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Diagram of thermal runaway.

Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, leading (in the normal case of an exothermic reaction) to a destructive result. It is a kind of positive feedback.

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Chemical engineering

In chemical engineering, thermal runaway is a process by which an exothermic reaction goes out of control, often resulting in an explosion. It is also known as a "runaway reaction" in organic chemistry.

Thermal runaway occurs when the reaction rate increases due to an increase in temperature, causing a further increase in temperature and hence a further increase in the reaction rate. It has contributed to industrial chemical accidents, most notably the 1947 Texas City disaster from overheated ammonium nitrate in a ship's hold, and the disastrous release of a large volume of methyl isocyanate gas from a Union Carbide plant in Bhopal, India in 1984. Thermal runaway is also a concern in hydrocracking, an oil refinery process. Thermal runaway may result from exothermic side reaction(s) that begin at higher temperatures, following an initial accidental overheating of the reaction mixture. This scenario was behind the Seveso disaster, where thermal runaway heated a reaction to temperatures such that in addition to the intended 2,4,5-trichlorophenol, poisonous 2,3,7,8-tetrachlorodibenzo-p-dioxin was also produced, and was vented into the environment after the reactor's rupture disk burst.[1]

Thermal runaway is most often caused by failure of the reactor vessel's cooling system. Failure of the mixer can result in localized heating, which initiates thermal runaway. Similarly, in flow reactors, localized insufficient mixing causes hotspots to form, where thermal runaway conditions occur. Incorrect component installation is also a common cause. Many chemical production facilities are designed with high-volume emergency venting to limit the extent of injury and property damage when such accidents occur.

Some laboratory reactions must be run under extreme cooling, because they are prone to hazardous thermal runaway. For example, in Swern oxidation, the formation of the sulfonium chloride must be performed in a cooled system (–30 °C), because at room temperature, the reaction undergoes thermal runaway explosively.[2]

The UK Chemical Reaction Hazards Forum[3] publishes previously unreported chemical accidents to assist the education of the scientific community, with the aim of preventing similar occurrences elsewhere. Almost 150 such reports are available to view at the present time (Jan 2009).

Electronics

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Bipolar transistors

Leakage current increases significantly in some bipolar transistors (notably germanium-based bipolar transistors) as they increase in temperature. Depending on the design of the circuit, this increase in leakage current can increase the current flowing through the transistor and with it the power dissipation. This causes a further increase in C–E current. This is frequently seen in a push–pull stage of a class AB amplifier. If the transistors are biased to have minimal crossover distortion at room temperature, and the biasing is not made temperature dependent, as the temperature rises, both transistors will be increasingly turned on, causing current and power to further increase, eventually destroying one or both devices.

To avoid thermal runaway the operating point of BJT should be Vce ≤ 1/2Vcc

If multiple bipolar transistors are connected in parallel (which is typical in high current applications) one device will enter thermal runaway first, taking the current which originally was distributed across all the devices and exacerbating the problem. This effect is called current hogging. Eventually one of two things will happen, either the circuit will stabilize or the transistor in thermal runaway will be destroyed by the heat. Hence current hogging term is related to thermal runaway.

Power MOSFETs

Power MOSFETs display increase of the on-resistance with temperature. Power dissipated in this resistance causes more heating of the junction, which further increases the junction temperature, in a positive feedback loop. (However, the increase of on-resistance with temperature helps balance current across multiple MOSFETs connected in parallel and current hogging does not occur). If the transistor produces more heat than the heatsink can dissipate, the thermal runaway happens and destroys the transistor. This problem can be alleviated to a degree by lowering the thermal resistance between the transistor die and the heatsink. See also Thermal Design Power.

Microwave heating

Microwaves are used for heating of various materials in cooking and various industrial processes. The rate of heating of the material depends on the energy absorption, which depends on the dielectric constant of the material. The dependence of dielectric constant on temperature varies for different materials; some materials display significant increase with increasing temperature. This behavior, when the material gets exposed to microwaves, leads to selective local overheating, as the warmer areas are better able to accept further energy than the colder areas—potentially dangerous especially for thermal insulators, where the heat exchange between the hot spots and the rest of the material is slow. These materials are called thermal runaway materials. This phenomenon occurs in some ceramics.

Batteries

When handled improperly, some rechargeable batteries can experience thermal runaway, resulting in overheating. Sealed cells will sometimes explode. Especially prone to thermal runaway are lithium-ion batteries. Reports of exploding cellphones occasionally appear in newspapers. Laptop batteries from Apple, HP, Toshiba, Lenovo, Dell and other notebook manufacturers were recalled because of fire and explosions.[4] [5] [6] The Pipeline and Hazardous Materials Safety Administration (PHMSA) of the U.S. Department of Transportation recently established regulations regarding the carrying of certain types of batteries on airplanes because of their instability in certain situations. This action was partially inspired by a fire on a UPS airplane.[7]

Electronics and tropical environments

Many electronic circuits contain provisions against thermal runaway. This is most often seen in transistor biasing arrangements. However when equipment is used above its designed ambient temperature, thermal runaway can in some cases still occur. This occasionally causes equipment failures in tropical countries, and when air cooling vents are blocked.

Digital electronics

The leakage currents of transistors increase with temperature. In rare instances, this may lead to thermal runaway in digital electronics. This is not a common problem, since leakage currents make up a small portion of overall power consumption, so the increase in power is fairly modest - for an Athlon 64, the power dissipation increases by about 10% for every 30 degrees Celsius[8]. For a device with a TDP of 100 W, for thermal runaway to occur, the heat sink would have to have a thermal resistivity of over 3 K/W (kelvins per watt), which is about 6 times worse than a stock Athlon 64 heat sink. (A stock Athlon 64 heat sink is rated at 0.34 K/W, although the actual thermal resistance to the environment is somewhat higher, due to the thermal boundary between processor and heatsink, rising temperatures in the case, and other thermal resistances..) A heat sink with a thermal resistance of over 0.5 to 1 K/W would result in the destruction of a 100 W device even without thermal runaway effects.

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