Liquid oxide surface energy

Although fused oxides and halides have been less extensively studied than liquid metals, surface energies have been determined for a number of such compounds. In the absence of models for estimating the surface energy of oxide or halide mixtures, this quantity must be determined experimentally. 

This equation shows that the surface energy per atom correlation between the surface energy per atom and the energy of sublimation (or evaporation) is expected provided mi is constant. Such correlations hold well for the solid and liquid surfaces of metallic bodies and also for the liquid surfaces of oxides and halides (see Figures 4.1,4.9 and 4.10). 

Sessile drop experiments are also used to measure the effects of temperature on liquid surface energies. Because the temperature coefficient dliquid metals and oxides is usually a very small, negative, value (—0.05 to —0.5 mJ.m-2.K-1), a temperature rise of several hundred degrees is necessary to produce decreases in the surface energy that can be reliably detected by measurements of drop profiles. Even in this case, the error on the temperature coefficient lies between 30% and 100% (see Section 4.1.1). 

The surface energy of molten oxides has been studied less extensively than that of pure liquid metals. For instance, the surface energy of molten AI2O3, which is the most widely studied oxide, has been measured by 12 teams (Ikemiya et al. 1993) while that of Fe has been measured by 28 (Keene 1993). One reason for this difference is the experimental difficulties arising from the high melting point of many oxides but... 

The beneficial effects of Ni and Pd imply a decrease in the liquid/solid interfacial energy values, as discussed in Chapter 5. Work by researchers using other families of braze alloys has shown that the wetting of both steel and Ni alloy components by Ni brazes is promoted by the presence of B and Si which can flux, that is cause chemical reactions to disrupt the surface oxide, (Amato et al. 1972). 

The change in nature of the oxidized surface can be followed with immersion liquids other than water. By increasing the oxygen content of a carbon black (Le. Spheron 6) up to 12%, Robert and Brusset (1965) obtained an increase of the energy of immersion in methanol from 140 to as much as 390 mJ nT2 (practically the same ratio as that observed with water), whereas the energy of immersion in n-hexadecane remained nearly constant, around 100 mJ m-2. 

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