Highly aligned continuous mullite nanofibers: Conjugate...
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Highly aligned continuous mullite nanofibers: Conjugate electrospinning fabrication, microstructure and mechanical properties Xiaolei Song, Wensheng Liu ⇑ , Juan Wang, Shuheng Xu, Bing Liu, Qingshan Cai, Yunzhu Ma ⇑ State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China article info Article history: Received 3 July 2017 Received in revised form 17 September 2017 Accepted 15 October 2017 Available online 16 October 2017 Keywords: Ceramics Nanofibers Conjugate electrospinning Microstructure Mechanical properties abstract Continuous mullite nanofibers with highly aligned structure were fabricated by conjugate electrospin- ning technique combined with the sol-gel method. Aluminum acetate stabilized with boric acid and tet- raethyl orthosilicate (TEOS) were used as raw materials. Aligned and continuous as-spun nanofibers were prepared first. After calcining at 1000 °C, the obtained mullite nanofibers retained decent alignment and continuous structure despite the decomposition of PVP and other organics. Such fibers were composed of Al 4 B 2 O 9 phase and amorphous SiO 2 . The elastic modulus of a single mullite nanofiber was 12.27 ± 1.77 GPa and the tensile strength of the aligned mullite nanofiber bundle was 32.21 ± 3.73 MPa. Ó 2017 Elsevier B.V. All rights reserved. 1. Introduction Mullite fibers, with outstanding physical and chemical stabili- ties, are widely used in areas of filtration, catalyst, insulation and also used as reinforcements for resins, metals and ceramics [1–3]. They have been converted into textiles, such as yarn, sleevings or fabrics, to expand their applications [1,4]. In order to produce mullite textiles, continuous mullite fibers with aligned structure are required. In recent years, extensive researches has been focused on ceramic nanofibers due to their better flexibility, lar- ger surface-area, lower thermal conductivity and other superior properties, compared with the commercial ceramic fibers [5–7]. Therefore, it is an effective approach to use mullite nanofibers to produce mullite textiles with improved performance. And con- sequently, preparing aligned and continuous mullite nanofibers with good flexibility and high tensile strength is of great significance. In the past two decades, electrospinning has drawn increasing attention for its unique advantages in producing continuous cera- mic nanofibers [8,9]. Compared to other modified devices of elec- trospinning, conjugate electrospinning is more effective to prepare aligned nanofibers [10,11]. During conjugate electrospin- ning, two oppositely placed metal needles are connected to posi- tive and negative voltages, respectively. Positively and negatively charged fibers eject from the two needles will attract and stick to each other. Continuous and aligned fibers are obtained after col- lecting the merged fibers using a rotating drum. Based on the pre- vious literatures, little research work has been done regarding fabricating the aligned continuous mullite nanofibers, especially by the means of conjugate electrospinning. Thus, the current work was focused on such a topic. The microstructure and mechanical properties of the aligned continuous mullite nanofibers were investigated. 2. Experimental procedure Aluminum acetate stabilized with boric acid (Al(OH) 2 (OOCCH 3 ) 1/3H 3 BO 3 ) and tetraethyl orthosilicate (TEOS) were used as the sources of alumina and silica. Deionized water and alcohol were used as the solvents. Polyvinyl pyrrolidone (PVP, Mn = 130,000) was used as the polymer additive. Aluminum acetate, TEOS, deion- ized water and ethanol with the corresponding mass percentages of 15, 7.2, 38.9 and 38.9 wt% were mixed and stirred for 12 h at 40 °C to prepare the precursor sol. A 16% PVP alcohol solution and the precursor sol were mixed with a mass ratio of 3:1 to obtain the spinning solution. Fig. 1 shows the schematic diagram of the conjugate electro- spinning setup for collecting aligned as-spun nanofibers. The angle https://doi.org/10.1016/j.matlet.2017.10.055 0167-577X/Ó 2017 Elsevier B.V. All rights reserved. ⇑ Corresponding authors. E-mail addresses: [email protected] (W. Liu), [email protected] (Y. Ma). Materials Letters 212 (2018) 20–24 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Transcript of Highly aligned continuous mullite nanofibers: Conjugate...
