Metals for Special Uses
have been used since prehistory to obtain ever better tools. The development of the scientific and technological knowledge makes more specific the use of metals. Here, some particular uses of metals will be described. The different metals are arranged according to an economic classification which reflects mainly the use, the occurrence and the economic values of metals.
1.1 Iron (Fe)
Iron and its alloys
are by far the most important metals from the point of view of production and use, as resulted in the classification of metals in ferrous (iron and its alloys) and non-ferrous metals (all the other metals). The most important use of iron is the steel
production. According to their applications, steels are divided in three main groups. Constructional steels: these are used for the manufacture of machine parts, motor cars, building elements, sky scrapers, ship bridges, war instruments (cannons, tanks, etc). Chromium and nickel are the main alloying elements used in this category. Tool steels: they are used for the manufacture of tools (lathe knives, chisels, cutters, etc); addition of manganese makes these types of steel useful for machines such as rock crushers and power shovels, which must withstand extremely hard use. Special-quality steels include corrosion
resistant and acid resistant steels. Stainless steel, which contains chromium and sometimes nickel and manganese, is a hard strong alloy that resists to heat and corrosion. Stainless steel is used for such things as jet engines, knives, forks, spoon and kitchen equipments.
1.2 Precious metals
Silver (Ag), Iridium (Ir), Gold (Au), Osmium (Os), Palladium (Pd), Platinum (Pt), Rhodium( Rh), Ruthenium (Ru).
Owing to its chemical and physical properties, gold is employed as an alloy
with other metals, not only in jewellery. From a technological point of view, gold exhibits two usefull properties: good corrosion
resistance and good electrical conductivity
. These properties allow use of gold in the electronic industry to produce electrical contacts, spring contacts, bonding wires, solder alloys, bonding wires, bumping wires, electroplating, and sputtering targets. Gold is also a useful brazing material. Its use for dental prostheses is dropping down and more than 50% of gold production is employed in jewellery.
Pd Rh Ru Ir:
In addition to the typical jewellery uses (born in the last years) these metals are employed as catalysts
. Rhodium is an alloying agent used to harden platinum and palladium. Such alloys are used for furnace windings, thermocouples
(link to document 1_2_thermocouples), elements, bushings for glass fibre production, and laboratory crucibles. Plated rhodium produced by electroplating or evaporation is exceptionally hard and is used for optical instruments.
most of the annual production is consumed by two dominant categories: catalytic converters and fine jewellery. Together, these two applications consume more than 70% of the world's supply of platinum. Currently, platinum is also used in electronic devices like i-Pods and computer hard-discs. Asian manufacturers are using the metal to create flat-panel glass in computer and television screens.
Because of the strong demand for platinum, investors and consumers have driven up the price to almost double that those of gold.
1.3 Refractory metals
Hafnium (Hf), Chromium (Cr), Niobium (Nb), Molybdenum (Mo), Rhenium (Re),Tantalum (Ta), Titanium (Ti), Tungsten (W), Vanadium (V), Zirconium (Zr).
it claims very high melting point
and electrical conductivity
, thus is used for high temperature applications. For instance, the filaments in light bulbs are made of tungsten. Tungsten can react with carbon to make a very strong and very resistant material, called tungsten carbide; the latter is used to produce cutting tools and wear-resistant tools for metalworking, drills for oil-wells and mining. These applications account for more than 60% of the tungsten consumed in the USA each year.
the excellent resistance to chemical agents makes tantalum a very useful metal to produce laboratory equipments, also for high temperatures. Owing to tantalum high price, it is often better to cover with a tantalum layer equipments made of other metals. Tantalum has also low toxicity and high bio-inertness thus, in the past, it was used to produce prostheses; recently it has been replaced by the cheaper titanium.
hafnium is mainly used together with nickel to form superalloys
. It is used to make rods to control nuclear fission reactions. This is one of the most important uses of hafnium.
Titanium is widely used to manufacture prostheses because of its very high corrosion resistance and because it is able to form a titanium dioxide layer when suffering a mechanical stress; this layer creates a surface passivity, that is fundamental for the bio-compatibility. Moreover, titanium is X-ray-transparent, thereby a prostheses made with this metal doesn’t interfere with clinical exams. An interesting titanium property, useful in dentistry, is the total absence of metallic taste. Finally, titanium is very light: its specific weight
is very low (about a quarter than the gold one), then prostheses results light and portable. Pacemakers and defibrillators are frequently made of titanium. Recently, this metal has been used also in jewellery; titanium jewel is lightweight and feels warm to the touch; it is easily adopted by the wearer, and its sober colour is very stylish.
