Abstract
Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) perovskite-type oxide is currently one of the most promising materials for applications in solid-oxide fuel cells (SOFCs), protonic ceramic fuel cells (PCFCs), oxygen separation membranes, and catalytic membranes for methane conversion. BSCF powder synthesis has received considerable attention and new synthesis methods have been proposed to obtain nanoscale powders with high chemical homogeneity. In this study, BSCF perovskite powder has been successfully prepared by microwave assisted combustion in aqueous solution. The synthesized powder was characterized by DSC, BET, XRD, and SEM. BSCF powder presented phase homogeneity, high specific surface area (9.93 g/m2) and nanometric crystallite size (23nm). The microwave sintering has been conducted in different conditions of dwell time and temperature. The influence of the temperature and the dwell time on microstructure was evaluated by optical microscopy. The results showed that microwave sintering could achieve the same densification compared to conventional sintering with only 10% of processing time. This shorter processing time has resulted in a reduction of grain size of up to 46.5%, compared to conventional sintering.
MIEC; BSCF; microwave assisted combustion synthesis; microwave sintering
1 Introduction
Ba0.5Sr0.5Co0.8Fe0.2O3-δ
(BSCF) is a mixed ionic-electronic conducting (MIEC) oxide which has received
attention as a promising material in advanced technologies, such as, solid-oxide
fuel cells (SOFCs) or protonic ceramic fuel cells (PCFCs) cathodes, and membrane
reactors in air separation and catalytic conversion of methane11. Zhou W, Ran R and Shao Z. Progress in understanding and
development of Ba0.5Sr0.5CoFe0.2O.
0.83-δ based cathodes for intermediate-temperature
solid-oxide fuel cells: A reviewJournal of Power Sources. 2009; 192(2):231-246.
http://dx.doi.org/10.1016/j.jpowsour.2009.02.069.
http://dx.doi.org/10.1016/j.jpowsour.200...
,22. Sunarso J, Baumann S, Serra JM, Meulenberg WA, Liu S, Lin YS, et
al. Mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen
separation. Journal of Membrane Science. 2008; 320(1-2):13-41.
http://dx.doi.org/10.1016/j.memsci.2008.03.074.
http://dx.doi.org/10.1016/j.memsci.2008....
. The BSCF compound was originally developed to
application in oxygen permeable ceramic membranes. In this application, an oxygen
partial pressure gradient between the membrane sides acts as the driving force for
the oxygen transport across the membrane. The oxygen permeation process through
these membranes involves oxygen exchange on the membrane surfaces and migration of
oxygen anions through the crystal lattice via oxygen vacancies until to reach the
opposite side and the exchange reverse process occurs11. Zhou W, Ran R and Shao Z. Progress in understanding and
development of Ba0.5Sr0.5CoFe0.2O.
0.83-δ based cathodes for intermediate-temperature
solid-oxide fuel cells: A reviewJournal of Power Sources. 2009; 192(2):231-246.
http://dx.doi.org/10.1016/j.jpowsour.2009.02.069.
http://dx.doi.org/10.1016/j.jpowsour.200...
2. Sunarso J, Baumann S, Serra JM, Meulenberg WA, Liu S, Lin YS, et
al. Mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen
separation. Journal of Membrane Science. 2008; 320(1-2):13-41.
http://dx.doi.org/10.1016/j.memsci.2008.03.074.
http://dx.doi.org/10.1016/j.memsci.2008....
3. Zeng P, Chen Z, Zhou W, Gu H, Shao Z and Liu S. Re-evaluation of
BaSr0.5CoFeO.
0.50.80.23-δ perovskite as
oxygen semi-permeable membraneJournal of Membrane Science. 2007;
291(1-2):148-156.
http://dx.doi.org/10.1016/j.memsci.2007.01.003.
http://dx.doi.org/10.1016/j.memsci.2007....
