Defects zigzag

IrB lrBo.7 No Trigonal prism Defect zigzag chains... 

If there is no defect present, the temporal change of this zigzag region is period-2. The teinporsl change of the domain boundary is chaotic (i.e. it has a positive Lyapunov exponent). 

Regime II Defect Diff ision the zigzag pattern is modulated quasi-periodically in time. The chaos in the domain boiindary is still localized but it can now move about in space. For r 0.1, the phase change Regime I —> Regime II occurs at about a 3.82. 

The possible fatigue failure mechanisms of SWCNT in the composite were also reported (Ren et al., 2004). Possible failure modes mainly include three stages, that is, splitting of SWCNT bundles, kink formation, and subsequent failure in SWCNTs, and the fracture of SWCNT bundles. As shown in Fig. 9.12, for zigzag SWCNT, failure of defect-free tube and tubes with Stone-Wales defect of either A or B mode all resulted in brittle-like, flat fracture surface. A kinetic model for time-dependent fracture of CNTs is also reported (Satapathy et al., 2005). These simulation results are almost consistent with the observed fracture surfaces, which can be reproduced reasonably well, suggesting the possible mechanism should exist in CNT-polymer composites. 

Fig. 9.12 Results of molecular mechanics simulations (a) A Stone-Wales defect (A mode) in a zigzag SWCNT, (b) a Stone-Wales defect (B mode) in a zigzag SWCNT, the bonds with highest potential energy are indicated by arrows. Propagating cracks in (c) A defect-fiee zigzag tube, and (d) defect-lfee armchair tube. Fracture mode of armchair tube with (e) Stone-Wales defect (A mode), and (f) Stone-Wales defect (B mode). Fracture mode of zigzag tube with (g) Stone-Wales defect (A mode), and (h) Stone-Wales defect (B mode) (Huynh et al., 2002. With permission from Wiley)...

Figure 14.17 Idealized representation of defect-free (n,m)-SWNTs with open ends. Left a metallic conducting (10,10)-tube (armchair) middle a chiral, semiconducting (12,7)-tube right a conducting (15,0)-tube (zigzag). Armchair and zigzag tubes are achiral.

For the binary alkaline earth sihcide SiSr, two different structures have been reported. One contains one-dimensionally extended zigzag chains beside isolated Si" atoms [73]. Schafer et al. prepared a modification with the same composition, which instead contained isolated sUicide units of ten atoms. In these units, planar hexagons are substituted in the 1-, 2-, 4-, and 5-ring positions by four additional Si atoms. An isostructural compound was found for germanium as well, but showing defects in this unit in the positions 1, 2, 4, and 5. Both materials could not be obtained from stoichiometric approaches, and their formation obviously is coupled to strontium excess [69] (Fig. 3). 

The attempt of systematic theoretical investigation of the influence of defects on geometrical configuration and electronic structure of carbon zigzag and armchair nanotubes is undertaken. 

The graphitization stage (HTT > 2000°C) corresponds to complete dewrinkling of the layers, which become stiff and nearly perfect within each LMO area (stage 4 in Figure 4). Rapid development of three dimensional order then occurs because the defects situated at the boundaries of the zigzag domains have been suddenly removed. 

In view of wideness of information connected with the influence of impurities on geometric and physical characteristics of nanotubes [3], here are presented only results of investigations of the local defects in zigzag (12,0)-nanotubes to appear after removing one or two neighboring carbon atoms is presented. Especial attention is paid to research of vacancies as more widespread kind of breach of regularity in arrangement atoms in nano tubes. We use for vacancies the... 

Polyvinylidene fluoride is a semicrystalline polymer (35-70% crystallinity) with an extended zigzag chain.f Head-to-tail addition of VDF dominates, but there are head-to-head or tail-to-tail defects that affect crystallinity and properties of PVDF. It has a number of transitions, and its density alters for each polymorph state. There are four known states, named as a, (3, y, and 8, and a proposed state. The most common phase is a-PVDF, which exhibits transitions at -70°C (y), -38°C (p), 50°C (ot"), and 100°C (a ). 

Poly(vinyl fluoride) is a semicrystalline polymer with a planar, zigzag conformation.The degree of crystallinity can vary significantly from 20-60% and is a function of defect structures. Commercial PVF is atactic, contains approximately 12%o head-to-head linkages, and displays a peak melting point of about 190°C (52,53,62,63). Poly(vinyl flouride) displays several transitions below the melting temperature. Lower Tg occurs at — 15 to —20°C and upper Tg is in the 40-50°C range. Two other transitions at 80 and 150°C have been reported. 

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