Diborides

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). 

Boron nitride and titanium diboride coatings are used on graphite for the evaporation of Hquid aluminum. 

The most important of these is the diboride, TiB2, which has a hexagonal stmeture and lattice parameters of a = 302.8 pm and c = 322.8 pm. Titanium diboride is a gray crystalline soUd. It is not attacked by cold concentrated hydrochloric or sulfuric acids, but dissolves slowly at boiling temperatures. It dissolves mote readily in nitric acid/hydrogen peroxide or nitric acid/sulfuric acid mixtures. It also decomposes upon fusion with alkaU hydroxides, carbonates, or bisulfates. 

Titanium diboride, typically 96—98% pure, may also be made by the electrolysis of mineral mtile dissolved in mixed electrolytes,... 

Ti02/Na2C02/Na2AlF2/NaCl/Na2B40, at 1050°C (20). Very fine titanium diboride may be made by a gas-phase plasma process in which titanium tetrachloride and boron trichloride are reacted in a hydrogen gas heated by a d-c plasma (21). 

Supphers of titanium diboride include Micron Metals, Atomergic Chemmetals, Cerac, and Noah Technologies, in the United States, and ElektroschmeUwerk Kempten, Herman C. Starck, and RTZ Chemicals in Europe. 

Borides. Zirconium forms two borides zirconium diboride [12045-64-6] ZrB2, and zirconium dodecaboride [12046-91 -2] The diboride... 

The diboride has a hexagonal stmcture and melts at 3245°C (136) it is considered to have the best oxidation resistance of all the refractory hard metals. The dodecaboride has a cubic stmcture. 

Ceramics (qv) such as those in Table 12 find high temperature use to over 800°C (32). Advanced ceramics finding interest include alumina, partially stabilized zitconia, siUcon nitride, boron nitride, siUcon carbide, boron carbide, titanium diboride, titanium carbide, and sialon (Si—Al—O—N) (33) (see... 

C14B tetrachromium boride [12006-81-4] ">1680 TaB2 tantalum diboride [12007-35-1] 3100... 

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