Reaction-boronizing sintering

Figure 113. Schematic illustration of the reaction boronizing sintering process (Takagi, 1993) [359],...

Pressureless sintered SiC bodies made from ultrafine SiC powders which are themselves highly milled Aecheson powders or are made from vapor-phase reactions of silanes with hydroearbons. Such ultrafine powders are usually mixed with carbon and boron sintering aids in order to achieve the necessary high densities. 

Annual production of powdered BN is ca 180—200 metric tons per year and its cost is 50—250/kg, depending on purity and density. The price of cubic boron nitride is similar to that of synthetic diamond bort. Hot-pressed, dense BN parts are 3—10 times more expensive than reaction-sintered parts. 

A pellet is pressed of an intimate mixture of finely divided reactants and reaction induced either by arc melting and high-T annealing or by solid-state sintering in an electrical or high-frequency furnace. Isolating the borides from reactive container components can be a problem. The use of boron nitride liners has proved effective. In some cases the protective liner is made of sintered boride containing the same elements as the boride in preparation. 

Boron trifluoride-diethyl ether (7.1 g, 0.05 mol) is added during 5 min to the previously prepared solution of (IV-methylanilino)lithium. The pale-yellow slurry is then refluxed for 3 hr. At the end of this time, the mixed solvent is concentrated to approximately one-half of its original volume by distillation from the reaction vessel. The reaction mixture is cooled to 0° and subjected to a vacuum filtration through a 9-cm Buchner funnel to afford 22.0 g of a tan precipitate. This solid is placed in 200 mL of toluene, which is then brought to a boil the mixture is subjected to a hot vacuum-filtration in order to remove the lithium fluoride co-product. A considerable reduction in yield may result unless a steam-jacketed Buchner funnel or hot sintered-glass filter is used. Upon cooling, the toluene filtrate deposits white crystals, which are collected by filtration. Yield 13.7 g (83%) of tris(N-methyl-anilino)borane, mp 211-213° (lit. mp 210° 1 214-216°3). ... 

The recently developed materials of pure SiC can be manufactured by pressure sintering or reaction sintering of a mixture of SiC powder with graphite in silicon vapour. High-dcnsity SiC (98% of theoretical density) can also be prepared by sintering submicron powdered j3-SiC with small additions of boron and carbon at 2000 "C in an inert atmosphere. The effect of these additions is explained by surface effects (Prochazka, 1975). 

Boron carbide powders are preferred (reaction sintering gives porous material). 

I. Slynthesis via fusion of the elements entails such high heats of formation that the reaction temperatures become veiy high. As a result, there is interaction with the material of the vessel and the product boride becomes contaminated. On the other hand, all borides may be prepared by sintering the appropriate metal with amorphous boron powder, which should be as pure as possible (commercial grades now available contain 97-99% B). The reaction mixtures should be heated in alumina crucibles (W or Mo crucibles or boats may also be used) in vacuum (an argon atmosphere is occasionally also used). The reaction, which is always exothermic, starts at temperatimes of 700-1200 C the highest temperature may lie above 2000°C. In some cases, sintering under pressure in carbon tubes (mentioned as a possible method of synthesis for silicides—see p. 1796) can be used. 

Another reason for the poor sinterability is the extraordinarily high vapor pressure of boron oxides and suboxides. Since boron carbide powders are generally coated by a B2O3 layer [172] which quickly reacts to form boric acid, H3BO3, in humid atmosphere, vapor phase reactions are active at higher temperatures, in particular above 1500"C, providing a fast transport of boron compounds. Redox reactions such as... 

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