Sintering of Samarium-doped ceria powders prepared by a glycine-nitrate process

Solid State Ionics 192 (2011) 580–583

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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i

Sintering of Samarium-doped ceria powders prepared by a glycine-nitrate process Ruifen Tian a,b, Fei Zhao a,c, Fanglin Chen c, Changrong Xia a,⁎ a

CAS Key Laboratory of Materials for Energy Conversion, Department of Material Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China b College of Chemistry and Material Science, Anhui Normal University, Wuhu, Anhui, 241000, China c Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA

a r t i c l e

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Article history: Received 13 August 2009 Received in revised form 26 July 2010 Accepted 10 August 2010 Available online 6 September 2010 Keywords: Sm doped ceria Sintering Glycine-nitrate process Solid oxide fuel cells Low temperature SOFC

a b s t r a c t This work investigates the sintering characteristics and electrical properties of SDC electrolytes prepared with glycine-nitrate-combustion derived powders, which are often used for fabricating dense SDC electrolyte films using a dry-pressing process. The compact powders show the highest sintering rate at 730 °C and are sintered to 90.7% of theoretical density at 1300 °C for 5 h. However, using the powder as the precursor, SDC films can be sintered to be much denser (about 99% of the theoretical density) even at 1250 °C when the electrolyte layers are co-sintered with NiO-SDC substrates. In addition to increased density, increasing the sintering temperature causes enlargement of crystalline parameters, which is possibly due to the reduction of Ce4+ to Ce3+. The reduction is associated with the formation of oxygen vacancies, which might cause defect-association and thus promote the activation energy for oxygen-ion conduction. It is found that the conductivity increases with the sintering temperature to its maximum when SDC is sintered at 1300 °C. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Samaria doped ceria (SDC) is often used as the electrolyte for lowtemperature solid oxide fuel cells (SOFCs) for its superior conductivity to that of stabilized zirconia. It also presents good compatibility with SOFC electrode materials such as La1 − xSrxMnO3 (LSM), Sm0.5Sr0.5CoO3 (SSC) and Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF). It is therefore used as the inter-layer for intermediate temperature SOFC with zirconia electrolytes. SDC powders can be synthesized by solid-state reaction [1], co-precipitation [2,3], sol–gel [4], or glycine-nitrate [5] method. The glycine-nitrate process is a self-combustion method, which is relatively inexpensive. The SDC powders are formed instantly when the glycine burns, resulting in fine powders with foam-like structures [5]. The powders are often used for preparing thin membranes of dense SDC electrolyte films using a dry-pressing technique, which is a simple and cost-effective method to fabricate ceramic membranes [6]. The powders are also used in composite electrodes such as Ni-SDC, LSM-SDC, SSC-SDC, and BSCF-SDC [7–9]. In addition, the powders are used to fabricate the SDC inter-layers. In these applications, the powders are usually sintered at high temperatures to form the expected structures. The temperature can be as low as 850 °C and as high as 1500 °C [10,11]. Although the glycine-nitrate-derived SDC

⁎ Corresponding author. Tel.: +86 5513607475; fax: +86 5513601592. E-mail address: [email protected] (C. Xia). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.08.003

powders have numerous applications which require different sintering temperatures, their sintering characteristics have not been studied. In this work, the microstructures and conductivities of typical SDC ceramics derived from the SDC powders obtained from the glycine-nitrate process are investigated as a function of the sintering temperatures. 2. Experimental Sm0.2Ce0.8O1.9 (SDC) powders were prepared by the glycinenitrate process [5]. Ce(NH4)2(NO3)6 and Sm(NO3)3 were mixed with a molar ratio of 4:1 and dissolved in distilled water to form a SDC solution. Glycine was then added to the solution. The molar ratio of glycine to nitrate was 2. The solution was subsequently stirred and heated on a hot plate. As water was evaporated, the solution converted to a viscous gel and eventually ignited to flame, resulting in yellow fine powders. The powders were collected and fired at 600 °C for 2 h. With 5% polyvinyl alcohol (PVA) solution as adhesive, pellets were prepared by uniaxially pressing the powders under 300 MPa. The pellets were 13 mm in diameter and 1 mm in thickness. Sintering behavior of the compact powders was studied in air via dilatometry (Netzsch DIL 402C) with a heating rate of 5 °C/min. The pellets were sintered at temperatures in the range from 1000 to 1500 °C for 5 h with a heating rate of 5 °C/min. The ionic conductivities were measured with two-probe impedance spectroscopy using an electrochemical workstation (IM6e Zahner) over 450 °C–800 °C, the typical temperatures for low-and-intermediate temperature SOFCs. Prior to the impedance

