DTA Instrumentation

The plot shows the baseline shift that occurs at the glass transition temperature, T, exothermic peaks for crystallization and cross-linking (or curing), an exothennic peak (off scale) for oxidative decomposition, and an endothermic peak for melting of the polymer. A similar thermal plot would be obtained by DSC analysis. (Courtesy of TA Instruments, New Castle, DE, www.tainstmments.com.) 

When a physical change takes place in the sample, heat is absorbed or generated. For example, when a metal carbonate decomposes, CO2 is evolved. This is an endothermic reaction heat is absorbed and the sample temperature decreases. The sample is now at a lower temperature than 

Modem DTA instrumoits have the aWlity to change atmospheres fron inert to reactive gases, as is done in TGA. As is the case with TGA, the appearance of the DTA thermal curve depends on the particle size of the sample, sample packing, the heating rate, flow charactoistics inside the furnace, and other factors. Thermal matching between the sample and the reference is often improved by diluting the sample with the inert reference, keeping the total masses in each cmcible as close to each other as possible. 

The peak area in DTA is related to the enthalpy change, AH to the mass of sample used, m and to a large number of factors like sample geometry and TC. These other factors result in the area. A, being related to the mass and AH by an empirically determined calibration constant, K  

Unfortunately, K is highly temperature dependent in the DTA experiment, so it is necessary to calibrate the peak area in the same temperature region as the peak of interest. This may require multiple calibration standards and can be time consuming. As we shall see, the calibration constant K for DSC is not temperature dependent therefore, DTA is usually used for qualitative analysis, while DSC is used for quantitative measurements of AH and heat capacity. 

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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. 

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). 

The crystallization behaviour has been described by Oyumi et al. (1987a). An amorphous phase, unstable at room temperature, can be prepared under kinetic conditions by rapid removal of solvent (acetone or acetonitrile) or rapid cooling of the melt. It initially appears as a transparent waxy material that transforms into a crystalline material over about an hour. It can also be prepared on a DTA instrument by cooling from the melting point of the crystalline material (406 K) to the temperature range 333-290 K. On the other hand, crystals suitable for single crystal structure determination (carried out by the same authors) can be grown by slow evaporation from the same solvents. 

A DTA instrument is designed to measures temperature differences between sample and reference as illustrated in Figure 10.2. A sample and reference are symmetrically placed in a furnace. The temperature difference between the sample and reference are measured by two thermocouples one is in contact with the underside of the sample holder (also called the crucible), the other is in contact with the underside of the reference holder. The reference should be made from a material that satisfies the following conditions it does not undergo thermal events over the operation temperature range, does not react with any component in the instrument, and has similar thermal conductivity and heat capacity to the sample being examined. 

Figure 10.2 Differential thermal analysis (DTA) instrumentation. VTs and VTr are the thermocouple voltages for measuring sample and reference temperatures, respectively. (Reproduced with permission from R.F. Speyer, Thermal Analysis of Materials, Marcel Dekker, New York. 1993 Taylor Francis Group Ltd.)...

The other common category of calorimetry is differential methods, in which the thermal behavior of the substance being measured is compared to that of a reference sample whose behavior is known. In differential scanning calorimetry (DSC), the instrument measures the difference in power needed to maintain the samples at the same temperature. In differential thermal analysis (DTA), the samples are heated in a furnace whose temperature is continuously changed (usually linearly), and the temperature difference between the sample and the reference sample as a function of time can yield thermodynamic information. DSC and DTA are most commonly used for determining the temperature of a phase transition, particularly for transitions involving solids. In addition, DSC experiments can yield values for the enthalpy of a phase transition or the heat capacity. Commercial DSC and DTA instruments are available. 

DTA Instrumentation. The differential thermograph used in this study has been described in detail (1, 2, 3). The microcalorimeter cell which used 0.005 gram of sample was used. The thermograms were recorded on an x-y recorder with the differential temperature, A T, on the y-axis and the sample temperature, T, on the x-axis. The sample temperature was measured with the same thermocouple as the AT. This produces thermograms with peak locations independent of heating rate. The heating rate was 4°C./minute. The calorimeter was calibrated with zone-purified dotriacontane, AHr + AHf = 51.7 cal./gram. 

A thermocouple generates an electrical potential which is roughly proportional to the difference in temperature between the two junctions (Seebeck effect), and is well suited for differential temperature measurements (10). It may also be used for absolute and relative temperature measurements by keeping one junction, the reference junction, at constant temperature. Thermocouples normally used in DTA instruments are shown in Table 6.1. The temperature limits listed are for relatively accurate measurements with... 

Although most DTA instruments have only one furnace, to increase the number of samples that can be run each day several furnaces may be used in conjunction with the sample holder, amplifier, and recording system. In fact, an instrument that contains four different furnaces (44) has been described. 

A number of DTA instruments are described here an attempt is made to include only those instruments which possess some novelty in design or that have made important contributions to the development of DTA instrumentation. 

DTA instruments for high-pressure hydrogenation reactions have been described by various Japanese investigators (76, 77). Bousquet et al, (78) described a high-pressure DTA furnace and sample holder which could be... 

The determination of DTA curves from microgram quantities of sample has previously been described by Mazieres (4) (Section 2). A more comprehensive review of micro-DTA instrumentation is that by Sommer and Jochens (84). In this review, the entire area of high-temperature microscopy, coupled with DTA measurements, is discussed in detail. In most of the instruments described, the thermocouple junction acts as a heater and temperature detector, as well as the sample holder. 

Present-day DTA instruments are capable of automatic operation in that after the sample has been manually inserted the temperature rise is controlled by a temperature programmer which will turn off the instrument after a preselected temperature limit is attained. When the furnace has been cooled... 

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