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Materials Chemistry and Physics 113 (2009) 803–815

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

ynthesis of pure and Sr-doped LaGaO3, LaFeO3 and LaCoO3 and Sr,Mg-dopedaGaO3 for ITSOFC application using different wet chemical routes

. Kumara, S. Srikantha,∗, B. Ravikumarb, T.C.Alexb, S.K. Dasb

National Metallurgical Laboratory-Madras Center, CSIR Madras Complex, Chennai 600113, IndiaNational Metallurgical Laboratory, Jamshedpur 831007, India

r t i c l e i n f o

rticle history:eceived 17 April 2008eceived in revised form 29 July 2008ccepted 14 August 2008

eywords:xideshemical synthesisowder diffraction

a b s t r a c t

Pure and Sr-doped LaGaO3, LaFeO3 and LaCoO3 and Sr,Mg-doped LaGaO3 were synthesized by various wetchemical routes, namely combustion, co-precipitation and citrate-gel methods. The effect of the variousprocess parameters on the phase purity, particle size and surface area and morphology of the synthesizedpowders were determined by XRD, simultaneous TG-DTA, laser light scattering, BET and scanning elec-tron microscopy. The stability of the synthesized pure phases in oxidizing and reducing atmosphere wasalso studied by thermogravimetry. It was observed that pure and Sr-doped single perovskite phases oflanthanum ferrite, cobaltite and gallate and Sr,Mg-doped lanthanum gallate could be synthesized by com-bustion and citrate-gel methods under suitable process conditions. Synthesis using the co-precipitation

hermogravimetric analysis method yielded incomplete reaction irrespective of the calcination temperature adopted. The citrate-gel method yielded better powder properties in terms of particle size and morphology and surface areacompared to combustion synthesis. It was found that pure and Sr-doped lanthanum ferrite, lanthanumcobaltite, lanthanum gallate and Sr,Mg-doped lanthanum gallate were stable in the oxidizing atmosphere.In the reducing atmosphere, pure and Sr-doped lanthanum ferrite and Sr,Mg-doped lanthanum gallatewas found to be stable at least during the timeframe of the thermogravimetric experiment whereas pure

cobal

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and Sr-doped lanthanum

. Introduction

The present state of the art solid oxide fuel cells (SOFC) areased on (Y2O3)ZrO2 solid electrolyte and operating in the tem-erature range of 800–1000 ◦C [1]. High temperature of operationf the SOFC is dictated by the ionic conductivity requirement of theolid electrolyte (>0.1 S cm−1). However, the higher temperature ofperation of these solid oxide fuel cells results in several mate-ial problems, including requirement of high temperature sealingnd therefore a high cost. From the materials and cost point ofiew, it is advantageous to operate the fuel cells at intermedi-te temperatures of 600–800 ◦C, which retains the advantages ofnternal reforming and co-generation but imposes less stringentequirements on the materials. The main limitation of operatinghe solid oxide fuel cell at lower temperature is to find alternate

xygen ion conducting electrolyte materials, which have compara-le ionic conductivities at these lower temperatures. In addition tohe conductivity requirements of the solid electrolyte, thermal andhemical compatibility with the anode and cathode as well as sta-

∗ Corresponding author. Tel.: +91 44 22542523; fax: +91 44 22541027.E-mail address: s srikanth 99@yahoo.com (S. Srikanth).

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254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2008.08.047

tite was partially reduced in hydrogen atmosphere.© 2008 Elsevier B.V. All rights reserved.

ility under both oxidizing and reducing conditions are the otherssential requirements.

Since the discovery of high and exclusive oxygen ion conduc-ivity in doped cubic zirconia by Kiukkola and Wagner [2] in 1957,everal other materials with high oxygen ion conductivity has beeneported in the literature [1,3–15]. Oxygen ion conductivity has noteen restricted to the cubic fluorite structure, but has been found

n perovskite and pyrochlore structures [1,3–6]. One of the promis-ng electrolyte candidate for a low or intermediate temperatureOFC (ITSOFC) is La(Sr)Ga(Mg)O3−ı known as LSGM which has aerovskite structure and ionic conductivity several times higherhan (Y2O3)ZrO2 in the temperature range of 600–800 ◦C [10–14].lthough La(Sr)Ga(Mg)O3−ı has a thermal expansion coefficientimilar to (Y2O3)ZrO2 and therefore the same cathode and anodeaterials as that used in the conventional cell can in principle

e used, the chemical compatibility between these materials areot well established [3,15]. It has been reported that ZrO2 reactsith lanthanum oxide and forms several stable interoxide com-

ounds and therefore zirconia-based material is not suitable forse as anode when LSGM is used as an electrolyte [15]. Further,r-doped lanthanum manganite, which is used as a cathode inhe conventional SOFC offers very high interfacial resistance athe cathode–electrolyte interface [16–19]. Lanthanum cobaltite and

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04 M. Kumar et al. / Materials Chem

anthanum ferrite are being considered as potential cathode mate-ials for SOFC applications [1,9,20–30]. Lanthanum cobaltite has aigher electronic conductivity but lower thermal expansion com-atibility [23–28,31], whereas lanthanum ferrite (LSF) has higherhermal expansion compatibility but lower electronic conductivity28–31]. It has been reported that mixed La(Sr)Fe(Co)O3−ı (LSCF)as the optimum combination of electronic conductivity and ther-al expansion compatibility with LSGM [31–36].Knowledge of the preparation of pure single-phase ceramic

owders with controlled particle size and morphology and surfacerea is essential for their subsequent processing. Most commoneramic processing techniques such as slip and tape casting, extru-ion, screen printing, calendaring and electrophoretic depositionequire fine particle size and high surface area to form stable aque-us and non-aqueous suspensions with high solid loading. Theonventional solid-state reaction method is in general employedo prepare mixed oxide compounds because of its lower manu-acturing cost and simplicity. Several investigators [14,18,31,37–43]ave employed a solid-state reaction method to synthesize per-vskite oxides pertinent to IT-SOFC. The main drawback of the solidtate route is the long diffusion distances and slow reaction kineticsindering the formation of pure single phases and lack of controlf particle size and surface area. In general, solid-state synthesisequires a temperature higher than 1500 ◦C and time period inxcess of 48 h.

