RECENTLY, nanostructured oxides have gained a great deal of. Journal

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1 Journal J. Am. Ceram. Soc., 91 [10] (2008) DOI: /j x r 2008 The American Ceramic Society Electrical Conductivity of Submicrometer Gadolinia-Doped Ceria Sintered at 10001C Using Precipitation-Synthesized Nanocrystalline Powders Pandurangan Muralidharan, z Seung Hwan Jo, z and Do Kyung Kim*,w Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon , Republic of Korea A simple synthetic strategy has been implemented to obtain low-temperature sintered fine grain size gadolinia-doped ceria, Ce 0.9 Gd 0.1 O 1.95, (CGO) electrolyte pellets with a high density using weakly agglomerated particles of calcined nanopowders synthesized by a homogeneous precipitation process. The precipitants used were diethylamine (DEA process) and ammonium hydroxide in neutral precipitation (NP process). X-ray diffraction patterns revealed the single-phase crystalline CGO of a fluorite-type structure. The crystalline powder was directly synthesized from solution by the DEA process at room temperature, whereas the NP process powder was crystallized upon hydrothermal treatment at an elevated temperature. Transmission electron microscopy images showed homogeneously dispersed spherical-shaped particles of B5 nm size for nanopowders calcined at 3001C for 4 h. A high densification range from B96% to 99% of the theoretical was achieved for the nonconventionally low-temperature sintered pellets at 10001C from weakly bonded particles of CGO nanopowders calcined at 3001C for 4 h without any sintering aid. The dense CGO pellets sintered at 10001C for4hwithanaveragegrainsizeofb nm exhibited a promising high electrical conductivity of S/cm (DEA process) and S/cm (NP process), measured at 6501C, and low activation energy E a.the electrical conductivities of fine grain size low-temperature sintered CGO pellets are comparable with the literature reports of sintered pellets using sintering aids, and high-temperature sintered CGO pellets above 13001C with a larger grain size. I. Introduction RECENTLY, nanostructured oxides have gained a great deal of importance in several applications ranging from chemical storage to drug delivery and catalysis as they exhibit a wide functional diversity, and enhanced or different properties. 1 5 One of the most rapidly growing areas of investigation is the intermediate-temperature solid oxide fuel cells (IT-SOFC) with an operating temperature of C. 6 9 The lower operating temperature allows greater flexibility in the fabrication of electrodes, cell interconnectors, reduced thermal degradation, and thermal cycling stress, which in turn increase the long-term stability of the cell. The doped ceria solid solutions have been identified as one of the most promising electrolytes because of their sufficient high oxygen ion conductivity at around 6001C compared with yttria-stabilized zirconia. In addition, these solid R. Cutler contributing editor Manuscript No Received December 18, 2007; approved July 14, This work was financially supported by the Core Technology Development Program for Fuel Cells of the Ministry of Commerce, Industry, and Energy in Korea (Grant No ) and the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF J09701). *Member, The American Ceramic Society. w Author to whom correspondence should be addressed. dkkim@kaist.ac.kr z Contributed equally for this work solutions possess a reasonably good chemical compatibility that is of special interest for use in doped ceria solid electrolytes in the fabrication of IT-SOFC. 6 9 In general, a doped ceria solid solution is easy to prepare through solid-state reactions, but it is difficult to obtain high densification below 15001C In addition, ceria shows a strong tendency to undergo reduction during high-temperature sintering, which retards densification due to the oxygen release and, as a consequence, lowers the electrical conductivity Therefore, it has become a challenging task for researchers to find a better synthesis process to prepare fine powders that can easily attain full densification at a low temperature with a fine grain size and characteristic electrical conductivity The samples obtained can be easily cofired with the other components, including the anode, the cathode, and the interconnector materials, to yield an IT-SOFC system with lesser effort, and that is cost-effective. As a result, different solution-based synthetic routes have been established for their synthesis, apart from the conventional solid-state reaction process. It has been recognized that the solution-based synthesized single or multioxide powders were found to possess high sintering activity, a high surface area, well-defined chemical compositions, and a homogeneous distribution of the elements. Even then, the powders need to be typically sintered at a high temperature above 13001C to obtain full densification The most important inherent property of these powders is strong agglomeration of particles, which may reduce efficient utilization of the nanocrystalline characteristic of the powder and may retard the densification kinetics. 