Hafnium

Atomic Number:	72	Melting Point:	2233 ºC
Atomic Symbol:	Hf	Boiling Point:	4603 ºC
Atomic Weight:	178.49 amu	Density:	13310 kg/m³
Atomic Radius:	156.4 pm	Oxidation States:	4
Covalent Radius:	150 pm	Electron Configuration:	[Xe]6s²4f¹⁴5d²
van der Waals Radius:	--	State of Matter:	solid

History

(Hafinia, Latin name for Copenhagen) Many years before its discovery in 1932 (credited to D. Coster and G. von Hevesey), Hafnium was thought to be present in various minerals and concentrations. On the basis of the Bohr theory, the new element was expected to be associated with zirconium.

It was finally identified in zircon from Norway, by means of X-ray spectroscope analysis. It was named in honor of the city in which the discovery was made.

It was originally separated from zirconium by repeated recrystallization of the double ammonium or potassium fluorides by von Hevesey and Jantzen. Metallic hafnium was first prepared by van Arkel and deBoer by passing the vapor of the tetraiodide over a heated tungsten filament.

Properties

Hafnium is a ductile metal with a brilliant silver luster. Its properties are considerably influenced by presence of zirconium impurities. Of all the elements, zirconium and hafnium are two of the most difficult to separate. Although their chemistry is almost identical, the density of zirconium is about half of hafnium. Very pure hafnium has been produced, with zirconium being the major impurity.

Hafnium is resistant to concentrated alkalis, but at elevated temperatures reacts with oxygen, nitrogen, carbon, boron, sulfur , and silicon. Halogens react directly to form tetrahalides.

Sources

Hafnium is found combined in natural zirconium compounds but does not exist as a free element in nature. Minerals that contain zirconium, such as alvite, thortveitite and zircon, usually contain between 1 and 5 percent hafnium. Hafnium and zirconium have nearly identical chemistry, which makes the two difficult to separate. About half of all hafnium metal manufactured is produced by a by-product of zirconium refinement. This is done through reducing hafnium tetrachloride with magnesium or sodium in the Kroll Process.

Uses

Because the element not only has a good absorption cross section for thermal neutrons (almost 600 times that of zirconium), but also excellent mechanical properties and is extremely corrosion-resistant, hafnium is used for reactor control rods. Such rods are used in nuclear submarines.

Hafnium is used in gas-filled and incandescent lamps, and is an efficient getter for scavenging oxygen and nitrogen. It is used as the electrode in plasma cutting because of its ability to shed electrons into air.

Recently, hafnium has been put into development of newer nuclear weapons by the U.S. government. There has also been recent speculation about the possibility of using nuclear isomers of hafnium (created by neutron bombardment) to construct small low yield weapons with extremely simple triggering mechanisms known as the hafnium bomb.

Hafnium has been successfully alloyed with iron , titanium , niobium , tantalum , and other metals. Hafnium carbide is the most refractory binary composition known, and the nitride is the most refractory of all known metal nitrides (m.p. 3310C). At 700 degrees C hafnium rapidly absorbs hydrogen to form the composition HfH1.86. The nuclear isomer Hf-178-2m is also a source of energetic gamma rays, and is being studied as a possible power source for gamma ray lasers. Hafnium Oxide is a candidate for High-K gate insulators in future generations of integrated circuits.

Isotopes