Materials Research Bulletin 46 (2011) 2398–2402

Preparation and mechanical properties of silicon oxycarbide fibers fromelectrospinning/sol–gel process

Xiaofei Wang, Cairong Gong *, Guoliang Fan

School of Materials Sciences and Engineering, Tianjin University, 300072 Tianjin, PR China

A R T I C L E I N F O

Article history:

Received 28 April 2011

Received in revised form 15 July 2011

Accepted 24 August 2011

Available online 1 September 2011

Keywords:

A. Ceramics

B. Sol–gel chemistry

C. Electron microscopy

D. Mechanical properties

A B S T R A C T

Silicon oxycarbide (SiOC) fibers were produced through the electrospinning of the solution containing

vinyltrimethoxysilane and tetraethoxysilane in the course of sol–gel reaction with pyrolysis to ceramic.

The effect of the amount of spinning agent Polyvinylpyrrolidone (PVP) on the dope spinnability was

investigated. At a mass ratio of PVP/alkoxides = 0.05, the spinning sol exhibited an optimal spinnable

time of 50 min and generated a large quantity of fibers. Electrospun fibers were characterized by Fourier

transform infrared spectroscopy (FTIR), thermo gravimetric analysis–differential scanning calorimetry

(TGA–DSC), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The SEM

results revealed that the SiOC fibers had a smooth surface and dense cross-section, free of residue pores

and cracks. The XPS results gave high content of SiC (13.99%) in SiOC fibers. The SiOC fibers prepared at

1000 8C had a high tensile strength of 967 MPa and Young’s modulus of 58 GPa.

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1. Introduction

Increasing industrial and scientific interest in advancedcomposites has stimulated the wide spread investigation ofapplying non-oxide ceramic fibers as high strength materialsand composites due to their favorable properties including lowdensity, excellent specific strength, high thermal conductivity, andresist to high temperature and oxidation [1–3]. The high cost ofnon-oxide fibers, however, has so far greatly hampered theexpectation of applications. With excellent mechanical properties,oxidation resistance and low cost for production, silicon oxycar-bide (SiOC) fibers are a promising alternative candidate tosubstitute the non-oxide fibers used at a moderate temperature(<1200 8C) [4,5]. Pyrolyzing the hydrolysis products (gel) oforganosilicon esters may produce silicon oxycarbide phases [6–8]. The sol–gel process has now been developed into one of themost ideal routes to inorganic fibers because of adjustable fibercompositions, lower formation temperatures and relatively lowcost [9]. Until now, only limited success on the preparation of SiOCfibers by using the sol–gel process has been reported. To get aspinnable sol, Kamiya et al. [10] introduced methyltriethoxysilane(MTES, CH3Si(OC2H5)3) into an enthanol solution of TEOS, and SiOCglass fibers were prepared by heat-treating the gel fibers. Ruanet al. [11] reported a fabrication of SiOC fibers by pyrolyzing thehydrolysis products (gel) from the solution containing vinyltri-

* Corresponding author. Tel.: +86 22 27404226; fax: +86 022 27404724.

E-mail address: gcr@tju.edu.cn (C. Gong).

0025-5408/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2011.08.052

methoxysilane (VTMS, ViSi(OMe)3) and TEOS along the sol–gelprocess. The SiOC fiber presented a tensile strength of 776 MPa.Chen et al. [12,13] fabricated SiOC fibers using an acetone solutionof VTMS with cellulose acetate as a spinning agent. However, theabove mentioned conventional sol–gel processes have difficultiesto prepare ultrafine fibers and to control the fiber morphology aswell.

