Oxygen Permeability of Calcia-Stabilized Zirconia at Low Oxygen Partial Pressures
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col CO2
Zr02 (CaO) solid
= 1 atm) electrolyte
(P6
=10-7 to
(Po
tube
2
10-12 atm),
2
The oxygen permeability of calcia-stabilized zirconia is proportional to p'6 2- 1 /4• From the permeability measurements, the conduction properties of the electrolyte were obtained. SINCE the pioneering work of Kiukkola and Wagner" on Zr02-based solid electrolytes, solid-oxide galvanic cells have been extensively studied by a number of investigators. An important application of the Zr02based oxide solid electrolyte is as an oxygen probe for monitoring oxygen in liquid steel. However, this electrolyte shows significant permeability to oxygen at steelmaking ternperaturea.?" This could be a major source of error during emf measurements by the oxygen probe. Prior to the present study, the oxygen permeability of calcia-stabilized zirconia at comparatively high oxygen pressures (1 to 10- 3 atrn) was studied by the authors ," using the following cell. O2
Zr02 (CaO)
solid (Po = 1 atm) electrolyte 2 tube
N2 - O2
the defect equilibrium Eqs. [2] and [3], respectively, which should describe the introduction of electronic defects into the electrolyte structure. 02(g) + 2 Vo' 20 0
= 02(g)
= 20 0 +
4 h:
[2]
+ 2 Vo' + 4 e'
[3]
where 0 0 is a normal oxygen ion on an oxygen site, Va' is a vacancy at an oxygen site having two positive charges relative to the normal site occupation, h' is a positive hole, and e' is an excess electron," The introduction of a new parameter, PEEle' would facilitate the discussion of the specific cases which arise from Eq. [1 V
[4] (Po
2
=
3
0.39 to 10- atm).
In the present study, the permeability cell is represented schematically as,
It was found that the oxygen permeability for this
r»:»
Zr02 (CaO)
cell is proportionaUo (1The purpose of the present study is to determine the oxygen permeability at low oxygen partial pressures.
(Po
= 2
1 atrn)
solid electrolyte tube
(Po
=
10- 7 to 10-12 atm).
2
The oxygen partial pressure used in this study satisfy the condition,
THEORETICAL The oxygen permeability due to p-type and n-type electronic conduction is given by:2 Jo = ~ [aO (P' 1/4 2 4 F2L tEl 02 -
P'6
2
_
P"
02
1/4) _
-1/4)]
aO (P' 8
02
[5]
-1/4
[1]
where J 02 is the oxygen flux (mole em -'2 S-l), R is the gas constant, T is temperature, F is the Faraday constant, L is the thickness of the electrolyte wall, and u;B and Us are the p-type and n-type electronic conductivities at P 0 2 = 1 atm, respectively. 14 The proposed P and P 02 -1/4 dependencies for the p-typeand n-type electronic conductivities result from
0/
MASANORI IWASE and TOSHISADA MORI are Research Associate and Professor, respectively, Department of Metallurgy, Kyoto University, Sakyo-ku, Kyoto 606 Japan. Manuscript submitted March 24,1978. METALLURGICAL TRANSACTIONS B
where the parameters, Pe and Pe , denote respectively the oxygen partial pressure at which the p-type and ntype electronic conductivities are equal to the ionic conduct ivlty,">" Combining Eqs. [1] and [5],
[6] Equation [6] should be applicabl
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