Quenching rate temperature dependence

Any refractory material that does not decompose or vaporize can be used for melt spraying. Particles do not coalesce within the spray. The temperature of the particles and the extent to which they melt depend on the flame temperature, which can be controlled by the fueLoxidizer ratio or electrical input, gas flow rate, residence time of the particle in the heat zone, the particle-size distribution of the powders, and the melting point and thermal conductivity of the particle. Quenching rates are very high, and the time required for the molten particle to soHdify after impingement is typically to... 

Hamiltonian does not give rise to any crystalline order in the system. By employing models hke this, the quench-rate and chain-length dependence of the glass transition temperature, as well as time-temperature superposition, similar to experiments [23], were investigated in detail. 

The advantage of the simulations compared to the experiments is that the correspondence between the tracer diffusion coefficient and the internal states of the chains can be investigated without additional assumptions. In order to perform a more complete analysis of the data one has to look at the quench-rate and chain-length dependence of the glass transition temperature for a given density [43]. A detailed discussion of these effects is far beyond the scope of this review. Here we just want to discuss a characteristic quantity which one can analyze in this context. 

The quantum yield for the formation of the cycloaddition product has been found to be temperature dependent, increasing by a factor of approximately three as the temperature is lowered from 65 ( = 0.24) to 5°C ( = 0.69). Photolysis of mixtures of the olefin and f/my-stilbene in the presence of sensitizers yielded no cycloaddition product (42) but rather only m-stilbene. This suggests that the cycloadduct is produced via a singlet reaction. This conclusion is supported by the fact that tetramethylethylene quenches fluorescence from the /rans-stilbene singlet. A plot of l/ (42) vs. 1/[TME] (TME = tetramethylethylene) is linear. The slope of this plot yields rate constants for cycloadduct formation which show a negative temperature dependence. To account for this fact, a reversibly formed exciplex leading to (42) was proposed in the following mechanism<82) ... 

The so-called glass transition temperature, Tg, must be considered below this temperature the liquid configuration is frozen in a structure corresponding to equilibrium at Tg. Around Tg a rather abrupt change is observed of several properties as a function of temperature (viscosity, diffusion, molar volume). Above 7 , for instance, viscosity shows a strong temperature dependence below Tg only a rather weak temperature dependence is observed, approximately similar to that of crystal. Notice that 7 is not a thermodynamically defined temperature its value is determined by kinetic considerations it also depends on the quenching rate. 

The structural stability of mixed-metal hemoglobin hybrids also has allowed us to study low-temperature electron transfer in this system. We first reported the temperature dependence of triplet-state quenching within the [ (ZnP), Fe (H20)P] hybrids, which we attributed to the ZnP Fe P ET reaction [7d]. The rate constant dropped smoothly as the temperature was lowered from room temperature to 200 K. Below this temperature the rate constant remained roughly constant with a tunnelling rate constant of kt 9 s (Fig. 7). 

Fig. 7. Temperature dependence of the triplet-state quenching rate constant (k,) for the [a(Zn), PiFe +HjO)] hybrid. Adapted from Ret [7d]...

Then, they depend also on the viscosity of the system. Specific diffusion control is characteristic of fast reactions like fluorescence quenching. In polymer formation, specific diffusion control is responsible for the acceleration of chain polymerization due to the retardation of the termination by recombination of two macroradicals (Trommsdorff effect). Step reactions are usually too slow to exhibit a dependence on translational diffusion also, the temperature dependence of their rate constants is of the Arrhenius type. 

The ratio of the intensities of the two bands in delayed emission spectrum should thus be independent of the efficiency of triplet formation and of all triplet quenching processes. It should also be independent of the intensity of absorption Ia. The rate of emission is then given as Aed[T1 where Aed is temperature dependent. 

The temperature dependence of the quenching rate constant k(T) as measured by Lindinger et al.141 can be obtained by a thermal average over the velocity dependence of the cross section. Good agreement is obtained for both He(2lS ) and He(23S) +Ar (see Fig. 48). ... 

Figure 48. Temperature dependence of total quenching rate constant for He + Ar. Solid curves represent measurements by Lin-dinger et al. MI dashed curves are calculated from optical potentials due to other authors.100 102...

Fig. 14 Temperature dependence of yield stress, cry, and plastic flow stress, crpf, for quenched and physically aged PMMA. Strain rate is 2 x 10-3 s-1 (From [32])...

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