Available online at ScienceDirect

ScienceDirectJ. Mater. Sci. Technol., 2014, -(-), 1e5

Synthesis of Non-oxide Porous Ceramics Using Random Copolymers as

Precursors

Xiaoqian Wang1,2), Kewei Wang1,2), Jie Kong3), Yiguang Wang1,2)*, Linan An4)

1) State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China2) Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University,

Xi’an 710072, China3) Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an 710072, China4) Department of Materials Science and Engineering, Advanced Materials Processing and Analysis Center,

University of Central Florida, Orlando, FL 32816, USA[Manuscript received November 9, 2013, in revised form December 31, 2013, Available online xxx]

* Corresp29 88491005-03JournalLimited.http://dx

Please

In this paper, we reported a novel method for synthesis of non-oxide porous ceramics by using randomcopolymers as precursors. A silazane oligomer and styrene monomer were used as starting materials, whichwere copolymerized at 120 �C to form random polysilazaneepolystyrene copolymers. The copolymers werethen pyrolyzed at 500 �C to obtain porous ceramics by completely decomposing polystyrene (PS) andconverting polysilazane (PSZ) into non-oxide SieCeN ceramics. The obtained material contained a bi-modelpore-structure consisting of both micro-sized and nano-sized pores with very high surface area of more than500 m2/g. We also demonstrated that the pore structure and surface area of the materials can be tailored bychanging the ratio of the two blocks. Current results suggest a promising simple method for making multi-scaled porous non-oxide materials.

KEY WORDS: Non-oxide porous ceramics; Polymer-derived ceramics; Random copolymer

1. Introduction

Non-oxide porous ceramics have attracted increasing interestsin recent years[1e5]. This new class of materials exhibit manyadvantages over their oxide counterparts, including low density,better chemical inertness and high thermal stability[6,7], thuspromising for applications in extreme environments, such assolideliquid separation, gas and liquid filtration, purification,catalysts supports, sound and shock absorption, and thermalinsulation[8]. Previously, non-oxide porous ceramics were pri-marily synthesized by using “conventional” ceramic materialsvia following techniques: replica, sacrificial template, directfoaming techniques, freezeedrying and solegel method[1,9,10].While a variety of porous materials have been prepared, thesetechniques are limited in terms of the complex fabrication pro-cesses and the lack of flexibility in pore structure manipulations.

onding author. Prof., Ph.D.; Tel.: þ86 29 88494914; Fax: þ864620; E-mail address: wangyiguang@nwpu.edu.cn (Y. Wang).02/$e see front matter Copyright� 2014, The editorial office ofof Materials Science & Technology. Published by ElsevierAll rights reserved..doi.org/10.1016/j.jmst.2014.04.008

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Recently, non-oxide porous ceramics have also been synthe-sized by thermal decomposition of polymeric precursors (so-called polymer-derived ceramics, PDCs). The technique caneasily achieve multi-scaled porous structures even in one step.For example, nano-scaled pores can be self-formed during thedecomposition of the chemical bonds such as CeH, SieH, andNeH in polymer precursors[11,12]; the mesopores can be ob-tained by designing the molecular structure of the preceramicpolymer[13,14]; and the large-scaled pores can be realized byusing template technique[14e16]. In addition to the flexible pro-cessing, polymer-derived ceramics also exhibited a set of uniquestructural and functional properties[17e25], which makes porousPDC very promising for widespread applications.In this paper, we report a new facile approach to prepare non-

oxide porous SieCeN ceramics by using random copolymers asprecursors. The basic idea of the technique is illustrated in Fig. 1.It includes following basic steps: (i) synthesizing copolymerscontaining polysilazane blocks which can be converted to non-oxide ceramics and polystyrene blocks which will becompletely decomposed to form pores; (ii) cross-linking thecopolymer to form preceramic precursor; and (iii) pyrolyzed theprecursor to form porous ceramics. By this technique, the innerstructure, size and distribution of the pores can be designed andcontrolled by tailoring the amount and size of the two blocks.

nce & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.04.008

Fig. 1 Schematic illustration of the proposed synthesis procedure for non-oxide porous ceramics by using random block copolymer as the precursor.

