Low-Temperature, Efficient Synthesis of Highly Crystalline Urchin-like Tantalum Diboride Nanoflowers

Urchin-like tantalum diboride (TaB2) nanoflowers were successfully synthesized via a high-efficiency and energy-saving methodology, molten-salt and microwave co-modified boro/carbothermal reduction, using less expensive B4C as a reducing agent. By taking advantage of the synergistic effects of the molten-salt medium and microwave heating conditions, the onset formation temperature of TaB2 was drastically reduced to below 1000 °C, and phase-pure powders of TaB2 nanoflowers were obtained at temperatures as low as 1200 °C within only 20 min. Notably, the present temperature conditions were remarkably milder than those (>1500 °C for several hours) required by conventional reduction methods, which use the strong, but expensive, reducing agent, elemental boron. The resulting urchin-like TaB2 nanoflowers consisted of numerous uniform single-crystalline nanowires with lengths up to 4.16 μm, and high aspect ratios >10. This result indicated that the as-synthesized urchin-like TaB2 nanoflowers possessed high specific surface area and anisotropic morphology, which were favorable not only for sintering, but also for toughening their bulk counterparts.


Introduction
As one of the most important ultra-high-temperature ceramics (UHTCs) for potential applications at >2000 • C, tantalum diboride (TaB 2 ) has attracted extensive interest in recent decades due to its rapidly rising demand for high-end structural applications in harsh environments, such as the propulsion systems in new-generation space vehicles, combustion chambers of rockets, and the leading edge of reentry hypersonic missiles [1][2][3]. Under such extreme operating conditions, the frequent occurrence of sharp heating-cooling cycles results in high thermal stress in TaB 2 -based protective materials, greatly challenging their thermal shock resistance as well as the relevant main factor, fracture toughness [4,5]. Thus, it is of significant importance to prepare monolithic TaB 2 with sufficient fracture toughness.
To overcome the poor toughness of many ceramics, the introduction of a secondary reinforcement phase with an anisotropic (e.g., rod-or plate-like) morphology into the ceramic matrix has been demonstrated to be effective [6,7]. Nevertheless, due to the ultrahigh service temperature, the number of the second-phase candidates applicable to TaB 2based materials was extremely limited. Considering this, "self-toughening" was regarded as the only feasible way to enhance the fracture toughness of TaB 2 -based materials [8]. Thus, their powder counterparts, consisting of highly crystalline anisotropy-shaped TaB 2 particles, were perceived to be desirable for preparation of high-performance TaB 2 -based materials [9][10][11][12].
Unfortunately, with the boro/carbothermal reduction methodology conventionally utilized for the industrial preparation of TaB 2 and other raw powders of UHTCs as well as TaB 2 , it was still a challenging task to endow the powder product with anisotropic morphology and the resulting potential for toughness enhancement [13,14]. Additionally, the preparation of TaB 2 powders via conventional thermal reduction commonly necessitates severe temperature conditions, including high firing temperature (>1500 • C) and long dwelling time (dozens of hours), which not only dramatically increases corresponding energy and time consumptions, but also seriously deteriorates the sinterability of the powder product. For instance, Guo et al. synthesized TaB 2 with amorphous morphology using the conventional borothermal reduction (BTR) at temperatures as high as 1550 • C for one hour, by using highly expensive elemental boron as both B source and reducing agent [13]. Zhang et al. successfully prepared TaB 2 powders by heating the raw powders of Ta 2 O 5 , B 4 C and graphite at 1600 • C, albeit both needle-like and rounded particles coexisted in the final product [15]. In summation, the current thermal-reduction-based methodology for mass preparation of TaB 2 powders suffers from the drawbacks of high preparation cost and poor quality of products, which are mainly attributable to the intrinsically low diffusion coefficient of TaB 2 and the resulted low rates of both synthetic reaction and crystal growth [16].
In this view, the prospect of introducing a liquid phase was deemed promising for improving the diffusion rate of TaB 2 , and therefore enhancing its synthesis and epitaxial crystal growth. For instance, Ren et al. exploited low-melting-point Ta-B-C-O precursors to supply the boro/carbothermal reduction reaction for synthesizing TaB 2 with liquid medium. However, no formation of anisotropic TaB 2 was not reported despite the requirement of a high processing temperature of 1500 • C was still demanded, which was considered to be mainly due to the decomposition loss of the low-melting-point organic-based medium, before the temperature desirable for crystal growth of TaB 2 was attained [17]. As a result, inorganic salt with a relatively higher melting point was regarded as a promising medium for solid-state synthetic reaction of high-temperature materials, including TaB 2 . For instance, Ran et al. utilized the molten-salt-assisted BTR method to prepare rod-like TaB 2 powders after a one-hour dwelling period at a reduced temperature of 1000 • C, albeit the requirement of a large amount of boron powders resulted in high material costs [18].
In addition, microwaves have also been verified to be capable of effectively accelerating not only synthetic reactions, but also crystal growth of high-temperature materials. Moreover, it was attainable for the microwave field to cooperate with the molten-salt field, therefore jointly enhancing the thermodynamic favorability of synthetic reactions and facilitating self-assembly of anisotropic-shaped crystals [19,20]. Accordingly, in our previous research, microwaves were utilized to further modify the molten-salt-assisted BCTR method to prepare ZrB 2 and ZrB 2 -SiC powders with anisotropic morphologies under much milder conditions [21,22].
Consequently, in this work, molten-salt and microwave comodified boro/carbothermal reduction was developed for synthesizing TaB 2 , using Ta 2 O 5 and B 4 C as raw materials. The effects of various important parameters on the synthesis of TaB 2 were elaborately regulated to save energy and efficiently prepare single-phase TaB 2 powders with high specific surface area and anisotropic morphology.

