Quasi-isothermal method

Sorenson, O.T. Quasi-isothermal methods in thermal analysis. Thermochim. Acta 1981, 50,163-175. 

Fig. 41. Apparent desorption energy for Pb from GaAs as a function of coverage, deduced using the quasi-isotherm method of analysis.

Figure 34. The Q-TG and Q-DTG thermodesorption mass loss curves of benzene from the alumina oxide sample made using the programmed quasi-isothermal method.

Figure 9.34 (a) The comparison of the reversing heat capacities of poly(lactic acid) (PLA) resulting from different the time domains in the melting region by quasi-isothermal method of temperature-modulated DSC in the frame of vibrational and liquid Cp [55]. (See text for full caption.)... 

The experiments described above for the cure studies of epoxy resins (a typical three-dimensional network) were restricted to isothermal experiments (or quasi-isothermal experiments in the case of MDSC). As discussed later, these are the most reliable methods for generating kinetic information, but suffer from the length of time taken to generate data and, furthermore, in some systems the heat flow may be vanishingly small. 

A more general method of using integrated forms involves using the standard procedure where one waits until the TSR experiment is completed and then proceeds to smooth the (X, T, t) surface as described in previous discussion. On the smoothed surface one identifies the operating lines for the system and uses them in an integral method of data interpretation. This removes the restriction employed in the hydrolysis study, that the system be at quasi-isothermal conditions, and makes the method more general. 

Figure 7. Fractal dimension method of calculation on the basis of Q-TG thermodesorption of 1-butanol from La-montmorillonite surface under the quasi-isothermal conditions.

A new method for the calculation of diffusion coefficients from Q-TG data has been presented. The programmed thermodesorption of polar and nonpolar liquids from aluminium oxide and montmorillonite-Na and -La samples under quasi-isothermal conditions has been studied. The results from above methods were compared with literature data and good correlation was obtained. 

Quasi-isothermal TMDMA data for PEN are shown in Fig. 6.51 for slow cold crystallization at 418 K [42]. The method consists of dynamic mechanical analysis, DMA (see Sect. 4.5.4), to which temperature modulation was added. The insert is a... 

A more detailed analysis of the kinetics of the glass transition was carried out by quasi-isothermal TMDSC [66,67]. Figure 6.132 is a representation of plots of the reversing, apparent heat capacities of some of these samples after extrapolation to zero modulation amplitude. All traces of hysteresis are absent. The same methods as described in Sects. 6.3.1 and 6.3.2 were used for the analyses of the glass transitions. For the amorphous PET, the analysis is shown in Figs. 4.129-133 and 6.119-121. 

A sample can also be observed under isothermal conditions where the mass change is recorded as a function of time at a predetermined temperature. A third method of TG analysis is the jump method [4]. In this case the sample is held at a fixed temperature for a period of time until the temperature is discontinuously changed (or jumped), where again the mass change is observed as a function of time. The application of the jump method to the study of reaction rate kinetics is discussed in Section 5.3.3. If the chemical reaction under investivation proceeds slowly then the linear heating programme may be replaced by a stepwise programme so that the experimental conditions become quasi-isothermal [5]. 

A more complex system of temperature and atmosphere control used in the study of decomposition rates is quasi-isothermal and quasi-isobaric thermogravimetry [6]. In this method a heating programme linearly increases the sample temperature until a mass change is detected. On detection of a mass... 

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... 

Thermoanalytical methods comprise calorimetry (Differential Scanning Calorimetry, DSC) and thermogravimetry (TG) [3]. Mostly, the temperature of the sample is increased linearly and peaks of the sample temperature in comparison to a reference sample are recorded (Differential Thermal Analysis, DTA). In thermogravimetry the mass loss is observed. In this way the content of adsorbed liquids can be determined and the desorption kinetics can be observed. For special tasks also a quasi-isothermal treatment may be applied [4]. The most important standards on such methods are summarized in Table 2. 

The method of compensation is advantageous, since it permits measurements to be carried out under quasi-isothermal conditions, thereby avoiding heat loss from the calorimeter to the surroundings by heat transport processes. Furthermore. 

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