Effect of sintering aid (CoO) on transport properties of nanocrystalline Gd doped ceria (GDC) materials prepared by co-precipitation method
Introduction
Gd and Sm doped ceria based materials are well known oxygen ion conductors and have been extensively studied from the research and technological view point for the development of solid oxide fuel cell (SOFC). High sintering temperatures (1400–1600 °C) of ceria based materials have been the major issues which induce atomic diffusion of electrode–electrolyte inter-layers in SOFC cells, leading to the deterioration of cell properties. In past decades various efforts have been made to reduce the sintering temperature of the ceria based materials by co-doping/addition of sintering aids such as MnO2, Mn2O3, Co3O4, CoO, and Fe2O3. [1], [2], [3], [4], [5], [6], [7], [8]. Various results have shown the cobalt oxide as an alternate sintering aid since it has high volatility and is able to reduce sintering temperature more effectively than any other metal oxides [6], [9], [10], [11], [12], [13], [14]. Kleinlogel et al. [15] observed that the optimum CoO additive concentration to be 1 to 5 mol% for GDC, where the sintering temperature of GDC lies between 875 °C and 1000 °C. In previous work of our group [16], the sintering temperature of Gd doped ceria (GDC) has been successfully reduced to 850 °C by synthesizing the material in co-precipitation method. CoO was added with GDC materials as sintering aid by using deposition precipitation method [16]. As far as electrical transport properties of these materials are concerned, there was a little known about sintering additive effect. Different authors have observed that the conductivity of doped ceria remains same up to 3 mol% of CoO and it drops above 5 mol% of CoO [12], [17]. Reduction of electrical conductivity in GDC co-doped with transition metal up to 5% is also observed elsewhere [3], [6]. In a recent study, Han et al. [18] observed that the sintering aid (CoO) does not have any effect on electrical conductivity of GDC material. Fagg et al. [7] and Schmale et al. [8] claim that the Co addition does not affect the ionic conductivity but increases the electronic conductivity and oxygen permeability in “Pr” and “Gd” doped cerias. Hence, with various contradicting results, the effect of sintering aids on electrical transport properties is yet not clear.
In this work, ionic transport properties of 1, 3 and 5 mol% CoO co-doped GDC nanomaterials (prepared by co-precipitation method) have been studied by correlating the electrical and dielectric properties. The space charge effect at the grain boundaries is also considered to understand the effect of sintering aid (CoO) on the electrical properties of GDC.
Section snippets
Experimental
The nanocrystalline Gd (10 mol%) doped ceria (GDC0.1) was prepared by co-precipitation method, the details of which was given elsewhere [19]. 1–5 mol% of CoO was co-doped in GDC0.1 by using deposition precipitation method [16]. The dried precursors/precipitates of all the samples were calcined at 450 °C for 3 h to get the single phased materials. The Phase and morphology of the materials were studied by X-ray diffraction (PW 3830) and scanning electron microscopy (XL-30 FEG ESEM). The specific
Structural and morphological study
Fig. 1 shows the powder X-ray diffraction patterns of Gd doped cerias with and without CoO, which were calcined at 450 °C for 3 h. The materials are formed with the cubic fluorite structure without any additional impurity peak, as seen in the earlier reports on CoO co-doped GDC [16], [20]. As discussed in the previous studies [16], [20], even though some of cobalts might be present in oxide phase, the corresponding peaks were not observed due to detection limit of X-ray diffractometer. The
Conclusions
The CoO co-doped GDC samples, prepared by deposition precipitation method, exhibited higher grain interior conductivity (about three times) and grain boundary conductivity (about two to five times) compared to that of GDC, due to incorporation of Co2+ in the Ce4+ sites. Bulk conductivity does not increase further with CoO content above 1 mol% due to the solubility limit of Co in the lattice. Grain boundary conductivity (σgb) and activation energy (Egb) in the co-doped samples are being
Acknowledgements
This work was supported by the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea and institutional program of KIST.
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