N.H. Vasoya

Structural and elastic properties of Ca-substituted LaMnO3at 300 K

The compositional dependence of structural and elastic properties of Ca2+ substituted manganite perovskite system with a general formula La1−xCaxMnO3 (x = 0.00, 0.25, 0.33, 0.50 and 1.00) have been studied by means of x-ray diffraction, scanning electron microscopy and ultrasonic pulse transmission technique at 300 K. The elastic moduli of the specimens have also been computed and these are corrected to the void free state. The lattice parameters calculated theoretically are in agreement with those determined experimentally. The structural–elastic properties correlations have been studied. The observed enhancement of elastic constants up to x = 0.33 and reduction with a further increase in calcium concentration has been explained in the light of change in the strength of interatomic bonding caused by change in interatomic distances, microstructure and electronic configuration. The applicability of the heterogeneous metal mixture rule for theoretical estimation of elastic constants has been tested.

Raman and Mossbauer Spectroscopy and X-ray Diffractometry Studies on Quenched Copper–Ferri–Aluminates

Four spinel ferrite compositions of the CuAl(x)Fe(2-x)O4, x = 0.0, 0.2, 0.4, 0.6, system prepared by usual double-sintering ceramic route and quenched (rapid thermal cooling) from final sintering temperature (1373 K) to liquid nitrogen temperature (80 K) were investigated by employing X-ray powder diffractometry, (57)Fe Mossbauer spectroscopy, and micro-Raman spectroscopy at 300 K. The Raman spectra collected in the wavenumber range of 100-1000 cm(-1) were analyzed in a systematic manner and showed five predicted modes for the spinel structure and splitting of A1g Raman mode into two/three energy values, attributed to peaks belonging to each ion (Cu(2+), Fe(3+), and Al(3+)) in the tetrahedral positions. The suppression of lower-frequency peaks was explained on the basis of weakening in magnetic coupling and reduction in ferrimagnetic behavior as well as increase in stress induced by square bond formation on Al(3+) substitution. The enhancement in intensity, random variation of line width, and blue shift for highest frequency peak corresponding to A1g mode were observed. The ferric ion (Fe(3+)) concentration for different compositions determined from Raman spectral analysis agrees well with that deduced by means of X-ray diffraction line-intensity calculations and Mossbauer spectral analysis. An attempt was made to determine elastic and thermodynamic properties from Raman spectral analysis and elastic constants from cation distribution.

Photocatalytic Degradation of Aqueous Nitrobenzene Solution Using Nanocrystalline Mg-Mn Ferrites

MgxMn1-xFe2O4 (x = 0.0, 0.2, 0.4, 0.5, 0.6 0.8 and 1.0) spinel ferrite system was synthesized by the chemical co-precipitation route. Subsequent characterization of synthesized Mg-Mn ferrites was carried out by X-ray powder diffraction and transmission electron microscopy to study the structural and textural properties of photocatalysts. Porosity, surface area and equivalent surface free energy of different Mg-Mn ferrite photocatalysts were calculated. The photocatalytic activity of synthesized photocatalysts was evaluated by degradation of nitrobenzene in aqueous medium under ultraviolet light irradiation. The results demonstrated that the percentage degradation of nitrobenzene was decreased with increase in Mg concentration (x) from x = 0.0 0.5 and further increase in concentration from x = 0.6 1.0 results increase in percentage degradation of NB. This dissimilarity in the percentage degradation of NB may be due to the change in grain morphology, optical energy band gap, role played by d-electrons and porosity as a function of Mg-substitution for Mn2+ in the system. The percentage degradation was further confirmed by chemical oxygen demand (COD) analysis.

