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

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

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    View Article Online / Journal Homepage / Table of Contents for this issue3D 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/b920627bThree-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 g1 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.710

    A variety of macroporous materials as powders, films and

    patterned substrates have been successfully fabricated by using

    polymer beads as a sacrificial template.1114

    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 foraState 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 Kims lab under a co-advisorprogram.

    This journal is The Royal Society of Chemistry 2010reducing 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, 28532857 | 2853

    http://dx.doi.org/10.1039/b920627bhttp://pubs.rsc.org/en/journals/journal/JMhttp://pubs.rsc.org/en/journals/journal/JM?issueid=JM020014

  • Scheme 1 Procedure for fabrication of the patterned 3D macroporous

    SiCN ceramic patterns.

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    View Article OnlineSynthesis 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 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

    35 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 1C min1

    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 min1 and a heating rate of 10 C min1.

    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 packing2854 | J. Mater. Chem., 2010, 20, 28532857process, 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 about34 mm.15 Subsequently, after PS spherePVSZ 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 The Royal Society of Chemistry 2010

    http://dx.doi.org/10.1039/b920627b

  • 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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    View Article Onlinethe parallel line patterns of the PS spherePVSZ 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 exactTg 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 imagesFig. 2 Differential scanning calorimetry curve of PS spheres. Heating

    rate: 10 K min1.

    This journal is The Royal Society of Chemistry 2010in 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 sizeFig. 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, 28532857 | 2855

    http://dx.doi.org/10.1039/b920627b

  • 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 g1

    0 44360 371120 337

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    View Article Online(1330 nm) of the ceramic product exhibited a 10% shrinkagefrom 1.5 mm of the PS spheres, much smaller than the typical

    shrinkage of 2030%. 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 g1 for the

    sample from non-annealed template, to 371 m2 g1 from the

    60 min annealed template, and to 337 m2 g1 from the 120 min

    annealed template. The apparent surface areas seem to generally

    be dominated by the presence of micropores (

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    http://dx.doi.org/10.1039/b920627b

    3D macroporous SiCN ceramic patterns tailored by thermally-induced deformation of template3D macroporous SiCN ceramic patterns tailored by thermally-induced deformation of template3D macroporous SiCN ceramic patterns tailored by thermally-induced deformation of template3D macroporous SiCN ceramic patterns tailored by thermally-induced deformation of template3D macroporous SiCN ceramic patterns tailored by thermally-induced deformation of template3D macroporous SiCN ceramic patterns tailored by thermally-induced deformation of template3D macroporous SiCN ceramic patterns tailored by thermally-induced deformation of template

    3D macroporous SiCN ceramic patterns tailored by thermally-induced deformation of template3D macroporous SiCN ceramic patterns tailored by thermally-induced deformation of template3D macroporous SiCN ceramic patterns tailored by thermally-induced deformation of template

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