| The nanocomposite Nd2Fe14B/ a -Fe magnet is a new kind of magnetic material developed since the end of 1980s. This kind of magnet consist of two phases: magnetically hard Nd2Fe14B and soft a -Fe. The rotation of magnetic moments in the a -Fe phase is controlled by Nd2Fe14B phase through exchange coupling effect which gives this kind of magnet high magnetic energy product (BH)m. This kind of magnet has 1000KJ/m3 (BH)m received by calculation, which is much higher than the (BH)m of other kinds of magnets. So this kind of magnet is called "The magnet of next era". But there is a large gap between the (BH)m received in the lab and the (BH)m received by calculation. The reason is that the theory (BH)m is calculated from ideal model, in which the shape of grains is regular, the distributing of two phase grains is homogenous and the dimension of all the grains is tens of nanometers. And at present the most popular way of producing nanocomposite permanent magnet is RQC, that is, rapid quenching and crystallization. The micro structure of magnet produced by RQC is different from the ideal model not only in the shape and distributing of grains but also in the dimension of grains. This difference leads to the gap of magnetic properties between the (BH)m received in the lab and the (BH)m received by calculation. In view of this condition the paper focuses on the micro structure and magnetic properties of crystallization phase transformed from amorphous NdFeB alloys under magnetic field in order to find new way of manufacturing nanocomposite permanent magnet with higher (BH)m.The first step is to ascertain component of alloy and transform the alloy into amorphous phase. Then transform amorphous phase into nanocomposite crystallization phase under different crystallizing temperature and different magnetic field. Finally analyze the microstructure of crystallization phase by XRD and TEM; analyze the magnetic properties of crystallization phase by VSM. Such conclusions can be drawn from the results: 1 . During the DTA analysis of amorphous Nd8Fe65Co10B17 alloy two peaks can befound. The first 680℃ peak represents the crystallizing of amorphous alloys.And the second 870℃ peak represents the decomposition of metastable2. During the crystallization course of amorphous NdgFe65Co10B17 alloy magnetic field promotes the decomposition of metastable Nd2Fe23B3. The reason is that Nd2Fe23B3 phase gains the energy from magnetic field and decomposes into Nd2Fe14B and a -Fe phase.3.By analyzing the microstructure of crystallization samples it can be found that magnetic field makes grains smaller, makes grain distributing more regular and makes remanent amorphous phase reduced. The thermodynamic analysisshows that magnetic field changes Gibbs free energy of the system and increase the rate of forming nucleus which leads to the experimental results above.4. At the low crystallizing temperature (650C) there is strong exchange coupling effect between magnetically hard phase and soft phase., and the magnet has high magnetic properties. With the increasing of crystallizing temperature the exchange coupling effect becomes weak because of grains' growing, and the magnetic properties of magnet becomes bad. Because magnetic field can make grains smaller it is useful to improve magnetic properties of the magnet by the participating of magnetic field during the crystallization. It can be found that the magnetic properties improves with the 30% increasing in (BH)m when magnetic field reaches 10T. |
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