简介概要

Development of high-performance dry-pressed anisotropic permanent ferrite for La-Ca-Sr-Co system

来源期刊：Rare Metals2020年第1期

论文作者：Dai-Man Zhu Zhao-Wen Geng Rong-Ming Liu Xiao-Wen Zhou Li-Ying Jia Guo-Hui Hu Qian Wang Bing-Shan Li

文章页码：89 - 94

摘要：In this work,the BMS-12 pre-sintered material was used as the basic material,and the optimal secondary additive addition amount was found by the wet pressing process.Then,the anisotropic ferrite material was prepared by dry-pres sing method,and the effects of camphor adhesives,calcium stearate lubricant and powder particle size on the magnetic properties were studied.The experimental results show that the appropriate addition amount of camphor adhesive and calcium stearate lubricant is 0.6 wt%and 0.8 wt%,respectively.High remanence and coercivity can be obtained for particle size of 0.85-1.00 μm.Superior dry-pressed magnet ferrite was prepared with typical properties of magnetic induction intensity of B_r=421 mT,induction coercivity of H_cb=296 kA·m^-1,intrinsic coercivity of H_cj=369 kA·m^-1 and maximum energy product of（BH）_max=33.3 kJ·m^-3,which have reached the level of TDK corporation product of FB5 D.This shows that the dry-pressed magnet of La-Ca-Sr-Co system has higher remanence and intrinsic coercivity than the traditional wetpressed Sr-ferrite.

详情信息展示

稀有金属(英文版) 2020,39(01),89-94

Development of high-performance dry-pressed anisotropic permanent ferrite for La-Ca-Sr-Co system

Dai-Man Zhu Zhao-Wen Geng Rong-Ming Liu Xiao-Wen Zhou Li-Ying Jia Guo-Hui Hu Qian Wang Bing-Shan Li

National Engineering Technology Research Center for Magnetic Materials,BGRIMM

BGRIMM Magnetic Materials and Technology(Fuyang) Co.,Ltd.

作者简介：*Rong-Ming Liu e-mail:liurongming@magmat.com;

收稿日期：8 June 2018

基金：financially supported by the National Natural Science Foundation of China (No.51401023);the National Basic Research Program of China (No.2014CB643702);Youth Innovation Fund Project of BGRIMM Technology Group (Nos.QCJ201809,QC-201724 and QCJ201820);

Development of high-performance dry-pressed anisotropic permanent ferrite for La-Ca-Sr-Co system

Dai-Man Zhu Zhao-Wen Geng Rong-Ming Liu Xiao-Wen Zhou Li-Ying Jia Guo-Hui Hu Qian Wang Bing-Shan Li

National Engineering Technology Research Center for Magnetic Materials,BGRIMM

BGRIMM Magnetic Materials and Technology(Fuyang) Co.,Ltd.

Abstract：

In this work,the BMS-12 pre-sintered material was used as the basic material,and the optimal secondary additive addition amount was found by the wet pressing process.Then,the anisotropic ferrite material was prepared by dry-pres sing method,and the effects of camphor adhesives,calcium stearate lubricant and powder particle size on the magnetic properties were studied.The experimental results show that the appropriate addition amount of camphor adhesive and calcium stearate lubricant is 0.6 wt%and 0.8 wt%,respectively.High remanence and coercivity can be obtained for particle size of 0.85-1.00 μm.Superior dry-pressed magnet ferrite was prepared with typical properties of magnetic induction intensity of B_r=421 mT,induction coercivity of H_cb=296 kA·m^-1,intrinsic coercivity of H_cj=369 kA·m^-1 and maximum energy product of(BH)_max=33.3 kJ·m^-3,which have reached the level of TDK corporation product of FB5 D.This shows that the dry-pressed magnet of La-Ca-Sr-Co system has higher remanence and intrinsic coercivity than the traditional wetpressed Sr-ferrite.

Keyword：

Dry-press molding; Permanent magnetic ferrite; La-Ca-Sr-Co system; Magnetic properties;

Received： 8 June 2018

1 Introduction

M-type ferrite is an important permanent magn_etic material and widely ^used in automobile motors,h_ome appliances and _other fields ^because of its low p^rice,excellent chemical _stability,high _coercive f_orce and magnetocrystalline anisotropy[1-⁷]._According to the molding process,permanent magnet ferrite magnets can be classified in^to wet a_nd dry presses [ 8, 9, 10] .During w_e^tpress molding,the magnetic particles are dispersed i_n_a solvent such as water or other organic liquid_s.As themselves not blocking,the magnetic particles are easy to rotate when the magnetic field is loaded;therefore,th^eir easy axis of magnetization and magnetic field direction are same,resulting in their high performance.However,the molding equipment is c_omplicated an_d req_uires a pumping system,and the products need grinding and cannot be processed into small pieces,which makes the processing cost very high [ 11] .Dry-press molding is a process with filling dry an^d broken fine powder into the molding cavity and plus an orientation magnetic field to form.Owing to dry-press without dehydration and low equipment costs,it is especial^ly suitable to produce _anⁱsotropic devices and small devices with high production efficienc^y which are produced difficult^ly using wet-press methods [ 12, 13, 14] .

