CRTA-controlled rate

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). 

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. 

CRTA Controlled transformation rate DLI (1) Direct laser ionisation ... 

Figure 10.17. Development of the BET-mtrogen surface area during the dehydration, by CRTA, of a sample of fine gibbsite, 1 pm gram size. CRTA conditions controlled pressure indicated in mbar on the curve rate of dehydration, 11.4 mg IT1 g"1 (Rouquerol and Ganteaume, 1977).

It has already been stated that SCTA envelopes the original CRTA techniques. Essentially, there are two forms of CRTA - the quasi-isothermal/quasi-isobaric methods (Paulik) and the constant rate method (Rouquerol). The Paulik method is based on thermogravimetry (TG) and is known as Q-TG . It depends on a control system that maintains a constant rate of mass loss and the measured parameter is temperature as a function of time. CRTA relies on using a transducer to monitor the pressure of evolved gas in a continuously evacuated chamber. The sample is heated in such a way as to maintain the monitored gas pressure constant. Since the pressure is maintained constant, the rate of gas pump-off is maintained constant and thus the rate of mass loss (when a single gas is evolved) is also maintained constant. Hence, CRTA is effectively vacuum thermogravimetry. The similarity between the Paulik and Rouquerol techniques is immediately apparent. Both maintain the reaction rate constant and both control the pressure of the evolved species in the reaction environ ment. Hence, the differences in the techniques are purely semantic. These techniques allow a precise... 

Over the last decade, it has been internationally accepted that the generic title for TA techniques whereby the heating rate is controlled using a feedback loop as some function of the rate of reaction or the chemical or physical process under study should be sample controlled thermal analysis (SCTA) and this title now embraces Q-TG, CRTA, SIA, and dynamic rate thermal analysis. 

In controlled transformation rate thermal analysis (CRTA), instead of controlling the temperature (as in conventional thermal analysis (Fig. 2.8a)), some other physical or chemical property X is modified, which is made to follow a pre-determined programme X = f(t) under the appropriate action of temperature (Fig. 2.8b) [7]. Heating of the sample may be controlled by any parameter finked to the rate of thermally activated transformations, such as total gas flow (EGD control constant decomposition rate thermal analysis [199]), partial gas flow (EGA... 

A valuable approach for measuring thermal degradation kinetic parameters is controlled-transformation-rate thermal analysis (CRTA) - a stepwise isothermal analysis and quasi-isothermal and quasi-isobaric method. In this method, some parameters follow a predetermined programme as functions of time, this being achieved by adjusting the sample temperature. This technique maintains a constant reaction rate, and controls the pressure of the evolved species in the reaction environment. CRTA is, therefore, characterised by the fact that it does not reqnire the predetermined temperature programmes that are indispensable for TG. This method eliminates the nnderestimation and/or overestimation of kinetic effects, which may resnlt from an incomplete understanding of the kinetics of the solid-state reactions normally associated with non-isothermal methods. 

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