NASA Proposes Zeolite Coats for Contamination Monitoring
By Lisa A. Karter
Jet Propulsion Laboratory, Pasadena, CA–NASA is proposing to coat quartz-crystal microbalances (QCM) with a zeolite to increase the QCM`s sensitivity as a contamination monitor.
A QCM is used to measure concentrations of contaminants in the air, in vacuums, and in special scientific and industrial atmospheres. Zeolite would increase the quantities of volatile contaminants adsorbed and would then increase the QCM`s ability to monitor contamination. The zeolite formulation and the technique for applying it to a surface was discovered in 1966. An aluminum electrode on the sensing crystal of a commercial QCM is coated with the zeolite to the required thickness–a compromise– the thickness should be great enough to provide adequate range for collection of contaminants, but not so great that the mass of the coat saturates the response of the QCM. For example, on a typical QCM with a useful saturation mass of 3 mg/cm2 a zeolite coat 40 nm thick would collect 0.2 mg/cm2.
The enhanced ability of a zeolite-coated QCM to collect volatile materials at room temperature would eliminate the need for a temperature-controlled (cooled) QCM. This would be an advantage for monitoring air and other gases containing water because condensation of water on a cooled QCM gives rise to a contamination reading, which could mask the reading of another contaminant. In addition, the zeolite coat could retain the sample of volatile material(s) for later analysis.
Beyond the increase in adsorbance, the other unique adsorption and desorption characteristics to the zeolite coats could also enable the use of QCMs in ways not otherwise possible, according to JPL. For example, a zeolite-coated QCM could be operated in conjunction with a heater for use as an in situ thermogravimetric-analysis (TGA) device. As a TGA, heating would cause a slow, controllable desorption that could be measured by the response of the QCM. The desorption features (that is, peaks in the rate of desorption) should be more easily separable than in the case of a coated temperature-controlled QCM, when desorption is typically too fast to permit accurate characterization of its thermal-response transient. n
Editor`s Note: For more information on this technology, contact JPL`s Technology Reporting and Communication Office at (818) 354-2240. Use the following identification number when asking for information: NPO-19496.