Raw Materials
The raw materials of tantalum oxide (Ta 2 O 5 ; purity > 98.5%; average particle size of 0.39 µm; Shanghai Aladdin Bio-Chem Technology Co., Ltd., Tianjin, China) and boron carbide (B 4 C; purity > 98.0%; average particle size of 3.74 µm; Mudanjiang Jingangzuan Boron-Carbide Co., Ltd., Mudanjiang, China) were used as received for synthesis of TaB 2 . Salts of sodium chloride and potassium chloride (NaCl and KCl; purities > 99.9%; Bodi Chem. Co., Ltd., Tianjin, China) were used as a reacting medium. All above reagents were directly used without further purification or modification.
For compensating for the volatilization loss of boron species during the synthetic reaction of TaB 2 , certain excess amounts of B 4 C were added into the raw material powders (according to reaction (1)). For the reacting medium, the molar ratio between NaCl and KCl was fixed at 1:1. The batch compositions and processing parameters for the molten-salt and microwave-assisted boro/carbothermal reduction (MSM-BCTR) and the conventional BCTR are listed in Table 1. The corresponding products are labeled as MSMBC-(1-7).

Experiment Procedure
Similar to a typical MSM-BCTR process for synthesizing TaB 2 , the reactants, consisting of Ta 2 O 5 /B 4 C and the salt mixture of NaCl/KCl in a weight ratio of 1:2, were initially mixed in a corundum crucible. Then, the crucible, contained by a SiC saggar, was positioned in the heating area of a microwave furnace (HAMiLab-V3000; 3 kW; 2.45 GHz; Changsha Longtech Co., Ltd., Changsha, China). The space between the saggar and crucible was filled with green silicon carbide powders to facilitate the microwave heating of the raw material powders, and the operating temperature was monitored in real-time by an infrared thermometer. Afterwards, in a flowing argon atmosphere, the raw material powders were heated at a constant rate of 10 • C/min to the preset temperature range of 1000-1200 • C, then held for 0-20 min before cooling naturally to the ambient temperature. The resultant powders were ground and repeatedly rinsed with deionized water to completely remove the residual salt, followed by overnight drying in a vacuum oven at 80 • C, before undergoing the following characterization and testing.

Characterization and Testing
Phase compositions of the powder products were examined by X-ray diffraction (XRD, Empyrean, PANalytical, Almelo, The Netherlands) with CuKα radiation (λ = 0.1542 nm). The morphology and microstructure of as-obtained TaB 2 powders were characterized using a fieldemission scanning electron microscope (FE-SEM, Nova400NanoSEM, Philips, Amsterdam, The Netherlands) equipped with an energy dispersive spectrometer (EDS) and transmission electron microscope (TEM, JEM-2100UHRSTEM, JEOL, Tokyo, Japan), and their mean aspect ratio was estimated based on the measurements of at least 200 well-grown rod-like particles.

