PAPER www.rsc.org/materials | Journal of Materials Chemistry

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3D macroporous SiCN ceramic patterns tailored by thermally-induceddeformation of template

ZuoYi Xiao,†a Anjie Wanga and Dong-Pyo Kim*b

Received 2nd October 2009, Accepted 5th January 2010

First published as an Advance Article on the web 11th February 2010

DOI: 10.1039/b920627b

Three-dimensional (3D) macroporous SiCN ceramic patterns with tailored window size and pore shape

were fabricated by thermal deformation of a close-packed polystyrene (PS) sphere template, which

was obtained by applying consecutive capillary force and centrifugation in a packing process of a few

hours. Subsequent infiltration of a viscous preceramic inorganic polymer under strong centrifugal force

was followed by pyrolysis at 800 �C to decompose the sacrificial PS sphere-packed template. In this

work, the window sizes among the interconnected macropores were controlled in the range of 258 to

740 nm by tailoring the shape of the packed PS spheres (diameter 1.5 mm) by annealing above the glass

transition temperature (Tg) of the PS spheres for different periods. The pore shapes changed from

circular to hexagonal, and the BET surface area of the samples was reduced from 443 to 337 m2 g�1 with

a thinner network skeleton. This approach should be useful in combining a low pressure drop with high

external surface area for microfluidic applications with 3D porous structures.

Introduction

Ordered macroporous materials have been widely studied for

applications in photonic band gap (PBG) materials,1 catalytic

supports,2 adsorbents,3 chromatographic materials,4 membranes,5

and chemical sensors.6 Many methods have already been reported

for producing highly ordered 3D macroporous materials with

pore sizes ranging from 50 nm to several micrometres.7–10

A variety of macroporous materials as powders, films and

patterned substrates have been successfully fabricated by using

polymer beads as a sacrificial template.11–14

The challenge in fabrication of monolithic microscale catalyst

supports for high temperature use is to combine properties such

as high surface area per unit volume, stability at high tempera-

tures, and acceptable pressure drop. The requirements of high

surface area per unit volume and high-temperature stability can

be met by macroporous ceramic materials, as in SiC-based

ceramic patterns with 3D macroporous structures fabricated by

the soft lithography technique, as previously reported.2 Such

obtained 3D macroporous patterns could be used in a high-

temperature ceramic microreactor.15 For these applications, the

parameters of pore and window sizes in the ordered 3D macro-

porous material are both very important, significantly affecting

mass transfer capability and the pressure drops that are likely to

occur during their usage.16 Our previous works reported the

controlled pore characteristics, including window size, of 3D

porous SiCN structures simply by selection of PS spheres or silica

beads. However, when large diameter spheres were used for

aState Key Laboratory of Fine Chemicals, Dalian University ofTechnology, 158 Zhongshan Road, Dalian, 116012, P. R. ChinabDepartment of Fine Chemical Engineering and Chemistry and GraduateSchool of Analytical Science and Technology, Chungnam NationalUniversity, Daejeon, 305-764, South Korea. E-mail: dpkim@cnu.ac.kr;Fax: +82-42-823-6665; Tel: +82-42-821-7684

† ZuoYi Xiao worked at Professor Kim’s lab under a co-advisorprogram.

This journal is ª The Royal Society of Chemistry 2010

reducing the pressure drops in catalytic applications, it was

disadvantageous that the obtained 3D porous structure with

large windows drastically reduced the external surface area for

immobilizing the catalysts. Therefore, the present work proposes

a new approach, involving thermally-induced deformation of the

sacrificial polymer beads, toward minimizing pressure drop while

maintaining a high external surface area. In addition, tailoring

the geometrical structure of the colloidal crystals should be

useful for photonic crystal performance.17

Herein, we describe a thermally-deformed template method

for fabricating high-quality 3D macroporous patterns with

tailored window size and pore shape. A centrifugation method

was utilized to accelerate the formation of the packed template of

PS spheres in a PDMS mold, as well as filling the voids among PS

spheres. By this method we improved the quality of the packed

template and porous structure. In particular, the window size of

the resulting 3D macroporous SiCN ceramic was controlled by

the thermally-induced deformation of the sacrificial polymer

bead-packed template, which adjusts the shape of the PS spheres,

by simply annealing the original PS sphere-packed template

above Tg for different periods.

Experimental

Chemicals

Styrene monomer, dicumyl peroxide and 2-methoxyethanol

were purchased from Aldrich. Poly(vinylpyrrolidone) (PVP) was

acquired from Fluka, 2,20-azobis(isobutyrylnitrile) (AIBN) was

obtained from ACROS Organics Corporation and poly-

(vinylsilazane) (PVSZ) was received from KiON Corporation.

