Cadmium in Alloys
Although cadmium has only limited use as a pure metal it forms many binary and more complex alloys which have useful properties for many commercial applications.
Most commercial alloys containing cadmium fall into two major groups, where
(a) the presence of cadmium improves some feature of the alloy. Small amounts of cadmium can improve the hardness and wear resistance, mechanical strength, fatigue strength, castability and electrochemical properties of a number of alloys. Cadmium is added principally to alloys based on copper, tin, lead and zinc although several others benefit from its presence.
(b) lower melting points are obtained. These alloys range from the low melting point eutectic ('fusible') alloys to high melting point non-eutectic alloys used in metal joining.
A eutectic alloy melts to become a free running liquid at a single temperature which is lower than the melting point of any of its component elements. The lowest melting point eutectic cadmium alloy fuses at 46.81' C and also contains bismuth, lead, indium and tin. The rapid fusing characteristics of these and the near-eutectic alloys are useful in many temperature sensitive applications. The non-eutectic alloys melt over a range of temperatures and have a 'pasty' range in which they can be easily worked and shaped as solders, fillers and brazes.
These two alloy groups will be considered in more detail:
(a) Alloys with special features
(i) Copper-cadmium alloys which have almost double the mechanical strength and wear resistance of pure copper, yet still retain 90 per cent of its conductivity, contain between 0.8 to 1.2 per cent cadmium. In addition, copper-cadmium alloys can be severely cold rolled without making them susceptible to softening during soldering. Major uses for such alloys include telephone drop wires, contact wire and catenary strand for railway overhead electrification, tinsel conductor for flexible telephone cords, special cables for military and aerospace uses and electrical components such as contact strips. Copper-cadmium alloys can withstand service temperatures as high as 150'C; as a result they are used in both domestic and automotive radiators and fittings.
(ii) A zinc alloy containing 0.1 per cent cadmium improves the mechanical properties of rolled, drawn or extruded zinc. Zinc alloys containing cadmium in the range 0.025 to 0. 1 5 per cent are used as sacrificial anodes in the corrosion protection of structural steelwork immersed in seawater.
(iii) Lead alloys with up to 0.075 per cent cadmium are sometimes used as sheaths for cables subject to cyclic stress.
(iv) Tin-based white metal bearing alloys with up to 1 per cent cadmium have improved tensile and fatigue strength for use in marine engines and gearboxes.
(v) Precious metal alloys for jewellery incorporate cadmium for improved hardness and strength. Levels of up to 5 per cent cadmium in gold-silver-copper alloys make Greek gold, a greenish-tinged gold.
(vi) Silver electric contacts incorporating 10 to 15 per cent cadmium or cadmium oxide are useful in many heavy duty electrical applications such as relays, switches and thermostats. The presence of cadmium gives some degree of arc-quenching and improves resistance to material transfer and electric erosion.
(b) Joining and fusible alloys
The tin-lead-bismuth-cadmium alloy which melts at 7OO C is more commonly known as Woods Metal and is used in the bonding of metallized ceramic and glass components to metal frames and chassis where higher soldering temperatures are not possible. Other cadmium containing solders are useful in light electrical and electronic assemblies and when it is necessary to make a lower temperature second joint adjacent to a conventional tin-lead solder joint.
(i) Intermediate temperature soldering alloys Cadmium alloyed with silver, zinc and/or tin makes excellent solders with a tensile strength two to three times greater than most common solders in this temperature range. They are also often used in applications where temperature sensitivity prohibits the use of silver solders with higher tensile strength. Zinc-cadmium alloys are useful for soldering aluminium whilst cadmium-zinc-tin alloys are used for soldering magnesium.
(ii) Silver-brazing alloys Cadmium is an important component in quaternary alloys with silver, copper and zinc in the lower temperature range of brazing alloys.
They are distinguished by their ability to produce high strength joints at working temperatures more than 1 000 C below the melting points of the ternary silver-copper-zinc brazing alloys.
(iii) Fusible alloys Low temperature fusible alloys containing cadmium employ their low melting points and rapid fusing or solidifying characteristics in a variety of uses. Heat sensitive fusible links in fire safety devices or kilns and ovens can activate control mechanisms when they melt at specific temperatures. Woods metal in water sprinkler valves automatically activates the water supply as it melts when the local temperature exceeds 700 C.
Proof castings in these alloys can be made from wood, plastic or plaster foundry patterns for short run dies for sheet metal and thermoplastics.
The alloys are used to firmly mount glass lenses during grinding operations. Metal fabrication operations on complex or delicate assemblies are also easy when they are encased in a supporting jacket of metal. This technique also produces wrinkle-free bends in pipes and tubes.
Similarly, jet engine turbine blades and vanes are machined whilst held by such low melting point alloys (shown in Fig. 1).
In addition to the major uses of cadmium there are a wide variety of uses which consume only small amounts of cadmium. These minor uses, however, often have considerable technological and social importance.
Photovoltaic cells, based on thin-films of either cadmium telluride (CdTe) or cadmium sulphide (CdS), generate an electric current when light fails upon them. These devices are used in solar cells and have an important role in the development of cost-effective solar power installations.
Cadmium sulphide photoconductive cells are used in photographic exposure meters for cameras where the CdS cell acts as a light-sensitive variable resistor. The CdS cell is also used as a highly sensitive photo-receptor in electrophotographic systems in photocopiers. Other cadmium compounds are used to improve the stability of developed photographic images and so help preserve them during long-term storage.
Cadmium compounds, such as cadmium sulphide, cadmium tungstate, cadmium borate and cadmium silicate, are essential in the preparation of light-emitting phosphors that are activated by electron beams. These phosphors have long lifetimes and high reliability and are useful in applications such as colour displays, x-ray instruments, luminescent dials and fluorescent lamps. Phosphors based upon cadmium/zinc sulphide are used in cathode ray tubes.
Cadmium mercury telluride is an important semiconductor for infra-red imaging systems in defence, space and search and rescue applications.
Cadmium salts of organic acids are used as catalysts in the production of a wide variety of organic materials.
Thin-film transistors based on cadmium selenide have been developed for switching applications in matrix and alphanumeric displays.
Silver-indium-cadmium alloys are used in control rods for some pressurised water reactors in nuclear power generation. These rods absorb free neutrons and so control the process. In other nuclear engineering applications, cadmium metal sheet is used for shielding by similar neutron absorption.
Fig.1 Atypical work holding application of a low melting point alloy - an encapsulated turbine blade.