DC Mode Research Articles

A Polarity Switching Technique for the Efficient Production of Sodium Hypochlorite from Aqueous Sodium Chloride Using Platinum Electrodes

A novel, periodic polarity-switching technique is developed for the electrolytic production of sodium hypochlorite from aqueous sodium chloride. The technique employs a flow-through cell having two identical platinum electrodes. Polarities of these electrodes are periodically swapped. The main purpose of this step is to prevent the growth of platinum oxide film and thereby retain high initial activity of the electrodes throughout the operation. The performance of this mode of operation is evaluated using the cumulative productivity and current efficiency of the cell as indicators. Typical ranges of parameters used are as follows: concentration of Cl– ions, 0.1–0.5 M; switching frequency, 0.01–1 Hz; and current density, 100–700 A m–2. The productivity of the switching mode is 5–6 times higher and its current efficiency is 15%–20% higher than that of the DC mode. The switching frequency has a strong influence on both the cumulative production rate and the average current efficiency and an optimal switching ...

Correlation between plasma electrolytic oxidation treatment stages and coating microstructure on aluminum under unipolar pulsed DC mode

Alumina coatings were deposited on 6061 aluminum alloy substrates using plasma electrolytic oxidation (PEO) in an alkaline silicate electrolyte. Different electrical parameters, including frequency, duty cycle, and current density were applied to obtain PEO coatings. Scanning electron microscopy (SEM) and microhardness tests were used to investigate the coating microstructure and properties. For each sample, the voltage–time response of the PEO process was obtained and compared with surface morphology, and coating cross sections and thicknesses. Special consideration was given to the connection between the electrical parameters, different stages of the PEO process, and the morphology and microstructure of the coatings. Applying different electrical parameters changed the duration and ratio of the PEO stages in the voltage–time response curve and affected the growth rate, surface morphology, microstructure and microhardness of the coatings. We report the new finding of a correlation between the stage of the PEO process and the microstructure and morphology of the coating.

Transition of Electron Field Emission to Normal Glow Discharge

The field emission (FE) properties of single walled carbon nanotube (SWCNT)/graphene hybrid samples were investigated for high currents in dc mode and pulsed mode with pulse-ON times between 0.2 and 400 ms and duty cycles between 1% and 80% with stainless steel anodes. Our dc measurements showed a transition to glow discharge (GD) already at currents larger than 10 mA. The pulsed \(I\) – \(V\) characteristics with up to 400-mA maximum current were limited by the cathode resistance, consisting of the emitting layer resistance and the contact resistance, which was found to be between 1.1 and 1.4 \(\text{k}{\Omega }\) . A constant-voltage behavior arises after subtracting the cathode resistance. This constant-voltage behavior, the observed glowing during FE, and time-resolved measurements give evidence for the transition of Fowler–Nordheim-type FE to FE enhanced GD. FE enables a soft transition to normal GD without the need of an ignition voltage. The buildup dynamic and the time constants of the transition to GD were evaluated and discussed. The FE and GD behaviors were also compared using copper and molybdenum anodes.

Control of particle flux and energy on substrate in an inverted cylindrical magnetron for plasma PVD

Inverted cylindrical magnetrons (ICMs) are often used in dc, pulsed dc or mid-frequency ac mode for coating complex objects with thin films deposited by plasma PVD. Since in such a configuration the substrate is inherently surrounded by the target and hence by the plasma, the energy flux of the impinging particles represents the main contribution to the substrate heating. This can readily constitute a limiting factor in the deposition process, especially when it is not possible to cool and bias the substrate. This work concerns a dc-driven ICM configuration subjected to several constraints: not only is the substrate surface area small by comparison to the cathode surface area, but its imposed potential is the ground one, thus itself constituting the anode surface of the considered setup. Several important substrate heating factors are highlighted and, in order to reduce the most prominent of them, a means to raise the plasma potential is proposed. This is achieved by positively polarizing two additional electrodes with respect to the ground. This additional surface generates a redistribution of the current and consequently regulates the electron flux on the substrate. The results are shown as a function of bias applied on the auxiliary electrodes and discussed in terms of the impact on the substrate heating.

Kinetic simulation of direct-current driven microdischarges in argon at atmospheric pressure

A one-dimensional, implicit particle-in-cell Monte Carlo collision model is used to simulate the plasma kinetic properties at a steady state in a parallel-plate direct current argon glow microdischarge under various operating conditions, such as driving voltage (30–1000 V) and gap size (10–1000 µm) at atmospheric pressure. First, a comparison between rf and dc modes is shown for the same pressure, driving voltage and gap spacing. Furthermore, the effect of gap size scaling (in the range of 10–1000 µm) on the breakdown voltage, peak electron density and peak electron current density at the breakdown voltage is examined. The breakdown voltage is lower than 150 V in all gaps considered. The microdischarge is found to have a neutral bulk plasma region and a cathode sheath region with size varying with the applied voltage and the discharge gap. In our calculations, the electron and ion densities are of the order of 1018–1023 m−3, which is in the glow discharge limit, as the ionization degree is lower than 1% . The electron energy distribution function shows a two-energy group distribution at a gap of 10 µm and a three-energy group distribution at larger gaps such as 200 µm and 1000 µm, emphasizing the importance of the gap spacing in dc microdischarges.

