| Silver is the best metal for thermal conductivity and conductivity,and also has good ductility.It is mostly used for the preparation of electronic devices with high electrical conductivity requirements.Silver is a precious metal with a high degree of light reflectance and is often used for making decorations and collections.Graphite silver composites have good electrical conductivity and self-lubricating properties and are often used as wear-resistant electrical contact materials.Silver is one of the first metals to achieve industrial electroplating applications.The cyanide silver plating process has been used for nearly 200 years,and about 90%of the silver plating lines still use toxic cyanide systems.Therefore,the development of a green and non-cyanide silver plating process is of great significance.In this paper,through a large number of experiments,a new composite-free coordination system with 5,5-dimethylhydantoin as the main complexing agent was successfully developed.The composition,process parameters and process performance characteristics of the bath were determined.The microstructure,crystal structure and elemental composition of the coatings were characterized by SEM,XRD and XPS.The mechanism of silver electrodeposition at the surface of the glassy carbon electrode was studied by using the method of cyclic voltammetry and chronoamperometry.The effects of temperature and additives on the mechanism of electrocrystallization were investigated.A graphite silver composite coating was prepared by electrodeposition and the composite coating was characterized.The main findings of this paper are as follows:(1)The new non-cyanide silver plating with complex coordination system Through a large number of experiments to determine the new process of the bath composition and process parameters:silver nitrate 30 g/L,the main complexing agent 5,5-dimethylhydantoin(DMH)80 g/L,auxiliary coordination agent nicotinic acid(NA)40 g/L,conductive salt potassium carbonate 30 g/L,additive X-Ag(containing surfactant SDS and brightener PEI-1500)5 to 10 ml/L,adjusted to pH = 10.5 by using potassium hydroxide solution or dilute nitric acid solution;in the process parameters of 45?55 0C,0.2?1.7 A/dm2,magnetic stirring conditions,you can get smooth,bright,uniform silver plating.The non-cyanide silver plating bath has good stability,a wide current density range,easy adjustment of the plating solution,high current efficiency,bright plating,and good adhesion to the copper substrate.The electrolyte has a current efficiency of about 99%,a dispersion capacity of 56.8%,and a deep plating capability of up to 100%.(2)Silver coating properties and characterization The silver coating has a micro-hardness of 83 HV and a resistivity of 1.83??·cm,which is higher than that of pure silver(1.65??·cm).Scanning electron microscopy(SEM)observations showed that the coating crystals were fine and uniform;X-ray diffraction(XRD)results showed that the silver coating was a face-centered cubic crystal structure with preferred orientation of the(220)crystal plane;X-ray photoelectron spectroscopy(XPS)shows that the surface of the silver coating has S,O,C,Ag and other elements.After the coating is Ar+-etched 100 nm,it is an Ag element and a very small amount of C element,and the silver is all 0-valence.(3)The mechanism of silver electrodeposition on GCE in this complex coordination system The results show that,the electrodeposition of silver is a diffusion controlled irreversible electrode process according to three-dimensional instantaneous nucleation mechanism.When the step potential shifted from-750 mV to-825 mV,the peak deposition current Im was increased and the induced nucleation time tm shorten,the kinetic parameters obtained by calculating show that the diffusion coefficient(D)was basically constant,ranged(7.31 ±0.34)×10-5 cm2·s-1 and the active nucleation sites density(N0)increased from 3.26x105 cm-2 to 10.2×105 cm-2.The morphology for the initial deposition of Ag verified its three-dimensional instantaneous nucleation mechanism.Temperature and additives do not affect the electrocrystallization mechanism of silver.(4)Preparation of Graphite Silver Composites with electroplating Electrodeposited Ag-5%C composite coatings were successfully deposited.In the bright silver plating electrolyte,the use of sodium methyl cellulose(CMC)as a dispersant for graphite was found.When the CMC concentration was 100 mg/L and the graphite concentration was 8?10 g/L,Ag-5%C composites was obtained;graphite particles are embedded in the silver coating.The Ag-5%C composite coating presents a silver-gray macroscopic appearance.The microhardness was 109.6 HV and the resistivity was 2.92 ??·cm.Although it was higher than the silver coating,it was lower than the Ag-5%C composite prepared by the chemical coating method.The Ag-5%C composite coating has better wear resistance than the pure silver coating. |
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