Composite fine particles, conductive paste, and conductive film

#This invention relates to composite fine particles comprising a nickel base core and an oxide coating, an electrically conductive paste using the particles, and an electrically conductive film obtained therefrom.

#In the current trend toward the size reduction of portable equipment and digital electric appliances, it is under study to reduce the size and increase the capacitance of multilayer ceramic capacitors. For the size reduction and capacitance increase of multilayer ceramic capacitors, it is most effective to reduce the thickness of dielectric layers and internal electrode layers in the multilayer structure. To reduce the thickness of internal electrode layers, the conductive paste from which they are made must contain finer metal particles. Among the currently available internal electrode layers, the thinnest layers have a thickness of the order of 1 micron and are made of a conductive paste containing metal particles of a single metal element such as nickel, silver or palladium with a mean particle size of about 1 micron. There is a good possibility of reducing the thickness of internal electrode layers to about 0.3 micron. To form such thin layers, presumably the metal particles in the conductive paste must have a mean particle size of less than about 0.2 micron.

#However, mainly three problems arise in reducing the size of metal particles. A first problem is that finer metal particles suffer greater thermal shrinkage. During the firing of multilayer ceramic capacitors, cracks and other defects occur due to the differential thermal shrinkage between dielectric layers and internal electrode layers. A second problem arises from the greater surface area of finer metal particles, which permits metal oxidation during binder burnout from ceramic material. A third problem is due to the ferromagnetism of nickel. As nickel particles are finely divided, they become single domain particles having strong coercivity. Due to magnetic interaction, such particles tend to agglomerate together. A number of solutions have been proposed to overcome these three problems.

#Examples of the invention are given below by way of illustration and not by way of limitation.

#Nickel sulfate hexahydrate, 431 g, was weighed and dissolved in pure water to form 1.7 liters of a nickel sulfate solution. With stirring at room temperature, 50 g of trisodium phosphate dodecahydrate was added to 0.8 liter of a 48 wt % aqueous solution of sodium hydroxide. Thereafter, the nickel sulfate solution was slowly added thereto over about one hour. The temperature was raised to 95° C., 1 liter of a 60 wt % aqueous hydrazine solution was added thereto over about 30 seconds. After reaction for 20 minutes, the temperature was lowered below 40° C. and 11 g of nickel sulfate hexahydrate was added. The excess of alkali was neutralized with hydrochloric acid to pH 9. The particles thus obtained were filtered and washed with a solution of stearic acid in ethanol for coating the particles on surfaces thereof with stearic acid. The particles were heat treated in an inert atmosphere (Ar) at 600° C. for 4 hours.

#There were obtained composite particles whose composition was analyzed by ICP. The results are shown in Table 1. The particle size distribution was determined by the Fisher Subsieve Sizer method. The size of crystallites was determined by taking an X-ray diffraction diagram and calculating according to Scherrer method. The outer appearance of particles was observed by photomicrography using SEM. The volume resistivity was determined by packing a cell of predetermined volume with particles and measuring the resistance of the cell by a four-terminal method. The results are shown in Table 2.