Most industrial establishments and scientific laboratories require the use of harsh environment gas sensors. In such setups, the operating environments for the sensors can be as high as 400 °C, which, solely (or associated with high pressure and/or severe vibration) will disqualify most room temperature gas sensing materials such as polymers, biofilms, and quartz, as candidates for harsh environment gas sensors due to degradation and unrecoverable damage to their functionality. Currently, there are very few materials that can withstand the tortures of high or ultra-high temperatures without damage or oxidation. As a result, research has focused on metal oxides and piezoelectric crystal resonators for high temperature gas sensing as they are either oxidized or stable at high temperature. To be specific, high temperature piezoelectric materials such as langasite (LGS), langatate, have attracted researchers’ attention due to their outstanding high temperature performance. Nevertheless, the LGS resonators have some inherent shortfalls; for instance, they are too bulky to be integrated with outer read-out circuits. Therefore, there is a strong demand for miniaturized LGS micro-electromechanical systems (MEMS) resonators with high sensitivity for in situ real environment gas sensing.
Previous studies showed that such a system has been developed; however, the sensitivity of the LGS gas sensor is still incompatible with other existing gas sensors, despite it being optimized. Fortunately, this can be overcome by introducing an extra sensing layer to capture more gas molecules and thus increase the mass of the adsorbed gas. In this view, researchers from the University of North Texas, USA: Dr. Chen Zhang, Dr. Abhishek Ghosh and Professor Haifeng Zhang together with Dr. Wei Jiang and Dr. Guoan Wang at the University of South Carolina and Dr. Felix Wu at the US Department of Energy, proposed the idea of incorporating metal nano oxides, i.e. ZnO nanorods, as the extra sensing layer to overcome the aforementioned shortfalls, in the harsh environment (high temperature) gas sensing applications. Their work is currently published in the research journal, Smart Materials and Structures.
In their approach, the miniaturized LGS MEMS micro-cantilever beam with one end fixed to the substrate was firstly structured by wet chemical etching. Afterwards, the LGS MEMS micro-cantilever beam was deposited with ZnO nanorods. The fabricated sample was firstly tested at different temperatures to determine the optimum operating temperature, and then tested by monitoring CO2 gas concentration in air, N2, and a mixed gas environment.
The authors reported that in comparison to other major resonator-based gas sensors, the proposed ZnO-enhanced LGS MEMS micro-cantilever beam gas sensor displayed commendable gas sensing sensitivity in terms of the resonant frequency drift of the LGS MEMS resonator. Further, they noted that the nature of the cross-sensitivity of ZnO nanorods could be explicitly addressed by PCA technology, where important features could be extracted to distinguish different gas species and/or concentrations.
In summary, the study presented the design and fabrication of a miniaturized LGS MEMS micro-cantilever beam resonator that could be adopted for harsh environment gas sensing applications. Generally, the approach employed involved deposition of ZnO nanorods chemically on the resonator surface. Remarkably, the proposed MEMS LGS resonator can be utilized for harsh environment applications where high temperature can hamper most room temperature gas sensors. In a statement to Advances in Engineering, the authors highlighted that if combined with appropriate machine learning technology, the proposed gas sensor can be a good alternative to expensive, bulky gas chromatography devices.
Chen Zhang, Wei Jiang, Abhishek Ghosh, Guoan Wang, Felix Wu, Haifeng Zhang. Miniaturized langasite MEMS microcantilever beam structured resonator for high temperature gas sensing. Smart Materials and Structures, volume 29 (2020) 055002 (12pp).