Fabrication of boron carbide fibers consisting of connected particles by carbothermal reduction via electrospinning

Materials Letters 254 (2019) 158–161

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Fabrication of boron carbide fibers consisting of connected particles by carbothermal reduction via electrospinning Masaki Kakiage a,b,⇑, Taiju Kobayashi c a Institute for Fiber Engineering, Shinshu University (IFES), Interdisciplinary Cluster for Cutting Edge Research (ICCER), Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan b Division of Molecular Science, Graduate School of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan c Department of Textile Science and Technology, Graduate School of Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan

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Article history: Received 10 May 2019 Received in revised form 24 June 2019 Accepted 8 July 2019 Available online 9 July 2019 Keywords: Boron carbide (B4C) Fiber Poly(vinyl alcohol) Electrospinning Carbothermal reduction

a b s t r a c t We successfully fabricated crystalline boron carbide (B4C) fibers consisting of connected particles by carbothermal reduction. The condensed boric acid (H3BO3)-poly(vinyl alcohol) (PVA) product fibers were prepared from a H3BO3-PVA/dimethyl sulfoxide solution with glycerin added by electrospinning. The fibrous B4C precursor consisting of boron oxide and carbon components was prepared by the thermal decomposition of the electrospun condensed H3BO3-PVA product fibers in air. Crystalline B4C fibers consisting of connected particles were fabricated by the heat treatment of the fibrous B4C precursor at 1400 °C in an Ar flow. Ó 2019 Elsevier B.V. All rights reserved.

1. Introduction One-dimensional structural ceramics, such as fibers and whiskers, are remarkable materials owing to their unique morphology and properties. Boron carbide (B4C) is an important nonoxide ceramic with attractive properties and superior functions [1]. The machining of B4C is extremely difficult owing to its high hardness. Thus, a direct synthesis route is required for the fabrication of B4C fibers. The fabrication of B4C fibers by various methods was previously reported [2–6]; however, these methods require specialized techniques and/or raw materials. The general manufacturing process of B4C is the carbothermal reduction of boron oxide (B2O3) at a high temperature of approximately 2000 °C, for which the overall reaction is

2B2 O3 þ 7C ! B4 C þ 6CO:

ð1Þ

In this study, we attempted to fabricate B4C fibers by conventional carbothermal reduction. We successfully synthesized crystalline B4C powder with little free carbon at a low temperature of 1200–1250 °C by using a condensed product prepared from boric acid (H3BO3) and an organic compound with a number of hydroxyl groups (a polyol) and by the thermal decomposition of the con⇑ Corresponding author. Present address: Division of Molecular Science, Graduate School of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan. E-mail address: [email protected] (M. Kakiage). https://doi.org/10.1016/j.matlet.2019.07.028 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.

densed H3BO3-polyol product in air [7–13]. A condensed H3BO3polyol product was prepared by dehydration condensation with borate ester (B–O–C) bond formation. A B4C precursor consisting of finely dispersed B2O3 and carbon components was obtained by the thermal decomposition of the condensed H3BO3-polyol product in air. This approach has been applied to the low-temperature synthesis of other boride powders [13–17]. Poly(vinyl alcohol) (PVA), which is a polymer polyol and thus has superior spinnability, is commonly used for the preparation of a condensed product with H3BO3 in our low-temperature synthesis of B4C powder [8,11]. The B2O3/carbon structure (B4C precursor) formed by the thermal decomposition of a condensed H3BO3-PVA product in air consists of nanosize B2O3 particles and a carbon matrix [8,11]. Thus, it is expected that a fibrous B4C precursor with a nanodispersed B2O3/ carbon structure can be formed from condensed H3BO3-PVA product fibers. Electrospinning was selected as the spinning method of a H3BO3-PVA solution because the electrospinnability of a H3BO3-PVA solution was previously reported [18,19]. In this study, we fabricated B4C fibers by carbothermal reduction starting from electrospun condensed H3BO3-PVA product fibers.

