A novel spray-pyrolysis technique to produce nanocrystalline lanthanum strontium manganite powder

Article abstract of DOI:10.1111/j.1551-2916.2005.00152.x

Nanocrystalline, single phase, and highly homogeneous La _0.84Sr_0.16MnO₃ (LSM) powder was prepared by a unique spray-pyrolysis process for solid oxide fuel cell applications. Atomization of a citrate-nitrate precursor solution consisting of La ³⁺, Sr²⁺, and Mn²⁺ ions in the molar ratio 0.84:0.16:1.0, which can initiate a controlled exothermic anionic oxidation-reduction reaction leading to a self-propagating auto-ignition (self-ignition) reaction within individual droplets led to the conversion of the precursor to their corresponding single-phase LSM powder. Characterization of the as-sprayed and calcined products by X-ray powder diffraction, thermal analysis, and microstructural analysis confirmed the formation of nanocrystalline single-phase LSM powder by this process.

Full text of DOI:10.1111/j.1551-2916.2005.00152.x

J. Am. Ceram. Soc., 88 [4] 971–973 (2005)
DOI: 10.1111/j.1551-2916.2005.00152.x
ournal
J
A Novel Spray-Pyrolysis Technique to Produce Nanocrystalline
Lanthanum Strontium Manganite Powder
Abhoy Kumar, Parukuttyamma Sujatha Devi,w Abhijit Das Sharma, and Himadri Sekhar Maiti
Electroceramics Division, Central Glass and Ceramic Research Institute, Kolkata 700 032, India
Nanocrystalline, single phase, and highly homogeneous
reaction utilizes metallic nitrates, acetates, etc. as precursor solu-
tion.8–10 In this context, it is worth noting the work of Mancic
et al.,11 where a urea–nitrate combustion mixture has been used
as a precursor for spray pyrolysis to produce high Tc supercon-
ducting oxides.
La0.84Sr0.16MnO3 (LSM) powder was prepared by a unique
spray-pyrolysis process for solid oxide fuel cell applications.
Atomization of a citrate–nitrate precursor solution consisting of
La31, Sr21, and Mn21 ions in the molar ratio 0.84:0.16:1.0,
which can initiate a controlled exothermic anionic oxidation-re-
duction reaction leading to a self-propagating auto-ignition
(self-ignition) reaction within individual droplets led to the con-
version of the precursor to their corresponding single-phase
LSM powder. Characterization of the as-sprayed and calcined
products by X-ray powder diffraction, thermal analysis, and mi-
crostructural analysis confirmed the formation of nanocrystal-
line single-phase LSM powder by this process.
A modified citrate process named as an ‘‘auto-ignition proc-
ess’’ was developed earlier in our laboratory to prepare LaM-
nO3-based oxides.12–14 In this process, the precursor solution
consisting of metal nitrates and citric acid was allowed to po-
lymerize completely, resulting in gel formation and subsequent
auto-ignition of the gel within the container itself, leading to the
formation of micron-sized powders. Since the burning process
occurred within the glass container (generally a glass beaker),
there was no way of heat dissipation from the confined volume,
leading to particle agglomeration and inhomogeneity. Further-
more, upgrading this process for large-scale powder production
was found to be impractical. Consequently, for large-scale, eco-
nomical, and continuous production of LSM powder for SOFC
applications, a novel spray-pyrolysis process was developed that
combines the features of auto-ignition and spray-pyrolysis proc-
ess, where a controlled exothermic anionic oxidation–reduction
reaction between the anions of metal salt and citric acid leads to
a self-propagating auto-ignition (self-ignition) reaction occur-
ring within the droplets.
I. Introduction
OLID oxide fuel cell (SOFC) technology has gained consid-
S
erable attention recently as the most efficient, versatile, and
less polluting power-generating system.1–3 The electrical con-
ductivity, chemical and thermal stability at the operating tem-
peratures, and the electrochemical performance of the cell
components are the key factors controlling the cell efficiency
of an SOFC power pack. Among the different cell components
of SOFC, till today, Sr-doped lanthanum manganite (LSM) is
considered as the preferred cathode material for the modern
high-temperature SOFC.1–6 The electrochemical properties of
LSM cathode layers depend largely on the microscopic features
of the triple point boundaries (TPB), which in turn depend on
the starting precursor powder characteristics such as particle
size, shape, and their distribution.5,6 Hence, optimization of the
structure and composition of the cathode layer is of prime im-
portance in enhancing the electrochemical performance of the
cathode and thereby the overall performance of the cell.
II. Experimental Procedure
Lanthanum nitrate hexahydrate (Indian Rare Earth, Kerala,
India, 99.9%), Strontium nitrate (E Merck India Limited,
Mumbai, India, 98%), Manganese (II) acetate tetra hydrate
(E Merck India limited, 99.5%), and citric acid monohydrate
(Merck Limited, Mumbai, India, 99.5%) were used for the
preparation of the precursor solutions as described earlier.12,13
Briefly, for the laboratory-scale preparation of the LSM pow-
der, aqueous solutions of lanthanum nitrate, strontium nitrate,
and manganous (II) acetate were mixed in a beaker to have
(La1Sr):Mn molar ratio of 1:1. To this mixed solution, calcu-
lated quantity of citric acid monohydrate was added to obtain a
fixed C/N ratio of 0.75. This mixed solution was heated on a
magnetic stirrer (2001751C) with continuous stirring and was
allowed to evaporate. The solution became viscous, turned into
a gel that slowly foamed, swelled, and finally burnt on its own
with glowing flints that propagated to the entire volume of the
gel container resulting in the formation of finely dispersed ash
powder. Similarly, for preparing the precursor solution for spray
pyrolysis, the components were mixed as described above but
polymerization was arrested at an intermediate stage to control
the viscosity of the solution suitable for spraying. The pH of the
solution soon after the addition of citric acid was found to be
around 1.54 and the viscosity of the mixed solution at 251C was
around 1.22 (70.05) cps. After polymerization, the pH of the
final precursor solution dropped to o1.0 while the viscosity of
the final precursor solution increased to 1.30 (70.05) cps. Such a
precursor solution was atomized through a 0.70-mm bore dou-