Ethanol was acquired from Daejung Chemical and Materials

Corporation. Poly(dimethylsiloxane) (PDMS) precursor and

curing agent (Sylgard 184) were supplied by Dow Corning. All

the chemicals were used without further purification.

J. Mater. Chem., 2010, 20, 2853–2857 | 2853

Scheme 1 Procedure for fabrication of the patterned 3D macroporous

SiCN ceramic patterns.

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Synthesis of PS spheres

PS spheres were prepared by the following procedure. A mixture

of 9.7 g styrene monomer, 1.8 g PVP and 0.1 g AIBN was

dissolved in a mixed solvent of 8.8 g 2-methoxyethanol and 79 g

ethanol, then refluxed at 70 �C for 16 h under mechanical stirring

with a speed of 250 rpm in an N2 atmosphere. Then 1.5 mm PS

spheres were homogeneously obtained after centrifugation,

washing with ethanol and drying at 60 �C. A suspension of 1 wt%

PS spheres was formed by dispersing the centrifuged spheres in

a mixture of water and ethanol (volume ratio ¼ 1 : 5).

Fabrication of the patterned 3D macroporous SiCN ceramic

A PDMS mold with 35 mm wide, 8 mm deep and 10 mm long

concave patterns was made from a SU-8 photoresist master

obtained by photolithography. The PDMS microchannels were

made by bonding the PDMS mold onto a silicon wafer after

plasma treatment for 1 min. Subsequently, 1000 ml PS sphere

solution was dropped into the reservoir connected to the inlet of

the PDMS microchannels. PS spheres in the suspension were

initially infiltrated into the PDMS microchannels for 0.5 h by

capillary force, and then packed by centrifugal force at 1000 rpm

for about 4 h. The resultant PS sphere-packed templates in the

PDMS mold were kept on a hotplate at 110 �C for varying

periods of time (0, 30, 60, 90, 120 and 135 min) to induce shape

deformation of the PS spheres. In order to infiltrate the voids

among the deformed PS spheres, viscous PVSZ mixed with

3–5 wt% of the thermal initiator (dicumyl peroxide) was injected

into the reservoir. The PS sphere-packed template was then

quickly filled with the preceramic polymer under strong centrif-

ugal force for 1 h. Then, the PS sphere-preceramic polymer

composite in the PDMS mold was cured at 90 �C for 12 h in

a glove-box under N2 atmosphere. The PDMS mold was then

peeled off very carefully. In addition, the torn PDMS debris was

removed by dipping in tetrabutylammonium fluoride (TBAF,

1.0 M) in THF for 20 min.15 Pyrolysis was carried out in a tube

furnace under N2 atmosphere by heating at a rate of 1 �C min�1

to 800 �C, then kept at this temperature for 3 h. This resulted in

the patterned 3D macroporous SiCN ceramic.

Characterization

The morphologies of the PS sphere-packed template and the 3D

macroporous SiCN ceramic patterns were examined with scan-

ning electron microscopy (SEM, a JEOL JSM-840 scanning

electron microscope) and optical microscopy (SV 32, Sometech).

The thermal properties of the PS spheres were determined by

using a thermogravimetric analyzer (TGA, TA instruments High

Resolution TGA2950), at up to 200 �C in air with a flow rate of

70 ml min�1 and a heating rate of 10 �C min�1.

Results and discussion

3D macroporous microstructured materials have usually been

fabricated by using polymer beads as the sacrificial template.

Scheme 1 shows the procedure for patterning 3D macroporous

SiCN ceramics, in which a series of packing, infiltration,

consolidation and template removal steps yield spherical pores

left by the polymer beads. At the beginning of the packing

2854 | J. Mater. Chem., 2010, 20, 2853–2857

process, the capillary force drives the PS sphere suspension into

the microchannels quickly due to a small pressure drop. Then,

the beads are arranged into the crystal structure inside the

microchannels as the solvent is allowed to evaporate at room

temperature. However, the suspension flow rate is drastically

reduced by the increased pressure drop due to the extended PS

sphere-packed area along the microchannels, which needs

a longer, overnight process to obtain the bead-packed template

by use of only a capillary method.15 The capillary force-induced

packing process often involves unexpected structural defects

such as grain boundaries, dislocations, and vacancies, since the

beads in the suspension have to overcome the friction with the

inner walls of the PDMS mold.

