Mechanism Transitions

Facilitates GI transit/mechanical intestinal allodynia Cardiac arrhythmia... 

The matter of predicting flow pattern transitions is still in a developing state. In some cases the transition mechanisms are reliably understood, other mechanisms suggested are more in the nature of speculations.. .. If the ability to predict these transition conditions is to become more reliable and more generalized, it will require considerable additional research into mechanisms and into the basic fluid mechanical factors that enter into these mechanisms. 

Figure 3.7 Schematic view of the relative locations of transition mechanisms. (From Dukler and Taitel, 1991b. Copyright 1991 by University of Houston, Houston, TX. Reprinted with permission.)...

Pattern transition in vertical adiabatic flow. Upward vertical flow has been studied intensively, both because of the simplicity of the geometric condition and the relevance in applications. The map shown in Figure 3.4 is the result of rather recent and relevant studies into the interpretation of regime transition mechanisms. In this figure, the transition between bubbly flow and slug flow occurs be-... 

The morphologies of the large ionic clusters observed in these simulations rather suggest free chain end folding to produce rudimentary lattice structure as a possible pre transitional mechanism. 

Theoretical Modelling of the Phase Transition Mechanism of Hydrogen-Bonded Ferroelectrics... 

Because of the prediction of coexistence of order/disorder in [26], Dalai et al. [27] reinvestigated the phase transition mechanism with an ultra-high resolution NMR method, which is based on magic angle spinning (MAS) and is described in detail by Dalai et al. in Chap 2 of this volume. The advantages of this method as compared to conventional NMR techniques are that it is ... 

Abstract This chapter describes the experimentai compiement of theoretical models of the microscopic mechanism of ferroelectric transitions. We use the hydrogen-bonded compounds as examples, and attempt to show that the new experimental data obtained via recently developed high resolution nuclear magnetic resonance techniques for solids clearly support the hypothesis that the transition mechanism must involve lattice polarizability (i.e. a displacive component), in addition to the order/disorder behaviour of the lattices. 

Fig. 9 Temperature dependence of 5iso for the NMR peaks in squaric acid in the close vicinity of the phase transition temperature left panel). Note that the change starts as a smooth curve, followed by a jump caused by the first-order character of the phase transition. This is considered evidence for the coexistence of an order/disorder and displacive character in the phase transition mechanism [20]. The right panel gives a comparison between theoretical and experimental data. For details, see text...

Fig. 12 Temperature dependence of the average MAS peaks at 14.1 Tesla in the vicinity of the antiferroelectric transition at 372 K. Note that the value above the transition differs considerably from its value below the transition temperature, thus yielding clearer evidence for the role of the displacive component in the transition mechanism [26]...

Fig. 15 Temperature dependence of the isotropic chemical shift for a 80% deuterated crystal of KD2PO4. Notice the anomaly around Tc, pointing to the existence of a displacive component in the transition mechanism [25]...

Fig. 20 Temperature dependence of 8 so left), and expanded view around Tn (right) without the jump for ADA around its phase transition temperature at 216 K. Note the large anomaly around Tn, indicating the presence of a displacive component in the transition mechanism [27]...

Fig. 23 Temperature dependence of 8iso for ADP around Tn 148 K. Note the anomaly in the isotropic chemical shift around 148 K, emphasizing the role of the displacive component in the phase transition mechanism...

Recently, perfect softening of the ferroelectric TO mode was confirmed by Takesada et al., so ST018 is considered to be a displacive-type ferroelectric [11]. Kvyakovskii [ 12] gave an explanation for the phase transition mechanism. Part of the theory is summarized below. 

However, it seems that possibilities of this model to explain some of the experimental results were not fully explored. Also, this model shows some weaknesses. First, it predicts a value of Tc in DKDP (310 K) that is higher than the experimental value (229 K). Second, the calculated rate of the proton transfer in the PE phase is too low to explain the width of the CP in the neutron scattering spectra of KDP [62]. Third, and most important, because the model assumes that dipoles of PO4 groups lie along the c-axis, it cannot to explain static and dynamic properties of the transverse polarization fluctuations, whose importance in the phase transition mechanism has been supported by several experimental results [8,20,28,31-33]. 

Abstract This chapter comments on the motivations and the methods of crystallographic studies at low temperature. Cry-crystallography is a brunch of Crystallography, a science that is too often confused with a technique. On the other hand, the scientific background to study crystal phases at low temperature is here provided, together with a survey of many possible techniques that provide complementary or supplementary information. Several apphcations are discussed, in particular in relation with highly accurate studies like electron density determination or phase transition mechanisms. 

---