5 research outputs found
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X-ray Reflectivity Studies of Atomic-Level Surface-Segregation in a Liquid Eutectic Alloy of AuSn
X-ray reflectivity studies reveal atomic-level surface-segregation at the free surface of the eutectic Au71Sn29 liquid alloy. The surface-segregation extends up to three layers, in which the top layer is almost a pure monolayer of Sn, the second layer is almost a pure monolayer of Au and the third layer appears to be slightly enhanced in Au. Although the surface-segregation concentration profiles can be qualitatively accounted for by the theories of Defay-Prigogine and Strohl-King, they cannot satisfactorily account for the measured surface tension.Engineering and Applied Science
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Surface Structure of Liquid Bi and Sn: An X-Ray Reflectivity Study
X-ray reflectivity measurements of the liquid Bi surface are presented and analyzed together with previous liquid Sn results. Published measurements on liquid Ga, In, and K all exhibit a single strong maximum at a wave-vector transfer of the order of the reciprocal of an atomic-diameter, due to surface-induced layering. In contrast, both Sn and Bi exhibit—in addition—a weak broad peak at much smaller wave-vector transfers. This feature is an unambiguous signature of an enhanced electron density in the near-surface region. Possible ways of modeling this enhancement are presented. Once the different surface-roughening effects of thermal capillary waves are accounted for, the surface structure factors of Sn and Bi are remarkably similar. The principal difference between the two is that the depth of the layering below the surface is more than ∼40% larger for Bi than for Sn. This is considerably larger than the ratio of their covalent radii which is only ∼10%. No theoretical explanation can be offered at this time for the surface structure difference between Sn and Bi and other elemental liquid metals studied to date: Ga, In, and K.Engineering and Applied Science
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Crystalline Surface Phases of the Liquid Au-Si Eutectic Alloy
A two-dimensional crystalline layer is found at the surface of the liquid eutectic Au82Si18 alloy above its melting point TM=359 °C. Underlying this crystalline layer, we find a layered structure, 6–7 atomic layers thick. This surface layer undergoes a first-order solid-solid phase transition occurring at 371 °C. The crystalline phase observed for T>371 °C is stable up to at least 430 °C. Grazing-incidence x-ray diffraction data at T>371 °C imply lateral order comprising two coexisting phases of different oblique unit cells, in stark contrast with the single phase with a rectangular unit cell found for low-temperature crystalline phase, 359 °C<T<371 °C.Engineering and Applied Science