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A facile method to prepare ZrC nanofibers by electrospinning and pyrolysis of polymeric precursors
Ceramics International (xxxx) xxxx–xxxx
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
A facile method to prepare ZrC nanofibers by electrospinning and pyrolysis of polymeric precursors ⁎
XueYu Tao , ShiXiang Zhou, Jie Ma, ZhiMei Xiang, RuiLin Hou, JingJing Wang, XiYao Li School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, China
A R T I C L E I N F O
A BS T RAC T
Keywords: ZrC nanofibers Polyvinylpyrrolidone Polymeric precursor Electrospinning
Zirconium carbide (ZrC) is one of the most attractive ultra-high temperature ceramics due to its excellent properties. ZrC nanofibers were fabricated via electrospinning and pyrolysis of a novel polymeric precursor, Polyzirconosaal (PZSA), with the addition of polyvinylpyrrolidone (PVP) as the spinning aid. The polymer PZSA was prepared from the chemical reaction between Polyzirconoxane (PZO) and Salicyl alcohol. The as-spun PZSA/PVP fibers were converted to ZrC nanofibers with a diameter ~200 nm after carbothermal reduction at 1300 °C in argon. The obtained ZrC nanofibers maintained its excellent fibrous morphology. The microstructures exhibited that nanoscale ZrC particles dispersed in the fibers containing free carbon. The average crystallite size of ZrC particles using Scherrer method was 42 nm. The obtained ZrC nanofibers were characterized by XRD, SEM and TEM. The current material would be particularly useful for applications such as catalyst support, filters, gas storage, supercapacitors, and phase change material support in thermal management systems.
1. Introduction Zirconium carbide (ZrC) is a typical member of ultra-high temperature ceramics (UHTCs) family [1,2]. ZrC has attracted considerable attention due to its superior properties, such as high melting point (3420 °C), high hardness (25 GPa) and good chemical stability, which lead to potential applications in extremely harsh environment [3–5]. Generally, there are various methods for the preparation of ZrC ceramics, such as carbothermal reduction of ZrO2, mechanical alloying and self-propagating high-temperature synthesis. However, these processes require high temperature and very long time. The shape and size of the final product are difficult to control [6–8]. Recently, the polymeric precursor method has been developed for preparing advanced ceramics by pyrolysis of the polymers [9–11]. This method can prepare complex shapes (films, fibers, coatings) and ceramic matrix composites in a wide variety of nonoxide systems. Ceramic nanofibers have become the focus of the scientific research for the past decades due to their unique physical and chemical properties leading to potential application in the areas of mesoscopic physics and fabrication of nanoscale devices and more. Ceramics nanofibers can be prepared by a variety of methods, among which electrospinning attracts a lot of attention due to its efficiency and simplicity. Electrospinning is a versatile method to produce ultra-long and one-dimensional ceramic fibers with controllable diameter, mor-
⁎
phology, and composition [12]. By controlling the precursory components and processing parameters of this technique, the nanoparticles could be assembled within the electrospun fibers, which can meet the requirements of application in various fields such as filtration, energy storage, thermal storage and so forth [13–15]. Electrospinning has been used to produce ZrN [16], SiC [17], ZrO2 [18], SiBCN [19], ZrB2 [20,21], ZrC [22,23] and TiC nanofibers [24]. However, there are only limited reports on the fabrication of zirconium carbide nanofibers by electrospinning. Cui et al. [22] synthesized ZrC ultra-thin fibers via electrospinning the solution by dissolving zirconium acetyl acetonate, phenolic resins in a solvent mixture of ethyl alcohol and 2,4-pentanedione. Li et al. prepared ZrC nanofibers by electrospinning using Polyzirconoxane (PZO) as zirconium source and polyacrylonitrile (PAN) as the spinning aid and primary carbon source [25]. Ghelich et al. prepared ZrC nanofibers via the self-condensation of Zr(OPr)4 and PVP through electrospinning by combination of carbothermal reduction [26]. However, these ZrC precursor fibers were prepared via just blending of zirconium and carbon sources, and polycondensation of zirconium alkoxides. Therefore, this strategy will not work for producing continuous ceramic fibers using a single process of spinning and pyrolysis. To overcome these limitations, we developed a simple strategy to prepare ZrC nanofibers by taking advantage of polymeric precursor method and electrospinning technique employing chemical reaction of PZO and Salicyl alcohol (SA). The product Polyzirconosaal
Corresponding author. E-mail address: [email protected] (X. Tao).
http://dx.doi.org/10.1016/j.ceramint.2016.12.005 Received 23 September 2016; Received in revised form 14 November 2016; Accepted 1 December 2016 0272-8842/ © 2016 Published by Elsevier Ltd.
