Spherical ultrafine copper powder is an important material for the manufacture of multilayer ceramic capacitors (MLCC). This paper aims to explore and study new methods for the preparation of ultrafine copper powder for MLCC.
By investigating the literature on the method of controlling the copper powder for synthesizing MLCC electrodes, a new process for the preparation of copper powder by pre-reduction of glucose-hydrogen reduction was carried out. A new two-step reduction process for the preparation of ultrafine copper powder by liquid phase reduction was carried out. In order to improve the oxidation resistance of copper powder, the surface modification of copper powder SiO_2-B_2O_3 composite film was studied.
The specific contents are summarized as follows: Generally, the preparation of Cu_2O is controlled by additives (gelatin, PVP, PEG 400, TritonX-100, etc.) to control the particle size and morphology. Many preparation methods use high-energy ray irradiation, seeding or hydrothermal method. . In this study, spherical and square Cu_2O particles with uniform particle size and good dispersibility were prepared by using no additives and only by the feeding method and temperature control during the precipitation process, which is more economical and simpler than additive control. The particle size of Cu_2O is between 0.5 and 2 μm. The content and particle size of impurity elements in Cu_2O particles change regularly with the change of reaction temperature and reactant concentration. After the Cu 2 O powder was reduced at 200 to 270 ° C in a hydrogen stream, most of the glucose contained in the Cu 2 O was removed. The obtained copper powder particles maintain the original morphology and particle size distribution characteristics of the Cu 2 O particles, the particle size is slightly reduced, the tap density is about 2 g/cm 3 , the dispersibility is good, and the impurity content is less than 1%. In the liquid phase two-step reduction method, Cu(II) is first reduced to Cu_2O, and then Cu_2O is reduced to metal copper powder by adding hydrazine hydrate in two steps. The hydrazine hydrate is added in two parts at a lower temperature (nucleation stage) and a higher temperature (growth stage).
The experimental results show that the two-step reduction, the addition of hydrazine hydrate in two steps and the control of the addition rate of hydrazine hydrate are beneficial to the uniform growth of copper particles; Cu_2O is not substantially reduced by hydrazine hydrate when mixed with hydrazine hydrate at low temperature, and can be reduced during the heating phase. It is reduced and forms a copper crystal nucleus; the suitable temperature of the copper crystal nucleus during the growth phase is 84 °C. The different addition amounts of the two-step hydrazine hydrate result in different particle size distributions of the finally obtained ultrafine copper powder; the essence of the two-step addition of hydrazine hydrate is to separate the nucleation and growth process of the copper particles. The addition of proper amount of PVP is beneficial to the smooth surface of the copper powder and effectively prevent the agglomeration of the ultrafine copper powder particles; the addition of other additives can significantly change the morphology of the ultrafine copper powder particles. The copper powder prepared under the optimized conditions has a particle size of 1.0-2.5 μm and a spherical shape, and has the characteristics of high tap density, high purity and good dispersibility. Suitable for preparing MLCC electrodes. Copper powder with larger particle size was prepared by seed crystal growth method. The particle size of copper powder was determined by the amount of copper seed crystal and Cu_2O. The reaction temperature has a significant effect on the dispersibility of the copper powder, and the suitable reaction temperature is about 60 ° C; the dispersant PVP also has a certain influence on the dispersibility of the copper powder. In the surface modification of copper powder, the surface of the copper powder is coated with a layer of SiO 2 -B 2 O 3 film by hydrolysis of the silicate silicate in the boric acid solution to improve the oxidation resistance of the copper powder.
The experiment shows that the higher the amount of ethyl silicate, the higher the temperature and the longer the reaction time, the better the high temperature oxidation resistance of the modified copper powder; the trace amount of H_3BO_3 can significantly improve the high temperature oxidation resistance of copper powder; water and ammonia The effect of the amount on the high-temperature oxidation resistance of copper powder is not obvious; after the wet copper powder is washed, the anti-oxidation effect is worse; the dissolution of boric acid with isopropanol is better than the dissolution of boric acid with ethanol, and the anti-oxidation effect of the modified copper powder is better. Under the optimized process conditions, the modified copper powder has an oxidation initiation temperature of 110 ° C higher than that of the copper powder, and the sintering temperature is also greatly reduced, and the modified copper powder still has good conductivity, indicating that the surface coating is modified. Copper powder can be used to prepare MLCC electrodes.