Preparation Of Monodispersed Copper Powder For MLCC Electrode Via Hydrogen Reduction Of Cu₂O Powder

This dissertation aims at developing a new synthesis process of copper powder with uniform partical size and good dispersibility suitable for MLCC (multilayer ceramic capacitors) electrode. A novel process, including Cu2O preparation, Al(OH)3coating, low temperature hydrogen reduction and high temperature densification, has been proposed on the basis of thorough review of large amount of references. The features of this process are as follows. Morphology and size control of copper powder are transformed into the control of Cu2O particles. The Cu2O particles of0.5μm to3.2μm in diameter are prepared by glucose reduction of Cu(Ⅱ) and their morphology and size are controllable. The sintering of copper powders at high temperature is avoided by coating Al(OH)3on the surface of Cu2O particles. The porous copper particles prepared by hydrogen reduction at low temperature are transformed into compact copper particles by densification at high temperature. The details are summarized as follows.The effects of feeding modes of CuSO4, NaOH and glucose on performance of Cu2O particles and stability of the process were investigated. It has been found that the Cu(OH)2precursor with good thermal stability and the Cu2O particles with high dispersibility, spherical morphology, uniform size and good repeatability can be prepared by so-called "fractional alkali feeding mode", in which mode Cu(OH)2precursor was first prepared by adding NaOH solution to CUSO4solution step by step, and then Cu2O particles are obtained by glucose reduction of Cu(OH)2precursor. Fractional alkali feeding mode is determined as the basic process for preparation of Cu2O particles.As for the fractional alkali feeding mode, effects of reaction temperature, glucose and NaOH concentration on the morphology and size of the Cu2O particles were investigated systematically. The results indicate that spherical Cu2O particles with good dispersibility and uniform size usually were prepared using Cu(OH)2as the precursor. However, when the glucose concentration was lower than0.50mol/1or the NaOH concentration was higher than5.00mol/1, the Cu2O particles were grown into octahedron. The cubic and octahedral Cu2O particles were single crystals which obeys diffusion-growth mechanism, and the spherical Cu2O particles were polycrystals which obeys aggregation mechanism. The size of the spherical Cu2O particles varied regularly with the variation of reaction temperature and reactant concentration:The size of the Cu2O particles decreased with the increase of the reaction temperature and glucose concentration, and decreased with the increase of the NaOH concentration. There is a liner relationship between the final particle density and the above influencing factors in the system.Al(OH)3was selected as the coating layer material in the coating process. The experimental results indicated that there were two coating patterns, nuclei coating and film coating, and the latter was evidently superior to the former. The film coating can be realized by continuous alkali dripping method. The initial pH value, Ostwald ripening pH, reaction temperature and NaOH feeding rate had great importance on the coating effect while the ripening time showed tiny effect. The optimal coating technical conditions were as follows:the initial pH value was3.5to4.0, the ripening pH value5.00to7.00, the reaction temperature60℃or80℃, and the NaOH feeding rate was below5ml/min when NaOH concentration was0.50mol/1. In this study, when the coating amount of Al(OH)3was too low, the obstructing from high-temperature sintering could not be achieved. On the contrary, when the coating amount of Al(OH)3was too high, the difficulty during acid washing was increased. Therefore, the appropriate coating amount of Al(OH)3was determined as2%～3%.In the process of hydrogen reduction of Cu2O powder, it is found that the particle size in the range of0.6μ.m to1.5μm and the coating layer had little influence, but reduction temperature had remarkable influence on the reduction rate of Cu2O particles. The appropriate reduction temperature for the Al(OH)3/Cu2O powder was175℃. The powders obtained by hydrogen reduction were porous and so their densification was necessary for the MLCC electrode paste usage. The copper powder with A1(OH)3coating obtained at different densification temperature inherited the morphology of the Cu2O precursor and had high dispersibility. With the elevated densification temperature, the particle shank, the specific area decreased, the tap density increased, the crystallinity was improved and the anti-oxidation enhanced. As a typical example, through hydrogen reducion of Cu2O particles with1.85μm in diameter at175℃, porous copper powders with1.70μm in diameter,3.52g/cm3in tap density and125℃in initial oxidation temperature in the air was obtained, further through the densification at700℃, they were transformed into compact copper powders with1.58μm in diameter,4.10g/cm3in tap density and175℃in initial oxidation temperature in the air, which was suitable for the MLCC electrode paste usage.