Full-rate cooperative communications with spatial diversity for half-duplex uplink relay channels

This paper proposes a full-rate cooperative communication technique with spatial diversity for single-carrier transmissions. In order to achieve both the spatial diversity and the full-rate, data streams are simultaneously transmitted through all direct and relay channels with different phase rotation and cyclic delay patterns. The phase rotation and cyclic delay patterns are derived in the sense of minimizing interlayer interference and a corresponding destination structure is presented for decoupling each layer at the destination. Simulation results show that the proposed technique achieves spatial diversity without sacrificing spectral efficiency and outperforms conventional full-rate cooperative communication techniques.     

CdSe-sensitized inorganic–organic heterojunction solar cells: The effect of molecular dipole interface modification and surface passivation

CdSe-sensitized heterojunction solar cells composed of mesoscopic TiO₂/CdSe/P3HT (poly-3-hexylthiophene) were constructed, and the negative molecular dipole of 4-methoxybenzenethiol (MBT) and the ZnS passivation layer were used as interface modifiers to improve device performance. Through the interface modification between TiO₂/CdSe and P3HT using MBT and by ZnS surface passivation, the power conversion efficiency of the modified solar cell was greatly enhanced from 1.02% to 1.62% under 1 sun illumination.     

Shape-controlled fabrication of polypyrrole microstructures by replicating organic crystals through electrostatic interactions

Polypyrrole (PPy) microstructures with diverse shapes were synthesized in an aqueous inorganic salt medium including organic crystals and pyrrole (Py). A series of sulfobenzoic acid salt forms with various cations (K⁺, Na⁺, Li⁺, NH₄⁺) in different positions (para, meta, ortho) of the sulfonate group on the benzene ring were used to form organic crystals as sacrificial templates. Using these crystals, we produced five different shapes of PPy microstructures (hexagonal microplates, curled nanofibers, lozenge-shaped microplates, rigid rods, parallelogram microplates), which replicated the shapes of the organic crystal templates through electrostatic interaction between the anionic crystal surfaces and the cationic PPy chains. In contrast, PPy that was polymerized without crystals showed bulky agglomerates of 200-500 nm size. The electrical properties were dictated by the molecular structures of the organic salt molecules used. While the highest conductivity (200.3 Scm⁻¹) was observed in PPy using crystals of para-linked 4-sulfobenzoic acid monopotassium salt, the lowest conductivity (0.8 Scm⁻¹) was observed in PPy prepared in the presence of crystals of ortho-linked 2-sulfobenzoic acid monoammonium salt.     

The electrochemical behavior of an enzyme biosensor electrode using an oxyfluorinated pitch-based carbon

A glucose sensor electrode was prepared by thermally treating a pitch-based carbon material. Oxyfluorination was used to modify the surface of the prepared carbon to induce the formation of hydrophilic functional groups. A glucose oxidase enzyme was effectively loaded onto the surface of the oxyfluorinated carbon and was more sensitive in glucose sensing because of the effects of the improved interfacial affinity between the electrode and the glucose oxidase. The introduced hydrophilic functional groups were examined using XPS analysis. In current–voltage measurements, a higher current was observed in the samples prepared with a higher oxygen content. In addition, a clear redox peak was observed in the surface modified samples. These results can be attributed to efficient electrical resistance measurement by easy electron transfer during glucose sensing. An efficient glucose sensor electrode was prepared using pitch-based carbon, which has beneficial electrical properties, and oxyfluorination, which improves the surface interface.     

Synthesis of ultra-thin polypyrrole nanosheets for chemical sensor applications

Ultra-thin polypyrrole nanosheets (UPNSs) are fabricated by organic crystal surface-induced polymerization (OCSP) of pyrrole in an aqueous suspension containing hydrated crystals of sodium decylsulfonate (C₁₀SO₃Na) below the Krafft temperature using FeCl₃ as an oxidant. The hydrated C₁₀SO₃Na crystals are used as templates through electrostatic binding of the cationic polypyrrole (PPy) chains oxidized by Fe(III) ions on the anionic C₁₀SO₃Na crystal surface. The resulting UPNSs have a single layer thickness of ∼21 nm, widths between 2 and 6 μm, and lengths greater than 10 μm. The UPNSs are composed of a single continuous PPy domain. Moreover, the UPNSs exhibit higher conductivity (30.6 Scm⁻¹) and longer conjugation lengths than the PPy nanoparticles (2.4 Scm⁻¹) prepared using emulsion polymerization. We systematically investigate the UPNSs as gas sensors for detecting and quantifying toxic gases such as HCl and NH₃. The UPNSs exhibit much higher gas sensitivity and faster response times compared with the PPy nanoparticles.     

