Synthesis and Application of Porous Sm0. 2Ce0. 8O1. 9 Nanocrystal

Sep 4, 2009 - Compared with the SDC powders obtained by the traditional combustion synthesis method (SDC_CB), SDC_SR powders showed better ...
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Synthesis and Application of Porous Sm0.2Ce0.8O1.9 Nanocrystal Aggregates Qiang Liu, Fei Zhao, Xihui Dong, Chenghao Yang, and Fanglin Chen* Department of Mechanical Engineering, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed: June 18, 2009; ReVised Manuscript ReceiVed: July 27, 2009

Highly porous Sm0.2Ce0.8O1.9 (SDC) powder was synthesized by a novel PVA assisted self-rising approach (SDC_SR). The as-synthesized SDC powders were composed of 3-5 nm nanocrystals, which displayed extremely low filled density and high BET surface area. Compared with the SDC powders obtained by the traditional combustion synthesis method (SDC_CB), SDC_SR powders showed better sinterability and higher ionic conductivity. 1. Introduction Compared with the traditional bulk materials, metal oxide nanocrystals, due to their unusual optical, electronic, and catalytic properties, have recently attracted much interest.1,2 Ceria-based solid solutions have been considered as promising electrolytes for intermediate temperature (450-650 °C) solid oxide fuel cells (IT-SOFCs) due to their excellent oxygen-ion conductivity compared to yttria stabilized zirconia (YSZ).3 Among the doped-ceria materials (e.g., Sr2+, Ca2+, Y3+, La3+, Gd3+, and Sm3+), the Sm0.2Ce0.8O1.9 system (SDC) shows the highest electrical conductivity (specific to ionic conductivity), since Sm3+ doping induces the least distortion of the parent lattice when oxygen vacancies are created in the CeO2 lattice for charge compensation.4,5 Solid SDC electrolytes for SOFCs should be highly dense to avoid combustion between the fuel at the anode and oxygen at the cathode. In addition, the thickness of the electrolyte membrane should be as thin as possible to reduce the ohmic resistance.6 To get the dense and thin electrolyte membrane, various techniques have been developed recently, such as the advanced chemical/physical vapor deposition method.7,8 However, these film deposition techniques are complicated and expensive. The traditional dry-pressing technique is a costeffective and simple method for electrolyte membrane fabrication.6,9,10 The use of fluffy and highly porous powders with a low bulk density makes it possible to apply a small amount of powders uniformly distributed in a hardened metal die to successfully obtain thin electrolyte membranes.6 To prepare such fine SDC powders with good sinterability, numerous synthetic routes have been developed, including solid-state reactions,11 hydrothermal,12 and wet-chemical coprecipitation processing techniques.13 Recently, the combustion method has been introduced for the preparation of ceramic powders with homogeneous nanocrystals.14 Different organic compounds can be used as fuel, such as glycine,15,16 citric acid,17 urea,18 and polyvinyl alcohol (PVA).19 A key feature of the combustion process is that the heat required to trigger the chemical reaction is provided by the reaction itself and not by an external source. In this report, highly porous SDC powders are synthesized by a novel self-rising approach, which is an extension of the combustion synthesis. Macroscopically, the as-synthesized powders are extremely loose, which makes it easier to obtain * Corresponding author. Phone: 803-777-4875. Fax: 803-777-0106. E-mail: [email protected].

