Fine particulate silver powder and production method thereof

Abstract

An object of the present invention is to provide a silver powder which is an unprecedentedly fine particulate silver powder, has a dispersibility more approximate to monodispersibility resulting less aggregation, and has a low impurity content. To achieve the above object, a fine particulate silver powder having unprecedented powder properties in which: a. the average particle diameter D IA of the primary particles obtained by image analysis of a scanning electron micrograph is 0.6 μm or less; b. the crystallite diameter is 10 nm or less; c. the sintering starting temperature is 240° C. or less; and d. the carbon content is 0.25 wt % or less is obtained by allowing a silver ammine complex aqueous solution S 1 to flow in a certain flow path (hereinbelow referred to as “first flow path”) providing a second flow path b which joins the first flow path a on its way and allowing an organic reducing agent and additives S 2 , if required, to flow into the first flow path a through the second first flow path b, and contacting and mixing them at the joining point of the first flow path a and the second flow path b to allow reduction and deposition of particles, and washing the particles with an excess amount of an alcohol.

1 . A fine particulate silver powder having low aggregation properties, characterized in that the silver powder has the following powder properties in which: 
 a. the average particle diameter D IA  of the primary particles obtained by image analysis of a scanning electron micrograph is 0.6 μm or less;    b. the aggregation degree represented by D 50 /D IA  using the average particle diameter D IA  of the primary particles and the average particle diameter D 50  by laser diffraction scattering particle size distribution measurement is 1.5 or less;    c. the crystallite diameter is 10 nm or less; and    d. the content of organic impurities is 0.25 wt % or less on the basis of the amount of carbon.    
   
   
       2 . The fine particulate silver powder according to  claim 1 , wherein the sintering starting temperature is 240° C. or less.  
   
   
       3 . A production method of a fine particulate silver powder comprising obtaining an aqueous solution of silver ammine complex by mixing and reacting an aqueous silver nitrate solution with ammonia water, reducing and depositing silver particles by adding a reducing agent thereto and performing filtering, washing and drying the particles, characterized in that the silver particles are reduced and deposited by contacting and mixing aqueous solution of silver ammine complex with an organic reducing agent, and maintaining the silver concentration to 1 g/l to 6 g/l, and the concentration of the organic reducing agent to 1 g/l to 3 g/l in the mixed solution, and the silver particles are then filtered, washed with water, and washed with an excess amount of a methanol solution.  
   
   
       4 . The production method of a fine particulate silver powder according to  claim 3 , characterized in that when the aqueous solution of silver ammine complex is contacted and mixed with the organic reducing agent, the silver ammine complex aqueous solution flows in a certain flow path (hereinbelow referred to as “first flow path”), a second flow path which joins the first flow path on its way is provided, and the organic reducing agent is allowed to flow into the first flow path through the second first flow path and is contacted and mixed with the aqueous solution of silver ammine complex at the joining point of the first flow path and the second flow path.  
   
   
       5 . The production method of a fine particulate silver powder according to  claim 3 , characterized in that the method employs an aqueous solution of silver ammine complex having a silver concentration of 2 to 12 g/l obtained by mixing and reacting a silver nitrate aqueous solution having a silver nitrate concentration of 2.6 to 4.8 g/l with ammonia water.  
   
   
       6 . The production method of a fine particulate silver powder according to  claim 3 , wherein a dispersant is contained in the organic reducing agent to be used.  
   
   
       7 . The production method of a fine particulate silver powder according to  claim 3 , wherein hydroquinone is used as the organic reducing agent.  
   
   
       8 . The production method of a fine particulate silver powder according to  claim 3 , wherein alcohol is used in an amount of 5 L or more per kg of the obtained silver particles.