Study On Preparation And Properties Of Bonded NdFeB/Sr-ferrite Hybrid Magnets

Wide attention has been attracted to the bonded magnets because of their advantages ofeasy molding special-shaped parts and displaying good uniformity of magnetic properties.Currently, bonded NdFeB and bonded ferrite are the widely used bonded magnets. BondedNdFeB magnets show excellent magnetic properties but poor thermal stability and expensive.While bonded ferrite magnets show low magnetic properties but high temperature stabilityand low price. The bonded hybrid magnets made of NdFeB and ferrite have the advantages ofcontinuously adjustable magnetic properties, good thermal stability and low cost, which canbe applied in different occasions.In this thesis, the effects of ball milling, particle size distribution, solvent, the mass ratioof magnetic powder (NdFeB and ferrite), binders, coupling agents and lubricant, molding andcuring process on the density, magnetic properties and compressive strength of the bondedhybrid magnets had been systematically investigated.. The main contents and results are asfollows:After ball milling, NdFeB powders are broken and the particle size is too small to beeasily oxidized, which will damage the magnetic properties. By adjusting the magneticpowder particle size to reasonable distribution, it will benefit for improving the, magneticproperties of the magnets. When NdFeB and ferrite powders were mixed with differentproportions, the temperature coefficient of coercive force for the hybrid magnets graduallyincreased. When the ferrite content reached to65wt.%, the temperature coefficient coerciveforce for the hybrid magnets was positive. At300K,, the actual coercivity of the hybridmagnets was larger than the calculated value by dilution law when the addition of NdFeBcontent exceeded42wt.%. while the actual coercive force was larger than the calculated valueby dilution law at375K when the the addition of NdFeB content was more than30.wt.%,Whether at300K or375K, the actual remanence of the hybrid magnets was smaller than thecalculated value by dilution law.Adopting large polarity, volatile acetone as solvent, it was easy to obtain hybrid magnetswith high magnetic properties Compared with silane coupling agent, the magnets could obtainhigher magnetic properties but lower compressive strength after coupling treatment onmagnetic powders with titanate coupling agent. The optimium dosage of titanate couplingagent was0.6wt.%, while the silane coupling agent was1wt.%. Mezzo epoxy binder washelpful to obtain high magnetic properties for the magnets, the higher the epoxy value, bebetter the compressive strength, and the optimum dosage of the binder was3wt.%. In the process of moulding, pressing temperature, pressing pressure and holding time hada great impact on the magnetic properties and compressive strength of the bonded hybridmagnets. The optimal parameters were77℃-900MPa-8minutes and the optimal properties asfollows: D=5.5g/cm3, Br=0.52T, Hci=740.48kA/m,(BH)max=39.82kJ/m3, σbc=185.98MPa.The bonded hybrid magnets were cured in the vacuum tube furnace vacuumed andpurged argon atmosphere had the best magnetic properties, but the compressive strength wassmaller than that of the hybrid magnets cured at ordinary resistor furnace. With increasingcuring temperature and curing time, the compressive strength and magnetic properties werefirstly increased and then decreased the optimized processing parameters was180℃for120min, and then cooled down to room temperature in the furnace.