Extended Range QCM


by Don Wallace and Scott Wallace


The use of quartz crystal microbalances (QCMs) for the measurement of the amount of mass flux that is deposited on a surface for space applications has historically been limited to the use of crystals having a resonant frequency of, at most, 15MHz, because of the difficulty of working with the small dimensions in thickness that are associated with such crystals. This has limited the lower mass flux measurement to approximately 10-11 g/cm2-s, or 0.20 ang/Hz, if the condensate density is near unity. Until recently, this has been a sufficiently precise measurement of molecular flux tosatisfy the needs of the experimenter. However, the growing need for the precise measurement of, for instance, the erosion/deposition rate of ion thrustors, the erosion rate on low-orbit satellites and the precise measurement of outgassing over long periods of time, has necessitated increasingly lower mass flux measurement, translating into higher mass sensitivities. With the trend toward reduced satellite size, there is a corresponding need to dimensionally miniaturize the QCMs in order to place them into even smaller spaces.

A new series of QCMs and TQCMs (thermoelectrically-cooled QCMs) with crystal frequencies upward to 25 MHz, has recently been developed. These are not only physically much smaller than earlier models, but also extend the mass sensitivity range upward by a factor of 4.84 over the 15MHz theoretical value of 5.102×108 Hz cm2/g to ~2.47×109 Hz cm2/g, lowering the limit of discernible condensate thickness measurement to approximately 0.04 ang/Hz. The new TQCMs also increase the effective delta T, i.e. the temperature differential between the hot and cold sides of the Peltier in the TQCMs, from 86 C to 120 C, causing the lower temperature of the crystals to be between -75 C and -100 C when the QCM is operated at ambient temperature. Tests conducted under simulated space environments using these new miniaturized QCMs will be the subject of this paper.