Graduation Year

2016

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Mahn-Huong Phan, Ph.D.

Committee Member

Hariharan Srikanth, Ph.D.

Committee Member

Denis Karaiskaj, Ph.D.

Committee Member

Jiangfeng Zhou, Ph.D.

Committee Member

Andreas Muller, Ph.D.

Keywords

films, gadolinium, magnetocaloric, microwires, refrigeration

Abstract

Magnetic refrigeration based on the magnetocaloric effect (MCE) is a promising alternative to conventional gas compression based cooling techniques. Understanding impacts of reduced dimensionality on the magnetocaloric response of a material such as Gadolinium (Gd) or its alloys is essential in optimizing the performance of cooling devices, which is also the overall goal of this thesis. We have determined, in the first part of the thesis, that laminate structures of pure Gd produced by magnetron sputtering have several disadvantages. The target material (pure Gd), ultra-high vacuum components, and the electrical energy it takes to run the manufacturing process are all very costly. To produce quality films requires a time and energy consuming chamber preparation (gettering) to produce films with a relative cooling power (RCP) of an order of magnitude smaller (~70 J/kg) than can be obtained with Gd-alloy microwires (~800 J/kg). The increased surface area for an array of wires as compared to a laminate structure allows for more efficient heat transfer. For all of these reasons, we turned the focus onto Gd-alloy microwires. In the latter part of this thesis, we have discussed the Gd-alloy microwires as a function of magnetocaloric parameters of magnetic entropy change, adiabatic temperature change, and refrigerant capacity (RC). We have demonstrated two effective methods for improving the RC of the microwires through creating novel biphase nanocrystalline/amorphous structures via thermal annealing and directly from adjusted melt-extraction. Through studying the effects of chemical doping, as well as studying arrays of microwires with a range of Curie temperature (TC) values, we have designed a new magnetic bed structure that has potential applications as a cooling device for micro-electro-mechanical systems and energy-conversion devices.

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