Hafnium compounds using

Hafnium is obtained as a by-product of the production of hafnium-free nuclear-grade 2irconium (see Nuclear reactors Zirconiumand zirconium compounds). Hafnium s primary use is as a minor strengthening agent in high temperature nickel-base superakoys. Additionally, hafnium is used as a neutron-absorber material, primarily in the form of control rods in nuclear reactors. 

Hafnium Carbide. The need of pure zirconium [7440-67-7] for nuclear reactors prompted the large-scale separation of hafnium [7440-58-6] from zirconium. This in turn made sufficient quantities of hafnium dioxide [12055-23-17, Hf02, or Hf metal sponge available for production of HfC for use in cemented carbides (see Hafniumand hafnium compounds). 

Hafnium tetrachloride is an important intermediate in production of hafnium metal. It also is used to prepare many hafnium compounds. 

Coating -by extrusion processes [PLASTIC PROCESSING] (Vol 19) -with LDPE [OLEFIN POLYMERS - POLYETHYLENE - LOW DENSITY POLYETHYLENE] (Vol 17) -PVC use piNYL POLYMERS - VINYLCITLORIDE POLYMERS] (Vol 24) -use ofhafmum [HAFNIUM AND HAFNIUM COMPOUNDS] (Vol 12) -use of lime and limestone QIME AND LIMESTONE] (V ol 15)... 

Willi improved means to separate the compounds of these two elements, future research will yield more details of specific hafnium compounds. The methods of separation used effectively include ion exchange techniques, a particularly effective one using u column of silica gel. w ith a solution of the tetrachlorides in methanol as feed and a 1.9 N HCI solution as eluant for zirconium. Separations also have heen accomplished through the distillation of the phosphorus oxychloride addition products. 

Minor industrial uses include the application of silver iodide as a smoke for the seeding of clouds to induce rainfall. Compounds used for obtaining some nonflammable plastics and cellulose are benzyltriphenyl-phosphoniumiodides and [2,-(acetyloxy)ethyl] triphenyl-phosphoniumiodides (see Flame RETARDANTS, HALOGENATED FLAME retardants) (142). The addition of iodine to an aromatic hydrocarbon such as -butylbenzene results in the formation of charge-transfer complexes that display outstanding effectiveness as lubricants for hard-to-lubricate metals (143), such as titanium or steels (see also LUBRICATION AND LUBRICANTS). Iodine is also used in the production of high purity metals such as titanium, silicon, hafnium, and zirconium (144). 

The corresponding hafnium compound behaves similarly. Attempts to make the titanium derivative using this method failed. Primarily titanium is reduced, as indicated by the color change of the solution from green to brown. Variation of the reaction conditions did not provide better results, in all cases only bi(undecamethylcyclohexasilanyl) could be identified among the reaction products. 

As exemplified by the reactions of Schemes 1 and 4, fluorotitanium compounds could open new possibilities for metal-catalyzed processes. Their fascinating structural diversity [7] as well as further catalytic possibilities in the field of olefin polymerizations [7i, 16] have been put forward by the pioneering work of Roesky, Noltemeyer and co-workers. Similar properties were also exhibited by the analogous zirconium and hafnium compounds [7b,i]. A Zr binaphtholate has already been successfully applied for the enantioselective allylstannylation of aldehydes [2f], Buch-wald and co-workers successfully used a chiral titanocene difluoride as precursor for the corresponding Ti(lII) hydride, a very efficient catalyst for the enantioselective hydrosilylation of imines [17]. 

The uses of zirconium and hafnium compounds based on their thermal and corrosion stability are described in related sections (see below). The most dramatic use of zirconium... 

Allenes have also been used to stabilize low-valent zirconium and hafnium compounds. This chemistry has been the subject of a recent review.74... 

The analogous titanium and hafnium compounds form active catalysts, too. Especially at higher temperatures the zirconium catalysts are more stable and active than the titanium or hafnium systems. The co-catalyst has a main influence. The most used co-catalyst is methylalumoxane (MAO). At lower temperature, fluorinated borane compounds activate the catalyst, too [17,18]. The structure of MAO is complicated. Sinn characterized MAO by different analytical methods and found it to be a cage with a formula of AlisO 12(0113)24 [19]. 

Zirconium has a high corrosion resistance and low cross-section for neutron capture (see Section 2.4) and is used for cladding fuel rods in water-cooled nuclear reactors. For this application, Zr must be free of Hf, which is a very good neutron absorber. The main use of pure Hf is in nuclear reactor control rods. Zirconium and hafnium compounds possess similar lattice energies and solubilities, and their complexes have similar stabilities this means that... 

Mah studied the enthalpy of formation of zirconium and hafnium carbides using combustion calorimetry. This author had previously studied [55MAH/BOY] the enthalpy of formation of zirconium and niobium carbides using the same technique. In the former study, Mah and Boyle [55MAH/BOY] had used an impure sample of uncertain composition. Mah was able to source much purer and well characterized samples of zirconium carbide and, as such, restudied the enthalpy of formation of this compound using the same technique. 

The discovery of the element hafnium provides a further example of a missed opportunity to turn scientific achievement into practical application. Hafnium was discovered at Niels Bohr s institute in Copenhagen in 1923 and quickly turned out to be fairly abundant and of potential industrial interest. However, no Danish chemical company seems to have taken an interest in the discovery. The two discoverers, Hevesy and Dirk Coster, sold the patent rights to the Philips firm in the Netherlands. Philips at once took out several patents on the use of hafnium compounds and developed methods for applying the metal in electrodes, filaments and fireproof enamels. ... 

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