Nd–Fe–B Magnetic Powders Prepared By Hydrothermal Method And Performance Optimization

Rare earth permanent magnet materials,especially Nd₂Fe₁₄B magnets,have been widely used in energy,transportation,machinery,medical,computer,home appliances,aerospace,aviation and other fields,due to their excellent magnetic properties.They are an important basic functional material supporting the modern society.In recent years,the preparation of high performance Nd₂Fe₁₄B magnets has attracted the attention of scientists.Reducing the grain size of Nd₂Fe₁₄B is a way to improve the magnetic properties.Therefore,the preparation of Nd₂Fe₁₄B magnetic powder with nano structure has become a research hotspot.The traditional preparation method of Nd₂Fe₁₄B magnetic powder is metallurgy.Metallurgical preparation process is complex,high energy consumption,long time,and mainly uses high purity rare earth elements as raw materials,which greatly increases the cost.Therefore,the search for a“Bottom-Up”chemical preparation of Nd₂Fe₁₄B based magnetic powder has aroused many researchers'interest.The hydrothermal method is a low cost,easy to operate and expandable method for preparing ultrafine powder.Up to now,there are few reports on the hydrothermal synthesis of Nd–Fe–B magnetic particles.Therefore,in this study,a method combining hydrothermal method and reduction diffusion method is proposed to synthesize Nd–Fe–B based magnetic particles with good magnetic properties,which provides a new idea for chemical preparation of Nd₂Fe₁₄B based magnetic particles.The Nd₂Fe₁₄B based magnetic particles were prepared by hydrothermal method combined with reduction-diffusion annealing.The chemical mechanism of the preparation process was studied.The results show that the preparation of Nd₂Fe₁₄B based magnetic powder by hydrothermal method consists of four steps.First,Nd–Fe–B precursor was prepared,and Nd(NO₃)₃·6H₂O,Fe(NO₃)₃·9H₂O and H₃BO₃reacted with Na OH to form amorphous Fe(OH)₃,Nd(OH)₃and Nd(BO₂)₃.Secondly,Nd–Fe–B oxide was prepared.In this process,Fe(OH)₃,Nd(OH)₃and Nd(BO₂)₃were oxidized to form Fe₂O₃,Nd Fe O₃and Nd BO₃.Then,Nd₂Fe₁₄B magnetic particles were obtained through reduction-diffusion annealing process.In this process,Nd–Fe–B oxides were reduced by CaH₂to form Fe,Nd H₂and amorphous B,and the three are then combined to form Nd₂Fe₁₄B phase.Finally,the by-product Ca O was removed by cleaning to obtain Nd₂Fe₁₄B based magnetic powder.The synthesized Nd₂Fe₁₄B particles are polycrystalline,the coercivity mechanism is the reversed domain nucleation control mechanism.Compared with other reported methods of preparing Nd–Fe–B precursors,this method has the advantages of lower formation temperature(<200°C),shorter preparation time,simple operation and low cost,and has great application potential.On the basis of the successful synthesis of Nd₂Fe₁₄B magnetic particles,the key factors affecting the preparation and magnetic properties of Nd₂Fe₁₄B magnetic particles were systematically studied.The high purity and good performance of Nd₂Fe₁₄B magnetic particles were obtained by changing the process parameters.The results show that the optimum parameters are p H=9?11;T=180?;t=6h;c_(??)=0.3 mol/L;n_(?:?)=1:1.2.Optimum magnetic energy:M_s=86.8 emu/g(86.8 Am²/kg),M_r=29.8 emu/g(29.8 Am²/kg),H_c=1.48 k Oe(117.7 k A/m).In addition,the protective atmosphere and the state of the sample during the reduction-diffusion reaction also play a decisive role in the synthesis of Nd₂Fe₁₄B phase.The Nd₂Fe₁₄B phase can only be obtained in a 5%H₂/Ar protective atmosphere by using a pellet sample.On the basis of optimizing the process parameters,the magnetic properties of the products were improved by adjusting the raw material composition.The effects of different contents of Nd and B elements on the composition and properties of Nd–Fe–B magnetic particles,and the effects of Dy content on Dy-doped Nd–Fe–B magnetic particles((Nd,Dy)–Fe–B)have been studied.The best contents of Nd,B and Dy were determined by characterizing the magnetic properties of the magnetic powder.The results show that the coercivity of magnetic particles with Nd₂₇Dy₈Fe₆₀B₅is the highest,H_c=6.9 k Oe(548.9 k A/m).At this point,M_s=70.3 emu/g(70.3 Am²/kg),M_r=37.5 emu/g(37.5 Am²/kg).Moreover,with the addition of Dy,the TSR of(Nd,Dy)₂Fe₁₄B magnetic particles decreases,making(Nd,Dy)₂Fe₁₄B magnetic particles more suitable for application in low temperature environment.With the increase of Dy content,the thermal stability of magnetic particles also improved.By using mechanical ball milling technology,the particle size of the mixture of Nd–Fe–B oxide and CaH₂was refined,and the Nd₂H₅/Nd₂Fe₁₄B/?–Fe composite magnetic particles with exchange coupling effect were obtained.The magnetic properties of the products were further improved.Meanwhile,the microstructure of the products was optimized by the improvement of this process.The results show that the particle size of Nd₂Fe₁₄B obtained is small and uniform at the proper milling speed.The Nd elements are more evenly distributed around the Nd₂Fe₁₄B particles,forming an ideal Nd-rich layer,which makes the magnetic particles have higher coercivity.With the addition of the ball milling process,the H_cincreased from 2.68 k Oe(213.2 k A/m)to 4.38 k Oe(348.4 k A/m),and(BH)_maxincreased from 2.44 MGOe(19.4 k J/m³)to 5.49MGOe(43.6 k J/m³).The ball milling speed also has a great effect on the properties of magnetic particles.With the increase of ball milling speed,the saturation magnetization and coercivity of magnetic particles both increased first and then decreased.The optimum milling speed is 350 rpm,in which case,M_s=106.21 emu/g(106.21Am²/kg),M_r=66.7 emu/g(66.7 Am²/kg),H_c=6.01 k Oe(478.1 k A/m).Through the ball milling of((Nd,Dy)–Fe–B)oxide and CaH₂,and also get the performance improved significantly(Nd,Dy)₂Fe₁₄B magnetic powder,magnetic powder performance indicators for:M_s=84.87 emu/g(84.87 Am²/kg),M_r=56.02 emu/g(56.02 Am²/kg),H_c=7.00 k Oe(556.8 k A/m).The(BH)_maxalso increased from 2.76 MGOe(21.9 k J/m³)to 3.56 MGOe(28.2 k J/m³).