At high temperatures, ZrO2 has the cubic fluorite structure we studied in class,

Question

At high temperatures, ZrO2 has the cubic fluorite structure we studied in class, where the Oxygen anions take the simple cubic positions and the Zirconium cations fill the body centered position (cubic interstice) in half the cells, leaving half the body centered sites open for a self-interstitial. It is technologically interesting as an oxygen sensor in applications such as internal combustion engines and as the original material for high temperature fuel cells. At low temperatures, it undergoes a phase transformation to its tetragonal or monoclinic polymorphs. To avoid this and the accompanying volume change that fractures parts, zirconia is doped to stabilize the structure so it can be used for engineering applications. Please answer the questions below given the ionic radii provided If CaO is added to ZrO2, would an oxygen vacancy or interstitial be created? Briefly explain your reasoning. (1 pt) If Y203 is added to ZrO2, would an oxygen vacancy or interstitial be created? Briefly explain your reasoning. (1 pt) Using radius ratios, explain why cubic ZrO2, where Zr sits in the cubic interstice, is unstable at room temperature? How do you think adding Y203 stabilizes the cubic structure? (2 pts) lonic Radius (nm) Ca 2 4 y*3 0-2 0.114 0.082 0.096 0.126

Explanation / Answer

For both CaO - ZrO2 and Y2O3 - ZrO2 oxygen vacancies are created Ca2+ or Y3+ do not occupy
the Zr+4 sites instead they sit in interstitial sites while O2- occupy the new sites which are formed.
So an oxygen vacancy is created by this transfer of O2- atom to this newly formed normal site.

At high temperatures Y2O3 is added in such a way that the cubic fluorite structure of ZrO2 is not
destroyed or in other words the structure should be maintained so in order to do this ratio of cation
to anion radius should be ideal. At room temperatures ZrO2 tends to transform into monoclinic
form which eventually increases the toughness of ZrO2 . So we add Y2O3 at high temperatures
to stabilize ZrO2 which will result in increased oxygen vacancies, remember high temperature
transistion phases are more stablized when cations like Ca+2 , Y3+ are added as they maintain
neutral charges on a whole.

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