Study On Functional Application Of Fe-based Amorphous And Amorphous-Nanocrystalline Alloys

Due to the essentially disordered atomic configurations and metastable energy states,amorphous alloys exhibit many unique physical and chemical properties.Widely adjustable compositions and atomic structures provide fascinating possibilities to further improve properties.In addition,significant number of low-coordination surface atoms and defect sites promise amorphous alloys a broad application prospect in catalyst fields.As an efficient,low-cost and environmentally friendly catalyst,Fe-based amorphous alloys also exhibit excellent soft magnetic properties such as low coercivity（H_c）and low core loss,which have attracted extensive attention.Further research has found that the catalytic and soft magnetic properties could be greatly promoted by introducing additional crystalline phases in the amorphous matrix due to the synergistic advantages of the crystalline and amorphous phases.The former originates from the"galvanic effect"between nanocrystals and amorphous matrix,while the latter can be attributed to the high saturation magnetic induction（B_s）and low magnetic anisotropy caused by nanocrystals precipitation.However,it is a key difficulty to control the nucleation and growth procedures of nanocrystals during annealing.The conventional casting process and annealing approaches induced amorphous-crystalline composites restrict galvanic cells effects and lead to the deterioration of soft magnetic properties because the generated crystalline phases are easily coarsened.In this dissertation,we report a controllable film deposition approach to construct biphasic amorphous film with hyperfine spinodal decomposition morphology in advance,and subsequently perform high temperature annealing to result ultra-fine amorphous-nanocrystalline Fe₇₆Si₈B₁₃Nb₃ catalyst with abundant interfaces.The biphasic amorphous film follows the mechanism of inhibiting grain growth during annealing,and forms fine nanocrystals of about 7.7±1.9 nm,thus obtaining composite structural materials with ultra-high interface density(2×10¹⁶ cm^-2).The designed ultra-fine Fe-based amorphous-nanocrystalline catalyst exhibits extraordinary dye degradation efficiency of 300 times than that of the commercial Fe powder.Especially,the outstanding catalytic performances of amorphous-nanocrystalline composite are achieved without the additional involvement of hydrogen peroxide assistance,which provides an environmental-friendly neutral catalytic condition and avoids the corrosive damage during commercial sewage-treatment.This study provides a distinct perspective to design and regulate catalytic performances by amorphous precursor with preexistent ultra-fine structures.Considering the abundance of active sites,high corrosion resistance,and low cost,Fe-based amorphous alloys may be used as electrochemical catalysts in renewable energy conversion.In this thesis,for the first time,Fe-based biphasic amorphous alloys are applied to the electrochemical nitrogen reduction reaction（NRR）.Surprisingly,the biphasic amorphous Fe₇₆Si₈B₁₃Nb₃ films show excellent NRR activity,with an ammonia yield of 101μg h^-1 mg_cat.^-1in sodium sulfate electrolyte.The ammonia yield is higher than most NRR catalysts,and even much higher than noble-metal catalysts.The abundant unsaturated coordination sites and defect sites in Fe-based amorphous alloys are the main reasons for the enhanced electrocatalytic activity.In addition,the high corrosion resistance of amorphous alloys enables them to overcome the instability of crystalline materials in electrocatalytic experiments.This study provides implications for the subsequent development of the low-cost noble-metal-free electrocatalysts.In this dissertation,the Fe₇₆Si₈B₁₃Nb₃ soft magnetic film with ultra-fine amorphous-nanocrystalline core-shell structure is achieved from the amorphous precursor with the nanoscale phase separation.The pre-existed nanoscale phase separation provides vast nucleation sites and interphase boundaries,which could generate high concentration of nanocrystals.It is worth noting that even during a long annealing process,the coarsening behavior of the grains is suppressed,resulting in grain size of～4 nm.In addition,the Fe-based amorphous-nanocrystalline alloy film with ultra-fine core-shell structure also breaks the mutually exclusive relationship between high B_s and low H_c in the soft magnetic material family.This study provides a new perspective to design amorphous-nanocrystalline soft magnetic alloy by amorphous precursor with dual phase structure,and importantly,it supplies fairly wide annealing time window to conveniently regulate the microstructures and soft magnetic properties of amorphous-nanocrystalline alloy.The results reported in this dissertation provide a new approach for the design of ultra-fine amorphous-nanocrystalline composite structures.With the biphasic amorphous alloy precursors,the microstructure can be easily adjusted even in a wide annealing time window,and then the optimized catalytic and soft magnetic properties can be achieved.In addition,the successful application of Fe-based biphasic amorphous alloys in electrochemical nitrogen reduction reactions suggests the potential to be a much-needed low-cost noble-metal-free electrochemical catalyst for renewable energy conversion.