Please cite this article as: Tao, X., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.12.005
Ceramics International (xxxx) xxxx–xxxx
X. Tao et al.
standard electrospinning device at room temperature (Fig. 1A). The electrospun fibers were collected on the aluminum foil panel. During the electrospinning process, parameters such as concentration of the solution, applied voltage, tip-to-collector distance, and feed rate were optimized to receive the consistent hybrid nanofibers. The spinning solution used is a 10 mass% solution. The diameter of the needle is 0.5 mm. The voltage applied was in the range of 14–21 kV, the flow rate was 0.5–1.1 mL h−1 and the collection distance was adjusted at 10–12 cm. Subsequently, the as-spun precursor fibers were carefully peeled off from the aluminum foil, transferred into a crucible for drying (Fig. 1B) . After being dried in a vacuum oven at 60 °C for 24 h, the electrospun precursors were subsequently heat-treated in a furnace for pyrolysis into final ZrC.
(PZSA) was utilized as a novel polymeric precursor to produce continuous ZrC ceramic fibers, with polyvinylpyrrolidone (PVP) as the spinning aid [21,26–28]. The solution for electrospinning was prepared by dissolving PZO, Salicyl alcohol in ethanol followed by the addition of PVP. The synthesized nanofibers were characterized by DSC-TG, FE-SEM, TEM and XRD techniques. The structure of PVP is shown as follows.
.
3. Results and discussion
2. Experimental
Fig. 2 is the SEM images of the green fibers electrospun from PZSA/ PVP spinning solution. The green fibers appear to be very smooth without any beads, and the diameters (~400 nm) of these fibers are uniform, which indicates that the PZSA/PVP spinning solution has very good spinnability and PVP is an excellent spinning aid for the preparation of fibers [26,30]. The electrospinning conditions for obtaining smooth fibers without any beads were at a positive voltage of 15 kV, with a mass flow rate of 0.8 mL h−1, and a working distance of 10 cm. This method combines the advantages of electrospinning and polymeric precursor techniques, thus the new method can facilely produce ZrC electrospun fibers with controllable chemical and morphological structures. The attractive feature is that the precursor PZSA was prepared by chemical reaction of PZO and Salicyl alcohol (SA) (Scheme 1). In this polymer, zirconium and carbon elements distributed uniformly at molecular level, not just by blending of the source of zirconium and carbon. Further, C=O groups of PVP are known to be strongly bonded with OH groups of the metalloxane polymers via hydrogen bonding [27]. Such C=O groups can be regarded as the “capping agent” for the OH groups of the metalloxane polymers, obstructing the condensation reaction [31]. There are some OH groups in the structure of PZSA and these OH groups can interact with C=O groups of PVP polymer to form hydrogen bonds, which increase the spinnability and stability of the precursor. Moreover, Incorporation of PVP in precursor solutions allows formation of target complex with ease, which is another benefit of PVP for fabricating fibers. Also, PVP has C=O groups, which can also coordinate metal atoms, offering homogeneity in metal atom distribution in the precursor fibers [27,32]. FTIR spectroscopy (Fig. 3 A) was carried out on the as-spun fibers. The peaks at 1597 and 1530 cm−1 are assigned to C=O and C=C in acetylacetonate ligand. The band at 757 cm−1 is assigned to C-H of
2.1. Preparation of PZO PZO was prepared according to the previous publications [29]. 9.0 g (0.09 mol) acetylacetone and then 12.2 g (0.12 mol) triethylamine was added drop wise into 260 mL methanol solution of 19.4 g (0.06 mol) zirconium oxychloride octahydrate (ZOC) below 5 °C at molar ratio of Et3N/ZOC=2.0. The reaction mixture was stirred at room temperature for 2 h and then evaporated. Addition of 150 mL THF and filtration of the precipitate followed by concentration of the filtrate gave a highly viscous solution. PZO was isolated as a white powder by adding the viscous solution. 2.2. Preparation of electrospinning solution In a typical procedure, 0.502 g PZO was dissolved into 15 mL ethanol to forma transparent solution at room temperature (25 °C). Then, 0.163 g Salicyl alcohol was added to this solution till the mixture became transparent. The chemical reaction between PZO and Salicyl alcohol was shown in Scheme 1. The molar ratio of the C/Zr within the spinning solution is set at 4.0, higher than the stoichiometric ratio. Finally, PVP was added into the solution under continuous magnetic stirring for 24 h at room temperature. The concentration of the spinning solutions was adjusted by changing the amount of PVP. The resulting stable homogeneous solution was employed for the electrospinning process. 2.3. Electrospinning and heat treatment of as-spun nanofibers The electrospinning solution was subsequently electrospun using a
Scheme 1. Synthetic route of the PZSA precursor.
