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Synthesis of nanosized gadolinium doped ceria solid solution by high energy ball milling
Powder Technology 214 (2011) 117–121
Contents lists available at SciVerse ScienceDirect
Powder Technology 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 / p ow t e c
Synthesis of nanosized gadolinium doped ceria solid solution by high energy ball milling Z. Khakpour a, A.A. Youzbashi b,⁎, A. Maghsoudipour a, K. Ahmadi b a b
Ceramic Department, Materials and Energy Research Center, P. O. Box: 14155-4777 Tehran, Iran Semiconductors Department, Materials and Energy Research Center, P. O. Box: 14155-4777 Tehran, Iran
a r t i c l e
i n f o
Article history: Received 15 November 2010 Received in revised form 31 July 2011 Accepted 1 August 2011 Available online 16 August 2011 Keywords: Gd doped Ceria nano-particles 20GDC solid solution Mechanical alloying and Raman spectroscopy
a b s t r a c t This paper reports the development of a new process for the synthesis of gadolinium-doped ceria (20GDC) solid solution nanoparticles, as a solid electrolyte for use in solid oxide fuel cells. It is based on high energy milling (mechanical alloying or MA) of CeO2 and Gd2O3 powders containing 10 mol% Gd2O3. The samples, obtained after different milling times of 10 to 60 h, were characterized by X-ray diffraction (XRD), BET method of the specific surface area measurement, electron microscopy and Raman spectroscopy. The fluorite-structured Ce0.8Gd0.2O1.9 solid solution nanopowder with the surface area of 16.86 m2/g and particle sizes below 50 nm was obtained by milling the oxide mixture for 30 h. Increase in the milling time beyond 30 h led to the smaller crystallite sizes and more agglomeration. The structural changes, as evidence for the formation of solid solution, which occurred during the course of the milling process, could be appropriately followed and discussed by Raman spectroscopy. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Ceria-based materials have been investigated as the promising alternative electrolytes for use in the solid oxide fuel cells (SOFC) for their superior ionic conductivities at intermediate temperatures, as compared to the zirconia-based materials [1–3]. Gadolinium oxidedoped fluorite structured cerium oxide (GDC) is a solid solution, which is formed by replacing the Ce 4+ sites of CeO2 lattice by Gd 3+ cations. The optimal composition of Gd in CeO2, giving the highest conductivity, is Ce1 − xGdxO2 − 0.5x (x = 0.1–0.2) [2]. There have been many attempts for the synthesis of this compound having nanosized particles with desired characteristics, to be used as a suitable raw material for the preparation of solid electrolyte. The factors influencing the ionic conductivity of the synthesized GDC solid electrolytes may include homogeneity in size, shape and structure of the particles constituting the starting powders [4,5]. The chemical routes like co-precipitation [6–8], sol–gel [9] and hydrothermal [10] methods have been extensively investigated for the production of GDC nanopowder materials. However, inhibiting factors like agglomeration of the extremely fine particles or crystallites, nonreproducibility of the synthesis process and the relatively higher costs of such methods have prevented their wide commercial applications. For the conventional solid state methods to be suitable for preparation of a homogeneous GDC solid solution, it requires that the starting materials have very small particle sizes, which could withstand long calcination times.
⁎ Corresponding author. Tel.: + 98 2616204131; fax: + 98 261 1888. E-mail addresses: [email protected], [email protected] (A.A. Youzbashi). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.08.001
High-energy milling or mechanical alloying (MA) is a well-known solid-state synthesis method that can be used to prepare a variety of materials, from alloys and inter-metallic compounds to ceramics and composites [11]. The mechanical alloying process is superior to both the conventional solid state reaction and wet-chemistry-based processing routes for ceramic powders for several reasons. It uses low-cost and widely commercially available oxides as starting materials and skips the calcination step at an intermediate temperature, leading to a simplified process [12]. MA has recently been used in successful preparation of the nano-sized oxide compounds such as Al2O3/ZrO2 composite [13], Nickel–Zirconia cermets for solid oxide fuel cell anodes [14], LiMn2O4 spinel powders for lithium ion batteries [15], nanocrystalline Cu–Cr supersaturated solid solution [16] and ferroelectric powders [12,17]. It is well known that mechanochemical reactions are promoted by the energy transfer from the milling bodies to the milled powder, leading to increase in bulk and surface energy, which in turn lowers the sintering temperature. It has been shown that the homogeneous powders with fine grain sizes and refined material structures can be obtained by the MA process. The present paper discloses the results of a study on the synthesis of 20GDC nanocrystalline powders by mechanical alloying process for the first time. The influence of milling time on the formation of nanocrystalline 20GDC powder solid solution was investigated. The products were thoroughly characterized by XRD and Raman spectroscopy and the results were appropriately discussed. 2. Experimental The starting materials, used for the 20GDC synthesis included stoichiometric mixture of CeO2 and Gd2O3 powders from Alfa Aesar with 99.99% purity, were according to the composition Ce0.8Gd0.2O1.9. The
