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Optimal Design of Materials for Energy Conversion


Collins, Lincoln Nash (2017) Optimal Design of Materials for Energy Conversion. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9X928B7.


The efficiency of fuel cells, batteries and thermochemical energy conversion devices depends on inherent material characteristics that govern the complex chemistry and transport of multiple species as well as the spatial arrangement of the various materials. Therefore, optimization of the spatial arrangement is a recurrent theme in energy conversion devices. Traditional methods of synthesis offer limited control of the microstructure and there has been much work in advanced imaging for these uncontrolled microstructures and optimizing gross features. However, the growing ability for directed synthesis allows us to ask the question of what microgeometries are optimal for particular applications. Through this work, we study problems motivated by metal oxides used in solar-driven thermochemical conversion devices designed to split water or carbon dioxide into fuels. We seek to understand the arrangement of the solid and porous regions to maximize the transport given sources and sinks for the gaseous oxygen and vacancies. Three related problems are investigated with the common theme of understanding the role of microstructure design.

We derive the transport equations for electrons and oxygen vacancies through ceria under an externally-applied electric potential in an oxygen environment using various balance laws and constitutive equations. From this, we obtain various thermodynamic potentials that take into consideration the thermal, chemical, and mechanical state of the material. Accordingly, we obtain a system of partial differential equations describing ambipolar diffusion. We present the applicability of strain-engineering as a way to design systems to improve the behavior of thermochemical conversion devices. We look at an idealized thin film of mixed conductor attached to an inert substrate with a thermal mismatch as a way to induce strain into the film. The resulting impact on equilibrium non-stoichiometry is analyzed using data describing non-stoichiometry in ceria as a function of oxygen pressure and temperature.

The optimal design of material microstructure for thermochemical conversion is addressed from two standpoints: the mathematical homogenization of associated transport models, and from topology optimization. We present the homogenization of coupled transport through porous media consisting of linearized Stokes flow, convective diffusion, and diffusion in the solid phase with interface reaction. Depending on the strength of the interface chemistry, different forms of effective behavior are described at the macroscale, and we gain insight into the impact cell-design and pore shape has on the behavior.

The topology optimization of a model energy-conversion reactor is then presented. We express the problem of optimal design of the material arrangement as a saddle point problem and obtain an effective functional which shows that regions with very fine phase mixtures of the material arise naturally. To explore this further, we introduce a phase-field formulation of the optimal design problem, and numerically study selected examples. We find that zig-zag interfaces develop to balance mass transport and interface exchange.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Optimal design, topology optimization, thermochemical conversion
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Materials Science
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Bhattacharya, Kaushik
Thesis Committee:
  • Ravichandran, Guruswami (chair)
  • Fultz, Brent T.
  • Minnich, Austin J.
  • Bhattacharya, Kaushik
Defense Date:9 June 2017
Record Number:CaltechTHESIS:06132017-164608976
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:10336
Deposited By: Lincoln Collins
Deposited On:15 Jun 2017 21:40
Last Modified:04 Oct 2019 00:17

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