Constant rate thermal analysis

Non-isothermal measurements (Chapter 2) have yielded valuable information about reaction temperatures and the successive steps in the removal of water from crystalline hydrates, e g. oxalates [14], sulfates [15-17]. DTA and DSC studies have sometimes provided additional information on the recrystallization of the dehydrated product [18]. The problems of relating kinetic parameters obtained by non-isothermal measurements to those from isothermal experiments are discussed in Chapter 5. The effects of heat transfer and diffusion of water vapour may be of even greater consequence in non-isothermal work. Rouquerol [19,20] has suggested that some of the above problems may be significantly decreased through the use of constant rate thermal analysis. 

Real C, Alcala MD, Criado JM (2004) Synthesis of silicon nitride from carbothermal reduction of rice husk by the constant-rate-thermal-analysis (CRTA) method. J Am Ceram Soc 87 75-78... 

Another interesting comparison of various thermal analysis techniques, namely adsorption microcalorimetry, thermoprogrammed desorption, and thermoprogrammed reaction using constant rate thermal analysis (CRTA), has been performed by Fesenko et al. in order to study the reactivity of zeolites in terms of the adsorption or desorption of base probe molecules [24]. As an example, CRTA was applied to the desorption of isopropylamine from Na-Y zeolite and its acidic form HY. 

CnRTA Constant rate thermal analysis (2) Direct inlet... 

Criado, J. M., and A. Ortega (1992). A study of the influence of particle size on the thermal decomposition of CaCOj by means of constant rate thermal analysis . Thermochimica Acta 195 163-167. 

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. 

Theory and kinetic analysis (38 entries). Many aspects of the theory of kinetic analysis were discussed (27 entries). Some papers were specifically concerned with discrimination of fit of data between alternative kinetic expressions or with constant reaction rate thermal analysis. Other articles (11 entries) were concerned with aspects of the fundamental theory of the subject and with the compensation effect. The content of papers concerned with kinetic analyses appeared to accept the common basis of the applicability of the rate equations listed in Table 3.3. 

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. 

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 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. 

FIGURE 2.6. Typical Curves Obtained from (A) Constant Heating Rate Tests, (B) Isothermal Tests, ( C) Differential Thermal Analysis and (D) Adiabatic Calorimetry... 

More advanced techniques are now available and section 4.2.1.2 described differential scanning calorimetry (DSC) and differential thermal analysis (DTA). DTA, in particular, is widely used for determination of liquidus and solidus points and an excellent case of its application is in the In-Pb system studied by Evans and Prince (1978) who used a DTA technique after Smith (1940). In this method the rate of heat transfer between specimen and furnace is maintained at a constant value and cooling curves determined during solidification. During the solidification process itself cooling rates of the order of 1.25°C min" were used. This particular paper is of great interest in that it shows a very precise determination of the liquidus, but clearly demonstrates the problems associated widi determining solidus temperatures. 

The sample is enclosed in a heavy walled bomb with an internal volume of approximately 1 to 1 ml. Although similar to a differential thermal analysis (DTA) test, the samples used are much larger, and the conditions of confinement allow the liquid to remain in contact with any decomposition products that form as vapors. Heat is applied so that the bath temperature increases at a constant rate and the temperature of both the heating bath and the sample are recorded continuously. When the temperature of the sample exceeds that of the bath, an exothermic reaction must be occurring in the sample, and this process is frequently accompanied by a detonation. (The bomb is equipped with a blow out disc to avoid any major damage to the equipment). In the more usual case the discrepancy between sample temperature and bath temperature increases with temperature, and the point at which this deviation is 5°F./min. is called the self-heating temperature. Typical values for some liquid materials of interest in the propellant field are listed in Table V. 

It is a common practice in thermal analysis literature to quote the apparent rate constant, k, which is convenient although only dimensionally correct for first-order reactions. 

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