18,19 One of the most efficient and practical ways to synthesize nanocrystalline particles of oxides with high sintering activity is possibly by the chemical precipitation method. The precipitation reactions involve various significant parameters including solvents, precipitants, and washing media to obtain weakly agglomerated particles and desirable morphology of nanopowders. 20 In the literature, nanocrystalline pure CeO 2 or doped ceria powders were synthesized through the precipitation process using various precipitants such hexamethylenetetramine, 17,21,22 diethylamine (DEA), 19 ammonium hydroxide 21,23 or hydrazine hydrate, 24 formic acid, 25 urea, 25 ammonium carbonate, 22,26,27 and oxalic acid. 28 It has been recognized from the literature reports that the nanocrystalline powders synthesized by the alkoxide process in organic solvents normally show a much higher reactivity, weak agglomeration during the drying stage, and high sintering activity. 18 The process that mostly uses hydrated metal nitrate as a precursor and organic precipitants can be easily dissolved into alcohols and precipitation occurs with precipitating anions (OH )fromthe hydrolysis of precipitants with hydrated molecular water of the metal salt. 19,29 Minimum amount of water is involved in the entire process, make it possible to obtain better dispersed powders, which is similar to the alkoxide method. Li et al. 22 have clearly defined a pathway for reaching a theoretical density of 99% for doped ceria synthesized by precipitating cerium and samarium nitrates using ammonium carbonate. These loosely agglomerated nanopowders could be

2 3268 Journal of the American Ceramic Society Muralidharan et al. Vol. 91, No. 10 sintered at 10001C for a wide range of samaria doping, which is similar to gadolina for conferring high ionic conductivity in ceria. While Li et al. demonstrated that it was possible to prepare submicrometer grains, they did not provide data for ionic conductivity in comparison with traditional methods for densifying doped ceria using transition metals as sintering aids One concern with submicrometer grains is that grain-boundary resistivity can be a significant impediment to obtaining high ionic conductivity. The purpose of the present work was to sinter weakly agglomerated nanopowders in order to obtain submicrometer grains and then compare the conductivity of these low-temperature sintered materials with coarser-grained materials that have been reported in the literature. The present work reports a simple synthetic approach to obtain weakly agglomerated phase-pure nanopowders with a high crystallinity of 10 mol% Gd-doped ceria solid solution synthesized by the neutral precipitation (NP) and DEA precipitation processes. The effects of different calcination temperatures and time periods were investigated to obtain nearly full densification and high sinterable pellets. The electrical conductivity properties for low-temperature (10001C) sintered pellets of fine grain materials with a relative density of B96% 99% were compared with the pellets sintered using a sintering aid, and pellets sintered at high temperatures, as reported in the literature. II. Experimental Procedure The precursor materials used for the synthesis were cerium nitrate hexahydrate (Ce(NO 3 ) 3 6H 2 O), gadolium nitrate hexahydrate (Gd(NO 3 ) 3 6H 2 O) (99.999% purity; Aldrich, St. Louis, MO), (C 2 H 5 ) 2 NH (DEA) (99.9% purity; Aldrich), ammonium hydroxide solution (Fluka, Buchs, Switzerland), hydrogen peroxide (35 vol%; Junsei, Tokyo, Japan), absolute ethanol (Baker, Phillipsburg, NJ), and high-purity deionized water (DI water). The chemicals were used as received without further purification. (1) Synthesis of Nanopowders by the NP Process The NP process was adopted to prepare the nanocrystalline 10 mol% Gd-doped CeO 2 powder. In the synthesis, mol Ce(NO 3 ) 3 6H 2 O and mol Gd(NO 3 ) 3 6H 2 O were dissolved in DI water to form a homogeneous clear solution and 35 vol% of cooled hydrogen peroxide was added slowly with continuous stirring. The mixed solution appeared to be brownish orange upon aging for 10 min. Then, ammonium hydroxide solution was added