Defect levels in chromium-doped silicon

The transient capacitance technique has been used to study the chromium-related levels in the silicon band gap. Chromium was diffused at temperature of 1100 and 1150°C for 0.5 and 3 hr. Five different levels at E_c?0.11 eV, E_c?0.21 eV, E_c?0.28 eV, E_c?0.36 eV and E_c?0.45 eV were obtained from the Arrheniu plots of the electron thermal-emission rates. The number of levels in the upper half of the band gap decreased from five to two with an increase of Cr-diffusion period. Two levels were located at E_c?0.20 eV (donor) and E_c?0.43 eV (acceptor). A donor level was also observed at E_v + 0.25 eV. The donor level was not affected by the diffusion condition. The majority carrier capture cross sections of the three dominant levels have been measured by the transient capacitance technique modified by the pulse transformer. The values were σ_n = 4.1 × 10^?15 cm² for the upper donor at E_c?0.20 eV, σ_n = 2.0 × 10^?16 cm² for the acceptor at E_c ?0.43 eV and σ_p = 9.1 × 10^?18 cm² for the lower donor at E_v + 0.25 eV, and were independent of temperature. The three dominant levels are due to distinct chromium centers.     

Changes in photovoltaic and dark electrical properties of hydrogenated amorphous silicon diodes induced by forward bias carrier injection

The photovoltaic and dark electrical properties of hydrogenated amorphous silicon diodes were changed by forward bias carrier injection for several hours. These changes were similar to photoinduced (PI) changes previously reported, and this result supports previous explanations for PI changes. The differences between these two types of change are also discussed.     

Optical topography: practical problems and new applications

We will briefly review the present status of optical topography and then discuss the method of improving practicality, i.e., the signal-to-noise (S/N) ratio and the spatial resolution in observations of higher-order brain functions. The optimum wavelength pair improved the S/N ratio sixfold for deoxyhemoglobin, and new configurations of light irradiation and detection positions doubled the spatial resolution. We also report on developing application fields of optical topography. This modality will bridge the gap between natural sciences, neuroscience, and pedagogy, and show actual real-time brain activity.     

Chemicofunctional membrane for integrated chemical processes on a microchip

Here we report a design and synthesis of a chemically functional polymer membrane by an interfacial polycondensation reaction and multilayer flow inside a microchannel. Single and parallel dual-membrane structures are successfully prepared by using organic/aqueous two-layer flow and organic/aqueous/organic three-layer flow inside the microchannel followed by an interfacial polycondensation reaction. By using the inner-channel membrane, permeation of ammonia species through the inner-channel membrane is successfully achieved. Furthermore, horseradish peroxidase is immobilized on one side of the membrane surface to integrate the chemical transform function onto the inner-channel membrane. Here substrate permeation through the membrane and subsequent chemical transformation at the membrane surface are realized. The polymer membrane prepared inside the microchannel has an important role in ensuring stable contact of different phases such as gas/liquid or liquid/ liquid and the permeation of chemical species through the membrane. Furthermore, membrane surface modification chemistry allows chemical transformation of permeated chemical species. These methods are expected to lead to development of complicated and sophisticated chemical systems involving membrane permeation and chemical reactions.     

Preparation of porous sheet composite impregnated with TiO<Subscript>2</Subscript> photocatalyst by a papermaking technique

Titanium dioxide (TiO₂) powder-containing sheet composites, called TiO₂ sheet, were prepared by a papermaking technique, and their photocatalytic efficiency was investigated. The TiO₂ powders were homogeneously scattered over the fiber-mix networks tailored within the catalyst sheet. Under UV irradiation, the TiO₂ paper could decompose p-hydroxyacetophenone (p-HAP), although the degradation efficiency by the TiO₂ sheet was lower than that by the TiO₂ powder. Scanning electron microscopy revealed that coverage of the TiO₂ particles inside the sheet by alumina binder which was used to improve the sheet strength caused the deterioration of the photocatalytic performance. Internal addition of alumina binder made the TiO₂ sheet porous and such a TiO₂ sheet exhibited high photocatalytic performance equivalent to that of TiO₂ powder. The porous structure of TiO₂ sheet might contribute to effective transport of p-HAP to the surface of TiO₂ particles inside the sheet, resulting in high degradation performance. In addition, TiO₂ sheet prepared using TiO₂ sol showed higher photocatalytic efficiency than TiO₂ powder and it was indicated that the porous sheet structure might provide suitable conditions for TiO₂ catalysis for photodecomposition.     

