The Study Of CO₂ Gas Sensor Based On Surface Acoustic Wave

Carbon dioxide?CO₂?,which is the main source of the greenhouse effect,does harm to the marine environment and human health,the CO₂ emission reduction has attracted widespread attention all over the world.However,with the continuous improvement of the industrialization,CO₂ gas emissions are still increasing year by year,the trend is expected to slow in 2035.Therefore,now the most important thing is the monitoring of CO₂ concentration in the people's living environment,and the CO₂ concentration exceeded the warning made,to prevent its effect on people's health hazards,which requires high quality of CO₂ gas sensor.Surface acoustic wave?SAW?technology is the origin in the 80 s of the 19 th century.In recent years,with the development of wireless communication,digital circuit technology and semiconductor manufacturing technology,surface acoustic wave device applications has joined the army with radar,television and video extended to mobile communications,wireless sensor,networking,etc.more broad fields.The temperature sensors and RFID tags,which are supported by the SAW technique,have been reported in foreign countries.The humidity and pressure sensor have been reported.But the gas detection is just beginning.In this paper,a CO₂ gas sensor is designed and fabricated.And the static,dynamic and stability of the sensor are measured.The sandwith structure of “L-Alanine/Ni/Graphene” is used as the sensitive membrane of the sensor.The main contents are as follows:1.According to the perturbation theory,a new mechanism of surface acoustic wave sensing is proposed,in which the sensitive capacitance is coupled to the static capacitance of the SAW device.The mechanism of SAW sensor based on electric loading effect caused by the changing of static capacitance has been studied.The equivalent circuit model is established for the response of the electric load response based on the Butterworth-van Dyke Modified?MBVD?model.The feasibility of the model is verified by experiments.2.The structure of " L-alanine /Ni/Gr" was designed as a sensitive membrane for the surface acoustic wave gas sensor.In the structure,L-Alanine is the main sensitive layer,which reflects different polarization form at different CO₂ concentrations;Ni as the catalyst;and graphene is used to to adsorb the CO₂ gas molecules due to its specific surface area of 2630m2/g.In this paper,the synergistic effect of the three kinds of sensitive membrane materials is studied.3.The one-port SAW resonator has been designed and fabricated as CO₂ sensor converter.The center frequency of the resonator is 434.12 MHz.The preparation process has been optimized according to the practical problems in the fabrication.The L-alanine and Ni layer are prepared by electrochemical deposition method,the Gr layer is prepared by DC-electrodeposition.And the each sensitive membrane has been characterized.4.The CO₂ sensor with a "L-Alanine/Ni/Gr" sandwich structure sensitive membrane,which is designed and fabricated in this paper,is tested by the test platform.The results show that: in the whole measurement range?CO₂ concentration is 0⁴0000ppm?,the sensor can achieve high resolution and accuracy.At low concentration region?0²000ppm?,the sensitivity of the sensor is 2.51MHz/?ppm-1m-2?;while in the high concentration region?2000⁴0000ppm?,the sensitivity of the sensor is 0.46MHz?ppm-1m-2?.At the same time,the static characteristics and dynamic characteristics of the sensor are calibrated.The anti-interference ability and long-term stability of the sensor are tested and analyzed.