Electrospinning is a simple, low cost and relatively highlyproductive method for generating fine inorganic solid fiber withdiameters range from tens of nanometers to tens of micrometers.Electrospinning has so far been employed to produce a numerousof materials including polymers, biological material [14,15] andceramics [16,17]. Although electrospinning of inorganic sols isrelatively straightforward and capable of generating ceramic fibersmade of various amorphous oxides, the viscosity changes withaging time, which makes it difficult to spin uniform fiberscontinuously. Therefore, preparation of a precursor solution withappropriate rheological properties is the key step for successfulelectrospinning [18]. To date, sols used for the formation of siliconoxycarbide usually had poor spinnability because of their difficultyin solidification. In this paper, polyvinylpyrrolidone (PVP) is usedto control the rheological properties of the solution. And thesolution is sufficiently stable to enable the continuing productionof fibers compared with the conventional sols in previous reports.

In the present work, we report fabrication of SiOC fibers bycombined sol–gel method and electrospinning techniques. Aspinning sol is prepared by simply co-dissolving the precursorand polymer with HNO3 in water. The polysiloxane (PSO) fibers areproduced by electrospinning of the spinnable solutions and after

Fig. 1. The variation of viscosity with time starting from the preparation of the

solution (a), and the section corresponding to a spinnable state in the solution (b).

Table 1The amount of PVP on preparation of the solution and spinnable time.

PVP (g) Time for preparation of the solution (h) Spinnable time (min)

0.8 2.5 10

0.9 3.0 30

1.0 4.0 50

1.1 2.5 20

1.2 2.5 <10

X. Wang et al. / Materials Research Bulletin 46 (2011) 2398–2402 2399

pyrolysis in flowing argon at 1000 8C, silicon oxycarbide (SiOC)fibers are obtained. The spinnability of the solution is studied inassociation with reaction conditions. In addition, SiOC fibers arecharacterized by structural analysis and mechanical properties ofthe SiOC fibers are also investigated.

2. Experimental

2.1. Preparations

The alkoxides including silicon tetraethoxide (TEOS, Si(OC2H5)4,Tianjin Jiangtian Chemicals, Tianjin, China) and vinyltrimethox-ysilane (VTMS, CH255CHSi(OCH3)3, Qufu Wanda Chemicals, Shan-dong, China) were used as raw materials. Nitrate acid (HNO3,Tianjin Jiangtian Chemicals, Tianjin, China) was used as catalysts.Polyvinylpyrrolidone (PVP; Tianjin tiantai Chemicals, Tianjin,China) was used as spinning agent. A typical solution was madewith VTMS/TEOS = 1.3, H2O/alkoxides = 3.5, HNO3/alkox-ides = 0.02 in mole ratios and PVP/alkoxides = 0.05 in mass ratioto the alkoxides (VTMS + TEOS). In a procedure, TEOS (10.0 g,0.048 mol), VTMS (9.4 g, 0.064 mol), nitrate acid (HNO3, 3.0 g,1 mol/L), distillated water (7.0 g) and PVP (1.0 g, Mw � 30,000 Da)were sequentially added into a 100 ml-flask under magneticallystirring (200 r/min), and a transparent solution was obtained. Aftercontinuous stirring for about 4 h, the transparent solutiontransformed gradually into a viscous sol. Fibers could be formedby pulling an immersed glass bar from the sol. The interval of thetime between the fibers could be drawn from the solution and thegel formation was defined as spinnable time (t).

The spinnable sol was loaded in a plastic syringe equipped witha 9 # gauge stainless steel needle, and the syringe was set on asyringe pump (Cole-Parmer Instrument Co.) with the feeding rateof 2.5 ml/h. A dc voltage of 15 kV was supplied by a dc high-voltagegenerator was connected to the needle. A stainless steel meshcovered with a piece of aluminum foil was used as a collector. Thedistance between the needle and the collector was 10 cm. Thepolysiloxane (PSO) fibers were formed as a result of electrostaticjetting. These fibers were subsequently converted into SiOC fibersafter pyrolysis in an alumina crucible in a tube furnace at 1000 8Cfor 1 h with a ramping rate of 5 8C/min under flowing argonprotection.

