Temperature-mechanical field phase

The state of polarization, and hence the electrical properties, responds to changes in temperature in several ways. Within the Bom-Oppenheimer approximation, the motion of electrons and atoms can be decoupled, and the atomic motions in the crystalline solid treated as thermally activated vibrations. These atomic vibrations give rise to the thermal expansion of the lattice itself, which can be measured independendy. The electronic motions are assumed to be rapidly equilibrated in the state defined by the temperature and electric field. At lower temperatures, the quantization of vibrational states can be significant, as manifested in such properties as thermal expansion and heat capacity. In polymer crystals quantum mechanical effects can be important even at room temperature. For example, the magnitude of the negative axial thermal expansion coefficient in polyethylene is a direct result of the quantum mechanical nature of the heat capacity at room temperature." At still higher temperatures, near a phase transition, e.g., the assumption of stricdy vibrational dynamics of atoms is no... 

The phase transition rate in the crystallization of polymeric materials is of the same order as the rates of the heat exchange processes accompanying crystallization. Consequently, the boundary between phases becomes spatially dispersed. This excludes the possibility of using methods based on the front transition model proposed for metals to calculate residual stresses in plastics.148 It is possible to split the general problem and to find the temperature-conversion field independently. Then, assuming that the evolution of temperature T(x,t) and degree of crystallinity a(x,t) in time t and in space (x is the radius vector of an arbitrary point in a body) is known, we can analyze the mechanical problem.143... 

What could be the reason for the failure of the additivity law Obviously one has to assume that, for multicomponent and/or multiphase systems, when one of the components (phases) is characterized by a viscosity at room temperature which is typical for low-molecular-weight liquids, the microhardness behaviour of the entire system should be different from the case in which all the components (phases) have TgS higher than room temperature because the mechanism of the response to the applied external mechanical field is different. In the latter case all the components (phases) plastically deform as a result of the applied external force. In the former... 

Thus, it seems to be of interest to examine the influence of stress-induced polymorphic changes on the microhardness. While in the case of f-PP two samples comprising the a or phase were characterized, here we wish to follow the microhardness behaviour during the a-j6 polymorphic transition caused by a mechanical field. For this purpose PBT has been selected as a suitable material because of its ability to undergo stress-induced polymorphic transition from the a (relaxed) to the P (strained) form. Bristles of commercial PBT with a diameter of about 1 mm were drawn at room temperature via neck formation (final diameter about 0.5 mm and draw ratio of 3.4) and thereafter annealed in vacuum at 200°C for 6 h with fixed ends (Fakirov etal., 1998). 

C. A composition in this two-phase field should have superior high-temperature mechanical properties. Greskovich [21] has synthesized ceramics in this phase field, and high-temperature stress rupture tests showed that they are the most stable silicon nitride ceramics among all of the other systems studied. 

In summary, we calculate that the low clinoenstatite is not stable under hydrostatic conditions and enstatite has a comparatively small stability field. The energy differences are so small that they are within the reliability of the simulations and thus the precise positions of the phase boundaries are not well located. The primary reason for this problem is the reliability of the potential models. Hence, calculating phase relationships represents the most difficult challenge for free energy minimization techniques. However, the simulations do provide valuable insights into the mechanisms of phase transitions and the effect of pressure and/or temperature on the crystal structures and the relative phase stabilities. 

In the Ni-Fe system at room temperature, the a phase extends from 0 to 7% Ni, then a. Fy mixtures from 7 to 50% Ni, and the y phase from 50 to 100% Ni. y-Phase alloys in the Ni-Fe system, known as Permalloys, exhibit a wide variety of magnetic properties, which may be controlled precisely by means of well-established technologies. Initial permeabilities up to 10 in an extremely wide temperature range, as well as coercive fields between 0.16 and 800 A/m, can be obtained (Chin Wemick, 1980). Induced anisotropy of 65-85% Ni alloys can be drastically varied by field annealing and mechanical deformation (slip-induced anisotropy) an order-disorder transformation occurs for Ni3Fe finally, preferential orientation can be induced in 50%Ni-50%Fe. 

Thus, all monomers of the ChMAA-n series fonn a monotropic liquid crystalline phase of the cholesteric type, whose temperature interval of existence depends on the rate of cooling. The liquid crystalline phase is unstable and is transformed to crystal phase so soon that X-ray examination of the mesophase structure becomes difficult. Nevertheless, polarization-optical studies have made it possible to draw certain conclusions as to the nature of the liquid crystalline phase of monomers. Cooling of isotropic melts of monomers results in a confocal texture which turns to a planar one when a mechanical field is superimposed on the sample, for example, by shifting a cover glass in the cell of the polarizing microscope (Figure 4). The observed planar texture exhibits the property of selective light reflection, which is typical of low-molecular cholesteric liquid crystals. 

Obviously, the involvement of the heterogeneity of the transition temperatures makes a sensible description of the phase transition in LCEs. However, one finds an even better match with the experimental data when considering only the heterogeneity of the mechanical field G instead aj = 0, (G) 0, 0). The... 

The heat-capacity and H-NMR experiments show that in the case of LCEs this critical point is smeared in both the temperature and mechanical stress directions for both side-chain and main-chain LCEs. The smearing of the critical point is a direct consequence of the heterogeneity of the locked-in mechanical fields and the phase-transition temperatures. 

The phase transitions in the CEC-DMF and CEC-DMAA systems under static conditions and in a shear field are studied. The mechanical field leads to an extension of the temperature - concentration region of the existence of the LC phase (fig. 5), a phenomenon that is due to the change of orientation of CEC macromolecules in solutions. 

This pattern of the curve was associated with two opposite processes in the system, namely, the orientation of macromolecules along the flow direction, which is favorable for phase transition, and the destruction of nuclei of the new phase by a mechanical field, a process that retards the formation of the LC phase. In the examined range of shear rates, the orientation processes dominate, thereby resulting in the elevation of the formation temperature (relative to static conditions) of the LC phase, as manifested in the elevation of the LC phase transition temperature. For comparison. Fig. 6 a shows the data for the PE - p-... 

The deformation of these systems increases the temperatures of heat-induced sepraration and decreases the temperatures of transition from region II to region III. As the shear rate is increased, the absolute value of AT increases for both transitions and achieves 7 K (AT is a difference between phase transition temperatures under dynamic and static conditions). This phenomenon may be explained by the breakdown of nuclei of a new phase under the action of the mechanic field, as was observed for a number of pelymer - solvent systems characterized by amorphous and crystalline phase separation (Vshivkov et al., 1998, Vshivkov Rusinova, 2001). 

A well known property of cholesteric phases is tiie extreme sensitivity of the pitch (or the reflection color) against weak external perturbations such as temperature, mechanical stress or electric fields. The pitch P (and therefore the reflection color Xmax disturbed considerably by small amounts of non-chiralic... 

In this study we have investigated the structural and interaction parameters of ternary water/octane/CiaEs system by means of SAXS. Phase behavior of this system was studied by Kahlweit et al. [9]. This system shows interesting phase behavior (Fig. 1). One can study the structures of low-temperature microemulsion (LTM) phase, middle-temperature lamellar (MTL) phase and high-temperature microemulsion (HTM) phase by changing temperature only, provided that the sample contains approximately more than 12 wt% of surfactant at equal volume fraction of water and oil. Bodet et al. have clarified the structural evolution of this system by means of pulsed-field gradient spin-echo NMR, quasi-elastic light scattering and freeze-fracture transmission electron microscopy [10]. Local structure of the bilayer and monolayer of the same system was also studied by Strey et al. [11]. Recently, we have studied the mechanism of the phase transition [12]. 

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