In order to overcome the disadvantages of solid-state synthesis,number of chemical synthesis techniques such as wet combustionynthesis [44–50], sol–gel [12,51], citrate-gel/Pechini [34,52–57]nd co-precipitation [18,58,59] methods have been reported in theiterature for the preparation of these oxides. These methods pro-ide mixing of the elements at an atomic scale accelerating purehase formation and since these methods employ lower calcinationemperatures, more control over the particle size and morphol-gy and surface area can be exercised. In the combustion process,lycine or urea is used as a fuel for combustion as well as to formomplexes with metal ions to increase solubility and prevent selec-ive precipitation (during water removal). The resultant oxide ashfter combustion is generally composed of very fine particles ofhe desired stoichiometry linked together in a networked struc-ure. This process produces oxide powders of good compositionalomogeneity in a short time, but forms agglomerates readily.

Co-precipitation method involves the precipitation of oxalatesrom a solution of metal nitrates at pH < 1 [60], and its subsequenthermal dissociation to form stoichiometric oxide compounds. Innother variant of the co-precipitation method, the respectivetoichiometric elemental nitrates are taken into solution and co-recipitated as hydroxides using ammonia as the precipitatinggent at an alkaline pH range. Several parameters such as pH, mix-ng rates, temperature and concentration have to be controlled toroduce the desired single phase. However, different rates of pre-ipitation of each individual compound may lead to inhomogeneitynd agglomerates are generally formed during the calcination treat-ent.The Pechini or citrate gel process involves two basic chemical

eactions: (i) complexation or chelation between metal ions anditric acid, and (ii) polyesterification of complexes with ethylenelycol. The complexation and poly-esterification reactions preservehe homogeneity of the metal salt solution in a gel. This polymer-zation reaction forms three-dimensional structures and minimizesegregation. The biggest advantage of this method is the high

urity and excellent control over the composition of the resultingowders. In all these methods, a subsequent calcination step is nec-ssary to completely drive out the organic products and achieve theesired single phase. Although some of these methods have beeneported in the literature for the synthesis of pure and Sr,Mg-doped

ctuhsa

nd Physics 113 (2009) 803–815

anthanum gallate, lanthanum ferrite and lanthanum cobaltite, noystematic and comparative study highlighting the effect of variousrocess parameters on the particle properties are available. Fur-her, very little information is available on the long-term stabilityf these compounds in reducing and oxidizing atmospheres.

In the present work, pure single phase lanthanum cobaltiteLaCoO3 i.e. LCO), lanthanum ferrite (LaFeO3, i.e. LFO), lan-hanum gallate (LaGaO3 i.e. LGO), Sr-doped lanthanumobaltite (La0.90Sr0.10CoO2.95 i.e. LSC), lanthanum ferriteLa0.90Sr0.10FeO2.95 i.e. LSF), and Sr- and Mg-doped lanthanumallate (La0.90Sr0.10Ga0.90Mg0.10O2.9 i.e. LSGM) powders werettempted to be prepared by combustion, co-precipitation anditrate-gel methods and their characteristics studied. The sta-ility of these pure phases in reducing, oxidizing and ambienttmospheres was also investigated.

. Experimental

.1. Materials

High purity lanthanum oxide, (99.99% pure from Central Drug House (CDH),ndia), strontium nitrate, (99% pure from Merck, India), metallic gallium 99.99%ACROS Organics, India), ferric nitrate nanohydrate, (98% pure from CDH), cobaltitrate hexahydrate, (99% pure, CDH), magnesium nitrate hexahydrate, (99% pure,DH), glycine (99% pure, CDH), oxalic acid (99.8% pure, S.D. Fine Chemicals, India),olyvinyl alcohol (CDH), liquid ammonia (25% solution, Merck), citric acid monohy-rate (99.9% pure, Merck) and ethylene glycol (99% pure, CDH) have been used astarting materials.

.2. Glycine-nitrate combustion method

In this technique, the various perovskite oxides were prepared by the com-ustion of the corresponding metal nitrate–glycine mixtures. Stoichiometricompositions of mixtures were used for the synthesis. This method involved rapideating of an aqueous concentrated solution containing the respective startingaterials to about 500 ◦C. The solution initially boiled, underwent rapid degrada-

ion and foaming followed by decomposition and vigorous generation of gases suchs CO2, N2, H2O. In order to avoid the spillage of the powder during combustion,ome amount of excess fuel was added. The combustion products were subjectedo simultaneous thermogravimetric (TG) and differential thermal analysis (DTA) inmbient air to determine their calcination behavior. The as-synthesized powdersere subjected to calcination in air at a temperature of 900 ◦C for 3 h to get the sin-

le phase for pure and Sr-doped lanthanum ferrite and cobaltite and 1500 ◦C for 3 hor lanthanum gallate and LSGM.

