In the present work, the surface tension of zirconium samples with different oxygen concentrations was measured by the oscillation droplet method using ESL at NASA MSFC. After being levitated and melted, the sample was excited by electrostatic field and damped freely to its equilibrium state. The oscillating frequency was extracted from video data to determine the surface tension.
The three oxygen concentrations of zirconium samples include zirconium with 57 ppm oxygen, and zirconium with 2.42 at.% oxygen. To prepare samples, raw materials were bought from Alfa Aesar. After a hole was drilled on one side of a 6.35 mm diameter x 12.7 mm length zirconium slug, zirconia powder was loaded in the hole and the hole was capped with a 3.175 mm diameter x 3.175 mm length zirconium slug. Then the capped slug was arc-melted into an ingot. The ingot was cut into two halves. One of the halves was sent to Luvak Inc. Boylston, MA for elemental analysis (Method: Inert gas fusion—ASTM E 1019-11). The other was potted in epoxy and cut into several 30-60 mg pieces using a diamond saw. The cut pieces were arc melted again to form near-spherical shape. The preparation process is shown in Fig. 3.
The zirconium samples with different oxygen concentrations were put into multiple melt cycles. A solid sample was heated and melted by a heating laser in vacuum chamber with pressure 10E-7 Torr. As soon as the sample temperature
Fig. 3 Sample preparation process
reaches the desired superheated temperature, about 100 K above melting point, the heating laser was modulated to put the sample on a thermal hold. The sequence of the thermal hold was randomized to minimize the effect of mass evaporation on the measured surface tension. A typical temperature-time plot is shown in Fig. 4. The mass loss of the tested samples is shown in Table 1 and it turns out to be less than 0.124% and average is 0.06%.
Figure 5 shows the change in the projected area in pixels of a sample during excitation and damping. Between the two vertical lines is the damping region, which is mixed with some noise. Since the Rayleigh equation  was derived based on assumptions that there is no external force and the amplitude is very small, more
Fig. 4 Temperature versus Time in one melt cycle. The blue straight lines give the number of oscillations the sample completed
Fig. 5 Sample oscillation and free damping accurate surface tension can be determined if the latter portion of the damping region is to be used.