Phase transformation exothermic heat

Although there are other ways, one of the most convenient and rapid ways to measure AH is by differential scanning calorimetry. When the temperature is reached at which a phase transition occurs, heat is absorbed, so more heat must flow to the sample in order to keep the temperature equal to that of the reference. This produces a peak in the endothermic direction. If the transition is readily reversible, cooling the sample will result in heat being liberated as the sample is transformed into the original phase, and a peak in the exothermic direction will be observed. The area of the peak is proportional to the enthalpy change for transformation of the sample into the new phase. Before the sample is completely transformed into the new phase, the fraction transformed at a specific temperature can be determined by comparing the partial peak area up to that temperature to the total area. That fraction, a, determined as a function of temperature can be used as the variable for kinetic analysis of the transformation. 

Differential thermoanalysis involves recording the temperature difference between an inert compound and the sample during heating. Such differences occur if reactions take place which either release (exothermic effect) or consume (endothermic effect) energy. These effects are recorded as peaks on a plot of the temperature difference versus the temperature. Such thermal effects are associated with the loss of adsorbed H2O and structural OH as in TGA and also with phase transformations. 

When the sample undergoes a transformation, it will either absorb (endothermic) or release (exothermic) heat. For example, the melting of a solid material will absorb heat, where that thermal energy is used to promote the phase transformation. The instrument will detect that the sample is cooler than the reference, and will indicate the transformation as an endotherm on a plot of differential temperature (AT) versus time.2 Figure 3.2 shows a typical DTA trace of the decomposition of dolomite. If the sample and reference are exposed to a constant heating rate, the x-axis is often denoted as tem-... 

The enthalpy values of dehydriding AIH3 have been determined. The a-phase dehydriding reaction was found to be endothermic with 6.0 1.5kJ (molH2) consistent with values reported elsewhere of 7.6kJ (molH2) [19]. Similar to early reports, the y-phase showed exothermic transformation to the a-phase followed by dehydriding to aluminum with a net endothermic heat of reaction of 1.0 0.5kJ (molH2). The P-phase, however, was found to dehydride directly to aluminum, which is a different result from results reported elsewhere [15, 16, 19]. 

Differential thermal analysis (DTA) measures the amount of heat released or absorbed by a sample as it is heated at a known rate." When the enthalpy change is determined, the method is called differential scanning calorimetry (DSC). The presence of exothermic or endothermic processes at certain temperatnres provides information about the nature of phase changes and chemical reactions occurring in the material as it is heated. DTA can often be used as a sensitive method for establishing the presence or absence of secondary phases in samples if these phases undergo phase transformations at known temperatures. ... 

A sample is continuously heated at a constant rate (e.g. 10 C min ) while two changes are recorded (1) the temperature difference between an inert compound and the sample with a thermocouple (differential thermo analysis DTA) and (2) the weight loss measured with a balance (thermogravimetry TGA) (Mackenzie, 1957 Smykatz-Kloss, 1974). With DTA, information is obtained about endothermic and exothermic phase transformations (see Fig. 1-2), whereas with TGA adsorbed water and structural OH can be measured. 

Eutectic arrest In a cooling (or heating) curve an approximately exothermal segment corresponding to the time interval during which the heat of transformation from the liquid phase of two or more conjugate solid phases is being evolved (or conversely). 

In the wet oxidation process, materials partially or completely dissolve into a homogeneous, condensed-phase mixture of oxygen and water, and chemical reactions between the material and oxygen take place in the bulk water phase. This condensed-phase makes wet oxidation an ideal process to transform materials which would otherwise be non-soluble in water to a harmless mixture of carbon dioxide and water. Since oxidation reactions are also exothermic, the high thermal mass of supercritical water makes this reaction medium better suited for thermal control, reactor stability, and heat dissipation. The purpose of this research was to establish a new method for selectively oxidizing waste hydrocarbons into new and reusable products. 

In the homogeneous phase under thermodynamically favorable temperature conditions, the formation of ammonia may be forced by employing other forms of energy, such as electrical energy or ionizing radiation. The principal difficulty with these so-called plasma processes, which also impedes their economic use, is that the energy supplied is useful only in part for ammonia formation. A greater part is transformed in primary collision and exothermic secondary processes into undesirable heat or unusable incidental radiation. 

When this sample was heated in an oxygen atmosphere, a larger exotherm occurred at 1050 C as a fraction of the AloOo support reacted with WO3 to form Al2(W04)3. The formation of Al2(W0 )3 was confirmed by X-ray diffraction measurements. Alumina not utilized in tungstate formation transformed predominantly to 0-AI2O3 only a trace of orAl203 was produced. Thus, the presence of the tungsten oxide surface phase inhibits the transition of 0-AI2O3 to orAl 2O3. 

A detailed literature summary and discussions of the thermal decomposition of HMX are presented by Boggs1151, and the general picture of the decomposition processes of HMX may be understood from [15]. When HMX is heated slowly, a single-stage mass loss process is observed the mass loss begins at 550 K and rapid gasification reaction occurs at 553 K. No solid residue remains above 553 K. Two endothermic peaks and one exothermic peak are seen the first endothermic peak at 463 K is the crystal transformation from (3 to 6 and the second endothermic peak at 550 K is the phase change from solid to liquid. The exothermic peak at 553 K is caused by the reaction accompanied by gas phase reaction. 

Metastable intermetallic compounds can be obtained from amorphous alloys after partial crystallization. Two exotherms are usually found when the corresponding alloys are heated in a DSC unit. The first exotherm originates from the crystallization of the metastable compound, the second exotherm corresponding to the transformation into the stable equilibrium phases. The metastable compound found are commonly obtained by heating to temperatures close to the first exotherm and subsequently cooling to room temperature. 

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