R.G. Kulkarni

PHOTOCATALYTIC DEGRADATION OF DYES AND ORGANIC CONTAMINANTS IN WATER USING NANOCRYSTALLINE ANATASE AND RUTILE TIO2

Abstract Nanocrystalline TiO2 was synthesized by controlled hydrolysis of titanium tetraisopropoxide. The anatase phase was converted to rutile phase by thermal treatment at 1023 K for 11 h. The catalysts were characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), Fourier-transform infrared absorption spectrophotometry (FT-IR) and N2 adsorption (BET) at 77 K. This study compare the photocatalytic activity of the anatase and rutile phases of nanocrystalline TiO2 for the degradation of acetophenone, nitrobenzene, methylene blue and malachite green present in aqueous solutions. The initial rate of degradation was calculated to compare the photocatalytic activity of anatase and rutile nanocrystalline TiO2 for the degradation of different substances under ultraviolet light irradiation. The higher photocatalytic activity was obtained in anatase phase TiO2 for the degradation of all substances as compared with rutile phase. It is concluded that the higher photocatalytic activity in anatase TiO2 is due to parameters like band-gap, number of hydroxyl groups, surface area and porosity of the catalyst.

Magnetic and electrical properties of Al3+-substituted MgFe2O4

The structural, magnetic and electrical properties of the Al3+-substituted disordered spinel system Mg(Fe2−xAlx)O4 have been investigated by X-ray diffraction, magnetization, a.c. susceptibility and electrical resistivity measurements. The cation distribution derived from the X-ray diffractometry data was found to agree very well with the cation distribution obtained through Mössbauer spectroscopy. The variation of saturation magnetization per formula unit as a function of aluminium context, x, has been satisfactorily explained on the basis of Neels collinear spin model and the slight discrepancy between the observed and calculated nB values can be explained in terms of a random canting model. The Néel temperatures calculated theoretically by applying molecular field theory agreed well with the experimentally determined values from thermal variation of susceptibility and electrical resistivity. An unusual metal-like thermo-electric behaviour was found for the compositions with x⩾ 0.3 which was attributed to the decrease in the Fe-Fe separation distance arising from aluminium substitution.

Structural, magnetic and magnetotransport properties of the La0.67Ca0.33Mn0.9Fe0.1O3 perovskite

Abstract Magnetization, AC susceptibility, Mossbauer spectroscopy, neutron diffraction (ND) and magnetoresistance measurements have been carried out on a polycrystalline perovskite, La0.67Ca0.33Mn0.9Fe0.1O3. Considerable changes occur in magnetic, transport and magnetoresistance properties with respect to the classic composition without Fe but no spin-glass like behavior is observed. ND data confirms that the compound crystallizes in the orthorhombic perovskite structure (space group Pbnm). The Mossbauer spectrum reveals that Fe substitutes for Mn as Fe3+ (S=5/2) and is antiferromagnetically coupled to the Mn host lattice. The substitution of Mn3+ by Fe3+ reduces the number of available hopping sites for the Mn eg(↑) electron and suppresses the double exchange (DE), resulting in the reduction of ferromagnetic exchange as evidenced by a decrease of about 125xa0K in the value of ferromagnetic ordering temperature (TC) and of about 183xa0K in the value of metal-to-insulator transition temperature (Tp) without Fe. This compound presents 98% of giant magnetoresistance ratio in the temperature range of 50–80xa0K at 40xa0kOe applied magnetic field. The competition between the ferromagnetic DE interactions and the coexisting anti-ferromagnetic super exchange interactions with the introduction of Fe3+ for Mn3+ drive the system supposedly into a random canted ferromagnetic state at low temperatures.

Magnetic properties of the mixed spinel CoFe2−xCrxO4

Abstract The crystal and magnetic properties of the Cr-substituted disordered spinel system CoFe2−xCrxO4 (x = 0–1.6) are investigated by means of X-ray, magnetization, Mossbauer effect and AC susceptibility measurements. Variations of magnetic moment per formula unit measured at 80 K with Cr-content obtained from magnetization and Mossbauer data for x = 0–0.6 display a discrepancy between them, thus exhibiting a significant canting on the B-site which is satisfactorily explained on the basis of the non-collinear spin ordering model of random canting. Magnetization and Mossbauer results also show a sudden change near x > 0.6 indicating a change in magnetic structure from non-collinear to possibly collinear for x = 0.8–1.6. The AC susceptibility indicates a paramagnetic → unstable ferrimagnetic → cluster spin glass-type transition as the temperature is lowered for x = 0–1.2. The further increase in x from x > 1.2 increases frustration and disorder in the system supressing the ferrimagnetic ordering and the system approaches a cluster spin glass-type ordering at x = 1.4–1.6 as reflected in AC susceptibility and Mossbauer spectra.

