New Phenomena Of Rare Earth Permanent Magnets Initiated By High Energy Ball Milling In Organic Medium And Their Application

The rare earth permanent magnets, including Sm–Co and Nd–Fe–B systems,are the most important permanent magnets widely applied in energy storage andConversion. Our country is rich in rare earth resource, the investigation of rare earthpermanent is meaningful for the effective usage of the rare earth resource. In thisthesis, high energy ball milling in organic solvent is applied in the preparation ofrare earth permanent magnets in an attempt to obtain new submicron or nanopermanent magnets. Detail works are as fellow.1.With heptane as the ball-milling medium and OA as the surfactant, RCo5(R=Sm, Y，Ce) are processed by surfactant assisted high-energy ball milling. In theforepart of the HEBM duration (t120minutes), the particles with coarse grainsthinned rapidly from bout several μm to about100–200nm. These particles aresingle crystalline and could be oriented in a magnetic field. While the coercivity istoo small due to the larger grain size and could not meet the demand of permanentmagnet. Extending mill time results in a further decrease of the particle’s thicknessto about a few tens nm but at an obvious slow speed. On the contrary, the sizes ofother two dimensions remain unchanged almost during the whole HEBM process.Hence submicron RCo5fakes with larger aspect ratio formed by thissurfactant-assisted HEBM are obtained. What more valuable is that these fakes arecrystallographically and magnetically anisotropic and the c-axes of CaCu5structure,also the easy magnetization axes, are oriented perpendicular to the fake’s surface.Well dispersive and magnetically anisotropic particles could be aligned by magneticfelds and could be used to prepared anisotropic bonded magnets. Surfactants play an important role in the formation of such high aspect ratio submicron fakes.Without surfactant, the fnely divided particles tend to agglomerate in order tominimize their surface energy. Milling in inert atmosphere or in organic solvent willresult in the formation of isotropic polycrystalline particles. Surfactants covering onthe particle could lower the energies of freshly cleaved surfaces, enabling long-rangecapillary forces and lowering the energy required for crack propagation. Henceapplication of surfactant could avoid agglomeration and cold welding during theHEBM. By surfactant assisted HEBM, coercivity of15kOe for SmCo5flake,2.4kOe for YCo5and3.3kOe for CeCo5are achieved along their easy directions.2.It is found for the first time that being high energy ball milled in heptanes forcertain time, the SmCo5alloy could disproportionate into SmH2+and Co. Heatingthe disproportionated powder in vacuum from room temperature to800℃, twodesorption processes, one between200℃and400℃and the other around about600℃, were observed. The desorbed gas is proved to be hydrogen by gaschromatography. The Heated product consists of mainly hexagonal SmCo7phasewith a TbCu7structure. The coercivity of SmCo7phase is larger than1T, being ableto meet the demand of permanent magnetic application. Excess high energy ballmilling results in the appearance of minor SmCoC2and cubic Co.Traditional hydrogenation disproportionation desorption recombination (HDDR)technique succeeds in fabricating bonded Nd–Fe–B magnet while encountersdiffculty in the fabrication of Sm–Co magnet, due to the higher thermodynamicstability of the Sm–Co compounds against the disproportionation by hydrogen. Toinduce the disproporationation of Sm–Co compounds, high pressure hydrogen underhigh temperature has to be employed. This research result presents a HDDR-likedtechnique that suitable for preparing Sm-Co bonded permanent and is expected tosolve the technique and security problems that in the conventional HDDR technique.