Thermal active effect

Figure 23 shows the coercivity dependence on temperature for FePt C cluster films with 45 vol. % C from 10 to 300 K. The films were annealed at different temperature for 10 min. The coercivity decreases with increase of measuring temperature, for example, dropping from 19 kOe at 10 K to about 13 kOe at 300 K for a film annealed at 750 °C for 10 min. This may be caused by a contribution of intrinsic temperature dependence of the anisotropy and magnetization, and thermal activation effects [49]. 

Whatever the reversal mechanism, the critical phenomenon which determines reversal occurs within a local critical volume under an effective critical field. Together with the experimental coercive field, the critical volume, associated with the determinant magnetization reversal process, is another important parameter characterizing coercivity [9, 11, 12]. The size of this critical volume may be derived from the analysis of thermal activation effects and we are thus led to discuss these effects briefly. 

In usual hard magnetic materials, thermal activation effects are the source of a coercive field decrease by typically 10% at room temperature. Importantly, they provide an experimental access to the critical volume for reversal, which may be identified with the activation volume, vact. 

In this section, we concentrate on the fundamental impact of particle size reduction on magnetization processes in individual particles. Although not directly related to coercivity, the classical effect of single domain particle formation is described. At small particle size, reversal by coherent rotation tends to be favoured with respect to nucleation/pinning-depinning finally thermal activation effects and macroscopic quantum tunnelling are discussed. 

Even below TB, the magnetization of nanoparticles may be strongly affected by thermal activation. Very small Co particles showing coherent rotation allowed thermal activation effects to be analyzed quantitatively [110]. From 40 mK up to 12 K, the coercive field was found to be a function of the expression theoretically expected for thermal activation, Tln(T/T0]213. 

As thermal activation effects are not negligible at room temperature, and as they must eventually override mechanical activation effects at some point, in Marx laboratory... 

The first term describes the dependence of the coercivity on the anisotropy field. In an ideal system, the phenomenological coefficient c would be one and is reduced to about 0.1 in a real system. The second term describes the thermal activation effects. denotes the anisotropy energy, is the molecular field and c is a phenomenological coefficient, which gives an account for the decrease of anisotropy and/or exchange interactions at defect position. The experimental data show that - has a quadratic-like behaviour which means... 

Amorphous Silicon. Amorphous alloys made of thin films of hydrogenated siUcon (a-Si H) are an alternative to crystalline siUcon devices. Amorphous siUcon ahoy devices have demonstrated smah-area laboratory device efficiencies above 13%, but a-Si H materials exhibit an inherent dynamic effect cahed the Staebler-Wronski effect in which electron—hole recombination, via photogeneration or junction currents, creates electricahy active defects that reduce the light-to-electricity efficiency of a-Si H devices. Quasi-steady-state efficiencies are typicahy reached outdoors after a few weeks of exposure as photoinduced defect generation is balanced by thermally activated defect annihilation. Commercial single-junction devices have initial efficiencies of ca 7.5%, photoinduced losses of ca 20 rel %, and stabilized efficiencies of ca 6%. These stabilized efficiencies are approximately half those of commercial crystalline shicon PV modules. In the future, initial module efficiencies up to 12.5% and photoinduced losses of ca 10 rel % are projected, suggesting stabilized module aperture-area efficiencies above 11%. 

GVD Coatings. As in PVD, the stmcture of the deposited material depends on the temperature and supersaturation, roughly as pictured in Figure 8 (12). In the case of CVD, however, the effective supersaturation, ie, the local effective concentration in the gas phase of the materials to be deposited, relative to its equiUbrium concentration, depends not only on concentration, but on temperature. The reaction is thermally activated. Because the effective supersaturation for thermally activated reactions increases with temperature, the opposing tendencies can lead in some cases to a reversal of the sequence of crystalline forms Hsted in Figure 8, as temperature is increased (12). 

Combination of effects of thermal activation and viscous drag then gives for an average dislocation velocity... 

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

Interestingly, this same effect has been observed for the addition of a rubber toughening agent to ethyl cyanoacrylate-based adhesives, as was reported previously. The rubber must contain enough latent acid functionality on the polymer backbone or in an additive to inhibit the thermally activated decomposition of the alkyl cyanoacrylate adhesive polymer. 

From the Arrhenius form of Eq. (70) it is intuitively expected that the rate constant for chain scission kc should increase exponentially with the temperature as with any thermal activation process. It is practically impossible to change the experimental temperature without affecting at the same time the medium viscosity. The measured scission rate is necessarily the result of these two combined effects to single out the role of temperature, kc must be corrected for the variation in solvent viscosity according to some known relationship, established either empirically or theoretically. 

Effects of Temperature on Ionic Reactions in TD/D2 CH4/ CD4. Observation that the methanium ion proton (deuteron) transfer sequence fails to exhibit a temperature coefficient within experimental uncertainties leads unavoidably to the conclusion that none of the reactions from 1 to 12 requires thermal activation between —78° and 25°C. From Equations I, II, III, appropriate steady state assumptions, and representing both neutralization steps by kX2, we find that... 

The literature in this field is confusing because of a somewhat haphazard method of nomenclature that has arisen historically. This is compounded by some mistakes in structure determination, reported in early papers, and which are occasionally quoted. The first part of this chapter deals with nomenclature and with a brief overview of early work. Subsequent sections deal with the formation and metabolism of di-D-fructose dianhydrides by micro-organisms, and the formation of dihexulose dianhydrides by protonic and thermal activation. In relation to the latter topic, recent conclusions regarding the nature of sucrose caramels are covered. Other sections deal with the effects of di-D-fructose dianhydrides upon the industrial production of sucrose and fructose, and the possible ways in which these compounds might be exploited. An overview of the topic of conformational energies and implications for product distributions is also presented. 

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