Effect of Oxygen Adsorption on Surface Tension of Liquid Aluminum

To ascertain the effect of oxygen adsorption on the surface tension of liquid aluminum, we conducted measurement under Ar-He mixture gas with oxygen partial pressure of 10^-2 Pa as functions of both temperature and the gas flow rate by the oscillating droplet method using electromagnetic levitation. When the gas flow rate was 2.0 L·min^-1, the surface tension was decreased continuously with elevating temperature. The slope of the temperature dependence became gradual at lower temperatures due to an increase oxygen adsorption. Under the gas flow rate of 1.0 L·min^-1, the surface tension displayed nearly identical temperature dependence at lower temperatures but showed an increase once followed by a decrease with increasing temperature. When the gas flow rate was further reduced to 0.4 L·min^-1, the temperature at which the increase in surface tension was observed at lowered compared to that under a gas flow rate of 1.0 L·min^-1. These unique phenomena were explained by considering the competition between effect of gas phase formation, such as Al₂O(g) and aluminum vapor, from the surface of liquid aluminum, which inhibits oxygen adsorption, and the counteracting effect of the atmospheric Ar-He gas flow. The increase in the surface tension was due to oxygen desorption to approach the pure state value. From the measurement results that oxygen desorbed at high temperature, the pure state value of the surface tension of liquid aluminum was proposed as σ=1128−0.3628(T−933) N·m^-1.