In this work, we efficiently employed two types of driving

force, capillary force and centrifugal force, to achieve a high-

quality bead-packed template within a few hours. Firstly, it was

necessary to form a short PS sphere pre-packed zone at the end of

PDMS microchannels under capillary force for 0.5 h, which

played an important role as a microfilter during the centrifugal

packing process. Otherwise, the beads in the suspension would

immediately flow out without the filtering membrane. Secondly,

the centrifugal force, controlled by varying the radial distance or

the angular frequency of rotation, was applied to the PS sphere

suspension in the reservoir, which accelerated the continuous

supply of the beads and forced them to come into intimate

contact with neighboring beads for facile generation of a well-

ordered PS sphere-packed template by a less time- and labor-

consuming process. Optical and SEM images in Fig. 1A and B

reveal the multiple line pattern with well-ordered PS spheres

along the 35 mm wide PDMS microchannels after 4 h centrifu-

gation, and the close-packing structure, respectively.

Centrifugation was also effectively applied in the infiltrating

process of the viscous preceramic polymer to complete the filling

of the voids among PS spheres in the template. Under strong

centrifugal force, it took only 1 h to infiltrate a template about

10 mm long in the PDMS mold. This is in contrast to our

previous work, in which the capillary force-driven process

needed a much longer time (�12 h) to fill a distance of about

3–4 mm.15 Subsequently, after PS sphere–PVSZ composite

patterns were solidified by thermal curing at 90 �C for 12 h in N2,

the PDMS mold was carefully peeled off. Due to the strong

adhesion between the cured inorganic polymer and silicon wafer,

This journal is ª The Royal Society of Chemistry 2010

Fig. 1 The template: (A) optical image and (B) SEM image. 3D macro-

porous ceramic material: (C) low-magnification and (D) high-magnification

image (inset: cross-sectional view).

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the parallel line patterns of the PS sphere–PVSZ composite were

left on the substrate. Finally, well-ordered 3D macroporous

SiCN ceramic patterns were obtained, since the PS spheres were

completely burned off at 800 �C in N2, as shown in Fig. 1C.

Fig. 1D clearly shows that the air pores were connected by the

windows with a diameter of around 250 nm.

In applications of ordered 3D macroporous structures, pore

characteristics will obviously determine mass transfer and pres-

sure drop behavior, as may be expected. The pore and window

sizes in the 3D macroporous material can be controlled by size of

the sacrificial beads as well as the as-made contact among the

beads in the template. Therefore, it is very useful to develop an

alternative approach for tailoring the interconnected windows of

3D macroporous material. In this context, we were strongly

interested in the fact that the shape of PS spheres could be

tunable by heating above Tg (�98 �C) for a given time (the exact

Tg value is determined by molecular weight of PS and sphere size

used).18 From our own differential scanning calorimeter

measurement in Fig. 2, the used PS spheres in this work started to

become soft at 104 �C. Therefore, the well-ordered PS sphere-

packed template was selectively annealed at 110 �C for different

periods of 30, 60, 90, 120 and 135 min, to adjust the contact area

among the neighboring PS spheres in the template. SEM images

Fig. 2 Differential scanning calorimetry curve of PS spheres. Heating

rate: 10 K min�1.

This journal is ª The Royal Society of Chemistry 2010

in Fig. 3 clearly show step-wise changes in the PS sphere shapes

during an annealing process. The as-made template formed by

centrifugal force with no thermal treatment revealed point

contacts among PS spheres (Fig. 3A). When annealed at 110 �C

for 30 min, necks among PS spheres were formed, as shown in

Fig. 3B. Then, PS beads were deformed into a non-spherical

shape as the template was thermally softened at 110 �C for

extended periods (Fig. 3C, D and E). The annealed PS spheres

gradually became hexagonal, and the lateral length between

adjacent deformed PS beads grew to 580 nm by 60 min heating,

and to 822 nm by 120 min, which is slightly less than 852 nm

(maximum of the contact area) calculated from a model (Fig. 4).

Here, the thermally-induced deformation did not disturb the

ordered arrangement and symmetry of PS spheres in the

template, although all voids between PS spheres were totally

filled when annealed for 135 min.

Fig. 5 shows the 3D macroporous SiCN ceramic structures,

which were successfully fabricated from the various deformed

templates. The conversion chemistry of PVSZ to the amorphous

SiCN ceramic phase at 800 �C in an anaerobic atmosphere has

been widely reported in the literature.19 The obtained pore size

Fig. 3 SEM images of PS sphere shapes in the well-ordered template by

annealing at 110 �C for different periods: (A) as-made, and annealing for

(B) 30 min, (C) 60 min, (D) 90 min, (E) 120 min and (F) 135 min.

