Preparation And Properties Of High Temperature Thin Film Thermocouples

The precise surface temperature measurement is crucial for aeroengines in order to validate the effectiveness of modeling and simulation of the thermo-mechanical behavior of hot section components,monitor the operating conditions and perform diagnostics.Thin film thermocouples（TFTCs）can provide fast response and precise in-situ temperature measurement for aeroengine with negligible disturbance of test environment.In this thesis,W-5%Re/W-26%Re,Pt-13%Rh/Pt and ITO-based thin film thermocouples were fabricated and their properties were characterized and discussed in detail.Firstly,W-5%Re/W-26%Re TFTCs were prepared by magnetron sputtering on Al₂O₃ ceramic substrates and their properties were characterized.The results showed that the electromotive force（EMF）of W-5%Re/W-26%Re TFTCs（63mm×1mm×1μm）increased linearly with the increasing temperature difference between hot junction and cold junction.When the temperature rose to around 600℃,abrupt changes in EMF and resistance of W-5%Re/W-26%Re TFTCs were observed simutaneously,leading to the failure of the TFTCs.In addition,the thermoelectric properties of the TFTCs were significantly improved by vacuum annealing.Compared with the as-deposited sample,the average Seebeck coefficient of W-5%Re/W-26%Re TFTCs after annealing at1000℃for 120 min increased from 2.02μV/℃to 11.19μV/℃.Secondly,in order to overcome the oxidation problem of metal-based TFTCs at high temperature,the protective layers were prepared and characterized.The Seebeck coefficients of the W-5%Re/W-26%Re TFTCs with Al₂O₃,ZrBN/Al₂O₃ and SiCN/SiC protective layers were 11.19μV/℃,10.72μV/℃and 10.70μV/℃,respectively.The thicknesses of all protective layers were nearly 3μm.Similarly,during the temperature rising from 500℃to 600℃,all tungsten-rhenium TFTCs failed and the effective protection time of different protective layers were approximately 10 min,11 min and 6min,respectively.After annealing at 1000℃for 6 h in air,the main crystal phases of the Al₂O₃/Zr BN-SiCN/Al₂O₃ composite protective layer prepared by reactive sputtering were Al₂O₃ and Al₂SiO₅,while AlZrO and B₂O₃ existed in the amorphous state.The thickness of the composite protective layer was approximately 750 nm.Crystalline oxide/amorphous oxide double-protection mechanism was formed inside the composite protective layer after annealing,leading to excellent oxidation resistance performance of the composite protective layer.As a result,the high-temperature stability and repeatability of Pt-13%Rh/Pt TFTCs（63mm×1mm×1μm）with Al₂O₃/ZrBN-SiCN/Al₂O₃ composite protective layer were significantly improved.In six static cycle calibrations with temperature up to 1000℃,the average Seebeck coefficients ranged from 9.51μV/℃to 9.58μV/℃.The fluctuation range was only 0.07μV/℃.The TFTCs remained in good condition after multiple calibration cycles.In other words,the life time of the TFTCs was more than 60 h in harsh environment.Finally,ITO/PtRh composite film with multilayer structure was introduced as a thermoelement to improve thermoelectric properties of ITO-based TFTCs.The ITO/PtRh:PtRh TFTCs（63mm×1mm×1μm）were prepared by magnetron sputtering on Al₂O₃ ceramic substrates and their thermoelectric properties were characterized and discussed.The results indicated that with the increasing temperature difference between hot junction and cold junction,the EMF and Seebeck coefficient of the ITO/PtRh:PtRh TFTCs increased nonlinearly up to 1000℃.Meanwhile,the TFTCs demonstrated excellent stability and repeatability.The average Seebeck coefficients of the TFTCs in three calibration cycles were 2.19μV/℃.The TFTCs remained in good condition after calibration cycles and the life time of the TFTCs was more than 30 h.