Refractory metals

Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The expression is mostly used in the context of materials science, metallurgy and engineering. The definition of which elements belong to this group differs. The most common definition includes five elements: two of the fifth period (niobium and molybdenum) and three of the sixth period (tantalum, tungsten, and rhenium). They all share some properties, including a melting point above 2000 °C and high hardness at room temperature. They are chemically inert and have a relatively high density. Their high melting points make powder metallurgy the method of choice for fabricating components from these metals. Some of their applications include tools to work metals at high temperatures, wire filaments, casting molds, and chemical reaction vessels in corrosive environments. Partly due to the high melting point, refractory metals are stable against creep deformation to very high temperatures.

H   He
Li Be   B C N O F Ne
Na Mg   Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs Ba La* Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra Ac** Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og
  * Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
  ** Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
  Refractory metals
  Wider definition of refractory metals[1]

Definition

Most definitions of the term 'refractory metals' list the extraordinarily high melting point as a key requirement for inclusion. By one definition, a melting point above 4,000 °F (2,200 °C) is necessary to qualify.[2] The five elements niobium, molybdenum, tantalum, tungsten and rhenium are included in all definitions,[3] while the wider definition, including all elements with a melting point above 2,123 K (1,850 °C), includes a varying number of nine additional elements: titanium, vanadium, chromium, zirconium, hafnium, ruthenium, rhodium, osmium and iridium. The artificial elements, being radioactive, are never considered to be part of the refractory metals, although technetium has a melting point of 2430 K or 2157 °C and rutherfordium is predicted to have melting point of 2400 K or 2100 °C.[4]

Properties

Physical

Properties of the refractory metals
Name Niobium Molybdenum Tantalum Tungsten Rhenium
Melting point K 2750 2896 3290 3695 3459
Boiling point K 5017 4912 5731 5828 5869
Melting point °C 2477 2623 3017 3422 3186
Boiling point °C 4744 4639 5458 5555 5596
Density g·cm−3 8.57 10.28 16.69 19.25 21.02
Young's modulus GPa 105 329 186 411 463
Vickers hardness MPa 1320 1530 873 3430 2450

The melting point of the refractory metals are the highest for all elements except carbon, osmium and iridium. This high melting point defines most of their applications. All the metals are body-centered cubic except rhenium which is hexagonal close-packed. Most physical properties of the elements in this group vary significantly because they are members of different groups.[5][6]

Creep resistance is a key property of the refractory metals. In metals, the starting of creep correlates with the melting point of the material; the creep in aluminium alloys starts at 200 °C, while for refractory metals temperatures above 1500 °C are necessary. This resistance against deformation at high temperatures makes the refractory metals suitable against strong forces at high temperature, for example in jet engines, or tools used during forging.[7][8]

Chemical

The refractory metals show a wide variety of chemical properties because they are members of three distinct groups in the periodic table. They are easily oxidized, but this reaction is slowed down in the bulk metal by the formation of stable oxide layers on the surface. Especially the oxide of rhenium is more volatile than the metal, and therefore at high temperature the stabilization against the attack of oxygen is lost, because the oxide layer evaporates. They all are relatively stable against acids.[5]

Applications

Refractory metals are used in lighting, tools, lubricants, nuclear reaction control rods, as catalysts, and for their chemical or electrical properties. Because of their high melting point, refractory metal components are never fabricated by casting. The process of powder metallurgy is used. Powders of the pure metal are compacted, heated using electric current, and further fabricated by cold working with annealing steps. Refractory metals can be worked into wire, ingots, rebars, sheets or foil.

Molybdenum alloys

Molybdenum based alloys are widely used, because they are cheaper than superior tungsten alloys. The most widely used alloy of molybdenum is the Titanium-Zirconium-Molybdenum alloy TZM, composed of 0.5% titanium and 0.08% of zirconium (with molybdenum being the rest). The alloy exhibits a higher creep resistance and strength at high temperatures, making service temperatures of above 1060 °C possible for the material. The high resistivity of Mo-30W, an alloy of 70% molybdenum and 30% tungsten, against the attack of molten zinc makes it the ideal material for casting zinc. It is also used to construct valves for molten zinc.[9]