2.2. Characterizations

The viscosity of solutions was measured with a viscometer(Brookfield DV-II, Middleboro, USA) at 25 8C using a rotatingvelocity of 25 r/min. Fiber strength was tested by using a fibermicro-tester (JSF08, Shanghai Zhongcheng Digitals Co Ltd.,Shanghai, China) with 10 mm gauge length and tensile speed of0.5 mm/min. Materials were also characterized by using a Fouriertransform infrared spectrometry (FT-IR; NICOLET 6700), a thermalgravity (TG) and (analysis-differential scanning calorimetry) DSCanalysis (NETZSCH STA 449 C, Germany) in flowing nitrogen at theheating rate of 5 8C/min, a scanning electron microscopy (SEM;XL30ESEM, Philips), and a X-ray photoelectron spectroscopy (XPS;PE, PHI-1600, US).

3. Results and discussion

3.1. Spinnability of the solutions

The ratio of (H2O/alkoxide <4) and (HNO3/alkoxides <0.04) arecrucial to the attaining of a spinnable solution from the(VTMS + TEOS) system according to the literature [9,11,19]. Thespinnable solution containing VTMS/TEOS = 1.3, H2O/alkox-ides = 3.5, HNO3/alkoxides = 0.02 in mole ratios and PVP/alkox-

ides = 0.05 in mass ratios to the alkoxides (VTMS + TEOS) wasformed after approximately 4 h of constantly stirring. The solutionwas able to electrospin fibers for 50 min before gelation. The timeinfluence on the viscosity change of the solution was measured byusing a rotating viscometer, as shown in Fig. 1. The viscosity of thesolution exhibits little change in the initial 13 h with h < 1.8 p.After 14 h, the viscosity increases dramatically to h = 230 p andthen gelation occurs. The viscosity corresponding to the spinnablestate is at h = 1.8 – 51.6 p. The range of viscosity is within that (10–100 p) for spinning silica gel fibers from the TEOS + H2O + HClsystem observed previously [20,21].

Poly(vinyl pyrrolidone) (PVP) is one of the typically spinningagents used in the solution for the electrospinning process toregulate the rheological properties of the solution. Our studyindicates that the amount of PVP has influence on both the time forpreparation of the solution and spinnable time, as listed in Table 1.The amount of PVP was changed with the mass ratio of PVP/alkoxides = 0.04–0.06 at VTMS/TEOS = 1.3, H2O/alkoxides = 3.5and HNO3/alkoxides = 0.02. The solution is spinnable at a moderateratio of PVP/alkoxides = 0.05, with t = 50 min. The spinnable time islonger than without using spinning agent. Moreover, at a lowerPVP/alkoxides = 0.04 or at a higher PVP/alkoxides = 0.06, thespinnable time becomes too short for electrospinning. This mightrelate to the N-alkyl substituted carboxylic amide groups as astrong hydrogen acceptor in PVP [22]. When less than 5 wt% of PVPis added, the hydrogen bond between silanol group of theintermediate species from silica alkoxide and the carboxylic amidegroups in PVP is formed, then the organic polymer is subjected tohydrolysis together with TEOS and the subsequent co-polycon-densation. While with the amount of PVP of over 5 wt%, thespinnable time becomes shorter. It might be attributed to thestructural features of PVP, which is a polymer with a long and softpolyvinyl chain, and polar groups [23]. PVP prohibits (HO)xSiO4�x

(x = 1–3) from the hydrolysate of TEOS bonding as a result of itssteric effect and the coordinative chemical bonding of C55N andC55O to TEOS.

200 400 600 800 10000

20

40

60

80

100

Temperature /°C

Res

idua

l mas

s /%

0

4

8

12

16

DSC

/mW

TG

DSC

Fig. 2. TG and DSC curves of PSO fiber from electrospinning in flowing nitrogen at

5 8C/min.

