4.7 Article

Entropy stabilized single-phase (Hf,Nb,Ta,Ti,Zr)B2 solid solution powders obtained via carboiboro-thermal reduction

Journal

JOURNAL OF ALLOYS AND COMPOUNDS
Volume 824, Issue -, Pages -

Publisher

ELSEVIER SCIENCE SA
DOI: 10.1016/j.jallcom.2020.153930

Keywords

Ultra-high temperature ceramics; Solid solution; Carbo/boro-thermal reduction; Micro-strain

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This report deals with the synthesis of entropy stabilized (ES) single-phase (Hf,Nb,Ta,Ti,Zr)B-2 powders, with specific surface area of about 1.6 m(2)/g, in a AIB(2)-type structure using individual transition metal (TM) oxides and, for the first time, elemental boron and carbon as precursors to feed the carbo/borothermal (CBT) reduction. Elemental B and C were intimately mixed into a mixture of five TM oxide powders, TM = Hf, Nb, Ta, Ti and Zr through an high energy planetary milling. ES single-phase TM diboride solid solution powders were obtained by a synthesis process consisting of a CBT reduction followed by solid solution formation. A B:C molar ratio = 1.27 (per 1 M mass of metals) was adjusted leading the CBT reduction to completion (i.e., full conversion of TM oxides), applying the synthesis temperature of 2123 K under vacuum. The micro-strain intended as deviation of some inter-planar distances of the entropy stabilized AIB(2)-type solid solution lattice, was analyzed by x-ray diffraction: a strong anisotropic micro-strain was found, and was attributed to the compositional disorder due to the coexistence of differing TM with different atomic radius. (C) 2020 Elsevier B.V. All rights reserved.

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