Studies of the Vaporization Mechanism of Ice Single Crystals

摘要

The kinetics of the vacuum sublimation of ice single crystals has been investigated by a vacuum microbalance technique in the temperature range minus;90 to minus;40deg;C. The vaporization coefficient agr;vequiv; (observed vaporization rate)divide;(theoretical maximum rate) and the activation enthalpy of sublimation,Dgr; Hs*, vary markedly with temperature in this range. At temperatures below about minus;85deg;C,agr;vequals;1andDgr; Hs*equals the thermodynamic enthalpy of sublimationDgr; Hsdeg;. Between about minus;85 and minus;60deg;C, agr;vdecreases slowly with increasing temperature,Dgr; Hs*lsim;Dgr;Hsdeg;. Between about minus;60 and minus;40deg;C, agr;vdecreases progressively more rapidly with increasing temperature andDgr; Hs*decreases to a highhyphen;temperature limiting value ofsime;12Dgr; Hsdeg;. The effects of various experimental parameters such as crystal orientation, doping with impurities and adsorbed gases on the ice vaporization kinetics have also been investigated. Neither grain boundaries nor crystal orientation has a measurable effect on the rate. Ice doped with monovalent impurities was found to vaporize at steadyhyphen;state rates that were uniformly lower over the entire temperature range of the study. Also, NH3(gas) and HF (gas), present in the ambient at pressuressim;10minus;3ndash;10minus;2torr, reduce and increase, respectively, the ice vaporization rate. The experimental results, along with previously reported physicalmdash;chemical properties of ice are used to arrive at a vaporization mechanism: Ice at equilibrium with the vapor has a surface population of a highly mobile species assumed to be water molecules hydrogen bonded to only one nearest neighbor. These energetic molecules are the source of the vapor flux leaving the surface. At sufficiently low temperatures, vacuum vaporization does not occur rapidly enough to alter this equilibrium surface population. Sublimation at higher temperatures, however, depletes the population to a progressively greater extent with increasing temperature. Thus the ratehyphen;limiting step in vaporization, which is thedesorptionof the mobile water molecules at low temperatures, changes to theirformationat high temperatures.