X. Wang et al. / Materials Research Bulletin 46 (2011) 2398–24022400

3.2. Thermal gravity and DSC analyses

The pyrolysis behavior of the PSO fiber was studied by TG andDSC analyses, as shown in Fig. 2. The TG curve is divided into threesteps: 35 wt% below 450 8C, 50 wt% between 450 and 600 8C and2 wt% among the temperature of 600–1000 8C. The first step ismainly attributed to the decomposition of PVP [24–26] along withthe release of small hydrocarbon molecules. The second step isrelated primarily to the removal of organics (principal weight loss)accompanied by polymerization and structural relaxation. Theshrinkage occurs with light weight loss in the third step is probablydue to the releasing of oxygen in the procedure of the densificationand crystallization of the amorphous SiOC ceramic. A correspond-ing exothermic peak centered at 732 8C can be clearly identified inDSC curve.

3.3. Fourier transform infrared spectra

Fig. 3 shows the FTIR spectrum of PSO fiber and SiOC fiber. Asshown in Fig. 3a, there is a strong absorption peak at 1040 cm�1

which corresponds to the stretching mode of Si–O–Si fromhydrolysis and condensation of the alkoxides (VTMS + TEOS).And absorption at 1650 cm�1 is referred to the vibrations of C55Ofrom PVP, which affords a strong support to the formation of

10001500200025003000

(b)

Wavenumber /cm-1

Inte

nsity

/a.u

.

770969

104 0

1279

14101601

1650282 0

292 0

(a)

Fig. 3. FTIR spectra of PSO fiber from electrospinning (a), and SiOC fiber after

pyrolysis at 1000 8C in argon (b).

hydrogen bond in the hybrids [22]. Moreover, the absorption bandsfor Si–CH at 1279 cm�1 and C55C at 1601 cm�1 well accord with theliterature [27]. These spectra characteristics reveal the incorpo-ration of vinyl groups into the PSO network existing as attachgroups bonded to Si in the Si–O. However, the absorption at770 cm�1 was still visible in the SiOC fiber Fig. 3b, which wasattributed to Si–C bonds from VTMS. A clear decrease on theabsorption peak at 1410 cm�1 for C–H from vinyl (bending mode)in SiOC fiber can be observed. Furthermore, the organic relatedbonds such as stretching mode of C–H (2820, 2920 cm�1) and H–CH55CH2 (969 cm�1), as existed in the PSO fiber have disappearedafter the pyrolysis process. This confirms that the pyrolyzed fibersare structured as a silicon oxycarbide phase rather than a mixtureof silicon oxide and carbon.

3.4. X-ray photoelectron spectroscopy

Curve-fitting of XPS-Si2p1/2 spectra from the SiOC fibers isshown in Fig. 4. Assuming that the SiOC fibers consist of SiO2, SiCand free C, the fraction of carbidic phase or SiC/(SiO2 + SiC) wt%was obtained from area of XPS-Si2p1/2. As expected, the XPS bindingenergy (BE) values for Si2p are 103.24 eV, which matching to bedetermined for the siloxy species [28]. Furthermore, SiO2 curve andSiC curve are 103.49 and 102.16 eV, respectively. Si2p1/2 BE value ofthe SiOC fibers is shifted toward lower value or toward the valuefor SiC crystal. These facts imply that carbidic bonds are formed inthe SiOC fibers. Additionally, the analyzed compositions of SiOCfibers at 1000 8C shows SiC is of 13.99 wt%, which is comparable tothe reported results [10]. The relatively high content of SiC mightcontribute to the high strength of SiOC fibers.

3.5. Scanning electron microscopy

The SEM images of PSO fibers and SiOC fibers are shown inFig. 5. According to Fig. 5a, electrospinning is able to produce thePSO fibers with the diameter less than 10 mm in a considerablelarge-scale. From Fig. 5b and c, it can be seen that the SiOC fiber hasa smooth surface and dense cross-section, free of residue pores andcracks after pyrolysis under argon protection at 1000 8C. Thefractural end shows the fiber has an elliptic cross-section, whichmight result from an inhomogeneous shrinkage during the quickevaporation of the electrospun fiber. Energy dispersive X-ray

Nam e. Pos. %Area

SiC 102.1 6 13.9 9 SiO2 103.4 9 86.0 1

99100101102103104105106107108

2000

3000

4000

5000

6000

7000

8000

9000

Binding Ener gy (eV)

Inte

nsity

(CPS

)

Fig. 4. XPS-Si2p1/2 spectrum of SiOC fiber derived from VTMS + TEOS.

