Abstract: Doped ceria is considered the most promising high-conducting electrolyte, alternative to the commercially used yttria-stabilized zirconia. The ceria fluorite structure is very tolerant to dissolution of lower valent metal ions. The present employed mechanical method, introduces comparison of mechanical-synthesized yttrium-doped ceria system, denoted as Ce1-xYxO2- δ(0.05≤ x≤0.4), which were successfully synthesized by both conventional solid-state and mechanochemical methods. In mechanochemical reaction, fine-grained powders with uniform grain size distribution were obtained. X-ray diffraction analysis showed all solid solutions were single phase with cubic fluorite structure. The electrical conductivity of sintered samples of calcium doped ceria compounds were investigated in air as a function of temperature using AC impedance spectroscopy. Impedance data showed that yttrium-doped ceria electrolyte is a good ionic conductor with conductivity as high as 10-3 ohm cm-1. Further characterization using Differential Thermal Analysis (DTA), Thermo Gravimetric Analysis (TGA) and Scanning Electron Microscopy (SEM) were carried out.
INTRODUCTION
Solid electrolytes based on cerium oxide have been the focus of significant research ever since the discovery of high oxide ion conductivity in these materials. Ceria-based electrolytes have collected much attention for the alternative of the YSZ as the electrolyte of the solid oxide fuel cell (Steele, 2000). The magnitude of electrical conductivity and the stability under reductive atmospheres for ceria-based oxides are greatly dependent on the kind and quantity of doping elements (Chen and Chen, 1996). The general principles of oxide ion conduction in ceramics have long been understood: O2¯ ions diffuse through the crystal lattice at a rate that depends primarily on the concentration, distribution and mobility of anion vacancies in the structure. Vacancies are most commonly introduced by creating solid solutions with cations (“aliovalent”) having formal valances that are reduced from that of the host phase.
Several studies concern powder preparation and densification of doped ceria materials. The main drawback of doped ceria materials synthesized via conventional solid-state method is the high sintering temperature. This has driven several research advancements in search of other synthesis methods to increase the sinterability of doped ceria materials at low temperatures. Rare-earth-doped ceria powders were synthesized with several wet chemistry approaches such as preparation by homogeneous co-precipitation (Thangadurai and Kopp, 2007), sol-gel (Huang et al., 1997) and hydrothermal methods (Yamashita et al., 1995). Likewise, in recent study by combustion-based methods it is possible to produce monophasic nanopowders with homogeneous microstructure at lower temperatures or shorter reaction times, if compared with other conventional methods like solid-state synthesis (Fu, 2009).
However, the present study is in contrast with the current advancement; compromises with the fact that previous research work has not shown significant focus on systematic study of mechanochemical synthesis route or better known as the ball milling method due to the main drawbacks encountered via mechanical synthesis route compared to other wet chemical approaches. The concept of mechanochemical reaction imposes ball milling in application of both impact action and shearing forces for efficient fine grinding, which subsequently lead to higher homogeneity of resultant powders. The current study is to address the above matter with the main purpose of contributing to the construction of solid-state method basis. In this study, synthesis of the Ce1-xYxO2-δ (0.05 ≤ x ≤0.4) powders and properties were thoroughly investigated to build up a window for fine microstructure-manipulation of the mechanical synthesized yttrium-doped ceria ceramics.
MATERIALS AND METHODS
Yttrium oxide-substituted ceria compounds were synthesized using two methods, i.e., conventional solid state and mechanochemical methods with starting material of CeO2 (99.9% Acros Organics) and Y2 O3 (98% Fluka-Garantie). All starting materials were dried at 500 - 600°C prior to weighing. Stoichiometric calculated amount of these materials were mixed. For mechanochemical method, appropriate total weight of stoichiometric mixtures of CeO2 with Y2O3, along with agate balls of diameter 10 mm, were placed in an agate bowl (99.9% SiO2). The mixture was then milled using a planetary ball mill (Model Pulverisette 4 vario-Planetary mill) for one hour in a suspension of ethanol at 1000 rpm. After milling, the slurry was then dried in oven at 60-70°C to evaporate off the ethanol. On the other hand, the conventional solid state method, mixtures of required molar ratios of materials were weighed and mixed manually using an agate mortar and pastle. The materials were subjected to heat treatments with variation in temperature and duration to ensure the formation of single phase materials.
Single phase materials were characterized by X-Ray Diffraction analysis (XRD) (Shimadzu diffractormeter XRD 6000, CuKα radiation) and scanned in the range of 10- 60° with a stepsize of 0.02° and scan rate of 0.1° per minute for higher resolution. Checkcell refinement program was used to obtain lattice parameters of the structure. The thermal events of samples were studied from room temperature to 1000°C on heat and cool cycles with heating rate of 10°C per minute by differential thermal analysis (DTA, Perkin-Elmer instrument with model DTA 7) and thermogravimetric analysis (TGA, Perkin Elmer thermogravimetric analyser model TGA7).
