Hafnium physical properties

Most hafnium compounds have been of slight commercial interest aside from intermediates in the production of hafnium metal. However, hafnium oxide, hafnium carbide, and hafnium nitride are quite refractory and have received considerable study as the most refractory compounds of the Group 4 (IVB) elements. Physical properties of some of the hafnium compounds are shown in Table 4. 

Table 4. Physical Properties of Some Hafnium Compounds...

The chemical and physical properties of Unq (or rutherfordium) are homologous with the element hafnium ( jHf), located just above it in group 4 (fVB) in the periodic table. It was first claimed to be produced artificially by the Joint Institute for Nuclear Research (JINR) located in Dubna, Russia. The Russian scientists used a cyclotron that smashed a target of plutonium-242 with very heavy ions of neon-22, resulting in the following reaction Pu-242 + jjjNe-22 —> jj, Unq-260 + 4 n-1 (alpha radiation). The Russians named Unq-260 kurcha-tovium (Ku-260) for the head of their center, Ivan Kurchatov. (See details in the next section, History. )... 

The coordination chemistry of zirconium(III) is an ill-defined and relatively unexplored area.19 Only one magnetically dilute zirconium(III) complex has been isolated, [ZrCl3(tfars)]-MeCN [tfars= 1, 2-bis(dimethylarsino)-3,3,4,4-tetrafluorocyclobutene (CFZ)2C2-(AsMe2)2]2° and the first structurally characterized zirconium(III) complex [ ZrCl3(PBu3)2 2] has just recently been reported.21 The known zirconium(III) complexes and their physical properties are listed in Table 2. Even less is known about the coordination chemistry of hafnium(III). Just one complex, HfCl3(py)4, has been reported.22... 

Physical Properties. Hafnium is a hard, heavy, somewhat ductile metal having an appearance slightly darker than that of stainless steel. The color of hafnium sponge metal is a dull powder gray. Physical properties of hafnium are summarized in Table 1. These data are for commercially pure hafnium which may contain from 0.2 to 3% zirconium. Although a number of radioactive isotopes have been artificially produced, naturally occurring hafnium consists of six stable isotopes (Table 2). Hafnium crystallizes in a body-centered cubic system which transforms to a hexagonal close-packed system below 2033 K. 

Chemists later developed a better understanding about the relationship of zirconium and hafnium. These two elements are as alike as any two elements in the periodic table. They have nearly identical chemical and physical properties. This similarity explains why it took so long to find hafnium. Chemists had probably discovered hafnium before 1923, but thought it was zirconium. The differences in the X-ray patterns of the two elements finally proved that hafnium was different from zirconium. 

The physical property of greatest interest for hafnium is how it responds to neutrons. A neutron is a very small particle found in the nucleus (center) of an atom. Neutrons are used to make nuclear fission reactions occur. Nuclear fission reactions take place when a neutron strikes a large atom, such as an atom of uranium. The neutron makes the atom break apart. In the process, a large amount of energy is released. That energy can be converted to electricity. 

The physical properties of hafnium dioxide are very similar to those of Zr02. Hafnium dioxide, Hf02, thin films have been prepared by CVD using the /S-diketonate complexes Hf(tfac)4 (tfac" 5) [32, 37] and Hf(acac)4 (acac 7) [37] as precursors. 

Because of its neuronic, mechanical, and physical properties, hafnium is an excellent control material for water-cooled, water-moderated reactors. It is found together with zirconium, and the process that produces pure zirconium produces hafnium as a by-product. Hafnium is resistant to corrosion by high-temperature water, has adequate mechanical strength, and can be readily fabricated. Hafnium consists of four isotopes, each of which has appreciable neutron absorption cross sections. The capture of neutrons by the isotope hafnium-177 leads to the formation of hafnium-178 the latter forms hafnium-179, which leads to hafnium-180. The first three have large resonance-capture cross sections, and hafnium-180 has a moderately large cross section. Thus, the element hafnium in its natural form has a long, useful lifetime as a neutron absorber. Because of the limited availability and high cost of hafnium, its use as a control material in civilian power reactors has been restricted. 

Modern devices like computers and cell phones are built from solids with very specific physical properties. For example, the integrated circuit that is at the heart of many electronic devices is built from semiconductors like silicon, metals like copper, and insulators like hafnium oxide. 

Hafnium tetrabromide [13777-22-5], HfBr, is very similar to the tetrachloride in both its physical and chemical properties. Hafnium tetraiodide [13777-23-6], Hfl, is produced by reaction of iodine with hafnium metal at 300°C or higher. At temperatures above 1200°C, the iodide dissociates to hafnium metal and iodine. These two reactions are the basis for the iodide-bar refining process. Hafnium iodide is reported to have three stable crystalline forms at 263—405°C (60). 

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. 

Wilk, G. D. Wallace, R. M. 1999. Electrical properties of hafnium silicate gate dielectrics deposited directly on silicon. Applied Physics Letters, 74, 2854-2856. 

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