survey of the quasi-ternary system

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Survey of the quasi-ternary systemLa0.8Sr0.2MnO3eLa0.8Sr0.2CoO3eLa0.8Sr0.2FeO3

F. Tietz*, I. Arul Raj 1, M. Zahid, A. Mai, D. Stover

Forschungszentrum Julich GmbH, IWV-1, D-52425 Julich, Germany

Abstract

An overview on the variation of the thermal expansion, the electrical conductivity as well as non-

stoichiometry of the oxide content as a function of composition within the quasi-ternary system La0.8Sr0.2MnO3ÀdeLa0.8Sr0.2CoO3ÀdeLa0.8Sr0.2FeO3Àd in air is presented. The various powders were synthesized

under identical conditions. The DC electrical conductivity values of the compositions at 800 C in air

vary in a wide range from 15 to 1338 S cmÀ1. The magnitude of electrical conductivity of the perovskites

is mainly determined by the percentage of cobalt in the compositions. A similar behaviour was observed forthe measured thermal expansion coefficients between room temperature and 1000 C in air, increasing from

10.9 to 19.4Â 10À6 K À1 as a function of cobalt content. Changes in the oxygen stoichiometry of the mate-

rials were characterized by temperature-programmed oxidation measurements.

Ó 2007 Elsevier Ltd. All rights reserved.

1. Introduction

ABO3 perovskites have attracted extensive interest owing to the need for the fundamentalknowledge on their physical properties for their application as heterogeneous catalysts in chem-

ical processes, as ceramic gas separation membranes and as electrodes in solid oxide fuel cells

(SOFC).

Currently attention is focussed on materials with compositions within the quasi-ternary

system La1À x Sr

x MnO3ÀdeLa1À x

Sr x

CoO3ÀdeLa1À x Sr

x FeO3Àd, since they exhibit electronic or

mixed conductivity as a function of composition, making them relevant for application as

* Corresponding author. Tel.: þ49 2461 615007; fax: þ49 2461 612455.

E-mail address: [email protected] (F. Tietz).1 On leave from Central Electrochemical Research Institute, Karaikudi 630006, India.

0079-6786/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.progsolidstchem.2007.01.028

Progress in Solid State Chemistry 35 (2007) 539e543www.elsevier.com/locate/pssc

mailto:[email protected]

http://www.elsevier.com/locate/pssc

http://www.elsevier.com/locate/pssc

mailto:[email protected]



cathode or cathode/interconnect contact material in SOFC, e.g. Refs. [1e7]. Published litera-

ture data on the physical properties of different compositions in this system reveal that different

authors adopt different synthesis and measurement procedures, thereby reporting data with

wide variation [3]. Among the La1À x Sr

x (Co,Fe)O3Àd, La1À x

Sr x

(Co,Mn)O3Àd and La1À x Sr

x

(Fe,Mn)O3Àd series, x ¼ 0.2 is reported to exhibit an acceptable electrical conductivity andmatching thermal expansion coefficient (TEC) [2e5] as well as good chemical compatibility

with yttria-stabilized zirconia electrolytes [6] under SOFC conditions. Therefore in this work

measurements on electrical conductivity, thermal expansion and changes in the oxygen stoichi-

ometry of compositions in the La0.8Sr0.2MeO3Àd (Me¼ Co, Fe, Mn) system are made under

identical synthesis and processing conditions in order to evaluate the variation of these proper-

ties as a function of temperature.

2. Experimental

Nineteen powders with different compositions within the La0.8Sr0.2MeO3Àd (Me¼Co, Fe,

Mn) system (black dots in Figs. 1e3) were synthesized by the Pechini method using nitrate

solutions of La, Sr, Co, Mn and Fe in the corresponding metallic ratios. A detailed description

of the synthesis process is given in Ref. [8]. The experimental procedures of the chemical and

X-ray diffraction (XRD) analyses as well as DC electrical conductivity measurements have

been described in Refs. [7,9]. The raw powders calcined at 600 C were uniaxially pressed

to form rectangular bars and sintered at 1300 C for 6 h. The thermal expansion between 30

and 1000 C was determined using a Netzsch DIL 402C dilatometer.

Changes in the oxygen stoichiometry of the compositions during heating-up at a p(O2) of 0.21 bar

were characterizedby temperature programmed reduction(TPR), using a flowing streamof He mixed

with 21% of oxygen. A more detailed description of the process can be found in Ref. [10].

Fig. 1. Contour plot of electrical conductivity of La0.8Sr0.2(Co,Fe,Mn)O3Àd perovskites at 800 C in air. The lines cor-

respond to a fitting of the experimental data with a full cubic function and have an interval of 100 S cm À1. In the legend

only the bold lines are listed.