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Fig. 1. (A) Schematic diagram of the preparation route for PZSA/ PVP fibers, and (B) digital photos of as-spun PZSA/ PVP fibers.
aromatic in SA ligand. In addition, we see peaks assigned to Zr-O at 649 and 578 cm−1 [10,29]. There is also significant broadening of the bands below 750 cm−1 assigned to the formation of the O-Zr-O network. This indicates that zirconium atoms have been successfully introduced into ZrC precursor polymer. The bands in the 3500– 3200 cm−1 can be assigned to -NH stretch of the PVP. PVP is present in the fibers and the bands at 1672 cm−1 and 1288 cm−1 are the characteristic peaks of PVP. The prominent band at 1672 cm−1 corresponds to C=O stretching vibration, 1288 cm−1 is due to C-N stretching vibration of the PVP [23]. The bands at 1483, 1458 and 1424 cm−1 correspond to the ring CH2 scissoring of PVP. The electrospun precursors need to be pyrolyzed to produce ZrC nanofibers. The crystalline structure of ZrC nanofibers heat-treated at 1300 °C was further investigated by XRD, and the results were shown in Fig. 3B. It is noted that all of the diffraction peaks in the XRD pattern match well with the cubic ZrC phase (JCPDS No. 35-0784). Five characteristic peaks at 2θ=33.1° (111), 38.4° (200), 55.4° (220), 65.9° (311), 69.3° (222) appeared in XRD pattern [29], and no other peaks are found, indicating the ZrC crystalline with cubic NaClstructure has been successfully synthesized. The sharp diffraction peaks implies the better crystallinity of ZrC grains. The PVP in composite fibers had been completely carbonized after heat treatment at 1300 °C for 2 h in argon. ZrC fibers are formed via the reaction as follows [26,29].
ZrO2(s) + 3 C(s) = ZrC(s) + 2CO(g)
Fig. 2. (A) and (B) SEM images of as-spun PZSA/PVP fibers at different magnifications.
Fig. 4(A) and (B) shows SEM images of the nanofibers after heat treatment at 1300 °C. As can be seen, the fibrous morphology of the
Fig. 3. (A) FTIR spectrum of the as-spun PZSA/ PVP fibers and (B) XRD pattern of pyrolyzed fibers heat-treated at 1300 °C.
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Fig. 4. SEM (A and B), TEM image (C and D), EDS (E) and Raman spectrum (F) of the pyrolyzed nanofibers heat-treated at 1300 °C.
fibers is well retained after the pyrolysis. But the surface of the nanofibers starts to become a little rough due to the removal of PVP [33] and crystallization of ZrC [30]. The ZrC fibers have a diameter of about 200 nm, which is smaller than the electrospun fibers because of shrinkage when the organic components were removed [30]. TEM investigation shows that ZrC fibers structure is well kept and nanosized hexagonal ZrC crystals are dispersed in the fibers. ZrC nanocrystals were embedded or engulfed by disordered carbon coming from Salicyl alcohol (Fig. 4(C) and (D)) [23], the average diameter of the fibers is ∼200 nm which is accordance with the SEM observation. These results confirm that the crystalline phases dispersed in the ceramic fibers uniformly and the ZrC crystalline is in the range of 30– 50 nm. A quantitative analysis by energy-dispersive spectroscopy (EDS) shows that ZrC nanofibers consisted of Zr, C and O elements
(Fig. 4(E)). It is known that in the preparation of UHTCs, it is very difficult to remove the carbon and oxygen residues entirely [20]. Raman spectroscopy was used to characterize the status of carbon in ZrC fibers. As shown in Fig. 4(F), two strong peaks centered at 1298 cm−1 and 1596 cm−1 were detected, which correspond to the D and G peaks of free carbon, respectively. The intensity ratio of the D to G band (ID/IG) was calculated to be 2.54, indicating the relative disorder of the carbon in ceramic fibers. Referring to XRD, SEM, TEM and Raman characterization, the obtained fibers at 1300 °C could be considered as nanoscale ZrC ceramic fibers. 4. Conclusions In summary, continuous ZrC nanofibers are successfully fabricated 4
