Reference materials for DTA

Differential This refers to the difference between two physical quantities (e.g. the temperatures of sample and reference material for DTA) under a controlled temperature program. 

Many instrument manufacturers produce simultaneous TG-DTA apparatus. The advantage of such apparatus is not only that the sample and experimental conditions are identical but also that standard reference materials for DTA and DSC can be used for temperature calibration (see Section 2.2.1.7). 

Sample mass, 75.65 mg apparatus, Shimadzu DT-2A DTA heating rate, 3 °C min DTA sensitivity (told deflection), 100 pV paper speed, 250 mm h reference material for melting-condensation, Si02, crucible, platinum thermocouple, Pt-PtRh. 

For Inorganic materials, the best reference material to use is a - AI2O3. Its heat capacity remains constant even up to its melting point (1930 °C.). What this means is that no thermal changes occur in R so that any change detected wUl be that of the sample, S. In DTA, we wjmt to measure ACp, but find that this is... 

Methodology appropriate for the measuring of DTA profiles has been extensively reviewed [12,13]. A schematic diagram illustrating the essential aspects of the DTA technique is shown in Fig. 3. Both the sample and reference materials are contained within the same furnace, whose temperature program is externally controlled. The outputs of the sensing thermocouples are amplified, electronically subtracted, and finally shown on a suitable display device. 

The calibration of DTA systems is dependent on the use of appropriate reference materials, rather than on the application of electrical heating methods. The temperature calibration is normally accomplished with the thermogram being obtained at the heating rate normally used for analysis [16], and the temperatures... 

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. 

Differential Thermal Analysis (DTA) A sample and inert reference material are heated at a controlled rate in a single heating block. This test is basically qualitative and can be used for identifying exothermic reactions. Like the DSC, it is also a screening test. Reported temperatures are not reliable enough to be able to make quantitative conclusions. If an exothermic reaction is observed, it is advisable to conduct tests in the ARC. 

In the combination TG-DTA normally two crucibles of identical shape and size are used, one for the sample and the other one for the inert reference material. The crucibles may be e.g. cylindrical, conical, plate-type or any other form which favors a good contact between the sample, thermocouple and sample holder (Fig. 8 j). The thermocouples are placed either on the bottom of the crucible or in the center of a cylindrical crucible. The latter type allows the element to be brought protected or unprotected into the center of the substance. 

For exact temperature evaluation the thermocouple must be calibrated with temperature standards. Today commercial temperature standards are available Nat. Bureau of Standards (NBS) Washington offers standard reference materials (No 759, 758, 760) as DTA-temperature standards 6a-e for 3 temperature ranges ... 

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). 

Differential Thermal Analysis (DTA) — A procedure for recording the difference in temp between a substance and a reference material, against either time or temp as the two specimens are subjected to identical temp regimes in an environment heated or cooled at a controlled rate. The record is the differential thermal or DTA curve the temp difference (A T) is usually plotted on the ordinate with endothermic reactions downward and time or temp on the abscissa increasing from left to right Differential Scanning Calorimetry (DSC) —... 

Figure 3.1 is a schematic of the differential thermal analyzer (DTA) design. The device measures the difference in temperature between a sample and reference which are exposed to the same heating schedule via symmetric placement with respect to the furnace. The reference material is any substance, with about the same thermal mass as the sample, which undergoes no transformations in the temperature range of interest. The temperature difference between sample and reference is measured by a differential thermocouple in which one junction is in contact with the underside of the sample crucible, and the other is in contact with the underside of the reference crucible.1 The sample temperature is measured via the voltage across the appropriate screw terminals (Vt,) and similarly for the reference temperature (Vrr) generally only one or the other is recorded (see section 3.5.1). Sample and reference... 

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... 

Differential scanning calorimetry, DSC, and differential thermal analysis, DTA show similar traces for T measurements although the property being measured is different. DSC measures the amount of heat required to increase the sample temperature over that required to heat up a reference material, normally an empty pan, to the same temperature. The variation in power necessary to maintain this level during a transition is monitored. DTA measures the difference in temperature between the sample and the reference material when both are heated at the same temperature rate. These techniques require a small amount of specimen, about 15 mg, and have... 

Techniques such as DTA, TG or DSG study the thermal behavior of a catalyst as it undergoes heating at a constant rate. For example, DTA relies on the measurement, as function of time, of the difference between the temperature of the sample (Tj) and that of a reference material (rj, upon heahng both materials in a furnace. AT = T — is related to the heat evolved (or absorbed) by the sample at a time when the sample undergoes an endothermic or exothermic reaction. In the TG and DTG methods, the variation with time of the sample mass (dm or dm/dt) is measured as a function of temperature while the sample is heated at a constant rate [7]. 

Differential thermal analysis (DTA) is a technique for measuring the difference in temperature between the substance under investigation and an inert reference material with respect to temperature or time. 

The original work extends the discussion to more complex reactions and the determination of activation energies and heats of reaction. However, the equations were developed for homogeneous reactions in solution and required twelve assumptions some of which are very difficult to satisfy when applied to dta studies of solid state reactions. These assumptions 2u e (/) the heat transfer coefficients and heat capacities of reactants and products are equal and constant, and (ii) that the temperature is uniform throughout the sample and reference material. Freeman and Carroll and Wendlandt have suggested simplifications in Borchardt and Daniels procedure. 

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. 

---