Heating rate analysis, varied

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. 

Stabilization of the samples was carried out imder air atmosphere in a chamber furnace (Nabertherm Controller Co.) using different temperatures and dwell times ranging from 180 to 270°C and 15-120 min, respectively. All runs were in discontinuous mode. Heating rate also varied from 1 to 4° C to obtain the best results. The range of temperatures was chosen according to the thermal behavior of PAN nanolibers got from DSC analysis. 

A Perkin-Elmer instrument (TGA 7) was used for the thermal analysis. The heating temperature was varied from 50 to 600°C at a rate of 10°C/min, and air was used as purging gas. These measurements provide information about the weight of the plasma-polymer coating per unit weight of the substrate. 

Thermogravimetric analysis (heating rate 10°C/min) of PPNA revealed that the polymer was stable to at least 220°C in an N2 atmosphere. (Figure 2) Differential scanning calorimetric measurements (heating rate 10°C/min) on several batches of PPNA indicated that Tg varied between 125-140°C. This variation may be a reflection of the amount of solvent (NMP) entrapped in the... 

Heat activated reactions show a variation of the position of the peak in the cure rate curves with varying heating rate. If the temperature rises during reaction, the reaction rate, dc/dt, will rise to a maximum value and then returns to zero as reaction completes. The temperature at which the reaction rate reaches its maximum is also the temperature of maximum deflection in differential thermal analysis. Dynamic DSC measurements at different heating rates are used to determine the activation energy of the material, because they show the effects both time and temperature have on the reaction. 

Some attention has been given to the NIK analysis of complex reactions (i.e., rate processes incorporating contributions from reversible, concurrent and consecutive reaction steps). The occurrence of complex reactions is detected in isothermal studies by Arrhenius plots that are curved or give two linear regions [93]. The shapes of plots of nr against reduced time also vary systematically with temperature. Experiments at different heating rates also show up complexities. 

In parallel, a small quantity of lyophilized samples placed into DSC pans was also equilibrated in each chamber at controlled RH to be submitted to DSC analysis. At equilibrium, the pans were sealed and analyzed. Glass-transition temperature was analyzed in a DSC 5200 (Seiko Instruments Inc., Chiba, Japan). Heating rate was 5°C/min and temperature range varied between — 20 and 80°C. The midpoint of the glass transition was considered as the characteristic temperature of the transition. Determinations were carried out in triplicate. 

The microkinetic analysis of the N2 TPD data from a multiply promoted Fe catalyst showed that the PFR model is mandatory for the the assessment of the influence of readsorption. It is important to derive kinetic parameters not only from the shape of the experimental TPD trace or from varying the heating rate, but to confirm the result by changing the reaction conditions such as the number of sites and the total flow rate of inert gas. 

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