Power-compensating DSC

The power compensating DSC (Figure 156) uses a sample and a reference. During a controlled temperature program the sample and the reference are held at equal temperatures by adjusting the heat supplied to them by separate... 

Figure 156. Power compensating DSC (picture Universitat de Lleida)...

Two types of DSC measurement are possible, which are usually identified as power-compensation DSC and heat-flux DSC, and the details of each configuration have been fully described [1,14]. In power-compensated DSC, the sample and reference materials are kept at the same temperature by the use of individualized heating elements, and the observable parameter recorded is the difference in power inputs to the two heaters. In heat-flux DSC, one simply monitors the heat differential between the sample and reference materials, with the methodology not being terribly different from that used for DTA. Schematic diagrams of the two modes of DSC measurement are illustrated in Fig. 9. 

The equipment for power-compensated DSC involves two parallel temperature measurement systems. Sample (ca. 50 mg) and reference in... 

Whereas the heat flux DSC measures the temperature difference between the sample and the reference sample, power-compensated DSCs are based on compensation of the heat to be measured by electrical energy. Here the sample and the reference are contained in separate micro-furnaces, as shown in Figure 10.6(b). The time integral over the compensating heating power is proportional to the enthalpy absorbed by or released from the sample. 

Figure 10.6 Schematic representation of (a) heat flux DSC and (b) power-compensated DSC.

FIGURE 2.12. Schematic Representation of Heat-flux DTA and Power Compensation DSC... 

In the DTA measurement, an exothermic reaction is plotted as a positive thermal event, while an endothermic reaction is usually displayed as a negative event. Unfortunately, the use of power-compensation DSC results in endothermic reactions being displayed as positive events, a situation which is counter to IUPAC recommendations [38]. When the heat-flux method is used to detect the thermal phenomena, the signs of the DSC events concur with those obtained using DTA, and also agree with the IUPAC recommendations. 

In a power compensation DSC (figure 12.3), the sample and the reference crucible holders consist of two small furnaces, As and Ar, each one equipped with a temperature sensor, Bs or Br, and a heat source, Cs or Cr. The furnaces... 

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). 

The power compensation DSC instrument was first described by Watson et al.3) and by O Neill4) and it was developed into a commercial instrument by the Perkin-Elmer Corporation. It utilises separate sample and reference holders of low thermal mass, with individual heaters and platinum thermometers, as shown schematically in Fig. 1. In addidion to controlling the average temperature the instrument employs a... 

Fig. 1. Power Compensation DSC. Schematic Cross-Section of Perkin-Elmer DSC Cell. (Reproduces from Thermal Analysis Newsletter, Perkin-Elmer Corp., No. 9 (1970))...

Figure 3.4 Typical power-compensated DSC trace glass transformation and devitrification of amorphous CdGeAsj. 6.86 mg of sample was heated at 20°C/min. Indicated exothermic and endothermic directions are those used in power-compensated DSC, but are reversed as compared to the convention used in this book.

For reversible transformations such as melting/solidification or the Q to (3 quartz inversion in silica, heat flux DSC and power compensated DSC can each be equivalently precise in determining the latent heat of transformation. Transformations of... 

The linear drop and exponential recovery shape of these transformations also appear in power-compensated DSC traces, but for different reasons. The temperature measuring device (RTD) measures its own temperature, which is influenced by all substances in the chamber, the housing, the sample crucible, as well as the melting sample. The device adds power to the sample side as needed to compensate for the cooling effect on the chamber due to sample melting. This energy requirement increases lineaxly since the setpoint sample temperature increases linearly. When melting is over, the need for extra heat flow to the sample chamber side drops exponentially as the chamber temperature quickly catches up to the setpoint. 

Irreversible transformations are those in which reactants do not reform from products upon cooling. Generally one of the reactants is in a metastable state, and only requires thermal agitation or the presence of a catalyst to initiate the transformation. Examples would be combustion of a fossil fuel or glass devitrification. Power-compensated DSC has a distinct advantage over heat-flux DSC in determining the kinetics of transformation from metastable phases. In these type of reactions,... 

To maintain the sample at the setpoint temperature during a self-feeding reaction in a power-compensated DSC, small sample mass (e.g. <10 mg) and excellent thermal contact between the sample and its container, as well as the container and the chamber, are required. Figure 3.15 shows the rather unusual effects of using excessive sample masses of glass in... 

Figure 3.15 Devitrification of amorphous CdGeAs2 in a heat flux and power-compensated DSC, heated at the same rate, as a function of sample temperature [6]. 61.4 mg and 41.4 mgs of glass were used in the heat-flux and power-compensated DSC s, respectively. Note that exothermic and endothermic directions are consistent with those used in power-compensated DSC, but reversed compared to the usual convention in this book.

For irreversible transformations (glass crystallization as example), the particle size in the sample container will effect the rate of reaction, more so in DTA than in power-compensated DSC (Figure 3.29). For spherical particles, the surface to volume ratio decreases by 3(r (where r is particle diameter) with... 

The intensity at peak maximum for the faster heating rates is greater than that for the slower heating rates, since for DTA the reference temperature increase is more rapid, while at the same time the sample strives to remain at the melting temperature. For faster heating rates in power-compensated DSC, the sample chamber temperature deviates more quickly from the rising setpoint, so the device compensates with more heat dissipation per unit time to the sample side. 

The superposition principle for heat flow as measured by power-compensated DSC should apply—just as it would be expected that the water flow into one tank from two pipes would be additive. Assuming Fourier s law holds (steady state heat flow proportional to temperature gradient), the temperature differences measured in DTA (and heat-flux DSC) are additive via contributions from multiple transformation sources within the sample material. 

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