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powders were mixed by ball-milling (attrition) in acetone with 150 rpm for 3 h using zirconia balls as the milling medium. After milling, the powders were dried at 100 °C for 24 h and deagglomerated. The obtained CeO2/Gd2O3 mixture was processed in a planetary ball mill with a rotary speed of 270 rpm at a ball to powder weight ratio of 10:1, using zirconia vial (60 ml) and balls (10 and 20 mm diameters). The samples were collected after different milling times of 10, 20, 30, 40 and 60 h. For the phase identification, the XRD profile was equipped with a Philips Cu Kα radiation (λ = 1.5404 Å). All XRD experiments were performed with step size of 0.02° and the step time of 2 s. The crystallite size and micro-strain were estimated from the resultant spectra using the Williamson–Hall method [18]. The lattice parameter (a) was determined by fitting the observed reflection with the least squares refinement program. Prior to the calculations from XRD peaks, the background was automatically removed and Kα2 radiation was stripped from the scans using X-pert High score software developed by PANalytical B.V. Company (Netherlands). Specific surface area measurements were performed by BET method using a Gemini 2360 surface area analyzer (Micrometritics). Raman spectra analysis of the milled powders was performed using a Nicolet Almega Dispersive Raman Spectrometer (laser: 532 nm of the Nd:YLF, exposure time: 6 s). Particle morphology and agglomeration state of the mechanically alloyed powders were observed via Vega Tescan electron microscope (SEM) after they were ultrasonically dispersed into ethanol and the suspensions were then spread on the surface of aluminum foil. Gold coating was performed before observation for better conductivity. 3. Results and discussion 3.1. XRD analysis Fig. 1(a) shows the XRD patterns of the starting materials that were mixed to give 20GDC (20 mol% Gd doped CeO2) solid solution. The XRD patterns of the mixture milled for the different times are given in Fig. 1(b). It is worth noting that the diffraction peaks are attributed to CeO2 and Gd2O3, even though their intensity is greatly reduced as compared with the unmilled ones. These patterns display gradual broadening of the peaks as the MA time increases. The broadening of the peaks corresponds to the microstructure refinement (grain size reduction and lattice strain rising) [11]. After milling up to 10 h, the Gd2O3 peaks disappeared and their monitoring became difficult (marked by arrows in the patterns). This could be, however, due to the limited or relatively low intensity of the identifiable Gd2O3 peaks compared to the CeO2 peaks and also the significant reduction of crystallite and particle sizes. It may also be due to the dissolution of Gd into the CeO2 lattice to form a solid solution. For this purpose, the variation of lattice parameter with MA times must be determined. The +4 3+ partial replacement of the host (rCe = 0.97 Å) by the guest (rGd = 1.053 Å) creates additional oxygen vacancies and forces the lattice to expand in the vicinity of the guest ion [19]. It was observed that by increasing of the milling time up to 30 h, evolution of the diffractograms in Fig. 2(b) reveals a shift in the CeO2 peaks, which could occur as a result of Gd dissolution in the ceria lattice. The lattice parameter of the 20GDC mixture milled for 30 h was determined to be 5.419 Å, which is close to the reported value (5.423 Å, Powder Diffraction File No. 75-0162). Heavy deformation during the milling introduces stacking faults on alloys and metals. The contribution of the stacking faults to the broadening/shift of the peak positions can further complicate the situation as it may cause the peak shift to occur in other ways [11]. To elucidate this, we milled pure ceria for 30 h and determined the lattice parameter. The value obtained (Table 1) clearly confirms that the shifts observed for the XRD peaks of the 20GDC powders after 30 h milling can be only due to the dissolution of Gd in the CeO2 lattice and formation of the solid solution. The results of the measurements of the lattice parameter after 40 and 60 h milling indicated that the increase of the milling time up to 60 h did not cause
Fig. 1. X-ray diffraction profiles of: (a) CeO2 and Gd2O3 as received powders and (b) 20 mol% Gd doped CeO2 powder processed by MA for different period times.
any further shift in the XRD peaks, and only the broadening effects and decrease in the intensity of the peaks could be observed (Fig. 1(b) and Table 1). Apparent grain sizes were determined by XRD profile analysis (dXRD in Table 1). The surface area of the milled powders was measured by the BET method. The particle sizes were also determined from the BET surface area values using the following equation, assuming that the particles are spherical with smooth surfaces and uniform sizes. dBET = 6 × 1000 = ðDth × SBET Þ
ð1Þ
