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MM: Fachverband Metall- und Materialphysik

MM 52: Phase Transformations I

MM 52.5: Vortrag

Donnerstag, 14. März 2013, 11:15–11:30, H26

Measurement of nucleation rates using fast scanning calorimetry on samples prepared by the Droplet Emulsion Technique (DET) — •Christian Simon, Joachim Bokeloh, and Gerhard Wilde — Westfälische Wilhelms-Universität, Institut für Materialphysik, Wilhelm-Klemm-Str. 10, 48149 Münster

The most difficult part in experiments on heterogeneous nucleation is the control of the heterogeneous nucleant. On one hand, parasitic nucleants like the container wall and impurities must be removed, and on the other hand the target nucleant phase must be kept at a constant potency. The DET is used to reduce the influence of impurities by dispersing a bulk volume to a fine dispersion of droplets. During the emulsification, a low-catalytic surface coating is applied to reduce the nucleation potency of the surface. The nucleation rate is evaluated by a statistical analysis that treats nucleation as a non-homogenous Poisson process and leads to model-free values of the nucleation rate. A fast scanning nano-calorimeter (10 K/s - 10000 K/s) is used to analyze the nucleation rate over a large number of repeated heating-cooling-cycles on a single droplet. We present experimental data on tin and a tin-bismuth alloy, which were both prepared by DET. Large values of undercooling were observed in tin. Indications of the formation of a metastable phase at large undercooling were observed in the tin-bismuth alloy. In additional experiments, the Sn-Bi alloys were only partially melted and the residual crystalline phase was used to act as heterogeneous nucleation-site.