Browsing by Author "Pombe, Joseph John"

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    The decomposition kinetics of the Lanthanide (III) Oxalato Halides, LnC2O4X (x = Cl, Br and Ln = Gd).
    (University of Dar es Salaam, 1994) Pombe, Joseph John
    The kinetics of the thermal decomposition of GdC2O4Br single crystal and GdC2O41 powder have been studied by a thermogravimetric method under the purging with drynitrogen gas. The infrared spectra were used to show that the prepared sample were of acceptable purity. The decomposition kinetics of GdC204Br were shown by isothermal studies to proceed according to the one-dimensional diffusion controlled mechanism (D) for the late part of the action and the 3- dimensional phase boundary reaction mechanism (R3) during the initial reaction. The three dimensional phase boundary reaction mechanism. Gives a typical splitting of the decomposition reaction resulting in two kinetic phases. This is presumably caused by a stable layer of the denser solid. Gd0Br. Which is formed around the reactant material, GdC204Br. The splitting occurs at te degree of conversion. a, between 0.81804 and 0.875. A larger value of the activation energy, Ea (R3(1) = 180 kj/mol was obtained during the first phase of the reaction and small value of Ea(R3(2) = 173.7 kJ/mol. Obtained above the degree of conversion a = 0.81804 according to the phase boundary reaction mechanism. The reaction rate however appears to be diffusion controlled above a = 0.31623 proceeding with the activation energy Ea(D) = 176 kJ/mol. The trend of the activation energies is Ea(R3(2) < Ea(R3(1), where Ea(R3(1)) are the activation energies obtained using the three-dimensional phase boundary reaction mechanism for the first and second kinetic phases respectively. The frequency factors obtained for R3(2) varied between 1.118 x 1011 and 1.856x1011 s-1, for R3(1) between 3.45x1011 and s-1, while for the mechanism D the frequency factor varies between 0.94x1011 and 1.364x1011s-1. It was also observed that the value of the frequency factor was decreasing as the weight of single crystal increases. Based on the int versus 1/T plots of the decomposition reaction of GdC204Br crystals. It is found that the reaction is mechanistically uniform between a = 0.2 and a = 0.9. The average activation energies obtained for 12mg and 43mg samples for this range of a values is 191.5kJ/mol. Non-isothermal kinetic studies were conducted using isothermal kinetic studies were conducted using isoconversional method at the heating rates of 1,2,6,8 and 10K/min. The activation energy obtained at the degee of conversion, a + 0.1 to a = 0.9 ranges from 191.3 to 160.6kJ/mol respectively. It is observed that as the value of a increases, the corresponding activation energy decreases. The activation energy at a = 0.1 [191.3kJ/mol] is in good agreement with the average activation energy [191.5 kJ/mol] obtained from isothermal method using Int Vs 1/T plots. The results obtained the isoconversional and isothermal methods seem to relate one another in special ways. The study show that the activation energy obtained between degree of conversions, a = 0.1 to a= 0.8, a = 0.4 to a= 0.9 (a> 0.3) and a= 0.9 in both isoconversional and isothermal methods, tends to correspond with the activation energies obtained during the first phase of the three dimensional phases boundary reaction mechanism [R3(1), one dimensional diffusion controlled reaction mechanism (D) and the second phase of the three dimensional phase boundary reaction mechanism [R3(2)] respectively. The decomposition kinetics of GdC204C1 powder is best described by a one-dimensional diffusion controlled mechanism which gives an activation energy of 200 kJ/mol. Based on the Int versus 1/T plots the decomposition reaction is found to be mechanistically uniform between a= 0.3 and a= 0.7 and the average activation energy obtained for this range of a values is 232.43 kJ/mol.