Effect of calcination temperature on Co(II) oxalate dihydrate-iron(II) oxalate dihydrate mixture DTA-TG, XRD, Mossbauer, FT-IR and SEM studies (Part II)

Effect of calcination temperature on the physico-morphological properties of CoC2O4.2H(2)O-FeC2O4-2H(2)O (1:2 mole ratio) mixture has been investigated using DTA-TG, XRD, Mossbauer effect (ME) spectroscopy, FT-IR and SEM measurements. DTA-TG results showed that the decomposition proceeds via four well-defined steps. The first is attributed to dehydration and formation of anhydrous mixture which decomposed in the second step to give CoC2O4-Fe2O3 mixture. In the third step, CoC2O4 decomposed and Co3O4-Fe2O3 mixture is formed. This mixture is thermally stable up to 920degreesC, then CO3O4 reduced to CoO, which reacts directly with alpha-Fe2O3 to form CoFe2O4, in the fourth step. ME spectra demonstrated that part of alpha-Fe2O3 is formed in a superparamagnetic doublet state in the early stages of decomposition. As the temperature increased to 800 degreesC, the crystallites are grown and the superparamagnetism disappeared. At 1000degreesC, a ferrimagnetic ordering ME spectrum fitted into two Zeeman sextets due to Fe3+ at the two distinct sites of CoFe2O4 inverse spinel has been obtained. XRD patterns of samples calcined at 250 and 280degreesC revealed none of XRD lines characteristic of alpha-Fe2O3, while the pattern of that fired at 1000degreesC confirmed the presence of single-phase cubic spinel structure with no evidence of impurities. FT-IR and SEM studies are consistent with the previous results. Kinetic analysis of the dehydration and the oxalate decomposition reactions was performed under non-isothermal conditions using different integral methods of analysis. The dynamic TG curves showed a sigmoid shape and obeyed the Avrami-Erofeev equation characteristic of the solid state nucleation-growth mechanism. The activation parameters were calculated and discussed. (C) 2003 Elsevier Science B.V. All rights reserved.