Compensation heat fluxes

LOW TEMPERATURE HIGH SENSITIVITY TEMPERATURE COMPENSATED HEAT FLUX SENSOR. 

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. 

Fig. 9 Schematic diagrams illustrating the sample cell configurations for (a) power-compensation and (b) heat-flux modes of DSC detection. Each cell system is contained in the furnace assembly, and the differential heat flow between sample and reference is monitored as the experimental observable and ultimately is plotted as a function of the system temperature.

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... 

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... 

Fig. 4.6. Schematic diagrams of the power-compensation and heat-flux modes of DSC measurement.

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. 

There are two types of differential scanning calorimeters (a) heat flux (AT) and (b) power compensation (AT). Subsequent sections of this experiment will not distinguish between the two types. In either type of calorimeter, the measurement is compared to that for a reference material having a known specific heat [16,17], As AT and AT have opposite signs there is some potential for confusion [3], e.g., at the melting point, Tm, Ts < Tr, and AT < 0, whereas Ts > Tr and AT > 0 because latent heat must be supplied (subscripts s and r refer to the sample and the reference material, respectively) [3]. 

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). 

Another measurement principle is the DSC, after Boersma [8]. In this case, no compensation heating is used and a temperature difference is allowed between sample crucible and reference crucible (Figure 4.5). This temperature difference is recorded and plotted as a function of time or temperature. The instrument must be calibrated in order to identify the relation between heat release rate and temperature difference. Usually this calibration is by using the melting enthalpy of reference substances. This allows both a temperature calibration and a calorimetric calibration. In fact, the DSC after Boersma works following the isoperibolic operating mode (see Section 4.2.2). Nevertheless, the sample size is so small (3 to 20 mg) that it is close to ideal flux. 

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... 

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,... 

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.

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. 

Most commonly used DSC instruments fall into two categories (Laye, 2002) power compensation (PC) instruments, for which the term DSC was coined when this type became available in 1963 (Wendlandt, 1974), and heat flux (HF) instruments (Figure 22.2). The latter are essentially quantitative DTA instruments classical DTA is a qualitative, or at best semi-quantita-tive, technique (Wright, 1984). 

Figure 22.2. Schematic diagrams of (A) a power compensation DSC and (B) a heat flux DSC. Heat sensitive plate heat flux plate. (Reproduced with permission from Hatakeyama and Quinn, 1994.)...

In contrast, DSC, designed in 1960 by Watson184 and O Neill,185 is a newer, more quantitative technique that does measure Ts and TR, but also measures very precisely the electrical energy used by separate heaters under either pan to make Ts = TR (this is power-compensated DSC, useable below 650° C). The power input into S minus the power input into R is plotted against Tr. High-temperature DSC (useful for TR > 1000°C) measures the heat fluxes by Tian-Calvet thermopiles rather than the electrical power, as a function of Tr. In a heat-flux DSC, both pans sit on a small slab of material with a calibrated heat resistance. The temperature of the calorimeter is raised linearly with time. A schematic DSC curve is shown in Fig. 11.80. 

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... 

Fig. 4.8.6. Arrangements of sample holder (5) and reference holder (R) in the heater of a DTA or DSC apparatus 1 DTA 2 heat-flux DSC 3 power-compensated DSC...

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