S. Ramana Murthy

Electrical conductivity of NiZn ferrites

The electrical conductivity σ of polycrystalline NiZn ferrites of various compositions was investigated from room temperature to the neighbourhood of the Curie temperature. The electrical conduction in these ferrites is explained on the basis of the hopping mechanism. Plots of log(σT) versus 103/T are almost linear and show a transition near the Curie temperature. The activation energy in the ferrimagnetic region is in general less than that in the paramagnetic region.

Temperature and composition dependence of the elastic moduli of NiZn ferrites

Abstract The elastic moduli of four different compositions of mixed NiZn ferrites in the polycrystalline form have been measured in the temperature range 90 – 580 K using the composite oscillator method. The values of Youngs modulus and the rigidity modulus are corrected to theoretical density using the method of Mackenzie. The data obtained on the mixed ferrites are found to be in reasonable agreement with Van Der Burgts values. It is observed that the longitudinal and shear velocities vary linearly with density as these mixed ferrites have nearly identical mean atomic weights. As in the cases of simple ferrites, the variation of Youngs modulus Y with temperature T (in kelvins) follows Wachtmans equation Y= y 00 − BTe −T 0 T where y00, b and T0 are constants. The rigidity modulus n is found to vary linearly with temperature T according to the equation n = n0 − bT where n0 and B are constants.

Dielectric behaviour of cobalt-zinc ferrites

Variation de la constante dielectrique et de tgδ avec la composition de Co x Zn 1−x Fe 2 O 4 (avec x=0 a 1)

Electrical conductivity of CoZn ferrites

Abstract The electrical conductivity of mixed CoZn ferrites was studied as a function of composition and temperature. The existence of the single-phase spinel structure was established by X-ray analysis of the specimens. The Seebeck coefficient for these ferrites was determined in order to discuss the conductivity which is explained in terms of the hopping mechanism. Plots of the logarithm of conductivity versus the reciprocal temperature are almost linear and show a transition near the Curie temperature. The activation energy in the paramagnetic region is higher than that in the ferrimagnetic region.

Dehydration of natural stilbite: An in situ FTIR study

Abstract The dehydration behavior of a natural stilbite sample from Poona (India) was investigated by in situ FTIR. The thermal induced variations of the water molecule bending (ν2) mode around 1653 cm-1, the stretching (ν3 and ν1) modes around 3587 and 3426 cm-1, and the corresponding second-order modes in the wavenumber region 4000.8000 cm-1 were followed as indicative of the dehydration process. The observed spectral variations indicate that stilbite undergoes a transformation at about 448 K due to the loss of half of the original content of water molecules. The rehydration of stilbite is partial in samples heated up to 630 K. Concerning both the dehydration and rehydration behaviors of stilbite, our results are in concert with those proposed in the literature. In addition, the growth of a new mode around 4550 cm-1 is observed in the temperature range 430-650 K and may indicate the presence of hydroxyl groups created by the breaking of the T-O-T linkages.

Effect of magnetic field and temperature on the elastic behaviour of cobalt-zinc ferrites

Abstract A study of the dependence of the elastic behaviour of polycrystalline mixed cobalt-zinc ferrites on temperature and magnetic field has been made using a composite oscillator method. As the temperature is increased the Youngs modulus attains a minimum at a temperature below the Curie point. Thereafter it exhibits a positive temperature coefficient up to the Curie point and decreases with further increase of temperature. This anomalous behaviour has been explained in terms of magnetic anisotropy energy. In the case of cobalt ferrite and Co 0.6 Zn 0.4 Fe 2 O 4 the Youngs modulus is found first to decrease and later to increase with increasing magnetic field, finally becoming constant at the saturation field. In contrast, in the case of Co 0.4 Zn 0.6 Fe 2 O 4 and Co 0.2 Zn 0.8 Fe 2 O 4 the Youngs modulus does not show an initial decrease but increases with increasing magnetic field. The observed ΔE effect in the case of cobalt ferrite has been interpreted in terms of two domain processes, i.e. domain rotation against uniaxial strain anisotropy and the movement of 90° boundary walls. A study of the effect of the simultaneous application of temperature and magnetic field on the elastic behaviour of mixed cobalt-zinc ferrites has also been carried out.

