Research On SAW Temperature Sensor With Wide Temperature Range

Surface acoustic wave(SAW)sensors are used in many areas of daily life due to their high sensitivity,small size,and good stability.At present,the research on SAW temperature sensors is mainly concentrated in the low temperature range.However,with the growth of social demand,temperature measurement in some special environments has become urgent:such as aerospace engines,power plant furnaces and other high-temperature and wide-temperature equipment health monitoring.Nevertheless,the application of SAW temperature sensors in a wide temperature range is mainly limited by the temperature characteristics of the piezoelectric substrate material.In this paper,SAW temperature sensors with LGS and CTGS piezoelectric substrates were designed.This article first studied the temperature characteristics of the SAW temperature sensor based on LGS.Three tangential Single-port SAW resonators are prepared on(0°,90°,80°),(90°,90°,36°),(0°,138.5°,26.6°)LGS substrate,and the corresponding resonant frequencies are 194 MHz,204 MHz,and 224.8 MHz.According to the dual resonator temperature sensor design proposed in the article,(0o,90o,80o)and(0o,138.6o,26.6o)resonators are connected in series to form the temperature sensor a,(90o,90o,36o)and(0o,138.6o,26.6o)resonators are connected in series to form the temperature sensor b.The temperature characteristics of the two sensors were tested from room temperature to 300°C.The results showed that the frequency of the(0o,90o,80o)and(90o,90o,36o)resonators first increased and then decreased as the temperature increased.The temperature transition points are at 178°C and 195°C,respectively;the relationship between the frequency and temperature of the(0o,138.6o,26.6o)resonator is monotonously decreasing,and the calculated temperature transition point is 18°C.At the same time,the temperature output expression of the corresponding sensor is obtained according to the fitting parameters,which proves the feasibility of the wide temperature test of the scheme.Looking for a piezoelectric material with a monotonous frequency-temperature curve as the piezoelectric substrate of the sensor is one of the methods to prepare a wide temperature range SAW temperature sensor,this paper selects CTGS as the focus of the research.The SAW resonator was fabricated on the(0o,90o,?)section and the temperature characteristics of 12 tangential CTGS were studied.The 12 kinds of tangential resonators all show the characteristics of symmetry about?=0°on the S₁₁curve,electromechanical coupling coefficient,and frequency temperature curve,which well verify the symmetry of the CTGS structure.Through the temperature test of 12kinds of devices at room temperature to 400?,it is found that the zero-frequency temperature coefficient cut type appears near?=38°.In addition,there are 5 kinds of tangential resonator frequency-temperature changes that are always monotonously decreasing.Among them,the tangential temperature transition point of(0°,90°,0°)is the lowest.Therefore,the tangential direction(0°,90°,0°)is finally selected as the piezoelectric substrate of the temperature sensor with a wide temperature range.Resonators with different structural parameters were prepared on the(0°,90°,0°)tangential CTGS to optimize the performance of the sensor.Through comparative experiments on devices with different interdigital logarithms,apertures,interdigital gaps,and metal film thicknesses,the final parameters were determined.The optimized device was used for high and low temperature testing.The experimental results showed that the frequency-temperature of the sensor showed a linear change in the range of-100??400?.In this temperature range,the temperature sensitivity obtained by linear fitting is as follows:146.84?/MHz,which has obvious advantages compared to LGS in this temperature range.The test data of a wider temperature range(-100??1100?)is fitted with a quadratic function,and the temperature coefficients TCF1 and TCF2 are extracted,and finally the temperature solution expression is determined according to the fitting parameters.The feasibility of the SAW temperature sensor is verified by the temperature test comparison with the thermocouple.