Nano-derived sensors for high-temperature sensing of H2, SO2 and H2S
There is also a substantial necessity for microsensors that can be applied inexpensively, have quick response time and low power consumption for sustained operation at high temperature. In order to develop a high temperature compatible microsensor, this work will discourse issues related to sensor stability, selectivity, and miniaturization. It has been shown that the integration of nanomaterials as the sensing material within resistive-type chemical sensor platforms increase sensitivity. Unfortunately, nanomaterials are not stable at high temperatures due to sintering and coarsening processes that are driven by their high surface to volume ratio. Therefore, new hydrogen and sulfur selective nanomaterial systems with potentially highly selective and stable properties in the proposed harsh environment were investigated. Different tungstates and molybdates (WO3, MoO3, MgMoO4, NiMoO4, NiWO4, Sr2MgWO6 (SMW), Sr2MgMoO6 (SMM), SrMoO4, and SrWO4) were investigated at the micro- and nano-scale, due to their well-known properties as the reversible absorbents of sulfur compounds. Different morphologies of aforementioned compounds as well as microstructural alterations were also the subject of the investigation. The fabrication of the microsensors consisted of the deposition of the selective nanomaterial systems over metal based interconnects on an inert substrate. This work utilized the chemi-resistive (resistive-type) microsensor architecture where the chemically and structurally stable, high temperature compatible electrodes were sputtered onto a ceramic substrate. The nanomaterial sensing systems were deposited over the electrodes using a lost mold method patterned by conventional optical lithography.
Development of metal based high temperature compatible electrodes was crucial to the development of the high temperature sensor due to the instability of typically used noble metal (platinum) based electrode material over ceramic substrates. Therefore, the thermal stability limitations of platinum films with various adhesion layers (titanium (Ti), tantalum (Ta), zirconium (Zr), and hafnium (Hf)) were characterized at 800 and 1200°C. Platinum (Pt)-zirconium (Zr)-hafnium (Hf) were investigated. The high-temperature stable composite thin film architecture was developed by sequential sputter deposition of Hf, Zr and Pt. In addition to this multilayer architecture, further investigation was carried out by using an alternative DC sputtering deposition process, which led to the fabrication of a functionally-gradient platinum and zirconium composite microstructure with very promising high temperature properties. The final process investigated reduced labor, time and material consumption compared to methods for forming multilayer architectures previously discussed in literature.
In addition to electrical resistivity characterization of the different thin film electrode architectures, the chemical composition, and nano- and micro-structure of the developed nanomaterial films, as well as sensing mechanism, were characterized by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS), atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), Raman spectroscopy, temperature programmed reduction (TPR) and transmission electron microscopy (TEM). The macro-configurations of the sensors were tested and analyzed for sensitivity and cross-sensitivity, response time and recovery time, as well as long term stability. The microsensor configuration with optimized nanomaterial system was tested and compared to a millimeter-size sensor platform in terms of sensitivity and accuracy. Electrochemical relaxation (ECR) technique was also utilized to quantify the surface diffusion kinetics of SO2 over the chosen sensor material surface. The outcomes of this research will contribute to the economical application of sensor arrays for simultaneous sensing of H2, H2S, and SO2.