Materials Letters 63 (2009) 168–170

Contents lists available at ScienceDirect

Materials Letters

j ourna l homepage: www.e lsev ie r.com/ locate /mat le t

Fabrication and characterization of porous unidirectional Si2N2O–Si3N4 composite

Shamiul Islam a,b, Min-Sung Kim a, Byong-Taek Lee a,⁎a Department of Biomedical Engineering and Materials, School of Medicine, Soonchunhyang University, 366-1 Ssyangyoung-dong, Cheonan, Chungnam 330-090, South Koreab Department of Display Materials Engineering, College of Engineering, Soonchunhyang University, Asan, Chungnam 336-745, South Korea

⁎ Corresponding author. Tel.: +82 41 570 2427; fax: +E-mail address: lbt@sch.ac.kr (B.-T. Lee).

0167-577X/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.matlet.2008.09.038

a b s t r a c t

a r t i c l e i n f o

Article history:

Porous unidirectional Si2N2 Received 15 July 2008Accepted 17 September 2008Available online 27 September 2008

Keywords:CeramicsComposite materialMicrostructure

O–Si3N4 composite was fabricated by in-situ nitriding of a porous unidirectionalSi substrate. The porous unidirectional Si substrate having a diameter of 450 μm, was prepared by formingethanol bubbles in a slurry which contained Si, Y2O3, Al2O3 and methylcellulose powder. After nitridation at1400 °C, the Si substrate was transformed into Si2N2O–Si3N4 composite and the pore surface of theunidirectional Si2N2O–Si3N4 composite was covered throughout with Si2N2O fibers, which had a diameter ofabout 55 nm. The Si2N2O fibers were orthorhombic single-crystals with an amorphous layer having athickness of about 1 nm. The compressive strength of the in-situ synthesized Si2N2O–Si3N4 composite wasabout 30 MPa.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Si-based porous ceramics have received a great deal of attentionbecause of their potential use as environmental filters for thepurification of polluted air and water. Among them, reaction-bondedSi3N4 (RBSN) ceramics have some advantages such as low cost of rawmaterials, easy control of dimensions, high strength retention andgood chemical stability at elevated temperatures, which make them agood candidate as a material for filtering air and water [1–3]. On theother hand, due to their excellent high-temperature strength andoxidation resistance, Si2N2O ceramics have been used as industrialceramics in severe environments [4–6]. It has been found that acomposite of Si2N2O and Si3N4 has a modified microstructure whichcombines the favorable properties such as the strength of Si3N4 andthe oxidation resistance of Si2N2O [7]. Hence a porous composite ofSi2N2O–Si3N4 could serve as a filter in severe environments [8].

Again, it is important for any environmental filter to have a highsurface area so that filtration efficiency can be increased. This can beachieved by increasing porosity and/or modifying the pore surface ofthe porous ceramic. However, if porosity is increased, then it will havea detrimental effect on themechanical property of the porous ceramic.Thus modifying the pore surface, such as, forming a large number offibers throughout the pore surface of the porous ceramic can increasethe efficacy of the filtration method.

In this work, first, the porous unidirectional Si substrate wasfabricated following a method which results in unidirectional pores[9]. After that, nitridation of the Si substrate was carried out at 1400 °Cin flowing N2+10% H2 gas which resulted in the formation of Si2N2Ofibers throughout the porous unidirectional Si2N2O–Si3N4 composite.

82 41 577 2415.

l rights reserved.

The morphology of the Si2N2O fibers in the porous Si2N2O–Si3N4

composite was investigated using transmission electron microscopy(TEM), and high-resolution transmission electron microscopy(HRTEM) techniques. The microstructure and the phase analysis ofthe Si substrate and the Si2N2O–Si3N4 composite were carried out bySEM and X-ray diffraction technique, respectively. The compressivestrength of the Si2N2O–Si3N4 composite was also reported.

2. Experimental

2.1. Porous unidirectional Si substrate preparation

As starting materials, Si powder (Permascand, Sweden, d50=7 μm,BET=1.2m2/g),methylcellulose (3000–5500 CPS, Yakuri Pure ChemicalsCo. Ltd, Kyoto, Japan) and sintering additives (6 wt.% Y2O3, d50=15 μm–

2wt.% Al2O3, d50=300 nm; Y2O3, DaejungChemicals &Metals Co., Seoul,Korea; Al2O3, AKP-50, Sumitomo Chemical Co., Tokyo, Japan) weremixed in ethanol (400ml) andwater. Themixerwas then ball milled for12husing siliconnitrideballmediawhichproduced aviscous slurry. Theas prepared slurry was then simultaneously stirred and heated to 70–80 °C for nearly 2 h. While the ethanol and water were evaporating outfrom the slurry at this temperature, at the same time, porous Si substratestarted to form at the bottom of the beaker. After complete evaporationof the ethanol and water, the porous body was taken out of the beakerand dried in the oven at 90 °C for 6 h to remove any traces of ethanol andwater. Thus the Si substrate was fabricated, which contained Si,methylcellulose, Y2O3 and Al2O3 powder.