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[11] E. Bernardo, G. Parcianello, P. Colombo, J.H. Adair, A.T. Barnes, J.R. Hellmann, B.H. Jones, J. Kruise, J.J. Swab, SiAlON ceramics from preceramic polymers and nano-sized fillers: application in ceramic joining, J. Eur. Ceram. Soc. 32 (2012) 1329–1335. [12] X.Y. Tao, X.Y. Wei, X.L. Chen, T. Zhao, Synthesis of nano-sized zirconia ceramics via preceramic polymer method, Refract. Ind. Ceram. 55 (2014) 63–66. [13] H.L. Hou, L. Wang, G.D. Wei, J.J. Chen, W.Y. Yang, B. Tang, F.M. Gao, Fabrication of TiO2/SiO2 hybrid fibers with tunable internal porous structures, Ceram. Int. 40 (2014) 16309–16316. [14] B.K. Brettmann, S. Tsang, K.M. Forward, G.C. Rutledge, A.S. Myerson, B.L. Trout, Free surface electrospinning of fibers containing microparticles, Langmuir 28 (2012) 9714–9721. [15] L. Lanotte, C. Bilotti, L. Sabetta, G. Tomaiuolo, S. Guido, Dispersion of sepiolite rods in nanofibers by electrospinning, Polymer 54 (2013) 1295–1297. [16] K. Nakane, S. Matsuoka, S. Gao, S. Yonezawa, J.H. Kim, N. Ogata, Formation of inorganic nanofibers by heat-treatment of poly(vinylalcohol)-zirconium compound hybrid nanofibers, J. Min. Metall. Sect. B: Metall. 49 (2013) 77–82. [17] P. Lu, Q. Huang, A. Mukherjee, Y.-L. Hsieh, Effects of polymer matrices to the formation of silicon carbide (SiC) nanoporous fibers and nanowires under carbothermal reduction, J. Mater. Chem. 21 (2011) 1005–1012. [18] C. Shao, H. Guan, Y. Liu, J. Gong, A novel method for making ZrO2 nanofibers via an electrospinning technique, J. Cryst. Growth 267 (2004) 380–384. [19] J. Wilfert, R. Von Hagen, R. Fiz, M. Jansen, S. Mathur, Electrospinning of preceramic polymers for the preparation of SiBCN felts and their modification with semiconductor nanowires, J. Mater. Chem. 22 (2012) 2099–2104. [20] F. Li, Z. Kang, X. Huang, G.J. Zhang, Synthesis of ZrB2 nanofibers by carbothermal reduction via electrospinning, Chem. Eng. J. 234 (2013) 184–188. [21] R. Ghelich, R.M. Aghdam, F.S. Torknik, M.R. Jahannama, M.K. Rad, Carbothermal reduction synthesis of ZrB2 nanofibers via pre-oxidized electrospun zirconium npropoxide, Ceram. Int. 41 (2015) 6905–6911. [22] X.M. Cui, Y.S. Nam, J.Y. Lee, W.H. Park, Fabrication of zirconium carbide (ZrC) ultra-thin fibers by electrospinning, Mater. Lett. 62 (2008) 1961–1964. [23] J.S. Atchison, M. Zeiger, A. Tolosa, L.M. Funke, N. Jӓckel, V. Presser, Electrospinning of ultrafine metal oxide/carbon and metal carbide/carbon nanocomposite fibers, RSC Adv. 5 (2015) 35683–35692. [24] L.F. Zhang, J.J. Hu, A.A. Voevodin, H. Fong, Synthesis of continuous TiC nanofibers and/or nanoribbons through electrospinning followed by carbothermal reduction, Nanoscale 2 (2010) 1670–1673. [25] F. Li, Z. Kang, X. Huang, G.-J. Zhang, Fabrication of zirconium carbide nanofibers by electrospinning, Ceram. Int. 40 (2014) 10137–10141. [26] R. Ghelich, R.M. Aghdam, M.R. Jahannama, Low temperature carbothermal reduction synthesis of ZrC nanofibers via cyclized electrospun PVP/Zr(OPr)4 hybrid, Int. J. Appl. Ceram. Technol. (2015) 1–7. [27] H. Kozuka, S. Takenaka, H. Tolita, M. Okubayashi, PVP-assisted sol-gel deposition of single layer ferroelectric thin films over submicron or micron in thickness, J. Eur. Ceram. Soc. 24 (2004) 1585–1588. [28] P.Q. Wang, D. Zhang, F.Y. Ma, Y. Ou, Q.N. Chen, S.H. Xie, J.Y. Li, Mesoporous carbon nanofibers with a high surface area electrospun from thermoplastic polyvinylpyrrolidone, Nanoscale 4 (2012) 7199–7204. [29] X.Y. Tao, X.Y. Wei, Q. Chen, W.Z. Lu, M. Ma, T. Zhao, Synthesis, characterization and thermal behavior of new preceramic polymers for zirconium carbide, Adv. Appl. Ceram. 112 (2013) 301–305. [30] J.Y. Li, Y. Sun, Y. Tan, F.M. Xu, X.L. Shi, N. Ren, Zirconium nitride (ZrN) fibers prepared by carbothermal reduction and nitridation of electrospun PVP/zirconium oxychloride composite fibers, Chem. Eng. J. 144 (2008) 149–152. [31] S. Linardos, Q. Zhang, J.R. Alcok, Preparation of sub-micron PZT particles with the sol-gel technique, J. Eur. Ceram. Soc. 26 (2006) 117–123. [32] A. Greiner, J.H. Wendorff, Electrospinning: a Fascinating method for the preparation of ultrathin fibers, Angew. Chem., Int. Ed. 46 (2007) 5670–5703. [33] L.F. Zhang, J.Y. Howe, Y. Zhang, H. Fong, Synthesis and characterization of zirconium tungstate ultra-thin fibers, Cryst. Growth Des. 9 (2009) 667–670.
