Oxide Growth at a Liquid Metal Surface


General Introduction

Chemical reactions at interfaces are of the fundamental and practical scientific interest. They sometimes exhibit both unusual kinetics and new phases that are unstable in the bulk. There are considerable differences between the ways oxidation develops in various materials. One commonly cited example is the surface of solid aluminum. Although the oxidation of Al is rather rapid in the presence of even trace amounts of molecular oxygen, the formation of a relatively thin surface oxide layer effectively passivates the bulk from further oxidation. By contrast, oxidation of metals like Fe proceeds well into the bulk. In spite of the fact that the free surfaces of liquid metals have recently attracted considerable attention because of the atomic ordering at the liquid-vapor interface [reference] there have been very few studies of their reactive properties. Oxidation of such surfaces are of particular interest because they lack the types of defects at which homogeneous nucleation occurs on solid surfaces namely steps, pits and dislocations. In addition, surface oxidation of liquid metals can drastically change the surface tension which will have a profound effect on the way the liquid metal wets different surfaces. This is important for practical processes such as soldering, brazing, casting etc.

The only two liquid metals for which the structure of the surface oxide has been studied by x-ray scattering technique are In and Ga, which were found to behave differently. Oxidation of the liquid Ga surface is similar to that of solid Al in that it saturates at a 5 depth to form a uniform layer protecting the metal from further oxidation. By contrast, oxidation of liquid In produces a rough oxide film from which there is negligible x-ray reflectivity signal. Grazing incidence diffraction (GID) of the Ga surface oxide did not reveal any Bragg peaks, indicating that this oxide is amorphous. A direct comparison with In is not possible since GID measurements were not done for the surface oxide to date.

Outline

Surface oxidation study is important because of: Chemical reactions (oxidation as an example) at liquid surfaces (especially liquid metals) have never been a subject of a systematic study, thus there are some questions:
Achievements

The surface oxidation of liquid Sn [reference] has recently been studied by x-ray scattering (reflectivity, off-specular diffuse scattering, grazing incidence diffraction).

The study reveals oxidation kinetics and formation of a previously unreported crystalline phase of SnO at the liquid-vapour interface of Sn. Our experiments reveal that the pure liquid Sn surface does not react with molecular oxygen below an activation pressure of ~5.0*10^-6 Torr. Above that pressure a rough solid Sn oxide grows over the liquid metal surface. Once the activation pressure has been exceeded the oxidation proceeds at pressures below the oxidation pressure threshold. The observed diffraction pattern associated with the surface oxidation does not match any of the known Sn oxide phases. The data have an explicit signature of the face-centred cubic structure, however it requires lattice parameters that are about 9% smaller than those reported for cubic structures of high-pressure phases of Sn oxides.

The further plan is to extend the oxidation study to binary alloys starting from a very important and interesting system AuSi. The surface oxidation of liquid AuSi eutectic alloy experiment is scheduled for August 2004, ChemMat-CARS, APS, Argonne, IL.



Some Literature References

  1. In: Oxide surfaces and metal/oxide interfaces studied by grazing incidence X-ray scattering
    G. Renaud, Surface Science Reports 32, 1 (1998)
  2. Symposium N on Ultrathin Epitaxial Oxides, E-MRS Spring Conference
    Sawatzky, This Solid Films 400, (2001)
  3. X-ray study of the oxidation of liquid-gallium surfaces
    M.J. Regan, H. Tostmann, P.S. Pershan, O.M. Magnussen, E. DiMasi, B.M. Ocko, M. Deutsch, Phys. Rev. B 55, 10786 (1997)
  4. Surface structure of liquid metals and the effect of capillary waves: X-ray studies on liquid indium
    H. Tostmann, E. DiMasi, P.S. Pershan, B.M. Ocko, O.G. Shpyrko, M. Deutsch, Phys. Rev. B 59, 783 (1999)
  5. A LEELS and Auger study of the oxidation of liquid and solid tin
    A.J. Bevolo, J.D. Verhoeven, M. Noack, Surface Science 134, 499 (1983)
  6. Synthesis and microstructure of metal oxide thin films containing metal nanoparticles by liquid phase deposition (LPD) method
    S. Deki, Yu Yu Ko Hnin, T. Fujita, K. Akamatsu, M. Mizuhata, A. Kajinami, 10th International Symposium on Small Particles and Inorganic Clusters, ISSPIC 10, V.16, 325 (2001)