.3. Co-precipitation method

In the co-precipitation method, fine particles of LCO, LFO, LSC, LSF, LGO andSGM were prepared by mixing stoichiometric amounts of the corresponding metalitrate and polyvinyl alcohol with oxalic acid as the precipitating agent. The amountf oxalic acid, polyvinyl alcohol and drying conditions were optimized in the initialxperiments. First, one molar solution of the respective nitrates and 2 M solution ofxalic acid along with 3 wt.% of polyvinyl alcohol were prepared. The homogenousolution was prepared by mixing the respective elemental nitrate solution with con-inuous stirring (the pH of the solution was maintained at 1), and to this, the preparedxalic acid solution was added. This yielded a precipitate of the respective oxalatehich was washed and dried at 80 ◦C. Simultaneous thermogravimetric and differ-

ntial thermal analysis experiments in static air were carried out on the precipitate.he dried precipitate was subjected to heating at a temperature of 900 ◦C for 3 h in anttempt to produce the crystalline pure phases. However, since calcination at 900 ◦Cid not yield pure phases for either of the oxides, a systematic calcination study inhe temperature range 900–1500 ◦C was taken up. Experiments were also under-aken with liquid ammonia as the precipitating agent in the pH range of 8.5–9.5.hese were then subject to the same calcination treatment as described above.

.4. Pechini method

In this technique, the pure phases were prepared by the mixing of the corre-ponding metal nitrates and citric acid in ethylene glycol. In the case of gallium,itrate was first prepared from the pure metal by dissolution in nitric acid and pre-

ipitation. A small amount of HCl (5% by volume) addition was found to acceleratehe dissolution of gallium. First, a 1-M solution of respective nitrates was preparedsing citric acid and ethylene glycol as sol and gel forming agents respectively. Theomogenous solution was prepared by mixing of all the respective elemental nitrateolution in ethylene glycol and water (1:1 ratio) as solvent with stirring and heatingt 60 ◦C. To this, the prepared 1 M citric acid dissolved in ethylene glycol was added

M. Kumar et al. / Materials Chemistry and Physics 113 (2009) 803–815 805

d) LSF

wagawSa

2

dyaur1mim

dttbuddfitd

3

Fig. 1. XRD pattern of (a) LCO, (b) LSC 9010, (c) LFO, (

ith continuous stirring and heating at 80 ◦C, to initially form a sol and finally getresin during heating. The resin was dried in air at 500 ◦C for 3 h to yield a dry

el. The resin as well as the dried gel was subject to simultaneous TG/DTA in staticir to study the progress of the reaction. Based on the TG/DTA results, calcinationas carried out at a temperature of 900 ◦C for 3 h to get a single phase for pure and

r-doped lanthanum ferrite and cobaltite and 1500 ◦C for 3 h for lanthanum gallatend LSGM.

.5. Characterization

The perovskite powders (LCO, LSC, LFO, LSF, LGO and LSGM) prepared by theifferent chemical routes were characterized by X-ray diffraction, particle size anal-sis, microscopy, thermogravimetric analysis in oxidizing and reducing atmospheresnd surface area measurement. The X-ray diffraction of the samples was carried out

sing a Siemens D-500 diffractometer with Co K� radiation (� = 1.79026) at a scanate of 1◦ min−1, particle size distribution using laser scattering technique (CILAS180) and surface area using a BET analyzer (Micromeritics ASAP 2020 V1.04 H). Theorphology of the synthesized powders was observed in the back-scattered electron

maging mode under a scanning electron microscope (JEOL JSM 840A) and stereoicroscope (Leica MZ6). The stability of the synthesized powders in controlled oxi-

3

Lr

Fig. 2. XRD pattern of (a) LCO, (b) LSC 9010, (c) LFO, (d) L

9010 and (e) LGO prepared by combustion method.

izing (99.9% pure O2) and reducing (80%N2 + 20%H2) atmospheres were studied byhermogravimetry (Thermoelectron Corporation, Versa Therm TGA). Simultaneoushermogravimetry and differential thermal analysis of all the samples synthesizedy the different chemical routes prior to calcination were carried out in static airsing a SEIKO (Model No. 320) simultaneous TG/DTA. The indexing of the X-rayiffraction patterns was carried out using the JCPDS (Joint Committee on Pow-er Diffraction Standards—International Centre for Diffraction Data, JCPDS-ICPDD)les. The line broadening analysis of the X-ray diffractograms was carried out usinghe software X-ray Diffraction Analysis (1992–93) and the lattice parameters wereetermined from the d-spacing manually.

. Results and discussion

.1. Phase purity

The X-ray diffraction pattern of LCO, LSC, LFO, LSF, LGO andSGM from combustion and Pechini methods is shown in Figs. 1–3,espectively. It is seen that the XRD pattern of all the materials

SF 9010 and (e) LGO prepared by Pechini method.

806 M. Kumar et al. / Materials Chemistry and Physics 113 (2009) 803–815

Fig. 3. X-ray diffractogram of Sr and Mg doped lanthanum gallate prepared through (a) combustion method and (b) Pechini method.

Table 1Phase purity of the various oxides prepared from co-precipitation method and calcined at different temperatures.

