Studies of the Evaporation Mechanism of Sodium Chloride Single Crystals

Abstract

The vacuum evaporation rate of the (100) face of pure sodium chloride single crystals has been measured with a vacuum microbalance in the temperature range 450°–650°C. The steady-state vaporization rates of pure crystals with dislocation densities of ∼1 × 106/cm2 were lower by about a factor of two than the maximum thermodynamic rate (ᾱυ ≈ 0.5). When the dislocation density was increased an order of magnitude (to ∼ 107/cm2) by straining the crystals, the steady-state evaporation rate increased to approximately the thermodynamic rate (ᾱυ ≈ 1.0). The relative vaporization rates of monomer and dimer were measured using a quadrupole mass spectrometer. Vaporization activation energies ED* = 53 ± 1 kcal/mole and ED* = 62 ± 1 kcal/mole were found for the monomer and dimer, respectively, in the temperature range of the experiments for low dislocation crystals. No dependence of the monomer to dimer ratio on the dislocation density was detected. The presence of the divalent cation impurity, Ca2+, in the NaCl crystals markedly reduced the vaporization rate and increased the activation energy of vaporization. Crystals doped with 300 ppm Ca2+ exhibited vaporization rates approximately an order of magnitude lower than the maximum thermodynamic rate (ᾱυ ≈ 0.1). The average activation energy of vaporization for these crystals was Ē* ≈ 63 kcal/mole as computed from the total rate of vaporization. The monovalent impurities, Br−, OH−, O2−, had no detectable effect on the vaporization rate. An evaporation mechanism was postulated which could account for the experimental observation.

Studies of the Sublimation Mechanism of Solids

“…It is well known (Somorjai and Lester, 1967;Lester and Somorjai, 1968) that the surface which ,vaporizes into vacuum becomes !trough", i.e.…”

“…of vapor) and the arsenic (E *-~ 142 .. ,kcal/niGleof vapor) H~rystal faces'; respectivelY~_ This l' esult indicates that the formation of the tetramer molecules is dependent on some physical-chemical or structural property of the particULar crystal face. Differences in dislocation densities (Lester and Somorjai, 1968), charge density distributions (Somorjai and Lester, 1967), or impurity concentrations (Somorja'i and Lester, 1967) in the (111) and (iii) faces may play an important role in determining the surface concentration of the vaporizing specieE~…”

“…It should be a listable" surface which retains its crystallographic orientation throughout the experiments (Somc)rjai and Stemple,1953)~· It is important to determine its dislocation density using etch-pit count techniques (Lester andSomorjai, 1968) and' -8-to investigate its surface topology using optical or electron microscopy (Searcy, 1968). It has been shown that the surface structure can markedly influence the evaporation rates of many solids (Lester and Somorjai, 1968;Rosenblatt and'Lee, 1968;Searcy, 1968) •. The bulk properties of the vaporizing crystal samples should also be well characterized.…”

“…Certain solids may vaporize by two different reaction paths -6-which can operate simultaneously at different surface sites (Lester and Somorjai, 1968).…”

“…Sodium chloride vaporizes according to the dominant net reaction (Lester and Somorjai) 1968;Berkowitz and Chupka, 1958) NaCl ( Dislocation densities were determined by counting chemically formed etch pits ona (100) cleavage face which was used in all 0f the vaporization studies. Most of the pure NaCl crystals had etch pit densities of 0.8 -4XI0 6 /cm 2 • We have assumed a constant proportionality between observed etch pits and dislocations which intersect the surface under observation and that the proportionality f'actor is close to one.…”