A Study Of Preparation Process Of Nickel Coated Graphite Powders By Electrodeposition And Nucleation Mechanism

Nickel coated graphite(Ni/graphite) powders as a type of composite materials possess dual properties of nickel and graphite, such as excellent magnetic conductivity, wear resistance, self-lubrication and durability under high temperature. Nickel coated graphite powders can be used as fillings in conductive silicon rubber, microwave absorption materials and electrical shielding materials. Traditional electroless deposition possesses the disadvantages of poor chemical stability of plating solution, complicated pretreatment pro cesses, high cost of production and heavy environmental pollution. Here, a method of electrodeposition was used to prepare Ni/graphite powders with compact and uniform Ni coating which has a certain thickness to avoid these disadvantages.Natural flake graphite with micron size was used as raw material, nickel was deposited on the surface of graphite powders by intermittent electrodeposition. The pretreatment process was optimized with electroless deposition carried out before electroplating to improve the effect of electrodeposition. The parameters of electroplating process were studied to choose appropriate conditions. The nucleation mechanisms of nickel electrodeposition under high current density w ere also investigated. The surface morphology and coating thickness of the composite powders were analyzed by scanning electron microscope and the surface elements were detected by energy dispersed X-ray spectrometer. The results are as follows:(1) In order to compare the activation effects of different methods, SEM was used to observe the surface morphology of graphite powders which were activated by different treatment methods, electroless plated and then electroplated. The reason why electroless deposition influenced electroplating effect was researched through the sedimentation experiment of graphite.(2) The optimal process conditions of electroplating Ni/graphite powders such as the current density, temperature, loading capacity, duty cycle, plating time were determined by electroplating efficiency, nickel content and surface morphology of the composite plating powders. The entire electroplating procedure included several cycle processes with precipitation, electroplating and stirring. The effective plating time was 30 min to fabricate Ni/graphite powders with high coverage rate and uniform coating. Improving the concentration of Ni Cl2 in the solution and area ratio anode vs. cathode were effective measures to prevent the anode passivation.(3) The open circuit potential and cathodic polarization curves were measured to study the influence of graphite powders on the electroplating bath during initial electroplating process. The potentiostatic transient step experiments were used to analyze the nucleation mechanism of nickel on graphite at large current density.(4) The appropriate amount of organic additives can effectively increase the cathodic overpotential and electrochemical reaction impedance, the probability of nickel electrical crystallization would increase and the nuclei size of Ni would decrease. As a result, the brightness of the Ni coating improved.