Search

This paper presents a thorough review of transmitarray devices particularly aiming antenna beamsteering, gathering some of the most relevant solutions published by the scientific community in the field. First, the background for realizing 1-D and 2-D antenna beamsteering with a transmitarray is introduced. Subsequently, several examples of unit-cells for transmitarray implementation and complete transmitarray designs presented in the literature are outlined. Each solution is analyzed in detail, identifying the nature of its layout, e.g., based on microstrip patches, frequency selective surfaces (FSS), or metamaterials (MMs), and the method employed to enable electronic reconfigurability, e.g., p-i-n diodes, varactor diodes, or microelectromechanical systems (MEMS). In addition, some models with the capability of controlling the wavefront polarization modes are also included herein since these are the base of hybrid transmitarrays, i.e., transmitarray with both electronic beamsteering and polarization control. Finally, all the models are compared against each other in order to highlight their benefits and limitations, summarizing their main characteristics, such as the frequency of operation and bandwidth, insertion loss, physical dimensions, and maximum beamsteering range, when available.

This work focuses on the effect of graphite powder grain size on the properties of carbon paper and proton exchange membrane fuel cell. In this paper, graphite powders of different grain sizes (0.5–30 μm) are added to carbon paper for proton exchange membrane fuel cell. Furthermore, the carbon paper is subjected to hydrophobic treatment at polytetrafluoroethylene (PTFE). The research results show that with the increase in graphite grain size, the cell performance increases at dry condition, the carbon paper containing 10 wt% PTFE and graphite powder grain size of 30 μm has the best performance, which power density of 892 mW cm−2 and limit current density of 2.40 A cm−2. With the increase in graphite grain size, the cell performance decrease at wet condition, the carbon paper containg 20 wt% PTFE and graphite powder grain size of 0.5 μm has the best performance, which power density of 524 mW cm−2 and limit current density of 1.84 A cm−2. The paper illustrates that reducing the grain size of graphite powder is beneficial to enhancing the water management ability of gas diffuse layer and reduces the performance loss at wet condition, while, increasing the gas transmission resistance and the performance loss at dry condition. • The innovative carbon paper structure regulation method. • The effect of grain size of carbon powder on carbon paper. • The effect of grain size on cell performance in different condition. [ABSTRACT FROM AUTHOR]

Oil impregnated paper in power transformers deteriorates in long-term operation, which leads to the variations of its dielectric characteristics. This paper focuses on investigating the generation mechanism of the dielectric processes in oil impregnated paper and their variations with thermal aging. Firstly, a set of thermally stimulated current tests with bias-voltages were conducted in distinguishing the dielectric processes and revealing their generation mechanism. Secondly, the standard thermally stimulated current spectrum and the M″ spectrum of 0–1000 h aged samples were joint analyzed in investigating the variations of dielectric processes in thermal aging process. Finally, additional experiments of moisture, acidity and microscopic observation were conducted in explaining these variations. It is concluded that three dielectric processes can be found in oil impregnated paper, i.e. the dipole polarization, the interfacial polarization and the electric conduction. These processes are all strengthened with thermal aging. The generation of molecules with strong polarity, the structural changes in insulating paper, and the generation of hydrogen ions are responsible for the strengthening of these processes sequentially. This investigation on distinguishing dielectric processes and obtaining their mechanism can promote further understanding on oil impregnated paper and provide information for insulation design in power transformers. [ABSTRACT FROM PUBLISHER]

Being an offline measurement technique, Polarization Depolarization Current (PDC) data is conventionally measured after the oil-paper insulation of the equipment reaches thermal equilibrium. Depending on the condition of the insulation, PDC data measurement time typically ranges from several minutes to few hours. It is understood that atmospheric conditions controls the value of ambient temperature. During field measurement, these conditions may not remain constant throughout the PDC recording process. Variation in ambient temperature disturbs the thermal equilibrium that the equipment may have attained at the start of the measurement procedure. Such a condition invariably affects the recorded dielectric response data. However, methods that are available for analyzing relaxation current assume that temperature equilibrium is maintained throughout the measurement period. Results presented in this paper show that dielectric response data is indeed affected if the ambient temperature increases/decreases monotonically. A method is also proposed in this paper using which the effect of such temperature variation on the recorded relaxation current data can be eliminated. This also ensures correct interpretation of PDC data for condition assessment. Analysis presented shows that the value of the equilibrium temperature does not affect the performance of the proposed technique. The applicability of the proposed methodology is first tested on laboratory samples. Thereafter, it is applied to data collected from a real life power transformer. [ABSTRACT FROM AUTHOR]

