Chiang Mai J. Sci. 2010; 37(2)
231
Chiang Mai J. Sci. 2010; 37(2) : 231-242
www.science.cmu.ac.th/journal-science/josci.html
Contributed Paper
A Barium-Calcium Silicate Glass for Use as Seals in
Planar SOFCs
Prachaya Namwong [a], Nattapol Laorodphan [b], Worapong Thiemsorn [b],
Manat Jaimasith [b], Anucha Wannakon [c] and Torranin Chairuangsri*[b]
[a] Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand.
[b] Department of Industrial Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand.
[c] National Metal and Materials Technology Center, Bangkok 12120, Thailand.
*Author for correspondence; e-mail: [email protected]
Received: 30 December 2009
Accepted: 26 February 2010
ABSTRACT
A barium-calcium silicate glass for use as seals in planar SOFCs with a chemical
composition of 40.78mol%BaO-4.36mol%CaO-0.62mol%Al2O3-0.83mol%B2O3-47.41mol%
SiO2 was studied. Its coefficient of thermal expansion (CTE) between 25-640oC, glass
transformation temperature (Tg) and softening temperature (Ts), determined by dilatometry,
were 10.6x10-6 oC-1, 646oC and 694oC, respectively. Isothermal devitrification heat treatment
at 800oC, which is in the intermediate operation temperature range of planar SOFCs, revealed
surface crystallization of various crystalline phases identified by XRD and SEM-EDS as
BaAl2Si2O8 (hexacelsian), BaSiO3 and BaCa2Si3O9. Kinetics of devitrification followed the
Johnson-Mehl-Avrami-Kolmogorov relationship with the Avrami exponent of 1.4. The overall
CTE of the obtained glass-ceramic after the isothermal devitrification heat treatment was
comparable to that of the glass. Adhesion test between the glass and the other components
including YSZ and stainless steel grade AISI430 revealed that good union was achieved. The
interfacial phenomena were studied by SEM-EDS and the mechanism of bonding were
discussed.
Keywords : silicate glass, seals, planar solid-oxide fuel cells, interfacial phenomena, devitrification,
thermal expansion, adhesion.
1. INTRODUCTION
SOFCs are a power source converting
chemical energy to electricity via an electrochemical reaction [1]. SOFCs have become
increasingly attractive to industries for a
number of reasons including high energyconversion efficiency [1,2] and very low
pollutant emission making them a more
environmentally friendly power source [1-3].
Two different designs, tubular and planar
SOFCs, are currently under development [4].
Planar SOFCs are expected to offer several
advantages including simple and cost-effective
manufacturing, mechanical robustness and
relatively short current path resulting in high
power density and efficiency [1,4]. Tubular
SOFCs do not require high temperature
232
Chiang Mai J. Sci. 2010; 37(2)
sealants [5], but planar SOFCs require hermetic
seals to separate fuel and oxidant contained
within the cell, and to bond cell components
together [1,4]. The operating condition of
planar SOFCs is in the temperature range of
600-1,000oC exposing to both oxidizing and
reducing conditions for thousands of hours
[4]. A key problem in fabrication of planar
SOFCs is therefore sealing of solid electrolyte
to anode with metallic interconnect for gas
tightness [6]. Development of sealant materials
for planar SOFCs is also particularly important
to achieve rapid start up time, which is a major
challenge for planar SOFCs in comparison
to PEM fuel cells [5]. The seals must be
chemically and mechanically compatible with
different oxide and metallic cell components,
should be electrically insulating, and can
Table 1. Coefficient of thermal expansion (CTE) of crystalline phases formed in several
glasses or glass-ceramics systems (adapted from Fergus [5]).
