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

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<ul><li><p>PAPER www.rsc.org/materials | Journal of Materials Chemistry</p><p>Publ</p><p>ishe</p><p>d on</p><p> 11 </p><p>Febr</p><p>uary</p><p> 201</p><p>0. D</p><p>ownl</p><p>oade</p><p>d by</p><p> CA</p><p>SE W</p><p>EST</p><p>ER</p><p>N R</p><p>ESE</p><p>RV</p><p>E U</p><p>NIV</p><p>ER</p><p>SIT</p><p>Y o</p><p>n 31</p><p>/10/</p><p>2014</p><p> 18:</p><p>46:5</p><p>9. </p><p>View Article Online / Journal Homepage / Table of Contents for this issue3D macroporous SiCN ceramic patterns tailored by thermally-induceddeformation of template</p><p>ZuoYi Xiao,a Anjie Wanga and Dong-Pyo Kim*b</p><p>Received 2nd October 2009, Accepted 5th January 2010</p><p>First published as an Advance Article on the web 11th February 2010</p><p>DOI: 10.1039/b920627bThree-dimensional (3D) macroporous SiCN ceramic patterns with tailored window size and pore shape</p><p>were fabricated by thermal deformation of a close-packed polystyrene (PS) sphere template, which</p><p>was obtained by applying consecutive capillary force and centrifugation in a packing process of a few</p><p>hours. Subsequent infiltration of a viscous preceramic inorganic polymer under strong centrifugal force</p><p>was followed by pyrolysis at 800 C to decompose the sacrificial PS sphere-packed template. In this</p><p>work, the window sizes among the interconnected macropores were controlled in the range of 258 to</p><p>740 nm by tailoring the shape of the packed PS spheres (diameter 1.5 mm) by annealing above the glass</p><p>transition temperature (Tg) of the PS spheres for different periods. The pore shapes changed from</p><p>circular to hexagonal, and the BET surface area of the samples was reduced from 443 to 337 m2 g1 with</p><p>a thinner network skeleton. This approach should be useful in combining a low pressure drop with high</p><p>external surface area for microfluidic applications with 3D porous structures.Introduction</p><p>Ordered macroporous materials have been widely studied for</p><p>applications in photonic band gap (PBG) materials,1 catalytic</p><p>supports,2 adsorbents,3 chromatographic materials,4 membranes,5</p><p>and chemical sensors.6 Many methods have already been reported</p><p>for producing highly ordered 3D macroporous materials with</p><p>pore sizes ranging from 50 nm to several micrometres.710</p><p>A variety of macroporous materials as powders, films and</p><p>patterned substrates have been successfully fabricated by using</p><p>polymer beads as a sacrificial template.1114</p><p>The challenge in fabrication of monolithic microscale catalyst</p><p>supports for high temperature use is to combine properties such</p><p>as high surface area per unit volume, stability at high tempera-</p><p>tures, and acceptable pressure drop. The requirements of high</p><p>surface area per unit volume and high-temperature stability can</p><p>be met by macroporous ceramic materials, as in SiC-based</p><p>ceramic patterns with 3D macroporous structures fabricated by</p><p>the soft lithography technique, as previously reported.2 Such</p><p>obtained 3D macroporous patterns could be used in a high-</p><p>temperature ceramic microreactor.15 For these applications, the</p><p>parameters of pore and window sizes in the ordered 3D macro-</p><p>porous material are both very important, significantly affecting</p><p>mass transfer capability and the pressure drops that are likely to</p><p>occur during their usage.16 Our previous works reported the</p><p>controlled pore characteristics, including window size, of 3D</p><p>porous SiCN structures simply by selection of PS spheres or silica</p><p>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</p><p> ZuoYi Xiao worked at Professor Kims lab under a co-advisorprogram.