|Atomic mass||178,49 u|
|Density||13,28 g/cm³ (at 25 °C)|
|Melting point||2506 K (2233 °C)|
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This paramagnetic transition metal with the symbol Hf shines in silver-grey and is located in the 4th subgroup of the periodic table. Its features are very similar to the neighbouring zirconium and it was one of the last stable elements in the periodic system to be discovered.
The chemical element hafnium was discovered in the city of Kopenhagen that was also known by its Latin name “Hafnia”. First clues to the existence of another element occurred during the examination of the Moselysch law found in 1912. The nuclear physicist Nils Bohr predicted in his work on the nuclear theory issued in 1922 that element 72 most likely would be similar to zirconium. The element finally was discovered in 1923 by means of x-ray spectroscopy of Norwegian zirconium. Further examinations revealed that hafnium always contains zirconium bearing minerals. Jantzen and Hevesy separated hafnium from zirconium through repeated crystallisation of diammonium- and dipotassium-fluorides. By causing a reaction with sodium elementary hafnium was extracted.
Hafnium is an element that is seldom found on the continental earth crust and it is comparable to bromine and caesium. The zircon minerals zircon and allendeit contain hafnium in small amounts, mostly at 2% of the zirconium rate. The zircon variety alvit is one of the few minerals that contain more hafnium than zirconium. The largest hafnium reservoirs are located in Australia and South Africa.
For the separation of hafnium from zirconium extraction techniques are applied, using the different solubility of particular salts of zirconium and hafnium in special solvents. Other means of separation are fractional distillation ion exchange. Later, the separated hafnium can be transferred into hafnium chloride by the Kroll-process. Elementary hafnium is extracted by reduction with sodium or magnesium, by applying the Van-Arkel-de-Boer-process hafnium of even higher purity can be produced.
Highly pure hafnium is soft and flexible, thus it can be forged or rolled. Contamination with oxygen, nitrogen or carbon will make the material brittle. Subsequent processing of hafnium is difficult due to the fact that the reactive base metal reacts with oxygen when heated. Further non-metals like nitrogen, carbon, boron and silicon form compounds. An oxide layer passivates the metal and protects it against further oxidation at room temperature, because of this oxide layer hafnium resists acids.
Due to its difficult and cost-intensive extraction hafnium is only used in small amounts, mainly in nuclear technology. Because of its reaction speed with oxygen and nitrogen it is used to remove smallest amounts of these materials from ultrahigh vacuum systems. When burning, the material emits a very bright light. Hafnium powered flashlights therefore have a very high light output. As an alloying element in other metals like niobium, tantalum, molybdenum and tungsten it increases stability.
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