s . __ + __ Ii!3 SOLiD STATE IONICS

ELSEVIER Solid State Ionics 99 (1997) 287-295

A dilatometric study of the La,.,Sr,_,MnO, sintering behaviour

A. Poirson, P. Decorse, G. Caboche, L.C. Dufour”

Universite’ de Bourgogne, Laboratoire de Recherches sur la Riactivite’ des Solides (L.RRS), UMR 5613 CNRS-UB, UFR Sciences et

Techniques, BP 400, 21011 Dijon cedex, France

Received 21 October 1996; accepted 26 March 1997

Abstract

The sintering behaviour of La,,,Sr,,,MnO, has been studied by dilatometry between 1100 and 1800 K in various oxygen potentials [pure oxygen, air and nitrogen (PO, = 1 X lo-’ bar)]. The starting material was prepared by spray pyrolysis of

aqueous solutions of nitrates. Two classical sinter stages, neck formation and grain growth respectively, were evidenced from dilatometric curves and this result was corroborated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses. The rate of densification was found to be directly dependent on oxygen partial pressure, the best conditions being in nitrogen. These results are discussed in terms of vacancy diffusion and oxygen stoichiometry linked to two factors: the initial (La+Sr)/Mn ratio and the Mn3+ /Mn4+ ratio determined both by temperature and oxygen partial pressure.

Keywords: Sintering; Dilatometry; La,,,Sr,,MnO,

1. Introduction

The electrical [l-4] and magnetic [5-71 properties

of the La,_,Sr,MnO,,, compounds have been inten- sively studied. The applications of such materials as cathodes or interconnectors in the high-temperature fuel cells (HTFC) have been noticed and tested recently. The current development of investigations on the magneto-resistive properties of these oxides attests this interest. La,.,Sr,.,MnO, is a compound relatively important in the La,_,Sr,MnO,,, series, candidate as cathode material of solid oxide fuel cells (SOFC). This compound presents a good compromise between high electrical conductivity [8] and good thermodynamic stability in oxidizing at-

*Corresponding author: Tel.: +33 3 8039 6153; fax: +33 3

8039 6132; e-mail: dufour@satie.u-bourgogncfr

mosphere and its thermal expansion coefficient is close to that of yttrium stabilized zirconia (YSZ), one of the classical high temperature solid elec- trolytes. As usually observed with solids, its electri- cal conductivity is strongly dependent on both density and microstructure and, therefore, the factors controlling its porosity and densification should be perfectly identified in order to improve the properties of this material. Only few articles, recently pub- lished, aim at understanding the sinter behaviour of the La 1 -3,MnO,,, oxides [&lo]. In this paper, a dilatometric study of the high temperature La,_,Sr,_,MnO, behaviour is reported showing clear- ly the various sinter stages. It is completed by scanning electron microscopy (SEM) observations and X-ray diffraction (XRD) analyses. For the sake of simplicity, we use the formal notation La,,,Sr,,,MnO, but it is obvious that the exact

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288 A. Poirson et al. / Solid State Ionics 99 (1997) 287-295

oxygen content depends on both the relative cationic concentrations and the thermodynamical conditions fixing the Mn4+ concentration excess.

2. Experimental

The La,,,Sr,,,MnO, powder (from S.S.C., Seattle, USA) was elaborated by using a spray pyrolysis way [l 11. An aqueous solution of lanthanum, strontium and manganese nitrates was prepared in the pro- portions 4:1:5 for the cationic concentrations. Fuel additive (glycine) was added in amount required for

complete combustion into metal oxides. The solution was spray-dried and calcinated in a continuous stream. The powder was formed at the end of this calcination.

According to the X-ray diffraction patterns (Fig. la and b) recorded with a curve position sensitive detector, Inel CPS 120, using the Cu Ko, radiation (h = 0.154051 nm) and to chemical analysis (Table l), the powder obtained in these conditions was single phase with a cationic ratio (La+ Sr)/Mn = 0.986, showing a slight excess of manganese. Im- purities like Al and Si were found in very small amount. The X-ray diffraction (XRD) pattern of the

In

,$j 1200 9

$ lOOO- &

c

2 800-

.r” 600-

: !x f 400-

Fig. 1. (a) X-ray diffraction pattern of the initial La, $r, ,MnO, powder. (5) X-ray diffraction pattern of La,,,Sr, ,MnO, after sintering at

