Electroless Plating Preparation Of Molybdenum - Copper Composites

In the investigation, the method of copper coating on the surface of the molybdenum powders with the electroless plating and technique of Mo/Cu composite powders sintering were studied with the aid of X-ray diffraction (XRD), scanning electron microcopy (SEM), transmission electron microcopy (TEM), auger electron spectrometer (AES) and X-ray photoelectron spectrometer (XPS). The influence of solution composition and plating conditions on electroless copper plating was studied. The reaction mechanism of electroless copper coating of molybdenum powders and the growth mechanism of copper growth on the surface of the molybdenum powders were suggested. Besides, the electroless deposition of copper on the surface of molybdenum powders by using silver as catalyst was investigated and the possible mechanism of electroless copper coating of molybdenum by using silver as catalyst was proposed. Finally, the technique of Mo/Cu composite powders sintering was researched primarily. The results are as follows:(1) Through pretreatment before electroless copper plating, the Molybdenum powders had better dispersity and catalytic activity. Through rational solution composition and plating conditions, Mo/Cu composite powders with a range content of copper from 15～85wt.% can be successfully obtained. The coating rate was fast and the composite powders with continuous lay of copper but rough surface were dispersive. The plating technique was that TEA and EDTA as complex agents, the 2, 2'-bipyridyl and PEG as double stabilizers, pH value 12～13, temperature 50～55℃, formaldehyde 24～28ml/L, metal salt 15g/L, whose mole ratio to the complex agent content was 1:5, ultrasonic and magntic stiring.(2) The reaction mechanism of electroless copper coating was mainly studied with the help of XPS, which shows the reduction PdCl₂ (activator) catalyst on the surface of molybdenum powders by SnCl₂ (sensitizer) to produce pure Pd⁰ clusters, which subsequently act as nucleation sites for copper depositon. The concept of XPS core-level binding energy(BE) shift due to small cluster size was utilized to predict the size of pure Pd⁰ clusters deposited on the surface of molybdenum powders, which were about 3～4 nm. Combination the reaction mechanism of electroless copper coating and the morphologies of the copper coated powders at the different stages in this experiment, the diffusion-shrinkage autocatalytic model was supposed for the growth mechanism over the surface.(3) The metallic Ag⁰ clusters acted as nucleation sites for the copper deposition and the copper coated molybdenum composite powders can be acquired successfully. With the analysis of XPS, the reaction mechanism of electroless copper coating of molybdenum powders as Ag catalysis was suggested. After the sensitization step, the reducing agent(H₂PO₂^-) in the bath immediately reduced the [Ag(NH₃)₂]⁺ ions adsorbed on the molybdenum to the metallic Ag⁰ clusters which subsequently act as nucleation sites for copper depositon.(4) The Mo/Cu composite powders were reducing treated for 60min at 350℃, which can improve the final densification of the compact. The Mo/Cu composite powders had good formability. The green compact of the relative density can reached up to 87%, however it needed higher forming pressure.(5) The preparation of Mo/Cu composite materials with liquid sintering and solid sintering methods was discussed, respectively. The liquid sintering method was not ideal way to fabricate the Mo/Cu composite materials, because after liquid sintering the phenomena of copper exudation and collapse were observed. After Mo/Cu green compacts were sintered to composite materials at temperature that was lower than the melting point of copper. The conductivity, heat conductivity, heat expansion coefficient and rigid were detected. The results showed that properties composites materials were excellent due to the special structure of Mo/Cu composite powders.