Mechanism of conductance switching: An optical investigation

Electronic conductance switching in devices based on thin films of an organic molecule has been studied. Switching between two conducting states has been induced by voltage pulse, while the states have been probed by optical and electrical measurements simultaneously. In situ optical measurements showed that electroreduction of molecules led to conductance switching and appearance of high-conducting state in the device. We could “write” or “erase” a state by applying electrical pulse and “read” it by measuring electronic absorbance and conductivity. The “write” and “read” processes have been carried out for many cycles to exhibit a correspondence between conductance switching and electrochromism.     

Obliging Institutions and Industry Evolution: A Comparative Study of the German and UK Wind Energy Industries

This paper firstly examines the relationship between state coordination and wind energy growth by comparing the differences between UK and German institutional configurations and state involvements in the 1990s. While the EU was calling for a unified regulatory framework for its member states, the UK and Germany adopted very different renewable energy policies. The evidence of the early 1990s shows that the British government employed “deregulation” with so limited state involvement that wind energy project developers faced thorny development problems, while the German government tried to require the electricity supply industry to purchase expensive wind power by “regulation” but encountered formidable resistance. Nevertheless, both the British and German administrations later could resolve these problems through new forms of state power. This suggests that, firstly, neither “deregulation” nor “regulation” is an effective means to develop wind power with increasing electricity liberalization and regionalization. Secondly, “obliging” regulations and state powers are vital to the policy outcome by harnessing the state's institutional capacity to smooth out technology innovation and diffusion.     

Bipolar injection as a cause of electrical switching phenomena in thin organic films

An investigation of electrical conductance switching phenomena in thin organic films has been carried out. For thin films of p-quaterphenyl and tetracene grown in high vacuum the electrical conductance, dynamic capacitance and temperature patterns corresponding to the distribution of electrical current density have been measured. The measurements were carried out in vacuum as well as in humid air. It is suggested that previously reported switching phenomena are caused by field-induced bipolar injection of charge carriers.     

Experimental investigation and flow visualisation of the resin transfer mould filling process for non-woven hemp reinforced phenolic composites

Resin transfer moulding (RTM) of glass fibre reinforced polymeric composites offers the advantages of automation, low cost and versatile design of fibre reinforcement. A replacement of glass fibres with natural plant fibres as reinforcement in polymeric composites provides additional technological, economical, ecological and environmental benefits. The resin transfer mould filling process has significant effects on different aspects, such as fibre wetting out and impregnation, injection gate design, “dry patch” and void formation. Flow visualisation experiments were carried out using a transparent RTM mould to develop a better understanding of the mould filling process for hemp mat reinforced phenolic composites. The mould filling of unreinforced phenolics was characterised by a “quasi-one-dimensional steady state” flow. In the case of hemp non-woven reinforced system, the mould filling process can be considered as the flow of fluids through porous media. “Fibre washing” was a typical problem encountered during the injection process, leading to poor property uniformity. In addition, a preferential flow path was usually created near the edges and corners of the mould. The path exhibited low flow resistance and caused the resin flow front to advance much faster in these regions. The edge flow disturbed the steady flow, leading to difficulties in venting arrangement and “dry patch” formation. The edge flow and fibre washing were alleviated by reinforcement manipulation so steady state flow could be achieved. The relationships between the filling time and injection pressure and between filling time and different fibre weight fractions have been established for certain specific injection strategies.     

HPGe detectors for low-temperature nuclear orientation

Using the Low-Temperature Nuclear Orientation (LTNO) technique one can study various interesting properties of atomic nuclei and nuclear decay which can be deduced from the measurements of the angular distributions of charged particles emitted during the decay. However, the use of particle detectors working in conditions of LTNO devices (which are generally not available commercially) is a necessary precondition for the realization of these experiments.

Planar HPGe detectors for detection of charged particles at “liquid helium” temperatures were developed and produced at NPI e . Relatively simple technology using vacuum evaporation and diffusion was employed. The performance of detectors at low temperatures was tested and their characteristics measured in a testing cryostat before using them in real experiments.

The HPGe detectors were extensively used in a whole range of LTNO experiments with various physical objectives — in offline (IKS Leuven) as well as online (CERN-ISOLDE, Louvain-la-Neuve — LISOL) experiments. In frame of the project “Meson-Exchange Enhancement of First-Forbidden Beta Transitions in the Lead Region”, the measurements of angular distribution of emitted β-particles allowed to determine experimentally the “meson-exchange currents” contribution to the β-decay. In the project “Isospin Mixing in NZ nuclei”, the isospin-forbidden β-transitions of the nuclei in region (A=50–100) were studied in order to obtain information on the isospin structure of the nuclear states. A new project looking for the possible presence of the tensor currents contribution to the β-decay is being prepared for the CERN-ISOLDE facility.     

