E. Ricci

Surface tension of liquid metals and alloys — Recent developments

Surface tension measurements are a central task in the study of surfaces and interfaces. For liquid metals, they are complicated by the high temperatures and the consequently high reactivity characterising these melts. In particular, oxidation of the liquid surface in combination with evaporation phenomena requires a stringent control of the experimental conditions, and an appropriate theoretical treatment. Recently, much progress has been made on both sides. In addition to improving the conventional sessile drop technique, new containerless methods have been developed for surface tension measurements. This paper reviews the experimental progress made in the last few years, and the theoretical framework required for modelling and understanding the relevant physico-chemical surface phenomena.

A new experimental method for the measurement of the interfacial tension between immiscible fluids at zero bond number

Abstract A new methodology (Pressure Derivative Method) for measuring the interfacial tension σ between immiscible fluid phases at Δρg = 0 (zero Bond number) is presented. This method, which is particularly suitable for measurements under microgravity conditions, is also applicable on earth to liquid pairs with a small density difference. The method consists, essentially, of calculating σ by a linear fitting of experimental capillary pressure and curvature data, collected during the formation of a drop of one of the two liquids inside the other one. The curvature data are derived from the injected drops volume; thus, any pair of fluids can be used, including opaque liquids, for which optical curvature measurements are not possible. By a specific mathematical treatment of the experimental data, an overall accuracy on the order of less than 1% has been obtained. During the drop growth, a maximum in the pressure is reached, which can also be used to calculate σ by the Maximum Bubble Pressure technique (MBP). This value can be utilized as an internal check on the interfacial tension data obtained by the method proposed.

Mechanism of oxidation/deoxidation of liquid silicon: theoretical analysis and interpretation of experimental surface tension data

Abstract A theoretical analysis of the behaviour of molten metals in the presence of oxygen is presented. A generalised Wagner approach has been adopted for molten metals, forming volatile oxides (Si, Sn, Al, etc.), in which the description of oxygen transfer from the gas phase to the condensed phase must account for the double contribution of molecular oxygen and oxygen linked as oxide, which leads to define the concept of ‘oxygen effective pressure’. From the analysis of the system at varying operating conditions, it was possible to relate the gas-phase composition at the surface, which is hardly measurable, to the composition in the feed or at the outlet. The application of this theory to the molten silicon-oxygen system is presented and the effectiveness of such a theory as a supporting tool for experimental work of capillary phenomena and crystal growth processes is discussed and verified.

Surface properties of Bi–Pb liquid alloys

Abstract The present work represents an experimentally based investigation of the applicability of a statistical mechanical theory in conjunction with a compound formation model (CFM) to describe surface properties of Bi–Pb liquid alloys. The surface tension of molten Pb, Bi and Bi–Pb alloys has been measured by the sessile-drop method over a temperature range. The results obtained show general agreement with other reported measurements on pure elements and their alloys as well as with calculated surface tension values. Based on the phase diagram evidence about the existence of the e-Pb3Bi intermetallic compound, the phenomenon of compound formation in Bi–Pb liquid alloys has been analysed through the study of surface properties (surface tension and surface composition) and microscopic functions (concentration fluctuations in the long-wavelength limit and chemical short-range order parameter) in the frame of CFM.

Influence of oxygen contamination on the surface tension of liquid tin

The surface tension of liquid tin has been measured by the sessile-drop technique as a function of temperature, in the range 232 ⩽T (°C) ⩽ 800 and under different atmospheres. It is shown that oxygen strongly affects the surface tension values and that, under “nominally” very clean conditions, a considerable scatter of experimental results occurs. This scatter can be explained by taking into account kinetic factors, especially those related to the gaseous fluxes around the molten drop. By this procedure, a number of experimental results can be singled out, which corresponds to “clean” surface conditions. On the basis of these results, the following expression for surface tension politherm is proposed: σ(mN m−1 = 581-0.13) (t-232).

Dynamic surface tension measurements on molten metal-oxygen systems: model validation on molten tin

Abstract A theoretical model on oxygen transport at the surface of liquid metals has been validated by dynamic surface tension measurements performed on liquid tin as test metal. The oxygen contamination conditions have been obtained at different oxygen partial pressures under low total pressure conditions (Knudsen regime), confirming that an oxide removal regime occurs under an oxygen partial pressure much higher than the equilibrium one (the “Effective Oxidation Pressure”). Experimental results are reported which give a new insight on the relative importance of the various processes due to the oxygen mass transport between the liquid metal and the gas phase. The critical aspects involved in surface tension measurements of liquid metals, related to the problem of liquid metal–oxygen interactions, are also carefully underlined.

Wetting and surface tension measurements on gold alloys

The surface tension and the density of some gold alloys have been measured and their wetting behaviour investigated in relation to different ceramic supports. Relationships between surface tension and density as a function of temperature are proposed.

Oxygen mass transfer at liquid-metal-vapour interfaces under a low total pressure

AbstractThe problem of oxygen exchange at the interface between a gas and a liquid metal is treated for systems under a “vacuum” (Knudsen regime, pressures lower than 1 Pa), where, due to the large mean free path of gas molecules in a vacuum, transport processes in the gas phase have no influence on the total interphase mass exchange, which is controlled by interface phenomena and by oxygen partition equilibrium inside the liquid. Owing to the double contribution of molecular O2 and volatile oxides to the oxygen flux from the surface, non-equilibrium steady-state conditions can be established, in which no variations in the composition of the two phases occur with time, as the result of opposite oxygen exchanges. The total oxygen and metal evaporation rates are evaluated as a function of the overall thermodynamic driving forces, and an account of the transport kinetics is given by using appropriate coefficients. A steady-state saturation degree sr, is defined which relates the oxygen activity in the liquid metal to the O2 pressure imposed and to the vapour pressures of volatile oxides. When metals able to form volatile oxides are considered, pressures of molecular O2 higher than those defined under equilibrium conditions have to be imposed in the experimental set-up in order to obtain a certain saturation degree, as a consequence of the condensation of the oxide vapours on the reactor walls. Effective oxidation parameters are determined, which define the conditions leading the liquid to a definite steady-state composition under a “vacuum” when it is out of equilibrium. The effective value of the oxygen pressure which corresponds to the complete oxygen saturation of the metal,

Surface tension and wetting behaviour of molten Bi–Pb alloys

P_{O_{2,} s}^E