Carbothermal reduction synthesis of ZrB2 nanofibers via pre-oxidized electrospun zirconium n-propoxide
Introduction
Zirconium diboride (ZrB2) is a kind of ultra high-temperature ceramic with a melting point of 3200 °C having high hardness, high electric conductivity, an outstanding wear and excellent corrosion resistance [1], [2]. The unique combination of these properties makes ZrB2 a promising candidate for using as thermal protection materials, cutting tools, high temperature electrodes, and molten metal crucibles [3], [4]. Although different methods of synthesis such as solid-state reaction [5], electrochemical [6], mechanochemical [7] and self-propagating high-temperature [8] have been applied for its synthesis, however barriers, such as high synthesis temperature, long production period, low purity, relatively large crystallite size and poor sinterability, are still considered to be the challenges [9], [10]. One-dimensional (1D) structures exhibit a number of unique features and properties, such as long length, high surface area, and hierarchically porous structure with excellent pore interconnectivity and enhanced electron and phonon transport properties. Hitherto, the integration of long fibers has strongly increased the crack resistance, elongation and thermal shock resistance, resulting in several advanced applications [11], [12].
Electrospinning as a simple and versatile technique has been broadly employed for preparation of the ceramic nanofibers with exceptional length, uniform diameter, and diversified compositions using a proper solution of the ceramic precursor and polymer, followed by calcination of the resulting mat to remove the polymer matrix [13]. Electrospinning has been employed to prepare oxide and non-oxide zirconium based nanofibers such as ZrB2 [14], ZrO2 [15], ZrC [16], [17], ZrN [13], [17] and also yttria stabilized zirconia (YSZ) [18]. Li et al. [14] synthesized ZrB2 nanofibers via electrospinning of the solution by dissolving of polyzirconoxane (PZO), boric acid and polyacrylonitrile (PAN) in N,N-dimethylformamide (DMF) as the solvent. Zirconium oxychloride octahydrate (ZrOCl2·8H2O), zirconium (IV) acetyl acetonate, zirconium oxychloride (ZrOCl3), water-soluble zirconium compound (ZC) and PZO are the most commonly used sources of zirconium to prepare Zr-based nanofibers [14], [15], [16], [17], [18].
Up to the present time, no reports have been published on the synthesis of the sols and moreover, precursor polymers that have good stability to prepare ZrB2 nanofibers by self-condensation of commercially available Zr(OPr)4, which appears to be a promising precursor for synthesizing of ZrB2. But this compound should be modified by acetyl acetone (acac) [19] or acetic acid (AcOH) [10] as bridging and chelating agents for formation of (Zr(acac)2) and Zr(OAc)2(OPr)2, by substitution of acac and OAc for OPr groups to interdict the hydrolysis of Zr(OPr)4 [20]. For preparation of electrospinnable solution of Zr(OPr)4, a non-aqueous solution is preferred to prevent of hydrolysis of Zr(OPr)4, therefore, PVP which considered to be an electrospun polymer with unique properties, such as solubility in ethanol, low toxicity, high complexing ability, and good film forming characteristics can be used to prepare the polymer/Zr(OPr)4 pre-electrospinning solution [21].
In this paper, fabrication capability of zirconium diboride (ZrB2) nanofibers by Zr, B, and C sources, such as zirconium n-propoxide (Zr(OPr)4), boric acid, and polyvinylpyrrolidone (PVP), respectively, was examined using electrospinning method followed by carbothermal reduction for the first time. The effect of internal and external supplemental sources of carbon (citric acid and graphite) and the following pyrolysis treatment on the morphology and final phase of the resulting products were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD).
Section snippets
Materials
Polyvinylpyrrolidone (PVP, MW=1,300,000), zirconium n-propoxide Zr(OPr)4 having concentration of 70 wt% in 1-propanol, were obtained from Aldrich Chemical Company. Boric acid (H3BO3), acetylacetone (C5H8O2), glacial acetic acid (CH3COOH), citric acid (C6H8O7) and absolute ethanol (C2H5OH) were supplied from Merck Chemical Company.
Preparation of the electrospinning solution
Preparation path of the electrospun PVP/Zr(OPr)4 hybrid nanofibers was schematically presented in the diagram of Fig. 1. Zr(OPr)4 with molarity of 0.0001 was modified
Results and discussion
Fig. 2(a)–(c) shows typical SE-SEM micrographs of the as-spun raw fibers having different carbon contents. Average diameter size of the resultant fibers with the C/Zr ratio of 4, 5.5 and 7 were 436±210 nm, 517±196 nm and 762±336nm, respectively. Considering the effect of solution concentration on spinnability of the fibers and their morphology, the electrospun fibers which obtained from the 6 wt% PVP solution with C/Zr=4.0 (Fig. 2(a)), contained some beads. It was found that the higher polymer
Conclusions
A new procedure for synthesizing zirconium diboride nanofibers having average diameter of 297±103 nm was presented. The electrospun PVP/Zr(OPr)4 was pyrolyzed in an oxidizing atmosphere, and further heat treated in an argon atmosphere for carbonization process. Single phase ZrB2 having continuous long fibrous structure was obtained when the optimized ratio of C/Zr was 5.5, also ZrO2 impurity was reduced and completely disappeared in the final product with increasing the C/Zr ratio. The rod-like
Acknowledgments
This work was supported by the Space Transportation Research Institute.
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