Thermal runaway describes a process that is accelerated by increased temperature , in turn releasing energy which further increases the temperature. Thermal runaway occurs in situations where a rise in temperature alters conditions in such a way as to cause a further rise in temperature, often with disastrous results. This is a kind of uncontrolled positive feedback .
In chemistry (and chemical engineering ), thermal runaway is strongly associated with exothermic reactions that are accelerated by temperature rise. In electrical engineering , thermal runaway is typically associated with increased current flow and power dissipation . Thermal runaway can occur in civil engineering , especially when the heat released by large amounts of treated concrete is not controlled. [ citation needed ] In Astrophysics , runaway nuclear fusion may lead to reactions in stars Nova and many types of supernova explosions, and in the normal evolution of solar-mass stars, also occur as a less dramatic event, the ” helium flash “.
Some climate researchers have recognized that a global mean temperature increase of 3–4 °C above the pre-industrial baseline could lead to a further uncontrolled rise in surface temperatures . For example, releases of methane , a greenhouse gas more potent than CO2 , from lakes , melting permafrost and continental margins may be subject to seabed clathrate deposits positive feedback .
Chemical Engineering
Thermal runaway is also called thermal explosion in chemical engineering , or runaway reaction in organic chemistry . It is a process by which an exothermic reaction spirals out of control: an increase in temperature causes the reaction rate to increase, leading to a further increase in temperature and therefore a more rapid increase in the reaction rate. It has contributed to industrial chemical accidents , most notably from the overheated ammonium nitrate in a ship’s hold in the Texas City disaster of 1947, and of xylene in a dryer at Kings Lynn in 1976.explosion . [3] The Frank–Kamenetsky principle provides a simple analytical model for a thermal explosion. Chain branching is an additional positive feedback mechanism that can cause the temperature to skyrocket due to the rapidly increasing reaction rate.
Chemical reactions are either endothermic or exothermic, as expressed by the change in their enthalpy. Many reactions are highly exothermic, so there is some level of risk of thermal runaway in many industrial-scale and oil refinery processes. These include hydrocracking, hydrogenation, alkylation ( Sn ), oxidation, metalation and nucleophilic aromatic substitution. For example, the oxidation of cyclohexane to cyclohexanol and cyclohexanone and the oxidation of ortho-xylene to phthalic anhydride have led to disastrous explosions when reaction control fails.
Thermal runaway can result from unwanted exothermic side reaction(s) that begins at high temperatures after initial accidental overheating of the reaction mixture. This scenario was behind the Seveso disaster, where thermal runaway heats the reaction to temperatures such that in addition to 2,4,5-trichlorophenol, the toxic 2,3,7,8-tetrachlorodibenzo-p-dioxene was also produced, and was ejected into the atmosphere after the bursting disc of the reactor exploded. [4]
Thermal runaway is most often caused by a failure of the reactor vessel’s cooling system. Localized heating can occur as a result of mixer failure, which initiates thermal runaway. Similarly, in flow reactors, hotspots are formed due to local insufficient mixing, with thermal runaway conditions leading to violent detonation of the reactor material and catalyst. Incorrect equipment component installation is also a common cause. Many chemical production facilities are designed with high volume emergency venting, a measure to limit the extent of injury and property damage if accidents like this occur.
On a large scale, it is unsafe to “charge and mix all the reagents”, as is done in a laboratory scale. This is because the volume of the reaction scales with the cube of the vessel size (V R ), but the heat transfer area scales with the square of the size (A R²), so that the heat output-to-area with size Ratio scales (V / Ar r). As a result, reactions that are easily sufficiently rapidly cooled in the laboratory can self-heat dangerously on the ton scale. In 2007, this type of erroneous process caused an explosion of a 2,400 US-gallon (9100 L)-reactor used to metalate methylcyclopentadiene with metal sodium, causing four lives and parts of the reactor being removed. Damage at a distance of 400 ft (120 m). [5] [6]Thus, industrial-scale reactions likely to be thermal runaway are preferably controlled by the addition of a reagent at a rate consistent with the available cooling capacity.
Some laboratory reactions must be run under extreme cooling, as they are very prone to dangerous thermal runaway. For example, in Swarn oxidation, sulfonium chloride must be formed in a cooled system (-30 °C), as the reaction undergoes explosive thermal runaway at room temperature.