very thin films using the dry-pressing method; furthermore, the powders are composed of very small SDC nanocrystals, which would be beneficial for the sintering process to achieve higher density at lower sintering temperatures. 2. Experimental Section Taking the synthesis of 20 mmol of Sm0.2Ce0.8O1.9 as an example, 6 g of PVA was first dissolved in 100 mL of hot water in a 250 mL beaker. Metal precursors (Sm(NO3)3 · 6H2O and Ce(NO3)3 · 6H2O) in the correct stoichiometric ratio were dissolved in 30 mL of water in another beaker together with 10 g of urea as the leavening agent in the self-rising approach (SDC_SR). Finally, the two solutions were mixed together, stirred for 30 min, and then dried in an oven at 50 °C. The dried precursor was then taken into a furnace for calcination. The heating rate was 1 °C/min to 160 °C. After dwelling at 160 °C for 3 h, it was then heated to 500 °C and held for 5 h. For comparison, SDC samples were also synthesized by the PVA combustion method (SDC_CB) without the addition of urea as the leavening agent, while all of the other conditions were kept the same. The powder X-ray diffraction (XRD) pattern was recorded on a D/MAX-3C X-ray diffractometer with graphite-monochromatized Cu KR radiation (λ ) 1.5418 Å), employing a scanning rate of 5°/min in the 2θ range of 20-80°. The structure and morphology of the synthesized product were characterized by scanning electron microscopy (SEM, FEI Quanta and XL 30) equipped with an energy dispersive X-ray (EDX) analyzer and transmission electron microscopy (TEM, Hitachi H-800, 200 kV). The grain size was determined using a linear intercept method by counting at least 50 adjacent grains in the SEM micrographs. High resolution TEM (HRTEM) and selected area electron diffraction (SAED) were conducted using a JEOL 2011 instrument. The samples were also characterized by nitrogen adsorption/desorption at -196 °C by using a NOVA 2000 series volumetric adsorption system. Simultaneous thermal analysis (thermogravimetry-differential scanning calorimetry, TG-DSC) was performed on a NETZSCH STA 409 instrument. Data was collected between room temperature and 500 °C at a heating rate of 2.5 °C/min under air atmosphere. AC impedance spectra of the sintered SDC pellets were conducted on a potentiostat/galvanostat with a built-in impedance analyzer (Versa STAT 3-400, Princeton Applied Research) in the frequency range of 0.05 Hz to 100 kHz between 500 and 800 °C. Both sides of the sintered SDC pellets were coated with

10.1021/jp905745w CCC: $40.75  2009 American Chemical Society Published on Web 09/04/2009

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SCHEME 1: Tentative Mechanism of the Self-Rising Approach

silver paste and heat-treated at 800 °C for 30 min before testing. In all of the measurements, the Ag lead resistance was subtracted by measuring the impedance of a blank cell. The conductivity is obtained using the following equation:

σ)

L RS

Figure 1. TG and DSC of (a) SDC_SR and (b) SDC_CB.

(1)

where σ is the conductivity, R is the resistance, L is the sample thickness, and S is the sample area. 3. Results and Discussion 3.1. The Self-Rising Approach. The word “self-rising” comes from the food industry, particularly referring to the selfrising flour, which is a mixture of an all-purpose flour with a leavening agent, usually the baking soda.20 At elevated temperatures, baking soda will decompose and release carbon dioxides, thus making the dough rise. Inspired by this strategy that can easily introduce pores/bubbles to bulk materials, we try to synthesize highly porous ceramic materials using the selfrising approach. Urea and PVA were chosen as the leavening agent and binder for the self-rising approach. PVA is nontoxic, sticky, and elastic, which will make it readily change shapes when gas starts to blow bubbles inside. The self-rising process is shown in Scheme 1. Urea was initially evenly distributed inside the PVA binder together with the metal precursors. As the temperature increases, urea, the leavening agent, would gradually decompose to NH3 and CO2, producing plenty of pores which make the size of PVA dough dramatically expand. At even higher temperatures, the organic PVA binders would be burnt off, resulting in highly porous metal oxides. 3.2. Study on the SDC Nanocrystals. The simultaneous TGDSC was conducted to help us understand the thermal behavior of the two precursor powders, indicating that the decomposition processes are very different. The starting temperature of weight loss is about 160 °C for SDC_SR (Figure 1a) which is much lower than that of SDC_CB (215 °C), as shown in Figure 1b; on the other hand, for SDC_CB, 30% weight loss occurred instantaneously around 215 °C, consequently producing one significant exothermic peak on the DSC curve (Figure 1b). As a comparison, SDC_SR showed gradual weight loss from 160 to 400 °C, while the DSC curve showed several broad exothermic peaks. On the basis of the above observation, the following conclusions can be made: (1) urea (the leavening agent) starts to decompose around 160 °C, which is the dwelling temperature needed to be taken to ensure that the self-rising process is slow and completed; (2) for the combustion synthesis, the drastic weight loss (at 215 °C) is due to the combustion of organic binders (PVA), and a substantial amount of heat was produced by the combustion reaction at a very short period of time.

Figure 2. XRD of (a) SDC_SR and (b) SDC_CB calcined at different temperatures.

