Preparation And Surface Modification Of Ultrafine Copper Powder For MLCC Electrode

The ultrafine copper powder is an important material in production of MLCC (multilayer ceramic capacitors). The main goal of this dissertation research was to explore and study new synthetic methods of copper powder for MLCC electrode. On the basis of thorough review of large amount of references on preparation of copper powder, two new approaches-glucose pre-reduction-H₂ reduction method and liquid phase two-step reduction method have been proposed and successfully utilized to control the size and morphology of the copper powder. Size control of copper particles by crystal seed method was also studied. In order to enhance the oxidation resistance of copper powder, the surface modification of copper powder by SiO₂-B₂O₃ composite thin film coating was studied. The details are summarized as follows.Additives, such as glutin, PVP, PEG 400, Triton X-100 were usually used to control the size and shape of Cu₂O particles. Some methods used high energy radiation, seeds or hydrothermal method. In this study, spherical and cubic Cu₂O particles with uniform size and good dispersibility were prepared by controlling the reaction temperature and the adding mode of reagents in the precipitation process, without any additives. The method is simple and effective. The average diameter of particles were 0.5～2μm. The average particles size and impurity element content of Cu₂O depended on the reaction temperature and the concentration of reagents. Most of glucose in Cu₂O was eliminated after the Cu₂O was reduced at 200～270℃. The copper particles kept the particle morphology and size distribution feature of Cu₂O, except their sizes slightly shrunk. The copper powder prepared by this way had spherical morphology, uniform size, good dispersibility and high purity (>99%). The tap density of the Cu powder is about 2 g/cm³.In the liquid phase two-step reduction method, the Cu( II) was first reduced to Cu₂O, and then was reduced to copper powder by hydrazine hydrate. Hydrazine hydrate was divided into two parts and added in at lower temperature (nucleation stage of copper crystal) and higher temperature (growth stage of copper crystal) respectively. The results showed that the two-step reduction, adding hydrazine hydrate by two stages and slow flow rate were in favor of uniform growth of copper particles. Cu₂O essentially can not be reduced by hydrazine hydrate at lower temperature, but can be reduced to form copper crystal nucleus at higher temperature. The appropriate temperature in growth stage of copper particles was 84℃. The amount of hydrazine hydrate in the two stages could control the size distribution of copper particle to a great extent. The essential of adding hydrazine hydrate in two stages was to separate the nucleation and growth stage of copper particles. Appropriate amount of PVP could effectively avoid the agglomeration phenomena of copper particles. The copper powder prepared under optimum conditions had spherical morphology, uniform size, good dispersibility and high tap density. All properties of copper powder meet the demands of MLCC electrode materials.Copper powder with larger particle size was prepared by crystal seed method. The amount of crystal seed and Cu₂O could effectively control the particle size of copper powders. The appropriate temperature was about 60℃. Dispersant PVP affected the copper particles to a certain extent.The hydrolyzation of tetraethyl orthosilicate (TEOS) in H₃BO₃ solution was adapted in the surface modification of copper powder, and copper particles was coated by a SiO₂-B₂O₃ thin film. The investigation results showed that the oxidation resistance of copper powder was improved with the increase of TEOS, temperature and reaction time. A small quantity of H₃BO₃ remarkably improved the oxidation resistance of copper powder. The amount of water and ammonia had little influence on the high temperature oxidation ratio of copper powder. Dissolution of H₃BO₃ with (CH₃)₂CHOH, the copper powder has higher oxidation resistance. Under optimum conditions, the oxidizing point of the modified copper powder increased about 110℃than the origin copper powder, and also its sinter temperature decreased remarkably. Furthermore, the modified copper powder had high electrical conductivity. The results showed that the modified copper powder could be used to prepare MLCC electrode.