Materials Phases identified at different temperatures

900◦C 1100◦C 1300◦C 1500◦C

LCO La2O3 Pure – –LSC 9010 La2O3 Pure – –LL 2SrOx

LL 2SrOx

nLw3cdt

m0

FO La(OH)3 La(OH)3

SF 9010 La(OH)3 + La2SrOx La(OH)3 + LaGO La(OH)3 La(OH)3

SGM 9191 La(OH)3 + La2SrOx + La2MgOx La(OH)3 + La

amely LaCoO3, La0.90Sr0.10CoO3, LaFeO3, La0.90Sr0.10FeO3−ı, andaGaO3, show the formation of a single phase and perfectly match

ith the JCPDS X-ray pattern files (25-1060 for LCO, 36-1392 for LSC,

7-1493 for LFO and LSF and 24-1102 for LGO). However, at loweralcination temperatures (900–1300 ◦C), the LGO and LSGM phasesid not form completely, whereas at higher temperature (1500 ◦C),he LSGM powder prepared from both combustion and citrate-gel

tLwwo

Fig. 4. XRD of (a) LCO, (b) LSC 9010, (c) LFO, (d) LSF 9010, (e) LGO

La(OH)3 –La(OH)3 –La(OH)3 –

+ La2MgOx La(OH)3 + LaSrGa3O7 La(OH)3 + LaSrGa3O7

ethods were found to contain about 5% of LaSrGa3O7 (JCPDS: 45-637) as an impurity phase. Synthesis of pure LSGM is reported

o be extremely difficult and traces of LaSrGa3O7 or La4Ga2O9 oraSrGaO4 phases are frequently detected [12,14,37–42] especiallyhen the solid-state synthesis is adopted [14,37–42]. However, Itas observed by us that increasing the doping of Mg in the B-sitef LGO stabilized the formation of the pure perovskite phase. A

and (f) LSGM 9191 prepared by co-precipitation method.

M. Kumar et al. / Materials Chemistry and Physics 113 (2009) 803–815 807

Table 2Crystallographic properties of materials prepared from different synthetic routes

Properties Crystal structure Calculated unit cell parameters (Å)3 Unit cell volume (Å)3 Calculated X-raydensity (g cc−1)

d50 particlediameter (�m)

a b c

LCO—GN Rhombohedral 5.4406 – 13.1444 336.9397 7.2314 6.87LSC9010—GN Rhombohedral 5.4469 – 13.1553 338.0153 7.0941 10.71LFO—GN Orthorhombic 5.5512 7.8567 5.5597 242.4814 6.6412 18.99LSF 9010—GN Orthorhombic 5.5426 7.8349 5.5430 240.7090 6.6793 15.96LGO—GN Orthorhombic 5.497 5.480 7.785 234.4910 7.2586 3.64LSGM 9191—GN Orthorhombic 5.5945 5.5140 7.7638 239.498 6.8504 3.08LCO—P Rhombohedral 5.4426 – 13.0901 335.7995 7.2931 1.17LSC9010—P Rhombohedral 5.4452 – 13.1413 337.4335 7.1063 0.24LFO—P Orthorhombic 5.5556 7.8689 5.5212 241.3673 6.5559 2.62LSF 9010—P Orthorhombic 5.5513 7.8326 5.5385 240.8201 6.5529 1.64LGO—P Orthorhombic 5.529 5.476 7.784 235.70 7.2306 1.93LSGM 9191—P Orthorhombic 5.4956 5.5309 7.7850 236.6277 6.9312 3.21L 337.0836 7.2652 2.40L 336.0387 7.1358 2.99

G n method.

stopocathiipaltpcdappbtatfpLoabashpS

X

�

wtAl

Fig. 5. Particle size distribution of the various oxides prepared by combustionmethod.

CO—CP Rhombohedral 5.4489 – 13.1102SC9010—CP Rhombohedral 5.4433 – 13.0963

N: Glycine nitrate combustion method, P: Pechini method and CP: Co-precipitatio

eparate study has been undertaken to study the composition andemperature range of stability of the single phase as a functionf doping at the A- and B-sites of LGO. A representative XRD ofure single phase La0.90Sr0.10Ga0.80Mg0.20O3−ı (the impurity phasef LaSrGa3O7 is absent here) obtained from the Pechini method andalcined at 1500 ◦C is also included in Fig. 3. Majewski et al. [61] havelso reported the formation of pure La0.90Sr0.10Ga0.80Mg0.20O3−ı byhe citrate-gel method when calcined at 1500 ◦C. Huang et al. [12]ave discussed in detail the formation of the secondary phases dur-

ng the synthesis of cubic perovskite LSGM phase by various routesn the light of phase equilibria in the relevant systems. For the co-recipitation method (using both oxalic acid and liquid ammonias the precipitating agents), all the samples (pure and Sr-dopedanthanum ferrite, cobaltite and gallate) calcined at 900 ◦C showedhe presence of additional hydroxide phases. Therefore, for the co-recipitation method, the synthesized powders were subjected toalcination in the temperature range 900–1300 ◦C for pure and Sr-oped lanthanum ferrite and cobaltite and 900–1500 ◦C for LGOnd Mg,Sr-doped LaGaO3 at an interval of 100 ◦C and the calcinedowder subjected to XRD. The phases identified at different tem-eratures of calcination during the synthesis of the various oxidesy co-precipitation are summarized in Table 1. It was observedhat single phases of several of the oxides (notably LFO, LSF, LGOnd LSGM) could not be synthesized by co-precipitation at calcina-ion temperatures up to 1300 ◦C for pure and Sr-doped lanthanumerrite and up to 1500 ◦C for lanthanum gallate. However, the co-recipitation method yielded pure single phases of LaCoO3 anda(Sr)CoO3 when calcined at 1300 ◦C. X-ray diffractograms of thexide phases synthesized by co-precipitation method and calcinedt 1300 ◦C are given in Fig. 4. Similarly, none of the pure phases coulde synthesized when liquid ammonia was used as the precipitatinggent despite carrying out calcination up to 1500 ◦C. Although theynthesis of yttria and gadolinia doped ceria by co-precipitationas been reported in the literature [58,59], the oxalate route of co-recipitation has not been reported for the synthesis of pure andr-doped lanthanum gallate, ferrite and cobaltite.