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Materials Letters 212 (2018) 20–24
Contents lists available at ScienceDirect
Materials Letters
journal homepage: www.elsevier .com/ locate/mlblue
Highly aligned continuous mullite nanofibers: Conjugate electrospinningfabrication, microstructure and mechanical properties
https://doi.org/10.1016/j.matlet.2017.10.0550167-577X/� 2017 Elsevier B.V. All rights reserved.
⇑ Corresponding authors.E-mail addresses: [email protected] (W. Liu), [email protected]
(Y. Ma).
Xiaolei Song, Wensheng Liu ⇑, Juan Wang, Shuheng Xu, Bing Liu, Qingshan Cai, Yunzhu Ma ⇑State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
a r t i c l e i n f o
Article history:Received 3 July 2017Received in revised form 17 September2017Accepted 15 October 2017Available online 16 October 2017
Keywords:CeramicsNanofibersConjugate electrospinningMicrostructureMechanical properties
a b s t r a c t
Continuous mullite nanofibers with highly aligned structure were fabricated by conjugate electrospin-ning technique combined with the sol-gel method. Aluminum acetate stabilized with boric acid and tet-raethyl orthosilicate (TEOS) were used as rawmaterials. Aligned and continuous as-spun nanofibers wereprepared first. After calcining at 1000 �C, the obtained mullite nanofibers retained decent alignment andcontinuous structure despite the decomposition of PVP and other organics. Such fibers were composed ofAl4B2O9 phase and amorphous SiO2. The elastic modulus of a single mullite nanofiber was 12.27 ± 1.77GPa and the tensile strength of the aligned mullite nanofiber bundle was 32.21 ± 3.73 MPa.
� 2017 Elsevier B.V. All rights reserved.
1. Introduction
Mullite fibers, with outstanding physical and chemical stabili-ties, are widely used in areas of filtration, catalyst, insulation andalso used as reinforcements for resins, metals and ceramics [1–3].They have been converted into textiles, such as yarn, sleevings orfabrics, to expand their applications [1,4]. In order to producemullite textiles, continuous mullite fibers with aligned structureare required. In recent years, extensive researches has beenfocused on ceramic nanofibers due to their better flexibility, lar-ger surface-area, lower thermal conductivity and other superiorproperties, compared with the commercial ceramic fibers [5–7].Therefore, it is an effective approach to use mullite nanofibersto produce mullite textiles with improved performance. And con-sequently, preparing aligned and continuous mullite nanofiberswith good flexibility and high tensile strength is of greatsignificance.
In the past two decades, electrospinning has drawn increasingattention for its unique advantages in producing continuous cera-mic nanofibers [8,9]. Compared to other modified devices of elec-trospinning, conjugate electrospinning is more effective toprepare aligned nanofibers [10,11]. During conjugate electrospin-
ning, two oppositely placed metal needles are connected to posi-tive and negative voltages, respectively. Positively and negativelycharged fibers eject from the two needles will attract and stick toeach other. Continuous and aligned fibers are obtained after col-lecting the merged fibers using a rotating drum. Based on the pre-vious literatures, little research work has been done regardingfabricating the aligned continuous mullite nanofibers, especiallyby the means of conjugate electrospinning. Thus, the current workwas focused on such a topic. The microstructure and mechanicalproperties of the aligned continuous mullite nanofibers wereinvestigated.
