Silver intermetallic phases

FIG. 24-7. Phase diagram for the binary system silver-strontium, showing the formation of four intermetallic compounds. 

Both silver and gold form ideal solid solutions with palladium. However, stoichiometric compositions with unique properties, such as in Hunter s preferred membrane composition of PdsAg, [31], might suggest the possibility of intermetallic compounds or ordered structures differing from that of the ideal solutions [35]. Palladium and copper also form ideal solid solutions, but in this system phase diagrams clearly show additional phases with crystal structures differing from the parent fee phase of the solid solutions. 

As shown in Fig. 9, the metal A is first plated or coated (by any process) on a substrate. Then the metal B is plated/coated on top of film A. Next a heat treatment is applied. If the melting temperature of metal A would be lower than that of metal B, the temperature should be slightly over the melting point of metal A. Then some of the metal A would melt, and the atoms of metal B would diffuse into the metal A at a very high speed, since one of the phases (phase of metal A) is a melted form. To enhance the antibacterial effect for tin plating used in food industries, tin was chosen as metal A. And as metal B, silver and copper were chosen independently. As a result, the process was used to produce tin-silver or tin-copper intermetallic compounds. The evaluation tests for the antibacterial effect (ISO 22196) confirmed that tin film did not show any antibacterial effect. On the other hand, the intermetallic compounds for the alloy films clearly showed antibacterial effects. This suggests that silver or copper could dissolve into the environment at a low concentration level, and that the silver or copper ions could make the antibacterial effect appear on materials surfaces. Fortunately, these well-known antibacterial metals could show antibacterial effects at very low levels [7]. 

Removal of silver using zinc must follow copper removal and softening . Copper forms intermetallic compounds with zinc, as does arsenic and tellurium. The presence of antimony and tin also affects the performance of the desilverising operation, in particular separation of the silver-zinc alloy phase. 

As shown in the zinc-silver phase diagram in Figure 12.9, there are five identified solid phases. The extremes a and r are solids solutions and there are three intermetallic compounds p, y and e. In practice the addition of molten zinc to lead bullion followed by cooling will result in the formation of these intermetaUics as solid precipitates, which are mainly of the e form but also contain r] towards the... 

Immersion silver dissolves into the tin phase of molten solder. The silver does not melt, but rather forms a solid solution. The dissolution rate of silver is in the range of 0.5 to 1.5 jum per sec. at standard soldering temperatures. Once the silver is dissolved, the underlying copper forms intermetallics with tin as discussed previously. Flux has little effect on silver, but can help clean trace contaminates and reduce surface tension. 

The term alloy can also apply to certain heterogeneous mixtures, such as the common two-phase solid mixture of lead and tin known as solder, or to intermetallic compounds, such as the silver-tin compound Ag3Sn that, in the past, was mixed with mercury in dmtal amalgam. 

---