BEHAVIOR OF QUARTZ-CRYSTAL MICROBALANCE IN NONADSORBED GASES AT HIGH-PRESSURES

The purpose of this work is to determine experimentally the frequency shift of a quartz crystal resonator in contact with an inert gas, over a wide range of density of the gas, and to evaluate the various facto rs affecting this observed behavior. Four different gases were used (A r, N-2, He, and H-2), allowing a wide range of densities to be studied , over a range of pressure of 0.5-50 atm. Three types of surfaces were used: gold, as obtained from the manufacturer of the quartz crystal; the same electroplated with a rough layer of nickel; the same nickel s urface polished mechanically. Hydrogen was not studied in combination with nickel because absorption of the gas occurs spontaneously at room temperature. The total frequency shift observed can be divided into c ontributions due to added mass, to the pressure, and to the product of density and viscosity. Under the present experimental conditions the effect of added mass is negligible, since adsorption does not occur. T he effects of pressure, density, and viscosity have been calculated fr om equations given in the literature. When all these are corrected for , there is a residual effect associated with the interaction of the fl uid with a rough surface. A detailed analysis of the effect of roughne ss on frequency cannot be made, since the roughness is of a random nat ure. Theories developed for a number of limiting cases indicated that the frequency shift associated with surface roughness should be propor tional to the density of the fluid. This relationship was found to hol d experimentally. The roughness affecting the frequency of the quartz crystal resonator cannot be related in a simple manner to the surface roughness determined by standard electrochemical methods or by STM. Ne vertheless, it will be possible to detect changes in surface morpholog y by their effect on the frequency in most cases.