CONTROLLED TRANSFORMATION RATE THERMAL ANALYSIS CRTA

0 CONTROLLED TRANSFORMATION RATE THERMAL ANALYSIS (CRTA) 

Conventional thermal analysis requires that the temperature of the sample follows some predetermined program as a function of time. Controlled transformation rate thermal analysis (CRTA), referred to as the 

A schematic of a set-up used for CRTA is shown in Fig. 18. The apparatus permits a thermal pathway to be eharaeterized at each point, not only by the sample temperature, but also by the rate of reaction, and more importantly by the gaseous environment. Control of the residual pressure above the sample at different values permits an understanding of this parameter. 

The XRD analysis indicated the final product to be y-CaS04 although diffraction peaks of calcium sulfate hemihydrate were observed, likely the result of spontaneous rehydration of 7-CaS04 into calcium sulfate hemihydrate. 

An original result ofthis study is that depending of the microstructure of the initial sample of g5 sum, the lattice of the intermediate ealcium sulfate hemihydrate varies. There appears to be no transformation of the gypsum lattice in the case of micro-sized needles of gypsum. There is a partial transformation in the case of centimeter-sized single eiystals of gypsum. 

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Controlled Transformation Rate Thermal Analysis (CRTA) Method [40,43]... 

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. 

Badens, E., Llewellyn, P., Fulconis, J. M., Jourdon, C., Veesler, S., Boistelle, R., and Rouquerol, F., Study of Gypsirm Dehydration by Controlled Transformation Rate Thermal Analysis (CRTA), J. Solid State Chem., 139 37 4 (1998)... 

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