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Updated on: July 15, 2010  
     
  Polycrystal of CoSb3  
 
 

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.

 

Introduction:
This beautiful scanning electron micrograph seen is the fracture surface of a polycrystal of CoSb3 containing Mn and Yb.

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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