1800 K.

A. Poirson et al. I Solid State Ionics 99 (1997) 287-295 289

Table 1

Cationic concentrations in the initial powder

La Sr Mn (La+Sr)/Mn

Theoretical composition

Experimental composition

0.8 0.2 1 1

0.795kO.015 0.198kO.003 1.007-t0.019 0.986+-0.016

as-received powder (Fig. la) showed an important overlapping of some peaks due to the broadening related to the very small mean size of the crystallites. This size distribution was measured by laser granulometry (Malvern 4700C) after ultrasonic treat- ment in pure water. It was narrow with a mean value of particle size of 140 mn (Fig. 2). As evidenced by scanning electron microscopy (JEOL JSM 64OOF), these particles were gathered in agglomerates of 50-100 pm (Fig. 3) by using a specific additive in order to improve both powder pressing and shaping. After sintering at 1790 K and as a result of the grain coarsening, the XRD pattern (Fig. lb) exhibited very fine peaks with positions and intensities as expected from the JCPDS file Nr 40-l 100 [12].

The samples were shaped and pressed into cylin- drical rods using special latex tubes and an isostatic press (Top Industrie) under a pressure of 300 MPa. The pressed rods were heated up to various tempera- tures between 1100 and 1790 K under nitrogen (P,*<lxlo-5 bar), air and oxygen (gas flow rate

Fig. 3. Agglomerates of initial La,,Sr,,,MnO, crystallites as

observed by scanning electron microscopy.

-10 1 h-r) with a rate of 2 K min-’ and, after 5 hours at constant high temperature, cooled down with a rate of 10 K min-' . The dilatometric curves were recorded with a dilatometer Netzsch TASC 41412.

40 1 I I I I _ 140nm.

& ,

m E 30 -

: z P

20 -

0 -4

I I I I 1 0 0.5 1 1.5 2 2.5 3

Diameter / pm.

Fig. 2. Powder size distribution as determined by laser granulometry.

290 A. Poisson et al. I Solid State Ionics 99 (1997) 287-295

3. Dilatometric results

3.1. Sintering behaviour in air as a function of temperature

in the regions II and III, two specific dilatometric curves were performed in the same conditions of rate as previously but keeping the sample at constant temperatures from 1070 K and 1580 K respectively with a plateau time of five hours.

A typical dilatometric curve of sintering between room temperature and 1790 K under air conditions (Fig. 4) is composed of three different regions.

Region I: between room temperature and around 1060 K, the only thermal expansion is visible leading to a coefficient of thermal expansion of 11.6 X 10e6 K-‘. Although the grain growth is not negligible up to 1060 K (Fig. 5a), no shrinkage was detectable and the sintering behaviour can be considered not being modified during this first part.

Recorded in the region II of sintering, the di- latometric curve corresponding to the plateau at 1070 K (Fig. 7) showed a classical sinter behaviour where the relative decrease in length AL/L, at constant temperature is a function of time. The first sintering stage due to neck formation accompanied by densifi- cation and identified by SEM (Fig. 5b) is in complete agreement with the dilatometric measurements and the experimental results reported by Van Roosmalen et al. [9].

Region II: between 1060 and 1470 K, shrinkage appears and sintering begins. As observed with crystallites treated at about 1310 K, the grain growth is more important in this region (Fig. 5b).

Region III: for temperatures higher than 1470 K, the shrinkage rate increases and the grain growth becomes very important (Fig. 5~). An estimation of the crystallite size was done from the SEM observa- tions at different temperatures (Fig. 6). It can be noticed that this size is little modified up to 1500 K but strongly grows between 1500 to 1820 K in total agreement with the dilatometric information.

Recorded in the region III of sintering, the di- latometric curve with the plateau at 1580 K (Fig. 8) clearly evidenced the two sintering stages. Up to 1470 K the first stage of sintering was present (region II, see above). Above this temperature the sinter behaviour changed, exhibiting a steeper shrinkage curve (region III). This second sinter stage due to grain growth was identified by SEM (Fig. 5c) and corroborated the study by Van Roosmalen et al [9]. However, during the sinter time at 1580 K, shrinkage carried on and died down denoting that the grain growth rate was smaller than the heating rate.

In order to better understand the sinter behaviour If the dilatomett-ic curves in Fig. 4 and Fig. 8 are

0.05, I I I , I I I , I I I , I I I , , ‘\’ I , I

dL/Lo : / /

/ 0 ~.....__.__.__.___ ._._.. _.-._ . ..** ,/” \ .* . .

*. -. / \ -0.05 -

,Y... ,’ ‘.

