High-aspect-ratio single-crystalline (HfxZr(1−x))B2 micron-rods: low-temperature, highly-efficient synthesis and oriented growth mechanism

Abstract

By taking advantage of a microwave and molten-salt co-assisted boro/carbothermal reduction method (MSM-BCTR) and cheap raw materials such as HfO₂/ZrO₂, B₄C and amorphous carbon, three types of (Hf_xZr_(1−x))B₂ (x = 0.25, 0.50 and 0.75) solid-solution powders with a single-crystalline rod-like morphology, high compositional uniformity, long lengths of 27.8–41.6 μm and high aspect ratios of 10.1–17.2 were successfully synthesized for the first time. And the required temperature conditions of 1100 °C for 20 min were significantly milder than that required by the molten-salt-assisted reduction method even using strong but high-price elemental boron as a reducing agent. These results demonstrated that the specific MSM-BCTR conditions characterized by molten-salt medium and microwave heating were capable of greatly enhancing not only the kinetics but also the thermodynamic favorabilities of the reactions for synthesizing HfB₂ and ZrB₂ as well as their solid solutions ((Hf_xZr_(1−x))B₂). More interestingly, the investigation on the crystalline growth mechanism of (Hf_xZr_1−x)B₂ indicated that under the MSM-BCTR conditions, residual amorphous carbon played an important role in enhancing the oriented growth of (Hf_xZr_(1−x))B₂, by preferential adsorption onto their {001} crystal planes and therefore greatly promoting corresponding specific surface energies. This study was expected to provide a feasible way of highly-efficiently and cost-effectively synthesizing ultra-high temperature ceramic powders with a highly-crystalline structure and uniform anisotropic morphology, which were favorable for promoting the oxidation resistance and toughness of their sintered products.