Chemical selection rules of single-phase high-entropy oxides

Tseng, Kuo-Pin

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https://hdl.handle.net/2142/108148 Copy

Description

Title

Chemical selection rules of single-phase high-entropy oxides

Author(s)

Tseng, Kuo-Pin

Issue Date

2020-05-05

Director of Research (if dissertation) or Advisor (if thesis)

Kriven, Waltraud M

Doctoral Committee Chair(s)

Kriven, Waltraud M

Committee Member(s)

• Zuo, Jian-Min
• Shoemaker, Daniel P
• Maaß, Robert

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• High entropy oxides
• Phase transformations
• Lanthanide oxides
• Powder X-ray diffraction
• High temperature
• Size mismatch
• Valence states
• Thermal expansion

Abstract

High-entropy oxides, as a novel research field in ceramics, have been found to present state-of-the-art improvement in various properties. These contributions could be achieved by multiple cations homogeneously occupying the same polyhedral sites, introducing severe lattice distortion throughout a structure. However, the mechanism of chemical selection rules for designing new high-entropy oxides was still unclear. Randomly mixed, multi-components usually form composites instead of a single-phase, solid solution. In this research, twenty high-entropy lanthanide candidates were synthesized and examined to explore the function of two potential parameters: (1) cation size mismatch, and (2) preferred valence states. The oxide candidates were synthesized by the polymeric steric entrapment method to ensure homogeneous mixing among the cations. The evolution of phase transformation and structural stability from room temperature up to ~2000C were examined in a quadrupole lamp furnace and conical nozzle levitator at synchrotron X-ray facilities. The thermal expansion behaviors of single-phase, high-entropy, lanthanide oxides were measured. Cation size mismatch and preferred valence configurations have significant influences on the formation of high-entropy oxides. In most of circumstances, mixing cations with excess threshold in size mismatch (δ > 7) caused the formation of secondary phase(s), leading to failure in forming stable, single-phase, high-entropy oxides. By choosing cations with different preferences in valence configurations, the final structure could be constructed for a prototype with a similar combination of oxidation states. Furthermore, merging cations with different valence states could trigger phase transformations/separations during heat treatments. However, in high-entropy oxides, the contribution from configurational mixing entropy was thought to be negligible. Understanding the function of cation size mismatch and preferred valence configurations can benefit the ceramic community in the future when designing high-entropy oxides.

Graduation Semester

2020-05

Type of Resource

Thesis

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http://hdl.handle.net/2142/108148

Copyright and License Information

Copyright 2020 Kuo-Pin Tseng

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