4. Jiang Q, Nordheden KJ and Stagg-Williams SM. Oxygen permeation
study and improvement of Ba0.5Sr0.5Co0.8Fe0.2Ox perovskite ceramic membranes.
Journal of Membrane Science. 2011; 369(1-2):174-181.
http://dx.doi.org/10.1016/j.memsci.2010.11.073.
http://dx.doi.org/10.1016/j.memsci.2010....
-55. Baumann S, Schulze-Küppers F, Roitsch S, Betz M, Zwick M, Pfaff
EM, et al. Influence of sintering conditions on microstructure and oxygen
permeation of Ba0.5Sr0.5Co0.8FeO (BSCF) oxygen
transport membranes. 0.23-δJournal of Membrane Science.
2010; 359(1-2):102-109.
http://dx.doi.org/10.1016/j.memsci.2010.02.002.
http://dx.doi.org/10.1016/j.memsci.2010....
. In particular, the BSCF perovskite is known by its
high oxygen diffusivity and extremely low surface resistance to oxygen reduction,
and incorporation into crystal lattice11. Zhou W, Ran R and Shao Z. Progress in understanding and
development of Ba0.5Sr0.5CoFe0.2O.
0.83-δ based cathodes for intermediate-temperature
solid-oxide fuel cells: A reviewJournal of Power Sources. 2009; 192(2):231-246.
http://dx.doi.org/10.1016/j.jpowsour.2009.02.069.
http://dx.doi.org/10.1016/j.jpowsour.200...
,22. Sunarso J, Baumann S, Serra JM, Meulenberg WA, Liu S, Lin YS, et
al. Mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen
separation. Journal of Membrane Science. 2008; 320(1-2):13-41.
http://dx.doi.org/10.1016/j.memsci.2008.03.074.
http://dx.doi.org/10.1016/j.memsci.2008....
,66. Baumann FS, Fleig J, Habermeier HU and Maier J. BaSr.
0.50.5Co0.8Fe0.2O3-δ
thin film microelectrodes investigated by impedance spectroscopySolid State
Ionics. 2006; 177(35-36):3187-3191.
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7. Baumann FS, Maier J and Fleig J. The polarization resistance of
mixed conducting SOFC cathodes: A comparative study using thin film model
electrodes. Solid State Ionics. 2008; 179(21-26):1198-1204.
http://dx.doi.org/10.1016/j.ssi.2008.02.059.
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-88. Ahmadrezaei M, Muchtar A, Muhamad N, Tan CY and Majlan EH.
Electrochemical and microstructural characteristics of nanoperovskite oxides
Ba0.5Sr.
0.5Co0.8Fe0.2O3-δ (BSCF) for
solid oxide fuel cellsCeramics International. 2013; 39(1):439-444.
http://dx.doi.org/10.1016/j.ceramint.2012.06.045.
http://dx.doi.org/10.1016/j.ceramint.201...
.
While the search for novel materials with improved transport and mechanical
properties have been continued, research efforts were also directed towards
optimization of the membrane architecture. An architecture composed of a dense layer
permeable only to oxygen and a porous layer of high specific surface area is usually
employed to increase the oxygen permeability. In order to maximize the oxygen
permeation flux, the membrane thickness is often reduced. However, with decreasing
membrane thickness, the mechanical strength is affected. Furthermore, the membrane
becomes surface limiting and the flux no longer increase in proportion as decreasing
membrane thickness. The most employed strategy is to use thick porous layer with
high permeability and high specific surface area, in order to improve the mechanical
properties and increase the available area to exchange reactions. Thereby, this
architecture development suggests the using of starting powders with nanometer
particles size and high specific surface area44. Jiang Q, Nordheden KJ and Stagg-Williams SM. Oxygen permeation
study and improvement of Ba0.5Sr0.5Co0.8Fe0.2Ox perovskite ceramic membranes.
Journal of Membrane Science. 2011; 369(1-2):174-181.
http://dx.doi.org/10.1016/j.memsci.2010.11.073.
http://dx.doi.org/10.1016/j.memsci.2010....
