| With the advantages of good sphericity, high crystallinity, strong resistance to oxidation, and small sintering shrinkage, nickel powders prepared by chemical vapor deposition (CVD) using NiCl2as raw material, are used mainly for preparing inner electrodes of base metal electrode multilayer ceramic capacitors (BME-MLCCs). However, the production of this product is still in blank in China till now.Matters adhered on the surface of powders, mainly reactant NiCl2that did not react and product HCl in the CVD process of nickle powders, must removed by washing. In the washing process, due to high activity of ultrafine nickel powders, the oxidation-hydrolysis of nickel will take place, resulting in oxygen content of nickel powders after washing on the high side. In addition, the development of BME-MLCC technology calls for sub-micron and even nanoscale powders with good monodispersion. Such as a Japanese company can produces the product which has a mean particle size100-200nm with all the particles being less than600nm.Therefore, in order to master the technology of preparing nickel powders for BME-MLCC, such difficult problems, as anti-oxidation, dispersion and precision classification of the nickel powders obtained by CVD method in aqueous medium must be solved, except for the CVD method itself.In order to solve the problems mentioned above, based on the experimental preparation of nickel powders by CVD method which uses NiCl2as raw material, the effect of surface coating on oxygen resistance and dispersibility were investigated systematically. Furthermore, their hydro-classifications were explored primarily through gravity and centrifugal sittling respectively. The main results obtained were as follows.(1) Adding benzotriazole (BTA) into the washing solution can effectively inhibit the oxidation-hydrolysis of nickel, consequently reducing oxygen content of the nickel powders after washing. FTIR analyses testified that the coordinations between the unsaturated nitrogen atoms in the triazole ring and the surface of nickel powders take place, forming a coating to protect nickel powders from oxidation. The effects of BTA concentration, pH and temperature on oxygen contents of nickel powders were investigated in the washing processes which use BTA as oxidation inhibitor. Under the conditions of BTA concentration0.5wt%,30℃and pH3.0, the oxygen contents of after washing CVD nickel powders decrease from no BTA added0.62wt%to as low as0.25wt%.(2) The dispersities of CVD nickel powders after washing were characterized by SEM, ultraviolet spectrophotometry and size analyse methods. The results showed that adding the anionic and non-ionic dispersing agent in the washing can improve the dispersion of CVD nickel powder. The optimum conditions determined were as follows: BTA concentration0.5wt%, PVP20000dosage0.01wt%, liquid/solid mass ratio30, temperature10℃, pH3. Under the condition described above, the mean size (D50) of the nickel powder decreased from uncoated2.28μm to1.03μm, demonstrating that the dispersity of the nickel prowder was significantly improved,(3) Centrifugation had a certain classification performance. Under conditions of centrifugation time5min, the speed of1000r/min, the recovery of fine particles (particle size <1μm) was12.03%. The accuracy of centrifugal classification is not enough to meet the needs of high-precision classification. Sedimentation classification in gravity field had a better performance. Under the condition of slurry density6wt%, sedimentation time5hours, the recovery of fine particles (particle size <1μm) was10.8%in water medium, and particle size of suspension was less than1μm. |
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