Researches On Fabrication And Properties Of In₂O₃/ITO High-Temperature Ceramic Thin-Film Thermocouples

During the process of design and verification of aeroengine, precise measurement of surface temperature and its distribution of turbine blade and combustion chamber are crucial, which enables the effectiveness evaluation of cooling design and thermal barrier coating of the hot components. Compared to thermocouple wires, thin film thermocouples have no damage to the structure of measured components, and it has faster response due to smaller thermal capacity and thinner size. Therefore, they have obvious technical advantages in surface temperature detection of the hot components. With the development of aeroengine towards high thrust and high thrust to weight ratio, the surface temperature of turbine blade and combustion chamber is getting higher than ever（generally exceeding 1100 ℃）, the conventional metallic thin film thermocouples are unable to meet the requirement, whereas In₂O₃/ITO thin film thermocouples, as wide band gap semiconductor ceramics, have the characteristics of higher temperature endurance, antioxidation and large Seebeck coefficient. These characteristics make it more suitable to measure higher temperature in harsh environments. In this paper, aiming at the surface temperature measurement of the hot components of aeroengines, the preparation and properties of In₂O₃/ITO high temperature ceramic thin film thermocouples were investigated, the influence of nitrogen doping and annealing treatment on the thermoelectric properties of In₂O₃/ITO high temperature ceramic thin film thermocouples was addressed.First of all, the preparation of RF sputtered In₂O₃ thin film was studied. With the increase of sputtering pressure and the proportion of nitrogen in the gas media, the oxygen vacancies of In₂O₃ is partially filled and the carrier concentration decreases, leading to resistivity of the thin film increased. With the increasing proportion of nitrogen in the gas media, the thin film tends to（222） preferential growth. The deposition rate of In₂O₃ thin film is increased with increasing sputtering power, the surface of the film becomes denser, and the resistivity is gradually reduced due to the decrease of the detects.Secondly, In₂O₃ thin film was heat treated with different annealing processes（atmosphere, vacuum/atmosphere, and nitrogen/atmosphere）. Indium nitride and（400） preferential growth of In₂O₃ were found after annealing. In vacuum/atmosphere annealing, the resistivity of the In₂O₃ thin film increases with increasing atmosphere annealing temperature. When the temperature rises to 1200 ℃, the thin film becomes denser due to the increase of the crystal size, and the resistivity of the thin film decreases due to the decrease of the lattice defects, the electron traps and the grain boundary scattering. While in the atmosphere annealing, the resistivity of the thin film increases with increasing annealing time due to the decrease of the carrier concentration. In nitrogen /atmosphere annealing, In₂O₃ thin film becomes denser with fewer defects when the nitrogen annealing temperature is increased, and consequently the resistivity of the thin films is reduced.Thirdly, In₂O₃/ITO thin film thermocouples（63 mm×1 mm×1 μm） were fabricated on the surface of Al₂O₃ ceramic, following the samples were heat treated with different annealing methods, and the thermoelectric properties of In₂O₃/ITO thin film thermocouples were studied by means of static calibration at the temperature range of 300 ℃-1000 ℃. The results show that the thermoelectric stability of In₂O₃/ITO thin film thermocouples is effectively improved with nitrogen/atmosphere annealing treatment. In nitrogen/atmosphere annealing, the average Seebeck coefficients of In₂O₃/ITO thin film thermocouples are gradually decreased due to nitrogen as valence band electron acceptor existing in the thin films with increase of the proportion of nitrogen in the gas media. Besides, the Al₂O₃ protecting layer can make the average Seebeck coefficient of In₂O₃/ITO thin film thermocouple be bigger, and the thermoelectric properties of In₂O₃/ITO thin film thermocouple are more stable after the first static calibration. The cubic polynomial curve-fitting of the fourth calibration shows that the average Seebeck coefficient of the thin film thermocouple is 132.05 μV/℃, the maximum error of temperature measurement is 0.96%, and the minimum error of temperature measurement is 0.10%.Finally, In₂O₃/ITO thin film thermocouple was fabricated on the surface of Ni-based superalloy, and it is composed of about 15 μm NiCrAlY buffer layer, about 1.5 μm thermal grown Al₂O₃ layer, about 12 μm Al₂O₃ insulating layer, about 1 μm In₂O₃/ITO thin film thermocouple layer（63 mm×1 mm×1 μm） and about 3 μm Al₂O₃ protecting layer. The sample was heat treated with nitrogen/atmosphere annealing, and the thermoelectric properties of In₂O₃/ITO thin film thermocouple were studied by means of static calibration at the temperature range of 300 ℃-1000 ℃. The results show that the thermoelectric properties of the thin film thermocouple are more stable after the first static calibration. The total time of the four static calibration is about 28 h, which shows that In₂O₃/ITO thin film thermocouple can exist in high temperature for at least 28 h. The cubic polynomial curve-fitting of the fourth calibration shows that the average Seebeck coefficient of In₂O₃/ITO thin film thermocouple is 123.55 μV/℃, and the error of temperature measurement of the thin film thermocouple is from 0.06% to 0.94%.