Study On Preparation, Structure And Magnetic Properties Of Nanostructured Permanent Magnetic Materials

Nanostructured permanent magnets have drawn considerable attention in recentyears due to their unusual scientific and technological values. Different fromconventional permanent magnets, they possess unique nanostructure and thereforeexcellent magnetic properties, good thermal stability, desirable corrosion resistance,and strong mechanical strength. However, as a new type of permanent magneticmaterial, such magnets need more in-depth and systematic investigation to promotethe process of practical application. In this thesis, three main subjects were selectedand investigated. Firstly, the nanocrystalline Tb-Fe-B melt-spun ribbons withultra-high coercivity were prepared, and the formation and evolution of Tb2Fe14Bc-axis crystal texture were discussed. Secondly, several kinds of nanoflakes andnanoparticles with good magnetic properties were fabricated via a top-down approach,and their magnetization and demagnetization behavior as well as their mechanism ofmagnetic hardening were investigated seriously. Thirdly, anisotropic Nd₂Fe₁₄B/α-Fenanocomposite permanent magnetic bulks were produced via a bottom-up approach,and their structure, magnetic properties as well as the exchange coupling effect werestudied.Tb-Fe-B permanent magnetic ribbons with nominal composition of Nd14Fe86-xBx(x=5.2⁶.8) were prepared by melt-spun technique (MS). The composition, structure,and magnetic properties of the ribbons were studied by means of XRD, SEM, VSM,and PPMS; and effects of wheel speed on the formation and evolution of the c-axiscrystal texture of Tb₂Fe₁₄B phase in the ribbons were investigated. Under optimalnominal composition and quenching conditions, the single phase Tb₂Fe₁₄B ribbonsbears fine and uniform grains and therefore coercivity as high as77.4kOe, which isamongst the the highest reported room temperature coercivity for permanent magnets.Moreover, it is found that the formation of Tb₂Fe₁₄B c-axis crystal texture in theribbons was mainly due to the directional solidification process, which was caused bythe different temperature gradients during the quenching process. Furtherinvestigation shows that the competition of two different types of temperaturegradients, which are perpendicular to each other, was responsible for evolution of thecrystallographic alignment in the ribbons with the wheel speed changing gradually.Tb-Fe-B, Dy-Fe-B, Nd-Fe-B, Sm-Fe-N, SmCo5, and Mn-Bi nanoflakes andnanoparticles with narrow particle size distributions have been prepared via surfactantassisted ball milling (SABM) and a subsequent size-selection process. Thecomposition, structure, and magnetic properties of the nanoflakes and nanoparticleswere studied by means of ICP, LPSA, XRD, SEM, TEM, VSM, and PPMS. Thesenanoflakes, with average diameter of about1μm and thickness of less than100nm, are of strong magnetic anisotropy and high coercivity values. Except Nd-Fe-Bnanoflakes, coercivity values of the other flakes prepared in this thesis are all higherthan15kOe. While, The nanoparticles with average particle sizes of30⁵0nm bearsgood permanent magnetic properties, coercivity values of the small nanoparticles withaverage particle sizes smaller than10nm reduces remarkably, which exhibit somecertain extent superparamagnetic characterization. Further chemical compositionanalyses of the Tb-Fe-B samples show that both the nanoflakes and nanoparticleskeep close composition to the stoichiometric Tb₂Fe₁₄B compound, though the Tb andB content drops gradually but slightly during the ball milling process.The coercivity of the the nanoflakes and the nanoparticles as a function of theirsize were studied. It is found that the coercivity of the Tb-Fe-B, Dy-Fe-B, Nd-Fe-B,and SmCo5samples reduces with the particle size, while the coercivity of theSm-Fe-N, and Mn-Bi samples increases first, then decreases with the reduction of theparticle size. According to the magnetization and demagnetization behaviorinvestagation, coercivity of the Tb-Fe-B nanoflakes and nanoparticles were allcontrolled by the domain wall pinning on the whole, while the coercivities ofSm-Fe-N nanoflakes and nanoparticles were dermined primarily by the nucleation ofreversal domain and the domain wall pinning, respectively.Anisotropic single phase Nd₂Fe₁₄B nanocrystalline permanent magnetic bulksand Nd₂Fe₁₄B/α-Fe nanocomposite permanent magnets have been prepared viaultrasonic chemistry reactive coating (UCRC) and spark plasma sintering techniques(SPS). The composition, structure, and magnetic properties of the magnets werestudied by means of XRD, SEM, TEM, VSM, and PPMS, and effects of compositionand sintering conditions on the magnetic properties of the magnets weresystematically studied. Under optimal sintering conditions, the Nd₂Fe₁₄B nanocrystalline permanent magnets bears magnetic properties of Hciof4.88kOe, Mrof2.89kGs, and (BH)maxof2.09MGOe. The Nd₂Fe₁₄B/α-Fe nanocompositepermanent magnet coating with nominal20wt.%iron nanoparticles (comparing to theweight of Nd-Fe-B nanoflakes) exhibits magnetic properties of Hciof6.45kOe, Mrof4.57kGs, and (BH)maxof4.87MGOe. Further investigations indicate that the strongexchange coupling between the magnetically hard phase and soft phase result inremarkable remanence enhancement in these magnets.