Molybdenum is used in mercury wetted reed relays, because molybdenum does not form amalgams and is therefore resistant to corrosion by liquid mercury.[10][11]

Molybdenum is the most commonly used of the refractory metals. Its most important use is as a strengthening alloy of steel. Structural tubing and piping often contains molybdenum, as do many stainless steels. Its strength at high temperatures, resistance to wear and low coefficient of friction are all properties which make it invaluable as an alloying compound. Its excellent anti-friction properties lead to its incorporation in greases and oils where reliability and performance are critical. Automotive constant-velocity joints use grease containing molybdenum. The compound sticks readily to metal and forms a very hard, friction resistant coating. Most of the world's molybdenum ore can be found in China, the USA, Chile and Canada.[12][13][14][15]

Tungsten and its alloys

Tungsten was discovered in 1781 by the Swedish chemist, Carl Wilhelm Scheele. Tungsten has the highest melting point of all metals, at 3,410 °C (6,170 °F).

Filament
Filament of a 200 watt incandescent lightbulb highly magnified

Up to 22% rhenium is alloyed with tungsten to improve its high temperature strength and corrosion resistance. Thorium as an alloying compound is used when electric arcs have to be established. The ignition is easier and the arc burns more stably than without the addition of thorium. For powder metallurgy applications, binders have to be used for the sintering process. For the production of the tungsten heavy alloy, binder mixtures of nickel and iron or nickel and copper are widely used. The tungsten content of the alloy is normally above 90%. The diffusion of the binder elements into the tungsten grains is low even at the sintering temperatures and therefore the interior of the grains are pure tungsten.[16]

Tungsten and its alloys are often used in applications where high temperatures are present but still a high strength is necessary and the high density is not troublesome.[17] Tungsten wire filaments provide the vast majority of household incandescent lighting, but are also common in industrial lighting as electrodes in arc lamps. Lamps get more efficient in the conversion of electric energy to light with higher temperatures and therefore a high melting point is essential for the application as filament in incandescent light.[18] Gas tungsten arc welding (GTAW, also known as tungsten inert gas (TIG) welding) equipment uses a permanent, non-melting electrode. The high melting point and the wear resistance against the electric arc makes tungsten a suitable material for the electrode.[19][20]

Tungsten's high density and strength is also a key property for its use in weapon projectiles, for example as an alternative to depleted Uranium for tank guns.[21] Its high melting point makes tungsten a good material for applications like rocket nozzles, for example in the UGM-27 Polaris.[22] Some of the applications of tungsten are not related to its refractory properties but simply to its density. For example, it is used in balance weights for planes and helicopters or for heads of golf clubs.[23][24] In this applications similar dense materials like the more expensive osmium can also be used.

The most common use for tungsten is as the compound tungsten carbide in drill bits, machining and cutting tools. The largest reserves of tungsten are in China, with deposits in Korea, Bolivia, Australia, and other countries.

It also finds itself serving as a lubricant, antioxidant, in nozzles and bushings, as a protective coating and in many other ways. Tungsten can be found in printing inks, x-ray screens, photographic chemicals, in the processing of petroleum products, and flame proofing of textiles.

Niobium alloys

Apollo CSM lunar orbit
Apollo CSM with the dark rocket nozzle made from niobium-titanium alloy

Niobium is nearly always found together with tantalum, and was named after Niobe, the daughter of the mythical Greek king Tantalus for whom tantalum was named. Niobium has many uses, some of which it shares with other refractory metals. It is unique in that it can be worked through annealing to achieve a wide range of strength and elasticity, and is the least dense of the refractory metals. It can also be found in electrolytic capacitors and in the most practical superconducting alloys. Niobium can be found in aircraft gas turbines, vacuum tubes and nuclear reactors.

An alloy used for liquid rocket thruster nozzles, such as in the main engine of the Apollo Lunar Modules, is C103, which consists of 89% niobium, 10% hafnium and 1% titanium.[25] Another niobium alloy was used for the nozzle of the Apollo Service Module. As niobium is oxidized at temperatures above 400 °C, a protective coating is necessary for these applications to prevent the alloy from becoming brittle.[25]

Tantalum and its alloys

Tantalum is one of the most corrosion resistant substances available.

