Anisotropic wetting of ZnO by Bi_2O_3 with and without nanometer-thick surficial amorphous films

摘要

Anisotropic formation of equilibrium-thickness Bi_2O_3-enriched surficial amorphous films (SAFs) on ZnO has been documented Luo J, Chiang Y-M. Acta Mater 2000;48 is to 4501. This study further explores anisotropic wetting of ZnO single crystals by Bi_2O_3-rich liquid with and without SAFs. For Bi_2O_3 on the ZnO {11 2-bar 0} surfaces wherein nanometer-thick SAFs are present in equilibrium with partial wetting drops, the measured (advancing) contact angle decreases with increasing temperature, but it stabilizes at approx 6 deg above approx 860 deg C. In contrast, the contact angle is virtually a constant on the {1 1-bar 00} surfaces where SAFs are not present. This observation suggests that wetting in the presence of nanoscale SAFs follows a generalized Cahn wetting model. Faceted ridge formation at the triple lines and associated pinning effects are observed. Observation and analysis of unique two-stage isothermal drop receding kinetics show that complete wetting does not occur up to 1050 deg C and that the receding contact angle is estimated to be approx 4 deg via two methods. Quantitative evaluation of a thermodynamic model shows that the observed "residual" contact angle of approx 4-6 deg and the extended SAF stability can be explained on the basis of a significant attractive London dispersion force.