The density of the synthesized pure phases was calculated fromRD data using the following equation [62]:

= zM

NV

here � is the density in g cc−1; z is the number of atoms inhe unit cell; M is the molecular weight of the material; N is thevogadro number and V is the unit cell volume in m3. The calcu-

ated lattice parameters and density of the synthesized pure phasesFig. 6. Particle size distribution of the different materials prepared by Pechinimethod.

808 M. Kumar et al. / Materials Chemistry and Physics 113 (2009) 803–815

Fm

flfa

3

LcfttLsrpoipacss3teia

Table 3Variation of particle size with calcinations temperature for materials prepared byco-precipitation method

Temperatures (K) Mean particle size (�m)

LL

gadtbsfeitcsdcaaitccss

3

3

pPcsspaccsm

TC

M

LLLLLL

ig. 7. Particle size distribution for the materials prepared by Co-precipitationethod.

rom XRD data are given in Table 2. It is found that the calculatedattice constant for LCO, LSC, LFO, LSF, LGO and LSGM preparedrom combustion, Pechini and co-precipitation methods are in closegreement with those reported in the respective JCPDS files.

.2. Particle size distribution and surface area

The particle size distribution of crystalline LCO, LSC, LFO, LSF,GO and LSGM powders synthesized by combustion, Pechini ando-precipitation methods is shown in Figs. 5–7, respectively. It wasound that the dry ground powders prepared by combustion syn-hesis possess particle size with d50 (size below which 50 vol.% ofhe powders exist) ranging from 7 to 19 �m for LCO, LSC, LFO andSF and around 3 �m for LGO and LSGM. For the powders synthe-ized by the citrate-gel method, the mean particle size was in theange of 0.5–3.0 �m. The mean particle size of powders from the co-recipitation method is also less than 3 �m. The mean particle sizef the powders prepared from the different routes is summarizedn Table 2. The larger particle size for the combustion synthesizedowders may be attributed to the high combustion temperaturettained during the process of synthesis. To study the effect of cal-ination temperature on particle growth, the single phase powdersynthesized by the co-precipitation method (LCO and LSC) wereubject to calcination in the temperature range of 900–1300 ◦C for

h and particle sizes measured. The mean particle sizes of these

wo oxides synthesized by co-precipitation and calcined at differ-nt temperatures are given in Table 3. It is seen that there is anncrease in mean particle size as a function of calcination temper-ture because of the growth of particles.

c(aoe

able 4omparison of surface area and pore size of powders prepared by different synthetic rout

aterials Combustion method Co-precipita

SA (m2/g) Pore size (nm) SA (m2/g)

CO 0.7547 7.6178 1.2900SC9010 1.8020 7.96402 1.9796FO 1.4724 5.2555 –SF 9010 1.24 7.4028 –GO 1.8281 7.5543 –SGM 9191 6.4137 11.7643 –

1173 1273 1373 1473 1573

aCoO3 2.40 2.50 2.74 2.81 3.65a0.90Sr0.10CoO3 2.99 4.66 5.17 5.40 5.94

The BET surface area obtained for the different crystalline sin-le phases from combustion, co-precipitation and Pechini methodsre given in Table 4. It is observed that the BET surface area of pow-ers synthesized by Pechini method is higher than that of the otherwo methods. Further, the crystalline LSGM powders synthesizedoth by combustion and citrate-gel methods yield a much higherurface area (despite the larger particle size) than that achievedor the other phases. This may be attributed to a lower surfacenergy and thereby a lesser tendency for agglomeration or sinter-ng of this phase. In general, the adsorption isotherm which showshe variation of adsorbate volume with relative pressure 0 < P/P◦ < 1,ontains information on the pore structure of materials. The poreize obtained from surface area measurement for the various pow-ers synthesized in this study is in the range of 5–11 nm for theombustion method, 30–58 nm for the co-precipitation methodnd 7–37 nm for the citrate-gel method. Sing and co-workers [63]lso observed mesopores in the size range between 2 and 50 nmn the materials prepared by them using the sol–gel method. It canherefore be inferred that the powders prepared by the wet chemi-al routes adopted in this study especially the co-precipitation anditrate gel routes are porous and have considerable internal poreurfaces. The internal surfaces of the pores also contribute to thepecific surface area of samples measured by BET.

.3. Microscopic studies

.3.1. Bulk powder morphology by stereomicroscopyThe bulk powder morphology of the as-synthesized pure LaCoO3

hase is shown in Fig. 8 for combustion, co-precipitation andechini methods, respectively. The morphology of the other purerystalline phases was similar. The stereo images reveal that thetructure obtained for products derived from combustion synthe-is (Fig. 8a) and citrate-gel method (Fig. 8c) is similar and appearorous and networked in nature, although in terms of particle sizend hardness, the combustion derived powders are hard and coarseompared to that synthesized by citrate gel method. Despite theoarser mean particle size for combustion synthesis, these powderstill showed some tendency for agglomeration of particles, whichay be attributed to the high in situ combustion temperature. In the

ase of co-precipitation, the morphology of precipitated powdersoxalate of respective element) appears porous and agglomerated,s can be seen in Fig. 8b. The agglomeration of particles dependsn the processing condition and calcination temperature. The pres-nce of agglomerated particles is detrimental to the final state

es

tion method Citrate-gel method (Pechini)

Pore size (nm) SA (m2/g) Pore size (nm)

57.6491 2.4832 7.733731.1802 3.5295 18.1324– 3.4533 12.5745– 2.2305 36.8628– 2.1680 8.5088– 7.0237 12.6343

M. Kumar et al. / Materials Chemistry and Physics 113 (2009) 803–815 809

stion,

ooaap

Fig. 8. As-synthesized powder morphology for (a) Combu

f sintering and introduces heterogeneities in the microstructuref the sintered ceramics that cannot be eliminated readily. Thesegglomerated particles act as defect centers [64] and therefore thegglomerates need to be eliminated by grinding. The wet grindingrocess is effective in reducing agglomeration.

