Dielectric heating thermal runaway

Differences in sample size and composition can also affect heating rates. In the latter case, this particularly applies when ionic conduction becomes possible through the addition or formation of salts. For compounds of low-molecular weight, the dielectric loss contributed by dipole rotation decreases with rising temperature, but that due to ionic conduction increases. When working under pressure, it is essential to measure pressure. This can be used for reaction control. If pressures fall beyond acceptable upper and lower limits or the rate of pressure rise exceeds a tolerable value, operating software should automatically shut down the machine. In combination with efficient cooling this approach can avoid thermal runaways near their onset. 

As was mentioned above, every efficient application of microwave energy to perform chemical syntheses requires reliable temperature measurement as well as continuous power feedback control, which enable heating of reaction mixtures to a desired temperature without thermal runaways. Moreover, power feedback control systems that are operated in the most microwave reactors enable a synthesis to be carried out without knowing the dielectric properties or/and conductive properties of all the components of the reaction mixture in detail. On the other hand, temperature control during microwave irradiation is a major problem that one faces during microwave-assisted chemical reactions. In general, temperature in microwave field can be measured by means of ... 

Whenever there is sufficient conductivity present in a dielectric to produce appreciable Joule heating in an applied field, the possibility of thermal runaway exists, for the accompanying rise in temperature will increase the conductivity still further. In alternating fields there may be additional heat generated through one or more relaxation processes, as described in Chapter 3, and this will hasten the onset of any thermal runaway condition. Whether thermal breakdown will eventually develop in this way or not will also depend on the rate at which heat is conducted away to the surroundings. The heat balance equation is expressed by the following continuity equation ... 

Thermal breakdown. The criterion for thermal breakdown is that the rate of heat generation in the dielectric, as a result of losses, must be greater than the rate of heat removal from the sample. Whenever this condition occurs the dielectric will heat up, which in turn will increase its conductivity, which causes further heating, etc. This is termed thermal breakdown or thermal runaway. 

In classical descriptions, thermal runaway is attributed to a strong increase of dielectric losses because of heating. So, the energy provided by microwave irradiation increases with temperature. The authors have shown it is possible to achieve thermal runaway with dielectric losses decreasing with temperature as a result of dimensional resonance or focusing effects of an electromagnetic held within the dielectric sample [128, 129]. 

In solids, the component of conduction must be added to the dielectric properties. This is especially critical in semi-conductive particles, like carbon based materials. Under microwave radiation, conductive particles loose energy through their displacement. This complicates the estimation of absorption energy, characterized by the equivalent dielectric conductivity, cj, and a loss parameter of a/uteQ. This term increases with increasing temperature. This effect, named thermal runaway, comphcates the control of homogeneous temperature heating. 

For many plastics the capacitance and dissipation factor, tan S, increase with increasing temperature. A cumulative effect may occur in which the thermal loss increases with increasing temperature. In this case, the rate of heating also increases with time under voltage. If the rate of dielectric heating exceeds the rate of cooling by thermal transfer, thermal runaway ultimately occurs. Thermal nm-away and breakdown may occur in a limited section finm which heat flows in a restricted path to the remainder of the material. Such a localized thermal breakdown is often called a Wagner breakdown. 

Thermal breakdown is caused by the fact that d.c. conductivity results in Joule heating. Under an a.c. field, there is additional energy dissipation, with heat being generated in the dielectric materials faster than it can be dissipated to the surroundings. The subsequent rise in temperature will lead to an increase in conductivity and dielectric loss, which eventually culminates in a runaway situation and thermal breakdown. The breakdown voltage, Ub. is proportional to the thermal conductivity of the materials, X, the function

flat disc and the heat transfer coefficient), and is inversely proportional to the angular frequency of the a.c. field to, the temperature coefficient of the loss factor T, the dielectric permittivily e, and the loss factor tan 5 ... 

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