| Amorphous and nanocrystalline soft magnetic composites with low coercivity,high permeability,and high resistivity meet the development requirements for miniaturization,high frequency,and high power of electronic devices and are increasingly expanding their applications.Amorphous and nanocrystalline soft magnetic powder with smooth surface,nearly spherical shape,low oxygen content,and fine particle size is the prerequisite for preparing soft magnetic composites with high performance.However,the current amorphous and nanocrystalline powder has poor morphology or achieve amorphous structure with difficulty due to low cooling rate.In this paper,by CFD(Computational Fluid Dynamics)simulation and finite difference method,the flow field structure,break-up mechanism of the melt,and cooling and solidification process of metal droplets were analyzed.Gas-water combined atomizing technology and equipment with high cooling rate were developed and built,which can prepare the powder with low oxygen,fine particle size,and spherical shape.The amorphous Fe73.5Si13B11Cr1C1.5(at.%)and nanocrystalline Fe80Cu1Nb2Si11-xB6Px(at.%)soft magnetic powder were prepared,the effects of P element on improving glass forming ability and refining grains were studied,and the soft magnetic characteristics and application of powder were evaluated.The main research results of this paper are concluded as follows:(1)The supersonic close-coupled gas atomization nozzles with different gas Mach number at the outlet were designed,and the gas flow field structure was analyzed.A gas recirculation zone existed under the melt delivery tube.At high atomization pressure,a bow-shaped Mach disk was formed downstream of the recirculation zone.The flow field of the nozzle with a high Mach number had less strong Mach disk and kept a long high velocity gas wake.The break-up process of metal liquid consisted of the fluctuation on the liquid surface,primary and secondary break-up.Under the high Mach number,high atomization pressure,and large mass flow ratio of gas to metal liquid,relatively finer powder with a median mass diameter of less than 30 μm could be prepared.(2)The cooling and solidification process of metal droplets in the gas field was calculated by finite difference method,which comprised of the liquid phase cooling,nucleation,recalescence,segregated solidification and solid phase cooling.The nozzle with higher Mach number had a greater cooling rate.At Mach number of 2.5,the cooling rate of droplets with a diameter of 80 μm was 3.50 × 104 K/s.The relationship between the grain diameter(Dg)and the cooling rate(T)was given by Dg=88.975·T-0.221.(3)The gas-water combined atomization system were developed by taking the respective advantage of gas atomization and water atomization,which was equipped with a first layer of gas atomization nozzle and a second layer of water atomization nozzle.Due to the presence of a large amount of nitrogen,the solidification of the droplet was postponed and enough time was rendered for spheroidization compared to water atomization.Besides,due to the presence of water jet,the cooling ability was enhanced compared to gas atomization.The powder prepared by gas-water combined atomization has nearly spherical shape,low oxygen content,and regulable particle size in a wide rage.Compared with water atomization,the particle size was significantly refined under the same water pressure.The mass median diameter could be less than 10 μm,the oxygen content decreased by 0.03 wt.%～0.25 wt.%,and the tap density increased by 0.5 g/cm3～1.0 g/cm3.(4)The Fe73.5Si13B11Cr1C1.5(at.%)amorphous powder was prepared using three atomization processes.Gas atomized Fe73.5Si13B11Cr1C1.5(at.%)powder(-60μm)had partially crystallized,whereas the water atomized powder and gas-water combined atomized powder were completely amorphous.Moreover,the sphericity of gas-water combined atomized powder was the best.The oxygen content of water atomized,gas-water combined atomized and gas atomized powder was 0.087 wt.%,0.059 wt.%and 0.012 wt.%,respectively;the coercivity(Hc)was 151.7 A/m,156 A/m,and 413.8 A/m,respectively;the saturation magnetic polarization intensity(Js)was 1.07 T,1.09 T and 1.12 T,respectively.After annealed under 793 K in nitrogen for an hour,the gas-water combined atomized magnetic powder cores had the best comprehensive performance,i.e.,effective permeability(μ3)was 26.9,the DC bias under DC magnetic field 7.96 kA/m was 83.3%,and the core loss was 258 mw/cm3 at f=100 kHz and Bm=0.05 T.Compared with other crystal metal magnetic powder cores,it had the lowest core loss,indicating qualified operation at high frequency.(5)Fe80Cu1Nb2Si11-xB6Px(at.%)powder was prepared by gas-water combined atomization.With the increase of P content,the glass forming ability of powder gradually increased,the first crystallization onset temperature(Tx1)and the second crystallization onset temperature(Tx2)were reduced,and the temperature intervalΔTx(=Tx2-Tx1)was narrowed.P could promote the nucleation of α-Fe(Si)and refine the grains.After annealed under 773 K in hydrogen for an hour,the average grain size inside Fe80Cu1Nb2Si6B6P5 powder was 20.3 nm,the Js and Hc was 1.42 T and 98 A/m,respectively.Corresponding magnetic powder cores had the best comprehensive performance,i.e.,μe was 23.3,the DC bias under DC magnetic field 7.96 kA/m was 90.5%,and the core loss was 495 mW/cm3 at f=100 kHz and Bm=0.05 T. |
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