Electrical conductivity of Y2O3-doped CeO2 based composite ceramics by spark plasma sintering: The effects of a second phase of CeAlO3
Introduction
Oxygen ion conducting ceramics have been extensively studied in the last decades due to their applications in solid oxide fuel cells (SOFCs), electrochemical sensors and other fields [1,2]. Y2O3-doped CeO2 (YDC) is an oxygen ion conductive electrolyte, which has an excellent oxygen ion conductivity in the intermediate temperature range of 500–700 °C and favorable fracture strength. It can maintain thermal and chemical stability during the long-term operation process [[3], [4], [5]]. However, it is difficult to further improve the electrical conductivity of YDC in the low temperature range (≤500 °C) due to the influence of its grain boundary (GB). Generally, the grain boundaries of polycrystalline doped ceria are the main barrier for ion migration. The GB electrical conductivity of doped ceria ceramics is always more than 2 orders of magnitude lower than that of the grain [6,7].
The main factors for this phenomenon can be attributed to the impurity blocking effect existing at the grain boundaries. During the sintering of doped ceria ceramics, impurities are usually inevitably introduced, which segregate at the grain boundaries, hinder the oxygen ion conduction and reduce the GB electrical conductivity [8,9]. Besides, the segregation of doped elements and the depletion of oxygen vacancies in the space charge region near the grain boundaries also result in low ionic conductivity at GB [[10], [11], [12], [13]].
In [[14], [15], [16]] it was demonstrated that the introduction of a small amount of Al2O3 into the doped CeO2 matrix can effectively reduce the sintering temperature, increase the hardness and improve its GB ionic conductivity. When the content of Al2O3 exceeds 20%, it can precipitate at the GBs of YDC matrix as a second phase. The uniform distribution of the insulating Al2O3 phase can inhibit the electronic conductivity of YDC through ‘electron capture’ mechanism. The formed Al2O3/YDC heterogeneous interface also has a beneficial impact on the GB electrical conductivity [[17], [18], [19]].
In recent years, many novel sintering methods of ceramics, such as microwave sintering and spark plasma sintering (SPS), have attracted much attention due to their great superiority over conventional sintering technology, such as extremely short sintering time and extremely low sintering temperature [[20], [21], [22]]. Chickalingam R et al. prepared Gd-doped CeO2 (GDC)-Al2O3 composite electrolyte by microwave sintering, in which needle-like and spherical nano-Al2O3 grains formed and its electrical conductivity was much higher than that of the sample prepared by conventional sintering [19]. Tatiana L et al. rapidly sintered and densified the nanocrystalline YDC powders by SPS at a lower sintering temperature (1000–1200 °C) [23]. SPS method greatly lowers the difficulty in preparing dense nano-ceramics. At present, there are a lot of research reports about the doped CeO2 ceramics sintered by SPS. However, the doped CeO2 based composite ceramics sintered by SPS have been seldom involved. So, a further research is necessary to understand the effect of SPS on the properties of CeO2-based ceramics.
In this experiment, 15 mol% Y2O3 doped CeO2 (YDC) nano-powders were prepared by a co-precipitation method. The YDC powder was mixed with an aluminum-containing sol, and then the composite powders were densified by SPS at 1300 °C for 5 min to prepare the corresponding composite ceramics. The phase composition, microstructure morphology and electrical conductivity of the composite ceramics were characterized and the influences of the Al(NO3)3 addition amount on the ion conductivity of samples were elaborated.
Section snippets
Sample preparation
Ce0.85Y0.15O1.925 (YDC) powders were synthesized by a co-precipitation method. The mixed solution of Ce(NO3)3·6H2O (Aladdin, 99.95%) and Y(NO3)3·6H2O (Aladdin, 99.5%) was prepared, and ammonium carbonate solution (0.1 mol/L) was used as a precipitant. The precipitate was washed three times with ethanol, and then dried at 80 °C for 24 h. Then the precursor powders were ground by an agate mortar for 0.5 h and then calcined at 700 °C for 2 h to obtain YDC nano-powders.
After that, Al(NO3)3.9H2O
Phase composition and microstructure
The XRD patterns of the composite ceramics of YDC-xmol% aluminum containing compound (x = 0, 10, 20 and 40, named as A0, A10, A20, A40) sintered by SPS are illustrated in Fig. 1 (The XRD patterns and EDS result of the powders for A20 and A40 are listed in Fig.S4 and Fig.S5). All samples exhibit the diffraction peaks corresponding to a main phase of YDC with cubic fluorite structure. A second phase of CeAlO3 begins to form for x ≥ 10, and its diffraction peak intensity gradually increases with
Conclusions
The composite powders of YDC-xmol% aluminum containing compound (x = 0, 10, 20 and 40, named as A0, A10, A20, A40) were densified by SPS process at 1300 °C for 5 min. All specimens consist of the main phase of YDC, while a second phase of CeAlO3 forms in A10, A20 and A40. The average grain size of the composite ceramics decreases from 1.7 μm to 0.34 μm as the Al(NO3)3 addition amount rises from 0 to 40%. The grain electrical conductivity of the samples increases with the rise in Al(NO3)3
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors gratefully acknowledge the financial support from the Yunnan Ten Thousand Talents Plan Young & Elite Talents Project.
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