AlxGa−xAs/GaAs heterostructure characterization by wet chemical etching

Selective wet chemical etching of the Al_xGa_1−xAs/GaAs system has been applied to heterostructure characterization. Samples of LPE grown AlGaAs/GaAs laser double-heterostructures and separate confinement heterostructures as well as antiresonant reflecting optical waveguides heterostructures were treated with “I2 solution” (I₂:KI:H₂O) and hydrochloric acid. These compounds selectively etch the ternary Al_xGa_1−xAs layers, but with different “threshold composition” x_th values (the x value is that above which the etching rate of a given compound increases sharply). Selectively etched samples have been examined by SEM. The experimental dependence of etching rate on the x value for I2 solution has been derived. From this dependence, the x composition of any ternary layer can be estimated simply. Observations were made of the “microscopic” properties of the heterostructure, such as the smoothness of the interfaces and the uniformity of layers. All imperfections resulting from the growth process, such as interface perturbations or compositional nonuniformity of layers, are clearly seen. An additional advantage of this etching technique is its simplicity. It allows quick examination of grown heterostructure for the selection of wafers for further processing.     

Attitudes of commercial motor vehicle drivers towards safety belts

Despite the fact that Hawaii has one of the highest seat belt use rates for passenger vehicles in the United States, and has had a mandatory seat belt use law since the 1980s, studies have shown that commercial motor vehicles (CMV) seat belt use rates are low. To better understand this phenomenon, a comprehensive survey of commercial vehicle drivers was conducted in Hawaii to ascertain attitudes and self-reported behaviors regarding seat belt use. A total of 791 drivers responded to a written questionnaire implemented at weigh stations and distributed to various trucking firms and transport centers. Approximately 67% reported that they use seat belts “always” when driving a CMV (commercial motor vehicle), yet when asked how often do other CMV drivers use seat belts, only 31% responded “always.” Interestingly, 86% of these same drivers reported that they use seat belts “always” when driving a personal vehicle. The major reason cited for non-use of belts was “frequent stops/inconvenience” (29%), and “not safety conscious” (23%). Notably, the self-reported use of safety belts is highest among operators of vans (88% said “always”), followed by buses (87% said “always”) and lowest among truck drivers (only 60% said “always”). In this paper, some of the differences between self-reported users and non-users are explored and a multivariate logit model was developed to predict the odds of belt use as a function of various factors.     

Cadmium tellutide etched surfaces

Studies of etched surfaces of single-crystal cadmium telluride (CdTe) are presented. A CdTe-electrolyte system is used for electrical measurements. The possibility of carrying out electrochemical processes to alter the etched surface is demonstrated. Auger electron spectroscopy and electron loss spectroscopy depth profiles are obtained for the etched and electrochemically treated CdTe surfaces. Disturbed layers whose stoichiometry varies with depth differently for p- and n-type CdTe were detected on the surfaces.

Precise interpretation of the data is quite complex, but the model for a “nonequilibrium metal/insulator/semiconductor” structure is adopted to explain the elimination of the “pinning” effect with a reduction in the surface layer thickness.     

AN INVESTIGATION ON THE OPTICAL PROPERTIES OF CARBON BLACK, FULLERITE, AND OTHER CARBONACEOUS MATERIALS IN RELATION TO THE SPECTRUM OF INTERSTELLAR EXTINCTION OF LIGHT

In this work, the optical properties in the UV-VIS of three carbon blacks having specific surface area from 145 to 7 m²/g and produced with furnace or thermal processes have been studied. The results have been compared to the optical properties (in the same spectral range) of C₆₀ fullerene photopolymer, to fullerite i.e., the carbon soot containing fullerenes and to a sample of carbonaceous matter containing carbyne. The scope of the work was to verify if any of the carbonaceous matter studied was able to match the interstellar extinction spectrum which shows a “bump” at 217.5 nm and which was originally attributed to interstellar graphitic particles. None of the materials studied has shown a peak at 217.5 nm. All the carbonaceous materials studied show maxima of absorption from 252 to 267 nm with the exclusion of C₆₀ photopolymer which has a completely peculiar and different spectrum from all other materials with three maxima at 271, 389 and 510 nm. All the carbon materials studied do not match the 217 nm peak, hence, cannot be considered the carrier of the interstellar “bump” but may be present in some circumstellar shells of late type stars, where the peaks at 240-250 nm have been recorded and attributed to a more ordered and partially graphitized carbonaceous matter. In the discussion, the matrix effect, the particle size, and the clumping of the material studied have been considered as cause of the shift of the peak to longer wavelength.

The carbon black samples have also been extracted with pentane or ethanol and the polycyclic aromatic hydrocarbons (PAH) identified have been discussed in the frame of recent works and in relation to the role that they play in the interstellar medium as potential carriers of the diffuse interstellar bands (DIBS).     

