Permanent Magnets And Magnetic Materials Modified Grain Boundary Energy, Corrosion Research

NdFeB permanent magnet materials applied to every field of our country,big to our country’s space program, small to people’s daily lives, with thedevelopment of more devices based on the function of NdFeB, there will bemore goods appeared around us.Our country is rich in rare earth resourcesreserve, NdFeB industry has a good foundation and will be bound to have abright future.But NdFeB industry’s independent intellectual property rights inour country at present is relatively less, high-end products fall behind the UnitedStates and Japan, more research was imperatived. In this paper, the preparationof the magnet was in the vacuum sintering furnace,was similar to the doublealloy method, the composition of matrix is:(PrNd)30Gd3Al0.8B1.03Fe余.I maxedvarying proportions MgO and magnetic powder prepared before in a VacuumGlove Box, and ball-milling a certain period of time, then put mixed powdercompressed into cylinder and magnetization,sintered s in vacuum sinteringfurnace according to the sintering curve.The magnetic performance of eachsample was tested by the NIM-10000H magnetic measurement device, thecorrosion resistance in the hygrothermal or electrochemical environment wastested by high temperature and high pressure reaction kettle and CSelectrochemical workstation.I systematically researched the influence of thegrain boundary phase composition of magnets and the grain boundarymicrostructure by analysis means of XRD, SEM and EDS,was aimed to findinner link between magnetic performance and percentage composition of MgO.NdFeB alloy magnetic performance and corrosion resistant property can beimproved because of moderate MgO.Magnet’s density,remanent magnetism andmagnetic energy product increased by0.527g/cm3,0.179T,74kJ·m-3whenadding0.2%of MgO,however,magnetic property would gradually decreasewhen adding too much MgO.Magnets emerged sintering incompletephenomenon when adding1%,1.2%of MgO, meanwhile there were lots of tinypore.The magnet coercive force adding0.4%of MgO is the largest, increased of 120KA·m-1compared with them without adding MgO.When corrosion timereached24hours, the weightlessness of magnets without adding MgO wereminimum,the weightlessness of magnets adding0.2%of MgO were thelargest;When corrosion time reached48hours,the weight loss of magnets adding0.4%MgO were minimum, reduced6.2g·cm-2compared with magnets withoutadding MgO,suggesting that the time had a larger effect on corrosion.By XRD analysis, the growth direction of main grain (006) grain of magnetadding0.2%of MgO was good, the grain orientation degree was good.SEMpictures showed a new phase in the magnets adding MgO, it had a strongpinning effect to magnetic domain and conducived to increase the coerciveforce;the grain boundary of magnets adding0.4%of MgO wre clear, had a thinlayer phase with rich neodymium,the main grains had been thininged. By EDSfound that adding MgO only existed in the grain boundary and did not enter themain grain phase, the oxygen content of rich neodymium phase was graduallyincreasing with the adding of MgO, the structure of rich neodymium phase hadalso been changed.Moderate MgO could refine the grain of main phase,prevented the formation of large grain, grain growth orientation and grainboundary phase was optimized, increased the wettability of magnetic powderand improved the density of magnets, there were main reasons to improve themagnetic property, but it could cause the opposite effect when adding excessiveMgO.By accelerating oxidation experiment of high temperature and highpressure and electrochemical corrosion experiment, adding MgO enhanced thecorrosion resistance of magnets in the electrochemical environment,not onlyadvanced the magnetic corrosion potential, reduced the corrosion current densitybut also protected the original passive film;Weightlessness experiments alsoshowed that a moderate amount of MgO can enhance the corrosion resistance ofmagnets in the hygrothermal environment, I observed magnet corrosion pitssignificantly reduction and stable compounds from the microstructure,thesecompounds cut off the diffusion of corrosive medium, which is beneficial tograin boundary stability.