2.2. Fabrication of Si2N2O–Si3N4 composite

To remove themethylcellulose binder from the Si substrate, a burn-out process was carried out at 1000 °C in air. After that, nitridationwas

Fig. 2. XRD profiles of the Si substrate after (a) burning out of methylcellulose at 1000 °Cand after (b) nitridation at 1400 °C for 8 h.

169S. Islam et al. / Materials Letters 63 (2009) 168–170

performedat 1400 °C inflowingN2+10%H2 gas at aflowrate of 50 cm3/min for 8 h, which transformed the Si substrate into Si2N2O–Si3N4

composite. Also, Si2N2O fibers were formed throughout the unidirec-tional pore surface of the Si2N2O–Si3N4 composite.

To identify the phases of the sample, X-ray diffraction (XRD, D/MAX-250, Rigaku, Tokyo, Japan) analysis was employed by crushingthe sample into powder. The microstructure and morphology of the Sisubstrate, Si2N2O–Si3N4 composite and Si2N2O fibers were observedby SEM (JSM-635, JEOL, Tokyo, Japan), TEM (JEM2010, Japan) andHRTEM (JEM2010, JEOL) techniques. Compressive strength wasmeasured by a Universal Testing Machine (Unitech™, R&B, Daejeon,Korea) with a cross head speed 0.5 mm/min. Load was applied parallelto the pore axis. On an average, five samples were taken for com-pressive strength measurement.

3. Results and discussion

The schematic diagram of the pore formation model and the SEM micrographs ofthe Si substrate after burning out of methylcellulose are shown in Fig. 1. Fig. 1(a) showsthe ball-milled slurry which contained dissolved methylcellulose particles andsuspended Si, Y2O3 and Al2O3 particles. The particles were homogeneously suspendedthroughout the liquid media. The slurry was then heated to a temperature of 70–80 °C,which caused a large number of bubbles to nucleate from the bottom of the glassbeaker, as in Fig. 1(b). Most of the bubbles formed were of ethanol, as the temperaturewas close to the boiling point of ethanol (79 °C). The bubbles created from the bottom ofthe glass beaker did not diminish until they reached the surface of the slurry. Whileethanol and water were evaporating from the slurry, all the suspended particles startedto settle down. However, since therewas continuous bubble formation from the bottomof the beaker, all the particles had to settle down between the available space of nearbybubbles and pile up to form a porous unidirectional body.

Hence, the bubble nucleation sites remained void and these void spaces resulted inunidirectional pores. Si particles remained bonded to each other because of the stickymethylcellulose particles, which acted as a binder material for the settled Si particles.Eventually, after complete evaporation of ethanol and water, the porous green body ofSi was obtained, as shown in Fig. 1(c). Fig. 1(e) shows the SEM images of the porousunidirectional Si substrate. The pores in Fig. 1(e) represent the bubble formation sites,i.e., the places fromwhere the bubbles came out continuously and left void spaces at theend of the operation. The pores were homogeneously distributed throughout theporous body. The longitudinal image in Fig. 1(f) confirms that the pores wereunidirectional. The average pore diameter was about 450 μm while the thickness ofthe frame was about 100 μm.

Fig. 1. Schematic diagram of pore formation model: (a) ball-milled slurry, (b) bubble formatithe Si substrate after burning out of methylcellulose: (e) cross-sectional and (f) longitudinal. (the web version of this article.)

Fig. 2 shows the XRD profiles of the Si substrate (a) after burning out ofmethylcellulose and (b) after nitridation of the Si substrate at 1400 °C for 8 h. The peakof Si and Y2O3was detected after burning out of methylcellulose from the Si substrate asshown in Fig. 2(a). However, Si2N2O, β-Si3N4 and α-Si3N4 phases were observed as themain peaks after nitridation as can be seen in Fig. 2(b). The formation of Si2N2O fiberswas due to the vapor phase reactions between the intermediate phase SiO and N2

during nitridation. SiO might have been produced at the Si–SiO2 interface from thereduction of SiO2 by the gaseous environment (N2+10% H2) [10,11].

