Hafnium thermal properties

Ceramic borides, carbides and nitrides are characterized by high melting points, chemical inertness and relatively good oxidation resistance in extreme environments, such as conditions experienced during reentry. This family of ceramic materials has come to be known as Ultra High Temperature Ceramics (UHTCs). Some of the earliest work on UHTCs was conducted by the Air Force in the 1960 s and 1970 s. Since then, work has continued sporadically and has primarily been funded by NASA, the Navy and the Air Force. This article summarizes some of the early works, with a focus on hafnium diboride and zirconium diboride-based compositions. These works focused on identifying additives, such as SiC, to improve mechanical or thermal properties, and/or to improve oxidation resistance in extreme environments at temperatures greater than 2000°C. 

Because the element not only has a good absorption cross section for thermal neutrons (almost 600 times that of zirconium), but also excellent mechanical properties and is extremely corrosion-resistant, hafnium is used for reactor control rods. Such rods are used in nuclear submarines. 

Because hafnium has a high absorption cross-section for thermal neutrons (almost 600 times that of zirconium), has excellent mechanical properties, and is extremely corrosion resistant, it is used to make the control rods of nuclear reactors. It is also applied in vacuum lines as a getter —a material that combines with and removes trace gases from vacuum tubes. Hafnium has been used as an alloying agent for iron, titanium, niobium, and other metals. Finely divided hafnium is pyrophoric and can ignite spontaneously in air. 

Zirconium and hafnium have very similar chemical properties, invariably occur together in nature, and are difficult to separate. Yet their absorption cross sections for thermal neutrons are very different ... 

ADE] Adenstedt, H. K., Physical, thermal and electrical properties of hafnium and high purity zirconium, Trans. Am. Soc. Met., 44, (1952), 949-973. Cited on page 83. 

Because of good corrosion resistance in both acids and bases, zirconium alloys are widely used in chemical plants. Commercial zirconium, as used primarily for corrosion resistance in the chemical industry [4], contains up to 4.5% hafnium, which is difficult to separate because of the similar chemical properties of zirconium and hafnium. TTie presence or absence of hafnium has no effect on the corrosion resistance, which is controlled by a very stable oxide. At ambient temperature, this passive oxide is 2-5 nm thick [4]. The pure metal low in hafnium (0.02% max) has a low thermal neutron capture, making it useful for nuclear-power applications. 

The presence of hafnium in zirconium does not significantly influence mechanical properties other than the thermal neutron cross-section. Moreover, hafniiun is a valuable metal for many applications. The source of hafnium comes as the byproduct in the production of zirconium. The nonnudear grades of zirconium alloys are also low in hafnium content. Consequently, the coimterparts of nuclear and normuclear grades of zirconium alloys are interchangeable in mechanical properties. However, specification requirements for nuclear materials are more extensive than those for nonnuclear materials. Only requirements for nonnuclear materials are given in Table 22.3 and Table 22.4. It can be seen that Zr 705 is the... 

M.M. Opeka, l.G. Talmy, E.J. Wuchina, J.A. Zaykoski and S.J. Causey Mechanical, Thermal, and Oxidation Properties of Refractory Hafnium and zirconium Compounds. Journal of the European Ceramic Society 19, 2405-2414(1999). 

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