Microporous and Mesoporous Materials 142 (2011) 754–758

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Microporous and Mesoporous Materials

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Correspondence

Preparation of ordered mesoporous silicon carbide monoliths via preceramicpolymer nanocasting

Xiaoyan Yuan a,b, Jingwen Lü b, Xingbin Yan a,⇑, Litian Hu a, Qunji Xue a

a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, Chinab Department of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130000, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 December 2010Received in revised form 20 January 2011Accepted 21 January 2011Available online 23 February 2011

Keywords:Silicon carbideMonolithMesoporousNanocasting

1387-1811/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.micromeso.2011.01.014

⇑ Corresponding author. Tel./fax: +86 931 4968055E-mail address: xbyan@licp.cas.cn (X. Yan).

Ordered mesoporous silicon carbide (SiC) monoliths have been synthesized using polycarbosilane asstarting preceramic polymer and mesoporous silica SBA-15 as start template. SiC-carbon compositemonoliths were generated via nanocasting, pressing, and subsequent pyrolysis under nitrogen at1000 �C and 1200 �C, respectively. Finally, the carbon template was removed through thermal treat-ment in an ammonia atmosphere to obtain SiC monoliths with ordered mesoporous structures. Inves-tigated by small-angle X-ray diffraction (SA-XRD), powder wide-angle X-ray diffraction (WA-XRD),transmission electron microscopy (TEM), field emission scanning electron microscope (FE-SEM), thenitrogen adsorption–desorption isotherm measurements and thermogravimetric analysis (TGA), theSiC monoliths display crack-free, ordered 2-dimentional hexagonal p6mm symmetry, high specificsurface areas and oxidation stability. The porous ceramic monoliths possess compression strengthof about 33.5 MPa, which is adequate for their use in several engineering applications.

� 2011 Elsevier Inc. All rights reserved.

1. Introduction

Porous monolithic materials have become very popular due tothe good combination of the compact integral structure and theporous microstructure. Large surface areas and multimodal poros-ities are great advantages in many fields, such as electrochemistry,energy storage, separation, chromatography and catalysis [1–4].Porous monoliths can, for instance, be used in flow through cata-lytic or separation systems give better performance and higherpermeability compared to packed columns [4,5]. Among the vari-ous porous monolithic materials, silicon carbide (SiC) monolithshave attracted great attention because of their excellent mechani-cal strength, oxidation resistance, thermal stability and functionalsemiconductor characteristics [6–8]. These characteristics makethem hold many potential applications in high-temperature catal-ysis [4,9], separation [5,10] and semiconductors [7]. Accordingly,much effort has been paid to the design and synthesis of SiCmonoliths that have tailored porosity and pore interconnectivity[11–14].

So far, porous SiC monoliths can be synthesized by the carbo-thermal reaction between carbon and silica at high temperatures.In this process, either porous carbon aerogel [11] or porous silicamonolith/aerogel [12–14] is used for the shape-memory templateand the framework of as-synthesized porous monolith is similar

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to the preformed carbon or silica template. However, because thecarbon or silica template is also be used as reactant to generatetarget SiC phase, a certain degree collapse of the porous frameworkis unavoidable during the high-temperature sintering. As a result,the preparation of ordered mesoporous SiC monolith by the carbo-thermal reaction is still failed.

PCS, polycarbosilane, is known as a good polymer precursor forpreparing SiC-based ceramics through pyrolysis. Because of theexcellent solubility in common organic solvents, PCS has good abil-ity to fill into the nano-scale channels of ordered porous templatesby nanocasting. Recently, ordered mesoporous SiC powders havebeen synthesized by nanocasting technique, using PCS as preceram-ic polymer [15–17]. Furthermore, the presence of Si-H groups inPCS and the melt-capability of PCS are helpful to the cross-linkingand shaping of PCS during the pyrolysis process, as a result of thegeneration of a compact integral SiC matrix, including fibers andmonoliths [18,19]. CMK-3 as a typical mesoporous carbon template,exhibits a highly ordered 2D hexagonal mesostructure with a highspecific surface area and uniform pore diameter. It should bementioned that, the choice of CMK-3 as the hard template is justi-fied by the connection of its hexagonal array of mesopores bymicropores in the walls, which permits the replication process[20]. The main advantage of CMK-3 is that the carbon frameworkwill not react with PCS-derived SiC during high-temperature pyro-lysis and it can be removed completely under ammonia at a hightemperature of 1000 �C [21]. In addition, PCS polymer will bemelted and cross-linked during the opening low-temperature ther-

