Hafnium

0.162 %

{ syn. }

5.206 %

18,606 %

27.297 %

13.629 %

35.1%

0%

Risk

Powder

Hafnium is a chemical element with the symbol Hf and atomic number 72 is named it after the Latin name of the city of Copenhagen, Hafnia, in which the element was discovered. It is a silver-gray lustrous, corrosion-resistant transition metal in the periodic table in group 4 (group 4) or titanium group.

Hafnium has very similar characteristics to the in the periodic table directly above preferred zirconium. Biological functions are not known, it does not normally occur in the human body and is not toxic.

• 8.1 hafnium ( IV) oxide
• 8.2 Further hafnium compounds

History

Hafnium was one of the last stable elements of the Periodic Table, which has been detected. The first indication of the existence of another element between lutetium and tantalum resulted from the Moseley's law in 1912 found. Henry Moseley tried in 1914, the unknown, but to be expected under this Act element with the atomic number 72 in samples of rare earth minerals to find ( lanthanides today). But he was so unsuccessful.

Niels Bohr predicted in his 1922 published work on the atomic theory that the lanthanide series with lutetium at an end and that its member must have 72 resemblance to zirconium. A year later, could be detected hafnium: 1923 it was discovered by Dirk Coster in Copenhagen and George de Hevesy by X-ray spectroscopy in Norwegian zircon. Further studies of other minerals showed that hafnium is always contained in zirconium minerals. The separation of zirconium succeeded Jantzen and Hevesy by repeated crystallization of diammonium and Dikaliumfluoride the two elements. Elemental hafnium could then be obtained by reduction with sodium.

Occurrence

Hafnium is a on earth not very common element with a content of 4.9 ppm of the Earth's continental crust. In the frequency it is therefore comparable with the elements cesium and bromine, and more often than the long-known gold and mercury. Hafnium does not appear dignified and so far only one mineral with the main ingredient hafnium is known, which was discovered in 1974 and named after this as Hafnon. In zirconium minerals such as, inter alia, zirconium and hafnium Allendeit but is often included as a second additive, wherein the amount of the most 2% of hafnium is Zirconiumgehaltes (1-5 weight percent hafnium). One of the rare minerals which contain more than hafnium zirconium, the zirconium variety Alvit [ (Hf, Th, Zr) SiO4 ].

Similar to zirconium are the main reservoirs of hafnium zirconium deposits in Australia and South Africa. The reserves are 1.1 million tons (calculated as hafnium ) estimated.

Production and representation

To obtain hafnium, it must be separated from zirconium. This is not possible during the manufacturing process, but is carried out in a separate process. For the separation extraction methods are applied. The different solubility of certain zirconium and Hafniumsalze is exploited in specific solvents. Examples of this are the different solubilities of the nitrates in tri-n- butyl phosphate and the thiocyanates in methyl isobutyl ketone. Other possible options are ion-exchange separation and fractional distillation of suitable compounds.

The separated hafnium may then in accordance with the Kroll process initially hafnium ( IV) chloride are transferred and then reduced with sodium or magnesium to elemental hafnium.

If even purer hafnium needed the van Arkel -de Boer method can be applied. This reacts during the heating under vacuum, first the hafnium with iodine to hafnium (IV ) iodide. This is decomposed on a hot wire back to hafnium and iodine.

Hafnium is only produced in small amounts on a scale of 100 tons. It is not specially made ​​, but is a by- product during the extraction of hafnium -free zirconium for the shells of the fuel rods.

Properties

Physical Properties

Hafnium is a silvery- shiny heavy metal of high density ( 13.31 g/cm3). It crystallizes in two different modifications depending on the temperature. In normal conditions it crystallizes in a hexagonal- close-packed ( α - Hf) and is thus isotypic to α -Zr, above 1775 ° C it goes into a body-centered cubic structure over ( β - Hf).

If hafnium in a high purity before, it is relatively soft and pliable. It is easy to be processed by rolling, forging and hammering. By contrast, if traces of oxygen, nitrogen or carbon in the material exists, it is brittle and difficult to process. The melting and boiling point of hafnium are 2227 ° C and 4450 ° C, the highest in the group (melting point: Titan: 1667 ° C, zirconium: 1857 ° C).

In almost all other properties of the metal resembles its lighter homologues zirconium. This is caused by the lanthanide contraction, similar atomic and ionic radii due ( Zr atomic radii: 159 pm, Hf: 156 pm ). An exception is the density at which zirconium of 6.5 g/cm3 having a significantly smaller value. A technically important difference is that hafnium 600 times better able to absorb neutrons. This is the reason that the hafnium has to be removed for the use of zirconium in nuclear power plants.

Hafnium is superconducting below the transition temperature of 0.08 K.

