There is nothing more practical than a good theory.
At the dawn of the last century, Niels Bohr predicted that the last rare-earth metal would be element number 72, similar to zirconium. IN 1923, D. Koster and D. de Hevesy discovered this element with the properties Bohr predicted in Scandinavian zircons. Zirconium ores are still the source of hafnium today. Hafnium takes its name from the ancient Roman name of Copenhagen, Hafnia... In the 21st century, annual global extraction of hafnium exceeds 70 tons, which is proportional to zirconium production. Most hafnium is mined in Australia and South Africa. The price of hafnium 99% purity is about $ 780 per kilogram.
Hafnium (symbol Hf) is a heat-resistant, silver-steel-colored metal. Its crystal lattice has a hexagonal structure and its density exceeds 13 g/cm³. The melting point is 2150 °C. The thermal neutron capture cross-section is about 100 barns, five hundred times that of zirconium. Therefore, zirconium used for nuclear power must be purified of hafnium. The natural isotope 174 Hf is α-active, with a half-life of about 2x1015 years.
|Atomic number||Atomic (molar) mass g/mol||Oxidation state||Density [g/cm3]||Melting point t ° C||Boiling point t ° C||Melting point kJ/kg|
Nuclear power engineering. Ability to capture neutrons determines the use of hafnium in control rods of fuel rods, special ceramics and glass (oxide, boride, oxocarbide, carbide, lithium hafnut, dysprosium hafnut). Ta-W-Hf alloy is the best for feeding fuel in gas-phase nuclear engines.
Optics. Owing to their heat resistance and very high degree of refraction HfO2 and HfF4 are used in fibre optics, astronomic and multilayer X-ray mirrors as well as in night vision devices.
Radioelectronics. Low work of electron yield (3.53 eV) allows to produce from Hf cathodes of powerful radio lamps and electron guns and effective thermionic electrogenerators.
Electrical Engineering. Along with heat resistance, the small work of electron output allows to use Hf in welding electrodes for argon, especially for welding of low-carbon steel inCO2 atmosphere where such electrodes appeared to be almost 4 times more resistant than tungsten ones. Let us also note a great economic effect of hafnium-tantalum alloy application for the electrodes of oxygen-flame and air-plasma metal cutting. The alloy has Hf - 77%, Ta - 20%, W - 2%, Ag - 0,5%, Cs - 0,1%, Cr - 0,4% with a ninefold service life compared to pure hafnium.
Alloys. Adding 1% Hf to aluminum significantly strengthens the alloy, increases the tensile strength, shear and torsion, improves resistance to vibration and shock Hafnium dramatically strengthens cobalt alloys, so necessary in modern processing industries. Hafnium carbide and boride (melting point 3250°C) are used in superhard alloys, in the construction of nuclear gas boilers and in the production of rocket nozzles as well as in wear-resistant coatings. HfB2+ Ni alloy is relevant as an exceptionally wear-resistant composite coating. The alloy of hafnium carbide 20% and tantalum carbide 80% is the most temperature-resistant material known (melting point 4216 °C).
Mechanical engineering. Titanium alloys alloyed with hafnium serve as the basis for parts of ship engines. Addition of hafnium to nickel makes the alloy stronger, increases corrosion resistance, facilitates welding and seals the weld. The addition of hafnium to tantalum leads to the formation of a protective film of complex oxides on the surface, which increases resistance to oxidation to heat shocks (thermal shock). Alloys of these two metals were used to create materials for jet engines - gas rudders, nozzles
Other applications. Hafnium is alloyed with such rare-earth elements as terbium and samarium when creating powerful permanent magnets. HfO2-based dielectrics with high dielectric constant are now successfully replacing silicon oxide in computers, allowing much higher fill densities on chips. Hafnium silicide dielectrics also have significant dielectric permittivity. Hafnium and scandium alloys are known in microelectronics as the basis for resistive films.
Content limits for controlled impurities, %, max.
Hf-178m2 is a byproduct of the nuclear power industry (spent absorbing hafnium rods in nuclear power plants). This isotope (half-life of 31 years) contains excess energy that can be released by irradiation of the nucleus. The energy contained in 1g. Hf-178m2 is equivalent to 50 kg of TNT. It can be used to "pump up" compact battlefield lasers and as a regulated gamma ray source, for example in defectoscopy and as a capacitive energy source for transportation.
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