dropwise slowly to increase the ph value to The solution was continuously stirred for 6 h and the precipitate was allowed to settle down. The precipitate was washed thoroughly using DI water by centrifugation. The powder obtained was heat treated hydrothermally at 1801C for 4 h under autogenous pressure to obtain a high-quality crystalline oxide. The chemical reactions can be expressed by the following equation: 0:9CeðNO 3 Þ 3 6H 2 O þ 0:1GdðNO 3 Þ 3 6H 2 O þ nh 2 O 2 þnh 4 OH!Ce 0:9 Gd 0:1 ðohþ 4 d =Ce 0:9 Gd 0:1 O 2 d nh 2 O þ NH 4 NO 3 þ nh 2 O (1) The whitish yellow precipitate obtained was washed thoroughly using DI water and ethanol by centrifugation and then dried at 601C for 12 h. The dried powder was ground and sieved to obtain homogeneous granules. (2) Synthesis of Nanopowders by the DEA Process A simple precipitation process was carried out using DEA as a precipitant to prepare nanocrystalline 10 mol% Gd-doped CeO 2 powders. In the procedure, mol Ce(NO 3 ) 3 6H 2 O and mol Gd(NO 3 ) 3 6H 2 O were dissolved in absolute ethanol with continuous stirring to form a clear homogenous solution. To the above solution, 0.25 mol DEA was added dropwise with continuous stirring. The solution immediately turned into a yellowish brown thickened slurry, which was stirred vigorously and precipitated at room temperature (RT). The chemical reaction can be expressed by the following equation: 0:9CeðNO 3 Þ 3 6H 2 O þ 0:1GdðNO 3 Þ 3 6H 2 O þðc 2 H 5 Þ 2 NH! RT Ce 0:9 Gd 0:1 O 2 d =Ce 0:9 Gd 0:1 O 2 d nh 2 O þðc 2 H 5 Þ 2 NH 2 NO 3 þ nh 2 O ð2þ A small amount of DI water was added to the precipitate and was allowed to stand for a few hours to ensure complete precipitation and then washed systematically with DI water and ethanol by centrifugation. The bright yellow powder obtained was dried at 601C for 12 h and sieved to obtain homogeneous granule. (3) Sintering and Characterization To investigate the optimum calcination conditions of CGO nanopowders for high densification at a low temperature, the powders were calcined at 3001,5001, and 8001C for 4 h and their sintering behavior was examined. The powders were uniaxially pressed with a cylindrical stainless mold without using any binder, followed by cold isostatic pressure under 200 MPa. The isostatically pressed pellet had an B2 mm thickness and B9.8 mm diameter. The CGO pellets were sintered in an electric furnace at 10001, 11001, 12001, and 13001C for 4 h in air, respectively, with a heating ramp of 51C/min. The phase compositions were characterized by an X-ray diffractometer (XRD, Rigaku, D/MAX-IIIC X-ray diffractometer, Tokyo, Japan), CuKa radiation (l nm at 40 kv and 45 ma). The crystallite size of the CGO powders was estimated from the Scherrer equation D 5 (0.9l)/(b cosy), where D is the crystallite size, l the wavelength of incident X-rays ( nm), b the half-width at full-maximum, and y the diffraction angle. To investigate decomposition behavior depending on the processes, the as-synthesized CGO nanopowders were analyzed by thermal analysis (TG/DTA, TA, SDT Q600 V 8.3, Newcastle, DE) in air at a constant heating rate of 51C/min. The particle size and selected area electron diffraction (SAED) were characterized by transmission electron microscopy (TEM, JEM 3010, Jeol; with an accelerating voltage of 300 kv, Tokyo, Japan). The relative density of the pellets under different sintering conditions was measured by the Archimedes method in DI water as an intrusion medium. The grain sizes and the morphology of the samples from each condition were characterized by a scanning electron microscope (FE-SEM Philips XL30 FEG, Eindhoven, the Netherlands) for the fractured surface of the sintered pellets. The surface area was determined using Brunauer Emmett Teller analysis (BET, TriStar 3000, Micromeritics, Norcross, GA) via nitrogen chemisorption at 77 K. The measured specific surface areas were converted to equivalent particle sizes according to the following equation: D BET ¼ =ðd th S BET Þ (3) where D BET (nm) is the average particle size, S BET the specific surface area expressed in m 2 /g, and d th the theoretical density of the solid solution oxide (g/cm 3 ). The nanopowders calcined at 3001C with smaller crystallite sizes and the powders prepared through both the processes were easy to consolidate into green pellets by the dry pressing method, and crack-free dense pellets were obtained using a heating ramp of 51C/min. For impedance analysis, the surfaces of the sintered pellets were polished, coated with platinum paint on either side, and calcined at 8001C for 2 h to facilitate stable contact of the electrode to the pellet surfaces. The platinum electrode-coated pellet was attached to a platinum mesh connected with platinum wires and sandwiched in a spring-loaded specimen holder. The electrical conductivity of the pellets was studied in the presence of air by ac impedance spectroscopy using a Solartron 1260

