Between DTA and DSC

Watson et al (128) apparently first used the term differential scanning calorimetry (DSC) to describe the instrumental technique developed (1963) by the Perkin-Elmer Corporation. This technique maintained the sample and reference materials isothermal to each other by proper application of 

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The practical distinction between DTA and DSC is in the nature of the signal obtained from the equipment. In the case of a differential thermal analyser it is proportional to the temperature difference,... 

In contrast, one finds many DSCs which are used only for qualitative DTA work on transition temperatures. The often-posed question of the difference between DTA and DSC is therefore easily answered DTA is the general term covering all differential thermal analysis techniques, while DSC must be reserved for scanning experiments that yield calorimetric information. 

For this summary, forms of thermal analyses under extreme conditions are described for the measurement of heat and temperature, as dealt within Sects. 4.1-4. The distinction between DTA and DSC seen in these methods is described in Appendix 9. In Appendix 10, DTA or DSC at very low and high temperatures and DTA at very high pressures are mentioned. This is followed by a discussion of high-speed thermal analysis which, in some cases, may simply be thermometry. Finally, micro-calorimetry is treated. One might expect that these techniques will develop in this century [1]. The numbers in brackets link to references at the end of this appendix. 

Boersma s DTA configuration, Fig. lb, can be considered as the missing link between differential thermal analysis and differential scanning calorimetry. Some even feel that this configuration is, in fact, a DSC instrument. This is the major reason behind the confusion as to the differences between DTA and DSC. 

DSC (Differential Scanning Calorimetry) has also been used in cement science investigations to some extent. It is based on a power compensated system. In this technique the reference and the sample imder investigation are maintained at a constant temperature throughout the heating schedule. The heat energy required to maintain the isothermal condition is recorded as a function of time or temperature. There are some similarities between DTA and DSC ineluding the appearance of thermal curves. DSC can be used to measure the heat capacities of materials. DSC measures directly the heat effects involved in a reaction. 

As shown in Figs. 4.8.4 and 4.8.5, typical DTA and DSC apparatus consist of the following basic components (1) furnace or heating device, (2) differential temperature detector, (3) amplifier, and (4) recorder. There are many modifications of this basic design. However, all instruments measure the difference in temperature (DTA) or enthalpy (DSC) between a sample and an inert reference material. 

Three kinds of sample holders are available for DTA and DSC (Fig. 4.8.6). Type 1 holders are commonly used for a DTA apparatus. In this type, the sample and reference holders are placed on the same metal block and heated by the same heater the temperature difference between the two holders is indicated by a thermocouple. Type 2 holders are generally used in a quantitative DTA (heat-flux DSC) apparatus. Both sample and reference holders are maintained at the same temperature by two individual internal heaters, which, in turn, are heated by the same main heater. The temperature difference between the sample and the reference material is measured by a thermocouple placed outside of the holders. Type 3 holders are customarily used for a power-compensated DSC apparatus. This apparatus has separate heaters for heating the sample and reference holders thus maintaining the sample and the reference... 

Different sample pans are used in DTA and DSC measurements for samples of various shapes and physical states. The sample pans are of two types open pans for solid samples and sealed pans for liquids or volatile samples. The materials used for making sample pans are mostly aluminum, silver, gold, stainless steel, and carbon. When aluminum and silver sample pans are used for samples containing water, the sample pans should be pretreated with water in a small pressure vessel to avoid reactions between the pans and water during subsequent heating. 

Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are the most widely used thermal analysis techniques. Both techniques have the same objective to examine thermal events in a sample by heating or cooling without mass exchange with its surroundings. The thermal events examined by DTA and DSC include solid phase transformation, glass transition, crystallization and melting. Differential emphasizes that analysis is based on differences between sample material and a reference material in which the examined thermal events do not occur. 

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 basic components of a DTA apparatus are a temperature-controlled furnace containing sample and reference cells and a pair of matched temperature sensors connected to recording apparatus, as indicated in Figure 1.1. The temperature sensors (usually thermocouples) are in contact with the sample and reference or their containers, and the output is amplified and recorded. DTA data may be plotted as a function of sample temperature, reference temperature (as is usually the case), or time. In both DTA and DSC, the measurement relies on the occurrence of a temperature difference between a sample and reference (AT) as a result of the thermal event in question. 

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. 

Quantitative correlations between kinetic parameters and sample mass and heating rate as well as dependent variables were derived for DTA and DSC by Krishnan et al. (179). 

Because of the large number of applications of the techniques of DTA and DSC, the applications described here will be concerned mainly with analytical chemistry problems. In this area, DTA and DSC can be used as a control or a routine tool for comparing similar but not identical materials. As a control technique, it may be used to distinguish between raw materials quickly and easily in those cases in which the treatment of the material must be modified if slight changes in the material are encountered. As a comparison technique, DTA and DSC may be used in some cases to detect materials that yield anomalous results by other tests. Lastly, by suitable calibration of the instruments, these techniques may be used for the quantitative estimation of a substance or mixture of substances, or for purity determinations (see... 

