Low temperature synthesis of TaB 2 nanorods by molten-salt assisted borothermal reduction

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1 Received: 12 August 2017 Accepted: 11 September 2017 DOI: /jace RAPID COMMUNICATION Low temperature synthesis of TaB 2 nanorods by molten-salt assisted borothermal reduction Tingting Wei 1,2 Zetan Liu 1,2 Donglou Ren 3 Xiangong Deng 1 Qihuang Deng 3 Qing Huang 3 Songlin Ran 2 1 School of Materials Science and Engineering, Anhui University of Technology, Maanshan, China 2 Key Laboratory of Metallurgical Emission Reduction & Resources Recycling, Ministry of Education, Anhui University of Technology, Maanshan, China 3 Engineering Laboratory of Specialty Fibers and Nuclear Energy Materials, Ningbo Institute of Industrial Technology, Chinese Academy of Sciences, Ningbo, China Correspondence Xiangong Deng, School of Materials Science and Engineering, Anhui University of Technology, Maanshan, China. dengxg@ahut.edu.cn and Songlin Ran, Key Laboratory of Metallurgical Emission Reduction & Resources Recycling, Ministry of Education, Anhui University of Technology, Maanshan, China. songlin.ran@gmail.com Abstract TaB 2 powders were synthesized by a molten-salt assisted borothermal reduction method at 900 C-1000 C in flowing argon using Ta 2 O 5 and amorphous B as starting materials. The results indicated that the presence of liquid phase, such as B 2 O 3 and NaCl/KCl, accelerated the mass transfer of reactant species and resulted in the complete finish of the reaction at low temperatures. The obtained TaB 2 powders exhibited a flow-like shape assembled from nanorods grow along [001] direction or c-axis. The morphology of the synthesized TaB 2 powders could be tailored by the amount of B 2 O 3 or NaCl/KCl. KEYWORDS low temperature, molten salt synthesis, nanorods, synthesis Funding information Higher Education Institutions of Anhui Province, Grant/Award Number: gxyqzd ; National Natural Science Foundation of China, Grant/ Award Number: , INTRODUCTION Transition-metal diborides (MB 2 ), such as ZrB 2, HfB 2, TiB 2, TaB 2, etc., are recommend as ultra-high temperature ceramics (UHTCs) for potential applications in high-temperature environment due to their high melting points, hardness, thermal and electrical conductivities, and good thermal shock resistance. 1-5 However, the intrinsic brittleness of ceramics makes MB 2 and MB 2 -based ceramic composites a low toughness of 2-6 MPam 1/2, which greatly reduces the reliability of the materials and limits their applications. 4 Recently, partially textured TiB 2 SiC composites were reported to have a high fracture toughness of 7.3 MPam 1/2, which was attributed to the formation of plate-like TiB 2 grains. 6 Similarly, ZrB 2 SiC-based composites with elongated ZrB 2 grains exhibited a fracture J Am Ceram Soc. 2018;101: wileyonlinelibrary.com/journal/jace 2017 The American Ceramic Society 45

2 46 WEI ET AL. toughness of 6.5 MPam 1/2. 7 Therefore, it seems that controlling the grain shape of MB 2 is an effective method to improve the toughness of MB 2 -based ceramics. Although nearly all MB 2 are hexagonal, it is difficult to obtain grains like platelet, rod, and flake in MB 2 ceramics especially in MB 2 -based composites. Normally, the presence of low melting point phases during sintering may induce anisotropic growth of MB 2 grains, 8,9 but it is detrimental to some other properties of ceramics, such as hardness and high-temperature strength. Texturing by strong magnetic field alignment (SMFA) or templated grain growth (TGG) is another potential method to enhance the toughness of MB 2 ceramics. However, due to the absence of MB 2 particles with elongated morphologies, textured ZrB 2 and HfB 2 -based ceramics via SMFA exhibited no elongated grains, despite anisotropic properties. 