Hafnium, alloying element

Niobium is important as an alloy addition in steels (see Steel). This use consumes over 90% of the niobium produced. Niobium is also vital as an alloying element in superalloys for aircraft turbine engines. Other uses, mainly in aerospace appHcations, take advantage of its heat resistance when alloyed singly or with groups of elements such as titanium, tirconium, hafnium, or tungsten. Niobium alloyed with titanium or with tin is also important in the superconductor industry (see High temperature alloys Refractories). 

Most LWR fuel rod cladding is made of Zircaloy (and its derivatives), which is an alloy of primarily zirconium and tin. Other alloying elements include niobium, iron, chromium, and nickel. Zircaloy was chosen because it has a very low cross section for thermal neutrons. Naturally occurring zirconium contains about l%-5% hafnium. The hafnium must be removed because it has a very high thermal neutron cross section and is often used in making control rods for reactors. The separation process used in the United States is a liquid-liquid extraction process. It is based on the difference in solubility of the metal thiocyanates in methyl isobutyl ketone. In Europe, a process known as extractive distillation is used to purify zirconium. This method employs a separation solvent that interacts differently with the zirconium and hafnium, causing their relative volatilities to change. This enables them to be separated by a normal distillation process. The separated zirconium is then alloyed with the required constituents. 

Rare earths have been used in conjunction with anodizing of aluminium in several ways. Firstly, they have been used in fundamental investigations of the anodic oxidation of aluminium alloys where the rare earth element (cerium, hafnium, neodymium and samarium) was employed as an alloying element. These smdies revealed the distribution of the alloying element in the anodic alumina and the outward migration rate of the rare earth cations relative to aluminium ions. It has been shown also that rare earths (particularly cerium) can inhibit ejection of ions from films during anodizing in alkaline electrolytes. 

The silvery, shiny, ductile metal is passivated with an oxide layer. Chemically very similar to and always found with zirconium (like chemical twins, with almost identical ionic radii) the two are difficult to separate. Used in control rods in nuclear reactors (e.g. in nuclear submarines), as it absorbs electrons more effectively than any other element. Also used in special lamps and flash devices. Alloys with niobium and tantalum are used in the construction of chemical plants. Hafnium dioxide is a better insulator than Si02. Hafnium carbide (HfC) has the highest melting point of all solid substances (3890 °C record ). 

As one might expect, yttrium is not without its competitors hafnium has been proposed as a replacement for it in certain iron-based alloys as have other elements, but in the context of the applications as described yttrium remains the preferred additive. 

Assay of beryllium metal and beryllium compounds is usually accomplished by titration. The sample is dissolved in sulfuric acid. Solution pH is adjusted to 8.5 using sodium hydroxide. The beryllium hydroxide precipitate is redissolved by addition of excess sodium fluoride. Liberated hydroxide is titrated with sulfuric acid. The beryllium content of the sample is calculated from the titration volume. Standards containing known beryllium concentrations must be analyzed along with the samples, as complexation of beryllium by fluoride is not quantitative. Titration rate and hold times are critical therefore use of an automatic titrator is recommended. Other fluoride-complexing elements such as aluminum, silicon, zirconium, hafnium, uranium, thorium, and rare earth elements must be absent, or must be corrected for if present in small amounts. Copper—beryllium and nickel—beryllium alloys can be analyzed by titration if the beryllium is first separated from copper, nickel, and cobalt by ammonium hydroxide precipitation (15,16). 

The decade of the 1980s witnessed the development of hundreds of new alloys, involving not only the traditional metals, but much greater use of the less common chemical elements, such as indium, hafnium, etc. In this encyclopedia, alloys of a chemical element are discussed mainly under that particular element, or in an entry immediately following.. Also check alphabetical index. 

Olher mudern getter materials include cesium-rubidium alloys, tantalum. titanium, zirconium, and several of the rare-earth elements, such as hafnium,... 

The need to produce reactor-quality hafnium stimulated advances in the analytical chemistry of that element. The most difficult problems were the detection of hafnium in zirconium and related alloys, and vice versa due to the very similar chemical behavior of the elements. 

The major use for zirconium is in the nuclear industry. Zirconium alloys (zircaloys) are used extensively as a cladding for nuclear (uranium oxide) fuel rods in water cooled reactors. Zircaloys were favoured over stainless steel cladding because they had a considerably lower neutron cross-section, appropriate thermal conductivity and both corrosion and mechanical resistance. As indicated, hafnium is an impurity in nearly all zirconium ores. Hafnium, however, has a much higher neutron cross-section than zirconium and, as such, the two elements must be separated prior to using zirconium in fuel rod cladding. For many years the separation was very difficult due to the chemical similarity of the two elements. Zirconium hydride is used as a moderator in nuclear reactors. 

Hafnium-like boron is known to be a neutron absorber or neutron moderator element, and, therefore, composites of boron carbide, B4C, and hafnium diboride, HfB2, can be considered as nuclear materials. These boron compounds after sintering and °B/"B isotopic ratio adapting are found to be heterogeneous polyphone cermets useful for nuclear applications (Beauvy et al. 1999). Boron acid obtained from the °B enriched boron trifluoride also was used in nuclear reactors (Shalamberidze et al. 2005). Amorphous boron powders enriched both in °B and "B, boron carbide, and zirconium diboride (ZrB2) powders and pallets labeled with °B isotope And applications in nuclear engineering too. The °B enriched Fe-B and Ni-B alloys are useful for the production of casks for spent nuclear fuel transfer and storage. 

The heats of formation A H for ordered stoichiometric alloys were determined with self-consistent linear-augmented calculations [53]. For 50 50% alloys of titanium, zirconium, and hafnium with the heavier 4d and 5d elements the agreement between the theory and experiment was of the order of the scatter of the experimental data. For instance, heat of formation was found for the chemical compound of RuZr to be equal to —0.75 eV/atom according to the calorimetric measurements AH = -0.79eV/atom. 

C. The element is found with zhco-nium and is extracted by formation of the chloride and reduction by the KroU process. It is used in tungsten alloys in filaments and electrodes and as a neutron absorber. The metal forms a passive oxide layer in air. Most of its compounds are hafnium(IV) complexes less stable hafnium(III) complexes also exist. The element was first reported by Urbain in 1911, and its existence was finally established by... 

Hafnium is usually present in zircon to the extent of 2-3 per cent, and its chemical properties are so similar to those of zirconium that the two elements remain together through the ore breakdown and preliminary chemical stages. The resulting metal would, in fact, be an alloy of zirconium... 

The tubes themselves must be fully characterized in order to ensure that the zirconium alloys, or magnesium alloys, do not contain impurities that may affect the performance of the fuel. For example, the presence of traces of neutron absorbers, like hafnium that always accompanies zirconium in nature, or elements that modify the corrosion resistance of zirconium, must be determined. The ASTM has outlined the specifications for seamless wrought zirconium alloy tubes that are used for nuclear fuel cladding (B811 2013). The exact technical details and analytical test procedures do not directly involve uranium and are beyond the scope of this book. 

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