Magnetic and electrical properties of aluminium and chromium co-substituted yttrium iron garnets

Abstract Polycrystalline yttrium aluminium-chromium iron garnets (Y 3 Al x Cr x Fe 5 − 2 x O 12 ) with varying Al-Cr substitution (0 ≤ x ≤ 1.0) have been prepared in the pellet form, and studied by X-ray diffraction, magnetization, a.c. susceptibility and electrical resistivity measurements. The lattice constants are determined and the applicability of Vegards law has been tested. The saturation magnetization (4π M s ) decreases linearly with increasing x from x = 0.1 to 1.0 indicating reduction in ferrimagnetic behaviour. Variation of saturation magnetic moment per formula unit at 300 K with x can be explained satisfactorily assuming the collinear spin ordering model. The Curie temperature ( T C ) decreases with increasing x which is consistent with the observed decrease in 4π M s with x . The activation energy ( E ) initially increases up to x = 0.6 and thereafter it levels off for further increase in x > 0.6.

Bulk magnetic properties of Co-Zn ferrites prepared by the co-precipitation method

The alternating current (a.c.) susceptibility versus temperature and magnetization measurements are reported for the disordered spinel ferrite system Znx Co1-x Fe2 O4 prepared by a wet chemical method before and after high temperature annealing. The low field a.c. susceptibility measurements indicate that the low temperature synthesis of wet prepared Co-Zn ferrites aids the formation of spin-clusters and thereby increases the magnetic inhomogeneity. The X-ray analysis shows that the samples are single phase spinels and the variation of lattice constant with zinc concentration deviates from Vegards law [1]. The high temperature annealing changes the wet prepared ferrites into the ordered magnetic structure of the ceramic ferrites.

The magnetic properties of the MgZn ferrite system by Mössbauer spectroscopy

Abstract Samples of the magnetism-zinc ferrite series ZnxMg1−xFe2O4 (x = 0.0 to 1.0) have been studied by the Mossbauer effect technique at 77 K. Mossbauer spectra for x = 0.0 to 0.6 suggest the existence of two hyperfine fields, one due to the Fe3+ tetrahedral ions (A-sites) and the other due to the Fe3+ octachedral ions (B-sites), while for x=0.7 it shows relaxation behaviour and for x⩾0.8 it exhibits a paramagnetic quadrupole doublet. The variation of nuclear magnetic fields at the A and B sites is explained on the basis of the Aue5f8B and Bue5f8B supertransferred hyperfine interactions. Analysis of the average Mossbauer line width as a function of zinc concentration suggests that the relaxation spectrum observed at x=0.7 (77 K) is possibly due to domain wall oscillations.

Mössbauer and magnetization study of the mixed spinel CoFe2−xCrxO4

Abstract Mossbauer and magnetization measurements are performed at 300 K on the mixed spinel CoFe2−xCrxO4 with a selective Fe-dilution of B-sublattice on increasing Cr concentration X from 0 to 1. Magnetization results for X = 0 to 1 exhibit a collinear ferrimagnetic structure, while Mossbauer results indicate that on increasing Fe-dilution of the B-sublattice, the collinear ferrimagnetic phase breaks down at X > 0.8 before reaching the ferrimagnetic percolation limit as a result of the presence of competing exchange interactions. These results are explained in terms of Villains model.

Magnetic properties of copper ferrite aluminates

Abstract The Al-substituted disordered spinel series CuFe 2- x Al x O 4 ( x = 0.0−1.6) has been synthesized and studied by X-ray diffraction, magnetization, ac susceptibility and Mossbauer effect measurements. The lattice constants are determined and the applicability of Vegards law has been tested. The variation of the saturation magnetic moment per formula unit measured at 77 and 300 K with the Al content is satisfactorily explained on the basis of Neels collinear spin ordering model for x = 0.0−1.0. The Mossbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe 3+ at the tetrahedral (A) and octahedral (B) sites for x ≤ 0.8. Mossbauer results confirm a collinear ferrimagnetic structure for x = 0.0−0.6 and suggest non-collinear behaviour for x ≥ 0.8. The Curie temperature decreases nearly linear with increase of Al concentration from x = 0.0 to 1.2.

Mössbauer study of the spinel system GexCu1−xFe2O4☆

Abstract The magnetic properties of the spinel series Ge x Cu 1− x Fe 2 O 4 ( X = 0 to 0.8) have been investigated by means of Mossbauer spectroscopy. Mossbauer spectra for X = 0.0 to 0.6 suggest the existence of two hyperfine fields, one due to the Fe 3+ tetrahedral ions ( A -sites) and the other due to Fe 3+ octahedral ions ( B -sites), while for X = 0.8 it shows the superposition of hyperfine field split spectra from A - and B -site ions and a broad central line spectrum. For 0.2 ⩽ X ⩽ 0.4, fast electron exchange among octahedral iron ions occurs as in Fe 3 O 4 . The variations of nuclear magnetic fields at the A - and B -sites are explained on the basis of A ue5f8 B and B ue5f8 B supertransferred hyperfine interactions.