Microwave heating
Microwaves are used for cooking and heating various materials in various industrial processes. The rate of heating of a material depends on the energy absorption, which depends on the dielectric constant of the material. The dependence of the dielectric constant on temperature is different for different materials; Some materials exhibit a significant increase with increasing temperature. This behavior, when the material is exposed to microwaves, selectively leads to local overheating, as the hot regions are able to accept more energy than the cold regions—potentially for thermal insulators. Dangerous are those where heat exchange occurs between hot spots. The rest of the material is slow. These materials are called thermal runaway materials . This phenomenon occurs in some ceramics.
Electrical engineering
Some electronic components develop a lower resistance or lower triggering voltage (for non-linear resistors) with an increase in their internal temperature. If the circuit conditions in these conditions clearly cause an increase in current flow, the increased power dissipation can further raise the temperature by Joule heating. A vicious cycle or positive feedback effect of thermal runaway can sometimes lead to failure in a spectacular fashion (such as an electrical explosion or fire). To prevent these hazards, well-designed electronic systems typically include current limiting protections such as thermal fuses, circuit breakers, or PTC current limiters.
To handle large currents, circuit designers can connect several low-capacity devices (such as transistors, diodes or MOVs) in parallel. This technique can work well, but is susceptible to a phenomenon called current hogging , in which the current is not shared equally across all devices. Typically, a device may have a slightly lower resistance, and thus draw more current, heating it more than its sibling devices, causing its resistance to drop further. Electrical loads end up funneling into a single device, which then rapidly fails. Thus, an array of devices cannot be stronger than its weakest component.
The current-hogging effect can be reduced by carefully matching the characteristics of each parallel device, or by using other design techniques to balance the electrical load. However, maintaining load balancing under extreme conditions may not be straightforward. Devices with an intrinsic positive temperature coefficient (PTC) of electrical resistance are less prone to current hogging, but thermal runaway can still occur due to poor heat sinking or other problems.
Many electronic circuits have special provisions to prevent thermal runaway. This is often seen in transistor biasing arrangements for high-power output stages. However, thermal runaway may still occur in some cases when equipment is used above its designed ambient temperature. This sometimes causes equipment failure in hot environments, or when the air cooling vents are blocked.
Semiconductors
Silicon shows a peculiar profile, in which its electrical resistance increases with temperature up to about 160 °C, then begins to decrease , and drops further upon reaching the melting point. This can lead to thermal runaway phenomena within the internal regions of the semiconductor junction; Resistance decreases in areas that heat up above this limit, allowing more current to flow through the hotter areas, in turn producing more heat than the surrounding areas, causing a rise in temperature. further increase and decrease in resistance. This leads to the phenomenon of current congestion and the formation of current filaments (similar to current hogging, but within the same device), and is one of the underlying causes of many semiconductor junction failures.
Bipolar Junction Transistors (BJTs)
The leakage current in bipolar transistors (especially germanium-based bipolar transistors) increases significantly with increase in temperature. Depending on the design of the circuit, this increase in leakage current can increase the current flowing through a transistor and thus power dissipation, leading to a further increase in collector-to-emitter leakage current. This is often seen in the push-pull stage of a class AB amplifier. If the pull-up and pull-down transistors are biased to minimum crossover distortion at room temperature, and the bias is not temperature-compensated, then both transistors will be sharply biased when the temperature rises, leading to a further increase in current and power. , and eventually destroying one or both devices.
A rule of thumb to avoid thermal runaway is to place the operating point of the BJT so that V ce 1/2V cc
Another practice is to mount a thermal feedback sensing transistor or other device on the heat sink to control the crossover bias voltage. As the output transistors heat up, so does the thermal feedback transistor. This in turn causes the thermal feedback transistor to turn on at a slightly lower voltage, reducing the crossover bias voltage, and therefore reducing the heat exerted by the output transistor.
If multiple BJT transistors are connected in parallel (which is typical in high current applications), there may be a current hogging problem. Special measures should be taken to control this specific vulnerability of BJTs.
In power transistors (which effectively consist of several small transistors in parallel), current hogging can occur between different parts of the transistor, with one part of the transistor getting hotter than the others. This is called second breakdown, and can result in transistor destruction even when the average junction temperature is at a safe level.