2. Experimental procedure
Aluminum acetate stabilized with boric acid (Al(OH)2(OOCCH3)�1/3H3BO3) and tetraethyl orthosilicate (TEOS) were used as thesources of alumina and silica. Deionized water and alcohol wereused as the solvents. Polyvinyl pyrrolidone (PVP, Mn = 130,000)was used as the polymer additive. Aluminum acetate, TEOS, deion-ized water and ethanol with the corresponding mass percentagesof 15, 7.2, 38.9 and 38.9 wt% were mixed and stirred for 12 h at40 �C to prepare the precursor sol. A 16% PVP alcohol solutionand the precursor sol were mixed with a mass ratio of 3:1 to obtainthe spinning solution.
Fig. 1 shows the schematic diagram of the conjugate electro-spinning setup for collecting aligned as-spun nanofibers. The angle
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Fig. 1. Schematic diagram of conjugate electrospinning setup for collecting aligned as-spun nanofibers.
X. Song et al. /Materials Letters 212 (2018) 20–24 21
between the two needles equipped on the syringes was 120�. Thedistance between the needle tips was 20 cm. During electrospin-ning, the positive and negative voltages were kept at +3.5 and�3.5 kV, respectively. The feeding rate of each syringes was 0.45ml/h. The rotating drum with a linear velocity of 1.8 m/s was usedto collect as-spun nanofibers (shown in the right in Fig. 1). Thefibers were heated to 800 �C at a heating rate of 5 �C/min andmaintained for 1 h, and then were calcined at 1000 �C for an extra1 h to obtain the mullite nanofibers. For tension test, as-spun fiberswere converted into fiber bundle along the alignment beforecalcination.
Fiber morphology was observed by scanning electron micro-scopy (SEM) (Quanta FEG 250, FEI). X-ray powder diffraction(XRD) data was recorded by a diffractometer (D8 Advance,Bruker). Fourier transform infrared (FT-IR) absorption spectrumwas recorded using a spectrometer (Model 6700, Nicolet). Thegrain structure was observed by transmission electron micro-scopy (TEM) (JEM-2100F, JEOL). The elastic modulus of a singlenanofiber was measured using atomic force microscope (AFM)(Multimode 8, Veeco) by the three-point bending method asreported by Ding [12]. The tensile strength of the nanofiberbundles was measured using a fiber tensile tester (XS (08)XT-3, Xusai).
3. Results and discussion
Fig. 2(a) shows that highly aligned continuous as-spun nanofi-bers with an average diameter of 1152 nm were obtained. Thougha small fraction of fibers were in disordered patterns, as evidencedin SEM micrographs with lower magnifications, the integral per-spective was good. After calcining at 1000 �C (Fig. 2(b)), the aver-age fiber diameter decreased to 589 nm due to thedecomposition of PVP and other organics, but these reactionsexerted little influence on the decent alignment and continuousstructure. These results revealed that fabricating highly alignedcontinuous mullite nanofibers by conjugate electrospinning waseffective.
As mentioned, a large proportion of boron was introduced byaluminum acetate. In Al2O3-SiO2-B2O3 ternary system, Al2O3tended to react with B2O3 to form Al4B2O9, instead of trans-
forming to c-Al2O3 or reacting with SiO2 to form mullite[13]. Thus, it was reasonable that the diffraction peaks in theXRD pattern in Fig. 2(c) were identified as Al4B2O9 phase.Besides, the broad hump between the 2h of 13� and 29� wasdetected as amorphous SiO2. Such phase composition was con-firmed by the FT-IR results (Fig. 2(d)). The insert in Fig. 2(d)listed the absorption bands and the corresponding assignments[13,14]. Structural units of BO3, AlO4 and AlO6 were the maincomponents of Al4B2O9 phase and the absorption band at1099 cm�1 was due to the stretching vibration of SiAO-Si inamorphous SiO2. TEM micrograph in Fig. 2(e) revealed thatthe Al4B2O9 grains (� 20 nm) were disconnected since theywere normally surrounded by amorphous SiO2. Thus, the graingrowth would be limited and the fibers structure at high tem-peratures would be stable. The interplanar spacings displayed inFig. 2(f) were 0.53 nm and 0.34 nm, corresponded to the (1 1 0)and (2 1 0) crystal planes of the Al4B2O9 phase, respectively.The insert ED pattern further confirmed the above analyses.Additionally, a thin amorphous layer was observed on fiber sur-face, indicating that the crystal grains had not grew to the sur-face layer.