Where, Dth is the theoretical density of the materials (g/m 3). Assuming that Gd 3+ cations are homogeneously distributed in the CeO2 lattice and occupied the Ce 4+ sites to form a solid solution of Ce0.8Gd0.2O1.9, theoretical density of the 20GDC solid solution was calculated to be 7.258 g/cm 3. The calculated particle size values for the powders milled up to 30 h are given in Table 1 (dBET). It can be observed that the dXRD and dBET values show appreciable discrepancies, indicating the presence of agglomeration. Below 30 h, however, the dXRD and dBET values seem to be closer, suggesting the increase of agglomeration rate with increase in the milling time. 3.2. Raman analysis Raman scattering is an excellent, nondestructive and rapid analytical technique for investigating the phonon spectra of materials [20]. Here, Raman spectroscopy was used to study the microstructure of Ce0.8Gd0.2O1.9 solid solution. To confirm the formation of gadoliniumxdoped ceria solid solution during the milling process, Raman spectroscopy of the pure ceria, gadolinium-doped ceria (with different milling times up to 60 h) and a mixture of CeO2 and Gd2O3 without any heat treatment and milling were investigated. The Raman
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Table 2 Raman line broadening (FWHM) and Raman shift of the CeO2–Gd2O3 samples after milling. Sample
Milling time (h)
Raman line broadening (FWHM)
Raman shift (cm− 1)
Ceria as received Ceria + 20 mol% Gd Ceria + 20 mol% Gd Ceria + 20 mol% Gd Ceria + 20 mol% Gd
– 20 30 40 60
42.11 49.76 73.35 40.33 –
461 461 458.9 459 459
much more important than the one associated with the grain sizeinduced non-stoichiometry only. In the present work, the Raman spectra of CeO2 (Fig. 2) shows a strong band at 461 cm− 1, which is typical for CeO2 and corresponds to the cubic phase. The vibration mode at 360 cm− 1 is, however, attributed to the band corresponding to the Gd2O3 cubic phase [22], presented in the CeO2 + Gd2O3 mixture. It can be seen that after milling, the Raman band at 461 cm− 1 shifts to the lower frequency and gets broader with increasing of the milling times. Zhang et al. [23] reported that lattice expansion by decreasing the particle size explains this systematic change in the Raman peak, which can be attributed to the increasing concentration of the point defect. Raman characterization of 20GDC at different milling times is summarized in Table 2. Maximum broadening (FWHM) and red shift belong to the powders milled for 30 h. By increasing the milling time up to 40 h, the FWHM value was decreased and no further shift was seen. This observation suggests that there is a little change in the strength of Ce–O bands and additional milling time does not change the solubility of gadolinium in ceria. Besides the f2g Raman mode, one additional peak at low intensity of about 560 cm–1 is also observed. According to McBride et al. [24] this can be related to the extrinsic oxygen vacancies introduced into the ceria lattice by substitution of Ce+ 4 ions and intrinsic oxygen vacancies due to the nonstoichiometry of ceria nanopowders. In the Raman spectra of the sample milled for 60 h, another peak of about 980 cm− 1 can be observed, which could be due to a structural change occurring as a result of longer times of milling. However, at present, no absolute explanation can be given for this observation. Fig. 2. Raman spectra of CeO2 as received and 20 mol% Gd doped CeO2 powder after milling at room temperature.
spectra of these materials are shown in Fig. 2. According to the literature, CeO2 with a fluorite structure has one Raman active triply degenerate F2g mode at 464 cm− 1. This mode presents the symmetrical stretching vibrations of CeO8 vibration unit and should be very sensitive to the oxygen sublattice disorder. Only the oxygen atoms concerning this mode of vibration are very sensitive to the oxygen sublattice disorder resulting from the processes leading to the grain-size-induced non-stoichiometry [20]. Kosacki et al. [21] give the evidence for the stronger influence of substitution disorder over the stoichiometric one. They also showed that the broadening in Gd-substituted CeO2 is indeed
3.3. Morphology and microstructure Observations via scanning electron microscopy revealed that the milling process affects the particle morphology and sizes significantly. Fig. 3 shows the SEM micrographs of the powders milled for different times. During the milling, the particles constantly are impacted and fractured, leading to a considerable reduction of the particle size as a result of the energy provided during the ball milling. Initially, they formed larger aggregates, which were then broken up in further steps of the milling process. Consequently, uniform grain size distribution is observed with the increasing of milling time. It can be seen that the milled powder after 10 h (Fig. 3(b)) contains larger particles and agglomerates but with the increasing of milling time up to 20 h, the
Table 1 Mean grain size, micro-strain and lattice constant calculated from the XRD patterns, specific surface area and mean particle size obtained from the analysis of the CeO2–Gd2O3 samples after milling. Sample
Milling time (h)
Mean grain size (nm) with XRD
Mean particle size (nm) with BET
Micro-strain (%)
Lattice constant (Å)
Specific surface area (m2/gr)
Ceria as received Ceria Ceria + 20 mol% Gd Ceria + 20 mol% Gd Ceria + 20 mol% Gd Ceria + 20 mol% Gd Ceria + 20 mol% Gd
0 30 10 20 30 40 60
– 60 85 50 38 30 20
– – 88 66 50 48 45
– 0.2 0.16 0.25 0.4 0.47 0.7
5.411 5.409 5.411 5.414 5.419 5.419 5.418
1.62 – 9.39 12.29 16.56 16.89 18.21
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Fig. 3. SEM micrographs of CeO2 powder before milling (a), 20 mol% Gd doped CeO2 powder milled for 10 h (b), 20 h (c), 30 h (d) and 60 h (e).
