System Identification of Base-Isolated Building using Seismic Response Data

Due to the complex nature of the excitation, and the inherent dynamics characteristics of restoring force of the base isolation systems, the response of base-isolated structures subject to strong earthquakes often experiences excursion into the inelastic range. Therefore, in designing base-isolated structures, the nonlinear hysteretic restoring force model of the base isolation system is frequently used to predict structural response and to evaluate structural safety. In this paper, the prediction error method system identification technique is used in conjunction with nonlinear state-space models for identification of a base-isolated structure. Using a variety of nonlinear restoring force models and bidirectional recorded seismic responses, several identification runs are conducted to evaluate the accuracy of the selected models. Several nonlinear restoring force models are utilized for the base-isolation system, including a multiple shear spring (MSS) model. Among all models used, results indicate that the trilinear hysteretic MSS model closely matches the actual hysteretic restoring force profile and time histories obtained directly from the observed data.     

GRAPES-3—A high-density air shower array for studies on the structure in the cosmic-ray energy spectrum near the knee

The change in the spectral index from about -2.7 to -3.1 at 3×10¹⁵ eV in the all-particle energy spectrum of primary cosmic rays is very significant for learning about the nature of cosmic sources of ultra-high energy particles and their acceleration and propagation in the galactic disk. Any observation of a fine structure in the spectrum would be important for improving our understanding of these physical processes. The GRAPES-3 air shower array has been designed to achieve higher precision in determination of various shower parameters to enable observation of any fine structure in the energy spectrum, if it exists. The details of the shower detectors, shower trigger and the data acquisition system are presented here along with estimates of trigger efficiencies from Monte Carlo simulations for primary photons (γ-rays) and several nuclei.     

Oxidation protective silicide coating on Mo-Si-B alloys

A MoSi₂ coating was successfully formed on a Mo-9Si-18B alloy, consisting of Mo₅SiB₂ (T₂) and Mo solid solution (Mo _ss ) phases, using pack cementation with Si. Isothermal and cyclic oxidation tests of pack-cemented Mo-9Si-18B alloys were performed at 1300 °C and 1500 °C. Steady-state oxidation rates at both temperatures are almost equal to those of pure MoSi₂. The MoSi₂ layer is completely transformed into Mo₅Si₃ (T₁) containing B after oxidation at 1500 °C for 24 hours. Thermal expansion of the T₁ phase is anisotropic, but a [001] texture in the growth direction for the columnar grains in the T₁ layer reduces thermal stresses generated around the phases. Evolution of T₁ layers during oxidation between 1300 °C and 1500 °C was investigated; their growth rate constants and the interdiffusion coefficient of Mo and Si in the Mo-Si-B system have been evaluated and compared with those in the binary Mo-Si system. Furthermore, we have studied phase transformations in a simpler system MoSi₂ vs T₂ using MoSi₂/T₂ diffusion couples. Layers of T₁ and MoB + T₁ were formed in the diffusion zone during oxidation at temperatures between 1400 °C and 1600 °C. This behavior is different from that of the pack-cemented Mo-9Si-18B alloy. Pack-cemented T₂ single crystals show a diffusion structure similar to that of MoSi₂/T₂ diffusion couples, but the ratio of layer thickness is different. Based on these diffusion results, a method for extending the lifetime of the MoSi₂ layer is proposed. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

AlGaN/GaN HEMTs With Thin InGaN Cap Layer for Normally Off Operation

AlGaN/GaN HEMTs with a thin InGaN cap layer have been proposed to implement the normally off HEMTs. The key idea is to employ the polarization-induced field in the InGaN cap layer, by which the conduction band is raised, which leads to the normally off operation. The fabricated HEMT with an In_0.2Ga_0.8N cap layer with a thickness of 5 nm showed normally off operation with a threshold voltage of 0.4 V and a maximum transconductance of 85 mS/mm for the device with a 1.9-mum-long gate. By etching off the In_0.2Ga_0.8N cap layer at the access region using gate electrode as an etching mask, the maximum transconductance has increased from 85 to 130 mS/mm due to a reduction of the parasitic source resistance.     

