稀有金属(英文版) 2020,39(01),48-54

Enhancement of magnetic properties of hot pressed/die-upset Dy-free Nd-Fe-B magnets with Cu/Nd coating by wet process

Jeehye Kwon Dajeong Lee Dayoung Yoo Seongkyu Park Hee-Ryoung Cha Hae-Woong Kwon Junggoo Lee Dongyun Lee

Department of Advanced Circuit Interconnection,Pusan National University

Department of Nano Fusion Technology,Pusan National University

Department of Nanoenergy Engineering,Pusan National University

Powder and Ceramics Department,Korea Institute of Machinery and Materials

Department of Materials Science and Engineering,Pukyong National University

作者简介：*Dongyun Lee e-mail:dlee@pusan.ac.kr;

收稿日期：17 April 2018

基金：financially supported by the National Research Foundation of Korea (No.2015R1A2A2A01 002795);the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Trade,Industry,and Energy, Republic of Korea (No.10080382);the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No.2018R1D1A1B07041358);

Enhancement of magnetic properties of hot pressed/die-upset Dy-free Nd-Fe-B magnets with Cu/Nd coating by wet process

Jeehye Kwon Dajeong Lee Dayoung Yoo Seongkyu Park Hee-Ryoung Cha Hae-Woong Kwon Junggoo Lee Dongyun Lee

Department of Advanced Circuit Interconnection,Pusan National University

Department of Nano Fusion Technology,Pusan National University

Department of Nanoenergy Engineering,Pusan National University

Powder and Ceramics Department,Korea Institute of Machinery and Materials

Department of Materials Science and Engineering,Pukyong National University

Abstract：

A grain boundary diffusion process(GBDP) was adopted to improve magnetic properties of Dy-free highly coercive Nd-Fe-B permanent magnet by coating thin layers of Nd and Cu in grain boundaries.For GBDP of Nd and Cu,Nd and Cu were coated by wet process,e.g.,electrochemical and electroless on Nd-Fe-B magnets,which was fabricated by hot-deformed/die-upset with meltspun specimen.Heat treatment was performed for 20 min at 600℃followed by several different cooling conditions.The cooling conditions after heat treatment were varied to understand distribution and micros tructural effects of Nd and Cu species in grain boundaries.The coercivity increased from 1.565 to 1.637 T in oil cooling rate but remanence decreased,while remanence jumped with little decrease in coercivity in furnace cooling.Micros tructure analyses suggested that the coercivity was closely related to the cooling rate as well as distribution of Nd.The mechanism of coercivity enhancement due to the cooling rate was discussed based on the results presented here and those in the literature.

Keyword：

Magnetic materials; Electrochemical deposition; Diffusion; Scanning electron microscopy; Coercivity;

Received： 17 April 2018

1 Introduction

Dy,a heavy rare-earth element,is known as an effective element ^to increase magnetic properties of Nd-Fe-Bmagnets such as coercivity,temperature stability,and s^o forth.Owing to the limited reserves and high cos_t of Dy,intensive effort was devoted to the development of Dy-fr_ee high-performance Nd-Fe-B magnets [ 1, 2, 3, 4, 5, 6] To produce a high-performance,Dy-free Nd-Fe-B permanent magnet to be used at high temperatures,stringent control of grain size and interfaces,especially intergranular phases,is an important factor in the manufacturing process [7-1 ^{[7,8,9,10,11,12,13]}] .There is a need to fabricate materials whic_h have grains with a consistent domain size aligned in a specific direction with good separation but very thin interfaces with nonmagnetic materials [ 14, 15] In this stud_y,a melt-spun powder,which is about～20μm in thickness and 100-300um in length with 40-50-nm _sized _grains,was used to produce domain-sized grains,while hot pressing and dieupsetting processes ^were designed t_o form Nd-Fe-Bmagnets with anisotropic grains[16-¹9].The focus w_as on the application of thin coating layers between the grains to produce well-separated interfaces using n^on-magneticspecies.By controlling the microstructures of the interfaces,the degradation in magnetic properties generated mainly by defects and excessiv_e grain growth of Nd-Fe-Bcan be restricte^d.Therefor_e,a well-known grain boundary diffusion process (GBDP) was used with details described elsewhere [ 20, 21, 22] .An increase i_n coercivity can be easily obtained through rare-earth element diffusion by using coating methods such as dipping,sputtering,evaporation,and electrodeposition[2₃,24].

