Trigger temperature

REDUCTION OF OTHER METAL HALIDES. The reduction of several other metal halides with sodium dispersions proved very successful, although higher temperatures were required for the reduction of some of the metals. In most instances, reduction did not occur imtil a specific threshold, or trigger, temperature was reached no attempt was made to determine whether this phenomenon was due to a potential barrier which required a high activation energy to overcome, or to some other thermodynamic and/or kinetic properties of the system. Attempts to correlate free energy data with threshold temperatures were unsuccessful. 

The initial tests of the sodium dispersion-metal halide system were made with ferric chloride, fortunately, and with a nonaqueous solvent in which ferric chloride was soluble. Thus, ferric chloride was present as a solution and, consequently, presented maximum surface for contact with the sodium particles. This feature, coupled with the lower activation energy requirements, permitted the reaction to proceed at temperatures well below room temperature and established the operability of the method. The success of the initial (ferric chloride) tests lent encouragement to tests on other metal systems and prompted continued investigations when the initial runs at lower temperatures failed. The discovery of the threshold, or trigger, temperature for nickel (II) chloride reduction paved the way for successful reduction of other metal halides such as manganese (II) chloride, cobalt (II) chloride, and cadmium bromide. 

Fixed Temperature Fixed temperature heat sensors operate when a sensor reaches a preset temperature. They are a variety of temperature settings. Because there is some mass in the sensors themselves, the fixed temperature sensors take some time to respond to conditions. The air surrounding the device will reach the trigger temperature at some time before the sensor elements do. The time lag depends on the device. Because of the lag, fixed temperature sensors are not suitable for fires that develop quickly. 

Numerous polymers have been proposed as shape-memory polymers (SMPs), and many of them are based on polyurethanes. This is because of the intrinsic versatility of segmented copolyurethane systems. By suitable choice of diisocyanate and macrodiol, a wide variation in properties may be obtained, allowing the possibility of tuning the shape-memory response to suit different applications. Usually they are phase-segregated materials. For example, a dispersed rigid phase (usually based on the diisocyanate) provides physical crosslinks, while the macrodiol provides a soft amorphous phase with low glass transition that provides the trigger temperature for shape recovery [63]. 

The softening temperature, which acts as the shape-memory trigger temperature (to be precise, Tmax), may be changed systematically by variation of the HS content [366-369] or the chain length of the macrodiol employed [63, 370, 371]. A feature of practical importance for biomedical applications of SMPs is the fact that these systems may be designed with trigger temperatures in the convenient range 20-50°C [63,366,369-372]. 

Ampacet claims that it has been able to lower the range of trigger temperatures over which azodicarbonamide masterbatches work, from the usual 190-205 °C down to 170-200 °C. Bayer sells a modified azodicarboxylic acid diamide-based blowing agent called Porofor ADC/Z-C2 for foaming crosslinked polyethylene. It is claimed to allow the manufacture of lower density foams at conventional oven temperatures, with shorter production cycles than standard ADC diamides. 

In the shape memory polyurethane, the hydrogen bonded hand segment phase is responsible for shape recovery." Multiwalled carbon nanotubes formed lydrogen bonds with polyurethene in participation in shape memory preservation." Carbon nanotube and ester-based polyurethane form shape memory nanoeomposites with low trigger temperature." ... 

In some film systems there can be an exothermic reaction, such that large amounts of heat are generated after the reaction has been triggered . Such systems are Pd-Sn, Al-Pd, and Al-Zr, which have increasingly higher triggering temperatures. Multilayer composite structures of these materials may be used to rapidly release heat. ... 

In most applications of high frequency induction methods the rate of energy input is relatively slow, with times of 10 s being not uncommon to reach the trigger temperature of 160-180°C. Not all body materials are suitable for this method and it cannot therefore be regarded as a universal technique. 

In nonindustrial settings, MCS substances are the cause of indoor air pollution and are the contaminants in air and water. Many of the chemicals which trigger MCS symptoms are known to be irritants or toxic to the nervous system. As an example, volatile organic compounds readily evaporate into the air at room temperature. Permitted airborne levels of such contaminants can still make ordinary people sick. When the human body is assaulted with levels of toxic chemicals that it cannot safely process, it is likely that at some point an individual will become ill. For some, the outcome could be cancer or reproductive damage. Others may become hypersensitive to these chemicals or develop other chronic disorders, while some people may not experience any noticeable health effects. Even where high levels of exposure occur, generally only a small percentage of people become chemically sensitive. 

The released energy might result from the wanted reaction or from the reaction mass if the materials involved are thermodynamically unstable. The accumulation of the starting materials or intermediate products is an initial stage of a runaway reaction. Figure 12-6 illustrates the common causes of reactant accumulation. The energy release with the reactant accumulation can cause the batch temperature to rise to a critical level thereby triggering the secondary (unwanted) reactions. Thermal runaway starts slowly and then accelerates until finally it may lead to an explosion. 

Since the power 7 is easier to detect in two than in three dimensions, the first MC study [62] sampled a two-dimensional MWD in a range of temperatures (that is, of (L)), so that a change in the degree of interpenetration should trigger a crossover from dilute to semi-dilute regime at some density 0. Evidently, indeed, from Fig. 4, the MWD follows the form of Eq. (16). At 0 one observes a power 7eff 1.300 0.005 which comes closely to the expected one. Above 0 one finds 7eff —> 1, and the distribution (11) becomes relevant. 

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