In recent years,for dry-pressed product_s,the demand has grown rapidly,the research on the performance has been strengthened domestically,and ^the technical level has been improved dr_astically.D_ry-pr^essed products with performance compared with FB5D ferrite material_s from ^TDK co_rporation have rarely beeⁿ reported in China [ 15, 16, 17] .It is because that the FB5D anisotropic permanent ferrite materials,with the magnetic properties of magnetic induction intensity of B_r=(41₅±10)mT,induction coercivity o^f H_cb=(254.6±11.9)kA·^m-₁,intrinsic coe^rci_vity of H_cj=(262.6±15.9)kA·m^-1 and maximum energy product of (_BH)max=(32.₆±1.6)kJ·m_-³,ha_ve t_he highest perform_ance among all the permanent magnet ferrite dry-pressed products in the world [ 18, 19] .

The aim of thi^s work focused on the en_hancement of magnetic properties of domestic dry-pressed products to narrow the gap with the international advance_d level,letting them to be competitive commercialized goods to mee_t market needs.Based on the BMS-12 pre-sintered ferrite powders developed by BGRIMM as basic materials,the research on the influence of secondary formulas,mole ratio of adhesive and lubricant and particle size on high-performance dry-pressed products for La-Ca-Sr-Co system was conducted.

2 Experimental

Powdered samples of the lanthanum ferrites of the formu_la La_0.6(Ca_0.4-xSr_x)O·n/2(F_e1-yCo_y)_2O3(10.⁰≤n≤12.0)were prepare_d using th_e weighted Sr_CO₃ (98 at%),_Fe₂O₃(99 at%),La_2O3₍98 at%),C_o2O3 (98 at%) and CaCO3(98 at%) as raw materials.The mixture is mixed and ballmilled for 2h wⁱth 10%of PVA (^poly_vinyl alcoh_ol,CAS#:9002-89-5) water ^solution in a water,pelletize_d,isotropical^ly d^ry-pressed intoΦ25 mm×15 mm compact,calcined at 1240℃in a muffle furnace with air atmosphere and cooled to room temperature.T_he co_arse powders were then wet ball-milled to the required size _with the addition of secondary additiv^es.The sl^urry was dried in a drying cabinet at 10⁵℃f^or 30 minut_es.Th_en,it was disintegrate_d and mixed with camphor ^and calcium stearate by a 3000 rpm high-speed cr_usher.Th_is mixture named as dry-pressed magnetic powder.The dry-p^ressed magnetic powder ^was pressed into a cylinder in a magnetic field of 637 _kA·_m^-1,_sintered in aⁱr at 1215℃for 2 h and cooled in t_he furnace.

The magnetic properties and the average particle size distribution (APD) of the samples were measured using a DMT-1 magnetic magmeter from China Academ_y of Sciences and a WPL-₂02 particle size teste_r,respectivel_y.An _APD-10X X-ray dif^fractometer (XRD) was ^used ^to analyze the phase of the magnetic powder wi^th X-ray source of Cu Ka target,a wavelength of 0.15418 nm,_a tube pressure of 40 kV,_a scan step length _of ⁰.002 nm and _a scan rate of 8.00 (°)·min_-1.The S-350₀N scanning electron microscope (SEM) w^as employed to study the morphology ^and particle diameter of the samples.

3 Results and discussion

3.1 Effects of secondary additives on properties of wetpressed samples

After determining the composition of the raw materials,the main factors affecting product performance are density _and orientation.The wet compression molding process makes the magnetic particles ea^sy to be oriented in the magnetic _field,and the process interference is _small,so itis easy to determine the best secondary formulation.Figure_s 1,2 a_nd 3 show the changes in remanence and intrinsic coercive force of the wetpressed sample with different additive amounts,respect_ively.

