الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 206 |  |  |
| | | from | 206 | | |
Abstract
SUMMARY
In the present thesis we have used DTA-TG, XRD and the Mossbaucr spectroscopy techniques to study the thermal decomposition behaviour of pure Cu00, Mg(II) and Fe(II) oxalates, and the oxalate mixtures of CuC204.1/21420 - FeC204.2H20 and MgC204.2H20 - FeC204.2H20. The kinetics of some of the decomposition steps were studied in view of various solid state reaction models and computational methods .In the first part of the present study, pure Cali), Mg(lI) and Fe(II) oxalates were prepared using A.R salts by the co-preciptation method and the prepared oxalates were calcined at different calcination temperatures for different duration of times.
DTA-TG studies showed that CuC204.1/21120 loses weight in one step directly to oxide ; CuO. The dehydration isn’t seperated from the decomposition and the two stages appear as one decomposition step. The decomposition starts at 90t, reaching a constant weight loss of 49.5% at 310t with the formation of CuO. The DTA curve shows a broad exothermic peak in the temperature range 270-310*C which closely corresponds to the weight change observed on the TG curve. MgC204.2H20 loses weight in two steps. The first step is the dehydration step, begins at 155t and characterized by an endothermic peak in the temperature range 155-190’C and reaching a constant weight loss of 23.5% at 195t. The anhydrous mgc204 is thermally stable up
153
to 370’C. In the second step, the anhydrous MgC204 decomposed exothermically to MgO. This step indicates a weight loss of 48.5% at 510.0 and it is interrupted in the DTA curve by a broad exothermic peak in the temperature range 425-495t due to air oxidation of CO to CO2. FeC204.2H20 decomposed to Fe2O3 in two steps, the first is the dehydration step, begnis at 140t, indicating a weight loss of 20% due to loss of two molecules of water at 185t. This step showed two endothermic peaks in the DTA curve in the temperature range 140-180t, which may indicate that the loss of the two water molecules occurs in two steps. The second decomposition step is exothermic and starts as soon as the first step is complete, indicating a weight loss of 34.5% at about 270t due to decomposition of anhydrous FeC2O4 to Fe2O3. This was accompanied by a very broad exothermic DTA peak in temperature range 185-250t corresponding to air oxidation of CO to CO2.
The X-ray diffraction patterns for pure Cu(II), Mg(II) and Fe(II) oxalates were obtained at different calcination temperatures and the results were compared with ASTM cards. The results obtained are being comparable with those obtained from DTA-TG studies.
The kinetics of the thermal decomposition reaction of Cu(II), Fe(II) and Mg(II) oxalates in air were studied using non-isothermal thermogravimetric technique and a critical comparison was made of three integral methods
154
includes Coats-Redfern, Ozawa and Diefallah’s composite method. The kinetic analysis was performed with reference to the different models of heterogeneous solid-state reactions and the activation parameters (E and log A) were calculated by computer program. The results show that the
decomposition reactions of the pure oxalates are best described by phase boundary (R2 and R3), the first order (F1) and random nucleation (A2) models,
whereas other models gave less satisfactory fit to the experimental data and
(E1) model gave the least fit. The kinetics of decomposition reaction of Cu(II)
II I II
oxalate was tiegettbed agguilittig 112J model whereas the second step decomposition of Fe(II) oxalate and the two decomposition steps of Mg(II) oxalate was described according to (R3) model. Applying these models in each system to Coats-Redfern and Ozawa methods, the activation parameters were calculated by (LR) analysis of data. The results showed that both composite methods of analysis gave identical values for the activation parameters, since this method involves a complete analysis of all non-isothermal curves into a single curve and allows the results obtained not only at different heating rates but also with different a values to be superimposed on this one master curve. The results shows that the activation parameters calculated according to Coats-Redfern method are in agreement with those obtained from composite method, but the Ozawa method gave a less satisfactory results.
The kinetic of the isothermal dehydration and decomposition reactions
Mar4-14 ’71441 were etntlierl in air nver the te.mne.ratnre. range 410-160T in
155
case of dehydration and over the temperature range 410-430’C for the decomposition. The oilt data were analyzed by the method of (LR) analysis according to the various kinetic equations listed in Table(I-1) developed for
the different theortical models of solid-state reactions. The results show that the best fit of data is obtained for (R3) and (A2) models, whereas other models gave less satisfactory fits. The calculation of activation parameters in
Accordance with the (R) model showed better aveement between isothermal
and thin It (A;) is 11801
obtained for the dehydration reaction indicate that the water molecules are coordinatively linked.
Comparing the activation parameters obtained under dynamic conditions for the thermal decomposition of anhydrous Fe(I!) and Mg(II) oxalate assuming (R3) model, using different integral methods, showed that the activation parameters of MgC204 are higher than that of FeC2O4, this is attributed to the smaller ionic raduis of Mg(II) ion (0.65 A) than that of Fe(1I) ion (0.76 A) and based on coulombic attraction, the Mg(II) ion will have a stronger bond to oxalate than that of Fe(II) ion.
157
dehydration step, and is complete at 192’C. It is characterized by an endothermic DTA peak in the temperature range 167-190C. The oxalate decomposition steps begin as soon as the dehydration steps complete. In the first oxalate decomposition step, FeC204 was decomposed to Fe203, and it is accompanieded by an exothermic DTA peak in the temperature range
195-250C due to air oxidation of CO to CO2, and it is compete at 260C. The
anhydrous MgC204 is thermally stable up to 360.C. Above 360.C, it
decomposes to MgO. This step is complete at 485’C, and is accompanied by a
broad exothermic DTA peak in the temperature range 435-480t due to the oxidation of CO to CO2. In general the DTA-TG behaviour of the mixture corresponds closely to that of pure components.
