C.K. Subramaniam

Electronic transport properties of conducting polymers and polymer blends

Abstract We present conductivity and thermopower data for polyaniline (PANI) blends with non-conducting polymers, making a comparison with polyacetylene and discussing general models for conduction. As expected, blending PANI with PET (polyester) reduces conductivity. More unexpectedly, blending with PMMA (polymethylmethacrylate) and PVC can increase conductivity, especially at lower temperatures; this increase is ascribed to lessening of insulating barriers around PANI particles in these blends. The temperature dependence of conductivity in the PANI blends is surprisingly similar to that seen in polyacetylene; it is well described in both types of material by a series combination of quasi-ID metallic resistivity and tunnelling (between small metallic islands for the blends and lightly-doped polyacetylene, and between extended metallic regions in highly-conducting polyacetylene). This heterogeneous model for conduction can also account for some puzzling features of the data in highly-conducting polyacetylene; the large interaction and localization effects seen at low temperatures despite conductivities similar to good metals, the similarity of temperature dependence despite large differences in conductivity magnitude, and the simple linear increase of thermopower with temperature despite the complex temperature dependence of conductivity. The thermopowers of the PANI/PET and PANI/PMMA blends are small and also increase as temperature increases.

Development of polymer electrolyte membrane fuel cell stack

Abstract The proton exchange membrane fuel cell (PEMFC) is one of the strongest contenders as a power source for space, electric vehicle and domestic applications. Since 1988 intensive research is being carried out at our centre to develop PEMFCs. The main RandD activities are: (i) to develop a method for the electrode preparation (ii) to enhance platinum utilisation using low platinum loading and (iii) to design multicell stacks. The results of RandD development of the above activities are discussed in this paper.

Conductivity and thermopower of blends of polyaniline with insulating polymers (PETG and PMMA)

We have measured the temperature dependence of the conductivity and thermoelectric power of conducting polyaniline (PAni) dispersed in PETG copolyester and in poly(methylmethacrylate) (PMMA). Near room temperature, the PAniPMMA blends (like unblended PAni) show a change to metallic sign for the conductivity temperature dependence, whereas this sign change does not occur for the PAniPETG blends. As temperature decreases, the PAniPMMA blends show a much smaller decrease of conductivity than the PAniPETG blends and even unblended PAni. A simple model involving metallic conduction in addition to tunnelling between metallic particles can give a good account of all the conductivity data. The thermopower of all the blends is small and increases as temperature increases.

On the magnetic susceptibility of CuOx

Abstract The magnetic susceptibility measurements were performed on CuO before and after quenching from high temperature. A large increase in the magnetic susceptibility is observed by quenching or consequent decrease in the oxygen content. It is argued that the large rise in the magnetization at temperatures much below the Neel temperature observed in certain samples may not be of impurity origin but results from the random freezing of isolated Cu 2+ ions near the oxygen defects.

Thermoelectric power of thallium-based 1212 superconductors with varying hole concentration

Abstract We have measured the thermoelectric power and resistivity of a series of Tl 0.5+ x Pb 0.5 − x Sr 2 Ca 1 − y Y y Cu 2 O 7 compounds in which the hole concentration can be varied to span the entire range from under-doped to over-doped, with T c increasing from zero to a maximum and then decreasing to very low values. The thermopower changes from very large positive values in the under-doped region to small negative values in the over-doped region, with a small negative temperature derivative except in the extreme under-doped limit. We also report the thermopower of a series of under-doped YBa 2 − x La x Cu 4 O 8 compounds, which shows little temperature dependence but decreases in magnitude as x decreases. Our data support the decrease of the thermopower magnitude as the hole concentration in the CuO planes increases which was found in other systems.

Thermoelectric power and resistivity of bulk HgBa2CuO4+y superconductors and the effects of annealing

Abstract We have measured the thermopower and resistivity of bulk superconducting samples of HgBa2CuO4+y (Hg-1201). Various annealing treatments under flowing oxygen and argon were carried out in order to change the oxygen content and thereby the charge-carrier density. As the oxygen content is increased, both the thermopower and resistivity decrease, and the thermopower becomes negative with a negative linear slope. The overall pattern of the thermopower and Tc variation is similar to that found recently for the Tl-1201 superconductor series.

