Comparative studies have been made to understand the role of different crystallographic site pref... more Comparative studies have been made to understand the role of different crystallographic site preferences of the substituted non-magnetic divalent metal ions in the magnetostrictive properties of cobalt ferrite, by substitution of Zn2+ and/or Mg2+ for Fe3+ in CoMgxFe2-xO4, CoZnxFe2-xO4, and CoMgx/2Znx/2Fe2-xO4 (0.0 < x < 0.2). Detailed Raman spectral and magnetic characterizations are made to extract the information on the tetrahedral/octahedral site preferences of Zn and Mg in the spinel lattice of cobalt ferrite. The structural, microstructural, magnetic, Raman spectral, and magnetostrictive parameters of the studied compositions show distinguishable variations for x< 0.1 and x> 0.1. Co-substitution of a small amount of Mg and Zn for Fe in CoMgx/2Znx/2Fe2-xO4 (x < 0.1) showed relatively larger strain sensitivity, [dλ/dH]max (-2.6 x 10-9mA-1 for x = 0.05), higher than that for the Mg-substituted samples (-2.05 x 10-9mA-1 for x = 0.05) and comparable to that for the Zn-substituted samples (-2.47 x 10-9mA-1 for x= 0.05), without much drop in the maximum value of magnetostriction, kmax (-189 ppm for x = 0.05) compared to that for the unsubstituted counterpart (-221 ppm). The results show that it is possible to obtain high strain sensitivity (at fields <50 kA/m), along with high magnetostriction strain at low magnetic fields (~250 kA/m), by tuning the distribution of the substituted cations in the tetrahedral and octahedral sites of the cobalt ferrite lattice.
The influence of size and crystallographic site preference of three non-magnetic isovalent metal ... more The influence of size and crystallographic site preference of three non-magnetic isovalent metal ions of larger (In3+), comparable (Ga3+) and smaller (Al3+) sizes, substituted for Fe3+ in the spinel lattice of CoFe2O4 on its magnetostrictive properties is compared. For the different compositions in CoFe2−xMxO4 (M = In3+, Ga3+, Al3+ and 0 ⩽ x ⩽ 0.3), significant changes in the structural and magnetic parameters are observed with the degree of substitution, due to the size and site preferences. Magnetic and Raman spectral studies revealed that Al3+ is substituted for Fe3+ at both octahedral and tetrahedral sites for all compositions, whereas In3+ and Ga3+ are substituted for Fe3+ at the tetrahedral site only for x ⩽ 0.2 and partly at the octahedral site for x > 0.2. Regardless of the differences in the ionic size, site preference and the magnetic properties, compositions in all three series with x = 0.1 showed almost equal magnitude of maximum magnetostriction (λmax = ~230 ppm), marginally higher than that of x = 0 (217 ppm). However, at higher substituted compositions, λmax is decreased with x, but the decrease is much faster for the Al-substituted compositions. The maximum strain sensitivity, [dλ/dH]max, is also found to be comparable for all three compositions. The comparable magnetostriction characteristics and high strain at low magnetic fields for different substituted compositions at low levels of substitution are attributed to the local structural distortions associated with the inhomogeneous distribution of the substituted ions in the spinel ferrite lattice. The studies suggest ways to optimise the magnetostriction properties of properly substituted sintered cobalt ferrite for applications in sensors and actuators.
