Date of Award

4-2013

Degree Name

Doctor of Philosophy

Department

Physics

First Advisor

Dr. Asghar N. Kayani

Second Advisor

Dr. John A. Tanis

Third Advisor

Dr. Sung G. Chung

Fourth Advisor

Dr. Muralidhar K. Ghantasala

Keywords

solid oxide fuel cells, oxygen ionic conductivity, cerium oxide, zirconium oxide, multilayer hetero-structures, nano-scale thin films

Abstract

Solid state electrolytes with high oxygen ionic conductivity at low temperatures are required to develop cost effective and efficient solid oxide fuel cells. This study investigates the influence of engineered interfaces on the oxygen ionic conductivity of nano-scale multilayer thin film electrolytes. The epitaxial Sm2O3 doped CeO2 (SDC) and Sc2O3 stabilized ZrO2 (ScSZ) are selected as the alternative layers for the proposed multilayer thin film electrolyte based on the optimum structural, chemical, and electrical properties reported in the previous studies. The epitaxial SDC(111)/ScSZ(111) multilayer thin films are grown on high purity Al2O3(0001) substrates by oxygen-plasma assisted molecular beam epitaxy. Prior to the deposition of multilayers, the growth parameters are optimized for epitaxial CeO2, ZrO2, SDC, and ScSZ thin films.

The epitaxial orientation and surface morphology of CeO2 thin films shows dependency on the growth rate. Epitaxial CeO2(111) is obtained at relatively high growth rates (>9 Å/min) at a substrate temperature of 650°C and an oxygen partial pressure of 2 × 10-5 Torr. The same growth parameters are used for the deposition of ZrO2 thin films. ZrO2 exhibits both monoclinic and cubic phases, which is stabilized in the cubic structure by doping with Sc2O3. The Sm and Sc evaporation rates are varied during the growth to obtain thin films of 15 mol % SmO1.5 doped CeO2 and 20 mol % ScO1.5 stabilized ZrO2, respectively. The SDC/ScSZ multilayer thin films are grown using the same growth parameters by varying the number of layers.

The SDC/ScSZ multilayer thin films show significant enhancement in the oxygen ionic conductivity in comparison to single layer SDC and ScSZ thin films. The increase in the oxygen ionic conductivity with the increase in number of layers can be attributed to lattice mismatch induced ionic conductivity along the interfaces. The 8-layer film exhibits the maximum oxygen ionic conductivity with one order of magnitude enhancement in the temperature range of 500-800°C compared to single layer thin films.

Comments

Fifth advisor: Dr. S. Thevuthasan

Access Setting

Dissertation-Open Access

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