Research Efficiency Of Nickel Coated Graphite Powders By Electrodeposition

It is powder plating that is a special kind of composite plating. Using compositeplating，some metal ions or alloy ions in the solution will be electroplated directly onthe cathode materials. The metal coatings play the role of decoration、anti corrosion、wear and other usage. On the basis of composite plating, powder plating has higherrequirements and more complex process. It refers to plating these metal ions onpowders which are suspended in the solution. These powders mostly are micron sized.The characteristics of powder plating determine the challenges of this method. Theyare mainly manifested as following aspects: cathodic efficiency is not high; hydrogenevolution on the cathode is too severe. Powders are too light and water insoluble,which are easily suspended in the bath surface not contacting the cathode. Nickelcoated graphite composite powders refer to plating nickel on micron sized graphiteusing Watts nickel plating solution. The powders have an extremely wide range ofapplications: silastic、NiMH batteries、electromagnetic wave materials and so on.Compared to general metals, the deposition potential of nickel is more negative.Therefore, it is easily plating nickel on the cathode, not on the graphite powders, nomatter how much polarization on the cathode. So we must find a suitable cathodicmaterial. Because powder plating method belongs to a kind of high–speedelectrodepositing method, a large cathodic current density is required. So hydrogenevolution is intense and anodic passivation will limit the export of cathodic currentdensity during the electrodepositing process, which is a large loss in the currentefficiency. How to achieve minimum hydrogen current and decrease plating nickelcurrent on the cathode, but to improve the current efficiency is a challenge. Based onthis, electrolyte compositions, surfactants, the anode process and the cathode processwere studied in detail in this paper. Composite powders have been carried on thecharacterization of morphology and composition analysis by means of SEM and EDX.The valuable results are as follows:（1） Parallel experiments and comparative experiments were given respectively, inpreparation of nickel coated graphite powders process. The optimal pH and the mostsuitable surfactant were obtained. Experimental results showed that, the optimum pHof the nickel coated graphite powder process was near2.0. At this time, graphite canhave the strongest adsorption of cationic. Graphite is positively charged which isconducive to move to the cathode. Anionic surfactant and non ionic surfactant wereselected. The optimal dose of sodium dodecyl sulfate was0.075g dm^-3，The optimal dose of alkyl phenyl ethers（OP10） was also0.075g dm^-3. The appropriate amount ofmixed surfactants helped to disperse the graphite powders, reduce metal deposited onthe cathode, and accelerate hydrogen bubbles escape.（2） Electrochemical polarization curves were measured with different cathodicmaterials （stainless steel and niobium） and different main salt concentrations. Theappropriate concentrations of main salt were obtained with different cathodicmaterials, respectively, so that the amount of nickel deposited on the cathode was atleast after the electroplating process, and at this time the current efficiency was themaximum. The concentration of the main salt should be chosen100g dm^-3whenstainless steel as the cathode. While, the concentration of the main salt should bechosen200g dm^-3when nibium as the cathode.（3） The role of sodium hypophosphite and its mechanism were studied inelectroplating process. It was found that cathodic polarization became large with theincreased content of sodium hypophosphite. This facilitated the deposition of nickelon the graphite powders. And with the temperature increased, the oxidation behaviorof hypophosphite became faster. The best content of sodium hypophosphite was35g dm^-3which can derive from the experiments.（4） Using cathodic current efficiency showed the current efficiency of nickelcoated graphite powders, the suitable cathodic current density and the temperature ofthe plating solution were achieved. The suitable cathodic current density was40A dm2, and the suitable temperature was5060℃.（5） The optimal conditions are as follows: NiSO46H2O200g dm^-3, sodiumsulfate40g dm^-3, Nickel chloride40g dm^-3, Boric acid30g dm^-3, sodiumhypophosphite35g dm^-3, sodium dodecyl sulfate0.075g dm^-3, OP10emulsifier0.075g dm^-3, pH2.0,5060℃,0.4A cm2. Intermittent stirring electroplating wasemployed. Cycle operation mode: electroplating （3min）—stirring （2min,800r min1）—settlement （2min）. When the effective plating time was30min, the coatingobtained was thick （1.52m）, the coating ratio was high （>70%）, the nickel contentwas also high （>55wt.%） and phosphorus content in the coating was low （<1%）.This paper first studied factors affecting the current efficiency of the nickelcoated graphite powders. The plating process of nickel coated graphite powders hasbeen developed. High quality of products have been obtained and manufacturingtechnique was friendly to environment which will lay a solid theoretical basis toproduce cheap and good nickel coated graphite powders.