The temperature coefficient of the surface tension of pure liquid metals
- PDF / 681,833 Bytes
- 8 Pages / 603.28 x 783.28 pts Page_size
- 39 Downloads / 191 Views
I.
H.
INTRODUCTION
THEsurface tension and the temperature coefficient of the surface tension have been measured for most of the metallic elements, usually by several workers. 1-66Many of the values reported in the literature are not in good agreement since the surface tension of the majority of liquid metals is strongly affected by surface active impurities such as oxygen and sulfur. This is particularly evident in the case of liquid Zn and Cd, for which differences in the reported values for the surface tension vary by as much as 15 pet. For example, the surface tension values for Zn at its melting temperature range from 720 mN/m to 820 mN/m. 33-4~ Furthermore, some of the data for Zn and Cd show a positive temperature coefficient of surface tension rather than a negative value such as is normally obtained for pure metals. Since the surface entropy is directly related to the value of the temperature coefficient of surface tension in single component systems, it has been proposed 39that a real difference exists in the sign of the surface entropy (and therefore in the degree of surface ordering) in Zn and Cd on the one hand and most of the remaining liquid metal surfaces on the other. In this work, the surface tension of liquid Fe, Co, Ni, Cu, Ag, Zn, Pb, Cd, and Sn have been measured as a function of temperature by the sessile droplet method and/or the levitated droplet method. The results of the present work and all available results from the literature have been analyzed for each of these metals in terms of a plot of surface tension temperature coefficient vs the measured value of the surface tension at the melting temperature. It will be shown that the experimental values of both surface tension and temperature coefficient are influenced in a systematic way by the presence of surface active impurities.
EXPERIMENTAL
Two techniques were employed to determine the surface tension of the liquid metals: the sessile droplet method and the levitated droplet method. The sessile droplet method has three main advantages: any atmosphere can be used; temperature control is simple to arrange; it is applicable even for dense metals. The levitated droplet method also has some important advantages: contamination from refractories is eliminated; a value for the density of the liquid metal is not required; it can be used for reactive metals. Surface tension measurements as a function of temperature were made for several pure liquid metals. The sessile droplet method was used for Fe, Ni, Cu, Ag, Zn, Pb, Cd, and Sn. The levitated droplet method was used for Fe, Co, Ni, Cu, Ag, and Sn. The purity of the metals employed in this study was 99.99 pct or better. Details of the experimental technique used for the sessile droplet method have been published elsewhere. 67 Alumina and graphite plates were used as the substrate material. The apparatus used for the levitation method is shown in Figure 1. Again, the detailed experimental technique has been described elsewhere. 13.69The metal samples were placed on a magnesia plate located on
Data Loading...
