Thermally stimulated processes

For thermally stimulated processes to occur, fulfillment of two general conditions is necessary ... 

R. Chen and Y. Kirsh, Analysis of Thermally Stimulated Processes, Pergamon Press, London, 1981. 

Chen, R.. and Y. Kirsch.. 4ntflys s of Thermally Stimulated Processes. Pergamon, New York, 1981. 

Chen, R. Kirsh, Y. "Analysis of Thermally Stimulated Processes", Pergamon Press, Oxford (1981), ISBN 0 08 0229301. 

Increased temperatures at the crack-tip due to crack propagation - enhances thermally stimulated processes... 

FIGURE 3. The response of the EE from particulate-filled Epon 828 epoxy to thermal stimulation following fracture. The bottom arrow indicates when the temperature increase began. This supports the concept of a thermally stimulated process. 

International Workshop on "Thermally Stimulated Processes," Montpellier, France, June 22-25, 1976. 

The stress—relaxation process is governed by a number of different molecular motions. To resolve them, the thermally stimulated creep (TSCr) method was developed, which consists of the following steps. (/) The specimen is subjected to a given stress at a temperature T for a time /, both chosen to allow complete orientation of the mobile units that one wishes to consider. (2) The temperature is then lowered to Tq T, where any molecular motion is completely hindered then the stress is removed. (3) The specimen is subsequendy heated at a controlled rate. The mobile units reorient according to the available relaxation modes. The strain, its time derivative, and the temperature are recorded versus time. By mnning a series of experiments at different orientation temperatures and plotting the time derivative of the strain rate observed on heating versus the temperature, various relaxational processes are revealed as peaks (243). 

When the temperature of a carbonate reservoir that is saturated with high-viscosity oil and water increases to 200° C or more, chemical reactions occur in the formation, resulting in the formation of considerable amounts of CO2. The generation of CO2 during thermal stimulation of a carbonate reservoir results from the dealkylation of aromatic hydrocarbons in the presence of water vapor, catalytic conversion of hydrocarbons by water vapor, and oxidation of organic materials. Clay material and metals of variable valence (e.g., nickel, cobalt, iron) in the carbonate rock can serve as the catalyst. An optimal amount of CO2 exists for which maximal oil recovery is achieved [1538]. The performance of a steamflooding process can be improved by the addition of CO2 or methane [1216]. 

Currently, there are three possible gas recovery processes for hydrates (1) thermal stimulation, (2) depressurization and (3) inhibitor injection. Very limited thermal stimulation and depressurization tests were reported for the Mallik natural gas hydrate reservoirs, together with numerical simulations (Moridis et al. 2004,... 

Romanian scientists compared one-electron transfer reactions from triphenylmethyl or 2-methyl benzoyl chloride to nitrobenzene in thermal (210°C) conditions and on ultrasonic stimulation at 50°C (lancu et al. 1992, Vinatoru et al. 1994, Chivu et al. 2006). In the first step, the chloride cation-radical and the nitrobenzene anion-radicals are formed. In the thermal and acoustic variants, the reactions lead to the same set of products with one important exception The thermal reaction results in the formation of HCl, whereas ultrasonic stimulation results in CI2 evolution. At present, it is difficult to elucidate the mechanisms behind these two reactions. As an important conclusion, the sonochemical process goes through the inner-sphere electron transfer. The outer-sphere electron transfer mechanism is operative in the thermally induced process. 

Of special interest for thermally stimulated relaxation (TSR) is how to remove the system from equilibrium and physical phenomena that can be measured (monitored) during the relaxation process. We restrict ourselves by considering the physical phenomena, although these can also take place in chemical and biological objects. Further, among physical process, we consider only those that involve redistribution of electronic charge carriers in semiconductors during the relaxation process. 

It is also necessary to note that the success of TSR techniques to obtain information on trapping states in the gap depends on whether or not the experiment can be performed under conditions that justify equation (1.2) to be reduced to simple expressions for the kinetic process. Usually, the kinetic theory of TSR phenomena in bulk semiconductors—such as thermoluminescence, thermally stimulated current, polarization, and depolarization— has been interpreted by simple kinetic equations that were arrived at for reasons of mathematical simplicity only and that had no justified physical basis. The hope was to determine the most important parameters of traps— namely, the activation energies, thermal release probabilities, and capture cross section— by fitting experimental cnrves to those oversimplified kinetic descriptions. The success of such an approach seems to be only marginal. This situation changed after it was reahzed that TSR experiments can indeed be performed under conditions that justify the use of simple theoretical approaches for the determination of trapping parameters ... 