2 X. Wang et al.: J. Mater. Sci. Technol., 2014, -(-), 1e5

2. Experimental

2.1. Starting materials

A liquid-phased polysilazane (PSZ) was purchased fromInstitute of Chemistry, Chinese Academy of Sciences, Beijing,China. Styrene (purity, 98.0%), toluene (purity, 99.5%) and n-hexane (purity, 97.0%) were obtained from Xingyue ChemicalCo. Ltd., Tianjin, China. Azodiisobutyronitrile (AIBN, 99%)was provided by J&K Scientific Ltd., Beijing, China. Tetrahy-drofuran (THF, analytically pure grade) was obtained fromAladdin Chemistry Co. Ltd., Shanghai, China. The chemicalswere used as-received without further treatment, except that THFwas distilled under reflux using sodium/benzophenone followedby drying under vacuum of 2 kPa (20 mbar) at 40 �C for 24 h.

2.2. Synthesis of copolymer

PSZ, styrene, AIBN and anhydrous toluene were filled into aflame-dried flask equipped with a Teflon stir bar, septum, andhigh-vacuum stopcock at room temperature under an inert argonatmosphere. The flask was then heated to 120 �C in an oil bathfor 4 h to form PSZePS copolymer. The obtained copolymerwas dissolved in THF, and then was slowly added to excess n-hexane to precipitate. After three times of dissolve-precipitationprocess, the copolymer was separated from the solution. Theresidual solvent in the filtered polymer was removed at roomtemperature for 24 h in a vacuum oven. The produced copolymerwas white powders with a yield of 35%e40%. Two kinds ofcopolymers were prepared with the ratios of PSZ to styrene of2:1 and 3:5, respectively, and labeled as PSZecoePS (2:1) andPSZecoePS (3:5).

2.3. Pyrolysis

The pyrolysis process was carried out at 500 �C in an aluminatube furnace (GSL-1600X, Hefei Kejing Materials TechnologyCo., Ltd., Hefei, China) with the protection of flowing high-purity argon. The heating and cooling rate was 2 �C/min andthe holding time was 2 h. The open porosities and the densitiesof the resultant ceramics were measured by Archimedes drainagemethod.

2.4. Characterization

The resultant porous ceramics were characterized by scanningelectron microscopy (SEM, JEOL 6700F, Tokyo, Japan) andenergy dispersive spectroscopy (EDS). The bonding structure ofthe copolymers was measured by Fourier transform infraredspectroscopy (FTIR, Nicolet, Beijing Second Optical Instrument

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Factory, Beijing, China) with KBr tablets; nuclear magneticresonance spectroscopy (NMR, INOVA-400, Varian, Inc., USA)was carried out using CDCl3 as solvent and tetramethylsilan(TMS) as internal standard. The thermal properties of the ob-tained copolymers were studied by using thermogravimetricanalysis (TGA, Q50, TA Instrument, USA) in a platinum cru-cible up to 800 �C under an argon atmosphere with a flow rate of40 mL/min and a heating rate of 10 �C/min, and using differ-ential scanning calorimeter (DSC, MD2910, TA Company,USA). The molecular weight and its distribution of the synthe-sized copolymers were investigated by gel permeation chroma-tography (GPC, Waters, USA) using THF as solvent.Simultaneous thermal analysis of TGA and DSC coupled withmass spectrometry was performed using a simultaneous ther-moanalyzer (STA 449 F3) coupled with a quadrupole massspectrometer (QMS 403 C Aëolos, Netzsch Group, Germany) inthe temperature range of 40e1400 �C with a heating rate of10 �C/min under argon atmosphere with a flow rate of 50 mL/min. The BrunauereEmmetteTeller (BET) surface area and poresize were measured by surface area and porosimetry analyzer(Gold APP Instrument Corporation, Beijing, China).