</p><p>This journal is The Royal Society of Chemistry 2010reducing the pressure drops in catalytic applications, it was</p><p>disadvantageous that the obtained 3D porous structure with</p><p>large windows drastically reduced the external surface area for</p><p>immobilizing the catalysts. Therefore, the present work proposes</p><p>a new approach, involving thermally-induced deformation of the</p><p>sacrificial polymer beads, toward minimizing pressure drop while</p><p>maintaining a high external surface area. In addition, tailoring</p><p>the geometrical structure of the colloidal crystals should be</p><p>useful for photonic crystal performance.17</p><p>Herein, we describe a thermally-deformed template method</p><p>for fabricating high-quality 3D macroporous patterns with</p><p>tailored window size and pore shape. A centrifugation method</p><p>was utilized to accelerate the formation of the packed template of</p><p>PS spheres in a PDMS mold, as well as filling the voids among PS</p><p>spheres. By this method we improved the quality of the packed</p><p>template and porous structure. In particular, the window size of</p><p>the resulting 3D macroporous SiCN ceramic was controlled by</p><p>the thermally-induced deformation of the sacrificial polymer</p><p>bead-packed template, which adjusts the shape of the PS spheres,</p><p>by simply annealing the original PS sphere-packed template</p><p>above Tg for different periods.Experimental</p><p>Chemicals</p><p>Styrene monomer, dicumyl peroxide and 2-methoxyethanol</p><p>were purchased from Aldrich. Poly(vinylpyrrolidone) (PVP) was</p><p>acquired from Fluka, 2,20-azobis(isobutyrylnitrile) (AIBN) was</p><p>obtained from ACROS Organics Corporation and poly-</p><p>(vinylsilazane) (PVSZ) was received from KiON Corporation.</p><p>Ethanol was acquired from Daejung Chemical and Materials</p><p>Corporation. Poly(dimethylsiloxane) (PDMS) precursor and</p><p>curing agent (Sylgard 184) were supplied by Dow Corning. All</p><p>the chemicals were used without further purification.J. Mater. Chem., 2010, 20, 28532857 | 2853</p><p>http://dx.doi.org/10.1039/b920627bhttp://pubs.rsc.org/en/journals/journal/JMhttp://pubs.rsc.org/en/journals/journal/JM?issueid=JM020014</p></li><li><p>Scheme 1 Procedure for fabrication of the patterned 3D macroporous</p><p>SiCN ceramic patterns.</p><p>Publ</p><p>ishe</p><p>d on</p><p> 11 </p><p>Febr</p><p>uary</p><p> 201</p><p>0. D</p><p>ownl</p><p>oade</p><p>d by</p><p> CA</p><p>SE W</p><p>EST</p><p>ER</p><p>N R</p><p>ESE</p><p>RV</p><p>E U</p><p>NIV</p><p>ER</p><p>SIT</p><p>Y o</p><p>n 31</p><p>/10/</p><p>2014</p><p> 18:</p><p>46:5</p><p>9. </p><p>View Article OnlineSynthesis of PS spheres</p><p>PS spheres were prepared by the following procedure. A mixture</p><p>of 9.7 g styrene monomer, 1.8 g PVP and 0.1 g AIBN was</p><p>dissolved in a mixed solvent of 8.8 g 2-methoxyethanol and 79 g</p><p>ethanol, then refluxed at 70 C for 16 h under mechanical stirring</p><p>with a speed of 250 rpm in an N2 atmosphere. Then 1.5 mm PS</p><p>spheres were homogeneously obtained after centrifugation,</p><p>washing with ethanol and drying at 60 C. A suspension of 1 wt%</p><p>PS spheres was formed by dispersing the centrifuged spheres in</p><p>a mixture of water and ethanol (volume ratio 1 : 5).</p><p>Fabrication of the patterned 3D macroporous SiCN ceramic</p><p>A PDMS mold with 35 mm wide, 8 mm deep and 10 mm long</p><p>concave patterns was made from a SU-8 photoresist master</p><p>obtained by photolithography. The PDMS microchannels were</p><p>made by bonding the PDMS mold onto a silicon wafer after</p><p>plasma treatment for 1 min. Subsequently, 1000 ml PS sphere</p><p>solution was dropped into the reservoir connected to the inlet of</p><p>the PDMS microchannels. PS spheres in the suspension were</p><p>initially infiltrated into the PDMS microchannels for 0.5 h by</p><p>capillary force, and then packed by centrifugal force at 1000 rpm</p><p>for about 4 h. The resultant PS sphere-packed templates in the</p><p>PDMS mold were kept on a hotplate at 110 C for varying</p><p>periods of time (0, 30, 60, 90, 120 and 135 min) to induce shape</p><p>deformation of the PS spheres. In order to infiltrate the voids</p><p>among the deformed PS spheres, viscous PVSZ mixed with</p><p>35 wt% of the thermal initiator (dicumyl peroxide) was injected</p><p>into the reservoir. The PS sphere-packed template was then</p><p>quickly