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

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 (HfxZr(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 ((HfxZr(1−x))B2). More interestingly, the investigation on the crystalline growth mechanism of (HfxZr1−x)B2 indicated that under the MSM-BCTR conditions, residual amorphous carbon played an important role in enhancing the oriented growth of (HfxZr(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.