Publication Date

2017

Document Type

Dissertation/Thesis

First Advisor

Dąbrowski, Bogdan

Degree Name

M.S. (Master of Science)

Department

Department of Physics

LCSH

Physics

Abstract

This thesis discusses results of study of hexagonal manganites of RMnO[sub 3+[delta]] (R= Ho, Y) and Y[sub 1-x]R[sub x]MnO[sub 3+[delta]] (R = Gd and Tb) for potential air-separation applications. Structural and oxygen content changes are presented as a function of synthesis and annealing conditions. Different methods were utilized to synthesize the materials such as the solid-state synthesis, sol-gel synthesis (by collaborators), and high impact mechanical milling. In case of RMnO[sub 3+[delta]] (R= Ho, Y), the stability boundary between stoichiometric Hex0 and oxygen loaded Hex1 and Hex2 phases has been considered through several methods such as in house and in situ X-ray diffraction and thermogravimetry that measure the oxidation and reduction processes at high temperatures in either air or oxygen. Accurate transition temperatures of phase transitions have been derived. Furthermore, changes of oxygen content, either incorporated or released have been deduced for the materials that were utilized, i.e., the oxygen storage capacity (OSC) has been derived. The temperatures of reversible structural transitions between Hex0 and Hex1 have been established to be 250°C and 290°C on cooling and heating respectively at rates of 0.1° per minute. Reducing the grain size by ball milling have only the small effect of improving the speed of the reduction or oxidation processes in case of HoMnO[sub 3+[delta]] and YMnO[sub 3+[delta]] , and the oxygen incorporation into the ball-milled sample was observed as higher oxygen content when comparing with the initial synthesis materials for the same heating or cooling speeds. However, very small grain size YMnO[sub 3+[delta]] synthesized by the sol-gel method showed a significant enhancement of OSC properties that were related to the faster transformation of Hex0 to Hex2 ([delta] [almost equal to] 0.41) in oxygen. The morphology of the powders was found as a crucial parameter for defining the material's OSC. Despite of that oxidizing the YMnO[sub 3+[delta]] compound in air and obtaining single phase Hex1 material was not possible. Synthesis of Y[sub 1-x]R[sub x]MnO[sub 3+[delta]](R=Tb³⁺, Gd³, Ca²⁺, and Mg²⁺ substitution for Mn) was attempted to find materials with increased OSC. During solid-state reaction at elevated temperatures single-phase polycrystalline samples of stoichiometric Y[sub 1-x]R[sub x]MnO[sub 3+[delta]](R=Gd, Tb) have been formed in the hexagonal P6₃cm structure only after firing under reducing conditions in agreement with predictions of tolerance factor rules. The samples were transformed between Hex0, Hex1 and a new phase that we identify as Hex3, which indicates the discovery of yet another highly oxidized phase. The Y/Tb compositions showed some of the highest known OSC despite using heavier Tb because of the much-increased values of interstitial excess oxygen achieved. Remarkably, only Hex0 -- Hex1 -- Hex3 transitions were observed, never detecting Hex2 in both cases of terbium and gadolinium. In the case of samples with Ca²⁺, and Mg²⁺⁺ no synthesis conditions were found for the formation of any Hex phase.

Comments

Advisors: Bogdan Dabrowski.||Committee members: Omar Chmaissem; Yasuo Ito.||Includes bibliographical references.||Includes illustrations.

Extent

ix, 63 pages

Language

eng

Publisher

Northern Illinois University

Rights Statement

In Copyright

Rights Statement 2

NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.

Media Type

Text

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