It is clear from the TG curves of both the self-rising and combustion methods that almost no weight loss could be observed at above 450 °C, suggesting the formation of crystalline SDC as the decomposed product. This is confirmed by the XRD results, as shown in Figure 2. The XRD patterns of SDC_SR (Figure 2a) and SDC_CB (Figure 2b) calcined at different temperatures for 5 h were very similar. It seems that urea did not have any significant influence on the crystallinity of the final products, and a calcination temperature of 500 °C was sufficient for the formation of pure SDC powders. Reflection peaks of both of the samples become sharper and narrower with the increase in the calcination temperature, indicating that the crystal size increases and the crystallinity of SDC becomes better defined during the calcination process. All of the calcined samples at 500 °C exhibit XRD peaks that correspond to the (111), (200), (220), (311), and (222) planes of a cubic fluorite structure of Sm0.2Ce0.8O1.9, as identified using the standard PDF Card (JCPDS 75-0158). The values of lattice parameter a calculated from the XRD spectra are 0.5421 and 0.5457 nm for SDC_SR and SDC_CB, respectively. Apparently, Sm3+ doping induces an expansion in the unit cell of CeO2 (a ) 0.5411 nm, JCPDS 34-0394) because of the larger size of Sm3+ (0.1079 nm) than Ce4+ (0.0971 nm).21 It is noted that the lattice TABLE 1: Summary of the Particle Sizes, BET Surface Areas, Pore Sizes, and Pore Volumes of SDC_SR and SDC_CB Powders dXRD dHRTEM dBET pore size pore volume BET (nm) (nm) (nm) (nm) (mL · g-1) (m2 · g-1) SDC_SR SDC_CB

3.9 9.2

3-5 7-16

7.1 24.0

5.8 4.1

0.111 0.047

54.4 16.1

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Figure 3. HRTEM and SAED of (a) SDC_SR and (b) SDC_CB calicined at 500 °C for 5 h.

Figure 5. BET analyses of (a) SDC_SR and (b) SDC_CB.

Figure 4. (a) SEM and (b) TEM of SDC_SR; (c) SEM and (d) TEM of SDC_CB.

parameter of the standard SDC powder (0.5433 nm, JCPDS 750158) is larger than that of SDC_SR and smaller than that of SDC_CB. The possible reason was that a little amount of samarium was incorporated into the lattice of ceria, which led to a small change in cell parameters for SDC_SR, while for SDC_CB, the violent combustion reaction introduced more Sm3+ into the crystal lattice.22 The average crystallite sizes D of the SDC samples were calculated from X-ray line broadening of the (111) reflections using Scherrer’s equation, and the results are listed in Table 1.

D)

0.9λ β cos θ

(2)

where λ is the X-ray wavelength (1.5418 Å), β is the line broadening at half the maximum intensity (fwhm) in radians, and θ is the Bragg angle.

Figure 6. Relative density and grain size of SDC pellets as a function of sintering temperature.

The calculated crystallite sizes are 3.9 and 9.2 nm for SDC_SR and SDC_CB, respectively, indicating a smaller average particle size for the self-rising method, which can be further confirmed by high resolution TEM (HRTEM) analysis. It is clearly seen from the bright field images (Figure 3) that the SDC_SR sample contains aggregated nanocrystals of 3-5 nm in size, whereas the SDC_CB sample contains networked nanoparticles of 7-16 nm. These observed particle sizes are in agreement with the particle sizes determined from the X-ray and the subsequent BET analysis. Similar to the XRD results, the selected area electron diffraction (SAED) patterns of both

