T. SKALAR et al.: NOVEL MATERIALS BASED ON La0.75SrxA0.25-xCr0.5Mn0.5O3 (A=Ba, Ca, Mg) AS FULL-CERAMICS ...
51–58
NOVEL MATERIALS BASED ON La0.75SrxA0.25-xCr0.5Mn0.5O3
(A=Ba, Ca, Mg) AS FULL-CERAMICS ANODES IN
HIGH-TEMPERATURE FUEL CELLS
NOVI MATERIALI NA OSNOVI La0.75SrxA0.25-xCr0.5Mn0.5O3
(A=Ba, Ca, Mg) KOT KERAMI^NE ANODE V
VISOKOTEMPERATURNIH GORIVNIH CELICAH
Tina Skalar, Marjan Marin{ek, Klementina Zupan
Faculty of Chemistry and Chemical Technology, Ve~na pot 113, 1000 Ljubljana
klementina.zupan@fkkt.uni-lj.si
Prejem rokopisa – received: 2017-07-17; sprejem za objavo – accepted for publication: 2017-09-08
doi:10.17222/mit.2017.120
Among alternative anode materials for high-temperature fuel cells, the complex ceramic oxide La0,75Sr0,25Mn0,5Cr0,5O3 (LSCM)
has recently shown good catalytic activity regarding fuel oxidation and sufficient stability in reductive environments at relatively
low steam-to-carbon ratios. However, the electrical and ionic conductivities of LSCM are lower compared to some other
perovskite materials. One of the possibilities to improve the conductivity of LSCM is in its composition variations, i.e., altering
the Sr-content, doping on the A-site of the perovskite with other ions (Ba, Ca and Mg), and varying the Mn-to-Cr ratio on the
B-site of the perovskite. In this paper, systems with the general formula La0.75SrxA0.25-xCr0.5Mn0.5O3 (A = Ba, Ca, Mg, x varies
between 0 and 0.25) are described. Within the investigated system, prepared materials after synthesis contain the perovskite
structure as a main crystallographic phase with relatively low additions of secondary phases. Any secondary phases are
undesired, because they may substantially influence the electrical properties of the final materials. In samples with relatively
high Sr-additions, a secondary Sr-rich phase Sr2CrO4 is also identified. Ca-doping may result in traces of CaCr2O4 phase in
as-synthesized samples, while Ba-doping may lead to BaCrO4 or BaCO3 phases with higher Ba-additions. The quantity of the
secondary phases may be controlled by calcination program or sintering conditions. Secondary phases, which may form
additional grains or liquid phase, also influence the development of microstructures during sintering. Within the investigated
compositions, the most promising materials are La0.75SrxCa0.25-xCr0.5Mn0.5O3 (x = 0.05–0.15), because they exhibit single-phase
microstructure with fine grains after sintering at 1200 °C. Materials with Ba- or Mg-additions form precipitates of secondary
phases at 1200 °C, which also remain present after sintering at higher temperatures.
Keywords: combustion synthesis, perovskite, thermal analysis, x-ray powder diffraction, quantitative microstructure analysis
La0,75Sr0,25Mn0,5Cr0,5O3 (LSCM) je med alternativnimi anodnimi materiali za visokotemperaturne gorivne celice pokazal dobro
katalitsko aktivnost za oksidacijo goriva ter zadovoljivo stabilnost v reduktivnih okoljih z relativno nizkim razmerjem med
vodno paro in ogljikom. Vendar pa ima LSCM v primerjavi z nekaterimi drugimi perovskiti nekoliko ni`jo elektri~no in ionsko
prevodnost. Ena od mo`nosti za izbolj{anje njegove prevodnosti je v variacijah sestave, npr. sprememba koncentracije Sr,
dopiranje na A mestu v perovskitu z drugimi ioni (Ba, Ca in Mg), variiranje razmerja med Mn in Cr na B mestu v perovskitu. V
prispevku obravnavamo sistem s splo{no formulo La0.75SrxA0.25-xCr0.5Mn0.5O3 (A = Ba, Ca, Mg, x variira med 0 in 0,25).
Materiali v preiskovanem sistemu po sintezi kot glavno fazo vsebujejo perovskit z relativno nizko vsebnostjo sekundarnih faz.
Kakr{nekoli sekundarne faze so neza`elene, ker lahko bistveno vplivajo na elektri~ne lastnosti kon~nih materialov. V vzorcih z
relativno visokim dodatkom stroncija smo identificirali s stroncijem bogato fazo Sr2CrO4. Dopiranje s kalcijem se v vzorcih po
sintezi lahko odrazi v nastanku faze CaCr2O4, medtem ko dopiranje z barijem pri vi{jih koncentracijah lahko vodi v nastanek
BaCrO4 ali BaCO3. Na vsebnost sekundarnih faz lahko vplivamo s pogoji kalcinacije oziroma sintranja. Sekundarne faze, ki
med toplotno obdelavo vzorca tvorijo dodatna zrna ali teko~o fazo, vplivajo na razvoj mikrostrukture med sintranjem. Med
preiskovanimi sestavami, so najbolj perspektivni materiali La0.75SrxCa0.25-xCr0.5Mn0.5O3 (x = 0,05 – 0,15), ker imajo enofazno
mikrostrukturo `e po sintranju pri 1200 °C. Sestave z dodatki Ba ali Mg pa pri 1200 °C vsebujejo izlo~ke sekundarnih faz, ki v
mikrostrukturi ostanejo tudi po sintranju pri vi{jih temperaturah.
