Boron 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 wide range of cutting-tool materials is available. Properties, performance capabilities, and cost vary widely (2,7). Various steels (see Steel) cast cobalt alloys (see Cobalt and cobalt alloys) cemented, cast, and coated carbides (qv) ceramics (qv), sintered polycrystalline cubic boron nitride (cBN) (see Boron compounds) and sintered polycrystalline diamond tbin diamond coatings on cemented carbides and ceramics and single-crystal natural diamond (see Carbon) are all used as tool materials. Most tool materials used in the 1990s were developed during the twentieth century. The tool materials of the 1990s... 

Sdicon carbide, hot pressed Alumina, dense sintered Boron nitride, hot pressed Sdicon nitride, hot pressed Boron carbide, hot pressed... 

The ceramic, polycrystalline siHcon carbide [409-21-2], SiC, is processed using P-siHcon carbide and boron (9). The boron is a sintering aid used at... 

When the structure of a metal changes, it is because there is a driving force for the change. When iron goes from b.c.c. to f.c.c. as it is heated, or when a boron dopant diffuses into a silicon semiconductor, or when a powdered superalloy sinters together, it is because each process is pushed along by a driving force. 

This is illustrated in Eig. 2.30 [2.167]. The surface was that of a fractured compact of SiC to which horon and carhon had been added to aid the sintering process. The aim of the analysis was to establish the uniformity of distribution of the additives and the presence or absence of impurities. The Auger maps show not only very non-uniform distribution of boron (Eig. 2.30a) but also strong correlation of boron with sodium (Eig. 2.30c), and weaker correlation of boron with potassium (Eig. 2.30b). Point analyses for points A and B marked on the images reveal the presence of sulfur and cal-... 

Fig. 2.30. SAM map offractured SiC after sintering with B addition [2.167], (a)-(d) elemental maps in boron, potassium, sodium, and oxygen, respectively. (E), (F) point analyses at points A and B, respectively.

Ceramic-coated disposable inserts, including silicon nitride, boron nitride, titanium nitride (TIN), titanium carbide (TIC) and sintered synthetic diamond ... 

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. 

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-... 

The hardness of boron carbide (carbon hexaboride) is not well defined because it is made as sintered compacts which have variable densities, compositions, and defect densities. It is very hard (up to 4400kg/mm2), and of relatively low density, so it has been used extensively as body-armor (McColm,... 

The high-temperature stability of SiC-based ceramics is well-known, and therefore its composite materials have been investigated for application to high-tem-perature structural materials [19-21]. However, well-known SiC-based fibers and matrix-materials stained with alkali salt are easily oxidized at high temperatures in air [22]. This would be a serious problem when these materials are used near the ocean or in a combustion gas containing alkali elements. In particular, a silicon carbide fiber containing boron (a well-known sintering aid for SiC) over 1 wt% was extensively oxidized under the above condition. In this... 

The filtrate is then warmed to room temperature, and hydrogen sulfide is bubbled into the solution until no more silver sulfide precipitates. This solid is separated by filtration through a medium-porosity, sintered-glass frit also connected to a previously evacuated, nitrogen-purged, and ice-salt-cooled 500-mL, two-necked, round-bottomed flask bearing an adapter fitted with a 2-mm vacuum stopcock. The filtrate is warmed to room temperature, and the stirred solution is evaporated under vacuum to give a solid product, which is further dried for another 1 lA hours. The yellow-brown crystalline solid, mp 4345°, is crude dihydro(isocyano)(trimethylamine)boron. Yield 16.0 g (75.8% based on trimethylamine-borane). 

In experiments run over a number of cycles, the activity was observed to increase after the first cycle, unlike the y-A Os counterpart which deactivated. Using BN, no Pt sintering occurred and this was ascribed to the high thermal conductivity of BN, ensuring that no local hot-spots were formed. On the basis of XPS, the locus of Pt particle attachment was proposed to be surface boron oxide impurities. Taylor and Pollard have compared the activities of silica (194 m g ) and boron nitride (7 m g ) supported vanadium oxide catalysts for propane oxidation. The use of boron nitride was reported to significantly... 

A. B. Sawaoka, New sintering processing of high-density boron nitride and diamond. In High Temperature Ceramics (G. Kostorz, ed.), pp. 41-58. Academic Press, London, 1989. 

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