Molten salt synthesis, formation mechanism, and oxidation behavior of nanocrystalline HfB2 powders

Abstract

Nanocrystalline HfB 2 powders were successfully synthesized by molten salt synthesis technique at 1373 K using B and HfO 2 as precursors within KCl/NaCl molten salts. The results showed that the as-synthesized powders exhibited an irregular polyhedral morphology with the average particle size of 155 nm and possessed a single-crystalline structure. From a fundamental aspect, we demonstrated the molten-salt assisted formation mechanism that the molten salts could accelerate the diffusion rate of the reactants and improve the chemical reaction rate of the reactants in the system to induce the synthesis of the high-purity nanocrystalline powders. Thermogravimetric analysis showed that the oxidation of the as-synthesized HfB 2 powders at 773-1073 K in air was the weight gain process and the corresponding oxidation behavior followed parabolic kinetics governed by the diffusion of oxygen in the oxide layer.

Silicothermic synthesis of phase‐pure HfB2 fine powder

“…In a conventional molten salt assisted synthesis process, the molten salts are usually mixed water soluble halides, and could be easily leached out by using only water from the reacted mass. 19 In this study, the melts involved in the reacted masses were mainly sodium silicates. It is common knowledge that the water solubility of sodium silicates depends on their modulus of SiO 2 / Na 2 O, generally speaking, the higher the modulus of SiO 2 / Na 2 O, the poorer the water solubility.…”

“…In a conventional molten salt assisted synthesis process, the molten salts are usually mixed water soluble halides, and could be easily leached out by using only water from the reacted mass 19 . In this study, the melts involved in the reacted masses were mainly sodium silicates.…”

“…Liu et al 19 concluded in their work on molten salt assisted synthesis of HfB 2 that the molten salts could accelerate the diffusion rate of the reactants and improve the chemical reaction rate of the reactants in the system to induce the synthesis of the high‐purity HfB 2 nanocrystalline powders. Obviously, in that case (conventional molten salt assisted route), the KCl/NaCl molten salts only facilitated the mass transfer as reaction media, and did not participate in the reaction process.…”

“…1−5 Potential applications of HfB 2 include hypersonic flight and atmospheric re-entry, rocket propulsion systems, cutting tools, wear resistance coatings, molten metal crucibles, plasma arc electrode and neutron absorber in nuclear reactors. 6−11 A variety of investigations on synthesis of HfB 2 have been reported, such as direct reaction between elemental boron and hafnium, 12 self-propagating synthesis, 13−16 boron thermal reduction, 17−19 carbo/borothermal reduction, 20 solgel assistant method, 21 sodium borohydride reduction 22,23 and molten salt-assisted synthesis, 19 among other reactions. However, the above reported techniques for preparing HfB 2 powders suffer from several drawbacks, such as high synthesis temperatures, expensive reaction precursors, flammable and explosive, and so on.…”

Effects of flake-shape and content of nano-mullite on mechanical properties and fracture process of corundum composite ceramics

“…One of the important ways to enhance the value of coal gangue is to use it as a raw material for the preparation of ceramic powders. In practice, both mullite and cordierite have been successfully prepared by using molten salt method [4][5][6][7], sol-gel method [8,9]and solid-phase method [10][11][12][13][14][15], with gangue as a raw material. In addition, these ceramic materials are used in various aspects, such as catalytic degradation [16,17], thermal insulation [18][19][20][21], and many others [22][23][24].…”