Access Type

Dissertation/Thesis

Date of Award

January 2019

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Stephanie L. Brock

Abstract

BINARY AND TERNARY TRANSITION METAL PNICTIDES NANOPARTICLES AND THEIR THREE DIMENSIONAL ASSEMBLIES: TOWARDS PROMISING MAGNETIC REFRIGERATION MATERIALS

by

MALSHA ANURADHI HETTIARACHCHI

May 2019

Advisor: Dr. Stephanie L. Brock

Major : Chemistry

Degree : Doctor of Philosophy

Bulk MnAs has been recognized as a potential MR material with large MCE. However, the magnetic transition of bulk MnAs suffers from a large thermal hysteresis of 6 K precluding efficient MR cycling, and the magnetic entropy change associated with the phase transition is limited to a narrow temperature range, making the temperature control window very small. Solid solutions of MnAs, synthesized by both cation and anion doping, are reported to reduce the hysteresis, enabling tuning of the optimal temperature range. Reducing the materials’ dimensions to the nano scale has the potential to enable formation of nano 3-dimensional graded macrostructures thereby greatly expanding the temperature window.

This dissertation research was focused on: (1) synthesis, characterization, and magnetic property evaluation of MnAsxSb1-x (x=0.1-0.9) nanoparticles, (2) three-dimensional assembly of discrete nanoparticles of well-established system Fe1.2Ni0.8P. As a sub-goal of objective (1), MnSb nanoparticle synthesis was carried out employing two strategies in solution-phase.

The slow heating approach yielded phase-pure, spherical shape MnSb nanoparticles ca. 13 nm in diameter with an amorphous manganese oxide shell around the MnSb core, which suppresses the saturation magnetization up to 0.04 BM/mol Mn. The NaBH4 addition method produced elongated head-tail MnSb nanoparticles with increased saturation magnetization that was twenty times higher than the former case. MnAsxSb1-x nanoparticles of over all x were synthesized by slight modification of the protocol developed for MnSb nanoparticle synthesis. The target compositions of x=0.1-0.9 appeared phase-pure in PXRD, but almost all the compositions were As deficient as revealed by elemental compositional analysis by XRF.

In order to achieve objective (2), at the outset, compositions of Fe1.2Ni0.8P discrete nanoparticles were selected. These nanoparticles were assembled into three-dimensional networks by oxidation-induced gelation, after functionalized by 11-MUA or 1-DDT. Discrete nanoparticle compositions of Fe1.2Ni0.8P were capable of forming solid, black monoliths after supercritical drying. All the assembled compositions were capable of upholding the initial crystallinity and morphology even after the gelation, and most importantly, most of these composites exhibit no significant alternation in the crystallinity and the morphology after the post-heat treatment. Magnetic properties of these composites were evaluated as a function of different heating conditions.

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