The Research Of Microwave Gas And Humidity Sensor Sensitization Based On Filter Design

In recent years,the microwave sensors are widely used in environmental safety,medical health,industrial measurement and food safety.Microwave sensors have the characteristics of passive detection,which simplifies the complexity of the measured system.Microwave sensors can be integrated into other microwave devices to realize the miniaturization and integration of sensors.Microwave signals have wireless transmission performance,which lead to microwave sensors more convenient to wireless detection and non-invasive detection.Moreover,microwave sensors have more characteristic parameters,lower power consumption,higher versatility and stability,and better system compatibility,which is more appropriate to the demand of sensors.Therefore,high performance microwave gas sensors and humidity sensors were fabricated by high performance microwave circuit design.The relationship between different characteristic parameters and sensor performance was investigated,and the matching mechanism between microwave circuits and sensitive materials was discussed.On the basis,the sensitivity mechanism of microwave sensors was researched and discussed.The details of the research are as follows:The sensitization of microwave gas sensors was realized by high performance filter design.Firstly,an ultra-narrow stop band filter was designed based on coupled-lines sections.The input impedances,reflection coefficients and S-parameters were calculated by the even-and odd-mode equivalent circuit to provide a theoretical basis for filter design.The electromagnetic field simulation software was used to determine the coating position of sensitive material,and WO₃@C was selected as the sensitive material to be coated on the gap of coupled-lines sections.The dielectric properties of WO₃@C were changed after adsorption of the target gas,which disturbed the electromagnetic filed distribution of coupled-lines sections and led to the magnitude variation of S₁₁ at the resonant frequency,and the sensing for target gas was realized.The fabricated microwave sensor has the lower detection limit for 10 ppm of NH₃ at room temperature（20℃）,and the sensitive mechanism was analyzed and discussed based on the equivalent circuit of coupled lines sections.The matching relationship between microwave circuits and sensitive materials was researched and discussed.The NH3 sensor was fabricated based on commercial graphite powder,and an optimization algorithm for the sensor response was proposed.By the electromagnetic field simulation of the microwave circuit,there was a transverse electric field between the coupled-lines sections.Based on the measurement results,it was found that the sensor response was mainly variation of magnitude caused by the change of conductivity.Therefore,the matching mechanism between microwave circuit and sensitive material was proposed by combining the characteristics of electromagnetic wave transmission and chemical properties of sensitive material.The graphite powder was selected as sensitive material for proving proposed matching machine.Combining the ultra-high adsorption capacity of commercial graphite powder and stronger electromagnetic field distribution on the coupled lines sections,the performance of gas sensor was further enhanced.The fabricated microwave gas sensor was sensitive at the room temperature（20℃）with the detection limit 1 ppm,high selectivity and humidity-resistance.Moreover,a novel sensor response calculation method with energy loss was proposed based on the energy attenuation generated by electromagnetic wave during the transmission of graphite powder.Similarly,the sensitive mechanism was analyzed by the equivalent circuit of the coupled lines sections.A microwave dual-mode differential humidity sensor was designed based on microwave differential circuit and commercial PVA,which used the advantage of multi-characteristic parameters of microwave sensors.To solve the baseline shifting of humidity sensors,the differential circuit structure was adopted,and the reference port was used to sensing the variation of environment.When the environment humidity changed,the dielectric constant and conductivity of PVA coated on the sensitive port would be changed,which led to the variation of frequency and magnitude for sensitive port.The reference port was used to realize baseline calibration by sensing the changing of environment.The proposed sensor has a good frequency and magnitude dual-mode response for 20%RH-85%RH,which the frequency and magnitude sensitivities were1.5MHz/%RH and 0.39d B/%RH,respectively.Moreover,the magnitude response was more sensitive at the lower humidity,which enhanced the lower frequency sensitivity at the lower humidity.The sensing performance of reference port for temperature was verified by measurement.A three-dimensional microwave sensor was designed based on waveguide filter structure,which overcame the quality factor limitation of planar circuits.The waveguide could provide a complete electromagnetic wave transmission mode with a high-quality factor because of the three-dimensional structure.A bandpass filter was designed and adopted as sensitive circuit.By the theoretical calculation and electromagnetic field simulation,the physical size and electromagnetic field distribution of resonant windows were determined.For the characteristics of resonant window structure,the In₂O₃/Al₂O₃ monolith was selected as sensitive material and placed at the strongest electric field of resonant windows.Combining the high-quality factor of waveguide cavity and the porous and multi-reactive sites of the graded monolith material,proposed gas sensor has the lower detection limit of 10 ppb and the good selectivity for NH₃ at the room temperature（20℃）.Meanwhile,the proposed gas sensor was more reconfigurable because the monolith material could be placed at the resonant window directly without coating,and different target gas sensing could be realized by replacing different sensitive material.As shown above,this paper was aiming at the development of high-performance microwave sensors and focusing on the sensitization by microwave filter design.The matching relationship between microwave circuits and sensitive material was researched,and the analysis method based on multi-characteristic parameters was proposed.The sensitive mechanism of microwave sensors was revealed,and important reference was provided for the development and application of microwave sensors technology.