3 October 2008 Submicrometer Gadolinia-Doped Ceria 3269 impedance/gain-phase analyzer (Farnborough, UK) interfaced with a computer-controlled program for data acquisition. The impedance spectra were measured over a frequency range of 1 10 MHz as a function of temperature from 1501 to 6001C. III. Results and Discussion (1) Chemistry of the NP and DEA Synthesis Processes The preparation of CGO nanopowders through the NP and DEA processes was demonstrated to proceed through different stages of chemical reactions. In the NP process, initial addition of a strong oxidizing agent of 35 vol% H 2 O 2 to the precursor solution oxidized the cerium salt of Ce 31 ions (Lewis base) to favorable Ce 41 (Lewis acid). In addition, the presence of excess amount of NH 4 OH in the medium maintained the ph value at , which facilitated the Ce 41 ions to undergo strong hydration rapidly. These hydrated Ce 41 ions can form complexes with H 2 O and OH to yield Gd-doped [Ce(H 2 O) x (OH) y ] (4 y)1, where (x1y) is the coordination number of Ce 41 ions. 21 Subsequently, polymerization of this hydroxide is known to occur, and both can serve as precursors for the formation of an oxide nanopowder. The deprotonation of the hydroxide complexes can readily occur in the presence of polar molecules (H 2 O) by acquiring protons away from the hydroxide complex to form a mixture of Gd-doped Ce(OH) 4 /Gd-doped CeO 2 nh 2 O, which is a hydrous oxide rather than a definite compound. Further, it undergoes dehydration to yield Gd-doped CeO 2 nh 2 O(no2). Thus, the color of the solution changes to dirty yellowish due to the ligand field changes, followed by dehydration of Gd-doped CeO 2 nh 2 O to yield Gd-doped ceria powder, which is bright yellow or whitish yellow depending on the dehydration temperature. In the DEA process, followed by formation of Ce 41 ions (Lewis acid) oxidized from Ce 31 (Lewis base) in the base medium, the kinetics of the reactions for the formation of Gddoped [Ce(H 2 O) x (OH) y ] (4 y)1 complexes and deprotonation of the hydroxide complexes occurred reasonably faster. As a result, formation of the hydroxide complexes did not occur during the entire precipitation process, although the participating anion OH was involved, which is generated through the hydrolysis of DEA with the molecular water of the cerium salt. In the initial stage of precipitation, the colorless precipitant solution immediately changed to yellowish brown on addition of DEA, rapidly forming a thickened slurry, followed by precipitation, without the formation of a pinkish purple color corresponding to hydroxide complexes that have indicated the faster kinetics of the reaction. 21 Thus, this phenomenon may indicate that the kinetics of the reaction toward Gd-doped [Ce(H 2 O) x (OH) y ] (4 y)1 and deprotonation were significantly faster. It is known that ethanol is a solvent with weaker polarity than water and, therefore, such a rapid deprotonation reaction can only occur through other constituents of the reaction medium. As a result, DEA plays a significant role in the formation of hydrogen bonds with hydrated water of the metal salts and exhibits a strong inclination to acquire protons away from the hydroxide complexes. This is an advantageous stage in the process of DEA as a precipitant for the preparation of a weakly agglomerated nanocrystalline Gddoped CeO 2 powder. Accordingly, using DEA as a precipitant, the synthesized CGO powder yielded B98% with ultrafine homogenously dispersed particles and weakly agglomerated particles. (2) XRD The XRD patterns of the as-synthesized, and the CGO powders calcined at different temperatures synthesized by the NP and DEA processes are shown in Fig. 1. It can be observed that the XRD patterns confirmed the single-phase nanocrystalline CGO of the fluorite-type structure, which is directly synthesized from the solution by the DEA process at RT. On the other hand, crystallization occurred for the powder synthesized by the NP process at an elevated temperature after the hydrothermal treatment at 1801C for 4 h. The XRD patterns of CGO powders are Fig. 1. X-ray diffraction patterns of the as-synthesized and calcined CGO powders synthesized through the neutral precipitation (NP) and diethylamine (DEA) processes. in good agreement with the JCPDS data # of a typical fluorite-type structure. In Fig. 1, the powders calcined at 3001C exhibit a relatively low intensity and