Klju~ne besede: analiza zgorevanja, perovskit, termi~na analiza, rentgenska difrakcija prahu, kvantitativna analiza mikrostruk-
ture
1 INTRODUCTION
Among fuel cells, their high-temperature form is the
most efficient (>70 % with fuel regeneration) for gaseous
fuels’ conversion directly into electrical power and one
of the cleanest way to produce electricity due to their
low greenhouse-gas emissions.1–3 However, the perfor-
mance and stability of a SOFC are critically dependent
on the activity and structural stability of various cell
components, including the electrolyte, anode, cathode,
and interconnect. The most significant technical barriers
currently addressed relate to anode and cathode.4
Conventionally used materials for the anodes in ceramic
fuel cells are Ni/YSZ composite materials, due to their
high electrical conductivity and electro-chemical activity
for fuel oxidation reaction. Nevertheless, Ni has several
weaknesses, among others a tendency to coke in a
hydrocarbon environment and irreversible poisoning of
the catalyst by sulphur.5 The major challenges in the
selection of these types of fuel-cell anodes include the
poisoning issue and carbon deposition,6 surface diffusion
of adsorbed reactant gases and charge transfer at the
Materiali in tehnologije / Materials and technology 52 (2018) 1, 51–58 51
UDK 67.017:544.344.016.2:549.641 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(1)51(2018)
electrode-pore phase boundary. Regarding the develop-
ment of the anode material, it is essential to be able to
engineer the mass and charge-transfer through the bulk
through the development of appropriate, efficient pro-
cessing and fabrication techniques.
Conductive oxides including perovskites with nomi-
nal formula LaxSr1-xMn0,5Cr0,5O3- (LSCM) are known to
have reasonable catalytic activity towards the oxidation
of hydrogen and methane, in addition to the good coke
tolerance and improved sulphur-poisoning resistance.7
Lanthanum substitution in lanthanum chromite oxides by
alkali earth elements (specifically Ca, Sr, and Ba) signi-
ficantly affects their electrochemical performance.8,9 The
redox stability of doped chromites is probably due to the
high chemical stability of Cr3+ at the B-site of the perov-
skite among the first-row transition-metal system
(Cr3+>Fe3+>Mn3+>Co3+).10,11 According to Deleebeeck
and co-authors, the elimination of Sr in
La1-xSrxMn1-yCryO3± improves electronic conductivity in
a wet hydrogen atmosphere and also enhances thermo-
chemical stability.12 Relatively high strontium content
(x = 0.2) also results in poorer catalytic activity, while
catalytic activity towards hydrogen oxidation improves
with increasing manganese content (y = 0.4 – 0.6).13
Therefore, the question of an optimal perovskite material
composition remains open. Microstructure development
and final electrical properties strongly depend on the
presence of isolative secondary phases formed in perov-
skite during the preparation process and/or subsequent
sintering. Several routes are possible to prepare complex
oxide powders, including solid-state reaction,14 the
chelating method,15 gel casting,16 and co-precipitation17.
Among the various preparation techniques, combustion
synthesis18–21 enables the preparation of ultrafine soft
agglomerated metal oxide powder. The secondary phase
present after synthesis can be controlled by subsequent
calcination.
In the present contribution, materials with mixed
doping at the A site of the perovskite by earth alkali
elements were prepared for the first time. A series of
complex metal oxide materials with general formula
La0.75SrxA0.25-xCr0.5Mn0.5O3 (A = Ba, Ca, Mg, x varies
between 0 and 0.25) were prepared by combustion syn-
thesis. The thermal behaviour of the citrate-nitrate gels
was determined with thermogravimetric analyses. After
synthesis, samples were pressed into pellets and sintered
at different temperatures. The crystalline phase forma-
tion was followed by X-ray powder diffraction and the
relationship between the presence of the secondary phase
in the complex metal oxide and microstructure deve-
lopment was studied in relation to output chemical
composition.
2 EXPERIMENTAL PART
Samples with the nominal composition
La0.75SrxA0.25-xCr0.5Mn0.5O3 (A = Ba, Ca, Mg, x varies
between 0 and 0.25) were prepared by modified citrate-
nitrate combustion reaction (Table 1). Stoichiometric
amounts of metal nitrates La(NO3)3⋅6H2O, Sr(NO3)2,
Ca(NO3)2, Mg(NO3)2, Ba(NO3)2, Cr(NO3)3⋅9H2O and
Mn(NO3)2⋅4H2O and citric acid (analytical reagent
grade) were dissolved with minimum quantities of water.
