The existence of hafnium (Hf) was predicted many years before it was first identified in zircon from Norway, by means of X-ray spectroscopic analysis in 1923 by Coster and von Hevesey. Before that time, it was previously thought to be present in various minerals and concentrations. Based on the Bohr theory, Hafnium was expected to be associated with zirconium. Most zirconium minerals contain 1 to 5% hafnium.
Von Hevesey and Jantzen originally separated hafnium from zirconium by repeated recrystallisation of the double ammonium or potassium fluorides. Van Arkel and deBoer were the first to produce metallic hafnium by passing the vapour of the tetraiodide over a heated tungsten filament. Most hafnium metal produced is made by reducing the tetrachloride with magnesium or with sodium known as the Kroll Process.
Hafnium and zirconium are two of the most difficult elements to separate. Their chemistries are almost identical, except for there densities where hafnium is double that of zirconium. It is not surprising that zirconium is the major impurity in hafnium and vice versa.
Hafnium is ductile and has a brilliant silver lustre. The metal has a close-packed hexagonal crystal structure. It exhibits excellent mechanical properties and has extremely good corrosion resistance. However its properties are greatly influenced by the extent of zirconium impurity.
Fine divided hafnium powder is pyrophoric and can spontaneously ignite in air.
Hafnium is resistant to concentrated alkalis, but will react with oxygen, nitrogen, carbon, boron, sulphur and silicon, with halogens directly reacting to form tetrahalides at elevated temperatures.
Hafnium has good absorption cross-section for thermal neutrons (approximately 600 times that of zirconium).
Due to its good neutron absorbing properties, it is used in control rods in nuclear reactors.
Hafnium carbide is the most refractory of all the binary materials, while hafnium nitride is the most refractory of all the metal nitrides.
Other applications that hafnium is used in include:
• Gas-filled and incandescent lamps
• As a getter for oxygen and nitrogen
Primary author: AZoM.com