a broader peak width, which may be due to the smaller particle size. Further calcination at higher temperatures of 5001 and 8001C for 4 h resulted in enhanced peak intensity and sharpness, pointing out the rapid crystallite growth at that calcination temperature. The growth of the crystallite-size CGO powder during calcination has been investigated, and the results obtained from the X-ray line broadening of the (111) peak are presented in Table I. Thus, Table I presents a comparison of the processing conditions and calculated crystallite size from the XRD analysis of calcined samples at 3001, 5001, and 8001C. Also, the specific surface area S BET and the average particle size from the BET surface area data obtained for the powder calcined at 3001C, synthesized by the NP and DEA processes, are presented. The calculated values complement each other, and confirm that the powders are not strongly aggregated. The difference between the NP and DEA processes lies in the nonexistence and existence of crystallization at RT, respectively. In the NP process, hydrothermal treatment was attempted to crystallize the amorphous phase of the as-precipitated precursor powder. On the other hand, in the DEA process, crystallization occurred at RT, which indicates the case and convenience of this process for application for a large-scale synthesis. It can be recognized from Table I that the growth of the crystallite sizes of CGO nanopowder depends on calcination temperatures. The crystal growth is negligible at lower calcination temperatures around 5001C and, above this temperature (8001C), the crystallite size increases rapidly to 26.2 and 27.3 nm for NP and DEA, respectively; the growth process may be diffusion related. 19 (3) TG/DTA The thermal analysis data for the as-synthesized CGO nanopowders from the NP and DEA processes are shown in Fig. 2.

4 3270 Journal of the American Ceramic Society Muralidharan et al. Vol. 91, No. 10 Table I. The Crystallite Size and Surface Area of the CGO Nanopowders Calcined at Different Temperatures Produced from the NP and DEA Processes Process Precipitation temperature (1C) Precipitant Crystallization process Crystallite size (calcination temperatures for 4 h) Size/surface area 3001C 5001C 8001C NP 25 NH 4 OH 1801C, 4 h (hydrothermal) D XRD (nm) S BET (m 2 /g) D BET (nm) 4.6 DEA 25 (C 2 H 5 ) 2 NH 2 D XRD (nm) S BET (m 2 /g) D BET (nm) 5.2 CGO, Ce 0.9 Gd 0.1 O 1.95 ; NP, neutral precipitation; DEA, diethylamine. The precipitate obtained by the NP process after hydrothermal treatment and drying at 601C for 12 h showed a total weight loss of 14.42%. This weight loss is lower than that corresponding to the decomposition of Ce 0.9 Gd 0.1 (OH) 4 /Ce 0.9 Gd 0.1 O 2 d 2H 2 O (B17.3%) and indicates that the samples consist of a partially hydrated form of Gd-doped ceria, i.e. Ce 0.9 Gd 0.1 O 2 d nh 2 O. The precipitate obtained by the DEA process after drying at 601C for 12 h showed a total weight loss of 7.52%, as shown in Fig. 2(b). The weight loss is lower than that corresponding to the decomposition of Ce 0.9 Gd 0.1 (OH) 3 d (B9.95%) or Ce 0.9 Gd 0.1 (OH) 4 d /Ce 0.9 Gd 0.1 O 2 d 2H 2 O(B17.3%). This indicates that the samples consist of either a partially hydrated form of Gddoped CeO 2 nh 2 O (for which a 7.52% weight loss on decomposition corresponds to n 5 B0.86) or a mixture of phases, e.g. Ce 0.9 Gd 0.1 O 2 d /Ce 0.9 Gd 0.1 O 2 d nh 2 O. In Fig. 2(a), the DTA curve shows a sharp exothermic peak at 1631C, which may correspond to the residual transformation of amorphous to crystalline CGO nanopowder. On the other hand, this was scarcely observed in the case of DEA-synthesized powder (Fig. 2(b)) and may be attributed to the complete crystalline phase formed under the synthesis temperature condition. From the thermal analysis, it can be expected that CGO powder from the DEA process may have good crystallinity and weak agglomeration compared with the NP process-synthesized powder at RT. (4) TEM Figure 3 shows the HR-TEM images and the SAED patterns of the CGO nanopowders synthesized by the NP and DEA processes and calcined at 3001C for 4 h. The CGO nanopowders obtained by both the processes showed an B5 nmsizeofspherical-shaped particles and a homogenous dispersion state. The HR-TEM and SAED images confirmed that CGO nanoparti- Fig. 2. TG/DTA curves of the as-synthesized CGO powders of (a) neutral precipitation and (b) diethylamine. Fig. 3. HR-TEM and inset selected area electron diffraction patterns (SAED) of CGO nanopowders by (a) NP and (b) DEA, calcined at 3001C for 4 h; Inset SAED, the materials are indexed to the cubic fluorite structure [1 (111); 2 (200); 3 (220); 4 (311)].

