Definitions

hafnium

hafnium

[haf-nee-uhm, hahf-]
hafnium, metallic chemical element; symbol Hf; at. no. 72; at. wt. 178.49; m.p. about 2,227°C;; b.p. 4,602°C;; sp. gr. 13.31 at 20°C;; valence +4. Hafnium is a lustrous, ductile, silvery metal with a hexagonal, close-packed crystalline structure. Its chemical properties are almost identical to those of zirconium, the element directly above it in Group 4 of the periodic table. The two elements are among the most difficult to separate—zirconium is almost always an impurity in hafnium and affects its physical properties. Finely powdered hafnium can spontaneously ignite in air; because of this reactivity the metal has found use in the manufacture of light bulbs and vacuum tubes as a scavenger for small amounts of oxygen and nitrogen. Hafnium reacts directly with the halogens to form tetrahalides, and when heated it reacts with carbon, boron, sulfur, and silicon. Hafnium carbide is a refractory material with an extremely high melting point. Hafnium metal is produced by the Kroll process, in which a hafnium tetrahalide is reacted with magnesium or sodium metal. Because it is a good neutron absorber, hafnium metal is often used for nuclear reactor control rods. It has been alloyed with several other metals, among them iron and titanium. Hafnium is found widely distributed in nature, usually in association with zirconium minerals such as zircon. The existence of hafnium was suspected for many years before it was demonstrated (1923) through X-ray spectroscopic analysis by Dirk Coster and Georg von Hevesy. They named the element for Hafn, Latin for Copenhagen, the city where they had made the discovery.
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. It was predicted by Mendeleev in 1869 and in 1914 Henry Moseley noticed that element 72 was still missing. Hafnium became the second-to-last element with stable isotopes to be discovered, when it was found by Dirk Coster and Georg von Hevesy in 1923 in Copenhagen, Denmark, who originally named it Hafnia after the Latin name of "Copenhagen".

Hafnium is used 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. The main use of hafnium is complementary to that of zirconium in nuclear power plants. Zirconium is used for its low neutron capture rate in fuel rods, while hafnium is used for its high neutron capturing rate in control rods. Some superalloys used for special applications contain hafnium in combination with niobium, titanium, or tungsten.

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 (element 57) in that spot.

The exact placement of the elements and the location of missing elements was done by determining the specific weight of the elements and comparison of chemical and physical properties. The x-ray spectroscopy done by Henry Moseley in 1914 showed a direct dependency between spectral line and effective nuclear charge, which determines the place within the periodic table. With this method he determined the number of lanthanides and showed the gaps in the atomic number sequence at numbers 43, 61, 72, and 75..

The discovery of the gaps lead to a extensive search for the missing elements. Several people claimed the discovery after Henry Moseley predicted the gap in the periodic table for the then-undiscovered element 72 in 1914. Georges Urbain claimed that he found element 72 in the rare earth elements in 1907 and published his results on celtium in 1911. Neither the spectra nor chemical behaviour matched with the later found element, and therefore the claim was turned down after a long standing controversy. The controversy was partly due to the fact that the chemists favoured the chemical techniques which lead to the discovery of celtium while the physicists relied on the use of the new x-ray spectroscopy method, which proved that the the substances of Urbain did not contain element 72.

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. Other separation method was used for the first preparation of metallic hafnium 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.

By 1923, four predicted elements were stil missing. 43 (technetium) and 61 (promethium) are radioactive elements and are only present in trace amounts in the enviroment, which made 75 rhenium and 72 Hafnium the last two missing non radioactive elements. As Rhenium was discovered in 1925, hafnium was the next to last element with stable isotopes to be discovered. The Faculty of Science of the University of Copenhagen uses in its seal a stylized image of hafnium.

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 thermal neutron-capture cross-section, and nuclei of several hafnium isotopes can each absorb multiple neutrons.

Isotopes

At least 34 isotopes of hafnium have been observed, ranging in mass number from 153 to 186. The five stable isotopes are in the range of 176 to 180. The least stable of the synthetic is 153Hf with a half-life of 400 ms, and the most stable is 174Hf with a half-life of 2.0 petayears (1015 years).

The nuclear isomer 178m2Hf is also a source of cascades of gamma rays whose energies total 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 this pure isotope could release 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.