Many important uses have been found for tantalum owing to this property, particularly in the medical and surgical fields, and also in harsh acidic environments. It is also used to make superior electrolytic capacitors. Tantalum films provide the second most capacitance per volume of any substance after Aerogel, and allow miniaturization of electronic components and circuitry. Many cellular phones and computers contain tantalum capacitors.

Rhenium alloys

Rhenium is the most recently discovered refractory metal. It is found in low concentrations with many other metals, in the ores of other refractory metals, platinum or copper ores. It is useful as an alloy to other refractory metals, where it adds ductility and tensile strength. Rhenium alloys are being used in electronic components, gyroscopes and nuclear reactors. Rhenium finds its most important use as a catalyst. It is used as a catalyst in reactions such as alkylation, dealkylation, hydrogenation and oxidation. However its rarity makes it the most expensive of the refractory metals.[26]

Advantages and shortfalls

Refractory metals and alloys attract the attention of investigators because of their remarkable properties and promising practical usefulness.

Physical properties of refractory metals, such as molybdenum, tantalum and tungsten, their strength, and high-temperature stability make them suitable material for hot metalworking applications and for vacuum furnace technology. Many special applications exploit these properties: for example, tungsten lamp filaments operate at temperatures up to 3073 K, and molybdenum furnace windings withstand to 2273 K.

However, poor low-temperature fabricability and extreme oxidability at high temperatures are shortcomings of most refractory metals. Interactions with the environment can significantly influence their high-temperature creep strength. Application of these metals requires a protective atmosphere or coating.

The refractory metal alloys of molybdenum, niobium, tantalum, and tungsten have been applied to space nuclear power systems. These systems were designed to operate at temperatures from 1350 K to approximately 1900 K. An environment must not interact with the material in question. Liquid alkali metals as the heat transfer fluids are used as well as the ultra-high vacuum.

The high-temperature creep strain of alloys must be limited for them to be used. The creep strain should not exceed 1–2%. An additional complication in studying creep behavior of the refractory metals is interactions with environment, which can significantly influence the creep behavior.

