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

Bria Storr,Shane A Catledge,Vijaya Rangari,Deepa Kodali,Paul A Baker,Kallol Chakrabarty

doi:10.3390/ceramics4020020

Bria Storr, Shane A Catledge + Show 4 more

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https://doi.org/10.3390/ceramics4020020

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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.

Highlights

The recent spotlight on high entropy alloys is driven by outstanding properties such as enhanced fracture toughness, tensile strength, as well as corrosion and high temperature oxidation resistance [1]
In 2016, the fabrication of single-phase high-entropy borides (HEBs) in the hexagonal AlB2 structure was first reported [2], representing the first non-oxide high-entropy ceramic fabricated in bulk form
Pressure consolidated disk samples of the precursor powder mixture were exposed to the MW plasma to achieve disk temperatures of 2000 ◦ C and 1800 ◦ C for 1 h; the resulting XRD patterns are shown in Figure 1a,b, respectively

Summary

Introduction

The recent spotlight on high entropy alloys is driven by outstanding properties such as enhanced fracture toughness, tensile strength, as well as corrosion and high temperature oxidation resistance [1]. In 2016, the fabrication of single-phase high-entropy borides (HEBs) in the hexagonal AlB2 structure was first reported [2], representing the first non-oxide high-entropy ceramic fabricated in bulk form. They have a uniquely layered hexagonal crystal structure with alternating rigid 2D boron nets and high-entropy 2D layers of metal cations characterized by mixed ionic and covalent metal–boron bonds. Because of the potential for entropy stabilization in ceramics, high entropy ultra-high temperature ceramics (e.g., borides and carbides) are being considered as candidates for materials in extreme conditions, such as is needed in hypersonic or atmospheric re-entry vehicles, rocket propulsion, etc. Because of the potential for entropy stabilization in ceramics, high entropy ultra-high temperature ceramics (e.g., borides and carbides) are being considered as candidates for materials in extreme conditions, such as is needed in hypersonic or atmospheric re-entry vehicles, rocket propulsion, etc. [5]

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