Thermopower and resistivity of La-doped thallium 1201 and bismuth 2204 superconductors

We have measured the thermopower and resistivity of two series of cuprate superconductor, Tl0.5Pb0.5Sr2-xLaxCuO5 and Bi2Sr2-xLaxCuO6+y, which possess a single CuO2 plane. In each series, the hole concentration can be varied by changing the La content, permitting investigation of the overdoped, maximum-Tc and underdoped regimes. The thermopower follows a pattern similar to that seen in superconductor series with more than one CuO2 plane, namely an approximately linear decrease as temperature increases well above Tc. However, the thermopower shows a minimum above Tc followed by a maximum as temperature increases for samples with a small thermopower and high Tc. The magnitude of thermopower as a function of Tc is in general agreement with the universal behaviour found for other cuprate superconductors.

Crystal structure and anisotropy of iodine-intercalated Bi2Sr2Can−1CunOx

The electronic state and structural configuration of the intercalated iodine species in stage-1, I-Bi2Sr2Can−1CunOx (n = l, 2), have been studied through polarization-resolved Raman and129I Mössbauer spectroscopy. The polarization dependence of the Raman spectra and the Mössbauer measurement confirmed the dominant species to be triiodide ions, I3−, with alignment of these linear molecules either along thea- orb-axis in the host crystals. Transport measurements such as thermoelectric power and Hall coefficient clearly indicated that hole carriers are doped into the CuO2 planes upon intercalation, by whichTc of the host superconductor is changed. Furthermore, based on resistivity measurements in a magnetic field, we suggest that the iodine intercalation leads to a decrease of the anisotropy both in normal and superconducting states, suppressing the extremely two-dimensional character of the Bi2Sr2Can−1CunOx systems.

Logarithmic temperature dependence of the resistivity of Nd2−xCexCuO4−y

Abstract Single phase compounds of Nd 2− x Ce x CuO 4− y (0.14⩽ x ⩽0.18) were made by the standard solid state reaction. The as-prepared nonsuperconducting samples were further annealed in various reducing atmospheres to induce superconductivity. All of the samples, except that with a concentration x =0.18, exhibited superconductivity after an annealing treatment at 850°C for 6 h in a vacuum of 10 -5 Torr. Magnetisation measurments confirmed the Meissner fraction to be around 20% for these compounds. Resistivity measurements were performed on samples which were annealed under different conditions. The resistivity showed a ln T dependence, which was prominent in those samples in which superconductivity was not observed. In the samples showing zero resistivity, the ln T term is absent or present with a much smaller coefficient than in the nonsuperconducting samples.

Double layer energy storage in graphene - a study

An alternate energy storage device for high power applications are supercapacitors. They store energy either by pure electrostatic charge accumulation in the electrochemical double layer or as pseudo capacitance from fast reversible oxidation reduction process. However, they have low energy density. The electrodes in the Electrochemical Double Layer Capacitors (EDLC) are made of high surface area carbon. The carbon that can be used range from activated carbon to Graphene, with varying particle size, surface area, pore size and pore distribution. The main emphasis in the development of EDLCs is fabrication of electrodes having high surface area which would enhance the storage density of the EDLC. The EDLCs are assembled with different electrolytes which determine the operational voltage. Solid electrolytes can also be used as electrolyte and have an advantage in that we can avoid electrolyte leaks and are easy to handle. This would improve the reliability. They can also be shaped and sized to suit the application. The perflurosulfonic acid polymer as electrolyte has been used by various groups for EDLC application. The perflurosulfonic acid polymer possesses high ionic conductivity, good thermal stability, adequate mechanical strength and excellent chemical stability. The EDLCs, which are based on high-surface area carbon materials, utilize the capacitance arising from a purely non-Faradaic charge separation at an electrode/electrolyte interface. Carbon is widely used for many practical applications, especially for the adsorption of ions and molecules, as catalyst supports and electrode materials. The chemical characteristics of carbon determine the performance in all these applications. It is now possible to synthesize one-, two-, or three-dimensional (1-, 2-, or 3-D) carbons. Thus, carbon materials are very suitable candidates for super capacitor electrodes. We can overcome some of the problems in activated carbon like varying micro or meso pores, poor ion mobility due to varying pore distribution, low electrical conductivity, by using Graphene. Many forms of Graphene have been used by various groups. Graphene nanoplates (GNP), with narrow mesopore distribution have been effectively used to enhance charge storage performance. It has been found that graphene shows smaller decrease in storage capacity with increasing scan rate.