See also

References

  1. ^ "International Journal of Refractory Metals and Hard Materials". Elsevier. Retrieved 2010-02-07.
  2. ^ Bauccio, Michael; American Society for Metals (1993). "Refractory metals". ASM metals reference book. ASM International. pp. 120–122. ISBN 978-0-87170-478-8.
  3. ^ Metals, Behavior Of; Wilson, J. W (1965-06-01). "General Behaviour of Refractory Metals". Behavior and Properties of Refractory Metals. pp. 1–28. ISBN 978-0-8047-0162-4.
  4. ^ Davis, Joseph R (2001). Alloying: understanding the basics. pp. 308–333. ISBN 978-0-87170-744-4.
  5. ^ a b Borisenko, V. A. (1963). "Investigation of the temperature dependence of the hardness of molybdenum in the range of 20–2500°C". Soviet Powder Metallurgy and Metal Ceramics. 1 (3): 182. doi:10.1007/BF00775076.
  6. ^ Fathi, Habashi (2001). "Historical Introduction to Refractory Metals". Mineral Processing and Extractive Metallurgy Review. 22 (1): 25–53. doi:10.1080/08827509808962488.
  7. ^ Schmid, Kalpakjian (2006). "Creep". Manufacturing engineering and technology. Pearson Prentice Hall. pp. 86–93. ISBN 978-7-302-12535-8.
  8. ^ Weroński, Andrzej; Hejwowski, Tadeusz (1991). "Creep-Resisting Materials". Thermal fatigue of metals. CRC Press. pp. 81–93. ISBN 978-0-8247-7726-5.
  9. ^ Smallwood, Robert E. (1984). "TZM Moly Alloy". ASTM special technical publication 849: Refractory metals and their industrial applications: a symposium. ASTM International. p. 9. ISBN 978-0-8031-0203-3.
  10. ^ Kozbagarova, G. A.; Musina, A. S.; Mikhaleva, V. A. (2003). "Corrosion Resistance of Molybdenum in Mercury". Protection of Metals. 39 (4): 374–376. doi:10.1023/A:1024903616630.
  11. ^ Gupta, C. K. (1992). "Electric and Electronic Industry". Extractive Metallurgy of Molybdenum. CRC Press. pp. 48–49. ISBN 978-0-8493-4758-0.
  12. ^ Magyar, Michael J. "Commodity Summary 2009:Molybdenum" (PDF). United States Geological Survey. Retrieved 2010-04-01.
  13. ^ Ervin, D. R.; Bourell, D. L.; Persad, C.; Rabenberg, L. (1988). "Structure and properties of high energy, high rate consolidated molybdenum alloy TZM". Materials Science and Engineering: A. 102: 25. doi:10.1016/0025-5416(88)90529-0.
  14. ^ Oleg D., Neikov (2009). "Properties of Molybdenum and Molybdenum Alloys powder". Handbook of Non-Ferrous Metal Powders: Technologies and Applications. Elsevier. pp. 464–466. ISBN 978-1-85617-422-0.
  15. ^ Davis, Joseph R. (1997). "Refractory Metalls and Alloys". ASM specialty handbook: Heat-resistant materials. pp. 361–382. ISBN 978-0-87170-596-9.
  16. ^ Lassner, Erik; Schubert, Wolf-Dieter (1999). Tungsten: properties, chemistry, technology of the element, alloys, and chemical compounds. Springer. pp. 255–282. ISBN 978-0-306-45053-2.
  17. ^ National Research Council (U.S.), Panel on Tungsten, Committee on Technical Aspects of Critical and Strategic Material (1973). Trends in Usage of Tungsten: Report. National Research Council, National Academy of Sciences-National Academy of Engineering. pp. 1–3.CS1 maint: Multiple names: authors list (link)
  18. ^ Lassner, Erik; Schubert, Wolf-Dieter (1999). Tungsten: properties, chemistry, technology of the element, alloys, and chemical compounds. Springer. ISBN 978-0-306-45053-2.
  19. ^ Harris, Michael K. (2002). "Welding Health and Safety". Welding health and safety: a field guide for OEHS professionals. AIHA. p. 28. ISBN 978-1-931504-28-7.
  20. ^ Galvery, William L.; Marlow, Frank M. (2001). Welding essentials: questions & answers. Industrial Press Inc. p. 185. ISBN 978-0-8311-3151-7.
  21. ^ Lanz, W.; Odermatt, W.; Weihrauch3, G. (7–11 May 2001). KINETIC ENERGY PROJECTILES: DEVELOPMENT HISTORY, STATE OF THE ART, TRENDS (PDF). 19th International Symposium of Ballistics. Interlaken, Switzerland.
  22. ^ Ramakrishnan, P. (2007-01-01). "Powder metallurgyfor Aerospace Applications". Powder metallurgy : processing for automotive, electrical/electronic and engineering industry. New Age International. p. 38. ISBN 81-224-2030-3.
  23. ^ Arora, Arran (2004). "Tungsten Heavy Alloy For Defence Applications". Materials Technology. 19 (4): 210–216.
  24. ^ Moxson, V. S.; (sam) Froes, F. H. (2001). "Fabricating sports equipment components via powder metallurgy". JOM. 53 (4): 39. Bibcode:2001JOM....53d..39M. doi:10.1007/s11837-001-0147-z.
  25. ^ a b Hebda, John (2001-05-02). "Niobium alloys and high Temperature Applications" (PDF). Niobium Science & Technology: Proceedings of the International Symposium Niobium 2001 (Orlando, Florida, USA). Companhia Brasileira de Metalurgia e Mineração. Archived from the original (pdf) on 2008-12-17.
  26. ^ Wilson, J. W. (1965). "Rhenium". Behavior and Properties of Refractory Metals. Stanford University Press. ISBN 978-0-8047-0162-4.