Figure 1 shows that when the amount of La₂O₃+Co₂O₃added is 0.⁵ wt%(the ratio of La2O3 to Co_2O3 is consta_nt),and th_e amount of SiO₂ added is 0.5 w_t%(_i.e.,the mass ratio of SiO₂ to the pre-sintered powders is 0.5%),as the content of CaCO3 _increases from 0.65 wt%to 1.05 wt%,^the coercive force continues to decrease,but the remanence increases significantly from 43⁸ to 44⁵ mT when the content of CaCO₃is less than ⁰.8 w_t%,and the remanence is rapidly decreasing when the content of CaCO₃ is greater than 0.8 wt_%.Therefore,higher comprehensive magnetic properties can be obtai_ned when the content of CaCO3 _is 0.8 wt%.

In Fig.2,the amount of CaCO3 added is 0.8 wt%,and the amount of La₂O₃+Co2O3 _added is 0.5 wt%(the ratio of La₂O₃_to Co₂O₃ is a constant),as the SiO₂ con_tent increases from 0.3 wt%to 0.5 wt%,the r_emanence increases continuousl^y to 445 mT,and ^then the SiO₂content further increases t^o 0.7 wt^%,th_e^remanence rapidl^y drops to 425 m_T,but the addition amount of _SiO₂increases from 0.3 wt%to 0.7 w^t%.T_h^erefore,higher comprehensive magnetic properties can be obtained when the content of SiO₂ is 0.5 wt_%.

Fig.1 Changes in remanence (B_r) and intrinsic coercive force (H_cj)of samples wit_h CaCO₃ additive amounts

Fig.2 Changes in remanence (B_r) and intrinsic coercive force (H_cj)of samples with Si_O2 additive amounts

Fig.3 Changes in remanence (B_r) and intrinsic coercive force (H_cj)of samples with (La₂O₃+Co₂O₃) additive amount_s

The addition of CaCO₃ is intended to lower the sintering temperature and increase the magnet density ^and remanence.CaCO₃ is thermally decomposed a_t about 800℃to form calcium oxide and reacts with silica to form a grain boundary phase CaSiO₃,^which suppresses abnormal grain growth and _increases coercive force._According to the chemical reaction CaCO₃₊SiO₂=C_aSiO₃+CO_2↑,the ideal weight ratio of silica to calcium carbonate is 0.64.In the actua_l application proce_ss,to increase remanen_ce,the amount of calcium carbonate addition will b^e little more,but not excessive;otherwis_e,there will be abnormal gra_in growth which leads to deterioration of magnetic propert_ies.

In Fig.₃,the amount of CaCO₃ added is 0.8 wt%,the amount of ^SiO₂_added is 0.5 wt%,a_nd the amount of La₂O₃₊Co₂O₃ add_ed is changed from 0 wt%to 1.0 w_t%(the ratio of La₂O₃ to Co₂O₃_w^as constant).Wⁱth th_e increase in La₂O₃+C_o2O₃ conte_nt from 0 wt%to0.6 wt%,_the coercive force slightl^y decrease_s,but the remanence increases significantly,fr^om 4³5 ^to 443 m_T.When t^he content of _La₂O₃+Co₂O₃ increases from0.6 wt%to 1.0 wt%,remanence and coercive force begin to drop sharply.It ca_n be concluded that the content o^f La₂O₃₊Co₂O₃ ad_dition of 0.5 wt%can g_et higher comprehensive magnetic properties.

The purpose of La₂O₃+_Co₂O₃ com_pound addition is to increase the content of La and _Co on th^e M-phase boundary in order to improve the magnetic properties.But when _the amount of La a_nd Co added is too much,La and Co ions will not be able to enter the ferrite lattice,in which La₂O₃,_LaFeO₃ a_nd CoFe₂O₄ pha_ses will appe_ar in t^he sample,resulting in decreased performance[20_].

3.2 Effects of adhesives on properties of dry-pressed samples

In the dr^y-press molding proces_s,in order to facilitat_e the molding and _to improve the degree of orientation,it is necessary to add an _adhesive and a lubricant into the magnetic powder._Camphor and calcium stearates are commonly used as adhesives and lubricants,respectivel_y.

Table 1 shows the results of orthogonal tests on calcium stearate and camphor.When the calcium stearate content is in the range of 0.4⁵ wt%-0.75 wt%,the relative^ly hi_gh remanence and coercivity can be obtained and _the remanence is insensitive to the changes in calcium stearate content in this ran_ge.When the content of calcium stearate further increases from 0.75 ^wt%to 0.90 wt%,th_e remanence decreases.^When t_he content of camphor increases from 0.5 wt%to 1.3 wt%,t_h^e remanence grad_ually decreases and the coercivity basically does not chan_ge.

