Microstructure and Properties of Al-6061 Alloy by Equal Channel Angular Extrusion for 16 Passes

The experimental researches on Equal Channel Angular Extrusion (ECAE) process of commercial available Al-6061 alloy were conducted and the grain refinement after ECAE processing was investigated. Sixteen passes of ECAE processing at room temperature were conducted and the relationship of grain refinement with extrusion pass was established. The property enhancements after ECAE processing including ultimate tensile strength and Vickers microhardness were investigated to determine the effects of the number of ECAE passes on the mechanical properties of the extruded samples. The research presents a whole picture of ECAE processing of the alloy for up to 16 passes.     

The behavior of angle-ply laminated cylindrical shells with viscoelastic interfaces in cylindrical bending

The behavior of a simply supported angle-ply laminated cylindrical shell in cylindrical bending with viscoelastic interfaces is studied. The Kelvin–Voigt model is adopted to represent the character of interfaces. State-space formulations are developed based on the exact elasticity equations, and in particular, a variable substitution technique is used to derive the state equations with constant coefficients. Since the behavior of this structure under static loading is time-dependent, the power series expansion technique is used to approximate the variations of physical variables with time. The response for a laminated shell with viscous interfaces is also investigated as a particular case. Results show that the displacements as well as the maximum stresses in the panel increase rapidly with time. Thus, the imperfect bonding properties should be considered carefully in the design of laminated structures.     

ZnO metal-semiconductor-metal ultraviolet photodetectors with Iridium contact electrodes

ZnO epitaxial films were grown on sapphire (0001) substrates by using rf plasma-assisted molecular beam epitaxy. Metal-semiconductor-metal (MSM) photodetectors with Iridium (Ir) electrodes were then fabricated. It was found that Schottky barrier heights at the non-annealed and 500-C-annealed Ir/ZnO interfaces were around 0.65 and 0.78-eV, respectively. With an incident wavelength of 370-nm and 1-V applied bias, it was found that the maximum responsivities for the Ir/ZnO/Ir MSM photodetectors with and without thermal annealing were 0.18 and 0.13-A/W, respectively. From transient response measurement, it was found that time constant - of the fabricated photodetectors was 22-ms. For a given bandwidth of 100-Hz and 1-V applied bias, we found that noise equivalent power and corresponding detectivity D- were 6-10.     

Catalytic chemical vapor deposition of carbon nanotubes using Ni-La-O precursors

Carbon nanotubes (CNT) are synthesized by catalytic chemical vapor deposition with different compositions of Ni-La-O catalyst precursors obtained by citric acid complexometry. Only two compounds: LaNiO₃ (perovskite-type crystal structure, hexagonal system) and La₂NiO₄ (spinel-type crystal structure, orthorhombic system) in the obtained Ni-La-O catalyst precursors have the ability to grow CNT. Moreover, CNT obtained with the two different crystal structure catalyst precursors have different characteristics: different yield, pattern and oxidation resistance performance.     

Thermoluminescence properties of Ce-doped LiSr4(BO3)3 phosphor

Borates LiSr₄(BO₃)₃ were synthesized by high-temperature solid-state reaction. The thermoluminescence (TL) and some of the dosimetric characteristics of Ce³⁺-activated LiSr₄(BO₃)₃ were reported. The TL glow curve is composed of only one peak located at about 209 °C between room temperature and 500 °C. The optimum Ce³⁺ concentration is 1 mol% to obtain the highest TL intensity. The TL kinetic parameters of LiSr₄(BO₃)₃:0.01Ce³⁺ were studied by the peak shape method. The TL dose response is linear in the protection dose ranging from 1 mGy to 1 Gy. The three-dimensional thermoluminescence emission spectra were also studied, peaking at 441 and 474 nm due to the characteristic transition of Ce³⁺.     

An overview of a hybrid crack element and determination of its complete displacement field

The hybrid crack element (HCE) is one of the most accurate and convenient finite elements for the direct calculation of the stress intensity factor (SIF) and coefficients of the higher order terms of the Williams expansion. It is formulated from a simplified variational functional using truncated asymptotic crack tip displacement and stress expansions and interelement boundary displacements compatible with the surrounding regular elements. However, the exclusion of the rigid body modes in the truncated asymptotic displacements creates jumps between these displacements and element compatible boundary displacements. In this study, an overview of the HCE is given. Furthermore, the rigid body modes excluded in its formulation are recovered by minimizing the jumps via a least squares method. Limitations of the boundary collocation method (BCM) widely used for predicting these terms, as well as the complete displacements are also investigated.     

Decentralized cognitive MAC for opportunistic spectrum access in ad hoc networks: A POMDP framework

We propose decentralized cognitive MAC protocols that allow secondary users to independently search for spectrum opportunities without a central coordinator or a dedicated communication channel. Recognizing hardware and energy constraints, we assume that a secondary user may not be able to perform full-spectrum sensing or may not be willing to monitor the spectrum when it has no data to transmit. We develop an analytical framework for opportunistic spectrum access based on the theory of partially observable Markov decision process (POMDP). This decision-theoretic approach integrates the design of spectrum access protocols at the MAC layer with spectrum sensing at the physical layer and traffic statistics determined by the application layer of the primary network. It also allows easy incorporation of spectrum sensing error and constraint on the probability of colliding with the primary users. Under this POMDP framework, we propose cognitive MAC protocols that optimize the performance of secondary users while limiting the interference perceived by primary users. A suboptimal strategy with reduced complexity yet comparable performance is developed. Without additional control message exchange between the secondary transmitter and receiver, the proposed decentralized protocols ensure synchronous hopping in the spectrum between the transmitter and the receiver in the presence of collisions and spectrum sensing errors     

Broadband patch antenna with low cross-polarisation

A broadband differential-fed patch antenna with good radiation pattern is proposed. Unlike conventional differential-fed patch antennas, the proposed antenna employs a folded plate pair as the differential feeding scheme. To demonstrate the devised feeding approach, impedance bandwidth of up to 74% (SWR<2) has been achieved. More importantly, symmetric radiation pattern, low-cross polarisation level and stable radiation pattern are realised within the band     

Self-adaptive biasing technique for microwave bipolar amplifier linearisation

A novel linearised bipolar amplifier topology, integrating both reduction of baseband distortion and self-adaptive biasing techniques, is presented. The linearising circuit suppresses more than 25 dB of the spectral regrowth for the first-order sidelobes and reduces up to 36% DC bias power of the amplifying transistor     

Additional Y3+ or Al3+ Dopants on the Ferromagnetism of Co Doped Ceria Diluted Magnetic Oxide

Al2O3 and Y2O3 have been respectively chosen for additional dopants to investigate the influence on the ferromagnetism of Co doped CeO2 bulk. Results indicate that ferromagnetism (FM) of Co doped CeO2 decrease with additional Al3+. Accordingly, certain amount of Y3+ can readily be incorporated into the lattice of CeO2 with the decrease of its grain size as well as some pores formation, leading to an enhancement of FM with a positive correlation between magnetization and Y3+ doping content. This experimental result is helpful both in understanding FM origination in diluted magnetic oxide (DMO) as well as to improve the moments of DMO applicable in spintronic devices.