Document Type

Dissertation

Degree

Doctor of Philosophy (PhD)

Department

Materials Science and Engineering

First Advisor's Name

Benjamin Boesl

First Advisor's Committee Title

Committee Chair

Second Advisor's Name

W. Kinzy Jones

Second Advisor's Committee Title

Committee Co-Chair

Third Advisor's Name

Surendra K. Saxena

Third Advisor's Committee Title

committee member

Fourth Advisor's Name

Jin He

Fourth Advisor's Committee Title

committee member

Fifth Advisor's Name

Gerard Bourne

Fifth Advisor's Committee Title

committee member

Keywords

In Situ Electron Microscopy, Focused Ion Beam, Magnetocaloric Materials, Twin Boundary

Date of Defense

7-8-2016

Abstract

The objective of this research was to develop a novel technique for mechanical treatment to manipulate the microstructure of Nickel-Manganese-Gallium Hesuler alloys to increase anisotropy, which can lead to higher magnetocaloric properties. Ni2+xMn1-xGa intermetallics have the potential to be employed in magnetic refrigeration devices including residential refrigerators, heat pumps, and air conditioning. Solid-state magnetic refrigeration systems are smaller, quieter, and reduce energy consumption by 20% compared to existing conventional vapor-cycle refrigeration devices which rely on harmful hydro-fluorocarbon gases and pump millions of tons of greenhouse gases into the atmosphere. The magnetic refrigeration market is predicted to reach US$ 315.7 Million by 2022.

Magnetic refrigeration systems can also be used in electronic systems and the space industry. The current state-of-the-art magnetic refrigeration systems use expensive rare earth elements including Gadolinuim (Gd). The need to replace Gd and other rare earth elements with cheaper and more available elements led to other alloys including Ni-Mn-Ga. By understanding the processing-microstructure-property relationship of Ni-Mn-Ga alloy, it is possible to manipulate the microstructure in order to obtain higher refrigeration capacity. It is a promising alternative to rare earth elements and improves national security by minimizing foreign dependence on the import of rare earth metals.

This novel in situ study establishes that twin boundaries can be manipulated in a polycrystalline Ni-Mn-Ga alloy. This results in a change in magnetocrsytalline anisotropy, which leads to a higher magnetic cooling power. Mechanical loading in a preferred direction, traditionally referred to as a training process, was able to move the twin boundaries, and the combination of focused ion beam imaging linked specific movement with mechanical loading. This technique, in situ monitoring process, can be utilized to devise training procedures for future iterations of magnetocaloric and shape memory alloys.


Identifier

FIDC000753

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