Development of acoustic wave sensors for continuous monitoring of mercury vapour in an alumina refinery

The research aim of this project was to develop a sensor capable of detecting mercury in gaseous flow in alumina refineries for a concentration range from 1 to 10 mg/m3 at temperatures of 20-80 °C. The objectives were set to monitor the mercury in an industrial environment. <br><br>Quartz crystal microbalances (QCMs) with gold films were investigated and utilized in mercury sensing. When mercury amalgamates with thin gold film it changes the frequency of the device and the rate of the frequency change corresponds to the mercury concentration. The researcher’s goal was to improve the device sensitivity via gold film surface modification. In a QCM, the sensing mechanism is predominantly surface-dominated. Surface features of small grain size, large aspect ratio and, high porosity are required to realize a highly sensitive QCM. Gold covered nano-features on the surface of QCM electrodes were employed for increasing the surface to volume ratio of the device to enhance its sensitivity. <br><br>Three different methods were employed for the formation of nanostructured films which were covered with gold to form the mercury sensing layers. Nanofeatured thin films were successfully fabricated on the QCM electrode surfaces using thermal evaporation, anodization and nanocarving processes. These processes were chosen as they were compatible with QCM standards and the rigid structure of QCM allowed and facilitates such surface manipulations. <br><br>Thermal evaporation was used for the deposition of nanostructured molybdenum oxide (MoO3). These nanostructures comprised of hexagonal nanoplatelets with the majority of the side dimensions of less than 250 nm and thicknessed of less than 20 nm. Anodization process was used on QCMs’ titanium films electrodes. The nanopores of diameters in the range of 10-20 nm, pores length of 300-600 nm and inter-gaps in the order of 20-40 nm were obtained. Nanocarving process was used for carving the titanium thin films with H2 at elevated temperatures. Nano-rods of diameters 20-150 nm, lengths of up to 600 nm, and spacing in the range of 70-150 nm were observed.<br><br>The author employed an extensive range of characterisation equipment to understand films’ properties. In addition, different techniques were employed to correlate the gas sensing behavior to the films’ micro/nano characteristics. These analysis techniques include Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) which allowed visual analysis of the morphology of the thin films. Crystallization and grain structures were inspected by Transmission Electron Microscopy (TEM) analysis, crystallite phases were determined by X-Ray Diffraction (XRD) analysis, surface area of a material was assessed by Brunauer Emmett Teller (BET) analysis and determination of the presence of metals was carried out by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) analysis. <br><br>A computer controlled 4-channel gas calibration system was designed and developed to calibrate and test the sensors towards mercury, humidity and ammonia vapours. Compared to non-modified QCMs, the thermally evaporated nanostructures (MoO3) and anodized nonporous films (TiO2) showed larger sensitivities towards mercury vapours which were attributed to the large surface to volume ratio of these structures. The effects of interferants were studied and the optimum operating temperatures were obtained.<br>