The Investigation Of The Microstructure Of Sintered NdFeB And The Preparation Of Sintered NdFeB With High Coercivity

The microstructure of ingots and sintered magnets of NdFeB and the effective factors on the magnetic properties have been investigated by means of optical metallurgical analysis, SEM, EDAX, X-ray diffraction analysis, determination of oxygen by infrared analysis and the measurement of density and magnetic properties. The sintered NdFeB magnets with high coercivity have been prepared by traditional powder metallurgy, and the magnets with Br=l. 229T, H.j=12. 87kOe, (BH)粁=35. 2MGOe and Br=l. 162T, Hcj=17. 24kOe, (BH)MI=32MGOe have been gained.The results are as following:The ideal microstructure of NdFeB ingots is composed of fine Nd2Fe,4B lath uniformly divided by Nd-rich phase, without block Nd-rich phase, a -Fe and other nonmagnetic phases. The lower the total rare earth is, the slower the cooling rate is and the higher the casting temperature is, the easier a -Fe with dendritic and granular character separates out. The microstructure of NdFeB ingots prepared by strip cast is fine and uniform.Nd-rich phase with difference composition and form exits in NdFeB sintered magnets. The better the matrix distributes and the more regular the grains are, the higher the magnetic properties. The abnormal grain growth deteriorates the magnetic properties seriously, which has close relation with the size distribution of powder, the sintering temperature and time and the oxygen content, etc. The grain size has important effects on the magnetic properties. The finer the grain is, the higher the magnetic properties.The regularity and size distribution of powders have great effects on the magnetic properties. The powder produced by jet milling has bad regularity and size distribution, which are worse for the powder produced by ball milling. A method calculating alignment degree has been gained, the results achieved by which agrees with the alignment reflected by X-ray diffraction patterns. The oxygen content has close relations with the process. Generally speaking, the oxygen content can be reduced notably by jet milling with additions against oxide.The additions of some elements can improve the magnetic properties of sinteredNdFeB. When 3. 8%wt Dy is added, the coercivity can reach 18kOe. The addition of Nb forms the compounds: NbFe and Nb2Fe3, prevents the formation of a -Fe and refines the grains. Cu mainly distributes in Nd-rich phases, sometimes combining with Co which replaces Fe of matrix mainly. Dy203 that is added during milling improves the coercivity and prevents the grain growth.