Chemical Properties

Hafnium is a base metal, which when heated reacts with oxygen to form hafnium oxide. And other non-metals such as nitrogen, carbon, boron and silicon compounds form under these conditions. At room temperature, quickly forms a dense oxide layer that passivates the metal and protects against further oxidation.

In most acids, hafnium being resistant because of the passivation is effected under normal conditions. In hydrofluoric acid, it corrodes quickly in hot concentrated sulfuric and phosphoric acid enters noticeable corrosion. Hydrochloric acid - nitric acid mixtures, this also includes aqua regia, hafnium should only be exposed to short-term, even at room temperature, can be calculated at 35 ° C must with removal rates of more than 3 mm / year. In aqueous base, it is stable up to a temperature of about 100 ° C, the removal of material is generally less than 0.1 mm / year.

Isotopes

From a total of 35 hafnium isotopes and 18 Kernisomere of 153Hf to 188Hf are known. Natural hafnium is a mixing element consisting of a total of six different isotopes. The most common isotope is at a frequency of 35.08 % 180Hf. This is followed with 27.28% 178Hf, 177Hf with 18.61 %, with 13.62 % 179Hf, 176Hf 174Hf with 5.27% and 0.16 %. 174Hf is weakly radioactive, an alpha emitter with a half -life of 2.1015 years. The beta emitter 182Hf formed during planet formation with a half-life of 9 million years, the stable tungsten isotope 182W, which has been exploited to reduce the formation of the moon and the Earth's core to a period within the first 50 million years.

The isotope 177Hf and 179Hf can be detected by NMR spectroscopy.

The Kernisomer 178m2Hf is durable with a half-life of 31 years while sending the decay of a strong gamma radiation of 2.45 MeV. This is the highest energy, which emits a stable isotope for a long time. A possible application is to use this source in Kernisomer as powerful lasers. 1999 Carl Collins discovered that isomer may, at irradiation with X-rays its energy all at once. However, potential applications, such as explosives, unlikely.

Use

Due to his difficult recovery, hafnium is used only in small amounts. The main application is the core technology used in the hafnium control rod to regulate the chain reaction in nuclear reactors. The use of hafnium has a number of advantages over other possible neutron absorbing substances. The element is highly resistant to corrosion and at the nuclear reaction with the neutrons occur Hafniumisotope, which also have a high absorption cross-sections. Due to the high price often occurs only for military applications in question, such as reactors in nuclear submarines.

There are a few other uses. So hafnium reacts rapidly with small amounts of oxygen and nitrogen, and can therefore be used as a getter to remove small amounts of these substances from ultra-high vacuum systems. When burning the metal emits a very bright light. Therefore, it is possible to employ hafnium flash lamps with a particularly high light output. Several very stable and high-melting compounds, especially hafnium and hafnium carbide can be prepared from the elements.

In alloys with metals such as niobium, tantalum, molybdenum and tungsten, an addition of 2% hafnium strength-increasing effect. It forms particularly stable, high-melting and heat-resistant materials.

Safety

Like many other metals, hafnium is in a finely divided state highly flammable and pyrophoric. In a compact state, it is, however, not flammable. The metal is non-toxic. For these reasons, no special safety regulations for the handling of hafnium.

Compounds

Hafnium forms a series of compounds. These are usually salts or mixed crystals, and frequently have high melting points. The most important oxidation state of hafnium IV, but there are also compounds in lower oxidation states, from 0 to III, complexes in negative oxidation states known.

Hafnium ( IV) oxide

Hafnium ( IV) oxide is a very stable and high-melting solid. It has a high relative permittivity of 25 (compared to silica: 3.9). Therefore, a use as a high-k dielectric as an insulation of the control terminal (gate) for microprocessors is possible. By further reduction of the structure width, the leakage current to an increasingly larger problem, because the miniaturization of the CMOS structures requires thinner gate insulation. Less than 2 nm thickness of the undesired leakage current through the tunneling effect increases considerably. By using a high-k dielectric can be used for leakage current reduction, the thickness of the dielectric be increased again without causing performance degradation (reduction of the switching speed ) of the transistor. So Thicker dielectrics allow a further miniaturization.

More hafnium compounds

Hafnium carbide is one of the substances with the highest melting points at all. Together with hafnium and hafnium it is one of the hard materials.

There are known some halogen compounds of hafnium. In the oxidation state IV exist in both the fluoride ( HfF4 ) and chloride ( HfCl4 ), bromide ( HfBr4 ), and iodide ( HfI4 ). Hafnium ( IV ) chloride and hafnium (IV ) iodide play a role in the recovery of hafnium. In the lower oxidation states, only chlorine and bromine compounds, and hafnium (III ) iodide are known.

The Kaliumhexafluoridohafnat (IV) K2 [ HfF6 ] as well as the Ammoniumhexafluoridohafnat (IV) (NH4 ) 2 [ HfF6 ] can be used as both salts are readily soluble than the corresponding zirconium complexes for the separation of hafnium from zirconium.