Hafnium: Major Minerals, Chemistry Properties and Major Uses

Major Minerals

Hafnium is estimated to make up around 5.8 ppm of the Earth's upper crust by mass. It does not exist as a native element on Earth but is found as a solid solution with zirconium in natural zirconium compounds such as zircon, ZrSiO4, which usually has about 1%~ 4% of the Zr replaced by Hf. Infrequently, the Hf/Zr ratio increases during crystallization wto give the isostructural mineral hafnon (Hf,Zr)SiO4, with atomic Hf . Zr. A potential source of hafnium is trachyte tuffs containing the rare zircon-hafnium silicates eudialyte, Na15Ca6(Fe,Mn)3Zr3SiSi25O73(OH,O,H2O)3, or armstrongite, CaZrSi6O15· 2.5H2O.

The heavy mineral sand ore deposits of the titanium ore minerals ilmenite (FeTiO3) and rutile (TiO2) also produce most of the mined zirconium (zircon, ZrSiO4) and consequently also most of the hafnium. Zirconium is a good nuclear fuel-rod cladding metal, with the required properties of a very low neutron capture cross-section and good chemical stability at high temperatures. Nevertheless, due to hafnium's neutron-absorbing properties, the presence of hafnium impurities in zirconium would result in it to be far less suitable for nuclear reactor applications. Therefore an almost complete separation of zirconium and hafnium is essential for the use of zirconium in nuclear power applications. The manufacture of hafnium-free zirconium is the principal source for hafnium. About 50% of all hafnium metal produced is obtained as a by-product of zirconium refinement. The final product of the isolation is Hf(IV) chloride. The purified Hf(IV) chloride is converted to the metal by reduction with magnesium or sodium, as in the Kroll process.

Chemistry Properties

Hafnium is a shiny, silvery, ductile metal that is corrosion resistant and chemically comparable to zirconium (since it has the same number of valence electrons, being in the same group, but also to relativistic effects; the expected expansion of atomic radii from period 5~6 is practically canceled out by the lanthanide contraction) (Table 7.10). The physical properties of hafnium metal are substantially affected by zirconium impurities, particularly the nuclear properties, as these two elements are among the most difficult to separate due their chemical similarity. A noticeable physical difference between these metals is their density, with zirconium having about one-half the density of hafnium. The chemistry of hafnium and zirconium is similar that the two cannot be separated based on differing chemical reactions. The melting points and boiling points of the compounds and the solubility in solvents are the major differences in the chemistry of these twin elements.

Hafnium reacts in air forming a protective film, which prevents further corrosion. Hafnium(IV) oxide is the inorganic compound with the formula HfO2. Also known as hafnia, this colorless solid is one of the most common and stable compounds of hafnium. It is an electrical insulator with a band gap of 5.3 5.7 eV.  Hafnium(IV) oxide is fairly inert. It reacts with strong acids such as concentrated sulfuric acid and with strong bases.

At elevated temperatures, it reacts with chlorine in the presence of graphite or carbon tetrachloride to form hafnium tetrachloride. Hafnia has the same structure as zirconia (ZrO2). Unlike TiO2, which features six-coordinate Ti in all phases, zirconia and hafnia have seven-coordinate metal centers. Various crystalline phases have been experimentally observed, for example, cubic (Fm3m), tetragonal (P42/nmc), monoclinic (P21/c), and orthorhombic (Pbca and Pnma). In addition, it is known that hafnia may adopt two additional orthorhombic metastable phases (space group Pca21 and Pmn21) over a wide range of pressures and temperatures, probably being the sources of the ferroelectricity recently observed in thin films of hafnia. Thin films of hafnium oxides deposited by atomic layer deposition are usually crystalline. Since semiconductor devices benefit from having amorphous films present, hafnium oxide has been alloyed with aluminum or silicon (forming hafnium silicates), which possesses a higher crystallization temperature than hafnium oxide.

Major Uses

The nuclei of some Hf isotopes can each absorb multiple neutrons, which makes it a good element for usage in the control rods for nuclear reactors. The German research reactor FRM II (Forschungs-Neutronenquelle Heinz Maier-Leibnitz, research reactor Munich II) employs Hf as a neutron absorber. It is likewise common in military reactors, especially in the US naval reactors, though rarely found in civilian reactors, the first core of the Shippingport Atomic Power Station (a conversion of a naval reactor) being a prominent exception.

Isotopes of Hf and Lu (along with Yb) are also utilized in isotope geochemistry and geochronological applications, for example, in Lu - Hf dating. It is frequently employed as a tracer of isotopic evolution of Earth's mantle through time. This is because 176Lu decays to 176Hf with a half-life of about 37 billion years. In most geologic materials, the mineral zircon (ZrSiO4) is the principal host of Hf ( . 10,000 ppm) and is frequently the focus of Hf research in geology.

Hf is employed in alloys with Fe, Ti, Nb, Ta, and other metals. An alloy used for liquid rocket thruster nozzles, for example, used in the main engine of the Apollo Lunar Modules, is C103 that comprises 89% Nb, 10% Hf, and 1% Ti. Small additions of Hf improve the adherence of protective oxide scales on Ni-based alloys. It thus enhances the corrosion resistance in particular under cyclic temperature conditions that tend to break oxide scales by creating thermal stresses between the bulk material and the oxide layer.

Hf oxide-based compounds are practical high-k dielectrics, permitting a decrease of the gate leakage current, which enhances performance at these IC scales. Because of its heat resistance and its affinity to oxygen and nitrogen, Hf is a good scavenger for oxygen and nitrogen in gas-filled and incandescent lamps. In addition, Hf is employed as the electrode in plasma cutting due to its ability to shed electrons into air. Hf is used in high-temperature ceramics. Hf carbide and nitride are some of the most refractory materials identified, that is, they will not melt except under the most extreme temperatures, for example, for Hf nitride the melting point is 3310℃.

You may like