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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 8 0 5 6e8 0 6 1
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Preparation of lanthanum tungstate membranes by tapecasting technique
Manuel Weirich, Jonas Gurauskis*, Vanesa Gil, Kjell Wiik, Mari-Ann Einarsrud
Department of Materials Science and Engineering, Norwegian University of Science and Technology, Sem Sælandsvei 12,
NO-7491 Trondheim, Norway
a r t i c l e i n f o
Article history:
Received 14 August 2011
Received in revised form
13 September 2011
Accepted 14 September 2011
Available online 17 October 2011
Keywords:
Lanthanum tungstate
Tape casting
Porosity
Membrane
* Corresponding author. Tel.: þ47 73594079.E-mail address: jonas.gurauskis@materia
0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.09.083
a b s t r a c t
Lanthanum tungstate materials have been reported to show exceptional mixed proton and
electron conducting behaviour at elevated temperatures and making them attractive for
dense hydrogen gas separation membranes. In this work preparation of planar asymmetric
lanthanum tungstate membranes was addressed. For this purpose carbon black and rice
starch pore formers were evaluated for optimum substrate gas permeability. It was found
that carbon black pore former results in higher level of effective porosity. Stabilization of
fine lanthanum tungstate powder in ethanol based solvent media was carried out to find
out optimum surfactant quantity for tape casting slurry. By combining lanthanum tung-
state tapes with and without pore former defect free asymmetric membranes were
produced by conventional sintering.
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction Furthermore, these oxides exhibit ambipolar protoneelectron
Conventional methods used for pure hydrogen gas produc-
tion/separation will not meet the emerging hydrogen market
needs due to price and sustainability requirements [1]. One of
the promising alternatives is the use of dense ceramic
membranes [1e3]. These membranes present mixed electron
and proton conducting behaviour and can selectively separate
hydrogen at elevated temperatures without the need of any
external electric circuits. Although the research on dense
hydrogen gas separation membranes has been carried over
many years, only few compositions have been found which
present sufficient flux and make them reliable for industrial
scale applications [3].
Oxides of the lanthanum tungstate familywith the formula
LaW1/6O2 have been reported to show exceptional proton
conductivity ofw5 � 10�4 S/cm at 400 �C [4] and peaking up to
w5 � 10�3 S/cm at 900 �C [5] under wet conditions.
l.ntnu.no (J. Gurauskis).2011, Hydrogen Energy P
conductivities at temperatures above 800 �C [4,6]making them
attractive selection for hydrogen gas membrane applications.
Thus there has been growing interest from the scientific
community to evaluate the stability of this material and
possible employment in hydrogen gas separation systems
[5,7,8].
It is well known that apart from compositional depen-
dence the permeability of gas through the dense membrane
can be improved by architectural arrangement. By simply
decreasing the membrane thickness, the bulk diffusion
contribution can be reduced improving the gas flux. However
a reduction in membrane thickness below 100 mm prompts
the use of a support layer due to structural performance
conditions. This leads to an asymmetric-structured
membrane architecture composed of a thin dense func-
tional layer and porous structural support which should meet
the following requirements:
ublications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 8 0 5 6e8 0 6 1 8057
� Sufficient mechanical strength to withstand the stresses
induced during membrane operation and heating/cooling
cycles.
� High permeability.
� Similar thermal/chemical expansion coefficients as for the
functional membrane layer.
� Low or no reactivity with dense functional membrane layer
under membrane operation conditions.
In order to comply with the first two requirements, the
support layer should be optimized to present high level of
effective permeable porosity with as high as possible density.
As a reference value, porosities around 30 vol.% used in
anodes in anode supported solid oxide fuel cells (SOFC) [9,10]
can be taken as the minimum percentage needed to reach
physical gas permeability in a porous structural membrane
support. While in order to accomplish with the last two
requirements it is recommended to use the same phase
material for both structural and dense functional layers [11].
