In situ viscosity measurements of liquid Fe–S alloys at high pressures

Abstract

The viscosities of liquid Fe₇₃S₂₇ and Fe₉₀S₁₀ alloys were measured by combining a falling-sphere viscometer based on Stokes’ law with the technique of synchrotron radiography to up to 9.4 GPa and 2023 K. A high-speed CCD camera (125 frames/s) enabled the precise terminal velocity of the falling sphere to be determined over pressure and temperature ranges higher than those of previous studies. The effect of pressure on the viscosities was quite small, whereas temperature had a large effect. The apparent activation energies of Fe₇₃S₂₇ and Fe₉₀S₁₀ were quite similar, which indicates that the effect of sulfur content on liquid Fe alloys was negligible. The small effects associated with pressure and sulfur content on the viscosities of liquid Fe alloys show that iron is the dominant component of the viscous flow unit under high pressures. The addition of sulfur may not have a serious effect on the viscous flow of the Earth's outer core.

Keywords:

• iron–sulfur alloy
• viscosity
• multianvil
• synchrotron
• radiography

Acknowledgements

K.-I.F. thanks A. Nozawa, H. Kaneko, and Y. Ueda for their assistance in the experiments. This work was partly supported by a Grant-in-Aid for Scientific Research (20340154) from the Japanese Society for the Promotion of Science to K.-I.F. The experiments were performed under contract at SPring-8 (proposal no. 2003A0688-ND2-np, 2006B1276-ND2b-np, and 2008B2212-ND2b-np).