Power MOSFETs
Power MOSFETs typically increase their resistance with temperature. In some circumstances, the power dissipated in this resistance causes overheating of the junction, which further increases the junction temperature in the positive feedback loop. As a consequence, there are stable and unstable regions of operation for power MOSFETs. [7] However, increasing resistance with temperature helps to balance the current in multiple MOSFETs connected in parallel, so current hogging does not occur. If a MOSFET transistor produces more heat, the heatsink can dissipate, so thermal runaway can still destroy the transistor. This problem can be mitigated to an extent by reducing the thermal resistance between the transistor die and the heatsink. See also thermal design power.
Metal Oxide Varistors (MOVs)
Metal oxide varistors generally develop low resistance when heated. If connected directly to an AC or DC power bus (a common use for protection against power transistors), a MOV that has developed a low trigger voltage can slide into catastrophic thermal runaway, possibly resulting in a short explosion or May end in fire. [8] To prevent this possibility, the fault current is usually limited by a thermal fuse, circuit breaker, or other current limiting device.
Tantalum capacitor
Tantalum capacitors are, under certain circumstances, prone to self-destruction by thermal runaway. The capacitor typically consists of a sintered tantalum sponge acting as the anode, manganese dioxide cathode, and a dielectric layer of tantalum pentoxide which is created by anodizing the tantalum sponge surface. It may happen that the tantalum oxide layer has weak spots that undergo dielectric breakdown during a voltage spike. The tantalum sponge then comes into direct contact with manganese dioxide, and increased leakage current leads to localized heating; Typically, this drives an endothermic chemical reaction that produces manganese(III) oxide and regenerates the tantalum oxide dielectric layer (self-healing).
However, if the energy dissipated at the failure point is high enough, a self-sustaining exothermic reaction can begin, similar to the thermite reaction, with the metal tantalum as the fuel and manganese dioxide as the oxidizer. This undesirable feedback will destroy the capacitor, producing smoke and possibly flame.
Therefore, tantalum capacitors can be freely deployed in small-signal circuits, but applications in high-power circuits must be carefully designed to avoid thermal runaway failures.
Digital logic
The leakage current of a logic switching transistor increases with temperature. In rare instances, this can cause thermal runaway in digital circuits. This is not a common problem, as leakage currents typically make up a small fraction of the total power consumption, so the increase in power is quite modest – for the Athlon 64, power dissipation increases by about 10% for every 30 °C. Is. [10] For a device with a TDP of 100 W, to be thermal runaway, the heat sink must have a thermal resistivity of more than 3 K/W (Kelvin per watt), which is about 6 times worse than the stock. Athlon 64 heat sink. (The 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 the processor and the heatsink, rising temperatures in the case, and other thermal resistances. [citation needed ]) however, an insufficient heat sink with a thermal resistance of more than 0.5 to 1 K/W would result in the destruction of a 100 W device even without thermal runaway effects.
Batteries
When handled improperly, or if manufactured faulty, some rechargeable batteries can experience thermal runaway resulting in overheating. If the safety vents are overwhelmed or non-functional, the sealed cells will sometimes burst violently. [11] Lithium-ion batteries are particularly prone to thermal runaway, which is most pronounced in the form of lithium polymer batteries. [ citation needed ] Newspapers occasionally report cellphone explosions. In 2006, batteries from Apple, HP, Toshiba, Lenovo, Dell and other notebook makers were recalled due to fires and explosions. [12] [13] [14] [15]The Plumbing and Hazardous Substances Safety Administration’s (PHMSA) Department of Transportation has established regulations regarding carrying certain types of batteries on airplanes because of their instability in certain situations. This action was partly inspired by a fire in the cargo bay on a UPS airplane. [16] One of the possible solutions is to use safer and less reactive anode (lithium titanates) and cathode (lithium iron phosphate) materials – thereby avoiding the cobalt electrodes in many lithium rechargeable cells – together with ionic liquids. based non-flammable electrolytes. ,
Astrophysics
Runaway thermonuclear reactions in stars can occur when nuclear fusion is ignited in conditions under which the pressure exerted by the upper layers of the star is much greater than the thermal pressure, a condition that allows a rapid rise in temperature. manufactures. Such a scenario can arise in stars that contain degenerate matter, in which the electron degeneracy pressure instead of the normal thermal pressure does most of the work of supporting the star against gravity, and the stars are exploding. In all cases, imbalance occurs prior to fusion ignition; Otherwise, fusion reactions would naturally be controlled to counteract the temperature change and stabilize the star. When the thermal pressure is in equilibrium with the overlapping pressure, So a star would respond to an increase in temperature and thermal pressure by expanding and cooling by initiating a new exothermic reaction. A runaway reaction is possible only if this reaction is interrupted.