To study the role of unsaturation in the surfactant molecule on the thermal conductivity of magne... more To study the role of unsaturation in the surfactant molecule on the thermal conductivity of magnetite nanofluids, four different fatty acid (stearic, oleic, linoleic, and linolenic acids with different degree of unsaturation) coated magnetite nanoparticles of comparable size are prepared and dispersed in toluene. It is found that the nanofluid with the saturated fatty acid coated nanoparticles show larger viscosity than the fluid with the unsaturated fatty acid coated particles at all concentrations. Thermal conductivity studies show enhancement only above a critical concentration for all fluids. The critical concentration for thermal conductivity enhancement varies with the surfactant, possibly due to the difference in the degree of aggregation of the nanoparticles in the fluid, because of the difference in the conformation of the surfactant molecules on the nanoparticle’s surface. The experimental thermal conductivity follows the Maxwell model at higher concentrations. From the overall studies, it is observed that the thermal conductivity of the fluids with aggregated or assembled nanoparticles shows slightly larger enhancement than that of the fluids with isolated particles. However, in the presence of a magnetic field, the fluids with isolated nanoparticles showed relatively larger enhancement, possibly due to the easy response of the isolated magnetite nanoparticles to the applied field.
Thermal conductivity enhancement of nanofluids of oleic acid coated magnetite nanoparticles dispe... more Thermal conductivity enhancement of nanofluids of oleic acid coated magnetite nanoparticles dispersed in four different base fluids (toluene, xylene, mesitylene, kerosene) is studied to understand the role of the solvent (base fluid). From the correlation of the thermophysical properties of the base fluid with the thermal conductivity of the corresponding nanofluid, it is found that the nanofluid with the base fluid of lower intrinsic thermal conductivity and dielectric constant shows relatively larger enhancement in the thermal conductivity. A linear increase in the thermal conductivity with increasing viscosity is observed for all four nanofluids studied. The concentration dependent thermal conductivity studies showed enhancement only above a particular concentration, within the sensitivity of the measurement, and this critical concentration is different for the different nanofluids. The nanofluid with kerosene showed the lowest critical concentration for thermal conductivity enhancement compared to the other nanofluids. The difference between the experimental thermal conductivity and the calculated value using the Maxwell model is found to depend on the critical concentration. By assuming the critical concentration as the zero concentration, it is found that all the studied nanofluids almost follow the Maxwell model of thermal conductivity. Thus, for the dispersions of the same oleic acid coated magnetite nanoparticles, the base fluid affects the critical concentration for thermal conductivity enhancement, probably due to the interfacial effects arising from the surfactant-solvent interactions.
Page 1. The origin of ferromagnetism in the two different phases of LaMn0.5Co0.5O3: evidence from... more Page 1. The origin of ferromagnetism in the two different phases of LaMn0.5Co0.5O3: evidence from x-ray photoelectron spectroscopic studies This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2001 J. Phys.: Condens. ...
Cobalt-ferrite (CoFe2O4) based materials are suitable candidates for magnetomechanical sensor app... more Cobalt-ferrite (CoFe2O4) based materials are suitable candidates for magnetomechanical sensor applications owing to a strong sensitivity of their magnetostriction to an applied magnetic field. Zn-doped cobalt-ferrites, with nominal compositions CoFe2−xZnxO4 (x = 0–0.3), were synthesized by auto-combustion technique using Co- , Fe- , and Zn-nitrate as precursors. X-ray spectra analysis and Transmission electron microscopy studies revealed that the as-prepared powders were comprised of nano-crystalline (~25–30 nm) cubic-spinel phase with irregularly-shaped grains morphology along with minor impurity phases. Calcination (800 °C for 3 h) of the precursor followed by sintering (1300 °C for 12 h) resulted in a single phase cubic-spinel structure with average grain size ~2–4 μm, as revealed from scanning electron micrographs. The magnitude of coercive field decreases from ~540 Oe for x = 0 to 105 Oe for x = 0.30. Saturation magnetization initially increases and peaks to ~87 emu/g for x = 0...
