Silver powder made of silver particles, each to which fine silver particles adhere and process of producing the same

Abstract

This invention is a silver powder having a low-temperature sintering performance and dispersibility, which allows the powder particles to be agglomerated to a small degree and be nearly in the monodisperse state. Employed is silver powder of fine silver particles each to which fine silver particles adhere, wherein fine silver particles of nano-order particle size are adhered to the surface of each silver powder particle. The powder particles of the silver powder of fine silver particles each to which fine silver particles adhere have excellent dispersibility. In the production of the silver powder of fine silver particles each to which fine silver particles adhere, a process of including the steps of: adding a silver nitrate and a neutralizing agent into a slurry of silver powder in a dispersing medium; dissolving the mixture while stirring to allow fine silver oxide particles to be precipitated on the surface of each silver powder particle; washing the resultant silver powder; and exposing the fine silver oxide particles to UV rays to reduce the same to fine silver particles.

1 . Silver powder made of silver particles, each whose center part is regarded as a core material characterized in that: 
 silver particles each being finer than the silver particle of said center part adheres to said core material.    
     
     
         2 . The silver powder made of silver particles each to which fine silver particles adhere according to  claim 1 , wherein the silver powder is substantially spherical.  
     
     
         3 . The silver powder made of silver particles each to which fine silver particles adhere according to  claim 2 , wherein the silver powder has the following powder characteristics a to c: 
 a. an average particle size of primary particles D IA  obtained by image analysis of a scanning electron microscope is 0.6 μm or less;    b. a degree of agglomeration represented by D 50 /D IA , where D IA  is said average particle size of primary particles and D 50  is the average particle size obtained by laser diffraction scattering particle size distribution measurement method, is 1.5 or less; and    c. a crystallite size is 10 nm or less.    
     
     
         4 . The silver powder made of silver particles each to which fine silver particles adhere according to  claim 2 , characterized in that the silver powder has the following powder characteristics a to d: 
 a. an average particle size of primary particles D IA  obtained by image analysis of scanning electron microscope is 0.6 μm or less;    b. a degree of agglomeration represented by D 50 /D IA , where D IA  is said average particle size of primary particles and D 50  is the average particle size obtained by laser diffraction scattering particle size distribution measurement method, is 1.5 or less;    c. a crystallite size is 10 nm or less; and    d. a content of organic impurities is 0.25% by weight or less in terms of amount of carbon.    
     
     
         5 . The silver powder made of silver particles each to which fine silver particles adhere according to  claim 1 , wherein each of particles of the silver powder is substantially flat.  
     
     
         6 . The silver powder made of silver particles each to which fine silver particles adhere according to  claim 1 , wherein sinterable temperature is 170° C. or less.  
     
     
         7 . A process for producing the silver powder made of silver particles each to which fine silver particles adhere according to  claim 1 , comprising the steps of: 
 bringing a silver powder into contact with a solution containing a silver complex, which is obtained by mixing silver nitrate and a complexing agent and dissolving the mixture while stirring; and    adding a reducing agent into the solution to allow fine silver particles to be precipitated on the surface of each silver powder particle.    
     
     
         8 . The process for producing the silver powder made of silver particles each to which fine silver particles adhere according to  claim 7 , wherein the complexing agent is a sulfite salt or an ammonium salt.  
     
     
         9 . A process for producing the silver powder made of silver particles each to which fine silver particles adhere according to  claim 1 , comprising the steps of: 
 adding a silver nitrate and a neutralizing agent into a slurry of silver powder in a dispersing medium;    dissolving the slurry mixture while stirring to allow fine silver oxide particles to be precipitated on the surface of each silver powder particle;    washing the resultant silver powder; and    irradiating with UV rays to reduce the fine silver oxide particles to fine silver particles.    
     
     
         10 . The process for producing the silver powder made of silver particles each to which fine silver particles adhere according to  claim 9 , wherein the neutralizing agent is any one or two or more selected from the group consisting of sodium hydroxide, potassium hydroxide and aqueous ammonia.  
     
     
         11 . A process for producing the silver powder made of silver particles each to which fine silver particles adhere according to  claim 1 , comprising the step of: 
 using a silver powder of nearly spherical powder particles which is obtained in the steps of: preparing an aqueous solution of a silver complex by mixing and reacting an aqueous solution of silver nitrate and a complexing agent;    contact-mixing an organic reducing agent with said aqueous solution of a silver complex; allowing silver particles to be precipitated by reduction in the solution after the mixing, while keeping the silver concentration at 1 g/l to 6 g/l and the organic reducing agent concentration at 1 g/l to 3 g/l; separating the precipitated silver particles through a filter; and    washing the silver particles in water and then in an alcoholic solution.    
     
     
         12 . A process for producing the silver powder made of silver particles each to which fine silver particles adhere according to  claim 1 , comprising the steps of: 
 using silver powder of nearly spherical powder particles which is obtained in the steps of: preparing an aqueous solution of a silver complex by mixing and reacting an aqueous solution of silver nitrate and a complexing agent;    contact-mixing an organic reducing agent with said aqueous solution of a silver complex; allowing silver particles to be precipitated by reduction in the solution after the mixing, while keeping the silver concentration at 1 g/l to 6 g/l and the organic reducing agent concentration at 1 g/l to 3 g/l; separating the precipitated silver particles through a filter; and    washing the silver particles in water and then in an excess amount of alcoholic solution.