by combination of polymeric precursor chemistry and electrospinning technique. PZSA and PVP are used as zirconium source and spinning aid, respectively. ZrC nanofibers are obtained by pyrolyzing the as-spun green fibers at 1300 °C for 2 h under argon. The obtained ZrC nanofibers via carbothermal reduction treatment at 1300 °C exhibit an average diameter of 200 nm while preserving the fibrous morphology. TEM results show that ZrC nanoparticles disperse in the fiber uniformly containing free carbon and the average size is about 42 nm. The electrospun ZrC nanofibers in the form of freestanding nonwoven textile may serve as an ideal precursor for the synthesis of highly porous carbide-derived carbon materials, which would be particularly useful for applications such as catalyst support, filters, gas storage, supercapacitors, and phase change material support in thermal management systems. Additionally, this reported method could be utilized to prepare other metal carbides/borides nanofibers. Acknowledgments This research was supported by the financial support of the National Natural Science Foundation of China (51402354) and “the Fundamental Research Funds for the Central Universities” (2015QNA07) and “the Fundamental Research Funds for the Central Universities” (2015XKZD01). References [1] A. Momozawa, R. Tu, T. Goto, Y. Kubota, H. Hatta, K. Komurasaki, Quantitative evaluation of the oxidation behavior of ZrB2-15 vol% SiC at a low oxygen partial pressure, Vacuum 88 (2013) 98–102. [2] A.H. Sari, V.M. Astashynski, E.A. Kostyukevich, V.V. Uglov, N.N. Cherenda, Alloying of austenitic steel surface with zirconium using nitrogen compression plasma flow, Vacuum 115 (2015) 39–45. [3] D. Zhao, H.F. Hu, C.R. Zhang, Y.D. Zhang, J. Wang, A simple way to prepare precursors for zirconium carbide, J. Mater. Sci. 45 (2010) 6401–6405. [4] M.S. Song, B. Huang, M.X. Zhang, J.G. Li, Formation and growth mechanism of ZrC hexagonal platelets synthesized by self-propagating reaction, J. Cryst. Growth 310 (2008) 4290–4294. [5] H.Y. Ryu, H.H. Nersisyan, J.H. Lee, Preparation of zirconium-based ceramic and composite fine-grained powders, Int. J. Refract. Met. Hard Mater. 30 (2012) 133–138. [6] Y.J. Yan, Z.R. Huang, X.J. Liu, D.L. Jiang, Carbothermal synthesis of ultra-fine zirconium carbide powders using inorganic precursors via sol-gel method, J. Sol.Gel Sci. Technol. 44 (2007) 81–85. [7] S.C. Zhang, G.E. Hilmas, W.G. Fahrenholtz, Pressureless densification of zirconium diboride with boron carbide additions, J. Am. Ceram. Soc. 89 (2006) 1544–1550. [8] Z. Chen, W.P. Wu, Z.F. Chen, X.N. Cong, J.L. Qiu, Microstructural characterization on ZrC doped carbon/carbon composites, Ceram. Int. 38 (2012) 761–767. [9] X.Y. Tao, Z.M. Xiang, S.X. Zhou, Y.B. Zhu, W.F. Qiu, T. Zhao, Synthesis and characterization of a Boron-containing precursor for ZrB2 ceramic, J. Ceram. Sci. Technol. 7 (2016) 107–112. [10] X.Y. Tao, Z.M. Xiang, S.X. Zhou, Y.B. Zhu, W.F. Qiu, Synthesis of a soluble preceramic polymer for ZrC using 2-Hydroxybenzyl alcohol as carbon source, Adv. Appl. Ceram. 115 (2016) 342–348.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
A facile method to prepare ZrC nanofibers by electrospinning and pyrolysis of polymeric precursors ⁎
XueYu Tao , ShiXiang Zhou, Jie Ma, ZhiMei Xiang, RuiLin Hou, JingJing Wang, XiYao Li School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, China
A R T I C L E I N F O
A BS T RAC T
Keywords: ZrC nanofibers Polyvinylpyrrolidone Polymeric precursor Electrospinning
Zirconium carbide (ZrC) is one of the most attractive ultra-high temperature ceramics due to its excellent properties. ZrC nanofibers were fabricated via electrospinning and pyrolysis of a novel polymeric precursor, Polyzirconosaal (PZSA), with the addition of polyvinylpyrrolidone (PVP) as the spinning aid. The polymer PZSA was prepared from the chemical reaction between Polyzirconoxane (PZO) and Salicyl alcohol. The as-spun PZSA/PVP fibers were converted to ZrC nanofibers with a diameter ~200 nm after carbothermal reduction at 1300 °C in argon. The obtained ZrC nanofibers maintained its excellent fibrous morphology. The microstructures exhibited that nanoscale ZrC particles dispersed in the fibers containing free carbon. The average crystallite size of ZrC particles using Scherrer method was 42 nm. The obtained ZrC nanofibers were characterized by XRD, SEM and TEM. The current material would be particularly useful for applications such as catalyst support, filters, gas storage, supercapacitors, and phase change material support in thermal management systems.
