Controlled-rate thermal analysis

We have just seen how to avoid (either manually or automatically) spurting of the sample on evacuation, at room temperature. However, when the sample temperature is raised there is a further risk of spurting and of uncontrolled changes of the sample itself. The technique for overcoming these problems follows the general principle of controlled rate thermal analysis (CRTA) (Rouquerol, 1970,1989), where the heating... 

Some of the best-known examples of this type of reaction are the thermal decomposition of hydroxides to give active oxide-hydroxides and oxides (see Section 10.3.6). Another example is the calcination of a carbonate (e.g. CaC03). In fact, BET areas of up to 500 m2 g-1 can be produced by the calcination of an aluminium trihydroxide. But, unless the heat treatment is carefully regulated, as in controlled rate thermal analysis (CRTA), the pore structure of the active product tends to be highly heterogeneous (Rouquerol and Ganteaume, 1977). 

The samples were outgassed prior to each experiment using Controlled Rate Thermal Analysis or CRTA [2]. A constant pressure of 10 mbar was used for the thermal preparation up to a final temperature of 250°C. 

Ex-situ X-Ray Diffraction and Controlled Rate Thermal Analysis (CRTA)... 

The X-Ray analysis were performed on a INEL 120 diffractometer with a Mo wavelength (A = 0.70926 A). The dehydration process is investigated here by a heating using a Controlled Rate Thermal Analysis (CRTA). 

The adsorption up to 50 bars was carried out by means of a Tian-Calvet type isothermal microcalorimeter built in the former CNRS Centre for Thermodynamics and Microcalorimetry. For these experiments, around 2 g of sample was used which were outgassed by Controlled Rate Thermal Analysis (CRTA) [7]. The experiments were carried out at 30°C (303 K). Approximately 6 hours is required after introduction of the sample cell into the thermopile for the system to be within 1/100 of a degree Celsius. At this point the baseline recording is taken for 20 minutes. After this thermal equilibrium was attained, a point by point adsorptive dosing procedure was used. Equilibrium was considered attained when the thermal flow measured on adsorption by the calorimeter returned to the base line. For each point the thermal flow and the equilibrium pressure (by means of a 0-70 bar MKS pressure transdueer providing a sensitivity of 0.5% of the measured value) were recorded. The area under the peak in the thermal flow, Q eas, is measured to determine the pseudo-differential... 

This definition is more complicated than the first one and, in reality, although proposed to the ICTAC membership, was never officially endorsed by the ICTAC Council. It still does not embrace the case of sample-controlled thermal analysis, although SCTA is referred to in that 1991 booklet, in the form of controlled rate thermal analysis (CRTA). 

Temperature may not always be raised in a linear fashion. In the case of CRT A (Controlled Rate Thermal Analysis), the heating rate is varied in such a manner as to produce a constant rate of mass loss. Alternatively a sinusoidal temperature rise is superimposed on the linear rise this is known as Modulated TG and allows the continuous calculation of activation energy and pre-exponential factor during a run. Sometimes a Temperature Jump (or stepwise isothermal) " is used, where temperature is held constant for a time, then jumped rapidly to a higher constant temperature (usually quite close in temperature). All of these procedures are supposed to help in the determination of kinetics of reaction. Another system accelerates the temperature rise when no mass loss is experienced, i.e. between reactions. The rate is slowed to a low value during mass loss. Some manufacturers call this High Resolution TG and an example follows. 

M. Reading, Controlled Rate Thermal Analysis and Beyond , in Thermal Analysis - Techniques and Applications, ed. E. L. Charsley and S. B. Warrington, Royal Society of Chemistry Special Publications, No. 117, Cambridge, 1992, pp. 126-155. 

One of the approaches described in this paper entails keeping the reaction rate and partial pressure of product gas constant during calcination by changing the tempo ature appropriately by means of a feedback loop. This technique has its origins in Controlled Rate Thermal Analysis (CRTA), which was developed by Rouquraol [4] to provide improved kinetic data and higher resolution in thermal analysis. He showed that constant reaction rate conditions could be of benefit also in preparing materials with specifiable surface areas. 

In the last decades several high-resolution techniques have been introduced. These techniques are event-controlled , i.e. when a thermal event (decomposition, evaporation, oxidation, etc.) occurs a change in measuring condition is introduced. Such event-controlled techniques are termed controlled rate thermal analysis (CRTA) [7] or reaction-controUed thermal analysis (RCTA) [195]. Nomenclature in the pertinent literature is confusing [7, 196]. Scheme 2.1 gives an overview of the relations between the methods which all aim at increasing the resolution of closely occurring thermal events. 

Fig. 2.8. (a) Principle of conventional thermal analysis (temperature controlled) (b) principle of controlled rate thermal analysis (Z-controlled). After Rouquerol [7]. Reprinted from Thermochimica Acta 144, J. Rouquerol, 209-224 (1989),... 

For further information the reader is referred to some recent reviews on thermogravimetry [80,214, 215], in particular related to polymers [216], and on controlled rate thermal analysis and related techniques [195,196] many textbooks are available (cfr. Bibliography). 

CrRTA Controlled rate thermal analysis DIP Direct inlet (insertion) probe... 

Arii, T., and Fujii, N., Controlled Rate Thermal Analysis Kinetic Study in Thermal Dehydration of Calcium Sulfate Dihydrate, J. Anal, and Appl. Pyrolysis, 39 129-143 (1997)... 

Criado, J. N., Ortega, A., and Gotor, F., Correlation Between the Shape of Controlled Rate Thermal Analysis Curves and the Kinetics of Solid State... 

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