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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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