Ind. Eng. Chem. Res. 2006, 45, 1863-1868
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Precipitation of Silver Powders in the Presence of Ethylenediamine Tetraacetic Acid Eduardo Villegas Ortega and Dimitrios Berk* Department of Chemical Engineering, McGill UniVersity, 3610 UniVersity Street, Montreal, Que´ bec, H3A-2B2 Canada
Silver powders can be produced by the reduction of a silver salt in an aqueous solution. The manipulation of the rates of the reaction allows the control of the properties such as size and size distribution of the produced particles. In the present work, we studied the addition of ethylenediamine tetraacetic acid (EDTA) as a complexation agent to moderate the rate of the reduction of silver cations with hydrazine as the reducing agent. The experiments were carried out in a batch reactor at different concentrations of the reactants. In the absence of EDTA, the reaction between hydrazine and the silver cations is extremely fast. It was found that, in the absence of seeds, the reaction starts by the precipitation of the free silver ions followed by a period where any reduced elemental silver forms sub-micrometer particles by nucleation. The addition of seeds promotes the growth of particles and suppresses nucleation. When the concentration seeds are greater than 5.4 × 10-1 mM, no nucleation is apparent. This allows the determination of the reaction rate between hydrazine and silver cations. At 20 °C, the empirical rate law for the reduction of silver was rAg ) 9.35 [Ag]1.3 [N2H4]0.7 [EDTA]-1.1 in mmol of Ag/L-min. Introduction Silver powders are widely used in several applications of the electric and electronic industries. One of the most important applications is in their role as the active part of conductive inks and pastes used in electronic components. As the electronic and microelectronic industries have grown considerably during the past decades, the use of silver powders has increased accordingly. The suitability of powders for a certain industrial application is dictated by their properties. The properties of silver powders are strongly influenced by the particle size, shape, and size distribution.1 Different methods of production will give powders having different properties. Therefore, to manufacture particles with desired size, shape, and distribution, careful selection of the production techniques is necessary. There are several methods for the production of silver powders. These can be based on chemical, physical, electrochemical, and thermal principles. One of the most popular methods is the precipitation of silver by the reduction of a silver salt in an aqueous solution. The salt most widely studied is silver nitrate, but others such as silver carbonate and silver chloride have also been used.2,3 One of the most common reducing agents is hydrazine (N2H4), which has been tested by several researchers for the reduction of silver salts.3-5 Hydrazine presents several advantages, the main one being its high reducing potential. Additionally, the reduction with hydrazine has high production yields as 1 mol of hydrazine reduces 4 mol of silver.3 As will be seen in the discussion of the results of this study, the rate of the reaction between hydrazine and silver cations is extremely fast. The concentration of silver in the aqueous reaction mixture diminishes within 30 s when the experiment is carried out in a batch reactor even at very low reactant concentrations. In addition to forming various agglomerated particles, solid silver coats available surfaces (mirror effect). Because of this, complexation agents have been a popular alternative for the control of the rate of this reaction. * To whom correspondence should be addressed. Fax: (514) 3986678. E-mail:
[email protected].
Ammonia, the thiosulfate ion, and ethylenediamine tetraacetic acid (EDTA) have been characterized as strong complexation agents for silver that may affect the silver precipitation process.5-8 In particular, ethylenediamine tetraacetic acid is a promising compound for the complexation of silver ions. EDTA exists in solution as the ethylenediamine tetraacetate (EDTA4-) anion. One mole of EDTA can form complexes with 1-4 mol of silver(I), but under an excess of EDTA the one-to-one complex (Ag EDTA3-) prevails.8 This complex has a formation constant of 1.93 × 104 M-1, and the relevant solubility products are KS0 ) [Ag+]4[Y4-] ) 9.0 × 10-18 M5 and KS1 ) [Ag+]3[AgY3-] ) 1.74 × 10-12 M.49 When exposed to an excess of silver cations, the Ag4Y species is formed as a precipitate. The production of silver particles by precipitation is a very complex process influenced by the rates of the reaction of silver ions with the reducing agent, nucleation, growth, and agglomeration. The manipulation of any of these rates will bring about a modification in the properties of the resulting silver powders. Normally in crystallization, the rates of nucleation and growth depend on the supersaturation of the solute. In the case of the precipitation of silver, the supersaturation is not defined (in fact, it is infinitely large) since metallic silver is insoluble in an aqueous medium. Any metallic silver that is produced by the reaction contributes to the nucleation or growth of the particles. Therefore, control of the precipitation process requires knowledge of the kinetics of the chemical reaction, which is the source of supersaturation. Therefore, the main objective of the work presented in this article is to study the reduction of silver with hydrazine in the presence of EDTA at different conditions of reactant, EDTA, and seed concentrations. Materials and Methods The reactants used in the silver precipitation experiments were all of reagent grade. Silver nitrate and hydrazine hydrate were provided by Aldrich Co., USA. EDTA was purchased from Fischer Ltd. of Montreal, Quebec. For the preparation of all the solutions, distilled water was used. Regarding the control
10.1021/ie050516x CCC: $33.50 © 2006 American Chemical Society Published on Web 02/11/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 6, 2006
of pH, ammonium hydroxide, nitric acid, and commercial buffers of pH 4, 6, 7, 8, and 10 were used. In addition, 1 M solution of sodium hyposulfate was used to quench the reaction. The experiments were carried out in a 1-L round-bottom flask. The temperature was measured in every experiment with a thermometer, and in some experiments, a constant-temperature bath was employed. The reactor was also equipped with a stirrer and a pH probe. Initially, the silver nitrate solution (various concentrations) was placed in the reactor. The complexation agent and buffer, when used, were added to the silver nitrate solution. At a given time, counting time equal to zero, the hydrazine solution was added to the reactor. In the case of batch operation, the addition of hydrazine took
