 2 O 3 . Figure 13 shows the average sensitivity (The temperature difference was 400K) of WRe26-In 2 O 3 TFTCs at different annealing. The sensitivity coefficient of the TFTCs reached 186.1 µV/K at air annealing for 8 h.
Micromachines 2020, 11, x 10 of 13 where the ΔV is the thermoelectric voltage difference between the WRe26 and In2O3. Figure 13 shows the average sensitivity (The temperature difference was 400K) of WRe26-In2O3 TFTCs at different annealing. The sensitivity coefficient of the TFTCs reached 186.1 µV/K at air annealing for 8 h. Prepared WRe26-In2O3 TFTC was static calibrated in a high temperature after the optimal annealing process was determined. Figure 14 shows the temperature stability test of TFTC in muffle furnace. The WRe26-In2O3 TFTC and K-type thermocouples were raised from room temperature to 773 K and kept for two hours, and heated to 1000 K for twenty minutes. The heating rate was set at 10 °C/min. Then, TFTC was naturally cooled to room temperature. Figure 15 is a static thermoelectric voltage curve of WRe26-In2O3 TFTCs with the temperature difference up to 612.9 K. The hot junction of the thermocouple was 1000 K (the temperature of cold junction was 387.1 K), the thermoelectric voltage reached 123.6 mv. The average sensitivity coefficient was 201.6 µV/K. We have found the optimal annealing process at this magnetron sputtering process, but the Seebeck coefficient of In2O3 in the literature is about −200 µV/K, and the Seebeck coefficient of WRe26 is about 20 µV/K. So the sensitivity of the WRe26-In2O3 TFTCs is about 220 µV/K in theory. There was a little difference between the prepared TFTC and the theoretical thermoelectric output. This is mainly because the source of the In2O3 target was different, and so the Seebeck coefficient of In2O3 was also a little different. The Seebeck coefficient of In2O3 was highly affected by the quality In2O3 film. Prepared WRe26-In 2 O 3 TFTC was static calibrated in a high temperature after the optimal annealing process was determined. Figure 14 shows the temperature stability test of TFTC in muffle furnace. The WRe26-In 2 O 3 TFTC and K-type thermocouples were raised from room temperature to 773 K and kept for two hours, and heated to 1000 K for twenty minutes. The heating rate was set at 10 • C/min. Then, TFTC was naturally cooled to room temperature. Figure 15 is a static thermoelectric voltage curve of WRe26-In 2 O 3 TFTCs with the temperature difference up to 612.9 K. The hot junction of the thermocouple was 1000 K (the temperature of cold junction was 387.1 K), the thermoelectric voltage reached 123.6 mv. The average sensitivity coefficient was 201.6 µV/K. We have found the optimal annealing process at this magnetron sputtering process, but the Seebeck coefficient of In 2 O 3 in the literature is about −200 µV/K, and the Seebeck coefficient of WRe26 is about 20 µV/K. So the sensitivity of the WRe26-In 2 O 3 TFTCs is about 220 µV/K in theory. There was a little difference between the prepared TFTC and the theoretical thermoelectric output. This is mainly because the source of the In 2 O 3 target was different, and so the Seebeck coefficient of In 2 O 3 was also a little different. The Seebeck coefficient of In 2 O 3 was highly affected by the quality In 2 O 3 film.

Conclusions
In this study, a WRe26-In2O3 TFTC was reported. The WRe26-In2O3 TFTCs were successfully fabricated on the Si3N4 substrate by magnetron sputtering in order to improve the thermoelectric performance of the thermocouple. The properties of In2O3 films and the thermoelectric voltage properties of the WRe26-In2O3 TFTCs under different annealing processes were studied. The properties of In2O3 films at different annealing processes were analyzed by SEM, XRD, and XPS. The optimal annealing process of the TFTCs under this sputtering method was proposed. The WRe26-In2O3 TFTCs had ideal performance at the 1000 °C air annealing for 8 h. It was achieved that the average sensitivity of the WRe26-In2O3 TFTCs could reach 201.6 µV/K at a temperature difference of 612.9 K, which can maintain a stable output for 2 h at 773 K and 20 min for 1000 K.

Conclusions
In this study, a WRe26-In2O3 TFTC was reported. The WRe26-In2O3 TFTCs were successfully fabricated on the Si3N4 substrate by magnetron sputtering in order to improve the thermoelectric performance of the thermocouple. The properties of In2O3 films and the thermoelectric voltage properties of the WRe26-In2O3 TFTCs under different annealing processes were studied. The properties of In2O3 films at different annealing processes were analyzed by SEM, XRD, and XPS. The optimal annealing process of the TFTCs under this sputtering method was proposed. The WRe26-In2O3 TFTCs had ideal performance at the 1000 °C air annealing for 8 h. It was achieved that the average sensitivity of the WRe26-In2O3 TFTCs could reach 201.6 µV/K at a temperature difference of 612.9 K, which can maintain a stable output for 2 h at 773 K and 20 min for 1000 K.

Conclusions
In this study, a WRe26-In 2 O 3 TFTC was reported. The WRe26-In 2 O 3 TFTCs were successfully fabricated on the Si 3 N 4 substrate by magnetron sputtering in order to improve the thermoelectric performance of the thermocouple. The properties of In 2 O 3 films and the thermoelectric voltage properties of the WRe26-In 2 O 3 TFTCs under different annealing processes were studied. The properties of In 2 O 3 films at different annealing processes were analyzed by SEM, XRD, and XPS. The optimal annealing process of the TFTCs under this sputtering method was proposed. The WRe26-In 2 O 3 TFTCs had ideal performance at the 1000 • C air annealing for 8 h. It was achieved that the average sensitivity of the WRe26-In 2 O 3 TFTCs could reach 201.6 µV/K at a temperature difference of 612.9 K, which can maintain a stable output for 2 h at 773 K and 20 min for 1000 K.