Sreemanta Mitra

Magnetodielectric Effect in Graphene-PVA Nanocomposites

Graphene-polyvinyl alcohol (PVA) nanocomposite films with thickness of 120 μm 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 T. The effect was explained as arising because of Maxwell–Wagner polarization as applied to an inhomogeneous 2D, two-component composite model. This type of nanocomposite may be suitable for applications involving nanogenerators.

Tunneling conduction in graphene/(poly)vinyl alcohol composite

Graphene/(Poly)vinyl alcohol (PVA) composite film with thickness 60 μm was synthesized by solidification of a PVA solution comprising of dispersed graphene nanosheets. The close proximity of the graphene sheets enables the fluctuation induced tunneling of electrons to occur from one sheet to another. The dielectric data show that the present system can be simulated to a parallel resistance-capacitor network. The high frequency exponent of the frequency variation of the ac conductivity indicates that the charge carriers move in a two-dimensional space. The sample preparation technique will be helpful for synthesizing flexible conductors.

Magnetodielectric effect in nickel nanosheet-Na-4 mica composites

Nickel nanosheets of thickness 0.6 nm were grown within the nanochannels of Na-4 mica template. The specimens show magnetodielectric effect at room temperature with a change of dielectric constant as a function of magnetic field, the electric field frequency varying from 100 to 700 kHz. A decrease of 5% in the value of the dielectric constant was observed up to a field of 1.2 T. This is explained by an inhomogeneous two-component composite model as theoretically proposed recently. The present approach will open up synthesis of various nanocomposites for sensor applications.

Ferromagnetic Behavior of Ultrathin Manganese Nanosheets

Ferromagnetic behavior has been observed experimentally for the first time in nanostructured manganese. Ultrathin (∼0.6-nm) manganese nanosheets were synthesized inside the two-dimensional channels...

Magnetodielectric effect in CdS nanosheets grown within Na-4 mica

CdS nanosheets of thickness 0.6 nm were grown within the interlayer spaces of Na-4 mica. Magnetization measurements carried out in the temperature range 2–300 K showed the composites to have weak ferromagnetic-like properties even at room temperature. The saturation magnetization (MS) at room temperature was found to be higher than that reported for CdS nanoparticles. The higher value of MS may be ascribed to the presence of a large number defects in the present CdS system, due to a large surface to volume ratio in the nanosheets as compared to that of CdS nanoparticles. The nanocomposites exhibited a magnetodielectric effect with a dielectric constant change of 5.3% for a magnetic field of 0.5 T. This occurred due to a combination of magnetoresistance and Maxwell-Wagner effect as delineated in the model developed by Catalan.

Non-linear temperature variation of resistivity in graphene/silicate glass nanocomposite

Graphene/glass nanocomposite was synthesized by gelation of the glass in a solution with dispersed graphene sheets. Electrical transport measurements were carried out on pellets formed by cold pressing of composite powders. Resistivity showed a nonlinear increase as a function of temperature in the range 300–400 K. This has been explained as arising due to the phonon spectra of the glass affecting the movement of electrons in graphene. Raman studies confirmed the presence of phonons in the silicate glass phase. The dielectric relaxation spectra of the composites at different temperatures are consistent with the above mechanism of the electron–glass phonon interaction.

Nanoindentation studies on silver nanoparticles

Nanodimensional metallic silver was grown by electrodeposion technique in a semi solid polymer matrix of polyacrylamide. The whole structure looks like dendronic. The average particle diameter of the as grown metallic silver is 13 nm. Nanoindentation study of these nanoparticles shows modulus and hardness value as 103.93 GPa and 3.12 GPa respectively.

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.

Enhanced magnetic anisotropy of nickel nanosheet prepared in Na-4 mica

Nanosheets of nickel with thickness equal to 0.6 nm have been grown within the interlayer spaces of Na-4 mica. The sheets are made up of percolative clusters of nanodisks. Magnetization characteristics indicate a superparamagnetic behavior with a blocking temperature of 428 K. The magnetic anisotropy constant as extracted from the coercivity data has been found to be higher than that of bulk nickel by two orders of magnitude. This is ascribed to a large aspect ratio of the nickel nanophase. The Bloch exponent is also found to be considerably different from that of bulk nickel because of a size effect. The Bloch equation is still found to be valid for the two dimensional structures.