Sensing behaviour of some nanocomposite systems

Silver nanoparticles of diameters 3.4 to 13.2 nm grown at the interfaces between silicate glass and some oxide crystallites exhibited about six orders of magnitude reduction in resistivity for a relative humidity change from 25% to 80%. Sn-SnO2 nano core-shell structure prepared within a gel-derived silica glass film by electrodeposition technique followed by heat treatment showed large change in resistivity as a function of humidity. The resistivity also changed due to gas flow of CO2, C2H5OH and NO2, respectively. The latter arose because of reduction/oxidation of Sn4+/Sn2+ species present at the shell layer of the nanostructures. Nickel nanosheets of thickness ~0.6 nm grown within the interlayer spaces of Na-4 mica crystallites showed a change of dielectric permittivity (5%) for an applied magnetic field of 1.2 Tesla. An inhomogeneous model was used to explain this behavior. Two dimensional CuO phase was grown within the channels of diameter ~5 nm of mesoporous SiO2 structure. A magnetodielectric (MD) parameter M.D. of 4.4% was obtained in this case. BaTiO3 nanoparticles of diameter ~25 nm having pores with diameter 10 nm showed multiferroic behavior which arose due to the presence of oxygen vacancies as a result of large surface area present. An M.D. parameter of 11% was found. Similarly mesoporous LiNbO3 of 10 nm diameter showed an M.D. parameter of ~4.5% at a magnetic field 1 Tesla. A giant magnetocapacitance effect with a value of 44% at 1.5 T was observed in nickel zinc ferrite (NZF) impregnated mesoporous silica. A magnetocapacitance of 51% at magnetic field 1.7 T was found in the case of nanocomposites comprising of iron ion containing silica based nanoglass and mesoporous silica. In the last two examples the behavior was explained on the basis of Catalan model of space-charge polarization with extracted values of magnetoresistance of the NZF and nanoglass phases being 58%

Magnetodielectric effect of Graphene-PVA Nanocomposites

Graphene-Polyvinyl alcohol (PVA) nanocomposite films with thickness $120 \mu m$ were synthesized by solidification of PVA in a solution with dispersed graphene nanosheets. Electrical conductivity data were explained as arising due to hopping of carriers between localized states formed at the graphene-PVA interface. Dielectric permittivity data as a function of frequency indicated the occurrence of Debye-type relaxation mechanism. The nanocomposites showed a magnetodielectric effect with the dielectric constant changing by 1.8% as the magnetic field was increased to 1 Tesla. The effect was explained as arising due to Maxwell-Wagner polarization as applied to an inhomogeneous two-dimensional,two-component composite model. This type of nanocomposite may be suitable for applications involving nanogenerators.Comment: 13 pages, 11 figure

STUDY OF ELECTRICAL,MAGNETIC, MAGNETODIELECTRIC PROPERTIES OF NANODIMENSIONAL GLASSES AND THEIR NANOCOMPOSITES

The thesis deals with the synthesis of nanodimensional silica based glasses containing either lithium ions or iron ions. The effect of nanosize on the ionic conductivity and /or magnetic behaviour of the glass concerned have been investigated. Also, the magnetodielectric properties resulting from these structural peculiarities have been delineated.The research was carried out under the supervision of Prof. D Chakraborty, MLS and Prof. A K Nandi, PSU under SMS [School of Materials Sciences]The research was conducted under CSIR fellowship and research grant. Instrumental facilities was extended from Nano Science and Technology Initiative program of the Department of Science and Technology, New Delh

Study of dielectric relaxation process in nanocomposite of Li₂O−SiO₂ nanoglass-CuO nanoparticles

Dielectric behaviour of a nanocomposite consisting of 23Li₂O⋅77SiO₂ nanoglass within the pores of compacted CuO nanoparticles was studied. Real and imaginary parts of dielectric permittivity of the material were measured over the temperature range 313 to 363 K. The results indicated a relaxation behaviour. The data were explained by a space charge polarization model developed in the case of a laminar conductor. The activation energy of the relaxation process was in close agreement with that of lithium ion conduction in the nanoglass

Magnetodielectric effect in composites of nanodimensional glass and CuO nanoparticles