Results
As confirmed by our previous researchers [21,22], the present MSM-BCTR method was capable of greatly accelerating the BCTR reaction for synthesizing boron-based UHTCs at reduced temperatures. Therefore, the syntheses of TaB 2 were performed at a relatively low temperature range of 1000-1200 • C, for an identical dwelling time of 20 min, with batch powder composition n(B 4 C)/n(Ta 2 O 5 ) = 2.9, and weight ratio between salts and reactants of m s /m r = 2.0. Specifically, as the firing temperature reached 1000 • C (Figure 1), although there existed a lot of un-reacted Ta 2 O 5 and B 4 C, as well as the intermediate product of TaO 2 (produced by partial reduction of Ta 2 O 5 ), in the final product (MSMBC-1), a minor amount of TaB 2 could also be identified. This result indicated that the expected formation reaction of TaB 2 (reaction (1)) was initiated at a temperature lower than 1000 • C, which was dramatically lower than that (up to 1550 • C) required by the conventional BCTR method using an identical type of reducing agent [9]. Subsequently, on increasing the temperature to 1100 • C (MSMBC-2), the intensities of the diffraction peaks belonging to TaB 2 increased, while those of the residual reactants and intermediate product accordingly decreased, revealing the rising temperature had a positive effect on enhancing the synthesis of TaB 2 . More interestingly, upon further increasing temperature to 1200 • C, all observable diffraction peaks of the product (MSMBC-3) were indexed to TaB 2 , demonstrating that phase-pure TaB 2 powders were successfully obtained. This result verified that, although the temperature was as low as 1200 • C, the synthetic reaction of TaB 2 was still highly efficient, remarkably superior to that (at least one hour) in the conventional BCTR using the stronger reducing agent, boron [18]. Moreover, the additional amount of reducing agent was deemed another key factor that greatly influenced the synthetic result of TaB 2 , because insufficient dosages of reducing agent would have impeded the expected completion of the synthetic reaction, while excessive dosages would have induced carbon residue in the final product, seriously and irreversibly deteriorating its purity and sinterability. Given this, the following experiments were performed using different addition amounts of B 4 C in the original powder batches. On one hand, as shown in Figure 2, in sharp contrast with MSMBC-3 (composed of single-phase TaB 2 ), the sample (MSMBC-4) prepared under identical processing conditions, but with a slightly less amount of B 4 C (with the ratio of n(Ta 2 O 5 )/n(B 4 C) decreased from 2.9 to 2.8), contained a trace amount of TaO 2 but no B 4 C or C, indicating the expected synthetic reaction of TaB 2 failed to reach completion due to the insufficient amount of reducing agent. On the other hand, upon increasing the ratio of n(Ta 2 O 5 )/n(B 4 C) to 3.0, there existed no oxygen-containing impurity but a minor amount of B 4 C remained in the product (MSMBC-5), implying the present addition amount of reducing agent was excessive. So, it was reasonable to conclude that, under the present MSM-BCTR conditions, the optimal molar ratio between Ta 2 O 5 and B 4 C was approximately 2.9, which was favorable for the completion of the expected synthetic reaction, while minimizing the content of carbon-containing residue in the final TaB 2 product powders. Such a prominent achievement of low-temperature and high efficiency preparation of single-phase TaB 2 powders was regarded to be closely related to the specific MSM-BCTR conditions, characterized by microwave heating and salt medium. For the purpose of clarifying their independent impacts on the synthesis of TaB 2 , the following experiments were conducted under the optimized processing conditions, without either microwave heating or molten-salt medium, and then compared. On one hand, in the absence of salt medium (corresponding XRD patterns were presented by Figure 3), there existed a small amount of Ta 2 O 5 in addition to TaB 2 in the product (MSMBC-6), revealing the molten-salt medium had an obvious positive effect on accelerating the synthetic reaction of TaB 2 . On the other hand, in the case of conventional heating (without microwave), multiple types of oxygen-containing impurities (including Ta 2 O 5 , TaO 2 and TaO), as well as unreacted B 4 C, remained in the powder product (MSMBC-7). It was worth noting that TaC, as a common byproduct of TaB 2 synthesized by the conventional BCTR method, was not traced in this sample, although the corresponding synthetic reaction (reaction (2)) remained thermodynamically possible. Such seemingly contradicting results could be explained by the fact that, although reaction (2) prevailed to form TaC, it would be subsequently converted into TaB 2 via reaction (3), which was concurrently enhanced by the molten-salt medium. Therefore, it is reasonable to consider that the combined effects of molten-salt medium and microwave heating had significantly combined effects on the synthesis of TaB 2 , including (1) accelerating the targeted reaction by promoting the diffusion coefficients of reacting species and (2) enhancing the thermodynamic favorabilities of the reactions for synthesizing TaB 2 as well as converting intermediate TaC into TaB 2 .