ABSTRACT Tetrabutylammonium salt of [PW12O40](3-) and its transition metal (Mn2+, Fe2+, Co2+ and ... more ABSTRACT Tetrabutylammonium salt of [PW12O40](3-) and its transition metal (Mn2+, Fe2+, Co2+ and Cu2+) substituted compounds are characterized by elemental analysis, thermogravimetric and spectroscopic techniques. Thermal stability of these compounds decreases after metal ion substitution, but the cobalt substituted compound is relatively more stable. The cobalt substituted compound is found to be a good catalyst for the oxidation of cyclohexanol and the catalytic activity of the transition metal substituted compounds decreases in the order: Co2+ much greater than Mn2+ &gt; Fe2+ greater than or equal to Cu2+. Detailed studies of the reaction in the presence of Co2+ and Mn2+ substituted compounds reveal that the transition metal ions are initially oxidized to a higher oxidation state which then act as the catalytic centre for the reaction. The enhanced catalytic activity of Co2+ substituted compound is due its oxidation to Co4+ in the presence of H2O2 which is evidenced from EPR and UV-VIS spectroscopic studies.
ABSTRACT The La–Zn substituted hexagonal strontium aluminate, Sr1−xLaxAl12−xZnxO19, with the magn... more ABSTRACT The La–Zn substituted hexagonal strontium aluminate, Sr1−xLaxAl12−xZnxO19, with the magnetoplumbite structure and having five different coordination environments for Al with different symmetries, is investigated using 27Al solid-state NMR to get detailed information on the sites of substitution of Zn and the associated changes in the local coordination environments of Al. The objective of the study was to get information on the local structural variations in the isostructural La–Co substituted strontium ferrite, Sr1−xLaxFe12−xCoxO19, showing enhanced magnetic performance on substitution. The NMR studies on the aluminate give direct evidence for the sites of substitution and the changes in the local coordination environments. It is found that Zn is substituted at the 2a and 4f2AlO6 octahedral sites. However, an interesting observation from the NMR studies is the stabilization of the Al site occupancy at the penta-coordinated 2b site over the distorted tetrahedral 4eAl site, without any substitution at these sites. Large changes in the quadrupolar coupling constant of the 2a and 4e sites are observed between x = 0.2 and 0.3, corresponding to the compositional region showing higher performance in the case of Sr1−xLaxFe12−xCoxO19, indicating the role of distortion of local coordination environments on suitable substitution in controlling the performance parameters.
Rapidly growing technological interests on colossal magnetoresistive (CMR) materials have stimula... more Rapidly growing technological interests on colossal magnetoresistive (CMR) materials have stimulated scientists worldwide for a thorough investigation of the various aspects of physical properties of the rare-earth manganite system viz. R 1− x A x MnO 3 [1], [2], [3] and [4] ...
Particle & Particle Systems Characterization, 2014
ABSTRACT Core–shell bimetallic Au@Ni nanoparticles, with gold cores and thin nickel shells with o... more ABSTRACT Core–shell bimetallic Au@Ni nanoparticles, with gold cores and thin nickel shells with overall size less than 10 nm, are synthesized and stabilized in pure cubic (fcc) and hexagonal (hcp) phase. Due to their unique crystal, electronic, and geometric structure, they show interesting magnetic and chemical properties. The Au@Ni fcc is magnetic, whereas Au@Ni hcp is non-magnetic. Both the bimetallic nanostructures are stable to surface oxidation until 150 C and show excellent catalytic activity for p-nitrophenol reduction reaction.
