X-Message-Number: 24588
Date: Fri, 3 Sep 2004 21:15:04 -0700 (PDT)
From: Doug Skrecky <>
Subject: Model Unscrambles Complex Crystallization Puzzle

Source:   National Institute Of Standards And Technology Date:   2004-09-03

To the wonderment---and the befuddlement---of scientists, the patterns
that form as plastics, metals and many other materials crystallize can
vary incredibly, ranging from sea-urchin-like spheres to elaborate
tree-like branches.

Now, Hungarian and National Institute of Standards and Technology
scientists report in the September issue of Nature Materials* that they
have developed a way to predict the polycrystalline microstructures that
will form as complex liquid mixtures cool and solidify. Ultimately, the
team's new simulation tool could help manufacturers of everything from
plastic bags to airplane wings to design new products with improved
strength, durability and other properties.

Images generated with the team's mathematical model match up almost
feature for feature with the seemingly random crystal patterns formed in
experiments as temperatures or other processing variables are modified.
The model accurately predicts how both impurities (or additives) and
process differences affect the sizes, shapes and orientations of crystals
that form during the so-called supercooling process.

Whether initiated by "dirt" or by processing conditions, the resulting
patterns can be strikingly similar. This "duality in the growth process,"
notes NIST's James Warren, may help explain why polycrystalline growth
patterns are so prevalent in polymers and other materials derived from
complex mixtures.

Findings based on the model indicate that instabilities along the
boundary between liquid and solid areas during solidification effectively
clash with the otherwise orderly process of crystallization. Tiny
crystals-in-the-making move and position themselves along the growth
front, assuming an orientation peculiar to the energy conditions at their
location. Varying local conditions produce crystals in seemingly
disordered arrays, accounting for the rich diversity of microstructural
patterns.

Laszlo Granasy, of Hungary's Research Institute for Solid State Physics
and Optics, led the research effort. *L. Gr n sy, T. Pusztai, T.
B rzs nyi, J.A. Warren, and J.F. Douglas. A general mechanism of
polycrystalline growth. 2004. Nature Materials advance on-line
publication, Aug. 8, 2004.

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