Covalent Functionalization of Carbon Nanohorns with Porphyrins: Nanohybrid Formation and Photoinduced Electron and Energy Transfer

The covalent attachment of carbon nanohorns (CNHs) to α‐5‐(2‐aminophenyl)‐α‐15‐(2‐nitrophenyl)‐10,20‐bis(2,4,6‐trimethyl‐phenyl)‐porphyrin (H₂P) via an amide bond is accomplished. The resulting CNH–H₂P nanohybrids form a stable inklike solution. High‐resolution transmission electron microscopy (HRTEM) images demonstrate that the original dahlia‐flowerlike superstructure of the CNHs is preserved in the CNH–H₂P nanohybrids. Steady‐state and time‐resolved fluorescence studies show efficient quenching of the excited singlet state of H₂P, suggesting that both electron and energy transfer occur from the singlet excited state of H₂P to CNHs, depending on the polarity of the solvent. In the case of electron transfer, photoexcitation of H₂P results in the reduction of the nanohorns and the simultaneous oxidation of the porphyrin unit. The formation of a charge‐separated state, CNH^?––H₂P^?+, has been corroborated with the help of an electron mediator, hexyl‐viologen dication (HV²⁺), in polar solvents. Moreover, the charge‐separated CNH^?––H₂P^?+ states have been identified by transient absorption spectroscopy.     

Charging characteristics and electric field distribution on alumina as affected by triple junctions in vacuum

This paper describes the dependence of the charging characteristics on the electric field distribution on the alumina (Al/sub 2/O/sub 3/) surface as affected by the triple junction in vacuum. For HV electrical insulation design of vacuum interrupter, surface flashover in vacuum is very important problem to be solved. Attention should be paid to the fact that the insulation characteristics on the dielectric surface are strongly influenced by field emission of electrons from the triple junction and the accumulated charges on the dielectric surface. In order to clarify the charging mechanism, we measured the charging characteristics for various types of triple junctions. In particular, we focused on the influence of the electric field distribution along the solid dielectrics and near the cathode triple junction (CTJ) on the charging characteristics. The results confirmed that the electric field distribution strongly affected the 2-dimensional (2D) distribution of the surface charge on the dielectric sample. Consequently, it was found that positive charging was generated on alumina, when the incident angle of the electric line of force on the alumina surface became >60/spl deg/.     

Change of dielectric property with water-treed region

By dividing a water-treed XLPE sheet sample into a non-degraded layer and a water-treed one, relative permittivity ε_r2' and dielectric loss factor ε_r2^" of the water-treed layer have been estimated using an equivalent circuit. The way of changes in ε_r2' and ε_r2^" with the water tree length r estimated by the pulsed electroacoustic method has been discussed based on a Maxwell-Wagner-Sillars (MWS) model. It has been concluded that the MWS model gives a reasonable fit to the experimental data providing that the water conductivity in voids lies between 1×10^-3 S/m and 8×10^-3 S/m, and the minor and major axis ratio of the ellipsoids, to which water-filled voids are compared, lies in the range of 1:20 to 1:50     

Modified photo-sensors for grating-image type micro-encoder

New integration technique for grating-imaging-type encoder is proposed and the sensor for the encoder has been fabricated using Si micromachining technology. The sensor consists of the Si grids and line photodiodes. The light emission through the Si grids and the light detection with the four-phase-shifted line photodiodes make the optical system compact. Moreover, another photodiode array of the M-series arrangement is installed on the same substrate for the zero-point detection.     

160 GHz actively modelocked semiconductor laser

A 163.66 GHz optical pulse train was generated by a semiconductor modelocked laser integrated with a high-mesa EA modulator, which enables direct synchronisation with a 81.83 GHz electrical signal. A timing jitter of 0.15 ps and low amplitude modulation noise are achieved.     

Novolac epoxy resins and positron annihilation

Four kinds of epoxy resins: cresol novolac, tris-hydroxyphenylmethane, tetramethylbiphenol, and bisphenol A, were cured with phenol novolac epoxy resins. Characteristics of these epoxy compounds were studied by the positron annihilation lifetime (PAL) technique. Glass transition temperatures, thermal expansion coefficients, and volume of intermolecularspace holes among polymer chains were obtained from the lifetime, τ₃, of the long-lived component of ortho-positronium. It was revealed that, at the glass transition temperature, T_g, the volume of the hole created among polymer chains expanded 1.4 times the volume at room temperature. The smaller flexural modulus of tris-hydroxyphenylmethane than that of the other samples was explained by the volume of intermolecular-space holes obtained from τ₃. Aging effects were seen in the data of the intensities, I₃, of ortho-positronium, which became smaller after heating the samples above T_g. I₃ and τ₃ were strongly affected by the density of cross-linkings, and their chemical structures. The larger the density of cross-linkings, the smaller I₃ and higher T_g were obtained. Epoxy compounds with the higher water-absorption rates had larger intermolecular-space holes. © 1993 John Wiley & Sons, Inc.