Modification of space charge transport in nanocrystalline cerium oxide by heterogeneous doping

In the search for new materials for energy conversion and storage technologies such as solid oxide fuel cells, nano-ionic materials have become increasingly relevant because unique physical and transport properties that occur on the nanoscale may potentially lead to improved device performance. Nanocrystalline cerium oxide, in particular, has been the subject of intense scrutiny, as researchers have attempted to link trends in electrical conductivity with the properties of space charge layers within the material. In this thesis, efforts designed to intentionally modify the space charge potential, and thus the space charge profiles and the macroscopic conductivity, are described.Nanocrystalline CeO2 thin films with a columnar microstructure were grown by pulsed laser deposition. A novel heterogeneous doping technique was developed in which thin NiO and Gd203 diffusion sources were deposited on the ceria surface and annealed in the temperature range of 7008000C in order to diffuse the cations into the ceria layer exclusively along grain boundaries. Time-offlight secondary ion mass spectrometry (ToF-SIMS) was utilized to measure the diffusion profiles. A single diffusion mechanism, identified as grain boundary diffusion, was observed. Using the constant source solution to the diffusion equation, grain boundary diffusion coefficients on the order of 10-15 to 10-13 cm2/s were obtained for Ni, as well as Mg diffusion emanating from the underlying substrate. Microfabricated Pt electrodes were deposited on the sample surface, and electrical measurements were made using impedance spectroscopy and two-point DC techniques. The asdeposited thin films displayed a total conductivity and activation energy consistent with reference values in the literature. After in-diffusion, the electrical conductivity decreased by one order of magnitude. Novel electron-blocking electrodes, consisting of dense yttria-stabilized zirconia and porous Pt layers were fabricated in order to deconvolute the ionic and electronic contributions to the total conductivity. In the as-deposited state, the ionic conductivity was determined to be pO2-independent, and the electronic conductivity displayed a slope of -0.30. The ionic transference number in the as-deposited state was 0.34.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Materials Science and Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2009

Author dc:creator

• Litzelman, Scott J

Advisor dc:contributor.advisor

• Harry L. Tuller.

• Materials Science and Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/46681

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/46681