For nuclear submarines
| Atomic Number: | 72 | Atomic Symbol: | Hf |
| Atomic Weight: | 178.49 | Electron Configuration: | 2-8-18-32-10-2 |
| Shells: | 2,8,18,32,10,2 | Filling Orbital: | 5d2 |
| Melting Point: | 2150oC | Boiling Point: | 5400oC |
| Description: | Silver colored transition metal. |
| Uses: | Used in reactor control rods because of its ability to absorb
neutrons. |
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. Most zirconium minerals contain 1 to 5 percent hafnium.
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. Almost all hafnium metal now
produced is made by reducing the tetrachloride with magnesium or with sodium
(Kroll Process).
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 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.
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.
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.
Handling
Finely divided hafnium is phosphoric 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).
Costs
The price of the metal is in the broad range between $100/lb and
$500/lb, depending on purity and quantity. The yearly demand for hafnium in the
U.S. now exceeds 100,000 lb.