Self heat rate analysis

Figure 12-11. Self-heat rate analysis. ARC data are shown along with a fitted model obtained by assuming the following kinetic parameters reaction order = 1, activation energy = 31.08 kcal/mol, and frequency factor = 2.31 El 2 min ...

The ARC system is often operated in a stepwise heat-wait-search modus. After heating to a certain temperature, the system is stabilized for a pre-defined time until the calorimeter starts seeking for a temperature increase caused by first decomposition processes. If the temperature increase surpasses a pre-defined threshold (e.g. 0.01 K/min) the furnace temperature follows the sample temperature in the adiabatic mode and the calorimeter tracks the adiabatic temperature rise due to the self-heating of the sample. If the threshold is not surpassed after a certain period of time, the calorimeter proceeds with the next temperature step. In comparison to DSC analysis ARC measurements are significantly more sensitive, usually by a factor of 100 or more. Sensitivity is as low as 0.5mW/g and self-heating rates of 0.01 K/min can be detected. 

The sample is enclosed in a heavy walled bomb with an internal volume of approximately 1 to 1 ml. Although similar to a differential thermal analysis (DTA) test, the samples used are much larger, and the conditions of confinement allow the liquid to remain in contact with any decomposition products that form as vapors. Heat is applied so that the bath temperature increases at a constant rate and the temperature of both the heating bath and the sample are recorded continuously. When the temperature of the sample exceeds that of the bath, an exothermic reaction must be occurring in the sample, and this process is frequently accompanied by a detonation. (The bomb is equipped with a blow out disc to avoid any major damage to the equipment). In the more usual case the discrepancy between sample temperature and bath temperature increases with temperature, and the point at which this deviation is 5°F./min. is called the self-heating temperature. Typical values for some liquid materials of interest in the propellant field are listed in Table V. 

The temperature of the air bath is set at a proper value, i.e., the nominal T,. of the run, by means of the temperature dial on the air bath on the basis of the selfheating property of the liquid tested to give estimated rates of increase in temperature of 1.25 K/h in the adiabatic self-heating test. It is usually a temperature 20 30 K lower than the EOT, which is measured by thermal analysis, such as DTA or DSC, of each liquid in the case of liquids tested herein, with the result that the T, ranged from 27.29 °C for THPN to 100.6 °C for DTBP. As to TBPB, the T,. ranged from 73.18 to 79.62 °C. 

Another more elaborate pyrolysis—chromatography—apparatus was described by MacLaury and Schroll (146), which permitted heating rates from 5 C/m to 5000°C/S. It consisted of a Chemical Data Systems geological sample and analysis system and a gas chromatograph. This system is a self-contained bench-top instrument that provides a means of trapping volatiles from a DSC 100 Pyroprobe solids pyrolyzer. The Pyroprobe uses a platinum... 

The insulated exotherm test (lET) is essentially a form of differential thermal analysis (DTA) on the gram scale. The sample and an inert reference material are held in identical containers and heated at a constant rate, enclosed in an internally-lagged Dewar flask (see Figure 3.2 on page 30). The temperature of the sample and the temperature difference between the sample and the reference are recorded as functions of time. Self-heating of the sample relative to the inert reference can be determined under conditions of low heat loss. 

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