Molybdenum dichalcogenides

Scrosati B (1988) Molybdenum and Tungsten Dichalcogenides (M0X2 and WX2). In Finklea HO (ed) Semiconductor Electrodes (Studies in Physical and Theoretical Chemistry 55) Elsevier... 

The application of molybdenum disulphide and other dichalcogenides has become important in electrical brushes, especially in spacecraft, and its electrical properties are of considerable interest. It is therefore surprising to find that there is no clear agreement about its electrical conductivity. 

For space use, the voltages and current densities are usually low, and there has been extensive study of systems in which the brush and slipring materials are metallic, and lubrication is provided by molybdenum disulphide or other dichalcogenides. 

Surface films containing molybdenum disulphide have therefore been very little used for electrical contacts, and the usual technique is to use conducting compacts containing molybdenum disulphide or other dichalcogenides. Table 12.13 lists some of the compacts which have been used. 

The crystai form is a hexagonal lamellar structure, as shown in Figure 14.2. This has a general resemblance to that of molybdenum disulphide and the other transition metal dichalcogenides, but the main difference lies in the fact that each layer consists of a repeated hexagonai arrangement of carbon atoms, whereas in molybdenum disulphide and its analogues the layers consist of molecules, or assemblies of two different atoms. 

Both Tsuya and Ducas found that in moist air the friction of tungsten diselenide rose more than that of molybdenum disulphide. More surprisingly Ducas found that the friction of other dichalcogenides fell in moist air. His results are listed in Table 14.8. 

The other major difference between the various synthetic dichalcogenides lies in their electrical conductivities, as shown in Table 14.1. These figures should be considered relative rather than absolute, since values quoted by different investigators have differed by factors of over two hundred . Overall the lowest resistivity is that of niobium diselenide, and this has led to many investigations of its potential for use in situations, such as high vacuum, where graphite cannot be used. In brush compositions, however, molybdenum disulphide has generally been more successful, and this subject has been considered in more detail in Chapter 12. 

There is very little information available about corrosion risks with the other lubricating dichalcogenides. Spalvins mentioned that the friction and endurance of a sputtered tungsten disulphide film deteriorated considerably when tested in the atmosphere instead of in vacuum, and the deterioration was associated with formation of sulphuric acid and corrosion of the substrate. These results are not very different from those obtained with molybdenum disulphide in the same period, and there seems to be no reason to expect the problem to be any greater or any less with the other dichalcogenides. 

Eg S 1 eV). Molybdenum and tungsten dichalcogenides are, as we have seen, good semiconductors, but they can only form alkali intercalation compounds. Only organic solvents which are very stable against reduction can be used as electrolytes when elec-... 

The preparation of layered transition-metal dichalcogenides has been one of the areas of emphasis in this research (9). Molybdenum disulfide, an... 

Metathetical precursor reactions are well suited to the synthesis of solid-solution compounds. This approach offers a diverse set of precursors and has the advantage over other methods in that both mixed cation and mixed anion reagents are readily obtained. Mixed metal chloride precursors of molybdenum and tungsten, for example, have been found to be efficacious in the preparation of solid-solution dichalcogenides such as (Mo,W)S2. 

Selwyn LS, McKinnon WR, von Sacken U, Jones CA (1987) Lithium electrochemical cells at low voltage. Decomposition of Mo and W dichalcogenides. Solid State Ionics 22 337-344 Hearing RR, Stiles JAR, Brandt Klaus (1979) Lithiimi molybdenum disulphide battery cathode. US Patent 4,224,390, 23 Sep 1980... 

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