In-situ measurement of thermodiffusion in liquid alloys

Thermodiffusion (or Soret effect) describes the formation of a concentration gradient induced by a temperature gradient in binary or multicomponent mixtures. This crosscoupling effect influences the homogeneity of doped semiconductors and grown crystals as well as the microstructure formation in alloys [1-2]. The calculation of thermodiffusion by molecular dynamic simulation is very sensitive to the specific potential [3]. Recently improved theories were proposed but the database to validate models for thermodiffusion in liquid alloys is scarce. Therefore, there is a need for reliable measurements. For the measurement of thermodiffusion coefficients in liquid alloys, the concentration profile of a sample in a temperature gradient is usually analysed after solidification. We present a method using X-ray radiography in combination with a high temperature furnace to determine the concentration profile in a vertical capillary. 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 monitor its development and verify that a steady state has been reached. In a first experiment we studied Al-Ni which is a highly non-ideal binary system with a large difference in X-ray absorption. In liquid Al78.5Ni21.5 Nickel was found to migrate to the cold side. References [1] M. B. Sassi, S. Kaddeche, M. Lappa, S. Millet, D. Henry, H. B. Hadid, J. Cryst. Growth 458 (2017) 154. [2] E. Jafar-Salehi, M. Eslamian, and M. Saghir, Eng. Sci. Technology 19 (2016) 511. [3] E. V. Levchenko, A. V. Evteev, T. Ahmed, A. Kromik, R. Kozubski, I. V. Belova, Z.-K. Liu, G. E. Murch, Philos. Mag. 96 (2016) 3054