Control of the leakage current in SrTiO3 films by acceptor doping

Stoichiometric SrTiO3 (STO) films doped with Fe or Cr were prepared by r.f. magnetron sputtering technique. The effects of Fe or Cr doping in the SrTiO3 films were studied on the leakage current property which was discussed by defect chemistry. The experimental results can be explained by a model in which oxygen vacancies are the key defects responsible for the leakage current. Acceptor doping, with a small concentration of Fe or Cr, has led to a substantial improvement to 10–9 order in the leakage current density. Above the concentration of 0.010.02 mol% Fe2O3, Cr2O3, however, as the concentration increased, the leakage current increased. These acceptors in Ti4+ site are expected to electrically compensate for donor species such as oxygen vacancies, thereby reducing the concentration of mobile carriers that contribute to electrical conduction. Consequently, acceptor doped STO films have been shown to be superior to undoped films for applications requiring high leakage resistance, such as dynamic random access memory capacitors. © 1998 Chapman & Hall     

Electrophysicochemical characteristics of a waterpen point coronadischarge

Some interesting electrophysicochemical characteristics of waterpen point-to-plate discharges with DC and AC power have been investigated in a room atmosphere. The metal point electrode of a conventional point-to-plate air gap is severely bombarded by energetic ions from the discharge region near the point. Concurrently, the temperature of the point is greatly increased. There are different characteristics for the waterpen point discharge, because the surface temperature of the waterpen point stays low due to the evaporation of the moisture from the waterpen point. In this paper, the discharges and electrophysicochemical characteristics of a waterpen point and a Pt metal point have been studied to evaluate the differences. It was found that mists and sprays are dispersed from the waterpen point due to the bombardment of ions. Also, a positive corona is effective for mist dispersion, a negative corona disperses sprays, and AC corona disperses both mists and sprays. The AC discharge of the waterpen point generated more ozone of 56 ppm, while it was the negative DC discharge of the Pt point that produced more ozone of 30 ppm. The AC discharge of the waterpen point produced 0.11 ppm at 10 kV, and the Pt point produced 0.10 ppm at 15 kV. The DC discharge of the both points showed no NO formation     

High-resolution venography of the brain using magnetic resonance imaging

The purpose of this study was to evaluate a non-flow related magnetic resonance imaging method to visualize small veins independent of arteries in the human brain. A long TE, high-resolution 3D gradient echo MR acquisition was used to highlight venous information. The method is based on the paramagnetic property of deoxyhemoglobin and the resulting phase difference between veins and brain parenchyma at long echo times. The MR magnitude images were masked with a phase mask filter to enhance small structure visibility.. Venous information down to sub-pixel vessel diameters of several hundred microns is visible. Venous data are displayed in an angiographic manner using a minimum intensity projection algorithm. Both superficial veins and deep white matter veins are visible. The method has been successfully applied in volunteers. Preliminary results in patients with cerebral arteriovenous malformations indicate its potential in clinical applications. The proposed method is easy to implement and does not require administration of a contrast agent or application of specially designed rf pulses to highlight the veins. Rather it exploits the intrinsic magnetic properties (BOLD-effect) and the prolonged T ^_2* of venous blood. The method may be of diagnostic potential in the assessment of arteriovenous malformations or other vascular venous lesions. © 1998 Elsevier Science B.V. All rights reserved.     

Phase separation in segmented polyurethanes derived from mixtures of polyether polyols with different functionality

A series of segmented polyurethanes containing 60 wt° of hard segments (HS) was prepared from MDI (4,4-diphenylmethane diisocyanate) ethylene glycol and mixtures of a polyoxyethylene end-capped polyoxypropylene triol and a polyoxyethylene end-capped polyoxypropylene diol. The effects of the content of polyether diol in polyether polyols on phase separation and properties was investigated by dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC) and investigation of tensile properties. The DSC and DMA results indicate that the polyurethane derived from only polyether triol exhibits obvious phase separation and that the HS is immiscible with the SS, but that the HS is compatible with the HS for the polyurethane derived from polyether diol. As the content of polyether diol increases, the compatibility between HS and SS increases. As the content of polyether diol increases, the tensile strength. elongation. toughness and tear resistance of the polyurethanes increases. but their moduli decrease. The modulus-temperature dependence in the temperature region of –30 to 65 °C increases as the polyether diol content increases.     

Method to derive ocean absorption coefficients from remote-sensing reflectance

A method to derive in-water absorption coefficients from total remote-sensing reflectance (ratio of the upwelling radiance to the downwelling irradiance above the surface) analytically is presented. For measurements made in the Gulf of Mexico and Monterey Bay, with concentrations of chlorophyll-a ranging from 0.07 to 50 mg/m(3), comparisons are made for the total absorption coefficients derived with the suggested method and those derived with diffuse attenuation coefficients. For these coastal to open-ocean waters, including regions of upwelling and the Loop Current, the results are as follows: at 440 nm the difference between the two methods is 13.0% (r(2) = 0.96) for total absorption coefficients ranging from 0.02 to 2.0 m(-1); at 488 nm the difference is 14.5% (r(2) = 0.97); and at 550 nm the difference is 13.6% (r(2) = 0.96). The results indicate that the method presented works very well for retrieval of in-water absorption coefficients exclusively from remotely measured signals, and that this method has a wide range of potential applications in oceanic remote sensing.     