Further reading

  • Levitin, Valim (2006). High Temperature Strain of Metals and Alloys: Physical Fundamentals. WILEY-VCH. ISBN 978-3-527-31338-9.
  • Brunner, T (2000). "Chemical and structural analyses of aerosol and fly-ash particles from fixed-bed biomass combustion plants by electron microscopy". 1st World Conference on Biomass for Energy and Industry: proceedings of the conference held in Sevilla, Spain, 5–9 June 2000. London: James & James Ltd. ISBN 1-902916-15-8.
  • Spink, Donald (1961). "Reactive Metals. Zirconium, Hafnium, and Titanium". Industrial & Engineering Chemistry. 53 (2): 97–104. doi:10.1021/ie50614a019.
  • Hayes, Earl (1961). "Chromium and Vanadium". Industrial & Engineering Chemistry. 53 (2): 105–107. doi:10.1021/ie50614a020.
Brightray

Brightray is a nickel-chromium alloy that is noted for its resistance to erosion by gas flow at high temperatures. It was used for hard-facing the exhaust valve heads and seats of petrol engines, particularly aircraft engines from the 1930s onwards. It was developed by Henry Wiggin and Co at Birmingham.

As well as its use as a coating, it is also used in wire and strip form for electrical heating elements.

The original Brightray alloy was composed of 80% nickel / 20% chromium. This alloy is still in use today as Brightray S and can be used at temperatures up to 1050°C. Several other variants are now available. These include nickel-iron-chromium Brightray F that offers better resistance to both reducing and oxidizing environments. Brightray C is a nickel-chromium alloy with rare-earth additions to extend its lifetime under fluctuating temperatures, particularly with heating elements that are being continually switched on and off.

Diffusion bonding

Diffusion bonding or diffusion welding is a solid-state welding technique used in metalworking, capable of joining similar and dissimilar metals. It operates on the principle of solid-state diffusion, wherein the atoms of two solid, metallic surfaces intersperse themselves over time. This is typically accomplished at an elevated temperature, approximately 50-70% of the absolute melting temperature of the materials. Diffusion bonding is usually implemented by applying high pressure, in conjunction with necessarily high temperature, to the materials to be welded; the technique is most commonly used to weld "sandwiches" of alternating layers of thin metal foil, and metal wires or filaments. Currently, the diffusion bonding method is widely used in the joining of high-strength and refractory metals within the aerospace and nuclear industries.

Evaporating dish

An evaporating dish or watch glass is a piece of laboratory glassware used for the evaporation of solutions and supernatant liquids, and sometimes to their melting point. Evaporating dishes are used to evaporate excess solvents – most commonly water – to produce a concentrated solution or a solid precipitate of the dissolved substance.

Most are made of porcelain or borosilicate glass. Shallow glass evaporating dishes are commonly termed "watch glasses", since they resemble the front window of a pocket watch. Some used for high-temperature work are of refractory metals, usually of platinum, owing to its non-reactive behaviour and low risk of contamination.

The capacity of evaporators is usually small – in the range 3–10 ml. Larger dishes, up to 100 ml, are different in shape, and are more hemispherical.

The evaporator is used most often in quantitative analysis.

In the determination of silicon content in an organic sample, a small and accurately-measured quantity of a substance is added to the large amount of sulfuric acid, then heated in an evaporating dish. The dish is heated with a Bunsen burner, until only stable precipitate remains, which contains the silica content. The dish is then closed and heated at high temperature until completely clean, fused silica is produced. Comparison of the initial weight of the substance and that of the fused silica allows the content of silicon in the sample to be determined.

The shape of the evaporating dish encourages evaporation in two ways:

The shell is relatively flat. A relatively large liquid surface promotes evaporation.

If heated in a flask or beaker, a part of the evaporated liquid condenses on the vessel walls and flows back into the solution. This does not happen in a dish.When heating liquid in an evaporating dish, the low walls encourage splashes and so stirring or swirling of evaporating liquids is considered bad practice, owing to the risk of spillage.

Evaporation, especially in production quantities rather than merely for analysis, is now mostly performed in a rotary evaporator. This is preferred because it works much faster and may be used under vacuum, avoiding unwanted reactions with the atmosphere and allowing control of noxious fumes. Evaporation under vacuum also reduces the severity of bumping and violent ebullition.

FLiNaK

FLiNaK is the name of the ternary eutectic alkaline metal fluoride salt mixture LiF-NaF-KF (46.5-11.5-42 mol %). It has a melting point of 454 °C and a boiling point of 1570 °C. It is used as electrolyte for the electroplating of refractory metals and compounds like titanium, tantalum, hafnium, zirconium and their borides. FLiNaK also could see potential use as a coolant in the very high temperature reactor, a type of nuclear reactor.

