V. K. Sachdev

n-Type conduction in Pb doped Se–In chalcogenide glasses

This paper reports on the p to n transition in Pb doped Se–In chalcogenide glasses. Measurements of thermoelectric power in the temperature range 300 K⩽T⩽315 K, dc conductivity in the temperature range 100⩽T⩽300 K, and optical band gap (Egopt) have been carried out for Se75In25−xPbx (x=0,5,10,15) samples. The p-n transition occurs with very low addition of Pb impurity (5 at. %). The conductivity and pre-exponential factor also change by five to six orders of magnitude with Pb doping. Results are explained on the basis of the formation of ionic Pb–Se bonds, instead of covalent bonds. Formation of ionic bonds disturbs the equilibrium between the charged defect states of Se–In glass and unpins the Fermi level and thus leads to n-type conduction in these glasses.

High Pressure Studies on n-Type Se–In–Pb Chalcogenide Glasses

The measurement of dc conductivity as a function of pressure in the range 0 ≤ P < 2.0 GPa for Se 75 In 25-x Pb x (x = 0, 5, 10, 15) glasses are reported. n-type Se-In-Pb glasses show a maximum in the resistance versus pressure. Differential Scanning Calorimetric (DSC) studies have also been carried out for the measurement of the glass transition temperature (T g ). The results have been explained in terms of the presence of fourfold coordinated Pb atoms in the Se-In host matrix.

Synergic effect of graphene and MWCNT fillers on electromagnetic shielding properties of graphene–MWCNT/ABS nanocomposites

Graphene–MWCNT/ABS nanocomposites have been prepared using a solvent-free dry tumble mixing process followed by hot compaction. The combined effect of using two nanofillers (graphene and MWCNTs) on the electrical conductivity and EMI shielding properties of the nanocomposites has been studied. A unique conductive network of MWCNTs and graphene formed due to their different geometrical shapes, besides the high aspect ratios of the MWCNTs. This served to improve the electrical conductivity and the electromagnetic shielding properties. FESEM results for freeze-fractured pellets have confirmed the enhancement in conductivity and shielding of the graphene–MWCNT/ABS nanocomposites and, furthermore, the escape of EM waves through windows/gaps consisting of filler-depleted areas containing ABS. The addition of 1 wt% MWCNTs to a graphene–ABS nanocomposite resulted in a synergistic effect on the EM shielding properties.

Pre-localized MWCNT network for a low percolation threshold in MWCNT/ABS nanocomposites: experiment and theory

Nanocomposites of well-dispersed multi walled carbon nanotubes (MWCNTs) in an acrylonitrile–butadiene–styrene (ABS) matrix were prepared by dry tumble mixing and a subsequent hot compression technique. The preparation method is analysed here to assist in the explanation and understanding of the experimental observations. The vision of the processing method has been confirmed using scanning electron microscopy. As a result of decent dispersion and pre-localization of MWCNTs in the ABS matrix, the electrical conductivity of the nanocomposite as a function of filler content exhibits a distinctive percolation behavior with a percolation threshold of only 0.0005 volume fraction. The high aspect ratio of MWCNTs enables the creation of interconnected networks within the ABS matrix at a very low nanofiller loading, while their high inherent electrical conductivity yields nanocomposites with a high bulk electrical conductivity. To explain the conductivity behaviour, statistical percolation theory and GEM theory are employed. The model parameters elaborated show that both models can adequately describe the behaviour. A comparative analysis of the applicability of percolation theory and GEM theory for expressing the conductivity behavior of the MWCNT/ABS composites with other research works has been performed. Finally, the electrical conductivity of MWCNTs based on the data in this work and elsewhere is evaluated. The universal exponent t is found to be 1.93 for a best-fit value of percolation threshold of 0.000485 vol. fraction of MWCNTs in this work. The D shore hardness results have revealed a random increase in hardness and density with an increasing MWCNT content.

EMI shielding of MWCNT/ABS nanocomposites in contrast to graphite/ABS composites and MWCNT/PS nanocomposites

Acrylonitrile–butadiene–styrene (ABS) nanocomposites filled with multiwall carbon nanotubes (MWCNTs) are prepared through localized conductive patterning, using dry powder tumble mixing succeeded by hot compression technique. Electrical conductivity and complex permittivity together with electromagnetic interference shielding effectiveness (SE) are investigated in 8–12 GHz frequency range. Abrupt increase in dc conductivity by number of orders of magnitude on addition of 5 × 10−4 volume fractions of MWCNTs marked as percolation threshold, has adequately described the conductivity behavior with statistical parameters of percolation and GEM models. The SE of these nanocomposites increases with increase of MWCNTs content. SE ∼ 1 dB at 12 GHz produced with 0.05 wt% MWCNTs grows to ∼40 dB on addition of 5 wt% MWCNTs. SE exhibit marginal dependence or even independence on frequency for most of the compositions with 0–5 wt% MWCNTs. SE reflection (SEref) and absorption (SEabs) both are increasing with the rise of MWCNTs content. However increase in SEabs is faster than SEref in addition to its dominance. Complex dielectric constants in 8–12 GHz frequency range have been calculated by means of S11/S12 parameters using Nicolson Ross Weir algorithm. The variation of the SE as a function of dc conductivity which has rarely been examined in literature, investigated and compared with other polymer composites here.