A recent article published in Materials investigated the impact of thermochemical treatments on the electrical conductivity of SrTiO3-derived ceramics: A-site-deficient Sr0.85La0.10TiO3−δ (S85L10) and cation-stoichiometric Sr0.85Pr0.15TiO3+δ (S85P15).
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Background
Donor-doped SrTiO3 ceramics are promising for fuel electrodes, interconnects, and supports of solid oxide fuel and electrolysis cells (SOFC/SOEC) due to their excellent stability, appropriate thermomechanical characteristics, and acceptable electrical properties.
A minimum level of electrical conductivity is necessary for ceramics used in solid oxide cells to minimize ohmic losses during operation. However, inadequate conductivity can limit the electrochemical performance of the electrode.
Undoped SrTiO3 is a wide bandgap semiconductor with small electrical conductivity. However, donor-type partial replacements of Sr2+ sublattice (with rare-earth cations Ln3+) or Ti4+ sublattice (with Nb5+ and Ta5+) can significantly increase n-type electronic conductivity under reducing atmospheres.
The electrical conductivity of ceramics varies widely due to differences in fabrication and thermochemical processing conditions. This study explored the impact of thermochemical treatments on the microstructure and electrical conductivity of two SrTiO3-derived ceramics with moderate donor-type replacement in the strontium sublattice and varying defect profiles: S85L10 and S85P15.
Methods
S85L10 and S85P15 titanates were fabricated via a solid-state reaction process using SrCO3, TiO2, La2O3, and Pr6O11 as starting materials. The precursor mixtures were calcined at 900-1300 °C, raising the temperature in steps of 100 °C. Each temperature was maintained for five hours, and the mix was reground in a mortar between steps.
The prepared powders were ball-milled in ethanol for four hours, dried, and pressed into disk-shaped pellets. These green pellets were sintered in the air, followed by reductive treatment under 10 % H2-N2 gas flow or straightaway in a reducing 10 % H2-N2 environment.
Powdered ceramic samples were characterized using X-ray diffraction (XRD) and thermogravimetric analysis in flowing air or 10 % H2-N2 gas mix at 25-1100 °C. Microstructural analysis was conducted using scanning electron microscopy (SEM).
Additionally, the electrical conductivity of the prepared ceramics was determined using the four-probe direct current method at varying temperatures (300-1000 °C) in a 10 % H2-N2 environment and oxygen partial pressures at 900 °C in an H2-H2O-N2 environment.
Pseudo-four-probe alternating current impedance spectroscopy was also performed to record the changes in the conductivity of ceramic specimens during isothermal redox cycling between air and a 10 % H2-N2 environment.
Results and Discussion
The synthesized S85L10 powder exhibited a pure phase with a cubic perovskite-type structure after calcination at 1300 °C. This structure remained preserved post-sintering and thermochemical processing under varying conditions; XRD detected no phase impurities.
Sintering S85L10 in air at 1700 °C facilitated grain growth (5-31 µm) and densification, with the microstructure remaining intact after reduction in a 10 % H2-N2 atmosphere at 1300-1500 °C. The S85L10 ceramics were dense, with a residual porosity of 4-5 %.
XRD results for S85P15 specimens sintered at 1350 °C in air or a 10% H2-N2 environment demonstrated the development of single-phase perovskite ceramics, though the crystal lattice symmetry was not fully cubic. The sintering temperature influenced the microstructure, similar to S85L10 samples.
Sintering S85P15 in air at 1700 °C led to significant grain growth (10-38 µm) and densification, which remained unchanged after subsequent reduction at 1500 °C. The ceramics had a relative density of 90 %. However, direct sintering at 1500 °C in a reducing atmosphere reduced grain size significantly (0.8-3.5 µm).
Irrespective of the reduction extent, resistive grain boundaries defined the semiconducting behavior and a lower electrical conductivity of S85P15 ceramics with fine grain size. The resistive nature of grain boundaries was attributed to the specific defect chemistry of cation-stoichiometric Sr1−xLnxTiO3±δ titanates.
Oxidized porous S85P15 ceramics exhibited faster reduction kinetics than S85L10 ceramics at temperatures below 1000 °C, highlighting the importance of extended structural defects in cation-stoichiometric Sr1−xLnxTiO3±δ titanates. Alternatively, the reductive pretreatment of porous S85L10 ceramics at a high temperature of 1300 °C facilitated low-temperature equilibration kinetics on redox cycling.
Conclusion
The study examined the electrical conductivity of A-site-deficient S85L10 and cation-stoichiometric S85P15 ceramics under reducing environments at temperatures ≤ 1000 °C. The ceramics were processed under different thermochemical conditions to produce varying microstructures and reduction levels.
Sr1−1.5xLnxTiO3−δ titanates demonstrated better suitability for the interconnect applications when densified through sintering and well-reduced. Alternatively, Sr1−xLnxTiO3+δ titanates are more suitable as SOFC/SOEC fuel electrodes when fabricated under oxidizing conditions and reduced in situ at temperatures similar to operational conditions.
However, using a pre-reduced Sr1−1.5xLnxTiO3−δ as an electrode in a solid oxide cell might enhance the conductivity of the porous electrode in combination with appropriate equilibration kinetics.
Journal Reference
Bamburov, A., Kravchenko, E., & Yaremchenko, A. A. (2024). Impact of Thermochemical Treatments on Electrical Conductivity of Donor-Doped Strontium Titanate Sr(Ln)TiO3 Ceramics. Materials. DOI: 10.3390/ma17153876, https://www.mdpi.com/1996-1944/17/15/3876
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