Research On Composition, Structure, Preparatioin Of Rare-earth-cobalt Based High Temperature Permanent Magnetic Material

Owing to their excellent magnetic properties and thermal stability, Rareearth-Cobalt-based permanent magnets play an important role in both military andhigh technology applications. So far, the commercial Rare earth-Cobalt-based sinteredmagnet are1:5type and2:17type magnets, such as SmCo5and Sm(CoFeCuZr)z,respectively. As the raise of requirement for magnetic properties and operatingtemperature in applications, more attention has been paid on this area. Under thiscircumstance, the present work had been done, focusing on the composition, structureand preparing methods of the Rare-earth-Cobalt-based magnet, in order to achieve thebetter magnetic properties at elevated temperature. This thesis contains the followingaspects:1) preparation of bulk nanocrystalline SmCo5magnet with strong magneticanisotropy;2) phase structure, intrinsic magnetic properties and thermal stability ofTmCoxand Sm1-xTmxCo5alloys;3) synthesis of new Sm(CoFeCuZr)zmagnets bymeans of nano-powders doping method.The full dense and anisotropic SmCo5and PrCo5nanocrystalline magnets hadbeen prepared by hot-pressed and hot-deformed method using Spark Plasma Sintering(SPS) technology. The influence of the preparing conditions on the crystallographictexture, microstructure and magnetic properties had been studied systematically. Theresults indicate that, as the increase of deformation rate, c-axis crystal texture,remanence (Br) and magnetic energy product ((BH)max) are all notably increased. Forinstance, the energy product of SmCo5enhances from6.1MGOe for the isotropicmagnet to14.7MGOe after a deformation of70%. The coercivities (Hcj) of themagnets show a peak behavior as the increase of deformation, which reaches amaximum value of52.95kOe after deformed for70%. During the hot deformation, thegrains in the magnet change gradually from equiaxed shape to plate shape, while thesize grows up from nanometer to sub-micrometer. In view of these results, theformation of c-axis texture has been attributed to the rotation of grains and sliding ofgrain boundaries.TmCoxand Sm1-xTmxCo5alloys with CaCu5structure had been prepared byinduction melting and subsequent annealing. The phase composition and intrinsicmagnetic properties at different temperatures were investigated. XRD tests reveal thatthe Sm1-xTmxCo5alloys consist of mainly1:5phase and some TmCo2phase less than5.0wt.%. The Curie temperature of Sm1-xTmxCo5alloys increases while the saturation magnetization and anisotropy field decreases as the increases of Tm dopingamount. Tm doped Sm1-xTmxCo5alloys show positive coefficient of saturationmagnetization between30and200℃, and the coefficient increases with increased Tmcontent. The temperature coefficient of anisotropy field of the alloys is howeverreduced by more Tm doping. As for the TmCoxalloys, XRD results show that thereare small amounts of R2Co7and R2Co17phases in addition to the1:5main phaes.TmCo5.6, containing91wt.%of1:5phase, has the best phase quality which will getworse when departing from this composition. The saturation magnetization ofTmCo5.6alloy increases when the temperature goes up, corresponding to a positivecoefficient of saturation magnetization of+0.0345%/℃between30~200℃.Moreover, Sm0.8Tm0.2Co5.2and TmCo5.6nanocrystalline magnets were prepared bySPS, which show the positive coefficient of remanence of+0.0156%/K and+0.066%/K in the range of30~300℃and30~100℃, respectively.Based on a systematically research about the preparing process ofSm(CobalFe0.09Cu0.09Zr0.03)7.69, the optimal heat treatment processing conditions hasbeen determined as: the sintering temperature is1230℃, the solution temperature is1185℃, the aging process is annealing at840℃for12h, cooling to420℃at0.4℃/min and keeping for10h and then cooling in the furnace. The obtained magneticproperties are: Ms=10.67kGs，Br=9.89kGs，Hcj=40.78kOe，(BH)max=23.66MGOe. Bymeans of this process, Sm, Co, and Fe nano-powders were doped inSm(CobalFe0.09Cu0.09Zr0.03)7.69magnet to modify its properties. It shows that, exceptcoercivity, the saturation magnetization, remanence and maximum energy product areall increased by the increase of Sm nano-powder. TEM observation reveals that Smnano-powders can reduce the size and destroy uniformity of the cellular structure.Doping of Co nano-powder makes the saturation magnetization increases and thecoercivity decreases monotonously. The remanence and maximum energy productfirstly increase and then drop. It shows the optimized properties at2.0wt.%, of whichthe Ms=11.06kGs， Br=10.33kGs， Hci=30.21kOe,(BH)max=25.47MGOe. TEMobservation shows that Co doping could increase the size of cellular structure andinduce the decline of coercivity.The saturation magnetization was enhanced when increase the content of Fenano-powder. The coercivity, remanence and maximum energy product increasefirstly and then drop. The optimized doping content is1.0wt.%, and the magneticproperties are: Ms=11.33kGs，Br=10.12kGs，Hci=46.36kOe，(BH)max=24.31MGOe. TEM observation shows that the cell boundary becomes broad after Fe doping, whichwill weaken the pinning effect of domain walls and thus reduce the coercivity.In addition, the influence of doped Cu nano-powders on magnetic properties ofSm(CobalFe0.08Cu0.10Zr0.03)7magnet had also been studied. The results show that Cunano-powder could increase the coercivity of the magnets, while other properties havebeen decreased. TEM results show that the doped Cu nano-powder could increase thedensity of flake-shaped Zr-rich phase which could cause the increase of coercivity.