Comparative studies have been made to understand the role of different crystallographic site pref... more Comparative studies have been made to understand the role of different crystallographic site preferences of the substituted non-magnetic divalent metal ions in the magnetostrictive properties of cobalt ferrite, by substitution of Zn2+ and/or Mg2+ for Fe3+ in CoMgxFe2-xO4, CoZnxFe2-xO4, and CoMgx/2Znx/2Fe2-xO4 (0.0 < x < 0.2). Detailed Raman spectral and magnetic characterizations are made to extract the information on the tetrahedral/octahedral site preferences of Zn and Mg in the spinel lattice of cobalt ferrite. The structural, microstructural, magnetic, Raman spectral, and magnetostrictive parameters of the studied compositions show distinguishable variations for x< 0.1 and x> 0.1. Co-substitution of a small amount of Mg and Zn for Fe in CoMgx/2Znx/2Fe2-xO4 (x < 0.1) showed relatively larger strain sensitivity, [dλ/dH]max (-2.6 x 10-9mA-1 for x = 0.05), higher than that for the Mg-substituted samples (-2.05 x 10-9mA-1 for x = 0.05) and comparable to that for the Zn-substituted samples (-2.47 x 10-9mA-1 for x= 0.05), without much drop in the maximum value of magnetostriction, kmax (-189 ppm for x = 0.05) compared to that for the unsubstituted counterpart (-221 ppm). The results show that it is possible to obtain high strain sensitivity (at fields <50 kA/m), along with high magnetostriction strain at low magnetic fields (~250 kA/m), by tuning the distribution of the substituted cations in the tetrahedral and octahedral sites of the cobalt ferrite lattice.
The influence of size and crystallographic site preference of three non-magnetic isovalent metal ... more The influence of size and crystallographic site preference of three non-magnetic isovalent metal ions of larger (In3+), comparable (Ga3+) and smaller (Al3+) sizes, substituted for Fe3+ in the spinel lattice of CoFe2O4 on its magnetostrictive properties is compared. For the different compositions in CoFe2−xMxO4 (M = In3+, Ga3+, Al3+ and 0 ⩽ x ⩽ 0.3), significant changes in the structural and magnetic parameters are observed with the degree of substitution, due to the size and site preferences. Magnetic and Raman spectral studies revealed that Al3+ is substituted for Fe3+ at both octahedral and tetrahedral sites for all compositions, whereas In3+ and Ga3+ are substituted for Fe3+ at the tetrahedral site only for x ⩽ 0.2 and partly at the octahedral site for x > 0.2. Regardless of the differences in the ionic size, site preference and the magnetic properties, compositions in all three series with x = 0.1 showed almost equal magnitude of maximum magnetostriction (λmax = ~230 ppm), marginally higher than that of x = 0 (217 ppm). However, at higher substituted compositions, λmax is decreased with x, but the decrease is much faster for the Al-substituted compositions. The maximum strain sensitivity, [dλ/dH]max, is also found to be comparable for all three compositions. The comparable magnetostriction characteristics and high strain at low magnetic fields for different substituted compositions at low levels of substitution are attributed to the local structural distortions associated with the inhomogeneous distribution of the substituted ions in the spinel ferrite lattice. The studies suggest ways to optimise the magnetostriction properties of properly substituted sintered cobalt ferrite for applications in sensors and actuators.
To study the role of unsaturation in the surfactant molecule on the thermal conductivity of magne... more To study the role of unsaturation in the surfactant molecule on the thermal conductivity of magnetite nanofluids, four different fatty acid (stearic, oleic, linoleic, and linolenic acids with different degree of unsaturation) coated magnetite nanoparticles of comparable size are prepared and dispersed in toluene. It is found that the nanofluid with the saturated fatty acid coated nanoparticles show larger viscosity than the fluid with the unsaturated fatty acid coated particles at all concentrations. Thermal conductivity studies show enhancement only above a critical concentration for all fluids. The critical concentration for thermal conductivity enhancement varies with the surfactant, possibly due to the difference in the degree of aggregation of the nanoparticles in the fluid, because of the difference in the conformation of the surfactant molecules on the nanoparticle’s surface. The experimental thermal conductivity follows the Maxwell model at higher concentrations. From the overall studies, it is observed that the thermal conductivity of the fluids with aggregated or assembled nanoparticles shows slightly larger enhancement than that of the fluids with isolated particles. However, in the presence of a magnetic field, the fluids with isolated nanoparticles showed relatively larger enhancement, possibly due to the easy response of the isolated magnetite nanoparticles to the applied field.
