Molybdenum

Chemical Formula

Topics Covered

Background

Originally molybdenum was confused with graphite and lead ore, and was not prepared till 1782 by Hjelm in the impure state. Molybdenum does not occur native, and is obtained mainly from molybdenite (MoS₂). Other minor commercial ores of molybdenum are powellite (Ca(MoW)O₄) and wulfenite (PbMoO₄). It may also be recovered from copper and tungsten operations as a by-product.

The metal is prepared from the powder made by the hydrogen reduction of purified molybdic trioxide or ammonium molybdate. Molybdenum the metal is silvery-white, and very hard. However, it is softer and more ductile than tungsten and is readily worked or drawn into very fine wire. It cannot be hardened by heat treatment, only by working. It exhibits a high elastic modulus and a very high melting point. Above temperatures of 760°C (1400°F) molybdenum the metal forms an oxide that evaporates as it is formed and its resistance to corrosion is high. It has a low thermal expansion and its heat conductivity is twice that of iron. It is one of the few metals that has some resistance to hydrofluoric acid.

Key Properties

Molybdenum is a refractory metal typically used in high temperature applications. Key properties include:

· Low co-efficient of thermal expansion (5.1x10^-6 m/m/°C) which is about half that of most steels

· Good thermal conductivity

· Good electrical conductivity

· Good stiffness, greater then that of steel (Young’s Modulus 317MPa)

· High melting point (2615°C)

· Good hot strength

· Good strength and ductility at room temperature

· High density (10.2 g/cm³)

Its ability to withstand high temperatures and maintain strength under these conditions are responsible for the fact that molybdenum finds most of its application at elevated temperatures. In fact, it can work at temperatures above 1100°C (in non-oxidising conditions), which is higher than steels and nickel-based superalloys.

When exposed to temperatures in excess of 760°C in air rapid oxidation can result. Under these conditions, the oxide layer sublimes and the base metal is attacked. Thus, molybdenum performs best in inert of vacuum environments.

Applications

Molybdenum Metal

Molybdenum metal is used in:

· Alloying agent – contributing hardenability, toughness to quenched/tempered steels. It also improves the strength of steels at high temperatures (red-hardness).

· In nickel-based alloys (such as Hastelloys^®) and stainless steels it imparts heat-resistance and corrosion-resistance to chemical solutions.

· Electrodes for electrically heated glass furnaces and forehearths.

· Nuclear energy applications, as missile and aircraft parts (where high temperature resistance is vital).

· As a catalyst in the refining of petroleum.

· As a filament material in electronic/electrical applications.

· As a support members in radio and light bulbs.

· Arc resistant electric contacts.

· Thermocouple sheaths

· Flame- and corrosion-resistant coatings for other metals (generally arc deposited for metallising).

Molybdenum Compounds

Molybdenum and its compounds are used in:

· Molybdenum sulphide and selenites are used as a high temperature lubricant in favour to petroleum based oils, due to its superior high temperature resistance.

· Sodium molybdate (Na₂MoO₄) in the anhydrous form is used as a dry powdered fertiliser.

· Calcium molybdate (CaMoO₄), Molyte, molybdic oxide, molybdenum-chromium are used as sources of molybdenum for steels.