Refractory metals niobium

Nitrogen and carbon are the most potent solutes to obtain high strength in refractory metals (55). Particulady effective ate carbides and carbonitrides of hafnium in tungsten, niobium, and tantalum alloys, and carbides of titanium and zirconium in molybdenum alloys. 

Physical Properties. Molybdenum has many unique properties, leading to its importance as a refractory metal (see Refractories). Molybdenum, atomic no. 42, is in Group 6 (VIB) of the Periodic Table between chromium and tungsten vertically and niobium and technetium horizontally. It has a silvery gray appearance. The most stable valence states are +6, +4, and 0 lower, less stable valence states are +5, +3, and +2. 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

Use of low-temperature molten systems for electrolytic processes related with tantalum and niobium and other rare refractory metals seems to hold a promise for future industrial use, and is currently of great concern to researchers. The electrochemical behavior of tantalum, niobium and titanium in low-temperature carbamide-hilide melts has been investigated by Tumanova et al. [572]. Electrodeposition of tantalum and niobium from room/ambient temperature chloroaluminate molten systems has been studied by Cheek et al. [573],... 

Niobium (also known as columbium) is a soft, ductile, refractory metal with good strength retenti on at high temperature, and a low capture cross-section for thermal neutrons. Itis readily attacked by oxygen and other elements above 200°C. CVD is used to produce coatings or free standing shapes. The properties of niobium are summarized in Table 6.8. 

The refractory metals for which CVD is commonly used to produce free-standing shapes are tungsten, niobium, rhenium, tantalum, molybdenum, and nickelb lb lb l (see Ch. 6). Shapes presently produced include rods, tubes, crucibles, manifolds, ordnance items, nozzles, and thrust chambers. They are usually deposited on a disposable mandrel of copper, molybdenum, or graphite which is subsequently machined off or removed chemically by etching. 

The carbothermic reduction processes outlined so far apply to relatively unstable oxides of those metals which do not react with the carbon used as the reductant to form stable carbides. There are several metal oxides which are intermediate in stability. These oxides are less stable than carbon monoxide at temperatures above 1000 °C, but the metals form stable carbides. Examples are metals such as vanadium, chromium, niobium, and tantalum. Carbothermic reduction becomes complicated in such cases and was not preferred as a method of metal production earlier. However, the scenario changed when vacuum began to be used along with high temperatures for metal reduction. Carbothermic reduction under pyrovacuum conditions (high temperature and vacuum) emerged as a very useful commercial process for the production of the refractory metals, as for example, niobium and tantalum, and to a very limited extent, of vanadium. 

The melting points of chromium (1857 °C) and of manganese (1244 °C) are considerably lower than that of alumina. The heat requirement in respect of the reactions leading to the formation of these metals was calculated by considering alumina melting as the objective. The objective changes when aluminothermy is applied to the production of the refractory metals niobium and tantalum. Niobium melts at 2468 °C and tantalum at 3020 °C. Thus, when these metals are the products, the heat requirements for reaching temperatures in excess of 2500 °C and 3050 °C have to be calculated. 

Sample. This source places no restrictions on target material. Clusters of metals, produced. For example, polyethylene and alumina have been studied as well as refractory metals like tungsten and niobium. Molecular solids, liquids, and solutions could also be used. However the complexity of the vaporization process and plasma chemistry makes for even more complex mixtures in the gas phase. To date the transition metals(1-3) and early members of group 13 (IIIA) and 14 (IVA)( 11-16) have been the most actively studied. 

Because several of the superalloys contain very little iron, they are closely related to some of the non-ferrous alloys. Some of the second- and third-row transition metals possess many of the desirable properties of superalloys. They maintain their strength at high temperatures, but they may be somewhat reactive with oxygen under these conditions. These metals are known as refractory metals, and they include niobium, molybdenum, tantalum, tungsten, and rhenium. 

Hafnium is used in control rods for nuclear reactors. It has high resistance to radiation and also very high corrosion resistance. Another major application is in alloys with other refractory metals, such as, tungsten, niobium and tantalum. 

In the cuse of refractory metals, coalings generally arc silicidcs, applied by pack cementation or slurry processes. Typical silieidc compositions arc Si—20Cr-2DFe for niobium alloys and MoSiy for molybdenum alloys. 

Other physical properties of molybdenum are given under Chemical Elements. See also summary of properties of refractory metals under Niobium. 

In metallurgy, niobium is classified as a refractory metal, along with tungsten, tantalum, and molybdenum. A comparison of the four metals is given in the accompanying table. 

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