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 to satisfy 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.