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Abstract: Anisotropic formation of equilibrium-thickness Bi2O3-enriched surficial amorphous films (SAFs) on ZnO has been documented [Luo J, Chiang Y-M. Acta Mater 2000;48:4501]. This study further explores anisotropic wetting of ZnO single crystals by Bi2O3-rich liquid with and without SAFs. For Bi2O3 on the ZnO 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 ∼6° above ∼860°C. In contrast, the contact angle is virtually a constant on the 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°C and that the receding contact angle is estimated to be ∼4° via two methods. Quantitative evaluation of a thermodynamic model shows that the observed “residual” contact angle of ∼4–6° and the extended SAF stability can be explained on the basis of a significant attractive London dispersion force. [Copyright &y& Elsevier]

Abstract: Several grain boundary complexions (grain boundary interfacial phases) have been identified in TiO2 bicrystals by high-resolution transmission electron microscopy (TEM) and aberration-corrected scanning TEM (STEM). An intrinsic grain boundary with no apparent impurity segregation was observed in an undoped TiO2 bicrystal. In a WO3-doped TiO2 bicrystal, WO3 second-phase particles formed along the boundary, with a nominally clean, intrinsic-type grain boundary in between the particles. In a CuO-doped bicrystal, a remarkable series of three distinct grain boundary complexions with abrupt structural transitions was discovered coexisting at the grain boundary, and the existence of a fourth equilibrium complexion at the annealing temperature was implied. Thus, the WO3- and CuO-doped TiO2 bicrystals exhibit dramatically different solute partitioning behavior which can be understood in terms of the relative interphase boundary energies of these two systems. STEM–electron energy loss spectroscopy and energy-dispersive X-ray spectroscopy analysis of the nanoscale lens-shaped films of amorphous material in the CuO-doped TiO2 bicrystal demonstrated an excess of CuO, as expected, yet also revealed the unintentional presence of SiOx. The multiple grain boundary complexions in CuO-doped TiO2 offer an explanation for the CuO-enhanced grain growth and sintering of TiO2 that has been reported in the literature. Conversely, the intrinsic grain boundary complexion observed in WO3-doped TiO2 is consistent with previous work showing that WO3 has no effect on grain boundary mobility in TiO2. A phenomenological thermodynamic model is proposed to explain the physical origin of these observed grain boundary complexions and the abrupt, first-order complexion transitions that are believed to occur upon cooling of the CuO-doped TiO2 bicrystal. [Copyright &y& Elsevier]