particles' and agglomerates' sizes decrease (Fig. 3(c)).With further increasing the milling time, the microstructure becomes finer and more homogenous (Fig. 3(d)). Fig. 3(e) shows that with the increasing of milling time up to 60 h, the agglomerated particles, composed of nanoparticles, are more pronounced. The formation of solid solutions (both equilibrium and metastable) during mechanical alloying may be influenced by several factors, which is described as follows: Thermodynamic barriers have been discovered to be effective in the formation of a solid solution by MA process. Considering the formation of the disordered A (B) solid solution from a mixture of pure A and B elements (as the standard state), the Gibbs' free energy change can be presented as [22]: S
S
S
ΔG = ΔHM −TΔS
ð2Þ
where, ΔH SM and ΔS S are the enthalpy and entropy of mixing, respectively. T is the temperature at which a solid solution is formed. Also, for the formation of solid solution from the elemental powders,
ΔS S can be calculated with the assumption of configurational entropy of mixing: S
ΔS = RðxA ln xA + xB ln xB Þ
ð3Þ
where, R is the universal gas constant, and xA and xB are the mole fractions of elements A and B, respectively [25]. Entropy of mixing cannot be ignored when the particle size is less than a few hundreds of atoms, a case applicable to MA process where the particles could become finer [26]. The largest reduction in free energy occurs when the particles' size is in atomic level. Moreover, in MA process, plastic deformation occurs as well. Plastic deformation, in fact, refines the particle and grain size and increases the grain boundary area. The decreased particle size reduces the diffusion distances between the particles and facilitates pipe diffusion. Diffusion is further aided by the increased defect density and a local rise in temperature. The combination of all these effects would permit the diffusion to occur
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efficiently in the interfacial regions of the nanocrystalline grains to form solid solution. Many attempts have been made for the synthesis of various oxide systems by MA process. For example, J. Z. Jiang and coworkers investigated the synthesis of α–Fe2O3–SnO2 solid solution [27]. The high defect concentration and the chemical enthalpy of Fe3+–O 2−–Sn4+ interfaces between nanostructured α–Fe2O3 and SnO2 regions were suggested as the driving force for the formation of solid solution during the milling. In a similar attempt for the synthesis of stabilized zirconia by MA process [28], surface area and particle size were found to act as the major factors effective in the formation of stabilized zirconia solid solution. According to the results obtained in the present work, the influence of milling time on the decrease of crystallite size or the increase of surface area was more considerable up to 30 h (Table 1). In addition, the XRD results indicated that the powders undergo an increase in microstrain during various milling times (Table 1). Since the crystalline defects and structural disorders considerably favor diffusion and atomic rearrangements, they can, therefore, explain the increase in chemical reactivity of the powders during the milling. The results of the lattice constant calculated from the XRD study showed that the diffusion of Gd 3+ into the CeO2 phase occurred only after 10 h of milling. When the milling time increases, gadolinium ions diffuse further into the CeO2 phase, and after 30 h of milling, a solid solution is formed. Therefore, powder milling up to 30 h can cause the reduction of grain sizes, to an extent that facilitates the diffuse of Gd 3+ ions into the CeO2 phase where, finally it leads to the formation of 20GDC solid solution. 4. Conclusions Mechanical alloying of the powder mixture of CeO2-20 mol% Gd system was investigated and the obtained products were characterized by XRD, SEM, BET and Raman spectroscopy. The conclusions are as follows: 1- High-energy ball milling was found as a new process for the synthesis of GDC solid solution. 2- The powder product was pure, having homogenous microstructure and very fine particles containing nanocrystallites in the range of 50 nm. 3- A red shift, which is observed in the Raman spectra of the samples after 30 h of milling, could confirm the formation of solid solution. Moreover, a peak, which could be assigned to the presence of intrinsic and extrinsic oxygen vacancies, was also detected through the Raman studies. References [1] N.Q. Minh, Ceramic fuel cells, J. Am. Ceram. Soc. 76 (3) (1993) 563. [2] B.C.H. Steele, Appraisal of Ce1 − yGdyO2 − y/2 electrolytes for IT-SOFC operation at 500 °C, Solid State Ionics 129 (2000) 95.
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[3] S.M. Haile, Fuel cell materials and components, Acta Mater. 51 (2003) 5981. [4] J.G. Li, T. Ikegami, Y. Wang, T. Mori, Reactive ceria nanopowders via carbonate precipitation, J. Am. Ceram. Soc. 85 (9) (2002) 2376. [5] I. Kosacki, T.Suzuki. Petrovsky, H.U. Anderson, Electrical conductivity of nanocrystalline ceria and zirconia thin films, Solid State Ionics 136–37 (2000) 1225. [6] J.G. Li, T. Ikegami, T. Mori, T. Wada, Reactive Ce0.8RE0.2O1.9 (RE = La, Nd, Sm, Gd, Dy, Y, Ho, Er and Yb) powders via carbonate coprecipitation. 1. Synthesis and characterization, Chem. Mater. 13 (9) (2001) 2913. [7] F.Y. Wang, B.Z. Wan, S. Chang, Study on Gd3+ and Sm3+ co-doped ceria-based electrolytes, J. Solid State Electrochem. 9 (2005) 168. [8] T.S. Zhang, J. Ma, L.B. Kong, P. Hing, Y.J. Leng, S.H. Chan, J.A. Kilner, Sinterability and ionic conductivity of coprecipitated Ce0.8Gd0.2O2 − δ powders treated via a highenergy ball-milling process, J. Power Sources 124 (2003) 26. [9] J.X. Zhu, D.F. Zhou, S.R. Goo, J.F. Ye, X.F. Hio, X.Q. Cao, J. Meng, Grain boundary conductivity of high purity neodymium-doped ceria nanosystem with and without the doping of molybdenum oxide, J. Power Sources 174 (2007) 114. [10] A.I.Y. Tok, S.W. Du, F.Y.C. Boey, W.K. Chong, Hydrothermal synthesis and characterization of rare earth doped ceria nanoparticles, Mater. Sci. Eng., A 466 (2007) 223. [11] C. Suryanarayona, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001) 1. [12] L.B. Kong, J. Ma, H. Huang, R.F. Zhang, W.X. Que, Barium titanate derived from mechanochemically activated powders, J. Alloys Compd. 