Hafnium carbide preparation

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 carbide can be prepared industrially from hydrided hafnium sponge at 1500—1700°C or from Hf02 at 2000—2200°C by carburization in vacuum in the presence of hydrogen. The resulting carbide contains almost the theoretical quantity of carbon, 6.30 wt % C, of which a maximum of 0.1 wt % is unbound. 

Hafnium carbide has a melting point of 3890°C and exhibits extreme hardness and good electrical conductivity. It is used as an oxidation-resistant coating for composites and as a coating for superalloys [10]. The material is prepared by the CVD of a mixture of hafnium tetrachloride, methane and hydrogen [193, 194]. 

The most simple methods for preparation of hafnium carbide and its composites are the follows. The powder of HfO was thermally treated with Mg in molar ratio 5 4 under a CH flow ranging from 800 to 950 °C [4]. The effective high temperature coating for carbon fiber reinforced carbon and carbon fibre reinforced silicon carbide was prepared with use of HfC [5]. For this purpose hafnium carbide layers were obtained in a thermally simulated chemical vapor deposition (CVD) reactor on nonporous substrates by reaction of hafnium tetrachloride, methane and addition of hydrogen (Eq. 10.1) ... 

Hafiiium carbide powder is prepared by the reaction of Hf02 with carbon at 1800-2200°C in hydrogen by the carburization of hafoium sponge by the carburization of hafiiium hydride at 1600-1700°C, or by plasma CVD. Hafnium carbide coatings are deposited by CVD, evaporation or sputtering (see Chs. 14 and 15). 

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). 

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). 

While not pyrophoric, the porous copper metal monolith is readily combustible by the application of a flame in air. In addition to copper, this method has been appUed to prepare other nanoporous metals such as iron, cobalt, nickel, and tin, as well as carbides of chromium, titanium, and hafnium (Chap. 14). This method is especially powerful as it appears to be applicable to a wide variety of metals and affords articles that are mrmolithic. 

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