Fast heating rates

Table I presents the results of "isothermal" simultaneous thermoanalytical (STA) runs, at 573 K and 773 K, for all three products. Similar data, at a fixed heating rate is shown in Table II. One of the crucial parameters is the temperature of maximum weight loss rate, corresponding to the time when dehydrochlorination of PVC starts becoming important. This temperature is close to 573 K in all cases. In fact, at a relatively fast heating rate, almost no decomposition occurs at temperatures under 563 K. If the materials are heated at 573 K for a prolonged period, complete dehydrochlorination takes place, but no further stages of PVC decomposition occur. None of the three materials investigated decomposes completely until a temperature of ca. 773 K is attained. Even then only a certain fraction of the entire mass of the samples is volatilised, due to the presence of inorganic fillers in their composition.

The rate at which the sample is heated up is very important, especially in the case of slow or complex reactions. Fast heating rates shift the reactions to higher temperatures and decrease the resolution when several reactions follow each other closely. 

The West rod atomizers were marketed for a while, but their production has also ceased. These atomizers had the advantages of simplicity, low power requirements (less than half that required by a furnace) and fast heating rate (2000 K S l)- They were, however, considered to be extremely prone to interferences. This was attributed to the rapid cooling of the atoms once they had left the filament. This was partially overcome by setting the light beam so that it grazed the surface of the rod. 

O. Senneca, P. Salatino and R. Chirone, A fast heating-rate thermogravimetric study of the pyrolysis of scrap tyres. Fuel, 78, 1575-1581 (1999). 

As noted in Section I, the CS process is characterized by high temperatures, fast heating rates, and short reaction times. These features make CS an attractive method for commercial synthesis, with the potential for new materials and lower costs, compared to conventional methods of furnace synthesis. 

The thermite process may be the original inspiration of combustion synthesis (CS), a relatively new technique for synthesizing advanced materials fl-om powder into shaped products of ceramics, metallics, and composites. Professor Varma and his associates at Notre Dame contributed the article Combustion Synthesis of Advanced Materials Principles and Applications, which features this process that is characterized by high temperature, fast heating rates, and short reaction times. 

It is interesting to compare the softening of the material near Tg measured by H, as a function of temperature, with the corresponding DSC traces for amorphous PET and PMMA (Fig. 3.3). These experiments demonstrate that both methods, H and DSC, yield a similar measure of Tg. The apparent difference in the Tg values obtained ( 10°C) is a result of the fast heating rate used in the DSC determination in contrast to the quasi-static measurement in the case of H. 

Figure 4. When the densification mechanism has a higher activation energy than the coarsening mechanism, a fast heating rate to high firing temperatures (rapid sintering) can lead to the production of high densities.

Seven CH2N2O2 species have been formed at around 200 fs of simulation time. These results are similar to those identified in thermal decomposition experiments. [51,53] A further N-NO2 bond breaking then follows the decomposition (1) and (2) above. From (1), this leads to the formation of CH2N and NO2. These pathways are remarkably similar to those predicted previously by Melius from the decomposition of nitramines at fast heating rates. [70]... 

Turn the heater on and set it at about 30. This produces a fairly fast heating rate - 3 to 4 C/min. For the best... 

The first application of a quartz crystal for use as a nonisothermal thermobalance was reported by Henderson ei al. (66). This was accomplished through the use of a minicomputer to characterize the temperature-frequency relationship for the crystal and to correct numerically the frequency-temperature-mass relationship to obtain the TG curve of the sample. One advantage of such a system is that very fast heating rates may be employed due to small sample size (t-40 jug typical) and high sensitivity. Thin films of sample exhibit rapid gas diffusion and thus permit thermal equilibrium to be maintained at heating rates of l00°C/min or greater. Applications of this thermobalance include the evaluation of thin films and coatings as well as various polymer studies of pyrolysis, flammability, and so on. 

Tf and the various glass transition temperatures, such as T g(onset), T g(middle) etc. are also defined in Figure 9.08(A). The typical behaviour of both enthalpy and Cp are shown for a fast heating rate, where Qh Qc at the bottom of the figure. Humps in heat capacity plots of the type shown in Figure 9.08(A) (bottom) occur when the heating rates are very much... 

An explanation may be offered to take account of this behavior. The escape of gas by-products is hindered by the low xerogel permeability. The produced gas cannot escape easily. Its pressure increases, and thus the xerogel expands. A fast heating rate can lead to a cracked xerogel. To avoid this phenomenon, the thermal schedule must be optimized. However, more experiments are necessary to confirm this proposed explanation. 

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