3. Results and Discussion

The formation of the copolymers was first analyzed by FTIRspectrometry. It is seen (Fig. 2(a)) that both copolymers containpeaks corresponding to NeH bond, SieH group, SieNeSi bondand SieCH3 group stretching at 3377, 2127, 800 and1255 cm�1, respectively. However, peaks corresponding to theC]C in raw materials of polysilazane and styrene disappear inthe copolymers. It is indicated that the two substances havecross-linked together by the vinyl polymerization. The concen-tration of PSZ and PS in the copolymers can be estimated fromthe relative intensity of the representative peaks. Here, SieCH3

signal and benzene ring single were used for PSZ block and PSblock, respectively. It was found that the ratios ofISi�CH3=Ibenzene ring are 0.74 and 0.83 for PSZecoePS (3:5) andPSZecoePS (2:1), respectively, suggesting that the formercontains more PS than the later. This is consistent with ouroriginal design.Further characterization of the copolymers was carried out by

using 1H NMR (Fig. 2(b)). It is seen that the benzene ringprotons, SieCH3 groups and silicon hydrogen bonds (SieH)were observed at chemical shifts of 7.08 � 10�6e7.18 � 10�6,0 and 4 � 10�6, respectively. The peak for C]C around5.6 � 10�6e6.2 � 10�6 was not found, confirming that thepolymerization reaction happened between the vinyl in PSZ andstyrene. The peak at 1.6 � 10�6 was assigned to eCH2e pro-tons, which was derived from the polymerization of vinyl inpolysilazane. By using the silicon hydrogen bond (SieH) at thechemical shift of 4.15 � 10�6 as an internal reference, it is found

nce & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.04.008

Fig. 2 FTIR spectra (a) and 1H NMR spectra (b) of the synthesizedcopolymers; (c) GPC traces of the copolymers and PSZ; (d)proposed chemical structure of the copolymers.

X. Wang et al.: J. Mater. Sci. Technol., 2014, -(-), 1e5 3

that the integral area of benzene ring protons (d ¼ 6.6 � 10�6e7.5 � 10�6) in PSZecoePS (3:5) is much higher than that inPSZecoePS (2:1), clearly confirming that PSZecoePS (3:5)contains more PS segments than PSZecoePS (2:1).Fig. 2(c) shows the GPC traces of the obtained copolymers

and the starting PSZ. Compared to the starting PSZ, the obvious

Fig. 3 (a) TGA curves of PS, PSZ and the copolymers; (b) DSC curves o

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lower elution volumes of copolymers indicate that they havehigher molecular weights after polymerization at 120 �C for 4 h.The PSZecoePS (3:5) shows the lowest elution volume,thus the highest average molecular weight (Mn), which iscalculated to be 69,100 g/mol based on the Mn value of the PSZof 1210 g/mol. The high molecular weight of PSZecoePS (3:5)is likely due to the high feed ratio of styrene monomer. Morestyrene monomers can enhance the polymerization, leading to alarger molecular weight.Based on the aforementioned results, we can infer the chem-

ical formula and equation that best characterize the copolymer asshown in Fig. 2(d), where the values of m and n increase withincreasing reaction time and temperature.The thermal decomposition of the copolymers and the starting

PSZ and PS were analyzed by using TGA and DSC. Fig. 3(a)shows the TGA curves of PS, PSZ, and two copolymers. It isseen that pure PS is completely decomposed at around 400 �C,while the pyrolysis temperature of pure PSZ is around 800 �C.For the copolymers, the pyrolysis temperatures of the PSZecoePS (2:1) decreases to 600 �C; and that of PSZecoePS (3:5)further decreases to 550 �C. The results clearly reveal that theexistence of PSZ delays the pyrolysis of PS. The pyrolysis res-idue of pure PS is only 2% at 500 �C while it increases to nearly8% for PSZecoePS (3:5) and 33% for PSZecoePS (2:1) at thistemperature, respectively. The weight loss data can be used tocalculate the mass percentage of the PSZ segments in the co-polymers. Assuming that the percentage content of PSZ seg-ments is x and PS 1ex, according to the residue shown in TGAcurves at 500 �C, PSZ concentration can be estimated as follows:

PSZ�co� PSð3 : 5Þ 2%ð1� xÞ þ 80%x ¼ 8%

x ¼ 7:7% 1� x ¼ 92:3%

PSZ�co� PSð2 : 1Þ 2%ð1� xÞ þ 80%x ¼ 33%

x ¼ 39:7% 1� x ¼ 60:3%

DSC curves (Fig. 3(b)) reveal that an exothermic peak appearsat about 200 �C. This peak is attributed to the curving peaks ofPSZ segments in copolymer due to the dehydrogen reaction ofsilicon hydrogen bonds. The exothermic peak at 200 �C disap-pears in the second heating cycle. It is believed that the PSZ hasbeen fully cured after the first heating cycle. The endothermicpeak on the heating curve at around 255 �C and exothermic peakon the cooling curve at around 245 �C are believed to be themelting and crystallization peaks of the PS segments in copol-ymer. Combined with GPC curves, it is proved that the randomcopolymerization happens between PSZ and styrene. The twostages of degradation and small molecules extravasation on

f PSZecoePS (2:1); (c) TGAeMASS analysis of PSZecoePS (3:5).

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Fig. 4 SEM images of the ceramics synthesized from PSZecoePS (2:1) (a); and PSZecoePS (3:5) (b); (c) EDS of the ceramics synthesized fromPSZecoePS (3:5).

Fig. 5 Nitrogen isothermal adsorptionedesorption spectra and pore sizedistribution of PSZecoePS (3:5).

4 X. Wang et al.: J. Mater. Sci. Technol., 2014, -(-), 1e5

TGAeMASS curve of PSZecoePS (3:5) are shown in Fig. 3(c),which further supports the molecular structure of the PSZecoePS copolymers.The morphologies of pyrolyzed ceramics are characterized

using SEM, as shown in Fig. 4(a) and (b). All the products showporous structures. The pore size in PSZecoePS (3:5) is largerthan that in PSZecoePS (2:1). The EDS of PSZecoePS (3:5)analysis shows that the specimens contain Si, C and N elements.The content of Si, C and N is at 29.26%, 53.58% and 14.51%,respectively. The open porosities were measured to be 54.28%and 46.97% for PSZecoePS (3:5) and PSZecoePS (2:1),respectively; while the respective bulk densities are 0.74 and0.83 g/cm�1, respectively.The nitrogen isothermal adsorptionedesorption method was

used to test the BET of the obtained porous ceramics. The resultsare shown in Fig. 5, based on which the surface area and poresize were calculated and listed in Table 1. It is seen that bothceramics exhibit very large BET surface area and meso-scaledpores. BET surface area and mesopore size of specimen PSZecoePS (3:5) are larger than those of PSZecoePS (2:1), sug-gesting the more content of the PS segments which lead to thatthe longer the chains, thus the larger the specific surface area andpore size. In low pressure region, as the nitrogen partial pressureincreases, the adsorption volume increases rapidly. Relative

Table 1 Properties of the synthesized ceramics

Sample Temperature(�C)

Surface area(m2/g)

Pore volume(ml/g)

Averagepore width

(nm)

PSZecoePS(2:1)

500 484 0.0085 0.43

PSZecoePS(3:5)

500 524 0.0096 0.45

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pressure under 0.1 can be explained as a multi-layer adsorption.The pore size distribution in this region shows that the surfacearea mainly comes from the micropores. Relative pressure above0.1 may be considered that there is a small amount of mesoporeswhich results in the increase of the adsorption capacity. It can beconcluded from the size and shape of the hysteresis loop thatthere exists capillary condensation phenomenon. The adsorptionand desorption curves show a better overlap, which indicates alittle amount of mesopores in the ceramic structure.

4. Conclusion

Non-oxide porous ceramics have been successfully synthe-sized by using random block copolymer PSZecoePS as pre-cursors. The formation and pyrolysis process of the copolymerwere analyzed using various techniques. The obtained materialsshowed bi-model pore structure consisting of both micro-sizedand nano-sized pore structures. By tailoring the ratio of thestarting materials, the pore size and specific surface area can bechanged. The obtained products have very high surface areas ofmore than 500 m2/g. The materials have a great potential formany extreme environmental applications. In addition, thistechnique is promising for synthesizing various porous ceramicswith controlled structures.

AcknowledgmentsThis work was financially supported by the National Natural

Science Foundation of China (Grant Nos. 21174112 and51242009), the Research Fund of State Key Laboratory of So-lidification Processing (Grant No. 82-TZ-2013), and the project“111” (B08040). J. Kong thanks the grant of the New CenturyExcellent Talents of Education Ministry of China (NCET-11-0817).

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