Thermal hafnium

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. 

Hafnium oxide 30—40 mol % titanium oxide ceramics (qv) exhibit a very low coefficient of thermal expansion over the temperature range of 20—1000°C. A 45—50 mol % titanium oxide ceramic can be heated to over 2800°C with no crystallographic change (48). 

Hafnium Halides. Hafnium tetrafluoride, HfF, can be prepared by careful thermal decomposition of ammonium fluorohafnate in an... 

Hafnium-free zirconium is particularly weU-suited for these appHcations because of its ductiHty, excellent oxidation resistance in pure water at 300°C, low thermal neutron absorption, and low susceptibiHty to radiation. Nuclear fuel cladding and reactor core stmctural components are the principal uses for zirconium metal. 

Another trend which can be anticipated from electronegativity data, is drat the bromides will be more easily decomposed thermally than the chlorides, and the iodides more readily than the bromides. A typical comparison is between the compounds of titanium and those of hafnium, which play a significant role in vapour deposition. 

The oxidation rate of niobium in air from 800°C to above 1000°C can be decreased by alloying e.g. with hafnium, zirconium, tungsten, molybdenum, titanium or tantalum . However, the preferred fabricable alloys still require further protection by coating . Ion implantation improves thermal oxidation resistance of niobium in oxygen below 500°C . 

Balog, M., Schieber, M., Michman, M., andPatai, S., Zirconium and Hafnium Oxides by Thermal Decomposition of Zirconium and Hafnium Diketonate Complexes in the Presence and Absence of Oxygen, J. Electrochem. Soc., 126(7) 1203-1207 (1979)... 

Apart from the reactions described above for the formation of thin films of metals and compounds by the use of a solid source of the material, a very important industrial application of vapour phase transport involves the preparation of gas mixtures at room temperature which are then submitted to thermal decomposition in a high temperature furnace to produce a thin film at this temperature. Many of the molecular species and reactions which were considered earlier are used in this procedure, and so the conclusions which were drawn regarding choice and optimal performance apply again. For example, instead of using a solid source to prepare refractory compounds, as in the case of silicon carbide discussed above, a similar reaction has been used to prepare titanium boride coatings on silicon carbide and hafnium diboride coatings on carbon by means of a gaseous input to the deposition furnace (Choy and Derby, 1993) (Shinavski and Diefendorf, 1993). 

Although the surface zirconium complex has a lower thermal stability than the hafnium one, the nature of the gases released during thermolysis is similar, except for isobutene (hardly observed for zirconium) and isopentene (observed only for hafnium). These results suggest that isobutene resulting from ji-alkyl transfer of hafnium (Scheme 11.1) is inserted in the Hf-CHs bond. 

Hafnium dioxide is a high temperature refractory material. It is used for control rods in nuclear reactors. It has high stability and high thermal neutron absorption values. It also is used in special optical glasses and glazes. 

Grigg and co-workers (310) recently examined the 1,3-APT reaction of various aldoximes (270) (R or R = H) with divinyl ketone (Scheme 1.56). While ketoximes 270 (R = R) form a mixture of adducts, 271 and 272 via nitrone 273, the aldoximes selectively afford 272 (as a mixture of endo and exo diastereoisomers). Under the thermal reaction conditions, the oxime starting materials can undergo ( /Z) isomerization, while the nitrone intermediate was expected to be unaffected and the isolated cycloadducts showed no interconversion via cycloreversion. Thus, the increasing selectivity for endo-212 [via ( )-273, R = H] over exo-212 [via (Z)-273, R = H] with the increasing size of the aldoxime substituent was attributed primarily to the inhibition of oxime isomerization by steric clash between R or R and the oxime OH. In contrast, Lewis acid catalysis, in particular by hafnium (iv) chloride, of the cycloaddition of various aldoximes with this dipolarophile gave exo-271 exclusively (216). 

Zirconium(IV) and hafnium(IV) halides form adducts with a variety of monodentate and polydentate amines. Typical complexes are listed in Table 3. In general, these compounds are air- and moisture-sensitive white solids (yellow when the halide is iodide), thermally stable and insoluble in most organic solvents. 

J-Butylperoxy complexes of bis(pentamethylcyclopentadienyl)hafnium(IV) [(C5Me5)2Hf(OOBu )R 204 R = Me, Et, Ph] have been prepared from the reaction of Bu OOH with (C5Me5)Hf(H)(R), and thermally decompose to give the mixed alkoxide (C5Me5)Hf(OBu )-(OR). The X-ray crystal structure of (204 R = Ph) indicates a monodendate r-butylperoxy ligand.631... 

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. 

A systematic diffraction study was made with both neutrons and x-rays of metal- hydride systems in the composition range of 2 to 66.5 atomic % hydrogen of hafnium, titanium, and zirconium, and a nuclear null-matrix consisting of 62 atomic % titanium and 38 atomic % zirconium, with emphasis on the metal-rich regions. A nuclear null-matrix as defined here consists of two or more types of nuclei in which some of the nuclei scatter thermal neutrons 180° out of phase with others, such that the resultant structure factor is zero. 

The thermal conductivities of transition metal carbides increase with increasing temperature, an unexpected phenomenon that has been investigated extensively on titanium, zirconium, hafnium and carbides and carbonitrides. Previous studies have reported a linear increase of the thermal conductivity with temperature, but more recent investigations have revealed a nonlinear relationship. Carbon... 

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