Defect Thermodynamic and Transport Properties

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ORIGINAL PAPER

Defect thermodynamic and transport properties

of nanocrystalline Gd-doped ceria

S. Surble & G. Baldinozzi & M. Doll & D. Gosset &

C. Petot & G. Petot-Ervas

Received: 7 September 2007 /Accepted: 2 October 2007 /Published online: 29 October 2007# Springer-Verlag 2007

Abstract Nanocrystalline CeO2-doped (5, 7.5, 10, and

15 mol%) Gd2O3 powders, with a particle size of about

17 nm, were synthesized through the combustion of glycine/

nitrate gels. Dense nanocrystalline materials were obtainedby hot uniaxial sintering. Optical microscopy, scanning

electron microscopy and transmission electron microscopy

examinations, as well as X-ray diffraction analyses, have

allowed us to characterize these polycrystals. The grain sizes,

included between10 and 80 nm, depend on both the sintering

temperature and the amount of dopant. A comparison of the

transport properties of these nanocrystalline samples to the

values obtained with coarsened grained materials of same

composition shows that the ionic conductivity passes through

a maximum for mean grain sizes included between 300 and

500 nm. Furthermore, an enhancement of the ionic conduc-

tivity is observed when the amount of dopant increases. This

was attributed to a grain-size-dependent gadolinium segrega-

tion at the periphery of the grains confirmed by X-ray

photoelectron spectroscopy characterizations.

Keywords Yttria-doped Ceria .Nanocrystalline materials .

Segregation . Ionic conductivity. SOFC

Introduction

Solid oxide fuel cells are regarded to be among the cleanest

energy conversion systems. However, to be economically

competitive, it is necessary to lower their temperature

operation in the range 500600 C. This will allow not only

to minimize the cost of fabrication through the use of less

expensive materials for interconnections, heat exchangers,

and structural components but also to improve theperformance of the cells due to a lower degradation rate

of components. To realize these requirements, this implies

new system concepts and the use of electrode and

electrolyte materials with high oxygen ion conductivity,

good mechanical stability, and higher performance at the

electrode levels (solid electrolyte/electrode material inter-

face) to make the electrode over potential losses acceptable.

In the last decade, nanocrystalline materials are considered

to have a high potential to overcome these constraints. They

possess a wide range of original properties, including

enhanced mechanical properties and chemical reactivity,

useful in various technological fields, such as in catalysis

and in fuel cells. Nevertheless, to date, their long term be-

havior is unknown, their physico-chemical stability is not

fully understood yet, and the impact of this change of scale

may have on the transport properties remains unclear.

The aim of this paper was then to gain an understanding

on the influence of the nanostructure on the transport

properties. Gd-doped dense nanocrystalline ceria was

chosen due to its higher ionic conductivity as compared to

yttria-stabilized zirconia and to its enhanced electrode

reactions due to the catalytic effect of the Ce+3/Ce+4

couple.

Its electrical conductivity has been characterized in direct

comparison with polycrystalline specimens whose grain

sizes were in the micrometer range. The set of results is

analyzed, taking into account the segregation of gadolinium

at the periphery of the grains.

Sample synthesis and characterization

The starting materials were Gd-doped-CeO2 nanocrystalline

powders prepared by the combustion technique. A mixture

Ionics (2008) 14:3336

DOI 10.1007/s11581-007-0169-9

Paper presented at the 11th EuroConference on the Science and

Technology of Ionics, Batz-sur-Mer, France, 915 Sept. 2007.

S. Surble : G. Baldinozzi : M. Doll : D. Gosset: C. Petot:

G. Petot-Ervas (*)

Research Group CNRS/SPMS- Ecole Centrale Paris,

92295 Chtenay Malabry/SRMA-CEA Saclay,

91191 Gif sur Yvette, France

e-mail: [email protected]


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[1] of glycine and gadolinium nitrate (Gd(NO3)3.6H2O) was

added to an aqueous solution containing 0.1 mol of Ce

(NO3)3.6H2O to obtain the desired Ce/Gd molar ratio and a

molar ratio of 2 between nitrate ions and glycine. This ratioallows to control the combustion temperature near 1,100 C

and to achieve a uniform doping of the obtained powders

while controlling their size. Highly homogeneous and non-

aggregated powders were obtained after calcinations at 500 C

in air. X-ray diffraction shows that the powders are a single

phase and that the grain sizes is below 17 nm. Transmission

electron microscopy (TEM) observations confirm this result

(Fig. 1). They show that the grains consist of well-

crystallized single crystals having regular shapes and sizes

between 5 and 17 nm.

Calcined powders were sintered into pellets under

250 MPa uniaxial pressure at 750 C in WC dies. A seriesof samples (10 mol% Gd2O3) was also sintered at 850 C to

study the influence of the sintering temperature. The

polycrystals have densities >95% of the theoretical density.

Scanning electron microscopy (SEM) shows that all samples

have an average grain size less than 100 nm, which depends

on the amount of dopant. As an example, Fig. 2 displays the

microstructures of two samples sintered at the same

temperature but doped with a different amount of Gd (7.5

and 15 mol% Gd2O3). This figure clearly shows that the

more doped sample exhibits the smaller grain size, a

tendency which is observed throughout all the compositions.

