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Preparation of (Nd,Pr)FeB Nanocrystalline Permanent Magnets and Their Microstructure and Magnetic Properties

Author: ChaWuSheng
Tutor: TuMingZuo
School: Sichuan University
Course: Materials Science
Keywords: Nanocrystalline permanent magnets Rare earths content Dopant of Co and Zr Preparing process Microstructure Magnetic properties
CLC: TM273
Type: PhD thesis
Year: 2004
Downloads: 389
Quote: 5
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Abstract

According to the summarization and analysis of the exchange coupling principle for nanocrystalline composite permanent magnets and the influence factors of their magnetic properties, several series of iron-based bonded magnets have been prepared by using melt spinning and post annealing method. By means of XRD, AFM, DTA, and TEM, the dependence and mechanism of the preparing process parameters, the rare earths content, the substitution of Nd by Pr, and the addition of Co and Zr on the microstructure of alloy, the intrinsic magnetic feature of magnetic phases, and the magnetic properties of bonded magnets have been systematically investigated.The crystallinity of as-spun alloy significantly influences the microstructure of crystallized ribbons. For the (Nd0.8Pr0.2)10.5Fe77.5Co5Zr1B6, the as-spun alloy ribbons rapidly quenched at 24, 26, 28, and 30 m/s consist of amorphous phases and fine crystallites. The higher the quenching speed is, the lower the alloy crystallinity is, For the as-spun ribbons with the crystallinity of about 30%, which were quenched at 26 m/s, the heterogeneous nucleation of new grains occurs. The nucleation of a <WP=7>solid precipitate from the matrix occurs most easily on boundary already present in the structure and the amount of nuclei is largest. Then a fine and homogenous microstructure and an excellent combination of magnetic properties are obtained. Beyond the 30% of crystallinity, the amount of nuclei decreases and the crystallites present grow easily, which results in an inhomogeneous microstructure. Below 30%, the homogeneous nucleation occurs in the ribbons and the amount of nuclei reduces, which results in an uniform but a coarse grain structure. The annealing temperature and annealing time directly determine the properties of bonded magnets,too. Crystallized at 700℃ for 10 minutes, the magnetic phases precipitate completely and their grains are fine and uniform. The significant interaction among the magnetic grains makes the combination of properties excellent. If annealing temperature<700℃, or time<10min, the magnetic phases precipitate incompletely. If annealing temperature>700℃, or time>10min, the magnetic phases precipitate completely, but the grains grow furthermore. Both of them make the exchange couplings weakened and the magnetic properties degraded.With the decrease of rare earths content for the (Nd0.8Pr0.2)xFe88-xCo5Zr1B6 alloy, the volume fraction of soft magnetic phase and the remanence Br of bonded magnets increase., but both the intrinsic coercivity Hci and the maximum energy product (BH)m decrease. Atom fraction of 10% is the critical rare earths content for the magnetic properties change. Below this value, the magnetic properties, especially Hci, are very low. This experimental result is generally consistent with the calculated result using the model of volume fraction of soft magnetic phase coupled completely. According to this model suggested by us 20% is the maximum volume fraction of soft magnetic phase completely coupled with the hard magnetic phase. Beyond this theoretical value, there may exist some extra soft magnetic phases which are not coupled with the hard magnetic phase. These soft magnetic phases become the nuclei of the reverse magnetic domain and consequently initiate a cascade-type demagnetization process of the assembly.With the increase of Pr content of the (NdxPr1-x)10.5Fe77.5Co5Zr1B6 series alloy, <WP=8>the Hci of bonded magnets increases, but the Br decreases. The (BH)m reaches the maximum value of 70.6 kJ/m3 at x=0.8. For these alloys, the optimum substituted fraction of Nd by Pr is 0.8, in which the best combination of properties is obtained. The higher intrinsic coercivity of Pr-based magnets is derived from the 30% higher anisotropy field of Pr2Fe14B than that of Nd2Fe14B. With the substitution of Nd by Pr the alloy has a lower crystallization temperature and a lower crystallization energy of amorphous, which means that Pr makes the alloy instabler in crystallization kinetics. So the crystallization transition and the grain g

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