/ ‘.

,/

: -0.1 -

/’ 2;

\ ; \

/ -0.15 -

1’ i

,/’ /

1;

-0.2 - i

-,,i

1, I

~.__._______.____.________ -_ - -0.25 * ’ ’ ’ ’ ’ ’ ’ r 1 s ’ ’ ’ ’ ’ s m ’ ’ m s B ’ ’

0 200 400 600 800 1000 1200

1800

T 1600

1400

1200

1000

800

600

400

200

IK

Time I min

Fig. 4. Dilatometric curve between room temperature and 1800 K.

A. Poirson et al. I Solid State Ionics 99 (1997) 287-295 291

Fig. 5. (a) SEM micrograph of the La,,Sr,,MnO, crystallites after treatment in air up to 1020 K in the conditions of the

dilatometric study. (b) SEM micrograph of the La,,Sr,,,MnO,

crystallites after treatment in air up to 1300 K in the conditions of

the dilatometric study. (c) SEM micrograph of the La,,Sr, ,MnO,

crystallites after treatment in air up to 1500 K in the conditions of

the dilatometric study.

compared, it can be noticed that, while shrinkage carries on around 1600 K, it stops immediately as soon as temperature becomes constant around 1800

K. It can be assumed that, at 1800 K, the grain growth rate is at least equal to the heating rate, but not at 1600 K. Densification measurements were done for each sample by using the theoretical density given in the JCPDS file Nr. 40-l 100. The densifica- tion of the samples sintered around 1100, 1600 and 1800 K is 54, 78 and 95% respectively. At heating rate constant, the densification increases with sinter- ing temperature as expected from the above results.

3.2. InJEuence of the atmosphere on the sintering behaviour

Different sintering behaviours were observed in air, nitrogen and oxygen (Fig. 9). Whatever atmos- phere conditions, the three regions described above appeared still but with specific features. In nitrogen the characteristic temperatures were shifted to lower values and, therefore, the sintering processes in the region III were favoured. In pure oxygen, the sintering processes in the region II were kinetically improved. While the sintering started at about 1000 K in pure oxygen and in air, it started at 875 K in nitrogen. Shrinkage kinetics was quite similar for nitrogen and air, but was less efficient for oxygen. The change in sintering stage appeared at about 1460 K for both oxygen and air conditions and at 1320 K for nitrogen. The shrinkage maximum was obtained at the same temperature in oxygen and air. In nitrogen, shrinkage was more important and its maximum value was reached at lower temperature, 1675 K. That means that sintering in nitrogen leads to higher densification.

These experiments performed by using a support in pure alumina enabled to evidence the influence of oxygen potential on the solid state reaction between Al,O, and La,.,Sr,,,MnO,. For a plateau tempera- ture of around 1800 K in oxygen, no variation of ALIL, was detected, but, in air, there was a very slight contraction of the solid probably due to this starting solid state reaction with the Al,O, support. This assumption was corroborated in nitrogen atmos- phere where the dilatometric curve reversed around 1600 K into a large contraction shoulder due to the formation of LaAlO,, phase clearly identified by EDX and XRD (Fig. 10).

During the cooling, ALIL, decreased with tem- perature (Fig. 11). Whatever the atmosphere, two

292 A. Poirson et al. I Solid State Ionics 99 (1997) 287-295

E = 2500 -

: ‘5

.E 2000 -

2 aI e 1500-

g z

IOOO-

500 -

2000

l I i I I .:I I I *=I I I I I 400 600 800 1000 1200 1400 1600 1800

Temperature I K

Fig. 6. Evolution of the grain size of the La,,Sr,,MnO, crystdlites as a function of the treatment temperature in air

-0.015~100 0 100 200 300 400 500 600 700 800

Time I min

Fig. 7. Dilatometric curve with temperature plateau at 1070 K for 5 h.

slopes in the ALIL, vs. T curves were observed. Under oxygen, the values of 14X 10m6 K-’ and 11.6X 10P6 K-’ of the thermal expansion coefficient were found for temperatures higher or lower than 1050 K respectively. Under air and nitrogen, the same values were measured but with a transition temperature of 1170 K. It can be deduced that the presence of pure oxygen favours a high temperature phase having a thermal expansion coefficient higher

than that of low temperature with a value of 11.6 X 10e6 K-’ already reported [3].