High temperature thermoelectric power study on calcium substituted lanthanum manganites

Compositional and temperature dependent thermoelectric power measurements have been performed on the polycrystalline manganite perovskite system: La1?xCaxMnO3+? (x = 0.00, 0.25, 0.33, 0.50 and 1.00), in the paramagnetic state (T > Tc) (300?673?K). The Seebeck coefficient (?) has been used to calculate the Fermi energy, the activation energy (E?), the Mn4+-ion concentration and the oxygen content (?), for different compositions. The observed decrease in E? with increase in Ca-concentration and a large value of E? as compared with a relatively high temperature sintered material of the same compositions have been explained in the light of the decrease in the interatomic distances and grain growth with Ca-substitution. It is shown that [Mn4+][Mn3+] ion concentration is not the only factor that governs conduction in the manganite perovskites. The significantly small values of ? (?22??V?K?1) at high temperature can be explained on the basis of (Mn3+?Mn3+) charge disproportionation into stable Mn2+?Mn4+pairs in the system rather than on the basis of nominal Mn3+/4+ valence arguments. Finally, nearly temperature independent variation of ?, charge-carrier concentration at high temperature and low value of mobility are hallmarks of small polaron hopping.

Crystal Defects and Cation Redistribution Study on Nanocrystalline Cobalt-Ferri-Chromites by Positron Annihilation Spectroscopy

Positron lifetime and Doppler broadening measurements were carried out on nanocrystalline (grain size ~60–65 nm) samples of the Cr3

Master curve generation and modeling of ac conductivity for Mn0.7+xZn0.3SixFe2–2xO4 spinel ferrite system

The compositional dependence of ac conductivity (σac), real (σ′) and imaginary (σ′′) parts of complex electric conductivity (σ*) was investigated as a function of temperature (T) and frequency (f) for Mn0.7+xZn0.3SixFe2–2xO4, x=0.0, 0.1, 0.2 and 0.3 spinel ferrite system. The compositional dependence of lattice constant values suggested that the most of the substituted Si4+-ions reside at grain boundaries and only a few Si-ions are inside grains. The variation of σac(x, f, T) is explained on the basis of segregation and diffusion of Si4+ ions at grain boundaries and grains, respectively, and the electrode effect. Thermal variation of ac conductivity at fixed frequency suggested two different mechanisms which could be responsible for conduction in the system. It is found that σ* is not the preferred presentation for dielectric data and the scaling process of real part of conductivity by normalized frequency and the scaled frequency were found unsuccessful. The fitting results of ac conductivity data with p...

Observation of bimodality in nanocrystalline cobalt - ferri - chromites

In this communication we present detail analysis of particle size distribution curves, differential size distribution and cumulative undersize distribution, recorded for nano particles of spinel ferrite system, CoCrxFe2-xO4 (x = 0.0, 1.1 and 2.0), synthesized by chemical co-precipitation technique. It is found that the distribution is bimodal as well as not mono disperse. Observed bimodality has been explained in the light of Ostwald ripening. Various parameters have been determined and the role of Cr3+ - substitution in governing signature of distribution patterns has been discussed.

Improvement in elastic properties of CuAl0.4Fe1.6O4 spinel ferrite by rapid thermal cooling

The elastic properties of spinel ferrite composition, CuAl{sub 0.4}Fe{sub 1.6}O{sub 4}, quenched from final sintering temperature of 1373 K to liquid nitrogen temperature (∼ 80K) have been studied by means of X-ray powder diffractometry and pulse echo-overlap technique (9 MHz) at 300 K. The magnitude of elastic constants is found to enhance by 15% compared to slowly-cooled counterpart. The observed mechanical strengthening has been discussed in the light of compressive stress on the surface, with tensile stresses at interior regions and corresponding changes in structural parameters. The B{sub o}/G{sub o} ratio indicates the brittle nature of CuAl{sub 0.4}Fe{sub 1.6}O{sub 4}.

Improvement in elastic properties of CuAl0.4Fe 1.6O4 spinel ferrite by rapid thermal cooling

The elastic properties of spinel ferrite composition, CuAl{sub 0.4}Fe{sub 1.6}O{sub 4}, quenched from final sintering temperature of 1373 K to liquid nitrogen temperature (∼ 80K) have been studied by means of X-ray powder diffractometry and pulse echo-overlap technique (9 MHz) at 300 K. The magnitude of elastic constants is found to enhance by 15% compared to slowly-cooled counterpart. The observed mechanical strengthening has been discussed in the light of compressive stress on the surface, with tensile stresses at interior regions and corresponding changes in structural parameters. The B{sub o}/G{sub o} ratio indicates the brittle nature of CuAl{sub 0.4}Fe{sub 1.6}O{sub 4}.