System
Ba-Si-O
Reference
[5]
Crystalline phases
Barium silicate (BaSiO3)
Barium orthosilicate (BaSi2O5)
Ba-Al-Si-O
Ba-Ca-Si-O
[5]
[5]
[6]
with Zn
Ba-Mg-Ca-Al-B-O
[3]
with ZrO 2 as seeds
Mg-Si-O
[5]
Ca-Si-O
[5]
7-8
2-3
Celsian; orthocelsian (BaAl2Si2O8)
5-7
(in addition or instead of those found in Ba-Si-O system)
[5]
Sr-Al-Si-O
[5]
Ba-Ca-Al-B-Si-O
[1]
12-14
(in addition or instead of those found in Ba-Si-O system)
Barium silicate (Ba2Si2O4)
no information
Ensteatite (MgSiO3)
no information
Barium magnesium silicate (BaMgSiO4)
no information
Barium magnesium silicate (Ba2MgSiO 5)
no information
Barium zinc silicate (BaZn2Si 2O 7)
no information
BaZrO 3
7.9
no information
10.4
Gehlenite (Ca2Al2SiO7)
no information
Nepheline (AlNaSiO4)
no information
Corundum (Al 2O 3)
no information
Ensteatite (MgSiO3)
7-9
Wollastonite (CaSiO3)
Calcium orthosilicate (Ca2SiO 4)
Mg-Al-Si-O
14
Celsian; monocelsian (BaAl2Si2O8)
Kurchatovite (CaMgB2O5)
[16]
9-13
Celsian; hexacelsian (BaAl2Si 2O8)
CaZrO 3
Na-Ca-Al-B-Si-O
( C-1 x 106)
(in addition or instead of those found in Ba-Si-O system)
Barium calcium orthosilicate (Ba3CaSi 2O 8)
Ba-Mg-B-Si-O
α , CTE
o
4-9
10-14
Cordierite (Mg 2Al 4Si 5O 18)
1
Hexacelsian (SrAl 2Si2O8)
8-11
Monocelsian (SrAl 2Si2O8)
3
Orthocelsian (SrAl 2Si 2O 8)
5-8
Barium chromate orthorhombic (BaCrO 4) by reactions
αa = 16.5
with chromia-forming alloys
αa = 33.8
αa = 20.4
Chiang Mai J. Sci. 2010; 37(2)
233
survive thermal cycling between room and
SOFC operation temperature [4].
Glass or glass-ceramic may offer best
perspectives as seals for planar SOFCs
because of the inertness to oxidizing and
reducing conditions, as well as thermal and
mechanical stability. Various glasses or glassceramics base on borates, phosphates and
silicates have been investigated [1-17]. Tg of
the glass seals should be as low as possible
considering relaxation of residual stresses in
glass created by the coefficient of thermal
expansion (CTE) mismatch during service [2].
However, viscosity and flow properties of
glasses in a supercooled liquid state must
also be considered for fabrication and
optimum Tg is therefore required. The CTE
of glasses or glass-ceramics for use as seals in
planar SOFC must be in a range of 9 10-6 to
13 10-6 oC-1 [8,10,12-15].
Long term thermal stability and chemical
stability of glasses are important characteristics
and have been tested in several glass systems
[1-17]. Acceptable leak rate after long term
thermal stability and chemical stability tests
regarding to both glass/YSZ and glass/
interconnect metal interfaces was reported
only in a few particular glass systems e.g. SrCa-Ni-Y-B silicate [14,15]. Rapid and
progressive crystallisation of glass to glassceramic during cell operation can deteriorate
the fluidity of the glass-ceramic because of
thermal expansion mismatch [2]. Most of
borate, phosphate and silicate glasses were
crystallized either during joining or prolong
stability test, which may cause poor thermal
or chemical stability during operation of the
planar SOFCs. Table 1 summarised the
crystalline phases formed in several glass
systems including their CTE.
In the present work, a Ba-Ca silicate glass
was studied on its kinetics of devitrification
at 800 o C, which is in the intermediate
operation temperature of the planar SOFCs.
Types of crystalline phases formed by
devitrification were identified and their effects
on the CTE of the glass were examined.
Adhesion of this glass to the YSZ and the
stainless steel grade AISI430 has been tested.
Finally, the possibility of using this glass as seals
in planar SOFCs and its long term stability
were discussed.
