Boron-tungsten

I. ELEMENTAL BORON, TUNGSTEN BRONZES, AND METAL SULFIDES... 

Figure 7.13 Calibration curves for boron, tungsten and molybdenum from 0.0 to 4.0gg/ml (ppm) using scandium Sc as an internal standard. The units on the Y axis are flask numbers of standards 0.0, 1.0, 2.0, 3.0 4.0 ppm metal respectively...

Boron trichloride (BCI3). It boronizes tungsten at 900 to lOOO C. 

These data evidence that tungsten and boron hydrides are the most promising hydrogen accumulators of the hydrides studied the former being efficient in volume accumulation, the latter in weight one. The capacity of CeoHeo is close to that of boron tungsten. Both the hydrides may meet the demands of BMW car production to the mass capacity of hydrogen accumulators, namely it is to be not less than 10 %. 

Alloys are metallic substances containing two or more elements which are miscible when molten and do not separate when solidified. They may be liquid or solid. This mixture of elements, usually but not necessarily metals, allows careful manipulation of strength, melting point, corrosion resistance, magnetic, thermal, electrical, and other properties steel, for example, is an alloy of iron and carbon often present with nickel, chromium, copper, aluminium, boron, tungsten, manganese, cobalt, silicon, and other elements. 

Boron/tungsten filament endless solid two comm. vapor phase 3.2.2... 

The hot fiber (wire) CVD process has been commercially used for 30 years to produce continuous sheath/core bicomponent boron/tungsten and silicon carbide/carbon fibers. Since they are continuous fibers, they are discussed in Chapter 3.3. More recently, this process was used to produce discontinuous, i.e., short, experimental sheath/core diamond/carbon fibers by depositing a thick diamond sheath on short pieces of a potentially carbon fiber. 

Figure 8 contrasts a boron/tungsten fiber with a pure boron fiber. Aside from the >10x difference in diameter, the differences in surface texture are noteworthy. The surface of the pure boron fiber made by high pressure LCVD is smooth. Its strength is 7.5 GPa and its modulus is 400 GPa. In contrast, the surface texture of the boron/tungsten fiber is "nubby". Its strength is 3.6 GPa and its modulus is 400 GPa. In summary, the tensile strength of boron fibers is related to their surface uniformity. 

Figure 7. Schematic diagram of the CVD boron/tungsten fiber process. Redrawn from M. L. Dorf, Product bulletin, Textron Specialty Materials. Lowell, MA.

Figure 8. Boron fibers made by hot filament and by laser assisted CVD. This illustration compares the fiber diameter and surface character of a sheath/core boron/tungsten fiber (A,C) with that of a pure boron fiber (B,D). Reproduced from F. T. Wallenberger and P. C. Nordine, Strong, Small Diameter Boron Fibers by Laser Assisted Chemical Vapor Deposition, Materials Letters, 14 [4] 198-202 (1992). With permission from Elsevier Publishers (1992).

The technology of manufacturing sheath/core bicomponent boron/ tungsten, boron/carbon, silicon carbide/tungsten, and silicon carbide/ carbon fibers is 40 years old and the relationships between process variables, structures, and properties have been authoritatively described in important review articles. One article deals mainly with their preparation [33] another correlates process variables with structures [34], and one explores potential correlations between structures and properties [30]. 

Table V. Polished and unpolished boron/tungsten fibers [after 35]...

Commercial boron/tungsten fibers are, in practical terms, limited to fiber diameters of 100-140 pirn, and strength levels up to 4.8 GPa. Pure boron fibers can be made with diameters of >6 Ijm and a strength levels 7.6 GPa, i.e., with 1.6x the maximum strength at 0.06-0.04x the diameter of the former. High specific properties (strength or modulus divided by density) are... 

Boron/tungsten fiber applications include the use of filaments and of boron/tungsten fiber reinforced prepreg tape, aluminum matrix composites, and boron/graphite structures. The major applications for these structures are found in the aerospace market and about 25% in sporting goods markets [36]. SiC/carbon fiber reinforced products include aluminum, titanium, and ceramic matrix composites. Major applications for these structures are also found in the aerospace market, minor uses in the industrial market [37]. 

Key boron/tungsten iiber reinforced epoxy tapes are used in the aerospace market in selected applications on the F-14 and F-15 fighters, the B-1 bomber, the Blackhawk series Sea Stallion... 

Commercial boron/tungsten fibers are derived directly from the vapor phase. Commercial silicate glass fibers and most commercial silica glass fibers are derived from their melts, but some silica fibers, as discussed in Chapter 5, can be derived from viscous aqueous solutions. Glass fibers are therefore derived directly from a liquid phase. Commercial ceramic and carbon fibers are produced from solid precursor, or green, fibers which, in turn, are derived from a melt, dispersion, or viscous solution. This section of the book deals with fibers which are derived from solid precursor fibers. 

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