Oxygen lifetime parameters

The behavior of practically all luminescent materials is sensitive to various parameters of physical and chemical origin. The excited state lifetimes and average intensities of the fluorescence and/or phosphorescence of these materials are modulated, for example, by temperature, oxygen, pH, carbon dioxide, voltage, pressure, and ionic strength. Consequently, the luminescence of various materials could be used, in principle, to monitor parameters of interest in medicine, industry, research, and the environment. 

Significant curvature may be observed in the case of lifetime- (and intensity-) based sensors, mainly when the relation knri [Parameter]) is not linear. Figure 9.4 shows this type of nonlinear behavior for a fiberoptic oxygen sensor. The figure shows Stern-Volmer-type plots (r l versus [02]) at four different temperatures. The curvature is caused by the inability of the carrier to transport oxygen proportionally to the equilibrium partial pressure of oxygen. 

Table 10.3. Mean Lifetimes (r) in Solvents Purged by Nitrogen (N2), Air, and Oxygen (O2) and Sensing Parameters (Changes in Phase Angle, AO, and in Modulation, Am, "Air - N2 ) of Potential Oxygen Sensors 33 ...

This second-order decomposition of hydrogen peroxide has been studied independently, and its rate parameters are known. The activation energy is ca. 48 keal., and its lifetime is about 1 second at about 900°K. At temperatures of ca. 450° to 550°C., it proceeds at a sufficiently rapid rate to be responsible for initiating the normal explosion limit, which one finds for stoichiometric hydrocarbon-oxygen mixtures. 

Figure 8.9 Variation with quenching temperature of, (a) positron lifetime, (b) Doppler broadened lineshape parameter, I, and (c) oxygen deficiency, as obtained from weight loss (+) and Tc (o) measurements. From Bharathi et al. [54].

Vacancy-related complexes which were generated in silicon p -n diodes by irradiation with 6 MeV electrons in the temperature range of 350-800 K have been studied by means of deep level transient spectroscopy. Such defects are of interest because of their possible application in controlling the carrier lifetime in silicon power devices. Electronic parameters of defects incorporating up to three vacancies and an oxygen atom have been detemiined. Total introduction rate of radiation-induced defects increased about twice upon raising the irradiation temperature from 350 to 675 K. 

The explicit correlation between changes in the minority carrier life time for p -n diodes upon irradiations at different temperatures and changes in the concentration of vacancy and vacancy-oxygen complexes is established. The information obtained in the present work on the electronic parameters of the V 0 complexes and their introduction rates can be used for careful controlling the carrier lifetime and, therefore, the switching characteristics of silicon power devices. 

Although in principle all free radical species are detectable by ESR spectroscopy, in practice detection may be difficult or impossible under a given set of experimental conditions. Problems with detection of a particular species will reflect m netic and/or kinetic factors. For example, oxygen-centered species such as OH, Oj, and RO [75] and sulfur-centered species such as thiyl radicals (RS ) [76] cannot be detected directly in fluid solution because of extreme anisotropy in their magnetic parameters which makes their ESR signal amplitudes vanishingly small. To detect radicals such as these, it is necessary to immobilize them in frozen solutions or to resort to indirect methods of detection (see below). The same applies to radicals that have a short lifetime. 

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