Investigation On The Structure Regulation And Properties Of One-dimension Nanoporous Materials Based Upon Al-Ni Eutectic

One-dimensional nanomaterials(nanowire,nanotube,nanoribbon,etc.)have the advantages of excellent electron transmission efficiency,high specific surface area,showing broad application prospects in the fields of catalysis,microwave absorption,energy storage.Among which,nanowire materials are more widely concerned.Templates and active agents are usually used in the preparation of nanowires,which are easy to pollute and affect their structures and properties.Therefore,the preparation,structure and property regulation of nanowires has been an important research direction in the field of one-dimensional nanomaterials.Eutectic alloys are widely used in engineering,and the second phase in fibrous eutectic alloys(such as Al-Ni)is often grown into micron or nanowire shapes.In this dissertation,based on Al-Ni system,a variety of ribbon-like,powder-like and bulk precursor alloys were prepared through alloy design,microalloying and solidification control.A series of one-dimensional nanoporous materials were developed by further using dealloying technology,and their magnetic,waveabsorbing and electrocatalytic properties were studied.The correlation between structure and properties was explored by combining in situ and ex situ characterizations.The main contents are as follows:Firstly,Ni@NiO nanoporous nanowires were prepared by chemical dealloying of neareutectic Al96Ni4 precursor in alkaline solution using rapid solidification control and dealloying inheritance effect.The nanostructured Ni was spontaneously oxidized to form a core-shell structure(Ni as the core and NiO as the shell).In addition,the diameter of the nanowires can be effectively controlled by changing the Cu roller speed(cooling rate)during the preparation of the precursor.Magnetic measurements show that Ni@NiO nanoporous nanowires exhibit unique magnetic properties with smaller saturation magnetization and stronger coercivity compared to bulk Ni,micron-sized Ni powders and nanoscale Ni particles.The decrease in saturation magnetization is attributed to the presence of surface dielectric NiO shell,which is confirmed by prolonging the dealloying time in alkaline solution to aggravate surface oxidation.And the enhanced coercivity can be attributed to the reduced exchange stiffness due to the surface non-colinear magnetic order,residual stresses caused by dealloying,enhanced magnetic anisotropy of ferromagnetic Ni covered by antiferromagnetic NiO,and the anisotropy of nanowire structure.Secondly,on the basis of the above work,kilogram-scale Al-Ni eutectic alloy powders were fabricated by gas atomization method,and porous Ni@NiO microspheres were prepared by chemical dealloying.In view of the influence of the thickness of NiO shell on the electromagnetic properties,a series of Ni@NiO composite materials were obtained by regulating the ratio of two phases in Ni@NiO microspheres by heat treatment.The magnetic measurements show that the saturation magnetization and remanent magnetization of Ni@NiO composites decrease with the extension of heat treatment time(oxidation degree),which is related to the increase of NiO content in the composites.It is worth noting that with the extension of heat treatment time,the microwave absorption capacity of Ni@NiO composites shows a regular increase and then attenuation,and the effective absorption bandwidth of 3.6 GHz is achieved when the matching thickness is 1.3 mm.This enhanced microwave absorption performance can be attributed to the synergistic interaction of magnetic Ni core with dielectric NiO shell,as well as nanoporous structure and nanowires morphology.Thirdly,on the basis of binary precursor.Al97.8Ni2Rh0.2 precursor alloy was designed by adding a trace amount of Rh to Al-Ni eutectic alloy.The structure and composition analysis of the precursor alloy show that the two-phase microstructure formed after eutectic solidification.in which the α-Al phase was used as the template to restrict the formation and growth of Al3Ni nanowires,and the added Rh was segregated into Al3Ni nanowires to form Al3(Ni.Rh).In-situ X-ray diffraction(XRD)and ex-situ characterizations were performed to reveal the dealloying mechanism of the ternary Al-Ni-Rh precursor as well as the formation of RhNi NPNWs.In the dealloying process.the α-Al phase was completely corroded,and the selective corrosion of Al and Ni in Al3(Ni.Rh)led to the formation of nanoporous RhNi alloy nanowires.In other words.the nanowire morphology of Al3(Ni.Rh)can be retained in the dealloying products,which verifies the existence of dealloying inheritance effect.In addition,the diameter of RhNi nanowires can be regulated by changing the cooling rate during the solidification of Al-Ni-Rh eutectic.More importantly,the catalyst shows excellent hydrogen evolution reaction(HER)activity and stability comparable to commercial Pt/C in both acidic and alkaline electrolytes(acidic media:5.6 mV at a current density of 10 mA cm-2,alkaline media:41.1 mV at a current density of 10 mA cm-2).The enhanced mechanism of electrocatalytic performance was further explored by density functional theory(DFT)calculations.Finally,the strategy is extended from binary alloy to multicomponent alloy preparation.Five noble metals(Au.Pt,Pd,Rh and Ir)were introduced into Al-Ni eutectic alloys in equal proportions by eutectic reaction and microalloying,and more than thirty Al-Ni-X precursor alloys were obtained under normal solidification conditions.Structural characterization and composition analysis show that all the added noble metal elements can segregate into the second phase to form Al3(Ni,X).and the addition of alloying elements does not affect the morphology of the second phase(all grow into nanowires with large aspect ratio).Furthermore,a series of nanoporous alloy nanowires with different compositions and large aspect ratio were prepared by two-step dealloying process(the highest aspect ratio can reach 16000).Moreover,the diameter of nanowires can be adjusted by changing the cooling rate during the preparation of precursors,and the composition of the final sample can be regulated by adjusting the proportion of noble metals introduced.The electrocatalytic properties of oxygen reduction reaction(ORR)and methanol oxidation reaction(MOR)of the nanoporous alloy nanowires were investigated by electrochemical tests.The results show that compared with commercial Pt/C,PtNi and PtPdIrNi samples exhibit excellent ORR and MOR catalytic activity and stability.At the same time,the methanol oxidation process of commercial Pt/C and nanoporous Pt-based nanowires was investigated by in-situ Fourier transform infrared spectroscopy,and the catalytic mechanism and reaction path of catalysts were revealed.