Whisker mechanism

The carbon whisker mechanism can be blocked by the use of noble metal catalysts because these metals do not dissolve the carbon. 

The morphology of the carbon on the surface can assume several forms a two-dimensional film or so-called whisker carbon, which is formed when the carbon dissolves in the supported metal catalyst, diffuses through the metal, and forms a growing filament that lifts the metal from the catalyst surface. Whisker carbon is typically associated with Ni-based catalysts because carbon is soluble in Ni at reforming conditions. Whisker carbon tends to form at higher temperatures, low steam to hydrocarbon ratios and higher aromatic content of the feeds. This type of carbon formation may be minimized by the use of precious metals as catalysts, because these metals do not dissolve carbon. On a nickel surface, the whisker mechanism can be controlled by sulfur passivation. 

Low alloy steels which are not protected by a dense oxide layer may be attached by metal dusting. Carbon which is formed via the whisker mechanism reacts with alloy components to carbides resulting in a disintegration of the alloy (Figure 5.13) into dust, which is blown away in the gas flow. 

Carbon can be formed via the whisker mechanism as for methane (reaction (9)), but it may also be formed by thermal cracking of the hydrocarbons (reaction (10)) taking place above 600-650 C. In fact, a steam reformer without a catalyst would operate like a pyrolysis furnace (steam cracker) for ethylene production. 

In all of these processes it is possible to increase the yield of whiskers by a dding metallic impurities, and the sublimation process requires such additions. The vapor—Hquid—sohd (VLS) growth mechanism is often thought to be involved. 

Toughening for whisker-reinforced composites has been shown to arise from two separate mechanisms frictional bridging of intact whiskers, and pullout of fractured whiskers, both of which are crack-wake phenomena. These bridging processes are shown schematically in Figure 13. The mechanics of whisker bridging have been addressed (52). The appHed stress intensity factor is given by ... 

Experimentally it has been shown that both frictional bridging and whisker pullout play an important role in toughening industrially manufactured composites. Such investigations confirm that to maximize toughness via both mechanisms requires a high volume fraction of whiskers and a high composite modulus to whisker modulus ratio. For example, consider the effect of 20 vol % SiC whisker E = 500 GPa) reinforcement of various matrices on the toughness as presented in Table 7 (53). 

Short Random Fibers. The whiskers bridging mechanics given in equations 21 through 25 apply also to short random fiber bridging mechanisms. The bridging terms come from (44) ... 

The term s plastic, polymer, resin, elastomer, and reinforced plastic (RP) are some-what synonymous. However, polymer and resin usually denote the basic material. Whereas plastic pertains to polymers or resins containing additives, fillers, and/or reinforcements. Recognize that practically all materials worldwide contain some type of additive or ingredient. An elastomer is a rubberlike material (natural or synthetic). Reinforced plastics (also called composites although to be more accurate called plastic composites) are plastics with reinforcing additives, such as fibers and whiskers, added principally to increase the product s mechanical properties. 

Alternatives to activated tungsten wire emitters are also known, but less widespread in use. Cobalt and nickel [44,47] as well as silver [48] can be electrochemi-cally deposited on wires to produce activated FD emitters. Mechanically strong and efficient emitters can be made by growing fine silicon whiskers from silane gas on gold-coated tungsten or tantalum wires of 60 pm diameter. [45] Finally, on the fracture-surface of graphite rods fine microcrystallites are exposed, the sharpness of which provides field strengths sufficient for ionization. [49]... 

Becher, P.F. and Tiegs, T.N. (1987). Toughening behavior involving multiple mechanisms whisker reinforcement and zirconia toughening. J. Am. Ceram. Soc. 70, 651-654. 

The enhanced strength of whiskers and natural fibers, by comparison to the strength of materials of the same composition in another morphology, could be a coincidental in these crystalline synthetic and mineral fibers, a particular crystal direction is parallel to the direction of the applied stress. However, the inverse diameter-strength relationship indicates that factors other than crystal structure contribute to the mechanical strength of fibrous materials. 

Cook, J. (1970). Mechanical testing of whiskers. Composites, March, 176—180. 

Evans, C. C. (1972). Whiskers. M B Monograph Series in Mechanical Engineering, No. 8, Mills and Boon, Ltd., London. 

Webb, W. W., H. D. Bartha, and T. B. Shaffer (1966). Strength eharacteristics of whisker crystals, macrocrystals and microcrystals, pp. 329-354. In Strengthening Mechanisms, Metals and Ceramics. Burke, J. J., N. L. Reed, and V. Weiss, Eds. Syracuse University Press, New York. 

Fig. 2.23 The VLS mechanism of unidirectional growth of a fiber. Vapor condenses on a substrate and continues to localize at the site, providing the necessary material for whisker growth.

The theoretical mechanical strength of perfect carbon fibers is between 14 and 20 X 10 psi, while the Young s modulus for graphite whiskers was calculated as 145 X 10 . Diamond (the most dense crystalline C modification at 3.5 g/cc, compared with graphite at 2 to 2.22 g/cc) has a Young s... 

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