Study On The Oxidation Behavior Of Fine Copper Powder In Air And Its Surface Modification

Copper powder has excellent electric and thermal conduction and low price. Nowadays it is widely used in modern industry and defense-related technology such as conducting paint, electrostatic dissipation stuff, microelectronics material, electromagnetic interference (EMI) shielding, advanced lubricant, etc. However micron or nano size copper powder is very active and can be oxidized by air easily. After being oxidized, copper powder will lose lots of its fine properties.In this paper, the morphology and structure changes of fine copper powder during its oxidization between 150-600℃were observed. Then the processes of the oxidation of fine copper powder were proposed. Based on these models, new kinetic equations were deduced to simulate the oxidation process. The analog results agree well with the data gained from the oxidation of copper.Results show the oxidation patterns are not the same in different temperature ranges. In low temperature range (150-300℃) the oxide film is compact, the reacting resistances mainly come from the penetration of copper ion through the crystal lattice and "minipore" of oxide; the weight-gain was composed of two parts: parabolic part and logarithmic part, and the logarithmic part increases with increasing the temperature. In middle temperature range (250-400℃), there are a lot of "minipores" in the oxide, the oxidation follows the logarithmic rate law. In high temperature range (500-600℃), the huge inner stresses in the oxide will induce many cracks which are benefit for the oxidation, the oxidation follows the linear rate law in the beginning and then its speed slows down when most of the copper is oxidized.To improve copper powder's oxidation resistance, several methods were tried to coat copper powder with polyaniline, and the products made were of different morphologies. Results show that polyaniline coating is benefit to improve the oxidation resistance of copper powder.Firstly, non-catalytic coating method was tried, Ammonium persulfate (APS) was added into aniline/copper particles suspension. Results show that copper will be corroded by APS during the process. To avoid this shortage, before the polymerization, copper powder was pretreated by surfactant, oxidation, inhibitor Benzotriazole (BTA). Results show that the surfactants cann't protect copper powder thoroughly and promote the formation of coating layer, but pre-oxidation and BTA pretreatment are effective. In the pre-oxidation method, the final coating layer is compact and smooth. And BTA pretreatment method could form fiber liked polyaniline coating layer.In catalytic oxidation method, APS and H₂O₂ solution was added into aniline/copper particles suspension to produce polyaniline coating on copper powder. In the method, it isassumed that the main reaction is Cu²⁺ ionis produced by the reaction Cu+S₂O₈^2-=Cu²⁺+2SO₄^2-, and with the catalysis of Cu²⁺, aniline was oxidized by hydrogen peroxide to form the polyaniline coating layer on copper particles.The polyaniline/copper composites undergo weight loss stage and then weight-gain stage in air. The weight loss stage may correspond to the loss of bound water, oligomer and the decomposition of polyaniline. The weight increases rapidly with decomposition of polyaniline. The half oxidize polyaniline could improve the starting oxidation temperature from 200℃to 335℃, and the protection can continue to 400℃.In another experiment, copper particles were coated by ferric oxide. Result shows that the oxidation resistance can only be improved a little by this modification, and some copper was oxidized into cuprous oxideAt last, fine copper powder was treated by potassium rhodanate solution andβ-CuSCN coating layer was produced. It is found that theβ-CuSCN coating film has two layers: the outer is block coating layer, the inner is a thin three dimensional polymer layer composed of close-packedβ-CuSCN units. It is the inner layer that directly protects the copper; the outer one could repair the inner layer as it is damaged. The thermal analysis shows that theβ-CuSCN layer can apparently improve the oxidation resistance of copper powder. After the treatment the oxidation starting temperature of copper powder could be improved from 200℃to around 390℃.The coating layer forming reaction is 4Cu+O₂+4SCN^-+2H₂O=4CuSCN+4OH^-. Adding acid could remove the alkaline generated and benefit the generation of CuSCN, but if it is too much, copper will be corroded into Cu²⁺. The optimal H⁺: SCN^- ratio is 1:1.