There is nothing more practical than a good theory. V.I. Lenin.
At dawn of last century, Niels Bohr predicted that element number 72, similar to zirconium, would be last rare-earth metal. In 1923, D. Coster and D. de Hevesi discovered this element with properties predicted by Bohr in Scandinavian zircons. And today, zirconium ores remain source of hafnium. Name hafnium received from ancient Roman name of Copenhagen — Hafnia. In 21st century, annual global hafnium production exceeds 70 tons — proportional to production of zirconium. Most hafnium mined Australia and South Africa. Price hafnium 99% purity about $780 per kilogram.
Hafnium (symbol Hf) — heat-resistant silver-steel color metal. Its crystal lattice has hexagonal structure, density exceeds 13 g/cm³. Melting point 2150 °C. Thermal neutron capture cross section is about 100 barn, five hundred times larger than that of zirconium. Therefore, zirconium used for nuclear energy must be cleared of hafnium. Natural isotope 174 Hf is α-active; its half-life is about 2x1015 years.
|Atomic number||Atomic (molar) mass g/mol||Oxidation state||
|Melting point t °С||Boiling point t °С||Heat of fusion kJ/kg|
Nuclear energy. Ability to capture neutrons causes use of hafnium in control rods of fuel rods, special ceramics and glass (oxide, boride, oxocarbide, carbide, lithium hafnate, dysprosium hafnate). Ta-W-Hf alloy — best for fuel supply in gas-phased nuclear engines.
Optics. Due to heat resistance and very high degree of refraction, HfO2 and HfF4 are used in fiber optics, astronomical and multilayer x-ray mirrors, as well as night vision devices.
Radio electronics. Low electron work function (3.53 eV) allows production of high-power radio tubes and electron guns and effective thermionic electric generators from Hf.
Electrical engineering. Along with heat resistance, small electron work function allows Hf to be used in production of electrodes for argon welding, especially for welding low-carbon steel in CO2 atmosphere, where such electrodes were almost 4 times more resistant than tungsten. We also note great economic effect of use of hafnium-tantalum alloy for electrodes of oxygen-flame and air-plasma cutting of metals. Alloy Hf — 77%, Ta — 20%, W — 2%, Ag — 0.5%, Cs — 0.1%, Cr — 0.4%, nine-fold service life compared to pure hafnium.
Alloys. Addition of 1% Hf to aluminum significantly strengthens alloy, increases the tensile, shear and torsional strength, improves resistance to vibration and shock loads. Hafnium dramatically strengthens cobalt alloys, so necessary in modern processing industries. Hafnium carbide and boride (melting point 3250 °C) are used as part of superhard alloys, in construction of gas-phase atomic boilers and in manufacture of rocket nozzles, as well as wear-resistant spraying. HfB2 + Ni is relevant as extremely durable composite coating. Alloy of hafnium carbide 20% and tantalum carbide 80% is considered most heat-resistant of known materials (melting point 4216 °C).
Engineering. Hafnium-doped titanium alloys form basis of marine engine parts. Addition of hafnium to nickel strengthens alloy, increases corrosion resistance, facilitates welding and seals weld. Addition of hafnium to tantalum leads to formation of complex oxides on surface of protective film, which increase oxidation resistance to heat exchange (thermal shock). Based on alloys of these two metals, materials were created for jet engines — gas rudders, nozzles, etc.
Other applications. Rare earth elements such as terbium and samarium are doped with hafnium when creating powerful permanent magnets. Today, HfO2 dielectrics with high dielectric constant are successfully replacing silicon oxide in computers, allowing much higher chip density. Hafnium dielectric silicide also have significant dielectric constant. Hafnium and scandium alloys are known in microelectronics as basis of resistive films.
Content of controlled impurities,%, no more
Hf-178m2 — by-product of nuclear energy (spent absorption hafnium rods in nuclear power plants). This isotope (half-life of 31 years) contains an excess of energy that can be released by irradiation of nucleus. energy contained in 1 g of Hf-178m2 is equivalent to 50 kg of TNT. This can be used to «pump» compact combat lasers and as adjustable source of gamma rays, for example, in flaw detection and as capacious energy source for transport.
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