Low Ohmic Losses Research Articles

Measuring TE1 mode Losses in Terahertz Parallel-Plate Waveguides

The parallel-plate waveguide (PPWG) has drawn considerable research interest [1–12], since it was first reported to support the undistorted propagation of broadband terahertz (THz) pulses. From that time, the transverse-electromagnetic (TEM) mode of the PPWG has been the popular choice for measurements due to its low loss, ease of quasi-optic coupling, and negligible dispersion as a result of the lack of a cutoff. Unlike the TEM mode, the lowest order transverse-electric (TE1) mode has a cutoff frequency, and thus this mode was largely unexplored in the THz regime until recently [13, 14]. This recent work showed that the cutoff frequency could be moved to lower frequencies to reduce the dispersion by increasing the plate separation, while matching the input beam size to the plate separation to realize dominantly single-mode propagation. This same work predicted the possibility of realizing ultra-low ohmic losses in the dB/km range by again utilizing the TE1 mode of this waveguide. These low ohmic losses could permit long-distance transport of THz radiation. However, with such long propagation distances a new concern arises: energy leakage out of the unconfined sides of the PPWG due to diffraction. In fact, this would be the dominant loss mechanism in the case under consideration, where the ohmic losses are virtually negligible. We addressed this diffraction problem in a recent article [15], where we showed that it is possible to inhibit diffraction losses for the TE1 mode by using a waveguide with slightly concave plates. Via a simple “bouncing plane wave” analysis, we demonstrated how to determine an ideal radius of curvature for the inner surfaces, for a waveguide operating at a given THz frequency. This is governed by a confocal condition, R 1⁄4 2bv .

Enhanced slow light in superconducting electromagnetically induced transparency metamaterials

We characterized and compared the electromagnetically induced transparency (EIT) response of superconducting niobium nitride (NbN) and NbN–Au hybrid metamaterials. In our design, the two resonators in a unit cell have strong coupling and are directly excited under terahertz (THz) radiation. A stronger slow light effect was achieved using superconducting metamaterials than hybrid metamaterials. The enhanced slow light effect could be attributed to the remarkably low Ohmic loss and strong interaction of the resonators.

Klystron-like cavity with mode transformation for high-harmonic terahertz gyrotrons

A novel cavity scheme of a gyrotron is proposed and investigated. As it provides low Ohmic losses and high mode selectivity, it can be especially prospective for realization in gyrotrons operating in the THz frequency range. Numerical simulations show that it allows three-fold increase in the efficiency of the low-relativistic 500 GHz fourth-harmonic gyrotron as compared to conventional two-section scheme with modes transformation.

Electrode Material of Carbon Nanotube/Polyaniline Carbon Paper Applied in Microbial Fuel Cells

Abstrac t—Microbial fuel cells (MFCs) are promising clean energy sources for simultaneous recycling of organic waste while harvesting electricity. As for the different effects of electrode materials on the power performance of MFCs, a new material of electrode, carbon nanotube/polyaniline carbon paper (CNT/PANI carbon paper) was utilized and compared with other traditional carbon paper/cloth in this study. Results show that a lower ohmic loss and a better power performance were executed by using CNT/PANI carbon paper. These findings further exhibit its feasibility to improvement of power performance in MFCs.

A 670 GHz gyrotron with record power and efficiency

A 670 GHz gyrotron with record power and efficiency has been developed in joint experiments of the Institute of Applied Physics, Russian Academy of Sciences (Nizhny Novgord, Russia), and the University of Maryland (USA) teams. The magnetic field of 27–28 T required for operation at the 670 GHz at the fundamental cyclotron resonance is produced by a pulsed solenoid. The pulse duration of the magnetic field is several milliseconds. A gyrotron is driven by a 70 kV, 15 A electron beam, so the beam power is on the order of 1 MW in 10–20 ms pulses. The ratio of the orbital to axial electron velocity components is in the range of 1.2–1.3. The gyrotron is designed to operate in the TE31,8-mode. Operation in a so high-order mode results in relatively low ohmic losses (less than 10% of the radiated power). Achieved power of the outgoing radiation (210 kW) and corresponding efficiency (about 20%) represent record numbers for high-power sources of sub-THz radiation.

