Hafnium

Hafnium is a chemical element with atomic number 72 that is in group 4 of the periodic table of elements and is symbolized as Hf.

It is a shiny, silvery-grey transition metal, chemically very similar to zirconium, being found in the same minerals and compounds, and being difficult to separate. It is used in alloys with tungsten in filaments and in electrodes. It is also used as a nuclear reactor control rod material due to its neutron absorbing ability. Recently, it has become the material used to manufacture the transistors of the processors of the well-known Intel brand.

Main properties

It is a ductile, shiny, silvery metal that is resistant to corrosion; chemically very similar to zirconium. These two elements have the same number of electrons in their outer shells and their ionic radii are very similar due to lanthanide contraction. That is why they are very difficult to separate (geological processes have not separated them and in nature they are found together) and there are no other chemical elements that are more similar to each other. The only applications for which it is necessary to separate them are those in which they are used for their neutron absorbing properties, in nuclear reactors.

Hafnium carbide (HfC) is the most refractory binary compound known, with a melting point of 3,890 °C, and hafnium nitride (HfN) is the most refractory of all known metal nitrides, with a melting point of 3310 °C. Mixed carbide of hafnium and tantalum (Ta4HfC5) is the multiple compound with the highest known melting point, 4215 °C.

Hafnium is resistant to concentrated bases, but halogens can react with it to form hafnium tetrahalides (HfX₄). At high temperatures it can react with oxygen, nitrogen, carbon, boron, sulfur and silicon.

Applications

Hafnium is used to make control rods used in nuclear reactors, such as those found in nuclear submarines, because the neutron capture cross section of hafnium is about 600 times that of zirconium, giving it a high neutron absorption capacity, and also has very good mechanical properties, as well as high resistance to corrosion. Other applications:

• In gas and incandescent lamps.
• In catalysts for metal polymerization.
• To remove oxygen and nitrogen from vacuum tubes.
• In iron alloys, titanium, niobium, tantalin and other metal alloys.
• In January 2007, it was announced as a fundamental part of a new microprocessor technology, developed separately by IBM and Intel, to replace silicon but only in the transistor's dielectric gate, the rest of the device continues to use silicon that is the traditional base material. [1][2]
• One of its derivatives, in particular hafnio oxide, has an intermediate refractive index between silicon and air. This compound is used in the transition between these two interfaces on silicon phone devices, thus reducing the losses due to reflections.

HISTORY

Its existence was predicted, using Bohr's theory, which would be associated with the zirconium, and finally found in the zircon by analysis with X -ray spectroscopy in Norway.

Photo recording of X-ray emission lines features of some elements.

In his report on The Periodic Law of Chemical Elements, in 1869, Dmitri Mendeleev had implicitly predicted the existence of a heavier analogue of titanium and zirconium. At the time of its formulation, in 1871, Mendeleev believed that the elements were ordered by their atomic mass and placed lanthanum (element 57) in the place that was below zirconium. The exact placement of the elements and the location of the missing elements was made by determining the specific gravity of the elements and comparing the chemical and physical properties.

X-ray spectroscopy performed by Henry Moseley in 1914 showed a direct dependence between the spectral line and the effective nuclear charge. This led to the nuclear charge, or atomic number of an element, being used to determine its place on the periodic table. Using this method, Moseley 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 led to an extensive search for the missing elements. In 1914, several people claimed the discovery after Henry Moseley predicted the hole in the periodic table for the then-unknown element 72. Georges Urbain claimed to have found element 72 in the rare earth elements in 1907 and published his results on it. Celtium in 1911. Neither the spectra nor the chemical behavior he claimed matched the element found later, so his claim was rejected after a long controversy. The controversy arose, in part, because chemists favored the chemical techniques that led to the discovery of celtium, while physicists favored the new method of X-ray spectroscopy. which showed that the substances discovered by Urbain did not contain element 72. In 1921, Charles R. Bury suggested that element 72 must resemble zirconium and therefore was not part of the rare earth group of elements. By early 1923, Niels Bohr and others agreed with Bury. These suggestions were based on Bohr's theories of the atom, which were identical to those of the chemist Charles Bury, Moseley's X-ray spectroscopy, and the Friedrich Paneth's chemical arguments.

Encouraged by these suggestions and by the reappearance in 1922 of Urbain's claims that element 72 was a rare earth element discovered in 1911, Dirk Coster and Georg von Hevesy were motivated to search for the new element in minerals of zirconium. Hafnium was discovered by both in 1923 in Copenhagen, Denmark, validating Mendeleev's original 1869 prediction. It was eventually found in Norwegian zircon by X-ray spectroscopy analysis. The location of the discovery led to the element being named after the Latin name for "Copenhagen", Hafnia , the hometown of Niels Bohr. Today, the Faculty of Science at the University of Copenhagen uses a stylized image of the hafnium atom on its sign.

Hafnium was separated from zirconium by repeated recrystallization of ammonium or potassium double fluorides by Valdemar Thal Jantzen and von Hevesey. Anton Eduard van Arkel and Jan Hendrik de Boer were the first to prepare metallic hafnium by passing hafnium tetraiodide vapor over a heated tungsten filament in 1924. This process for the differential purification of zirconium and hafnium is still used in today.

