Heat flux DSCs

Figure 157. Disk-type heat flux 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 experimental set up for heat flux DSC is very similar to that for calorimetric or Boersma DTA. Thus heat flux DSC will have the same freedom from the thermal properties of the sample and slower response times associated with Boersma DTA. DSC will generally have better resolution, as illustrated in Figure 11.18. Finally, as has been discussed earlier, by measuring the power differential, DSC is making a direct... 

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.

Classical DTA has been developed into heat-flux DSC by the application of multiple sensors (e.g., a Calvet-type arrangement) or with a controlled heat... 

Danley, R. L. (2003). New heat flux DSC measurement technique. Thermochimica Acta. 395, 201-208. 

Dong, H. B. and Hunt, J. D. (2001). A numerical model for two-pan heat flux DSC. Journal of Thermal Analysis and Calorimetry. 64,167-176. 

OPEN SYSTEM HEAT CAPACITY INTERNAL PRESSURE Heat flux DSC,... 

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). 

Boersma5) showed that quantitative calorimetric data could be obtained from a modified DTA instrument in which the sample and reference are in separate containers connected by a controlled thermal resistance, and with external thermocouples. In such an instrument the sample-reference temperature difference can be related to the heat flow, and this is the basis of heat flux DSC. The DuPont 910 DSC is based on a further development of this principle, and it is illustrated by Fig. 2. 

Fig. 2. Heat Flux DSC. Schematic Cross-Section of DuPont 910 DSC cell. (Reproduced from Product Bulletin, 910 DSC, DuPont Instruments, DuPont Co.)...

The sample containers most commonly used are cylindrical pans pressed from pure aluminium foil. Alternative materials are used for very high temperatures or corrosive substances, and hermetically sealed pans to withstand several atmospheres pressure can be used for volatile materials. Some heat flux DSC instruments are available which are capable of operation at high pressures, by means of containment of the DSC cell within a pressure vessel. 

In the CSM laboratory, Rueff et al. (1988) used a Perkin-Elmer differential scanning calorimeter (DSC-2), with sample containers modified for high pressure, to obtain methane hydrate heat capacity (245-259 K) and heat of dissociation (285 K), which were accurate to within 20%. Rueff (1985) was able to analyze his data to account for the portion of the sample that was ice, in an extension of work done earlier (Rueff and Sloan, 1985) to measure the thermal properties of hydrates in sediments. At Rice University, Lievois (1987) developed a twin-cell heat flux calorimeter and made AH measurements at 278.15 and 283.15 K to within 2.6%. More recently, at CSM a method was developed using the Setaram high pressure (heat-flux) micro-DSC VII (Gupta, 2007) to determine the heat capacity and heats of dissociation of methane hydrate at 277-283 K and at pressures of 5-20 MPa to within 2%. See Section 6.3.2 for gas hydrate heat capacity and heats of dissociation data. Figure 6.6 shows a schematic of the heat flux DSC system. In heat flux DSC, the heat flow necessary to achieve a zero temperature difference between the reference and sample cells is measured through the thermocouples linked to each of the cells. For more details on the principles of calorimetry the reader is referred to Hohne et al. (2003) and Brown (1998). 

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... 

A heat-flux DSC trace of the boiling of water is shown in Figure 3.12. The shape of the peak differs from that of fusion in that there is a broad leading edge. 

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,... 

Under slower heating rates in heat-flux DSC, the deviation of sample temperature from the setpoint during a self-feeding reaction may be maintained adequately small so as to be neglected. If the furnace feedback control is set to act based on the temperature of the sample (that is the sample temperature thermocouple is the control thermocouple), then the control system may be able to allow the transforming sample to heat itself at a constant rate, and the heat input from the furnace will retreat as needed. 

An example of a heat-flux DSC trace of a lambda transformation is shown in Figure 3.17. The endothermic trend ap-... 

Figure 3.17 Heat-flux DSC trace at 10°C/min of the ferromagnetic to paramagnetic lambda transformation at 354°C in nickel (dotted line). The Curie temperature indicated by the DSC trace is 346°C (dot-dashed line).

The TA Instruments heat-flux DSC design (Figure 3.23c and Figure 3.5), where the sample and reference rest on elevated platforms of a constantan disk, also has minimal baseline float since the high thermal conductivity of the disk has an effect similar to that of the nickel block. The latter device is generally more calorimetric than the nickel block design. By mea-... 

---