Directional Solidification of Solvent/Polymer Alloys

In directional solidification, a thin, quasi-2D alloy sample is pulled though a temperature gradient from the warm side, where the alloy is molten, into the cold side, where the alloy is solid. Above a certain pulling speed, the planar solidification front goes unstable to a cellular structure which typically has a periodicity of a few 100 microns. At faster speeds, this structure is unstable to a dendritic morphology.

By using transparent organic materials that mimic metals, it is possible to observe the solidification process and its dynamics between two glass slides under the microscope. We are using an alloy which is a mixture of a low molecular weight solvent and a polymer as solute. The slow diffusion makes it possible to observe growth that is typically seen only at very rapid solidification rates.

The figures on the side show micrographs of the growing cellular and dendritic structure obtained with Hoffmann modulation contrast . The solid is growing downward into the liquid. The dendrites all have the same crystalline orientation, i.e., the whole structure is a single crystal. Both images are taken at identical conditions (succinonitrile/poly(ethylene oxide), 1%wt, MW 4500, G=10 K/cm,v=2.7 _m/sec, width=1.54mm). The only difference is the crystalline orientation of the grains. The first picture shows a grain which is growing in the <111> plane. In this orientation, the surface tension in the plane is nearly isotropic (small crystalline anisotropy), which leads to a seaweed or dense branching morphology with perpetual tip splitting. The lower picture shows a traditional dendrite growing along a principal axis of the cubic succinonitrile crystal.

Currently we are focusing on three topics:

• Weed growth mechanics and transition from weed to doublon. This includes the measurement of the diffusion field and studies of the tip splitting dynamics.
• Investigation of Superdendrites. At high concentrations or growth velocities, the interface typically forms sharp triangular shapes known as superdendrites.
• Measurent of rapid solidification (5mm/sec) for a system with large segregation coefficient. Development of a reflux distillation system to achieve purities of better than 99.999%.

People

• Eberhard Bodenschatz
• Brian Utter, former Graduate Research Associate, currently a postdoc at Duke University. (Brian's home page)
• Rolf Ragnarsson, former Graduate Research Associate

Papers

• "An experimental apparatus and sample preparation for directional solidification," B. Utter, R. Ragnarsson, and E. Bodenschatz, (in preparation, 2002)

• "Doublon growth in solidification," B. Utter and E. Bodenschatz, (to be submitted to Phys. Rev. Lett., 2002)

• "Dynamics of low anisotropy morphologies in directional solidification," B. Utter and E. Bodenschatz, (accepted, Physical Review E, 2002)

• "Alternating tip splitting in directional solidification", B. Utter, R. Ragnarsson, and E. Bodenschatz, Physical Review Letters 86, 4604 (2001). (link to article)

• "Periodic Tip Splitting Instability," B. Utter, R. Ragnarsson and Eberhard Bodenschatz, internal report (Sept, 1999). gzipped PS (390 KB) or PDF (380 KB)

• "Superdendrites in Directional Solidification of Polymer-Solvent Mixtures," R. Ragnarsson, B. Utter, and E. Bodenschatz,in Phase Transformations and Systems Driven Far From Equilibrium, Mater. Res. Soc. Symposium Proceedings 481, Mater. Res. Soc., Warrendale (PA), USA (1998). MRS Proceedings

This work is/was funded by the Alfred P. Sloan Foundation Foundation and the National Science Foundation through the Cornell Center for Materials Research.