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samples (inset of Figure 3) do not have an apparent difference in the spot intensity, indicating the same crystallization at the same temperature. The measured interplanar spacings (dhkl) from SAED patterns of both samples are in good agreement with the values in the standard data JCPDS 75-0158. 3.3. Highly Porous and Extremely Low Filled Density. Shown in Figure 4 are the SEM and TEM images of SDC powders obtained by the two different methods. The SEM image of SDC_SR (Figure 4a) shows a network-like morphology in large range, and plenty of voids were distributed inside a foamlike structure and a weak force among them were observed, which is consistent with the TEM results (Figure 4b). These hollow pores should be produced in the self-rising process through urea decomposition to gases inside the PVA binder. The theoretical density of SDC is about 7.15 g · cm-3, while the filled density of the SDC_SR foam-like powder is only 0.0438 g · cm-3, i.e., 0.61% of the bulk value, implying that the apparent volume of this foam-like powder is about 165 times that of the same amount of bulk material. Consequently, it is possible to make thin membranes by the dry-pressing method using the extremely loose SDC powders.6 The SEM (Figure 4c) and TEM (Figure 4d) of SDC_CB displays thin plate-like morphologies, with a filled density of 0.0962 g · cm-3, which is about twice that of SDC_SR. Figure 5 shows the nitrogen adsorption and desorption isotherms of the 500 °C calcined SDC_SR and SDC_CB samples. For SDC_SR, it exhibits a type IV with H4-shaped hysteresis loops. The BET surface area and pore volume are 54.4 m2/g and 0.111 cm3/g, respectively. BJH calculations reveal that the pore size distribution is centered around 2.8 nm (inset of Figure 5). Comparatively, the SDC_CB sample displays a type IV isotherm with H1 hysteresis loops in the relative pressure range of 0.85-0.98. The high relative pressure indicates a large pore size in these samples. The pore size distribution determined by the BJH method shows a broad distribution in the range 2-18 nm (inset of Figure 5). Correspondingly, the BET surface area is only about 16.1 m2/g, which is much smaller than that of SDC_SR. The correlation between external surface area and the particle size can be established according to the following equation:23

Figure 7. Microstructure of SDC_SR (left column) and SDC_CB (right column) pellets sintered at (a,b) 1300 °C, (c,d) 1400 °C, and (e,f) 1500 °C for 5 h.

For face-centered Sm0.2Ce0.8O1.9: a ) b ) c ) 0.5433 nm d)

388 S

(3)

where a, b, and c are cell parameters (Å), S is the BET surface (m2 · g-1), and d is the average particle size (nm). The calculated particle sizes are in good agreement with the trends determined from X-ray line broadening and HRTEM analysis (see the summary in Table 1). 3.4. Sintering. Shown in Figure 6 is the densification behavior of the SDC powders processed via the two different methods. Due to the smaller particle size, SDC_SR shows better sinterability than SDC_CB, especially at relatively lower sintering temperatures (1300 °C). With an increase in sintering temperature, progressive densification developments together with a gradual increase in grain size have been observed. Nearly theoretical density was achieved for both samples when the sintering temperature was increased to 1500 °C, which can be further confirmed by the microstructure evolution (Figure 7). At 1300 °C, the pores exist in the form of continuous open porosity and exhibit a homogeneous distribution, which is much more significant in SDC_CB samples. At 1400 °C, a relative density of 97.3 and 96.6% can be achieved for SDC_SR and

Figure 8. Impedance plots of SDC_SR and SDC_CB pellets sintered at 1500 °C, measured at (a,c) 600 °C and (b,d) 700 °C. The gi, gb, and ge stand for grain interior, grain boundary, and electrode-electrolyte surface effects, respectively.

SDC_CB samples, respectively, while the grain growth is still less pronounced. When the sintering temperature is raised to 1500 °C, nearly all of the pores are eliminated from the sintered SDC_SR bodies, and significant grain growth occurs. A few pores close to the larger grains which are located at the grain boundaries or at the triple points are identified in the SDC_CB samples. The sintering temperature achieved in this work is not as low as that reported by Ma et al.,19 who have obtained dense SDC_CB pellets even at 1300 °C using the similar PVA combustion method. The discrepancy may be due to the different PVA ratio used as fuel for the combustion process. 3.5. Conductivity. Shown in Figure 8 are the impedance plots measured at 600 and 700 °C in air of the SDC_SR and SDC_CB pellets sintered at 1500 °C, respectively. The ac

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Figure 9. σgi, σgb, and σall of SDC_SR and SDC_CB pellets measured at (a) 600 °C and (b) 650 °C in air versus sintering temperature.