Dynamic digital photorefractive memory for optoelectronic neural network learning modules

Neural network modules based on page-oriented dynamic digital photorefractive memory are described. The modules can implement two different interconnection organizations, fan-out and fan-in, depending on their target network applications. Neural network learning is realized by the real-time memory update of dynamic digital photorefractive memory. Physical separation of subvolumes in the page-oriented photorefractive memory architecture contributes to the low cross talk and high diffraction efficiency of the stored interconnection weights. Digitally encoded interconnection weights ensure high accuracy, providing superior neural network system scalability. Module scalability and feedforward throughput have been investigated based on photorefractive memory geometry and the photodetector power requirements. The following four approaches to extend module scalability are discussed: partial optical summation, semiparallel feedforward operation, time partitioning, and interconnection matrix partitioning. Learning capabilities of the system are investigated in terms of required interconnection primitives for implementing learning processes and three memory-update schemes. The experimental results of Perceptron learning network implementation with 900 input neurons with digital 6-bit accuracy are reported.     

Optical monitoring of silver-based transparent heat mirrors

Both three-layer (TiO(2)-Ag-TiO(2)) and five-layer (TiO(2)-Ag-TiO(2)-Ag-TiO(2)) heat mirrors with optimum transmission in the visible and good reflectance in the IR have been designed by admittance diagram techniques. The mirrors were fabricated successfully by optical monitoring. An interesting anomalous layer was found and explained, and its equivalent refractive index and thickness are 2.015 - i0.016 and 2.56 nm, respectively.     

Characterization of centrifugally atomized Al-12Si-5Fe-3Cu-1Mg(wt%) alloy powder and extruded rod

The Al-12Si-5Fe-3Cu-1Mg(wt%) alloy was rapidly solidified by centrifugal atomization. The microstructural characteristics of rapidly solidified powder and the microstructure changes with heat treatment were investigated in terms and related to powder size. The microstructures of the powder consisted of dendritic -Al, eutectic phase, Cu-rich phase, and needle-like intermetallic compounds. These phases were much finer than that of ingot cast structure and the size decreased with increasing cooling rate. The X-ray diffraction of the atomized powders revealed the presence of non-equilibrium 3-(AlFeSi) intermetallic phase. This phase appeared to transform to an equilibrium -(AlFeSi) phase by heating at temperatures above 470°C. The extruded rod which was hot extruded at 360°C with an extrusion ratio of 40:1 also revealed the presence of the -(AIFeSi) intermetallic phase. Using DSC, the exothermic peak due to precipitation from the supersaturated -Al matrix was observed in the range of 200–250°C during continuous heating of atomized powder, and the size of the peaks increased with decreasing powder size.     

Characteristics of sol-gel derived PZT thin films with lead oxide cover layers and lead titanate interlayers

The sol-gel derived PbZr_0.53Ti_0.47O₃ (PZT) films were fabricated on the bare Pt/Ti/SiO₂/Si substrates or the same substrates coated by the PbTiO₃ (PT) interlayers. The post-deposition annealing temperature and time were optimized when the PbO cover layers and PbO vapour-containing atmosphere were compared with each other and adopted as the method to diminish the lead-loss problem during the high-temperature post-deposition annealing. The X-ray diffraction patterns, microstructures, and electrical properties such as relative permittivity, _r, remanent polarization, P _r, and coercive electrical field, E _c, were investigated in relation to the annealing conditions. The PZT films deposited on the bare Pt/Ti/SiO₂/Si substrates under the PbO vapour-containing atmosphere showed better electrical properties. This indicates that the PbO vapour-containing atmosphere may be the better method of lead-loss-prevention to process the lead-containing films rather than the PbO cover layer method. The electrical characteristics of the PZT films, _r=1150, a dissipation factor of 0.039, P _r=26 C cm^–2, and E _c=40.5 kV cm^–1 were measured at 1 kHz. When PZT films were deposited on substrates coated by the PT layers, PZT-PT films with single perovskite phase were derived by post-deposition annealing at 500 °C for 1 h. However, the relative electrical properties are very poor, i.e. E _r=160, P _r=2.0 C cm^–2 and E _c=75 kVcm^–1. The optimum combination for preparing PZT-PT films is a 40 nm PbTiO₃ interlayer and annealing conditions of 6 h at 550 °C in a PbO vapour-containing atmosphere; the derived films exhibit electrical properties of E _r=885, P _r=21.5 C cm^–2 and E _c=64 kV cm^–1. The combination of inserting a PT interlayer and annealing in a PbO vapour-containing atmosphere can prevent the formation of electrical short paths. In this case, nearly pin-hole-free PZT films can be grown on the PT (interlayer) /Pt/Ti/SiO₂/Si substrates. It is believed that it is possible to prepare the PZT films with nano-scale uniformity, reproducible quality, which may be worth considering for commercial applications.     

Microstructural evolution of potassium titanate whiskers during the synthesis by the calcination and slow-cooling method

K₂Ti₆O₁₃ whisker was synthesized by the calcination and slow-cooling method, and the microstructural evolution of whiskers, and the relations between the phase and the layer structure, were investigated. The amount of K₂O in the starting powder mixture played a key role in forming a layer structure, which can be obtained by addition of excess K₂O. During calcination, rod-like K₂Ti₆O₁₃ particles with a layer structure were formed by reactions between K₂O and TiO₂. During slow-cooling, the K₂O-rich liquid phase reacted with K₂Ti₆O₁₃ layers near the boundary and k₂Ti₄O₉ whiskers were produced by the splitting of layers within rod-like k₂T₆O₁₃ particles. k₂Ti₄O₉ whiskers were reformed into k₂T₆O₁₃ whiskers by treatment in boiling water and reheating. The synthesized k₂T₆O₁₃ whiskers had a clean surface and a length exceeding 100 m.