Typical Composition of Technical Molybdic Oxide

Because of the abundance of naturally-occurring molybdenite, there is little real incentive for the synthesis of molybdenum disulphide, but it has been synthesised in small quantities. In most earlier work syntheses have been carried out only for research purposes, either to investigate the synthesis reactions themseives or to compare the properties of natural and synthetic material. Larger quantities seem to have been synthesized only when a country with insufficient natural sources wanted to ensure a reliable indigenous supply. 

Several different processes have been used, the simplest being by the reaction of hydrogen sulphide with molybdenum pentachloride, or the reaction of sulphur vapour with molybdic oxide or molybdenum metal. The last of these processes has been called the SHS process (Self-Propagating High-Temperature Synthesis) and Russian workers have reported that the product is less contaminated with impurities and has almost identical lubricating properties to natural molybdenum disulphide. The crystal structure is considered in more detail later, but it seems probable that the initial product of syntheses has a disordered 

The differences between synthetic and naturally-occurring molybdenum disulphide are considered later. Unless otherwise specified, information in this book relates to the hexagonal crystal form obtained from natural molybdenite. 

Molybdenum is the element of atomic number 42, and its atomic weight is 95.95. It is a shiny grey metal, resembling steel in appearance, but with an unusually high melting point of 2610 C. It has seven known natural stable isotopes whose mass numbers are 98, 96, 92, 95, 100, 97 and 94 in decreasing order of abundance. It was first isolated by P.J. Hjelm in 1782 from molybdenite, which he converted to oxide and then reduced by heating with charcoal  

Molybdenum is in Sub-Group VI A/B of the Periodic Table, and in the second series of transition elements. Transition elements are those which have an incomplete inner orbit in their atomic structure (see Table 3.1), and such an incomplete orbit is less stable than a filled orbit. The result is that the transition elements, and their compounds, show resemblances to each other and peculiarities in comparison with non-transition elements. It is therefore interesting that a number of compounds of other transition elements have been studied for solid lubricant use, and some of them have been found to be very effective, but no-one has yet shown any particular relationship between transition element structures and lubricating performance. The electron orbital assignments for these various elements are shown in Table 3.1. 

---

Table 2.6 Typical Composition of Technical Molybdic Oxide...

---