2 Experimental

Nanocrystalline Nd-Fe-B magnets we^re fabricated with commercially available MQU-F M^QU powder (Molycorp Magnequench,_Nd13.6_{Fe73.6Co6.6Ga0.6B5.6)} vi^a a hot pressing process to obtain highl^y anisotropic dense bulk magnets.Nd-Fe-B cylindrical and anisotropic magnets (φ=7mm,h₌6mm) were formed by hot pressing followed by a die-upsetting proces_s.Hot pressiⁿg was performed at655^℃under 3.43 kN with a constan_t deformation ra^te of0.22 mm·^s-1 in ^vac_uum,and die u_psetting was perfo_rmed at 655^℃w_ith a constant defo_rmation rate (0.35 mm·s^-1,9.8 kN).After die-upsetting,the magnet was cooled with Ar gas.Subsequently,the magnet was cut into pieces with sizes of 3 mm×3 mm×1 _mm.O_xides and impurities formed on the surface of the magnet during fabrication process were removed by chemical etching using a 2 vol%nital solution.

To enhance the coercivity in GBDP,the magnet was coated with an Nd-Cu alloy.The alloy was formed by electroplating of Nd on the magnet using a potentiostat(CompactstatTM,IVIUM Tech.,Netherlands) followed by electroless deposition of Cu and heat treatment to form the Nd-Cu alloy.To optimize the plating conditions,the applied potential,plating time,and plating temperature were varied.Pt mesh and saturated Ag/AgCl were used as the counter electrode and reference electrode,respectively,for Nd coating with aqueous Nd(NO3)3 (Sigma-Aldrich,99.9%) as the Nd precursor.An aqueous CuSO4 solution(Sigma-Aldrich,99.0%) was used as the Cu precursor for electroless deposition of Cu.In order to prepare a eutectic Cu-Nd alloy,the ion concentrations of Nd and Cu precursors were,respectively,1.25 and 3.00 mmol,and plated for 60 and 30 s,respectively.The temperatures of both electrolytes were maintained at 25℃using a thermocirculation system (SaeHan Ultrasonic Co.,Korea).The magnet coated with Cu-Nd alloy was heat treated at600℃(at a heating rate of 17℃·min-1) for 25 min in high vacuum (～1x10-5 Pa).The magnetic properties of the as-fabricated and treated samples were measured using a vibrating sample magnetometer (VSM,MicroMagTM3900).The microstructures of the samples were observedvia field-emission scanning electron microscope (FESEM,Carl Zeiss AG,SUPRA40 VP) equipped with a backscattered electron (BSE) detector.To ascertain how cooling rate affects the elemental distribution in the grain boundaries,transmission electron microscopy (TEM,TALOSF200X) was performed.The chemical compositions of the samples were analyzed using energy dispersive spectroscopy (EDS).

3 Results and discussion

A Nd-Cu system was chosen in this study because it forms lower melting point eutectic alloys,and is amenable to electrochemical and/or electroless deposition methods,which are cost-effective and suitable for mass production.Interfacial microstructur_es could be controlle^d b^y heattreatment condition_s,and in particular,on cooling rat^e after GBDP.It is well known that the change of interfacial microstructures with non-magnetic materials has a large influence on the magnetic properties of the material (_e.g.,coercivit^y and remanence).Nd and Cu ^were deposited by electrochemical and electroless methods,r_espectively,followed by heat treatment in vacuum at 600℃for25 min.They were cooled down ^to room _temperature in three different ways:(1) air-expos_ed furnace cooling;(2)air-closed furnace cooling;and (3) oil cooling.Heattreating temperature is essential to grain growth _and therefore cannot be changed by a large amoun_t.As a result,the cooling parameter was varie_d,and it was found that the Nd-rich phases became well distributed through the grai_n boundaries with relatively fast cooling rate _(i.e.,oil cooling) compared _with the other t^wo methods.Coercivity increased from 1.565 t_o 1.⁶37 T in oil cooling compared to air-expose_d furnace cooli_ng,but decreased from 1.565 to1.358 T in case of air-closed furnace cooling.However,the remanence and maximum energy product ((BH)ma_x)increased with slow cooling because of the large jump in remanence,_which will be later discussed in detai_l.In order to identify the phases during the GBDP and cooling process,TEM analysis of interfacial struct^ures was performed.Heat-treatment temperature and time for GBDP ^of NdCu coatings significantly affect the gr_ain size of the NdFe-B magnet,^which directly influences the coercivity a_nd remanence of the magnet.Therefore,_attempts ^were made to minimize grain growth during heat treatment [ 20] .To optimize heat-treatment conditions,annealing processes were performed over a wide range of temperatures starting at 500°C,since the Nd-Cu eutectic point is～520℃,up to 800℃with different holding time and cooling rates.Figure 1 shows the fracture surfaces of Nd-Fe-B magnets at different annealing temperatures for 25 min.Figu_re 1a depicts the fracture surface f^or the as-fabricated magnet,and it can be seen that grain growth starts to occur at500℃and it significantly increases aft^er 7⁰0℃.At800℃,the size of the grain has grown by sub-micromet_er.In Fig.1g,it is clear that increased g_rain size leads to a reduction in magnetic properties such as coercivity and remanence.Controlling heat-treatment time and temperature is an effective way to prevent grain growth.As a result,600℃was determined to be the most appropriate at a temperature of GBDP.

