Thermal history measurements

Although vitreous siUca is a simple, single-component glass, its properties can vary significantly, depending on thermal history, the type and concentration of defects, and impurities. Vitreous siUca can, however, be one of the purest commercially available glassy materials. In synthetic vitreous sihcas, for example, total metal contamination is typically measured ia the 50—100 ppb range. Even at such a low level of impurities, differences ia properties, such as uv-transmission, are observed for various siUcas. 

Semiconducting Ceramics. Most oxide semiconductors are either doped to create extrinsic defects or annealed under conditions in which they become non stoichiometric. Although the resulting defects have been carefully studied in many oxides, the precise nature of the conduction is not well understood. Mobihty values associated with the various charge transport mechanisms are often low and difficult to measure. In consequence, reported conductivities are often at variance because the effects of variable impurities and past thermal history may overwhelm the dopant effects. 

The influence of the thermal history on the location and intensity of the a peak of PTEB can be observed in Fig. 18. It can be seen that the intensity of this relaxation measured on both E" and tanS bases decreases considerably on passing from the liquid crystalline sample... 

The use of dilute polymer solutions for molecular weight measurements requires the macromolecules to be in a true solution, i.e., dispersed on a molecular level. This state may not be realized in certain instances because stable, multimolecular aggregates may persist under the conditions of "solution" preparation. In such cases, a dynamic equilibrium between clustered and isolated polymer molecules is not expected to be approached and the concentration and size of aggregates are little affected by the overall solute concentration. A pronounced effect of the thermal history of the solution is often noted under such conditions. 

The glass transition temperature can be measured in a variety of ways (DSC, dynamic mechanical analysis, thermal mechanical analysis), not all of which yield the same value [3,8,9,24,29], This results from the kinetic, rather than thermodynamic, nature of the transition [40,41], Tg depends on the heating rate of the experiment and the thermal history of the specimen [3,8,9], Also, any molecular parameter affecting chain mobility effects the T% [3,8], Table 16.2 provides a summary of molecular parameters that influence the T. From the point of view of DSC measurements, an increase in heat capacity occurs at Tg due to the onset of these additional molecular motions, which shows up as an endothermic response with a shift in the baseline [9,24]. 

Figure 7.5 shows the relative endotherms of PVDF/PMMA blends as a function of PVDF content. A relative endotherm is defined here as the ratio of a melt endotherm at a certain PVDF blend composition to the endotherm of the pure annealed PVDF in DSC measurements. The DSC measurements are at a constant heating rate of lOC/min. For annealed and solution-cast blends, the relative endotherms are in general greater than those of quenched blends of the same composition. Endotherms can be detected at all compositions when the blends are annealed. The quenched PVDF shows only 75% endotherm relative to the annealed PVDF. No endotherm can be detected when the PVDF content is lower than 50 wt % in quenched PVDF/PMMA blends. Obviously the thermal history of the blends has a tremendous effect on the final performance of the material. 

I. 4, Most physical and chemical properties of gelatin are measured on aqueous solutions and are functions of the source of collagen, method of manufacture, conditions during extraction and concentration, thermal history. pH, and chemical nature of impurities or additives. 

A parallel setup is generally recommended, in which a different NMR tube is used for each measurement temperature (Table D3.1.1). However, when the amount of sample is limited, it may be necessary to use a serial rather than parallel experimental design. In a serial measurement, after measuring a sample at the first (lowest) temperature, it is transferred to the next warmest tempering block, held at the measurement temperature for the appropriate incubation time, and then remeasured. The process is repeated until the entire temperature range has been covered. Note that the solid fat content of a given sample is a function of thermal history, so serial and parallel measurements may give dissimilar results. 

The rheological behaviour of thermotropic polymers is complex and not yet well understood. It is undoubtedly complicated in some cases by smectic phase formation and by variation in crystallinity arising from differences in thermal history. Such variations in crystallinity may be associated either with the rates of the physical processes of formation or destruction of crystallites, or with chemical redistribution of repeating units to produce non-random sequences. Since both shear history and thermal history affect the measured values of viscosity, and frequently neither is adequately defined, comparison of results between workers and between polymers is at present hazardous. 

Direct correlation between thermal radiation spectroscopic techniques and thermocouple temp measurements have been obtained for a pyrot mixt containing the agent CS and the simulant 1-methylaminoanthraquinone (Ref 28). Using rapid-scan spectroscopy for time resolution of the reaction, together with two-line analysis and max radiant energy wavelength techniques, the spatial, temporal and thermal history is documented... 

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