Surface tensions of Undercooled liquids Aluminum, Tantalum, Tungsten and Niobium

Abstract

ABSTRACT Measurement of the surface tension of refractory liquid metals up to date is a difficult task. Values of the surface tension of liquid metals, in general, in the undercooled region are lacking. Here we have calculated the surface tensions of undercooled liquids Aluminium (Al) and the three refractory metals Tantalum (Ta), Tungsten (W) and Niobium (Nb) using our previously reported statistical thermodynamic equation. The calculations have been carried out below and above the melting temperature (Tm) of the metal thus covering the undercooled region by about 300 K below Tm. For Al, there was a notable jump at the melting point upon cooling where the surface excess entropy changed by 30%. This stems from the increased ordering at the surface upon cooling. The observed abrupt change indicates a first order surface phase transition at Tm from a pure liquid surface to a solid like surface structure upon cooling. For the other metals studied, the change was less pronounced. From the calculated value of the fraction of surface broken bond, it is concluded that the surface structure of the studied metals resemble that of simple metals. The calculated surface tensions above the melting point of the metals show good agreement with measured ones. Below the melting point of the metals, measurements are lacking that emphasise the importance of our calculations.