Preparation And Reaction Process Of NdFeB Rare Earth Permanent Magnetic Super Fine Powers Synthesized By Chemistry Method

Sintered NdFeB magnet has been widely applied in information and communication,energy-saving motors,automation,power generation,biomedicine,electric vehicles and other fields because of it’s the most powerful magnetic properties.The magnetic energy product of commercial NdFeB magnets is close to the theoretical value,but the coercivity still has big gap from the theoretical value.NdFeB magnets are prepared by the powder metallurgy method.There are many preparing steps and complicated processes.Among these processes,the jet milling process is a key bottleneck to block improvement of the coercivity,because NdFeB powder with fine size and narrow distribution couldn’t be obtained by using jet milling that has been widely used in NdFeB production.Since the coercivity is sensitive to structure.The size distribution and the morphology of the magnetic powder obviously influenced on the microstructure of the sintered magnet.Therefore,it’s imperative to find a new milling path,which can produce nice NdFeB powder with a narrow size distribution and uniform morphology,to take place of the present milling technology.In response to this problem,this study introduced chemical method into the NdFeB powder production.it’s expected to obtain the NdFeB magnetic powder with controllable morphology and size by using chemical method.This is a new idea for the preparation of NdFeB powders.The chemical method eliminates the processes of strip,casting,hydrogenation-disproportionation crushing,jet milling,and saves working hours and production costs.Therefore,study on the NdFeB powder pepared by chemical method has fundamental and applicable significance.In this paper,the NdFeB intermediate(Fe3O4 and Nd,B ligands)was firstly prepared by wet chemical method,and then the metal ligand was reduced to NdFeB oxide by primary reduction annealing,finally the NdFeB powders with Nd2Fe14B main phase were obtained by secondary reduction annealing.The main research results are shown as follows:The monodisperse NdFeB intermediates with nanosize have been prepared by wet chemical method.The intermediate consists of spherical Fe3O4 nanoparticles and a flocculent complex.The complex is a metal acetylacetonate organic ligand.Nd,Fe and B atomic ratio of the NdFeB intermediate can be controlled by tuning molar ratio of the precursor.The influences of chemical synthesis process(including solvent,temperature and time)on the morphology and size of the synthesized NdFeB intermediate were studied.The best synthesis process is at 320℃ for 120 min in oleylamine.Under this reactive parameter,the NdFeB intermediate with uniform morphology,the average size of about 12 nm,and surround by flocculent complex could obtained.The NdFeB powders composed of Nd2Fe14B phase can be obtained after two-step reduction annealing.The reduction reaction mechanism during annealing treatment has been analyzed by using XRD,DSC and other characterization.The results are as follows:The primary reduction annealing converts the Fe3O4 and Nd,B organic ligand into NdFeO3,B2O3,Nd2O3 and α-Fe.The reactive processes at different temperatures during the secondary reduction annealing are:at 501℃,B2O3+3CaH2=B+3CaO+3H2↑;at 678℃,Nd2O3+3CaH2=2NdH2+3CaO+H2↑,FeNdO3+3CaH2=NdH2+Fe+3CaO+2H2↑;at 895℃,2NdH2+14Fe+B=Nd2Fe14B+2H2↑.The effects of secondary reduction annealing process(including annealing temperature and CaH2 dosage)on the morphology and properties of the synthesized NdFeB powders were investigated.The best secondary reduction annealing process was obtained.When the weight ratio of NdFeB oxide to CaH2 is 1:1 and the temperature is at 940℃ for 120 min,the NdFeB powders with main phase Nd2Fe14B and uniform morphology can be prepared.Magnetic properties of the NdFeB powders are:Hc=5.4 kOe,Ms=76.3 emu/g,(BH)max=31.2 kJ/m3.