Thermal activation energies, determination

Sharma et al. [153] have devised a gentle accelerated corrosion test using a kinetic rate equation to establish appropriate acceleration factors due to relative humidity and thermal effects. Using an estimate for the thermal activation energy of 0.6 eV and determining the amount of adsorbed water by a BET analysis on Au, Cu and Ni, they obtain an acceleration factor of 154 at 65°C/80% RH with respect to 25 °C/35-40% RH. 

The copolymerization of epoxides with cyclic anhydrides is a thermally activated reaction. Table 6 gives a survey of the thermodynamic parameters. The activation energies determined by different authors are in good agreement and vary between 52.8 and 64.9 kJ/mol, depending on the monomer used. Exceptions are only the... 

In this model,110 it was assumed that all C 2H bonds perform thermally activated rotational jumps within energy landscapes on the surface of a cone. Specifically, six basins were supposed to be separated by six energy barriers at positions 0, 60,..., 300° around the axis of the cone. For each cone, the barriers were drawn anew from the distribution of activation energies determined for TOL in DS.12,19 Further, it was assumed that all positions on the surface of the cone, except for the barriers, have the same energy, i.e., a random-barrier model was considered. The thermally activated jumps lead to a random new position in one of the two neighboring basins. This means that several back-and-forth jumps occur over relatively low energy barriers until relatively high barriers are crossed. In other words, many... 

Figure 4.9 Morse potential energy curves for chloromethane and its ions. The curves are calculated using the activation energy determined from data in Figure 4.8. The high-temperature data is for unimolecular dissociation via the curve crossing on the approach side of the molecule. Only the VEa is negative and dissociation occurs in the Franck Condon transition. The thermal energy dissociation occurs through the thermal activation of the molecule, as is the case for all DEC(l) molecules.

Table 8.2 Field effect hole mobilities //, threshold voltages Fj, and channel lengths L of the samples A to F (see Table 1). The values were determined according to method A. denotes the thermal activation energy of the field effect mobility determined from the slopes of the plots in Figure 8.8.

Fig. 45 Linear plots of the signal intensities for the two principal fragments (a) N2 (28 amu) and (b) the phenyl radical (76 amu), as a function of laser fluence. The dark line shows a least-squares fit of the data to a thermal model for emission, assuming an activation energy of 2.1 eV (the polymer thermal activation energy). The gray dots in (a) indicate the area of the N2 fast peak determined from curve fits to the TOF data. REPRINTED WITH PERMISSION OF [Ref. 324], COPYRIGHT (1999) American Institute of Physics...

TABLE 3.12 Thermal Transition and Activation Energies Determined by Chemiluminescence Under Nitrogen... 

There is, however, no direct experimental evidence for this hypothesis on the contrary, single-crystal photoconductivity measurements by Dedman and Lewis [64] suggest that thermal exciton formation is the first step in decomposition. Formation of the exciton for lead azide occurs with a thermal activation energy of 122 kJ/mole, which is very close to the experimental values of 123 kJ/mol and 125 kJ/mol for thermal decomposition, implying that exciton formation is the rate-determining step in decomposition. 

In more general terms, the thermal decomposition of coal is a complex process (Stein, 1981 Solomon et al., 1992). Activation energies determined by experimental techniques indicate that the decomposition rate(s) is (are) controlled by the scission of carbon-carbon covalent bonds and the like (POutsma, 1987). In fact, the concepts that bond scission during coal pyrolysis can be induced by other means (McMillen et al., 1989) or can be influenced by cross-linking are sound and deserve consideration in the light of coal complexity and the potential interference of the primary products with one another as well as with the vestiges of the original coal. 

In most covalent NCS it is found that AE, the thermal activation energy of the conductivity is about half the magnitude of the optical energy gap. This means that Ep is not far from the center of the mobility gap. Does this mean that these materials are intrinsic In the case of crystalline semiconductors the word intrinsic is used to mean that the conduction properties are not affected by the presence of localized impurity states. The position of Ep is then determined by the equality... 

Further below, time-dependent deformation (creep) iiutiated by climb will be extensively discussed. In this section, an example of dislocation climb is illustrated. Figure 3.70 shows dislocation climb in an AI2O3-YAG specimen. Here, climb was assisted by thermal activation. Such a dislocation network, resulting from the reaction of dislocations from the basal and pyramidal slip systems, involves dislocation climb. It is a diffusion-controlled deformation mode characterizing creep deformation and, in this particular case, the activation energy determined is Q = 670 kJ/mol. 

To be able to compare thermal stabUity between polymers of different structures, it is necessary to rely on some standardized system, sucdi as the temperature of half-decomposition Tyi). The temperature of halfdecomposition is defined as the temperature at whicJi the polymer loses half of its weight when heated in vacuo for 30 min. Experimentally, Ty2 can be conveniently determined by thermal gravimetry (TG). From the TG curves obtained at different scan speeds, an Arrhenius plot at constant weight-loss ratio is derived. The pre-exponential factor and activation energy determined are then used to calculate Ty2. 

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