Niobium diselenide

K.J. Reynolds, G.L. Frey, and R.H. Friend, Solution-processed niobium diselenide as conductor and anode for polymer light-emitting diodes, Appl. Phys. Lett., 82 1123-1125, 2003. 

Many layer-lattice compounds can intercalate additional metal atoms of the same element as comprised in the original structure (e.g. niobium in niobium diselenide), but molybdenum disulphide will not do so. The behaviour may be determined by the availability of electrons suitably oriented to form bonds with the additional metal atoms, although it seems unlikely that this single factor applies to all intercalation effects. 

The effect of intercalating like metal atoms is of course to change the atomic ratios, and for example it has been reported that niobium diselenide can intercalate additional niobium atoms to a composition of Nb, jSej There will also be corresponding changes in the crystal lattice parameters, and these are discussed in relation to lubrication properties in Chapter 14. 

It is usual to state " that molybdenum disulphide is a p type semiconductor, while niobium diselenide is a conductor, However Mikhailov has shown that pure molybdenum disulphide is a conductor and that only specimens having a developed film of oxidised material on the surface of the lamellae show semiconductor properties. Correspondingly a composite containing 15% was found to have a specific contact resistance of only 0.4 m.ohm.cm. compared with 0.7 m.ohm.cm ... 

There is a curious anomaly in the performance of these materials which may be related to the complex effects of purity and temperature reviewed in Chapter 4. Niobium diselenide (Nb 862) is a better conductor than molybdenum disulphide, but apart from one report in which the test conditions were unconventional , compacts containing molybdenum disulphide have generally performed better than those containing niobium diselenide, in terms of wear, electrical resistance and electrical noise. Apart from the compacts listed in Table 12.13, the superiority of molybdenum disulphide was also confirmed in contacts with silver and graphite . 

Their crystal structures have been mentioned briefly in connection with intercalation in Section 14.2. All five compounds can be obtained in the layered hexagonal crystal form, and most are also found in rhombohedral or trigonal form. The compounds of the Group 6 metals, molybdenum and tungsten, as well as niobium diselenide, have a hexagonal form similar to that of molybdenum disulphide, in which the metal atoms in one layer are displaced sideways from those in the layers immediately above and below. This structure results in the widest interlamellar spacing, the easiest interlamellar shear, and the lowest friction. 

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. 

Kirner reported the successful use of plasma spraying for niobium diselenide, niobium ditelluride, and a mixture of tungsten disulphide and silver, but the performances in high vacuum and high temperature were inferior to those obtained with molybdenum disulphide. There has been a great deal of Russian work on the... 

The temperature limits are more curious. There seems to be no evidence from other published literature for the much higher temperature performance in vacuum of the niobium diselenide and tungsten diselenide than tungsten disulphide, and the very inferior performance of molybdenum disulphide. Similarly, there is no obvious reason... 

Niobium diselenide Poorer lubricant than molybdenum disulphide, but better electrical conductivity Electrical contacts... 

Niobium diselenide (NbSc2) is sometimes used as a lubricant at high temperatures. It does not break down at temperatures up to about 2372°F (1300°C). Niobium silicide (NbSi2) is used as a refractory material. A refractory material is one that can withstand very high temperatures. 

The examples of the described methods applications can be found in the works by Nikolskaya et al. [14] where DC of lithium in niobium diselenide of layered structure 2H-NbSe2 has been determined by GSPM and SSCAM. For calculations. 

This material has the metallic type of conductivity and extremely flexible crystalline lattice (the lattice can expand along the c axis, doubling the parameter), and can be easily pressed into pellets. The degree of lithium-ion reduction is about 0.7 (i.e., lithium in the crystal is present in 70 % as atoms and 30 % as ions). Thus, layered niobium diselenide is the perfect material for testing the proposed models. In order to eliminate the pores, pellets have been impregnated with molten paraffin-polyethylene alloy imder vacuum in all cited works. 

Toroshchina El, Ravdel BA, Tikhonov KI (1991) On the Phase-Transitions and Phase-Composition in Cathode Reduction of 2H-Niobium Diselenide. Solid State Ionics 48 267-269. doi 10.1016/0167-2738(91)90041-9... 

Nikolskaya EY, Tikhonov Kl, Semenovkobzar AA, YanaM AA, Rotinyan AL (1981) Electrochemical Reduction and Oxidation of Niobium Diselenide in propylene carbonate. J Appl Chem-USSR 54 742-745... 

Nikolskaya EY, Tikhonov Kl (1981) The Effects of Solvents and Electrolytes on Electrochemical Niobium Diselenide Reduction. Sov Electrochem 17 1079-1083... 

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