Thermal decomposition endothermic transition

Experimental studies on the thermal decomposition and combushon processes of AP have been carried out and their detailed mechanisms have been reported.P-iil Fig. 5.1 shows the thermal decomposition of AP as measured by differential thermal analysis (DTA) and thermal gravimetry (TG) at a heating rate of 0.33 K s"f An endothermic peak is seen at 520 K, corresponding to an orthorhombic to cubic lattice crystal structure phase transition, the heat of reaction for which amounts to... 

Fig. 7.7 Differential thermal analysis (DTA) in N2 flow of C60H36 (top trace) showing a broad endothermic transition at 412°C, just at the onset of the decomposition.C(j0D36 shows two endothermic transitions at 379°C and 465°C. The second transition being the onset of the decomposition...

A DuPont TA-9900 thermal analyzer system, attached to a DuPont Data Unit, was used to obtain the DSC thermogram of EDTA over a range of 50 to 350°C. The thermogram shown in Figure 2 is fairly complicated, showing possible water loss at low temperature. The melting endotherm exhibited a maximum at 250.9°C, and the noise in the baseline following the endothermic transition is indicative of the thermal decomposition of the compound. 

The thermal decomposition of a number of organic acids has been studied by DTA by Wendlandt and Hoiberg (60,6l). Since ihe acids were decomposed in an argon atmosphere, only endothermic peaks were observed in the DTA curves. These peaks were caused by such reactions as dehydration, decarboxylation. sublimation, decomposition, and phase transitions from the solid to the liquid state. The maximum peak temperatures for the phase transitions were 10 to 30 higher than the reported melting-point temperatures. The DTA curves for some of the acids are given in Figure 7.33. 

The thermal decomposition characteristics of micron-sized aluminum powder and potassium perchlorate mixtures were studied with thermal analytical techniques by Pourmortazavi and coworkers [108]. The results showed that the reactivity of aluminum powder in air increases as the particle size decreases. Pure aluminum with 5 pm particle size has a fusion temperature of about 647 °C, but for 18 pm powder this temperature is 660 °C. Pure potassixun perchlorate has an endothermic peak at 300 C corresponding to a rhombic-cubic transition, a fiision temperature around 590 °C and decomposes at 592 °C. DTA curves for an AI5/KCIO4 (30 70) mixture show a maximum peak temperature for thermal decomposition at 400 °C. Increasing the particle size of aluminum powder increases the ignition temperature of the mixture. The oxidation temperature is increased by increasing the aluminum content of the mixture. 

For better comprehension of thermal decomposition process, the TG-DTA curves for MgAl-LDH are presented in Figure 20.5. It can be observed that the typical LDH decomposition is characterized by two endothermic transitions involving the following consecutive or overlapping steps dehydration, dehydroxylation, decomposition of anions, and segregation of oxides. 

The sample temperature is increased in a linear fashion, while the property in question is evaluated on a continuous basis. These methods are used to characterize compound purity, polymorphism, solvation, degradation, and excipient compatibility [41], Thermal analysis methods are normally used to monitor endothermic processes (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, and chemical degradation) as well as exothermic processes (crystallization and oxidative decomposition). Thermal methods can be extremely useful in preformulation studies, since the carefully planned studies can be used to indicate the existence of possible drug-excipient interactions in a prototype formulation [7]. 

Measurements of thermal analysis are conducted for the purpose of evaluating the physical and chemical changes that may take place in a heated sample. This requires that the operator interpret the observed events in a thermogram in terms of plausible reaction processes. The reactions normally monitored can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. 

All decomposition reactions are endothermal except that of FeU04, presumably because this is the only reaction which involves oxidation of the double oxide. No significant diflFerence was noted in the DTA or TGA curves of the two NiU04 phases. It is interesting to note the alternating pattern in the decomposition reactions of the uranates. The iron, nickel, and zinc double oxides tend to decompose directly into their constituent oxides, while the manganese, cobalt, and copper compounds decompose to other double oxides. The pattern is not carried over into the decomposition temperatures. In this instance, the thermal stability of the double oxides appears to vary directly with the characteristic transition element oxidation states Gr(III) > Mn, Go (III, II) > Ni, Zn(II) > Gu(II, I). The iron compounds constitute a definite exception to this pattern. 

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