Refractory metals carbide-forming

Stable oxides, such as those of clrromium, vanadium and titanium cannot be reduced to the metal by carbon and tire production of these metals, which have melting points above 2000 K, would lead to a refractoty solid containing carbon. The co-reduction of the oxides widr iron oxide leads to the formation of lower melting products, the feno-alloys, and tlris process is successfully used in industrial production. Since these metals form such stable oxides and carbides, tire process based on carbon reduction in a blast furnace would appear to be unsatisfactory, unless a product samrated with carbon is acceptable. This could not be decarburized by oxygen blowing without significairt re-oxidation of the refractory metal. 

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. 

Several other types of atomizer have been developed. Some of these are based on the design of the West rod, but others have made tubular atomizers from extremely refractory metals such as tungsten, tantalum and molybdenum. This latter class of atomizers tend to be made in-house by some laboratories and, at present, do not have any commercial suppliers. They have the advantage of being inert and non-porous so there is little interaction with the analyte, so that they can be used for the determination of elements which form refractory carbides. However, after extended use and in the presence of some acids, many of these atomizers become brittle and distorted. 

They are normally cast in the form of brick and are sometimes bonded to assure stability. The outstanding property of these materials is their ability to act as insulators. The most important are fireclay (aluminum silicates), silica, high alumina (70-80% ALjOj), mullite (clay-sand), magnesite (chiefly MgO), dolomite (CaO-MgO), forsterite (MgO-sand), carbon, chrome ore-magnesite, zirconia, and silicon carbide. (2) Characterizing the ability to withstand extremely high temperature, e.g., tungsten and tantalum are refractory metals, clay is a refractory earth, ceramics are refractory mixtures. 

Catalysts for the chemical industry have to be characterized with respect to their trace impurities and major components. Not only is their composition when they are used initially in chemical reactors important, but also their alteration in the course of time. As carbide forming elements such as V and Ti are often used, atomic absorption spectrometry could be problematic. This also applies to catalysts for exhaust gas detoxification in cars. Noble metals such as Pt, Pd and Rh are fixed on alumina supports often also containing cerium compounds. Both for the determination of the stoichiometry but also for the monitoring of the noble metal contents in used catalysts, AAS suffers from problems because of the need for sample dissolution as well as for the requirement to determine refractory oxide forming elements. 

Rhenium (Re) differs from the other refractory metals (Nb, Ta, Mo and W) in that it has an hep structure, and does not form carbides. Because it does not have a ductile-to-brittle transition temperature. Re retains its ductility from subzero to high temperatures. In addition, it can be mechanically formed and shaped to some degree at room temperature. It also has a very high modulus of elasticity that, among metals, is second only to those of Ir and Os. Compared with other refractory metals. Re has superior tensile strength and creep-rupture strength over a wide temperature range. 

Figure 9. Schematic diagram showing the proposed nucleation mechanism diamond nuclei form on a carbide interlayer on a carbide-forming refractory metal substrateJ Initially, carburization consumes all available C to form a carbide surface layer. A minimum C surface concentration required for diamond nucleation cannot be reached on the substrate surface. With increasing carbide layer thickness, the C transport rate stows and the C surface concentration increases. When the C surface concentration reaches a critical level for diamond nucleation, or a surface C cluster attains a critical size, a diamond nucleus forms. (Reproduced with permission.)...

Low resistance, reliable, temperature-stable ohmic contacts are a prerequisite for the commercialization of SiC device technology. Still, these contacts are not yet satisfactory for a variety of reasons the annealing temperature is too high, the contacts penetrate too deep, they deteriorate when devices operate at elevated temperature, and the contact resistance is quite high. The problem of stable contacts to SiC may be resolved by using refractory metals. Refractory metals can be used to form both carbides and silicides. Silicides appear to provide a stable resistance if carbides are not present [1]. Contact systems based on such metals as nickel, chromium, titanium, cobalt and tungsten have been demonstrated for n-type SiC. Most contact systems for p-type SiC are Al-based and this imposes a limitation on the operating temperature of SiC devices with p-type contacts. 

Vanadium also forms a very stable carbide VC, and carburization of this metal is part of the corrosion reactions of vanadium based alloys contacted with liquid lithium as well as sodium. Vanadium alloys with contents of titanium have an even higher affinity to form solid carbides by absorbing of carbon from liquid metals. In systems in which vanadium titanium alloys and stainless steels are in contact with the same lithium or sodium, carbon migrates from the steel to the refractory metal alloy, thus passing the alkali metal serving as a transport medium The free energies of formation of the alkali acetylides are compared with the values of several metal carbides in Table V. 

Stable compound formation will always cause a depressive effect. Typical examples are the lowering of alkaline earth metal absorbances in the presence of phosphate, aluminate, silicate and some other oxo anions, the low sensitivity of metals which form thermally stable oxides (refractory oxide elements), and the depression of the calcium signal in the presence of proteins. In addition, some refractory oxide elements may also form stable carbides, especially in rich hydrocarbon flames. 

Refractory metals are used as carbide formers (vanadium) in alloys that contain insufficient chromium to form a protective layer of Cr203 (M0O3 or WO3 in Mo or W containing alloys) or as solution strengthening elements in Co-based alloys (Mo or W) [8]. 

Decomposition of batch materials can produce extremely large quantities of gases such as CO2, SO3, NOx, H2O, etc. Reactions with metals in contact with the melt can generate oxygen, carbon dioxide, or hydrogen by electrolytic reactions. Corrosion of refractories can open previously closed pores to the melt, releasing the gas contained in those pores into the melt. Residual carbon in refractories, or carbide refractories such as SiC, can react with oxide melts to form CO2 or CO. The products of all of these reactions can agglomerate to form bubbles. 

Electrical properties of MeC H films, deposited by reactive sputtering from metal or metal carbide targets in hydrocarbon atmosphere, have been described elsewhere [32]. In case of Ta as a typical refractory, carbide-forming metal, the electrical behavior can be divided into three categories, depending on the metal content ... 

One disadvantage of the carbon rod analyzer is the tendency of certain metals, such as barium, strontium, and tungsten, to form refractory carbides at high temperatures. If metal carbides are formed, the metal is removed from the sample vapor and a loss of sensitivity for the metal occurs. 

The stmctures of the so-called interstitial carbides (formed by heating C with fi -block metals having > 130 pm, e.g. Ti, Zr, V, Mo, W) may be described in terms of a close-packed metal lattice with C atoms occupying octahedral holes (see Fig. 6.5). In carbides of type M2C (e.g. V2C, Nb2C) the metal atoms are in an hep lattice and half of the octahedral sites are occupied. In the MC type (e.g. TiC and WC), the metal atoms adopt a cep stmcture and all the octahedral holes are occupied. These interstitial carbides are important refractory materials they are very hard and infusible, have melting points >2800 K and, in contrast to the acetylide... 

Along with above-considered s,p elements (Be, B, C, N, O, Al, etc.) that can enter as impurities into transition metal carbides and nitrides or form solid solutions with them, one of the most widespread impurities found in refractory compounds is H, which is present in the atmosphere during compound synthesis and enters into the composition of final products. Moreover, transition metal hydrides are sometimes used as starting agents (Pavlov, Zainulin and Alyamovsky, 1976) in the production of carbides or nitrides and their solid solutions. 

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