Free-standing shapes

Molybdenum is a high-strength refractory metal, although recrystallizes above 950°C with accompanying reduction in mechanical properties. It is easily fabricated. Its properties are summarized in Table 6.6. CVD is commonly used for the production of molybdenum coatings and free-standing shapes. 

Niobium (also known as columbium) is a soft, ductile, refractory metal with good strength retenti on at high temperature, and a low capture cross-section for thermal neutrons. Itis readily attacked by oxygen and other elements above 200°C. CVD is used to produce coatings or free standing shapes. The properties of niobium are summarized in Table 6.8. 

The refractory metals for which CVD is commonly used to produce free-standing shapes are tungsten, niobium, rhenium, tantalum, molybdenum, and nickelb lb lb l (see Ch. 6). Shapes presently produced include rods, tubes, crucibles, manifolds, ordnance items, nozzles, and thrust chambers. They are usually deposited on a disposable mandrel of copper, molybdenum, or graphite which is subsequently machined off or removed chemically by etching. 

Kaplan, R., and Tuflfias, R., Fabrication of Free Standing Shapes by Chemical Vapor Deposition, Proc. of AES Int. Symp. on Electroforming/Deposition Forming, Los Angeles, CA (Mar. 1983)... 

It is only since the end of World War 11 that the CVD of graphite began to expand rapidly as researchers realized the potential of this technique for the formation of coatings and free-standing shapes. The importance and impact of pyrolytic graphite have been growing ever since. 

Coatings with athickness of 1 mm or more are now routinely produced. After removing the substrate, a free-standing shape remains with good integrity and properties similar to single-crystal diamond as reviewed in Sec. 5.6. 

Commercial prockiction of free-standing shapes up to 1 mm thick by Norton, General Electric, and others... 

Advances in deposition techniques have now made possible the fabrication of free-standing shapes such as the 7.6 cm (3 ) diameter hemispherical dome for IR-window applications shown in Fig. 13.10 which may eventually replace the present IR materials. 

By properly designing furnace hardware and controlling the deposition process, pyrolytic graphite can be manufactured commercially as solid plate, cones, tubes and other free-standing shapes. Because the deposition process relies, in part, on diffusion of the hydrocarbon gas to the heated mandrel, it also is possible to densify porous... 

The commercially available forms of pyrolytic graphite include plate stock, tubes and free-standing shapes. In addition, pyrolytic graphite is used commercially to densify porous structures such as carbon felts, fabrics, composites of carbon/graphite yams and that can withstand the deposition temperature and are compatible with pyrolytic graphite. It also is used to coat conventional (bulk) graphites where increased resistance to oxidation and chemical attack are desired. 

In summary, we have shown how two very similar Mo and Nb oxide mesostructured systems leads to formation of either continuous macroporous structures with extended tube lengths or free-standing torroids with limited extension in the third dimension. We attribute this difference to relative rates of condensation versus nucleation in each system. A unique TEM-induced reaction involving evolution of shape was also discussed and demonstrates that many of the new mesostructures with order on the macroscopic level may also show dynamic behavior. 

In this section advanced developments of the PLD technique are shortly described, e.g., the combinatorial approach and laser MBE. Because of the increasing research interest in ZnO-based nanostructures, the established PLD (at background gas pressure 10-4—3 mbar) was extended to much higher pressures of 50-200 mbar to grow arrays of free-standing ZnO nanowires. This unique high-pressure PLD process allows the growth of ZnO-based nanostructures with controlled shape and diameter and excellent optical properties. 

The particularly intriguing result of this laser chemical deposition was that the deposited boron formed as a free-standing, albeit narrow fiber. The shape of the... 

When well-defined, less Lewis-acidic metathesis polymerization catalysts are used to polymerize COT, a lower level of detectable sp defects are formed. Also, although the polyacetylene produced is still insoluble, the reaction proceeds slowly enough to allow manipulation of the liquid reaction solution before hardening. In this way, one can obtain films in a desired shape and location, e.g., on a semiconductor [123]. This procedure was found to result in better electrical contact than can be obtained when a free-standing film prepared via the Shirakawa route is simply pressed against an electrode. 

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