The aim of this work was to prepare asymmetrical planar
La6�xWO12�v (LWO) membranes with dense thin w50 mm
functional layers deposited on structural porous support of
the same composition. An industrially scalable process based
on individual tape casting of colloidal LWO suspensions was
chosen. LWO powder with nanosized particles was employed
in order to obtain final microstructure with small grain size
being less vulnerable to critical defect formation within the
dense thin functional layer [12]. The two pore formers with
different particle characteristics used to generate porosity
within the structural support were carbon black and rice
starch. Evaluation of the physical gas flow through the porous
structural support with different types and amount of pore
former was addressed. LWO membranes were fabricated
using dense and optimized porous composition tapes by
lamination into asymmetric structure.
Fig. 1 e Sequence used for tape casting slurry preparation.
2. Materials and methods
2.1. Material preparation
The La6�xWO12�v powder with a La:W ratio of 5.6 (further
denoted as LWO56) was prepared by spray pyrolysis (CerPo-
Tech AS) [13,14]. The powder has a specific surface area of
about 8 m2 g�1 and a primary particle size of 100 nm with
a mean agglomerate size of w1 mm. Reagent grade ethanol
(Sigma Aldrich) was used as the solvent carrier. Dolacol D1003
surfactant as 30 wt.% solution in ethanol was provided by
Zschimmer & Schwarz GmbH. Polyvinyl butyral B-98 (Butvar,
Solutia Inc.) was used as binder. Polyethylene glycol 400 (PEG-
400) and dibutylphthalate were provided by Merck Chemicals
KGaA and used as plasticizers. The carbon black powder used
was a charcoal activated provided by Merck Chemicals KGaA
and rice starch DR-LA was provided by REMY industries.
Samples of w1 mm thickness and 13 mm diameter for
microstructural observation and substrate permeability
measurementswere compacted by cold uniaxial pressing. The
compositions were prepared adding 35 and 40 vol.% of the
different pore formers (when needed) to LWO56 powder and
ball milling in ethanol to obtain microstructural uniformity.
Prior to sintering, a calcining step at 500 �C for 1 h and pre-
sintering step at 1100 �C for 1 h were carried out. Sintering of
the samples was done in air at 1410 �C with a dwell time of 2 h
with heating and cooling rates of 2 �C/min.
Slurries for tape casting were prepared using the
optimum surfactant content and following the sequence
shown in Fig. 1. Stabilization of LWO56 powder was done by
an initial ball milling step. Plasticizers were introduced to
the slurry during the 2nd milling step and when needed pore
former was added. PVB binder was introduced to the slurry
in diluted state (relation 1/5 in weigh) in ethanol previously
discounting the surplus ethanol from initial solvent quan-
tity. After binder addition, the suspension was slowly rolled
to reach good homogeneity and to eliminate air bubbles
introduced during previous slurry preparation steps. Tape
casting was done on Mylar film using doctor blade carrier
with blades adjusted to 100 mm for tapes of LWO56 and to
250 mm for LWO56 tapes with pore former. The casted tapes
were dried in air at room temperature for 24 h. Lamination
of the tapes was done using hydraulic hot press Carver
Model 4122CE. Optimized lamination conditions were found
to be 15 min dwell time at 85 �C and w40 MPa pressure.
Asymmetric membranes were obtained by placing a single
LWO56 tape layer without pore former on top of five layers
of LWO56 tape with optimized pore former content. In the
case of tape casted samples, the initial calcination step at
500 �C with the dwell time of 1 h was used to eliminate the
organic components used in the tape casting process. When
pore former was used, the additional calcination step was
performed at 1100 �C. Sintering of the samples was done at
1410 �C with a dwell time of 4 h and with heating and cooling
rates of 2 �C/min.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 8 0 5 6e8 0 6 18058
2.2. Material characterization
Particle size distribution was evaluated by laser diffraction in
aqueous media using a Mastersizer 2000 (Malvern Instru-
ments, UK). The starting powders and the obtained sintered
microstructures were investigated with a Hitachi S3500N
scanning electron microscopy (SEM).
Permeability through the structural substrates was evalu-
ated using the sintered pellet samples prepared by cold
uniaxial pressing. Samples were fitted to the experimental
setup shown in Fig. 2 using rubber O-rings. Synthetic air gas
flow throughout the porous substrate was recorded by using
overpressure on the primary side within the range of 0.75e1.5
bar and leaving to equilibrate for 15 min. The permeability
coefficient was determined using a rearranged form of Darcy’s
law [15]:
k ¼ v$m$DhDP
(1)
where k e permeability factor (m2); n e superficial velocity (m/
s); m e fluid viscosity of the gas (Pa$s); Dh e thickness of the
substrate (m); DP e pressure drop across the substrate (Pa).
