Hafnium

Hafnium is a chemical element that has the symbol Hf and atomic number 72. A lustrous, silvery gray tetravalent transition metal, hafnium resembles zirconium chemically and it is found in zirconium minerals. Hafnium is used in tungsten alloys in filaments and electrodes, in integrated circuits as a gate insulator for transistors, and as a neutron absorber in control rods in nuclear power plants.

Notable characteristics
Hafnium is a shiny silvery, ductile metal that is corrosion resistant and chemically similar to zirconium. The physical properties of hafnium are markedly affected by zirconium impurities, and these two elements are among the most difficult ones to separate. A notable physical difference between them is their density (zirconium being about half as dense as hafnium), but chemically the elements are extremely similar.

The most notable physical property of hafnium is that it has a very high neutron-capture cross-section, and several isotopes of hafnium nuclei can absorb multiple neutrons. This makes hafnium a good material for use in the control rods for nuclear reactors. Its neutron-capture cross-section is about 600 times that of zirconium. (Other elements that are good neutron-absorbers for control rods are cadmium and boron.)

Separation of hafnium and zirconium becomes very important in the nuclear power industry, since zirconium is a good fuel-rod cladding metal, with the desirable properties of a very low neutron capture cross-section, and a good chemical stability at high temperatures. However, because of hafnium's neutron-absorbing properties, hafnium impurities in zirconium would cause it to be far less useful for nuclear reactor materials applications. Thus a nearly-complete separation of zirconium and hafnium is necessary for their use in nuclear power.

Hafnium carbide is the most refractory binary compound known, with a melting point >3890 °C, and hafnium nitride is the most refractory of all known metal nitrides, with a melting point of 3310 °C. This has led to proposals that hafnium or its carbides might be useful as construction materials that are 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 2.45 MeV 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.

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 incandescent 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 employed in gate insulators in the 45 nm generation of integrated circuits from Intel, IBM and others . Hafnium oxide-based compounds are practical high-k dielectrics, allowing reduction of the gate leakage current which improves performance at such scales.
 * 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&mdash;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.

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 existence of a gap in the periodic table for a yet to be discovered element 72 was predicted by Henry Moseley in 1914. Hafnium was named for the Latin name Hafnia for "Copenhagen", the home town of Niels Bohr. It was discovered by Dirk Coster and Georg von Hevesy in 1923 in Copenhagen, Denmark, validating the original 1869 prediction of Mendeleev. Soon thereafter, the new element was predicted to be associated with zirconium by using the Bohr theories of the atom, and it was finally found in zircon through X-ray spectroscopy analysis in Norway.

Hafnium was separated from zirconium through repeated recrystallization of the 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 tetra-iodide 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.

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 as a by-product of zirconium refinement. This is done through reducing hafnium(IV) chloride with magnesium or sodium in the Kroll process.

A major source of zircon (and hence hafnium) ores are heavy mineral sands ore deposits, pegmatites particularly in Brazil and Malawi, and carbonatite intrusions particularly the Crown Polymetallic Deposit at Mount Weld, Western Australia. A potential source of hafnium is trachyte tuffs containing rare zircon-hafnium silicates eudialyte or armostrongite, at Dubbo in New South Wales, Australia.

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 (see Dragon's Breath for a demonstration). Compounds that contain this metal are rarely encountered by most people. The pure metal is not considered toxic, but hafnium compounds should be handled as if they are toxic because the ionic forms of metals are normally at greatest risk for toxicity, and limited animal testing has been done for hafnium compounds.