Fig. 3 shows the SEM micrographs and EDS profiles of Si2N2O–Si3N4 compositewhich was nitrided at 1400 °C in flowing N2+10% H2 gas. Fig. 3(a) shows the cross-sectional image of the nitrided composite. Si2N2O fibers can be seen in the pore surfaceof the Si2N2O–Si3N4 composite body in Fig. 3(b). The longitudinal image of the Si2N2O–Si3N4 composite body is shown in Fig. 3(c).The dark contrast represents the frame partwhile the relatively bright contrast shows the pore region (covered with Si2N2O fibers)

on starts at 70–80 °C, (c) green body of silicon-methylcellulose and SEM micrographs ofFor interpretation of the references to color in this figure legend, the reader is referred to

Fig. 3. SEM micrographs of porous Si2N2O–Si3N4 composite: (a) cross-sectional, (b) enlarged image of (a), (c) longitudinal, (d) enlarged image of (c) taken from the region marked Rand EDS profiles (e,f) of pore and frame of the regions marked S and T in (c), respectively.

170 S. Islam et al. / Materials Letters 63 (2009) 168–170

in the Si2N2O–Si3N4 composite. It is clear that Si2N2O fibers were formed throughoutthe entire unidirectional pore surface of the composite body. The enlarged image inFig. 3(d), which was taken from the regionmarked with R in Fig. 3(c), shows that a largenumber of network type Si2N2O fibers were formed after nitridation at 1400 °C. TwoEDS profiles, Fig. 3(e,f) were taken from the region marked with S (pore) and T (frame)in Fig. 3(c), respectively. Si2N2O fibers were also formed in the frame part, because of theporous framewhich allowed (N2+10% H2) gases to pass through and is evident from thepeaks of Si, N and O in Fig. 3(f). The compressive strength of the porous unidirectionalcomposite was measured after nitridation of the Si substrate at 1400 °C and thedirection of the measurement was parallel to the unidirectional channels. The averagecompressive strength of the nitrided composite was about 30 MPa. Such high strengthvalue could be attributed to the thick frame (100 μm) of the composite.

TEM and HRTEM observation was carried out to clearly identify the Si2N2O fibersand the results are shown in Fig. 4. The HRTEM observationwas obtained for the [1–10]zone axis of the Si2N2O fibers. The diameter of the Si2N2O fiber was about 55 nm as canbe seen from Fig. 4(a). The electron diffraction pattern in Fig. 4(b), which was takenfrom the center of the Si2N2O fiber, confirmed that the Si2N2O fibers were single crystal.

Fig. 4. (a) TEM, (b) electron diffraction pattern and (c) HRTEMmicrographs of the Si2N2Ofiber.

The HRTEM micrograph in Fig. 4(c) shows highly crystalline Si2N2O fiber with anamorphous layer having a thickness of about 1 nm.

4. Conclusions

In summary, porous unidirectional Si substrate was first fabricatedusing an easy method, which employed ethanol bubbles as a poreforming agent in a viscous slurry. The average diameter of the porousunidirectional Si substrate was about 450 μm while the framethickness was about 100 μm. After nitridation of the Si substrate at1400 °C, a large number of network type, single crystal Si2N2O fiberswere observed in the pore region of the nitrided body. Si2N2O fiberswere formed throughout the unidirectional pores of the Si2N2O–Si3N4

composite which could further enhance the filtration efficiency. Thediameter of the Si2N2O fibers was about 50 nm and the compressivestrength of the Si2N2O–Si3N4 composite was about 30 MPa.

Acknowledgements

This work was financially supported by a grant from the Center forAdvanced Materials Processing (CAMP) of the 21st Century FrontierR&D Program funded by the Ministry of Commerce, Industry andEnergy (MOCIE), Republic of Korea.

References

[1] Wang CM, Emoto H, Mitomo M. J Am Ceram Soc 1998;81:1125–35.[2] Lee BT, Kim HD. Mater Sci Eng A 2002;333:306–13.[3] Bressiani JC, Izhevskyi V, Bressiani HA. Mater Res 1999;2:165–72.[4] Lee BT, Kim HD. Mater Sci Eng A 2004;364:126–31.[5] Xie RJ, Mitomo M, Kim W, Kim YW. J Am Ceram Soc 2000;83:3147–52.[6] Xie RJ, Mitomo M, Xu FF, Zhan GD, Bando Y, Akimune Y. J Eur Ceram Soc

2002;22:963–71.[7] EmotoH,MitomoM,WangCM,HirosturuH, Inaba T. J EurCeramSoc1998;18:527–33.[8] Lee BT, Paul RK, Lee CW, Kim HD. Mater Lett 2007;61:2182–6.[9] Song HY, Islam S, Lee BT. J Am Ceram Soc 2008;91:3125–7.[10] Chollon G, Hany R, Vogt U, Berroth K. J Eur Ceram Soc 1998;18:535–41.[11] Scheffler M, Peppel E, Woltersdorf J, Greil P. Mater Chem Phys 2003;80:565–72.