Five or six reactant solutions were mixed to form a reac-
tion solution and then preserved at 60 °C under vacuum
(p = 5–7 mbar) until it transformed into a gel reactive
mixture (at least 6 hours). The citrate/nitrate molar ratios
in the starting solutions were 0.18, respectively. After
water evaporation, the gel was then slightly crushed in an
agate mortar and uni-axially pressed into pellets ( = 12
mm, h = 30 mm, p = 17 MPa). These samples were
placed on a corundum plate and ignited at the top of the
pellet with a hot tip to start an auto-ignition reaction that
travels as a reaction zone throughout the pellet. During
the exothermic combustion reaction, high temperatures
are reached in a very short reaction time, yielding a
fine-powdered product.
Table 1: Compositions, sample notations and secondary phase present
in as prepared samples
Sample composition Sample name
Secondary phase
in as prepared
sample
La0.75Mg0.25Sr0Cr0.5Mn0.5O3 Mg25Sr0 /
La0.75Mg0.20Sr0.05Cr0.5Mn0.5O3 Mg20Sr5 Sr2CrO4
La0.75Mg0.15Sr0.10Cr0.5Mn0.5O3 Mg15Sr10 Sr2CrO4
La0.75Mg0.10Sr0.15Cr0.5Mn0.5O3 Mg10Sr15 Sr2CrO4
La0.75Mg0.05Sr0.2Cr0.5Mn0.5O3 Mg5Sr20 Sr2CrO4
La0.75Ca0.25Sr0Cr0.5Mn0.5O3 Ca25Sr0 CaCrO4
La0.75Ca0.20Sr0.05Cr0.5Mn0.5O3 Ca20Sr5 /
La0.75Ca0.15Sr0.10Cr0.5Mn0.5O3 Ca15Sr10 /
La0.75Ca0.10Sr0.15Cr0.5Mn0.5O3 Ca10Sr15 Sr2CrO4
La0.75Ca0.05Sr0.20Cr0.5Mn0.5O3 Ca5Sr20 Sr2CrO4
La0.75Ba0.25Sr0Cr0.5Mn0.5O3 Ba25Sr0 BaCrO4,
La0.75Ba0.20Sr0.05Cr0.5Mn0.5O3 Ba20Sr5 BaCO3
La0.75Ba0.15Sr0.10Cr0.5Mn0.5O3 Ba15Sr10 Sr2CrO4
La0.75Ba0.10Sr0.15Cr0.5Mn0.5O3 Ba10Sr15 Sr2CrO4
La0.75Ba0.05Sr0.20Cr0.5Mn0.5O3 Ba5Sr20 Sr2CrO4
La0.75Sr0.25Cr0.5Mn0.5O3 Sr25 Sr2CrO4
The thermal behaviour (TG, DTG) of the reactive
gels prior to the combustion was followed by thermo-
gravimetric analyses (Netzsch STA 449 F3) at a heating
rate of 10 K min–1. The synthesized powders were milled
in an agate mortar, un-axially pressed into pellets
(100 MPa), and sintered at various temperatures
(1200 °C, 1300 °C, 1400 °C and 1500 °C) for 1 h. The
calcined and sintered samples were analysed with a
PANalytical X’Pert PRO MPD apparatus. Data were
collected in the range 2 from 20° to 90° in steps of
0.033°. For the determination of the microstructure, the
sintered tablets were polished (diamond pastes 3 μm and
0.25 μm), thermally etched, and subsequently analysed
with an FE-SEM Zeiss ULTRA plus. The quantitative
analyses of the microstructures were performed on
digital images (the images were digitized into pixels with
T. SKALAR et al.: NOVEL MATERIALS BASED ON La0.75SrxA0.25-xCr0.5Mn0.5O3 (A=Ba, Ca, Mg) AS FULL-CERAMICS ...
52 Materiali in tehnologije / Materials and technology 52 (2018) 1, 51–58
255 different grey values) using Axiovision4.8 image-
analysis software.
3 RESULTS AND DISCUSSION
The citrate-nitrate gel combustion described above is
used as a technique for the preparation of complex metal
oxides La1-xSrxA0,25-xMn0,5Cr0,5O3±, in which the stron-
tium at the A site is partly substituted by Ba, Ca, or Mg,
and x varies between 0 and 0.25.
