Amorphous interface

In the next section, we will see that for the commonly encountered symmetric amorphous interface that... 

Fluorescence decay measurements have also allowed one to propose a qualitative model of film occupancy in which both amorphous sites and crystal-line-amorphous interfaces would be occupied by ester molecules, with predominance of the former ones [291]. 

The first term in Eq. (5.11) reflects the gain in the bulk energy while the second term accounts for the variation in the total free energy associated with the replacement of the substrate/amorphous and vapour/amorphous interfaces (dAi) by the substrate/crystal and vapour/crystal interfaces. The last term represents the increase in the total free energy due to the increase in the crystal/amorphous interface (dA2). Taking into account Eq. (5.11) and the expressions for dV, dAi and dA2 given above, dG/dN can be expressed as ... 

Current research interest is in the solid phase epitaxial regrowth of amorphous Si using laser processing. RBS has been used to follow the melting and recrystallization of the crystal-amorphous interface (12). This is accomplished by monitoring the backscattered spectrum+with the substrate oriented in a direction that will allow the He to channel along the crystal planes. 

Fig. 7.8. TEM cross-section of an undoped PLD ZnO thin film on 3C-SiC buffered Si(lll), grown at 0.016mbar O2 and 620°C. The SAD pattern (inset) was taken from the circled area. The HRTEM image (right) of the interface shows residual 3C-SiC and an amorphous interface layer. Images by G. Wagner, Leipzig...

The dose required for amorphization is a function of the kinetics of simultaneous dynamic recovery processes. The recovery process is accelerated at elevated temperatures and, in many cases, is greatly increased by radiation-enhanced defect migration. These simultaneous recovery processes may be associated with defect recombination or annihilation, epitaxial recrystallization at crystalline-amorphous interfaces (Carter and Nobes 1991), or nucleation and growth recrystallization in the bulk of the amorphous state. For any crystalline solid, there is a critical temperature, above which the rate of amorphization is less than the rate of recovery, thus amorphization cannot occur. However, Tc also depends on the energy and mass of the incident beam, as well as the dose rate. 

Nonequilibrium molecular dynamics studies of the indentation process show a transition to the amorphous phase in a region a few atomic layers thick surrounding the lateral faces of the indentor [42], as has been suggested by experimental results [43]. This possibility has also been suggested by modeling of the crystalline-amorphous interface [40]... 

From the observations on branched PEO and alkanes more general conclusions can be drawn about the overcrowding problem at the crystalline—amorphous interface in polymers and about the mechanism of chain deposition during crystal growth. Thus, e.g., although for the Y-shaped alkane the energeti-... 

Evidence for the effect of chain ends on interfacial energy is provided by a number of experiments. The melting points of short PEO-containing diblocks [26] and triblocks [27,28], Tm=47-51 °C,is low compared to perfectly crystalline PEO (Tm=76 °C). This is due to the positive free energy of formation for the block copolymers of the amorphous layer from the melt, and of the crystal-line/amorphous interface. The end interfacial theory can be analyzed in terms of the theories for melting points of low molecular weight polymers [29,30]. 

Measurements of the growth velocity, vg of the crystal-amorphous interface are shown in Fig. 10.6. The measured velocities extend over nearly ten orders of magnitude and can be characterized by a single-activation energy, EA = 2.76 eV, (Olson and Roth 1988) so that... 

The adsorption of the oxygen atom is the first stage to control the formation of the Si/Si02 interface [70]. The amorphous interface that in experimental conditions contains impurities can not be accurately represented by a perfect crystalline model. Despite its simplification, such models hopefully can help to understand and control the material [70]. We also hope that the knowledge of the adsorption of other adsorbates can help building new materials for the future electronic devices. 

These questions are the basis of polymer morphology, which may be defined as the study of the structure and relationships of polymer chains on a scale large compared with that of the individual repeat unit or the unit cell, i.e. on the scale at which the polymer chains are often represented simply by lines to indicate the path of the backbone through various structures. In addition to the four questions above, morphology is concerned with such matters as the directions of the chain axes with respect to the crystallite faces and with the relationship between the crystallites and the non-crystalline material, a particular aspect of which is the nature of the crystalline-amorphous interface. Sections 5.2-5.5 are concerned... 

I2, T2) attributed to positrons trapped at the crystalline-amorphous interface had T2 0.32 ns and h exhibited a precipitous decrease from about 58% to about 50% at the yield point, followed by recovery back to about 58%. This phenomenon interpreted as indicating interfacial loss of defects occurs during the initial deformation process and then some unknown recovery process takes place subsequently. 

In another work, Velasco-Santos et al. measured reasonably large increases in modulus from 0.71 GPa for a methyl-ethyl-methacrylate copolymer to 2.34 GPa at 1 wt% arc-MWNT. This corresponds to a reinforcement of d T/d Ff 272 GPa which is on a par with the value (scaled) for PVA composites from Cadek et al However, in this work no nucleation of crystallinity was observed. This suggests that good stress transfer can be obtained at an amorphous interface, depending on the polymer. 

The semi-crystalline HDPE being modeled is initially of a sphernlitic morphology described in Chapter 2. It is made up of a 3D packing of crystalline lamellae and their attached amorphous layers as idealized in Fig. 9.25(a). The basic elements of the spherulite are two-phase composite inclusions that consist of integrally coupled crystalline lamellae and their associated amorphous layers between lamellae. Owing to their large aspect ratio, the composite inclusions are modeled as infinitely extended sandwiches with a planar crystalline/amorphous interface as shown in Fig. 9.25(b). Each composite inclusion I is characterized by its interface normal rt and the relative fractional thicknesses and / = 1 / of... 

We assume that there is no relative slippage at the crystalline/amorphous interface. Then the interface compatibility condition demands velocity continuity across the crystalline/amorphous interface. These compatibility conditions in conjunction with incompressibility in both phases require definite continuity conditions on strain-rate and spin components in the inclusion between the crystalline and amorphous components. Moreover, the crystalline/amorphous interface also enforces shear-traction equilibrium across the interface. More complete statements of the compatibility, continuity, and incompressibility constraints necessary for the full implementation of the model can be found elsewhere (Lee et al. 1993a). 

The configuration equilibrium observed in solution can be displaced when one isomer can be removed from the equilibrium, by, for example, crystallization. For example, when 1,4-poly(butadienes) of high trans content are dosed with trace amounts of an all-trans poly(butadiene), a decrease in the trans content is initially observed. Subsequently, however, the trans content increases again. It is assumed that a trans isomerization occurs at the crystalline-amorphous interface, whereby the longer trans sequences are incorporated into the crystal lattice and thereby removed from the equilibrium. A new equilibrium is then established by producing more new trans sequences. 

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