http://ocw.mit.edu/courses/materials-science-and-engineering/3-091sc-introductio
ID: 1719092 • Letter: H
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http://ocw.mit.edu/courses/materials-science-and-engineering/3-091sc-introduction-to-solid-state-chemistry-fall-2010/reactions-and-kinetics/24-diffusion/
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Professor Donald R. Sadoway is the John F. Elliott Professor of Materials Chemistry at the Department of Materials Science and Engineering, Massachusetts Institute of Technology. We will be studying some of his lectures from his "Solid State Chemistry" course provided through MITOpenCourseware Please watch the following video from 45min onwards to learn about applications of diffusion theory to the design of exhaust gas oxygen sensors used in vehicles. You may wish to watch the entire video to reinforce what you have learned so far. etic Please answer the following questions after watching the video: Explain how the addition of Calcium to Zirconium Oxide results in vacancies in the crystal structure. a) b) Why are vacancies important to the operation of the oxygen sensor?Explanation / Answer
a) ZrO2 adopts a monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at higher temperatures. The volume expansion caused by the cubic to tetragonal to monoclinic transformation induces large stresses, and these stresses cause ZrO2 to crack upon cooling from high temperatures. When the zirconia is blended with calcium and calcium oxides, the tetragonal and/or cubic phases are stabilized. Hence addition of calcium / calcium oxides results in vacancies in the crystal structure.
b) If there are no or less no. of vacancies in the zirconia oxide oxygen sensor , the amount of oxygen absorbed by the ceramic will be severely reduced and would reduce the performance of the oxygen sensor.
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