Fig.1 SEM images of fracture surfaces of heat-treated samples with a no heat treatment,b 500℃,c 600℃,d 650℃,e 700℃,and f 800℃for 25 min followed by oil cooling;g relationship between coercivity and grain size

As mentioned previously,electrodeposition and electroless deposition methods were both used to form a NdCu alloy on the surface of the magnet.It is likely that hydroxide films could be produced on the surface during electrodeposition.This process occurs by the formation of metal hydroxide as described in Reactions (1) and (2),and subsequent formation of the oxide form by heat treatment as described in Reaction (3) [ 25, 26] .

If non-magnetic Nd oxides are uniformly distributed on the grain boundaries by the heat treatment,the overall coercivity of the magnet can increase.Matsuura et al. [ 27] reported that a NdO phase is formed by oxidation of Ndrich phases in the Nd-Fe-B matrix.Then,the oxygen can diffuse out from NdO phase to the matrix by heat treatment at above 350℃and cause decomposition from the NdOphase to a-Nd and Nd2O3.They argued that the x-Nd phase possibly exists as an amorphous state distributed between the matrix and Nd2O3,reducing the reverse magnetic domain generation.Therefore,it is speculated that the NdOgenerated during electrodeposition has a somewhat positive effect on increasing the coercive force.It was very hard to measure how many oxides formed in between grains because they were formed as very thin layers and in very small amount.

However,it was previously observed that electrolysis of water occurs at the working electrode during electrodeposition,which generates oxygen and hydrogen molecules.Decomposition of the main phase,Nd,Fe14B,is described by Reaction (4) and is followed by the formation of Fe2O3soft magnets,as show in Reaction (5),reducing overalcoercivity [ 8] .

In addition,hydrogen might be reduced by the reaction shown in Reaction (6),and then it can react with Nd and Nd2Fe14B phases to form NdHx and Nd2Fe14BHx phases.The products of those reactions,NdHx,Nd2Fe14BHx,are likely to cause lattice expansion.Volume expansion due to the formation of Nd(OH)3 causes cracking of the magnet [ 28] .

To reduce oxygen and hydrogen absorption,electroless plating of Cu was performed,and the Nd-Cu alloy was formed with electrochemically deposited Nd and electrolessly deposited Cu at an elevated temperature in a high vacuum of about 1.3×10-4 Pa.

It is well known that the coercivity of Nd-Fe-B magnets can be controlled by their grain sizes,distribution of Ndrich phases,and thickness of the Nd-rich phases which can be converted to amorphous phases through heat treatment.In this study,the influence of cooling rate on the shape ofthe grain boundary around MQU-F powder,which is closely related with the magnetic properties,is observed.Samples were heat treated at 600℃for 25 min with three different cooling mechanisms with different rates (air-exposed furnace cooling;air-closed furnace cooling;and oil cooling).The samples were analyzed with BSE and VSMmeasurements to understand the relationship between magnetic properties and chemical composition of the grain boundaries,as shown in Fig.2.For comparison,SEM was also performed in addition to VSM of the as-fabricated specimens.Figure 2a-d shows representative BSE images of Nd-Fe-B magnets with different heat treatments.

As shown in Fig.2a,b,e,it is clear that the coercivity and remanence of the Nd-Fe-B magnets both increase by heat treatment without Nd/Cu coating.This is probably due to the redistribution of Nd-rich phases around grain boundaries of the as-fabricated Nd-Fe-B magnets facilitated by increased available energy.However,the Nd-Cucoated specimens show different results,even though the same heat energies were applied,cooling rates to room temperature directly influence the magnetic properties.In Fig.2c,d,specimens with furnace cooling and oil cooling show different distributions of Nd-rich phases (bright contrasted structures) along the grain boundaries of the MQU-F powder.A thin,band-like structure is formed along the boundary of the MQU-F powder under oil cooling.Interestingly,remanence meaningfully increases,while coercivity drops when furnace cooling was used.Onthe other hand,a slight increase in coercivity and significant decrease in remanence are observed for the oil cooling process.Consequently,it can be concluded that the redistribution of Nd-rich phases in thin-band structures around grains of Nd-Fe-B magnets improves their magnetic properties (Fig.2b-d).

