Materials Science Forum Vols. 475-479

Abstract: The effects of the ratio of Y to Dy as well as the effect of Nd and Co substitutions on magnetic properties in [Ndx(YDy)0.5(1-x)]2.2Fe14-yCoyB ribbons melt-spun at 22 m/s have been systematically studied. (Y1-zDyz)2.2Fe14B ribbons with a ratio z of 0.25 or 0.5 simultaneously obtains a smaller temperature coefficient of remanence (α ) and coervicity (β ) which are much smaller than those of Nd-based Nd2Fe14B ribbons. In [Ndx(YDy)0.5(1-x)]2.2Fe14-yCoyB ribbons, Nd substitution (x=0 to 0.8) can improve the maximum energy product (BH)max of annealed ribbons but degrades the temperature stability of the magnetic properties. The ribbons with x=0.4 and y=0 yield a (BH)max of 8.7 MGOe. For these ribbons, the α and β are -0.07 and -0.31 %/°C in the temperature range of 27 to 127°C, respectively. Increasing Co (x) from 0 to 3, slightly decreases coercivity Hcj from 21.5 to 16.3 kOe, but keeps the (BH)max in the range of 8.6 to 10.2 MGOe. The optimal sample with x=0.5 and y=1.5 obtains a (BH)max of 10.2 and 5.0 MGOe at 27 and 250°C, respectively. Its α and β are -0.11 and -0.30 %/°C, respectively. These results show that studied ribbons are very promising to develop into high temperature isotropic bonded magnets capable of operating at or above 180°C.

Abstract: The Nd2Fe14B/α-Fe bulk exchange-spring magnets have been prepared by spark plasma sintering melt spun Nd9.8Dy0. 4Fe78.4Co5.6B5.8 flakes under different temperatures and pressures. It was found that higher sintering temperature improved the densification of the magnets, while deteriorated their magnetic properties simultaneously. An increased compressive pressure can restrain the grain growth remarkably and then leads to better magnetic properties and higher density for the magnet at same sintering temperature. XRD analysis showed that with the increase of sintering pressure, some peaks indicating c-axis texture such as (006) and (105) became dominant. As a result, the bulk magnet exhibited higher remanence and maximum energy product than starting powders.

Abstract: Two-phase nanocrystalline ribbons were prepared by melt-spun and subsequent heat treatment. Excellent magnetic properties of the bonded magnet were achieved as follows: Br = 0.67 T, JHc = 664.3 kA·m-1, (BH) max = 75.0 kJ·m-3. The mean grain size of the ribbons heat-treated at 710 °C/ 4 min is about 50 nm. After heat-treated the ribbons at 750 °C/ 4 min, the mean grain size is about 100 nm.

Abstract: In this paper, flakes of NdFeB cast alloys were prepared by using the strip casting technique. Microstructure and composition of phases in NdFeB SC flakes were studied by SEM and energy spectra. Especially, the influences of cooling rate on the microstructure of SC flakes were discussed, helping us to master strip casting technology. The results show that the cooling rate plays an important role in obtaining the perfect microstructure of SC flakes, which thickness is supposed not less than 0.32mm in these studies.

Abstract: Using the part exchange-coupling model, the exchange-coupling interaction between magnetically soft and hard grains in Nd2Fe14B/α-Fe nanocomposite and its effect on the effective anisotropy of the material have been investigated. By testing six different functions, series curves of effective anisotropy constant versus the mean grain size were obtained. The calculation results show that the effective anisotropy of the composite increases slightly with decreasing grain size, reaches the maximum at about 23nm, and then decreases abruptly. The calculated curves resemble the experimental results of the coercivity versus grain size in nanocomposite.

Abstract: The addition of manganese into Nd9Fe85-xMnxB6（X=0,0.5,1） was found to enhance the magnetic properties of a -Fe/Nd2Fe14B magnets. After optimal annealed, the coercivity and remanence ratio increased from 0.34MA/m to 0.4MA/m and from 0.70 to 0.72, respectively, for the increase of x from 0 to 1. The XRD patterns indicated the addition of manganese could promote crystallization of as-spun samples. Moreover, the XRD spectra showed almost same grain sizes obtained for the optimal annealed samples. Thermomagnetic analysis (TMA) measurement indicated that the doping of Mn reduces the Tc of the Nd2Fe14B phase and raises that of a-Fe phase in the system. It is believed the exchange coupling between the Nd2Fe14B phase and a-Fe phase was partly changed by Mn addition, which is the reason for the enhanced magnetic properties and the change of Tc.

Abstract: In the present work, we investigated the hard magnetic properties of Nd-Fe-B magnets prepared by the decrepitation technique. The microstructure of (Nd0.935Dy0.065)14.5Fe79.4B6.1 HD - magnet was observed and the average grain size is about 6-15 µm. The HD alignment degree f is about 93%. Most rich-Nd phase can be found at grain boundaries of main phase, but a few rich – Nd phase occurs within main phase grains. The magnetizing field dependence of the hardmagnetic properties for the (Nd0.935Dy0.065)14.5Fe79.4B6.1 were investigated. The value of magnetizing field for the saturation coercivity is smaller than the value of the saturation coercivity, showing that the coercivity for the (Nd0.935Dy0.065)14.5Fe79.4B6.1 HD-magnet is controlled by the nucleation mechanism. Meanwhile, the temperature dependence of coercivity for HD magnet Nd14.5Fe79.4B6.1 was analyzed.

Abstract: Giant negative magnetoresistance (MR) up to 15 % were observed for nano-scaled grains of Ni1-xMnx(0.23<x<0.3) in high fields up to 30 T. The crystal grain size (≈5nm) was obtained by an adequate annealing condition of 630 K, 200hrs. The sample resistance decreased linearly with magnetization increases in each magnetization range for different x, and decreased with the same slope even for different x. The observed negative MR is plausibly explained by the scattering of carriers by the anti-parallel spin pairs of residual Mn-Mn clusters.

Abstract: Ni-Zn ferrite ceramics were prepared from the blend of nano-sized powders and ultra-fine particles at the relative low sintering temperature (≤1200°C). The microstructure of the as-sintered samples characterized by SEM indicated that this process obstructed abnormal grain growth which often appeared in the Ni-Zn ferrite ceramics sintered from the nano-sized powders. The result of VSM measurements showed that the ratio of nano-sized powders to ultra-fine particles effected the initial permeability (μi) of the as-sintered samples. Optimum initial permeability can be obtained by adjusting the ratio as 1:1.