2. MATERIALS AND METHODS
2.1 Materials
A BaCa silicate glass was prepared by
melting the mixture of raw materials including
reagent grade BaCO3, CaCO3, H3BO3 and
SiO2 in an alumina crucible at 1550oC for
1 hour in a box furnace. The molten glass
was poured into a metal mould and annealed
in a box furnace at 550oC for 1 hour followed
by furnace cooling. The chemical composition
of the glass was analysed by x-ray fluorescence
(XRF) method using a Philips, Magix Pro,
x-ray fluorescence spectrometer as given
in Table 2. Due to limitation of the XRF
technique which cannot directly analysed the
content of boron, the B 2O 3 content was
obtained by subtracting the sum of mol% of
other oxides from 100%.
Table 2. Chemical composition of the glass from XRF.
mol%
BaO
CaO
Al2O3
B2O3*
SiO2
40.78
4.36
0.62
0.83
47.41
* B2O3 content was obtained by subtracting the sum of mol%
of other oxides from 100%
234
2.2 Measurement of Thermal Properties
The glass was cut by a Struers, Labotom3, cutting machine using an alumina abrasive
wheel and polished as cylindrical specimens
with 0.5 cm in diameter and 2.0 cm in length.
The specimens were heated in a Linsed L75
dilatometer at a heating rate of 5oC/min from
room temperature up to the softening point.
The CTE, T g and T s were consequently
determined.
2.3 Devitrification and Kinetics Study
The glass was cut as cubes of 1.0 cm
1.0 cm 1.0 cm. The specimens were then
isothermally heat-treated at 800oC, which is
in the intermediate operation temperature
range of planar SOFCs, in a box furnace for
50 to 250 hours to devitrify crystalline phases.
Because of the difficulty on fast cooling to
stop the reaction without cracking, furnace
cooling was applied with acceptance of
transformation by continuous cooling within
the furnace. To study kinetics of isothermal
transformation, the Johnson-Mehl-AvramiKolmogorov (JMAK) equation [18] was
applied i.e. ln[-ln(l - f )] = lnk + nln(t ) where;
f = transformed volume fraction, k = a
constant related to the nucleation rate and the
velocity of the transforming interface, t = time,
and n = the Avrami exponent. Cross sections
of the specimens after devitrification heat
treatment at 800oC were prepared by cutting
the specimens, grinding on silicon carbide
papers down to 1,000 grits and polishing by
alumina paste down to 0.04 μm. They were
consequently examined under an Olympus,
SZ40, stereomicroscope to determine the
transformed volume fraction (f) at a
corresponding time (t). A linear relationship
between ln[-ln(1-f )] versus ln(t) could then
be plotted and the Avrami exponent (n) could
be determined from the slope.
Chiang Mai J. Sci. 2010; 37(2)
2.4 Crystalline Phase Identification
Cross sections of the specimens were
further used for crystalline phase identification
by x-ray diffractometry (XRD) and scanning
electron microscopy (SEM) with energydispersive x-ray spectrometry (EDS). A
Philips, X’ Pert, x-ray diffractometer was used
with a Cu Kα source and a scanning rate of
1.2 degrees per minute. A JEOL 5910LV
scanning electron microscope with an
Oxford, ultrathin window EDS detector was
utilized and operated at 15-20 keV. The x-ray
data acquisition was controlled by an Oxford,
Inca, software operated under Microsoft
Windows.
2.5 Adhesion Test
The glass was cut as square plates of 0.5
cm 0.5 cm 0.1 cm. A commercial stainless
steel grade AISI430 or TISI SST430 was cut
as plates of 0.5 cm 0.5 cm x 0.15 cm. YSZ,
which was prepared by the National Metals
and Materials Technology Center (MTEC),
Bangkok, Thailand, was obtained under a
collaboration and cut as plates of 0.5 cm x
0.5 cm 0.16 cm. The glass plates were placed
in between the stainless steel and the YSZ
plates like a sandwich. They were consequently
heated to 800oC with a heating rate of 2oC/
min in a box furnace under air atmosphere
and held for 1 hour followed by furnace
cooling. The joint were then cold-mounted in
an epoxy resin, cross-sectioned by a Struers,
Accutom-2, high precision cutting machine
using an alumina abrasive wheel, and polished
by alumina paste down to 0.04 μm. The glass/
stainless steel and glass/YSZ interfaces were
consequently examined by SEM-EDS.
