Silver Powder: Categories, Uses and Preparation Methods


Categories and uses of silver powder

Silver powder with many varieties can be classified based on the physical and chemical properties as well as market needs. And different silver powder of each category is applied to different fields, which aim to maximize its unique advantages and reduce costs. Main physical and chemical parameters to measure silver powder are: average particle size, particle size distribution, surface area, shape, apparent density, tap density, grain size, heat shrinkage property, purity, and impurities distribution and species, etc.

Categories based on physical property:

a. In terms of particle size of silver powder, maximum particle size is smaller than 100nm (0.1μm) known as nano-meter silver metal powder; average particle size is of 0.1μm~10μm known as micro silver powder; average particle size is greater than 10μm known as coarse silver powder. And particle size above is laser particle size. Nano-meter silver powder has been primarily used as an antibacterial and ultra-low temperature thermal conductivity materials in thick-film conductor paste; micro silver powder is mainly used in thick-film conductor paste and powder metallurgy; coarse silver powder can only be used as base materials for silver alloys.

b. Silver powder can be classified as weak flocculent aggregated silver powder, strong flocculent aggregated silver powder, highly dispersed silver powder and monodisperse silver powder according to the degree of dispersion. There is a strong link or welding generating between particles of strong flocculent aggregated silver powder in manufacturing process (chemical and thermodynamic reasons), without dispersing into small agglomerates and a few single particles by external force. Such silver powder cannot be used in powder metallurgy and common thick-film conductor paste because it causes poor fineness and uneven system of paste.

Currently, weak flocculent aggregated silver powder is mainly used in thick-film conductor paste and powder metallurgy. Such silver can be dispersed into small agglomerates and a few single particles through external force (rolling and mixing), so thick-film paste generally uses it as dispersants. Highly dispersed silver powder, in low degree of flocculent aggregation and most existing in the form of single particle, is an ideal conductive filler of thick-film conductor paste, which can ensure fineness, definition and low shrinkage rate in the sintered or drying process as well as form uniform dense layers. Monodisperse silver powder exists in the form of single particles with no aggregation, but this is an ideal case which is difficult to achieve. In general, electron micrograph of powder just represents a partial field of view, not the entire powder.

c. Silver powder can be classified as microcrystalline silver powder, polished silver powder, silver flake, spherical silver powder according to shapes. Polished silver powder and silver flake are both from chemical deposit powder through the machining, with the radius-thickness ratio to a certain extent called silver flake.

Thick-film silver conductor paste is divided into high-temperature sintering (Ag, Agpd, Agpt) with sintering peak temperature above 450°C and low-temperature sintering drying or curing (≤250°C). High-temperature sintered silver conductor paste generally forms conductive lines, electrodes or welding area in heat-resistant substrates such as ceramics, glass and so on, with liquid-phase reduction powder widely used; low-temperature drying or curing silver conductor material uses organic polymers as binder phase, including conductive paint, paste and adhesive. With contact area, silver flake and polished silver powder are commonly used in order to maximize electric conductivity of silver.

Preparation methods of silver powder

There are five preparation methods as following: a. liquid-phase reduction method; b. silver compounds thermal decomposition method (AgNO3, CH3-COOAg and Ag2CO3, etc.); c. electrolytic method; d. atomization method; e. plasma evaporation condensation method.

As silver is a precious metal and non-perishable as well as it is easy to return to the elemental state, silver powder prepared by liquid-phase reduction method is primary used in powder metallurgy and thick-film conductor paste. Silver powder with different physical and chemical properties can be prepared by selection of reduction materials, the use of surfactants as well as control of reaction process. Polished silver powder and silver flake machine micro silver powder obtained by liquid-phase reduction method generally via ball milling (stirred ball milling, roller ball milling, vibratory milling, high energy ball milling, etc.), properties of silver powder thus obtained depend on liquid-phase deposition powder, processing and the use of additives. Silver powder prepared by thermal decomposition, atomization and plasma evaporation condensation is wide in particle size distribution. Although silver powder with reasonable particle size distribution can be obtained by precision control, it is not usually used as conductive filler for thick-film conductive paste because such silver powder has too large silver crystal grain and poor sintering activity as well as its surface property is quite different from that of liquid-phase reduction method.