| Summary: | 碩士 === 國立交通大學 === 材料科學與工程學系所 === 101 === This thesis is divided into two sections. The first section is the characterizations of Nd2Fe14B Permanent magnets, and the second section is the development of first order reversal curve (FORC) program along with its application to the Nd2Fe14B analysis.
Nd2Fe14B permanent magnets have been extensively used in motors, alternators, and some mechanical machines nowadays. Therefore, numerous research efforts have been contributed to optimize Nd2Fe14B’s magnetic properties to meet the technology needs. Using three types of Nd2Fe14B (strip-casting(SC), commercial MQB(MQ), and commercial sintered magnet(N48)) provided by Metal Industry Research and Development Centre(MIRDC), their compositions, phases, (BH)max, macro-magnetic, and meso-magnetic properties were investigated to realize the inter-dependencies among these important factors.
Energy Dispersive Spectroscopy (EDS) was used to probe the elemental contributions to the compounds. Afterwards, the compounds were ground into size ranging from 44 to 74(μm) for the measurement of high energy X-ray powder diffraction (XRD) at National Synchrotron Radiation Research Center (NSRRC) with crystallographic analysis. A theoretical Nd2Fe14B crystallographic model was constructed by the Rietveld refinement program to compare with the experimental XRD patterns, and the grain sizes of the three investigated compounds were estimated using the Scherrer Equation. Meanwhile, the macro-magnetic properties were analyzed by a Vibrating Sample Magnetometer (VSM), and followed by the calculations of the maximum magnetic energy product ((BH)max) to evaluate their potentials for industrial applications. The resulst show that the MQ exhibits a poor tetragonal structure due to the use of an extremely fast cooling rate (106 K/second), resulting in a grain size approximately half of those of the SC and N48, as well as leading to a (BH)max one order magnitude smaller than the other two. Despite that both SC and N48 present fully developed Nd2Fe14B tetragonal structures close to the theoretical model, the x-ray data identify a second phase in the latter. The second phase of the N48 promoted the properties finitely, but the increased squareness have raised the BHmax by 40% compared to the SC.
In the second section of the thesis, the Nd2Fe14B compounds were analyzed by FORC using the VSM facility in our lab. A friendly software program was formulated to create an auto-running script specifically for the FORC measurements in the VSM. With the GUI (Graph User Interface) developed in the VSM, a measuring sequence can be constructed with the script. From FORC analysis, the three Nd2Fe14B compounds appear to comprise both soft and hard ferromagnetic components, a phenomenon unexpected from such a venerable permanent magnet, and possible reasons were provided to explain the nature. The coupling and interacting strengths of the two components appear to vary with the sample condition and possess an intimate connection to the macroscopic properties. The study delivers a picture of local magnetization reversal dynamics for the Nd2Fe14B, and highlights the strong sensitivity of the compound’s application potential to the nature of soft and hard components.
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