3

pms

Fig. 9. Back-scattered electron images of (a) LCO, (b) LSC 9010, (c) LFO, (d) L

(b) Co-precipitation and (c) Pechini synthesized LaCoO3.

.3.2. Morphology by scanning electron microscopyBack scattered electron images of the pure crystalline phases

repared by combustion, co-precipitation and citrate-gel (Pechini)ethods are shown in Figs. 9–11, respectively. The combustion

ynthesized pure phases (Fig. 9) have discrete particles with differ-

SF 9010, (e) LGO and (f) LSGM 9191 prepared by combustion method.

810 M. Kumar et al. / Materials Chemistry and Physics 113 (2009) 803–815

y co-p

ep(Iutpcmfiiade

pd

3

faF

Fig. 10. Morphology of pure (a) LCO and (b) LSC 9010 prepared b

nt morphologies and a porous and spongy appearance. The purehases (LCO and LSC) synthesized by the co-precipitation methodFig. 10) shows high porosity and a tendency for agglomeration.n the case of co-precipitation, wherein a wet chemical processingsing starting materials of oxides and nitrate salts was adopted,he powder morphology is significantly affected by the processingarameters. These include precipitation conditions, washing pro-edure and heating conditions in a static reactor. In the citrate-gelethod, at the same calcination temperature, the powders are of

ner size, exhibit a more uniform morphology and appear porousn nature (Fig. 11). The powders from citrate gel (Pechini) methodppear to be discrete and have a lesser tendency to agglomerationespite the finer size (Fig. 11). It is clear that the three methodsxhibit varied microstructures in terms of particle size and mor-

atcgi

Fig. 11. SEM-BSE images of (a) LCO, (b) LSC 9010, (c) LFO, (d) LSF 90

recipitation method as seen by back-scattered electron imaging.

hology as shown in Figs. 9–11. The agglomeration of particlesepends on the processing condition and calcination temperatures.

.4. Thermogravimetric and differential thermal analysis

The thermogravimetric and differential thermal analysis plotsor all precursor samples prepared by combustion, co-precipitationnd citrate-gel methods prior to calcination are displayed inigs. 12–17. The various stages of removal of organics, combustion

nd decarboxylation reactions, crystallization, phase transforma-ions and the stabilization of the pure phase at higher temperaturesan be inferred from these thermal analysis plots. The thermo-ravimetric plots of the combustion products shown in Fig. 12ndicate no further weight loss for the pure and Sr-doped lan-

10, (e) LGO and (f) LSGM 9191 prepared by Pechini method.

M. Kumar et al. / Materials Chemistry and Physics 113 (2009) 803–815 811

F

tf5atcpaSf3eLmwoa

F

BDptettn9olcr

ig. 12. Thermogravimetric pattern of materials prepared by combustion method.

hanum ferrite and cobaltite phases (a minor weight loss is noticedor LSC) indicating the completion of the combustion reaction at00 ◦C. However, although the ignition temperature is 500 ◦C, thectual temperature of the product will be much higher because ofhe combustion process. Berger et al. [50] have also reported theompletion of the combustion reaction when the initiation tem-erature was 500 ◦C through simultaneous TG/DTA of the LaCoO3lanine-based precursor. The corresponding DTA plots for pure andr-doped lanthanum ferrites show no thermal effects. Howeveror Sr-doped lanthanum cobaltite, a minor endothermic effect at00 ◦C and an exothermic reaction is noticed at 740 ◦C and a smallndothermic peak at 920 ◦C is observed for both pure LCO andSC. The exothermic reaction at 740 ◦C associated with a minor

ass loss can be attributed to the residual combustion reactionhereas the endothermic effects at around 300 and 920 ◦C with-

ut any mass loss are possibly due to structural transformations,lthough the nature of structural transformations are not clear.

ig. 13. Differential thermal analysis of materials prepared by combustion method.

egto

Fig. 14. TG plots for materials prepared by co-precipitation method.

erger et al. [50] observed a minor endothermic effect in theTA of pure and Sr-doped lanthanum cobaltite at 320 ◦C (peakosition at 345 ◦C), which they incorrectly attribute to a crys-allization process (crystallization process is exothermic). Stolent al. [54] attribute the maximum in heat capacity observed inheir measurements on LaCoO3 at 257 ◦C K to a continuous lowo high spin state electronic transition. Berger et al. [50] also didot notice any structural transformation in lanthanum cobaltite at20 ◦C in their TG/DTA result. However, Popa and Kakihana [55]bserved a minor endothermic reaction with no associated massoss in their DTA of LaCoO3 at ∼875 ◦C. Tikhonovich et al. [65]arried out TG/DTA on Sr(Co,Fe)O3−ı in air in the temperatureange 200–1000 ◦C. They also observed an explicit endothermic

ffect at 890 ± 10 ◦C associated with a drastic decrease in the oxy-en content (decrease in ı up to 0.02) supporting the structuralransformation at this temperature. Specific heat measurementsn this phase up to 900 ◦C would have provided conclusive evi-

Fig. 15. DTA plots for materials prepared by co-precipitation method.