5 October 2008 Submicrometer Gadolinia-Doped Ceria 3271 submicrometer grains. The best densities at 10001C in the present work were in the range of 97% of the theoretical density and also resulted in submicrometer grains. It is likely that Li et al. 22 obtained better packing of the powders due to their lower surface areas or that their processing method or composition aided them to obtain a slightly higher density. Neither method, however, results in easily processable powders for use during lowtemperature sintering due to the high surface areas. The effect of the calcination dwell period of the nanopowders was studied at 3001C for different dwelling times of 2, 4, and 8 h and sintering at 10001C for 4 h; approximately similar relative densification of CGO nanoparticles was observed. Thus, it may be concluded from the above results that the optimum calcination condition is around 3001C for 4 h to obtain high densification of CGO pellets. Fig. 4. Relative density of the CGO nanopowders sintered at different temperatures using the nanopowders calcined at 3001C for4h. cles show structurally uniform polycrystallinity with regular periodicity of the lattice structure. The SAED pattern of CGO powder observed from the DEA process was comparatively clearer than that from the NP process. In accordance with the SAED pattern and the thermal analysis, as shown in Fig. 2, it is clear that the DEA process-synthesized nanopowders enable small-sized CGO particles with high crystallinity even under a low-temperature calcination condition. The EDX results obtained from the TEM measurement confirmed the chemical composition of 10 mol% Gd-doped in CeO 2 nanopowders. (5) Density of Sintered CGO The surface area of the powder calcined at 3001C was m 2 /g, which is too high for practical processing methods such as dry pressing, isostatic pressing, tape casting, tape calendaring, dip coating, etc. Li et al. 22 calcined their powder at 7001C in order to reduce the surface area to m 2 /g, which is still too high for standard processing. The powders compacted uniaxially and isostatically to a green density of B51% due to their weak agglomerates, which resulted in excellent packing without a binder. Figure 4 shows the sintered density of CGO pellets heated at 10001, 11001, 12001, and 13001C using the calcined nanopowders at 3001, 5001, and 8001C for 4 h, synthesized by the NP and DEA processes. In addition, to obtain the appropriate powder characteristics for high densification, the pellets were sintered at 10001C from the calcined nanopowders at 3001C at different dwelling times. From Fig. 4, CGO nanopowders calcined at 3001 or 5001C for 4 h from the NP process and sintered at a temperature range of Cfor4hshowed a relative density of 496% without any sintering aid. However, as the calcination temperature was increased to 8001C, the densification significantly retarded, which can be attributed to the rapid increase in crystallite size during high-temperature calcination of CGO powder as is evident from the XRD analysis in Table I. Similarly, CGO nanopowders from DEA showed a relative density of 496% under the calcination condition of 3001C for 4 h. As the calcination temperature increased to 5001C, a slight decrease of densification was observed. However, significant retardation of densification was mainly observed at the calcination temperature of 8001C due to a rapid increase of crystallite size. It can be clearly observed that a high relative density of 496% was achieved for the low-temperature sintered pellets with a fine grain size using DEA and NH 4 OH as the precipitants. On the other hand, Li et al. 22 were able to obtain a sintered density of 99% of theoretical at 10001C, resulting in (6) SEM Figure 5 shows SEM micrographs of fractured CGO pellets sintered at 10001C from the (a) the NP and (b) the DEA processsynthesized nanoparticles. The micrograph images of sintered pellets revealed negligible porosity, equiaxed grains, and clearly resolved grain boundaries, which are typical microstructural features of highly sintered pellets. The average size of grains in the sintered pellets ranges approximately from 150 to 300 nm. The low-temperature sintered pellets with a high density of 496% determined by the Archimedes method were evidenced by the dense microstructure morphology in Fig. 5. In addition, this high densification at a low temperature can be explained by the small crystallite size, homogeneously dispersed particle morphologies, and weak agglomeration of the calcined nanopowders as evident from XRD and TEM analyses. (7) Electrical Transport Properties Complex impedance spectra for the sintered pellet (DEA process) at 10001C, measured at 2001, 5001, and 6001C in air, are shown in Figs. 6(a) and (b). The presence of three depressed semicircles within the frequency range measured is evident. The high-frequency region depressed semicircle corresponds to the Fig. 5. SEM micrographs of fractured CGO pellets sintered at 10001C for 4 h from (a) the NP and (b) the DEA processes.

