Investigation On The Manufacture Technology And Microstructure And Magnetic Properties Of Nd₂Fe₁₄B/α-Fe Nanocomposite Magnets

Nanocomposite NcbFe₁₄B/α-Fe permanent magnets have attracted considerable attention in the last two decades, because of an enhanced remanence, a relatively high coercivity, and a high maximum energy product (BH)_max. The excellent magnetic properties of these materials are known to arise from the exchange coupling interaction between magnetically hard and soft phases. The theoretical energy product of nanocomposite materials is about 1000kJ/m³, which is larger than that of any other single phase Nd-Fe-B magnets. However, the experimental value obtained is still relatively low, the phenomenon is attributed to the difference between the microstructure of real nanocomposite magnets and the theoretical models, such as the grain size, the volume fraction and distribution of soft and hard magnetic phase, the interface structure and the crystalline orientation etc. Hence, how to control the microstructure of nanocomposite magnets is a key process to obtain excellent magnetic properties.The effects of alloy compositions, preparation devices and techniques on the magnetic properties of nanocomposite Nd₂Fe₁₄B/α-Fe materials were investigated in this paper. Excellent magnetic properties which can be comparable to that of MagQuench magnetic powder are expected to obtain. The results are followed below.The optimum composition proportion is Nd₁₀Fe_75.9B_6.4Co₅Zr_2.7, which was obtained by studying systematically the effects of preparating techniques and contents of B,Zr and Nd on the microstructure and magnetic properties of nanocomposite materials for NdFeBCoZr system .The substitution of Pr, MR(mixed rare earth) for Nd on nanocomposite (Nd_1-xPr_x)₁₀Fe_75.9Co₅Zr_2.7B_6.4 alloy were studied. The results showed that the optimal Pr content is x=0.25 . The magnetic properties decreased 2% for the MR substitution, which demonstrated that the MR substitution for Nd in industrial product is feasible to obtain composite good magnetic properties.Quenching conditions and crystallization techniques are the main factors which affect the magnetic properties of nanocomposite materials. The physical model was firstly set up for the convenience of optimizing the quenching speed(Vx). On the basis of the theoretical analysis of relation of melt temperature with time: T = T₀ + (T_m-T₀)exp(-at/r_lc_plh), and the heating-transfer physical model of melt quenching for single wheel, the relationships between cooling rate and ribbon thickness(h) are obtained in the following:On the condition of Newton cooling, the cooling rate has an inverse proportion to the thickness of thin ribbons:dT/dÏ„=α/ρ_lC_plh(Tm-To)On the condition of ideal and middle cooling rate, the cooling rate has an inverse proportion to the square of ribbon thickness:dT/dÏ„=λ₁(Tm-To)ρ_lC_plh²The velocity range of the rapid wheel was achieved from the technique condition to form amorphous alloy and make the fine contact between the ribbon and wheel surface. The term is followed:(1/Tm-To)(dT/dÏ„)_glass≤V_x≤(p+F)â–³HR₀/αρ_slâ–³TThe velocity range of the rapid wheel is 14-54ra/s, which provides a theoretical guidance for the optimization of quenching parameters. The best magnetic properties are always corresponding with the proper wheel velocity. In other words, good magnetic properties can be obtained for proper proportion of amorphous phase and crystall phase. The optimum quenching rate of the optimum magnetic property in the NdFeBCoZr system is mainly related to the content of Co,Zr alloying elements, and hardly to Nd,Fe,B alloying elements. The inhomogenous distribution of the composition and grains is the main reason to deteriorate the magnetic properties by arc-melt quenching technique. The one dimension heating-transfer model was set up for the solidification of melt. The mathematical form about the distribution of Zr was calculated. The main reason for composition inhomogeneity is the existing solidification layer in the Cu crucible wall simulated by chemistry method.The inhomogeneity of grain distribution prepared by arc-melting quenching technique was studied. The results show that the inhomogenous thickness of the ribbons was the main reasons resulting in the uneven microstructure.An improvement of the arc-melting quenching device was conducted in order to resolve the problem of inhomogeneity induced by arc-melting quenching process, but the effects are not remarkable.The melt quenching device by induction furnace with the potential industrial application was manufactured to substitute the arc-melting furnace. By using this device the optimal magnetic properties for bonded Nd₁₀Fe_75.6B_6.7Co₅Zr_2.7 magnets were obtained, that is Br=0.734, H_cj=710kA/m, (BH)_max=88kJ/m³, which come up to the properties of MQP-B magnetic powder.The crystallization technique of quick increasing temperature rate, keep the temperature equably and quick decreasing temperature rate is favorable to obtain excellent magnetic properties for nanocomposite materials. Comparing the large scale crystallization urnace, it was found that the magnetic properties for the sample annealed using small scale furnace were better. The reason is that the latter can meet the requirement for optimized annealing technique.The effect of Zr element on the crystallization behavior, and the mechanism of action for the amorphous layer on the grain boundary were discussed. A model of "coherent coupling effect" on the amorphous phase layer on the grain boundary was proposed. On the basis of the model, the proper thickness of the boundary layer was calculated for 2nm, which is consistent to the previous report. Two aspects about the effect of the boundary layer were summarized. The grain growth is inhibited by the layer, the other is that the exchange coupling interaction between the Nd₂Fe₁₄B phase andα-Fe phase were strengthened by the coherent relationship of the boundary layer.