Endothermic processes phase changes

By convention, tabulated values of heats of phase changes are always specified in the endothermic direction. Consequently, the reverse processes have the same magnitude but the opposite sign ... 

A schematic representation of the combustion wave structure of a typical energetic material is shown in Fig. 3.9 and the heat transfer process as a function of the burning distance and temperature is shown in Fig. 3.10. In zone I (solid-phase zone or condensed-phase zone), no chemical reactions occur and the temperature increases from the initial temperature (Tq) to the decomposition temperature (T ). In zone II (condensed-phase reaction zone), in which there is a phase change from solid to liquid and/or to gas and reactive gaseous species are formed in endothermic or exothermic reactions, the temperature increases from T to the burning surface temperature (Tf In zone III (gas-phase reaction zone), in which exothermic gas-phase reactions occur, the temperature increases rapidly from Tj to the flame temperature (Tg). 

The Thermal Decomposition of /3-HMX , Proc7thSympExpl Pyrots, FIRL, Phila (1971) [DTA analysis of /3-HMX in air at 1 atm using a heating rate of 2°/min revealed the endothermic process of the irreversible cryst phase change from /3 to 6 at 192°, and the violent decompn of <5 -HMX at 276°. Integration of the dx/dt or dT/dt curves for the activation energy exponent yields the data shown in Table 4... 

A second method of process intensification for recuperative reactors is to enhance performance by using a reactive coolant or heating medium, since the heat effects associated with reactions are usually much larger than those available with phase changes or simpler heating and cooling procedures. The coupling of an exothermic auxiliary heat source reaction with the desired endothermic reaction, or vice... 

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. ... 

The attaining of Teq temperature by the sample is not controlled solely by the heat source of the pyrolyser but also by the sample properties and the pyrolyser construction. This is caused by the variations in the process of heat transfer to the sample from the heat source. For example, the sample characteristics such as the mass m and the specific heat c will influence the increase of the sample temperature by the formula A T = Q/(m c). In addition, phase changes and exothermic and endothermic chemical reactions in the sample may play an important role in temperature rise. To diminish the variations determined by these processes, a very small sample size is recommended. A study done on the temperature variation of a filament pyrolyser [4] showed a decrease of the nominal temperature of the filament for the first part of the THT, when the sample load increased, due to heat absorption by the sample. 

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. 

DSC is a thermal analysis technique that is used to measure the temperatures and energy flows related to transitions in materials as a function of time and temperature.These measurements provide qualitative and quantitative information about physical and chemical changes that involve endothermic or exothermic processes or changes in heat capacity. Any event, such as loss of solvent, phase transitions, crystallization temperature, melting point, and degradation temperature of the plastic sample, result in a change in the temperature of the sample. The systems available cover a wide temperature range, e g., -60°Cto>l,500°C. 

However, in many other cases, the sign of AH is no help. An exothermic process occurs spontaneously under certain conditions, whereas the opposite, endothermic, process occurs spontaneously under other conditions. Consider the following examples of phase changes, dissolving salts, and chemical changes. 

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