Fig. 4 A scheme for estimating the largest contact area between neigh-

boring PS spheres.

J. Mater. Chem., 2010, 20, 2853–2857 | 2855

Fig. 5 SEM images of 3D macroporous SiCN ceramic products with

different morphology, fabricated from the well-ordered PS sphere-

packed templates by annealing at 110 �C for different periods: (A)

as-made, and annealing for (B) 30 min, (C) 60 min, (D) 90 min and (E)

120 min. (F) A cross-sectional view (60 min; inset: high magnification).

Table 1 Pore characteristics of 3D macroporous SiCN ceramic productsobtained from the PS templates annealed at 110 �C for different periods

Annealing time/min BET surface area/m2 g�1

0 44360 371120 337

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(�1330 nm) of the ceramic product exhibited a �10% shrinkage

from 1.5 mm of the PS spheres, much smaller than the typical

shrinkage of 20–30%. This was interpreted to mean that the

pyrolysis in the confined space enhanced the ceramic yield by

allowing gaseous product to redeposit on the framework.

Furthermore, the pore shape changed from originally circular to

hexagonal, as shown in Fig. 5D. In the SEM images, the uniform

skeleton of the pore structure in the underlying layers was

observable through the windows of the top layer, and was

interconnected through the air necks. Moreover, the window size

was homogeneously tailored because the contact parts among PS

spheres in the template could not be filled with the inorganic

polymer. As shown in Fig. 5A, the as-made template by

centrifugal force with no thermal treatment had windows of

258 nm in the 3D macroporous material. From SEM images,

annealing at 110 �C for extended periods increased the window

size to 535 nm for 60 min, and to 740 nm for 120 min. The

window sizes were a little smaller than the contact area among PS

spheres in the packed template as expected, which could be

attributed to the �10% shrinkage during the pyrolysis step.

In addition, the wall thickness of the pore network was

reduced from 522 nm (the as-made sample) to 104 nm (annealed

at 110 �C for 120 min). Table 1 presents the typical pore char-

acteristics of the 3D macroporous ceramic products. Generally,

BET surface areas gradually decreased from 443 m2 g�1 for the

sample from non-annealed template, to 371 m2 g�1 from the

60 min annealed template, and to 337 m2 g�1 from the 120 min

annealed template. The apparent surface areas seem to generally

be dominated by the presence of micropores (<2 nm) in the solid

2856 | J. Mater. Chem., 2010, 20, 2853–2857

network, rather than the external surface area of the macro-

porous skeleton, similarly to our previous report on PVSZ

derived macroporous SiCN ceramic products.20 Therefore, the

ceramic product with the thicker walls presented a higher

porosity by the presence of micropores within the solid network.

Herein, we can speculate that, in our previous work, a packed

template of large PS spheres with diameter of 10 mm led to a 3D

macroporous material with large windows (�1100 nm), but

strongly sacrificed the external surface area per unit volume,

which is a major site for deposition of the catalysts.15 In this

work, the use of small PS spheres with 1.5 mm diameter favors

higher external surface area in the 3D macroporous SiCN

ceramic products, and the tailored window size from 258 to

740 nm was advantageous for releasing the pressure drop.21

Finally, the developed 3D macroporous patterns would be useful

in designing a monolithic catalyst support with reduced pressure

drop and high external surface area, in particular, suitable for

a chip-based microreactor device.

Conclusion

A facile, rapid method was developed to fabricate 3D macro-

porous SiCN ceramic patterns with the tailored window size and

pore shape, by annealing close-packing PS spheres at 110 �C for

30–120 min to induce the shape deformation of PS spheres,

followed by preceramic polymer infiltration and pyrolytic

conversion at 800 �C. Centrifugal force could be effectively used

as a complementary driving force to closely pack the PS spheres

into PDMS microchannels, as well as to infiltrate the viscous

preceramic polymer into the voids among PS spheres. The

thermal manipulation of the PS sphere-packed template resulted

in several interesting changes in the obtained ceramic products.

The window sizes were controlled in the range of 258 to 740 nm,

and the pore shapes were modified from circular to hexagonal, with

a surface area decrease from an initial 443 m2 g�1 to 337 m2 g�1.

This approach provides a route to combine a small pressure drop

with high external surface area in 3D porous network structures.

Acknowledgements

This work was supported by the Creative Research Initiatives

(CRI) project (20090063607) administered by the Korean

Ministry of Education, Science and Technology.

Notes and references

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Mesoporous Mater., 2009, 117, 208.

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