This experimental method, as well as the formal kinetics of the process, is closely related to trap level spectroscopy by thermally stimulated release of trap charge carriers. 

Thermally stimulated discharging of electrets provides spectroscopic information similar to trap level spectroscopy most importantly, their density and the activation energy required for the relaxation process to proceed. 

A detailed discussion of the statistical thermodynamic aspects of thermally stimulated dielectric relaxation is not provided here. It should suffice to state that kinetics of most of the processes are again complicated and that the phenomenological kinetic theories used to described thermally stimulated currents make use of assumptions that, being necessary to simplify the formalism, may not always be justified. Just as in the general case, TSL and TSC, the spectroscopic information may in principle be available from the measurement of thermally stimulated depolarization current (TSDC). However, it is frequently impossible to extract it unambiguously from such experiments. 

For most experiments on nonisothermal TSR, simple cooling of the sample to the desired initial temperature and a linear increase in T after excitation are sufficient to obtain TSC and TSL glow curves. Some techniques require more elaborate heating cycles, the details of which depend on the relaxation mechanism under study and on whether it is necessary to discriminate between simnltaneously occurring processes, e.g., thermally stimulated depolarization and thermally stimulated conductivity (see Chapter 2). 

The po and Pi ratio in equation (2.3) determines which of two factors—namely, equilibrium or nonequilibrium (due to emission from traps) carriers—dominate in the relaxation process. That is, the depolarization current contains two maximum one is related to release of carriers from trap the origin of the other lies in the change of conductivity with temperature [14-18]. Although only one of the peaks mentioned contains information about trap parameters, it is possible to discriminate between simultaneously occurring processes, e.g., thermally stimulated depolarization and thermally stimulated dielectric relaxation. 

A description of the emission and capture processes at a trap will be useful before discussing the various experimental methods. Figure 1 depicts the capture and emission processes that can occur at a center with electron energy ET. The subscripts n and p denote electron and hole transitions, and the superscripts t and differentiate between thermally and optically stimulated processes. It is assumed here that only thermal capture processes are occurring. 

Hydrate dissociation is of key importance in gas production from natural hydrate reservoirs and in pipeline plug remediation. Hydrate dissociation is an endothermic process in which heat must be supplied externally to break the hydrogen bonds between water molecules and the van der Waals interaction forces between the guest and water molecules of the hydrate lattice to decompose the hydrate to water and gas (e.g., the methane hydrate heat of dissociation is 500 J/gm-water). The different methods that can be used to dissociate a hydrate plug (in the pipeline) or hydrate core (in oceanic or permafrost deposits) are depressurization, thermal stimulation, thermodynamic inhibitor injection, or a combination of these methods. Thermal stimulation and depressurization have been well quantified using laboratory measurements and state-of-the-art models. Chapter 7 describes the application of hydrate dissociation to gas evolution from a hydrate reservoir, while Chapter 8 describes the industrial application of hydrate dissociation. Therefore in this section, discussion is limited to a brief review of the conceptual picture, correlations, and laboratory-scale phenomena of hydrate dissociation. 

From the extensive experimental and model development work performed at CSM (during a period of over 15 years), it has been demonstrated that a heat transfer controlled model is able to most accurately predict dissociation times (comparing to laboratory experiments) without any adjustable parameters. The current model (CSMPlug see Appendix B for details and examples) is based on Fourier s Law of heat transfer in cylindrical coordinates for the water, ice, and hydrate layers, and is able to predict data for single- and two-sided depressurization, as well as for thermal stimulation using electrical heating (Davies et al 2006). A heat transfer limited process is controlled by the rate of heat supplied to the system. Therefore, a measurable intermediate (cf. activated state) is not expected for heat transfer controlled dissociation (Gupta et al., 2006). 

It should be noted that in the above examples significant energy migration can occur between thermal stimulation and recombination since the electron is free to diffuse throughout the sample. Other electron-hole recombination processes occur, however, in which the excited states of the charge carrier are not to be found in the delocalized bands. An example of this, recently noted in the literature, concerns the TL emission from oligoclase feldspar in which the thermal stimulation involves the transition of the electron to a localized, intermediate state - see Figure 8... 

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