,88. Ahmadrezaei M, Muchtar A, Muhamad N, Tan CY and Majlan EH.
Electrochemical and microstructural characteristics of nanoperovskite oxides
Ba0.5Sr.
0.5Co0.8Fe0.2O3-δ (BSCF) for
solid oxide fuel cellsCeramics International. 2013; 39(1):439-444.
http://dx.doi.org/10.1016/j.ceramint.2012.06.045.
http://dx.doi.org/10.1016/j.ceramint.201...
9. Niehoff P, Baumann S, Schulze-Küppers F, Bradley RS, Shapiro I,
Meulenberg WA, et al. Oxygen transport through supported
Ba0.5Sr0.5CoFeO membranes.
0.80.23-δSeparation and Purification
Technology. 2014; 121:60-67.
http://dx.doi.org/10.1016/j.seppur.2013.07.002.
http://dx.doi.org/10.1016/j.seppur.2013....
10. Kovalevsky AV, Kharton VV, Snijkers FMM, Cooymans JFC, Luyten JJ
and Marques FMB. Oxygen transport and stability of asymmetric
SrFe(Al)O3−δ - SrAl composite
membranes. 2O4Journal of Membrane Science. 2007;
301(1-2):238-244.
http://dx.doi.org/10.1016/j.memsci.2007.06.028.
http://dx.doi.org/10.1016/j.memsci.2007....
-1111. Hong WK and Choi GM. Oxygen permeation of BSCF membrane with
varying thickness and surface coating. Journal of Membrane Science. 2010;
346(2):353-360. http://dx.doi.org/10.1016/j.memsci.2009.09.056.
http://dx.doi.org/10.1016/j.memsci.2009....
.
Consequently, BSCF powders synthesis has become a subject of great relevance and the
BSCF perovskite has been synthesized by several methods: conventionally by solid
state reaction1212. Tan L, Gu X, Yang L, Jin W, Zhang L and Xu N. Influence of
powder synthesis methods on microstructure and oxygen permeation performance of
BaSr0.5CoFeO.
0.50.80.23-δ perovskite-type
membranesJournal of Membrane Science. 2003; 212(1-2):157-165.
http://dx.doi.org/10.1016/S0376-7388(02)00494-5.
http://dx.doi.org/10.1016/S0376-7388(02)...
,1313. Burriel M, Niedrig C, Menesklou W, Wagner SF, Santiso J and
Ivers-Tiffée E. BSCF epitaxial thin films: Electrical transport and oxygen
surface exchange. Solid State Ionics. 2010; 181(13-14):602-608.
http://dx.doi.org/10.1016/j.ssi.2010.03.005.
http://dx.doi.org/10.1016/j.ssi.2010.03....
, combustion synthesis
reaction1414. Kovalevsky AV, Yaremchenko AA, Kolotygin VA, Shaula AL, Kharton
VV, Snijkers FMM, et al. Processing and oxygen permeation studies of asymmetric
multilayer Ba0.5Sr0.5Co0.8FeO membranes.
0.23-δJournal of Membrane Science. 2011;
380(1-2):68-80. http://dx.doi.org/10.1016/j.memsci.2011.06.034.
http://dx.doi.org/10.1016/j.memsci.2011....
,1515. Liu B, Zhang Y and Zhang L. Characteristics of
Ba0.5Sr0.5CoFe - La.
0.80.2O3-δ0.9Sr0.1Ga0.8Mg0.2O3−δ
composite cathode for solid oxide fuel cellJournal of Power Sources. 2008;
175(1):189-195.
http://dx.doi.org/10.1016/j.jpowsour.2007.09.088.
http://dx.doi.org/10.1016/j.jpowsour.200...
, co-precipitation method88. Ahmadrezaei M, Muchtar A, Muhamad N, Tan CY and Majlan EH.