Fig. 3(a) and (b) display the 3D image of a single nanofiberwith a diameter of 612 nm suspended over a groove and a rep-resentative force curve obtained by AFM. The elastic moduluswas calculated to be 12.27 ± 1.77 GPa, which was much smallerthan that of the micron-scale mullite fibers. This difference wasattributed to the higher surface-to-volume ratio and the tinycracks of the nanofibers [15]. The low modulus value indicatedthe good flexibility of the nanofibers. This was confirmed inFig. 3(c) since the aligned mullite nanofibers could be easilywrapped on a glass rod without any breakage. One reason forthe good flexibility was that the small Al4B2O9 grains had minoreffect to impede deformation. Another reason should be thatthe atoms in amorphous SiO2 were less constrained and theirbond energy was not too strong, thereby making the nanofiberseasy to deform.
The schematic diagram and photograph showing the tensiontest of the aligned fiber bundle is illustrated in Fig. 3(d) and a typ-ical stress-strain curve was displayed in Fig. 3(e). The elastic regionwas not smooth due to the slippages of inner fibers. The post-failure region was not straightly down because of the incomplete
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Fig. 2. (a) SEMmicrograph of as-spun nanofibers, (b) SEMmicrograph, (c) XRD pattern, (d) FT-IR spectrum, (e) TEMmicrograph and (f) HR-TEMmicrograph of aligned mullitenanofibers prepared at 1000 �C. The inserts in (a) and (b) show the corresponding fiber diameter distribution histograms, and in (d) and (f) show the FT-IR band assignmentsand the ED pattern, respectively.
22 X. Song et al. /Materials Letters 212 (2018) 20–24
fracture of the fiber bundle (shown in the insert in Fig. 3(e)). Suchfeatures were caused by the disordered nanofibers discussedabove. However, the tensile strength of the fiber bundles, whichmeasured to be 32.21 ± 3.73 MPa, was still much higher than thatof the random electrospun ceramic fiber mats reported in Ref. [6].Beyond the aligned structure, the fracture morphology of a singlefiber (Fig. 3(f)) revealed that few defects existed in the fiber, whichalso contributed to the high tensile strength.
4. Conclusions
This study presented the fabrication, microstructure andmechanical properties of the highly aligned continuous mullitenanofibers. Conjugate electrospinning was confirmed to be aneffective fabrication method. Al4B2O9 formed, instead ofc-Al2O3 or mullite phase, due to the presence of B2O3. The goodflexibility was attributed to the fine Al4B2O9 grains and the
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Fig. 3. (a) 3D AFM image of a single nanofiber suspended over a groove, (b) a representative force curve obtained by AFM, (c) photograph of aligned mullite nanofiberswrapped on a glass rod, (d) the schematic diagram and photograph showing the tension test, (e) a typical stress-strain curve of the fiber bundle and (f) the fracturemorphology of a single fiber. The insert in (e) shows the incomplete fracture of the fiber bundle.
X. Song et al. /Materials Letters 212 (2018) 20–24 23
presence of amorphous SiO2. The high tensile strength wasascribed to the aligned structure and the defects-free featureof the nanofibers.
Acknowledgements
The authors gratefully acknowledge the financial support fromthe ‘‘Chang Jiang Scholars Program” of the Ministry of Education ofChina (Grant no. T2011119).
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Highly aligned continuous mullite nanofibers: Conjugate electrospinning fabrication, microstructure and mechanical properties1 Introduction2 Experimental procedure3 Results and discussion4 ConclusionsAcknowledgementsReferences
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