Temperature solid phase transition

Above r, PDMS is in the amorphous state, and there are no indications in the literature concerning the possible appearance of a mesophase in it. The first information on phase transitions in polydiethylsiloxane (PDES) and polydipropylsiloxane (PDFS) was published in [17], in which low-temperature solid-phase transitions were detected in these polymCTS, but the existence of a mesophase was not found. A mesophase which appears after melting (>270°K) and persists up to =3(X)°K was found in a more detailed study of PDES [18] above 300 K, the sample turned into an amorphous melt. Beatty et al. called the observed mesophase a viscocrystalline state and attempted to characterize it by several physical methods (calorimetry, NMR, dielectric relaxation, optical observations). However, neither the structure nor the physical properties of this mesomorphic state have been studied completely enough. The mesomorphic state in PDFS was first detected and studied in our research [19, 20]. There is no information on the following representatives of linear polysiloxanes in the literature at present. 

The phase transitions are accompanied by an uneven change in the molecular mobility (Table 4.2) [25]. The data in Table 4.2 indirectly indicate the significant conformational disorder not only in the transition to the mesomorphic state, but also in the low-temperature solid-phase transition. 

Unlike melting and the solid-solid phase transitions discussed in the next section, these phase changes are not reversible processes they occur because the crystal stmcture of the nanocrystal is metastable. For example, titania made in the nanophase always adopts the anatase stmcture. At higher temperatures the material spontaneously transfonns to the mtile bulk stable phase [211, 212 and 213]. The role of grain size in these metastable-stable transitions is not well established the issue is complicated by the fact that the transition is accompanied by grain growth which clouds the inteiyDretation of size-dependent data [214, 215 and 216]. In situ TEM studies, however, indicate that the surface chemistry of the nanocrystals play a cmcial role in the transition temperatures [217, 218]. 

Presents temperature-dependent solid-solid phase transition. 

The dependence on the temperature of the specific resistance (Q/cm) of the pure MEPBr and MEMBr complexes, and a 1 1 mixture there of, as obtained in Ref. [73], is listed in Table 4. It is remarkable that within the complex phases consisting of Br2 and either pure MEP or MEM the change of specific resistance at the liquid —> solid phase transition amounts to about one order of magnitude, where as the value is only doubled in the 1 1 mixture. The table also indicates that MEMBr complexes possess higher melting temperatures. 

Figure 4.4 Heat capacity of N as a function of temperature. A solid phase transition occurs at 35.62 K, the melting temperature is 63.15 K, and the normal boiling temperature is 77.33 K.

Experience indicates that the Third Law of Thermodynamics not only predicts that So — 0, but produces a potential to drive a substance to zero entropy at 0 Kelvin. Cooling a gas causes it to successively become more ordered. Phase changes to liquid and solid increase the order. Cooling through equilibrium solid phase transitions invariably results in evolution of heat and a decrease in entropy. A number of solids are disordered at higher temperatures, but the disorder decreases with cooling until perfect order is obtained. Exceptions are... 

Figure 8.23 (Solid + liquid) phase diagram for (. 1CCI4 +. yiCHjCN), an example of a system with large positive deviations from ideal solution behavior. The solid line represents the experimental results and the dashed line is the ideal solution prediction. Solid-phase transitions (represented by horizontal lines) are present in both CCI4 and CH3CN. The CH3CN transition occurs at a temperature lower than the eutectic temperature. It is shown as a dashed line that intersects the ideal CH3CN (solid + liquid) equilibrium line.

P8.4 The (solid + liquid) phase diagram for (.Yin-C6Hi4 + y2c-C6Hi2) has a eutectic at T = 170.59 K and y2 = 0.3317. A solid phase transition occurs in c-CftH at T— 186.12 K, resulting in a second invariant point in the phase diagram at this temperature and. y2 — 0.6115, where liquid and the two solid forms of c-C6H12 are in equilibrium. A fit of the experimental... 

From the more recent reports cited below, further references to the extensive literature concerned with calcite decomposition may be traced. Other modifications of CaC03 (aragonite and vaterite) undergo solid phase transitions to calcite at temperatures of 728 K and 623—673 K respectively [733], below those of onset of decomposition (>900 K). There is strong evidence [742] that the reaction... 

The heat capacity function for the solid phase Is from Fink (4. Fink points out that although (U,Pu)02 UO2, and ThC>2 have solid-solid phase transitions, the available data (4) make It impossible to determine the existence of a similar phase transition for Pu02 If additional high-temperature measurements indicate the presence of a solid-solid phase transition, the heat capacity of Pu02 between the phase transition and 2701 K may be significantly higher. 

The uncertainties in the condensed-phase thermodynamic functions arise from (1) the possible existence of a solid-solid phase transition in the temperature range 2160 to 2370 K and (2) the uncertainty in the estimated value of the liquid heat capacity which is on the order of 40%. While these uncertainties affect the partial pressures of plutonium oxides by a factor of 10 at 4000 K, they are not limiting because, at that temperature, the total pressure is due essentially entirely to O2 and 0. 