2. Experimental procedure PVA was supplied by Kuraray Co., Ltd., Japan, for which the degrees of polymerization and hydrolysis were 300 and 98.2 mol%, respectively. H3BO3 (99.5%), glycerin (99.0%), and dimethyl

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sulfoxide (DMSO, 99.0%) were purchased from Wako Pure Chemical Industries, Ltd., Japan, and used as received. PVA (9 wt% based on DMSO) and glycerin (10 wt% based on PVA) were dissolved in DMSO by stirring and heating at 120 °C for 1 h, which was followed by cooling at room temperature (RT). Glycerin is a plasticizer with B–O–C formation ability [7,12]. H3BO3 (H3BO3:PVA (hydroxyl group of PVA) molar ratio of 1:4 and H3BO3:glycerin molar ratio of 1:1) was added to the solution by stirring at RT and then heating at 60 °C for 1 h, which was followed by cooling at RT. The condensed H3BO3-PVA product fibers were prepared by electrospinning. The prepared H3BO3-PVA/DMSO solution was electrospun using a Kato Tech NEU Nanofiber Electrospinning Unit. The voltage and flow rate were fixed at 20 kV and 2.0 mL/h, respectively. The inner diameter of the nozzle (stainless-steel needle) and the distance between the needle tip and the rotating collector were 0.50 mm and 15 cm, respectively. The collector, which was covered with aluminum foil, was rotated at a speed of 20 m/min. The temperature throughout the process was 45 °C. The resulting nonwoven fibrous mats were dried at 120 °C for 12 h in vacuum. Thermal decomposition of the electrospun condensed H3BO3-PVA product fibers in air was performed to eliminate the excess carbon component [7–17]. The obtained condensed H3BO3-PVA product fibers were thermally decomposed at 640 °C for 2 h in air. The obtained fibrous precursor was placed in a graphite boat and heated at 1200–1400 °C for 1–10 h in an Ar flow (200 mL/min) at a heating rate of 10 °C/min.

3. Results and discussion The H3BO3-PVA/DMSO solution without glycerin was an inhomogeneous gel and had no electrospinnability. The preparation of the spinning solution by blending glycerin as a plasticizer enabled electrospinning. Fig. 1(a) shows the scanning electron microscope (SEM) image of a nonwoven fibrous mat prepared from the H3BO3-PVA/DMSO solution with glycerin added by electrospinning. Homogeneous fibers with the average diameter of 2 lm were observed. When excess glycerin was added to the H3BO3-PVA/ DMSO solution, the adhesion between the fibers with incomplete solidification was observed (Fig. S1). The results of the thermogravimetric measurements (Fig. S2) and attenuated total reflectance Fourier transform infrared measurements (Fig. S3) of the obtained electrospun fibers corresponded to those of a previously prepared condensed H3BO3-PVA product with B–O–C bonds [8,11,20], indicating the formation of B–O–C bonds in the electrospun fibers. Thus, the condensed H3BO3-PVA product fibers were prepared by electrospinning the H3BO3-PVA/DMSO solution with added glycerin. The electrospun condensed H3BO3-PVA product fibers were thermally decomposed in air to form the fibrous B4C precursor. The thermal decomposition in air is performed to control the amount of carbon to the stoichiometric C/B2O3 ratio required for carbothermal reduction. The C/B2O3 ratio of the obtained thermally decomposed product was 3.2, which is slightly less than the stoichiometric C/B2O3 ratio required for carbothermal reduction given by Eq. (1) (C/B2O3 = 3.5) [7,8,11,12]. Fig. 1(b) shows the digital microscopic image of the thermally decomposed product prepared from the electrospun condensed H3BO3-PVA product fibers. The fibrous structure of the electrospun condensed H3BO3-PVA product fibers [Fig. 1(a)] remained after thermal decomposition in air. The microstructure of the thermally decomposed product consisted of nanosize B2O3 particles and a fibrous carbon matrix (Fig. S4). Consequently, the fibrous B4C precursor consisting of finely dispersed B2O3 and carbon components, resulting in a higher reactivity, was prepared by the thermal decomposition of the electrospun condensed H3BO3-PVA product fibers in air.

