| Permanent magnetic materials can transform electromagnetic energy to mechanical energy. Because of the energy transforming function and a lot of physical effects, varies of functional apparatus can be made from permanent magnetic materials. The commonly used permanent magnetic materials include ferrite, AlNiCo and rare-earth magnets. Rare-earth magnets contain SmCo5, Sm2Co17 and NdFeB et.al. The NdFeB is so-called "The King of Magnets" due to its outstanding magnetic properties of high remanence, high coercivity and high energy product.Nanocomposite permanent material is a new type one, which is composed of soft phase and hard phase with nanoscale microstructure. According to the exchange-coupling theory, nanocomposite permanent material can have the high remanence of soft phase and the high coercivity of hard phase at the same time. It is expected to be a new general permanent material with high properties. For the actual material prepared by experiment, remanence increases greatly, but the coercivity drops a lot and the energy product have great difference with the value predicted by the theory. Our main work includes:1. Nanocomposite materials NdgFeg7Bs and (Nd0.7Dy0.3)8Fe87B5 were prepared by the melt-spinning and subsequent annealing technique. The crystallization behaviors of the samples were studied by Differential scanning calorimeter (DSC) measurement. The phase components of powder speciments were analyzed with X-ray diffraction (XRD). Transmission electron microscopic (TEM) was used to observe the microscopic structure and to evaluate the grain sizes. The magnetic properties of the annealed ribbons were measured by Vibrating sample magnetometer (VSM). The main results are as follows: (1) The properties of nanocomposite material reply on the heat-treated conditions sensitively. At the annealing temperature 720C and the heating rate 420 C/min, when the annealing time is 3min, the optimal properties of Nd8Fe87B5 are as following: Mr=797.86Gs, Hci=4238.71Oe, (BH)max=8.94MGOe, Mr/Ms=0.68, and the demagnetic curves have the sigle hard phase characteristics. (2)The M-T curves of optimum heat-treated (Nd0.7Dy0.3)8Fe87B5 have been investigated and the curie temperature reaches 603K, which is 15 K higher than that of single phase Nd2Fe14B. (3) Compared to non-doped Nd8Fe87B5 sample, The coercivity and the reduced remanence of (Nd0.7Dy0.3)8Fe87B5 increased while the remanence and the energy product decreased.2. Using the part exchange-coupling model, the exchange-coupling interaction between magnetically soft and hard grains in Nd2Fe14B/a -Fe and their effects on the effective anisotropy of nanocomposite materials have been investigated. When the grain size D is smaller, the exchange-coupling interaction is strong enough that the grain dependence of the effective anisotropy between soft grain and hard grain can be calculated by the model of Herzer. When D is larger, there exist part-exchange-coupling interaction at the interface of the grain, and its thickness is Lex/2. By testing six different functions, series of curves of effective anisotropy constant versus grain size and volume fraction of the soft phase were obtained. The calculation results show that the effective anisotropy constant decreases with the reduction of grain size and increases with the decrease of the volume fraction of the soft phase. |
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