Quasi-equilibrium thermogravimetry (variant of CRTA) is put to use as an express method of thermoanalytical screening for volatile compounds. During the experiments for P—T relationship calculations (running with several calibrated standard sample holders) the non-volatile (polymerized) residue is formed (and is decomposed with further temperature rising). Thermogravimetric data are used for the calculation of the kinetic parameters for the polymerization reaction, taking place (concurrently with the evaporation) in the melt of the studied volatile compound.
For the analysis of experimental data, an elemental process model is suggested, which considers the kinetic and thermodynamic parameters of the reaction, its topochemical mechanism, and the mass exchange between the reaction zone and the external medium. The proposed model is fully based on the principal assumption that the duration of the experiment is much greater than the relaxation time for all the heat transfer processes in the sample volume.
It is shown that the total differential of the function of the amount of conversion versus temperature and time (α=f(T, t)) is equal to zero non-isothermal kinetics at constant heating rate. Hence, the mathematical expression used in the literature for the rate of the non-isothermal transformation,
, is not valid.
A mathematical model for the conjugate formulation of the working conditions of the Q-derivatograph is created. There are no simplifying assumptions about the heat transfer processes for reacting active media and for their thermal interaction with the inert surroundings in this model. The energetic significance of the key components of the transfer and the redistribution of the fluxes is calculated by means of special balance relations. The influence of the kinetic and thermal reaction parameters on the nature of the establishment of the constant transformation rate is demonstrated. The accompanying heat phenomena are studied in detail. The essential differences in the behaviour of the TA system during endo and exothermal transformations are discovered. The exothermal reactions introduce significant irregularities, up to sharp oscillations.
The effect of the dispersity of the sample on the topochemical mechanism and on the kinetic parameters of thermolysis (As→Bs + Cg) under non-isothermal conditions is discussed. For correctness of the experiment, a ratio of maximum and minimum particle diameters ≤ 6 is a necessary condition. The topochemical mechanism of the process should be determined separately for the initial and final stages.
Dehydratation kinetics of aquo-acido complex EDTA chelates CaML · nH2O (M= =Mn, Co, Ni, Cu, Zn, Cd, Ca,n=2 ... 5) were studied under non-isothermal conditions. The existence of two isokinetic temperatures (β1=42°,β2=216°) has been detected with a reliability of 95 %. The compensation effect found is explained by the change in the number of bonds formed in the sequence of equal-type dehydratation reactions of analogously built-up, but not isostructural compounds.
Authors:G. V. Gavrilova, N. V. Kislykh, and V. A. Logvinenko
The decompositions of the clathrate compounds [M(NCS)2(4-MePy)4]·nG (whereM=Mn, Co, Ni or Cd;G=4-methylpyridine (4-MePy), benzene or xylenes) were studied on a Q-derivatograph under quasi-equilibrium conditions and with linear heating. These clathrates can be divided into two groups, in which the loss of guest is either (I) accompanied by destruction of the host complex, or (II) occurs before decomposition of the host complex. Kinetic parameters were obtained.