R. Tian et al. / Solid State Ionics 192 (2011) 580–583

measurements, platinum electrodes were prepared by painting platinum paste onto both sides of the sintered pellets and then firing at 800 °C for 2 h. The SDC powders were also investigated by X-ray diffraction using CuKα radiation (D/Max-γA, Japan). Densities of the sintered pellets were measured using the Archimedes method. Average grain size was determined by linear intercept method from at least 100 randomly selected grains. Microstructures of the ceramics were observed through a scanning electron microscope (SEM, Hitch 650).

3. Results and discussion 3.1. XRD XRD analysis shows that the SDC powders fired at 600 °C have a single-phase fluorite structure (Fig. 1a), which is consistent with the previous reports [5]. The crystallite size of the powder is estimated to be 20.2 nm using the X-ray line broadening technique performed on the (422) diffraction peak of ceria lattice using the Scherrer equation [12]. The specific surface area of the powder is 36.93 m2/g. The XRD patterns of the SDC pellets sintered at 1000, 1100, 1200, 1300, 1400 and 1500 °C are analyzed by X'Pert Highscore software to calculate the lattice parameters. Shown in Fig. 1b is the dependence of unit cell parameters versus sintering temperatures. The lattice parameter increases gradually with the sintering

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temperature. Similar results have been reported elsewhere for doped ceria [13,14]. Zhang et al. found that the lattice constant of Gd0.2Ce0.8O2−δ and Gd0.1Ce0.9O2−δ prepared by the solid solution method increased with increasing sintering temperature and reached the maximum at 1600 °C. Since Gd3+ ion had a larger radius (1.05 Å) than Ce4+ ions (0.97 Å), they attributed the increase in the lattice constant to the increased dissolution of Gd2O3 in CeO2 and the maximum lattice constant at 1600 °C to complete the dissolution. Although the radius of Sm3+ (1.04 Å) is also larger, this might not be applicable here since SDC powder has formed fine fluorite structure even it is fired at 600 °C (Fig. 1a). And there are no observable peaks corresponding to single Sm2O3 phase. Therefore, SDC solid solution is formed when it is fired at 1000–1500 °C. The formation of solid solution has been also demonstrated in literature with Raman spectra. McBride et al. studied the Raman spectra of La, Pr, Nd, Eu, Gd, and Tb doped ceria [15]. They inferred that the main Raman band at around 460 cm−1 was the only allowed lattice mode (F2g) of fluorite metal dioxides. Souza et al. studied the Raman spectra of Sm0.3Ce0.7O1.85 and Sm0.2Ce0.8O1.9 powders fired at 500 to 1100 °C [16]. Typical bands correlating to F2g were observed. The increase of lattice parameter with the sintering temperature is possibly due to the reduction of Ce4+ ion to Ce3+. Since the radius of Ce3+ ion (1.14 Å) is larger than that of Ce4+ ion (0.97 Å), the increase of Ce 3+ ion in SDC solid solution would also lead to lattice expansion. The Ce3+ or at least part of the Ce3+ formed at high temperature is stable when the temperature is decreased down to the room temperature [17]. The reduction results in not only lattice expansion, but also oxygen vacancies formation. ×

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2CeCe + OO = 2Ce′Ce + VO + 1 = 2O2

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The vacancy formation has been previously reported [18]. It was observed that the intensity of vacancy-related Raman bands increased with the calcination temperatures. The increase in the sintering temperature resulted in the formation of oxygen vacancies. Similar results had been concluded by Im et al. on Gd0.1Ce0.9O2−δ samples heat-treated from 1100 to 1400 °C [19]. Therefore, it can be concluded that the increase in the lattice constant is not due to the increased dissolution of Sm3+ in CeO2. This result is similar to that concluded by Vries et al. on the ThxCe1 −x O2−δ system [14]. 3.2. Sintering behaviors

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Fig. 1. (a) X-ray patterns of SDC powder calcined at 600 °C for 2 h; (b) Variation of unit cell parameters for SDC pellets with sintering temperatures.

