利用一階迴轉曲線研究釹鐵硼永久磁石

Abstract

此份論文主要分兩部分。第一部分為描述釹鐵硼永久磁石性質，而第二部分為一階迴轉曲線的程式撰寫以及在釹鐵硼永久磁石中的應用。 近來，釹鐵硼磁石廣泛被運用在馬達、發電機以及各樣類型的機件中。因此，如何最佳化釹鐵硼磁石之磁性質被廣泛地研究。本論文將使用財團法人金屬研究發展中心所提供的三種釹鐵硼材料(分別是strip casting(SC)粉末、商用化MQB(MQ)粉末與商用化燒結磁石(N48))，研究材料成分、結構、巨觀磁性以及介觀磁性這些重要性質之間的相互關係。 實驗上，使用能量分布儀(EDS)來分析材料中的元素成分。之後，將材料研磨至粒徑大小為44~74(μm)，利用國家同步輻射(NSRRC)高能量的X光粉末繞射，對材料結構進行檢測，並計算Nd2Fe14B硬磁相之理論峰值加以比對，使用Scherrer Equation計算各個樣品中Nd2Fe14B相結晶之尺寸。同時，以振動樣品磁量儀(VSM)測量其巨觀磁性，並計算其最大磁能積來評估其作為永久磁石的潛力。結果顯示，MQ由於冷卻速度極快(106 K/second)，故具有較差的四方晶體結構，這導致其晶粒尺寸大約只有SC與N48的一半，同時也促使其最大磁能積(BH)max小於另外兩者約一個數量級。儘管SC與N48的XRD圖形幾乎與Nd2Fe14B四方晶體結構的理論繞射圖形相等，但仍然有其他相存在於材料中。N48的雜相較明顯，此雜相促使其磁滯曲線具有較好的方正性，以至於其最大磁能積遠大於SC。 在第二部分中，以本實驗室中的震動樣品磁量儀分析這三種釹鐵硼材料。主要是以自寫的自動化程式來驅動震動磁量儀的使用者圖形介面，使用這個程式可以自動化產生量測的序列。從一階迴轉曲線分布圖的分析可以知道這三種釹鐵硼材料中都具有軟鐵磁相與硬鐵磁相，此二磁相之間的交互作用隨著樣品的不同而影響其巨觀磁性。此研究傳遞出釹鐵硼磁石局部磁化量迴轉區域之圖像。

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.