ble fluid nozzle. The flow rate of the precursor solution was
Various processing techniques are currently available for the
preparation of LSM powder samples.1–7 In recent times, spray
pyrolysis has been identified as a powerful technique to synthe-
size a wide variety of ceramic materials.8–10 This process has
several advantages such as simplicity in operation, ability to
produce high purity, chemically homogeneous end products,
and above all ease of continuous operation. Nevertheless, it is
difficult to control the morphology of the particles by spray
pyrolysis since particle morphologies are affected by the nature
of the starting solution, concentration, and droplet size and res-
idence time in the reactor. Additionally, due to the differences in
the rates of precipitation of the metals as salts or hydroxides
from solution, inhomogeneity within the individual particles is
often encountered while producing multicomponent ceramic
powders. It may be noted that most of the spray-pyrolysis
R. Cutler—contributing editor
Manuscript No. 10991. Received April 26, 2004; approved August 31, 2004.
Supported by Ministry of Non-conventional Energy Sources, Govt. of India.
wAuthor to whom correspondence should be addressed. e-mail: psujathadevi@cgcri.
res.in
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Vol. 88, No. 4
maintained at 26 mL/min and the air gauge pressure was kept at
140 kPa during spraying. The shear stress applied by the flowing
air disintegrates the precursor solution into fine droplets. The
solution was directly sprayed into the hot chamber of the
pyrolyzer unit (SM Scientech, Kolkata, India). The inlet tem-
perature of the spray unit was maintained at 5001C during
spraying. The atomized citrate–nitrate precursor droplets dehy-
drated and dried to the critical state, and thereafter auto-ignition
reaction was initiated as evidenced by the flashing or sparking of
the particles during ignition within the chamber. The auto-ig-
nited particles were simultaneously carried by flowing hot air
and were separated from the drying gas using a cyclone sepa-
rator. The black-colored oxide particles were collected at the
collection chamber. The separated drying gas or carrier gas was
exhausted to a gas scrubber to remove the evolved gaseous
products. The as-sprayed black powder samples were calcined at
7501C for 4 h for further studies.
Thermogravimetric analysis (TGA) and differential thermal
analysis (DTA) of the citrate–nitrate gel were carried out on a
NETZSCH STA 409 C instrument (Bavaria, Germany) with a
heating rate of 101C/min. The phase identification of the as-pre-
pared material and the calcined powders was confirmed by the
evaluation of the X-ray powder diffraction (XRD) data collect-
ed at a scan rate of 21/min on a Philips PW 3207 diffractometer
(Holland) with CuKa radiation. The morphology of the powder
was examined by scanning electron microscopy (SEM) using a
Leo 430i scanning electron microscope (Cambridge, UK). The
particle size distribution of the calcined powder was analyzed
using a Sedigraph 5100 unit (Micromeritics Instruments Corp.,
Norcross, GA).
Fig. 2. Powder X-ray diffraction patterns of (a) lab-synthesized ash
powder, (b) as-sprayed powder, (c) lab synthesized powder calcined at
7501C/4 h, (d) sprayed powder calcined at 7501C/4 h, and (e) sprayed
powder calcined at 12501C/4 h.
thermal change in the TGA and DTA of the as-sprayed powder
confirmed complete decomposition of the gel during spray
pyrolysis. The minor weight change (15%) in the TGA and
the weak exothermic peaks in the DTA of the as-sprayed pow-
der corresponds to the decomposition of the unburnt carbon-
aceous material.
(2) Effect of Temperature on Phase Formation
In order to investigate the phase evolution of sprayed powder,
ex-situ XRD measurements were carried out on samples heat-
treated at different temperatures (Fig. 2). All the high intensity
reflections in the as-sprayed powder were identical to the re-
ported pseudo-cubic phase of LSM, confirming the formation of
single phase LSM in the as-sprayed stage itself (Fig. 2(b)). On
the contrary, the lab-synthesized as-burnt powder from the same
precursor solution was mostly amorphous (Fig. 2(a)). The XRD
patterns of the 7501C calcined lab-synthesized powder and the
sprayed powder are compared in Figs. 2(c) and (d). The absence
of SrCO3 in the sprayed powder compared with lab-synthesized
powder confirmed the better homogeneity achieved through
spray pyrolysis when compared with lab-scale synthesis. The
minor exothermic peak seen in the DTA of the gel sample above
6001C may be due to the decomposition of SrCO3 .The expected
structural changes from orthorhombic to rhombohedral were
also evident in the XRD pattern (Fig. 2(e)) of the 12501C/4 h
calcined sprayed sample.
III. Results and Discussion
(1) Thermal Analysis of Gel and Ash
In order to understand the thermal decomposition characteris-
tics of the starting precursor, gel samples were collected prior to
auto-ignition and their decomposition nature was studied using
TGA/DTA (Figs. 1(a) and (b)) and compared with that of the
as-sprayed powder (Figs. 1(c) and (d)). As evident from Fig. 1,
major decomposition of the gel occurred in the temperature
range of 1801–1951C, giving rise to a very sharp intense exo-
therm in the DTA around 1871C and a nearly vertical weight
loss curve in the TGA plot. This type of single step decompo-
sition indicates a self-propagating auto-ignition reaction occur-
ring during the burning of the gel samples. Hence, it is expected
that atomization of such a precursor solution would induce
auto-ignition reaction within individual droplets in the spray
chamber that would reduce the external energy needed for the
pyrolysis reaction. The broad exotherm around 4001C in the
DTA of both gel and as-sprayed powder may be due to the ox-
idation of the remaining unburnt carbon. The absence of a sharp
(3) Particle Size Distribution and Morphology of the
Particles
In Fig. 3, particle size distribution of the as-sprayed powder is
compared with that of the 7501C/4 h calcined powder. Contrary
Fig. 1. Thermal decomposition characteristics of the La0.84Sr0.16MnO3
precursor samples (a) TGA of the gel (b) DTA of the gel (c) TGA of the
as-sprayed powder, and (d) DTA of the as-sprayed powder.
Fig. 3. Particle size distribution of the as-sprayed and calcined spray-
pyrolized La0.84Sr0.16MnO3 powder sample.

