A.B. Kaiser

Thermoelectric power and conductivity of different types of polypyrrole

We have measured the thermoelectric power and conductivity as a function of temperature of a wide range of polypyrrole samples, including a film of soluble polypyrrole synthesized chemically, and wrinkled films synthesized using indium–tin oxide electrodes; other samples investigated include high-conductivity polypyrrole films synthesized at different temperatures and current densities, films grown on nonconducting substrates, and polypyrrole gas sensors. The thermoelectric powers are remarkably similar and metal-like for the medium and high conductivity samples but show nonzero extrapolations to zero temperature for wrinkled samples. The temperature dependence of conductivity tends to be greater for samples of lower conductivity. In contrast to polyaniline and polyacetylene, a crossover to metallic sign for the temperature dependence of conductivity at higher temperatures is not observed in any of our samples; the fluctuation-induced tunnelling and variable-range hopping expressions account for nearly all our conductivity data except for low-temperature anomalies.

Temperature-dependent conductivity of conducting polymers exposed to gases*

We have made measurements of the effect of water vapour, oxygen and nitrogen on the temperature-dependent conductivity of polypyrrole gas sensors. Oxygen and nitrogen had little effect, while water vapour decreased conductivity and increased conductivity temperature dependence, consistent with decreased hopping between localized states. No evidence was found of any metallic term in the conductivity of our polypyrrole samples, in strong contrast to the case of polyaniline. We analyze previous conductivity measurements for polyaniline exposed to the same gases, showing that the conductivity remains consistent with the general expression for polymer conductivity with a significant metallic term.

Comparison of electronic transport in polyaniline blends, polyaniline and polypyrrole

Abstract The conductivity of PAni/PETG copolyester blends shows the expected reduction compared to unblended PAni, but PAni/PMMA (and PAni/PVC blends at lower temperatures) show a larger conductivity. SEM images indicate that the different behaviour of the blends appears to be associated with greater granularity retained by the PAni particles dispersed in PETG. The conductivities of the more highly conducting PAni blends, like PAni-CSA and its blends with PMMA measured by other authors, show a change to metallic sign for the temperature dependence near room temperature — simple models combining hopping/tunnelling with metallic conduction are consistent with the conductivity except at very low temperatures. The thermopower of PAni blends shows a remarkable similarity for blends with different insulating polymers and widely varying conductivity. For polypyrrole samples of very different conductivity, the change to metallic sign for conductivity near room temperature does not occur, indicating a much smaller ‘metallic’ resistance contribution. Nevertheless, the thermopower of polypyrrole, like that of PAni and PAni blends, is of metallic size and generally increases with temperature, as expected for metallic diffusion thermopower.

Temperature dependence of conductivity in ‘metallic’ polyacetylene

Abstract Compiling experimental data on the conductivity of ‘metallic’ polymers (i.e. those in which the conductivity remains finite in the zero-temperature limit), we distinguish two general types of behaviour: small temperature dependence with a flattening at low temperatures, and larger temperature dependence with increased slope at low temperatures. The first type suggests that there are continuous metallic paths, with conduction limited by regions behaving as disordered metals in parallel with regions involving tunnelling or hopping. The second type is also consistent with this model if there is an additional conductivity term arising from electron-electron interactions, as expected if the disordered metal barrier region is near the metal-semiconductor transition. Our simple heterogeneous model is able to give a good fit to the conductivity temperature dependence of all the samples.

Heterogeneous model for conduction in conducting polymers and carbon nanotubes

Abstract The electronic transport properties of organic conducting polymers show remarkably similar behaviour patterns, and some aspects of this behaviour are also seen in mats of single-wall carbon nanotubes and bundles of (SN) x polymer fibres. Data for these materials provide further evidence for a series heterogeneous model for conduction, which can explain the crossover in the sign of the temperature dependence of the conductivity from nonmetallic to metallic; the model can also account for the metallic behaviour of thermoelectric power and the peak in the optical conductivity as a function of frequency observed in the polymers. The linear thermopower in highly-conducting polyacetylene and polyaniline indicates that there is no overall change in the nature of the electronic states near the crossover in conductivity temperature dependence, and also suggests an absence of significant superconductivity arising from the electron-phonon mechanism.

Simulation of Diffusion and Adsorption in Zeolites

Abstract We show that Poisson-distributed event times are required to correctly simulate diffusion and adsorption in zeolites. The main microscopic assumptions of the diffusion model are: the zeolite is a periodic array of sites, each of which may contain only one molecule; and transport is achieved by sorbate molecules randomly “jumping” from one site to an adjacent site. Three simulation schemes for moving the molecules are compared; two of the schemes exhibit significant deviations from expected behavior, showing that seemingly reasonable schemes can introduce subtle correlations that have observable consequences. The preferred third scheme implements Poisson-distributed event times and exhibits no significant deviations from expected behavior-even for transient conditions and high occupancies. The simulation technique is then extended to independent Poisson-distributed events of more than one type. Specifically, adsorption events are added to the simulation, resulting in Langmuirian adsorption and concentration independent intralattice diffusion, in agreement with experimental results for some zeolites. The simulation correctly reflects the microscopic assumptions, thus placing this type of simulation on a sound theoretical foundation.

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.

Tunneling conduction in polyacetylene nanofiber

We report the first low temperature measurements of I-V characteristics of polyacetylene nanofibers. Our data provide (i) no temperature dependence below 10-30K, (ii) no significant change of MR in magnetic field up to H=6T, to within the accuracy of the data. These are signatures of a novel conduction mechanism: the creation of charged soliton-antisoliton pairs by quantum-mechanical tunneling.

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.