Fig. 1. (a) SEM image of electrospun condensed H3BO3-PVA product fibers and (b) digital microscopic image of fibrous precursor prepared by thermal decomposition of electrospun condensed H3BO3-PVA product fibers in air.

Fig. 2(a) shows the X-ray diffraction (XRD) patterns of the product fibers obtained by the heat treatment of the fibrous precursor at 1200–1400 °C for 5 h in an Ar flow. Peaks corresponding to a rhombohedral B4C crystal were observed at a low temperature of 1200 °C owing to the higher reactivity of the fibrous precursor. The B4C peak intensity ratio increased with increasing heat treatment temperature [Fig. S5(a)]. Fig. 2(b) shows the XRD patterns of the product fibers obtained by the heat treatment of the fibrous precursor at 1400 °C for 1–10 h in an Ar flow. The B4C peak intensity ratio increased with increasing heat treatment time up to 5 h and became constant above 5 h [Fig. S5(b)]. Consequently, crystalline B4C with little free carbon was synthesized above 5 h. Fig. 3 shows the digital microscopic image and SEM images of the product fibers obtained by the heat treatment of the fibrous precursor at 1400 °C for 5 and 10 h in an Ar flow. A unique fiber structure consisting of connected microparticles was observed for both products, indicating the formation of crystalline B4C fibers consisting of connected particles from the condensed H3BO3-PVA product fibers. The morphology of the particles in the fibers was similar to that of B4C particles obtained from a condensed H3BO3-PVA product [8,11]. The B4C fibers consisting of connected particles were also observed for the products obtained by heat treatment at 1200 and 1300 °C, but less well developed fibers and lower connectivity of B4C particles were observed in the product (Fig. S6). The morphology of the obtained B4C fibers [Fig. 3 (a)] was reflected in that of the fibrous B4C precursor [Fig. 1(b)]. Therefore, the fabrication of B4C fibers is achieved by the formation

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Fig. 2. XRD patterns of product fibers obtained by heat treatment of fibrous precursor at (a) 1200–1400 °C for 5 h and (b) 1400 °C for 1–10 h in an Ar flow.

Fig. 3. (a) Digital microscopic image and (b, c) SEM images of product fibers obtained by heat treatment of fibrous precursor at 1400 °C for (a, b) 5 and (c) 10 h in an Ar flow.

of the fibrous B4C precursor prepared by the thermal decomposition of the electrospun condensed H3BO3-PVA product fibers in air. The B4C fibers consisting of connected submicrometer-size particles were fabricated by heat treatment at 1400 °C for 5 h [Fig. 3(b)]. The size of the particles making up the fibers increased with increasing heat treatment time [Fig. 3(c)] because the grain growth of particles was promoted.

4. Conclusions We fabricated crystalline B4C fibers consisting of connected particles by carbothermal reduction from condensed H3BO3-PVA product fibers via electrospinning in this study. The condensed H3BO3-PVA product fibers were prepared from a H3BO3-PVA/ DMSO solution with glycerin added by electrospinning. The fibrous B4C precursor consisting of B2O3 particles and a carbon matrix was prepared by the thermal decomposition of the electrospun condensed H3BO3-PVA product fibers in air. Fiber formation was promoted by heat treatment at 1400 °C, and crystalline B4C fibers consisting of connected particles were fabricated.

Declaration of Competing Interest The authors declare that they have no known competing financial interests. Acknowledgement This work was partly supported by a Grant-in-Aid for Young Scientists (B) (JP16K21067) from the Japan Society for the Promotion of Science (JSPS). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.07.028. References [1] F. Thévenot, J. Eur. Ceram. Soc. 6 (1990) 205–225. [2] D. Zhang, D.N. Mcilroy, Y. Geng, M.G. Norton, J. Mater. Sci. Lett. 18 (1999) 349– 351.

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