To obtain a better insight into the course of the
citrate-nitrate combustion reaction of various reactive
gels, a series of thermo-analytical tests is conducted
(Figure 1). TG in combination with EGA may be used to
study thermal stabilities of the reactive gel precursors
and to unveil the reaction mechanism by determining
volatile products of the citrate-nitrate combustion. It is
evident that the citrate-nitrate combustion reaction
proceeds over several consecutive steps. The first two
intervals of mass losses (150–180 °C and 200–300 °C,
respectively) are directly related to the citrate-nitrate
combustion itself, as described in the literature.22,23 This
citrate-nitrate combustion is accompanied by the release
of volatile products H2O, CO2, CO and NO and is very
similar in all investigated systems. The third step of mass
loss (300–380 °C) is a consequence of the so-called
citrate after-burning, where some residual citrate reacts
with oxygen from surrounding atmosphere. Peak tempe-
ratures of the first two intervals of mass losses differ
slightly if Ba2+, Ca2+ or Mg2+ ions are added to the
original mixture of metal nitrates (La, Sr, Cr, and Mn)
and citrate. It is well known from the literature that some
metal ions (especially transition elements) exhibit a
catalytic effect towards the citrate-nitrate combustion.24,25
However, by comparing the TG curves of the investi-
gated systems it appears that slight substitution of
original metal ions with Ba2+, Ca2+ or Mg2+ hinder the
combustion reaction meaning that these three ions do not
act catalytically. Solid residue (ash) above the citrate
after-burning temperature is a mixture of nano-sized
particles predominantly of the perovskite phase with
minor additions of some secondary phases. These secon-
dary phases (normally unreacted nitrates, formed carbo-
nates or mixed oxides) are further decomposed or trans-
formed during subsequent heating. In a combustion
system, the subsequent heating occurs inside the com-
bustion wave since citrate-nitrate combustion reaction
provides temperatures around 1000 °C. During the
investigated LSCM synthesis, very few secondary phases
are formed. In contrast, additions of Ba2+, Ca2+ or Mg2+
always resulted in the formation of secondary phases.
After the addition of Ba2+ into the reactive precursor, the
decomposition of a secondary phase may be found in
temperature ranges 560–580 °C and 695–715 °C.
Decomposition between 560–580 °C is accompanied by
NO release, implying that some Ba2+ ions are not com-
pletely chelated with citrate but rather partially remain in
the reactive gel as Ba(NO3)2. Subsequent decomposition
between 695–715 °C is accompanied by O2 release,
which suggests that Ba-containing Ruddlesden–Popper
phases are also formed during the combustion reaction.
The addition of Ca2+ or Mg2+ ions into initial precursor
mixtures after citrate-nitrate combustion causes the for-
mation of some undesired Ca-containing or Mg-contain-
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Materiali in tehnologije / Materials and technology 52 (2018) 1, 51–58 53
Figure 1: Thermal analysis of reactive gels (Ba25Sr0, Ca25Sr0, Mg25Sr0 and Sr25). Large graph represents mass losses; small graph inserts are
QMS signals for 30, 32, and 44 fragments
ing carbonates, which are decomposed through CO2
release into mixed oxides at 595–630 °C or 655–675 °C,
respectively. Thus, final as prepared ashes do not include
metal nitrates or metal carbonates. Instead, other than the
predominantly present perovskite phase, the as prepared
samples may contain minor amounts of Ba-, Ca- or
Mg-rich mixed oxides. Some of these secondary oxides
are subsequently dissolved in the main perovskite phase
during material sintering (LSMC or LSCCM systems);
however, in the case of Ba- or Mg-doping inclusions of
secondary phases are present also after sintering.
As-synthesized samples are not well crystalized and
the main phase corresponded to perovskite crystal struc-
ture. In samples with relatively higher strontium content
(x = 0.15 to 0.25), one of so-called Ruddlesden Popper
phases Sr2CrO4 was detected. The formation of a stron-
T. SKALAR et al.: NOVEL MATERIALS BASED ON La0.75SrxA0.25-xCr0.5Mn0.5O3 (A=Ba, Ca, Mg) AS FULL-CERAMICS ...
54 Materiali in tehnologije / Materials and technology 52 (2018) 1, 51–58
Figure 3: SEM micrographs of Mg15Sr10 and Ba15Sr10 after sintering at 1200 °C (left column) and after sintering at 1500 °C (right column)
Figure 2: X-ray powder diffraction patterns of all samples after sintering at 1200 °C and 1500 °C
tium-rich secondary phase may be expected during syn-
thesis in an oxygen-deficient atmosphere.26 In samples
Ba25Sr0 (x = 0) and Ba20Sr5 (x = 0.05), BaCrO4, and
BaCO3 were identified respectively, while in sample
Ca25Sr0 traces of CaCrO4 were found. In samples con-
taining magnesium, no secondary magnesium phases
were detected after the synthesis.
After 1-hour sintering at various temperatures (in the
range from 1200 °C to 1500 °C) in all samples, the
perovskite phase turns out to be well crystallized. In all
compositions containing magnesium (x = 0 to 0.20) after
sintering at 1200 °C there are two possible secondary
phases Mg2MnO4 and/or Mn2CrO4, and they remain in
the samples after sintering as high as 1500 °C. In sam-
ples containing barium (x = 0 to 0.10) barium secondary
phases BaMnO4 and BaCrO4 were found, indicating that
higher calcination temperatures do not increase the
solubility of Ba in LaCrO3.27 In samples with higher
strontium content (x = 0.20), traces of La2O3 appear after
sintering at higher temperatures (1300 °C to 1500 °C). In
samples containing calcium (x = 0 to 0.15), no particular
calcium or strontium secondary phases were found after
sintering at 1200 °C and also at higher sintering tempera-
tures. In samples with calcium, the presence of secon-
dary phases was negligible after the sintering at 1200 °C,
which makes compositions in which strontium is partly
substituted with calcium favourable in comparison to
compositions containing magnesium or barium.
