Refractory metals concentration

Relatively little work has been done on the effects of refractory-metal additions on the oxidation of Al203-forming alloys. In superalloys, where the refractory-metal concentration is rarely above 10 weight percent, three effects have been observed. One effect is beneficial and arises since these elements can be considered to be oxygen getters in comparison to the base metals (Ni and Co) and they can, therefore, promote the selective oxidation of aluminium and chromium. The other two effects are deleterious. One occurs because the refractory elements decrease the diffusion coefficients of the elements needed to be selectively oxidized. The other arises because of the relatively poor protectiveness of the oxides of the refractory metals, which makes their presence in the external scale undesirable. Refractory metals... 

Chlorination. In some instances, the extraction of a pure metal is more easily achieved from the chloride than from the oxide. Oxide ores and concentrates react at high temperature with chlorine gas to produce volatile chlorides of the metal. This reaction can be used for common nonferrous metals, but it is particularly useful for refractory metals like titanium (see Titanium and titanium alloys) and 2irconium (see Zirconium and zirconium compounds), and for reactive metals like aluminum. 

HI is a strong reducing acid with a negative pH. Even though it is a common reagent in organic chemistry, corrosion data of materials in HI acid at elevated temperatures are limited. Table 4.6 shows a summary of the available data. Noble and refractory metals have shown low corrosion rates, but the temperatures at which the data were taken are lower than those in the Bunsen reaction environment. The corrosion mechanism of H2SO4 depends on temperature and concentration. Within the... 

Fig. 16. Effect of NajW04, NH4Re04, and Na2Mo04 concentrations on the refractory metal (a) and phosphorus (b) content of electroless NiWP, NiReP, and NiMoP alloy films (T. Osaka et al., 1992 [38]).

Trace determinations in different metal bases (e.g. lead, gallium and indium) can make use of pre-concentration based on a partial dissolution of the matrix (see Ref. [318]). Refractory metals such as molybdenum, niobium, tungsten and tantalum... 

Metals are important resources and have a wide range of applications. Metals are often extracted from ores. Once the ore is mined, the metals must be extracted, usually by chemical or electrolytic reduction. Pyrometallurgy uses high temperatures to convert ore into raw metals, while hydrometalluigy employs aqueous chemistry for the same purpose. The methods used depend on the metal and their contaminants. Most metals are obtained by hydrometallurgical processes such as aqueous acids or alkalis are predominantly used to dissolve the metal oxides, sulfides, or silicates. Electrowinning and solvent extraction are frequently used to recover and concentrate the metals. A limited number of high-temperature molten salts have also been used for the recovery of refractory metals, such as titanium and aluminum, from their ores... 

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.)...

There are some exceptions to the generalizations above. The very refractory metals, e.g., W, Ta, etc., are not reduced to an atomic vapor in the flame and are therefore not accessible to flame AAS. In this case ICP is usually preferable. If the analyte concentration approaches the fig/L level or below, the precision becomes much poorer. In such cases other more sensitive methods are often preferable, for example, furnace AAS. If many metals must be determined in each sample, ICP emission spectroscopy is often more rapid, though it is more expensive and it requires a higher level of operator skill than flame AAS. 

The cathodes spots are the localized current centers, which appear on the cathode when significant current should be provided but the cathode carmot be heated enough as a whole. The most typical cause of cathode spots is the application of metals with relatively low melting points. The cathode spots can also be caused by low arc currents, which are only able to provide the necessary electron emission when concentrated to a small area. The cathode spots also appear at low gas pressures (<1 Torr), when metal vapor from the cathode provides atoms to generate positive ions bringing their energy to the cathode to sustain the electron emission. To provide the required evaporation, cmrent is concentrated in spots at pressures <1 Torr and currents 1-10 A, such spots occur even on refractory metals. 

In contrast to the sulphides of most of the transition metals, sulphides of the refractory metals have quite tight stoichiometry, similar to Cr203, although, in the cases of the refractory-metal sulfides and oxides, the defects appear on the anion sub-lattice. Figure 6.1 compares the rates of oxidation and sulphidation for several of the transition and refractory metals. The low rates of sulphidation of the refractory metals are thought to be due to the low concentrations of defects in the sulphide structures. 

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