Reaction Mechanism And Permanent Magnetic Properties Of RE₂Fe₁₄C Materials Prepared By Mechanochemical Method

Magnetism and magnetic materials are essential to our daily life.The rare earth permanent magnetic materials have attracted much attention owing to their high energy density.Among all the rare earth permanent magnetic materials,the Nd-Fe-B based permanent magnets with excellent hard magnetic properties were extensively used in generators,hybrid vehicles and electronic communication devices.Unfortunately,the relatively low coercivity of Nd-Fe-B based materials limited their applications in strong magnetic field.So far,the well-known method to increase coercivity is substituting Nd with heavy earth element such as Dy and Td,which may inevitably increase the cost of the materials.Actually,the RE₂Fe₁₄?RE=Pr,Nd?compound which has similar crystal structure to Nd₂Fe₁₄B compound has excellent intrinsic magnetic properties as well.Even the RE₂Fe₁₄C has a lower saturation magnetization and Curie temperature than those of boride,the higher magnetocrystalline anisotropy makes it a probable candidate for Dy/Tb containing Nd₂Fe₁₄B permanent magnetic materials with high coercivity.However,the complex manufacture process of RE₂Fe₁₄C compound is the biggest obstacle for the industrial production of RE₂Fe₁₄C materials.The Pr₂Fe₁₄C and Nd₂Fe₁₄C phase is equilibrium structure below 800?and 880?.If the annealing process was carried out beyond the critical temperature,the RE₂Fe₁₄C phase will transform into the magnetically soft RE₂Fe₁₇C_x?x=0³?phase.However,under this critical temperature,the nucleation and growth of 2:14:1 phase are too sluggish,which will prolong the annealing time and increase the energy consumption.Therefore,the sintering process of Nd-Fe-B materials is not suitable for RE-Fe-C.The mechanical alloying and melt-spinning method to prepare the RE-Fe-C materials are still at the stage of experimental exploration.Based on this problem,a facile and novel mechanochemical method was used to prepare a series of RE-Fe-C magnetic materials.The phase evolution?microstructure and magnetic properties were studied systematically,the process optimization was performed at the meantime.The major research contents and results obtained are as follows:?1?The Pr-Fe-C based materials were synthesized by a mechanochemical method.The Pr-Fe alloy was disproportionated into PrH_2+?,Fe₇C₃ and?-Fe phases after high-energy ball milling in the heptane solution.Subsequently,the Pr₂Fe₁₄C-based magnetic materials were obtained during a vacuum annealing process.Addition of small amounts of boron into initial Pr-Fe alloy could promote the formation of 2:14:1 and suppress the formation of magnetically soft phases.The two-step recombination in RE-Fe-C system and the phase evolution during the gradient annealing process was discovered and analyzed.The coercivity of Pr-Fe-C materials papered by the alloy containing a small amount of B element is approaching2 T.The excessive Pr and C content of innital alloy will deteriorate the remanence and coercivity of the annealed sample,respectively.The optimal initial alloy composition was confirmed as Pr₃Fe₁₄B_0.1 by fine turning the Pr,Fe and B radio of initial alloy.?2?The effects of solvent species on disproportionated reaction and magnetic performance of annealed samples were investigated.The amount of H₂ released from as-milled samples was increased when the disproportion reaction was more completed.And the best permanent magnetic properties were obtained when the alloy milled in alkane with high H/C ratio.Meanwhile,the effects of various annealing temperature and holding time on the microstructure and magnetic properties were studied.It is found that the high annealing temperature and long holding time will lead to the decomposition of 2:14:1 phase and the excessive grain growth.Tuned the rate of dehydrogenation and recombination reaction by varying the heating rate of annealing process and discussed the effect of the heating rate on magnetic properties of the annealed sample.Based on the results from SEM images and Henkel plots,the size,morphology and IGEC can be enhanced by tuning the annealing conditions.The optimum magnetic properties were achieved with remanent magnetization of 7.35 kG,coercivity of 16.2 kOe and maximum energy product of 12.3 MGOe.?3?The high-abundance rare earth La and Ce substituted Nd₂Fe₁₄C hard magnetic alloys were prepared by mechanochemical method.The enhancements of microstructure and magnetic permanence were achieved by adjusting the substituted concentration of high-abundance rare earth.SEM results show that the remarkable difference in the morphology between La and Ce substituted Nd_2.5Fe₁₄B_0.08 sample.The shape of grain is tend to be spherical of the Ce containing sample,however,the increasing the La content leads to the appearance of square shape grains.The magnetic properties measurements show an appropriate substitution of La for Nd in the initial alloy increase the coercivity without reducing the remanence.However,substitution of Nd by Ce leads to deterioration of the magnetic properties.The substitution energies of Nd replaced by La and Ce in the 4g sites of Nd₂Fe₁₄C structure were obtained first-principles calculation.According to the experimental and calculation results,the La tends to be expelled from 2:14:1 phase to form more minor phase which may acts as the pinning center.In contrast,the Ce atoms could steadily site in Nd₂Fe₁₄C structure,which deteriorates the intrinsic hard magnetic properties.The synchronization of the grain refinement and the pinning effect lead to the improvement of coercivity from 15.6 kOe to 18.6 kOe when the 10 at.%La substitution for Nd in initial alloy Nd_2.5Fe₁₄B_0.08.