简介概要

Enhancing magnetic properties of anisotropic NdDyFeCoNbCuB powder by applying magnetic field at high temperature during hydrogen desorption

来源期刊：Rare Metals2010年第5期

论文作者：P.De Rango

文章页码：480 - 485

摘要：Anisotropic powder was prepared with precursor（NdDy）-（FeCoNbCu）-B sintered magnets by hydrogen decrepitation,desorption,and subsequent annealing treatment.The hydrogen desorption was performed in magnetic fields of 0,1,3,and 5 T.The orientation of tetragonal phase grains of the powder was evaluated from the hysteresis loops measured by extraction magnetometer.Residual hydrogen content of the powder was evaluated by thermal-magnetic analysis.The powder with Hcj,Br,and（BH）max of 1138 kA·m-1,1.029 T,and 172.5 kJ·m-3,respectively,was achieved under the condition of the magnetic field of 3 T.Magnetic properties of the powder,especially,the remanence of the powder,are enhanced upon magnetic fields,which is due to better orientation of powder particles and less residual hydrogen in the powder resulted from the magnetic field during the hydrogen desorption process.

Rare Metals 2010,29(05),480-485

Enhancing magnetic properties of anisotropic NdDyFeCoNbCuB powder by applying magnetic field at high temperature during hydrogen desorption

P.De Rango D.Fruchart

作者简介：LUO Jianjun,E-mail:luojianj@c-nin.com;

收稿日期：22 September 2009

基金：the French Embassy in Beijing for provision of a collaborative research grant as part of a co-research program under the frame of LIA-LAS2M between Northwestern Polytechnic University-Xi'an,China and CNRS-Grenoble,France;

Enhancing magnetic properties of anisotropic NdDyFeCoNbCuB powder by applying magnetic field at high temperature during hydrogen desorption

Abstract：

Anisotropic powder was prepared with precursor(NdDy)-(FeCoNbCu)-B sintered magnets by hydrogen decrepitation,desorption,and subsequent annealing treatment.The hydrogen desorption was performed in magnetic fields of 0,1,3,and 5 T.The orientation of tetragonal phase grains of the powder was evaluated from the hysteresis loops measured by extraction magnetometer.Residual hydrogen content of the powder was evaluated by thermal-magnetic analysis.The powder with Hcj,Br,and(BH)max of 1138 kA·m-1,1.029 T,and 172.5 kJ·m-3,respectively,was achieved under the condition of the magnetic field of 3 T.Magnetic properties of the powder,especially,the remanence of the powder,are enhanced upon magnetic fields,which is due to better orientation of powder particles and less residual hydrogen in the powder resulted from the magnetic field during the hydrogen desorption process.

Keyword：

permanent magnets;hydrogen absorbing materials;anisotropy;magnetic measurements;magnetic properties;

Received： 22 September 2009

1. Introduction

High-energy anisotropic Nd-Fe-B powders have been strongly required for high-performance bonded magnets.Therefore,it is most attractive to fabricate coercive and anisotropic powders using waste sintered Nd-Fe-B magnets by hydrogen decrepitation(HD),desorption,and subsequent annealing treatment by which their coercive and textured characteristics can be largely restored in the powders[1-2].

HD has been well developed as an effective technique to fabricate Nd-Fe-B type powders using sintered bulk magnets or textured ingots because magnets and ingots can be crushed into powders without damaging the textured microstructure by using this technique[1,3].However,intrinsic coercivity(H_cj)and maximum energy product((BH)_max)of Nd-Fe-B magnets decrease sharply upon hydrogenation[4],which is correlative to the decrease of magnetocrystalline anisotropy(H_A)of Nd₂Fe₁₄BH_x:magnetocrystalline anisotropy(H_A)of Nd₂Fe₁₄BH_x decreases from 8 to 2 T when x value increases from 0 to 4[5].Therefore,hydrogen should be removed from the lattice of Nd₂Fe₁₄BH_x for recovering magnetic properties of Nd-Fe-B magnets.This process is known as hydrogen desorption.H_cj and(BH)_maxof sintered Nd-Fe-B magnets prepared by hydrogen desorption powders can be well recovered with the decrease of residual hydrogen in the magnets[4,6].Hydrogen atoms can escape more easily from Nd₂Fe₁₄BH_x phase than from Nd-rich phase hydride at less than 650°C in vacuum[7].In addition,Nd-rich phase hydride alloying with Co,Cu,etc.,is more stable;hydrogen atoms are more difficult to be desorbed until the temperature is up to 1000°C in vacuum[7].