Fig. 2 shows the sintering curves varying from dilatometry study. The peak at around 460 °C is possibly attributed to the loss of PVA, which is added as an adhesive for fabricating the pellet. A maximum shrinkage rate is observed at about 730 °C. The pellet shrinks till 1350 °C. Further sintering leads to small expansion, which might also be due to cerium reduction. The average grain sizes are estimated from the SEM pictures by selecting at least 100 randomly selected grains. The average grain size is 91 nm for SDC sintered at 1000 °C. The surface view of the sample (Fig. 3a) shows a highly porous structure. When the sintering temperature increases to 1300 °C, the grain size grows to 294 nm. The surface structure becomes rather dense and only a few porous holes can be seen (Fig. 3b). The grain size is about 1 μm when it is sintered at 1500 °C. Shown is Fig. 3c is the cross-sectional view of an SDC ceramic sintered at 1300 °C. Isolated pin-holes and several open holes can be observed and the sample is not fully dense. Shown in Fig. 3d is the cross-sectional view of a single cell sintered at 1250 °C with SDC electrolyte and NiO/SDC anode. The electrolyte layer is much denser than that shown is Fig. 3c. Reports have shown that the electrolytes are dense when they are co-sintered at 1250–1450 °C

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1

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Fig. 2. Densification behaviors of SDC powders sintered in air via dilatometry at a heating rate of 10 °C/min. The inset curve is the dependence of linear shrinkage rate on temperature.

with anodic substrates [20,21]. The high density might be caused by the co-firing process in which the sintering rate of the electrolyte layer is enhanced by the NiO-SDC substrate.

3.3. Conductivities and activation energies Fig 4a presents the Arrhenius plots of Sm0.2Ce0.8O1.9 pellets sintered at different temperatures. Activation energies and relative densities are also shown in Fig. 4b. Both activation

energies and relative densities increase with the sintering temperatures. The pellet possesses a relative density of 90.7% with a sintering temperature of 1300 °C. Conductivity in air at 600 °C via sintering temperature is also presented in Fig 4b. The conductivity at 600 °C increases with sintering temperature from 0.0133 S cm−1 at 1000 °C to 0.0154 S cm−1 at 1300 °C. Increasing temperature beyond 1300 °C does not further improve the conductivity. On the contrary, it causes decline in conductivity, and the conductivity is only 0.0056 S cm−1 when the SDC pellet is sintered at 1500 °C. Conductivity usually increases with density and decreases with the activation energy. XRD analysis suggests that the increase of sintering temperature results in the formation of oxygen vacancies. The increase of electrical conductivities below 1300 °C may be attributed to the improvement of densification. The decline of electrical conductivities beyond 1300 °C could be explained by the association of oxygen vacancies, which is of high concentration due to the reduction of cerium ions at high sintering temperature.

4. Conclusions Fine Sm0.2Ce0.8O1.9 powders were synthesized by the glycinenitrate process. Increasing the sintering temperature promoted the densification of SDC electrolytes and enlarged the crystalline parameters, which were associated with the formation of oxygen vacancies. The sintering process could be enhanced by a co-sintering process with a co-pressed NiO-SDC compact. The highest ionic conductivity of 0.0154 S cm−1 was achieved at 600 °C, when the SDC pellet was sintered at 1300 °C.

Fig. 3. SEM micrographs of the surface views of SDC pellets sintered at (a) 1000 °C and (b) 1300 °C and SEM micrographs for the cross-section views of (c) SDC pellet sintered at 1300 °C and (d) single cell sintered at 1250 °C.

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Acknowledgements

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This work is supported by the Natural Science Foundation of China (10979046 and 50730002) and the US National Science Foundation (CMMI 1000068).

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Fig. 4. (a) Arrhenius plots of Sm0.2Ce0.8O1.9 pellets sintered at 1000, 1100, 1200, 1300, 1400 and 1500 °C for 5 h. (b) Relative densities, activation energies, and electrical conductivities in air at 600 °C as a function of the sintering temperature.

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