(Invited) Nano-Derived Microsensors for Monitoring Gas Species in Harsh-Environments

The objective of our work is to develop nano-derived, micro-scale sensors to monitor various gases within extreme or harsh-environments.  These environments include both high temperature and pressure applications, where sensors are required to monitor emissions in coal gasifiers and turbine generators.   The environment may also include applications in extreme cold and low pressure applications, where sensors are required to monitor chemical leaks outside space stations, vehicles and satellites.    This review includes both of these two extreme temperature and pressure environments and the nano-enhanced electrochemical and electromechanical sensor arrays developed to for these applications. 

In the case of the high-temperature emission sensing, nano-derived materials to detect gases such as H₂, SO_x, and H₂S within high-temperature environments were investigated up to 1000°C. The micro-sensors investigated in this work are based upon a chemi-resistive sensor platform. The functionality of these micro-sensors is dependent upon the incorporation of refractory MOS nanomaterials within thin-film electrode patterns.  The selective nanomaterials used for this work were synthesized by hydrothermal process. A range of baseline binary (WO₃ and MoO₃) and complex ternary tungstate_, molybdates, and zirconate compositions were successfully synthesized by co-precipitation and/or hydrothermal methods. 

The sensing materials were deposited on macro- and micro-sensor platforms, and the chemi-resistive response of the sensing materials were investigated for SO₂ and H₂S exposure at levels of 100 to 3000 ppm and 50 to 300 ppm, respectively.  The electrode used was an interdigitized electrode (IDE) pattern.  Both CO and H₂cross-sensitivity tests were also included.

The sensing nanomaterials were also deposited over high-temperature stabile Pt IDEs, which were developed at WVU with dimensions in the millimeter to micron size range set upon a polished alumina substrate. The resistance change as a function of exposure time, temperature, and gas composition was monitored. The measurements were conducted at temperatures from 600 to 1000 °C, where enhanced sensitivities as high as 50-90% were achieved.  Post-mortem microstructural and compositional analysis including XRD, XPS, SEM and EDS was completed to better understand both the sensing and degradation mechanisms.

In the case of the low temperature and pressure leak sensors (for space applications), a micro-Surface Acoustic Wave (mSAW) sensors was investigated, which depends upon molecular interaction of the leaked species (such as fuel or life-support gasses) with the nano-enhanced surface of the sensor.  In this case, the sensor would be dependent upon both mechanical and chemical interaction of the leaked specie with a micron-thick membrane, which would result in a high-frequency vibration.  The mSAW structure is based upon a pair of inter-digitized electrodes (IDEs) set on a piezoelectric material, where a mechanical wave can be launched perpendicular to the surface onto the surface of the material.  The interaction of the leaked fuel (or oxidant) with the membrane-supported mSAW sensor results in a frequency phase shift of the resonant frequency.   The work describes the effect of the various electrode architectures (which includes bus bar height, finger dimensions, finger spacing and delay length) on the sensing response.  In addition, the effect of key aspects of the piezoelectric film thickness, microstructure, and roughness, and further nano-enhancement to the sensor surface, was also investigated.  The sensors were tested on a simulant gas between -150 to 200°C from standard pressure to 10^-2Torr.     

Acknowledgements:

The high-temperature emissions research was funded by US Department of Energy University Coal Research (UCR) program under contract no. DE-FE0003872.  The space leak sensor work was sponsored by NASA Goddard Space Flight Center under grant no. NNX10AD17A and prime contract n. NNG10CR16C.  The authors would also like to WVU Shared Facilities.