DTA and DSC Curves

DTA and DSC curves are commonly recorded by plotting the temperature or enthalpy differences, respectively, against temperature (T) or time (t). 

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Figure 10.9 Examples of sample baseline interpolation for different types of differential thermal analysis (DTA) and DSC curves. (Reproduced with kind permission of Springer Science and Business Media from M. Brown, Introduction to Thermal Analysis, Kluwer Academic Publishers, Dordrecht. 2001 Springer Science.)...

Clearly, it would be desirable if the area under the peak was a measure of the enthalpy associated with the transition. However, in the case of DTA, the heat path to the sample thermocouple includes the sample itself. The thermal properties of each sample will be different and uncontrolled. In order for the DTA signal to be a measure of heat flow, the thermal resistances between the furnace and both thermocouples must be carefully controlled and predictable so that it can be calibrated and then can remain the same in subsequent experiments. This is impossible in the case of DTA, so it cannot be a quantitative calorimetric technique. Note that the return to baseline of the peak takes a certain amount of time, and during this time the temperature increases thus the peak appears to have a certain width. In reality this width is a function of the calorimeter and not of the sample (the melting of a pure material occurs at a single temperature, not over a temperature interval). This distortion of peak shape is usually not a problem when interpreting DTA and DSC curves but should be borne in mind when studying sharp transitions. 

TG, DTA and DSC curves of the cadmium peroxotitanate complex Cd2[Ti2(02)20(0H)6] H20 were recorded and used to determine the isothermal conditions suitable for obtaining the intermediate samples corresponding to the phases observed during the thermal decomposition. The experimental results were used to propose a mechanism of thermal decomposition of the investigated compound to CdTiOa. The aim of this study has been to determine the optimum conditions for obtaining CdTiOa with well-defined crystallinity [133],... 

The treatments at 300 ev, low and nediun doses, are also in the anomalous transport regime, whereas that at high dose is in the steady state regime. In addition, at this high dose, a thermal spike mechanism proposed for low ion energies, could induce a morphological modification, as shown in the IR spectra by diminished cis bands and increase in aromatic carbon (Fig. 7b) and also sustained by DTA and DSC curves (Figures 4 and 5. ... 

Table I shows the Tg values obtained in this work on standard polystyrene samples in most cases, the values obtained from DTA data are slightly higher than those obtained by DSC. Since these data were to be used for detailed comparisons with data obtained by others as well as by ourselves on styrene block copolymers, we first compared our data on anionically polymerized standard polystyrenes with those obtained on other anionically polymerized polystyrenes by other workers also using DTA and DSC techniques. This comparison is shown graphically in Figure 1. In Figure 1, the solid curve is drawn through a combination of our data with that obtained by Wall et al. (25) using a Dupont 900...

As an amorphous polymer, lignin undergoes chain segment motion upon heating. This motion, a glass transition, is characteristic of all amorphous polymers, and is indicated by an endothermic shift in the DTA or DSC curves. This glass transition is accompanied by abrupt changes in free volume, heat capacity, and thermal expansion coefficient. 

DTA and DSC are so similar in their working principles and in characteristics of their curves that we often do not distinguish between these two techniques in their applications for materials characterization. These two techniques are especially useful in characterization of polymeric materials, as well as in characterization of inorganic materials. Some typical applications of DTA and DSC are introduced in this section. 

The determination of the heat of transition (or reaction) or the mass of the reactive sample from the area of the curve peak is a widely used procedure in DTA and DSC. Expressed very simply,... 

The expression used for K depends on the type of instrumeni employed and the method of recording the DTA or DSC curves. If the variables of AT sensitivity and recorder chart speed are included, the expression for K can be written as (103)... 

As previously discussed in Chapter 5, the DTA or DSC curve consists of a series of peaks in an upward or downward direction on the AT or heat-flow axis. The positions (on the temperature or X axis), shape, and number of peaks are used for purposes of qualitative identification of a substance, while the areas of the peaks, since they are related to the enthalpy of the reaction, are used for quantitative estimation of the reactive substance present or for thermochemica determinations. Because of the various factors which aflect the DTA or DSC curve of a sample, the peak temperatures and the shape of the peak are rather empirical. Generally, however, the curves are reproducible for any given instrument, so that they can be useful in the laboratory. By use of various calibration substances, the areas enclosed by the curve peaks can be related to heats of reaction, transition, polymerization, fusion, and so on. Or, if the heat of the reaction is known, the amount of reacting substance can be determined. 

The specific heat of a substance can be determined conveniently and rapidly using the techniques of DTA and DSC (173, 88). The method (173) is illustrated by the DuPont DSC curves for a-alumina, as given in Figure 7.62. A curve for the empty sample container is first run, as indicated by the upper curve. The sample is then placed in the sample container and its curve recorded, using the same instrument adjustments. The relationship between the blank (empty container) and the sample (empty container plus sample) then is... 

The thermal properties of explosives and propellanl compositions are widely studied by DTA and DSC. Fauth (47) recorded the DTA curves of some hydrazine, guanidine and guanidinium picrates, slyphnates, and sulfates. The decomposition temperatures found were generally considerably lower than those reported in the literature. Other picrates, those with thallium, ammonium, tetramethylammonium, and letraethylammonium, were studied by Stammler (27). David (28) and Bohon (29) examined the thermal behavior of explosives and propellants under various external pressures up... 

The results from DTA and DSC experiments are displayed as a thermal analysis curve in which the instrument signal is plotted against temperature - usually the sample temperature - or time. Figure 3 shows some of the terminology relating to the results from DSC experiments. The description heat flow is frequently used for the instrument signal. Analysis of the thermal analysis curve is carried out using the instrument software. Of particular importance is the extrapolated onset temperature which is... 

Wendlandt identified some 16 variables which influence the results from DTA and DSC experiments. Whilst many are attributable to the design of the equipment or to the inherent properties of the sample there remains a core of variables where the practitioner is able to exert some control. Sample preparation and containment, heating rate and atmosphere all come within this core and even small refinements in technique can often enhance the quality of the results. Two somewhat extreme examples illustrate the need for careful control of experimental technique. The first is the well-known example of the decomposition of an oxalate to carbonate and CO. This is an endothermic process but the thermal analysis curve will show an exotherm due to combustion of CO if there is a trace of air remaining in the apparatus. The second example illustrates how the use of a crucible with an ill-fitting lid may give rise to ambiguous results. Thus, in the study of emulsion explosives the vaporisation of the sample... 

Once the main feutures of the DTA or DSC curve have been established, and baseline discontinuiUes have been examined, attention con be directed at the correlation of the endothermic or exothermic peaks with thcnnol events in the sample. 

C) Flow rate. The flow rate of the atmosphere affects the decomposition temperature of the sample, precision of determination of temperature, flatness of the baseline and peak areas of DTA or DSC curves. 

From the melting endothermic peak and the crystallization exothermic peak of DTA or DSC curves, transition temperatures can be determined. In order to erase the effect of the previous thermal history, such as superheating, super-... 

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