Controlling microstructure of nanocrystalline thermoelectrics through powder processing

Bismuth Telluride and its solid solutions are currently front running thermoelectric materials because of their high figure of merit. When processed via mechanical alloying to obtain nanocrystalline structures, their efficiency is increased dramatically, due to enhanced phonon scattering at grain boundaries. However, the excess free energy of these interfaces renders them inherently susceptible to grain growth, therefore there is a need for materials with enhanced thermal stability. Despite this, little is known about the relevant processing science of these materials with respect to mechanical alloying and powder consolidation. This shortcoming is addressed here via systematic study of the processing-structure relationships that govern these processing operations. Firstly, during mechanical alloying, the primary mechanism of mixing between elemental constituents is revealed, as well as the limitations to subsequent grain refinement. The resultant behaviour is unique in the literature on mechanical alloying, due to the unusual thermal and thermodynamic properties of the compound and its elements, rendering deformation-induced heating effects especially prevalent. Next, during sintering operations of the powders, the kinetics of grain growth and porosity evolution were studied. By quantifying these processes, a thermal budget map for the nanocrystalline compound is constructed, to allow predictive powder and guidance of both processing and device operation at elevated temperatures. Finally, based on the improved understanding in processing science and thermal stability of these materials, a new class of thermally stable composites is engineered, with improved thermal stability, and hence enhanced thermoelectric properties.

Department dc:contributor.department

Massachusetts Institute of Technology. Department of Materials Science and Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2014

Author dc:creator

• Humphry-Baker, Samuel A

Advisor dc:contributor.advisor

• Christopher A. Schuh.

• Materials Science and Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/90085

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/90085