The spinel formation reaction was investigated using X-ray diffraction patterns and Mossbauer spectroscopy for samples calcined at different calcination temperatures. The X-ray analysis gave results compared well with those obtained using DTA-TG techniques. For samples of CuC204.1/2H20 - FeC204.2H20 and MgC204.2H20 - FeC204.2H20 mixtures calcined at 250 and 300C for 5 min respectively, the X-ray diffraction patterns showed no lines, which may be due to the formation of very fine particles of ferric oxide having colloidal dimensions. The X-ray analysis showed that at the end of the decomposition of CuC204.1/2H20 - FeC204.2H20 and MgC204.2H20 - FeC204.2H20 systems, a partial formation of spinet ferrite was proceeded with MgC204.2H20 - FeC204.21120 mixture, whereas no ferrite spinel was
158
formed in case of CuC204.1/2H20 - FeC204.2H20 mixture . Samples of both mixtures calcined at higher temperatures (800’C for 2hr.) show incomplete formation of copper ferrite in case of CuC204.1/2H20 - FeC204.2H20 mixture, due to the presence of a mixture of the pure oxides, whereas with MgC204.2H20 - FeC204.2H20 mixture a complete formation of magnesium ferrite spine! was observed.
Using the Mossbauer spectoscopy for samples of CuC204.1/2H20 -FeC204.2H20 and MgC204.2H20 - FeC204.2H20 mixtures calcined at different temperatures (as those used in the XRD study), it is found that , the Mossbauer spectrum not changed by the dehydration of the mixture and gave the same MOssbauer parameters for both hydrated and dehydrated samples. For a samples of CuC204.1/2H20 - FeC204.2H20 and MgC204.2H20 -FeC204.2H20 mixtures calcined at 250C and 300C for 5 min respectively, the MOssbauer spectrum for CuC204.1/2H20 - FeC204.2H20 mixture showed no more than two lines owing to quadrupole interaction, due to the formation of very fine particles having colloidal dimensions of ferric oxide. It must be noted that this result is also in agreement with the XRD patterns obtained for the calcined mixture under the same conditions, in which the diffraction pattern showed no lines, and this demonstrating the amorphous nature of the colloidal ferric oxide formed under these conditions. For MgC204.2H20 -FeC204.2H20 mixture the MOssbauer spectrum showed the six-lines patterns associated with magnetically oriented iron(III) oxide. For samples of both
159
CuC204.1/2H20 - FeC204.21-120 and MgC204.2H20 - FeC204.2H20 mixtures calcined at 320.0 and 500C respectively the MOssbauer spectrum showed the six-lines pattern associated with magnetically oriented iron(III) oxide with some ferrite formation in case of MgC204.2H20 - FeC204.2H20 mixture. For samples of both mixtures calcined at 800.0 for 2 hrs , the MO.ssbauer
spectrum of CuC204.1/2H20 - FeC204.2H20 mixture showed the six-lines
Pattern characteristic of ferric oxide, in addition to the lines spectrum
cciffcr fuck spinet, whereas in case of MA04,214S) -
FeC204.21-120 mixture the Mossbauer spectrum showed the six-lines pattern associated with the ferrite spinel. The Mossbauer parameters were calculated and compared with that obtained in the literature.
The kinetics of the thermal decomposition reactions of CuC204.1/2H20 - FeC204.2H20 and MgC204.2H20 - FeC204.21120 systems aimed at production of ferrite spinels (CuFe204 and MgFe204) were studied using non-isothermal thermogravimetric technique and a critical comparison was made of three integral methods includes Coats-Redfern, Ozawa and Difallah’s composite method. The kinetic analysis was proceeded as previously mentioned in case of pure oxalates, and the results showed that, the decomposition of oxalate mixtures were best described by phase boundary (R2 and R3), the first order (Fi) and Random nucleation (A2, A3 and A4) models, whereas other models gave less satisfactory fit to the experimental data. The kinetic of the thermal decompsition reaction of CuC204.1/21120 -
160
FeC204.2H20 mixture was described assuming (R2) model, whereas in case of MgC204.2H20 - FeC204.2H20 mixture, all steps were described assuming (R3) model except for the second dehydration step which was described
assuming (A3) model as this model gave best fit to the experimental data than if the R3 model. The results shows that there is a good agreements between values of activation parameters calculated according to composite method with
those of Coats-Rfdfern and Ozawa methods, except in case of last
hompsitioo TIiP l0 6olh rigs win GIN method pi lgss
satisfactory results compared with the other two methods.
The activation parameters accompanied the dehydration of the mixed oxalates under investigation , indicate that the water molecules are
coordinatively linked.
On comparing the results obtained for activation parameters calculated according to R2 model for the first oxalate decomposition step in CuC204.1/2H20 - FeC204.21-120 mixture (in which only FeC204 was decomposed) with those of pure FeC204 assuming also R2 model, it is found that the activation parameters in the mixture decreased from that in case of
161
parameters than those of pure CuC204 decomposition assuming R2 model, this may be due to the ferric ion seems not to be operating in the electron transfer mechanism.
On comparing the activation energies obtained under different dynamic conditions for the oxalates decomposition steps in MgC204.2H20 - FeC204.2H20 mixture, it was found that the activation energies obtained for the second oxalate decomposition step (in which MgC204 decomposed) arc higher than those of the first oxalate decomposition step (in which FeC204 decomposed), this may be attributed to smaller ionic radius of Mg(II) ions compared with that of Fe(II) ions.