Electrochromic applications

“Chromogenic” materials, i.e. compounds, especially oxides, of polyvalent elements with coloration dependent on oxidation state, are of high practical interest since they allow the construction of “electrochromic” devices whose optical properties, light absorption and reflection, can be controlled by an external voltage. After a presentation of the basic chromogenic reactions, a systematic overview of reported electrochromic thin-layer systems is given, examples are discussed, and the probable first large-scale electrochromic application, an antiglare rear view mirror, is outlined. The trend of patents indicates that the ideal electrochromic system for various applications is still to be found.     

Biomemory device composed of mutant azurin thin films modified by site-directed mutagenesis

A bioelectronic device composed of self-assembled mutant azurin thin films with memory function has been developed for molecular electronics. Azurin was recombined with a cysteine residue to enhance the stability of self-assembled protein on gold surface. The thin films of protein on gold substrate were investigated by surface plasmon resonance spectroscopy, scanning tunneling microscopy and Raman spectroscopy, respectively. The memory characteristics, including the “read”, “write” and “erase” functions of self-assembled azurin layer, were obtained with three distinct electrical states of azurin layers by cyclic voltammetry. These results show the proposed biomemory device as a step towards the protein based nanobiochip.     

The use of energy windowing to discriminate SNM from NORM in radiation portal monitors

Energy windowing is an algorithmic alarm method that can be applied to plastic scintillator-based radiation portal monitor (RPM) systems to improve operational sensitivity to certain threat sources while reducing the alarm rates from naturally occurring radioactive material. Various implementations of energy windowing have been tested and documented by industry and at Pacific Northwest National Laboratory, and are available in commercial RPMs built by several manufacturers. Moreover, energy windowing is being used in many deployed RPMs to reduce nuisance alarms and improve operational sensitivity during the screening of cargo. This paper describes energy windowing algorithms and demonstrates how these algorithms succeed when applied to “controlled” experimental measurements and “real world” vehicle traffic data.     

Preparation of Langmuir-Blodgett “built-up” multilayer films of a lightly substituted model aromatic, anthracene

It is generally considered almost impossible to make Langmuir-Blodgett “built-up” multilayer films of conjugated (aromatic) molecules unless their aromaticity is very heavily diluted with aliphatic substituents or molecules; this severely limits the scientific and technological applications of such films. For example, even to handle anthracene-derivative monolayers on water (without any attempt to transfer them to a substrate) has in the past required the anthracene to be substituted with an aliphatic side chain containing at least twelve carbon atoms; this prevents instability of the water-borne monolayer. As a result, the interesting and useful electrical and optical properties which conjugated multilayers would be expected to possess (because of the extended molecular π electron systems) are heavily diluted by aliphatic properties. We now report the successful deposition of high quality multilayers of lightly substituted anthracene derivatives, with aliphatic side chains as short as four CH₂ units. Improved techniques may well permit even shorter side chains to be used. We describe the deposition methods in detail and give experimental evidence that high quality films from one monolayer thick (about 1.5 nm) to 500 monolayers were indeed obtained; the interesting electrical and optical properties of these films, and their unique structure, will be discussed in detail at a later date. If these preparation techniques can be extended to other conjugated systems, as seems likely, the range of highly organized structures which can be constructed by the Langmuir-Blodgett method has been greatly increased, and the prospects for its technological use (in applications ranging from electronic devices to high temperature superconductors) have been improved.     

Imaging X-ray fluorescence spectroscopy: laboratory measurements

Following on from the proof-of principle measurements of Martin et al. (X-ray Spectrom. 28 (1999) 64) we further describe the development of an imaging X-ray fluorescence (IXRF) spectrometer with no moving parts. Our laboratory system is based on a microchannel plate (MCP) “lens”, a CCD X-ray detector with good sub-keV quantum efficiency and a conventional electron bombardment X-ray source. We have used this equipment to form images of a standard XRF target, demonstrating that “elemental maps” (images of the target in the characteristic X-rays of one particular element) may be formed with sub-millimetre resolution. In addition to fluorescent X-rays, we detected X-rays which had been Bragg reflected from the polycrystalline aluminium substrate of the target. It is possible that the resulting “Bragg images” may be exploited to measure spatially varying strain, manifested as lattice distortion, introduced, for example, by thin films deposited on the surface of a sample.     

Comparison groups in road safety studies: An analysis

When the safety effect of some treatment is to be estimated, one has to answer the question: “what would have been the ratio of the expected number of accidents ‘before’ and ‘after’ had the treated group been left without treatment? ” In retrospective observational studies, the count of accidents on a “comparison group” is often used to provide an answer. In consequence, the estimate of the safety effect depends on which of several comparison groups is chosen. It is therefore important to have guidance for this choice. The empirical inquiry in the companion paper led to several conclusions. The explanation of what has been noted by the examination of data requires analysis and is taken up in this paper.     