812 M. Kumar et al. / Materials Chemistry a

Fp

d[ut1aacirh

Fp

sttmtamotetNtCcs2t7woattttrTs1Lta

ig. 16. Thermogravimetric plot of the precursor resins (prior to calcination) pre-ared by Pechini method.

ence on the phase transformation. Unfortunately, Stolen et al.54] have carried out heat capacity measurements on LaCoO3 onlyp to 727 ◦C. A rhombohedral to orthorhombic phase transforma-ion has also been reported in La0.58Sr0.4Co0.2Fe0.8O3−ı between250 and 1400 ◦C [57]. The thermogravimetric behavior of LGOnd LSGM suggest that the combustion reaction is not completet 500 ◦C and two separate stages of mass loss corresponding to

ombustion reactions are observed between 800 and 1000 ◦C. Thiss also substantiated by the broad exothermic peak in the cor-esponding temperature range in the DTA (e and f of Fig. 13). Aigher calcination temperature will therefore be required for the

ig. 17. Differential thermal analysis of the precursor resins (prior to calcination)repared by Pechini method.

pobinfitmg5tgppmdtpm2samttiereab

nd Physics 113 (2009) 803–815

ynthesis of pure and Sr,Mg-doped lanthanum gallate comparedo lanthanum ferrite and cobaltite. The thermogravimetric plot ofhe co-precipitated products shown in Fig. 14 shows three stages of

ass loss for all the precursors; the first up to 150 ◦C due to dehydra-ion, the 2nd steep mass loss between 250 and 500 ◦C and the thirdt 700 ◦C. The corresponding DTA (Fig. 15) indicates an endother-ic peak at 150 ◦C and a large exothermic peak for the 2nd stage

f mass loss (two large peaks in the case of LSC). The transforma-ion depicted in the TG at 700 ◦C is not associated with any heatffects in the DTA. The general trend of TG/DTA is similar for allhe co-precipitated precursors (except for LSC) used in this study.akayama et al. [57] have carried out a detailed investigation on

he calcination behavior of co-precipitates of La2(C2O4)3·nH2O andoC2O4·mH2O using thermogravimetry and IR spectra of samplesalcined at several temperatures. They also observed the decompo-ition to occur in three steps: dehydration up to 150 ◦C, an abruptnd stage of weight loss at 270 ◦C attributed to decomposition ofhe oxalate and finally the decomposition of the carbonates above00 ◦C. The results obtained in the present study are consistentith the observations of Nakayama et al. except for the third stage

f carbonate decomposition. The decomposition of carbonates isssociated with an endothermic effect which was not detected inhe present study. Nakayama et al. observed that high calcinationemperatures (>1200 ◦C) was required to synthesize LaCoO3 fromheir oxalate precursors by co-precipitation. This is consistent withhe present study where a calcination temperature of 1300 ◦C wasequired to synthesize pure and Sr-doped LaCoO3. Although theG plots obtained for the various oxalate precursors showed con-tancy in mass beyond 1000 ◦C, XRD of the calcined products at300 ◦C (1500 ◦C for LGO and LSGM) (Fig. 4) show that except forCO and LSC, pure phases are not formed by co-precipitation (withhe use of both oxalic acid and liquid ammonia as precipitatinggents). However, interestingly, Huang et al. [12] could synthesizeure crystalline La0.9Sr0.1Ga0.8Mg0.2O2.85 (minor amounts of sec-ndary phases were observed even here) through the sol–gel routey sintering the amorphous gel calcined at 600 to ∼1370 ◦C for 72 hn air and vacuum. Further, they observed no mass loss or DTA sig-al above 500 ◦C in their TG-DTA measurements on the gel (formed

rom acetate precursor solution peptized with ammonium hydrox-de and dried) although they had to sinter the gel calcined at 600o ∼1370 ◦C for 72 h to get the crystalline phase. The thermogravi-

etric and differential thermal analysis results on the amorphousel synthesized by the citrate-gel method (prior to calcination at00 ◦C) in this study are shown in Figs. 16 and 17, respectively. Thehermal analysis behavior for the precursors derived by the citrateel method is very similar to that observed for the co-precipitatedrecursors, except that the reactions occur over a narrower tem-erature range. The thermogravimetric plot shows three stages ofass loss for all the precursors; the first up to 150 ◦C due to dehy-

ration, the 2nd steep mass loss between 200 and 400 ◦C and thehird between 500 and 700 ◦C depending upon the nature of therecursor. The corresponding DTA (Fig. 17) indicates an endother-ic peak at 150 ◦C and two overlapping exothermic peaks for the

nd stage of mass loss between 200 and 400 ◦C (the two peaks areeparated in the case of LGO). The 3rd stage of mass loss is notccompanied by any heat effects, except for LGO which shows ainor exothermic effect at ∼700 ◦C. The 1st stage of mass loss and

he associated endothermic effect is due to moisture loss whereashe 2nd stage of mass loss associated with large exothermicitys due to the oxidation reactions. Polini et al. [53] as well as Tas

t al. [56] have carried out simultaneous TG-DTA on the LSGMesins synthesized by the Pechini method (using citric acid andthylene glycol) prior to calcination. Polini et al. [53] observedsimilar thermal analysis behavior on the LSGM foam obtained

y sol–gel citrate synthesis and dried at 200 ◦C overnight (three

M. Kumar et al. / Materials Chemistry and Physics 113 (2009) 803–815 813

Fm

stcomr[cmm6ttpdlsletcfmaittrsoaAhttitff

Fs

OiAopttiahsshows the formation of La2CoO4 (72-0937). The stability of thesecompounds in reducing atmosphere has not been reported in theliterature earlier.