Microstructure and morphology of as-prepared phase pure TaB 2 powders (MSMBC-3) were characterized. According to the FE-SEM observation (presented in Figure 4), similar to the TaB 2 synthesized by Shah et al. [23], the particles in this sample exhibited the uniform morphology of urchin-like nanoflowers, which consist of numerous loosely agglomerated nanowires with average diameters and lengths of 410 nm and 4.16 µm, respectively, and a high aspect ratio (>10). Furthermore, the results of EDS-mapping analysis evidence that elements of Ta and B in an atomic ratio close to 2:1, were homogeneously distributed over the as-observed urchin-like nanoflowers, verifying that they were TaB 2 and the as-synthesized TaB 2 powders resulting from the MSM-BCTR were of high purity. For further revealing the microstructure of as-observed rod-like particles, TEM examination was also conducted. The HRTEM image (Figure 5b, taken in the area defined by a black-dotted circle in Figure 5a) of a well-defined nanowire in an urchin-like nanoflower, shows that a highly arranged atomic lattice and constant interplanar spacings, along the mutually perpendicular directions, of 0.324 nm and 0.265 nm, which matched well with the interplanar spacings of (100) and (001) crystal planes of TaB 2 crystal, respectively. In addition, the SAED pattern (Figure 5c) further verifies that this representative nanowire has a single-crystalline structure, and its anisotropic morphology is formed by epitaxial growth along the [010] direction of the hexagon-system TaB 2 crystal. Therefore, it is reasonable to conclude that the TaB 2 powders resulting from the MSM-BCTR method generally had a low extent of aggregation, large specific surface area, single-crystalline nature, and highly oriented anisotropic morphology, which were very favorable for enhancing not only sinterability but also the integral structural performance represented by fracture toughness of their bulk counterparts [24][25][26]. The achievement of low-temperature, highly efficiency preparation of such high-quality TaB 2 powders can be mainly attributed to the combined effects of molten-salt medium and microwave heating.
Above all, compared to the conventional thermal reduction method, the MSM-BCTR method possessed versatile significant merits in the preparation of TaB 2 and other raw material powder for UHTCs as well as TaB 2 , including the low material cost, ultra-high efficiency, low energy consumption, and the potential in optimizing sinterability as well as the overall mechanical properties (especially fracture toughness) of their bulk counterparts, which may shed substantial light on further development of the methodology for industrial production of UHTCs powders.

Conclusions
By taking advantage of the highly efficient and energy-saving approach of molten-salt and microwave comodified boro/carbothermal reduction, phase-pure TaB 2 powders were successfully prepared by firing the less expensive raw powders of Ta 2 O 5 /B 4 C at a reduced temperature of 1200 • C for only 20 min, which were not only remarkably milder than those (firing temperature > 1250 • C and dwell time of dozens of hours) required by the conventional thermal reduction method using the same type of reducing agent, but also much more efficient (at least several hours) even than the methods using a vast amount of strong, but expensive, reducing agent of elemental boron. Moreover, the as-obtained TaB 2 powders exhibited a single-crystalline structure, well-grown morphology of an urchin-like nanoflower, large specific surface area, low agglomeration extent of nanowires with high aspect ratio of >10, and great potential in improving sinterability and toughening their bulk counterparts. Such promising results were mainly attributable to the synergistic effects of molten-salt medium and microwave heating. Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: For anyone who is interested with the data of this article, please contact with corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.