1. Introduction Zirconium carbide (ZrC) is a typical member of ultra-high temperature ceramics (UHTCs) family [1,2]. ZrC has attracted considerable attention due to its superior properties, such as high melting point (3420 °C), high hardness (25 GPa) and good chemical stability, which lead to potential applications in extremely harsh environment [3–5]. Generally, there are various methods for the preparation of ZrC ceramics, such as carbothermal reduction of ZrO2, mechanical alloying and self-propagating high-temperature synthesis. However, these processes require high temperature and very long time. The shape and size of the final product are difficult to control [6–8]. Recently, the polymeric precursor method has been developed for preparing advanced ceramics by pyrolysis of the polymers [9–11]. This method can prepare complex shapes (films, fibers, coatings) and ceramic matrix composites in a wide variety of nonoxide systems. Ceramic nanofibers have become the focus of the scientific research for the past decades due to their unique physical and chemical properties leading to potential application in the areas of mesoscopic physics and fabrication of nanoscale devices and more. Ceramics nanofibers can be prepared by a variety of methods, among which electrospinning attracts a lot of attention due to its efficiency and simplicity. Electrospinning is a versatile method to produce ultra-long and one-dimensional ceramic fibers with controllable diameter, mor-
⁎
phology, and composition [12]. By controlling the precursory components and processing parameters of this technique, the nanoparticles could be assembled within the electrospun fibers, which can meet the requirements of application in various fields such as filtration, energy storage, thermal storage and so forth [13–15]. Electrospinning has been used to produce ZrN [16], SiC [17], ZrO2 [18], SiBCN [19], ZrB2 [20,21], ZrC [22,23] and TiC nanofibers [24]. However, there are only limited reports on the fabrication of zirconium carbide nanofibers by electrospinning. Cui et al. [22] synthesized ZrC ultra-thin fibers via electrospinning the solution by dissolving zirconium acetyl acetonate, phenolic resins in a solvent mixture of ethyl alcohol and 2,4-pentanedione. Li et al. prepared ZrC nanofibers by electrospinning using Polyzirconoxane (PZO) as zirconium source and polyacrylonitrile (PAN) as the spinning aid and primary carbon source [25]. Ghelich et al. prepared ZrC nanofibers via the self-condensation of Zr(OPr)4 and PVP through electrospinning by combination of carbothermal reduction [26]. However, these ZrC precursor fibers were prepared via just blending of zirconium and carbon sources, and polycondensation of zirconium alkoxides. Therefore, this strategy will not work for producing continuous ceramic fibers using a single process of spinning and pyrolysis. To overcome these limitations, we developed a simple strategy to prepare ZrC nanofibers by taking advantage of polymeric precursor method and electrospinning technique employing chemical reaction of PZO and Salicyl alcohol (SA). The product Polyzirconosaal
Corresponding author. E-mail address: [email protected] (X. Tao).
http://dx.doi.org/10.1016/j.ceramint.2016.12.005 Received 23 September 2016; Received in revised form 14 November 2016; Accepted 1 December 2016 0272-8842/ © 2016 Published by Elsevier Ltd.
Please cite this article as: Tao, X., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.12.005
Ceramics International (xxxx) xxxx–xxxx
X. Tao et al.
standard electrospinning device at room temperature (Fig. 1A). The electrospun fibers were collected on the aluminum foil panel. During the electrospinning process, parameters such as concentration of the solution, applied voltage, tip-to-collector distance, and feed rate were optimized to receive the consistent hybrid nanofibers. The spinning solution used is a 10 mass% solution. The diameter of the needle is 0.5 mm. The voltage applied was in the range of 14–21 kV, the flow rate was 0.5–1.1 mL h−1 and the collection distance was adjusted at 10–12 cm. Subsequently, the as-spun precursor fibers were carefully peeled off from the aluminum foil, transferred into a crucible for drying (Fig. 1B) . After being dried in a vacuum oven at 60 °C for 24 h, the electrospun precursors were subsequently heat-treated in a furnace for pyrolysis into final ZrC.
(PZSA) was utilized as a novel polymeric precursor to produce continuous ZrC ceramic fibers, with polyvinylpyrrolidone (PVP) as the spinning aid [21,26–28]. The solution for electrospinning was prepared by dissolving PZO, Salicyl alcohol in ethanol followed by the addition of PVP. The synthesized nanofibers were characterized by DSC-TG, FE-SEM, TEM and XRD techniques. The structure of PVP is shown as follows.
.
