Fundamental Research And Novel Preparation Technology Of Fibrous Ultra-fine Porous Copper Powder

Due to its characteristics of small particle size, high surface activity, excellent electrical and thermal conductivity, ultra-fine copper powder is widely used in powder metallurgy products, diamond tools, electrical carbon, lubricants, electromagnetic shielding coatings, conductive adhesive and other industries. In recent years, China's copper consumption increased rapidly, while the domestic production of copper powder, whether quantity or quality can not meet the market demand, especially high-performance ultra-fine copper powder used in the area of electromagnetic shielding coatings, conductive adhesive and other emerging areas. Compared with foreign countries there are still a wide gap. Therefore, the development of a high performance ultra-fine copper powder with promising industrial application has important practical significance, meanwhile provides a new approach for the reasonable use of copper resources and recovery of copper from copper-bearing scrap. This study accords to the basic ideas in modern chemistry that the material structure determines morphology and function, and claims that designing the molecular structure of copper oxalate precipitation from the aspect of the coordination chemistry and structural chemistry, and preparing specific morphology powder taking advantage of the growth habit of itself. So under the guidance of tutor, the author invented a novel preparation technology of fibrous ultra-fine porous copper powder using "ligand intervening synthesis-thermal decomposition method". This paper mainly describes the new preparation method of fibrous copper powder and the fundamental research.Firstly, according to the law of conservation of mass and law of simultaneous equilibrium, thermodynamic equilibrium mathematical models of copper ions and oxalate was established in Cu (Ⅱ)-NH3-NH4+-C2O42-Cl--H2O system, and the corresponding logarithms concentration of metal ions-pH value-ligand concentration three-dimensional diagrams, as well as the percentage content of copper species distribution are also drawn. And the effects of the total oxalate concentration, total ammonia concentration and pH value on the equilibrium system was studied, determined the best pH range for morphology control.Secondly, soluble copper salt as raw material, ammonium oxalate (or oxalic acid) as precipitating agent, the precursor powders with different phase, particle size and morphology were firstly synthesized. And their molecular structures were studied and identified by X-ray diffraction and infrared spectroscopy. The study found that the ammonia as a coordination buffering agent, regulators of the molecular structure of copper oxalate salt and solution pH value, played a crucial role in molecular structure and morphology. In addition, the impacts of other reaction conditions, such as feeding methods, feeding rate, anion type, reaction temperature, reactant concentration and aging time on the precursor powder particle size, morphology and dispersion were researched. The optimum conditions preparing rod-like precursor powder with fine particle size, crystallinity and dispersion were concluded:the copper chloride as raw materials, using reverse feeding means, controlling the feeding rate of 2.22～3.33 mL/min, reaction temperature 40～50℃, the initial reaction concentration of 0.6～0.8 mol/L, solution pH value of 8.0～8.2, aging time of 60 min.Thirdly, crystal growth habits and morphology control mechanisms of copper oxalate powders with different morphologies were studied from the perspective of the crystal structure. Copper oxalate has a two-dimensional flaky molecular structure, in which perpendicular to the direction of the molecular plane, L ligands can possibly connect with the copper atoms. These ligands can further constitute a long chain, and these long chains stack in certain rules and form one-dimensional crystal particles. Considering ammonia copper oxalate salt, the functional group NH3 will substitute L-ligand in the molecular chain, and these coordinated NH3 groups enable the growth of primitive [...(NH3) Cu-C2O4-Cu(NH3)...] in the axial direction to form one-dimensional crystals.Fourthly, the thermal decomposition mechanism of various morphological precursor prepared under different pH in nitrogen atmosphere was researched depending on the TG-DTA curves and X-ray diffraction analysis. And the precursor reaction steps were determined in the thermal decomposition process. Moreover, for the first time the thermal decomposition kinetics and reaction mechanisms of the rod-like copper powder in an atmosphere of nitrogen and carbon dioxide were studied respectively, which provided a theoretical basis for copper powder morphology change by controlling the atmosphere in the thermal decomposition process. The results show that decomposition kinetics control mechanism of copper oxalate changes with the thermal decomposition atmosphere. In the nitrogen atmosphere, decomposition kinetics of copper oxalate belongs to chemical reaction control (random nucleation and subsequent growth mechanism, n= 2/3), decomposition activation energy E is 104.55 kJ/mol (Achar differential method), and 103.61 kJ/mol (coats-redfern integral method); while in carbon dioxide atmosphere decomposition kinetics fo copper oxalate belongs to diffusion control (two-dimensional diffusion (cylindrical symmetry) control mechanism), decomposition activation energy E is 105.74 kJ/mol (Achar differential method), and 123.59 kJ/mol (coats-redfern integral method).According to the kinetics study results, we focused on the research of morphology inheritance of the rod-like precursor in thermal decomposition process under nitrogen, sealed nitrogen and carbon dioxide atmosphere. Results show that the carbon dioxide atmosphere can effectively inhibit thermal decomposition reaction rate, in which fibrous centered cubic structure ultra-fine copper powder can be prepared. On this basis, the effects of decomposition conditions on the properties of copper powder were studied systematically in the thermal decomposition process of rod-like copper oxalate, and obtained the best thermal decomposition conditions:controlling thermal decomposition temperature of 330℃～360℃, heating time of 60min, holding time of 30min, thermal decomposition atmosphere 90% CO2+10%H2, gas flow 1.2 L·min-1. Through analyzing pore structure and pore size distribution of fibrous ultra-fine copper powder prepared under the best conditions, it was shown that the pore structure of copper was mainly plate-shaped open-hole structure, with pore size of macropore, mesopore and miropore. Lastly, by studying the anti-oxidation method of fibrous ultra-fine porous copper powder, a gas adsorption and passivation method by using ordinary-purity carbon dioxide is firstly proposed to meet with the anti-oxidation requirements in storage and transport process at room temperature. In addition, using BTA coated copper powder processing method, anti-oxidation temperature of fibrous ultra-fine copper powder can be increased to 200℃, with oxidation resistance better than the outsourcing electrolytic copper powder.