In-situ measurement of thermodiffusion in liquid alloys

Abstract

The Thermodiffusion (or Soret effect) describes the formation of a concentration gradient induced by a temperature gradient in binary or multicomponent mixtures. This crosscoupling effect of heat and mass transfer influences the homogeneity of doped semiconductors and grown crystals as well as the microstructure formation in alloys. The calculation of thermodiffusion by molecular dynamic simulation can be very sensitive to the specific potential [1]. Recently improved theories were proposed but the database to validate models for thermodiffusion in liquid alloys is scarce. Therefore, reliable measurements are needed. For transparent materials, in-situ methods are well established whereas for liquid alloys it was only possible to analyze the solidified samples. We present a method using X-ray radiography in combination with a high temperature furnace to determine the concentration profile in a vertical capillary [2]. In alloys with sufficient contrast in X-ray absorption this technique allows for in-situ observation which excludes disturbances by solidification and reveals possible error sources as e.g. free surfaces. Furthermore, these measurements are time-resolved. Measuring the concentration distribution at different points in time allows us to verify that a steady state has been reached and makes a simultaneous measurement of thermodiffusion and diffusion possible. This is demonstrated with an aluminum-rich Al-Ni melt where nickel is found to migrate to the cold side. The derived interdiffusion coefficient is compatible with independent measurements of interdiffusion in liquid Al-Ni. [1] E. V. Levchenko, A. V. Evteev, T. Ahmed, A. Kromik, R. Kozubski, I. V. Belova, Z.-K. Liu and G. E. Murch, Philos. Mag., 96, 3054 (2016). [2] E. Sondermann, F. Kargl, and A. Meyer, Phys. Rev. Lett., 123, 255902 (2019).