2013 IR&D Annual Report

Development of Novel Silicon Clathrates for Energy Harvesting and Storage,

Principal Investigators
Kwai S. Chan
Michael A. Miller

Inclusive Dates: 01/01/12 – 07/01/13

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 improvement for industrial applications.

Graphic: Hybrid carbon-silicon Type I clathrate framework
 Hybrid carbon-silicon Type I clathrate framework with guest atom A inside the cage structure designed via first-principles computations. Similar structures were designed and synthesized by substituting Si framework atoms with nitrogen atoms.

Approach — The objectives of this research project are to:

  1. develop novel silicon clathrates by substituting clathrates framework and guest atoms using 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-sized 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 using small-sized atoms such as C or N as substitution atoms on the framework that is stabilized by alkaline or alkaline earth guest atoms, A, inside the cage structure of the framework, see illustration. Several hybrid carbon-silicon clathrates and one hybrid carbon-nitrogen clathrate were synthesized and confirmed by experimental techniques. First-principles computations indicated that new clathrate materials can be tailored to exhibit a wide range of electronic properties and have potential as either electronic or thermoelectric materials. Two patent applications related to the hybrid carbon-silicon and carbon-nitrogen clathrates are pending.

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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 10 technical divisions.