Frequency Domain Spectroscopy (FDS) is a powerful tool to study the dielectric properties of oil-paper insulation, which is widely used under high electric field strength. The motion of charge carriers under high electric field strength will cause new dielectric characteristics in FDS curves, from which more information could be extracted for insulation state assessment. In this paper, FD spectra are measured under high electric field strength. The electric field dependence of the loss peak in middle frequency and the characteristics of dielectric loss in low frequency are studied. With the influence of trapping on the motion of charge carriers under electric field being considered, charge carriers hopping polarization model and charge carriers motion dependent conductance model are proposed. The trap energy level of oil-paper insulation decreases when the moisture content increases, thus, the motion characteristics of charge carriers change, leading to decreasing loss peak in middle frequency and inverse dielectric loss characteristics in low frequency. These two models introduce the influence of traps on carrier motion into the dielectric response and enable one to gain a better understanding of the dielectric properties of oil-paper insulation under high electric field strength. [ABSTRACT FROM AUTHOR]

Abstract: Porous conducting carbon paper with its unique combination of properties acts as the backing material of electrode in a fuel cell. It not only assists in the flow of electrons and reactant gases but also acts as an effective support for the electrolyte and the catalyst layer. The electrically conducting porous carbon paper was prepared by adopting a modified process of preparation which involves molding together several carbon fiber preforms in the form of laminates rather than a single preform. The method was found to influence the characteristics of the paper significantly and resulted in improved performance of the unit fuel cell employing the laminated paper as electrode. The I–V performance of the fuel cell using carbon paper formed by molding single ply showed a peak power density of 573mW/cm2 as compared to that of 722mW/cm2 for three ply laminates, a value very close to that of achieved by using Toray (Japan) carbon paper (782mW/cm2) under identical operating conditions. [Copyright &y& Elsevier]

Abstract: The polarization phenomenon of papers containing different amounts and different types of inorganic fillers during corona charging was studied using a dose-charging method. This method enables to investigate the phenomena in the conditions close to the real ones. It revealed that the paper composition affects the polarization tendency during the corona charging of paper. The filler influences the paper''s dielectric constant and hence its electrical capacitance, charging potential and polarizing electric field. It was shown that paper polarization depends mainly on the electric field and this is caused by the orientation polarization and the role of the filler origin and its concentration in the polarization was surprisingly low. Polarization temperature dependences enable to propose the paper polarization mechanism. Paper polarizability is dominated by cellulose and the moisture content in it, and the liability of cellulose and hydrogen bonds hydroxyl groups. [Copyright &y& Elsevier]

Abstract: A dosed charging–discharging method was applied to an investigation of the paper charging properties which may be important in toner transfer in electrophotographic printers and copiers. This investigation method, which is new for paper substrates, made it possible to reveal some characteristics of paper charging and discharging processes: the dependence of charge acceptance on charging intensity, which is different for coated and uncoated papers, the change in paper charging properties during charging, unconventional charge acceptance and potential decay dependence on thickness at higher relative humidity, and the role of paper polarization in the discharge of paper. [Copyright &y& Elsevier]

An analysis of the observational polarization properties of the decameter spikes is presented in the paper. It is shown that decameter spikes possess high degree of circular polarization with average value of about 60%. In the frames of "leading spot" theory we associated the spikes activity with a certain active region on the solar disk and determined the mode of the emission. Supposing plasma emission mechanism we link and determine coronal plasma and fast electron beam parameters. [ABSTRACT FROM AUTHOR]

This paper describes the unique characteristics of the oil-paper insulation system utilized in power transformers, their influence on the dielectric response and means for proper modeling of these characteristics. Dielectric response measurement between windings of a power transformer is used to estimate the water content of the solid insulation. By measuring the dielectric response over a wide frequency range different properties of the insulation materials can be determined. These properties can be modeled in a laboratory so that measured response can be compared to modeled response with known properties. This comparison enables the estimation of properties of the materials such as water content and conductivity. The influences of temperature, aging and insulation geometry are described. Case studies are presented which demonstrate the validity of the test method. The influence of frequency range, temperature, aging byproducts, oil conductivity and insulation material type on the accuracy of the moisture estimation is discussed. [ABSTRACT FROM PUBLISHER]

