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 ${\mathrm{(ᾱ}}_{\upsilon }\text{\hspace{0.17em}≈\hspace{0.17em}0.5)}$ . 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 ${\mathrm{(ᾱ}}_{\upsilon }\text{\hspace{0.17em}≈\hspace{0.17em}1.0)}$ . The relative vaporization rates of monomer and dimer were measured using a quadrupole mass spectrometer. Vaporization activation energies $E_D\text{*\hspace{0.17em}=\hspace{0.17em}53\hspace{0.17em}±\hspace{0.17em}1}\mathrm{kcal}/\mathrm{mole}$ and $E_D\text{*\hspace{0.17em}=\hspace{0.17em}62\hspace{0.17em}±\hspace{0.17em}1}\mathrm{kcal}/\mathrm{mole}$ 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 ${\mathrm{(ᾱ}}_{\upsilon }\text{\hspace{0.17em}≈\hspace{0.17em}0.1)}$ . The average activation energy of vaporization for these crystals was $\text{Ē*\hspace{0.17em}≈\hspace{0.17em}63}\mathrm{kcal}/\mathrm{mole}$ 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.