Surface tension of zinc: The positive temperature coefficient
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(dynes/cm)
=
8 3 9 . 7 - 125.7 e -°'°°941(T-419'5~.
The temperature (T) is in d e g r e e s centigrade. A total of 130 experimental values were determined with a correlation index of 0.98 with the equation. Comparisons of this r e s u l t with others in the literature are discussed.
IN
1966, D. W. G. White reported a positive temperature coefficient for the surface tension of molten zinc. 1 Positive temperature coefficients for surface tension had been reported previously for cadmium, 2-4 copper,5'6 and some iron alloys;5,7 however, White's work was the f i r s t report of experimental data on the positive temperature coefficient for zinc. White used a sessile drop system. The surface tension of liquid zinc reported h e r e i n was measured using an improved maximum-bubble-pressure system d u r i n g investigations on the wetting and adhesive properties of solders and soldering systems. The p u r p o s e of this report is t o confirm the positive surface tension-temperature coefficient of molten zinc as reported by White and t o show that this is not due to the high vapor pressure of zinc in the molten state. EXPERIMENTAL Many methods have been used for measuring surface tension, but for work with liquid metals at elevated temperatures only two warranted serious consideration. T h e s e are the drop shape, or s e s s i l e drop method, and the maximum-bubble-pressure (MBP) method. The MBP method was chosen because it has s e v e r a l features that make it attractive for measuring surface tension of liquid metals and alloys over a wide temperature r a n g e . Some advantages of the MBP method are that 1) theoretically, it is accurate to a few tenths of a percent, 2) it does not depend on the contact a n g l e with a substrate, 3) it r e q u i r e s only a close approximation of the density of the liquid metal, and 4) because it is quasi-dyn a m i c in operation, a freshly formed surface is involved for each measurement. The last advantage reduces the influence of surface contamination. The MBP apparatus used is described elsewherefl The system uses a modified form of the Sugden W. L. FALKE is Research Chemist, A. E. SCHWANEKE is Supervisory Physical Scientist, and R. W. NASH is Physical Science Technician, Rolla Metallurgy Research Center, Bureau of Mines, U.S. Department of the Interior, Rolla, MO 65401. Manuscript submitted June 2 4 , 1976. METALLURGICAL
TRANSACTIONS B
twin-capillary method9 w h e r e two capillaries of different r a d i i are placed side by side at the same d e p t h in the liquid. The original Sugden method used concentric capillaries. The twin-tube method overcomes the problem of accurately measuring the d e p t h of capillary immersion. Fig. 1 presents a schematic of the MBP system used. Improvements made in this system over other MBP systems described in the lite r a t u r e1° include 1) internally ground s i l i c a capillary tubes, which reduce the influence of "tip effects" for nonwetting liquid metals, 2) an electronic differential pressure transducer having a rapid response to rep
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