Pellets for electrical property measurement were cold pressed and sintered at a temperature range of 1200-1500°C for optimization purposes. Gold paste electrodes were then fired on at 200-600°C. The electrical properties were determined by AC impedance spectroscopy using a Hewlett Packard Impedance Analyzer, HP4192A in the frequency range of 5 Hz to 13 MHz. Measurements were made from 200 to 800°C by incremental steps of 50°C on a heating cycle with 30 min stabilization time. Scanning Electron Microscopy (SEM) analysis was carried out using SEM JEOL JSM-6400 operated at 15 kV, with working distance of 13 mm.
RESULTS AND DISCUSSION
Ce0.9Y0.1O1.95 was obtained as a single phase material after calcinations at temperature range 1200-1500°C in solid state reaction. In mechanochemical reaction, pure phase of Ce0.9Y0.1O1.95 was obtained after calcinations at 1100°C for 96 h. The XRD spectra of the CeO2 based oxide powders doped with Y2O3 concentration of 10% were analyzed. Figure 1 and 2 show the phase evolution of Ce0.9Y0.1O1.95 prepared via solid-state synthesis and mechanochemical reactions, respectively. The Bragg peaks of unreacted Y2O3 phase was found in the XRD spectra of compounds prepared using conventional solid-state method after calcined at temperature range of 800- 1200°C.
The Y-doped CeO2 prepared using mechanochemical method showed strong XRD Bragg peaks which subsequently indicated a fluorite type phase with the space group Fm3m, to which CeO2 belongs at a lower temperature compared to that of synthesized using conventional solid-state method. X-ray diffraction analysis also showed all doped ceria powders obtained in Ce1-xYxO2-δ (0.05≤x≤0.4) solid solutions prepared via mechanical synthesis (Fig. 3).
The lattice constants of Ce1-xYxO2-δ samples decrease linearly with increasing yttrium content in the range of x = 0.05 - 0.4.
The results further suggest that all the doped ceria samples of this work were ceria-based solid solutions. This further indicates combination with an increase in the lattice parameter with decreasing particle size appears to be general and was observed for Ce1-xYxO2-δ samples prepared via mechanochemical route.
The results of resultant powders synthesized by both mechanical approaches demonstrated a much lower synthesis temperature at 1100-1200°C, compared to previous study (Levy and Fouletier, 1984). It was also found that the 2θ values of the doped ceria shift slightly towards higher angles when x varies from 0.05 to 0.4 as can be seen from Fig. 3. Substitution of a smaller radius cation into ceria system testifies that yttrium is well into the crystal lattice of ceria fluorite structure which is tolerant to dissolution of lower valent metal ions (Mogensen, 2000).
The electrical conductivity of doped ceria is much dependent on the ionic radius and the concentration of dopants. Domains have also indicated that yttrium-doped ceria system belongs to the fluorite structure evidenced by the XRD spectra.
| Fig. 1: | XRD diffraction patterns of Y2O3 doped CeO2 system prepared via conventional solid-state method Δ denotes the un-reacted Y2O3 |
| Fig. 2: | XRD diffraction patterns of Y2O3 doped CeO2 system prepared via mechanochemical method Δ denotes the un-reacted Y2O3 |
| Fig. 3: | Powder XRD (CuKα) patterns of Ce1-xYxO2-δ prepared via mechanical method at scan rate of 0.1° min-1 |
A replacement of Y2O3 into CeO2 system caused the almost linear decrease of cell parameter and is in good agreement with effective ionic radii considerations (Shannon, 1976).
This recent work has highlighted a wider range of yttrium dopant concentration in ceria system compared to previous researches conducted (Li et al., 2008).
Thermal analysis was conducted to investigate the thermal events for Ce1-xYxO2-δ (0.05 ≤ x ≤ 0.4) samples. The results showed no observable significant thermal events which verifies that all compounds are thermally stable at temperature range of room temperature up to 1000°C.
In order to investigate the conductivity performance Y-doped ceria systems prepared via conventional solid-state and mechanochemiccal methods, the conductivity measurement below 1073 K in air was conducted. The measured conductivity data were analyzed using the traditional Arrhenius equation:
| (1) |
where, E is the activation energy of electrical conduction, k the Boltzman’s constant, T the absolute temperature and σO the pre-exponential factor. Conductivity values were extracted from AC impedance data and summarized in Fig. 7. Optimizations on conductivity measurement Fig. 6 were carried out and the results indicated that the density of the sample have a great impact on its conductivity. A broadened semicircle with low-frequency spike was obtained for the temperature range 450 -600°C. The spike became less pronounced at higher temperatures. At 400°C, the associated capacitance of the semicircle has a value of 1.14 x 10-11 F cm-1, which could be typical for bulk component. At higher temperatures, the predominant feature is low-frequency spike inclined at ~ 45° to the horizontal axis. Its associated capacitance of ~ 10-6 F cm-1 is characteristic of ionic polarization phenomena at the blocking electrodes, thus supporting the idea that the conduction was purely or predominantly ionic.
