Negative temperature coefficient, and

The slow combustion reactions of acetone, methyl ethyl ketone, and diethyl ketone possess most of the features of hydrocarbon oxidation, but their mechanisms are simpler since the confusing effects of olefin formation are unimportant. Specifically, the low temperature combustion of acetone is simpler than that of propane, and the intermediate responsible for degenerate chain branching is methyl hydroperoxide. The Arrhenius parameters for its unimolecular decomposition can be derived by the theory previously developed by Knox. Analytical studies of the slow combustion of methyl ethyl ketone and diethyl ketone show many similarities to that of acetone. The reactions of methyl radicals with oxygen are considered in relation to their thermochemistry. Competition between them provides a simple explanation of the negative temperature coefficient and of cool flames. 

Whereas the RTD exhibits a small positive temperature coefficient, the thermistor has a large negative temperature coefficient and the resistance/temperature relationship is highly non-linear. The latter is typically ... 

At 273 K it is about 2000 times less (for M = O2) than the proportionality constant for O + NO. The latter has a small negative temperature coefficient, and plots of log /q.co and log 7o,no versus T show that the two pre-exponential factors are similar. The rate coefficient ks-j has also been found to be very much smaller than the corresponding coefficient for O + NO at 300 K, and the evaluation to be given below indicates that fe 7 also has an activation energy of about 4 kcal. mole. The mechanism of light emission is therefore closely related to the mechanism of combination. 

Three requirements must be fulfilled before any mechanism can be accepted. Firstly, it must be capable of explaining the mode of formation of the reaction products secondly, it must be acceptable from thermo-kinetic considerations and finally it must be capable of explaining phenomena such as the negative temperature coefficient and periodic cool flames. It was recently pointed out [38] that many mechanisms have been proposed in recent years which do not take the second consideration into account. Whilst this is a valid criticism, the reverse is also true, i.e. many mechanisms have been suggested which are based on inaccurate thermo-kinetic considerations and have not been confirmed experimentally. In any event, the system under consideration must be defined by experiment, which in this case requires extensive knowledge of the kinetics, the yields and nature of the products formed and their variation with the extent of reaction and the reaction conditions. Modern techniques have allowed the system to be reasonably well defined in these terms and this has led to two principal theories regarding chain-propagation. 

The reaction rate does not show any marked negative-temperature coefficient, and the apparent activation energy calculated from the slope of the log (d[02 ] /dt) versus 1/T graph was about 14 kcal. mole . ... 

The Arrhenius graph for the combustion of this compound possesses a region of negative temperature coefficient, and the cool flame limits have been determined [45]. 

Thermistors are resistors with a temperature-dependent value of their resistance. There are three types of thermistors CTT (critical temperature thermistors), NTC (negative temperature coefficient), and PTC (positive temperature coefficient) thermistors. Their thermal behavior is shown in Figure 9.7. 

The core reactivity is controlled by control rods in the core and reflectors. A completely independent and redundant reserve shutdown system provides a diverse reactivity control capability using boron pellets stored in hoppers above special channels in the core. The inherent features that control reactivity and thus heat generation, include a strong negative temperature coefficient, and the single phase, neutronically inert cool. ... 

Thermistors are semiconductor resistors that have resistor values that vary over a wide range. They are available with both positive and negative temperature coefficients and are used for temperature measurements and control systems, as well as for temperature compensation. In the latter they are utilized to offset unwanted increases or decreases in resistance due to temperature change. 

Owing to the small thermal capacities of core and coolant in a gas-cooled FBR, the initial rate of temperature rise in the event of a loss-of-flow transient will be greater than in the metal-cooled reactor. The combination of a negative temperature coefficient and the absence of a voidage reactivity effect, however, result in a more rapid negative feedback. 

A design goal of the SSR was that it would be self-regulating with regard to power excursions (negative temperature coefficient) and thus not require control rods during reactor operation. Nevertheless, some type of shutdown mechanism must be present in the system to keep the reactor subcritical prior to reaching the deployment site and also to allow for shutdown of the reactor prior to depletion of its fuel... 

Materials with large thermal conductivity at low temperatures generally have a negative temperature coefficient—and vice versa for those with low conductivity at low temperatures. 

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