Correlation between electron spin resonance,electrical conductivity and optical absorption edge of co-evaporated thin films of the dielectric system SiO/V2O5

Measurements of electron spin resonance, d.c. and a.c. electrical conduction and optical absorption of thin film samples of SiO/V₂O₅ prepared by a co-evaporation process at a pressure in the range 8×10⁻⁶ to 3×10⁻⁵ torr are reported. It is found that the spin density of the mixed system decreases by about one order of magnitude compared with that of an SiO film deposited under similar conditions. This reduction correlates with the d.c. electrical conductivity which also decreases as the V₂O₅ content of the complex SiO/V₂O₅ films increases. At the same time the optical energy gap also decreases. At lower temperatures (down to 198 K), a.c. conductance measurements give evidence of hopping conduction. D.c. conductance shows a transition from hopping conduction to free-band (extended state) conduction at about 263 K.     

A mechanistic model for the effect of stress ratio on the LEFM fatigue crack growth behavior of metals and alloys—II. Crack-brittle materials

A recently proposed mechanistic model for the effect of stress ratio, R, on the LEFM (long) fatigue crack growth behavior of “crack-ductile” materials is extended here to explain and predict similar behavior under similar conditions of “crack-brittle” materials characterised by the presence of “static” modes of fatigue fracture in stages II and III. It is shown that in these materials the stage I behavior is similar, but the stages II and III behave differently from crack-ductile materials. Mechanism-based existence of two types of stage II curves characterised respectively by “ pure shear mode ” (SM-II) and “mixed-mode” (MM-II), both plotting linear but having different slopes, is introduced. It is shown that while stage SM-II is insensitive, stage MM-II is significantly sensitive to R, in the same material. Similar to stage I, another “ moving pivot-point ” exists at the transition from SM-II to MM-II, which slides down the “ master shear-curve ” with increasing R. Assuming a critical K_max for the initiation of static modes, a critical R for saturation of these modes, and Paris-type growth relations, a quantitative predictive model containing growth equations for stages SM-II and MM-II, has been developed. Stage III is discussed only qualitatively. Reasonably good agreement was found between predicted curves at selected R-values and a relatively large volume of experimental data for steels, Al-alloys and Ti-alloys. This simple, alternative model may be used for obtaining quick, fairly accurate and conservative estimates of R-influenced crack growth rates for design applications in preference to crack-closure which frequently requires elaborate and tedious experimental procedures.     

Isothermal transformation of low-carbon microalloyed steels

The microstructures of isothermal transformation products and time–temperature–transformation (TTT) diagrams of two low-carbon microalloyed steels have been investigated by the means of dilatation and microscopy. The results show there are three independent “C-shape” curves in the TTT diagrams of low-carbon microalloyed steels, namely, the polygonal ferrite/pearlite transformation “C-shape” curve, the massive ferrite transformation “C-shape” curve and the bainitic transformation “C-shape” curve, respectively. The effects of the carbon and Mo content on massive ferritic transformation are discussed.     

On new types of non-crystalline metallic composite

The structure and mechanical properties of new types of non-crystalline metallic composites, namely “glass-quasi-crystal”, “glass-disclinated nanocrystal” and “quasi-crystal(-glass)-disclinated nanocrystal” composites are theoretically examined. In particular, a theoretical model is proposed which effectively describes the relationship between plastic deformation and the growth of the glassy phase in metallic “glass-quasi-crystal” composite materials. Here also basic features of both the structure and the mechanical properties of the “glass-disclinated nanocrystal” and “quasi-crystal(-glass)-disclinated nanocrystal” composites are theoretically examined. It is shown that such composites are characterized by a very high yield stress.     

A dynamic measure of order in structural glasses

Although some patterns of physical behavior are common in the glass transition and in the properties of supercooled liquids and glasses (characteristic viscoelasticity, temperature dependence of viscosity and relaxation times, property evolution through “physical aging”, difficulties in performing equilibrium measurements or simulations, etc.), it is difficult to arrive at a definition of the glass transition which distinguishes it from other phenomena exhibiting similar features. The present paper addresses this problem by defining a dynamical measure of order involving the average “shape” of particle trajectories in supercooled liquids. This dynamic order parameter should provide a measure of “closeness” to the glass transition and some indirect insights into the physical nature of supercooled liquids and glasses. Arguments are given that the proposed dynamic measure of order [“generalized capacity”, C(T)] is related to the temperature-dependent “effective hydrodynamic radius” R_H(T) measured in supercooled liquids and model numerical calculations are given to support this view. Some consequences of the intermittent particle motion at low temperatures for stress relaxation are also discussed.