Crystalline transition rate

In this study, the crystalline transition rate was additionally calculated from the obtained X-ray diffraction patterns using a peak method to be given as... 

Crystalline polyimide powders, 304 Crystalline transition temperature. See Melting temperature (Tm) Crystallization rate, for processing semicrystalline polymers, 44 CTE. See Coefficient of thermal expansion (CTE)... 

Investigators of works listed here as Refs 1, 2 3 have been concerned mostly wirh rates of propagative burning in gaseous mixtures and also for some solid-state systems in which neither fusion nor crystalline transitions were considered. Jackson (Ref 4) investigated the burning of quaternary compositions... 

Studies of more crystalline systems would be useful in completely defining the mechanism of the medium enhancement of radiationless transition rates. 

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... 

Kunitake has also studied the hydrolysis of -nitrophenyl esters in the presence of aqueous bilayer vesicles. The rate of inter-vesicle S2 hydrolysis by a cholesteryl derivative of 4(5)-imidazole carboxylic acid was much slower than the intra-vesicle counterpart.(32) Rate-limiting substrate transfer was responsible for the lower rates and was influenced by the crystalline-liquid crystalline transition.(33)... 

Ziegler-Natta catalyst systems being mostly heterogeneous in nature, adsorption reactions are most likely to occur in such polymerizations and feature in their kinetic schemes (Erich and Mark, 1956). A number of kinetic schemes have thus been proposed based on the assumption that the polymerization centers are formed by the adsorption of metal alkyl species on to the surface of a crystalline transition metal halide and that chain propagation occurs between the adsorbed metal alkyl and monomer. In this regard the Rideal rate law and the Langmuir-Hinshelwood rate law for adsorption and reaction on solids assume importance see Problem 9.4). 

A disadvantage of amorphous alloys is their metastable character which makes them transform into the stable crystalline state as a function of temperature and time. In calorimetric experiments the amorphous-to-crystalline transition is revealed by an exothermic heat effect. Typical traces obtained using a differential scanning calorimeter are shown for amorphous Gd064Co0 36 in fig. 51. The dependence of the crystallization temperature Tx on the heating rate s implies that there is a risk of crystallization taking place even at room temperature after an extended period (s - 0). This is particularly likely when Tx is rather low, and it may have consequences for practical applications. 

Polymer chemists use DSC extensively to study percent crystallinity, crystallization rate, polymerization reaction kinetics, polymer degradation, and the effect of composition on the glass transition temperature, heat capacity determinations, and characterization of polymer blends. Materials scientists, physical chemists, and analytical chemists use DSC to study corrosion, oxidation, reduction, phase changes, catalysts, surface reactions, chemical adsorption and desorption (chemisorption), physical adsorption and desorption (physisorp-tion), fundamental physical properties such as enthalpy, boiling point, and equdibrium vapor pressure. DSC instruments permit the purge gas to be changed automatically, so sample interactions with reactive gas atmospheres can be studied. 

Fig. 21 a Schematic illustration of the new approach for the formation of microperfo-rated single crystals deposition of the ACC mesh from the CaCOs solution, oriented nucleation at the imprinted nucleation site, and the amorphous-to-crystalline transition of the ACC film on the engineered 3D templates. (Reproduced from [247], 2003, American Association for the Advancement of Science)... 

One of the specific features of these polymer systems is a low mobility of the macromolecules and correspondingly slow phase transition rates. This enables one to use, in analyzing such systems, composite phase diagrams showing all the types of phase equilibria inherent in a given system. Extension of this principle to the systems "rigid-chain- polymer-solvent" makes it possible to construct a phase diagram which combines (a) equilibrium with the formation of a crystalline phase, (b) equilibrium with the formation of liquid-crystalline phases, and (c) equilibrium with the formation of amorphous (liquid) phases. 

Let us consider what can happen when an electron in a crystalline solid absorbs a photon and jumps to an excited state. The transition rate for such an excitation is given by Fermi s golden rule (see Appendix B, Eq. (B.63)) ... 

In thermogravimetric analysis (TG), the weight changes are determined as the sample is heated at a uniform rate. It differs from the semistatic or static method in which the sample is held at a constant temperature for a required period of time. In concrete investigations, TG is commonly used with DTA to follow the hydration reactions. The first derivative of change of mass (DTG) can also be used for identification purposes as it yields sharper peaks. TG cannot detect crystalline transitions as they do not involve weight losses. 

In thermogravimetry, the sample is heated, often at about 10°C min in a thermobalance instrument as described below. Only those changes that affect the mass of the sample will affect the measurements, so that condensed phase changes such as melting or crystalline transitions cause no mass change. The rate of change of mass, dm/dt depends on the amount of sample present, and the reaction rate constant at the experimental temperature. With solids it is... 

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