337 (2002) 226. [13] C. Santos, M.H. Koizumi, J.K.M.F. Daguano, F.A. Santos, C.N. Elias, A.S. Ramos, Properties of Y-TZP/Al2O3 ceramic nanocomposites obtained by high-energy ball milling, Mater. Sci. Eng., A 502 (2009) 6. [14] T. Augusto Guisard, Restivo, S. Regina Homem de Mello-Castanho, Nickel-Zirconia cermet processing by mechanical alloying for solid oxide fuel cell anodes, J. Power Sources 185 (2008) 1262. [15] W.T. Jeong, J.H. Joo, K.S. Lee, Synthesis of spinel LiMn2O4 powders for lithium ion battery by mechanical alloying of Li2O2 and Mn2O3, J. Alloys Compd. 358 (2003) 294. [16] S. Sheibani, S. Heshmati-Manesh, A. Ataie, Structural investigation on nanocrystalline Cu–Cr supersaturated solid solution prepared by mechanical alloying, J. Alloys Compd. 495 (2010) 50. [17] J.S. Forrester, J.S. Zobec, D. Phelan, E.H. Kisi, Synthesis of PbTiO3 ceramic using mechanical alloying and solid state sintering, J. Solid State Chem. 177 (2007) 3553. [18] G.K. Williamson, W.H. Hall, X-ray line broadening from filed aluminium and wolfram, Acta Metall. 1 (1953) 22. [19] L. Minervini, M.O. Zacate, R.W. Grimes, Defect cluster formation in M2O3-doped CeO2, Solid State Ionics 116 (1999) 339. [20] G. Gouadec, Ph. Colomban, Raman spectroscopy of nanomaterials: how spectra relate to disorder, particle size and mechanical properties, Prog. Cryst. Growth Charact. Mater. 53 (2007) 1. [21] I. Kosacki, T. Suzuki, H.U. Anderson, Ph. Colomban, Raman scattering and lattice defects in nanocrystalline CeO2 thin films, Solid State Ionics 149 (2002) 99. [22] N. Dilawar, Sh. Mehrotra, D. Varandani, B.V. Kumaraswamy, S.K. Haldar, A.K. Bandyopadhyay, A Raman spectroscopic study of C-type rare earth sesqioxides, Mater. Charact. 59 (2008) 462. [23] F. Zhang, S.W. Chan, J.E. Spanier, E. Apak, Q. Jin, R.D. Robinson, I.P. Herman, Cerium oxide nanoparticles: size-selective formation and structure analysis, Appl. Phys. Lett. 80 (2002) 127. [24] J.R. McBride, K.C. Hass, B.D. Poindexter, W.H. Weber, Raman and x-ray studies of Ce1-xRExO2-y, where RE = La, Pr, Nd, Eu, Gd and Tb, J. Appl. Phys. 76 (1994) 2435. [25] D.R. Gaskell, Introduction to Metallurgical Thermodynamics, third ed. McGrawHill, Washington, NY, 1981. [26] A.Y. Badmos, H.K.D.H. Bhadeshia, The evolution of solutions: a thermodynamic analysis of mechanical alloying, Metall. Mater. Trans. A 28A (1997) 2189. [27] J.Z. Jiang, R. Line, S. Morup, K. Nielsen, F.W. Poulsen, F.J. Berry, R. Clasen, Mechanical alloying of an immiscible α–Fe2O3–SnO2 ceramic, Phys. Rev. B 55 (1) (1997) 11. [28] D. Michel, F. Faudot, E. Gaffet, L. Mazerolles, Stabilized zirconia prepared by mechanical alloying, J. Am. Ceram. Soc. 76 (11) (1994) 2884.
Contents lists available at SciVerse ScienceDirect
Powder Technology 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 / p ow t e c
Synthesis of nanosized gadolinium doped ceria solid solution by high energy ball milling Z. Khakpour a, A.A. Youzbashi b,⁎, A. Maghsoudipour a, K. Ahmadi b a b
Ceramic Department, Materials and Energy Research Center, P. O. Box: 14155-4777 Tehran, Iran Semiconductors Department, Materials and Energy Research Center, P. O. Box: 14155-4777 Tehran, Iran
a r t i c l e
i n f o
Article history: Received 15 November 2010 Received in revised form 31 July 2011 Accepted 1 August 2011 Available online 16 August 2011 Keywords: Gd doped Ceria nano-particles 20GDC solid solution Mechanical alloying and Raman spectroscopy
a b s t r a c t This paper reports the development of a new process for the synthesis of gadolinium-doped ceria (20GDC) solid solution nanoparticles, as a solid electrolyte for use in solid oxide fuel cells. It is based on high energy milling (mechanical alloying or MA) of CeO2 and Gd2O3 powders containing 10 mol% Gd2O3. The samples, obtained after different milling times of 10 to 60 h, were characterized by X-ray diffraction (XRD), BET method of the specific surface area measurement, electron microscopy and Raman spectroscopy. The fluorite-structured Ce0.8Gd0.2O1.9 solid solution nanopowder with the surface area of 16.86 m2/g and particle sizes below 50 nm was obtained by milling the oxide mixture for 30 h. Increase in the milling time beyond 30 h led to the smaller crystallite sizes and more agglomeration. The structural changes, as evidence for the formation of solid solution, which occurred during the course of the milling process, could be appropriately followed and discussed by Raman spectroscopy. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Ceria-based materials have been investigated as the promising alternative electrolytes for use in the solid oxide fuel cells (SOFC) for their superior ionic conductivities at intermediate temperatures, as compared to the zirconia-based materials [1–3]. Gadolinium oxidedoped fluorite structured cerium oxide (GDC) is a solid solution, which is formed by replacing the Ce 4+ sites of CeO2 lattice by Gd 3+ cations. The optimal composition of Gd in CeO2, giving the highest conductivity, is Ce1 − xGdxO2 − 0.5x (x = 0.1–0.2) [2]. There have been many attempts for the synthesis of this compound having nanosized particles with desired characteristics, to be used as a suitable raw material for the preparation of solid electrolyte. The factors influencing the ionic conductivity of the synthesized GDC solid electrolytes may include homogeneity in size, shape and structure of the particles constituting the starting powders [4,5]. The chemical routes like co-precipitation [6–8], sol–gel [9] and hydrothermal [10] methods have been extensively investigated for the production of GDC nanopowder materials. However, inhibiting factors like agglomeration of the extremely fine particles or crystallites, nonreproducibility of the synthesis process and the relatively higher costs of such methods have prevented their wide commercial applications. For the conventional solid state methods to be suitable for preparation of a homogeneous GDC solid solution, it requires that the starting materials have very small particle sizes, which could withstand long calcination times.