Figure 10. (a) Temperature dependence and (b) Arrhenius plots of σall for SDC_SR and SDC_CB pellets sintered at different temperatures.

impedance of an ionic conductor contains contributions from the grain interior (gi), the grain boundaries (gb), and the electrode-electrolyte interfaces (ge).24 These can be represented in a complex plane by three successive arcs. At 600 °C, the gi, gb, and ge effects can be clearly identified, as shown in Figure 8a and c. However, as the measuring temperature increased to 700 °C, the arcs of gi and gb shift to higher frequencies, resulting in the disappearance of the impedance arcs corresponding to gb at higher frequencies (Figure 8b and d).25 These changing trends have also been observed in other SDC pellets sintered at 1300 and 1400 °C. The effects of the sintering temperature on the gi ionic conductivity (σgi), gb ionic conductivity (σgb), and total ionic conductivity (σall) of the SDC_SR and SDC_CB measured at 650 and 600 °C in air are presented in Figure 9. The values of σgi, σgb, and σall display similar changes, increasing with sintering temperature for both samples. It is noted that, with the increase in temperatures, σgb increases much faster and gradually becomes the dominant influence for sample SDC_CB. The increment of σgb in the SDC_CB samples from 1300 to 1500 °C is 4 times larger than that for the SDC_SR sample measured at 600 °C, and about 6 times larger at 650 °C. Compared to σgb, the value of σgi is less dependent on sintering temperature for both samples. It is widely accepted that the relative density, grain size, and level of impurities (siliceous) are the three major factors responsible for the σgb behavior.18 Due to the fact that the SDC powders are synthesized by a solution-based combustion method, the influence of silicon impurities would be very limited. Considering the relative density versus temperature (Figure 6), the fast increment of σgb in SDC_CB samples is likely due to the densification of the SDC_CB ceramics. It is also found that, at 1300 and 1400 °C, the SDC_SR samples have a much larger σgb value than that of SDC_CB measured at 600 and 650 °C. Compared to larger SDC_CB grains (Figure 6b) after sintering at 1300 and 1400 °C, smaller SDC_SR grains (Figure 6a) are advantageous to increase the number of grain

boundaries in the sintered specimens, allowing the impurities to be dispersed to a larger extent and resulting in a reduction of the amount of impurities located in the grain boundaries, thereby facilitating the movement of oxygen vacancies across the grain boundaries. Therefore, the smaller grain size is beneficial to the enhancement of its ionic conductivity (σall). Consistent with this speculation, for the SDC_SR pellets, after being sintered at 1500 °C for 5 h, a significant grain size increase can be observed, and the average grain size has surpassed that of SDC_CB. Consequently, SDC_CB samples sintered at 1500 °C show a larger gb conductivity than that of SDC_SR samples. Shown in Figure 10a is the measured σall of both SDC_SR and SDC_CB pellets in air, which displays a monotonous increase with measuring temperature. The maximum value of σall is attained for the SDC_SR sample sintered at 1500 °C, which offers a value of 0.081 S · cm-1 at 800 °C, in the upper range of the reported data for doped ceria.18 Compared with SDC_CB, SDC_SR shows higher conductivity: even the SDC_SR sample sintered at 1300 °C has a conductivity larger than that of SDC_CB sintered at 1400 °C. These conductivity measurements show that the PVA self-rising synthesis is an effective method to prepare doped ceria powder having excellent electrical performance. Figure 10b shows the Arrhenius plots of ln(σT) versus 1/T for the as-obtained SDC powders sintered at 1300-1500 °C. The apparent activation energy for oxygen ion immigration was calculated from the following equation:

1n(σT) ) -

Ea 1000 · + ln σ0 1000R T

(3)

The calculated results are shown in the inset of Figure 10b. It is clear that the activation energy depends on the temperature range and there is no significant difference in the activation energy for samples sintered at 1300 and 1400

Porous Sm0.2Ce0.8O1.9 Nanocrystal Aggregates °C. For fully densified ceramics, it has been reported that the average Ea for the grain boundary is higher than that for the grain interior.26 Therefore, the lower Ea for SDC_CB than that for SDC_SR sintered at 1500 °C can be understood by the contribution from the grain boundary conductivity (σgb), which has a drastic increase after sintering at 1500 °C for SDC_CB samples (Figure 9). 4. Conclusions Compared with the traditional combustion synthesis method, the self-rising approach is more efficient to get nanocrystal metal oxides with extremely low filled density and high BET surface area. The average particle size of the as-synthesized SDC_SR powders is 3-5 nm, with a surface area of 54.4 m2 · g-1 and a filled density of only 0.61% of the bulk material. The self-rising strategy can be readily extended to prepare highly porous materials composed by nanoparticle aggregates of other single or mixed metal oxides. The fine grain size of SDC_SR shows better sinterability than the SDC_CB sample. Further, sintered SDC_SR samples display a higher conductivity than that of SDC_CB. SDC_SR pellets sintered at 1500 °C show a conductivity value of 0.081 S · cm-1 at 800 °C and 0.031 S · cm-1 at 700 °C. Acknowledgment. The financial support of the Department of Energy (Contract No. DE-FG36-08GO88116) is acknowledged gratefully. References and Notes (1) Cozzoli, P. D.; Pellegrino, T.; Manna, L. Chem. Soc. ReV. 2006, 35, 1195. (2) Norris, D. J.; Efros, A. L.; Erwin, S. C. Science 2008, 319, 1776. (3) Mogensen, M.; Sammes, N. M.; Tompsett, G. A. Solid State Ionics 2000, 129, 63.

