Differential thermal analysis power compensation

The Nomenclature Committee of the International Confederation for Thermal Analysis (ICTA) has defined DSC as a technique in which the difference in energy inputs into a substance and a reference material is measured as a function of temperature whilst the substance and reference material are subjected to a controlled temperature program. Two modes, power compensation DSC and heat flux DSC, can be distinguished depending on the method of measurement used1 . The relationship of these techniques to classical differential thermal analysis (DTA) is discussed by MacKenzie2). 

Fig. 1 Schematic diagrams of the (A) differential thermal analysis (DTA) (B) power-compensated DSC and (C) heat-flux DSC cells. (From Ref adapted from DuPont Instruments Systems Brochure.)...

Historically, DSC is a development of differential thermal analysis (DTA) and both techniques have a common origin in the measurement of temperature. The fundamental concept of both techniques is sim-ple-to measure thermal changes in a sample relative to a thermally inert reference as both are subjected to a controlled temperature program. In classical DTA, the temperature difference between sample and reference is measured as a function of temperature in classical DSC, the energy difference between sample and reference is measured as a function of temperature. Hence, DSC is simply quantitative DTA , or more precisely, DSC is a combination of DTA and adiabatic calorimetry. DSC is the more recent technique and was developed for quantitative calorimetric measurements over a wide temperature range from subambient to 1500 C. DTA is not appropriate for such precision measurements and has been progressively replaced by DSC, even for high-temperature measurements, as the major thermal anal-ysis/calorimetric technique. DSC is a differential calorimeter that achieves a continuous power compensation between sample and reference. 

Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are similar techniques. They measure change in the heat capacity of a sample. These techniques can be used to determine various transition temperatures (T , Tg, T , Tp, etc.), specific heat, heat of fusion, percent crystallinity, onset of degradation temperature, induction time, reaction rate, crystallization rate, etc. A DSC instrument operates by compensating electrically for a change in sample heat. The power for heating is controlled in such a way that the temperature of the sample and the reference is the same. The vertical axis of a DSC temperature scan shows the heat flow in cal/s. 

One of these techniques that brought into science the name DSC, called today power compensation DSC, was created by Gray and O Neil at the Perkin-Elmer Corporation in 1963. The other technique grew out of differential thermal analysis (DTA), and is called heat flux DSC. Differential thermal analysis itself originates from the works of Le Chatelier (1887), Roberts-Austen (1899), and Kurnakov (1904) (see Wunderlich, 1990). It needs to be emphasized that both of these techniques give similar results, but of course, they both have their advantages and disadvantages. 

Figure 10.4 Differential scanning calorimetry (DSC) instrumentation design (a) heat flux DSC and (b) power compensation DSC. A, furnace B, separate heaters and C, sample and reference holders. (Reproduced with permission from E.L. Charsley and S.B. Warrington, Thermal Analysis Techniques and Applications, Royal Society of Chemistry, Cambridge, UK. 1992 Royal Society of Chemistry.)...

The term differential scanning calorimetry has become a source of confusion in thermal analysis. This confusion is understandable because at the present time there are several entirely different types of instruments that use the same name. These instruments are based on different designs, which are illustrated schematically in Figure 5.36 (157). In DTA. the temperature difference between the sample and reference materials is detected, Ts — Tx [a, 6, and c). In power-compensated DSC (/), the sample and reference materials are maintained isothermally by use of individual heaters. The parameter recorded is the difference in power inputs to the heaters, d /SQ /dt or dH/dt. If the sample is surrounded by a thermopile such as in the Tian-Calvet calorimeter, heat flux can be measured directly (e). The thermopiles surrounding the sample and reference material are connected in opposition (Calvet calorimeter). A simpler system, also the heat-flux type, is to measure the heat flux between the sample and reference materials (d). Hence, dqjdi is measured by having all the hot junctions in contact with the sample and all the cold junctions in contact with the reference material. Thus, there are at least three possible DSC systems, (d), (c), and (/), and three derived from DTA (a), [b), and (c), the last one also being found in DSC. Mackenzie (157) has stated that the Boersma system of DTA (c) should perhaps also be called a DSC system. 

Thermal analysis is not really one subject, because the information gained and the purposes for which it can be used are quite varied. The main truly thermal technique is differential scanning calorimetry (DSC). The heat input and temperature rise for the material under test are compared with those for a standard material, both subjected to a controlled temperature programme. In power compensation DSC the difference in heat input to maintain both test pieces at the same temperature is recorded. In heat flux DSC the difference in heat input is derived from the difference in temperature between the sample and the reference material. Heat losses to the surroundings are allowed but assumed to depend on temperature only. 

A Power Compensation Differential Scanning Calorimeter and Thermal Analysis Software, American Laboratory Magazine, January 1996. 

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