Helium shines in red giant stars
When stars in the 0.8–2.0 solar mass range deplete the hydrogen in their cores and become red giants, the helium that accumulates in their cores reaches degeneracy before igniting. When the degenerative core reaches a critical mass of about 0.45 solar masses, helium fusion ignites and goes off in a runaway fashion, known as a helium flash, briefly reducing the star’s energy output to normally 100. Increases it billion times. About 6% of the core is quickly converted to carbon. [17] While the release is sufficient to convert the core back to normal plasma after a few seconds, it does not obstruct the star, [18] [19]Nor does it change its brightness instantly. The star then shrinks, leaving the red giant phase and continuing its evolution into a stable helium-burning phase.
Novi
A nova results from runaway hydrogen fusion (via the CNO cycle) in the outer layer of a carbon-oxygen white dwarf star. If a white dwarf had a companion star from which it could collect gas, material would accumulate in a surface layer perturbed by the dwarf’s intense gravity. Under the right conditions, a sufficiently thick layer of hydrogen is eventually heated to a temperature of 20 million K, igniting runaway fusion. The surface layer is destroyed by the white dwarf, increasing the brightness by a factor of the order of 50,000. However, the white dwarf and companion remain intact, so the process can be repeated. [20] A very rare type of nova can occur when the outer layer that ignites is made of helium. [21]
X-ray burst
In line with the process leading to the nova, degenerate matter can also accumulate on the surface of a neutron star that is accumulating gas from a close companion. If a sufficiently thick layer of hydrogen accumulates, the ignition of the fledgling hydrogen fusion can cause an X-ray burst. As with nova, such bursts tend to recur and can also be triggered by helium or carbon fusion. [22] [23] It has been proposed that in the case of a “superburst”, the runaway breakdown of heavy nuclei accumulated in iron cluster nuclei through photodissociation rather than nuclear fusion may contribute most of the energy of the explosion. [23]
Type Ia Supernova
A Type Ia supernova results from runaway carbon fusion in the core of a carbon-oxygen white dwarf star. If a white dwarf, composed almost entirely of degenerative material, could gain mass from a companion, the rising temperature and density of material in its core would ignite carbon fusion if the star’s mass reaches the Chandrasekhar limit. . This causes an explosion that completely obliterates the star. The brightness increases by a factor of over 5 billion. One way to gain additional mass would be to accumulate gas from a massive star (or even main sequence) companion. [24] A second and apparently more common mechanism to generate the same type of explosion is the merger of two white dwarfs. [24] [25]
Pair-instability supernova
A pair-instability supernova with a mass, 130–250 solar masses, is believed to be the result of runaway oxygen fusion in the core of a low to moderate metallicity star. [26]According to the theory, such a star would form a large but relatively low-density core of nonfusing oxygen, the weight of which is supported by the pressure of gamma rays generated by the extreme temperatures. As the core further heats up, gamma rays eventually begin to exceed the energy threshold required for collision-induced decay in electron-positron pairs, a process called pair production. This causes a drop in pressure within the core, causing it to shrink and heat further, producing more pairings, another pressure drop, and so on. The core begins to undergo gravitational collapse. At some point it ignites the runaway oxygen fusion, releasing enough energy to annihilate the star. These explosions are rare, perhaps one per 100,000 supernovae.
Comparison to non-runaway supernovae
Not all supernovae are triggered by runaway nuclear fusion. Type Ib, Ic, and Type II supernovae also undergo core collapse, but because they have exhausted the supply of atomic nuclei capable of undergoing exothermic fusion reactions, they can also form into neutron stars, or stellar black holes in high-mass cases. Kind of fall. , powering explosions by the release of gravitational potential energy (mainly through the release of neutrinos). It is the absence of runaway fusion reactions that allows such supernovas to leave behind compact stellar remnants.