In 1923, six predicted elements were still missing from the periodic table: 43 (technetium), 61 (promethium), 85 (astatine), and 87 (francium) are radioactive elements and are only present in trace amounts in the environment, thus making elements 75 (rhenium) and 72 (hafnium) the last two unknown non-radioactive elements.

Abundance and production

Cast tip of a hafnio consumable electrode used in a beam of unconsolidated electrons, a cube of 1 cm and an oxidized lingo of hafnio recast by electron beam (from left to right).

It is always found together with zirconium in its same compounds, but it is not found as a free element in nature. It is present, as mixtures, in zirconium minerals, such as zircon (ZrSiO₄) and other varieties of it (such as albite), containing between 1 and 5% hafnium.

Heavy mineral sands deposits of ilmenite and rutile titanium ores produce most of the mined zirconium, and thus also most of the hafnium.

Zirconium is a good nuclear fuel rod cladding metal, with the desirable properties of a very low neutron capture cross section and good chemical stability at high temperatures. However, due to the neutron absorbing properties of hafnium, hafnium impurities in zirconium would make it much less useful for nuclear reactor applications. Therefore, an almost complete separation of zirconium and hafnium is necessary for their use in nuclear power. Hafnium-free zirconium production is the main source of hafnium.

Hafnio oxidized ligotes that have effects from a thin optical film.

The chemical properties of hafnium and zirconium are nearly identical, making the two difficult to separate. Approximately half of all metallic hafnium produced is obtained as a byproduct of zirconium purification. This is done by reducing hafnium tetrachloride (HfCl₄) with magnesium or sodium in the Kroll process.

The methods used for the first time - fractional crystallization of ammonium fluoride salts or the fractional distillation of chloride - have not proven suitable for production on an industrial scale. Following the choice of zirconium as a material for nuclear reactor programs in the 1940s, it was necessary to develop a separation method. Liquid-liquid extraction processes with a wide variety of solvents were developed and are still used for the production of hafnium. Approximately half of all metallic hafnium manufactured is produced as a by-product of zirconium refining. The final product of the separation is hafnium(IV) chloride. Purified hafnium(IV) chloride is converted to the metal by reduction with magnesium or sodium, as in the Kroll process.

[1100^oC] Hf + 2MgCl2}}}" xmlns="http://www.w3.org/1998/Math/MathML">HfCl4+2Mg→1100orCHf+2MgCl2{displaystyle {ce {HfCl4 + 2Mg - 2005,[1100^oC] Hf + 2MgCl2}}}[1100^oC] Hf + 2MgCl2}}}" aria-hidden="true" class="mwe-math-fallback-image-inline" src="https://wikimedia.org/api/rest_v1/media/math/render/svg/aa72f37e998474c30ff0a68803a831241768a664" style="vertical-align: -1.005ex; margin-top: -0.433ex; width:34.997ex; height:4.509ex;"/>

Further purification is accomplished by a chemical transport reaction developed by Arkel and de Boer: In a closed vessel, hafnium reacts with iodine at temperatures of 500 degrees Celsius (932 °F), forming hafnium iodide(IV); at a tungsten filament of 1700 degrees Celsius (3092.0 °F) the reverse reaction occurs preferentially, and the chemically bound iodine and hafnium dissociate into the native elements. The hafnium forms a solid coating on the tungsten filament, and the iodine can react with the additional hafnium, resulting in a constant turnover of iodine and ensuring that the chemical balance is maintained in favor of hafnium production.

[500^oC] HfI4}}}" xmlns="http://www.w3.org/1998/Math/MathML">Hf+2I2→500orCHfI4{displaystyle {ce {Hf + 2I2 - 2005[500^oC] HfI4}}}}}[500^oC] HfI4}}}" aria-hidden="true" class="mwe-math-fallback-image-inline" src="https://wikimedia.org/api/rest_v1/media/math/render/svg/ac2cfa9d8a39e3fc014956d17bc17a4d7b22bbed" style="vertical-align: -1.005ex; margin-top: -0.433ex; width:20.228ex; height:4.509ex;"/>

[1700^oC] Hf + 2I2}}}" xmlns="http://www.w3.org/1998/Math/MathML">HfI4→1700orCHf+2I2{displaystyle {ce {HfI4-negative[1700^oC] Hf + 2I2}}[1700^oC] Hf + 2I2}}}" aria-hidden="true" class="mwe-math-fallback-image-inline" src="https://wikimedia.org/api/rest_v1/media/math/render/svg/f3f0df30a8edc787cb1735a3dc40ac05af6e924f" style="vertical-align: -1.005ex; margin-top: -0.432ex; width:21.05ex; height:4.509ex;"/>

Precautions

It is necessary to be careful when working with hafnium because when it is divided into small particles it is pyrophoric and can ignite spontaneously in contact with air. Compounds containing this metal are rarely in contact with most people, and the pure metal is not particularly toxic, but all its compounds should be handled as if they were toxic (although early evidence does not seem to indicate a very high risk).