Transformation range

The indicator method is especially convenient when the pH of a weU-buffered colorless solution must be measured at room temperature with an accuracy no greater than 0.5 pH unit. Under optimum conditions an accuracy of 0.2 pH unit is obtainable. A Hst of representative acid—base indicators is given in Table 2 with the corresponding transformation ranges. A more complete listing, including the theory of the indicator color change and of the salt effect, is also available (1). 

Transformations in the Solid State. From a practical standpoint, the most important soHd-state transformation of PB involves the irreversible conversion of its metastable form II developed during melt crystallization into the stable form I. This transformation is affected by the polymer molecular weight and tacticity as well as by temperature, pressure, mechanical stress, and the presence of impurities and additives (38,39). At room temperature, half-times of the transformation range between 4 and 45 h with an average half-time of 22—25 h (39). The process can be significantly accelerated by annealing articles made of PB at temperatures below 90°C, by ultrasonic or y-ray irradiation, and by utilizing various additives. Conversion of... 

Abstract The dirhodium(II) core is a template onto which both achiral and chiral ligands are placed so that four exist in a paddle wheel fashion around the core. The resulting structures are effective electrophilic catalysts for diazo decomposition in reactions that involve metal carbene intermediates. High selectivities are achieved in transformations ranging from addition to insertion and association. The syntheses of natural products and compounds of biological interest have employed these catalysts and methods with increasing frequency. 

Cold bending of ferritic materials shall be done at a temperature below the transformation range. 

Hot bending shall be done at a temperature above the transformation range and in any case within a temperature range consistent with the material and the intended service. 

This chapter is an update (2003 to present) of the main applications of Bi(III) Lewis acids in organic synthesis developed and, in some cases, co-developed, by French and Portuguese research groups. Thus, in this chapter, the preparation of Bi(III) catalysts and their application to chemical transformations ranging from electrophilic addition to cyclization reactions, will be reviewed. The development of new environmentally friendly chemical processes, using Bi(III) reagents and catalysts, with direct application to steroid chemistry and related compounds will also be considered. 

KuRKjian, C. R. Relaxation of torsional stress in the transformation range. Phys. Chem. Glasses, a paraitre (1963). 

The corresponding diastereomeric adducts of /J-hydroxyphosphines 9 and 10 were obtained, as unseparable mixtures (Table 3). The diastereomeric excesses observed in this transformation ranged from 20 to 56%. 

Stabilization is a result of spontaneous structural changes, through which the substance passes from the non-equilibrium state into the metastable one. The transformation range is a temperature region within which the changes occur at a measurable rate. The transformation point appears on the temperature dependence curve of the particular property at a temperature where the time required for a distinct change in property is commensurable with the period for which the glass is held at this temperature. 

From the point of view of rheology, above the transformation range, glass behaves as a Newtonian liquid, that is, when deformed by external force the shear stress T is proportional to the gradient of local velocity v ... 

Considerable attention has been paid in the literature to the viscosity of glass in the transformation range, in particular in connection with annealing during which internal stresses are to be eliminated. In the transformatin range, the viscosity of glass is a function of temperature as well as time. Viscosity changes as a function of time... 

The theoretical explanation of the Fulcher-Tammann relation is based either on the free volume theory or on the concept of the temperature-dependent size of structural units taking part in viscous flow (cf. Doremus, 1973). The Fulcher-Tammann relation predicts a rapid increase of viscosity to an infinite value at To, while in fact in the transformation range approaches a constant and relation (6) is thus complied with. 

FIG. 35. The time dependence of viscosity for soda-— lime—silica glass in the transformation range (Lillie, 1933). 

Some more recently published studies (e.g. Hennicke and Tomsu, 1970 Tomsu, 1974) stress the viscoelastic behaviour of refractories at high temperatures on the basis of extensive experiments. According to these authors, slow deformations take place even under very low loads without any perceptible yield value. The viscoelastic behaviour has the result that the deformation is partially reversible but not immediately after the stress has been removed. Such properties can be expected especially with materials whose glassy phase has a viscosity corresponding to the transformation range, or with materials polycrystalline in character, free from flux. 

Figure 22.5 Pd K-edge EXAFS Fourier transforms for Pd/USY measured in the TPR (r.t.-773 K). Fourier transforms range, 30—120 nm-1.

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