Evaluation and optimization of mass transport of redox species in silicon microwire-array photoelectrodes

Physical integration of a Ag electrical contact internally into a metal/substrate/microstructured Si wire array/oxide/Ag/electrolyte photoelectrochemical solar cell has produced structures that display relatively low ohmic resistance losses, as well as highly efficient mass transport of redox species in the absence of forced convection. Even with front-side illumination, such wire-array based photoelectrochemical solar cells do not require a transparent conducting oxide top contact. In contact with a test electrolyte that contained 50 mM/5.0 mM of the cobaltocenium(+/0) redox species in CH(3)CN-1.0 M LiClO(4), when the counterelectrode was placed in the solution and separated from the photoelectrode, mass transport restrictions of redox species in the internal volume of the Si wire array photoelectrode produced low fill factors and limited the obtainable current densities to 17.6 mA cm(-2) even under high illumination. In contrast, when the physically integrated internal Ag film served as the counter electrode, the redox couple species were regenerated inside the internal volume of the photoelectrode, especially in regions where depletion of the redox species due to mass transport limitations would have otherwise occurred. This behavior allowed the integrated assembly to operate as a two-terminal, stand-alone, photoelectrochemical solar cell. The current density vs. voltage behavior of the integrated photoelectrochemical solar cell produced short-circuit current densities in excess of 80 mA cm(-2) at high light intensities, and resulted in relatively low losses due to concentration overpotentials at 1 Sun illumination. The integrated wire array-based device architecture also provides design guidance for tandem photoelectrochemical cells for solar-driven water splitting.

Thick-Copper-Buried Inductors Using Anodized Aluminum Package Substrates

Spiral inductors were fabricated by burying thick metals in radio frequency (RF) package substrates. The inductors consist of a 40-μm-thick copper spiral buried in an anodized aluminum substrate and an overpass supported by a benzocyclobutene layer on top of the substrate. Due to their low ohmic loss, significantly high Q factors were obtained at 2 GHz: a Q factor of over 100 with 4.2-nH inductance (2.5-turn spiral) and a Q of 60 with 7.8-nH inductance (3.5-turn). The results demonstrate that the substrate-buried thick metal process can be a viable technique to meet the demands on cost-effective high-Q inductors compatible with conventional thin-film processes for RF integrated circuits.

A Novel Slow-Wave Structure—Folded Rectangular Groove Waveguide for Millimeter-Wave TWT

A novel slow-wave structure (SWS) called the folded rectangular groove guide, its electromagnetic characteristics, and the beam-wave interaction are presented. This structure, evolved from a conventional rectangular groove guide by bending the groove along its longitudinal direction, can operate with the sheet electron beam or the circular electron beam. From our calculations, this SWS possesses larger interaction impedance and lower ohmic loss than those of a folded rectangular waveguide with the same size. This circuit can produce output power of over 260 W with a sheet electron beam in the 3-dB bandwidth range of 88.5.5-98.5 GHz when the cathode voltage and beam current are set to 19.45 kV and 150 mA, respectively, whereas the folded rectangular waveguide can produce output power over 245 W in the 3-dB bandwidth range of 84-89 GHz.

A 0.6-V 30 GHz CMOS Quadrature VCO Using Microwave 1:1:1 Trifilar Transformer

A 30 GHz trifilar-transformer-coupled quadrature voltage-controlled oscillator (QVCO) is proposed in this letter using 0.13 μm CMOS technology. The trifilar transformer provides dc decoupling as well as ac mutual coupling from drain, gate, and source nodes. The mutual coupling increases the overall quality factor while the gate/drain-to-source coupling provides quadrature coupling. The proposed 1:1:1 trifilar transformer realization has lower ohmic loss than the conventional trifilar transformer realization and is especially suitable for high-frequency applications. As a result, the proposed 30 GHz QVCO has a -195 dBc/Hz figure of merit (FOM) and a maximum 41.6 dB sideband rejection (SBR) at a 0.6 V supply.

Low-loss terahertz metamaterial from superconducting niobium nitride films

This paper reports a type of low Ohmic loss terahertz (THz) metamaterials made from low-temperature superconducting niobium nitride (NbN) films. Its resonance properties are studied by THz time domain spectroscopy. Our experiments show that its unloaded quality factor reaches as high as 178 at 8 K with the resonance frequency at around 0.58 THz, which is about 24 times that of gold metamaterial at the same temperature. The unloaded quality factor keeps at a high level, above 90, even when the resonance frequency increases to 1.02 THz, which is close to the gap frequency of NbN film. All these experimental observations fit well into the framework of Bardeen-Copper-Schrieffer theory and equivalent circuit model. These new metamaterials offer an efficient way to the design and implementation of high performance THz electronic devices.

Sine Waveguide for 0.22-THz Traveling-Wave Tube

A novel slow-wave structure called sine waveguide has been proposed to develop a wideband high-power terahertz radiation source. The sine waveguide evolves from a rectangular waveguide oscillating with sinusoid along its longitudinal direction. This letter reports the electromagnetic characteristics of the sine waveguide and its effective surface plasmon amplification mechanism. From our calculation, this circuit structure possesses low ohmic losses and reflection and can be applied to produce terahertz waves ranging from 0.2 to 0.25 THz with several hundreds of watts. Moreover, the maximum gain and interaction efficiency may reach 37.7 dB and 9.6%, respectively.

