| Sintered Nd-Fe-B magnet is a new generation of rare earth permanent magnetic material with outstanding magnetic properties and high cost performance,and has been applied widely in various hi-tech fields.However,the disadvantages of low coercivity,poor thermal stability,and easy corrosion seriously restrict its further development and application.In addition to the magnet composition,microstructure is also an important factor influencing the properties of sintered Nd-Fe-B magnets.Therefore,how to improve its overall properties through adjusting microstructure of the magnet is of both theoretical and practical significance.In this dissertation,aiming to adjust and modify the microstructure of sintered Nd-Fe-B materials,the effects of intergranular phase modification on the magnetic properties,corrosion resistance and thermal stability were systematically investigated.An ideal distribution model of nano-sized additives on the surface of magnetic powders was proposed.Several possible influencing mechanisms of different nano-sized additives on grain size and distribution of the main phase Nd2Fe14B,on the interface structure of main phase/Nd-rich phase,and on the properties of magnet were revealed on the basis of thermodynamic calculation.Meanwhile,the variation law of the properties and microstructure of magnet with additive content of ultrafine powders were made clear by intergranular addition.Furthermore,through investigating the variation of temperature coefficient with intrinsic property and microstructral parameter,the design principle of sintered Nd-Fe-B magnets with high thermal stability was suggested,and the magnet with low temperature coefficient was successfully prepared.The main results are as follows:Both the magnetic properties and corrosion resistance of sintered Nd-Fe-B magnets can be improved simultaneously by the intergranular phase modification of nano-sized additives.The results show that the additions of various nanopowders can all improve the coercivity Hcj of magnets.The intergranular additions of Cu,SiO2,ZnO and AIN nanopowders can refine the grains of main phase effectively and make them distribute more homogeneously,increasing the nucleation field of reversed domain.The intergranular additions of Cu,Zn,SiO2 and ZnO nanopowders can also improve the characteristics of Nd-rich phase,and make it distribute more homogeneously around the main phase,weakening the magnetic exchange coupling interaction. The intergranular additions of Dy2O3 nanopowders can harden the grain surface of the main phase.The intergranular additions of Cu,Zn,SiO2 and ZnO nanopowders can promote the sintering densification of magnets,increasing the magnet density effectively,thereby improving the remanence Br and magnetic energy product(BH)max;whereas the additives of Dy2O3 and AlN nanopowders played an opposite role.Furthermore,all the intergranular additions of Cu, Zn,SiO2,ZnO and AlN nanopowders can increase the electrode potential of the intergranular phase and optimize the microstructure,consequently inhibiting intergranular corrosion and thus improving the corrosion resistance of magnet.On the basis of thermodynamic analysis,the influencing mechanism of various nanopowders addition on the composition and the physico-chemical characteristics of the intergranular Nd-rich phase have been described.It is found that the added Cu and Zn nanopowders reacted mainly with the intergranular Nd-rich phase to form new grain boundary phase or dissolved into the Nd-rich phase during sintering.Both of them do not enter the main phase.Thermodynamic calculations show that SiO2 and ZnO nanopowders added into grain boundaries react with Nd-rich phase and form Nd2O3 particles,as well as the elements Si or Zn at the sintering temperature.These Nd2O3 particles can inhibit the grain growth of the main phase,while Si and Zn can improve the physico-chemical characteristics of the Nd-rich phase, consequently optimizing the microstructure.According to the Miedema theoretical models for the regular melt,Dy formed through reduction reaction between Dy2O3 and the Nd-rich phase can easily diffuse into the main phase grain surface to form Dy2Fe14B with higher magnetocrystalline anisotropy field,playing a role of surface magnetic hardening.The high performance,low cost sintered Nd-Fe-B magnet has been prepared through intergranular addition of ultrafine powders recycled during jet-milling process.The influencing mechanism of additive content of ultrafine powders on the properties and microstructure is also revealed.The results show that the composition of ultrafine powders consists mainly of rare earth elements of Nd,Pr and Dy.The intergranular addition can improve the magnetic properties, especially the coercivity Hcj.The intergranular addition of ultrafine powders can decrease the diffusion mass transfer rate of solute during sintering,hindering the grain growth of the main phase,and thus improving the nucleation field of reversed domain.The intergranular addition of ultrafine powders is helpful to the formation of Dy2Fe14B and Pr2Fe14B with higher magnetocrystalline anisotropy field on the grain surface of the main phase,playing a role of surface magnetic hardening,consequently improving the coercivity Hcj of magnet significantly. When the additive content is below 5 wt%,the magnetic energy product(BH)max can also be improved.Moreover,the intergranular addition of ultrafine powders also results in the decrease of the temperature coefficient of coercivityβand the irreversible flux loss hirr,which improves the thermal stability of sintered Nd-Fe-B magnets.The quantitative relationship between temperature coefficient and intrinsic property and microstructral parameter has been established,which provide a theoretical guidance for designing low temperature coefficient magnet.Sintered Nd-Fe-B magnet with high thermal stability has also been prepared.The relationship between temperature coefficient of coercivityβand magnetocrystalline anisotropy field HA and microstructral parameter c/Neff have been established through theoretical analysis,it is found that the higher HA is and the bigger c/Neff is, the smallerβis.Based on this,the sintered Nd-Fe-B magnet with low temperature coefficient have been successfully prepared through composition design and microstructure optimization,βis only -0.385%/℃at the temperature interval of 20～150℃.Its coercivity Hcj can reach 557 kA/m at 220℃,indicating that the magnet possesses very high thermal stability.Furthermore, according to the theoretical relationship between highest operating temperature andβ,the influence of temperature coefficient of coercivity for sintered Nd-Fe-B magnets on the highest operating temperature is described.At the same Hcj,the highest operating temperature increases significantly with the decrease ofβ,suggesting that decreasingβis an effective method to prepare magnet with high thermal stability.
|
PDF Full Text Request