⁎ Corresponding author. Tel.: + 98 2616204131; fax: + 98 261 1888. E-mail addresses: [email protected], [email protected] (A.A. Youzbashi). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.08.001
High-energy milling or mechanical alloying (MA) is a well-known solid-state synthesis method that can be used to prepare a variety of materials, from alloys and inter-metallic compounds to ceramics and composites [11]. The mechanical alloying process is superior to both the conventional solid state reaction and wet-chemistry-based processing routes for ceramic powders for several reasons. It uses low-cost and widely commercially available oxides as starting materials and skips the calcination step at an intermediate temperature, leading to a simplified process [12]. MA has recently been used in successful preparation of the nano-sized oxide compounds such as Al2O3/ZrO2 composite [13], Nickel–Zirconia cermets for solid oxide fuel cell anodes [14], LiMn2O4 spinel powders for lithium ion batteries [15], nanocrystalline Cu–Cr supersaturated solid solution [16] and ferroelectric powders [12,17]. It is well known that mechanochemical reactions are promoted by the energy transfer from the milling bodies to the milled powder, leading to increase in bulk and surface energy, which in turn lowers the sintering temperature. It has been shown that the homogeneous powders with fine grain sizes and refined material structures can be obtained by the MA process. The present paper discloses the results of a study on the synthesis of 20GDC nanocrystalline powders by mechanical alloying process for the first time. The influence of milling time on the formation of nanocrystalline 20GDC powder solid solution was investigated. The products were thoroughly characterized by XRD and Raman spectroscopy and the results were appropriately discussed. 2. Experimental The starting materials, used for the 20GDC synthesis included stoichiometric mixture of CeO2 and Gd2O3 powders from Alfa Aesar with 99.99% purity, were according to the composition Ce0.8Gd0.2O1.9. The
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powders were mixed by ball-milling (attrition) in acetone with 150 rpm for 3 h using zirconia balls as the milling medium. After milling, the powders were dried at 100 °C for 24 h and deagglomerated. The obtained CeO2/Gd2O3 mixture was processed in a planetary ball mill with a rotary speed of 270 rpm at a ball to powder weight ratio of 10:1, using zirconia vial (60 ml) and balls (10 and 20 mm diameters). The samples were collected after different milling times of 10, 20, 30, 40 and 60 h. For the phase identification, the XRD profile was equipped with a Philips Cu Kα radiation (λ = 1.5404 Å). All XRD experiments were performed with step size of 0.02° and the step time of 2 s. The crystallite size and micro-strain were estimated from the resultant spectra using the Williamson–Hall method [18]. The lattice parameter (a) was determined by fitting the observed reflection with the least squares refinement program. Prior to the calculations from XRD peaks, the background was automatically removed and Kα2 radiation was stripped from the scans using X-pert High score software developed by PANalytical B.V. Company (Netherlands). Specific surface area measurements were performed by BET method using a Gemini 2360 surface area analyzer (Micrometritics). Raman spectra analysis of the milled powders was performed using a Nicolet Almega Dispersive Raman Spectrometer (laser: 532 nm of the Nd:YLF, exposure time: 6 s). Particle morphology and agglomeration state of the mechanically alloyed powders were observed via Vega Tescan electron microscope (SEM) after they were ultrasonically dispersed into ethanol and the suspensions were then spread on the surface of aluminum foil. Gold coating was performed before observation for better conductivity. 3. Results and discussion 3.1. XRD analysis Fig. 1(a) shows the XRD patterns of the starting materials that were mixed to give 20GDC (20 mol% Gd doped CeO2) solid solution. The XRD patterns of the mixture milled for the different times are given in Fig. 1(b). It is worth noting that the diffraction peaks are attributed to CeO2 and Gd2O3, even though their intensity is greatly reduced as compared with the unmilled ones. These patterns display gradual broadening of the peaks as the MA time increases. The broadening of the peaks corresponds to the microstructure refinement (grain size reduction and lattice strain rising) [11]. After milling up to 10 h, the Gd2O3 peaks disappeared and their monitoring became difficult (marked by arrows in the patterns). This could be, however, due to the limited or relatively low intensity of the identifiable Gd2O3 peaks compared to the CeO2 peaks and also the significant reduction of crystallite and particle sizes. It may also be due to the dissolution of Gd into the CeO2 lattice to form a solid solution. For this purpose, the variation of lattice parameter with MA times must be determined. The +4 3+ partial replacement of the host (rCe = 0.97 Å) by the guest (rGd = 1.053 Å) creates additional oxygen vacancies and forces the lattice to expand in the vicinity of the guest ion [19]. It was observed that by increasing of the milling time up to 30 h, evolution of the diffractograms in Fig. 2(b) reveals a shift in the CeO2 peaks, which could occur as a result of Gd dissolution in the ceria lattice. The lattice parameter of the 20GDC mixture milled for 30 h was determined to be 5.419 Å, which is close to the reported value (5.423 Å, Powder Diffraction File No. 75-0162). Heavy deformation during the milling introduces stacking faults on alloys and metals. The contribution of the stacking faults to the broadening/shift of the peak positions can further complicate the situation as it may cause the peak shift to occur in other ways [11]. To elucidate this, we milled pure ceria for 30 h and determined the lattice parameter. The value obtained (Table 1) clearly confirms that the shifts observed for the XRD peaks of the 20GDC powders after 30 h milling can be only due to the dissolution of Gd in the CeO2 lattice and formation of the solid solution. The results of the measurements of the lattice parameter after 40 and 60 h milling indicated that the increase of the milling time up to 60 h did not cause
Fig. 1. X-ray diffraction profiles of: (a) CeO2 and Gd2O3 as received powders and (b) 20 mol% Gd doped CeO2 powder processed by MA for different period times.