3. RESULTS AND DISCUSSION
The linear thermal expansion curve of
the glass obtained from dilatometry is given
in Figure 1. The CTE of glass between 25640oC was 10.6x10-6 oC-1, Tg was 646oC and
Ts was about 694oC.
Chiang Mai J. Sci. 2010; 37(2)
235
Figure 1. Linear thermal expansion curves of the glass and the glass-ceramic obtained from
dilatometry.
Figure 2 was taken under the stereomicroscope showing the cross sections of the glass
specimens after isothermal devitrification
heat treatment at 800oC for 50, 100, 150, 200
and 250 hours, respectively. The crystalline
phases appeared as the white, opaque layer
thickening into the apparently dark, translucent
amorphous glass. The thickness of the
crystalline layer at corresponding heat treating
time (t) was measured and calculated in term
of the transformed volume fraction (f ) as
given in Table 3. Plots between f versus t and
between ln[-ln(1-f )] versus ln(t ) according to
the JMAK relationship in the equation 1 were
performed as given in Figures 3 and 4. The
slope obtained from the plot of ln[-ln(1-f )]
versus ln(t ) in Figure 4 is 1.4, which is the
Avrami exponent (n) following the JMAK
relationship.
Figure 2. Micrographs taken from the stereomicroscope showing cross sections of the
glass specimens after isothermal devitrification heat treatment at 800oC for (a) 50 hours,
(b) 100 hours, (c) 150 hours, (d) 200 hours and (e) 250 hours.
236
Chiang Mai J. Sci. 2010; 37(2)
Table 3. The transformed volume fraction (f ) at corresponding heat-treating time (t ).
t (hours)
ln(t )
f
1-f
ln[-ln(1-f )]
50
8.0
0.157
0.843
-1.767
100
8.7
0.334
0.666
-0.900
150
9.1
0.566
0.434
-0.181
200
9.4
0.721
0.279
0.244
Figure 3. A plot between the transformed volume fraction (f ) versus heat treating time (t ).
Figure 4. A plot between ln[-ln(1-f )] versus ln(t ) following the JMAK equation.
Chiang Mai J. Sci. 2010; 37(2)
Tg and Ts of the glass, which are 646oC
and 694 o C, respectively, are below the
intermediate operation temperature at 800oC.
This could lead to adequate flowability of the
glass during bonding as suggested by Geasee
[7]. The values of CTE of glass i.e. 10.6x10-6
o -1
C is in the range applicable for planar
SOFCs [8,10,12-15]. Isothermal devitrification
study in Figure 2 revealed that devitrification
mainly occurred as surface crystallization,
by which the apparently white, opaque crystals
nucleated at the free surface and grew
directionally into the apparently dark, translucent amorphous glass. Shape retention of this
glass was attained at 800oC, which is the
intermediate operation temperature of planar
SOFCs. At this temperature, only slightly
distortion was only observed after heat treating
time after 100 hours. Theoretically, the plot
between f versus t should follow the typical
S-shape growth curve [19] as shown by the
dot line in Figure 3. However, the intercept
on the t axis obtained by extrapolating the
trend line, as shown by the dash line in
237
Figure 3, can be considered as an incubation
period for growth of nuclei with adequate
size to be observed. The incubation period
of this glass is approximately 9.6 hours. Pask
et al. [20] and Matusita et al. [21] suggested
that the value of the Avrami exponent (n)
will be between 1-2 if the growth is onedimensional. The Avrami exponent in this
work is in this range i.e. 1.4 indicating onedimentional growth. This is in agreement with
the microstructural observation as surface
crystallisation in Figure 2.