Abstract: Porous conducting carbon paper has been identified as the most suitable material to be used as a backing material for the fuel cell electrode. The surface of carbon fiber, the major constituent of the carbon paper was modified by: (1) removing the functional groups by heat cleaning process and (2) coating the non-functionalized carbon fiber with multi-walled carbon nanotubes (MWCNTs). This has a marked influence on the fiber–matrix interactions during later stages of processing of carbon paper that helped in controlling its various characteristic properties. Using the carbon paper formed with CNT coated carbon fiber as electrode, the maximum power density achieved from a unit fuel cell was found to be 783mW/cm2 as compared to 630mW/cm2 when the paper was formed with normal fiber. [Copyright &y& Elsevier]

Sustainable power generation for portable electronic devices is of high interest due to increasing energy demands. Miniaturized biofuel cells are well-reported for this purpose where ubiquitous fluids, like glucose or lactate, can be leveraged. However, such biofuel cells need enzymes to complete the catalysis, leading to many prevalent issues like the longevity of enzymes, complicated immobilization, additional mediators etc. Therefore, non-enzymatic glucose biofuel cells (NEGBFCs) are gaining interest due to their cost-effectiveness, high repeatability and excellent stability. However, based on our best knowledge such glucose biofuel cells are not reported in microfluidic environment, and therefore their potential is not exploited. Realization of the functionality of such NEGBFCs in a microscale is crucial to replace them with their enzymatic counterparts. In this work, for the first time, a microfluidic non-enzymatic glucose biofuel cell (μNEGBFC) is developed on a paper-based platform using metallic catalysts. The catalysts for electrocatalytic energy generation is fabricated using electrodeposition method and their detailed physicochemical characterizations are done. The μNEGBFC is capable of producing a power density of 12.5 μW/cm2 with a stable potential of 410 mV. This green power output can be used to operate low-power micro-devices and lab-on-chip sensors in a more continual and viable manner. [Display omitted] • A simple, cost-effective electrodeposition technique was used for electrode fabrication. • Gold deposited anode and copper deposited cathode were used as electrodes. • A paper-based Microfluidic Non-Enzymatic Biofuel Cell (μNGBFC) was constructed. • The developed μNGBFC generated a power density of 12.5 μW/cm2. • The μNGBFC was capable of retaining 56% of the initial power after 72 h. [ABSTRACT FROM AUTHOR]

In the recent paper [JQSRT 141 (2014) 14-23] it is stated that the scattering matrix element 34 for the random mixture in the framework of geometrical optics is equal to zero, i.e. no circular polarization can be born from the initial linear polarization in the random mixture, because in the geometrical optics all phase effects are neglected. However, the phase shift between parallel and perpendicular components in total internal reflection leads to the birth of circular polarization. In this work we investigate this phenomenon and show that the effect of the phase shift is really not negligible and matrix element 34 can notably differ from zero. [ABSTRACT FROM AUTHOR]

In the estimation of moisture content of the inverted-type oil-immersed current transformer (IOCT) insulation system, the interfacial polarization between oil-immersed insulation paper and semi-conductive paper should be considered. The classical model functions of frequency-domain dielectric spectroscopy (FDS) are no longer applicable. Therefore, the planar layered two-phase system model is applied in this paper. First, the FDS of oil-immersed paper and semi-conductive paper with certain moisture content is measured using Alpha-A broadband dielectric spectrometer. Then, the FDS of oil-immersed insulation paper is explained using the Havriliak–Negami (H–N) model function and the amendatory H–N model function considering that the dc conductance is used to explain the FDS of semi-conductive paper. The two functions are substituted into the planar layered two-phase system model. The model parameters are fit by means of least-squares technique. Finally, the numerical relationship between the chosen characteristic parameter and the moisture content of the IOCT insulation system is obtained by least-squares technique. In engineering, one can use the results considering the influence of interfacial polarization to estimate the moisture content of the IOCT insulation system. [ABSTRACT FROM AUTHOR]

Arithmetic logic unit (ALU), a core component of a processor, is one of the thrust areas of the current research. Presently, ALU is designed by transistor-based CMOS technique and its individual components are placed in different layers. The current design is affected by the limitations of Moore's law and design complexity. At present, 'Quantum cellular automata electro-spin (QCA-ES)' technology is widely accepted technology as an alternative of 'CMOS' to minimize the above discussed problems. In this research paper, the design of a novel multilayer portable, dynamic, fault-tolerant, power-efficient, thermally stable reversible ALU is proposed which is explored through QCA-ES. All the arithmetic and logical components of ALU are separately placed in different layers. Area density, delay, fault tolerance and thermal stability are investigated. A specific type of gate, known as reversible gate (modified 3:3 'TSG' gate), is used in this proposed design with QCA technology to get the optimized design ALU with low occupied area, complexity, delay and power dissipation. Investigation of a fault-free design and saturated amplitude level (of output) change with respect to temperature increment in the proposed device are also discussed in this paper. Not only the thermal stability (up to 6 k temperature) but also an investigation on cell complexity of the 100% fault-free (against multiple cell-omission, cell-displacement, cell-orientation change and extra cell deposition), multilayer nano-device is represented in this work. 'QCA-Designer' software is used in this research work to design and develop layout of the proposed components in quantum-field and find out the occupied area, delay and complexity of proposed design. 'QCA-Pro' software is used for getting the value of dissipated power. [ABSTRACT FROM AUTHOR]

