Differential scanning calorimeters compensation DSCs

DTA as well as power compensation DSC instruments, and is called temperature modulated DSC, or TMDSC. The following trade marks are used by different TA instrument manufacturers for their temperature modulated differential scanning calorimeters Modulated DSC (MDSC ) of TA Instruments Inc., Oscillating DSC (ODSC ) of Seiko Instruments Inc., Alternating DSC (ADSC ) of Mettler-Toledo Inc. and Dynamic DSC (DDSC ) of Perkin-Elmer Corp. 

Adiabatic calorimeters are complex home-made instruments, and the measurements are time-consuming. Less accurate but easy to use commercial differential scanning calorimeters (DSCs) [18, 19] are a frequently used alternative. The method involves measurement of the temperature of both a sample and a reference sample and the differential emphasizes the difference between the sample and the reference. The two main types of DSC are heat flux and power-compensated instruments. In a heat flux DSC, as in the older differential thermal analyzers (DTA), the... 

In a power compensation differential scanning calorimeter (DSC), the base of the sample holder unit is in direct contact with a reservoir of coolant (Figure 2.12). The sample and reference holders are indiv idually equipped with a resistance sensor, which measures the temperature of the base of the holder, and a resistance heater. Upon detection of a temperature difference between the sample and reference, due to a phase change in the sample, electrical power is supplied so that the temperature difference falls below a threshold value, typically <0.01 K. 

As noted in Section 4.6, one of the measuring systems, the sample side, is filled with the substance to be tested, but the other measuring system, the reference side, remains empty or contains a reference material. The thermophysical properties of the reference material must be known. To fulfill the symmetry condition, its heat capacity should at least be able to compensate that of the sample in the vicinity of the transition temperature (reaction temperature) of the sample. In the temperature range of the transition (reaction), the reference material should, of course, show no transition whatsoever - that is, it must be inert (inactive). With such conditions, the measured functions of a differential scanning calorimeter (DSC) transform from those of Figure 6.17 to those plotted in Figure 6.18. Here it should be mentioned that the heat flow rate into a system on an endothermic event is defined as positive in thermodynamics, but the temperature difference is negative in such a case. 

Another type of DSC, the power-compensated differential scanning calorimeters (see Section 7.9.4.2), follows in principle the model shown in Figure 6.15, but instead of one, there are two furnaces for the sample and the reference system, respectively. The current in the heaters (furnaces) (T) is so controlled that the average temperature of the two systems always matches the preset temperature. 

Figure 4 is a schematic representation of a Perkin-Elmer DSC 2 differential scanning calorimeter, whose principles are given by Gray [17]. The calorimeter head is constituted of two identical plates in continuous contact with a cold source and heaters that supply the required energy to impose the programmed temperature Tp to the plates. The heaters run independently, and an electronic system compensates for temperatures between the plates. 

Figure 1 Different types of differential scanning calorimeters, (a) Three-dimensional cylindrical calorimeter (Tian-Calvet). (b) Three-dimensional calorimeter with power compensation, (c) Two-dimensional plate-like calorimeter, (d) Scheme of a twin-chip sensor (Mettler Toledo Flash 1 DSC ) for fast scanning calorimetry.

Differential scanning calorimetry (DSC) can be performed in heat compensating calorimeters (as the adiabatic calorimetry), and heat-exchanging calorimeters (Hemminger, 1989 Speyer, 1994 Brown, 1998). 

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. 

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