Sintering of Pure Borides

Owing to the high sintering temperatures employed, losses of material (by volatilization of boron or boride) and grain growth are observed. In order to limit these losses, the part to be sintered can be embedded in a powder of the same boride. Sintering of pure refractory borides requires > 0.7T, (Tj = absolute melting temper-... 

Reaction between elemental boron and metal powder is the simplest route to produce boride powder. It can have excellent control on the stoichiometry of the resultant boride. Although this route results in pure ZrB /HfB but cannot be employed for industrial production because the starting materials used are expensive. This route can be exploited to get dense shapes of borides by hot pressing or spark plasma sintering of mixed powder (metal and boron), if the reaction with die is avoided (Tamburini et al., 2008). To avoid any reaction with the die, the graphite die is coated with boron nitride. 

Two different types of reactors are used depending on the product synthesized. The first type can maintain pressures up to 150 atm, and is widely used for production of powders in gasless and gas-solid systems. Carbides, borides, silicides, intermetallics, chalcogenides, phosphides, and nitrides are usually produced in this type of reactor. The second type, a high-pressure reactor (up to 2000 atm), is used for the production of nitride-based articles and materials, since higher initial sample densities require elevated reactant gas pressures for full conversion. For example, well-sintered pure BN ceramic with a porosity of about 20-35% was synthesized at 100 to 5000-atm nitrogen pressure (Merzhanov, 1992). Additional examples are discussed in Section III. 

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. 

In 1958, Hall [141] discussed the desirability of preparing a cemented diamond composition analogous to WC and hinted that experiments to produce polycrystalline diamonds were underway. But it was not until 1970, when he reported details of his procedures [142], that he established experimentally practical pressure and temperature fields where pure diamond powder can be sintered within times ranging from several days down to about one second. He mentions hard refractory materials like borides, carbides, nitrides and oxides as suitable binders. 

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