Heat capacity evolution

FIGURE 4.6 Heat capacity evolution of the crystallization of pregelatinized waxy com starch at 52°C. (From De Meuter, P., Rahier, H., and Van Mele, B., Ini. J. Pharm., 192, 77, 1999. With permission.)... 

The information available in the heat capacity evolution is a key factor for the correct interpretation of the heat flow signal. The results indicate that the heat flow phase angle contains interesting information regarding the rheological state of the reacting material and especially about the occurrence of relaxation phenomena. 

In this study, ab initio calculations have been performed to evaluate firstly, the influence of cationic disorder on internal energy variation (U) and excess of heat capacity evolution and secondly, the relative density behavior. The cationic disorder parameter versus temperature curve is deduced as a by product, according to an effective thermodynamic model using both a regular solution and a quadratic form of the internal energy [22]. Results are compared to available experimental data. 

As already indicated, Tian s equation supposes (1) that the temperature of the external boundary of the thermoelectric element 8e, and consequently of the heat sink, remains constant and (2) that the temperature Oi of the inner cell is uniform at all times. The first condition is reasonably well satisfied when the heat capacity of the heat sink is large and when the rate of the heat flux is small enough to avoid the accumulation of heat at the external boundary. The second condition, however, is physically impossible to satisfy since any heat evolution necessarily produces heat flows and temperature gradients. It is only in the case of slow thermal phenomena that the second condition underlying Tian s equation is approximately valid, i.e., that temperature gradients within the inner cell are low enough to be neglected. The evolution of many thermal phenomena is indeed slow with respect to the time constant of heat-flow calorimeters (Table II) and, in numerous cases, it has been shown that the Tian equation is valid (16). 

The next step was the introduction of ion implantation to dope Si for thermometers. Downey et al. [66] used micromachining to realize a Si bolometer with an implanted thermometer. This bolometer had very little low-frequency noise. The use of thermometers doped by neutron transmutation instead of melt doping is described by Lange et al. [67], The evolution of bolometers sees the replacement of the nylon wires to make the conductance to the bath, used by Lange et al. with a micromachined silicon nitride membrane with a definite reduction in the heat capacity associated to the conductance G [68],... 

Experimental Methods In Differential thermal analysis (DTA) the sample and an inert reference substance, undergoing no thermal transition in the temperature range under study are heated at the same rate. The temperature difference between sample and reference is measured and plotted as a function of sample temperature. The temperature difference is finite only when heat is being evolved or absorbed because of exothermic or endothermic activity in the sample, or when the heat capacity of the sample changes abruptly. As the temperature difference is directly proportional to the heat capacity so the curves are similar to specific heat curves, but are inverted because, by convention, heat evolution is registered as an upward peak and heat absorption as a downward peak. 

Latent heat of vaporization (cal/gm) Density (gm/cm3) Thermal conductivity (cal/s-1/ cirr /KT1) Stoichiometric heat evolution in air per unit weight of fuel (cal/gm) Heat capacity (cal/gm-1/ K->)... 

Alpha-beta (a-jS) transitions of the condensed forms of Si02 quartz, try-dimite, and cristobalite may all be regarded as lambda transformations. Their kinetics are higher than those of quartz-trydimite, quartz-cristobahte, and quartz-coesite, which are first-order transformations. Figure 2.7 plots in detail the evolution of enthalpy, entropy, heat capacity, and volume at the transition zone... 

COMMENTS CONCERNING PARAMETERS OF THE SHORT-RANGE ORDER EVOLUTION DETERMINED FROM THE DATA ON KINETICS OF A HEAT-CAPACITY RELAXATION FOR Lu-H ALLOY... 

As noted in Ref. [6], two fitting parameters, tj and x2, are chosen for more high-quality reproduction of measuring data. Nevertheless, in fact, the sense of these parameters is more intimate. Characterization of both heat-capacity and residual-resistivity relaxation kinetics (following the short-range order evolution kinetics) for... 

The evolution in calorimetry technology has also led to the development of protocols for quantitative analysis (Buckton and Darcy 1999). Fiebich and Mutz (1999) determined the amorphous content of desferal using both isothermal microcalorimetry and water vapour sorption gravimetry with a level of detection of less than 1 per cent amorphous material. The heat capacity jump associated with the glass transition of amorphous materials MTDSC was used to quantify the amorphous content of a micronised drag substance with a limit of detection of 3 per cent w/w of amorphous... 

In the summation the specific heats of the substances which are produced with evolution of heat are reckoned positive. The temperature coefficient of the heat of reaction is therefore equal to the change in the heat capacity of the system, consequent on the reaction. The heat of reaction increases with temperature when the substances formed in the reaction have a smaller heat capacity than the substances which disappear in the reverse case it decreases with temperature. For endothermic reactions in which Q is negative, an increase in Q means a diminution in the numerical value of the heat of reaction, and conversely. 

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