Current work showed that the conductivity measurement on parent Ce0.9Y0.1O1.95 samples which were sintered at 1500°C (Fig. 7), demonstrates a relative comparison of Y-doped ceria system synthesized via both methods. It can be observed that the Y-doped ceria prepared via mechanochemical reaction showed comparable and higher conductivity at 600°C (2.32 x 10-3 ohm-1 cm-1) than that prepared via conventional solid-state method (9.56 x 10-3 ohm cm-1) (Fig. 4, 5).
Conductivity of the electrolyte is closely related to the sintered density, dopant type and concentration, grain boundary structure (second phase) and the distribution of dopant (doping homogeneity) in the parent lattice (Balazs and Glass, 1995).
| Fig. 4: | The compositional dependence of lattice constant of Ce1-xYxO2-δ solid solutions prepared via conventional solid-state reaction |
| Fig. 5: | The compositional dependence of lattice constant of Ce1-xYxO2-δ solid solutions prepared via mechanochemical reaction |
It has been shown very recently that the conductivity is also greatly affected by the grain size of the sintered body, which enhanced property control and conduction mechanism understanding of the electrolyte material. Nanostructured ceramic materials are known to exhibit higher sinter activity than coarse grained powders which translates into considerably lower processing temperatures (Li et al., 2002). The results of the demonstrated conductivity properties is in strong agreement with that reported by Balazs and Glass (1995) which noted that ionic conductivity yttrium-doped ceria reaches a maximum at a doping level of 10 mol% of Y2O3.
| Fig. 6: | Optimizations on conductivity measurement of Ce0.9Y0.1O1.95 at temperature range of 1200 -1500°C |
| Fig. 7: | Arrhenius plot of Ce0.9Y0.1O1.95 prepared via solid-state and mechanochemical reactions |
The best conductivity of the Ce0.9Y0.1O1.95 studied (σ600°C ~ 2.32 x 10-3 S cm-1) prepared via both mechanical methods, is close to the previously reported (Levy and Fouletier, 1984) (σ600°C ~ 2.37 x 10-3 S cm-1) and one order of magnitude higher than that of the most commonly used solid electrolyte, stabilized zirconia at the corresponding temperatures (σ600°C ~ 10-4 S cm-1).
Nevertheless, it is noted that nanocrystalline materials turned out to be difficult to benefit from this superior advantage due to high tendency agglomeration of nanocrystals. As a consequence, the temperatures required for sintering such agglomerated materials to nearly full density, although lower than that for microcrystalline materials, were still rather high. Current mechanical approach has addressed the above matter with sintered resultant powder prepared via mechanochemical reaction demonstrated higher homogeneity of particles.
| Fig. 8: | Scanning electron micrographs of Ce 0.9 Y0.1 O1.95 prepared via (a) conventional solid-state method and (b) mechanochemical method |
The powder samples which were pelletized and sintered at 1400°C overnight with a programmed heating and cooling rate of 5°C min-1 exhibited a density of ~ 4.78 g cm-3 (~ 70% of the theoretical density). The morphology and microstructure of sintered pellets were studied. Particle size analysis revealed that fine-grained (mean = 4.315 μm) powders with uniform grain size distribution were obtained via mechanochemical reaction. Figure 8 shows typical micrographs of samples prepared via solid-state and mechanochemical reactions.
The solid-state reaction method yields larger particle size and intergranular porosity due to high temperature preparations. It is obvious that the mechanochemical reaction gives smaller particles and aggregates compared to conventional high temperature solid-state reactions, which correlates with the higher density value of the pellet attained. Resultant powder of Ce0.9Y0.1O1.95 which was synthesized via this method appeared denser with lower intergranular porosity (Fig. 8 a, b) compared to that prepared via conventional solid-state method. This however appears to be a drawback in nanocrystalline materials whereby subsequence of inhomogeneous pressure distribution after uniaxial pressing due to high friction forces in the nanocrystalline powder or large shrinkage upon extraction, resulted in an inhomogeneity in the final ceramic.
CONCLUSION
The yttrium-doped ceria solid solutions, Ce1-xYxO2-δ (0.05 ≤ x ≤ 0.4) with fluorite structure were prepared using mechanical synthesis. The results of X-ray diffraction showed that powder crystallite was single phase with cubic fluorite structure. It is apparent that the employment of mechanochemical synthesis route has been proven promising in terms of phase stability and microstructure homogeneity of Y-doped ceria system. The best conductivity of the Ce0.9Y0.1O1.95 studied (σ600°C ~ 10-3 S cm-1) prepared via both mechanical methods, is relatively comparable with previous researches utilizing wet chemical approach. This further testifies that mechanical approach’s reliability in terms of microstructure-manipulation of yttrium-doped ceria ceramics.
ACKNOWLEDGMENT
The authors wish to thank Yayasan Khazanah for the funding through the Khazanah Watan Scholarship and the Ministry of Science and Technology for the financial support through Research Grants.