Hafnium
 |
Atomic Number: |
72 |
|
Melting
Point: |
2233 ºC |
Atomic Symbol: |
Hf |
|
Boiling
Point: |
4603 ºC |
Atomic Weight: |
178.49
amu |
|
Density: |
13310
kg/m 3
|
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
Hafnium has a total of 31 isotopes. Of these 31 isotopes, six are considered
stable. The remainder of the isotopes are radioactive. All of these radioactive
isotopes are man-made. Half-lifes of the radioactive isotopes range from one
second to about two years and have various modes of radioactive decay. The decay
modes exhibited by the hafnium isotopes encompass all major decay modes
including electron capture, beta-plus emission, beta-minus emission, and alpha
decay.
Hazards
Finely divided hafnium is pyrophoric and can ignite spontaneously in air.
Care should be taken when machining the metal or when handling hot sponge
hafnium.
Exposure to hafnium should not exceed 0.5 mg/hr. (8 hour time-weighted
average - 40-hour week).