4. Discussion

The sintering behaviour of La,,Sr,,,MnO, may be governed by two types of factors: on the one hand, the size of the ionic radius of the diffusing

A. Poirson et al. I Solid State Ionics 99 (1997) 287-295 293

0.05

dL/Lo

0

800 1000

Time / min

1600

T/K

1400

800

600

Fig. 8. Dilatometric curve with temperature plateau at 1580 K for 5 h.

-0.25 ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’

Time / min

Fig. 9. Dilatometric curves observed in air, oxygen and nitrogen respectively.

species, on the other hand, the defect chemistry concluded that the change in the respective sizes of induced both by the substitution of strontium for the cations due to the change in the strontium content lanthanum and by the subsequent non-stoichiometry is not the main factor. Most likely the defect in oxygen [S-lo]. chemistry do play the major role for sinterability.

Concerning the first factor, the substitution of The presence of strontium in the lattice increases the

strontium for lanthanum does not modify the ionic Mn4+ content and it can be assumed that the diffusivity in the series of the La,_,Sr,MnO,,, transport phenomena determining the aptitude of oxides [3]. This result can be expected if we consider La,_,SrJvInO,,, for sintering are basically depen- that the radius of La3+ is little different from that of dent on the Mn4+/Mn3+ ratio, that is on the Mn4+ Sr2+ (0.132 and 0.140 nm respectively). It can be content. All the experimental results in literature and

294 A. Poirson et al. I Solid State Ionics 99 (1997) 287-295

l lines 3 LaAIO, 1200 -

._

5 I ~ 1000

t

.e 800 f!

T 600

or”‘..‘“““...‘.““” m s’rm IS’ 20 30 40 50 60 7o 2 6 8o

Fig. 10. Comparative X-ray diffraction patterns of a not polluted (a) and polluted (b) zone around the Al,O,-La, ,Sr,,,MnO, contact after

high temperature treatment in nitrogen. The additional lines to the La,&, ,MnO, pattern are due to LaAlO,.

-0.2, , , , , , , , , , , , , , , I , I I , I I ,

““,: #&-p: -0.22

_o.23; /.LY ;

-0.24 s - ’ ’ m m ’ ’ - ’ s ’ I m ’ ’ t a ’ m ’ 600 800 1000 1200 1400 1600 1800

T/K

Fig. 11. Cooling curves of dilatometry.

in this study show that decreasing the Mn4+ content improves the sinterability of this type of materials. In air or in oxygen, higher the temperature is, smaller the Mn4+ content is, according to the decrease of oxygen non-stoichiometry with temperature [2,3]. Thus, temperature must be high enough to permit a good densification in air. In other words, as 6 in

La0.8%.,Mn0,~, can decrease with increasing tem- perature, the concentration of cationic vacancies gradually decreases and the heating rate has to be

sufficiently low in order to make possible the ionic diffusion throughout bulk before the total disappear ante of the cationic vacancies.

With oxygen partial pressure increasing, Mn4+ content increases and the sinterability decreases. This is shown by the results of the dilatometric study where sintering begins at higher temperature in oxygen and air than in nitrogen. Nevertheless, in pure oxygen, the concentration of cationic vacancies strongly increases and the first sintering stage is

A. Poirson et al. I Solid State Ionics 99 (1997) 287-295 295

favoured and simultaneously the second sintering stage is penalized as confirmed by the dilatometric curves. On the contrary, under nitrogen, at low oxygen partial pressure (1 X 10e5 bar), the second stage of grain growth is promoted.

als Chemistry of the Research Institute of Hydro- quebec (IREQ, Quebec, Canada) for their support (Contracts Nr. 93051757 and 9551105 1). They also thank Francois Morin, Guy Belanger, from IREQ, and Michel Vareille, from LRRS, for their assistance with this work.

5. Conclusion References

Two sintering stages of La,.,Sr,.,MnO, have been clearly evidenced by coupling dilatometry measure- ments and SEM observations. Below 1470 K, the neck formation is the rate limiting step of the sintering while, above this temperature, the processes of grain growth dictate the sinter behaviour of the material. To optimize the densification, the rate of heating should be low around 2 K mm’. When temperature reaches 1800 K, time does not play an important role. In fact, rate of sintering phenomena and heating rate are closely connected and the sinter time may be short. The best conditions should be to proceed under an atmosphere poor in oxygen and to restore the stoichiometry of the sample by annealing at lOOOK in air. Lastly, if necessary, the possible interaction with alumina has to be carefully con- trolled.

Acknowledgments

The authors gratefully thank the Regional Council of Burgundy (France) and the Department of Materi-

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