Electrochemical and microstructural characteristics of nanoperovskite oxides
Ba0.5Sr.
0.5Co0.8Fe0.2O3-δ (BSCF) for
solid oxide fuel cellsCeramics International. 2013; 39(1):439-444.
http://dx.doi.org/10.1016/j.ceramint.2012.06.045.
http://dx.doi.org/10.1016/j.ceramint.201...
,1616. Toprak MS, Darab M, Syvertsen G and Muhammed M. Synthesis of
nanostructured BSCF by oxalate co-precipitation as potential cathode material
for solid oxide fuels cells. International Journal of Hydrogen Energy. 2010;
35(17):9448-9454.
http://dx.doi.org/10.1016/j.ijhydene.2010.03.121.
http://dx.doi.org/10.1016/j.ijhydene.201...
, complexing method55. Baumann S, Schulze-Küppers F, Roitsch S, Betz M, Zwick M, Pfaff
EM, et al. Influence of sintering conditions on microstructure and oxygen
permeation of Ba0.5Sr0.5Co0.8FeO (BSCF) oxygen
transport membranes. 0.23-δJournal of Membrane Science.
2010; 359(1-2):102-109.
http://dx.doi.org/10.1016/j.memsci.2010.02.002.
http://dx.doi.org/10.1016/j.memsci.2010....
,1111. Hong WK and Choi GM. Oxygen permeation of BSCF membrane with
varying thickness and surface coating. Journal of Membrane Science. 2010;
346(2):353-360. http://dx.doi.org/10.1016/j.memsci.2009.09.056.
http://dx.doi.org/10.1016/j.memsci.2009....
,1212. Tan L, Gu X, Yang L, Jin W, Zhang L and Xu N. Influence of
powder synthesis methods on microstructure and oxygen permeation performance of
BaSr0.5CoFeO.
0.50.80.23-δ perovskite-type
membranesJournal of Membrane Science. 2003; 212(1-2):157-165.
http://dx.doi.org/10.1016/S0376-7388(02)00494-5.
http://dx.doi.org/10.1016/S0376-7388(02)...
, and others1717. Shao Z, Zhou W and Zhu Z. Advanced synthesis of materials for
intermediate-temperature solid oxide fuel cells. Progress in Materials Science.
2012; 57(4):804-874.
http://dx.doi.org/10.1016/j.pmatsci.2011.08.002.
http://dx.doi.org/10.1016/j.pmatsci.2011...
. Moreover, the use of microwave heating together with
these synthesis methods has emerged as a novel route to produce materials in
nanoscale1717. Shao Z, Zhou W and Zhu Z. Advanced synthesis of materials for
intermediate-temperature solid oxide fuel cells. Progress in Materials Science.
2012; 57(4):804-874.
http://dx.doi.org/10.1016/j.pmatsci.2011.08.002.
http://dx.doi.org/10.1016/j.pmatsci.2011...
. On the other
hand, the sintering process is important for membrane consolidation and
microstructural development. The control of this stage is indispensable to retain a
thinner microstructure in dense layer aiming to improve the mechanical properties of
these membranes. Regarding the porous layer, the control of grain size aiming to
avoid grain growth, results an increase of membrane specific surface55. Baumann S, Schulze-Küppers F, Roitsch S, Betz M, Zwick M, Pfaff
EM, et al. Influence of sintering conditions on microstructure and oxygen
permeation of Ba0.5Sr0.5Co0.8FeO (BSCF) oxygen
transport membranes. 0.23-δJournal of Membrane Science.
2010; 359(1-2):102-109.
http://dx.doi.org/10.1016/j.memsci.2010.02.002.
http://dx.doi.org/10.1016/j.memsci.2010....
,88. Ahmadrezaei M, Muchtar A, Muhamad N, Tan CY and Majlan EH.
Electrochemical and microstructural characteristics of nanoperovskite oxides
Ba0.5Sr.