6 3272 Journal of the American Ceramic Society Muralidharan et al. Vol. 91, No. 10 Fig. 7. Comparison of total electrical conductivity versus 1000/T plots for the sintered pellets at 10001C for 4 h from the neutral precipitation (NP) and diethylamine (DEA) processes calcined nanopowders at 3001C for 4 h, with data on high-temperature sintered pellets reported in the literature. grain bulk resistance (R b ) and the depressed semicircle in the middle-frequency region represents the resistance of a grainboundary (R gb ) response. The low-frequency region depressed semicircles may be attributed to electrode resistance (R elect ). It is observed that the depressed semicircles shift to higher frequencies with increasing temperature. The inset in Fig. 6(a) represents the schematic plot of an idealized impedance spectrum associated with an equivalent circuit model compared of two serial RC elements. As can be observed clearly from the impedance spectra, the semicircles are depressed and hence a constant phase element (CPE) is used instead of pure capacitance, as shown in the inset of Fig. 6(a). It can be observed clearly from Fig. 6(b) that the grain-boundary semicircle reduces with increasing temperature and disappears entirely at 6001C. Therefore, this indicates that the contribution of grain-boundary resistance to the present CGO electrolytes synthesized through DEA and NP is negligible at the operating temperature of the IT-SOFC. The conductivity data were determined using the resistance obtained from the simulated and fitted impedance data with an equivalent circuit model (as inset, Fig. 6(a)) using the nonlinear least-squares fitting program of the Z-view software. The equivalent circuit had two serial (R parallel with CPE) elements; one represents the bulk and the other is related to the grain boundary of the material. The electrode polarization at a low frequency can be modelized by a CPE elect. Thus, such a simple model matches the CGO fine grain material perfectly in the entire temperature range. The total resistance of the electrolyte is given by R t 5 (R b )1(R gb ). Figure 7 shows the calculated total electrical conductivity as a function of temperature following the Arrhenius law in the following equation: s ¼ðs o =TÞ expð E a =RTÞ (4) Fig. 6. Impedance spectra of the sintered CGO pellet at 10001C for 4 h, measured at (a) 2001C, and (b) 6001, and 5001C in the inset, from the DEA process-calcined nanopowders at 3001C for 4 h. for NP and DEA pellets sintered at and 13001C, and compared with the data of high-temperature sintered pellets reported in the literature by Steele, 7 Huang et al., 16 Zhang et al., 28 and Zha et al. 36 Table II presents a summary of the conductivity data and activation energies for CGO processed by a variety of methods and sintered at various temperatures. The conductivities of submicrometer CGO at 6501C prepared by sintering at 10001C (this study) are comparable to the data for the same composition prepared from coarser-grained materials as indicated in Table II. The submicrometer CGO had a slightly higher conductivity than the coarser-grained materials prepared in this study. While there has been some discussion in the literature 42,43 about microdomain formation decreasing conductivity as the sintering temperature increases to cause heterogeneous grain growth, there are ample data in the literature to show excellent conductivity for coarse-grained doped ceria. 28,35,39 In addition, submicrometer CGO with an excellent conductivity has been prepared at temperatures as low as 9001C using Co to enhance diffusion. 41 There appears to be little advantage to the present process over commercially available materials. The low-temperature sintered pellets with a fine grain size of B nm demonstrated high electrical conductivity comparable with the pellets sintered using sintering aids, including Fe 2 O 3,Bi 2 O 3,andCo 2 O 3, and data on conventional high-temperature sintered pellets as reported in the literature (Table II). Among the reports, Steele s data 7 for a commercial high-purity CGO powder produced by Rhodia (Cranbury, NJ), and a hightemperature sintered pellet at 14001C showed higher electrical conductivity, which is almost equivalent to the total electrical conductivity data obtained from low-temperature (10001C) sintered pellets of the DEA and NP processes. Similarly, their data on activation energies (E a ) are also comparable. In addition, the electrical conductivity data for low-temperature (10001C) sintered pellets of the DEA and NP processes are comparable in Table II with the sintered pellets using Co 2 O 3 Fe 2 O 3, and Bi 2 O 3 as sintering aids Thus, in the present work, it should be noted that the equivalent high electrical conductivity is obtained for low-temperature sintered pellets at 10001C, which demonstrates the importance and simplicity of the synthesis techniques implemented. Zha et al. 36 noted a change in activation energy at 6501C, whereas the present data show a break occurring at 4001C. The activation energies for electrical conduction for the

