| Nanocomposite magnets consisting of a hard magnetic phase with high anisotropy and a soft magnetic phases with high saturation magnetization have been widely studied experimentally and theoretically owing to their unusually high remanence,energy product and low cost.The high remanence in nanocomposites arises from the exchange coupling between the magnetically hard and soft phases.In this dissertation,the influences of alloy compositions and crystallization techniques on the magnetic properties and microstructure for nanocomposite Nd2Fe14B/α-Fe permanent magnets have been investigated clearly.X-ray diffraction (XRD),differential scanning calorimeter(DSC),transimission electron microscope (TEM),three-dimensional atom probe(3DAP),vibrating sample magnetometer(VSM) and Mrssbauer spectroscopy have been employed.Special attention has been paid to the effects of Zr,Nb and Ga on the crystallization behavior,microstructure,magnetic properties and temperature stability for nanocomposite Nd2Fe14B/α-Fe magnets. Pulsed magnetic field was employed when annealing,and the effects of manetic annealing on the exchange coupling,microstructure and magnetic properties for nanocomposite Nd2Fe14B/α-Fe magnets have been studied in detail.The results show that:The addition of Zr changed the crystallization behavior of amorphous phase, inhibited the grain growth,and thus enhanced the exchange coupling bwteen magnetically soft and hard phases for nanocomposite Nd9.5Fe79-xCo5ZrxB6.5(x=0～4) magnets.The temeperature stability and irreversible flux aging loss were improved by proper Zr addition.The optimal magnetic properties for nanocomposite Nd9.5Fe76Co5Zr3B6.5 magnet are:α=-0.13%/℃,β=-0.35%/℃andδirr=-4.50%, respectively. The addition of Nb element improved obviously the coercivity of nanocomposite Nd9.5Fe79-xCo5NbxB6.5(x=0～3) magnets.Nb addition made grains smaller and led to the homogenization and regularization of grains,which is benifical to reduce inner dispersal magnetic field and improve the thermal stability of the alloys.An intergranular NbFeB phase with Nb-enriched atoms between magnetic phases was observed by 3DAP technique,which is the main reason for microstructure refinement and magnetic properties improvement for nanocomposite Nd9.5Fe77Co2Nb2B6.5 alloy.Proper Ga addition improved the Curie temperature of nanocomposite Nd8.5Fe77.6-xCo5Zr2.7GaxB6.2(x=0～1) alloys,thus the temperature coefficient and irreversible flux aging loss were improved.The optimal magnetic properties of Jr= 0.73T,iHc=643kA/m,(BH)max=82kJ/m3,α=-0.095%/℃,β=-0.35%/℃andδirr =-4.06%were obtained for nanocomposite Nd8.5Fe77Co5Zr2.7Ga0.6B6.2 alloy.The melt-spun Nd8.5Fe77Co5Zr2.7Ga0.6B6.2 alloy ribbons were annealed in a pulsed magnetic field.The results revealed that the optimal annealing temperature was degraded and the nucleation rate was elevated by magnetic annealing.The magnetic annealing also led to a refined microstructure and an enhanced exchange coupling between magnetically hard and soft phases.The optimal magnetic properties of iHc= 586kA/m,Jr= 1.01T and(BH)max=138kJ/m3 were obtained for nanocomposite Nd8.5Fe77Co5Zr2.7Ga0.6B6.2 alloy ribbons annealed with a pulsed magnetic field at 670℃for 4rain.The(BH)max was enhanced by 15%compared with that of samples without magnetic annealing.The melt-spun Nd8.5Fe77Co5Zr2.7Ga0.6B6.2 alloy ribbons were annealed in a pulsed magnetic field at temperatures near the Curie temperature of the alloy.The results demonstrated that the remanence and the maximum energy product were obviously improved when annealed below the Curie temperature of the alloy.The (BH)max was enhanced by 24.8%compared with that of samples without magnetic annealing at 300℃.It provides a new way to improve the magnetic properties of nanocomposite permanent magnets produced by pulsed magnetic annealing at temperatures below the Curie point of the alloys.
|
PDF Full Text Request