X. Yuan et al. / Microporous and Mesoporous Materials 142 (2011) 754–758 755

molysis stage. In our synthesis, we believe that, the relatively slowheating rate (1 �C min�1) is benefited to the cross-linking amongPCS molecules, especially for the molecules located within themicropores of CMK-3 and surrounding CMK-3 particles. Thus,through the subsequent polymer-to-ceramic conversion and tem-plate-removing process, the connection of hexagonal array of SiCmesopores and the integrity of monolithic SiC can be reserved, asa result of the generation of ordered mesoporous SiC monoliths.

Therefore, we describe here a facile preceramic polymer-de-rived synthesis of mesoporous SiC monoliths with ordered hexag-onal p6mm symmetry via pressing. In our synthesis process, PCSwas used as the preceramic polymer and highly ordered mesopor-ous SBA-15 silica was employed as the starting template. The latterwas converted into its negative replica CMK-3 carbon template,which was subsequently casted with PCS using a liquid-phaseimpregnation of the tetrahydrofuran (THF) solution of PCS. Pressedthe powdery polymer-carbon composites into tablet-like mono-liths and then pyrolyzed under argon atmosphere at 1000 �C and1200 �C, respectively, to generate SiC-carbon composite monoliths.Subsequently, template removed to generate ordered mesoporousSiC monoliths. The structures at different temperatures werestudied using Transmission Electron Microscopy (TEM), Smallangle X-ray diffraction (SA-XRD) and Wide-angle X-ray diffraction(WA-XRD). Surface area and pore size were inferred from nitrogenadsorption/desorption measurements.

2. Experimental

2.1. Synthesis of hard templates

Mesoporous silica SBA-15 template was prepared by hydrother-mal synthesis method according to established procedures [22].And mesoporous carbon CMK-3 template was synthesized by thenanocasting method using sucrose as a precursor and mesoporoussilica SBA-15 as a hard template according to the literature [23]. Adetailed explanation on the synthesis and characterization of thehard templates is given in the ESI.�.

2.2. Synthesis of mesoporous SiC monoliths

Mesoporous SiC monoliths were synthesized by the nanocastingmethod using PCS as preceramic polymer and mesoporous CMK-3as hard template. In a typical process, 1.0 g of CMK-3 was placedin a Schlenk flask, dehydrated at 150 �C under a vacuum for 4 h,flushed with argon and cooled down to room temperature. Thedried CMK-3 was added in a 10 ml THF solution of 1.5 g PCS andthe mixture was stirred at RT for 1 day. After that, the THF solventwas evaporated under vacuum to generate a black powdery com-posite, which was grinded in an onyx mortar and were sieved bythe 800 mesh sieve. Subsequently, the powders were equally di-vided into two parts and each was added into a stainless steelmould to press into a tablet-like monolith under a pressure of5 MPa. The polymer-carbon composite monoliths were transferredinto a horizontal ceramic tube furnace and subjected to the thermaltreatment in Ar atmosphere at a heating rate of 1 �C min�1 up to1000 �C and 1200 �C and kept 2 h at this temperature, respectively,to generate SiC-carbon composite monoliths. After this polymer-to-ceramic conversion process, the composite underwent a final ther-mal treatment in an ammonia atmosphere from RT to 1000 �C(2 �C min�1) and kept at this temperature for 10 h to remove thecarbon CMK-3 template and generate the ordered mesoporous SiCmonoliths. The monolithic samples pyrolyzed at different tempera-tures were denoted as M-SiC-1000 and M-SiC-1200, respectively.