1.4 Gallium (Ga), Indium (In), Thallium (Tl)
it is mainly used for semiconductor
production; at this aim it is combined with phosphorous and arsenic.
low melting alloys represent the major application of indium. Indium is added to solder alloys, mostly composed of tin and lead, to increase fatigue
resistance, and to improve malleability
at low temperature. Some indium alloys (usually based on bismuth in combination with lead, cadmium, tin and indium) are used in fire control systems: restraining links that hold alarm, water valve, and door operating mechanisms are soldered with a low-melting alloy. A rise of temperature sufficient to melt the alloy results in the system coming into operation.
1.5 Nichel (Ni), Lead (Pb), Copper (Cu), Tin (Sn), Zinc (Zn)
this metal is characterized by a low melting point
and a good alloying capability, so it is used as soldering element. Tin is used to coat other metals in order to prevent corrosion or other chemical actions (tin cans are made of tin coated steel).
stainless steel production accounts for over 50% of nickel consumption; the addition of nickel makes the steel lighter and more resistant, excellent for different tools, both for domestic or industrial uses., Nickel-based superalloys
are used in aerospace industry thanks to their stress and corrosion
resistance at temperatures higher than 1000°C.
because of its excellent electrical conductivity
and thermal conductivity
about 50% of the world consumption of copper is for electrical wires or pipes for thermal exchange devices. About 30% of copper world production is used for alloying. Copper alloys are usually cold-worked. Depending on its high cost, copper is being replaced in several uses, for example copper wires and cable for telecommunications are being displaced by microwave technology and fibre optics.
zinc coating provides a very good corrosion protection for steel structures. Different methods can be used to coat with zinc a steel manufacture. One of these methods consists in placing the steel component in a molten zinc bath at the temperature of 450°C: at this temperature the diffusion process forms a layer of iron-zinc alloy on the steel surface. After this step an electrolytic galvanizing process is added: an electric current causes the deposition of a polished coating, thick about 5-25 µm. Another method is the spray galvanizing process: zinc is melted in a gas-oxygen flame and propelled by compressed air from a distance of 15 cm, obtaining a coating thick about 0.1 cm. Recently Zn-Ti-Cu alloys have been used in the manufacture of roof drainage for covering buildings: they makes surfaces protected from the rigors of nature and give them aesthetic appeal.
1.6 Boron (B), Chromium (Cr), Manganese (Mn), Silicon (Si)
chromium and manganese improve the corrosion resistance of the alloys
at which they are added. Chromium is useful both for ferrous and non ferrous alloys, while 90% of manganese is used for ferrous alloys. Chromium oxide (chromite, Cr2O3) is used as refractory
material, to produce bricks, laboratory equipments, gun components.
silicon is a semiconductor. This means that an electrical flow can be controlled by using silicon parts. Therefore, silicon is very important in electrical industry. Silicon parts are used in computers, transistors, solar cells, LCD screens and other semiconductor devices.
LG has just announced that it has created a prototype eBook reader that relies on solar cells which are just 0.7 mm thick and which weighs only 20 grams.
As regards to high pure silicon, its employ in solar cells has to be cited. A distinction can be made between solar cells for terrestrial applications and solar cells for extraterrestrial applications. In the latter, the efficiency (output per square meter) has to be major and they have to resist at hard use conditions, so they need the purest silicon. In the case of terrestrial photovoltaic cells
, the needed silicon can be less pure. At present, about only 30% of the module cost results from the silicon wafer, while the other 70% of the cost comes from manufacturing of silicon wafers and module (metals, glass, machining, etc). Because the module dimension is not reducible, the silicon cost doesn’t influence the cost of the whole structure.
a relatively large quantity of boron is used as additive in pyrotechnic mixtures. Boron filaments have been developed to increase mechanical resistance of rigid materials that have to preserve their lightness, for example in the manufacture of commercial and military aircrafts. Thin boron films are used in the field of nuclear technology, in neutron
counters, while boron powder dispersed in polyethylene
plastic is used for shielding against thermal neutrons in thermal reactors.