Nano-silver powder for electronic inks requires regular morphology, small particle size, uniform particle size distribution, good dispersibility, and high tapped density. The physical method is simple, but it demands high equipment standards, leading to high costs. Due to the limitations of existing technology, it is challenging to produce large quantities of metal nano-powders with uniform shape and size. In chemical methods, there are many controllable factors, and by controlling reaction conditions, silver powders with different properties can be produced. This is currently the most promising method for large-scale industrial production. The main chemical methods include liquid-phase reduction, microemulsion, and precipitation reduction methods.
1. Liquid-phase reduction method
The liquid phase reduction method involves adding inorganic or organic reducing agents to an aqueous solution of silver salts, which form a uniform liquid phase. By controlling the conditions of the reduction reaction, silver ions are reduced to elemental silver. To produce ultra-fine silver powder, it is necessary to incorporate a certain amount of dispersant or protective agent into the reduction system to minimize agglomeration of silver particles. After reduction, the obtained silver powder is filtered, washed, and dried to yield nanoscale silver powder.
2. Microemulsion Method
Microemulsions are formed by two immiscible liquids and represent a thermodynamically stable, isotropic, transparent or translucent dispersion system. Microscopically, they consist of tiny droplets of one or two liquids stabilized by an interfacial membrane of surfactants. These stable droplet units form a miniature reactor with diameters ranging from tens of nanometers. When a reducer solution and silver nitrate solution are simultaneously mixed in these tiny reactors, nanosilver powder is spontaneously reduced. The droplet liquid membrane is surrounded by surfactants, preventing the aggregation of the formed nanosilver crystals. The microemulsion method is a special technique within the liquid-phase reduction methods, both occurring under pure liquid-phase systems. However, microemulsions involve two immiscible solvents, resulting in silver powder with a nanoscale size and finer, more uniform particle size.
3. Sedimentation Reversion Method
The precipitation reduction method converts silver nitrate into silver precipitate, and by adding a reducing agent in a liquid phase environment, it yields high-purity nanosilver powder. Silver powder can also be obtained directly through thermal decomposition.