0.5Co0.8Fe0.2O3-δ (BSCF) for
solid oxide fuel cellsCeramics International. 2013; 39(1):439-444.
http://dx.doi.org/10.1016/j.ceramint.2012.06.045.
http://dx.doi.org/10.1016/j.ceramint.201...
,1616. Toprak MS, Darab M, Syvertsen G and Muhammed M. Synthesis of
nanostructured BSCF by oxalate co-precipitation as potential cathode material
for solid oxide fuels cells. International Journal of Hydrogen Energy. 2010;
35(17):9448-9454.
http://dx.doi.org/10.1016/j.ijhydene.2010.03.121.
http://dx.doi.org/10.1016/j.ijhydene.201...
,1818. Wang H, Tablet C, Feldhoff A and Caro J. Investigation of phase
structure, sintering, and permeability of perovskite-type BaSr.
0.50.5Co0.8Fe0.2O3-δ
membranesJournal of Membrane Science. 2005; 262(1-2):20-26.
http://dx.doi.org/10.1016/j.memsci.2005.03.046.
http://dx.doi.org/10.1016/j.memsci.2005....
19. Salehi M, Clemens F, Pfaff EM, Diethelm S, Leach C, Graule T, et
al. A case study of the effect of grain size on the oxygen permeation flux of
BSCF disk-shaped membrane fabricated by thermoplastic processing. Journal of
Membrane Science. 2011; 382(1-2):186-193.
http://dx.doi.org/10.1016/j.memsci.2011.08.007.
http://dx.doi.org/10.1016/j.memsci.2011....
-2020. Baumann S, Serra JM, Lobera MP, Escolástico S, Schulze-Küppers F
and Meulenberg WA. Ultrahigh oxygen permeation flux through supported
BaSr0.5CoFeO.
0.50.80.23-δ membranesJournal of
Membrane Science. 2011; 377(1-2):198-205.
http://dx.doi.org/10.1016/j.memsci.2011.04.050.
http://dx.doi.org/10.1016/j.memsci.2011....
. The use of microwave energy in ceramic materials
sintering has been widely studied over the last decades. Some of the most important
advantages of microwave heating are volumetric energy absorption and enhancement of
the mass transport. Therefore, higher heating rates and shorter processing time can
be employed in microwave heating. Thus, the final microstructure can be better
controlled, resulting in materials with thinner microstructure and better functional
properties in comparison to conventional heating methods2121. Clark DE, Folz DC, Folgar CE and Mahmoud MM. What is microwave
processing. In: Clark DE, Folz DC, Folgar CE and Mahmoud MM, editors. Microwave
solutions for ceramic engineer. Ohio: John Wiley & Sons,
2005.
22. Charmond S, Carry CP and Bouvard D. Densification and
microstructure evolution of Y-Tetragonal Zirconia Polycrystal powder during
direct and hybrid microwave sintering in a single-mode cavity. Journal of the
European Ceramic Society. 2010; 30(6):1211-1221.
http://dx.doi.org/10.1016/j.jeurceramsoc.2009.11.014.
http://dx.doi.org/10.1016/j.jeurceramsoc...
23. Menezes RR, Souto PM and Kiminami RHGA. Microwave hybrid fast
sintering of porcelain bodies. Journal of Materials Processing Technology. 2007;
190(1-3):223-229.
http://dx.doi.org/10.1016/j.jmatprotec.2007.02.041.
http://dx.doi.org/10.1016/j.jmatprotec.2...
24. Oghbaei M and Mirzaee O. Microwave versus conventional
sintering: a review of fundamentals, advantages and applications. Journal of
Alloys and Compounds. 2010; 494(1-2):175-189.
http://dx.doi.org/10.1016/j.jallcom.2010.01.068.
http://dx.doi.org/10.1016/j.jallcom.2010...
-2525. Bykov YV, Rybakov KI and Semenov VE. High-temperature microwave
processing of materials. Journal of Physics. D, Applied Physics. 2001;
34(13):R55-R75. http://dx.doi.org/10.1088/0022-3727/34/13/201.
http://dx.doi.org/10.1088/0022-3727/34/1...
