Abstract:
Silver powders containing particles with a different morphology are disclosed. The silver particles are spherically-shaped with an open structure surface. The majority of the spherically-shaped silver particles have one or more silver plates attached to each of the majority of the spherically-shaped silver particles. Also provided is a process for making these silver particles. The silver particles formed are particularly useful in electronic applications.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention is directed to silver particles with unique morphologies and a process for making them. These silver particles are particularly useful in electronic applications. 
       BACKGROUND OF THE INVENTION 
       [0002]    Silver powder is used in the electronics industry for the manufacture of conductor thick film pastes. The thick film pastes are screen printed onto substrates forming conductive circuit patterns. These circuits are then dried and fired to volatilize the liquid organic vehicle and sinter the silver particles. 
         [0003]    Printed circuit technology is requiring denser and more precise electronic circuits. To meet these requirements, the conductive lines have become narrower in width with smaller distances between lines. The silver powder particles necessary to form dense, closely packed, narrow lines must be as close as possible to monosized, dense packing spheres. Most existing spherical particles have smooth surfaces. The use of powders comprised of such particles results in having limited latitude when sintering. 
         [0004]    Many process currently used to manufacture metal powders can be applied to the production of silver powders. For example, thermal decomposition processes, electrochemical processes, physical processes such as atomization or milling and chemical reduction process can be used. Thermal decomposition processes tend to produce powders that are spongy, agglomerated, and very porous whereas electrochemical processes produce powders that are crystalline in shape and very large. Physical processes are generally used to make flaked materials or very large spherical particles. Chemical precipitation processes produce silver powders with a range of sizes and shapes. 
         [0005]    Silver powders used in electronic applications are generally manufactured using chemical precipitation processes. Silver powder is produced by chemical reduction in which an aqueous solution of a soluble salt of silver is reacted with an appropriate reducing agent under conditions such that silver powder can be precipitated. Inorganic reducing agents including hydrazine, sulfite salts and formate salts can produce powders which are very coarse in size, are irregularly shaped and have a large particle size distribution due to aggregation. Organic reducing agents such as alcohols, sugars or aldehydes are used with alkali hydroxides to reduce silver nitrate. The reduction reaction is very fast; hard to control and produces a powder contaminated with residual alkali ions. Although small in size (&lt;1 μm), these powders tend to have an irregular shape with a wide distribution of particle sizes that do not pack well. It is difficult to control the sintering of these types of silver powders and they do not provide adequate line resolution in thick film conductor circuits. 
         [0006]    Therefore, there is a need to produce particles with morphologies that result in the particles being more easily sintered and a process to readily make them. 
       SUMMARY OF THE INVENTION 
       [0007]    This invention provides a silver powder comprising spherically-shaped silver particles, each silver particle comprising non-spherical silver components 20-200 nm in size assembled to form an open-structure surface, wherein the d 50  particle size is from about 2.5 μm to about 6 μm. Further provided is this silver powder in which the majority of the spherically-shaped silver particles further comprise one or more silver plates with lateral dimensions of 100-2000 nm attached to each of the majority of the spherically-shaped silver particles. 
         [0008]    There is also provided a process for making the silver powder comprising spherically-shaped silver particles, the process comprising:
       (a) preparing an acidic aqueous silver salt solution comprising a water soluble silver salt dissolved in deionzed water;   (b) preparing an acidic reducing and surface morphology modifier solution comprising:
           (i) a reducing agent selected from the group consisting of an ascorbic acid, an ascorbate and mixtures thereof dissolved in deionzed water;   (ii) nitric acid;   (iii) a first surface morphology modifier selected from the group consisting of citric acid, citrate salts and mixtures thereof; and   (iv) a second surface morphology modifier selected from the group consisting of water soluble copper compounds that are sources of Cu 2+  ions;   
           (c) maintaining the acidic aqueous silver salt solution and the acidic reducing and surface morphology modifier solution at the same temperature, wherein that temperature is in the range of about 20° C. to about 65° C., while stirring each solution; and   (d) mixing the acidic aqueous silver salt solution and the acidic reducing and surface morphology modifier solution over a period of less than 10 seconds with no stirring to make a reaction mixture, maintaining the reaction mixture at the temperature of (c) and after 3 to 7 minutes stirring the reaction mixture for 2 to 5 minutes to produce the silver powder particles in a final aqueous solution.       
 
         [0017]    Also provided is the above process further comprising:
       (e) separating the silver powder particles from the final aqueous solution;   (f) washing the silver powder particles with deionized water; and   (g) drying the silver powder particles.       