10,11 Few reports concerned textured MB 2 ceramics via TGG method, for which single crystal MB 2 seeds with controlled morphology is required. Therefore, the synthesis of MB 2 powders with elongated morphology is quite important for improving toughness of MB 2 ceramics by texturing with either SMFA or TGG. At present, a variety of methods have been performed to synthesize MB 2 powders with special morphologies. For example, plate-like ZrB 2 grains were synthesized by in situ solid/liquid reaction using Zr and B powders mixed with transition metal (Mo, Nb, Ti, or W) and Si powder. 12 Hexagonal-prism-like or rod-like ZrB 2 particles were synthesized by a sol-gel route. 13,14 Hexagonal columnar ZrB 2 powders were obtained from irregular commercial powder in a glassy mixture of ZrO 2,B 2 O 3, and C using microwave heating technique. 15 Nevertheless, high temperature (1320 C-1550 C) and vacuum is unavoidable. Very recently, rod-like ZrB 2 powders were prepared at 1200 C in flowing argon by a novel molten-salt and microwave coassisted carbothermal reduction method. 16 In this communication, a molten-salt assisted borothermal reduction method was developed to synthesize TaB 2 nanorods at 900 C C under ambient pressure. 2 EXPERIMENTAL PROCEDURES TaB 2 powders were synthesized by the borothermal reduction of Ta 2 O 5 (99.99% purity, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) with amorphous B (B95, d lm, 95.82% purity; Dandong Chemical Engineering Institute Co. Ltd., Dandong, China) as a reductant. The SEM images of Ta 2 O 5 and B powders were shown in Figure S1. The molar ratio of Ta 2 O 5 /B was 3:22 according to reaction (1). A mixture of NaCl/KCl with a molar ratio of 1:1 were used as molten salts to tailor the morphology of the powders. The mass ratio of salts and the reactant powders was set as 0:1, 10:1, 20:1 and 30:1, respectively. The mixtures of salts and powders were ground by hand for 30 minutes with an agate mortar and pestle, and then placed in an Al 2 O 3 crucible inside a tubular furnace. The mixture was heat-treated at 800 C-1000 C for 1 hour under a flowing argon gas with a heating rate of 10 C/min. After natural cooling to room temperature, the product in the crucible was boiled repeatedly in distilled water and filtered to remove NaCl/KCl and B 2 O 3 produced by the reduction reaction. After washed by absolute ethanol for several times, the product was dried at 60 C for characterization. 3Ta 2 O 5 þ 22B ¼ 6TaB 2 þ 5B 2 O 3 ðlþ (1) The phase composition of as-synthesized powders was evaluated by X-ray diffraction (XRD, Ultima IV, Rigaku, Japan) with Cu-Ka radiation (k = A). The morphology of the particles was examined by filed-emission scanning electron microscope (FE-SEM, Nova NanoSEM, NPE207, FEI, USA) and filed-emission transmission electron microscopy (FE-TEM, Technai G2 F20, FEI, USA). 3 RESULTS AND DISCUSSION Figure 1 shows XRD patterns of samples synthesized at 800 C-1000 C with a mass ratio of 10:1 for NaCl/KCl salts and the reactant powders. When the temperature is 800 C, poor crystallized TaB 2 phase appears in Figure 1A. When the temperature increases to 900 C (Figure 1B) and 1000 C (Figure 1C), the crystallinity of TaB 2 phase is greatly improved and TaB 2 is the only crystalline phase. Traditionally, it is difficult to obtain pure ceramic powders via a solid-sate reaction with stoichiometric reactants due to their low speed of diffusion and impurities are always present in the product. As a result, the excess of a reactant, the enhancement of synthesis temperature and the prolongation of reaction time are essential for a solid-sate reaction, which normally lead to the residual of unreacted reactant and large particle size of the product. The molten salt method provides a liquid circumstance for the reaction and the presence of molten salts accelerate the