Group 4 element

Group 4 is a group of elements in the periodic table.

It contains the elements titanium (Ti), zirconium (Zr), hafnium (Hf) and rutherfordium (Rf). This group lies in the d-block of the periodic table. The group itself has not acquired a trivial name; it belongs to the broader grouping of the transition metals.

The three Group 4 elements that occur naturally are titanium, zirconium and hafnium. The first three members of the group share similar properties; all three are hard refractory metals under standard conditions. However, the fourth element rutherfordium (Rf), has been synthesized in the laboratory; none of its isotopes have been found occurring in nature. All isotopes of rutherfordium are radioactive. So far, no experiments in a supercollider have been conducted to synthesize the next member of the group, either unpenthexium (Uph, element 156) or unpentoctium (Upo, element 158), and it is unlikely that they will be synthesized in the near future.

Group 5 element

Group 5 (by IUPAC style) is a group of elements in the periodic table. Group 5 contains vanadium (V), niobium (Nb), tantalum (Ta) and dubnium (Db). This group lies in the d-block of the periodic table. The group itself has not acquired a trivial name; it belongs to the broader grouping of the transition metals.

The lighter three Group 5 elements occur naturally and share similar properties; all three are hard refractory metals under standard conditions. The fourth element, dubnium, has been synthesized in laboratories, but it has not been found occurring in nature, with half-life of the most stable isotope, dubnium-268, being only 29 hours, and other isotopes even more radioactive. To date, no experiments in a supercollider have been conducted to synthesize the next member of the group, either unpentseptium (Ups) or unpentennium (Upe). As unpentseptium and unpentennium are both late period 8 elements it is unlikely that these elements will be synthesized in the near future.

Haynes International

Haynes International, Inc., headquartered in Kokomo, Indiana, is one of the largest producers of corrosion-resistant and high-temperature alloys. In addition to Kokomo, Haynes has manufacturing facilities in Arcadia, Louisiana, and Mountain Home, North Carolina. The Kokomo facility specializes in flat products, the Arcadia facility in tubular products, and the Mountain Home facility in wire products. In fiscal year 2018, the company's revenues were derived from the aerospace (52.1%), chemical processing (18.2%), industrial gas turbine (12.0%) and other (12.3%) industries. The company's alloys are primarily marketed under the Hastelloy and the Haynes brands. They are based on nickel, but also include cobalt, chromium, molybdenum, tungsten, iron, silicon, manganese, carbon, aluminum, and/or titanium.

Inconel

Inconel is a family of austenitic nickel-chromium-based superalloys.Inconel alloys are oxidation-corrosion-resistant materials well suited for service in extreme environments subjected to pressure and heat. When heated, Inconel forms a thick, stable, passivating oxide layer protecting the surface from further attack. Inconel retains strength over a wide temperature range, attractive for high temperature applications where aluminum and steel would succumb to creep as a result of thermally induced crystal vacancies. Inconel's high temperature strength is developed by solid solution strengthening or precipitation hardening, depending on the alloy.Inconel alloys are typically used in high temperature applications. Common trade names for Inconel Alloy 625 include: Inconel 625, Chronin 625, Altemp 625, Haynes 625, Nickelvac 625 and Nicrofer 6020. Inconel Alloy 600 include: NA14, N06600, BS3076, 2.4816, NCr15Fe (FR), NiCr15Fe (EU) and NiCr15Fe8 (DE). Inconel 718 include: Nicrofer 5219, Superimphy 718, Haynes 718, Pyromet 718, Supermet 718, and Udimet 718.

Kanthal (alloy)

Kanthal is the trademark for a family of iron-chromium-aluminium (FeCrAl) alloys used in a wide range of resistance and high-temperature applications. Kanthal FeCrAl alloys consist of mainly iron, chromium (20–30%) and aluminium (4–7.5 %). The first Kanthal FeCrAl alloy was developed by Hans von Kantzow in Hallstahammar, Sweden. The alloys are known for their ability to withstand high temperatures and having intermediate electric resistance. As such, it is frequently used in heating elements. The trademark Kanthal is owned by Sandvik Intellectual Property AB.

