Intermetallic compounds, oxidation

Whereas finely divided cobalt is pyrophoric, the metal in massive form is not readily attacked by air or water or temperatures below approximately 300°C. Above 300°C, cobalt is oxidized by air. Cobalt combines readily with the halogens to form haUdes and with most of the other nonmetals when heated or in the molten state. Although it does not combine direcdy with nitrogen, cobalt decomposes ammonia at elevated temperatures to form a nitride, and reacts with carbon monoxide above 225°C to form the carbide C02C. Cobalt forms intermetallic compounds with many metals, such as Al, Cr, Mo,... 

Sohd rocket propellants represent a very special case of a particulate composite ia which inorganic propellant particles, about 75% by volume, are bound ia an organic matrix such as polyurethane. An essential requirement is that the composite be uniform to promote a steady burning reaction (1). Further examples of particulate composites are those with metal matrices and iaclude cermets, which consist of ceramic particles ia a metal matrix, and dispersion hardened alloys, ia which the particles may be metal oxides or intermetallic compounds with smaller diameters and lower volume fractions than those ia cermets (1). The general nature of particulate reinforcement is such that the resulting composite material is macroscopicaHy isotropic. 

Type 315-This has a composition that provides a similar oxidation resistance to type 309 but has less liability to embrittlement due to sigma formation if used for long periods in the range of 425 to 815°C. (Sigma phase is the hard and brittle intermetallic compound FeCr formed in chromium rich alloys when used for long periods in the temperature range of 650 to 850°.)... 

In this chapter synthesis of zinc ferrites, intermetallic compounds, and metal-oxides. 

The electrodeposition of Cr in acidic chloroaluminates was investigated in [24]. The authors report that the Cr content in the AlCr deposit can vary from 0 to 94 mol %, depending on the deposition parameters. The deposit consists both of Cr-rich and Al-rich solid solutions as well as intermetallic compounds. An interesting feature of these deposits is their high-temperature oxidation resistance, the layers seeming to withstand temperatures of up to 800 °C, so coatings with such an alloy could have interesting applications. 

The major types of interferences in ASV procedures are overlapping stripping peaks caused by a similarity in the oxidation potentials (e.g., of the Pb, Tl, Cd, Sn or Bi, Cu, Sb groups), the presence of surface-active organic compounds that adsorb on tlie mercury electrode and inhibit the metal deposition, and the formation of intermetallic compounds (e.g., Cu-Zn) which affects the peak size and position. Knowledge of these interferences can allow prevention through adequate attention to key operations. 

Only two processes for the manufacture of Be are of industrial importance (i). the thermal reduction of BeF2 using Mg, and (ii) the electrolysis of BeCl2 in a molten chloride electrolyte. Direct reduction of the oxide is ineffective because of its thermodynamic stability only Ca reduces BeO to the metal unfortunately, Ca cannot be used since it forms a stable intermetallic compound with Be, BejjCa. 

Several preparative methods do not use elemental mixtures. Group IIA-Pt intermetallic compounds have been prepared by reacting platinum metal with the group-IIA oxide under hydrogen or ammonia at 900-1200 C. Beryllium metal reacts with neptunium fluoride under vacuum at 1100-1200°C to form BC 3Np. 

For the spectra of Ni, peaks corresponding to Ni oxide and Ni metal are observed in the as-prepared sample [28-30]. After the etching with Ar, however, the peak of Ni metal is predominant. This implies that the state of Ni in the Ni-Zn nanoclusters is metallic, although their surface was oxidized under the atmospheric conditions. On the other hand, the identification of Zn state is difficult because the peak positions of Zn and ZnO in ESCA spectra are very close to each other. Furthermore, the B/Ni ratio determined by ESCA was increased with increasing Zn added e.g., Ni B = 73.3 26.7 and 60.6 39.4 for Zn/Ni = 0.0 and 1.0, respectively. Because no crystalline structure was found except for Ti02 from both electron and X-ray diffraction patterns of the respective samples, it can be concluded that formed nanoclusters were amorphous. Ni-Zn nanoclusters would be composed of amorphous intermetallic compounds through the... 

Figure 15.1 Comparison of the hot hardnesses of three strong oxides and the strong intermetallic compound, Ni3Al. Yttrium aluminum garnet is the hardest of all oxides at high temperatures.

An interesting, peculiar laboratory preparative reaction may finally be mentioned. This is based on the very high stability of the intermetallic compounds of actinides (and lanthanides) with the platinum family metals. The combined reduction capability of Pt with H2 (coupled reduction, see 6.7.2 fi) can be used to obtain, from its oxide, the platinide of the actinide metal. The An-Pt intermetallic compound can then be decomposed by heating in vacuum and the actinide can be obtained by distillation. 

Two other methods have been used successfully to prepare very pure Cm metal. A rather unique one is thermal decomposition of the intermetallic compound PtjCm produced by hydrogen reduction of curium oxide in the presence of Pt (36, 82). The second method, the method of choice for gram-scale preparations of very pure Cm metal, involves reduction of curium oxide with Th metal (8, 83) in an apparatus... 

In alkaline electrolyzers, Ni is the only elemental cathode that can be used. It is generally considered as a fairly good electrocatalyst, but in facts it exhibits two shortcomings (i) its activity decreases with time [cf. the AVtterm in Equation (7.16)] especially under conditions of intermittent electrolysis and (ii) shutdown of industrial cells (for maintenance) leads to Ni dissolution at the cathode since this electrode is driven to more positive potentials by short-circuit with the anode. These shortcomings can be alleviated if Ni cathodes are activated, that is, if they are coated with a thin layer of more active and more stable materials. Activation has been attempted with a variety of materials from sulfides to oxides, from alloys to intermetallic compounds. 

The vapour pressure ratio of actinides to noble metals is also the basis of the actinide metal preparation by thermal dissociation of intermetallic compounds. Such intermetallic compounds of An and noble metals can be prepared by hydrogen reduction of a mixture of an An oxide and a finely divided noble metal (Pt, Ir.. in the absence of noble metals, hydrogen reduction of An oxides is impossible. Am and Cm metals have been obtained by thermal dissociation of their intermetallic compounds with Pt and Ir High purity Th and Pa, the least volatile actinide metals, can be prepared by thermal dissociation of their iodides, which form readily by reaction of iodine vapour with car-... 

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