Preparation of carbon microparticle assemblies from phenolic resin using an inverse opal templating method

Three-dimensionally (3D) well-ordered carbon microparticle assemblies with different particle morphologies were fabricated by infiltration of phenolic resin solution into SiO₂ inverse opal structures and subsequent carbonization. The effect of the phenolic resin solution concentration and the carbonization temperature on the morphology of the fabricated carbon microparticles was investigated. At a carbonization temperature of 1000 °C, carbon microparticles with interlocked bridges were obtained when the concentration of phenolic resin solution is 40 wt% and hollow carbon microparticles with opened window channels were obtained at a concentration of 30–35 wt%. When the carbonization temperature was decreased to 500 °C, carbon microparticles with interlocked bridges also were observed, even when the phenolic resin concentration was 30 wt%. The structures and properties of the carbon microparticles and their assemblies were characterized using SEM, XRD, and N₂ adsorption.     

Disintegration process of yttria-stabilized zirconia ceramics using hydrothermal conditions

Capability of the recycling of high strength and high fracture toughness yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) sintered body utilizing “low-temperature annealing degradation” phenomenon was investigated. Hydrothermal treatment was employed to induce the phase transformation from tetragonal to monoclinic zirconia and to disintegrate the Y-TZP sintered body. 3 mol% Y₂O₃–ZrO₂ specimens sintered at 1,550 °C and more were disintegrated without leaving the original appearances when the treatment temperature was between 200 °C and 400 °C. The size of the disintegrated fragments of Y-TZP sintered body was much affected by hydrothermal treatment conditions. Only with hydrothermal treatment and simple ball milling, the sintered body was pulverized into the primary particle level. This technique is expected to apply to a sustainable recycling system for the zirconia ceramics, which restrains an energy consumption compared to crushing zirconia using mechanical procedures.     

Ns-kriging based microstructural optimization applied to minimizing stochastic variation of homogenized elasticity of fiber reinforced composites

This paper describes an approximate microstructural optimization using the ns-kriging (noise-resistant smoothed kriging) method for minimizing the maximum stochastic variation of homogenized elastic properties of a composite material caused by microscopic uncertainties of component materials. Since evaluation of a stochastic characteristic of a homogenized material property such as expectation or variance will involve a high computational cost and its results include inaccuracy in using the Monte Carlo simulation, an approximation-based optimization technique is useful for solving the optimization problem considering the multiscale stochastic problem. Especially, the ns-kriging will work well in case of using inaccurate data for an unknown objective function. In order to investigate applicability and effectiveness of the proposed ns-kriging based approach to the optimization problem, it is applied to the cross-sectional shape optimization of fiber in a unidirectional FRP. From the numerical results, validity and effectiveness of the proposed approach are confirmed.     

Analysis of Dispatching Rules of AGV Systems with Multiple Vehicles

We study automated guided vehicle systems that contain multiple vehicles. Each vehicle transports an item from a certain point to its destination. The inter-arrival time of items to each node is assumed to be independently and geometrically distributed. Each node has an infinite buffer capacity, and thus no item is lost. The deadhead time of the empty vehicle and the transportation time of the loaded vehicle are assumed to be arbitrarily distributed. We propose two empty vehicle dispatching policies, and numerically calculate the waiting time distributions of the items for each policy using a state-space reduction technique for Markov chains. Several examples are given to demonstrate the advantages and disadvantages of each policy.     

Friction and frictional tracks on thick silica films prepared by vacuum deposition

Friction and frictional tracks on 2 µm thick SiO₂ films evaporated on polymethylmethacrylate (PMMA) substrate were investigated. Diamond spherical sliders of radius 30 and 100 µm, respectively, were slid on these coatings under a load of 50 to 200g at a sliding speed of 15 cm min^–1. The static and dynamic friction coefficients for SiO₂ films were found to be 0.1 and 0.06, respectively, depending on the load and radius of the slider. For lower load and small slider radius the tracks on SiO₂ film were groove-like, and whisker-like cracks regularly grew from the edges of the tracks. For higher loads and larger slider radius, semicircular cracks in the film were regularly found behind the slider, but in thicker film (6 µm thick), circular cracks occurred. The origin of these cracks is discussed in terms of a tension zone produced around the contact area between the slider and the substrate under frictional force.