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| Single Crystal
Orderly arrangement
of atoms with one orientation |
Polycrystal
Multiple domains of orderly arrangement of different
orientations separated by regions of "disorderly"
arrangement called grain boundaries. Each of the
domains is a single crystal and is typically called
a grain.
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Overview:
Most crystalline materials can be classified in to two
categories - single crystals (periodic arrangement of
atoms of one orientation) or polycrystals (see 2-D cartoon
below). The Au nanoparticle and the nanowires seen in
the other images are single crystals. The material seen
in the photograph consists of a large number of single
crystals of different orientations and hence the name
polycrystal. Each of the crystals is called a grain
and the boundary between them is called a grain boundary.
Going Deeper:
CoSb3 , the material in the image, belongs
to a class of materials called skutterudites, which
are suitable candidates for high temperature thermoelectric
applications. Thermoelectrics
are materials that have a high electrical conductivity
and low thermal conductivity. They are typically used
to convert an applied thermal gradient (and hence the
requirement of low thermal conductivity) to a voltage
gradient and hence flow of current (and therefore the
requirement of high electrical conductivity to prevent
Joule heating) or vice-versa. In the material processed
in Prof. Umarji's lab., structural voids in the crystal
of CoSb3 have been filled with Mn / Yb acting
as rattlers. The rattling motion of these filler atoms
reduces the thermal conductivity by scattering phonons,
thereby increasing the thermoelectric efficiency of
these materials.
For more information visit Prof. Umarji's
faculty
page.
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