The air viscosity used for the calculations was
1.85 � 10�5 Pa$s [16]. The superficial velocity of the gas used
was calculated using the volumetric flow rate through the
membrane and the permeated surface:
v ¼ Qvol
A(2)
where Qvol e volumetric flow rate (m3/s); A e sample surface
(m2).
Prior to tape casting, diluted suspensions (5 vol.%) of
LWO56 powder were prepared with varying Dolacol D1003
surfactant content and the effect on suspension stability was
evaluated using rheological characterization. Rheological
characterization was performed using rotational rheometer
Mars III (Haake, Karlsruhe) equipped with double cone-and-
plate test geometry (titanium double cone of 1�,⌀ ¼ 60 mm). Suspensions were prepared by ball milling for
24 h and the viscosity measurements were carried under
controlled shear rate mode. Obtained dynamic viscosity
values were employed to evaluate optimum surfactant
content for LWO56 powder.
Fig. 2 e Experimental setup used for physical gas flux
measurements through the porous substrates.
3. Results and discussion
To produce ceramicswith controlledmicrostructure (porosity,
pore size and pore morphology) requires control of the
morphology of the pore former and the size distributions of
the pore formers. Both parameters will affect in the pore size
distribution and morphology of the pores in the final ceramic
microstructure. Fig. 3 shows micrographs of carbon black
(Fig. 3a) and rice starch (Fig. 3b) used as the pore formers.
Carbon black exhibits the largest and most heterogeneous
particles sizes with elongated morphology. The particle size
for the carbon black powder shows very broad distribution
with particles within 5e250 mm length range (Fig. 3a). The rice
starch has smaller particles and presents isometric and non-
spherical particles with about 2 mm in size. Soft agglomer-
ates up to 10 mm in size were observed for the rice starch
(Fig. 3b).
Fig. 4 shows the average particle size distribution in terms
of relative volume corresponding to the carbon black and rice
starch pore formers. Both pore formers present unimodal size
distribution but as also observed from the SEM micrographs,
the carbon black powder contains a larger quantity of particles
with average size larger than 20 mm. Further, for carbon black
a residual tail in the range of �6 mm is observed. The particle
size distribution for the carbon black is broader than for starch
Fig. 3 e SEM micrographs of as received pore formers; (a)
carbon black and (b) rice starch.
Fig. 4 e Particle size distribution of as received carbon black
and rice starch pore formers.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 8 0 5 6e8 0 6 1 8059
with amean particle sizew30 mmwhile themean particle size
for rice starch is 6 mm.
Both, SEM and particle size distribution measurements,
confirm the significant differences between the two pore
formers selected, not only in morphology but also on average
particle size and size distribution. Therefore, it is expected
that the samples with different pore formers will have
completely different microstructures.
A comparison of the permeability coefficients as a function
of pore former content for the two pore formers is given in
Fig. 5. For carbon black, a considerably higher permeability
was obtained compared to rice starch with identical vol.%
pore former addition. The permeability coefficients ranged
from w0.8 � 10�14 to 1.9 � 10�14 m2 for the substrates con-
taining carbon black as the pore former. These values are
Fig. 5 e Permeability coefficients for structural substrates
prepared using carbon black and rice starch pore formers.
within the range of values corresponding to conventional
structural anode layers used in SOFCs [17]. Samples prepared
with rice starch show almost 4 times lower permeability
values with nearly no gas diffusion for the lowest porosity
percentage evaluated. This demonstrates that carbon black
pore former results in high volume of effective porosity which
contributes to the physical gas permeability through the
substrate.
Following these results, carbon black pore former with
35 vol.% addition was selected for further work. Fig. 6 shows
typical fracture SEM micrographs of the porous microstruc-
ture from sample with 35 vol.% of carbon black content
(Fig. 6a) and a sample without pore former (Fig. 6b). Both
samples demonstrate suitable microstructures and could be
used as porous structural support and dense functional
membrane layers, respectively.