2.3. Characterization

Powder small-angle X-ray diffraction (SA-XRD) and Wide-angleX-ray diffraction (WA-XRD) patterns were achieved using aPhilipps X0Pert PRO X-ray diffraction system (Cu Ka radiation,0.15406 nm). Transmission electron microscopy (TEM) imagesand the corresponding energy diffused spectrum (EDS) were ob-tained on a JEM-2010 instrument (JEOL), using an acceleratingvoltage of 200 kV. All samples were first dispersed in ethanol withthe aid of sonication and then collected using carbon-film-coveredcopper grids for TEM analyses. The morphology of the final prod-ucts was observed on a field emission scanning electron micro-scope (FE-SEM, JSM 6701F). Nitrogen adsorption–desorptionisotherm measurements were performed on a Micrometitics ASAP2020 volumetric adsorption analyzer at �196 �C. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specificsurface area of each sample and the pore-size distribution was de-rived from the adsorption branch of the corresponding isothermusing the Barrett-Joyner-Halenda (BJH) method. The mesopore vol-ume was estimated from the amount adsorbed at a relative pres-sure of P/P0 = 0.99. The macroporosity of the SiC monoliths wasmeasured by mercury intrusion porosimetry using a MicromeriticsAutopore 9500 apparatus. Raman spectra were performed on a mi-cro-Raman spectroscopy (JY-HR800) using the excitation wave-length of 532 nm. The mechanical compressive strength of themesoporous SiC monoliths was evaluated using a universal tensiletesting machine (SHIMADZU Universal Testing Machine AGS-X5kN) at room temperature. The oxidation stability was evaluatedby thermogravimetric analysis (TGA) from RT to 1000 �C at10 �C min�1 under air atmosphere.

3. Results and discussion

After the nanocasting and removing THF, the morphology ofdried PCS-carbon composite powders is similar to that of the car-bon CMK-3 template. The powdery material exhibits monodi-spersed particles with a uniform size of about 1 lm. Thissimilarity in the surface morphology between PCS-carbon compos-ite and CMK-3 template confirms that the surface morphology ofthe mesoporous carbon host is retained in the composite. This alsoconfirms the fact that the PCS polymer displays an excellent abilityto fill the nanopores of the carbon template and to be retained in-side the nanopores of the template.

As the PCS powders stick together easily under pressure, pow-dery PCS-carbon composite can be pressed into tablet-like mono-liths without any binder. Through the pyrolysis under argonatmosphere and subsequent heat-treatment in ammonia, meso-porous SiC monoliths were obtained. Photograph of our typical,as-synthesized M-SiC-1200 sample shows a good bulk macroscopicappearance without any crack to be seen by the naked eye (Fig. 1a).The monolith is stable, even after being immerged into water andsonicated for several minutes using a low-power ultrasonic bath,there is no crumbling peeling off from the monolithic body.Because the sizes/or shapes of the monoliths can be easily adjustedby choosing moulds with different inner diameters/or shapes, thesynthetic approach can be useful for the large-scale industrial pro-duction of ordered mesoporous SiC monoliths. Scanning electronmicroscopy (SEM) images (Fig. 1b and c) show that the structureof the M-SiC-1200 sample is constructed from interconnectedparticles.

Evidence of the maintenance of ordered hexagonal mesoporesfor the M-SiC-1000 and M-SiC-1200 samples is provided by TEMimages. As shown in Fig. 2, two samples both exhibit characteristicarrangement of cylindrical channels for 2-D hexagonal p6mm sym-metry in large domains, which is similar to the structure of original

756 X. Yuan et al. / Microporous and Mesoporous Materials 142 (2011) 754–758

mesoporous SBA-15 silica. To provide information about chemicalcomposition, the M-SiC-1200 sample was analyzed by energy dis-persive X-ray (EDX) spectroscopy. The signals in the EDX spectrum(ESI.�, Fig. S3) indicate the appearance of Si, C, O and N in the sam-ple matrix, which correspond to the following composition: Si,48.10 wt.%; C, 45.16 wt.%; O, 6.72 wt.%; N, 0.02 wt.%. It means thatthe sample is mainly composed of silicon and carbon. It should bementioned that, it is quite common that polymer-derived non-oxide ceramics contain a small quantity of oxygen. In addition,the weak nitrogen signal might come from the formation of surfacenitride during the thermal treatment at 1000 �C in ammonia atmo-sphere. Thus, an empirical formula of Si1.0C2.2O0.25 can be obtained.However, it should be mentioned that the elemental compositionsbased on the EDS data have certain errors. Sufficient samplingpoints, oil contaminants and carbon coated the TEM grids all canresult in errors. Additionally, in order to gain further informationon the overall structure of the mesoporous SiC monoliths, weinvestigated the Raman spectrum of the M-SiC-1200 sample. Itshows two well-discernible peaks which are the typical featuresof disordered graphite-like carbon, namely the D-peak located ataround 1348 cm�1, and the G-peak centred around 1610 cm�1

(ESI.�, Fig. S4). Such diffuse peaks are in general found for SiC-based materials containing free carbon phase [24]. This indicatesthat there is a certain amount of free carbon in the as-synthesizedmesoporous SiC monolith.

