Low-Temperature Combustion Synthesis and Characterization of

Ce1-xPrxO2-δδδδ Solid Solutions

Cheng Peng1, a, Zhen Zhang1 , Donglin Huang1 and Yanli Liu2 1 College of Chemistry, South China University of Technology, Guangzhou 510640, China

2College of Materials Science and Engineering, Hunan University, Changsha 410082, China

a chengpen@scut.edu.cn

Keywords: Nanocrystalline powders, Solid electrolyte, Combustion synthesis, Cerium oxide.

Abstract. Pr2O3-doped ceria nanopowders were synthesized by a nitrate-citrate combustion process. This

route is based on the gelling of nitrate solutions by the addition of citric acid and ammonium hydroxide,

followed by an intense combustion process due to an exothermic redox reaction between nitrate and

citrate ions. XRD analysis showed that no impurity were observed up to x=0.3 in Ce1-xPrxO2-δ systems.

The influence of ignition temperature on the characteristics of the powders was studied. The change of the

crystal structure with the content of doped Pr was investigated. The highest ionic conductivity,

σ600°C=2.45×10-3S/cm, was found for the composition of x=0.15.

Introduction

Yttria-stabilized zirconia (YSZ) is the most important material to be used as solid electrolyte in solid

oxide fuel cells (SOFCs) operating at high temperatures [1-3]. Such high temperature would lead to many

technological problems, such as mechanical instability, materials aging, and undesirable chemical

reaction between cell components (electrolyte, electrode and interconnecting materials). If the operation

temperature could be lowered to 800°C, it will prolong the life-span of cells and reduce the production

cost [4,5]. Among the new oxides considered as possible alternative to YSZ, the doped cerium oxide

turned out to be among the most promising ones. Many researches have been focused on this system [6-8].

The majority of past work has been undertaken on doped-ceria using traditional solid-state techniques

which has caused some troubles [9]. For example, the density of the samples is less than 95% of the

theoretical. Moreover, the samples should be sintered at 1600-1650°C, which is costly and can allow the

loss of ceria due to the high mobility of Ce4+ at these temperatures.

In this paper we present results of a systematic study of the structure, electrical conductivity and

thermophysical properties of Ce1-xPrxO2-δ solid solutions prepared by gel-combustion method.

Experimental Procedure

According to the stoichiometric ratios of Ce1-xPrxO2-δ, Ce(NO3)3 and Pr(NO3)3 solutions were mixed,

citric acid was then added in a proportion of 2 mol/mol of metal atom and the pH of the solution was

adjusted to pH≈8 by adding ammonium hydroxide. The solution was vaporized in a water bath at 60-70°C

and finally it became a transparent gel. The resulting gel was heated on a hot plate until it turned into a

black viscous mass, which on continued heating burned due to a vigorous exothermic reaction. Shallow-

yellowish ashes obtained after combustion were treated at 500°C in a muffle furnace. The product was

pressed into pellets. Finally, the pellets were sintered at 1300°C for 10h and cooled to room temperature.

The thermal decomposition in air of the dried gel precursor was investigated by means of differential

thermal analysis (DTA) and thermogravimetric analysis (TGA) in the temperature range of 25- 800°C

with a heating rate of 10°C /min in an oxidizing atmosphere (N2 + 21% O2, flow rate 50ml/min).

The room and high temperature powder X-ray diffraction patterns of the ultrafine powders were

obtained with CuKα1 radiation. Cell parameters were calculated by the CELL program [10]. The

reflection from the (111) plane was used for the determination of the average crystallite size with the

Scherrer formula: D=0.9λ/βcosθ, where λ is the wavelength of the X-rays, θ is the diffraction angle,

β=(β2m-β

2s)1/2 is the corrected halfwidth, βm, of the (111) reflection in samples of Ce1-x PrxO2-δ, and βs is

the halfwidth of the (111) reflection in a standard sample of CeO2 (D~100nm).

Key Engineering Materials Vols. 368-372 (2008) pp 253-255Online available since 2008/Feb/11 at www.scientific.net© (2008) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.368-372.253

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The sintered pellets were coated with platinum. The

electrical conductivity of the coated pellets was

measured at different temperatures using a frequency

response analyzer.

Results and Discussion

Fig. 1 shows the XRD spectra of the fluorite-phase

evolution for Ce0.8Pr0.15O2-δ during gel-combustion as a

function of heat-treatment temperature. As can be seen,

no evidence of impurity appeared in the XRD data at

160°C, but compared with that of well-crystallized CeO2 the peak width is wide and the intensity is weaker. This

suggests that at this temperature, which is significantly

lower compared with traditional solid-state techniques

[11] and lower than that used in hydrothermal synthesis

[12], the samples can form a cubic fluorite structure. It is

also observed in Fig.1 that with increasing ignition

temperature the half-width of diffraction peaks become

narrower and their intensity gets stronger.

Fig.2 shows the XRD patterns of Ce1-xPrxO2-δ sys-

tems, no impurity were observed up to x=0.3. Its crystal

structure was calculated using the CELL program. The

results indicated that all the samples can be indexed to

cubic structure. Fig. 3 shows the change of the calculated

cell parameter and unit volume with the substitution x.