Elastic behaviour of MnZn ferrites

Abstract A series of Mn-Zn ferrites was prepared by a double sintering method. Samples were sintered at various temperatures to obtain different porosities. Ultrasonic velocities were determined by using a pulse transmission method at 10 MHz and the values of the elastic moduli were obtained from the measured sound velocities. The observed experimental results have been explained on the basis of microstructural changes.

Elastic behaviour of mixed manganese-magnesium ferrites

of the interatomic and interionic forces in them. Further, a detailed investigation of the dependence of elastic moduli on magnetic field or AE effect is interesting from the stand point of explaining the domain process occurring in the ferrite material. Thus, a study of the dependence of the elastic behaviour of Mn-Mg ferrites on composition and magnetic field at room temperature has been undertaken. The results are presented in this paper. Youngs modulus (E) and the rigidity modulus (g) of polycrystalline Mn-Mg ferrites were measured by using the composite oscillator [3]. Mixed Mn-Mg ferrites having the chemical formula Mnl_xMgxFe20 4 (x = 0 to 1) are prepared by the double sintering method. X-ray diffraction patterns for these mixed ferrites have been obtained using CuKa radiation. The lattice constants for ferrites were calculated and it is observed that the variation of lattice constants with composition is linear (Table I). Bulk densities of samples are determined from volume and weight measurements in air at room temperature (30 ° C) are found to be 95% of the corresponding X-ray densities in all the cases. The experimental values of the elastic moduli obtained on porous specimens at room temperature are corrected to theoretical density using the method of Mackenzie [4]. The corrected values of Youngs modulus (E) and rigidity modulus (/.t), together with calculated values of the bulk modulus (k), compressibility (fl) and Poissons ratio (o) are presented in Table I. Following Woosters work [5] the experimental data are interpreted in terms of the binding forces between the atoms. It can be observed from Table I that an addition of 0.25 tool magnesium ferrite to manganese ferrite has increased the value of the E by about 15%. Similarly an addition of 0.1 mol manganese ferrite to magnesium ferrite has resulted in an increase in E of about 5%. The value of the rigidity modulus is increased by about 15% and 9%, respectively, Further addition of magnesium ferrite to manganese ferrite has also resulted in an increase of the values of E and g. This shows that an addition of a small quantity of magnesium ferrite to manganese ferrite or manganese ferrite to magnesium ferrite has resulted in an increase in the binding forces between the atoms. It can also be observed from the table that the values of k and tr are smaller for mixed magnesium-manganese ferrites of various compositions than for magnesium ferrite.

Ultrasonic studies of zeolites during dehydration

Abstract The synthetic zeolites of the heulandite group were prepared using the microwave hydrothermal method at 120°C for 30 min. The powders were characterised using X‐ray diffraction. The zeolites were annealed at 60, 80 and 100°C for 1 h. Dehydration studies were conducted on prepared zelolite in the temperature range 300–800 K using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). A conventional ultrasonic pulse transmission technique was used to measure the longitudinal ultrasonic velocity and attenuation at 1 MHz in the temperature range 300–500 K. The accuracy of ultrasonic velocities and attenuation measurements was 0·01 and 0·2% respectively. The observed ultrasonic velocity changes were compared with FTIR, DSC and TGA studies, and it was found that the one can study dehydration studies in zeolites by carrying out ultrasonic studies.

In-situ FTIR study of dehydration of natural Fe-stilbite

Thermally induced dehydration behavior of a naturally occurring iron bearing stilbite around Poona region, India has been studied by comparing with the one depleted in iron. In-situ FTIR spectroscopy in the wavenumber range 4000–10,000 cm −1 has been used to monitor the dehydration behavior in the temperature range 300–850 K. The thermal induced variations in the integrated peak areas of water modes indicate two step dehydration behaviors. During dehydration (in phase B) process growth of Si-OH bonds (4550 cm −1 ) formed by virtue of the breakage of T-O-T linkages is observed. A peak around 8650 cm −1 indicates the presence of iron ion in stilbites as Fe 2+ . Further, abrupt disappearance of this mode indicates Fe 2+ ion in extra framework sites. The presence of iron in stilbites increased the formation of Si-OH bonds in phase B and grossly did not effect the dehydration behavior of stilbites.