Quasi‐One Dimensional in‐Plane Conductivity in Filamentary Films of PEDOT:PSS

The mechanism and magnitude of the in‐plane conductivity of poly(3,4‐ethy‐lenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films is determined using temperature dependent conductivity measurements for various PEDOT:PSS weight ratios with and without a high boiling solvent (HBS). Without the HBS the in‐plane conductivity of PEDOT:PSS is lower and for all studied weight ratios well described by the relation $ \sigma = \sigma _0 {\rm exp}[- \left({{{{T_0}}\over{T}}} \right)^{0.5}] $ with T₀ a characteristic temperature. The exponent 0.5 indicates quasi‐one dimensional (quasi‐1D) variable range hopping (VRH). The conductivity prefactor σ₀ varies over three orders of magnitudes and follows a power law σ₀∝c^3.5_PEDOT with c_PEDOT the weight fraction of PEDOT in PEDOT:PSS. The field dependent conductivity is consistent with quasi‐1D VRH. Combined, these observations suggest that conductance takes place via a percolating network of quasi‐1D filaments. Using transmission electron microscopy (TEM) filamentary structures are observed in vitrified dispersions and dried films. For PEDOT:PSS films with HBS, the conductivity also exhibits quasi‐1D VRH behavior when the temperature is less than 200 K. The low characteristic temperature T₀ indicates that HBS‐treated films are close to the critical regime between a metal and an insulator. In this case, the conductivity prefactor scales linearly with c_PEDOT, indicating the conduction is no longer limited by a percolation of filaments. The lack of observable changes in TEM upon processing with the HBS suggests that the changes in conductivity are due to a smaller spread in the conductivities of individual filaments, or a higher probability for neighboring filaments to be connected rather than being caused by major morphological modification of the material.     

Transconductance and mobility of Si:B delta MOSFETs

Delta-doped MOSFETs have been fabricated in MBE-grown silicon using for the first time boron as the dopant within the delta layer. Current-voltage characteristics have been measured, and secondary ion mass spectrometry (SIMS) is used to confirm the location of the delta layer and the extent of layer broadening by diffusion during processing. Precise threshold voltages of the devices are difficult to determine since the devices (which all operate in depletion mode) take several volts to switch off. Transconductances of the devices have been measured, and it is shown how analysis of these results can yield estimates of the carrier mobility for transport along the delta layers despite the uncertainty in the threshold voltage. A clear transition is observed in the results which is attributed to the formation of a parasitic surface-channel field-effect transistor, providing conclusive evidence that the devices are conducting along a delta channel for part of the measured range of applied gate biases     

Concentric ring and jerusalem cross arrays as frequency selective surfaces for a 45° incidence diplexer

Typical crosspolar levels of ?33 dB in the reflection band and slightly lower levels in transmission were recorded for the concentric ring array in the 45° planes of the feed/array combination. The Jerusalem cross array gave higher levels in transmission. The band centre frequency ratios were 1.4 and 1.6.     

An on-line microcomputing system for the acquisition and processing of data from a fourier spectrometer

A microcomputing system has been developed to provide on-line data acquisition and analysis for a Fourier transform spectrometer, and the interface between the spectrometer and the microcomputer is described in detail. The system has been equipped with software which includes a fast Fourier transform programme for use in either conventional or dispersive Fourier transform spectroscopy, and programmes for calculating the optical constants and dielectric functions of solids from complex spectra obtained by dispersive Fourier transform spectroscopy.     

Optical switching of a GaAs triangular barrier switch grown by MBE

A 100 ?m-diameter GaAs triangular barrier switch (TBS) has been optically switched using a GaAlAs laser diode. The resulting transient response has been measured to have a leading edge of ~ 295 ps.     

Al2TiO5-ZrTiO4-ZrO2 Composites: A New Family of Low-Thermal-Expansion Ceramics

The characterization and properties of ceramic composites containing the phases Al₂TiO₅, ZrTiO₄, and ZrO₂ are described. The range of compositions investigated gives very low average thermal expansions (α_24–1000°C as low as −2.0 × 10⁻⁶°C⁻¹) and excellent high-temperature stability. The low thermal expansions are apparently due to a combination of microcracking by the titanate phases and a contractive phase transformation by the ZrO₂. The crystal chemistry and microstructure of the product are processing dependent. Although the composites represent a complex microcracking system, the low thermal expansions and high-temperature stability make them potential candidates for commercial applications requiring thermal shock resistance.     

Diffusion in Solids via Nuclear Track Analysis

Nuclear track analysis has been used to follow the diffusion of melted lithium hydroxide in pellets of aluminum oxide as a function of temperature and time. α particle tracks, created by thermal neutrons on an allyl diglycol carbonate monomer film, are visible under a high-power optical microscope after chemical etching in a sodium hydroxide solution. The number of tracks present at various depths within each pellet is proportional to the concentration of lithium hydroxide. Using a one-dimensional diffusion model with appropriate boundary conditions, values of the lithium hydroxide diffusion coefficient in the porous pellet have been determined. The diffusion coefficients, as well as their temperature dependency, are consistent with expectations for this type of system and it is concluded that nuclear track analysis is a viable technique for solid-state diffusion studies in appropriate materials.     

Measurement of Torsion from Multitemporal Images of the Eye Using Digital Signal Processing Techniques

Two methods of measuring ocular torsion from digital images of the eyes were developed and tested. One method measures torsion from the translation of two landmarks using a rectilinear coordinate system. The second method measures torsion from the translation of two landmarks using a polar coordinate system. The center for the polar sampling is the center of the pupil. After thresholding and filtering the images, landmark translation is measured from the interpolated peak in the normalized cross correlation of the reference landmark with the image.     

AUTOMATIC DELINEATION OF FUNCTIONAL RIVER REACH BOUNDARIES FOR RIVER RESEARCH AND APPLICATIONS

The river reach is a pervasive term within contemporary river research and applications. Yet, despite its prevalence, there is a notable lack of consistency in its definition. This paper identifies the presence of two broad types of reach definition within the academic literature, operational and functional, and argues that a functional definition is more suitable for applications within river research and management. A range of sequence zonation algorithms that were originally derived for geological well‐log analysis were compared for their ability to automatically identify functional reach boundaries. An analysis of variance‐based global boundary hunting algorithm was identified as the most suitable. To demonstrate the potential practical applications of automatic reach delineation methods, two case studies where functional reach boundaries have been identified were described: first, in a sequence of predicted sediment transport capacities for use in a reach‐based sediment transport model; and second, in a sequence of RHS Habitat Quality Assessment scores for identification of lengths of channel in need of habitat restoration efforts. Finally, the paper discusses how this type of functional reach identification procedure might be applied in other areas of river research and applications and how a multivariate version of a statistical zonation algorithm might prove useful in facilitating integrated catchment management by identifying reach boundaries common across all variables of interest in the system. Copyright © 2011 John Wiley & Sons, Ltd.