高性能Nd-Fe-B永磁材料的微结构与磁性能

【作者】 刘汉强；

【导师】 高汝伟；

【作者基本信息】 山东大学 ， 材料物理与化学， 2005， 博士

【摘要】 永磁材料已成为现代科学技术,如计算机技术、信息技术、航空航天技术、通讯技术、交通运输(汽车)技术、办公自动化技术、家电技术与人体健康和保健等的重要物质基础。现在常用的永磁材料主要有铁氧体、铝镍钴及稀土永磁。稀土永磁包括钐钴及钕铁硼等,其中钕铁硼具有创记录的高剩磁、高矫顽力和高磁能积,称为新一代稀土永磁或第三代稀土永磁。钕铁硼磁体按其制备工艺可分为烧结钕铁硼磁体和粘结钕铁硼磁体两类。在高新技术领域,对高性能钕铁硼磁体的需求日益增长。为满足市场对高性能钕铁硼磁体的需求,我们进行了高性能钕铁硼永磁材料的研究。无论对于烧结Nd-Fe-B系永磁材料,还是纳米复合粘结Nd-Fe-B永磁材料,其永磁性能和微结构有密切联系。我们的研究主要包括最近发展起来的纳米复合钕铁硼永磁材料和速凝工艺制备的烧结钕铁硼永磁材料。 1,纳米复合钕铁硼永磁材料的研究 纳米复合永磁材料由软、硬磁性相晶粒在纳米尺寸范围内复合而成。由软、硬磁性相组成的纳米复合永磁材料存在强烈的交换耦合相互作用,可同时具有硬磁性相的高矫顽力和软磁性相的高饱和磁化强度,理论磁能积能达到1MJ/m3(120MGOe)。同时这类材料中稀土含量较低,价格低廉,可望成为一种有广泛应用前景的新一代永磁材料。但多年的实验研究结果表明:纳米复合磁体的剩磁有很大提高,但是矫顽力下降太多,导致这类磁体的硬磁性能远低于理论值,限制了这类材料的发展和应用。 如何使纳米复合永磁材料在具有高剩磁的同时具有高的矫顽力是一个重要的研究课题。纳米复合永磁材料的矫顽力取决于软、硬磁性相的成分配比、内禀磁性(特别是各向异性)和微结构。添加元素可以改变软、硬磁性相的内禀磁性,改善微结构,提高矫顽力。对于纳米复合Nd-Fe-B永磁材料,在一定晶粒尺寸范围内,晶粒尺寸减小,晶粒的比表面积增加,单位体积的晶界数量增加,晶间交换作用加强,在提高剩磁的同时,提高了矫顽力。通过添加元素阻更多还原