Molybdenum

Molybdenum is a chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin molybdaenum, from Ancient Greek Μόλυβδος molybdos, meaning lead, since its ores were confused with lead ores. Molybdenum minerals have been known throughout history, but the element was discovered (in the sense of differentiating it as a new entity from the mineral salts of other metals) in 1778 by Carl Wilhelm Scheele. The metal was first isolated in 1781 by Peter Jacob Hjelm.Molybdenum does not occur naturally as a free metal on Earth; it is found only in various oxidation states in minerals. The free element, a silvery metal with a gray cast, has the sixth-highest melting point of any element. It readily forms hard, stable carbides in alloys, and for this reason most of world production of the element (about 80%) is used in steel alloys, including high-strength alloys and superalloys.

Most molybdenum compounds have low solubility in water, but when molybdenum-bearing minerals contact oxygen and water, the resulting molybdate ion MoO2−4 is quite soluble. Industrially, molybdenum compounds (about 14% of world production of the element) are used in high-pressure and high-temperature applications as pigments and catalysts.

Molybdenum-bearing enzymes are by far the most common bacterial catalysts for breaking the chemical bond in atmospheric molecular nitrogen in the process of biological nitrogen fixation. At least 50 molybdenum enzymes are now known in bacteria, plants, and animals, although only bacterial and cyanobacterial enzymes are involved in nitrogen fixation. These nitrogenases contain molybdenum in a form different from other molybdenum enzymes, which all contain fully oxidized molybdenum in a molybdenum cofactor. These various molybdenum cofactor enzymes are vital to the organisms, and molybdenum is an essential element for life in all higher eukaryote organisms, though not in all bacteria.

Nichrome

Nichrome (NiCr, nickel-chrome, chrome-nickel, etc.) is any of various alloys of nickel, chromium, and often iron (and possibly other elements). The most common usage is as resistance wire, although they are also used in some dental restorations (fillings) and in a few other applications.

Nicrosil

Nicrosil is a nickel alloy containing 14.4% chromium, 1.4% silicon, and (in some sources) 0.1% magnesium.Nicrosil is used as the positive leg of type N thermocouples. In this application another nickel alloy, Nisil, is used as the negative leg. The Nicrosil alloy in this case does not contain magnesium.

Nisil

Nisil is an alloy of nickel (95.5% wt.) and silicon (4.4% wt.) with traces of Mg (0.1% wt.),which is non magnetic.

Nisil melts at 1341 - 1420 °C, has a density of 8.58 g/cm³, and electrical resistivity of 0.365 Ω⋅mm2/m at 20 °C.

It is often used in conjunction with Nicrosil in type N thermocouples. In this use, it serves as the negative leg of the thermocouple. It offers higher thermoelectric stability in air above 1000°C (1830°F) and better oxidation resistance than type E, J and K thermocouples. It can not be exposed to sulphur-containing gases.

Refraction (metallurgy)

In metallurgy, refraction is a property of metals that indicates their ability to withstand heat. Metals with a high degree of refraction are referred to as refractory. These metals derive their high melting points from their strong intermolecular forces. Large quantities of energy are required to overcome intermolecular forces.

Some refractory metals include molybdenum, niobium, tungsten, and tantalum. These materials are also noted for their high elastic modulus and hardness.

René 41

René 41 is a nickel-based high temperature alloy developed by General Electric. It retains high strength in the 1200/1800°F (649/982°C) temperature range. It is used in jet engine and missile components, and other applications that require high strength at extreme temperatures. René 41 is considered a Nickel alloy based upon its chemical composition.

René 41 was used to create the outer shell of the Mercury space capsule, due to its ability to retain high strength at very high temperatures.

Rhenium

Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is one of the rarest elements in the Earth's crust. Rhenium has the third-highest melting point and highest boiling point of any stable element at 5903 K. Rhenium resembles manganese and technetium chemically and is mainly obtained as a by-product of the extraction and refinement of molybdenum and copper ores. Rhenium shows in its compounds a wide variety of oxidation states ranging from −1 to +7.