High-angle annular dark field (HADDF) images and EDS analyses are shown in Fig.3.The green color represents Fe and red color represents Nd.The slowest cooling rate shows rounded grains (Fig.3b) compared to elongated Nd-Fe-B grains in the control sample shown in Fig.3a.By comparing this with EDS in Fig.3f,it can be seen that less directional grains that have not yet been equiaxed were formed and thick Nd-rich phases accumulated (red dotted circles) at the interface of the Nd/Cu coating and the matrix.In Fig.3c,d,the relatively faster cooling rate shows a similar distribution of Nd-rich phases but it is continuously and evenly formed,and the matrix grains have maintained directionality.Therefore,it is confirmed that the Nd-rich phases are clumped together with slower cooling rates,while the Nd-rich phases are distributed along grain boundaries in faster cooling rate.However,it seems that the grain does not form properly close to the surface in the case of the fast-cooled specimens (upper part of the micrographs).If the surface is slightly melted at the time of heat treatment but not enough time is given for cooling,it seems that the grain is not formed properly on the surface (Fig.3d).These results show that an evendistribution of the Nd-rich phase can be achieved through the modification of cooling rates,but the coercivity cannot be tuned to the same extent.

Fig.2 BSE images of Nd-Fe-B magnets with different heat treatments:a no heat treatment (Sample A);b without coating,heat treated at600℃for 25 min followed by oil cooling (Sample B);c Nd/Cu coating,heat treated at 600℃for 25 min followed by furnace cooling (Sample C);d Nd/Cu coating,heat treated at 600℃for 25 min followed by oil cooling (Sample D);e relationship between coercivity and remanence under different heat-treatment conditions

Fig.3 HADDF TEM images and EDS maps with green and red representing Fe and Nd,respectively:a,e no treatment;b,f in-furnace coolic,g open-furnace cooling;d,h oil cooling

Fig.4 Schematic illustrations of effects of cooling rates on coercivity of Nd-Fe-B magnet:a relationship of grain boundary energies under partial or full wetting conditions;b illustration of the effect of cooling rate on wetting conditions

Figure 4 is a schematic diagram of a MQU-F powder with surface energies through the grain boundaries,which could be solid-liquid coexisting interfaces during heat treatment.It is assumed that low-melting point eutectic phases could be formed as a liquid droplet within the grain boundaries.The formation of liquid phases can be subpided in two cases:fully wetted or partially wetted in grain boundaries.Full wetting occurs when the contact angle between the liquid/solid interfaces is zero.The transition to full wetting occurs in partial wetting when energy of the grain boundary (σGB) of the MQU-F powder is higher than the solid-liquid energy(2σsL) [ 29, 30] .The transition from partial to full wetting is facilitated by increasing temperature [ 29, 30] .After GBDP,the Nd-rich eutectic components will be melted in the grain boundaries,and subsequently solidified by cooling process.With cooling rate changing,interfacial energies between phases will also be changed.A slower cooling rate will give enough time to form a crystalline solid that causes aσGB<2σSL condition,leading to a partial wetting state of the MQU-F powder boundaries.On the other hand,faster cooling conditions limit the amount of time for transformation of liquid to crystalline solid,and the material likely freezes as an amorphous state.

4 Conclusion

Nd and Cu were deposited on an Nd-Fe-B magnet with electrochemical and electroless deposition methods,respectively.The GBDP process enabled the formation of eutectic compositions of the Nd-Cu alloy and then facilitated its diffusion into interfaces within the magnet.After GBDP,it was possible to tune microstructural properties by controlling the cooling rates.As the cooling rates increased,Nd-rich phases were evenly distributed within the interfaces.Partial wetting states of Nd-rich phases were achieved with slow cooling rates,while this partial wetting could be changed to a full wetting state,σGB>2σSL,by increasing the cooling rate.Presumably,crystalized microstructures were not fully formed under high cooling rates and this caused an increase in coercivity(1.565-1.637 T).However,due to the lack of control of air exposure in the wetting process,oxidation was also observed.The GBDP with a wetting process in a fully controlled system to prevent atmospheric oxidation effects will likely provide an inexpensive path to increase both coercivity and remanence of this permanent magnet.

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