The small-angle X-ray diffraction (SA-XRD) patterns of theM-SiC-1000 and M-SiC-1200 samples are shown in Fig. 3a. It isclear seen that, after removal of the carbon template, a distinct dif-fraction peak can be observed in each XRD pattern, indexed as the(1 0 0) reflection of the 2-D hexagonal p6mm symmetry. The d100

value is calculated to be 8.21 nm for M-SiC-1000 and 8.42 nm forM-SiC-1200, respectively. As a consequence, the cell parameter isdeduced to be 9.48 nm for M-SiC-1000 and 9.72 nm for M-SiC-1200, respectively. Fig. 3b shows the typical wide-angle XRD

Fig. 1. Photograph of the M-SiC-1200 (a) and its cross-sectio

Fig. 2. TEM images of the M-SiC-1000 (a) and M-S

(WA-XRD) patterns of the M-SiC-1000 and M-SiC-1200 samples.The M-SiC-1000 has a very low crystalline b-SiC matrix and thecrystallization degree of b-SiC increases with the elevated pyrolysistemperature. In addition, the wide diffraction peaks indicate thatthe crystalline growth of b-SiC component might be restricted bynano-channels of carbon template.

The nitrogen adsorption–desorption isotherms measurementswere performed to characterize the porosity of the mesoporousSiC monoliths. As seen from Fig. 4, both isotherms exhibit typicaltype-IV curves with H1 hysteretic loops characteristic for mesopor-ous materials, with a clear capillary condensation step at a relativepressure (P/P0) of approximately 0.32–0.55, indicating the unifor-mity of mesopores. The capillary condensation step for theM-SiC-1200 is shifted to lower relative pressure, implying thesmaller mesopore size compared with that of the M-SiC-1000.The specific Brunauer-Emmett-Teller (BET) surface area is calcu-lated to be 632 m2 g�1 for the M-SiC-1000 and 564 m2 g�1 forthe M-SiC-1200, respectively. Moreover, the Barret-Joyner-Halen-da (BJH) mesopore size distribution for two samples, calculatedfrom the adsorption branch, both show a narrow distribution withan apparent average pore size of 2.55 and 2.48 nm, respectively.Moreover, the macroporosity of the SiC samples by mercuryporosimetry. The macropore surface area is 10.54 m2 g�1 for theM-SiC-1000 and 2.17 m2 g�1 for the M-SiC-1200, respectively.The macropore volume is 0.53 mL g�1 for the M-SiC-1000 and0.58 mL g�1 for the M-SiC-1200, respectively.

Table 1 summarizes the textural properties of samples. It isobvious that the M-SiC-1000 and M-SiC-1200 samples both displayhigh BET surface areas and high total pore volumes. Also, a de-crease in the surface area, the pore volume and the average poresize of the monolithic SiC samples with the increase of the pyroly-sis temperature is observed, which is mainly due to the shrinkageof CMK-3 framework combined with the densification of SiC com-ponents and similar results compared with that for mesoporous

n SEM images with low (b) and high (c) magnification.

iC-1200 (b), viewed from the [110] direction.

1.0 1.5 2.0 2.5 3.0

Inte

nsity

(a.

u.)

2theta (degree)

a

M-SiC-1000

M-SiC-1200

(100)

20 30 40 50 60 70 80

Inte

nsity

(a.

u.)

2theta (degree)

M-SiC-1200

M-SiC-1000

(111)

(220)(311)

b

Fig. 3. SA-XRD (a) and WA-XRD (b) patterns of the M-SiC-1000 and the M-SiC-1200, respectively.