Thermal conductivity enhancement of nanofluids of oleic acid coated magnetite nanoparticles dispe... more Thermal conductivity enhancement of nanofluids of oleic acid coated magnetite nanoparticles dispersed in four different base fluids (toluene, xylene, mesitylene, kerosene) is studied to understand the role of the solvent (base fluid). From the correlation of the thermophysical properties of the base fluid with the thermal conductivity of the corresponding nanofluid, it is found that the nanofluid with the base fluid of lower intrinsic thermal conductivity and dielectric constant shows relatively larger enhancement in the thermal conductivity. A linear increase in the thermal conductivity with increasing viscosity is observed for all four nanofluids studied. The concentration dependent thermal conductivity studies showed enhancement only above a particular concentration, within the sensitivity of the measurement, and this critical concentration is different for the different nanofluids. The nanofluid with kerosene showed the lowest critical concentration for thermal conductivity enhancement compared to the other nanofluids. The difference between the experimental thermal conductivity and the calculated value using the Maxwell model is found to depend on the critical concentration. By assuming the critical concentration as the zero concentration, it is found that all the studied nanofluids almost follow the Maxwell model of thermal conductivity. Thus, for the dispersions of the same oleic acid coated magnetite nanoparticles, the base fluid affects the critical concentration for thermal conductivity enhancement, probably due to the interfacial effects arising from the surfactant-solvent interactions.
Page 1. The origin of ferromagnetism in the two different phases of LaMn0.5Co0.5O3: evidence from... more Page 1. The origin of ferromagnetism in the two different phases of LaMn0.5Co0.5O3: evidence from x-ray photoelectron spectroscopic studies This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2001 J. Phys.: Condens. ...
Cobalt-ferrite (CoFe2O4) based materials are suitable candidates for magnetomechanical sensor app... more Cobalt-ferrite (CoFe2O4) based materials are suitable candidates for magnetomechanical sensor applications owing to a strong sensitivity of their magnetostriction to an applied magnetic field. Zn-doped cobalt-ferrites, with nominal compositions CoFe2−xZnxO4 (x = 0–0.3), were synthesized by auto-combustion technique using Co- , Fe- , and Zn-nitrate as precursors. X-ray spectra analysis and Transmission electron microscopy studies revealed that the as-prepared powders were comprised of nano-crystalline (~25–30 nm) cubic-spinel phase with irregularly-shaped grains morphology along with minor impurity phases. Calcination (800 °C for 3 h) of the precursor followed by sintering (1300 °C for 12 h) resulted in a single phase cubic-spinel structure with average grain size ~2–4 μm, as revealed from scanning electron micrographs. The magnitude of coercive field decreases from ~540 Oe for x = 0 to 105 Oe for x = 0.30. Saturation magnetization initially increases and peaks to ~87 emu/g for x = 0...
ABSTRACT Tetrabutylammonium salt of [PW12O40](3-) and its transition metal (Mn2+, Fe2+, Co2+ and ... more ABSTRACT Tetrabutylammonium salt of [PW12O40](3-) and its transition metal (Mn2+, Fe2+, Co2+ and Cu2+) substituted compounds are characterized by elemental analysis, thermogravimetric and spectroscopic techniques. Thermal stability of these compounds decreases after metal ion substitution, but the cobalt substituted compound is relatively more stable. The cobalt substituted compound is found to be a good catalyst for the oxidation of cyclohexanol and the catalytic activity of the transition metal substituted compounds decreases in the order: Co2+ much greater than Mn2+ &gt; Fe2+ greater than or equal to Cu2+. Detailed studies of the reaction in the presence of Co2+ and Mn2+ substituted compounds reveal that the transition metal ions are initially oxidized to a higher oxidation state which then act as the catalytic centre for the reaction. The enhanced catalytic activity of Co2+ substituted compound is due its oxidation to Co4+ in the presence of H2O2 which is evidenced from EPR and UV-VIS spectroscopic studies.