filled with the preceramic polymer under strong centrif-</p><p>ugal force for 1 h. Then, the PS sphere-preceramic polymer</p><p>composite in the PDMS mold was cured at 90 C for 12 h in</p><p>a glove-box under N2 atmosphere. The PDMS mold was then</p><p>peeled off very carefully. In addition, the torn PDMS debris was</p><p>removed by dipping in tetrabutylammonium fluoride (TBAF,</p><p>1.0 M) in THF for 20 min.15 Pyrolysis was carried out in a tube</p><p>furnace under N2 atmosphere by heating at a rate of 1C min1</p><p>to 800 C, then kept at this temperature for 3 h. This resulted in</p><p>the patterned 3D macroporous SiCN ceramic.</p><p>Characterization</p><p>The morphologies of the PS sphere-packed template and the 3D</p><p>macroporous SiCN ceramic patterns were examined with scan-</p><p>ning electron microscopy (SEM, a JEOL JSM-840 scanning</p><p>electron microscope) and optical microscopy (SV 32, Sometech).</p><p>The thermal properties of the PS spheres were determined by</p><p>using a thermogravimetric analyzer (TGA, TA instruments High</p><p>Resolution TGA2950), at up to 200 C in air with a flow rate of</p><p>70 ml min1 and a heating rate of 10 C min1.</p><p>Results and discussion</p><p>3D macroporous microstructured materials have usually been</p><p>fabricated by using polymer beads as the sacrificial template.</p><p>Scheme 1 shows the procedure for patterning 3D macroporous</p><p>SiCN ceramics, in which a series of packing, infiltration,</p><p>consolidation and template removal steps yield spherical pores</p><p>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</p><p>the microchannels quickly due to a small pressure drop. Then,</p><p>the beads are arranged into the crystal structure inside the</p><p>microchannels as the solvent is allowed to evaporate at room</p><p>temperature. However, the suspension flow rate is drastically</p><p>reduced by the increased pressure drop due to the extended PS</p><p>sphere-packed area along the microchannels, which needs</p><p>a longer, overnight process to obtain the bead-packed template</p><p>by use of only a capillary method.15 The capillary force-induced</p><p>packing process often involves unexpected structural defects</p><p>such as grain boundaries, dislocations, and vacancies, since the</p><p>beads in the suspension have to overcome the friction with the</p><p>inner walls of the PDMS mold.</p><p>In this work, we efficiently employed two types of driving</p><p>force, capillary force and centrifugal force, to achieve a high-</p><p>quality bead-packed template within a few hours. Firstly, it was</p><p>necessary to form a short PS sphere pre-packed zone at the end of</p><p>PDMS microchannels under capillary force for 0.5 h, which</p><p>played an important role as a microfilter during the centrifugal</p><p>packing process. Otherwise, the beads in the suspension would</p><p>immediately flow out without the filtering membrane. Secondly,</p><p>the centrifugal force, controlled by varying the radial distance or</p><p>the angular frequency of rotation, was applied to the PS sphere</p><p>suspension in the reservoir, which accelerated the continuous</p><p>supply of the beads and forced them to come into intimate</p><p>contact with neighboring beads for facile generation of a well-</p><p>ordered PS sphere-packed template by a less time- and labor-</p><p>consuming process. Optical and SEM images in Fig. 1A and B</p><p>reveal the multiple line pattern with well-ordered PS spheres</p><p>along the 35 mm wide PDMS microchannels after 4 h centrifu-</p><p>gation, and the close-packing structure, respectively.</p><p>Centrifugation was also effectively applied in the infiltrating</p><p>process of the viscous preceramic polymer to complete the filling</p><p>of the voids among PS spheres in the template. Under strong</p><p>centrifugal force, it took only 1 h to infiltrate a template about</p><p>10 mm long in the PDMS mold. This is in contrast to our</p><p>previous work, in which the capillary force-driven process</p><p>needed a much longer time (12 h) to fill a distance of about34 mm.15 Subsequently, after PS spherePVSZ composite</p><p>patterns were solidified by thermal curing at 90 C for 12 h in N2,</p><p>the PDMS mold was carefully peeled off. Due to the strong</p><p>adhesion between the cured inorganic polymer and silicon wafer,This journal is The Royal Society of Chemistry 2010</p><p>http://dx.doi.org/10.1039/b920627b</p></li><li><p>Fig. 1 The template: (A) optical image and (B) SEM image. 