Improving the hard magnetic properties by intragrain pinning for Ta doped nanocrystalline Ce-Fe-B alloys

To develop Ce based permanent magnets with high performance/cost ratio, Ta doping is was employed to enhance the magnetic performance of Ce-Fe-B alloys. For melt spun Cei7Fe78-xTaxB6 (x = 0-1) alloys, the coercivity Hc increases from 439 to 553 kA/m with increasing x value from 0 to 0.75. Microstructure characterizations indicate that Ta doping is helpful for grain refinement. A second phase of TaB2 is observed in Ce17Fe77.25Tao.75B6 alloy, which acts as the pinning center of the magnetic domains, resulting in the change of coercivity mechanism from nucleation type to nucleation +pinning type. The micromagnetic simulation confirms that non-magnetic particles within hard magnetic phase can increase the demagnetization field around them and it is crucial for preventing the further magnetization reverse by pinning effect. Take the advantage of Ta doping for enhancing the coercivity, Ce content of Ce-Fe-B alloy can be further cut down to increase the remanence Jr due to the reduced volume fraction of CeFe2 phase and increased Fe/Ce ratio. As a result, a good combination of magnetic properties with Hc = 514 kA/m, Jr = 0.49 T, and the maximum energy product (BH)max = 36 kJ/m^3 have been obtained in Ce15Fe79.25Tao.75B6 alloy. It is expected that the present work can serve as a useful reference for designing new permanent magnetic materials with low-cost.

A direct H2O2 production based on hollow porous carbon sphere-sulfur nanocrystal composites by confinement effect as oxygen reduction electrocatalysts

Carbon-sulfur composites have draw n in creasing interest in various fields including electrocatalysis because of their unique structures.However,carb on-sulfur composite with tiny sulfur nano crystals has still received little attention.Herein,hollow porous carb on sphere-sulfur composite(HPCS-S)which possesses excellent electrochemical performance towards H2O2 has been prepared for the first time via a simple silica template method.The 2-5 nm sulfur nan ocrystals being restricted in the cha nnel of the hollow porous carb on spheres are un der a strong compressive stress,which was further con firmed by high-resoluti on tran smissi on electr on microscopy(HRTEM)and GPA.The HPCS-S nano crystals show better con ductivity tha n amorphous sulfur clusters because of the reducti on of the steric hindrance which efficie ntly promotes the electron transportation.Consequently,the higher activity and selectivity towards the 2e^oxygen reduction reaction(ORR)to H2O2 in alkaline solution was obtained.The H2O2 selectivity rises from 20%to over 70%after the sulfur addition and the H2O2 production rate achieves 183.99 mmol-gcataiyst-1 with the Faradaic efficiency of 70%.Furthermore,performance enhancement mechanism was also investigated using the den sity functional theory(DFT)calculatio ns.After the in troduci ng of sulfur nano crystals,the appeara nee of S-S bond greatly decreases the overpotential compared with S-doping,which results in significant enhancement of the electrocatalytic property.Consequently,the HPCS-S can be an efficient H2O2 production electrocatalyst in alkaline solution.