mass transfer of reactant species, which resulted in the significantly decrease in the synthesis temperature and the particle size. In our previous reports, 17,18 pure CrB 2 and NbB 2 powders were successfully synthesized by the same borothermal reduction combined with molten salt method at a low temperature of 800 C. With the absence of NaCl/KCl salts, traces of CrB or NbB phase always existed in the product even at 1000 C. The above results proved the beneficial effect of molten salts on the synthesis of diboride powders. However, in the case of TaB 2,it seems that the absence of molten salt has less effect on

3 WEI ET AL. 47 FIGURE 1 XRD patterns of samples synthesized at (A) 800 C, (B) 900 C, and (C) 1000 C with a mass ratio of 10:1 for NaCl/KCl salts and the reactant powders [Color figure can be viewed at the phase composition of the product at 800 C-1000 C. As shown in Figure S2, nearly pure TaB 2 phase was synthesized by the borothermal reduction without the addition of NaCl/KCl salts and only trace of Ta 2 O 5 phase is found in the XRD patterns of the samples. It should be noticed that another resultant of the borothermal reduction is B 2 O 3 with a low melting point of 450 C. When the initial part of reactants begins to react, the B 2 O 3 product melts and serves as a liquid medium to accelerate the rest of the reaction to finish. Base on above discussion, it could say that the reaction process of borothermal reduction is similar that of the molten salt method. As is known to all, the type and the amount of the salt are the most important factors for the powder synthesis by molten salt method. The effects of the salts on each reaction are not the same, which resulted in the different XRD results between TaB 2 and NbB 2 or CrB 2 powders synthesized by the same borothermal reduction. Although pure TaB 2 powders were synthesized by the borothermal reduction without the addition of NaCl/KCl salts, the effects of NaCl/KCl amount were still investigated in the present paper considering their possible benefits on the particle size refinement and morphology control. Figure 2 displays the XRD patterns of the samples synthesized at 1000 C with different amount of NaCl/KCl salts. It is clear that pure TaB 2 phase is obtained only when the mass ratio of NaCl/KCl salts and the reactant powders ranges from 0:1 to 20:1. As the ratio increases to 30:1, the NaCl/KCl salts react with the reactants to form impurities such as Na 2 Ta 4 O 11,K 2 Ta 4 O 11, and Na 2 Ta 8 O 21. The results indicate that the suitable amount of the salts is essential important for the molten salt method to obtain product with a high purity. In our previous report, pure CrB 2 phase could only be synthesized at 1000 C under the mass ratio FIGURE 2 XRD patterns of samples synthesized at 1000 C with a mass ratio of (A) 0:1, (B) 10:1, (C) 20:1, and (D) 30:1 for NaCl/ KCl salts and the reactant powders [Color figure can be viewed at range from 5:1 to 10:1; either too much or too little NaCl/ KCl introduced impurities in the product. 17 Since B 2 O 3 also serves as liquid medium during the process of borothermal reduction, it is possible to tailor the product by adding more B 2 O 3 in the starting powders. However, the XRD patterns indicate that small amount of unreacted Ta 2 O 5 remains in the product when the weight of B 2 O 3 is 5 times that of the reactants (Figure S3B). It is easy to be understood because B 2 O 3 is one of the resultant of reaction (1) and the presence of B 2 O 3 will decelerate the reaction proceeding towards the right. Excess of B (5 wt%) in the starting powders slightly decreases the XRD intensity of Ta 2 O 5 phase but has nearly no effect on the purity improvement of the product due to the unremovable B (Figure S3C). As a result, B 2 O 3 is not a perfect material to tailor the size