Laboratory tests on mercury emission monitoring with resonating gold-coated silicon cantilevers.

To measure extremely low concentrations of mercury vapor in gases as encountered in flue gases of coal-fired power plants, accurate and reliable online and/or portable mercury detection systems are needed. As discussed in this communication, resonating silicon-based cantilevers coated with thin films of gold change their resonant frequency when exposed to mercury vapors and could serve as the basis for such sensing devices. Two different types of commercial AFM cantilevers, which differed by physical dimensions and surface finish, were coated with a 10 nm film of gold and were tested in streams of argon containing mercury. The argon flow rates ranged from 5.7 to 57.4 ml/min, carrying mercury vapors at concentrations between 37 and 700 microg/m3. The results show that smaller cantilevers (approximately 140 microm x 40 microm x 4 microm) with a resonant frequency of 270-275 kHz were sensitive to less than 10 picograms of mercury, whereas larger cantilevers (approximately 245 microm x 50 microm x 7 microm) with a resonant frequency of 155-165 kHz have a sensitivity about 10 times lower. The results indicate that the kinetics of mercury capture by the gold coating follows a simple power law-correlation with the mass change (delta m) being proportional to t(n), where t is the capture time and n depends strongly on the concentration of mercury in the gas. It is also demonstrated that the mercury can be stripped off the gold coating by heating to 350 degrees C, which would allowthe cantilevers to be regenerated and reused.