Since calcium stearate and camphor are organic substances,carbon dioxide can be formed during the sintering if the amount of addition is too much,which makes the density decrease and the magnetic properties such as Br,Hcj and (BH)max deteriorate [ 21] .Therefore,0.6 wt%calcium stearate and 0.8 wt%camphor are selected to obtain higher comprehensive magnetic properties.

3.3 Effects of particle size on magnet performance

Owing to the difficulty in uniform dispersing,the pinholes are generated in the magnet when camphor is added directly into the dry powder.In order to eliminate pinholes,it is necessary to dissolve the camphor in alcohol to make a solution into the dry powder,which will increase the cost [ 22] .In this work,to reduce the cost,the way of adding camphor and calcium stearate is modified and improved,which is called“adding black material”.The black material is prepared as follows:first weighting fine powder1000 g,calcium stearate 120 g and camphor 160 g,and then mixing them 2 times using high powder machine.Adding 5 wt%black material to the dry powder caneffectively avoid pinholes (Fig.6),and the performance changes with particle size were then studied.

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Table 1 Results of orthogonal tests on calcium stearate and camphor

In Fig.4,as the fine pwder size becomes smaller,the performance of the sintered magnet increases.When the particle size reaches 0.9μm,the performance reaches the highest value,the remanence of 421 mT and the coercive force of 369 kA·m-1.This is because as the particle size decreases,the polycrystalline particles decrease and the orientation degree increases,so the magnetic properties are improved.While the particle size is reduced to 0.85μm,the performance slightly decreases.It shows that the particle size of the powder has a certain influence on the secondary formulation.With smaller particle size,the activity of the particle is enhanced,which is caused by the relative excess of the addition.

The performance of the magnets can meet or exceed those of FB5D products in TDK corporation when theparticle size is in the range of 0.85-1.00μm which is a wide range for suitable industrial production.

3.4 Relations between structure and magnetic properties

Fig.4 Changes in remanence (Br) and intrinsic coercive force (Hcj)of samples with particle sizes

Fig.5 XRD patterns of powders and sintered magnet

Fig.6 SEM images of micromorphology of magnet based on observation plane a parallel and b perpendicular to orientation magnetic fⁱeld direction

4 Conclusion

Based on the study of the wet compression performance of BMS-12 products,a better secondary addition formula was obtained as the following:0.80 wt%CaCO3,0.50 wt%SiO2 and 0.50 wt%(La2O3+Co2O3).In order to improve the properties and processing characteristics of dry-pressed powders,camphor and calcium stearate were selected for orthogonal test to obtain reasonable amounts:0.6 wt%calcium stearate and 0.8 wt%camphor.Pinholes can be effectively reduced by improving the way in which camphor and calcium stearate are added.The performance of FB5D in TDK corporation can be achieved as Br=421mT,Hcb=296 kA·m-1,Hcj=369 kA·m-1 and(BH)max=33.3 kJ·m-3,within the particle size range of0.85-1.00μm.The orientation of the pressed and oriented magnetic powder is not destroyed during the sintering process,and the grain orientation grows.The crystal grains in the sintered magnet show a sheet shape with a high degree of orientation in the direction of the easy axis of magnetization.The normal direction of the tabular grainscoincides with the easy axis of magnetization,resulting in high properties of magnets.

Figure 5 shows XRD patterns of the powders and the sintered magnet.The normal direction of the measurement surface is parallel to the orientation magnetic field direction.According to exponential calibration of the spectrum,there is an obvious hexagonal crystal structure,which is the same as that of other M-phase ferrites [ 23, 24, 25, 26, 27, 28, 29, 30, 31] .The positions of peak in the spectrum of the powders are similar to those of the sintered magnet,while the intensity is very different.It is found that the intensity of the peaks such as(004),(006),(008) and (0014) of the sintered magnet is much higher.The reason is that the (001) face is the one corresponding to the easy axis of magnetization of(001)inM-type ferrite and its homogeneous crystal faces are as follows:(004),(006),(0⁰8) and (0014),indicating ^that the main diffraction peaks originate from these four crystal planes.Although the _grains of ^the powder s^ample ar_e randomly oriented,the crystal grains have a high degre^e of orientation along the easy axi^s.The micromorphology of the magnet is shown in Fig.₆.The crystal grains in the magnet appear sheet-like,and the normal direction of the tabular crystal grains coincides with the easy magnetization axis.The orientation of the pressed and oriented magnetic powder ⁱs not destroyed during the sintering p^rocess;therefore,the grains are oriented an_d grown,resulting in the high properties of the magnets.