and morphology of TaB 2 powders. Figure 3 illustrates the morphology of the TaB 2 powders synthesized at 1000 C with different amount of NaCl/ KCl salts. It is seen that all of the TaB 2 particles have the flower-like shape assembled from nanorods. The nanorods in Figure 3B and Figure 3C are more uniform and surface smooth than those in Figure 3A, and the rods in Figure 3C are greater in diameter than those in Figure 3B, indicating that the addition of NaCl/KCl salts obviously improved the crystal growth of TaB 2 nanorods. The introduction of more B 2 O 3 has a similar effect on the morphology of the synthesized TaB 2 powders, as shown in Figure S4. Figure 4A shows a typical TEM image of TaB 2 particles, confirming the existence of the nanorods with diameters of nm. The high-resolution TEM image in Figure 4B clearly exhibits the lattice fringe. The lattice fringe spacing is measured to be 0.32 nm, corresponding to the d-spacing of (001)

4 48 WEI ET AL. FIGURE 4 (A) TEM and (B) HRTEM image of samples synthesized at 1000 C with a mass ratio of 10:1 for NaCl/KCl salts and the reactant powders [Color figure can be viewed at FIGURE 3 SEM images of samples synthesized at 1000 C with a mass ratio of (A) 0:1, (B) 10:1, and (C) 20:1 for NaCl/KCl salts and the reactant powders planes of TaB 2. The above result reveals that TaB 2 particles grow along [001] direction or c-axis. An amorphous film with a thickness of 3-5 nm is found in the surface of crystallized TaB 2 nanorods, which is attributed to the spontaneous oxidation of TaB 2 in air. According to previous reports, there are two mechanisms, namely dissolution-precipitation and template formation mechanism, for the crystal growth in the process of MSS. The specific mechanism is relied on the difference in solubility of the reactants in the molten salts. Although the solubility of Ta 2 O 5 and B in molten B 2 O 3 or NaCl/KCl is unknown, the mechanism in this study might be deduced by comparing the morphology of the reactants with that of the product. For the template formation mechanism, there is a reactant which is much more soluble than another, the more soluble reactant dissolves into the salts and then quickly transports to the surfaces of the less soluble reactant where the reaction performs. As a result, the products retain the morphology of the less soluble reactant. In this study, the morphologies of the synthesized TaB 2 powders are obviously different from the raw Ta 2 O 5 and B powders, as shown in Figure 3 and Figure S1. Therefore, it is reasonable to conclude that the dissolution-precipitation mechanism dominated in the MSS process instead of the template formation mechanism. Guo et al synthesized TaB 2 powders using the same starting materials but no nanorods were found in the product, 22 which could be explained by the different pressures during the borothermal reduction processes. When the reaction was performed under vacuum, the liquid B 2 O 3 had no obvious effect on improving the mass transfer of reactant species due to the weightless environment. In addition, B 2 O 3 (l) has a high vapor pressure and could be easily pumped out in an active vacuum pump.

5 WEI ET AL CONCLUSIONS TaB 2 powders were synthesized by the borothermal reduction in Ta 2 O 5 with amorphous B in flowing argon. The B 2 O 3 product provided a liquid circumstance which enabled the completion of the reaction at a low temperature of 900 C-1000 C. The presence of liquid phase also induced the TaB 2 particle to grow along [001] direction or c-axis into nanorods. The addition of more B 2 O 3 or NaCl/ KCl in the starting powders benefited the crystal growth of the nanorods. ACKNOWLEDGMENTS This work was supported by the Program for the Outstanding Young Talents in Higher Education Institutions of Anhui Province (No. gxyqzd ) and the National