Vacuum Microbalance Apparatus for Rapid Determination of Low-Temperature Vaporization Rates
An apparatus has been constructed for the study of the vaporization rate of materials at temperatures below room temperature. The system employs a modified Cahn UHV RG Electrobalance®, mounted in a stainless steel vacuum chamber which can be cooled to liquid nitrogen temperature. For low-temperature studies, it is shown that a thermocouple can be connected to the sample without objectionably reducing balance sensibility. A radiant heater, operated by a high-gain proportional controller, maintains the sample temperature constant to within about 0.01 C. The rate of vaporization is monitored directly by employing a time derivative computer, which can measure rates as low as 10 µg/min. The computer makes it possible to investigate the vaporization kinetics of a material over three orders of magnitude change in vaporization rate, using a single sample, in a matter of hours. Thus, it is possible to know exactly when steady-state vaporization conditions have been established. Some initial results obtained on ice single crystals between −90 and −40 C are presented.
Finally, in addition to studying the vaporization of a material in vacuum, it is often of interest to investigate the effect of gases on the sublimation rate. It is possible with this equipment to introduce gases into the chamber at pressures up to 0.05 torr and use the direct rate readout to follow transient as well as steadystate effects.
KeywordsVaporization Rate Reference Voltage Torque Motor Sublimation Rate Voltage Follower
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