Hafnium dioxide

Hafnium carbide is inert to most reagents at room temperature, but is dissolved by hydrofluoric acid solutions which also contain an oxidising agent. Above 250°C, hafnium carbide reacts exothermically with halogens to form hafnium tetrahaUde, and above 500°C, with oxygen to form hafnium dioxide. At higher temperatures in a flow of hydrogen, hafnium carbide slowly loses some of its carbon. 

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. 

At room temperature, hafnium dioxide is slowly dissolved by hydrofluoric acid. At elevated temperatures, hafnium dioxide reacts with concentrated sulfuric acid or alkaU bisulfates to form various sulfates, with carbon tetrachloride or with chlorine in the presence of carbon to form hafnium tetrachloride, with alkaline fluorosiUcates to form alkaU fluorohafnates, with alkaUes to form alkaline hafnates, and with carbon above 1500°C to form hafnium carbide. 

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). 

It was shown that hafnium silicate thin films could be deposited by ALD using HtCl2[N(SiMe3)2]2 with H2O in the temperature range of 150-40010. As the deposition temperature increased, the Si content in the film increased but growth rate linearly decreased. However at low temperature, the Si content was as low as Si/(Hf+Si)=0.05 at ISOU, which was close to hafnium dioxide [4]. The surface mechanism of this ALD process was analyzed by in-situ FT-IR [6]. 

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 ). 

Compounds Hafnium chloride (HfCU) hafnyl chloride (HfOCU) hafnium dioxide (Hf02)... 

Riedel WR, Hirschberg AK, Kaul A, et al. 1979. Comparative investigations on the biokinetics of colloidal thorium, zirconium, and hafnium dioxides in animals. Environ Res 28 127-139. 

In finely divided form, hafnium is pyrophoric, igniting in air spontaneously. However, bulk metal reacts slowly in oxygen or air above 400°C. The rate of oxidation increases with temperature. The product is hafnium dioxide, Hf02. It combines with nitrogen, carbon, boron, sulfur and silicon at very high temperatures to form hafnium nitride HfN, hafnium boride HfB, hafnium sulfide HfSi2, respectively. Nitride formation occurs at 900°C. 

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. 

Hafnium dioxide may be prepared by heating the metal with air or oxygen... 

Hafnium dioxide reacts with chlorine in the presence of carbon at elevated temperatures to yield hafnium tetrachloride, HfCh. When ammonium hydroxide solution is added to an acid solution of hafnium dioxide, the hydrous oxide, Hf02 xH20 precipitates. 

Hafnium tetrachloride can be prepared (i) by chlorination of hafnium dioxide in the presence of carbon ... 

It also may be prepared by several other methods, such as (ii) reaction of carbon tetrachloride with hafiiium dioxide above 450°C (iii) heating a mixture of hafnium dioxide and carbon above 700°C and (iv) reaction of chlorine with hafnium at elevated temperatures. 

At elevated temperatures and in vapor phase, the tetrachloride reacts with air or steam forming finely divided hafnium dioxide, Hf02. When heated with boron trichloride and hydrogen to very high temperatures (above 2,000°C) hafnium diboride, H B2, a gray crystalline solid, forms. 

Hf02 (c, monoclinic). Roth and Becker3 measured the heat of combustion of hafnium, to form monoclinic hafnium dioxide, to be 271.5. 

Sulphuric Acid and Related Compounds. The vibrational spectra of H2SO4 and D2SO4 in the crystalline state have been measured at various temperatures. The cation-transference numbers of Li, Na, K, NH3, Ag, and Ba hydrogen sulphates, and of solutions of water and acetic acid, have been measured in 100% H2SO4 at 25 °C. The numbers obtained were of the order of 0.005, much smaller than was previously believed. The behaviour of hafnium dioxide and copper(ii) oxide in sulphuric acid has been studied and various compounds have been isolated. 

The physical properties of hafnium dioxide are very similar to those of Zr02. Hafnium dioxide, Hf02, thin films have been prepared by CVD using the /S-diketonate complexes Hf(tfac)4 (tfac" 5) [32, 37] and Hf(acac)4 (acac 7) [37] as precursors. 

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