Moreover, for a given composition, the grain size increaseswhen both the sintering temperature and the annealing time

are increased.

In Fig. 3, we have reported the microstructure of a

reference ceria sample doped with Gd (10 mol% Gd2O3) and

obtained by natural sintering of nanocrystalline powders at

1,400 C during 2 h [2]. These sintering conditions lead to a

final average grain size of about 500 nm (Fig. 3a).

Furthermore, the TEM image shows the absence of second

phase along grain boundaries.

Fig. 2 Microstructural characterization (SEM) of the Gd-doped ceria

polycrystals containing 7.5 (a) and 15 mol% Gd2O3 (b) sintered at

750 C under 250 MPa

Fig. 1 Microstructural charac-

terization (TEM) of Gd2O3(7.5 mol%)-doped ceria powder

synthesized by the combustion

technique. Left panel: bright

field image showing the narrow

distribution of the particle sizes.

Right panel: dark field image

showing that each grain is gen-

erally a single crystal

34 Ionics (2008) 14:3336


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Results

The electrical conductivity was characterized by complex

impedance spectroscopy [2, 6] using silver as an electrode

material (=g/R, where g=l/s). In Fig. 4, we have reported

the results obtained with nanocrystalline samples doped with

different Gd amounts and sintered according to the con-

ditions detailed above. These results are compared to those

of a reference sample (R) doped with 10 mol% Gd2O3(Fig. 3) and obtained by natural sintering at 1,400 C for 2 h.

These results clearly show that the ionic conductivity of the

nanocrystalline samples, with an average grain size lower than

150 nm, is smaller than the ionic conductivity of the reference

material R. They also show that the ionic conductivity

depends on both the grain size and the Gd composition.

As shown in Fig. 5, the ionic conductivity of polycrys-

talline specimens, whose grain sizes are in the micrometer

range, depends not only on the cooling rate at the end of

sintering (P8 cooled at a higher cooling rate than that of P6)

but also on both the grain size and the Gd amount, as the

Fig. 3 a SEM image of a fracture surface of a Gd (10 mol% Gd2O3)-

doped ceria reference polycrystal (R) sintered at.1,400 C for 2 h

showing the distribution of grain sizes and b TEM image showing the

absence of precipitates at grain boundaries

Fig. 4 Influence of the grain size andof the amount of gadolinium on the

ionic conductivity of nanocrystalline ceria. Comparison with the values

obtained with a polycrystal R prepared by conventional sintering

Fig. 5 Influence of the grain size and of the amount of gadolinium on

the ionic conductivity of microcrystalline Gd-doped ceria

Ionics (2008) 14:3336 35


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nanocrystalline materials. However, it should be noted that

the ionic conductivity of these samples increases as the

grain size decreases. Therefore, according to the results

reported in Fig. 4, the ionic conductivity goes through a

maximum that occurs at about 300500 nm for the CeO2-

doped (10 mol%) Gd2O3 polycrystals.

Discussion

To explain these results, it is important to take into account the

occurrence of kinetic demixing phenomena during the sinter-

ing process [2, 6]. The X-ray photoelectron spectroscopy

(XPS) analyses results reported in Table 1 show the influence

of the grain size, of the amount of Gd, and of the cooling rate

at the end of sintering on the amount of Gd segregated at the

periphery of the grains (depth close to 0.2 nm).

As already observed in previous studies [36], the

kinetic demixing effects are less pronounced when the

grain size decreases. It is also interesting to notice (Table 1)

that whatever the nominal amount of dopant in the sample,

for a given grain size, a close Gd enrichment in the first

monolayers is observed (P6, P8, P9). Furthermore, a slight

reduction of the kinetic demixing effect occurs when the

cooling rate at the end of sintering is faster (P6< P8).

When one considers the results in Table 1 and the ionic

conductivity values reported in Figs. 4 and 5, it seems

reasonable to conclude that the ionic conductivity of these

samples, mainly controlled by the grain boundaries in the

temperature range investigated, is affected by the segrega-

tion of Gd at the periphery of the grains. In the nano-

crystalline samples, the effects of kinetic demixing are

strongly reduced as shown previously in Mg-doped alumina

[5]. In fact, the ratio between surface area and volume is

much larger in these samples, characterized by very small

grain sizes. In this case, it is not possible to achieve the

strong Gd segregation observed in the microcrystalline

samples simply because of the large areas of the nano-grain

surfaces. Therefore, the present results obtained with the

ceria polycrystals doped with 10 mol% Gd2O3 show that an

optimum for the ionic conductivity is obtained for a grain

size small enough (300500 nm) to provide a significant

fraction of grain boundaries in the sample but large enough

to provide a significant enrichment of gadolinium (close to

Gd/Ce 0.9) near the grain boundaries.

Conclusion

In conclusion, a comparison between the ionic conductivity of

dense nanocrystalline Gd-doped ceria samples (grain size

Defect Thermodynamic and Transport Properties

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