Characteristcs Of Wireless Sensing With Surface Acoustic Waves Resonator For High Temperature Application

Due to its high sensing accuracy,fast response,passive and wireless capability,surface acoustic wave(SAW)sensors are widely used in aerospace,environmental monitoring,military security etc..For structural health monitoring(SHM)in harsh environment,the relationship between resonant performance of SAW sensors with temperature and strain was studied in the dissertation by using wireless interrogating measurement setup.The protective coatings were used to improve the failure of the SAW sensors operating in harsh environments.Then,SAW sensors based on piezoelectric thin films directly deposited on metallic substrate by sputtering were explored to eliminate the effect of cement.In order to meet the requirement of temperature wireless sensing application at high temperature,SAW sensors based on LGS single crystals was investigated.According to the FEM simulation results,two SAW one-port resonators were fabricated on LGS single crystals with Euler angle of(0?,138.5?,117?)and(0?,138.5?,27?).The relationships between resonant frequencies with temperature of the SAW sensors were studied from 20 °C to 600 °C.Different resonant frequency-temperature relationships of the SAW sensors fabricated on different oriented LGS single crystals were observed.For the SAW sensors fabricated on(0?,138.5?,27?)oriented LGS single crystal,the resonant frequency decreases monotonically with the increased of temperature.While,for the SAW sensors fabricated on(0?,138.5?,117?)oriented LGS single crystal,the resonant frequency increased with the increase of temperature from room temperature to about 200 °C and then decreased with the further increase of temperature.Although SAW sensors based on LGS single crystals can be operated in high temperature,the performance of the SAW sensors deteriorated gradually.In order to improve the failure of the SAW sensors operating in high temperature,AlN protective layer was introduced.The effect of the protective layer on the performance of the SAW sensors was studied by simulation and experiment this dissertation.The results implied that the SAW velocity and the electromechanical coupling coefficient(K2)of the SAW devices increase with the thickness of AlN protective layer.However,the temperature coefficients of frequency(TCF)of the SAW devices decreased with the thickness of AlN protective layer.The most important result was that no performance degradation of SAW devices was observed within 10 h experimental duration.So,the introduction of AlN protective layer greatly improved the failure SAW sensors in high temperature.Based on the studies about the LGS SAW resonators,the strain sensing characteristics in high temperature was achieved by an own designed wireless strain measurement system.The sensors made from(0?,138.5?,117?)and(0?,138.5?,27?)oriented LGS single crystal were characterized under strain from 20 °C to 500 °C.The resonant frequency shift caused by strain was extracted.The gauge factors of the two kinds of SAW sensors increase with temperature,and the SAW sensor made from(0?,138.5?,117?)cut LGS single crystal had a little larger gauge factor than that made from(0?,138.5?,27?)cut LGS single crystal.The relative linearity errors of the two kinds of sensors were less than 2%.The thermal hysteresis errors of the two kinds of sensors were less than 5% even at 500 ?C.So the SAW sensors have good performance for strain measurement at high temperature.In practical strain measurements,the wireless SAW sensors still should be fixed on the metallic components by cement.So,the results were affected by the stiffiness of the cement frequently.SAW sensors made from piezoelectric thin films which were directly deposited on metallic components were explored in the dissertation.In order to distinguish from the wireless SAW sensors made from LGS single crystals,the SAW sensors based on piezoelectric thin films on metallic components were named by the integrated wireless SAW sensor.The feasibility of the integrated wireless SAW sensor was analyzed by simulation at first in the dissertation.And then,the integrated wireless SAW sensor were made from AlN thin films deposited by mid-frequency magnetron sputtering on TC4 titanium alloy components.The performance of the integrated wireless SAW sensor was characterized.The acoustic velocity and electromechanical coupling coefficient of the integrated SAW resonators on the thickness of AlN thin film were presented.And the temperature dependence of the integrated SAW resonators on the thickness of AlN thin films was also discussed.The results showed that the resonant frequency of the integrated wireless SAW sensor decreased with temperature.The temperature coefficient of frequency decreased from 89.4 ppm/°C to 66.2 ppm/°C when the thickness of AlN thin film increased from 1.5 ?m to 3.0 ?m.The K2 increased by about 30% when the temperature increased from room temperature to 350 °C.The strain sensing characteristics of the integrated SAW resonators showed good performance.The strain hysteresis error was only 1.12% at 350 °C,which is far less than that of the SAW wireless strain gauge based on LGS single crystals.So,the integrated SAW wireless sensors are feasible.In summary,SAW wireless sensors were fabricated for harsh environment in the dissertation.The SAW wireless sensors based on LGS single crystals can work well till 600 °C by introducing AlN thin film as protective layer.Although the SAW wireless sensors made from LGS single crystals have good performance for strain measurement at high-temperature,the effected of cement on the sensing results still exist especially at high temperature.SAW wireless sensors made from AlN piezoelectric thin films directly deposited on TC4 titanium alloy components were explored.