Hafnium is a chemical element that has the symbol Hf and atomic number 72. A lustrous, silvery gray tetravalent transition metal, hafnium resembles zirconium chemically and it is found in zirconium minerals. Hafnium is used in tungsten alloys in filaments and electrodes, in integrated circuits as a gate insulator for transistors, and as a neutron absorber in control rods in nuclear power plants.
Hafnium is a shiny silvery, ductile metal that is corrosion resistant and chemically similar to zirconium. The physical properties of hafnium are markedly affected by zirconium impurities, and these two elements are among the most difficult ones to separate. A notable physical difference between them is their density (zirconium being about half as dense as hafnium), but chemically the elements are extremely similar.
The most notable physical property of hafnium is that it has a very high neutron-capture cross-section, and several isotopes of hafnium nuclei can absorb multiple neutrons. This makes hafnium a good material for use in the control rods for nuclear reactors. Its neutron-capture cross-section is about 600 times that of zirconium. (Other elements that are good neutron-absorbers for control rods are cadmium and boron.)
Separation of hafnium and zirconium becomes very important in the nuclear power industry, since zirconium is a good fuel-rod cladding metal, with the desirable properties of a very low neutron capture cross-section, and a good chemical stability at high temperatures. However, because of hafnium's neutron-absorbing properties, hafnium impurities in zirconium would cause it to be far less useful for nuclear reactor materials applications. Thus a nearly-complete separation of zirconium and hafnium is necessary for their use in nuclear power.
Hafnium carbide is the most refractory binary compound known, with a melting point >3890 °C, and hafnium nitride is the most refractory of all known metal nitrides, with a melting point of 3310 °C. This has led to proposals that hafnium or its carbides might be useful as construction materials that are subjected to very high temperatures.
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