Power Compensation, DSC

A power compensation DSC uses a radically different measurement method. With its development in 1963, the name DSC was coined [21]. In Fig. 4.58 a schematic of sample and reference arrangement is drawn and some typical parameters are given. 

The sample and reference thermometers and heaters are platinum resistance thermometers (see Sect. 4.1). Instead of relying on heat conduction from a single furnace, governed by temperature difference, reference and sample are heated separately as required by their temperature and the temperature difference between the two furnaces. The two calorimeters are each less than one centimeter in diameter and are mounted in a constant temperature block. This instrument is, thus, a scanning, isoperibol twin-calorimeter (see Sect. 4.2). 

Mathematically this simation without a AT loop is expressed by the upper three boxed equations in Fig. 4.60, where dQs/dt and dQs/dt are the heat-flow rates into the reference and sample calorimeters, respectively. The measured and true temperatures are represented by T and T. For simplicity, one can assume that the proportionality constant K is the same for the sample and reference calorimeters. Differences are assessed by calibration. Both bottom equations are then equal to the power input from the average temperature amplifier. Wav (ill W or J s ). 

The use of the upper, differential amplifier loop in Fig. 4.59 closes the AT loop. The temperature-difference signal is directly amplified and a corresponding power is used to compensate the imbalance between the sample and reference calorimeter temperatures so that only a small temperature difference remains, as indicated in Fig. 4.60. This differential-power signal is also sent to the computer as W, the 

One additional point needs to be considered. The commercial DSC is constructed in a slightly different fashion. Instead of letting the differential amplifier correct only the temperature of the sample calorimeter by adding power, only half is added, and an equal amount of power is subtracted from the reference calorimeter. This is accomplished by properly phasing the power input of the two amplifiers. A check of the derivations shows that the result does not change with this modification. 

A DSC curve plots the power input per unit time, which is proportional to the heat capacity of the sample, as a function of either the programmed temperature or time. The maximum sensitivity of a power compensation DSC is 35 mW. Temperature and energy calibrations are 

Compared with a heat-flux DSC, higher scanning rates can be used with a power compensation DSC, with a maximum reliable scanning rate of 60 K min. Maintaining the linearity of the instrument baseline can pose problems at high operating temperatures or in the sub-ambient mode. 

TMDSC is a recently suggested technique, and discussions of its attributes are continuing [14-23]. 

a perturbation in the form of an oscillating sine wave of known frequency is applied to the linear temperature control program. This variation can be applied in principle to heat-flux DSC and to power compensation DSC instruments. The thermal response is analyzed using Fourier transformation, with the component in-phase with the temperature oscillation thought to be caused by reversible or equilibrium changes in the sample, and the out-of-phase component associated with non-reversible changes. 

Experimental parameters Heating rate isothermal to 60 K min Heating rate isothermal to 10 K min Temperature modulation amplitude 0.01-10 K Temperature modulation period 10-100 s 

Therefore the raw heat flow signal can be viewed as a form of heat capacity. In practice, it reflects the changes occurring in heat capacity, and the absolute value is obtained when the 

The small furnaces of this system can be heated or cooled at very low rates to very high rates and are ideal for a range of different techniques, particularly fast scan DSC. Power-compensated DSC also permits true isothermal operation, since under constant temperature conditions both the sample and furnace are held isothermaUy. The temperature range of 

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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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