Thermal runaway phenomenon

The shutdown property of separators is measured by measuring the impedance of a separator while the temperature is linearly increased. Figure 7 shows the actual measurement for Celgard 2325 membrane. The heating rate was around 60 °C/min, and the impedance was measured at 1 kHz. The rise in impedance corresponds to a collapse in pore structure due to melting of the separator. A 1000-fold increase in impedance is necessary for the separator to stop thermal runaway in the battery. The drop in impedance corresponds to opening of the separator due to coalescence of the polymer and/or to penetration of the separator by the electrodes this phenomenon is... 

A phenomenon known as thermal runaway can occur when a large new lithium surface is produced on charge which reacts exothermically with components of the electrolyte. In some cases the temperature is raised sufficiently to melt some of the lithium, which then in turn reacts with more electrolyte, causing a further rise in temperature and eventually the device goes on fire. 

From these equations, it is clear that an increase in temperature reduces the relaxation time. When the dielectric loss factor increases with the increasing temperature, food would experience a phenomenon known as thermal runaway. When frozen food is thawed with higher microwave power, certain area of the food would be overheated while other areas remain much cooler. Hence, it is important to maintain low microwave power for thawing frozen food to have a uniform thawing. 

TABLE 4-5 Reactor Design Strategies to Prevent the Phenomenon of Thermal Runaway in Plug-Flow Tabular Reactors... 

Polarization of a VRLA cell at 2.40 V. The thermal runaway (TRA) phenomenon is manifested by changes in (a) current and voltage (b) positive and negative plate potentials (c) cell temperature... 

The difTerence in the mechanism of autoacceleration, depending on the size of the test tubes, changed the character of the PMMA produced in the reaction system. As shown in Figure 3, both M and MJM increased in the smallest test tube after the onset of autoacceleration. This is a typical phenomenon for the Trommsdorff effect, since it is led by the retardation of the termination reaction. Increase of M of the polymer, however, was not observed in the larger test tubes. Thermal runaway led to rapid decomposition of the initiator to produce more radicals transiently. This process produced more polymers having a lower degree of polymerization, lowering M,. 

Needless to say, that an insufficient thermal design of the batteiy assembty or an accidental slight overcharge may lead to the catastrophic destruction of the battery by the phenomenon of thermal runaway. The losses suffered recently by some battery and laptop computers manufacturers that needed to recall a large number of faulty batteries constitute an eloquent illustration, in economic terms, of the importance that this phenomenon may acquire. 

Nonlinear phenomena in any system require some type of feedback. The most obvious source of feedback in polymerization reactions is thermal autocatalysis, often called thermal runaway in the engineering literature. The heat released by the reaction increases the rate of reaction, which increases the rate of heat release, and so on. This phenomenon can occur in almost any reaction and will be important when we consider thermal frontal polymerization. 

In case of an inerease in operating temperature with a strong current or because of poorly-eontrolled or uncontrolled environmental conditions, the phenomenon leading to thermal runaway is linked primarily to the deeomposition of the SEI of the lithiated carbon electrode, leaving exposed the electrode, whieh deeomposes by reaction between the inserted lithium and the eleetrolyte. Gas formation occurs, once again leading to an increase in the internal pressure of the element. 

In Figures 3.8 and 3.10, it is possible to observe a hot spot. This phenomenon is frequently observed for strong exothermic processes and can possibly lead to runaway. Several criteria have been put forward to predict this behavior and related thermal effects, such as steady-state multiplicity. Several references have been devoted to this complex subject [137], which is out of the scope of this chapter. 

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