Effect of Sintering Atmosphere and Particle Size on the Ionic Conductivity of Gadolinia-Doped Ceria

Gadolinia-doped ceria has been intensively investigated over the last decades as a promising electrolyte and electrode material for intermediate-temperature solid oxide fuel cells [1]. The sintering behavior of this solid electrolyte in reducing atmosphere is different from that in air, because of the reduction of Ce⁴⁺ to Ce³⁺[2]. In this case, changes in the microstructure and in the related properties are expected to occur, due to variation in the oxygen-vacancy concentration.

               In this work, the effects of the sintering atmosphere and the particle size of powders on densification and ionic conductivity of Ce_0.9Gd_0.1O_2-δ were systematically investigated.  

               Ce_0.9Gd_0.1O_2-δ commercial powders (>99.5%, Fuel Cell Materials) with specific surface area ranging from 7.4 to 210 m².g^-1were used as starting materials. Cylindrical pellets were prepared by uniaxial and isopressing following by sintering at 1250°C for 2 h under different atmospheres.

               The average initial particle size of the powders was evaluated by nitrogen adsorption (Quantachrome, NOVA 1200) using the BET method. Characterization of the sintered materials was carried out by density measurements, thermodilatometry (Anter-1161, Unitherm), electronic Raman spectroscopy (InVia Raman microscope, Renishaw), electron paramagnetic resonance (Bruker-EMS), field-emission scanning electron microscopy (FEI, Inspect F50) and impedance spectroscopy (HP4192A). The mean grain size was estimated by the linear intercept method.

               The sintered pellets reached relatively high densities (93 to 97% of the theoretical value) independent on the sintering atmosphere. The particle size determined by nitrogen adsorption of the starting powders was: 112, 23  and 4 nm for powders, hereafter named S1, S2 and S3, respectively. The linear shrinkage curves show that good densification of gadolinia-doped ceria is attained depending on the initial size of the powder particles. However, for isothermal treatments the densification is homogenized  and all sintered pellets reached high values after 1250°C for 2 h.

All investigated properties changed to some extent with the sintering atmosphere and the initial particle size of the starting powders. The grain conductivity, for example, of Ce_0.9Gd_0.1O_2-δ pellets sintered in air and under inert atmospheres (argon and nitrogen) exhibit the same bulk conductivity not depending on the sintering atmosphere, in spite of the changes observed in the electron paramagnetic signal.

References

[1] H. Yahiro, Y. Eguchi, K. Eguchi, H. Arai, J. Appl. Electrochem. 18 (1988) 527.

[2] Z. He,  H. Yuan, J. A. Glasscock, C. Chatzichristodoulou, J. W. Phair, A. Kaiser, S. Ramousse, Acta Mater. 58 (2010) 1866.