Dissertation/Thesis Abstract

Nanostructured Materials For Magnetic Refrigeration
by Khattak, Khurram S., Ph.D., The George Washington University, 2017, 72; 10261344
Abstract (Summary)

Magnetic refrigeration based on the magnetocaloric effect (MCE) has attracted much attention because of its environmentally and operating energy cost advantages over vapor-compression refrigeration. In this regard, search for a suitable magnetic refrigerant material has been topic of intense research. Challenges faced in this regard are: (1). tailoring of transition temperature near room temperature, (2). finding a material with a large temperature change per unit of applied magnetic field, (3). materials with wide operating temperature, (4). either find materials that produce low hysteresis, or, add or make compositions to lower it, (5). easy and low-cost of fabrication of magnetocaloric regenerator (MR), (6) a minimal volume of the required (and costly) permanent magnet array, (7) corrosion resistance, nontoxicity, and high thermal conductivity.

The magnetocaloric effect (MCE) arises from changes in the magnetic order of materials. The most appreciable MCE can be expected in the vicinity of magnetic phase transitions induced by temperature and/or magnetic fields. The application of a magnetic field vector causes the magnetic moments of a magnetic material tend to align parallel to it, and the thermal energy released in this process heats the material. Reversibly, the magnetic moments become randomly oriented on magnetic field removal cooling down the material. The value of the MCE depends on the difference in the magnetic state before and after temperature or field induced phase transitions (PT). The largest MCE is expected at first-order transition (FOT) changes in magnetization though hysteresis effect, which lowers the efficiency of magnetic refrigerant, is greatest with this transitions. The MCE related to a second order transition (SOT) possesses significantly less hysteresis but MCE properties associated are also not that large.

Literature survey [Khattak 15] observed an enhancement of MCE properties by at least 40% when a material is fabricated as a nanostructure on account of broadening of Magnetocaloric curve, as well as reduction in its hysteresis. Different aspects such as the size, shape, chemical composition, structure and interaction of the nanostructure with the surrounding matrix and neighboring particles all have a profound effect on the magnetic behavior of a material. Moreover, after extensive literature survey [Khattak 15] it has been observed that at ambient temperature (between 260–340K) three families Gd5(Si xGe1_x)4, MnFe(P,As) and La(FexSi 1_x)13 stand out based on their MCE properties, each with their own shortcomings such as hysteresis loss, toxicity and fabrication difficulties respectively.

The purpose of this dissertation is to study each of the aforementioned families not just on a single criteria, but on all variables of MCE e.g. Curie Temperature Tc, Magnetic Entropy Change Δ|SM|, Adiabatic Temperature ΔTad and Relative Cooling Power RCP with missing variables calculated wherever possible for thorough analytical and comparisons purposes. Effects of impurities, heat treatment, synthesis methods and doping of different materials on MCE properties and hysteresis on the aforementioned families would be analyzed, with the aim of identifying the best sample composition and preparation methods for the nanostructure synthesis among each family.

Moreover one of basic hurdle is to find materials with wide operating temperature span. In this regard a new approach, where instead of a single material a combination of nanostructure materials in a matrix, is being proposed. Each material in the matrix has its Tc at different points on the desired temperature scale thus giving a wide range of operating temperature span. An added purpose of this dissertation is to propose a group of suitable materials used in combination in a matrix for enhanced MCE properties over a wide operating temperature.

Indexing (document details)
Advisor: Torre, Edward D.
Commitee: Ahmadi, Prof. Shahrokh, Harrington, Prof. Robert, Khawar, Awais, Nwokoye, Chidubem A., Torre, Prof. Edward D.
School: The George Washington University
Department: Computer Engineering
School Location: United States -- District of Columbia
Source: DAI-B 78/08(E), Dissertation Abstracts International
Source Type: DISSERTATION
Subjects: Computer Engineering, Materials science
Keywords: Adiabatic temperature, Curie temperature, Magnetic entropy, Magnetic refrigeration, Magnetocaloric effect, Superparamagnetism
Publication Number: 10261344
ISBN: 9781369668162
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