ABSTRACT The La–Zn substituted hexagonal strontium aluminate, Sr1−xLaxAl12−xZnxO19, with the magn... more ABSTRACT The La–Zn substituted hexagonal strontium aluminate, Sr1−xLaxAl12−xZnxO19, with the magnetoplumbite structure and having five different coordination environments for Al with different symmetries, is investigated using 27Al solid-state NMR to get detailed information on the sites of substitution of Zn and the associated changes in the local coordination environments of Al. The objective of the study was to get information on the local structural variations in the isostructural La–Co substituted strontium ferrite, Sr1−xLaxFe12−xCoxO19, showing enhanced magnetic performance on substitution. The NMR studies on the aluminate give direct evidence for the sites of substitution and the changes in the local coordination environments. It is found that Zn is substituted at the 2a and 4f2AlO6 octahedral sites. However, an interesting observation from the NMR studies is the stabilization of the Al site occupancy at the penta-coordinated 2b site over the distorted tetrahedral 4eAl site, without any substitution at these sites. Large changes in the quadrupolar coupling constant of the 2a and 4e sites are observed between x = 0.2 and 0.3, corresponding to the compositional region showing higher performance in the case of Sr1−xLaxFe12−xCoxO19, indicating the role of distortion of local coordination environments on suitable substitution in controlling the performance parameters.
Rapidly growing technological interests on colossal magnetoresistive (CMR) materials have stimula... more Rapidly growing technological interests on colossal magnetoresistive (CMR) materials have stimulated scientists worldwide for a thorough investigation of the various aspects of physical properties of the rare-earth manganite system viz. R 1− x A x MnO 3 [1], [2], [3] and [4] ...
Particle & Particle Systems Characterization, 2014
ABSTRACT Core–shell bimetallic Au@Ni nanoparticles, with gold cores and thin nickel shells with o... more ABSTRACT Core–shell bimetallic Au@Ni nanoparticles, with gold cores and thin nickel shells with overall size less than 10 nm, are synthesized and stabilized in pure cubic (fcc) and hexagonal (hcp) phase. Due to their unique crystal, electronic, and geometric structure, they show interesting magnetic and chemical properties. The Au@Ni fcc is magnetic, whereas Au@Ni hcp is non-magnetic. Both the bimetallic nanostructures are stable to surface oxidation until 150 C and show excellent catalytic activity for p-nitrophenol reduction reaction.
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Papers by Pattayil Joy
increase in the thermal conductivity with increasing viscosity is observed for all four nanofluids studied. The concentration dependent thermal conductivity studies showed enhancement only above a particular concentration, within the sensitivity of the measurement, and this critical concentration is different for the different nanofluids. The nanofluid with kerosene showed the lowest critical concentration for thermal conductivity enhancement compared to the other nanofluids. The difference between the experimental thermal conductivity and the calculated value using the Maxwell model is found to depend on the critical concentration. By assuming the critical concentration as the zero concentration, it is found that all the studied nanofluids almost follow the Maxwell model of thermal conductivity. Thus, for the dispersions of the same oleic acid coated magnetite nanoparticles, the base fluid affects the critical concentration for thermal conductivity enhancement, probably due to the interfacial effects arising from the surfactant-solvent interactions.
increase in the thermal conductivity with increasing viscosity is observed for all four nanofluids studied. The concentration dependent thermal conductivity studies showed enhancement only above a particular concentration, within the sensitivity of the measurement, and this critical concentration is different for the different nanofluids. The nanofluid with kerosene showed the lowest critical concentration for thermal conductivity enhancement compared to the other nanofluids. The difference between the experimental thermal conductivity and the calculated value using the Maxwell model is found to depend on the critical concentration. By assuming the critical concentration as the zero concentration, it is found that all the studied nanofluids almost follow the Maxwell model of thermal conductivity. Thus, for the dispersions of the same oleic acid coated magnetite nanoparticles, the base fluid affects the critical concentration for thermal conductivity enhancement, probably due to the interfacial effects arising from the surfactant-solvent interactions.