Synthesis Of Anisotropic Nanocrystalline Sm-Pr-Co Single Phaseand Sm-Pr-Co/Co Composite Magnet Powders

The synthesis of nanocomposite permanent magnet via bottom-up approach, compared with top-down approach, is a new method for the next generation magnets. Essential progress for hard magnet nanoparticles in the first-step product has been made by fabricating anisotropic nanoflakes via surfactant-assisted high energy ball milling. For the second step, the coating method is potential in assembling the soft nanoparticles into thin continuous soft-magnet films on the hard particles. However, many studies demonstrated that, there are still many problems in both fabrication methods. For example, there exists severe coercivity and remanence loss when the flake thickness is thinned into nano-scale dimension, the coated soft nanoparticles are too sparsely distributed to form continuous coatings, and it is extremely difficult to control the size of the soft nanoparticles below 10 nm. In this paper, the subjects about the control of the microstructure and properties of nanocrystalline Sm-Pr-Co single and Sm-Pr-Co/Co composite permanent powders were studied by employing X ray diffraction(XRD), field emission scanning electron microscopy(FESEM), and vibrating sample magnetometer(VSM) techniques as follows:In nonpolar milling medium with oleic acid as surfactant and heptane as solution, anisotropic Sm-Pr-Co nanoflakes have been fabricated by surfactant assisted high energy ball milling. The flakes have a maximum coercivity of 12.0-12.4 k Oe and an aspect ratio of 102-103 with thickness of tens to hundreds of nanometers and in-plane sizes of 0.5-10 μm. The flake formation mechanism was studied by comparision with the powders produced by dry milling and heptane-only wet milling.Studies on magnetic properties show that the coercivity of the Sm-Pr-Co flakes relies not only on the total milling energy, but also the energy injection process. A maximum coercivity of Hci=13.2 k Oe, along with a remanence enhancement was obtained at an optimum milling condition.By milling in polar milling medium with ethanol as solvent and PVP as surfactant, the polar milling condition has successfully been introduced into the fabrication of anisotropic permanent nanoflakes. The nanoflakes show a lower aspect ratio of 5-50 with reduced in-plane size of 0.2-2 μm. Studies on the magnetic properties show that this milling condition did not induce the severe remanence loss, or the decrease in coercivity when the flake thickness is thinned into nano scale. The reasons for the flake-morphology and magnetic-property differences in both milling medium have been discussed by analyzing the milling processes.The flake coercivity can be enhanced because of enhanced domain pinning after adding Sn Cl2·2H2O into the polar milling condition. A maximum coercivity of Hci =14.7 k Oe, so far the highest, were achieved under an optimum milling condition. This result provides a new route for the coercivity enhancement in the wet milling condition.Co nanoparticles with average sizes of 25-70 nm have been distributed homogeneously to form soft magnet coating on ball-milled anisotropic nanocrystalline Sm-Pr-Co flakes via appropriate low-rate electroless cobalt deposition. The composite powders show a strong(00l) hard-phase texture, a higher magnetization moment and remanence, but unfortunately a lower coercivity and energy product. Studies of the morphology dependence on coating parameters show that, the low-rate deposition favors the Co deposition on the surfaces of the hard particles. The formation mechanism of these continuous coatings was also studied by studying the coating evolutions.Pretreating the hard magnet particles in HCl solution promotes the Co nucleation and deposition on the hard powders, leading down sized cobalt particles of smaller than 10 nm in the dense/continuous coatings. As a result, enhanced remanence and maximum energy product have been observed in the Sm-Pr-Co/Co nanocomposite products.Variations of the coated cobalt size and magnetic properties as a function of deposited cobalt content were studied when coating hard particles with different sizes. The result shows that, the maximum energy product peaks at x=3 wt% with coated cobalt size of 9nm when coating microsized particles before milling, and at x=10 wt% with cobalt size of 5.5nm when coating the flakes after milling.