Process for making silver powder particles with small size crystallites

Abstract

The process for making silver powder particles with small size crystallites uses a combination of gum arabic and maleic acid with the reduction of a silver salt with ascorbic acid, Silver thick film paste containing these silver powder particles can be used in electronic applications to form electrodes for semiconductor devices and, in particular, solar cells.

What is claimed is: 
     
         1 . Silver powder particles made by a process comprising:
 (a) preparing an acidic aqueous silver salt solution comprising a water soluble silver salt dissolved in deionized water;   (b) preparing an acidic aqueous reducing and particle modifier solution comprising:
 (i) a reducing agent selected from the group consisting of ascorbic acid, ascorbates and mixtures thereof dissolved in deionized water; 
 (ii) nitric acid; 
 (iii) gum arabic; 
 (iv) maleic acid; and 
 (v) deionized water; 
   (c) maintaining said acidic aqueous silver salt solution and said acidic reducing and particle modifier solution at the same temperature, wherein said temperature is in the range of 10° C. to 65° C., while stirring each said solution during and after preparation;   (d) adding said acidic aqueous silver salt solution to said acidic aqueous reducing and particle modifier solution while stirring to make a reaction mixture and maintaining the temperature of said reaction mixture at said temperature of (c), wherein said silver powder particles precipitate and are contained within the resulting final aqueous solution; and   (e) increasing the temperature of said final aqueous solution to a temperature in the range of 65° C. to 80° C. while continuing stirring;   
       said silver powder particles comprising crystallites between 30 and 47 nm in size as determined by X-ray diffraction and the Scherrer formula; and said silver powder particles having d 50  in the range of 0.5 to 3.5 μm. 
     
     
         2 . The silver particles of  claim 1 , said process further comprising:
 (f) separating said silver powder particles from said final aqueous solution;   (g) washing said silver powder particles with deionized water; and   (h) drying said silver powder particles.   
     
     
         3 . The silver particles of  claim 1 , wherein said water soluble silver salt is silver nitrate and said reducing agent is ascorbic acid. 
     
     
         4 . The silver particles of  claim 1 , said acidic aqueous reducing and particle modifier solution further comprising a metal colloid selected from the group consisting of gold colloid and silver colloid. 
     
     
         5 . The silver particles of  claim 1 , wherein the temperature in process step (c) is in the range of 10° C. to 35° C. and the temperature in process step (e) is in the range of 65° C. to 75° C. 
     
     
         6 . A silver thick film paste comprising said silver powder particles of any of  claims 1 - 5 . 
     
     
         7 . The silver thick film paste of  claim 6 , said silver thick film paste comprising 65 to 90 wt % said silver powder particles and further comprising 0.1 to 8 wt % glass frit and 5 to 30 wt % organic medium, wherein the wt % are based on the total weight of the silver thick film paste. 
     
     
         8 . A semiconductor device comprising an electrode, said electrode, prior to firing, comprising said silver thick film paste of  claim 7 . 
     
     
         9 . The semiconductor device of  claim 8 , wherein said semiconductor device is a solar cell.