| Tungsten-copper composite materials is an outstanding functional materials with high melting point, high-hardness and low thermal expansion coefficient. However, it’s still bad at homogeneity of element distribution and compactness of encapsulation. Copper-coated tungsten composite powders have large specific surface area and perfect wettability, which led to well sintering property and formability during low-temperature sintering process. As a result, tungsten-copper composite materials with uniform distribution and fully dense will be obtained. Therefore, there are important practical significance in preparation of high quality copper-coated tungsten composite powder.In this study, copper-coated tungsten composite powder were prepared by electro less copper plating on tungsten particles and detected by SEM, EDS, XRD, etc. In addition, the electroless copper plating solutions were measured by electrochemical workstation and electrochemical mechanism of copper deposition on tungsten particles were discussed.The effects of the conditions such as plating temperature, copper sulfate mass and volume of formaldehyde and lOwt.% NaOH on deposition efficiency, deposition rate and composite powder quality were studied. The results shows that deposition efficiency increase first and then slightly decrease with the increase of plating temperature, and is consistent with the change of the volume of formaldehyde and lOwt.%NaOH and complexing agent mass ratio (EDTA-2Na: TART). Deposition rate increases with the increase of plating temperature and the mass of copper sulfate and tungsten particles. Traces of 2,2’-pyridine can effectively improve the glossiness and homogeneity of copper layer coated on tungsten particles. Response surface method was used for process optimization of copper-coated tungsten composite powder by electroless plating. A mathematical model is established and the optimum conditions for the biggest deposit efficiency are obtained as follow:45.75 ℃ for temperature,82.7 ml/L for sodium hydroxide solution (10 wt.%),13.87 ml/L for formaldehyde,14.33 g/L for disodium ethylenediamine tetraacetate (EDTA-2Na) and 12.33 g/L for potassium sodium tartrate (TART). In confirmation test, the deposit efficiency and deposition rate are 89.96%and 14.83 mg/min, respectively. The thickness, true density and relative density of cpper layer are 2.07μm,6.04 g/cm3 and 0.68, respectively. The composite powders with core-shell structure have excellent properties.Through electrochemistry polarization system testing on the electroless copper plating solutions, dynamic information were obtained and the influence law of polarization affected by conditions were analyzed. The results shows that diffusion coefficient of formaldehyde and copper ion are 1.035×10-7 m2/s and 7.872×10-8 m2/s, respectively, thus, diffusion control to canodic reaction is more than that to anthode reaction and overall reaction is mainly controlled by canodic reaction. Plating temperature, concentration of formaldehyde and lOwt.%NaOH have significant effect on anthode polarization, while copper ion concentration and complexing agent mass ratio (EDTA-2Na:TART) have significant effect on cathode polarization. Via complexation competition and discharge competition among different kinds of complex compounds, EDTA-2Na and TART affect the polarization. Traces of 2,2’-bipyridine (≤15 mg/L) can reduce warburg impedance, promote electron transfer, inhibit Cu2O and ultimately improve the coating quality. Through testing the open circuit potential, the change law of open circuit potential and deposition of copper layer were revealed. During electroless copper plating, the open circuit potential goes through five times change, that is, the initial value is about-600 mV, next goes through slight increase and rapid decline processes, then stabilizes for a little bit (about-870 mV), follows a serious ramp and finally reaches-690 mV, where the open circuit potential remains stable, accordingly, the thin-film stage of copper deposition ends and the bulk stage begins. |
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