To optimize the tape casting procedure, the stability of the
LWO56 powder in ethanol was evaluated by adjusting the
polymeric surfactant content. By adjusting the interaction of
the LWO56 particles in the solvent it is possible to promote
the repulsive interparticle forces and avoid the formation of
agglomerates. Furthermore this is crucial in order to increase
the solid phase loading within the colloidal system and thus
decrease the shrinkage and defect formation during the
solvent evaporation step. Fig. 7 shows dynamic viscosity
Fig. 6 e SEM micrographs of fracture surfaces of samples
sintered at 1410 �C; (a) LWO56 powder with 35 vol.% carbon
black and (b) LWO56 powder.
Fig. 7 e Dynamic viscosity values for 5 vol.% LWO56
suspension as a function of surfactant content.
Fig. 8 e SEM micrograph of fracture surface of LWO56
membrane sample prepared using individually casted
tapes.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 8 0 5 6e8 0 6 18060
values at a fixed shear rate of g ¼ 100 s�1 as a function of
surfactant content. The lowest viscosity values were
obtained for suspensions with 2.5 wt.% of surfactant related
to the LWO56 solid content. Higher addition of surfactant
significantly increased the viscosity. The observed increase
in viscosity indicates the buildup of interactions between the
solid particles within the colloidal system and can be attrib-
uted to two different phenomena. In conventional colloidal
system with solid phase particles within submicron range it
could be mainly attributed to cross linking due to excessive
polymeric surfactant content within the system [18]. But
taking into account that the primary particle size of the spray
pyrolyzed powder is within the nanometric range [13,14]
a further decrease in LWO56 agglomeration can lead to an
increase in viscosity. This would be expected in the case of
a ball milling process with the presence of effective surfac-
tant which would stop the re-agglomeration of the fine
particles. Although this decrease in agglomerate size would
be beneficial for the final microstructure, an observed steep
increase in viscosity (Fig. 7) would not allow preparation of
concentrated slurries. Taking into account this consider-
ation, a surfactant content of 2.5 wt.% was selected for
Table 1 e Slurry compositions for tape casting process.
Chemical Content (wt.%)
Without poreformer
With poreformer
Ethanol 27.5 24.3
Carbon black e 8.5
PEG 400 2.5 3.8
Dibutylphthalate 2.5 2.3
PVB B-98 5.0 4.5
LWO56 62.5 56.6
Dolacol 2.5a 2.5a
a Related to LWO56 weight content.
further preparation of concentrated LWO56 tape casting
slurries.
The tape casting slurry compositions are shown in Table
1. For the tapes with carbon black pore former the compo-
sition was adjusted to compensate for binder and plasticizer
content. This was needed to promote the lamination of
tapes and to eliminate the defects which appeared in dry
tapes with pore former addition. This confirms the obser-
vation by Sanson et al. [19] that the lack of binder and
plasticizers was especially clear in the case of carbon black
as a pore former. Casted tapes with adjusted compositions
showed defect free structure and were employed for the
lamination step.
Lamination of the tapes was done just above the glass
transition temperature of w78 �C for PVB binder. By
combining porous tapes (35 vol.% carbon black addition) and
a tape without pore former asymmetrical LWO56 membranes
were successfully produced (Fig. 8). The lamination process
applied resulted in defect free union between the individual
tapes without the presence of mass flow due to the excess
pressure. Dense w40 mm thick LWO56 functional layer was
obtained with overall membrane thickness reaching 300 mm.
4. Conclusions
Carbon black pore former results in higher level of effective
porosity within porous structural substrates compared to rice
starch. An optimized amount of black quantity of 35 vol.% to
lanthanum tungstate powder gave a permeability coefficient
of w0.8 � 10�14 m2. Minimum viscosity of the fine lanthanum
tungstate powder in ethanol was reached using 2.5 wt.%
Dolacol D1003 surfactant relative to LWO56 solid content. By
employing this amount of the surfactant in the tape casting
slurries, defect free lanthanum tungstate tapes with and
without pore former were obtained. By lamination of indi-
vidual tapes, defect free asymmetric lanthanum tungstate
membranes were prepared with dense w40 mm thick func-
tional layer.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 8 0 5 6e8 0 6 1 8061
Acknowledgements
Financial support from the Research Council of Norway (Grant
No. 195912/S10 and No. 191358) and Protia AS (Norway) is
gratefully acknowledged.
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