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17267 (4) Inaba, H.; Tagawa, H. Solid State Ionics 1996, 83, 1. (5) Xu, X. Y.; Xia, C. R.; Mao, G. L.; Peng, D. Solid State Ionics 2005, 176, 1513. (6) Xia, C. R.; Liu, M. L. J. Am. Ceram. Soc. 2001, 84, 1903. (7) Ge, X. D.; Huang, X. Q.; Zhang, Y. H.; Lu, Z.; Xu, J. H.; Chen, K. F.; Dong, D. W.; Liu, Z. G.; Miao, J. P.; Su, W. H. J. Power Sources 2006, 159, 1048. (8) Young, J. L.; Etsell, T. H. Solid State Ionics 2000, 135, 457. (9) Xin, X. S.; Lu, Z.; Huang, X. Q.; Sha, X. Q.; Zhang, Y. H.; Chen, K. F.; Ai, N.; Zhu, R. B.; Su, W. H. J. Power Sources 2006, 160, 1221. (10) Zuo, C. D.; Zha, S. W.; Liu, M. L.; Hatano, M.; Uchiyama, M. AdV. Mater. 2006, 18, 3318. (11) Backhaus-Ricoult, M. Solid State Sci. 2008, 10, 670. (12) Rambabu, B.; Ghosh, S.; Jena, H. J. Mater. Sci. 2006, 41, 7530. (13) Suda, E.; Pacaud, B.; Montardi, Y.; Mori, M.; Ozawa, M.; Takeda, Y. Electrochemistry 2003, 71, 866. (14) Brandon, N. P.; Skinner, S.; Steele, B. C. H. Annu. ReV. Mater. Res. 2003, 33, 183. (15) Wang, Q. G.; Peng, R. R.; Xia, C. R.; Zhu, W.; Wang, H. T. Ceram. Int. 2008, 34, 1773. (16) Peng, R. R.; Xia, C. R.; Fu, Q. X.; Meng, G. Y.; Peng, D. K. Mater. Lett. 2002, 56, 1043. (17) Tian, C. A.; Liu, J. L.; Cai, J.; Zeng, Y. W. J. Inorg. Mater. 2008, 23, 77. (18) Chen, M.; Kim, B. H.; Xu, Q.; Ahn, B. K.; Kang, W. J.; Huang, D. P. Ceram. Int. 2009, 35, 1335. (19) Ma, J. J.; Jiang, C. R.; Zhou, X. L.; Meng, G. Y.; Liu, X. Q. J. Power Sources 2006, 162, 1082. (20) McCamish, M. J. Chem. Educ. 1987, 64, 710. (21) Li, J. G.; Ikegami, T.; Mori, T. Acta Mater. 2004, 52, 2221. (22) Wang, Y. R.; Mori, T.; Li, J. G.; Yajima, Y.; Drennan, J. J. Eur. Ceram. Soc. 2006, 26, 417. (23) Liu, Q.; Cui, Z. M.; Ma, Z.; Bian, S. W.; Song, W. G.; Wan, L. J. Nanotechnology 2007, 18, 385605. (24) Li, H. B.; Xia, C. R.; Zhu, M. H.; Zhou, Z. X.; Meng, G. Y. Acta Mater. 2006, 54, 721. (25) Ma, J.; Zhang, T. S.; Kong, L. B.; Hing, P.; Chan, S. H. J. Power Sources 2004, 132, 71. (26) Li, C. X.; Xie, Y. X.; Li, C. J.; Yang, G. J. J. Power Sources 2008, 184, 370.

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