A microfabricated low cost enzyme-free glucose fuel cell for powering low-power implantable devices

In the past decade the scientific community has showed considerable interest in the development of implantable medical devices such as muscle stimulators, neuroprosthetic devices, and biosensors. Those devices have low power requirements and can potentially be operated through fuel cells using reactants present in the body such as glucose and oxygen instead of non-rechargeable lithium batteries. In this paper, we present a thin, enzyme-free fuel cell with high current density and good stability at a current density of 10 μA cm −2. A non-enzymatic approach is preferred because of higher long term stability. The fuel cell uses a stacked electrode design in order to achieve glucose and oxygen separation. An important characteristic of the fuel cell is that it has no membrane separating the electrodes, which results in low ohmic losses and small fuel cell volume. In addition, it uses a porous carbon paper support for the anodic catalyst layer which reduces the amount of platinum or other noble metal catalysts required for fabricating high surface area electrodes with good reactivity. The peak power output of the fuel cell is approximately 2 μW cm −2 and has a sustainable power density of 1.5 μW cm −2 at 10 μA cm −2. An analysis on the effects of electrode thickness and inter electrode gap on the maximum power output of the fuel cell is also performed.

Magnetic crossover effect in Nickel nanowire arrays

A crossover effect in the magnetic reversal mechanism within arrays of Nickel nanowires whose diameter varies from 15 to 100 nm is observed around 50 nm. Hysteresis loops and FMR measurements confirm that nanowire diameter controls effectively the nanowire easy axis as well as the magnetization reversal mechanism. This might be very interesting for spintronic devices based on current-induced domain motion such as non-volatile magnetic memory elements (MRAM) and low Ohmic loss devices.

Top-illuminated dye-sensitized solar cells with a room-temperature-processed ZnO photoanode on metal substrates and a Pt-coated Ga-doped ZnO counter electrode

We report on top-illuminated, fluorine tin oxide/indium tin oxide-free (FTO/ITO-free), dye-sensitized solar cells (DSCs) using room-temperature-processed ZnO layers on metal substrates as the working electrodes and Pt-coated Ga-doped ZnO layers (GZO) as the counter electrodes. These top-illuminated DSCs with GZO render comparable efficiency to those employing commercial FTO counter electrodes. Despite a lower current density, the top-illuminated DSCs result in a higher fill factor than conventional DSCs due to a low ohmic loss at the electrode/semiconductor interface. The effect of metal substrate on the performance of the resulting top-illuminated DSCs is also studied by employing various metals with different work functions. Ti is shown to be a suitable metal to be used as the working electrode in the top-illuminated device architecture owing to its low ohmic loss at the electrode/semiconductor interface, minimum catalytic activity on redox reactions and high resistance to corrosion by liquid electrolytes.

Spherical Photovoltaic Device With Tailored Emitter Structure

The design and fabrication of a spherical photovoltaic device with a tailored emitter are presented. A reactive ion etching process is used to thin the emitter, providing a progressively varying junction depth and sheet resistance along the spherical surface. The surface of the transparent emitter is passivated by a silicon nitride layer to improve the spectral response and the photocurrent. The tailored variation in emitter depth is also shown to provide a lower ohmic loss path for the current, compared to a uniformly thinned emitter, where the high sheet resistance adds to the series resistance, thereby degrading the fill factor of the device.

Experimental results on microwave pulse compression using helically corrugated waveguide

The paper presents new results on the development of a method to generate ultrahigh-power short-microwave pulses by using a known principle of compression (reduction in pulse duration accompanying with increase in pulse amplitude) of a frequency-swept wave packet propagating through a dispersive medium. An oversized circular waveguide with helical-corrugations of its inner surface ensures an eigenwave with strongly frequency dependent group velocity far from cutoff. These dispersive properties in conjunction with high rf breakdown strength and low Ohmic losses make a helically corrugated waveguide attractive for increasing microwave peak power. The experiments performed at kilowatt power levels, demonstrate that an X-band microwave pulse of 80 ns duration with a 5% frequency sweep can be compressed into a 1.5 ns pulse having 25 times higher peak power by optimizing the frequency modulation of the input wave packet.

