Mechanical stresses transformation

This class of smart materials is the mechanical equivalent of electrostrictive and magnetostrictive materials. Elastorestrictive materials exhibit high hysteresis between strain and stress (14,15). This hysteresis can be caused by motion of ferroelastic domain walls. This behavior is more compHcated and complex near a martensitic phase transformation. At this transformation, both crystal stmctural changes iaduced by mechanical stress and by domain wall motion occur. Martensitic shape memory alloys have broad, diffuse phase transformations and coexisting high and low temperature phases. The domain wall movements disappear with fully transformation to the high temperature austentic (paraelastic) phase. 

Transformations in the Solid State. From a practical standpoint, the most important soHd-state transformation of PB involves the irreversible conversion of its metastable form II developed during melt crystallization into the stable form I. This transformation is affected by the polymer molecular weight and tacticity as well as by temperature, pressure, mechanical stress, and the presence of impurities and additives (38,39). At room temperature, half-times of the transformation range between 4 and 45 h with an average half-time of 22—25 h (39). The process can be significantly accelerated by annealing articles made of PB at temperatures below 90°C, by ultrasonic or y-ray irradiation, and by utilizing various additives. Conversion of... 

When a compound that can form several modifications crystallizes, first a modification may form that is thermodynamically unstable under the given conditions afterwards it converts to the more stable form (Ostwald step rule). Selenium is an example when elemental selenium forms by a chemical reaction in solution, it precipitates in a red modification that consists of Se8 molecules this then converts slowly into the stable, gray form that consists of polymeric chain molecules. Potassium nitrate is another example at room temperature J3-KN03 is stable, but above 128 °C a-KNOs is stable. From an aqueous solution at room temperature a-KN03 crystallizes first, then, after a short while or when triggered by the slightest mechanical stress, it transforms to )3-KN03. 

The expression of the ET-1 gene is increased by growth factors and cytokines, including transforming growth factor-B (TGF-B) and interleukin 1 (IL-1), vasoactive substances including Ang II and vasopressin, and mechanical stress. Expression is inhibited by nitric oxide, prostacyclin, and ANP. 

The conditions and kinetic equations for phase transformations are treated in Chapters 17 and 20 and involve local changes in free-energy density. The quantification of thermodynamic sources for kinetically active interface motion is approximate for at least two reasons. First, the system is out of equilibrium (the transformations are not reversible). Second, because differences in normal component of mechanical stresses (pressures, in the hydrostatic case) can exist and because the thermal con-... 

At all levels of supermolecular structure, the transformations caused by mechanical stress stem from the anisotropy of the mechanical properties—by the existence of defects at the macroscopic level by aniso-... 

The transformations initiated by mechanical energy do not uniformly and necessarily pass through all three stages. The number and duration of the stages are determined by the nature and intensity of the mechanical stress. 

There is a small thermal hysteresis of the transition temperature, which depends on many parameters such as the rate of temperature change, mechanical stresses or crystal imperfections. From a crystal chemical view, the Ba-O framework evokes an interstitial for the central Ti4+ ion which is larger than the actual size of the Ti4+ ion. As a result, the serie of phase transformations takes place to reduce the Ti cavity size. Certainly, the radii of the ions involved impact the propensity for forming ferroelectric phases thus both PbTi03 and BaTi03 have ferroelectric phases, while CaTi03 and SrTi03 do not [5]. 

The notion that mechanical stress may cause solid-to-solid phase transitions or other physicochemical transformations is well known in materials sciences. However, examples of molecular crystals undergoing mechanically induced phase transitions or polymorph — pseudo-polymorph transformations between crystalline phases are not very abundant some recent entries in the literature can be found in Ref. 74. 

White beam synchrotron radiation was used in a topographical study [64] of the phase transition in [Ni(en)2 (NCS)2J single crystals. The short exposure times possible using this intense radiation enabled the strained centre of the crystal to be identified as the most imperfect zone, within which the nucleation and growth process was initiated. The phase transition commenced at defect sites. The role of generation and relaxation of mechanical stresses in influencing kinetics of solid state transformations was later reviewed [65]. 

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