One of the crucial tasks in the preparation of complex
oxide material as an anode in high-temperature fuel cells
is to achieve good connections among solid perovskite
grains without the appearance of secondary phases. At
the same time, pores in the electrode should be conti-
nuous. Consistent with the results of powder diffraction
in samples containing magnesium and barium grains of
secondary phases based on magnesium and barium were
observed in the microstructure after sintering at 1200 °C
and above (Figure 3). In Mg-containing samples,
smaller dark grains were ascribed to Mg-Mn and/or
Mn-Cr phase, while larger bright grains were ascribed to
perovskite. After sintering at 1500 °C, perovskite grains
formed dense regions with inclusions of secondary
phases. In Ba-containing samples after sintering at
1200 °C, a secondary liquid phase was present between
light grey perovskite round grains, while after sintering
at 1500 °C the morphology of perovskite grains is
changed significantly. The microstructure of sintered
samples consists mainly of cubic grains, surrounded by a
thin layer of secondary liquid phase, as it was described
in the literature for the case of La0.7Ba0.3Cr0.5Mn0.5O3-
after sintering at 1650 °C for five hours in air.28
The microstructures of the samples in which stron-
tium is partly replaced with calcium (Ca10Sr15 and
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Materiali in tehnologije / Materials and technology 52 (2018) 1, 51–58 55
Figure 4: SEM micrographs of Ca15Sr10 (left column) and Ca10Sr15 (right column) samples sintered at 1200 °C, 1300 °C, 1400 °C, and 1500 °C
Ca15Cr10) after sintering at temperatures from 1200 °C
to 1500 °C are presented in Figure 4. These samples
were selected since no secondary crystal phases were
identified by X-ray analysis after sintering at 1200 °C.
After sintering at 1200 °C for both samples, a homo-
genous microstructure was formed in which grains were
well connected to each other. At the same time, pores are
uniformly distributed in the microstructure and remain
opened to the surface allowing gaseous reactants to enter
and product to leave the reaction site at the potential
anode. After sintering at 1300 °C, porosity is apparently
reduced, and pores started to close. In Ca10Sr15 after
heating at 1400 °C, inhomogeneous microstructures with
dense areas of fine grains and dense areas of coarse
grains together with separate large pores can be
observed. In sample Ca15Sr10 the size of the grains is
much more uniform, and the sample is rather dense.
After sintering at 1500 °C, the microstructure of
Ca10Sr15 consisted of large grains and some pores at
grain borders, while sample Ca15Sr10 is fully dense and
some small grains of the secondary phase are present.
From these observations, it may be assumed that Sr sub-
stitution with Ca to some amount (x = 0.10, 0.15)
accelerates sintering at lower temperatures (1200 °C),
due to the low-temperature liquid phase formation.
Adding Ca to perovskite above or below certain concen-
trations (x = 0, 0.05, 0.20 and 0.25) does not work on the
densifying process in the same way. It is expected that in
mixed CaO-SrO-Cr2O3 system liquid phase forms at
lower temperatures than in specific CaO-Cr2O3 29 and
SrO-Cr2O3 19 systems.
Quantitative analysis of microstructures in all sin-
tered samples after sintering was also performed, while
only those results of samples containing calcium are
summarized in Table 2. Parameters dmax, dmin,  and d
are represented as maximum and minimum intercept
lengths in one direction, shape factor, and the diameter
of the area-analogue circle, respectively.
As expected, the green density of the samples
containing Ca is decreasing with a gradual strontium
replacement at the A site in the perovskite, since calcium
has a lower molecular mass than strontium. Sintered den-
sity depends greatly on the strontium substitution with
calcium as well as the sintering temperature. The highest
density (3.00 g cm–3) after sintering at 1200 °C achieved
sample with medium calcium content (Ca15Sr10), while
at the densification process of this sample is not so pro-
nounced higher sintering temperatures. After sintering at
temperatures above 1200 °C, the sintered density
increases faster in samples with strontium content higher
than calcium. Thus, in samples (x = 0.15 and 0.20), after
sintering at 1300 °C and 1400 °C, considerable increase
in density was noticed. After sintering at 1500 °C in
samples containing both calcium and strontium (x = 0 to
0.20). densities are rather high, and they are increasing
with strontium content (from 6.26 g cm–3 to 6.65 g cm–3).
Exceptions are the samples with no Ca (Ca25Sr0) or no
Sr (Sr25), where intensive densification starts as high as
1500 °C, meaning that liquid phase, which is formed in
the specific systems of CaO-Cr2O3 and SrO-Cr2O3, does
not accelerate sintering. Thus, final densities reached in
Ca25Sr0 or Sr25 are only 5.43 g cm–3 and 6.10 g cm–3,
respectively.