April 2005
Communications of the American Ceramic Society
973
ignition) reaction within individual droplets in addition to
controlling and maintaining compositional homogeneity. Since
inexpensive starting materials such as metal nitrates, acetates,
citric acid, and water are used, this could be an economic proc-
ess for producing mixed metal oxide nanopowders on a large
scale. One could easily vary the concentration of the precursor
solution and this could lead to increased production rate and
yield as well. We believe that these nanostructured LSM parti-
cles will improve the electrochemical performance of the cathode
layers and further work in this line is in progress.
IV. Conclusions
La0.84Sr0.16MnO3 powder was successfully prepared by a novel
auto-ignition-induced spray-pyrolysis technique by using a po-
lymerized precursor solution containing citric acid and metal
nitrates. The XRD analysis confirmed phase formation in the
as-sprayed powder itself. The as-sprayed particles appear to be
highly agglomerated with a broad size distribution as suggested
by the particle size analysis. Calcination on the other hand im-
proved the particle size distribution, where the particles are
mainly composed of well-dispersed nanocrystallites of 30–70-nm
size.
Acknowledgments
The authors thank the Director, CG&CRI for his kind permission to publish
this paper. A. Kumar is a recipient of the Junior Research Fellowship (JRF) of the
Council of Scientific & Industrial Research, Government of India. Technical sup-
port from the X-ray and SEM Divisions of CG & CRI is acknowledged.
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