Niobium disulphide

In the case of the equivalent niobium compounds, the same electronic effects are not present. He postulated that in pure stoichiometric niobium disulphide this results in poor lubrication. When good lubrication behaviour is observed, it is probably caused by additional niobium atoms intercalated between the lamellae, which contribute non-bonding electrons. On the basis of this theory, non-bonded atoms intercalated between the lamellae can increase the inter-lamellar spacing, whereas bonded intercalated atoms increase the resistance to inter-lamellar shear, and therefore the friction. However, an alternative interpretation is that certain intercalated atoms alter the interaction between the niobium atoms, allowing rearrangement to the 2H structure of molybdenum disulphide, and it is the favourable structure which provides good lubrication performance. 

Jamison found that low concentrations of intercalated copper or silver in niobium disulphides and diselenides promoted good lubricating performance. Higher concentrations increased the resistance to inter-lamellar shear, and therefore the friction, but improved high temperature performance due to the reduced intracrystalline shear and some sacrificial oxidation of the intercalated metals. 

However, pure niobium disulphide has a hexagonal structure in which the metal atoms in each layer are located directly above or below those in adjacent layers. Jamison showed that with this configuration niobium disulphide is not a good lubricant. He postulated that when niobium disulphide behaves as a good lubricant, additionai niobium atoms intercalated into the structure will have resulted in a change in eiectron bonding to favour the molybdenum disulphide structure. 

Grzesik Z, Mrowec S, Dabek J, On the defect mobility in niobium disulphide . 

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. 

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 ... 

Powder metallurgy techniques have been used to produce a very wide range of compacts containing molybdenum disulphide in such metals as mixed iron-palladium, iron-platinum , tantalum , iron-tantalum , molybdenum-tantalum , and molybdenum-niobium . The concentration of molybdenum disulphide in these compacts has risen to 90% compared with less than 35% in earlier materials. Composites containing nickel were found to be unsatisfactory because of high friction and wear. 

Martin and Murphy ° compared twenty-five different solid lubricant composites for use in small arms. The best performance was obtained with a composite of molybdenum disulphide in molybdenum with niobium and copper. This had a lower wear rate (0.224 x 10 mm /Nm) and lower coefficient of friction (0.05 - 0.15) than any of the twenty-one polymer-based composites. 

Suzuki et al ° made hot-pressed composites of molybdenum disulphide 80%, molybdenum dioxide 10% and niobium 10%. They were pressed at 25 MPa and 1500°C in carbon dies, and the flexural strength was 59 - 63 MPa and the elastic modulus 27.9 MPa. To improve the strength they added 5% of 304 stainless steel, and this gave a flexural strength of 69 - 80 MPa and elastic modulus 43.8 MPa. The coefficient of friction of the latter compact was 0.07 to 0.18 and the specific wear rate 2.2 x 10" mm Nm at 450°C in vacuum. 

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... 

The superconductivity and structure of some ternary molybdenum sulphides 258 the non-stoicheiometry of ZrS2 259 and the phase systems ZnCd-S, ZnHg-S, and CdHg-S260 have all been investigated. The reaction of carbon disulphide with the metals of the transition groups IV, V, and VI, has been studied.261 In most cases, the product of the reaction at 800—1000 °C is a sulphide (or more rarely a mixture of two sulphides), but in the case of the metals niobium and tantalum a mixture of carbides is produced. 

Riess and Hubert-Pfalzgraf " measured the temperature and concentration dependence of the NMR spectra of niobium and tantalnm pentamethoxides in nonpolar solvents like toluene, carbon disulphide, and octane at ambient temperature and at low temperatures (-11 to -74°C) and at different concentrations and confirmed that at low temperatures and higher concentrations methyl protons having peak intensity ratio 2 2 1 are obtained. This is consistent with the X-ray structure of [(MeO)4Nb(/r-OMe)2Nb(OMe)4]. The coalescence of these peaks occurs at higher temperature (10°C) and low concentration (0.01 m). 

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