【Abstract】 Permanent magnetic materials have become the most important substance foundations of modern science and technology such as computer technology, information technology, aviation and spaceflight technology, communication and transportation technology, office automation technology, household appliances and health care technology. The commonly used permanent magnetic materials include ferrite, AlNiCo and rare-earth magnets. SmCo and Nd-Fe-B are typical materials of the rare-earth magnets. Due to its outstanding magnetic properties of high remanence, high coercivity and high energy product, the Nd-Fe-B magnet is called the new generation permanent magnet, or the third generation rare-earth permanent magnet. Based on the different producing processes, the Nd-Fe-B magnet can be classified as sintered magnet and bonded magnet. In the high-tech field, the requirements of high performance Nd-Fe-B permanent magnetic material have increased, so we carried out the high performance Nd-Fe-B studying. The microstructure and magnetic properties are related with each other for both sintered magnet and nanocomposite material. Our studying mainly included recently developed nanocomposite permanent magnets and sintered Nd-Fe-B magnet prepared by strip casting (SC) process.1, Studying of nanocomposite Nd-Fe-B permanent magnetDue to the exchange-coupling interaction between magnetically soft and hard grains, nanocomposite permanent magnets can simultaneously keep both the high saturation magnetization of soft phase and the high coercivity of hard phase, and can have the high energy product. The theoretical energy product of nanocomposite magnets could be as high as 1MJm-3 (120MGOe), which is more than double of the energy product of thebest sintered Nd-Fe-B magnets. Besides, with the lower Nd content, nanocomposite magnets may be the candidates for the new generation of permanent magnets. However, up to now, the experimental energy product of nanocomposite magnets is still far below the theoretical value, and even lower than half of the energy product of best sintered Nd-Fe-B. This is caused by the severe decrease of the coercivity despite the obvious increase of the remanence, restricting its development and application.How to make the nanocomposite permanent magnets simultaneously possess both high saturation magnetization and high coercivity has become an important studying issue. The coercivity of nanocomposite permanent magnets depends on the ratio of soft/hard phase, the intrinsic property and the microstructure. Addition elements can improve intrinsic property, microstructure, increasing the coercivity. For nanocomposite material, when the grain size decreased under a certain range, the grain’s surface/volume ratio increased, the intergrain exchange-coupling interaction enhanced, and increased both of the coercivity and the remanence. We mainly studied the influences of microstructure and permanent magnetic properties by adding trace elements Co, Dy in nanocomposite Fe3B/Nd2Fe14B permanent magnetic material, so as to provide reference for producing high performance nanocomposite permanent magnetic material.The influences of the addition of trace elements Co, Dy in nanocomposite Fe3B/Nd2Fe14B permanent magnetic material on the crystallization temperature, Curie temperature . optimum annealing process, microstructure and hard magnetic properties have been studied by using the differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electronic microscope (TEM) and vibration sample magnetometer (VSM).In order to get high magnetic property, the annealing procedureshould make the amorphous material completely crystallize, and prevent from emergence of over-big grains. The annealing temperature and time are restricted each other. When the annealing temperature is lower and the annealing time is shorter, the sample will not be completely crystallized. When the annealing temperature is higher and the annealing time is longer, too big grains will be appeared.Studying results show that it is difficult to get the nanocomposite material with high hard magnetic properties for three-element alloy Nd4.5Fe77B18.5 and the addition of trace elements can enhance the intrinsic properties, improve the microstructure and increase the hard magnetic properties of nanocomposite material. The 1—3 at % addition of Co, Dy in Nd4.5Fe77B18.5 obviously decreases the crystallization temperature and the optimum annealing temperature, increases the Curie temperature of 2:14:1 phase, improves the microstructure and enhances the hard magnetic properties. The hard magnetic properties of Nd4.5Fe74Co3Dy1B18.5 nanocomposite are as follows: Br=1.06T, jHc =328KA/m, (BH)(?)=108. 9kJ/m2, which are higher 0. 22T(26%), 48 KA/m (17%) and 55. 6 kJ/m2 (104%) than that. of Nd4.5Fe77B18.5, respectively.2, Studying of sintered Nd-Fe-B permanent magnetRecently, sintered Nd-Fe-B magnets produced from strip casting (SC)alloys have been studied extensively due to their high permanent magnetic properties. The main process of SC technology is casting the melting metal liquid to the water-cooled copper wheel and the cooling speed is so high that the thickness of the strip flakes is about 0.3mm. By using this process, the alloy composition can be very close to that of Nd2Fe14B main phase. The main phase grain is finer, the thin Nd-rich phase is homogeneous distributed, and the α-Fe soft phase is avoided to form. The microstructure of the flakes satisfies the requirements forproducing high property sintered magnet. The alignment coefficient, the volume fraction, the distribution of grain size of the hard magnetic grains, the overall Oxygen and Carbon content of the sintered magnets are important factors influencing magnetic properties. The composition determines the volume fraction of the main phase, and the processes determine the alignment coefficient, the distribution of the grain size of the hard magnetic phase, the overall Oxygen and Carbon content of the sintered magnets. With adequate chemical composition, sintered magnets prepared from strip cast alloys have a higher alignment coefficient, low rare-earth content, high fraction of the hard magnetic phase and high permanent magnetic properties, which is the ideal microstructure for producing high performance magnet.With a fixed composition, the alignment coefficient and the distribution of grain size of the hard magnetic grains will be influenced by the alignment and microstructure of the strip cast alloy (the master alloy). In order to improve the permanent magnet properties of sintered Nd-Fe-B magnets, the alignment and the microstructure of strip-cast ribbons were studied. Combined SC process and hydrogen decrepitation (HD) process and rubber isostatic pressing (RIP) process, we produced high performance Nd-Fe-B magnets and compared its microstructure with ideal microstructure.For sintered Nd-Fe-B permanent magnets produced with SC process, with higher alignment degree, main-phase grain size is adequate and homogeneous distributed. At the same time Nd-rich phase homogeneous distributed as intergranular phase. The sintered magnet with higher remanence and coercivity has higher energy product.By adopting advanced production equipment, applying new preparing procedure, regulating process parameters, adjusting the composition and adding trace elements etc, the hard magnetic property of the magnets wasgreatly improved. Our main studying works are as follows:1) There exist a pronounced texture of the tetragonal T1 phase in the strips and the columnar grains exhibit apparent alignment along [OOL]. Both the cooling surface and the free surface of strips prepared at different wheel speeds were analyzed by XRD, the alignment degree of free surface and cooling surface were calculated. The alignment coefficient (?) changes with the wheel speed V and is highest at V = 1.5-2 m/s.2) The microstructure of the strip was investigated by backscattered scanning microscope with EDS. The influence of solidification rate on the microstructure of cast strip was studied. The microstructures of the strips vary significantly with the solidification rate. The thickness of T1 columnar grains is larger when the solidification rate is lower and the over-small isotropic microcrystalline appear on the cooling surface of the strip when the solidification rate is too high, both of which makes the solid-solid sinter and the main (2:14:1) phase grain unusually grow up during the sintering process, decreasing the hard magnetic properties of the magnet. The adequate wheel speed for obtaining optimum microstructure of the strip is about V=2.0m/s. The strip is composed of 2:14:1 columnar grains with thickness of 3μm separated by Nd-rich phase of 0.2—0. 5μm, and there’s no existence of dendrite α-Fe. This cast strip is the ideal starting material for preparing sintered magnet with high hard magnetic property.3) During the HD process, the hydrogen makes the flakes crack along the grain boundaries, which guarantees that the cracked particle is single crystal and the Nd-rich phase uniformly distributed between the main phase grain boundaries. The size distribution of magnetic powders is close to Gaussian distribution. The particle size is close to thethickness of the columnar grains.4) With decreased rare earth content in the alloy, the thickness of the Nd-rich phase decreased at the same time, but the distribution of the Nd-rich phase in the flakes is still homogenous. Fine main phase grain and homogenous distributed Nd-rich phase increased the coercivity.5) By using SC and HD processes to prepare the magnetic powder and RIP to prepare the compacts, suitably adjusting all process parameters, we obtained the high property magnet. The product’s uniformity and consistence are improved. The magnetic properties of sintered magnets prepared from strip cast ribbons with optimum microstructure are as follows: Br= 1.457 T(14. 57 kGs), jHc = 1048 kA/m, (BH)max = 408 kj/m3 (51.3 MGOe).更多还原