Chemistry

As a tetravalent transition metal, hafnium forms various inorganic compounds, generally in the oxidation state of +4. 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. Due to the lanthanide contraction zirconium and hafnium have nearly identical ion radii. The ionic radius of Zr4+ is 0.79 Ångström and that of Hf4+ is 0.78 Ångström. This similarity yields an nearly identical chemical behaviour and in the formation of similar chemical compounds.

The chemistry of hafnium is so similar to that of zirconium that a separation on chemical reactions was not possible, only the physical properties of the compounds differ. The melting points and boiling points of the compounds and the solubility in solvents is the major difference in the chemistry of twin elements.

Like zirconium hafnium reacts with halogens forming the tetrahalogen compound with the oxidation state of +4 for hafnium. Hafnium(IV) chloride and hafnium(IV) iodide have some applications in the production and purification of hafnium.

The white hafnium oxide (HfO2), with a melting point of 2812 °C and a boiling point of roughly 5100°C, is very similar to zirconia, but slightly basic.

Hafnium carbide is the most refractory binary compound known, with a melting point over 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.

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.

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.

Production

The placer deposits of the titanium ores ilmenite and rutile yield most of the mined zirconium and therefore also most the hafnium.

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 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 applications. Thus a nearly complete separation of zirconium and hafnium is necessary for their use in nuclear power. The production of hafnium free zirconium is the main source for hafnium.

Several details contribute to fact that there are only a few technical uses for hafnium. First the close similarity between hafnium and zirconium makes it possible to used zironium for most of the applications. Second, hafnium was first available as pure metal after the use in the nuclear industry for hafnium free zirconium in the 1950s. Further more the lowe abundance, and the difficult seperation technics necessary make it a scarce commodity.

Hafnium and zirconium have nearly identical chemistry, which makes the two difficult to separate. The first used methods of fractionated crystallisation of ammonium fluoride salts or the fractionated distillation of the chloride where not suitable for a industrial scale production. After zirconium was chosen as material for the nuclear reactor programme in the 1940s, a separation method had to be developed. Liquid-liquid extraction processes with a wide variety of solvents were developed and are still used for the production of hafnium. About half of all hafnium metal manufactured is produced as a by-product of zirconium refinement. The end product of the separation is hafnium(IV) chloride. The conversion to the matal is done through reducing hafnium(IV) chloride with magnesium or sodium in the Kroll process.

HfCl4 + 2Mg (1100 °C) → 2MgCl2 + Hf

Further purification is done by a chemical transport reaction developed by Arkel and de Boer. In a closed vessel hafnium reacts with iodine at temperatures of 500 °C forming hafnium tetraiodide, at a tungsten filament of 1700 °C the reverse reaction happens and the iodine and hafnium is set free. The hafnium forms a solid coating at the tungsten filament and the iodine can react with additional hafnium resulting a steady turn over.

Hf + 2I2 (500 °C) → HfI4
HfI4 (1700 °C) → Hf + 2I2

Applications

Most of the hafnium produced ends up in the production of control rod for nuclear reactors.

Nuclear reactors

The nuclei of several hafnium isotopes can each 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's. (Other elements that are good neutron-absorbers for control rods are cadmium and boron.) Excellent mechanical properties and exceptional corrosion-resistance properties allow its use in the harsh environment of a pressurised water reactors. The German research reactor FRM II uses Hafnium as neutron absorber.

Alloys

Hafnium is used in iron, titanium, niobium, tantalum, and other metal alloys. An alloy used for liquid rocket thruster nozzles, for example the main engine of the Apollo Lunar Modules is C130, which consists of 89% niobium, 10% hafnium and 1% titanium.

Small additions of hafnium increase the adherence of protective oxide scales on nickel based alloys. It improves thereby the corrosion resistance especially under cyclic temperature conditions that tend to break oxide scales by inducing thermal stresses between the bulk material and the oxide layer.

Other uses

The affinity to oxygen and its heat resitance makes hafnium a good scavenger for oxygen and nitrogen in gas-filled and incandescent lamps. Hafnium is also used as the electrode in plasma cutting because of its ability to shed electrons into air, The electronics industry discovered that hafnium-based compound can be 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.

The high energy content of Hf-178-m2 is the concern of a DARPA funded programs in the US. This program should 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 months at a time.

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.

See also

References

External links

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