3. Results and discussion
2. Experimental
Fig. 2 is the SEM images of the green fibers electrospun from PZSA/ PVP spinning solution. The green fibers appear to be very smooth without any beads, and the diameters (~400 nm) of these fibers are uniform, which indicates that the PZSA/PVP spinning solution has very good spinnability and PVP is an excellent spinning aid for the preparation of fibers [26,30]. The electrospinning conditions for obtaining smooth fibers without any beads were at a positive voltage of 15 kV, with a mass flow rate of 0.8 mL h−1, and a working distance of 10 cm. This method combines the advantages of electrospinning and polymeric precursor techniques, thus the new method can facilely produce ZrC electrospun fibers with controllable chemical and morphological structures. The attractive feature is that the precursor PZSA was prepared by chemical reaction of PZO and Salicyl alcohol (SA) (Scheme 1). In this polymer, zirconium and carbon elements distributed uniformly at molecular level, not just by blending of the source of zirconium and carbon. Further, C=O groups of PVP are known to be strongly bonded with OH groups of the metalloxane polymers via hydrogen bonding [27]. Such C=O groups can be regarded as the “capping agent” for the OH groups of the metalloxane polymers, obstructing the condensation reaction [31]. There are some OH groups in the structure of PZSA and these OH groups can interact with C=O groups of PVP polymer to form hydrogen bonds, which increase the spinnability and stability of the precursor. Moreover, Incorporation of PVP in precursor solutions allows formation of target complex with ease, which is another benefit of PVP for fabricating fibers. Also, PVP has C=O groups, which can also coordinate metal atoms, offering homogeneity in metal atom distribution in the precursor fibers [27,32]. FTIR spectroscopy (Fig. 3 A) was carried out on the as-spun fibers. The peaks at 1597 and 1530 cm−1 are assigned to C=O and C=C in acetylacetonate ligand. The band at 757 cm−1 is assigned to C-H of
2.1. Preparation of PZO PZO was prepared according to the previous publications [29]. 9.0 g (0.09 mol) acetylacetone and then 12.2 g (0.12 mol) triethylamine was added drop wise into 260 mL methanol solution of 19.4 g (0.06 mol) zirconium oxychloride octahydrate (ZOC) below 5 °C at molar ratio of Et3N/ZOC=2.0. The reaction mixture was stirred at room temperature for 2 h and then evaporated. Addition of 150 mL THF and filtration of the precipitate followed by concentration of the filtrate gave a highly viscous solution. PZO was isolated as a white powder by adding the viscous solution. 2.2. Preparation of electrospinning solution In a typical procedure, 0.502 g PZO was dissolved into 15 mL ethanol to forma transparent solution at room temperature (25 °C). Then, 0.163 g Salicyl alcohol was added to this solution till the mixture became transparent. The chemical reaction between PZO and Salicyl alcohol was shown in Scheme 1. The molar ratio of the C/Zr within the spinning solution is set at 4.0, higher than the stoichiometric ratio. Finally, PVP was added into the solution under continuous magnetic stirring for 24 h at room temperature. The concentration of the spinning solutions was adjusted by changing the amount of PVP. The resulting stable homogeneous solution was employed for the electrospinning process. 2.3. Electrospinning and heat treatment of as-spun nanofibers The electrospinning solution was subsequently electrospun using a
Scheme 1. Synthetic route of the PZSA precursor.
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Fig. 1. (A) Schematic diagram of the preparation route for PZSA/ PVP fibers, and (B) digital photos of as-spun PZSA/ PVP fibers.
aromatic in SA ligand. In addition, we see peaks assigned to Zr-O at 649 and 578 cm−1 [10,29]. There is also significant broadening of the bands below 750 cm−1 assigned to the formation of the O-Zr-O network. This indicates that zirconium atoms have been successfully introduced into ZrC precursor polymer. The bands in the 3500– 3200 cm−1 can be assigned to -NH stretch of the PVP. PVP is present in the fibers and the bands at 1672 cm−1 and 1288 cm−1 are the characteristic peaks of PVP. The prominent band at 1672 cm−1 corresponds to C=O stretching vibration, 1288 cm−1 is due to C-N stretching vibration of the PVP [23]. The bands at 1483, 1458 and 1424 cm−1 correspond to the ring CH2 scissoring of PVP. The electrospun precursors need to be pyrolyzed to produce ZrC nanofibers. The crystalline structure of ZrC nanofibers heat-treated at 1300 °C was further investigated by XRD, and the results were shown in Fig. 3B. It is noted that all of the diffraction peaks in the XRD pattern match well with the cubic ZrC phase (JCPDS No. 35-0784). Five characteristic peaks at 2θ=33.1° (111), 38.4° (200), 55.4° (220), 65.9° (311), 69.3° (222) appeared in XRD pattern [29], and no other peaks are found, indicating the ZrC crystalline with cubic NaClstructure has been successfully synthesized. The sharp diffraction peaks implies the better crystallinity of ZrC grains. The PVP in composite fibers had been completely carbonized after heat treatment at 1300 °C for 2 h in argon. ZrC fibers are formed via the reaction as follows [26,29].
ZrO2(s) + 3 C(s) = ZrC(s) + 2CO(g)
Fig. 2. (A) and (B) SEM images of as-spun PZSA/PVP fibers at different magnifications.
Fig. 4(A) and (B) shows SEM images of the nanofibers after heat treatment at 1300 °C. As can be seen, the fibrous morphology of the
Fig. 3. (A) FTIR spectrum of the as-spun PZSA/ PVP fibers and (B) XRD pattern of pyrolyzed fibers heat-treated at 1300 °C.
