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LOGVINENKO et al.: BISMUTH STEARATES
The study of thermal decomposition was performed by means of Derivatograph
Q-1500-D (MOM, Hungary). Quasi-isothermal heating was used both for the investi-
gation of the step-by-step character of thermal dissociation processes, for the attempt
to check the possible stable intermediate phases [6] and for kinetic study [7]. Two
sample holders (plate-like and standard open crucible) were used, sample mass was
200 mg, constant mass loss rates were 0.2 and 0.53 mg min–1, atmosphere – helium
(60 cm3 min–1). Linear heating was used for kinetic studies; sample mass was
50–60 mg, heating rate 2, 5 and 10°C min–1, helium flow 120 cm3 min–1.
Thermoanalytical studies indicated that decomposition processes are accompa-
nied by melting; the final products were both the bismuth oxide, and the dispersed
metal (the endothermic effect at 271°C can be attributed to the melting of bismuth,
the exothermic effects above 500°C can be connected with bismuth oxidation).
Thermogravimetric data were processed using the computer program ‘Netzsch
Thermokinetics’(version 2001.9d). So as the thermal decompositions are multi-step
processes for all salts, we select the convenient variant of calculation. Special pro-
gram module ‘model free’ allows processing several thermogravimetric curves, ob-
tained with different heating rates, without the information about the kinetic
topochemical equations. Program ‘ASTM E698’ [8] enables to calculate the averaged
values of activation energy and preexponential factor according to maximum rate
temperatures. Program ‘Friedman Analysis’ [9] allows calculating both the activation
energies, and preexponential factors (assuming a first-order reaction) for the every
experimental point of fractional conversion, program ‘Ozawa–Flynn–Wall Analysis’
[10–12] allows calculating only activation energies for every experimental point (in
the interval 0.02<α<0.98). The same set of experimental data is used further for
searching the topochemical equation (the selection from 16 equations (chemical reac-
tion on the interface, nucleation, and diffusion, Table 1)). This calculation is made by
the improved differential method of Borchardt–Daniels with multiple linear regres-
sion [13]. F-test is used for the search of the best kinetic description [14]. If the calcu-
lation results in two or three kinetics equations with near values of correlation coeffi-
cients (or F-test), but with noticeably different values of kinetics parameters, it is ra-
tionally to choose the equation with parameters values near to data of ‘model free’
module programs.
Results and discussion
The thermogravimetry under quasi-isothermal conditions denotes the multi-step
character of decomposition processes for all compounds. So as the molar masses of
the removing gases are different, reactions are apparently consecutive, we did not try
to divide the steps and did not go beyond the study of the first step only.
Bismuth stearate Bi6O4(OH)4(C17H35COO)6 nC17H35COOH
Data of two termogravimetric curves, obtained under linear heating (5 and 10°C min–1),
were processed by ‘Friedman Analysis’, ‘Ozawa–Flynn–Wall Analysis’ (Fig. 1) and
J. Therm. Anal. Cal., 74, 2003