强磁场中非晶晶化组织与性能研究

【作者】 王岩；

【导师】 齐民； 杨大智；

【作者基本信息】 大连理工大学 ， 材料学， 2004， 硕士

【摘要】 纳米复合Nd₂Fe₁₄B／α-Fe型磁体是八十年代末发展起来的新型永磁材料。这种磁体是由Nd₂Fe+（14）B相和α-Fe相在纳米尺度内复合而成的。α-Fe相中原子磁矩的转动受Nd₂Fe+（14）B相的控制，因而这种磁体具有磁能积高的优点，其理论预测磁能积高达1000KJ／m³，远高于目前使用的各类永磁材料，被称为下一代永磁体。但是，目前制备出的纳米复合NdFeB永磁材料，其磁能积与理论预测有很大的差距。原因是理论上的计算采用的都是理想模型：晶粒形状规则，大小均匀，都要达到几十纳米。而目前制备纳米复合永磁材料主要采用快淬方法得到非晶合金薄带，然后通过加热晶化来获得纳米晶复相组织。以这种方法制备的纳米晶微观组织与理论模型相比，无论是晶粒大小，晶粒形状还是晶粒分布都有很大差距，由此造成了磁性能的差距。本论文在这种背景下展开研究，考察强磁场中NdFeB非晶晶化组织与性能，探索新的工艺以进一步提高纳米复合永磁材料的磁能积。 首先，选择Nd-Fe-B基合金成分，并适当添加Co，制备成非晶合金；然后在强磁场中将非晶样品加热晶化处理，在不同的晶化条件下（外加磁场与晶化温度），制备出纳米晶材料；最后，通过XRD和TEM观察非晶晶化组织结构与形貌，利用振动样品磁强计（VSM）分析非晶晶化组织的磁性能。通过对实验结果分析，得到如下结论： 1．在Nd₈Fe₆₅Co₁₀B₁₇非晶合金的DTA分析中，发现两个放热峰。第一个680℃的放热峰是非晶晶化峰，生成Nd₂Fe₂₃B₃，Nd₂Fe（14）B和α-Fe晶化相；第二个870℃的放热峰是亚稳相Nd₂Fe₂₃B₃的分解，生成Nd₂Fe₁₄和α-Fe。 2．在非晶态Nd₈Fe₆₅Co₁₀B₁₇合金晶化的过程中，强磁场抑制了亚稳的Nd₂Fe₂₃B₃的析出，使其分解为稳定的Nd₂Fe₁₄B和α-Fe相。原因是Nd₂Fe₂₃B₃相从磁场中获得了能量，而越过势垒发生分解。 3．在对晶化相的微观形貌进行分析时发现，磁场使得晶粒变小，晶粒分布变得均匀，同时使残余非晶相的量减少。通过热力学分析认为，磁场的存在改变了体系的自由能，从而增大了形核率，使得晶粒细化，并且减少了残余非晶相的量。 4．在较低的晶化温度下（650℃），晶化析出的软磁相与硬磁相之间存在很强的交换耦合作用，材料具有很好的磁性能；当晶化温度升高后，由于晶粒长大，交换耦合作用被严重削弱，材料的磁性能也随之下降。由于磁场具有细化晶粒的作用，所以在晶化过程中施加磁场，可以有效地提高材料的磁性能。实验中发现，当外加磁场达到10T时，材料的磁能积提高了30％。更多还原

【Abstract】 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.更多还原