Nanocomposites comprising CuO particles of average diameter 21 nm coated with 5 nm silica glass containing iron ions were synthesized by a chemical route. An ion exchange reaction at the nanoglass/CuO interface produced iron-doped CuO with copper ion vacancies within the nanoparticles. Room temperature ferromagnetic-like behavior was observed in the nanocomposites. This was ascribed to uncompensated spins contributed by Fe ions with associated copper ion vacancies. A rather high value of magnetodielectric parameter in the range 16–26% depending on the measuring frequency was exhibited by these nanocomposites at a magnetic field of 10 KOe. This was caused by a magnetoresistance of 33% in the iron doped CuO nanoparticles. The experimental results were fitted to the Maxwell–Wagner Capacitor model developed by Catalan. These materials will be suited for magnetic sensor applications

Viscoelastic properties of graphene/PVA nanocomposite

Graphene/Polyvinyl alcohol (PVA) nanocomposite was synthesized with a graphene content of 1wt%. Viscoelastic properties were studied using a Dynamical Mechanical Analyzer. The composite showed a higher storage modulus as compared to PVA up to 275K. Thereafter there was a crossover to a lower value. The glass transition temperature for the composite was found to be smaller by 26K than that of pure PVA. This is believed to be occurring because of a reduction of surrounding entanglements of the polymer molecules due to the presence of graphene sheets

Enhancement of ionic conductivity in Li₂O–SiO₂ glass in nanodimensions grown within pellets of ZnO nanorods and magnetodielectric properties of these nanocomposites

23Li₂O·77SiO₂ glass with nanodimensions was grown within the pores of a pellet formed by ZnO nanorods of a diameter of 70 nm and lengths varying from 80 nm to 270 nm. The resistivity of the nanoglass was less than that of its bulk counterpart by more than three orders of magnitude. The activation energy was found to be 35% lower than that of the bulk glass. This was ascribed to an increase in free volume in the nanodimensional glass. The ac conductivity variation at high frequency showed an exponent of 0.5 indicating two dimensional movements of lithium ions in the nanoglass. Nanocomposites of lithia–silica nanoglass and CuO nanoparticles showed magnetodielectric property with dielectric permittivity at 500 kHz decreasing by 5.3% at 2 T. This was explained on the basis of space charge polarization and Hall Effect. Nanocomposites of lithia–silica nanoglass and ZnO nanorods exhibited a giant magnetodielectric effect with the dielectric permittivity increasing by 20% to 99% at 2 T depending on the measuring frequency. This was also ascribed to space charge polarization with the ZnO nanophase showing a negative magnetoresistance

Ferromagnetic-like behavior in nanosilica glass containing iron ions and giant magnetodielectric effect in composites of these glasses with mesoporous silica

Iron ion containing nanodimensional silica glass was grown within the 5 nm pores of mesoporous silica. The nanocomposite material showed ferromagnetic-like behavior at room temperature. This was ascribed to the presence of Fe²⁺ and Fe³⁺ ions within the nanodimensional glass phase and an antiferromagnetic superexchange interaction between them. The nanocomposite showed a large magnetodielectric coefficient (up to 51%) for a magnetic field of 1.7 T. The dielectric loss (tan δ) was found to be in the range 0.35 to 0.66. This was explained on the basis of Catalan’s model of space–charge polarization. From the theoretical fitting of experimental data, magnetoresistance of nanoglass phase was extracted to be 58% up to a magnetic field of 1.7 T

Nanoindentation measurements on nanostructured silver grown within a gel derived silica glass by electrodeposition

Mechanical Properties of silver nanoparticles were investigated. Sliver nanophase with particle diameters in the range 11.1 to 36 nm was grown in silica based gel derived glasses by an electrodeposition technique. Nanoindentation measurements were carried out on the silver particles. Hardness as well as modulus values decreased with a lowering of the silver particle diameter. This inverse Hall Petch Effect was analysed on the basis of Conrad-Narayan model of grain boundary sliding which involved silver diffusion along the grain surface. The activation energy extracted was in excellent agreement with the activation energy of grain boundary diffusion in bulk silver as reported earlier