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 aim here is simply to present an overview of the various features on offer. The range of instruments extends from differential scanning calorimeters in a suitcase for on-site use to spatially resolved micro-thermal analysis equipment for samples as minute as 2 x 2 fim. Between these rather extreme examples there is a wide choice of commercial DTA and DSC equipment which allows samples to be studied at temperatures ranging from — 150°C to about 1600°C. For higher temperature measurements (above 1600°C) the equipment becomes increasingly more specialised. The detailed specification of equipment is often difficult (sometimes impossible ) to decipher - there appears to be no common practice between manufacturers. Information can best be obtained by raising questions directly with the manufacturers. Even so, hands-on experience is to be recommended when choosing equipment. 

Very little further progress in instrumentation was made until the 1950s. A brief summary of the classical DTA and DSC operating system, instrumentation, rules of data treatment, and the differences between DSC and DTA are given in Appendix 9. The first milestone was reached by 1952 when about 1,000 research reports on DTA had been published. At that time, DT A was mainly used to determine phase diagrams, transition temperatures, and chemical reactions. Qualitative analysis (fingerprinting) was developed for metals, oxides, salts, ceramics, glasses, minerals, soils, and foods. 

Applications of DTA for Polymers. Table 2 (Ref 5, Chapt. l) describes some of the many applications of DTA and DSC. Both DTA and DSC can be used to determine the temperature of the transitions, cited in Table 2. However, the DSC peak area, in addition, gives quantitative calorimetric information (heat of reaction, transition, or heat capacity). DTA can only do so when calibration with a standard material allows the quantitative conversion of AT to heat flow and, ultimately, heat of transition (AH) or heat capacity (Cp). Also, the response of DTA with increasing temperature may be affected by poor heat transfer in the system, detector sensitivity, etc (4). For these reasons, when there is a choice between DSC and DTA, DSC is the preferred method. The illustrations shown below for applications of DSC in characterization of polymers also generally apply for DTA, with the limitations mentioned above. Therefore, DTA applications will not be considered here. Illustrations of polymer applications for DTA can be found in the Thermal Analysis section by Bacon Ke (6) in the previous edition of this encyclopedia. 

The important difference between the DTA and DSC systems is that in the latter the sample and reference are each provided with individual heaters. This makes it possible to use a null-balance principle. It is convenient to think of the system as divided into two control loops. One is for average temperature control, so that the temperature, Tq, of the sample and reference may be increased at a predetermined rate, which is recorded. The second loop ensures that if a temperature difference develops between the sample and reference (because of exothermic or endothermic reaction in the sample), the power input is adjusted to remove this difference. This is the null-balance principle. Thus, the temperature of the sample holder is always kept the same as that of the reference holder by continuous and automatic adjustment of the heater power. A signal, proportional to the difference between the heat input to the sample and that to the reference, dH/ d, is fed into a recorder. In practice this recorder is also used to register the average temperature of the sample and reference. 

The techniques of DTA and DSC are not identical. Let us consider the essential difference between them. In DTA, the heat changes within the material are monitored by measuring the difference in temperature (AT) between a sample and an inert reference. In a classical DTA equipment both the sample (S) and the reference (R) are heated by the same furnace (Fig. la). The temperature sensors are inserted directly into the sample and reference, while in a modification of classical DTA, called Boersma DTA (Fig. lb) they are in contact with the container but not with the materials under test. The temperature difference between the sample and the inert reference is recorded as a function of temperature (T) or time (t). In DSC, the sample and the reference materials are provided with their own separate furnaces as well as with their own separate temperature (T) sensors. In DSC, the sample and reference are maintained at identical temperatures by controlling the rate at which heat is transferred to them (Fig. Ic). 

DTA and DSC are related techniques that measure the same thermal events with different methods. Whereas DTA in the traditional use of the technique measures a difference in temperature, DSC monitors the difference in heat flow between a sample and a reference material as the material is heated or cooled (cfr. Chp. 2.1.1). Degradation processes may occur in a polymer which are not associated with the loss of volatiles. It is here that both DTA and DSC techniques are useful as they show whether any reactions are occurring which involve either heat evolution or absorption. 

For OIT determinations by DTA and DSC analyses pure oxygen at a flow rate 50 5 mL/min isothermal measurements between 150-230°C, aluminum pans, 2-8 mg samples as disk, onset temperature 60°C. 

The similarity between the DTA and DSC analyses lies in the fact that both evaluate temperature constantly, and the differences between them are that DTA performs a qualitative analysis of thermal events detected in the sample and DSC is able to quantify these events. However, DSC and DTA do not give absolute results because they measure heat flow dynamically, which hinder obtaining more accurate results since the analyses are not performed imder thermal equilibrium [21]. 

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