HAFNIUM


Atomic Number: 72

Atomic Symbol: Hf

Atomic Weight: 178.49

Electron Configuration: 32-10-2


(Hafinia, Latin name for Copenhagen) Hafnium was thought to be present in various minerals and concentrations many years prior to its discovery, in 1932, credited to D. Coster and G. von Hevesey. On the basis of the Bohr theory, the new element was expected to be assoacited with zirconium. It was finally identified in zircon from Norway, by means of X-ray spectroscope analysis. It was named in honor of the city in which the discovery was made. Most zirconium minerals contain 1 to 5% hafnium. It was originally separated from zirconium by repeated recrystallization of the double ammonium or potassium fluorieds by von Hevesey and Jantzen. Metallic hafnium was first prepared by van Arkel and deBoer by passing the vapor of the tetraiodide over a heated tungsten filament. Almost all hafnium metal now produced is made by reducing the tetrachloride with magnesium or with sodium (Kroll Process). Hafnium is a ductile metal with a brilliant silver luster. Its properties are considerably influenced by the impurities of zirconium present. Of all the elemtns, zirconium and hafnium are two of the most difficult to separate. Their chemistry is almost identical, however, the density of zirconium is about half that of hafnium. Very pure hafnium has been produced, with zirconium being the major impurity. Because hafnium has a good absorption cross section for thermal neutrons (almost 600 times that of zirconium), has excellent mechanical properties, and is extremely corrosion resistant, it is used for reactor control rods. Such rods are used in nuclear submarines. Hafnium has been successfully alloyed with iron, titanium, niobium, tantalum, and other metals. Hafnium carbide is the most refractory binary composition known,and the nitride is the most refractory of all known metal nitrides (m.p. 3310C). Hafnium is used in gas-filled and incandescent lamps, and is an efficient "getter" for scavenging oxygen and nitrogen. Finely divided hafnium is phosphoric and can ignite spontaneously in air. Care should be taken when machining the metal or when handling hot sponge hafnium. At 700C hafnium rapidly absorbs hydrogen to form the composition HfH1.86. Hafnium is resistant to concentrated alkalis, but at elevated temperatures reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. Halogens react directly to form tetrahalides. Exposure to hafnium should not exceed 0.5 mg/hr. (8 hour time-weighted average - 40-hour week). The price of the metal is in the broad range of $100 to $500/lb, depending on purity and quantity. The yearly demand for hafnium in the U.S. is now in excess of 100,000 lb.