The dissolution of the Pr2O3 in the fluorite lattice is

obvious from the increase in lattice constant and unit

volume. As can be seen in this figure, the lattice

parameter and unit volume increased sharply at x<0.15,

but it was maximal and almost constant at x ≥ 0.15. The

lattice expansion agreed perfectly with effective ionic

radii consideration (rPr3+=0.1266nm, rCe4+ = 0.1110nm)

[13]. The constant lattice parameter at 0.15 ≤ x ≤ 0.3

implies that the solubility limit of praseodymium is

somewhere between 0.15 and 0.3.

The average crystallite size, D, of Pr-doped ceria

powders were between 12-26 nm. The colossal surface

area of the ultrafine powder enhanced greatly the

momentum of sintering and the rate of diffusion was

improved and the path of diffusion was shorten. All these

factors contribute to accelerate the course of sintering.

The ultrafine substituted ceria powders were sintered

into pellets at 1300°C with apparent densities of over 95% of the theoretical value (Table 1).

Table 1 Lattice parameter, crystallite sizes and density of specimens used in this study.

Composition x a (nm) D(nm) Percent Bulk true density (%)

0.05

0.1

0.15

0.2

0.25

0.3

0.54129

0.54189

0.54190

0.54018

0.54071

0.54200

12

19

25

26

22

23

95.82

97.91

96.25

97.26

98.03

96.12

Fig.1 Thermal evolution of the powder

XRD spectrum of Ce0.85Pr0.15O2-δ powder.

Fig.2 XRD patterns of Ce1-xPrxO2-δ solid

solutions.

Fig.3 Lattice parameter and unit volume of

Ce1-xPrxO2-δ as a function of x.

254 High-Performance Ceramics V

Fig.4 is the temperature dependent XRD patterns of

the Ce1-xPrxO2-δ powder. No phase transition happened

from room temperature to 800°C and all diffraction peaks

systematically shift to lower angles when increasing

temperature. The 8 X-ray diffraction lines obtained at

different temperature between 20°≤2θ≤80° can be

indexed on a cubic unit cell and the calculated results

show that all the samples belong to F23 S.G.

Pure ceria oxide is basically a poor oxide ion

conductor (σ600°C ~10-5S/cm). Fig.5 shows that the ionic

conductivities are significantly enhanced in Ce1-xPrxO2-δ

solid electrolytes. Meanwhile, linear Arrhenius plots,

ln (σ, T) = ln A - Ea/kT, over a wide temperature range

indicate the presence of only one mode of oxide ion

conduction in the substitution range of Pr . The highest

ionic conductivity is found for the composition of

x=0.15,σ600°C =2.45×10-3S/cm and Ea=0.85eV. The

decrease in the ionic conductivity for x>0.15 is due to the

formation of defect associations.

Summary

A nitrate-citrate combustion route to synthesize

Ce1-xPrxO2-δ (x=0∼0.3) solid solutions has been

investigated. Ultrafine particles of uniform crystallite

dimension, ~ 12-26nm, were formed by this synthesis

technology. Because of the small particle size of the

doped ceria, the sintering temperature needed to obtain a dense ceramic pellet was reduced substantially

from 1600°C required for the corresponding materials prepared by conventional solid state methods, to

~1300°C. High temperature x-ray diffraction measurement showed that no phase transition happened

from room temperature to 800°C. The highest conductivity was found for the x=0.15 Pr substituted ceria

(σ600°C =2.45×10-3 S/cm, Ea=0.85 eV).

Acknowledgments

This work was supported by the Nature Science Young Teacher Fund of South China University of

Technology (No.B15-E5050650) and the Student Research Program of South China University of

Technology.

References

[1] O. Yamamoto: Electrochim. Acta. Vol. 45 (2000), p. 2423.

[2] B.C.H. Steele: J. Mater. Sci. Vol. 36 (2001), p. 1053.

[3] V. V. Kharton, A. P. Viskup, I. P. Marozau, E. N. Naumovich: Mater. Lett. Vol. 57(2003), p. 3017.

[4] A. B. Stambouli, E. Traversa: Renew. Sustain. Energy Rev. Vol. 6 (2002),p.433.

[5] W. Huang, P. Shuk, M. Greenblatt: Solid State Ionics, Vol. 100 (1997), p.23.

[6] G. Chiodelli, L. Malavasi, V. Massarotti: Solid state ionics, Vol. 176(2005), p.1505.

[7] D. Hirabayashi, A. Tomita, S. Teranishi: Solid state ionics, Vol. 176(2005), p.881.

[8] B. Zhu, I. Albinsson, C. Andersson: Electrochemistry Communications, Vol. 8(2006), p.495.

[9] L. E. Shea, J. McKittrick, O. A. Lopez: J. Am. Ceram.Soc. Vol.79(1996), p.3257.

[10] Y. Takaki, T. Taniguchi, K. Nakata and H. Yamaguchi: J. Ceram. Soc. Jpn. Vol. 97 (1989), p. 763.

[11] H.Yahiro, Y. Eguchi, K.Eguchi, H. Arai: J. Appl. Electrochem. Vol.18(1988),p.527.

[12] W. Huang, P. Shuk, M. Greenblatt: Chem. Mater. Vol.9(1997), p. 2240.

[13] R. D. Shannon: Acta Cryst. Vol. A32 (1976), p. 751.

Fig.4 High temperature PXD patterns of

Ce0.85Pr0.15O2-δ powder.

Fig.5 Arrhenius plots of the ionic

conductivity of Ce1-xPrxO2-δ solid

solutions.

Key Engineering Materials Vols. 368-372 255

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