any further shift in the XRD peaks, and only the broadening effects and decrease in the intensity of the peaks could be observed (Fig. 1(b) and Table 1). Apparent grain sizes were determined by XRD profile analysis (dXRD in Table 1). The surface area of the milled powders was measured by the BET method. The particle sizes were also determined from the BET surface area values using the following equation, assuming that the particles are spherical with smooth surfaces and uniform sizes. dBET = 6 × 1000 = ðDth × SBET Þ
ð1Þ
Where, Dth is the theoretical density of the materials (g/m 3). Assuming that Gd 3+ cations are homogeneously distributed in the CeO2 lattice and occupied the Ce 4+ sites to form a solid solution of Ce0.8Gd0.2O1.9, theoretical density of the 20GDC solid solution was calculated to be 7.258 g/cm 3. The calculated particle size values for the powders milled up to 30 h are given in Table 1 (dBET). It can be observed that the dXRD and dBET values show appreciable discrepancies, indicating the presence of agglomeration. Below 30 h, however, the dXRD and dBET values seem to be closer, suggesting the increase of agglomeration rate with increase in the milling time. 3.2. Raman analysis Raman scattering is an excellent, nondestructive and rapid analytical technique for investigating the phonon spectra of materials [20]. Here, Raman spectroscopy was used to study the microstructure of Ce0.8Gd0.2O1.9 solid solution. To confirm the formation of gadoliniumxdoped ceria solid solution during the milling process, Raman spectroscopy of the pure ceria, gadolinium-doped ceria (with different milling times up to 60 h) and a mixture of CeO2 and Gd2O3 without any heat treatment and milling were investigated. The Raman
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Table 2 Raman line broadening (FWHM) and Raman shift of the CeO2–Gd2O3 samples after milling. Sample
Milling time (h)
Raman line broadening (FWHM)
Raman shift (cm− 1)
Ceria as received Ceria + 20 mol% Gd Ceria + 20 mol% Gd Ceria + 20 mol% Gd Ceria + 20 mol% Gd
– 20 30 40 60
42.11 49.76 73.35 40.33 –
461 461 458.9 459 459
much more important than the one associated with the grain sizeinduced non-stoichiometry only. In the present work, the Raman spectra of CeO2 (Fig. 2) shows a strong band at 461 cm− 1, which is typical for CeO2 and corresponds to the cubic phase. The vibration mode at 360 cm− 1 is, however, attributed to the band corresponding to the Gd2O3 cubic phase [22], presented in the CeO2 + Gd2O3 mixture. It can be seen that after milling, the Raman band at 461 cm− 1 shifts to the lower frequency and gets broader with increasing of the milling times. Zhang et al. [23] reported that lattice expansion by decreasing the particle size explains this systematic change in the Raman peak, which can be attributed to the increasing concentration of the point defect. Raman characterization of 20GDC at different milling times is summarized in Table 2. Maximum broadening (FWHM) and red shift belong to the powders milled for 30 h. By increasing the milling time up to 40 h, the FWHM value was decreased and no further shift was seen. This observation suggests that there is a little change in the strength of Ce–O bands and additional milling time does not change the solubility of gadolinium in ceria. Besides the f2g Raman mode, one additional peak at low intensity of about 560 cm–1 is also observed. According to McBride et al. [24] this can be related to the extrinsic oxygen vacancies introduced into the ceria lattice by substitution of Ce+ 4 ions and intrinsic oxygen vacancies due to the nonstoichiometry of ceria nanopowders. In the Raman spectra of the sample milled for 60 h, another peak of about 980 cm− 1 can be observed, which could be due to a structural change occurring as a result of longer times of milling. However, at present, no absolute explanation can be given for this observation. Fig. 2. Raman spectra of CeO2 as received and 20 mol% Gd doped CeO2 powder after milling at room temperature.
