Study On Piezoresistive Stability And Temperature Effect Of High Temperature Thin Film Strain Gages

In recent years,the rapid development of aerospace and fourth-generation nuclear power technology has required structural components work for a long time under harsh environments such as high temperature,high air flow,high pressure,and rapid thermal alternation.These severe service conditions may cause catastrophic failure of structural components.Strain measurement is an important method to predict and evaluate the durability and life of components.Thin film gages fabricated directly on the surface of tested components have attracted wide interest for in-situ strain monitoring due to small size,high accuracy and stable performance.PdCr（Pd:Cr=87:13 wt%）thin film strain gages are promising to be used in both dynamic and static strain measurements of high-temperature components operated up to 1000 ℃ due to their excellent structural stability,desired linear strain vs.temperature characteristics,and the insensitivity to heating and cooling rates.However,the poor oxidation resistance of PdCr thin films results in the increased resistance at high temperatures,which leads to inaccurate strain measurement and even failure of strain gages.On the other hand,indium tin oxide（ITO）thin films（In₂O₃:Sn O₂=9:1 wt%）,with higher melting point and excellent oxidation resistance,can be used for the strain measurement at high-temperature components operated above 1000 ℃.However,it has been reported that the ITO thin film strain gages have a relatively high resistance temperature coefficient with obvious variation with temperature,which can cause the inaccurate strain measurement and limit their applications.In addition,regardless of metal thin films or ITO thin film strain gages,the temperature effect is another important issue that limits their large-scale use.How to realize in-situ temperature self-compensation is the key issue to improve the measurement accuracy and performing static strain measurements of thin film strain gages.In this thesis,PdCr alloy and ITO ceramics are used as sensitive grid materials of thin film strain gages.The size of the sensitive grid,the substrate of constant strain cantilever,and the strain testing device at high temperatures were designed.PdCr and ITO thin film strain gages were fabricated on the alumina constant strain beam via microfabrication technology.By preparing protective layers on the surfaces of the PdCr thin films,the important role of the double protective layer in realizing the excellent piezoresistive response of the PdCr thin film strain gages was clarified.Through the high temperature heat treatment subjected to as-fabricated ITO thin films,the mechanism of improving the high temperature resistance and piezo-resistance stability of the ITO thin films by adjusting the heat treatment atmospheres and temperatures was studied.Finally,based on the basic principle of the temperature effect of metal and semiconductor thin films,the apparent strain caused by the temperature effect was reduced by designing suspended films and nano-layered thin films for PdCr and ITO thin film strain gages,respectively.The purpose is to provide research ideas and technical paths for improving the measurement accuracy and service life of high temperature thin film strain gages by studying high temperature oxidation resistance,piezoresistance stability,and temperature compensation effects of PdCr and ITO thin film strain gages.The main research work is as follows:（1）The Al₂O₃ protective layer and the Al₂O₃/Al double protective layer were prepared on the surface of the PdCr thin films by an ion beam sputtering system.After heat treatment at 800 ℃ for 10 h,The composition and microstructure of PdCr thin film without protective layer,PdCr thin film with Al₂O₃ protective layer and PdCr thin film with Al₂O₃/Al double protective layer were studied by XRD,SEM and FIB.The effect of the oxide layer thickness on the resistance and piezoresistive response of PdCr thin film strain gages after heat treatment at 800 ℃ for 10 h was investigated.It was found that this marked improvement in the piezoresistive response and stability of PdCr thin film strain gage with Al₂O₃/Al double protective layer can be attributed to the excellent structure stability and the oxidation resistance provided by in-situ formed Al₂O₃interlayer combined with sputtered Al₂O₃ overcoat.The piezoresistive response of the ITO thin film strain gages was measured at 1200 ℃.（2）The as-fabricated ITO thin film strain gages were heat-treated in air and nitrogen at high temperature,respectively.The changes in the preferred orientation and grain size of ITO crystals after heat treatment were analyzed by XRD and SEM.In addition,the chemical states of the elements,resistivity,carrier concentration and Hall mobility in the ITO thin films after heat treatment in different atmospheres and different temperatures of N₂ were qualitatively and quantitatively analyzed.It was found that the excellent piezoresistive stability of ITO films annealed in nitrogen at 800°C was mainly due to more nitrogen doping into the ITO lattice,which stabilized the ITO grain size and increased the Sn⁴⁺content with good high temperature stability.（3）Based on the principle that the resistance variation of suspended PdCr thin films at high temperature was affected by temperature change rather than the thermal expansion of the substrate,suspended PdCr thin film was fabricated in order to perform in-situ temperature compensation for PdCr thin film fabricated on the substrate by micromachining technology.The variation of the resistance value of suspended PdCr film and unsuspended PdCr film with temperature was tested in the temperatures from25 ℃ to 380 ℃.The results indicated that it is feasible to use the suspended PdCr thin film to achieve the temperature compensation for the PdCr thin film strain gage in a certain temperature range.（4）The TCR was designed to approach zero by adjusting the thickness ratio of ITO and Pt films with opposite resistance temperature coefficients.ITO/Pt nanolaminated thin films with different thickness ratios of ITO and Pt were prepared by co-sputtering technology.The variation of resistance with temperature was studied.The results indicated that it is feasible to realize self-compensation of ITO film by nanolaminated structure.