Cubic cobalt

Figure 4.17 Regular layers Inside a cobalt nanoparticle larger than 20 nm in diameter, which are observed after the particle has been exposed to CO at the pressure of 1 bar and temperature 700 K. The light regions are the fine (approximately five atoms in thickness) hexagonal cobalt layers, dark region are the cubic cobalt layers [6].

In the case of cobalt, unstable cubic cobalt was identified as the product of the reduction of standard cobalt catalysts, while hexagonal cobalt was found as a product of the hydrogenation of cobalt carbide. Used cobalt catalysts show no carbide by x-ray examination. Bulk phase carbide decreases the activity of cobalt catalysts. Surface area measurements show no appreciable change when the cobalt of cobalt catalysts was converted to cobalt carbide. Carburization at conditions where free carbon is formed increases the area considerably. 

The internal field at Sn nuclei in face-centred cubic cobalt metal falls from -22 kG at 4-2 K to -10 kG at 500 K [248]. Above 800 K the field increases in value again, and it is believed that the smooth temperature-dependence curve (Fig. 14.15), which was drawn assuming that the value of the field at first decreases and then reverses in sign at about 700 K, to approach a new positive maximum at 1100 K, is genuine [249]. 

The two allotropes of cobalt were designated in the order of their discovery. Therefore the room temperature, hep, was originally called alpha and the medium temperature, fee, was named beta. In modern nomenclature, the hep phase was named epsilon, but the reader must always keep in mind that alpha may refer either to hexagonal or cubic cobalt. 

CoUey SE, Copperthwaite RG, Hutchings GJ, Terblanche SP, Thackeray MM Identification of body-centred cubic cobalt and its importance in cohydrogenation. Nature 339(6220) 129-130, 1989. 

Figure 2.3 Illustration of an asymmetric silica nanocoil catalyzed with a cubic cobalt oxide nanoparticle. Note the edges of a marquise-like cross-section.

A wide range of cutting-tool materials is available. Properties, performance capabilities, and cost vary widely (2,7). Various steels (see Steel) cast cobalt alloys (see Cobalt and cobalt alloys) cemented, cast, and coated carbides (qv) ceramics (qv), sintered polycrystalline cubic boron nitride (cBN) (see Boron compounds) and sintered polycrystalline diamond tbin diamond coatings on cemented carbides and ceramics and single-crystal natural diamond (see Carbon) are all used as tool materials. Most tool materials used in the 1990s were developed during the twentieth century. The tool materials of the 1990s... 

The electronic stmcture of cobalt is [Ar] 3i/4A. At room temperature the crystalline stmcture of the a (or s) form, is close-packed hexagonal (cph) and lattice parameters are a = 0.2501 nm and c = 0.4066 nm. Above approximately 417°C, a face-centered cubic (fee) aHotrope, the y (or P) form, having a lattice parameter a = 0.3544 nm, becomes the stable crystalline form. The mechanism of the aHotropic transformation has been well described (5,10—12). Cobalt is magnetic up to 1123°C and at room temperature the magnetic moment is parallel to the ( -direction. Physical properties are Hsted in Table 2. 

Cobalt cannot be classified as an oxidation-resistant metal. Scaling and oxidation rates of unalloyed cobalt in air are 25 times those of nickel. The oxidation resistance of Co has been compared with that of Zr, Ti, Fe, and Be. Cobalt in the hexagonal form (cold-worked specimens) oxidizes more rapidly than in the cubic form (annealed specimens) (3). 

Cobalt(Il) dicobalt(Ill) tetroxide [1308-06-17, Co O, is a black cubic crystalline material containing about 72% cobalt. It is prepared by oxidation of cobalt metal at temperatures below 900°C or by pyrolysis in air of cobalt salts, usually the nitrate or chloride. The mixed valence oxide is insoluble in water and organic solvents and only partially soluble in mineral acids. Complete solubiUty can be effected by dissolution in acids under reducing conditions. It is used in enamels, semiconductors, and grinding wheels. Both oxides adsorb molecular oxygen at room temperatures. 

CoNbOF5 [129] can also be considered an MeX3 type compound due to the steric similarity of cobalt and niobium ions. This compound crystallizes in tetragonal syngony with cell parameters a = 7.81 and c = 9.02 A (Z = 4 p = 3.19 g/cm3), and can be considered to have a distorted cubic Re03 structure. Both cobalt and niobium occur in the center of oxyfluoride octahedrons that are linked via their vertexes. 

Fig. 1.—A simple cubic framework of trisilver cobalticyanide. The circles, in increasing size, represent silver, cobalt, carbon, and nitrogen, respectively.

This study could be extended to the synthesis of iron nanoparticles. Using Fe[N(SiMe3)2]2 as precursor and a mixture of HDA and oleic acid, spherical nanoparticles are initially formed as in the case of cobalt. However, a thermal treatment at 150 °C in the presence of H2 leads to coalescence of the particles into cubic particles of 7 nm side length. Furthermore, these particles self-organize into cubic super-structures (cubes of cubes Fig. ) [79]. The nanoparticles are very air-sensitive but consist of zerovalent iron as evidenced by Mossbauer spectroscopy. The fact that the spherical particles present at the early stage of the reaction coalesce into rods in the case of cobalt and cubes in the case of iron is attributed to the crystal structure of the metal particles hep for cobalt, bcc for iron. 

The total surface areas determined by the N2 BET method for the calcined, supported catalysts are listed in Table II. The X-ray diffraction (XRD) results showed diffraction peaks from a cubic lattice with a unit cell distance of 6.1 A were present on all of the calcined catalysts. Both C03O4 and C0AI2O4 have structures consistent with that lattice spacing, making assignment of the type of crystalline cobalt species present on the alumina supports difficult. 

The spinel ferrites were fabricated by solid state reaction technique. Cobalt and Zinc ferrites CoxZnyFe204,(x=0.7,0.3,0.4,0.2 and y=0.3,0.7,0.6,0.8) were prepared by solid state reaction technique. The crystalline structure of the sample was investigated by X-ray diffraction(XRD). All samples show cubic spinel structure. The lattice parameter decreases with increasing cobalt content. Magnetic properties shows that the prepared sample exhibit ferromagnetic behaviour at room temperature. The saturation magnetization increases with increasing cobalt content. Curie temperature... 

Dl, particle size calculated from the Sherrer equation Sjh, specific surface area calculated assuming cubic particles and a density of LaCo03 equal to 7.29 D2, equivalent cubic particle size calculated from BET surface area. P, perovskite C, cobalt oxide. 

Spectra of the mixed spinels are interpretable in terms of X-ray diffraction studies of the same samples by Azdroff (16). Cobalt appears to be in the divalent condition, based upon the location of the principal maximum. Manganese appears in a higher valence state. The extended fine structure, which is supposed to be determined by the lattice, appears identical for all the spectra of Figs. 14 and 15 which are of truly cubic spinels, namely CosOi,... 

Fig. 14. Cobalt K-edge spectra of two mixed oxides compared to spectra of two common cobalt oxides. CoiMn40s is of hausmanite type tetragonal structure, CotO and CoNiMniOi are of cubic spinel structure.

Fio. 15. Manganese K-edge spectra of five mixed oxides, including two whose cobalt edge spectra were included in Fig. 14. Mni04 and CojMniOa are tetragonal others are cubic spinels. 

Cobalt disulfide is a black cubic crystal density 4.27 g/cm insoluble in water soluble in nitric acid. 

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