Details of Grant 

EPSRC Reference:	EP/H046690/1
Title:	Phonon Engineering of Nanocomposite Thermoelectric Materials
Principal Investigator:	Srivastava, Professor G
Other Investigators:
Researcher Co-Investigators:
Project Partners:
Department:	Physics
Organisation:	University of Exeter
Scheme:	Standard Research
Starts:	01 November 2010	Ends:	31 December 2013	Value (£):	326,408
EPSRC Research Topic Classifications:	Condensed Matter Physics Materials Characterisation
EPSRC Industrial Sector Classifications:	No relevance to Underpinning Sectors
Related Grants:
Panel History:	Panel Date Panel Name Outcome 25 Feb 2010 Physical Sciences Panel - Materials Announced
Summary on Grant Application Form
Thermoelectricity (TE) is the process of generating either electricity from heat engines or heating devices from electricity. Examples of modern TE applications include portable refrigerators, beverage coolers, electronic component coolers, infrared sensing, etc. Possible future applications of TE devices include efficient conversion of waste heat into usable energy, in improving the efficiency of photovoltaic cells, etc. TE materials have been investigated for several decades due to their energy efficiency. The efficiency of TE materials is defined by the figure of merit quantity Z. It is customary to express the usefulness of a thermoelectric material in terms of the dimensionless quantity ZT, where T is temperature. Larger values of ZT require high Seebeck coefficient S, high electrical conductivity, and low thermal conductivity. The thermal conductivity can be contributed by both carriers and phonons. An increase in S normally implies a decrease in electrical conductivity, and an increase in the latter normally implies an increase in carrier thermal conductivity. Thus, it is very difficult to increase Z in typical TE materials. Clearly, for higher values of Z, we require materials which are characterised by less efficient scattering of electrons and efficient scattering of phonons. Research over the past several decades has shown that SiGe and Bi-chalcogenides are good high-temperature and low-temperature TE materials, respectively. It is being realised that material choice with reduced dimensionality is a prudent strategy for increasing ZT relative to bulk values. This is because low dimensionality provides four new features, each of which can be helpful in increasing ZT: (1) increase in electronic density of states near Fermi level; (2) anisotropic effective mass tensor and contribution from multiple Fermi-energy ellipsoids; (3) reduction in thermal conductivity by increasing phonon scattering at the barrier-well interfaces, without as large an increase in electron scattering at the interface; and (4) increased carrier mobility at a given carrier concentration when quantum confinement conditions are satisfied, so that modulation doping and delta-doping can be utilised to some extent. An attempt to increase ZT using feature (2) requires achieving low-dimensional growth in a manner that makes best use of the anisotropic nature of the effective mass tensor, and may not be easily controllable. In contrast, features (1) and (3) are considered to be controllable and very effective in increasing ZT. The quantum size effects offered by the consideration of feature (1) will increase electron density of states at Fermi level, and exploiting the boundary scattering effects by the consideration of feature (3) will substantially reduce the thermal conductivity without much loss to the electrical conductivity. This proposal seeks to identify the key parameters for developing the phonon engineering of Si-based nanocomposite thermoelectric materials by undertaking a systematic state-of-the-art theoretical study of their enhanced ZT over a wide temperature range. The proposed research will be based on four key theoretical ingredients, all of which have been developed by the PI: a first-principles approach (using the pseudopotential and density-functional theories) for equilibrium geometry, electronic states, and phonons; the adiabatic bond charge model for phonons in systems with larger unit cells; an anharmonic elastic continuum model to treat phonon-phonon interactions; and a model phonon conductivity theory. The systems are investigations will be: (i) thin Si/Ge and Si/SiGe superlattices, (ii) Si nanowires embedded in Ge and SiGe matrices, and (iii) Si nanodots embedded in a Ge or SiGe matrix. Our study will also help us identify the role of dimensionality in the enhancement of the thermodynamic figure of merit over a large temperature range.
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Project URL:	http://newton.ex.ac.uk/research/qsyatems/gps
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Organisation Website:	http://www.ex.ac.uk