 
         [0021]    The above acidic reducing and surface morphology modifier solutions can optionally contain a dispersing agent. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0022]      FIG. 1  is a scanning electron microscope image at a magnification of 10,000 of the silver powder made in Example 1 and comprising spherically-shaped silver particles, each silver particle comprising non-spherical silver components 20-200 nm in size assembled to form an open-structure surface. The majority of the spherically-shaped silver particles further comprise one or more silver plates with lateral dimensions of 100-2000 nm attached to each of the majority of the spherically-shaped silver particles. The d 50  particle size is 2.9 μm. 
           [0023]      FIG. 2  is a scanning electron microscope image at a magnification of 10,000 of the silver powder made in Comparative Experiment 1 and shows larger silver components and the absence of silver plates attached to the silver particles when the second surface morphology modifier is not used. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    This invention provides a silver powder comprising silver particles and a process for making the silver powder comprising spherically-shaped silver particles. In the powder, each silver particle is comprised of non-spherical silver components 20-200 nm in size assembled to form an open-structure surface, wherein the d 50  particle size is from about 2.5 μm to about 6 μm. The structure of these particles is clearly shown in the scanning electron microscope (SEM) image of  FIG. 1  at 10,000 magnification. The SEM of  FIG. 1  also shows that the majority of the spherically-shaped silver particles further comprise one or more silver plates with lateral dimensions of 100-2000 nm attached to each of the majority of the spherically-shaped silver particles. The particles are described herein as spherically-shaped. It can be seen from the SEM images that the particles are generally spherical in shape but are not perfect spheres. The silver components making up a particle surface are evident as is the irregular and open surface that they form. The silver plates attached to the majority of the spherically-shaped silver particles are attached to or project outward from the surfaces. 
         [0025]    The process for forming the powder of the invention is a reductive process in which silver particles with controlled structures are precipitated by adding together an acidic aqueous solution of a water soluble silver salt and an acidic aqueous reducing and surface morphology modifier solution containing a reducing agent, nitric acid and two surface morphology modifiers. 
         [0026]    The acidic aqueous silver salt solution is prepared by adding a water soluble silver salt to deionized water. Any water soluble silver salt, e.g., silver nitrate, silver phosphate, and silver sulfate, can be used. Silver nitrate is preferred. No complexing agents are used which could provide side reactions that affect the reduction and type of particles produced. Nitric acid can be added to increase the acidity. 
         [0027]    The process can be run at concentrations up to 0.8 moles of silver per liter of final aqueous solution. It is preferred to run the process at concentrations less than or equal to 0.47 moles of silver per liter of final aqueous solution. These relatively high concentrations of silver make the manufacturing process cost effective. 
         [0028]    The acidic reducing and surface morphology modifier solution is prepared by first dissolving the reducing agent in deionized water. Suitable reducing agents for the process are ascorbic acids such L-ascorbic acid and D-ascorbic acid and related ascorbates such as sodium ascorbate. 
         [0029]    Nitric acid and the surface morphology modifier are then added to the mixture. The processes are run such that the pH of the solution after the reduction is completed (final aqueous solution) is less than or equal to 6, most preferably less than 2. This pH is adjusted by adding sufficient nitric acid to the reducing and surface morphology modifier solution and, optionally, to the acidic aqueous silver solution prior to the mixture of these two solutions and the formation of the silver particles. 
         [0030]    The surface morphology modifiers serve to control the structure of the silver particles. The first surface morphology modifier is selected from the group consisting of sodium citrate, citrate salts, citric acid and mixtures thereof. Sodium citrate is preferred. The amount of the first surface modifier used ranges from 0.001 gram of first surface modifier per gram of silver to greater than 0.25 gram of first surface modifier per gram of silver. The preferred range is from about 0.02 to about 0.25 gram of first surface modifier per gram of silver. The second surface modifier is selected from the group consisting of water soluble copper compounds that are sources of Cu 2+  ions when dissolved in water. Examples of such compounds include copper (II) nitrate, copper (II) acetate and copper (II) sulfate. Copper nitrate is preferred. The amount of the second surface modifier used ranges from 0.00001 gram of second surface modifier per gram of silver to 0.0050 gram of second surface modifier per gram of silver. The preferred range is from about 0.0006 to about 0.0024 gram of second surface modifier per gram of silver. 