0.0 0.2 0.4 0.6 0.8 1.0

80

100

120

140

160

180

200

220

240

260

280

300

Qua

ntity

Ade

sorb

ed (

cm3 /g

ST

P)

Relative Pressure (P/P0)

M-SiC-1000 M-SiC-1200

a

101 100

0

2

4

6

8

10

12

14

16

18

dvp/d

log(

Rp)

(cm

3 g-1nm

-1)

Pore diameter (nm)

M-SiC-1000 M-SiC-1200

b

Fig. 4. Nitrogen isotherms adsorption–desorption (a) and the corresponding pore-size distribution curves (b) of the M-SiC-1000 and M-SiC-1200, respectively.

Table 1Textural data and mechanical property of the samples.

Sample SBET

(m2 g�1)Meso-volume(cm3 g�1)

Pore size(nm)

Macro-volume(mL g�1)

DVa(%)

qb(gcm�3)

Total porosity(%)

Compression strength(MPa)

SBA-15 672 1.01 8.3 – – – – –CMK-3 1401 1.21 4.5 – – – – –M-SiC-

1000632 0.40 2.55 0.53 13.2 0.706 58.04 21.4

M-SiC-1200

564 0.30 2.48 0.58 20.9 0.742 53.57 33.5

a DV: means volume reduction (%) of the sample before and after pyrolysis.b q: means density of the resulting sample.

X. Yuan et al. / Microporous and Mesoporous Materials 142 (2011) 754–758 757

powdery SiC [15]. It can be verified by the increase of the volumereduction ratio and the density for the M-SiC-1200 compared withthose for the M-SiC-1000. In addition, as shown in Table 1, theaverage compressive strength for the M-SiC-1000 and the M-SiC-1200 is 21.4 MPa and 33.5 MPa, respectively. The values whichthe samples have in the same temperature treated both are muchhigher than that of porous SiC ceramics (2.19 MPa) reported by Yaoet al. [25] and porous SiOC monoliths (1.7 MPa) reported by Vaki-fahmetoglu et al. [26]. As the strength of a porous material dependsdirectly on the total pore volume [27], a higher amount of totalporosity has a lower strength. The total porosities of monolithicSiC samples are 58.04% and 53.57% for the M-SiC-1000 andM-SiC-1200, respectively (shown in Table 1). The relatively high

compressive strength in our system probably is due to the rela-tively dense matrix.

The oxidation behaviour at high-temperature of the M-SiC-1000and M-SiC-1200 sample was studied under air by means of TGA upto 1000 �C (shown in Fig. 5). The relatively high weight-loss be-tween 550 �C and 650 �C is due to the removal of free carbon com-pound. As the temperature continues to rise, curve presents aslight rising trend, which is because of mesoporous SiC surface oxi-dation. And the high rate of temperature (10 �C min�1) cannotmake sample completely oxidation, so the curve does not achievea steady state. The mesoporous SiC monolithic sample show thevery similar TGA curves compared with that for mesoporous pow-dery SiC [15]. Thus, the ordered mesoporous SiC monoliths can be

200 400 600 800 1000-100

-80

-60

-40

-20

0

Whi

tech

ange

(%)

Temperature (oC)

M-SiC-1000 M-SiC-1200

Fig. 5. TGA curves of the M-SiC-1000 and M-SiC-1200 in the temperature rangefrom RT to 1000 �C (heating rate, 10 �C /min; flowing air).

758 X. Yuan et al. / Microporous and Mesoporous Materials 142 (2011) 754–758

considered as stable supports to effectively increase the service lifeof devices that have to withstand harsh oxidative and thermalenvironments.

4. Conclusion

We have demonstrated a simple method for the synthesis ofmesoporous SiC monoliths, by using PCS as the starting preceramicpolymer and CMK-3 as the hard template. The as-synthesized SiCmonoliths show ordered 2-dimentional hexagonal p6mm meso-structures with high surface area, high pore volume and narrowpore-size distribution. Also, the monoliths exhibit good mechanicaland anti-oxidation properties. This approach may be useful for thelarge-scale industrial production of ordered mesoporous SiC-basedmonoliths, which would be the potential candidates for varioushigh-temperature applications, such as catalyst supports, sensors,energy storage and separation.

Acknowledgements

The authors acknowledge the support from the Top HundredTalents Program of Chinese Academy of Sciences, the NationalBasic Research 973 Program of China (2011CB706603) and theNational Nature Science Foundation of China (51005225).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.micromeso.2011.01.014.

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