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Temperature Induced Deflection of Yttria Stabilized Zirconia Membranes

Davis, Andrew Scott
http://rave.ohiolink.edu/etdc/view?acc_num=osu1338369600

Abstract Details

2012, Master of Science, Ohio State University, Mechanical Engineering.
Solid oxide fuel cells (SOFCs) are electrochemical energy conversion devices capable of producing electrical power with high efficiency and low emissions. SOFCs are characterized by ceramic electrolyte membranes which transport oxide ions in the range of temperatures between 600°C and 1000°C. In order to facilitate efficient, low-range temperature operation the electrolyte is typically made very thin, on the order of 40 µm. SOFCs also employ porous electrodes on either side of the electrolyte which are then placed in contact with current collectors and seals. In the fuel cell environment, with high temperatures, substantial thermal gradients, mechanical loading between layers, as well as the desire to be able to thermally cycle the cell, one of the layers or components must provide mechanical support. It is typical for either the anode or electrolyte to provide the necessary mechanical support. This thesis focuses on an electrolyte that is used for electrolyte-supported SOFC configuration. To address the need for mechanically robust electrolytes, NexTech Materials has developed the FlexCellTM electrolyte. This electrolyte design incorporates 40 µm thick conducting regions in a honeycomb pattern, and surrounding 200 µm thick stability regions. Various experiments on and determinations about this material and design must be made to ensure sufficient mechanical stability during fuel cell operation. Thermal stresses from high temperatures, temporal and spatial temperature gradients, and differential thermal expansion of contacting materials, are critical issues within SOFCs. The critical property related to these issues, coefficient of thermal expansion (CTE), was measured in this work. An apparatus to measure the CTE of the FlexCellTM electrolyte material was designed and implemented. The average CTE of 3 mol% Y2O3-ZrO2 (yttria stabilized zirconia or 3YSZ) was found to increase from 9 µm•m-1•°C-1 between room temperature (RT) and 180°C to nearly 11.5 µm•m-1•°C-1 from RT to about 650°C. A torch heating experiment was performed to study thermal gradients. For 200 µm thick YSZ electrolyte samples it was found that that heating experiment induced a directional dependence into unexpectedly large out-of-plane deflections. Digital image processing was used to capture deflection magnitudes which were compared against two finite element models. The two finite element models differed in how the through-thickness CTE variation was implemented. The first model contained a pseudo-continuous CTE gradient, whereas the second model contained an abrupt surface layer change in CTE. Both models were able to replicate the experimental results, thus pointing to CTE variation as the driving mechanism causing unexpected deflections in the experiments. Literature review and results from X-ray diffraction led to the conclusion that the tetragonal to monoclinic phase change in YSZ is the most likely reason for CTE change. It is proposed that the combination of heating and the initial tensile strain field are the root cause of this transformation.
Dr. Mark E. Walter (Advisor)
Dr. Brian D. Harper (Committee Member)
84 p.
Materials Science; Mechanical Engineering
SOFC; Solid Oxide Fuel Cell; CTE; Coefficient of thermal expansion; YSZ; phase

Recommended Citations

Citations

  • Davis, A. S. (2012). Temperature Induced Deflection of Yttria Stabilized Zirconia Membranes [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1338369600

    APA Style (7th edition)

  • Davis, Andrew. Temperature Induced Deflection of Yttria Stabilized Zirconia Membranes. 2012. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1338369600.

    MLA Style (8th edition)

  • Davis, Andrew. "Temperature Induced Deflection of Yttria Stabilized Zirconia Membranes." Master's thesis, Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1338369600

    Chicago Manual of Style (17th edition)

osu1338369600
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© 2012, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.