3D macro-</p><p>porous ceramic material: (C) low-magnification and (D) high-magnification</p><p>image (inset: cross-sectional view).</p><p>Publ</p><p>ishe</p><p>d on</p><p> 11 </p><p>Febr</p><p>uary</p><p> 201</p><p>0. D</p><p>ownl</p><p>oade</p><p>d by</p><p> CA</p><p>SE W</p><p>EST</p><p>ER</p><p>N R</p><p>ESE</p><p>RV</p><p>E U</p><p>NIV</p><p>ER</p><p>SIT</p><p>Y o</p><p>n 31</p><p>/10/</p><p>2014</p><p> 18:</p><p>46:5</p><p>9. </p><p>View Article Onlinethe parallel line patterns of the PS spherePVSZ composite were</p><p>left on the substrate. Finally, well-ordered 3D macroporous</p><p>SiCN ceramic patterns were obtained, since the PS spheres were</p><p>completely burned off at 800 C in N2, as shown in Fig. 1C.</p><p>Fig. 1D clearly shows that the air pores were connected by the</p><p>windows with a diameter of around 250 nm.</p><p>In applications of ordered 3D macroporous structures, pore</p><p>characteristics will obviously determine mass transfer and pres-</p><p>sure drop behavior, as may be expected. The pore and window</p><p>sizes in the 3D macroporous material can be controlled by size of</p><p>the sacrificial beads as well as the as-made contact among the</p><p>beads in the template. Therefore, it is very useful to develop an</p><p>alternative approach for tailoring the interconnected windows of</p><p>3D macroporous material. In this context, we were strongly</p><p>interested in the fact that the shape of PS spheres could be</p><p>tunable by heating above Tg (98 C) for a given time (the exactTg value is determined by molecular weight of PS and sphere size</p><p>used).18 From our own differential scanning calorimeter</p><p>measurement in Fig. 2, the used PS spheres in this work started to</p><p>become soft at 104 C. Therefore, the well-ordered PS sphere-</p><p>packed template was selectively annealed at 110 C for different</p><p>periods of 30, 60, 90, 120 and 135 min, to adjust the contact area</p><p>among the neighboring PS spheres in the template. SEM imagesFig. 2 Differential scanning calorimetry curve of PS spheres. Heating</p><p>rate: 10 K min1.</p><p>This journal is The Royal Society of Chemistry 2010in Fig. 3 clearly show step-wise changes in the PS sphere shapes</p><p>during an annealing process. The as-made template formed by</p><p>centrifugal force with no thermal treatment revealed point</p><p>contacts among PS spheres (Fig. 3A). When annealed at 110 C</p><p>for 30 min, necks among PS spheres were formed, as shown in</p><p>Fig. 3B. Then, PS beads were deformed into a non-spherical</p><p>shape as the template was thermally softened at 110 C for</p><p>extended periods (Fig. 3C, D and E). The annealed PS spheres</p><p>gradually became hexagonal, and the lateral length between</p><p>adjacent deformed PS beads grew to 580 nm by 60 min heating,</p><p>and to 822 nm by 120 min, which is slightly less than 852 nm</p><p>(maximum of the contact area) calculated from a model (Fig. 4).</p><p>Here, the thermally-induced deformation did not disturb the</p><p>ordered arrangement and symmetry of PS spheres in the</p><p>template, although all voids between PS spheres were totally</p><p>filled when annealed for 135 min.</p><p>Fig. 5 shows the 3D macroporous SiCN ceramic structures,</p><p>which were successfully fabricated from the various deformed</p><p>templates. The conversion chemistry of PVSZ to the amorphous</p><p>SiCN ceramic phase at 800 C in an anaerobic atmosphere has</p><p>been widely reported in the literature.19 The obtained pore sizeFig. 3 SEM images of PS sphere shapes in the well-ordered template by</p><p>annealing at 110 C for different periods: (A) as-made, and annealing for</p><p>(B) 30 min, (C) 60 min, (D) 90 min, (E) 120 min and (F) 135 min.</p><p>Fig....</p></li></ul>