DSC experiments

Methods and means cannot be separated from the ultimate aim (Emma Goldman) 

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To confirm that the matrix is amorphous following primary solidification, isothermal dsc experiments can be performed. The character of the isothermal transformation kinetics makes it possible to distinguish a microcrystalline stmcture from an amorphous stmcture assuming that the rate of heat released, dH/dt in an exothermic transformation is proportional to the transformation rate, dxjdt where H is the enthalpy and x(t) is the transformed volume fraction at time t. If microcrystals do exist in a grain growth process, the isothermal calorimetric signal dUldt s proportional to, where ris... 

Fig. 23. Evolution of the glass transition temperature of polychloroprene-aromatic hydrocarbon resin blends as a function of the resin content. values were obtained from DSC experiments.

Differential scanning calorimetry (DSC) experiments on the various dimeric carbocycles indicated that, depending on the length of the alkyl groups, thermal polymerization had occurred between 100 and 125°C as an abrupt, exothermic process. The narrow temperature range for each exotherm was suggestive of a chain reaction however, IR spectroscopy revealed the absence of acetylene functionalities in the polymerized material. Consequently, none of the substi-... 

Analysis of DSC experiments on various alkyl-substituted trimers gave even more disappointing results. Although more thermally resilient, these macrocycles polymerized with very broad exotherms. For the hexyl-substituted trimer, melting occurred around 150 °C, while polymerization extended from ca. 170 to 230°C. This pattern was thought to be indicative of a random polymerization process. Overall, polymerization of trimeric macrocycles occurred at sufficiently high temperatures that the resultant materials were intractable brown tars. 

Differential Scanning Calorimetry. A sample and an inert reference sample are heated separately so that they are thermally balanced, and the difference in energy input to the samples to keep them at the same temperature is recorded. Similarly to DTA analysis, DSC experiments can also be carried out isothermally. Data on heat generation rates within a short period of time are obtained. Experimental curves from DSC runs are similar in shape to DTA curves. The results are more accurate than those from DTA as far as the TMRbaiherm is concerned. 

The melting transition of ultra-pure metals is usually used for calibration of DSC instruments. Metals such as indium, lead, and zinc are useful and cover the usual temperature range of interest. Calibration of DSC instruments can be extended to temperatures other than the melting points of the standard materials applied through the recording of specific heat capacity of a standard material (e.g., sapphire) over the temperature range of interest. Several procedures for the performance of a DSC experiment and the calibration of the equipment are available [84-86]. A typical sensitivity of DSC apparatus is approximately 1 to 20 W/kg [15, 87]. 

As outlined in Section 2.2.4.1, impurities and catalysts may decrease To significantly a decrease of 100°C is not unusual. The material of construction of the sample cup may act as a catalyst, resulting in surface-induced decomposition which may even be promoted by the sample/surface area ratio in the DSC cup. Therefore, it is important to check if the substance is catalyzed during the DSC experiment and if such catalysis is representative of process conditions. Frequently, substances that are sensitive to catalysis are handled in passivated glass-lined reactors, receptacles, or containers. Another phenomenon to recognize is autocatalytic decomposition. Substances that are susceptible to autocatalytic decomposition have an induction period prior to initiation of rapid decomposition. The same holds for substances that contain inhibitors, which can be depleted. 

Data on the enthalpy change, AHd or AHr, established in testing homogeneous samples can be used in the extrapolation to large scale. In the DSC experiments, no pressure data are obtained. 

DSC experiments performed in closed vessels, with a heating rate of 5°C/min on samples of 10 to 20 mg, showed that the enthalpy of reaction was from 410 to 1175 kj/kg, a significant variation range. The corresponding adiabatic temperature rise related to the enthalpy of reaction results was about 200 to 580°C. This temperature range yields a pressure rise that would cer-... 

Keller, A., Stark, D., Fierz, H., Heinzle, E. and Hungerbiihler, K. (1997) Estimation of the Time to Maximum Rate Using Dynamic DSC Experiments. J. Loss Prev. Process Ind. 10, 31-41. 

In many respects, the practice of DSC is similar to the practice of DTA, and analogous information about the same types of thermally induced reactions can be obtained. However, the nature of the DSC experiment makes it considerably easier to conduct quantitative analyses, and this... 

Much more information can be obtained from the DSC experiment than simply an observation of the transition from a solid to a liquid phase. A plot of heat flow against temperature is a true depiction of the continuity of the heat capacity at constant pressure (Cp). If the entire temperature range of a given process is known, the physical state of a material will reflect the usefulness of that material at any temperature point on the plot. For polyethylene terephthalate (see Fig. 4.9), a stepshaped transition is interpreted as a change in Cp resulting from a... 

The versatility of the DSC method and the high speed of the experiments have costs in terms of accuracy. For example, the best accuracy in the determination of heat capacities of solids by DSC is typically 1% [3,248-250], at least one order of magnitude worse than the accuracy of the corresponding measurements by adiabatic calorimetry [251]. This accuracy loss may, however, be acceptable for many purposes, because DSC experiments are much faster and require much smaller samples than adiabatic calorimetry experiments. In addition, they can be performed at temperatures significantly above ambient, which are outside the normal operating range of most adiabatic calorimeters. 

Let us first address the question of the accurate measurement of the temperature of the sample in DSC experiments [252-255], As illustrated in figure 12.4, the programmed temperature, Tp, usually varies linearly with the time t, and can... 

To our knowledge, the question of the standard state corrections in DSC experiments has never been addressed. These corrections may in general be negligible, because most studies only involve condensed phases and are performed at pressures not too far from atmospheric. This may not be the case if, for example, a decomposition reaction of a solid compound that generates a gas is studied in a hermetically closed crucible, or high pressures are applied to the sample and reference cells. The strategies for the calculation of standard state corrections in calorimetric experiments have been illustrated in chapter 7 for combustion calorimetry. 

Another factor which suppresses melting of LiAlH is catalysts. Andreasen [67] used 1 min ball milling to disperse 2 mol%TiCl3 l/3AlCl3 in LiAlH. DSC experiments with the heating rates 3-5°C showed only two endothermic reactions, the... 

Figure 3.32 shows XRD patterns of (MgH -i-LiAlH ) composites after DSC testing up to 500°C. The primary phases present are Mg and Al. Peaks of MgO and (LiOH) HjO arise from the exposure of Mg and Li (or possibly even some retained LiH) to the environment during XRD tests. Apparently, XRD phase analysis indicates that a nearly full decomposition of original MgH and LiAlH hydride phases has occurred to the elements during a DSC experiment. In addition, no diffraction peaks of any intermetallic compound are observed in those XRD patterns. That means that no intermetallic compound was formed upon thermal decomposition of composites in DSC. Therefore, the mechanism of destabilization through the formation of an intermediate intermetallic phases proposed by Vajo et al. [196-198] and discussed in the beginning of this section seems to be ruled out of hand. 

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