ig. 18. Differential thermal analysis of the calcined precursors prepared by Pechiniethod.

tages of mass loss, endotherm at 200 ◦C corresponding to mois-ure loss and three exothermic peaks between 300 and 600 ◦Corresponding to the oxidation reactions). However, Polini et al.bserved an additional exothermic peak without any associatedass gain/loss at 1250 ◦C which they attributed to a solid state

eaction between the dissociated reactants. Tas and co-workers56] also observed a similar behavior in the TG/DTA of LSGM pre-ursors prepared by the citrate gel method. They observed a totalass loss of 55% in 3 stages up to 700 ◦C after which there was noass loss. They observed exothermic peaks at 350, 400, 480 and

00 ◦C corresponding to the various stages of oxidation. A calcina-ion treatment between 500 and 600 ◦C of the gel is expected to takehe oxidation reaction to completion and result in an amorphousroduct. The DTA plots of the gel calcined at 500 ◦C in this study isepicted in Fig. 18. Interestingly, the DTA of the pure and Sr-doped

anthanum cobaltite and pure lanthanum ferrite calcine do nothow any thermal effects at lower temperatures. As observed ear-ier, the DTA of the Sr-doped lanthanum cobaltite showed a minorndothermic peak at ∼1150 ◦C which is probably associated withhe rhombohedral→orthorhombic phase transformation in thisompound reported by Bucheler et al. [66]. For Sr-doped lanthanumerrite, a large exothermic peak at ∼350 ◦C and a minor exother-

ic deviation at ∼500 ◦C were noticed although these were notssociated with any mass loss. The exothermic reaction at 350 ◦Cs possibly due to the oxidation of the residual reactants whereashe reason for the minor exotherm at 500 ◦C is not clear. Inciden-ally, a second order magnetic order-disorder transition has beeneported by Stolen et al. [54] at 462 ◦C based on heat capacity mea-urements. The DTA for the LGO and LSGM samples shows the startf a minor endothermic effect at 320 ◦C (peak at 375 ◦C) withoutny associated mass effects. Again, the reason for this is not clear.lthough an orthorhombic to rhombohedral phase transformationas been reported to occur at around 147 ◦C [67] in pure LaGaO3,he endothermic effect observed is unlikely to be due to this struc-ural transformation since the crystallinity of the calcined product

s in doubt. Thermogravimetric studies on pure crystalline solid lan-hanum cobaltite and ferrite and Sr-doped lanthanum cobaltite anderrite; and Sr- and Mg-doped lanthanum gallate powders preparedrom Pechini method were also carried out in both oxidizing (pure

Fs

ig. 19. Stability of materials prepared from Pechini method in oxidizing atmo-phere.

2) and reducing atmosphere (80%N2 + 20%H2) to study their stabil-ty in these atmospheres. These results are shown in Figs. 19 and 20.ll the compounds were found to be stable in oxidizing atmospherever the temperature range of measurement (30–900 ◦C), whereasure and Sr-doped lanthanum cobaltite was found to dissociate inhe reducing atmosphere. The X-ray diffractograms of the respec-ive oxides after the TG run in the reducing atmosphere is depictedn Fig. 21. It was observed that both lanthanum cobaltite (LaCoO3)s well as Sr-doped LaCoO3 are partially reduced to Co+2 form inydrogen atmosphere. The X-ray diffraction pattern of both theseamples subjected to thermogravimetry under reducing conditions

ig. 20. Stability of materials prepared from Pechini method in reducing atmo-phere.

814 M. Kumar et al. / Materials Chemistry and Physics 113 (2009) 803–815

ermog

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Fig. 21. X-ray diffractogram of the oxides after th

. Conclusions

Various perovskite oxide phases relevant for ITSOFCpplications, i.e., lanthanum cobaltite (LaCoO3), lanthanumerrite (LaFeO3), lanthanum gallate (LaGaO3), Sr-doped lan-hanum cobaltite (La0.90Sr0.10CoO3−ı), lanthanum ferriteLa0.90Sr0.10FeO3−ı), and Sr- and Mg-doped lanthanum gal-ate (La0.90Sr0.10Ga0.90Mg0.10O3−ı) powders were prepared byombustion and citrate-gel methods and their characteristicstudied. Co-precipitation yielded pure phases only for lanthanumobaltite and Sr-doped lanthanum cobaltite. It was observed thathe powders of LCO, LSC 9010, LFO, LSF 9010, LGO and LSGM191 prepared by combustion and Pechini methods are discretehereas pure phases synthesized by co-precipitation have a

endency to agglomerate. From the viewpoint of particle size andurface area, powders synthesized by the Pechini method wereound to be superior. The various stages of reaction during theynthesis of all the above mentioned compounds by these wethemical methods were studied in detail by thermal analysisethods. The stability of these pure phases in oxidizing and

educing atmospheres was also investigated. It was observedhat all these compounds are stable in oxidizing atmosphere,hereas, both lanthanum cobaltite (LaCoO3) as well as Sr-oped LaCoO3 are partially reduced to Co+2 form in reducingtmosphere.

cknowledgements

The authors are thankful to the Council of Scientific andndustrial Research (CSIR) for supporting this work under theew Millennium Indian Technology Leadership Initiative program.ne of the authors (MK) is also thankful to CSIR for award-

ng a Research Associateship. Permission of Director, Nationaletallurgical Laboratory, to publish this paper is also acknowl-

dged.

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