. Accordingly, the aims of this present study are
microwave synthesis and sintering of BSCF perovskite in order to evaluate the
effectiveness of this heating method to control particles size of BSCF powder, and
densification and microstructural development of BSCF phase.
2 Experimental
All the reagents used in the experiments were acquired from Synth Analytical Reagents
and were used without further purification. BSCF powder was synthesized by microwave
assisted combustion synthesis. Stoichiometric amounts of metal nitrates
corresponding to BSCF phase (Ba(NO3)2,
Sr(NO3)2, Co(NO3)2.6H2O,
Fe(NO3)3.9H2O) and the fuel urea
((CO(NH2)2) were dissolved in deionized water into a
silica crucible at temperature of 80 °C. After that, the crucible was placed in a
domestic microwave oven (2.45 GHz, 900W) for 8 min. Firstly, the solution boils,
undergoes dehydration, and increases the concentration of regents in solution
followed by decomposition with the evolution of large amount of gases. Next, the
solution reaches the spontaneous combustion temperature, burns, instantly vaporizes,
and becomes a dark powder with friable appearance. The as-prepared powder was
analyzed by X-ray diffraction (XRD) using a SIEMENS D-5005 diffractometer with CuKa
radiation (λ = 0.15418 nm) and by differential scanning calorimetry /
thermogravimetry (DCS / TG) using a equipment Netzsch Sta 449c under heating rate of
10 °C/min. In order to crystallize BSCF phase, the powder was treated at temperature
of 900 °C according to data from DCS/TG analysis and, the treatment time was kept in
two hours according to some reports1414. Kovalevsky AV, Yaremchenko AA, Kolotygin VA, Shaula AL, Kharton
VV, Snijkers FMM, et al. Processing and oxygen permeation studies of asymmetric
multilayer Ba0.5Sr0.5Co0.8FeO membranes.
0.23-δJournal of Membrane Science. 2011;
380(1-2):68-80. http://dx.doi.org/10.1016/j.memsci.2011.06.034.
http://dx.doi.org/10.1016/j.memsci.2011....
,1515. Liu B, Zhang Y and Zhang L. Characteristics of
Ba0.5Sr0.5CoFe - La.
0.80.2O3-δ0.9Sr0.1Ga0.8Mg0.2O3−δ
composite cathode for solid oxide fuel cellJournal of Power Sources. 2008;
175(1):189-195.
http://dx.doi.org/10.1016/j.jpowsour.2007.09.088.
http://dx.doi.org/10.1016/j.jpowsour.200...
. Then, BSCF powder was subject to wet ball milling in
ethanol for 12 h in order to decrease agglomeration and to provide easier
densification in sintering stage. The structure and crystallinity of BSCF powder
were again characterized by XRD. The powder morphology was examined by scanning
electron microscope (SEM) using an equipment FEI Inspect s50. Specific surface area
was indirectly measured using ASAP 2020 through adsorption-desorption nitrogen
isotherms and estimated by BET method. Dilatometric analysis of a compacted sample
was performed in a contact dilatometer Netzsch 402C under heating rate of 5 °C/min
from 25 °C to 1050 °C.
As for the sintering stage, the synthetized BSCF powder was pressed in disk-shape
samples using a 10mm diameter stainless die under uniaxial pressure of 250MPa for 3
min. The sintering stage was conducted in several dwell times and temperatures,
using conventional and microwave heating. Table
1 shows the different conditions of dwell time and temperature used in
this study. It was used heating rates of 5 °C/min and 50 °C/min in conventional and
microwave sintering, respectively. The microwave equipment and its installation
scheme were reported in previous study2323. Menezes RR, Souto PM and Kiminami RHGA. Microwave hybrid fast
sintering of porcelain bodies. Journal of Materials Processing Technology. 2007;
190(1-3):223-229.
http://dx.doi.org/10.1016/j.jmatprotec.2007.02.041.
http://dx.doi.org/10.1016/j.jmatprotec.2...