On the other hand,Yamaguchi et al.[8-9]have observed remarkable effects of the magnetic field on the chemica equilibrium of systems containing ferromagnetic hydrides such as LaCo₅H_x systems:the applied magnetic field causes chemical equilibrium to shift towards the side that has greater magnetization so that magnetic energy is lowered De Rango et al.[10]have presented similar results obtained on NdFe_10.5V_1.5 compound where the formation of nitride is enhanced by applying a strong magnetic field,because Nd(Fe,V)₁₂ phase is paramagnetic whereas the nitride to be formed is ferromagnetic at the temperature of nitrogenation Liesert et al.[11]have shown that a high magnetic field is effective on Nd₂Fe₁₄B nuclei around Curie temperature during hydrogen disproportionation desorption recrystallisation(HDDR)process.

In the present paper,(NdDy)-(FeCoNbCu)-B powder was obtained from textured sintered bulk magnets by means of the technique of hydrogen decrepitation,desorption,and subsequent annealing treatment.We focused on the study about the effect of magnetic field during the hydrogen desorption process,intending to enhance magnetic properties of the powder.

2. Experimental

(Nd Dy)₁₅(Fe Co Nb Cu)₇₉B₆ sintered magnets used,which were manufactured by powder metallurgy technique,had H_ci,B_r,and(BH)_max of 2467.6 kA?m^-1,1.11 T,and 234 kJ?m^-3,respectively.Scanning electron microscopy(SEM),energy dispersive X-ray(EDX),and X-ray diffraction(XRD)analysis show that the tetragonal phase(φphase)is(Nd Dy)₂(Fe Co)₁₄B,and the Nd-rich phase is(Nd Dy)(Fe Co Cu)in the magnets.

A bulk of(Nd Dy)(Fe Co Nb Cu)B sintered magnets was crushed into pieces by hand,and then about 60 g of these pieces were charged into a furnace chamber and vacuumed to 1.0×10^-3 Pa.Then,the powder was prepared by following a process as shown schematically in Fig.1.Hydrogen gas(purity of 99.999%)was introduced into the chamber until the pressure of hydrogen inside reached 70 k Pa at150°C to crush the magnet pieces into powder.This decrepitated powder was then subjected to a hydrogen desorption process at 820°C for 1 h under magnetic fields of 0,1,3,and 5 T,respectively,in a superconductor magnet[7].The following annealing treatment was then performed at 510°C for 1 h.During the hydrogen desorption process,the magnetic field was applied in the heating stage from 450°C to820°C,dwelling stage at 820°C,and in the cooling stage from 820°C to 450°C(Section II in Fig.1).

After that,the powder was sealed inside quartz tubes for thermo-magnetic measurement,which is very helpful to evaluate the HDDR process of Nd-Fe-B alloys due to its sensitive reflection to disproportionation and recombination reaction between Nd₂Fe₁₄B and hydrogen[11].In this paper,the thermo-magnetic measurement was employed to reveal the residual hydrogen content in the powder according to the disproportionation reaction between Nd₂Fe₁₄B and hydrogen The microstructure of the powder was observed on a scanning electron microscope(JEOL SEM JSM5600LV).

