Hafnium oxide

The heavy mineral sand concentrates are scmbbed to remove any surface coatings, dried, and separated into magnetic and nonmagnetic fractions (see Separation, magnetic). Each of these fractions is further spHt into conducting and nonconducting fractions in an electrostatic separator to yield individual concentrates of ilmenite, leucoxene, monazite, mtile, xenotime, and zircon. Commercially pure zircon sand typically contains 64% zirconium oxide, 34% siUcon oxide, 1.2% hafnium oxide, and 0.8% other oxides including aluminum, iron, titanium, yttrium, lanthanides, uranium, thorium, phosphoms, scandium, and calcium. 

Reduction. Hafnium oxide can be reduced using calcium metal to yield a fine, pyrophoric metal powder (see Calciumand calciumalloys). This powder contains considerable oxygen contamination because of oxygen s high solubility in hot hafnium, and caimot be consoHdated into ductile metal. To obtain low oxygen ductile hafnium, the feed must be an oxygen-free halide compound such as hafnium tetrachloride or potassium hexafluorohafnate [16871-86-6]. 

Demand for hafnium has not shown significant growth since the late 1980s, nor has pricing changed. Hafnium oxide is priced at 80—120/kg depending on quality hafnium crystal bar about 180—220/kg and wrought plate, sheet, tube, and wine, from 250—500/kg. 

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). 

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. 

Hafnium Boride. Hafnium diboride [12007-23-7] HfB2, is a gray crystalline soHd. It is usually prepared by the reaction of hafnium oxide with carbon and either boron oxide or boron carbide, but it can also be prepared from mixtures of hafnium tetrachloride, boron trichloride, and hydrogen above 2000°C, or by direct synthesis from the elements. Hafnium diboride is attacked by hydrofluoric acid but is resistant to nearly all other reagents at room temperature. Hafnium dodecaboride [32342-52-2] has been prepared by direct synthesis from the elements (56). 

Hafnium Carbide. Hafnium carbide, HfC, is a dark gray brittle soHd. This carbide can be prepared by heating an intimate mixture of the elements or by the reaction of hafnium tetrachloride with methane at 2100°C, but is commonly produced by the reaction of hafnium oxide with lampblack... 

Hafnium Oxide. Two oxides of hafnium, hafnium monoxide [12029-22-0], HfO, and Hf02, are known to exist but only the dioxide is stable under ordinary conditions. Gaseous hafnium monoxide can be present at >2000° C, especially when the partial pressure of oxygen is low. Hafnium monoxide is probably the compound form in which oxygen is evolved when hafnium metal is melted in an electron-beam melting furnace. HfO(g) is the species observed mass spectrometricaHy when hafnium dioxide vaporizes. 

Hafnium dioxide is formed by ignition of hafnium metal, carbide, tetrachloride, sulfide, boride, nitride, or hydrous oxide. Commercial hafnium oxide, the product of the separation process for zirconium and hafnium, contains 97—99% hafnium oxide. Purer forms, up to 99.99%, are available. 

In the initial thiocyanate-complex Hquid—Hquid extraction process (42,43), the thiocyanate complexes of hafnium and zirconium were extracted with ether from a dilute sulfuric acid solution of zirconium and hafnium to obtain hafnium. This process was modified in 1949—1950 by an Oak Ridge team and is stiH used in the United States. A solution of thiocyanic acid in methyl isobutyl ketone (MIBK) is used to extract hafnium preferentially from a concentrated zirconium—hafnium oxide chloride solution which also contains thiocyanic acid. The separated metals are recovered by precipitation as basic zirconium sulfate and hydrous hafnium oxide, respectively, and calcined to the oxide (44,45). This process is used by Teledyne Wah Chang Albany Corporation and Western Zirconium Division of Westinghouse, and was used by Carbomndum Metals Company, Reactive Metals Inc., AMAX Specialty Metals, Toyo Zirconium in Japan, and Pechiney Ugine Kuhlmann in France. 

H+], calculation of, 192, see also Hydrogen ion Haber, Fritz, 151 Haber process, 140, 150 Hafnium, oxidation number, 414 Haldane, J. B. S., 436 Half-cell potentials effect of concentration, 213 measuring, 210 standard, 210 table of, 211, 452 Half-cell reactions, 201 Half-life, 416 Half-reaction, 201 balancing, 218 potentials, 452 Halides... 

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)... 

After extraction, the loaded solvent contains 6 g T1 zirconium as zirconium oxide with 0.2% hafnium oxide. The raffinate is left with 0.2 to 0.3 g l l of the oxides of zirconium and hafnium of this, 70-90% is hafnium oxide. This raffinate can act as a feed solution for the recovery of pure hafnium oxide. The loaded extractant, on the other hand, is subjected to a scrubbing operation with pure zirconium sulfate solution to eliminate any co-extracted hafnium. This scrubbing operation is essentially a displacement reaction ... 

A simplified series of reactions between a hafnium salt and sulfuric acid is given in Fig. 4.3. The reactions showcase important facets of thin-film synthesis (but do not address the precise identities of intermediates or complexities of aqueous hafnium chemistry.) In the first step, a hafnium oxide chloride crystal hydrate is dissolved in water to disperse small hafnium-hydroxo molecular clusters. Sulfato ligands are subsequently added in the form of sulfuric acid. Since sulfato binds more strongly than chloro, hafnium-hydroxo-sulfato aqueous species are created. Under mild heating, these species readily poly-... 

Figure 4.3. Chemical sequence representing deposition of hafnium oxide sulfates and generalized formulation for HafSOx. ...

Supplit, R Husing, N. Gross, S. Bernstorff, S. Puchberger, M. 2007. Hafnium oxide doped mesostructured silica films. Eur. J. Inorg. Chem. 2797-2802. 

Hafnium metal, analysis of, 13 87 Hafnium nitride, 13 89—90, 93 Hafnium oxide, 13 89, 93—94 reduction of, 73 84 Hafnium sulfides, 73 94 Hafnium tetrabromide, 73 93 Hafnium tetrachloride, 73 92 26 631 vapor reduction of, 73 84-85 Hafnium tetrafluoride, 73 90, 91 Hafnium tetrahydridoborate, 73 90 Hagen-Poiseuille expression/law, 27 726, 729 Hagen-Poiseuille flow, in microfluidics, 26 961... 

Cook. A.M. and Hiitter, R. Ametryne and prometryneas sulfur sources for bacteria. Appl Environ. Microbiol, 43(4) 781 -786,1982. Cooley. R.A. and Banks, H.O. The aqneous solubility of hafnium oxide by the radioactive isotope technique. J. Am. Chem. Soc, 73(8) 4022, 1951. 

Hafnium oxide (HfO ) Resists heat and corrosion, making it an ideal lining for refractory furnaces. 

Hafnium is obtained commercially from mineral zircon, which is zirconium orthosilicate [14940-68-2]. Zircon usually contains hafnium oxide, Hf02, in an amount that ranges between 1 to 2%. Zircon sand is separated from heavy... 

Hafnium tetrachloride reacts with water at room temperature, forming hafnium oxide chloride, HfOCb, and hydrochloric acid ... 

---