RADIUS

Random distribution vs local ordering in entropy stabilized ultra-high temperature layered ceramics

Principal Investigator: Frederic Monteverde

Involved personnel: Federico Saraga, Alex Sangiorgi

Starting date: 01/07/2020
Duration: 6 months
Call: Proposal nr. 20200101 – beamline EXAFS at Elettra – Sincrotrone Trieste S.C.p.A
Project Coordinator: Frederic Monteverde
Consortium: CNR-ISTEC, Materials Science and Engineering Department Missouri Science and Technology University, Elettra – Sincrotrone Trieste S.C.p.A

Entropy stabilized (ES) ceramics, while still in their embryonic development, have stimulated major interests in the scientific community since the first report of a bulk ES oxide, and are growing as the ceramic counterparts to the more established high-entropy alloys. Since then, studies have been thriving exponentially around this topic, with numerous efforts pushed toward the synthesis and densification of various families of ES ceramics, excited by the vast opportunity to explore a broadened new compositional space and their properties. Among them, ES diborides are most interesting due to their mixed covalent, metallic, and ionic bonds, with a layered hexagonal crystal structure consisting of a 2D high entropy mixing of metal/cation atoms separated by rigid covalently-bonded 2D boron layers. Dozens of ES-UHTCs in the diborides class have been synthesized but none of the reports to date have examined the local order, both structural and configurational, that may exist on the smallest crystalline length scale. In fact, different local distortions and ordering, depending on the TM, are expected, due to small variations that become averaged over longer sampling ranges typical of XRPD. A deeper understanding of the chemical/structural disorder will enable to a better insight of the properties, ultimately leading to the possible engineering of performance. Studies of the local environments of TMs in these layered diborides are limited to providing information about the average structure. X-ray absorption is better suited to collect information on a wide range of metals and in a single experiment with chemical selectivity. The EXAFS refinement will give us insight into the nearest neighbors in the crystal structure of the 3d metals when mixed in different proportions and chemical compositions.

Publications & Patents:

• F.Monteverde, F. Saraga, M. Gaboardi, Compositional disorder and sintering of entropy stabilized (Hf,Nb,Ta,Ti,Zr)B2 solid solution powders, https://doi.org/10.1016/j.jeurceramsoc.2020.04.026
• F.Monteverde, F. Saraga, Entropy stabilized single-phase (Hf,Nb,Ta,Ti,Zr)B2 solid solution powders obtained via carbo/boro-thermal reduction, https://doi.org/10.1016/j.jallcom.2020.153930