Local Atomic Structures at Grain Boundaries in Gadolinium Doped Cerium Oxides Tomomi Kosaka1*, Kiminori Sato2 1 Department of Chemistry, Tokyo Gakugei University, Tokyo, Japan, 2 Department of Environmental Science, Tokyo Gakugei University, Tokyo, Japan (Received February 28, 2010 ; Final form May 12, 2010) ABSTRACT 1. INTRODUCTION Gadolinium doped ceria (GDC) is one of the promising Gadolinium doped ceria (GDC) prepared by candidate for electrolyte 71-6/ of solid oxide fuel cell coprecipitation method was investigated by x-ray owing to high conductivity of oxygen ions in the diffraction (XRD), complex impedance measurement, intermediate temperature range (773-973K). and positron lifetime spectroscopy. The grain boundary Substitution from Ce4+ to Gd3+ produces oxygen conductivity was found to be lower than that of bulk vacancies for compensating electric charge. Since the conductivity. XRD revealed the fluorite structure ionic radius of Gd3+ is similar to that of Ce4+, gross indicating that gadolinium is successfully doped into distortion isn't expected in ceria host lattice. The excess cerium oxide. Prior to sintering, the vacancy-sized free oxygen vacancies lead to higher ionic conductivity than volume and nanovoid were observed at interfaces for undoped ceria. among crystallites. The vacancy-sized free volumes It has been suggested that oxygen ionic conductivity shrank with increasing sintering temperatures and of ceria based electrolyte is influenced by constituent finally got dominant. The results suggest that the grains. Sameshima et al. Ill reported that the grain sintering process of GDC follows the kinetics of resistance for the diffusion of oxygen ions is responsible vacancy-sized free volumes and nanovoids at interfaces. for ionic conductivity for rare-earth doped ceria prepared by coprecipitation method. In principle, high temperature sintering over at 1573K is required for GDC to achieve density more than 90% with less open porosity. Such high temperature causes a replacement Keywords: GDC, solid state electrolyte, sintering, from Ce02 to Ce203, which leads to produce positron lifetime spectroscopy micro-cracks along with the degradation of conduction efficiency. Takamura et al. /8/ observed an enhancement '•E-mail: [email protected] of electric conductivity under the pressure of Gigapascal 373 Vol. 29, Nos. 5-6, 2010 Local Atomic Structures at Grain Boundaries In Gadolinium for acceptor-doped ceria nanoparticles. They attributed 2.2. Characterization the enhancement of electric conductivity under high The lattice parameters of GDC were calculated from pressure to the densification of granular particles. diffracted peaks measured by Cu Κα X-ray diffractmeter Positrons preferentially localize at atomic disorder as (Ultima IV, Rigaku corp.) operated at 40kV, 50mA. The e.g., grain boundary 191, and provide us the information scattering angles were calibrated using Si powder (NIST on local atomic structures at annihilation sites /10-11/. 640c). The crystallite size of GDC was estimated from Some experiments by positron annihilation (111) diffraction peak by Scherrer's equation. The bulk spectroscopy predicted the presence of vacancy-like density was measured by the Archimedes method using defects and larger free volumes associated with ion exchanged water. The Pt electrodes were formed on interfaces among crystallites for zirconia-based the both side of GDC pellet by heating at 1473 Κ using materials /12-13/. Garay et al./l 3/ proposed that defects Pt paste (Tanaka Kikinzoku TR-7070). The oxygen in negatively charged grain-boundary space region ionic conductivity of the pellet was measured in air correspond to positron trapping sites, since positrons are from 673K to 973K conducted by complex impedance not trapped by positively charged spaces such as oxygen analyzer (Princeton applied research corp. VersaSTAT3) vacancies. in the frequency range from 100 μΗζ to 1MHz. The Here, our advances in the study of GDC by XRD, impedance data of GDC was analyzed using ZView complex impedance measurement, and positron lifetime software (Scribner Associates). spectroscopy are described/14/. We focus on the Positron lifetime spectroscopy was performed at evolution of local atomic structures at interfaces upon room temperature. The positron source (22Na), sealed in sintering to gain an insight into the densification a thin foil of Kapton, was mounted in a mechanism. sample-source-sample sandwich. Positron lifetime spectra (~ 106 coincidence counts) were recorded with a time resolution of 230 ps full-width at half-maximum 2. EXPERIMENTAL PROCEDURE (FWHM). The data were numerically analyzed using the 2.1. Materials POSITRONFIT code/16/. GDC was prepared by oxalate coprecipiation method /15/. The starting materials were Ce(N03)3-6H20, Gd(N03)3-6H20, H2C204'2H20. All 3. RESULTS AND DISCUSSION reagents