Fig. 5. SEM images of fibers from electrospinning. (a) PSO fibers observed at a lower magnification, (b) a SiOC fiber of fractural end at a higher magnification, (c) a SiOC fiber

observed at a higher magnification, (d) EDX spectrum collected from the marked region in c.

Table 2Tensile strength and Young’s modulus of the SiOC fibers by repetition measure-

ment.

No. Tensile strength

(MPa)

Young’s modulus

(GPa)

Standard

deviation (SD)

Seg.1 967.15 58.33

Seg.2 966.88 58.06

Seg.3 967.41 58.51 0.2

Seg.4 967.27 58.42

Seg.5 966.93 58.14

X. Wang et al. / Materials Research Bulletin 46 (2011) 2398–2402 2401

spectroscopy (EDX) analysis shows the existence of Si, C and O inthe SiOC fiber Fig. 5d. The atomic percentage of the elements are[Si] = 27.51 at%, [C] = 30.74 at% and [O] = 41.75 at%, correspondingto SiO1.52C1.12.

3.6. Tensile tests

There are several factors influencing the strength such asunevenness of the fiber surface, twisting of the fiber and accidentaltorsion during measurement. We therefore provide a standarddeviation (SD) value of the fiber by dividing a single fiber into fivesegments equally with repetition measurement as shown in Table2. The strength–strain curve of the SiOC fiber with a measuredforce of 27 cN at the break point is shown in Fig. 6. The SiOC fibershave a tensile strength of 967 MPa from the fiber diameter(11.7 mm) and moderate Young’s modulus of 58 GPa. The strengthof our fiber is higher than that (776 MPa) with diameter (26.8 mm)

2.01.51.00.50.00

200

400

600

800

1000

Stre

ngth

(MPa

)

Strain (%)

Fig. 6. A stress–strain curve for SiOC fibers derived from electrospinning.

under a similar alkoxides system (VTMS + TEOS) [11]. Moreover,the fiber strength is also slightly improved than that (940 MPa)with thickness (17.6 mm) derived from VTMS by a modified sol–gelmethod with secondary cellulose acetate (SCA) as the fiber-forming aid [12,13]. The higher strength is probably due to therelatively high content of SiC which come from vinyl in VTMS thatcan be crosslinked into infusible structures during pyrolysis.Furthermore, fiber diameter also has an effect on the tensilestrength of the fiber. A comparatively smaller diameter as a resultof electrospinning in the fiber formation might also contribute tothe higher mechanical properties.

4. Conclusions

Spinnable solutions are obtained from the sol–gel system oftetraethoxysilane (TEOS, Si(OC2H5)4) and vinyltrimethoxysilane(VTMS, CH255CHSi(OCH3)3) under aqueous conditions, acid cata-lysts and spinning agent. The amounts of spinning agent influencethe spinnability of the solution. The solution with a mass ratio ofPVP/alkoxides = 0.05 exhibited a maximum spinnable time (t) of50 min. Polysiloxane (PSO) fibers are electrospun from the solutionand pyrolyzed into SiOC fibers in an argon flow protection at1000 8C. The SiOC fibers reveal a smooth surface and dense cross-section, free of residue pores and cracks, and have a high content ofSiC (13.99%). The SiOC fibers have a high tensile strength of

X. Wang et al. / Materials Research Bulletin 46 (2011) 2398–24022402

967 MPa, fracture stress of 27 cN and moderate Young’s modulusof 58 GPa.

Acknowledgements

This study benefited from the Nanomaterials & PDCs Group, KeyLab of Advanced Ceramic and Machining Technology Ministry ofEducations, and State Key Laboratory of Precise Measurement andEquipment, Tianjin University, China. The authors thank Prof. Ya-LiLi for helpful discussions. This work was supported by the NationalNatural Science Foundation of China (No. 50706033).

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