         [0031]    In addition, a dispersing agent selected from the group consisting of ammonium stearate, stearate salts, polyethylene glycol with molecular weight ranging from 200 to 8000, and mixtures thereof can be added to the reducing and surface morphology modifier solution. 
         [0032]    The order of preparing the acidic aqueous silver salt solution and the acidic reducing and surface morphology modifier solution is not important. The acidic aqueous silver salt solution can be prepared before, after, or contemporaneously with the acidic reducing and surface morphology modifier solution. Either solution can be added to the other to form the reaction mixture. The two solutions are mixed quickly with a minimum of agitation to avoid agglomeration of the silver particles. By mixing quickly is meant that the two solutions are mixed over a period of less than 10 seconds, preferably of less than 5 seconds. 
         [0033]    In this process the acidic aqueous silver salt solution and the acidic reducing and surface morphology modifier solution are both maintained at the same temperature, i.e., a temperature in the range of about 20° C. to about 65° C. and each solution is stirred. When the two solutions are mixed to form the reaction mixture, the reaction mixture is maintained at that same temperature. 
         [0034]    After the reaction mixture is formed there is no agitation or stirring for a period of 3 to 7 minutes after which the reaction mixture is stirred for 2 to 5 minutes. The result is a final aqueous solution containing the silver particles. It is this final aqueous solution that has a pH less than or equal to 6, most preferably less than 2. 
         [0035]    The silver particles are then separated from the final aqueous solution by filtration or other suitable liquid-solid separation operation and the solids are washed with deionized water until the conductivity of the wash water is 100 microsiemans or less. The silver particles are then dried. 
         [0036]    The silver powder of this invention can be used in thick film paste applications, including thick films for front side metallization of photovoltaic solar cells. The structures of the silver particles of this powder and their surfaces will lend them to be more readily sintered. 
       EXAMPLES 
       [0037]    The following examples and discussion are offered to further illustrate, but not limit the process of this invention. Note that particle size distribution numbers (d 10 , d 50 , d 90 ) were measured using a Microtrac® Particle Size Analyzer from Leeds and Northrup. The d 10 , d 50  and d 90  represent the 10th percentile, the median or 50th percentile and the 90th percentile of the particle size distribution, respectively, as measured by volume. That is, the d 50  (d 10 , d 90 ) is a value on the distribution such that 50% (10%, 90%) of the particles have a volume of this value or less. 
       Example 1 
       [0038]    The acidic aqueous silver salt solution was prepared by dissolving 80 g of silver nitrate in 250 g of deionized water. This solution was kept at 25° C. while continuously stirring. 
         [0039]    The acidic reducing and surface morphology modifier solution was prepared by adding and dissolving 45 g of ascorbic acid to 750 g of deionized water in a separate container from the silver nitrate solution. 
         [0040]    This solution was kept at 25° C. while continuously stirring. 20 g of nitric acid was then added to the solution followed by the addition of 10 g of sodium citrate and 0.06 g of copper nitrate (Cu(NO 3 ) 2 ). 
         [0041]    After both solutions were prepared, the acidic aqueous silver nitrate solution was added to the acidic reducing and surface morphology modifier solution without any additional agitation or stirring in less than 5 seconds to make a reaction mixture. After five minutes, the reaction mixture was stirred for 3 minutes. 
         [0042]    The reaction mixture was filtered and the silver powder collected. The silver powder was washed with deionized water until a conductivity of the wash water was less than or equal to 100 microsiemans. The silver powder was dried for 30 hours at 30° C. 
         [0043]    As shown in the scanning electron microscope image of  FIG. 1 , the silver powder was comprised of spherically-shaped silver particles, each silver particle comprising non-spherical silver components 20-200 nm in size assembled to form an open-structure surface. The majority of the spherically-shaped silver particles further comprise one or more silver plates with lateral dimensions of 100-2000 nm attached to each of the majority of the spherically-shaped silver particles. The size of the silver components and silver plates making up the surfaces of the silver particles were obtained from the scanning electron microscope images. d 10 , d 50 , and d 90  were 2.0 μm, 2.9 μm and 4.8 μm, respectively. 
       Comparative Experiment 1 
       [0044]    Example 2 was made using the process described in Example 1 except that there was no second surface morphology modifier, i.e. no copper nitrate. The scanning electron microscope image of  FIG. 2  shows the resulting silver particles comprising larger size silver components and the absence of silver plates attached to any of the silver particles when the second surface morphology modifier is not used. The d 10  d 50 , and d 90  were 2.1 μm, 3.3 μm and 5.7 μm, respectively.