Table 2: Average values selected microstructure parameters, green
and sintering density for La0,75CaxSryCr0,5Mn0,5O3 sintered at 1200
°C, 1300 °C, 1400 °C, and 1500 °C.
sample dmax /μm
dmin /
μm Ngrains
Sgrain /
μm2
d /
μm 
sinter/
g cm–3
12
00
°C
Ca25Sr0 0.90 0.02 2223 0.09 0.31 0.80 2.21
Ca20Sr5 0.67 0.02 3290 0.05 0.23 0.82 2.45
Ca15Sr10 0.56 0.01 1736 0.04 0.20 0.82 3.00
Ca10Sr15 0.60 0.01 1097 0.04 0.21 0.82 2.70
Ca5Sr20 0.82 0.01 1018 0.07 0.27 0.80 2.74
Ca0Sr25 0.47 0.01 1419 0.01 0.10 0.83 2.55
13
00
°C
Ca25Sr0 1.11 0.02 1455 0.14 0.41 0.81 2.72
Ca20Sr5 0.99 0.01 677 0.13 0.37 0.79 3.27
Ca15Sr10 0.85 0.01 351 0.14 0.39 0.82 3.80
Ca10Sr15 0.86 0.01 143 0.15 0.39 0.81 4.11
Ca5Sr20 1.52 0.02 500 0.41 0.62 0.81 4.47
Ca0Sr25 0.76 0.01 291 0.07 0.26 0.80 3.40
14
00
°C
Ca25Sr0 1.42 0.02 525 0.33 0.57 0.82 3.59
Ca20Sr5 1.35 0.01 183 0.28 0.51 0.80 3.63
Ca15Sr10 1.73 0.02 756 0.38 0.62 0.81 5.26
Ca10Sr15 2.19 0.02 395 0.69 0.80 0.81 6.18
Ca5Sr20 3.28 0.03 340 1.66 1.23 0.82 6.68
Ca0Sr25 1.37 0.01 414 0.25 0.46 0.68 3.89
15
00
°C
Ca25Sr0 3.02 0.02 195 1.12 1.03 0.81 5.43
Ca20Sr5 2.50 0.01 72 0.80 0.81 0.78 6.26
Ca15Sr10 2.55 0.02 42 2.01 1.27 0.77 6.57
Ca10Sr15 2.65 0.03 36 2.07 1.25 0.80 6.60
Ca5Sr20 3.74 0.03 330 2.09 1.36 0.77 6.65
Ca0Sr25 1.75 0.01 80 0.49 0.53 0.84 6.10
Results of the quantitative microstructural analysis
are in good agreement with optical observations. Si-
ntered density increases and optically determined poro-
sity decreases with the increasing sintering temperature.
After sintering at 1200 °C samples Ca20Sr5, Ca15Sr10,
Ca10Sr15, and Ca5Sr20 also form relatively small
average grain sizes of 0.23, 0.20 μm, 0.21 μm 0.27 μm,
respectively. The fine-grained microstructure is favour-
able for ensuring good catalytic activity of the electrode.
Partial strontium substitution with calcium in sam-
ples with x = 0.05 to 0.20 resulted in moderate average
grain growth from 0.20 μm to 1.36 μm. In the sample
containing only calcium Ca25Sr0, average grain size
grows from 0.31 μm to 1.03 μm and in the sample with
no calcium Sr25 grains grow from 0.10 μm to 0.53 μm.
From these data, we may assume that only Sr or Ca
additions to LCM perovskite do not accelerate sintering
to a level as in the case in which both elements Ca and Sr
are added. According to the phase diagram, the
SrO–Cr2O3 secondary phase SrCrO4 forms a liquid phase
T. SKALAR et al.: NOVEL MATERIALS BASED ON La0.75SrxA0.25-xCr0.5Mn0.5O3 (A=Ba, Ca, Mg) AS FULL-CERAMICS ...
56 Materiali in tehnologije / Materials and technology 52 (2018) 1, 51–58
due to eutectic and peritectic reactions that promote
sintering, but only by adding Sr to perovskite under cer-
tain concentration x = 0.20.19,30 The Sr2CrO4-rich phase
presence in the as prepared materials was confirmed in
samples with higher Sr content (x = 0.15, 0.20 and 0.25),
which at higher temperatures first turns to SrCrO4 and
later forms a liquid phase to promote perovskite sin-
tering.20 With noticeable grain growth, the slightly de-
creased shape factor  indicates that grains become
diverse to ideal spheres, as already observed in the case
of combustion-derived LSCM ceramics.21
4 CONCLUSIONS
La0.75SrxA1-xMn0.5Cr0.5O3 perovskite materials in
which Sr was replaced with alkali earth ions Mg, Ca and
Ba (x = 0 to 0.25) were prepared using citrate-nitrate
combustion synthesis. The solid residue in the as-synthe-
sized samples contains the main perovskite phase with
some minor additions of secondary phases (formed car-
bonates or mixed oxides).