7 October 2008 Submicrometer Gadolinia-Doped Ceria 3273 Table II. Comparison of Sintering Temperature, Relative Density, Grain Size, Electrical Conductivity, and Activation Energy, E a Data of the Sintered CGO Pellets from the NP and DEA Processes Calcined Powders with the Literature Reports Sample composition Process Sintering temperature (1C) Relative density (%) Grain size (mm) Above 4001C E a (ev) Below 4001C s total (S/cm) 4001C s total (S/cm) 6501C Reference Ce 0.9 Gd 0.1 O 1.95 DEA B This work B NP B B Commercial (Rhodia) B B Steele 7 Sol gel B B Huang et al. 16 Oxalate B B B Zhang et al. 28 precipitation Oxalate B B Zha et al. 36 precipitation Commercial B Jo et al. 37 (Nextech, Worthington, OH) 1wt%Bi 2 O 3 -doped Ammonia B B Gil et al. 38 Ce 0.9 Gd 0.1 O mol% Fe 2 O 3 - doped Ce 0.9 Gd 0.1 O mol%Co 2 O 3 -doped Ce 0.8 Gd 0.2 O 1.9 2mol%Co 2 O 3 - doped Ce 0.8 Gd 0.2 O 1.9 precipitation Ammonium carbonate precipitation Commercial (Rhodia) Commercial (Rhodia) CGO, Ce 0.9 Gd 0.1 O 1.95 ; NP, neutral precipitation; DEA, diethylamine B B Zhang et al B B Kleinlogel and Gauckler B B Fagg et al. 41 materials prepared in this study are consistent with data as reported in the literature (see Table II). In Fig. 7, it can be clearly observed that there is a small change in slope around 4001C, and it could be associated with ðgd 0 Ce 2V00 OÞcomplexes, whereas above 4001C, the ðgd 0 Ce 2V00 OÞ complexes may be dissociated.7 Thus, there is a change in activation energy: 0.59 ev, which is less than the value of 0.75 ev for the data below 4001C, similar to Steele s report. 7 Figure 8 shows the total electrical conductivity and contributions of grain bulk and grain boundaries for the pellet sintered using the DEA process. Also, similar behavior was observed in the case of the NP process-sintered pellets. It can be seen from Fig. 8 that there is a slight decrease in the total electrical conductivity compared with the grain bulk conductivity due to the grain-boundary contribution. However, in the present work, the grain-boundary contribution in the decrease of the total electrical conductivity was insignificant compared with the literature reports, which include SiO 2 segregation at the grain-boundary area as an impurity in the CGO pellet. 43 As a result, the grain-boundary contribution related to the electrical conductivity of the present electrolytes can be mostly attributed to intrinsic blocking effects due to the space-charge region. 44 IV. Conclusion Nanopowders of Ce 0.9 Gd 0.1 O 1.95 were successfully prepared through a simple precipitation process using DEA and ammonium hydroxide as precipitants. The pure crystalline phase of CGO solid solutions was produced directly from solution using DEA at RT and at elevated temperature for the NP process. The synthesis process resulted in homogenously dispersed B5 nm size particles and weakly agglomerated particles of CGO nanopowders calcined at 3001C. High densification at the low-temperature sinterability at 10001C without any sintering aid was facilitated by the weak agglomerates and high-crystallinity nanopowders obtained from the simple synthetic methods. The densified submicrometer-grained high-purity CGO prepared in this study had conductivity and activation energies similar to coarser-grained high-purity CGO and submicrometer CGO prepared using impurities, as reported in the literature. The present study clearly shows that it is possible to make materials with clean grain boundaries with little resistance to electronic or ionic conduction. While the nanopowder synthesized using the DEA processing technique gives a high yield and appears to be scaleable, the high surface area of this powder presents a barrier to its use as an electrolyte in cofired low-temperature fuel cells. Fig. 8. Log st versus 1000/T plots (total, grain bulk, and grain-boundary conductivity) of the pellet sintered at 10001C for 4 h from the calcined nanopowder at 3001C for 4 h synthesized through the DEA process. References 1 J. Maier, Nanoionics: Ion Transport and Electrochemical Storage in Confined Systems, Nat. Mater., 4 [11] (2005).

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