, and the heating power was kept in 2.1KW. Bulk density of
the sintered discs was measured by water immersion technique based on Archimedes’
principle after two hours immersed in boiling water. Relative density was determined
using the theoretical density of BSCF phase. The dense ceramic disks were polished
and thermally attached 25 °C below the sintering temperature without dwell time and
under heating rate of 15 °C/min. The microstructure of samples was evaluated by
optical microscopy and the distribution and average grain size were estimated by
using Image J software considering projected area diameter of the measured grain
area.
3 Results and Discussion
3.1 Synthesis
XRD patterns of as-prepared powder and heat-treated BSCF powder are presented in
Figure 1. In as-prepared XRD patterns,
several basal reflections can be seen which makes the phases identifications
difficult; surely corresponding to the main phase and other secondary phases. On
the other hand, XRD pattern of heat-treated BSCF powder has presented complete
formation of cubic perovskite BSCF. The main reflection planes of BSCF phase are
indicated in Figure 1 according to some
reports1515. Liu B, Zhang Y and Zhang L. Characteristics of
Ba0.5Sr0.5CoFe - La.
0.80.2O3-δ0.9Sr0.1Ga0.8Mg0.2O3−δ
composite cathode for solid oxide fuel cellJournal of Power Sources. 2008;
175(1):189-195.
http://dx.doi.org/10.1016/j.jpowsour.2007.09.088.
http://dx.doi.org/10.1016/j.jpowsour.200...
,1616. Toprak MS, Darab M, Syvertsen G and Muhammed M. Synthesis of
nanostructured BSCF by oxalate co-precipitation as potential cathode material
for solid oxide fuels cells. International Journal of Hydrogen Energy. 2010;
35(17):9448-9454.
http://dx.doi.org/10.1016/j.ijhydene.2010.03.121.
http://dx.doi.org/10.1016/j.ijhydene.201...
. The average crystallite size was evaluated from
XRD broadening peak using the Scherrer’s equation2626. Mohebbi H, Ebadzadeh T and Hesari FA. Synthesis of
nano-crystalline NiO-YSZ by microwave-assisted combustion synthesis. Powder
Technology. 2009; 188(3):183-186.
http://dx.doi.org/10.1016/j.powtec.2008.04.095.
http://dx.doi.org/10.1016/j.powtec.2008....
which was found to be ~23 nm. The specific
surface area of BSCF powder and the corresponding size estimated by BET diameter
equation2727. Farhadi S, Momeni Z and Taherimehr M. Rapid synthesis of
perovskite-type LaFeO3 nanoparticles by microwave-assisted
decomposition of bimetallic La[Fe(CN)6]·5HO compound.
2Journal of Alloys and Compounds. 2009; 471(1-2):L5-L8.
http://dx.doi.org/10.1016/j.jallcom.2008.03.113.
http://dx.doi.org/10.1016/j.jallcom.2008...
were found to
be 9.93 m2/g and 107.8 nm, respectively.
Morphology and characteristics of BSCF powder can be seen in SEM micrographs, which is shown in Figure 2. The micrograph under magnification of 10000 X shown in Figure 2a presents a more general view of the synthesized powder, which presents a homogeneous morphology. Figure 2b shows a micrograph under magnification of 60000 X, where the particles of BSCF powder can be seen better, presenting a size smaller than 1 μm. Dilatometric analysis of a green disk-shape BSCF was made in order to observe the temperature of maximum densification rate of synthesized powder. Figure 3 shows the shrinkage curve and the first derivative curve. It can be seen by means of the first derivative curve that the maximum densification rate occurs around 975 °C. From this result were determined the sintering conditions to both sintering methods.