In sintered samples at 1200 °C containing magne-
sium (x = 0 to 0.20), there are two possible secondary
phases Mg2MnO4 and/or Mn2CrO4. These two secondary
phases are also present after sintering as high as
1500 °C. In samples containing barium (x = 0 to 0.10),
the secondary phases BaMnO4 and BaCrO4 are found,
indicating their relatively poor solubility in LaCrO3. In
Ca-doped samples in which calcium content is higher
than strontium (x = 0.05 to 0.10), the presence of
secondary phases after the sintering was negligible. This
makes Ca-doped LSCM compositions favourable, in
comparison to Mg-doped or Ba-doped LSCMs.
After sintering at 1200 °C in Ca-doped LSCMs,
relatively small grains of the main perovskite phase are
formed. At the same time, the LSCCM phase is conti-
nuous, and the pores remain open to the surface. Higher
sintering temperatures increase average grain sizes;
however, no grains of secondary phases may be found in
the materials’ macrostructure. In contrast, sintered
Ba-doped or Mg-doped LSCMs always contain unde-
sired secondary phases at the grain boundaries between
main perovskite grains.
Acknowledgment
We thank for the financial support of the Ministry of
Higher Education, Science and Technology of the Re-
public of Slovenia through grants P1–0175.
5 REFERENCES
1 J. D. Larminie, Fuel Cell Systems Explained, 2nd Edition. 2nd ed.
John Wiley & Sons, Chichester 2003
2 Rand DAJ, Dell RM. Hydrogen energy: challenges and prospects,
RSC Publishing, Cambridge 2008
3 S. C. Singhal, K. Kendall, High-temperature solid oxide fuel cells:
fundamentals, design, and applicatons. Elsevier Advanced Techno-
logy, New York 2003
4 A. Atkinson, S. Barnett, R. J.Gorte, J. T. D. Irvine, A. J. McEvoy, M.
Mogensen, S. C. Singhal, J. Vohs, Advanced anodes for high tem-
perature fuel cells, Nat. Mater., 3 (2004) 17–24, doi:10.1038/
nmat104
5 H. Hurokawa, T. Z. Sholklaper, C. P. Jacobson, L. C. De Jonghe, S. J.
Visco, Ceria nanocoating for sulphur tolerant Ni-based anodes of
solid oxide fuel calls, Electrochem.Solid State, 100 (2007) 135–138,
doi: 10.1149/1.2748630
6 S. Macintosh, R. J. Gorte, Direct hydrocarbon solid oxide fuel cells,
Chem. Rev., 104 (2004) 4845-4865, doi:10.1021/cr020725g
7 C. Sun, U. Stimming, Recent anode advances in solid oxide fuel
cells, J. Pow. Sources, 171 (2007) 247–260, doi:10.1016/j.jpowsour.
2007.06.086
8 S. P. Jiang, L. Liu, K. O. Ong, P. Wu, J. Li, J. Pu, Electrical
conductivity and performance of doped LaCrO3 perovskite oxides for
solid oxide fuel cells, J. Power. Sources, 176 (2008) 82,
doi:10.1016/j.jpowsour.2007.10.053
9 K. O. Ong, P. Wu, J. Li, S. P. Jiang, L. Liu, Optimization of electrical
conductivity of LaCrO3 through doping: A combined study of
molecular modeling and experiment, Appl. Phys. Lett., 90 (2007),
doi:10.1063/1.2431780
10 R. Koc, H.U. Anderson, Electrical conductivity and Seebeck coeffi-
cient of (La, Ca) (Cr, Co)O3, J. Mater. Sci., 27 (1992), 5477–5482,
doi: 10.1007/BF00541609
11 J. Yoo, A. Verma, A. J. Jacobson, T. A. Ramanarayanan (Ed.), Ionic
and Mixed Conducting Ceramics IV, The Electrochemical Society
Inc., 2001, 27
12 L. Deleebeeck, J. L. Fournier, V. Birss, Comparison of Sr-doped and
Sr-free La1 - xSrxMn0.5Cr0.5O3 ±  SOFC Anodes, Solid State Ionics,
181 (2010) 1229-1237, doi:10.1016/j.ssi.2010.05.027
13 L. Deleebeeck, J. L. Fournier, V. Birss, Catalysis of the hydrogen
oxidation reactions by Sr-doped LaMn1 - yCryO3 ±  oxides, Solid State
Ionics, 203 (2011), 69-79
14 S. Tao, J. T. S. Irvine, Synthesis and characterization of
La0.75Sr0.25Cr0.5Mn0.5O3– a redox stable, efficient perovskite anode
for SOFCs, J. Electrochem. Soc., 151 (2004) 252–259,
doi:10.1149/1.1639161
15 J. Wan, J. H. Zhu, J. B. Goodenough, La0.75Sr0.25Cr0.5Mn0.5O3- + Cu
composite anode running on H2 and CH4 fuels, Solid State Ionics,
177 (2006), 1211–121, doi:10.1016/j.ssi.2006.04.046
16 S. P. Jiang, L. Zhang, Y. Zhang, Lanthanum strontium manganese
chromite cathode and anode synthesized by gel-casting for solid
oxide fuel cells, J. Mater. Chem., 17 (2007), 2627–2635,
doi:10.1039/ b701339f
17 R. Pelosato, C. Cristiani, G. Dotelli, M. Mariani, A. Donazzi, I. N.
Sora, Co-precipitation synthesis of SOFC electrode materials, Int. J.