Discovered in 1908, rhenium was the second-last stable element to be discovered. It was named after the river Rhine in Europe.

Nickel-based superalloys of rhenium are used in the combustion chambers, turbine blades, and exhaust nozzles of jet engines. These alloys contain up to 6% rhenium, making jet engine construction the largest single use for the element. The second-most important use is as a catalyst: rhenium is an excellent catalyst for hydrogenation and isomerization, and is used for example in catalytic reforming of naphtha for use in gasoline (rheniforming process). Because of the low availability relative to demand, rhenium is expensive, with price reaching an all-time high in 2008/2009 US$10,600 per kilogram (US$4,800 per pound). Due to increases in rhenium recycling and a drop in demand for rhenium in catalysts, the price of rhenium has dropped to US$2,844 per kilogram (US$1,290 per pound) as of July 2018.

Tantalum

Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, its name comes from Tantalus, a villain from Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion-resistant. It is part of the refractory metals group, which are widely used as minor components in alloys. The chemical inertness of tantalum makes it a valuable substance for laboratory equipment and a substitute for platinum. Its main use today is in tantalum capacitors in electronic equipment such as mobile phones, DVD players, video game systems and computers.

Tantalum, always together with the chemically similar niobium, occurs in the mineral groups tantalite, columbite and coltan (a mix of columbite and tantalite, though not recognised as a separate mineral species). Tantalum is considered a technology-critical element.

Tantalum-tungsten alloys

Tantalum-tungsten alloys are in the refractory metals group, keeping their chemical and physical properties the same at high temperatures. The tantalum-tungsten alloys are characterized by their high melting point and the tension resistance. The properties of the final alloy are a combination of properties from the two elements: tungsten, the element with the highest melting point in the periodic table, and tantalum which has high corrosion resistance.The Tantalum-Tungsten alloys typically vary in their percentage of Tungsten. Some common variants are:

(Ta – 2.5% W) → also called 'tantaloy 63 metal.' The percentage of tungsten is about 2 to 3% and includes 0.5% of niobium. This alloy has a good resistance to corrosion and performs well at high temperatures. An example application is piping in chemical industries.

(Ta - 7.5% W) → also called 'tantaloy 61 metal,' has between 7 and 8% tungsten. The difference from this alloy to the others is that this alloy represents a high resilience modulus while maintaining its refractory properties.

(Ta - 10% W) → also called 'tantaloy 60 metal,' contains 9 to 11% tungsten. This alloy is less ductile than the other alloys and exhibits less plasticity. Applications include high-temperature, high-corrosion environments such as aerospace components, furnaces, and piping in nuclear plants.

Tungsten

Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. The name tungsten comes from the former Swedish name for the tungstate mineral scheelite, tung sten or "heavy stone". Tungsten is a rare metal found naturally on Earth almost exclusively combined with other elements in chemical compounds rather than alone. It was identified as a new element in 1781 and first isolated as a metal in 1783. Its important ores include wolframite and scheelite.

The free element is remarkable for its robustness, especially the fact that it has the highest melting point of all the elements discovered, melting at 3422 °C (6192 °F, 3695 K). It also has the highest boiling point, at 5930 °C (10706 °F, 6203 K). Its density is 19.3 times that of water, comparable to that of uranium and gold, and much higher (about 1.7 times) than that of lead. Polycrystalline tungsten is an intrinsically brittle and hard material (under standard conditions, when uncombined), making it difficult to work. However, pure single-crystalline tungsten is more ductile and can be cut with a hard-steel hacksaw.Tungsten's many alloys have numerous applications, including incandescent light bulb filaments, X-ray tubes (as both the filament and target), electrodes in gas tungsten arc welding, superalloys, and radiation shielding. Tungsten's hardness and high density give it military applications in penetrating projectiles. Tungsten compounds are also often used as industrial catalysts.

Tungsten is the only metal from the third transition series that is known to occur in biomolecules that are found in a few species of bacteria and archaea. It is the heaviest element known to be essential to any living organism. However, tungsten interferes with molybdenum and copper metabolism and is somewhat toxic to more familiar forms of animal life.

Periodic table forms
Sets of elements
Elements
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