Natural Science Foundation of China (No and ). The authors thank Mr. Huifeng Sun and Mr. Fanglv Qiu for their pre-experimental research on powder synthesis. ORCID Qihuang Deng Qing Huang Songlin Ran REFERENCES Fahrenholtz WG, Hilmas GE, Talmy IG, Zaykoski JA. Refractory diborides of zirconium and hafnium. J Am Ceram Soc. 2007;90: Opeka MM, Talmy IG, Zaykoski JA. Oxidation-based materials selection for 2000 C + hypersonic aerosurfaces: theoretical considerations and historical experience. J Mater Sci. 2004;39: Monteverde F, Scatteia L. Resistance to thermal shock and to oxidation of metal diborides sic ceramics for aerospace application. J Am Ceram Soc. 2007;90: Guo S-Q. Densification of ZrB 2 -based composites and their mechanical and physical properties: a review. J Eur Ceram Soc. 2009;29: Opeka MM, Talmy IG, Wuchina EJ, Zaykoski JA, Causey SJ. Mechanical, thermal, and oxidation properties of refractory hafnium and zirconium compounds. J Eur Ceram Soc. 1999;19: Ran S, Van der Biest O, Vleugels J. In situ platelet-toughened TiB 2 SiC composites prepared by reactive pulsed electric current sintering. Scripta Mater. 2011;64: Zou J, Zhang G-J, Kan Y-M. Formation of tough interlocking microstructure in ZrB 2 SiC-based ultrahigh-temperature ceramics by pressureless sintering. J Mater Res. 2009;24: Ran S, Van der Biest O, Vleugels J. ZrB 2 SiC composites prepared by reactive pulsed electric current sintering. J Eur Ceram Soc. 2010;30: Wu W-W, Wang Z, Zhang G-J, Kan Y-M, Wang P-L. ZrB 2 MoSi 2 composites toughened by elongated ZrB 2 grains via reactive hot pressing. Scripta Mater. 2009;61: Ni D-W, Zhang G-J, Kan Y-M, Sakka Y. Highly textured ZrB 2 - based ultrahigh temperature ceramics via strong magnetic field alignment. Scripta Mater. 2009;60: Ni D-W, Zhang G-J, Kan Y-M, Sakka Y. Textured HfB 2 -based ultrahigh-temperature ceramics with anisotropic oxidation behavior. Scripta Mater. 2009;60: Hu C, Zou J, Huang Q, Zhang G, Guo S, Sakka Y. Synthesis of plate-like ZrB 2 grains. J Am Ceram Soc. 2012;95: Zhang Y, Li R, Jiang Y, et al. Morphology evolution of ZrB 2 nanoparticles synthesized by sol-gel method. J Solid State Chem. 2011;184: Yang B, Li J, Zhao B, et al. Synthesis of hexagonal-prism-like ZrB 2 by a sol-gel route. Powder Technol. 2014;256: Ding Z, Deng Q, Shi D, et al. Synthesis of hexagonal columnar ZrB 2 powders through dissolution-recrystallization approach by microwave heating method. J Am Ceram Soc. 2014;97: Liu J, Huang Z, Huo C, et al. Low-temperature rapid synthesis of rod-like ZrB 2 powders by molten-salt and microwave Co-assisted carbothermal reduction. J Am Ceram Soc. 2016;99: Liu Z, Wei YN, Meng X, Wei T, Ran S. Synthesis of CrB 2 powders at 800 C under ambient pressure. Ceram Int 2017;43: Ran S, Sun H, Wei YN, Wang D, Zhou N, Huang Q. Lowtemperature synthesis of nanocrystalline NbB 2 powders by borothermal reduction in molten salt. J Am Ceram Soc 2014;97: Li Z, Lee WE, Zhang S. Low-temperature synthesis of CaZrO 3 powder from molten salts. J Am Ceram Soc. 2007; 90: Zhang S, Khangkhamano M, Zhang H, Yeprem HA. Novel synthesis of ZrB 2 powder via molten-salt-mediated magnesiothermic reduction. J Am Ceram Soc. 2014;97: Bao K, Wen Y, Khangkhamano M, Zhang S. Low-temperature preparation of titanium diboride fine powder via magnesiothermic reduction in molten salt magnesiothermic reduction in molten salt. J Am Ceram Soc. 2017;100: Guo W-M, Zeng L-Y, Su G-K, Li H, Lin H-T, Wu S-H. Synthesis of TaB 2 powders by borothermal reduction. J Am Ceram Soc. 2017;100: SUPPORTING INFORMATION Additional Supporting Information may be found online in the supporting information tab for this article. How to cite this article: Wei T, Liu Z, Ren D, et al. Low temperature synthesis of TaB 2 nanorods by molten-salt assisted borothermal reduction. JAm Ceram Soc. 2018;101: /jace.15231