Fabrication Of Porous Metal Copper By Dealloying And Study Of Porous Structure Via Synchrotron Radiation

The pore structure of porous metal makes it show very unique performance,such as low relative density,large specific surface area,high specific strength and strong permeability,which has been widely concerned by people.In particular,the porous metal prepared by the dealloying method has three-dimensional bi-continuous network structure of ligament and pore,which makes it an ideal candidate material in the field of battery electrode,catalyst,fuel cell,mechanical actuator and other engineering applications.However,the nano porous metal prepared by dealloying method has poor plasticity and low strength,and most of them are thin strips and flakes with very thickness.Therefore,how to use the dealloying method to prepare the porous metal bulk materials with relatively uniform pore structure,high porosity,high strength and plasticity has become the hotspot and difficulty of research and application at present.The study on the formation and evolution mechanism of porous structure during dealloying process has great significance to the precise control of porous structure.In this paper,micron-sized porous Cu was prepared by dealloying of immiscible Cu-Fe precursor alloy.The influence of composition and morphology of Cu-Fe precursor alloy and dealloying process on the microstructure of micron-sized porous Cu was studied,and the relationship between porous structure and mechanical properties was explored.The results show that the best dealloying technice of Cu-Fe alloy is:5 wt%,H₂SO₄ solution at 90 ^oC.With the increase of sintering temperature（600 ^oC-900 ^oC）of Cu-Fe precursor alloy,the pore size formed after dealloying increases gradually（1-4μm）;the parting limit of Cu-Fe alloy is 40 at%of Fe,and the porosity range of porous Cu is 39.54%-79.56%with the change of precursor alloy composition（Fe content:40-80 at%）.Moreover,the micron-sized porous Cu has excellent plasticity and strength.The compressive strength and tensile yield strength at room temperature increase with decreasing porosity and pore size.The compressive yield strength at room temperature increase from 3.91MPa to 58.62MPa,and tensile yield strength is 2.89-71.85MPa.In the process of compression deformation,the low angle grain boundary increases,the average grain size decreases and the number of twins increases,which leads to the plastic deformation behavior of porous Cu.The high-temperature compressive strength of micron-sized porous Cu does not decrease with increasing test temperature due to the softening mechanism,which is because of the formation of Cu O and Cu₂O during the high-temperature compression process.Porous metals with bimodal porosity from nanometer to micrometer sizes would lead to enhanced physical and chemical properties—the micron-sized pores can increase the effective diffusivity of ion transport within the porous media,and the nano-sized pores provide high specific surface area,enabling functionalities that are unique to nanoporous metals.A new Cu-Fe-Al ternary precursor alloy was selected according to the mixing enthalpy principle（whenΔH<0,the compatibility between elements is good,and whenΔH>0 is negative,the elements are immiscible）for the design of bimodal porous copper by a simple,one-step dealloying method.The effect of phase composition and initial microstructure of precursor alloys on the three dimensional（3D）morphology of bimodal porous metals were systematically studied.The characteristic parameters of the bimodal porous structure were quantified,and the mechanical properties were explored.The results show that the size of nanoporous structure depends on the proportion of noble and active components in the precursor alloy,while the size of microporous structure depends on the corresponding phase size of precursor alloy.The phase compostion of precursor alloys were determined by synchrotron X-ray diffraction refinement.The 3D morphology of four representative precursor alloys(Al_xFe_75-xCu₂₅,x=10,30,50 and 60)and corresponding bimodal porous Cu were directly visualized and quantified via advanced synchrotron X-ray nano-tomography.The relationship between the phases in the precursor alloy and the corresponding porous morphology is established:the micron-sized pores in bimodal porous Cu are formed by dissolving of Cu Fe Al phase with Body-center Cubic（BCC）and monoclinic crystal structure,and the nano-sized pores are formed by dealloying of Cu Fe Al phase with Face-center Cubic（FCC）and Cs Cl type（B2）crystal structure.This provides guidance for the precise control of the porous structure during dealloying of Cu-Fe-Al alloys,so that the porous structure can be designed according to the specific application to obtain the best performance.The moderate amount of nanoporous structure in bimodal porous Cu can improve the mechanical property.When the Al content is 10,20 and 30 at%in precursor alloys,the strength of bimodal porous Cu is 12.18MPa,8.64MPa and 6.62MPa respectively,which are higher than that of micron-sized porous Cu prepared by Cu-Fe alloy with the similar porosity.In this paper,in order to understand the dynamic mechanism of different competition and interdependence during the formation process of different porous structures,the 3D dynamic evolution of porous structures with time of Cu-Fe alloy and Cu-Fe-Al alloy during the dealloying process has been directly visualized and quantified via non-destructivei nterrupted in-situ advanced synchrotron X-ray nano-tomography.The formation mechanism of bimodal porous Cu was revealed.The results show that the dealloying mechanism of Cu-Fe alloy is a simple direct dissolution process of pure Fe phase in the acid corrosion environment,and the micron-sized porous Cu is formed after the Fe phase dissolved completely.In contrast,the dealloying mechanism of Cu-Fe-Al alloy is rather complicated,and it is not the same as that of nano porous metal during the formation process with uniform dealloying front（interface between corroded area and uncorroded area）.The Fe-rich phase and Cu-rich phase in the precurcor alloy will conduct sequential dealloying.The Fe-rich phase will take the priority of dealloying,and firstly forms the nanoporous structure,then the nanoporous structure dissolves gradually,finally forms micron-sized pores.After the Fe-rich phase fully dealloyed,the Cu-rich phase will start the dealloying process,forming nano-sized pores.In addition,interrupted in-situ X-ray near edge absorption spectroscopy（XANES）and oxidation valence distribution diagram show that part of Cu will be oxidized during dealloying process,and the oxidation products are mainly Cu²⁺.The 3D morphology of bimodal porous metal is precise controled,which lays a technical foundation for customizing the morphology of porous metal according to specific performance and application.