Studying For The High Property Of Sintered NdFeB Magnet On Microstructure, Magnetic Properties And Domain Structure

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, bonded magnet and thermal deformation magnet. At present, the sintered magnet's energy product is 444kJ/m³(55.8MGOe) (the international lab. research level) and 414 kJ/m³(52MGOe) (the mass production level), respectively. However, most of Nd-Fe-B production enterprises in China can only produce the magnet with lower energy product (usually the energy product is about 278—320kJ/m³(35-40MGOe)). Besides, both uniformity and consistence of the magnet are lower because of old equipment and out-of-date producing process. Up to now the Nd-Fe-B magnet prepared in China can't enter the main application field. Considering this situation described as above, we carried out the new process studying for the high property of sintered NdFeB magnet.Sintered NdFeB magnets are used in many fields due to their high magnetic product, low cost and good machinability. With the expending of their application area, NdFeB magnets with good combination property, especially high temperature resistance performance, are required by permanent magnetic motors. Sintered NdFeB magnets with high coercivity and low temperature coefficient, which could be used at high temperature, such as more than 200℃, should be required in many severe fields. Sintered Sin-Co magnets can be used at the temperature that more than 300℃, but cost more than NdFeB magnets, and have lower magnetic product. At the same time, sintered NdFeB magnets have better machinability than sintered Sin-Co magnets. So, sintered NdFeB magnets which can be used at 200～300℃are a significant area to be developed.The hard magnetic property of Nd-Fe-B magnet mainly depends on the Nd₂Fe₁₄B(2:14:1) main phase. Adjusting the component of the main phase can increase the intrinsic magnetic properties (such as anisotropy K₁), and improving the grain microstructure can enhance the macro-hard magnetic properties. We adopted advanced production equipment, applied new preparing procedure, regulated process parameters, adjusted the composition and added trace elements etc., so the hard magnetic property of the magnets was improved greatly. Our main studying works are as follows:(1)We have prepared and researched Nd_33.5Dy_0.99Fe_bal.Al_0.52Cu_0.1B_1.15(wt %) sintered magnet with high coercivity. We studied the influence of addition elements of Dy, Cu and A1 on the magnet microstructure and magnetic properties. The results showed that it is especially effective to compositely add Dy, Cu and Al for preparing the high coercivity magnets by conventional powder metallurgy. The addition of Dy not only increases the anisotropy of materials, but also decreases the grain size, prevents the formation of soft a-Fe in the alloys, effectively improves the microstructure, and also obviously enhances the coercivity of magnet. The addition of Cu increases coercive field owing to two reasons. One is that Cu enters the main phase, firstly occupies the J2 site, decreases the planar anisotropy that is beneficial to increase uniaxial anisotropy and enhance the coercivity, another is that owing to the grains prepared by adding Cu have finer sizes, the area per volume increases, and requires the Nd-rich phases more uniformly distribute for the high property magnet with low-Nd content. Because of the special lubrication of A1, the Nd-rich and Al-containing intergranular phases form in the magnet, which makes the grain boundary more clear and complete, effectively prevents the exchange-coupling interaction between the main phase grains, so as to enhances the coercivity of the magnet. The sintered NdFeB sample was examined by magnetic force microscope which revealed the domain structures at the surface. It was revealed that the mean Nd₂Fe₁₄B grain size was significantly larger than the average scale of the magnetic contrast. An explanation about this is that most Nd₂Fe₁₄B grains in sintered NdFeB alloy are dominated with the multidomain structures when the magnet is in thermally demagnetization state. (2) A new alloy of Nd_31.0Dy_1.08Fe_bal.Nb_0.5Al_0.34B_1.1(wt %) has been fabricated by HD and JM processes. The influence of the parameters of HD and JM processes on the microstructure of the magnetic magnet was studied. 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 a-Fe soft phase is avoided to form. The microstructure of the magnet satisfies the requirements for producing high property sintered magnet. The process is a new way to produce high property sintered magnet. The additions of some elements can improve the magnetic properties of sintered NdFeB. The addition of Nb forms the compounds: NbFe and Nb₂Fe₃, prevents the formation of a-Fe and refines the grains. The improvement of the magnetic properties of sintered Nd-Fe-B magnets with Nb addition is due to the improvement of the microstructure of magnets and as-cast alloys. In the magnets with Nb addition the size of Nd₂Fe₁₄B phase is homologous and the figure of Nd₂Fe₁₄B phase is regularer. The Nd-rich phase is distributed dispersively. Nb mainly distributes in Nd-rich phases, sometimes combining with Dy which replaces Nd of matrix mainly. Dy₂O₃ that is added during milling improves the coercivity and prevents the grain growth. The topography, the domain structure and the correlation between them are also investigated in this paper. The microstructure and the domain structure as well as the texture orientation are improved by the Dy, Nb and Al doping in the alloy. We studied the qualification for preparing the high performance permanent magnet, which includes the structure, magnetic properties and the relationship between them.At the same time, we studied the effect of aligning times on alignment degree of blank. The results show that repeating pulse field aligning can improve the blank's alignment degree.(3) As for the special demand of the permanent-magnet motor to the application, the technology for producing sintered NdFeB material of 35UH high coercive force is described. It adopt the liquid phase sintering technology by using the heavy rare earth instead of the light rare earth and reducing the oxygen content for a long time at the low temperature. Through the test the static performance has reached: Br=12.21kGs, Hci=19.49kOe, (BH)max=34.54MGOe at the same time. The study of the roles of Tb element in sintered NdFeB magnets shows that: the improvement of the magnetic properties of sintered NdFeB magnets with Tb addition is due to the improvement of the microstructure of the magnets. The magnet with Tb addition has good regularity and size distribution. It is shown that the addition of addition has good regularity and size distribution. It is shown that the addition of Tb results in grain refinement and enhancement of Hci and Hk/Hci. Because NdFeB sintered magnets are composed of grains withμm sizes and there exist non-magnetic boundary phases between grains, so the exchange coupling interaction is very weak and the coercivity of magnets is mainly determined by the long-range magnetostatic interaction. Results of experiment show that with lengthening of milling time, the grain size decreases, the long-range magnetostatic interaction reduces, coercivity increases and remanence enhances slightly. The remanence obviously enhances and the coercivity decreases for the aligned magnets comparing with the misaligned magnets. The starting field theory is in good agreement with this experimental result. In addition, the demagnetization curves show that the effects of applying alignment field when the magnetic powders were pressed on the properties of magnet are similar to the exchange-coupling interaction.