Jilin University: Research and Development of Reconfigurable Monolithic Microwave Gas Sensor for Ultra Sensitive Ammonia Detection

Microwave gas sensors (MGS) have received widespread attention due to their advantages of low power consumption, non-contact detection, and room temperature operation. However, the performance of sensors is limited by sensitive materials and microwave circuits. Traditional planar resonators have limited sensitivity due to their electromagnetic field distribution characteristics and low quality factor.

Recently, a research team from Jilin University proposed a reconfigurable rectangular waveguide MGS to achieve high sensitivity at low concentrations.

Compared with planar resonators, waveguide resonators have stronger electromagnetic fields, higher quality factors, and larger sensitive regions, which are beneficial for constructing high sensitivity MGS. In addition, the chamber can be used as an air chamber to simplify the sensing system. In order to further improve the sensitivity of rectangular waveguides, the research team designed a resonant window between two rectangular waveguides to optimize the frequency selection and quality factor of the waveguide circuit.When the length of the resonant window is 53.00 mm and the width is 5.00 mm, the quality factor is the highest. The resonant cavity with high quality factor is very sensitive to the change in dielectric constant caused by the adsorption of gas by sensitive materials. The electric (E) field is concentrated at the center of the resonant window, while the magnetic (H) field is concentrated at both ends of the resonant window. The presence of sensitive materials does not change the distribution of the electromagnetic field.

The overall material has large, medium and micropores with three-dimensional interconnections, facilitates gas adsorption and diffusion, and can be precisely placed in the strong electromagnetic field area of the cavity. The abundant acidic site of the Al2O3 integral material can effectively bind to alkaline gas, and researchers choose the Al2O3 integral material to detect NH₃.In order to further enhance the sensing properties of NH, In2O3/Al2O3 was synthesized by hydrothermal method. In2O3 loading enhanced the conductivity of Al2O3 and provided more oxygen vacancies (FIG. 3I) as the active site for gas adsorption. The research team used In2O3/Al2O3 monolithic material to prepare rectangular waveguide MGS to detect NH3, and the measurement results are as follows.In2O3/Al2O3 can detect NH3 from 10 ppb to 10 ppm and exhibits high sensitivity (116.1 dB ppm-1) at concentrations below 50 ppb. In order to further broaden the detection range of the sensor, the research team prepared sensors with different mass sensitive materials, 0.2g In2O3/Al2O3 increased the detection limit to 800 ppm, but further increases in mass will lead to reduced detection range.To demonstrate the reconfiguration of rectangular waveguide MGS, the researchers achieved a continuous response to NH3 by replacing the integral material of three components, indicating its significant reconfiguration, that is, replacing the integral material that is sensitive to different target gases can detect different gases, achieving chip sensing.

The quality factor of the circuit and the microstructure of the sensitive material play a crucial role in the sensing performance. Therefore, the researchers analyzed the NH3 sensing enhancement mechanism based on the hierarchical porous structure and the high quality factor waveguide resonator.The rectangular waveguide operates in TE10 mode, and the resonant window is a combination of inductive and capacitive diaphragm. The electromagnetic field is concentrated in the resonant window, and the sensitive material located in the center of the window interacts with the high-density electromagnetic wave to obtain high sensitivity.According to the gas diffusion theory, the hierarchical porous structure effectively increases the relative amount of gas molecules in the sensitive material. According to the Maxwell-Garnett equation, an increase in the amount of gas adsorption causes a significant change in the effective dielectric constant, which ultimately enhances the sensing performance.

All in all, the researchers introduced a new method of combining the waveguide cavity and the whole material to prepare the sensor, overcoming the problem of the low sensitivity of the planar resonator, simplifying the sensing system and improving the sensitivity of the MGS. This reconfigurable chip sensor has great potential for practical gas sensing applications, and this design principle can be extended to other monolithic materials for gas sensing.