Ashis Dey

Characterization and dielectric properties of polyaniline–TiO2 nanocomposites

Nanocomposites of polyaniline (PANI)–titanium dioxide (PANI–TiO2) are prepared from a colloidal sol of TiO2 nanoparticles. The dc and ac conductivities of samples with different concentrations of PANI have been investigated as a function of frequency and temperature. The dc conductivity follows three-dimensional variable range hopping. The ac conductivity has been interpreted as a power law of frequency. The temperature variation of the frequency exponent suggests a correlated barrier hopping conduction process in the nanocomposites. A very large dielectric constant of about 3700 at room temperature has been observed. An electric modulus presentation is used to interpret the dielectric spectra. The interface between polyaniline and TiO2 plays an important role in yielding a large dielectric constant in the nanocomposite.

Electrical transport and dielectric relaxation in Fe3O4?polypyrrole hybrid nanocomposites

The polymerization of pyrrole in an aqueous medium in the presence of nanodimensional Fe3O4 using ammonium peroxodisulphate (APS) as oxidant results in the formation of polypyrrole-Fe3O4 nanocomposites. Characterization of the composites was carried out by Fourier transform infrared spectroscopy, x-ray diffraction, scanning and transmission electron microscopy. The magnetization data exhibit a small hysteresis loop at room temperature. The Mössbauer spectra at room temperature reveal the doublet structure, characteristic of the superparamagnetic phase in magnetite (Fe3O4). The composite samples reveal ordered semiconducting behaviour. Polypyrrole is the dominating component in the transport process of the nanocomposites. A very large dielectric constant of about 11 000 at room temperature has been observed. The interface between polypyrrole and Fe3O4 plays an important role in producing a large dielectric constant in the composite.

Near-net-shape fibre-reinforced ceramic matrix composites by the sol infiltration technique

Abstract The infiltration of a ceramic fibre preform by a ceramic sol is a cost-effective method for the manufacture of ceramic matrix composites (CMCs). The ceramic sol after calcination forms the matrix and the discontinuous fibres act as the reinforcement agents. In this paper, a technique has been presented in which discontinuous mullite preform with 15 vol.% of fibre content was infiltrated with the stabilized zirconia–yttria (ZrO 2 ·10 wt.% Y 2 O 3 ) sol to fabricate near-net-shape CMCs. The effects of sol viscosity, number of infiltration, in situ deposition of carbon in the CMC samples, and calcination (in air and nitrogen atmosphere) temperature on physico-mechanical properties of fabricated CMCs were examined. The characterization of the preform and the developed CMCs were performed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD indicated the presence of both cubic (c) and tetragonal (t) zirconia in the CMCs calcined even at 1400 °C. The flexural strength of the CMCs (calcined in air at 1400 °C) when determined by the three-point bend test was found to be almost 14 MPa under a given set of experimental conditions, while calcination of the same materials at 1400 °C in nitrogen atmosphere (carbon-containing CMCs) exhibited a modulus value of almost 51 GPa. SEM indicated multiple fractures of the matrix, which gave rise to pseudo-ductility. This is also evident from the load–elongation curve of the three-point bend test. The reinforced fibres acted as crack arresters, which prevented the propagation of the cracks. SEM studies also indicated fibre pull-out in the fracture surface of the CMCs.

Impedance and dielectric spectra in zirconia?polypyrrole hybrid nanocomposites

Zirconia (ZrO2) nanoparticles have been synthesized by a novel two-reverse emulsion technique and combined with polypyrrole (PPY) to form ZrO2?PPY nanocomposites. The complex impedance and dielectric permittivity of ZrO2?PPY nanocomposite have been investigated as a function of frequency and temperature for different compositions. The optical band gap of zirconia decreases with increase in polymer concentration. Two electrical responses in the impedance spectra corresponding to the grain and grain boundary effects are analysed by an equivalent circuit. Activation energies are very small (35?70?meV). The samples exhibit a large dielectric constant and at room temperature it becomes about 12?000 at 0.7?KHz for the highest content of zirconia. The high dielectric response is mainly originated by Maxwell?Wagner polarization.

A novel technique for fabrication of near-net-shape CMCs

A sol-gel vacuum infiltration technique has been developed for the fabrication of near-net-shape ceramic matrix composites (CMCs) using discontinuous mullite fibre preform with 15 vol.% of fibre content and ZrO2.10 wt.% Y2O3 sol as the infiltrant. Effect of sol viscosity, number of infiltration and calcination temperature on physico-mechanical properties of fabricated CMCs were examined. Characterization of the fibre preform, matrix material (in the form of ceramic specimen without fibre) and the developed CMCs were performed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD indicated the presence of cubic (c) and tetragonal (t) zirconia in both the CMCs and the ceramic specimens calcined even at 1400°C. Flexural strength of the CMCs and the ceramic specimens (calcined at 1400°C), determined by the three-point bending test, was found to be about 14 mPa and 1.40 mPa, respectively. SEM indicated multiple fracture of the matrix which gave rise to pseudo elasticity. This is also evident from the load-displacement curve of the three-point bend test. SEM studies also indicated fibre pull-out in the fracture surface of the CMCs.