spectra of these materials are shown in Fig. 2. According to the literature, CeO2 with a fluorite structure has one Raman active triply degenerate F2g mode at 464 cm− 1. This mode presents the symmetrical stretching vibrations of CeO8 vibration unit and should be very sensitive to the oxygen sublattice disorder. Only the oxygen atoms concerning this mode of vibration are very sensitive to the oxygen sublattice disorder resulting from the processes leading to the grain-size-induced non-stoichiometry [20]. Kosacki et al. [21] give the evidence for the stronger influence of substitution disorder over the stoichiometric one. They also showed that the broadening in Gd-substituted CeO2 is indeed
3.3. Morphology and microstructure Observations via scanning electron microscopy revealed that the milling process affects the particle morphology and sizes significantly. Fig. 3 shows the SEM micrographs of the powders milled for different times. During the milling, the particles constantly are impacted and fractured, leading to a considerable reduction of the particle size as a result of the energy provided during the ball milling. Initially, they formed larger aggregates, which were then broken up in further steps of the milling process. Consequently, uniform grain size distribution is observed with the increasing of milling time. It can be seen that the milled powder after 10 h (Fig. 3(b)) contains larger particles and agglomerates but with the increasing of milling time up to 20 h, the
Table 1 Mean grain size, micro-strain and lattice constant calculated from the XRD patterns, specific surface area and mean particle size obtained from the analysis of the CeO2–Gd2O3 samples after milling. Sample
Milling time (h)
Mean grain size (nm) with XRD
Mean particle size (nm) with BET
Micro-strain (%)
Lattice constant (Å)
Specific surface area (m2/gr)
Ceria as received Ceria Ceria + 20 mol% Gd Ceria + 20 mol% Gd Ceria + 20 mol% Gd Ceria + 20 mol% Gd Ceria + 20 mol% Gd
0 30 10 20 30 40 60
– 60 85 50 38 30 20
– – 88 66 50 48 45
– 0.2 0.16 0.25 0.4 0.47 0.7
5.411 5.409 5.411 5.414 5.419 5.419 5.418
1.62 – 9.39 12.29 16.56 16.89 18.21
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Fig. 3. SEM micrographs of CeO2 powder before milling (a), 20 mol% Gd doped CeO2 powder milled for 10 h (b), 20 h (c), 30 h (d) and 60 h (e).
particles' and agglomerates' sizes decrease (Fig. 3(c)).With further increasing the milling time, the microstructure becomes finer and more homogenous (Fig. 3(d)). Fig. 3(e) shows that with the increasing of milling time up to 60 h, the agglomerated particles, composed of nanoparticles, are more pronounced. The formation of solid solutions (both equilibrium and metastable) during mechanical alloying may be influenced by several factors, which is described as follows: Thermodynamic barriers have been discovered to be effective in the formation of a solid solution by MA process. Considering the formation of the disordered A (B) solid solution from a mixture of pure A and B elements (as the standard state), the Gibbs' free energy change can be presented as [22]: S
S
S
ΔG = ΔHM −TΔS
ð2Þ
where, ΔH SM and ΔS S are the enthalpy and entropy of mixing, respectively. T is the temperature at which a solid solution is formed. Also, for the formation of solid solution from the elemental powders,
ΔS S can be calculated with the assumption of configurational entropy of mixing: S
ΔS = RðxA ln xA + xB ln xB Þ
ð3Þ
where, R is the universal gas constant, and xA and xB are the mole fractions of elements A and B, respectively [25]. Entropy of mixing cannot be ignored when the particle size is less than a few hundreds of atoms, a case applicable to MA process where the particles could become finer [26]. The largest reduction in free energy occurs when the particles' size is in atomic level. Moreover, in MA process, plastic deformation occurs as well. Plastic deformation, in fact, refines the particle and grain size and increases the grain boundary area. The decreased particle size reduces the diffusion distances between the particles and facilitates pipe diffusion. Diffusion is further aided by the increased defect density and a local rise in temperature. The combination of all these effects would permit the diffusion to occur
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efficiently in the interfacial regions of the nanocrystalline grains to form solid solution. Many attempts have been made for the synthesis of various oxide systems by MA process. For example, J. Z. Jiang and coworkers investigated the synthesis of α–Fe2O3–SnO2 solid solution [27]. The high defect concentration and the chemical enthalpy of Fe3+–O 2−–Sn4+ interfaces between nanostructured α–Fe2O3 and SnO2 regions were suggested as the driving force for the formation of solid solution during the milling. In a similar attempt for the synthesis of stabilized zirconia by MA process [28], surface area and particle size were found to act as the major factors effective in the formation of stabilized zirconia solid solution. According to the results obtained in the present work, the influence of milling time on the decrease of crystallite size or the increase of surface area was more considerable up to 30 h (Table 1). In addition, the XRD results indicated that the powders undergo an increase in microstrain during various milling times (Table 1). Since the crystalline defects and structural disorders considerably favor diffusion and atomic rearrangements, they can, therefore, explain the increase in chemical reactivity of the powders during the milling. The results of the lattice constant calculated from the XRD study showed that the diffusion of Gd 3+ into the CeO2 phase occurred only after 10 h of milling. When the milling time increases, gadolinium ions diffuse further into the CeO2 phase, and after 30 h of milling, a solid solution is formed. Therefore, powder milling up to 30 h can cause the reduction of grain sizes, to an extent that facilitates the diffuse of Gd 3+ ions into the CeO2 phase where, finally it leads to the formation of 20GDC solid solution. 4. Conclusions Mechanical alloying of the powder mixture of CeO2-20 mol% Gd system was investigated and the obtained products were characterized by XRD, SEM, BET and Raman spectroscopy. The conclusions are as follows: 1- High-energy ball milling was found as a new process for the synthesis of GDC solid solution. 2- The powder product was pure, having homogenous microstructure and very fine particles containing nanocrystallites in the range of 50 nm. 3- A red shift, which is observed in the Raman spectra of the samples after 30 h of milling, could confirm the formation of solid solution. Moreover, a peak, which could be assigned to the presence of intrinsic and extrinsic oxygen vacancies, was also detected through the Raman studies. References [1] N.Q. Minh, Ceramic fuel cells, J. Am. Ceram. Soc. 76 (3) (1993) 563. [2] B.C.H. Steele, Appraisal of Ce1 − yGdyO2 − y/2 electrolytes for IT-SOFC operation at 500 °C, Solid State Ionics 129 (2000) 95.
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