Hafnium (Hf), the heaviest of the three metals comprising the Group IV transition metals, is now in production. Because of the startling similarity in their chemical properties, zirconium and hafnium always occur together in nature. In their respective ability to absorb neutrons, however, they differ greatly, and this difference has led to their use in surprisingly different ways in nuclear reactors. Zirconium, with a low neutron-absorption cross section (0.18 barn), is highly desirable as a structural material in water-cooled nuclear reactor cores. Hafnium, on the other hand, because of its high neutron-absorption cross section (105 barns), can be used as a neutron-absorbing control material in the same nuclear reactor cores. Thus, the two elements, which occur together so intimately in nature that they are very difficult to separate, are used as individual and important but contrasting components in the cores of nuclear reactors.
Properties
Pure hafnium is a lustrous, silvery metal that is not so ductile nor so easily worked as zirconium; nevertheless, hafnium can be hot- and cold-rolled on the same equipment and with similar techniques as those used for zirconium. All zirconium chemicals and alloys may contain some hafnium, and hafnium metal usually contains about 2% zirconium. The melting point, 2222°C, is higher than that of zirconium, and heat-resistant parts for special purposes have been made by compacting hafnium powder to a density of 98%. The metal has a close-packed hexagonal structure. The electric conductivity is about 6% that of copper. It has excellent resistance to a wide range of corrosive environments.
Hafnium Alloys and Compounds
Hafnium forms refractory compounds with carbon, nitrogen, boron, and oxygen. Hafnium oxide, or hafnia, HfO2, is a better refractory ceramic than zirconia, but is costly.
Hafnium carbide, HfC, produced by reacting hafnium oxide and carbon at high temperature, is obtained as a loosely coherent mass of blue-black crystals. The crystals have a hardness of 2910 Vickers, and a melting point of 4160°C. It is thus one of the most refractory materials known. Heat-resistant ceramics are made from hafnium titanate by pressing and sintering the powder. The material has the general composition x(TiO2) • n (HfO2), with varying values of x and n. Parts made with 18% titania and 82% hafnia have a density of 7.197 kg/m3, a melting point at about 2204°C, a low coefficient of thermal expansion, good shock resistance, and a rupture strength above 68 MPa at 1093°C. Hafnium nitride, with a melting point of 3300°C has the highest melting point of any nitride and hafnium boride, with a melting point of 3260°C has a melting point higher than any other boride. The alloy Ta4HfC5 has the highest melting point of any substance known, about 4215°C.