540 F. Tietz et al. / Progress in Solid State Chemistry 35 (2007) 539e 543



3. Results and discussions

The chemical analysis data obtained on the powders calcined at 600 C for 3 h revealed

close agreement between the targeted and the obtained composition within the experimentallimitation. The XRD patterns obtained on the powders sintered at 900 and at 1100 C showed

Fig. 2. Contour plot of thermal expansion coefficients of La0.8Sr0.2(Co,Fe,Mn)O3Àd perovskites between room temper-

ature and 1000 C in air. The lines correspond to a fitting of the experimental data with a full cubic function.

Fig. 3. Contour plot of the oxygen stoichiometry change of La0.8Sr0.2(Co,Fe,Mn)O3Àd perovskites between 30 and

1000 C in a p(O2) of 0.21 bar. The lines correspond to a fitting of the experimental data with a full cubic function

and have an interval of 0.005 mol/formula unit. In the legend only the bold lines are listed.




that the perovskite phase crystallized as main phase at both temperatures. DC electrical conduc-

tivity measurements carried out at 800 C in air on the samples sintered at 1300 C for 6 h vary

in a wide range from 15 to 1338 S cmÀ1 and are presented in Fig. 1. These results were

discussed in detail previously [7,9].

The thermal expansion coefficients (TECs) were determined at temperatures between 30 and1000 C in air and are presented in Fig. 2. The lines shown in this figure are calculated contour

lines with equal TEC levels expressing the variation of the TEC as a function of the composi-

tion in the ternary system. The lowest TEC values were measured in the series La0.8Sr0.2(Mn,Fe)O3Àd. The variation of the measured TEC along this binary system was relatively

low and ranged from 10.9 to 12.3 Â 10À6 K À1 with the minimum observed for La0.8Sr0.2Fe0.5Mn0.5O3Àd.

An increase in cobalt content in the compositions led to a gradual increase in TEC values

reaching a maximum of 19.4Â 10À6 K À1 for La0.8Sr0.2Fe0.125Mn0.125Co0.75O3Àd. This increase

is similar to the behaviour observed in the electrical conductivity measurements [7]. A study of

the contour lines revealed that equal values of TEC follow a direction which is almost parallel

to the series La0.8Sr0.2(Mn,Fe)O3Àd. A given composition in the series La0.8Sr0.2(Co,Fe)O3Àd

exhibited higher TEC values than the corresponding composition in the series La0.8Sr0.2(Co,Mn)O3Àd. The TEC values for La0.8Sr0.2CoO3Àd, La0.8Sr0.2MnO3Àd and La0.8Sr0.2FeO3Àd

were 19.1, 11.8 and 12.2Â 10À6 K À1, respectively, showing the strong impact of cobalt on ther-

mal expansion. The TEC values of the compositions in the La0.8Sr0.2MnO3ÀdeLa0.8Sr0.2CoO3Àd series as reported in Ref. [2] differ from our results and are significantly smaller

than the values shown in Fig. 2. The values reported here, however, are in very good agreement

with other measurements on lanthanum manganites [11 and references therein].

The oxygen stoichiometry changes,d

, were determined by heating the powders up from30 to 1000 C in a gas stream with a p(O2) of 0.21 bar. Fig. 3 shows contour lines with

equal d values, calculated from the measured values. The measured values are rather low

in comparison to measurements in lower oxygen partial pressures (e.g. 10À4 bar) or to

materials with higher Sr contents. Because of these low stoichiometry changes, the results

have to be treated carefully, as the stoichiometry changes are at the detection limit of the

TPR method.

Nevertheless, some trends are obvious: For the binary systems La0.8Sr0.2(Mn,Co)O3Àd and

La0.8Sr0.2(Fe,Co)O3Àd, the oxygen non-stoichiometry d is increasing with increasing cobalt

content. The maximum value for d was measured for La0.8Sr0.2Fe0.25Mn0.75O3Àd, while the

minimum values were measured for La0.8Sr0.2MnO3À

d and La0.8Sr0.2Fe0.75Mn0.25O3À

d. In theTPR profiles (not shown here), there was a clear difference visible in the temperature depen-

dence of the oxygen release: While the La0.8Sr0.2(Fe,Co)O3Àd and La0.8Sr0.2MnO3Àd

compounds showed distinctive peaks for the oxygen release, the ternary compounds

La0.8Sr0.2(Fe,Co,Mn)O3Àd showed a more continuous, gradual release of oxygen.

4. Conclusions

The quasi-ternary system La0.8Sr0.2MnO3ÀdeLa0.8Sr0.2CoO3ÀdeLa0.8Sr0.2FeO3Àd has been

investigated using 19 different compositions to elucidate the dependence of electrical conduc-tivity, thermal expansion and changes of oxygen non-stoichiometry with composition. All three

properties show a strong influence of the cobalt content, the lowest values for these three prop-

erties were always found in the binary series La0.8Sr0.2MnO3ÀdeLa0.8Sr0.2FeO3Àd.




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