Impact of Spark Plasma Sintering Conditions on Ionic Conductivity in La1.95Sr0.05Zr2O7-δ Electrolyte Material for Intermediate Temperature SOFCs
The conventional route for the preparation of lanthanum zirconate pyrochlore (LSZO) via solid-state reactions requires multiple milling and high temperature calcination steps. Also, this method leads generally to an heterogeneity of the final product, whereas wet chemical route, which consists of mixing precursors in a solution, could improve compositional homogeneity and stoichiometry. In this work, we have synthesized nano-sized La1,95Sr0,05Zr2O7-d using an oxalic co-precipitation method. As impedance spectroscopy measurements require high densification, only spark plasma sintering (SPS) gives dense materials. Other sintering processes such as hot isostatic pressing induce a segregation of strontium at the surface of the pellet , and thereby decrease proton conductivities.
LSZO powders were densified using SPS apparatus under different sintering conditions: holding time, temperature and pressure. To maintain the same compacity for different grain sizes, starting powder materials were calcined at different temperatures in order to increase of the particle size. Thus several pellets with either different relative densities or grain sizes were obtained. The grain size increases with increasing of the sintering temperature. The proton conductivity behavior of those pellets was investigated by AC impedance spectroscopy under dry and wet atmospheres. The data were measured in the frequency range 0.1Hz – 6 MHz (Materials mates M2-7260 impedance analyzer) at intermediate temperatures 400-600°C. In order to verify the dependence of total resistance and capacitance, a DC-bias (UDC from 0 to 1V) was applied. The Nyquist diagrams were modeled by equivalent circuits based on resistors and constant phase elements (CPEs). The ionic conductivities are clearly dependent on grain sizes (see Figure 1). In order to elucidate this dependence, it will be necessary to assess a porosity correction equation. The activation energies, calculated using the Arrhenius equation, increase with increasing grain sizes. The proton conductivities are higher in wet atmosphere than dry atmosphere. For example, the ionic conductivities of 120 nm-LSZO are 2.45 × 10-5 S.cm-1 and 3.30 × 10-5 S.cm-1 under dry and wet atmosphere (5% H2O) at 600°C, respectively.
Figure 1 - Nyquist plots of impedance spectra for LSZO with different particle sizes at 600°C
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