The XRD spectrum of the glass after
isothermal devitrification at 800oC for 250
hours is given in Figure 5. Matching of major
peaks revealed types of crystalline phases as
BaAl 2 Si 2 O 8 (hexacelsian), BaSiO 3 and
BaCa2Si3O9, corresponding to the JCPDS
files No 88-1049, 21-0083, and 18-0162,
respectively. The broaden peak was found
between 2θ = 10-13 degrees corresponding
to the remaining amorphous glass in the
microstructure. Figure 6 shows results from
SEM-EDS. Three crystalline phases were
Figure 5. (a) XRD spectrum of the glass after isothermal devitrification at 800oC for
250 hours, (b)-(d) the JCPDS files No. 88-1049, 21-0083, and 18-0162, which are corresponding
to BaAl2Si2O8 (hexacelsian), BaSiO3 and BaCa2Si3O9, respectively.
238
identified by the backscattered electron image
(Figure 6(a) and (b) and the EDS spectra in
Figure 6(c) to 6(e).
During devitrification, plate-shape
BaAl2Si2O8 (hexacelsian) likely devitrified first
followed by equiaxed-shape BaCa2Si3O9 and
BaSiO3 in between. This is in agreement with
Fergus [5] who summarized that alkaline-earth
oxides do not dissolve in the celsian phase,
but rather form other phases.The BaAl2Si2O8
has the lowest backscattered electron intensity
in Figure 6. It is therefore believed that boron
possibly associated in the BaAl2Si2O8 crystal
structure but was not detected due to the
limitation of the EDS technique. The BaSiO3
has the highest backscattered electron intensity
due to the high Ba content. In Table 1, the
Chiang Mai J. Sci. 2010; 37(2)
CTE of BaAl2Si2O8 (hexacelsian) and BaSiO3
are (9-13)x10-6 oC -1 and (7-8)x10-6 oC -1 [5],
respectively. There is no information on the
CTE of the BaCa2Si 3O9 , but it could be
roughly estimated as comparable to the
Ba3CaSiO8 i.e. (12-14)x10-6 oC-1 [5]. Therefore,
it can be expected that devitrification of
hexacelsian BaAl2Si2O8 and BaCa2Si3O9 should
raise the CTE of the glass-ceramic, whereas
an opposite effect should be obtained from
crystalline BaSiO3. The overall CTE between
25-900oC of the glass-ceramic after long-term
devitrification at 800 oC for 250 hours is
10.8x10-6 oC-1, which is 1.61% higher than that
of the amorphous glass before devitrification.
The reason for an increase of the CTE of
the glass-ceramic can be attributed to the
Figure 6. (a) backscattered electron micrograph showing surface crystallization in the glass
after isothermal devitrification at 800oC from 250 hours, (b) backscattered electron micrograph
at higher magnification showing morphologies of the three devitrified crystalline phases
including BaAl2Si2O8 (hexacelsian), BaSiO3 and BaCa2Si3O9 , (c)-(e) corresponding EDS spectra
of BaAl2Si2O8 (hexacelsian), BaSiO3 and BaCa2Si3O9 together with the remaining glass,
respectively.
Chiang Mai J. Sci. 2010; 37(2)
higher volume fraction of the BaAl2Si2O8
(hexacelsian) and the BaCa2Si3O9. Because the
overall CTE of the glass-ceramic is still in the
range applicable for planar SOFCs, it is
believed that devitrification of this glass, when
used as seals, after prolong operation of the
fuel cells will not lead to a severe detrimental
effect on thermal stability.
Adhesion test of the stainless steel/glass/
YSZ sandwich joint at 800oC for 1 hour in a
239
box furnace under air atmosphere revealed
that good union could be achieved. The crosssectional microstructure of both glass/YSZ
and glass/stainless steel interfaces was shown
in Figures 7 and 8. Thick layers of devitrified
crystalline phases were found at both
interfaces. Fine crystalline layer was formed
adjacent to the interface and large plate-shape
crystalline layer was next to the amorphous
glass.
Figure 7. (a) Backscattered electron micrographs show cross-sectional microstructure at the
glass (G)/YSZ interface, (b) x-ray line scanning of SiKα, OKα, CaKα, BaLα, YKα and
ZrKα, respectively. The dot line indicates the position of the interface.
