The Crystallization Kinetics And Magnetic Properties Of FINEMET-type Soft Magnetic Alloys

Crystallization kinetics and magnetic properties of FINEMET-type soft magnetic alloy areinvestigated systematically in present thesis. Following are the main results:Amorphous ribbons of the alloys Fe_73.5Si_13.5B₉Cu₁Nb_3-xMo_x（x=1,2,3），Fe（73.5）Si_13.5B₉Cu₁Nb_3-xV_x（x=1,2）， Fe（73.5-x）Co_xSi_13.5B₉Cu₁Nb₃（x=10,30,50） andFe_73.5Si_13.5-xGe_xB₉Cu₁Nb₃（x=3,6） are successfully synthesized by the standard single copperwheel melt spinning technique in the air atmosphere. The crystallization kinetics ofamorphous ribbons have been investigated systematically by means of non-isothermalDifferential Scanning Calorimetry （DSC）. Our investigations indicate that crystallizationcharacteristic temperatures are shifted to the higher temperatures with increasing heating rate,which means the nanocrystallization process has kinetic effects. The crystallization activationenergies of amorphous ribbons have been calculated by using Kissinger, Ozava andAugis-Bennett models based on differential thermal analysis data. The local Avrami exponentn for primary crystallization is calculated using Johnson-Mehl-Avrami （JMA） equation. Thesignificant variation of local Avrami exponent n with crystallized volume fraction αdemonstrats that the primary crystallization kinetics of amorphous ribbons varies at differentstages. In the initial stage, the crystallization mechanism is bulk crystallization with threedimensional nucleation and grain growth controlled by diffusion at decreasing nucleation rate.In the following stage, it is surface crystallization with one dimensional nucleation and graingrowth at a near-zero nucleation rate.Amorphous ribbons with width of10mm and thickness of0.03mm forFe_73.5Si_13.5B₉Cu₁Nb₁Mo₂and Fe_73.5Si_13.5B₉Cu₁Nb₂V₁are successfully synthesized by thecustom single copper wheel melt spinning machine in the air atmosphere. The hysteresisloops of the toroidal shaped amorphous magnetic cores annealed at different temperatures aremeasured. The experimental results show that: the Fe_73.5Si_13.5B₉Cu₁Nb₁Mo₂amorphous coreannealed at470℃for45min firstly and then nanocrystallization at540℃for60minexhibits excellent soft magnetic properties. The following is the magnetic data measured at1kHz: Hm=199.1A/m，BS=0.4760T，HC=8.43A/m，Ps=4.708W/kg，Br=0.1908，μ=2.391mH/mï¼›Fe_73.5Si_13.5B₉Cu₁Nb₂V₁amorphous core annealed at470℃for45min firstly and thennanocrystallization at545℃for60minexhibits excellent soft magnetic properties. Thefollowing is the magnetic data measured at1kHz: Hm=197.88A/m，BS=0.690T，HC=6.933A/m，Ps=3.384W/kg，Br=0.1678，lamination factor=0.63。The soft magnetic properties ofFe_73.5Si_13.5B₉Cu₁Nb₂V₁nanocrystalline core at1kHz are better than that ofFe_73.5Si_13.5B₉Cu₁Nb₁Mo₂nanocrystalline core. The Fe_63.5Co₁₀Si_13.5B₉Cu₁Nb₃amorphous ribbons are first nanocrystallized by annealing at550℃for60min in vacuum and then converted topowders by milling with a planetary ballmill. The toroidal shaped soft magnetic cores with an outer diameter of20mm and an innerdiameter of10mm are prepared according to the following process: sieving-insulationcoating-cold molding-stress free annealing. The effects of powder size, binder content,molding pressure and stress relief annealing time on the density, the effective permeabilityand the quality factor of Fe_63.5Co₁₀Si_13.5B₉Cu₁Nb₃magnetic powder core are studiedsystematically. Our investigations indicate that the optimum process parameters ofFe_63.5Co₁₀Si_13.5B₉Cu₁Nb₃nanocrystalline magnetic powder core are listed as follows:insulation content is3%, forming pressure is174kN, annealing temperature is100℃,annealing time is11h. The corresponding magnetic properties show: the effectivepermeability μehas high frequency stability within the frequency range of5MHz andmaximum value of μeis55, the quality factor Q first increases to35at250kHz and thendecreases with increasing frequency.The heat treatment process of FINEMET core is two-step heat treatment （ordinary heattreatment and magnetic heat treatment）. The influence of the direction of the magnetic field,the strength of the magnetic field and the heat treatment temperature on the hysteresis loop ofthe core have been investigated systematically. The experimental results show that: the shapeof hysteresis loop gradually transformed into approximate straight line （permeability isconstant） with increasing transverse magnetic annealing temperature. Transverse magneticannealing can decrease the permeability and loss．The shape of hysteresis loop graduallytransformed into approximate rectangular with increasing longitudinal magnetic annealingtemperature. Longitudinal magnetic annealing can increase permeability and loss. The shapeof hysteresis loop of FINEMET core treated by Longitudinal and transverse compositemagnetic annealing have the characteristics both of them, so FINEMET core exhibit excellentsoft magnetic properties.