A thermodynamic study of 3 mole percent yttria stabilized zirconia

Ming-Yu Yin, Marquette University

Abstract

In recent years, fine powder YSZ starting materials have become available. These powders have significantly improved the fabrication of products with high purity and high density. The transformation toughening behavior found in the partially stabilized zirconia led to the special interest in 3Y-TZP(3 m/o $\rm Y\sb2O\sb3$-ZrO$\sb2)$ which contains primarily the tetragonal structure. Along with its ultra-high strength, its excellent thermal shock resistance provides another stimulus for investigating the electrical behavior of 3Y-TZP. 3Y-TZP also has a higher ionic conductivity at low temperature than other YSZ electrolytes. The high ionic conductivity is desirable for applications involving gas sensors and solid oxide fuel cell (SOFC). However, there is a dearth of information regarding the defect structure and nonstoichiometric thermodynamic behavior of 3Y-TZP. Techniques employed to obtain thermodynamic data were high temperature solid-state coulometric titration and high temperature constant composition measurements. The electronic transport behavior at low oxygen pressure was also investigated. In this study, the nonstoichiometric thermodynamic behavior of 3 m/o $\rm Y\sb2O\sb3$-ZrO$\sb2$ has been studied at temperature range 889 $\sim$ 1042$\sp\circ$C and the $P\sb{O\sb2}$ range $10\sp{-3.0}\sim10\sp{-23.4}$ atm. In the electrochemical cell technique $P\sb{O\sb2}$ was measured in situ. The oxygen pressure and temperature dependance of nonstoichiometry (i.e, x in $\rm Zr\sb{1-y}Y\sb{y}O\sb{2-y/2-x})$ was determined by the coulometric titration method. The nonstoichiometry, x, is given by the following expression $$x=3.36\times 10\sp{-3}\exp({-}1.72eV/kT)P\sbsp{O\sb2}{-1/4} - 8.40\exp({-}1.31eV/kT)P\sbsp{O\sb2}{+1/4}$$The concentration of electrons and holes are given as follows $$\eqalign{n&=1.99\times 10\sp{20}\exp({-}1.72eV/kT)P\sbsp{O\sb2}{-1/4}\cr p&=5.00\times 10\sp{23}\exp({-}1.31eV/kT)P\sbsp{O\sb2}{+1/4}\cr}$$Results of constant composition measurements was determined in the n-type region $(P\sb{O\sb2}<10\sp{-14} atm)$ with x larger than $1.39\times 10\sp{-6}.$ The relative partial molar enthalpy, $\Delta\bar{H}\sb{O\sb2},$ was determined to be 6.88 eV by the aforementioned method. The electron conductivity, $\sigma\sb{\rm n},$ was determined by steady-state oxygen permeation measurements. $\sigma\sb{\rm n}$ and the calculated expression for the electron mobility, $\mu\sb{\rm n},$ are given by the following equations $$\eqalign{\sigma\sb{n}&=4.30\times 10\sp5\exp({-}3.67eV/kT)P\sbsp{O\sb2}{-1/4}\cr \mu\sb{n}&=1.35\times10\sp4\exp({-}1.95eV/kT)\cr}$$ Under isothermal condition $\sigma\sb{\rm n}$ exhibits a $-$1/4 power dependence on oxygen partial pressure.

Recommended Citation

Ming-Yu Yin, "A thermodynamic study of 3 mole percent yttria stabilized zirconia" (January 1, 1993). Dissertations (1962 - 2010) Access via Proquest Digital Dissertations. Paper AAI9411529.
http://epublications.marquette.edu/dissertations/AAI9411529

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