Metastable defect models

As described above, there is a striking correlation between the creation and annealing kinetics of metastable defects in a-Si H and the motion of bonded hydrogen. There are essentially two classes of models proposed to both account for this agreement and to explain the microscopic mechanisms for hydrogen diffusion. One type of model assumes that the hydrogen is intrinsically mobile and moves from one bonded position to another... 

The following discussion concentrates on the creation of bulk dangling bond defects, which may not be the only process, but is almost certainly the dominant one. Dersch, Stuke and Beichler (1980) were the first to show that illumination causes an increase in the g = 2.0055 paramagnetic defect and concluded that the Staebler-Wronski effect was the creation of dangling bonds. The metastable defect creation and annealing is described by the potential well model shown in Fig. 6.1, except that the barrier is overcome by the recombination energy from... 

Hulbert [77] discusses the consequences of the relatively large concentrations of lattice imperfections, including, perhaps, metastable phases and structural deformations, which may be present at the commencement of reaction but later diminish in concentration and importance. If it is assumed [475] that the rate of defect removal is inversely proportional to time (the Tammann treatment) and this effect is incorporated in the Valensi [470]—Carter [474] approach it is found that eqn. (12) is modified by replacement of t by In t. This equation is obeyed [77] by many spinel formation reactions. Zuravlev et al. [476] introduced the postulate that the rate of interface advance under diffusion control was also proportional to the amount of unreacted substance present and, assuming a contracting sphere (radius r) model... 

Doyen [158] was one who theoretically examined the reflection of metastable atoms from a solid surface within the framework of a quantum- mechanical model based on the general properties of the solid body symmetry. From the author s viewpoint the probability of metastable atom reflection should be negligibly small, regardless of the chemical nature of the surface involved. However, presence of defects and inhomogeneities of a surface formed by adsorbed layers should lead to an abrupt increase in the reflection coefficient, so that its value can approach the relevant gaseous phase parameter on a very inhomogeneous surface. 

We will then examine other flexible polymer crystallization instances which may be interpreted, at least qualitatively, in terms of the bundle model. We will concentrate on crystallization occurring through metastable mesophases which develop by quenching polymers like isotactic polypropylene, syndiotactic polypropylene etc. In principle also hexagonal crystallization of highly defective polymers, and order developing in some microphase-separated copolymer systems could be discussed in a similar perspective but these two areas will be treated in future work. A comparison between the bundle approach and pertinent results of selected molecular simulation approaches follows. 

Amorphous metals can be prepared in a wide variety of stable and metastable compositions with all catalyti-cally relevant elements. This synthetic flexibility and the isotropic nature of the amorphous state with no defined surface orientations and no defect structure (as no long-range ordering exists) provoked the search for their application in catalysis [21]. The drastic effect of an average statistical mixture of a second metal component to a catalytically active base metal was illustrated in a model experiment of CO chemisorption on polycrystalline Ni which was alloyed by Zr as a crystalline phase and in the amorphous state. As CO... 

The application of this model to polydiacetylenes ( ) leads to different results since the chain structure must be different on either side of the defect, as shown in Figure 15. Bond-alternation defect must, therefore, be created in pairs at either end of a polybutatriene sequence. The creation of the new chain structure will require considerable energy and will be hindered by bulky, interacting side-groups. Thus, they are liable to be few in number, and, in view of the higher energy intrinsic and the relative instability of the polybutatriene structure, will be metastable. The occurrence of such defects will be favored in partially crystalline materials with small side-groups such as... 

The third threshold amplitude, Vq, is important for the precise meaning of the static friction Fj. Below this threshold the ground state of the undriven FKT model is the only mechanically stable state. For Vo > Vq additional metastable states appear. The first metastable state is not very different from the ground state. It can be described as a ground state plus a defect, which separates two equally sized domains. The motion of this defect allows sliding to occur. 

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