were purchased from Wako Pure Chemical industries LTD. The cerium and gadolinium nitrate Figure 1 shows the XRD pattern of GDC sintered at solutions (0.2mol/l) were mixed at a molar ratio of 1073 Κ for 6h in air. The XRD pattern indicates that gadolinium of approximately 0.2. The mixed solution GDC has a fluorite type structure without any other was dropped into a stirred oxalic acid solution phases. The peaks denoted as Pt indicate the diffraction (0.4mol/l) to produce oxalate coprecipitate. The from sample holder. All peaks of GDC are shifted to coprecipitate was filtered, washed with ion exchanged low angle region against to the corresponded ones of water in several times, and dried at 313K for 8h. The Ce02 (denoted as solid bar) indicating the expansion of oxalate coprecipitate was calcined at 873K in air for lh interplanar spacing for GDC by Gd doping. The lattice to form GDC. The GDC powders were uniaxially parameter calculated for GDC is 0.54245 ±0.00006nm. pressed into a pellet with 10mm diameter and 2mm Hong and Virkar /17/ reported that the lattice thickness under the pressure of 20MPa. The GDC parameter,a for GDC is given by following equation: pellets were sintered from 873K to 1273K for 6h in air. 4 a = — [ΧΪΙ + (1 - x)r2 + (1 - 0.25x)r3 + 0.2Sjcr4] χ 0.9971 374 Kosaka and Sato High Temperature Materials and Processes (111) 05 λ 3 u ^ (220 ) (311) .ΐπ 2 (Λ 1 (200) • ο J wL—X 20 40 60 80 Diffraction angle, 29/° (Cu Κα) Fig. 1: XRD pattern of GDC sintered at 1073K for 6h in air. For comparison, the data of Ce02 taken from JCPDS No.34-394 is added. where x, ru r2, r3, and rA are the concentration of If the relative density of GDC is close to theoretical one, gadolinium ion, the radii of the gadolinium ion we can determine whether oxygen vacancy is formed or (0.1053nm), cerium ion (0.097nm), oxygen ion not/17/. In the case of our GDC sintered at 1273K, the (0.138nm) and the oxygen vacancy (0.1164nm), relative density is 80%. It may be thus unsuitable to respectively. The factor, 0.9971, was introduced for directly discuss the structure of GDC with the results of correcting the discrepancy between the lattice parameter Archimedes method. Based on the results of XRD measured for pure ceria (0.541 lnm; JCPDS No.34-394) reported above and experimental evidence confirming and the calculated value using this equation when χ = 0. the oxygen vacancy model/18/, we believe that Using the calculated lattice parameter and this equation, substitution of Ce44 with Gd3+ partially occurs and the Gd composition in GDC is estimated to be 0.1969. oxygen vacancies are successfully formed. Additional EDS measurements however revealed the Gd The typical complex impedance of samples consists of composition of Π0.26, which is significantly higher two semicircles in the Nyquist plot (Figure 2(a)). The than above estimation. Further exploration is required impedance arc is interpreted with equivalent electrical on this matter. circuits (Figure 2(b)). One semicircle in high frequency 375 Vol. 29, Nos. 5-6, 2010 Local Atomic Structures at Grain Boundaries In Gadolinium (a) cj ω—> oo Ν H.. ω->0 \ Rl R{ +R2 ΖΊ a Bulk Grain boundary Ä R7 Λ Λ A Λ ΛΑ Γ^ννν— vvv Fig. 2: Illustrations of (a) the Nyquist plot of typical complex impedance for GDC, (b) the equivalent electrical circuit for interpreting complex impedance of GDC. range and the other in low frequency range may obtained. This value is close to the result (75 kJ /mol) correspond to the oxygen migration in the bulk and reported by Faber et al /19/. The activation energies of across the grain boundaries, respectively. Figure 3 grain boundary for GDC and Ce02 are relatively higher shows the Arrhenius plots of the ionic conductivity than that of bulk, suggesting that the diffusion of calculated from bulk and grain boundary resistance for oxygen ion is affected by the local structural disorders GDC and Ce02. The conductivity of grain boundary is at grain boundaries. lower than that of bulk in both samples. The bulk Figure 4 shows the results of positron lifetime conductivity of GDC at 773 Κ is estimated to be spectroscopy for GDC. Prior to sintering, two 3 1 4.0* 10" S cm" , which is 160 times higher than that of components τ\ (~ 260 ps) and τ2 (~ 500 ps) Ce02. The bulk activation energy of 78.9 kJ /mol is corresponding to a vacancy-sized free volume and a 376 Kosaka and Sato High Temperature Materials and Processes 1000/Τ (Κ ') Fig. 3: Arrhenius plots of the ionic conductivity calculated from bulk and grain boundary resistance for GDC and Ce02. 377 Vol. 29, Nos. 5-6, 2010 Local Atomic Structures at Grain Boundaries In Gadolinium 280 τ 1 Γ—,100 240 873 973 1073 1173 1273 1373 1473 sintering temperature / Κ 900 — I 1 1 1 50 • • • 800 40 700 30 j Ί/ - a it) .5 ; r ω 600 20 500 10 400 J 1 1 1 1 I Τ 873 973 1073 1173 1273 1373 1473 sintering temperature / Κ Fig.