Bellino, Martin G.

Lamas, Diego G.

Walsöe de Reca, Noemí Elisabeth

English

Qucosa - Publikationsserver der Universität Leipzig

Enhanced Ionic Conductivity in  Heavily Doped Ceria Nanoceramics  M. G. Bellino, D. G. Lamas  and N. E. Walsöe de Reca  CINSO (Centro de INvestigaciones en SOlidos) CITEFA-CONICET Juan Bautista de La Salle 4397, Villa Martelli  (B1603ALO) Buenos Aires, Argentina dlamas@citefa. gov.ar;walsoe@citefa.gov.ar  1. Introduction Nanostructured materials are widely investigated because they have a wide functional diversity and exhibit enhanced or different properties compared to bulk materials. Particularly, in case of nanostructured ceramics, the presence of a large fraction of grain boundaries (GB) can lead to unusual or better properties. One growing area of investigation is the ionic transport in nanostructured solid electrolytes (nanoionics) with important applications, solid oxide fuel cells (SOFCs), among them, since an intense work has been conducted to find new electrolyte and electrode materials to allow the reduction of  SOFCs operating temperature. Ceria-based ceramics have been regarded among the most promising electrolytes for intermediate temperature SOFCs (IT-SOFCs) since their ionic conductivity is higher than that of the traditionally used zirconia-based electrolytes. Transport properties of ceria-based electrolytes have been investigated, particularly the influence of microstructure and impurities in GB conductivity. This conductivity depends on several effects: i.e the specific GB conductivity considerably increases with decreasing grain size, for grain sizes < 3 μm in relatively pure Ce0.8Gd0.2O2-δ ceramics [1]. The true GB conductivity, σGB*, is a correction for the apparent GB conductivity, σGB, taking into account that only the width of the GBs perpendicular to the applied field should be considered. Therefore, σGB* is related to σGB by the expression: σGB* = (δGB/d)σGB, where δGB is the width of GBs and d is the grain size. The increasing of σGB* with decreasing grain size has also been observed by other authors in calcia-stabilized zirconia[2]. In all these cases, the maximum total ionic conductivity is the grain interior (intrinsic or bulk) conductivity, σGI, which is independent of microstructure. Differently, a remarkable enhancement in the total ionic conductivity has been found in nanostructured CeO2-Gd2O3 and yttria-stabilized zirconia (YSZ) thin films, between one or two order of magnitude compared to that of polycrystals with larger grain size or single-crystals[3]. However, this behavior has not been confirmed for nanostructured heavily-doped ceria or YSZ dense pellets yet. In the present work, the electrical properties of nanostructured CeO2-10 mol% Y2O3 and CeO2-10 mol% Sm2O3 ceramics have been studied as a function of grain size by electrochemical impedance spectroscopy (EIS). 2. Experimental Procedure: Commercial CeO2-10 mol% Sm2O3 (CSO) and CeO2-10 mol% Y2O3 (CYO) ultrafine powders (specific surface areas: 228 and 198 m2/g, respectively), were uniaxially pressed at 600 MPa into pellets (8 mm diameter) which were sintered by fast-firing process (dwell temperature: 700-1000°C and dwell time: 3 min.). Heating and cooling rates were 200°C/min. Fast-firing is a relatively non-conventional approach used to densify ceramics minimizing the grain growth during sintering. Reference specimen were © 2005, M. G. Bellino The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and ApplicationDiffusion Fundamentals 2 (2005) 47.1 - 47.2 1uniaxially pressed at 400 MPa into pellets (8 mm diam.) and sintered at 1300°C for 3 h. Crystalline phases and average crystallite size were evaluated by X-ray diffraction. Lattice parameters were determined by Rietveld analysis. The crystallite size was determined by the Scherrer equation. An ultrapycnometer in helium gas flow was used for density measurements. Microstructure and average grain size were studied by SEM. Ionic conductivity of samples was measured by EIS from 125 to 250°C in air. 3. Results and discussion  The total ionic conductivity was fitted as a function of temperature following the Arrhenius law: σ = (σο/T) exp(-Εa/kBT)  where, ΕB a: activation energy for ionic migration, kBB: Boltzmann’s constant and σο, pre-exponential factor, is a constant related to the density of charge carriers (oxide vacancies). Figures 1a and 1b display the Arrhenius plots for CYO and CSO ceramics, respectively. It can be observed that both, Ea and σο decrease with decreasing average grain size. For samples with the smallest grain sizes, a change in Ea was observed as a function of temperature. This change is observed at about 190°C in all cases. EIS data show that, at low temperatures, the total ionic σ of nanostructured heavily doped ceria solid electrolytes increases by about one order of magnitude compared to that of conventional microcrystalline material. This result  is attributed to the predominance of the GB conductivity in the nanostructured ceramics. It was also found that the increase of the total ionic σ is associated to a reduction of the Ea probably caused by an increase of the GB ionic diffusivity with decreasing grain size. For samples with the smallest average grain size (35-50 nm), a change in the transport mechanism vs T was established, attributed to a transition from bulk-controlled (at high T) to GB- controlled (low T) ionic conduction.  Figures 1a and b: Arrhenius plots: CeO2-10 mol% Y2O3 and CeO2-10 mol% Sm2O3ceramics, for different grain sizes.  References [1]  G. M. Christie, F. P. F. van Berkel, Solid State Ionics, 83 (1996) 17. [2]  M. Aoky, Y.-M. Chiang, I. Kosacki, J.-R. Lee, H. L. Tuller, Y. J. Liu, J. Am. Ceram. Soc. 79 (1996) 1169. [3]  T. Suzuki, I. Kosacki, H. U. Anderson, Solid State Ionics 151 (2002) 111.  Diffusion Fundamentals 2 (2005) 47.1 - 47.2 2

Enhanced ionic conductivity in heavily doped ceria nanoceramics

https://ul.qucosa.de/api/qucosa%3A14380/attachment/ATT-0/

Enhanced ionic conductivity in heavily doped ceria nanoceramics

Abstract

Similar works

Full text

Available Versions

Qucosa - Publikationsserver der Universität Leipzig