Polymorphic phase transition

Shock loading in most metals and alloys produces greater hardening than quasi-static deformation to the same total strain, particularly if the metal undergoes a polymorphic phase transition, such as is observed in pure iron [1]-[10]. Figure 6.1 compares the stress-strain response of an annealed... 

Keywords hydrogen accumulator, hydrated methane, polymorphism, phase transition, nanostructure of ice, modeling... 

At temperatures below the main transition, a basic equilibrium stracture is the subgel (crystalline) Lc phase. Its formation usually requires prolonged low-temperature incubation. In addition to the Lc phase, many intermediate stable, metastable, and transient lamellar gel structures are adopted by different lipid classes—with perpendicular or tilted chains with respect to the bilayer plane, with fully interdigitated, partially interdigitated, or noninterdigitated chains, rippled bilayers with various ripple periods, and so forth. (Fig. 1). Several polymorphic phase transitions between these structures have been reported. Well-known examples of polymorphic transitions are the subtransition (Lc- L ) and the pretransition (Lp/- Fp/) in phosphatidylcholines (33). Recently, a polymorphic transition that included rapid, reversible transformation of the usual gel phase into a metastable, more ordered gel phase with orthorhombic hydrocarbon chain-packing (so-called Y-transition) was reported to represent a common pathway of the bilayer transformation into a subgel (crystalline) Lc phase (62). 

Handa, T., Ichihashi, C., and Nakagaki, M. Polymorphic phase transition and monomolecular spreading of synthetic phospholipids. Progress Colloid Polym. ScL, 71, 26,1985. 

An interest in the crystals of amino acids and small peptides has reemerged with attention to the dynamic properties of these systems, the subtle kinetic factors determining their crystallization and polymorphism, phase transitions and anisotropic structural response to variation of temperature, pressure and an environment. The systems can serve as a unique interface between biology, chemistry, material science and nanotechnology of immediate and future importance. 

Figure 177. DSC curves showing polymorphic phase transitions of a triglyceride on storage, (a) Before storage (b) after 6-month storage at 25°C. (Reproduced from Ref. 709 with permission.)...

Accelerated stability studies are potentially problematic for at least three reasons. Stress testing conditions may exceed the temperature of a polymorphic phase transition or dehydration. The use of accelerated conditions may make a relaxation of a metastable phase more rapid due to the increased molecular mobility. Finally, the relative humidity of the station may be in the range sufficient to cause a pseudopolymorphic transition due to dehydration or hydration. It is sobering... 

Phase transitions (considered to be a part of physics) between one solid phase and another are similar to chemical solid state reactions. They have similar kinetics that are measured in the same way and in both cases chemical bonds are broken and remade. Polymorphic phase transitions are observed in many ceramic materials on heating, during which the crystal structures and properties change abruptly (not the composition). 

During polymorphic phase transitions the structures and properties of singlephase materials change discontinuously, which is of considerable interest in the... 

Figure 17.21 The two basic binary diagram elements. In the phase transition loop (left diagram) solution 1 and solution 2 can be solid and solid, solid and liquid, or liquid and vapor, respectively. and are melting temperatures, boiling temperatures, or polymorphic phase transition temperatures for pure A and B respectively. Three representative tie-lines are shown. In the solvus (right diagram), solution 1 and solution 2 can be two solids or two liquids. Two representative tie-lines are shown.

Parrinello M, Rahman A (1981) Polymorphic phase transitions in single-ciystals—a new molecular dynamics method. J App Phys 52 7182-7290... 

The thermosalient effect describes the situation whereby crystals jump or hop during a thermally induced polymorphic phase transition. The phenomenon is observable on a macroscopic level and occurs in both inorganic and organic crystals. The mechanism of hopping or jumping has not yet been elucidated. ... 

P. H. Poole, T. Grande, C. A. Angell, and P. F. McMillan, Polymorphic phase transitions in liquids... 

The phase diagrams in Figure 3.7 consider only SLE. In addition, solid-solid equilibria (e.g. polymorphic phase transitions), liquid-liquid equilibria such as a misdbihty gap in liquid state (hquid-liquid demixing) and also nonequilibrium phenomena (out-of-equilibrium states) can complicate melt phase diagrams and make their measurement more difficult. 

Retrograde solubility also occurs for organic substances. Examples are sulfaguani-dine and sparteine, a complicated natural product, both in water. Discontinuities in the solubility curve originating from polymorphic phase transitions are observed for enantiotropically related polymorphs. The transformation of one polymorph to the... 

Recently, the structural transitions without diffusion redistribution (polymorphic transitions) have been analyzed. We presented a general thermodynamic and kinetic analysis for a polymorphic phase transitions in a nanopowder. The corresponding model describes a size-induced change of critical temperatures of phase transitions in small, isolated nanoparticles, and application to the y-Fe to a-Fe polymorphic phase transformations gives the qualitative similar size-induced results [93, 98,99]. 

The rate of a polymorphic phase transition depends on nucleation and growth processes, which are related to the mobility of atoms/molecules in the solid state. The Avrami equation (Eq. 7) may be applied to describe the degree of transformation, X, as a function of time, f ... 

Table 4 (21) list the XRD results for the TiCu system (both ternary hydride and the binary alloy). The primary exothermic DSC peak for a-TiCu results in approximately equal amounts of y TiCu and 6-TiCu at about 680-700K, which is a polymorphic phase transition according to Roster and Herold (2). A polymorphic (polymorphous) phase transition is one where the... 

Additional data on the melting points and enthalpies of fusion of SmCh (Laptev et al., 1976) and EuCl2 (Kulagin and Laptev, 1976) have appeared. Both dichlorides undergo polymorphic phase transitions approximately 90° below their melting points. The calibrated enthalpies of transition (4 1 kcal/mol) are substantial and thermochemical cycles for these systems should be reexamined in light of these results. 

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