Single-Step Synthesis Process for High-Entropy Transition Metal Boride Powders Using Microwave Plasma

Abstract

A novel approach is demonstrated for the synthesis of the high entropy transition metal boride (Ta, Mo, Hf, Zr, Ti)B2 using a single heating step enabled by microwave-induced plasma. The argon-rich plasma allows rapid boro-carbothermal reduction of a consolidated powder mixture containing the five metal oxides, blended with graphite and boron carbide (B4C) as reducing agents. For plasma exposure as low as 1800 °C for 1 h, a single-phase hexagonal AlB2-type structure forms, with an average particle size of 165 nm and with uniform distribution of the five metal cations in the microstructure. In contrast to primarily convection-based (e.g., vacuum furnace) methods that typically require a thermal reduction step followed by conversion to the single high-entropy phase at elevated temperature, the microwave approach enables rapid heating rates and reduced processing time in a single heating step. The high-entropy phase purity improves significantly with the increasing of the ball milling time of the oxide precursors from two to eight hours. However, further improvement in phase purity was not observed as a result of increasing the microwave processing temperature from 1800 to 2000 °C (for fixed ball milling time). The benefits of microwave plasma heating, in terms of allowing the combination of boro-carbothermal reduction and high entropy single-phase formation in a single heating step, are expected to accelerate progress in the field of high entropy ceramic materials.

High Entropy Borides Synthesized by the Thermal Reduction of Metal Oxides in a Microwave Plasma

“…It was found that the small addition of H 2 (in addition to argon) helped to stabilize and concentrate the sample near the sample. A microwave power of 1.3–1.5 kW and chamber pressure of 2.4 × 10 4 –3.3 × 10 4 Pa was used to rapidly heat up the pellet (70–100 °C/min in this system) to 2000 °C, as also described in our previous works [ 15 , 16 ]. The precursor pellets were MW-plasma annealed using either BCTR or BTR at 2000 °C for 1 h.…”

Diborides of Multielement Transition Metals: Methods for Calculating Physical and Mechanical Characteristics

“…One of the possible ways to calculate the coefficient of thermal expansion is the "quasiharmonic approximation" method [14], based on determining the dependence of the total energy of the system on the parameters of the crystal lattice at different temperatures.…”

“…The force constant is determined using the pseudopotential method through the second derivative of the interatomic interaction energy with respect to the spatial variable. A model has been developed to identify the dependence of the vibration frequency on the volume of the unit cell at different temperatures [14][15][16][17].…”

Effects of Replacing Co2+ with Zn2+ on the Dielectric Properties of Ba [Zn1/3(Nb1/2Ta1/2)2/3]O3 Ceramics with High Dielectric Constant and High Quality Factor

“…For various microwave system requirements, in addition to the high quality of materials, it is also important to consider the price. Therefore, considering that the dielectric loss is extremely low, new low-cost dielectric materials with appropriate dielectric constants are needed [10][11][12][13][14][15][16].…”