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History

The 1869 periodic table by Mendeleev had implicitly predicted the existence of a heavier analog of titanium and zirconium, but in 1871 Mendeleev placed lanthanum in that spot.

The existance of a gap in the periodic table for an as-yet undiscovered element 72 was predicted by Henry Moseley in 1914.  Hafnium (Latin Hafnia for "Copenhagen", the home town of Niels Bohr)  was discovered by Dirk Coster and Georg Charles von Hevesy in 1923 in Copenhagen, Denmark, validating the original 1869 prediction of Mendeleev.  Soon after, the new element was predicted to be associated with zirconium by using the Bohr theory and was finally found in zircon through X-ray spectroscopy analysis in Norway.

It was separated from zirconium through repeated recrystallization of double ammonium or potassium fluorides by Jantzen and von Hevesey. Metallic hafnium was first prepared by Anton Eduard van Arkel and Jan Hendrik de Boer by passing hafnium tetraiodide vapor over a heated tungsten filament.  This process for differential purification of Zr and Hf is still in use today.

The Faculty of Science of the University of Copenhagen uses in its seal a stylized image of hafnium.

Characteristics

Hafnium is a lustrous, shiny silvery-gray,  tetravalent, ductile transition metal that is corrosion resistant and chemically similar to zirconium.  The properties of hafnium are markedly affected by zirconium impurities and these two elements are amongst the most difficult to separate.  A notable physical difference between them is their density (zirconium is about half as dense as hafnium), but chemically the elements are extremely similar.  Nevertheless, separation of them becomes important in the nuclear power industry, as Zr is common fuel-rod cladding alloy material, with the desirable properties of low neutron cross section and high chemical stability at high temperatures.   However, because of Hf's neutron absorbing properties, hafnium impurities in zirconium cause it to be far less useful for nuclear reactor materials applications.

Hafnium carbide is the most refractory binary compound known and hafnium nitride is the most refractory of all known metal nitrides with a melting point of 3310^oC.   This has led to proposals that hafnium or Hf carbides might be useful as construction materials subjected to very high temperatures.

The metal is resistant to concentrated alkalis, but halogens react with it to form hafnium tetrahalides.  At higher temperatures hafnium reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. 

The nuclear isomer Hf-178-m2 is also a source of cascades of gamma rays whose energies total to 2.45MeV per decay.  It is notable because it has the highest excitation energy of any comparably long-lived isomer of any element.  One gram of pure Hf-178-m2 would contain approximately 1330 megajoules of energy, the equivalent of exploding about 317 kilograms (700 pounds) of TNT.  Possible applications requiring such highly concentrated energy storage are of interest. For example, it has been studied as a possible power source for gamma ray lasers.

Occurrence

Hafnium is estimated to make up about 0.00058% of the Earth's upper crust by weight.   It is found combined in natural zirconium compounds but it does not exist as a free element in nature.  Minerals that contain zirconium, such as alvite [(Hf, Th, Zr) SiO₄·H₂O, thortveitite and zircon (ZrSiO₄), usually contain between 1 and 5% 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 (IV) chloride with magnesium or sodium in the Kroll process.

Applications

Hafnium is used to make control rods for nuclear reactors because of its ability to absorb neutrons (its thermal neutron absorption cross section is nearly 600 times that of zirconium), excellent mechanical properties and exceptional corrosion-resistance properties.

Other Uses

• In gas-filled and incandwscent lamps, for scavenging oxygen and nitrogen,
• As the electrode in plasma cutting because of its ability to shed electrons into air,
• and in iron, titanium, niobium, tantalum, and other metal alloys.
• A hafnium-based compound is a candidate for high-k dielectric gate insulators in future generations of integrated circuits.  Intel and IBM through separate laboratory tests found that certain hafnium compounds (eg. oxide, silicate) can be used as more effective insulators than silicon dioxide and are planning to use the element to produce faster and more energy efficient chips, by allowing circuitry scaling to be reduced to less than 45 nanometers.
• DARPA has been intermittently funding programs in the US to determine the possibility of using a nuclear isomer of hafnium (the above mentioned Hf-178-m2) to construct small, high yield weapons with simple x-ray triggering mechanisms—an application of induced gamma emission.  That work follows over two decades of basic research by an international community into the means for releasing the stored energy upon demand. There is considerable opposition to this program, both because the idea may not work and because uninvolved countries might perceive an imagined "isomer weapon gap" that would justify their further development and stockpiling of conventional nuclear weapons.  A related proposal is to use the same isomer to power Unmanned Aerial Vehicles, which could remain airborne for weeks at a time.

Compounds

Melting near 3890°C, hafnium carbide, HfC, has the highest melting point of any known two-element compound.  Hafnium nitride, HfN, also has a high melting point, around 3305°C. Other hafnium compounds include: hafnium chloride, HfCl₄, hafnium fluoride, HfF₄, and hafnium oxide, HfO₂.

Isotopes

Precautions

Care needs to be taken when machining hafnium because, like its sister metal zirconium, when hafnium is divided into fine particles, it is pyrophoric and can ignite spontaneously in air.

Compounds that contain this metal are rarely encountered by most people and the pure metal is not normally toxic but all its compounds should be handled as if they are toxic (although there appears to be limited danger to exposed individuals).