Magnetic properties of the powder were measured by an extraction magnetometer in terms of cylinder bonded magnets of 3 mm in diameter and 7 mm in length,which were prepared by mixing liquid epoxy resin with the powder,curing under a magnetic field of 5 T.With the help of the magnetic field,the powder particles in the bonded magnets rotated parallel to each other,and to macroscopic texture axis of the bonded magnets.Values of magnetic properties of the powder were corrected to the density of 7.5×10³kg?m^-3.

Fig.1.Technical procedure of the hydrogen decrepitation-desorption-annealing process for the HD powder.

3. Results and discussion

3.1. Magnetic properties of the powder

Fig.2 shows the variation of the magnetic properties versus magnetic field during the hydrogen desorption process.B_r of the powder increased from 0.99 to 1.029 T by about 4.2%,when the magnetic field increased from 0 to 3 T(BH)_max increased from 137.5 to 172.5 kJ/m³ by about 4.4%when the magnetic field increased from 0 to 3 T;while H_cjof the powder increased from 1106 to 1138 kA/m by about2.9%when the magnetic field increased from 0 to 3 T.Moreover,H_cj was much lower at a high magnetic field of 5T.The lower H_cj of the powder at a high magnetic field deduced that there were some other factors more important than the magnetic field affecting the H_cj of the powder during the hydrogen desorption process.

Fig.2.Magnetic properties of the powder with hydrogen desorption under magnetic fields:(a)Br;(b)(BH)max;(c)Hcj.

Enhancement of magnetic properties of anisotropic(Nd Dy)₁₅(Fe Co Nb Cu)₇₉B₆ powder may come from two possible mechanisms:one is that macroscopic magnetic effect contributed to the better orientation of the powder particles;the other is less residual hydrogen content in the powder,as will be discussed in the following sections.

3.2. Orientation of the powder particles

The ratio of M_r∥/M_r⊥has been employed to evaluate orientation of powder particles[7],where M_r∥and M_r⊥represent residual magnetization along and perpendicular to the magnetic direction of the powder,respectively.The larger the ratio of M_r∥/M_r⊥is,the better the anisotropy of powder is.

Fig.3 shows the hysteretic loops of the powder prepared under the magnetic fields of 0,1,3,and 5 T.It was mos impressed that M_r⊥of the powder decreased obviously with the increase of the magnetic fields from 0 to 5 T,although M_r∥of the powder did not change so much.

Fig.3.Hysteretic loops of the powder prepared under the magnetic fields of 0,1,3,and 5 T.

Fig.4 demonstrates that the ratio of M_r∥/M_r⊥of the powder increased by about 1.8 times with the increase of magnetic field from 0 to 5 T.This illustrates that M_r of the powder was enhanced due to the better orientation of(Nd Dy)₂(Fe Co)₁₄B grains,which resulted in fewer misaligned(NdDy)₂(Fe Co)₁₄B grains in the powder[12].

Furthermore,the orientation of powder particles during hydrogen desorption under magnetic field can be affected by the shape of powder particles.Mc Guiness et al.[13-14]have presented that hydrogen decrepitation of textured Nd-Fe-B sintered magnets happens along the direction perpendicular to the easy axis of magnets,resulting in flakelike pieces with the thinner direction along the easy axis.Fig.5shows the flake-like powder particles obtained in this test.This type of powder particles has large de-magnetic force.The applied magnetic field is considered to be resistive to the de-magnetic force,when the powder particles is orientated by the applied magnetic field during the hydrogen desorption in this test.

Fig.4.Anisotropy ratio of the powder versus the magnetic field during the hydrogen desorption process.

Fig.5.Flakelike morphology of the particles of the NdDyFe-Co Nb Cu B HD powder.

