Preparation Of Sm₂(Fe_0.9Co_0.1)₁₇N_x Magnetic Powder By Reduction Diffusion Method

Sm-Fe-N(Sm₂Fe₁₇N_x),as a rare earth permanent magnet material,has an anisotropic field of 20.7 MA/m（260 k Oe）and a Curie temperature of 749 K,which are higher than those of Nd-Fe-B(Nd₂Fe₁₄B),the king of permanent magnets,which has been widely used.The theoretical magnetic energy product of Sm-Fe-N is basically the same as that of Nd-Fe-B,up to 447 k J/m3（56.2 MGOe）.In addition,Sm-Fe-N is better than Nd Fe B in terms of corrosion resistance and oxidation resistance,and Sm-Fe-N is more abundant than neodymium and dysprosium reauired for Nd Fe B preparation.Therefore,the development and application of Sm-Fe-N materials are widely concerned by science and industry to meet the higher requirements of magnets for the rapid development of industry and the overall layout and balanced utilization of rare earth resources.However,the current Sm-Fe-N preparation process faces the problems of difficulty in obtaining pure phases,incomplete nitriding,and further improvement of properties.In this work,the samarium-iron alloy preparation process,nitriding process,as well as washing and debinding and ball-milling coating phosphorus process are investigated in order to produce Sm₂(Fe_0.9Co_0.1)₁₇N_x magnetic powder with sphere-like morphology,smooth surface,small and uniform particle size,and complete nitriding,which is mainly divided into the following aspects:（1）The Sm₂(Fe_0.9Co_0.1)₁₇ alloy was prepared by the reduction-diffusion method through the pretreatment of raw materials Fe₂O₃ and Sm₂O₃,hydrogen reduction of R-D precursors,and reduction-diffusion.The effects of Fe₂O₃ sintering temperature and ball milling time,R-D precursor hydrogen reduction,reduction-diffusion temperature,and reduction-diffusion time on the preparation of samarium iron alloy were investigated.The results show that by adjusting the above process parameters,the particle size control of samarium-iron alloy can be achieved,and the particle growth model of reduction diffusion is summarized.The Sm₂(Fe_0.9Co_0.1)₁₇ alloy was prepared by sintering and ball milling of Fe₂O₃ at 1000°C for 4h,hydrogen reduction of R-D precursor and reduction-diffusion at 1050°C for 2h.The particles of Sm₂(Fe_0.9Co_0.1)₁₇alloy were spherical with smooth boundary,particle size of 3.35±1.38μm,oxide layer with thickness of several tens to hundreds of nanometers,σ20=124 emu/g,H_cj=31Oe,T_c=287°C.（2）For the conventional nitrogen nitriding process of samarium iron alloy,the powder particle size,reaction time and reaction temperature were investigated,and the nitriding model of conventional nitrogen nitriding was summarized.It is found that larger particles are not easily nitrided completely,so it is necessary to increase the intensity of nitriding conditions,but this may trigger the decomposition of the nitrided phase,while on the other hand smaller nitrided particles will introduce more O elements due to the presence of the oxide layer,the presence of O elements will be oxidation reactions in the nitriding process or even become the main reaction in the nitriding process,these side reactions will generate Fe-Co,similar to the presence of Fe-Co soft magnetic phase will be greatly reduced to the performance of permanent magnetic powder.Based on this,a two-stage nitriding process optimization was proposed to achieve the controlled generation of Fe-Co phase while the sample was fully nitrided.The XRD pattern of the sample with 4h first-stage nitriding at 400°C and 2h second-stage nitriding at 480°C showed 0 peak intensity of Fe-Co phase and H_cj=2292 Oe.The nitriding of samarium iron alloy with ammonia-hydrogen mixture was also investigated,which is easier to nitride than nitrogen,but the excessive nitriding time leads to over-nitriding of the sample with nitrogen content x exceeding 3,which may lead to the collapse of the atomic layers in the crystal structure and the formation of amorphous phase.This may lead to the collapse of the atomic layers in the crystal structure,thus generating amorphous phases and affecting the final properties.The optimum nitriding time in the ammonia-hydrogen mixture is 2 h,where the nitrogen content is 2.9 and H_cj=2452 Oe.（3）For the washing and de-calcium process of Sm₂(Fe_0.9Co_0.1)₁₇N_x magnetic powder,the effect of solvent system and washing time was investigated.It was found that deionized water could not achieve complete washing of magnetic powder,while acetic acid solution with p H=5 could achieve de-calcium in more than 1h.The process of removing calcium by washing will have an erosive effect on the surface of the alloy,and with the washing process,H atoms will enter the lattice of the alloy,and too long washing time will lead to the appearance of Fe-Co phase,so the preparation process of samarium iron-nitrogen should be nitrided first and then washed.The H_cj of the dehydrogenated sample was increased from 2197 Oe to 2498 Oe by the treatment of Ar atmosphere at 200°C for 2 h.In addition,the performance of the sample treated with20 min of high energy pendulum grinding was greatly improved and the H_cj reached10305 Oe.Meanwhile,the samples were treated with phosphoric acid for phosphate coating.The results showed that a protective layer of phosphate was generated on the surface of the samples,which led to an increase in the oxidation resistance of the samples.