2012 IR&D Annual Report

Development of Novel Silicon Clathrates for Energy Harvesting and Storage, 18-R8279

Principal Investigators
Kwai S. Chan
Michael A. Miller

Inclusive Dates:  01/01/12 – Current

Background — Solid-state thermoelectric devices (TEDs) exhibit many attractive features for electrical power generation compared to traditional fuel-combustion systems, which include extraordinary long life, no moving parts, no emissions, and high reliability. To this end, the Type I and II clathrates of silicon and germanium alloys are attractive thermoelectric (TE) materials because they can be engineered to exhibit high thermal power, high electrical conductivity, and low thermal conductivity by scattering phonons without interrupting electron conduction. Despite these attributes, the figure of merit of current silicon clathrates is still below that of existing TE materials based on rare-Earth elements and needs further improvements for industrial applications.

Graphic: Unit cell of a Type I silicon-based clathrate with large guest atoms and small substituted framework atoms designed via first-principles computation.
Unit cell of a Type I silicon-based clathrate with large guest atoms and small substituted framework atoms designed via first-principles computation.

Approach — The objectives of this research project are to: (1) develop novel silicon clathrates by substituting clathrate framework and guest atoms with small-sized atoms, (2) characterize the thermoelectric properties, (3) develop a first-principles computational approach for modeling the effects of small-atom interactions, and (4) design and demonstrate a multilayered TED using the novel TE material. An innovative direct synthesis method and a traditional arc-melting method are used to synthesize Type I metal-silicon clathrates with small-atom substitution on the Si framework and guest-atom insertion within the cage structure. The thermoelectric properties of metal-silicon clathrate compounds in bulk and layer forms will be characterized with and without compressive stress. A computational methodology will be used to develop an understanding of the effects of small-atom substitution and encapsulation within the cage structure on the thermoelectric properties, and to design the desired multilayer architecture for optimum thermoelectric properties.

Accomplishments — A first-principles computational approach was used to design new silicon-based clathrates with small atoms, as shown in the illustration, as either guest or substitute framework atoms. The energy of formation of Type I clathrate compounds have been computed as a function of lattice parameter for various atom insertions or substitutions from a list of candidate elements. These energy calculations were used to identify Type I silicon-based clathrates that are amenable for synthesis and potentially good TE characteristics. Synthesis of selected silicon clathrate compounds is in progress. Preliminary results indicate that some of the compounds can be synthesized, but the yield and purity need further improvements.

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