4Tungsten, the 74th element of the periodic table, is named after the Swedish word for heavy stone. The chemical symbol of Tungsten (W) stands for its German name wolfram, though. The origin of this name goes back to medieval German smelters who first found tungsten-containing tin ores show much lower yield (History of Tungsten 2011).
Historically, the pure tungsten metal was first produced in 1781-82 by two Spanish metallurgical chemists, de D´Elhuyar brothers, and then published in 1783. The main tungsten ore (scheelite) is named after Carl Wilhelm Scheele for his contribution in the discovery of tungsten. He actually first published (in 1781) the results of his experiments on tungsten ores which led to the formation of an unknown acid; the main contribution of the de D´Elhuyar brothers was to reduce this acid under an oxidant atmosphere made by charcoal (History of Tungsten 2011).
Back to the 20th century science, tungsten can be described with its electronic structure known to be 4f145d46s2 giving this element the possibility to form various compounds with oxidation number of +4, e.g. WC (tungsten carbide) or +6, e.g. WO3 (tungsten oxide) and H2WO4 (tungstic acid). Possessing the highest melting point (3422 ± 15 °C) together with the lowest thermal expansion coefficient (4.32 to 4.68×10-6 K-1 at 25 °C) among metallic materials, and also a very low vapor pressure (8.15×10-8 at 2000 °C), tungsten is on the top list candidates of the engineering materials for high temperature applications (Tungsten Properties 2011).
Tungsten and its products are a collection of different superior physical and mechanical properties; one or more of the following strategies are determining in any material selection procedures:
- Thermal fatigue caused by non-uniform thermal expansion is considered to be one of the main failure mechanisms for multi-components systems; possessing very low thermal expansion coefficient, tungsten offers an excellent dimensional stability.
- Components intended to be used in high temperature applications are always susceptible to material loss due to the exponentially increased evaporation rate; due very low vapor pressure, tungsten is a promising candidate material for many high temperature applications.
- Together with the low vapor pressure, possessing uniquely low electrical resistivity (5.28 μΩ.Cm at 25 °ÂÂÂÂÂC) makes tungsten an excellent current carrying material at elevated temperatures (Tungsten Properties 2011).
- Connected to the intrinsic physical properties of tungsten, and principally related to the high electrical conductivity of tungsten, tungsten offers excellent heat conductivity (175 W.m-1K-1 at 25 °C). Owing such high conductivity together with its dimensional stability (due to low thermal expansion), tungsten is a promising material for heat sinking in integrated systems (Tungsten Properties 2011).
- Together with the superior mechanical properties, very high density is required for candidate materials for use in applications expecting to transfer the maximal momentum (or dissipate kinetic energy), such as anti-tank missiles. Tungsten possesses the highest density (19.25 gr.cm-3 at 25 °C) among the engineering materials. (Iridium and Osmium are known to have even higher densities – about 22.50 gr.cm-3 at 25 °C – but are not as commonly used as tungsten due to their high price.) Basically, tungsten is the key element for the so-called heavy alloys (Tungsten Properties 2011) (Ho 2007).
Generally speaking, in any material selection strategy in which tungsten (but mostly its compounds or alloys) is selected, a combination of one or more physical properties of tungsten together with its superior high temperature mechanical properties pulled this element to the top of the list. Due to its relatively high price, tungsten is not the best choice for low temperature applications. Taking into account that tungsten compounds can offer a broad range of mechanical properties, they will be discussed separately for particular applications.
Besides all conventional applications of tungsten (especially its compounds), new emerging high-tech applications mainly based on the intrinsic physical properties of tungsten, and in particular its electronic structure, have been developed strongly; in this context, critical catalysts for several important processes in refineries and full-cells can especially be highlighted. These applications will be discussed in details in the next chapters.
Further information about the study will follow shortly.