The nonmesogenic compound CB2 is described here, because it shows a reversible distortive solid-solid phase transition at 290.8 K (transition enthalpy 0.9 kj/mol) from the centrosymmetric low temperature phase I to the noncentrosymmetric high temperature phase II. The crystal structures of both solid phases I and II are very similar [45] as demonstrated in Fig. 2. The molecules are arranged in layers. The distances between the cyano groups of adjacent molecules are 3.50 A Ncyano-Ncyano and 3.35 A Ncyano-C ano for phase I and 3.55 A Ncyano-Ncyano and 3.43 A Ncyano-Ccyano for phase II. In the two... 

However at elevated temperatures (T2 > Tj, Figure 9) the increased entropy (TAS) associated with an open shell structure overcomes the ti —ti enthalpy of dimerisation associated with these distorted Ti-stacked structures and they undergo a solid-solid phase transition (Figure 9) The high temperature phase is typically associated with a Ti-stack of regularly spaced radicals which exhibit longer inter-radical S- S contacts (ca. 3.7 A). This process was first observed by Oakley60 in the DTA radical thiadiazolopyrazine-l,3,2-dithiazolyl 26, and a number of other derivatives have subsequently been identified which exhibit similar behaviour. These are compiled in Table 1. 

The sample temperature is increased in a linear fashion, while the property in question is evaluated on a continuous basis. These methods are used to characterize compound purity, polymorphism, solvation, degradation, and excipient compatibility [41], Thermal analysis methods are normally used to monitor endothermic processes (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, and chemical degradation) as well as exothermic processes (crystallization and oxidative decomposition). Thermal methods can be extremely useful in preformulation studies, since the carefully planned studies can be used to indicate the existence of possible drug-excipient interactions in a prototype formulation [7]. 

Temperature studies on the system Me NF-HF revealed four distinct phases Mc4NH2F3, mp 110°C Mc4NH3F4, mp 20°C (decomposes) Me NHjFg, mp — 76°C (decomposes) and Me NHyFg, mp — 110°C (decomposes) (Mootz and Boenigk, 1987). Most of these phases undergo solid-solid phase transitions. A crystal structure of the trifluoride shows two HFs attached to a central F with. .. p = 230.2 and 231.6 pm and with an... 

As the him is compressed, a transition to a solid him is observed, which collapses at higher surface pressure. The II versus A isotherms, below the transition temperatures, show the liquid to solid phase transition. These solid hlms have been also called condensed films. They are observed in such systems where the molecules adhere to each other through van der Waals forces very strongly. The Tl-A isotherm shows generally no change in II at high A, while at a rather low A value, a sudden... 

According to the well-known Landau theory, the eigenvector of the order parameter in any second order solid-solid phase transition transforms according to an irreducible representation of the space group of the parent phase state. Furthermore, the free energy F=U -TS can be expanded around the transition temperature Tc in terms of the scalar order parameter p, which... 

Near the transition temperature, SMAs also exhibit the curious effect of pseudoelasticity, in which the metal recovers (apparently in the usual manner) from an isothermal bending deformation when the stress is removed. However, the elasticity is not due to the usual elastic modulus of a fixed crystalline form, but instead results from strain-induced solid-solid phase transition to a more deformable crystalline structure, which yields to the stress, then spontaneously returns to the original equilibrium crystal structure (restoring the original macroscopic shape) when the stress is removed. 

The SMA effect can be traced to properties of two crystalline phases, called martensite and austenite, that undergo facile solid-solid phase transition at temperature Tm (dependent on P and x). The low-temperature martensite form is of body-centered cubic crystalline symmetry, soft and easily deformable, whereas the high-temperature austenite form is of face-centered cubic symmetry, hard and immalleable. Despite their dissimilar mechanical properties, the two crystalline forms are of nearly equal density, so that passage from austenite to a twinned form of martensite occurs without perceptible change of shape or size in the macroscopic object. 

In addition to the properties discussed above, there are several others whose determination would be of great use within a mesoscopic modeling framework. Among these are the melt curve TmeirTmeU(p), the temperature and pressure dependent specific heat, initial efforts towards understanding plastic response, and studies of solid-solid phase transitions. 

The use of a phase-change media circumvents many of these problems. A material slightly above its liquid/solid-phase transition temperature may be ejected from a jet-printing nozzle the droplet solidifies quickly upon contact with a cooler surface. The feature size will then depend more on the cooling rate and less on the material s wetting properties, because a frozen droplet cannot spread. In this situation, the substrate temperature controls the printed feature size for materials having excellent wetting properties. 

With respect to this equation, we have supposed that there is no solid-state transition and thus no heat of transition. The only constant-temperature heat effects are those of fusion at Tf and vaporization at TE. When a solid-phase transition occurs, a term AH,/ T, is added. 

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