Proton Insertion Properties in a Hybrid Membrane/Conducting Polymer Bilayer Investigated by AC Electrogravimetry

In this paper, original hybrid organic/inorganic membranes based on a poly(vinylidenefluoride--hexafluoropropylene) polymer associated with mesostructured modified silica and made through sol–gel techniques were characterized by using cyclic electrogravimetry and ac electrogravimetry. To perform these experiments, a polypyrrole–heteropolyanion doped mediator film was generated between the working electrode of the microbalance and the hybrid membrane (HM). This mixed conductor mediator film is necessary to provide proton transfer between the different interfaces and, therefore, proton transport inside the HM, which is only an ion conductor. These coupled techniques, due to their extraordinary sensitivity and precision, constitute an indirect way of understanding the insertion (at the interfaces)/diffusion (through the HMs) behavior of the HMs, where the transport is complex. Complementary to the ionic conductivity values, the characterization of the HMs with these novel techniques can explain the modest proton conduction and can determine if the limiting mechanism is the proton insertion or diffusion. Therefore, it represents a useful tool for studying the proton conduction mechanisms and, consequently, for designing HMs with low ohmic losses. These techniques can be extended to other proton exchange membrane fuel cell membranes and can constitute a general tool for characterizing the materials where the insertion of induces charge transfer.

A MICROSTRIP SMALL-SIZED ARRAY ANTENNA BASED ON THE META-MATERIAL ZEROTH-ORDER RESONATOR

A novel microstrip array antenna based on the meta- material zeroth-order resonator (ZOR) is presented in this paper. Considering both zeroth-order resonance and the array theory, a resonant array antenna has been designed and fabricated which has small dimensions both in length and width as well as an acceptable gain. Since the resonant frequency of the zeroth-order resonator is independent of the physical length of the resonator and determined only by LC values loaded in the resonator, therefore, antenna size can be reduced arbitrarily with no changes in the operation frequency and due to the array structure antenna gain can be enhanced. Additional beneflt of the ZOR antennas is lower ohmic losses as compared to the other small antennas and hence the e-ciency of the antenna is higher. The discrepancy between simulation and measurement determined by fabricating some of the simulated design and comparison between the results, has been taken into account for designing the flnal array. A small-sized two-elements ZOR array antenna with 50% reduction in size as compared to the conventional array with a acceptable gain and broad radiation pattern is tested at 4.25GHz. The proposed antenna has application in radio altimeters. The commercial softwares such as CST and HFSS were utilized for design and analysis of the structure.

Symmetrical and non-symmetrical coaxial-strip 3 dB directional couplers of decimetric frequency band

Numerical modeling and synthesis results of symmetrical and non-symmetrical coaxial-strip 3 dB directional couplers (DC) for the frequency band of 470…630 MHzpossessing low ohmic loss, good impedance matching (VSWR < 1.1), smooth characteristics in the frequency range of more than 30% and possibility of operating under the power levels of 2kW and more are presented. Theoretical results are confirmed by the data of experimental research of 3 dB coaxial-strip power divider with the 90 degrees phase shift between its outputs, which is built on the basis of a non-symmetrical coaxial-strip DC and is intended for the use in transmitting television antenna arrays of the decimetric wavelengths range.

The ohmic properties and current–voltage characteristics of the screen-printed silicon solar cells with porous silicon surface

The ohmic properties and current–voltage characteristics of the silicon solar cells with fire-through screen-printed Ag metal contacts on the etched porous silicon surface have been investigated. The porous silicon layer of approximately 85 nm thickness was electrochemically etched on the n + -Si surface prior to the deposition of the front Ag grid contacts. Then, Ag contacts are formed by suitably applying a fire-through step at a temperature typically between 700–825 ∘C for 2 min in air ambient followed by annealing step at a lower temperature of 450 ∘C for 5 min in nitrogen ambient. The specific contact resistance ( ρ c ) is one of the important parameters in studying the interfacial properties of the contact metalization system. The value of ρ c of the contact structure is determined using the extrapolation technique based on the three-point probe (TPP) method. It shows that the best value of ρ c is about 8.54×10 −4 Ω cm 2 is obtained for the contact structure which is an order less than the typical value of the ρ c of 3 mΩ cm 2 reported for the sintered Ag metal contacts. The sintering of the Ag contacts on the etched porous silicon at temperature above 800 ∘C in air ambient has resulted in considerable reduction in the value of ρ c of the contact structure. This improvement in ρ c is attributed to the improvement in the barrier properties of the contact structure upon sintering at high temperature. The value of ρ c follows a linear relationship with the surface doping level, N S > 10 19 atoms/cm 3 and improves with an increase in the sintering temperature. The ohmic losses of the fire through Ag contacts are assessed using the power loss algorithm and have shown that relative ohmic losses of the contact structure have improved with higher firing temperatures. These results suggest that fire-through Ag metal contacts on the porous silicon have good ohmic quality and can be expected to provide low ohmic losses.