Abstract: The sheet carrier concentrations, conduction band profiles and amount of free carriers in the barriers have been determined by solving coupled Schrödinger and Poisson equation self-consistently for coherently grown Al0.3Ga0.7N/GaN and Al0.3Ga0.7N/AlN/GaN structures on thick GaN. The Al0.3Ga0.7N/GaN heterojunction structures with and without 1nm AlN interlayer have been grown by MOCVD on sapphire substrate, the physical properties for these two structures have been investigated by various instruments such as Hall measurement and X-ray diffraction. By comparison of the theoretical and experimental results, we demonstrate that the sheet carrier concentration and the electrons mobility would be improved by the introduction of an AlN interlayer for Al0.3Ga0.7N/GaN structure. Mechanisms for the increasing of the sheet carrier concentration and the electrons mobility will be discussed in this paper. [Copyright &y& Elsevier]

The ability to freely control the polarization of X‐rays enables measurement techniques relying on circular or linear dichroism, which have become indispensable tools for characterizing the properties of chiral molecules or magnetic structures. Therefore, the demand for polarization control in X‐ray free‐electron lasers is increasing to enable polarization‐sensitive dynamical studies on ultrafast time scales. The soft X‐ray branch Athos of SwissFEL was designed with the aim of providing freely adjustable and arbitrary polarization by building its undulator solely from modules of the novel Apple X type. In this paper, the magnetic model of the linear inclined and circular Apple X polarization schemes are studied. The polarization is characterized by measuring the angular electron emission distributions of helium for various polarizations using cold target recoil ion momentum spectroscopy. The generation of fully linear polarized light of arbitrary angle, as well as elliptical polarizations of varying degree, are demonstrated. [ABSTRACT FROM AUTHOR]

The polarization of the acoustic field in the ocean waveguide environment is a unique property that can provide new ideas for locating and detecting the underwater target, so it is interesting to study the polarization. This paper extends the Stokes parameters to a broadband form, and uses the non-stationary phase approximation method to simplify the expressions, reducing the complexity of theoretical derivation. A physical phenomenon is observed where polarization exhibits significant variations concerning the sea surface, seafloor, source depth, and the source symmetrical depth. Simulation results demonstrate that the simplified equations using the non-stationary phase approximation are effective. Additionally, by normalizing the broadband Stokes parameters, the effects of horizontal range on the depth distribution law of polarization can be eliminated. Subsequently, using the normalized broadband Stokes parameters, the influence of environmental and source parameters on the depth distribution law of polarization is analyzed. The effectiveness of the non-stationary phase approximation and the range-independence property of the normalized broadband Stokes parameters are verified by processing RHUM-RUM experimental data. Based on the conclusions of this paper, it is expected that the polarization can be used for target depth estimation. [ABSTRACT FROM AUTHOR]

This paper investigates the discursive strategies employed by Oduduwa secessionists to construct polarization and otherness on Twitter. Using the socio-cognitive approach to CDA combined with social media CDA, the study illustrates how socio-cultural and spatiotemporal contexts are embedded in digital performances of resistance. Findings show that the secessionists employ four main discursive strategies, namely: (1) vitriolic socio-cognitive labels and coinages; (2) generalization and ethnocentrism; (3) language of threat; and (4) use of Yoruba language to legitimize their resistance, accentuate their ideological stances, construct polarization and otherness, and do social mobilization. These strategies are achieved via discursive, linguistic, and stylo-orthographic resources made available by digital technology. The paper concludes that the discursive strategies employed by the secessionists do not directly reflect polarization but are simply constitutive of it. [ABSTRACT FROM AUTHOR]

The article investigates how to pinpoint conduction and polarization in the oil and paper of a power transformer. It characterizes deterioration products at room temperature by the oil conductivity test according to the method described in International Electrotechnical Commission (IEC) 61620. It presents dielectric responses of transformers with different insulation conditions. It also recommends procedures to assess the condition of oil-paper interface and to characterize the aging of paper.