Effects of Two-stage Heat Treatment on Delayed Coke and Study of Their Surface Texture Characteristics

In the present study, surface texture features and chemical properties of two types of cokes, made from coal tar by either 1-stage heat treatment or 2-stage heat treatment, were researched. The relationship between surface texture characteristics and the chemical properties was identified through molecular weight distribution, insolubility of coal tar, weight loss with temperature increase, coking yield, and polarized light microscope analysis. Rapidly cleared anisotropy texture in cokes was observed in accordance with the coking temperature rise. Quinoline insolubility and toluene insolubility of coal tar increased with a corresponding increases in coking temperature. In particular, the cokes produced by the 2-stage heat treatment (2S-C) showed surface structure of needle cokes at a temperature approximately 50°C lower than the 1-stage heat treatment (1S-C). Additionally, the coking yield of 2S-C increased by approximately 14% in comparison with 1S-C.     

Anisotropic threshold stress intensity factor, KIH and crack growth rate in delayed hydride cracking of Zr-2.5Nb pressure tubes

The objectives of this study are to systematically investigate the delayed hydride cracking (DHC) velocity and the threshold-stress intensity factor, K _IH , of a Zr-2.5Nb pressure tube as a function of orientation and elucidate the cause of this anistropic DHC behavior. The DHC velocity as a function of orientation was determined using flattened cantilever beam specimens with 60 ppm H while the threshold-stress intensity factor K _IH , was evaluated as a function of orientation on the curved compact-tension (CT) and cantilever-beam (CB) specimens charged with hydrogen to 200 ppm H. To infer a difference in a stress gradient ahead of the crack tip as a function of orientation, tensile tests were conducted at temperatures ranging from room temperature (RT) to 560 °C using small tensile specimens of 2-mm-gage length taken from three directions of the tube. A textural change was investigated by comparing the inverse pole figures before and after DHC while the {10 7} pole figures were constructed to find out the growth pattern of the DHC crack as a function of orientation. Faster DHC velocity and lower K _IH were obtained over temperatures of 170 °C to 270 °C, when the DHC crack grew in the longitudinal direction of the Zr-2.5Nb pressure tube. The strain hardening after yielding and the extent of the textural change accompanied by DHC were higher in the longitudinal direction of the tube, suggesting a higher stress gradient ahead of the crack tip. Thus, the anisotropic DHC behavior of a Zr-2.5Nb pressure tube is discussed based on the stress gradient ahead of the crack tip governed by strain-hardening rate after yielding and a change in texture accompanied by DHC, and the distribution of the {10 7} hydride habit planes. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

A microfluidic gel valve device using reversible sol–gel transition of methyl cellulose for biomedical application

We have fabricated a microfluidic gel valve device that used reversible sol–gel transition of methyl cellulose (MC). A microheater and a microtemperature sensor were implemented in each microchannel in the gel valve device. Before evaluating the performance of the gel valve device, various properties of the MC solution were investigated using viscometer, spectrophotometer, and NMR. Gelation temperature was increased as the MC concentration was increased. Clear gel, an intermediate state between clear sol and turbid gel, was found at the temperature range from 30–40°C to 50–60°C. Temperature at each microchannel of the device was measured and the effect of the temperature difference on the valve operation was elucidated. In order to have normal operation of the gel valve, it was important to keep the temperature of the heated microchannel around 60°C while keeping the temperature of the flowing microchannel below 35°C. The temperature difference between two microchannels was about 23 K when fan forced cooling (FFC) method was used. For normal performance of the gel valve device, a temporary pause of fluid flow for at least 5 s was required to complete the local gelation in the microchannel. Stable gel valve performance was obtained at the flow rates larger than 5 μl/min. The gel valve device showed no leakage up to 2.07×10⁴ Pa.