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Fig. 4. SEM (A and B), TEM image (C and D), EDS (E) and Raman spectrum (F) of the pyrolyzed nanofibers heat-treated at 1300 °C.
fibers is well retained after the pyrolysis. But the surface of the nanofibers starts to become a little rough due to the removal of PVP [33] and crystallization of ZrC [30]. The ZrC fibers have a diameter of about 200 nm, which is smaller than the electrospun fibers because of shrinkage when the organic components were removed [30]. TEM investigation shows that ZrC fibers structure is well kept and nanosized hexagonal ZrC crystals are dispersed in the fibers. ZrC nanocrystals were embedded or engulfed by disordered carbon coming from Salicyl alcohol (Fig. 4(C) and (D)) [23], the average diameter of the fibers is ∼200 nm which is accordance with the SEM observation. These results confirm that the crystalline phases dispersed in the ceramic fibers uniformly and the ZrC crystalline is in the range of 30– 50 nm. A quantitative analysis by energy-dispersive spectroscopy (EDS) shows that ZrC nanofibers consisted of Zr, C and O elements
(Fig. 4(E)). It is known that in the preparation of UHTCs, it is very difficult to remove the carbon and oxygen residues entirely [20]. Raman spectroscopy was used to characterize the status of carbon in ZrC fibers. As shown in Fig. 4(F), two strong peaks centered at 1298 cm−1 and 1596 cm−1 were detected, which correspond to the D and G peaks of free carbon, respectively. The intensity ratio of the D to G band (ID/IG) was calculated to be 2.54, indicating the relative disorder of the carbon in ceramic fibers. Referring to XRD, SEM, TEM and Raman characterization, the obtained fibers at 1300 °C could be considered as nanoscale ZrC ceramic fibers. 4. Conclusions In summary, continuous ZrC nanofibers are successfully fabricated 4
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by combination of polymeric precursor chemistry and electrospinning technique. PZSA and PVP are used as zirconium source and spinning aid, respectively. ZrC nanofibers are obtained by pyrolyzing the as-spun green fibers at 1300 °C for 2 h under argon. The obtained ZrC nanofibers via carbothermal reduction treatment at 1300 °C exhibit an average diameter of 200 nm while preserving the fibrous morphology. TEM results show that ZrC nanoparticles disperse in the fiber uniformly containing free carbon and the average size is about 42 nm. The electrospun ZrC nanofibers in the form of freestanding nonwoven textile may serve as an ideal precursor for the synthesis of highly porous carbide-derived carbon materials, which would be particularly useful for applications such as catalyst support, filters, gas storage, supercapacitors, and phase change material support in thermal management systems. Additionally, this reported method could be utilized to prepare other metal carbides/borides nanofibers. Acknowledgments This research was supported by the financial support of the National Natural Science Foundation of China (51402354) and “the Fundamental Research Funds for the Central Universities” (2015QNA07) and “the Fundamental Research Funds for the Central Universities” (2015XKZD01). References [1] A. Momozawa, R. Tu, T. Goto, Y. Kubota, H. Hatta, K. Komurasaki, Quantitative evaluation of the oxidation behavior of ZrB2-15 vol% SiC at a low oxygen partial pressure, Vacuum 88 (2013) 98–102. [2] A.H. Sari, V.M. Astashynski, E.A. Kostyukevich, V.V. Uglov, N.N. Cherenda, Alloying of austenitic steel surface with zirconium using nitrogen compression plasma flow, Vacuum 115 (2015) 39–45. [3] D. Zhao, H.F. Hu, C.R. Zhang, Y.D. Zhang, J. Wang, A simple way to prepare precursors for zirconium carbide, J. Mater. Sci. 45 (2010) 6401–6405. [4] M.S. Song, B. Huang, M.X. Zhang, J.G. Li, Formation and growth mechanism of ZrC hexagonal platelets synthesized by self-propagating reaction, J. Cryst. Growth 310 (2008) 4290–4294. [5] H.Y. Ryu, H.H. Nersisyan, J.H. Lee, Preparation of zirconium-based ceramic and composite fine-grained powders, Int. J. Refract. Met. Hard Mater. 30 (2012) 133–138. [6] Y.J. Yan, Z.R. Huang, X.J. Liu, D.L. Jiang, Carbothermal synthesis of ultra-fine zirconium carbide powders using inorganic precursors via sol-gel method, J. Sol.Gel Sci. Technol. 44 (2007) 81–85. [7] S.C. Zhang, G.E. Hilmas, W.G. Fahrenholtz, Pressureless densification of zirconium diboride with boron carbide additions, J. Am. Ceram. Soc. 89 (2006) 1544–1550. [8] Z. Chen, W.P. Wu, Z.F. Chen, X.N. Cong, J.L. Qiu, Microstructural characterization on ZrC doped carbon/carbon composites, Ceram. Int. 38 (2012) 761–767. [9] X.Y. Tao, Z.M. Xiang, S.X. Zhou, Y.B. Zhu, W.F. Qiu, T. Zhao, Synthesis and characterization of a Boron-containing precursor for ZrB2 ceramic, J. Ceram. Sci. Technol. 7 (2016) 107–112. [10] X.Y. Tao, Z.M. Xiang, S.X. Zhou, Y.B. Zhu, W.F. Qiu, Synthesis of a soluble preceramic polymer for ZrC using 2-Hydroxybenzyl alcohol as carbon source, Adv. Appl. Ceram. 115 (2016) 342–348.
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