Enhancing the high-temperature durability of thin-film thermocouples with a multifunctional Al₂O₃/SiO₂ composite insulating layer

AbstractHigh-temperature thin-film thermocouples (TFTCs) are critical for temperature monitoring in extreme environments; however, conventional Al₂O₃ insulation layers suffer from crystallization-induced porosity and cracking above 1200 ℃, leading to Rh oxidation and rapid thermoelectric drift. To address this limitation, an Al₂O₃/SiO₂ composite insulating architecture was developed for R-type (Pt/PtRh13) TFTCs via reactive magnetron sputtering on Ni-based superalloy and ceramic substrates. The structural stability of SiO₂ at elevated temperatures, together with interfacial mullite formation induced by Al₂O₃-SiO₂ interdiffusion, enhances interfacial integrity and suppresses defect evolution. The composite layer exhibits a high resistivity of ∼1.2 × 106 Ω·m at 1200 ℃, two orders of magnitude higher than that of single-layer Al₂O₃, and remains stable over repeated thermal cycles. TFTCs incorporating this composite architecture demonstrate minimal thermoelectric drift (∼0.07 °C·h−1) and maintain measurement errors below 1% after 100 h of continuous operation at 1200 ℃. These results demonstrate that the proposed composite design effectively mitigates high-temperature degradation and provides a robust strategy for ultra-high-temperature thin-film sensing applications.