Because the inpidual decrepitated powder particles still hold the texture crystal structure originating from the bulk magnets by using hydrogen decrepitation[1-2,15],some of the powder particles will be bonded by liquid Nd-rich phase,which results from hydrogen desorption of hydride Nd-rich phase at high temperature.These particles finally form bigger powder particles during hydrogen desorption and the subsequent cooling process.As a result,the enhanced orientation of the powder particles by applying a magnetic field at high temperature above 450°C during the hydrogen desorption process comes from the fact that the reformed powder particles can keep the texture crystal structure of the origina bulk magnets with the help of the magnetic field during the hydrogen desorption process.This can be explained as follows:without the magnetic field,although keeping the textured crystal structure inside,the inpidual powder particles are distributed randomly when they undergo hydrogen desorption and subsequent cooling,resulting in less macroscopic orientation of tetragonal phase grains in the reformed powder particles.Contrarily,with the help of the magnetic field,the powder particles are rotated along the direction of the applied magnetic field during the hydrogen desorption process,forming reformed powder particles with textured crystal structure as that of the original bulk magnets.

3.3. Thermal-magnetic analysis and the residual hydro-gen content in the powder

Fig.6 shows thermo-magnetic measurement curves for the powder.In the case of the powder undergoing hydrogen desorption without magnetic field(0 T),a visible step a around 600°C was observed in the heating curve,which indicated the reaction between hydrogen and tetragonal phase:(Nd Dy)₂(Fe Co)₁₄B+H₂→(Nd Dy)H₂+α-(Fe Co)+FeB₂[16].

There was no obvious step found in the heating curves of the powder with hydrogen desorption under the magnetic field,which indicates that no reaction occurred between hydrogen and tetragonal phase.In other words,there should be no or less residual hydrogen remaining in the powder Therefore,it can be concluded that the magnetic field during the hydrogen desorption process was helpful in releasing hydrogen from the powder.

Fig.6.Thermal-magnetic measurement curves of the powder with hydrogen desorption under magnetic fields of 0,1,3,and 5 T.

The hydrogen desorption from the hydride(Nd Dy)-(Fe Co Nb Cu)-B powder includes two processes upon heating in vacuum:hydrogen atoms are released from tetragona hydride phase(Eq.(1))and Nd-rich hydride phase(Eq.(2)):(Nd Dy)₂(Fe Co)₁₄BH_x→(Nd Dy)₂(Fe Co)₁₄B+H₂(1)(Nd Dy Fe Co Cu)H_2.36→(Nd Dy Fe Co Cu)+H₂(2)

Here,(Nd Dy Fe Co Cu)is the Nd-rich phase of(Nd Dy)₁₅(Fe Co Cu)₇₉B₆magnets as mentioned above.

It has been reported that applied magnetic field is expected to cause chemical equilibrium to shift towards the side which has greater magnetization in the system including paramagnetic phase[8,10,17].The applied magnetic field may not be helpful in releasing hydrogen atoms from the tetragonal hydride phase(Eq.(1)),because the tetragona hydride has larger magnetization than that of the tetragona phase[15].Although magnetic properties of Nd-rich phase and Nd-rich hydride phase are not very clear until now,we may suppose that applying a magnetic field is helpful in making Eq.(2)shift towards the right side,which is beneficial to lowering the residual hydrogen content of the powder.

4. Conclusions

The magnetic properties(B_r and(BH)_max)of anisotropic(Nd Dy)₁₅(Fe Co Nb Cu)₇₉B₆powder prepared by hydrogen decrepitation-desorption-annealing were enhanced when the powder was hydrogen desorbed under the magnetic fields from 1 to 5 T.H_cj of the powder was not changed significantly with the magnetic field during the hydrogen desorption process.The anisotropic powder with H_cj,B_r,and(BH)_max of 1138 kA?m^-1,1.029 T,and 172.5 kJ?m^-3,respectively,was obtained.The enhancement of the magnetic performance of the powder with hydrogen desorption in the magnetic field comes from two possible mechanisms:one is the macroscopic magnetic effect resulting from better orientation of the powder particles;the other is less residual hydrogen content remaining in the powder.