Cracked oxide theory

This statement is in contrast to the cracked oxide theory, which surmises that oxides must be locally weak for a whisker to form. 

VII. WHISKER FORMATION AND GROWTH BASED ON CRACKED OXIDE THEORY... 

A. Conditions of Tin Whisker Growth—Cracked Oxide Theory (COT)... 

There are many intriguing issues concerning whisker growth such as, why do Sn films grow whiskers spontaneously at room temperature, whereas Au films do not There are also the issues of driving force, kinetics, and specific atomistic mechanism/s associated with whisker growth. The cracked oxide theory (COT), as initially proposed by K.N. Tu [54], addresses these necessary conditions somewhat differently than the above described recrystallization concept. 

Various crack advance theories have been proposed to relate crack propagation to oxidation rates and the stress-strain conditions at the crack tip, and these theories have been supported by a correlation between the average oxidation current density on a straining surface and the crack propagation rate for a number of systems [12,35]. There have been various hypotheses about the precise atom-atom rupture process at the crack tip—for example, the effect that the enviromnent has on the ductile fracture process (e g., the tensile ligament theory [36], the increase in the number of active sites for dissolution because of the strain concentration [37], the preferential dissolution of mobile dislocations because of the inherent chemical activity of the solute segregation in the dislocation core [38]). 

Dislocation theories were first proposed in 1953 by Eshelby [26] and Frank [25]. Recrystallization was first proposed by Ellis et. al. in 1958 [37]. The cracked oxide concept was proposed by K.N. Tu in 1994 [54]. 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

Theories of the oxidation of tantalum in the presence of suboxide have been developed by Stringer. By means of single-crystal studies he has been able to show that a rate anisotropy stems from the orientation of the suboxide which is precipitated in the form of thin plates. Their influence on the oxidation rate is least when they lie parallel to the metal interface, since the stresses set up by their oxidation to the pentoxide are most easily accommodated. By contrast, when the plates are at 45° to the surface, complex stresses are established which create characteristic chevron markings and cracks in the oxide. The cracks in this case follow lines of pores generated by oxidation of the plates. This behaviour is also found with niobium, but surprisingly, these pores are not formed when Ta-Nb alloys are oxidised, and the rate anisotropy disappears. However, the rate remains linear it seems that this is another case in which molecular oxygen travels by sub-microscopic routes. 

For quite some time, it was not clear whether the catalytic cracking process of these aluminosilicates involves the concerted action of both Lewis and Bronstedt acid sides, or only Lewis sites, or even the dissociation of the adsorbed hydrocarbons by surface metal ions (impurities).16,17,18 19 The need for oxide systems that allow evidence concerning the above catalytic theories was one of the driving forces for the efforts that have been done to coat a silica surface with aluminium compounds. One of the earlier studies, concerning both the reaction mechanisms of the aluminium... 

At room temperature, PP is close to its Tg(0-25°C) and well above its normal brittle-ductile transition temperature ( -30°C). However the presence of surface cracks in the photo-oxidized film is apparently sufficient to promote brittle failure at room temperature. According to the Griffith crack theory, once a critical crack length has been exceeded, a critical crack velocity is required to propagate the crack. If this velocity is not exceeded, cold drawing of the amorphous zones ensues. 

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