A Surface Acoustic Wave Mercury Vapor Sensor

An ST cut quartz 261 MHz surface acoustic wave (SAW) delay line mercury vapor sensor was designed, fabricated, and tested. A gold sensing film was sputtered onto the delay path of a SAW device to collect mercury for detection. The sensor\u27s ability to detect mercury was due to the strong interaction between gold and mercury, known as amalgamation. In the present work, a large number of gold films of various thicknesses were exposed to low concentrations of gaseous mercury. It was shown for thinner films (i.e. 25A) the total amount of mercury that could be absorbed was limited; however, the response time for this film was fast and the response slope was linear with respect to mercury concentration. It was also shown that the slope responses for the thicker films (i.e. 500A) were linear with respect to mercury concentration, but the response time and the response magnitude were significantly reduced. In the case of the thinner film, the decrease in frequency was attributed primarily to mass loading, while for the thicker film the changes in frequency were due to both mass loading and elastic stiffening. For the thick film the mass loading response (frequency decrease) was offset by the elastic stiffening response (frequency increase). This resulted in a decrease in response slope resolution with respect to mercury concentration. In conclusion the present work clearly indicates that a judicious choice of gold film thickness between these two thickness extremes results in a mercury sensor capable of very rapid detection of low mercury concentration levels with high resolution over a wide dynamic range