3.2 Sintering
The synthetized BSCF powder has presented excellent sinterability reaching relative densities superior to 97%. Figure 4 shows the dependence of the relative density and the average grain size on the temperature and the dwell time, for the two sintering methods. By means of comparison between Figure 4a (conventional sintering) and Figure 4b (microwave sintering), it can be noticed that the dependence of both average grain size and relative density on temperature follows a trend which seems to be independent of the sintering method. Nevertheless, although the relative densities in two heating methods were found in the same range, the average grain size reached in microwave sintering was about 44% smaller than conventional sintering. In case the variable is the dwell time, in short times sintering, there is a significant difference between relative densities in conventional (Figure 4c) and microwave sintering (Figure 4d). Under dwell time of 60 minutes in conventional heating and 6 minutes in microwave heating, the relative densities reached were 94.1 ± 0.1% and 88.9 ± 0.9%, respectively. However, this difference between heating methods is overcome in long times sintering. Concerning to microstructure, the microwave sintering retains grain size of up to 46.5% smaller than convectional sintering.
Dependence of relative density and average grain size with: i) temperature in (a) conventional heating and (b) microwave heating; ii) dwell time in conventional heating (c) and microwave heating (d).
Figure 5 shows four optical micrographs under magnification of 1000X corresponding to samples C1000-300 (a), M1000-30 (b), C1050-300 (c), and M1025-60 (d). Samples C1000-300 and M1000-30 have presented the same relative densities within the error tolerances; 94.7 ± 0.9% and 94.6 ± 0.1%, respectively. Comparing the micrograph of samples C1000-300 and M1000-30 , despite the same density, the effectiveness of microwave sintering on control of final microstructure of BSCF phase can be readily observed. The same effect occurs in samples C1050-300 and M1025-60, which presented relative density of 97.3% ± 0.6% and 97.3± 0.8%, respectively. In all these cases, microwave sintering resulted in a more refined microstructure than conventional sintering. This grain size reducing in microwave sintering can be attributed to shorter dwell time (10 times shorter) and higher heating rate (10 times higher), comparatively to conventional heating. Another advantage noted in microwave sintering is the effect in the grain size distribution. Figure 6 shows grain size distribution for each one of the microstructure presented in Figure 5. The use of microwave sintering resulted in a grain size distribution narrower than conventional sintering. This effect was observed in shorter dwell times and lower temperatures sintering (Figure 6a and Figure 6b) as well as in longer dwell times and higher temperature sintering (Figure 6c and Figure 6d). It is well known that the more homogeneous microstructures usually lead to better functional properties.
Optical micrographs under 1000X magnification for the samples C1000-300 (a), M1000-30 (b), C1050-300 (c) and M1025-60 (d).
Grain size distribution for the samples C1000-300 (a), M1000-30 (b), C1050-300 (c) and M1025-60 (d).
4 Conclusion
The BSCF perovskite was successfully synthesized using microwave-assisted combustion method followed by heat treatment. The synthesized powder shows a set of interesting characteristics to membrane development such as, phase homogeneity, high specific surface area, nanometric crystallite size, and submicrometric particle size. High densities can be reached even at temperature as low as 975°C. Microwave sintering has proven effectiveness in densification of BSCF perovskite. In the most of sintering conditions used, microwave sintering reaches the same densification of conventional sintering with only 10% of processing time. The processing time reduction has resulted in more homogeneous and finer microstructure related to size and grain size distribution. These results suggest the use of microwave synthesis and sintering in processing of oxygen separation and catalytic membranes as well as fuel cells cathodes in order to develop architecture with better oxygen permeation properties, improving the efficiency of these technologies.
Acknowledgements
The authors are grateful to the CNPq and the PPG-CEM/UFSCar for their financial support, and to the laboratory of synthesis and processing of ceramic materials (LASP) for their support given in the experimental part of this work.
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Publication Dates
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Publication in this collection
Jan-Feb 2015
History
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Received
02 Oct 2014 -
Reviewed
01 July 2015