Process for making silver powder particles with very small size crystallites

Abstract

The process for making silver powder particles with very 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; 
 (ii) nitric acid; 
 (iii) maleic acid; and 
 (iv) deionized water, wherein the pH of said acidic aqueous reducing and particle modifier solution is adjusted by the addition of a base to between 2.5 and 6; 
   (c) maintaining said acidic aqueous silver salt solution and said acidic reducing and surface morphology 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;   
       one or both of said acidic aqueous silver salt solution and said acidic aqueous reducing and particle modifier solution further comprising gum arabic and said silver powder particles comprising crystallites of size less than or equal to 32 nm as determined by X-ray diffraction and the Scherrer formula; and said silver powder particles having a d 50  in the range of 0.5 to 3.5 μm. 
     
     
         2 . The silver powder 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 powder particles of  claim 1 , wherein in said process said base used to adjust the pH of said acidic aqueous reducing and particle modifier solution is NaOH. 
     
     
         4 . The silver powder particles of  claim 1 , wherein in said process said water soluble silver salt is silver nitrate and said reducing agent is ascorbic acid. 
     
     
         5 . The silver powder particles of  claim 1 , wherein in said process said pH of said acidic aqueous reducing and particle modifier solution is adjusted to between 3 and 5. 
     
     
         6 . The silver powder particles of  claim 1 , wherein in said process said acidic aqueous reducing and particle modifier solution further comprising a metal colloid selected from the group consisting of gold colloid and silver colloid. 
     
     
         7 . The silver powder particles of  claim 1 , wherein in said process said temperature in step (c) is in the range of 10° C. to 35° C. and the temperature in step (e) is in the range of 65° C. to 75° C. 
     
     
         8 . A silver thick film paste comprising 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; 
 (ii) nitric acid; 
 (iii) maleic acid; and 
 (iv) deionized water, wherein the pH of said acidic aqueous reducing and particle modifier solution is adjusted by the addition of a base to between 2.5 and 6; 
   (c) maintaining said acidic aqueous silver salt solution and said acidic reducing and surface morphology 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;   
       one or both of said acidic aqueous silver salt solution and said acidic aqueous reducing and particle modifier solution further comprising gum arabic and said silver powder particles comprising crystallites of size less than or equal to 32 nm as determined by X-ray diffraction and the Scherrer formula; and said silver powder particles having a d 50  in the range of 0.5 to 3.5 μm. 
     
     
         9 . The silver thick film paste of  claim 8 , 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.   
     
     
         10 . The silver thick film paste of  claim 8 , wherein in said process said base used to adjust the pH of said acidic aqueous reducing and particle modifier solution is NaOH. 
     
     
         11 . The silver thick film paste of  claim 8 , wherein in said process said water soluble silver salt is silver nitrate and said reducing agent is ascorbic acid. 
     
     
         12 . The silver thick film paste of  claim 8 , wherein in said process said pH of said acidic aqueous reducing and particle modifier solution is adjusted to between 3 and 5. 
     
     
         13 . The silver thick film paste of  claim 8 , wherein in said process said acidic aqueous reducing and particle modifier solution further comprising a metal colloid selected from the group consisting of gold colloid and silver colloid. 
     
     
         14 . The silver thick film paste of  claim 8 , wherein in said process said temperature in step (c) is in the range of 10° C. to 35° C. and the temperature in step (e) is in the range of 65° C. to 75° C. 
     
     
         15 . A semiconductor device comprising an electrode formed by firing a silver thick film paste containing silver particles, said 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; 
 (ii) nitric acid; 
 (iii) maleic acid; and 
 (iv) deionized water, wherein the pH of said acidic aqueous reducing and particle modifier solution is adjusted by the addition of a base to between 2.5 and 6; 
   (c) maintaining said acidic aqueous silver salt solution and said acidic reducing and surface morphology 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;   
       one or both of said acidic aqueous silver salt solution and said acidic aqueous reducing and particle modifier solution further comprising gum arabic and said silver powder particles comprising crystallites of size less than or equal to 32 nm as determined by X-ray diffraction and the Scherrer formula; and said silver powder particles having a d 50  in the range of 0.5 to 3.5 μm. 
     
     
         16 . The semiconductor device of  claim 15 , 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.   
     
     
         17 . The semiconductor device of  claim 15 , wherein in said process said base used to adjust the pH of said acidic aqueous reducing and particle modifier solution is NaOH. 
     
     
         18 . The semiconductor device of  claim 15 , wherein in said process said water soluble silver salt is silver nitrate and said reducing agent is ascorbic acid. 
     
     
         19 . The semiconductor device of  claim 15 , wherein in said process said pH of said acidic aqueous reducing and particle modifier solution is adjusted to between 3 and 5. 
     
     
         20 . The semiconductor device of  claim 15 , wherein in said process said acidic aqueous reducing and particle modifier solution further comprising a metal colloid selected from the group consisting of gold colloid and silver colloid. 
     
     
         21 . The semiconductor device of  claim 15 , wherein in said process said temperature in step (c) is in the range of 10° C. to 35° C. and the temperature in step (e) is in the range of 65° C. to 75° C.