Impedance spectroscopy analysis of zirconia:8 mol% yttria solid electrolytes with graphite pore former
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Porous ZrO2:8 mol% Y2O3 sintered ceramics were prepared by adding graphite powder as pore former before sintering. The thermal elimination of graphite was evaluated by thermogravimetric analysis. Impedance spectroscopy analysis was carried out in the 5 Hz to 13 MHz frequency range in specimens sintered with and without pore former. The deconvolution of the impedance diagrams, [Z00 (o) Z0 (o)] and [Z00 (o) log f ], and the numerical residuals resulting from the subtraction of normalized impedance spectroscopy diagrams measured in specimens with and without pore formers were evaluated. A comparison of the impedance diagrams of samples sintered with and without pore former shows evidence of a modification of the electrical response caused by pores. The results show the unequivocal ability of the impedance spectroscopy technique to gauge microstructural modification caused by the presence of pores in ionic conducting solids.
I. INTRODUCTION
ZrO2:8 mol% Y2O3 polycrystalline ceramics are one of the most important electroceramic materials mainly because of their electrical properties above room temperature. They are largely used in commercial devices such as lambda oxygen sensors (for measuring the partial pressure of oxygen in gas exhaust and to optimize the air– fuel ratio in combustion engine vehicles for less fuel consumption and for pollution depletion) and in solid oxide fuel cells (as dense oxide ion conductor solid electrolyte, as porous cermet anode with Ni, and as porous composite cathode with lanthanum strontium manganite). Sintered polycrystalline ceramic materials consist basically of grains and interfaces, the latter being mainly composed of grain boundaries, segregated impurities, and pores. In oxide ions conducting solid electrolytes, the interfaces contribute to the reduction of the total electrical conductivity of a system by blocking charge carriers, oxide ion vacancies, at these intergranular regions. The grain boundaries in oxide ion conductors, which consist of one grain-boundary core and two adjacent space charge layers, are blockers to O2 charge carriers because of oxygen vacancy depletion in the space charge layer, and the blocking intensity decreases as the temperature increases.1–6 Pores are supposed to block the ionic transport across the grain boundaries by decreasing the conduction path width and constricting current lines, similar to the effect of impurities at grain boundaries.7 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0209
1780
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 5, May 2009 Downloaded: 19 Mar 2015
The impedance spectroscopy technique consists of applying to the electrode/specimen/electrode cell a lowamplitude alternating voltage V = V0 ejot, where o is the pulsation and V0 the voltage signal amplitude. The current output is I = I0 ej(ot+f), where f is the phase angle of the current versus voltage and I0 is the current signal amplitude. The impedance is Z(o) = V/I = Z0 ejf = Z0 (o) + j Z00 (o), where Z0 an
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