Research On Technology And Properties Of NdFeB Magnets Sintered In Microware Magnetic Field

This paper systematically discussed the fundamental, characteristics, research status and equipment of microwave sintering, and introduced the preparation and development of sintered NdFeB magnets, on the basis of which the effects of the parameters on the heating rate and sintering temperature were investigated. And by means of optical metallurgical analysis, X-ray diffraction analysis and the measurement of density and magnetic properties, the paper compared the properties and technology of NdFeB magnets sintered in microwave magnetic field with that of magnets sintered in conventional heating process.The result showed that microwave source's power, powder particle size and green compact's volume have great influence on the sintering process during the microwave magnetic sintering of NdFeB magnets. The highest sintering temperature got higher and the speed of temperature became faster, with the fining of the powder particle size or the increase of the microwave source's power or the decrease of the green compact's volume. The sintering time became shorter, with the fining of the powder particle size or the decrease of the microwave source's power or the increase of the green compact's volume. However, the green compact's density has little effect on the highest sinteringtemperature and time. After optimizing the technological technique, theparameters on microwave magnetic sintering of NdFeB magnets were chosen: the powder with average particle size of 3 m was isostatically pressed at 2000kg/cm2, and the shape of the green compact is 18 6mm. Then the green compact was microwave sintering in magnetic field produced by 2 kilowattpower of microwave source. The highest sintering temperature was 900 C, and the sintering lasted 70 seconds. The conventional sintered magnets were sintered at 1000 C for 1h. Then the sintered magnets heat treated at 800 C for 1h and aged at 600C for h. The density of microwave magnetic sintering NdFeB magnets could reach 7.59g/cm3, which was higher than density of NdFeB magnets sintered in conventional method.The microwave magnetic sintering can not only lower the sintering temperature and shorten the sintering time, but also decrease the micro-grain size of magnets and make intergranular phase and grain boundary of main phase distributed well. The abnormal grain growth was found in conventional sintered NdFeB magnets, which may be due to the greater particle size and uneven distribution of powder, and higher sintering temperature and longer sintering time. Of course, the abnormal grain growth would deteriorate the magnetic properties.The comparison between the magnetic properties of NdFeB magnet by microwave magnetic field sintering and those of magnet by conventional sintering technology shows that the Brwas improved 50%, the Hcb was improved 8%, and the Ms was improved 40%. The finer the grain is, the higher the magnetic properties are, such as the improvement of coercivity. According to the theory of remanence, the intensity of remanence is in direct proportion to the density of magnet and Ms. The improvement of remanence resulted from the interaction of density of magnet and Ms, but the Ms took the more important role.