The adhesion test revealed nucleation of
crystalline phase from the interfaces and
growth into the amorphous glass, comparable
to that observed at the free surface of glass
in isothermal devitrification experiment.
Because the incubation period for devitrification of this glass from kinetics study in Figure
3 was about 9.6 hours, the formation of these
devitrified crystalline phase layers is therefore
believed to occur not during joining, but
during furnace cooling overnight. At both
interfaces, fine crystalline BaCa 2 Si 3 O 9
devitrified first in the glass region adjacent to
the interface followed by plate-like BaAl2Si2O8
(hexacelsian) and equiaxed BaSiO3 as clearly
seen in Figures 7(b) and 8(b). At the glass/
YSZ interface, EDS x-ray line scanning in
Figure 7(c) revealed a Ca-rich area corresponding to the fine crystalline BaCa2Si3O9
zone. Mechanism of adhesion between the
240
Chiang Mai J. Sci. 2010; 37(2)
Figure 8. (a) Backscattered electron micrographs show cross-sectional microstructure at the
glass (G)/stainless steel (SS) interface, (b) x-ray line scanning of SiKα, OKα, CaKα, BaLα,
FeKα, AlKα and CrKα, respectively. The dot line indicates the position of the interface.
glass and the YSZ should involve the
formation of chemical bonding between Zr
in the YSZ and the oxygen atoms in the glass,
the extent of which could be within a few
layers of atoms. No diffusion of elements
from the glass toward the YSZ nor vice versa
was observed, which is in agreement to the
observation in the case of Ba aluminoborosilicate glasses by Sohn et al. [2, 17]. At the
glass/stainless steel, EDS x-ray line scanning
in Figure 8(c) revealed a Ca-rich area due to
the formation of fine crystalline Ca2BaSi3O9
similar to that observed at the glass/YSZ
interface. Moreover, a Cr-rich zone corresponding to the chromium oxide layer forming
on the porous steel surface was observed and
this led to a depletion of the Fe content in this
region. Mechanism of adhesion between the
glass and the stainless steel grade AISI430
should therefore involve both mechanical
bonding, whereby the soften glass filling into
the porous, oxidizing layer on the stainless steel
surface during heating, and chemical bonding,
whereby the Cr and possibly also Fe in the
metal oxide layer react with the oxygen atoms
in the glass. It was noted that Cr may form
BaCrO4 with Ba and O in glass at the glass/
stainless steel interface and can lead to a
detrimental effect because the CTE of
BaCrO4 is over 16.5x10-6 oC-1 [1]. However,
BaCrO4 was not observed after joining in this
study.
4. CONCLUSIONS
A barium-calcium silicate glass, promising
for use as seals in planar SOFCs, with the
chemical composition of 40.78mol%
B a O - 4 . 3 6 m o l % C a O - 0 . 6 2 m o l % A l 2O 3
-0.83mol%B2O3-47.41mol%SiO2 has been
studied. Its CTE between 25-900 o C is
10.6x10-6 oC-1 and Tg is 646oC. Devitrification
and kinetics study revealed surface crystalliza-
Chiang Mai J. Sci. 2010; 37(2)
tion of various crystalline phases including
BaAl 2 Si 2 O 8 (hexacelsian), BaSiO 3 and
BaCa 2Si 3O 9. Devitrification of this glass
should not lead to a severe detrimental effect
on thermal stability of planar SOFCs because
the overall CTE of the obtained glass-ceramic
is comparable to that of the parent glass.
Adhesion test revealed good union between
the glass and the other components including
YSZ and stainless steel grade AISI430.
ACKNOWLEDGEMENTS
The authors would like to thank the
National Metals and Materials Technology
Center, Bangkok, Thailand, for materials and
funding support. The authors gratefully
thank the Electron Microscopy Research and
Service Center, Faculty of Science, Chiang Mai
University, for electron microscopy facilities,
the Department of Geology, Faculty of
Science, Chiang Mai University, for XRF
facilities, and the Science Lab Center, Narasuan
University, for XRD facilities.
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