Hydrogen Energ., 38 (2013), 480–491, doi:10.1016/j.ijhydene.2012.
09.063
18 B. H. Sang, C. Pyeong-Seok, H. C. Yoon, L. Dokyol, L. Jong-Heun,
Preparation of La0.75Sr0.25Cr0.5Mn0.5O3- fine powders by carbonate
coprecipitation for solid oxide fuel cells, J. Power Sources, 195
(2010), 124–129, doi:10.1016/j.jpowsour.2009.06.078
19 E. M. Levin, H. F. McMurdie, Phase Diagrams for Ceramists, The
American Ceramic Society, 1975, 29
20 P. H. Duvigneaud, Factors Affecting the Sintering and the Electrical
Properties of Sr-Doped LaCrO3, J. European Cer. Soc., 14 (1994),
359–367, doi:10.1016/0955-2219(94)90073-6
21 K. Zupan, M. Marin{ek, Combustion Derived La1-xSrxMn0,5Cr0,5O3±
(x = 0.20, 0.25) Perovskite: Preparation, Properties, Characterization,
Materials and technology, 48 (2014), 885–891
22 M. Marin{ek, K. Zupan, J. Ma~ek, Citrate-nitrate gel transformation
behavior during the synthesis of combustion-derived NiO-yttria-
stabilized zirconia composite, J. Mater. Res., 18 (2003), 1551–1560
T. SKALAR et al.: NOVEL MATERIALS BASED ON La0.75SrxA0.25-xCr0.5Mn0.5O3 (A=Ba, Ca, Mg) AS FULL-CERAMICS ...
Materiali in tehnologije / Materials and technology 52 (2018) 1, 51–58 57
23 H. W. Wang, D. A. Hall, F. R. Sale, A thermoanalytical study of the
metal nitrate-EDTA precursors for lead zirconate titanate ceramic
powders, J. Therm. Anal., 41 (1994), 605–620, doi:10.1007/
BF02549337
24 S. T. Aruna, M. Muthuraman, K. C. Patil, Synthesis and properties of
Ni-YSZ cermet: anode material for solid oxide fuel cells, Solid State
Ionics, 111 (1998), 45–51, doi:10.1016/S0167-2738(98)00187-8
25 M. Marin{ek, J. Kemperl, B. Likozar, J. Ma~ek, Temperature Profile
Analysis of the Citrate-Nitrate Combustion System, Ind. Eng. Chem.
Res., 47 (2008), 4379–4386, doi:10.1021/ie800296m
26 H. Yokohawa, N. Sakai, T. Kawada, M. Dokiya, Chemical Thermo-
dynamic Considerations in Sintering of LaCrO3?-?Based Perovski-
tes, J. Electrochem. Soc. 138 (1991), 1018–1027, doi:10.1149/
1.2085708
27 J. H. Ouyang, S. Sasaki, T. Murakami, K. Umeda, Spark-plasma-
sintered ZrO2(Y2O3)-BaCrO4 self-lubricating composites for high
temperature tribological applications, Ceram. Int., 31
(2005), 543–553, doi:10.1016/j.ceramint.2004.06.020
28 L. Zhang, X. Chen, S. P. Jiang, H. Q. He, Y. Xiang, Characterization
of doped La0.7A0.3Cr0.5Mn0.5O3- (A=Ca, Sr, Ba) electrodes for solid
oxide fuel cells, Solid State Ionics, 180 (2009), 1076–1082,
doi:10.1016/jssi.2009.05.010
29 J. P. R. De Villiers, J. Mathias, A. Maun, Phase relations in the
System CaO- Cromium Oxide- SiO2 in Air, and Solid Solution
Relations along the Ca2SiO4- Ca3(CrO4)2 Join, Trans. Inns. Min.
Matall, Sect. C, 96 (1987), C55–C62
30 K. Zupan, M. Marin{ek, T. Skalar, Phase and microstructure
development of LSCM perovskite materials for SOFC anodes
prepared by the carbonate-coprecipitation method, Materials and
technology, 50 (2016), 743–748, doi: 10.17222/mit.2015.232
T. SKALAR et al.: NOVEL MATERIALS BASED ON La0.75SrxA0.25-xCr0.5Mn0.5O3 (A=Ba, Ca, Mg) AS FULL-CERAMICS ...
58 Materiali in tehnologije / Materials and technology 52 (2018) 1, 51–58