Activation energy crystallization

Semiconducting Properties. Sihcon carbide is a semiconductor it has a conductivity between that of metals and insulators or dielectrics (4,13,46,47). Because of the thermal stabiUty of its electronic stmcture, sihcon carbide has been studied for uses at high (>500° C) temperature. The Hall mobihty in sihcon carbide is a function of polytype (48,49), temperature (41,42,45—50), impurity, and concentration (49). In n-ty e crystals, activation energy for ioniza tion of nitrogen impurity varies with polytype (50,51). 

It was possible for two of the systems chosen that the nucleation and crystallization activation energies could be determined separately by distinguishing the induction period and crystal growth period in the overall crystallization process. Of the two hypotheses proposed for zeolite crystallization, in the gel phase or from the solution phase, the data support the latter hypothesis for crystal growth with the crystal-liquid surface enhancing the nucleation process in seeded systems. The precise mechanism of nucleation in unseeded systems remains to be determined. 

The non-isothermal crystallization activation energy can be derived by the combination of cooling rate and exothermic peak temperature (Tp), shown as the Kissinger method [22] in Equation (6). 

Mesoporous molecttlar sieve particles acted as nucleating agent accelerating the crystallization rate and decreasing the half-time of crystallization. The crystallization activation energy was redtrced as sieve content increased. Small spheruhtes and higher crystallinity are the other featirres of nucleation. ... 

Ozawa and Kissinger plots are the most commonly used equations to calculate non-isothermal kinetic data, such as Avrami constant, n and crystallization activation energy, Ea, respectively (Celikbilek et al., 2011 Kissinger, 1956 Ozawa, 1971 Prasad Varma, 2005). 

Fig. 21. Plot of Ink versus 1/T from which the values of crystallization activation energy are obtained (Prasad Varma, 2005)...

Table 2 Data collected from Thermal Analysis of composite materials where A H is the transition heat of melting or crystallization, Tm, Tc,m are temperatures of maximum of melting or crystallization peak or shoulder, and Tc,o is the extrapolated onset-temperature of dynamical crystallization. Activation energy is calculated from the...

Fig. 3. The diffusion coefficients of several single-crystal and polycrystaUine ceramic materials as a function of temperature where Pq represents the partial pressure of oxygen. The activation energy is obtained from the slope and the insert eg, for O diffusion in CaQ 86 186 2 Tl4 cation % Ca,...

Crystallization kinetics have been studied by differential thermal analysis (92,94,95). The heat of fusion of the crystalline phase is approximately 96 kj/kg (23 kcal/mol), and the activation energy for crystallization is 104 kj/mol (25 kcal/mol). The extent of crystallinity may be calculated from the density of amorphous polymer (d = 1.23), and the crystalline density (d = 1.35). Using this method, polymer prepared at —40° C melts at 73°C and is 38% crystalline. Polymer made at +40° C melts at 45°C and is about 12% crystalline. 

The interconversion of the two forms involves a configurational change from to Z at at least one double bond. The activation energy for this process is only about lOkcal/mol. The crystal stmcture for [14]annulene shows the to be present in the... 

Anhydrous NaC102 crystallizes from aqueous solutions above 37.4° but below this temperature the trihydrate is obtained. The commercial product contains about 80% NaC102. The anhydrous salt forms colourless deliquescent crystals which decompose when heated to 175-200° the reaction is predominantly a disproportionation to C103 and Cl but about 5% of molecular O2 is also released (based on the C102 consumed). Neutral and alkaline aqueous solutions of NaC102 are stable at room temperature (despite their thermodynamic instability towards disproportionation as evidenced by the reduction potentials on p. 854). This is a kinetic activation-energy effect and, when the solutions are heated near to boiling, slow disproportionation occurs ... 

Active Dissolution and Crystal Defects—Energy Considerations... 

The practical importance of vacancies is that they are mobile and, at elevated temperatures, can move relatively easily through the crystal lattice. As illustrated in Fig. 20.21b, this is accompanied by movement of an atom in the opposite direction indeed, the existence of vacancies was originally postulated to explain solid-state diffusion in metals. In order to jump into a vacancy an adjacent atom must overcome an energy barrier. The energy required for this is supplied by thermal vibrations. Thus the diffusion rate in metals increases exponentially with temperature, not only because the vacancy concentration increases with temperature, but also because there is more thermal energy available to overcome the activation energy required for each jump in the diffusion process. 

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