Fabrication And Property Research Of Au-Cu Alloy And Nanoporous Gold

Au-Cu alloys are widely used in electrical contact materials,jewelry materials,brazing materials,shape memory materials,and dental materials due to their excellent electrical conductivity,high corrosion resistance and oxidation resistance.Nanoporous gold prepared by dealloying using Au-Cu alloy has many special properties of nanomaterials,such as the quantum size effect,the surface effect,the small size effect and the macroscopic quantum tunneling effect.Moreover,the unique porous structure also has low density,high surface area and good permeability,so that nanoporous gold has a very high application value in electrocatalysis,biosensors and supercapacitors.In general,microstructure has a crucial influence on the properties of Au-Cu alloys and nanoporous gold.The Au-Cu alloy prepared by the traditional process has many casting defects and coarse dendrites,which is not conducive to subsequent processing.During the dealloying,nanoporous gold is prone to generate cracks,which impedes its practical use.To further improve the performance of Au-Cu alloys and nanoporous gold,in this work,Au-Cu alloys were fabricated by microalloying and permanent magnetic stirring.The SEM,EPMA,XRD,microhardness and potentiodynamic polarization curves were used to investigate the effects of different indium content and rotation speeds on the morohology,phase and properties of Au-Cu alloys.Moreover,a variety of AU25Cu75 precursor alloys were prepared by different methods,the effects of precursor microstructure on the cracks and pore size of porous gold were studied and the mechanism was also discussed.The major conclusions are presented as follows:1.Adding In elements with mass fractions of 1%,3%and 5%to Au-Cu alloys,respectively.Au-Cu alloys without and with indium mass fraction of 1%are single Au-Cu-based solid solution with face-centered cubic structure.The alloys with indium mass fraction of 3%and 5%consist of a precipitated phase and a maj or Au-Cu-based solid solution.With the increase of indium content,the dendrite structure of Au-Cu alloy is significantly refined,especially after the precipitation of the In-rich phase,the secondary dendrite arm spacing is reduced to 18.32 μm.2.By adding indium mass fraction up to 1%,the corrosion current density strikingly decrease to 2.30×10-8 A/cm2,which reveals indium addition can improve the corrosion resistance of Au-Cu alloys.However,as the indium mass fraction exceeds 3%,the precipitation of In-rich phase increases the micro-galvanic effect,thereby increasing the corrosion current density of the alloy to 9.16×10-8 A/cm2.The hardness values of Au-Cu alloys with different indium content exhibit better Hall-Petch relationship with the secondary dendrite arm spacing,HV=18.9+392.5λ2-0.5,R=0.99122.The Au-Cu alloy with indium mass fraction of 5%has the highest hardness value,which is 111 HV.3.The effects of different permanent magnet rotational speeds on the microstructure and properties of Au-Cu alloys were investigated.Compared to the Au-Cu alloy without permanent magnetic stirring,the secondary dendrite arm spacing is decreased by 32%at 300 r/min.As the rotation speed increases to 900 r/min,the magnetic induction intensity of the heart rapidly decreases to 52 Gs,and the secondary dendrite arm spacing increases to 25.1μm.4.The rotating permanent magnetic field can significantly improve the corrosion resistance of Au-Cu alloy,especially the alloy with 300 r/min,the corrosion current density decreases approximately three orders of magnitude.Moreover,with the application of the rotating permanent magnetic field,the dendrite segregation of Au element can be slightly improved,but the segregation of Cu is aggravated and the solute segregation coefficient SR(Cu)increases to 12.35.Using a microhardness tester,Au-Cu alloys with different rotation speeds were measured.The hardness of alloy obtained under 300 r/min is 137 HV,which is higher than other samples.5.Nanoporous golds were prepared by dealloying using Au25Cu75 alloy as a precursor.As-cast precursor prepared by the traditional method still has more terraces after long-time chemical etching,which seriously affects the performance of nanoporous gold.However,after dealloying,the precursor fabricated by permanent magnetic stirring forms a uniform ligament/pore structure.In addition,permanent magnetic stirring decreases the corrosion current of alloy to 9.96×10-7 A/cm2 in nitric acid solution,mitigates the internal stress and avoids the formation of microscopic cracks.Nanoindentation analysis shows that the mechanical properties of nanoporous gold have obvious ligament size effect.As the corrosion time increases,the ligament size of alloy increases form 17 nm to 28 nm,and the hardness decreases form 433 MPa to 65 MPa.6.The effects of Au-Cu precursor alloys obtained under different oxygen contents on the morphology and properties of porous gold were investigated.When annealing at an oxygen volume fraction of 2×10-6,a loose oxide layer is formed on the surface of Au-Cu alloy,of which main component is Cu2O.After dealloying,an etch pit nanoporous gold can be obtained,and the pit size is similar to the surface oxide grain size.However,as the oxygen volume fraction increases to 1×10-5 the surface oxide is a mixture of CuO and Cu2O,and a unique 3D porous gold can be formed after dealloying.7.Precursors were annealed at 500℃,600℃,700℃ and 800 ℃,respectively.As the heat treatment temperature elevates,the grain size of the surface oxide significantly increases,thereby enlarging the pore size of porous gold.The residual copper mole fraction of porous gold at 500℃ is 55%.As the annealing temperature increases to 800℃,the residual Cu mole fraction rapidly decreases to 33%.The cyclic voltammogram was used to study the electrocatalytic properties of 3D porous gold on H2O2.At-0.5 V,porous gold obtained under 500℃ has the largest reduction current,which is-1.231 mA/cm2.