Operation Patterns and Spectrum Characteristics of DC and AC Rotating Gliding Arc Plasma

Rotating gliding arc plasma reactor performs two operation modes according to supply power (dc and ac modes). According to the images of operating states and the emission spectrum of the two modes, the characteristics of dc mode and ac mode are well distinguished and investigated. The dc mode owns platform-type plasma region, high temperature, and energy consumption, suitable for fuel conversion field. The ac mode has a few advantages in gaseous organic pollutants treatment field, because of its relatively low gas atmosphere temperature, little by-products (NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) production, and low energy consumption.

Self-Heating in GaN Transistors Designed for High-Power Operation

DC and transient self-heating effects are investigated in normally off AlGaN/GaN transistors designed for a high-power operation. Electrical and optical methods are combined with thermal simulations; 2-μs-long voltage pulses dissipating about 4.5 W/mm are applied on four different transistor structures combining GaN or AlGaN buffer on an n-type SiC substrate with or without Ar implantation. Transistors with only 5% Al mass fraction in the buffer show almost a threefold increase in the transient self-heating if compared with devices on the GaN buffer. On the other hand, 2-μs-long pulses were found not to be long enough for the Ar-implanted SiC substrate to influence the device self-heating unless AlGaN composition changes. In the dc mode, however, both the buffer composition and Ar implantation significantly influence the self-heating effect with the highest temperature rise for the transistor having the AlGaN buffer grown on the Ar-implanted SiC. We point on possible tradeoffs between the transistor high-power design and the device thermal resistance.

Discharge characteristics of a penning ion source for compact neutron generator

We investigate the discharge characteristics of a penning ion source for a compact sealed neutron generator in DC mode. A measuring system consisting of console, vacuum gauges, and teslameter is established. By using the measuring system, the discharge current as a function of ion source voltage, gas pressure, and magnetic field is studied. The results show that the neutron generator can operate in a safe and steady state when the experimental parameters are as follows: ion source voltage of 1.2–2kV, gas pressure of 4×10−2–8×10−2Pa, and magnetic field of 0.3–0.5T. Within these ranges, the neutron yield of the generator can reach 2×108n/s.

Dynamic observation of phase transformation behaviors in indium(III) selenide nanowire based phase change memory.

Phase change random access memory (PCRAM) has been extensively investigated for its potential applications in next-generation nonvolatile memory. In this study, indium(III) selenide (In2Se3) was selected due to its high resistivity ratio and lower programming current. Au/In2Se3-nanowire/Au phase change memory devices were fabricated and measured systematically in an in situ transmission electron microscope to perform a RESET/SET process under pulsed and dc voltage swept mode, respectively. During the switching, we observed the dynamic evolution of the phase transformation process. The switching behavior resulted from crystalline/amorphous change and revealed that a long pulse width would induce the amorphous or polycrystalline state by different pulse amplitudes, supporting the improvement of the writing speed, retention, and endurance of PCRAM.

Detail Analysis of External Quantum Efficiency Measurement for Tandem Junction Solar Cells

The tandem junction solar cells require accurate external quantum efficiency (EQE) measurement to determine the current match condition for high efficiency approach. The EQE measurement of tandem-junction solar cells requires kinds of bias spectrum and voltage biasing. The optimization of bias spectra and adjusted bias voltage are necessary to minimize a measurement artifact due to the simultaneously measured response of the other subcell. Additionally, the shape of the spectral response curve is influenced under certain measurement conditions such as bias light and voltage bias. In this paper, the difference between AC and DC mode EQE measurements are compared and the influence of bias light and voltage biasing in EQE measurement are discussed. The bias light and voltage biasing should be applied for short-circuit condition of the subcell under test. To further improve the accuracy of EQE measurement, the heating due to the bias light illumination should be taken into consideration.

Detail Analysis of External Quantum Efficiency Measurement for Tandem Junction Solar Cells

The tandem junction solar cells require accurate external quantum efficiency (EQE) measurement to determine the current match condition for high efficiency approach. The EQE measurement of tandem-junction solar cells requires variation of the bias spectrum and voltage biasing. The optimization of bias spectra and adjusted bias voltage are necessary to minimize a measurement artifact due to the simultaneously measured response of another subcell. Additionally, the shape of the spectral response curve is influenced under certain measurement conditions such as bias light and voltage bias. In this paper, the difference between AC and DC mode EQE measurements are compared and the influence of bias light and voltage biasing in EQE measurement are discussed. The bias light and voltage biasing should be applied for short-circuit condition of the subcell under test. The measurement artifact has been thoroughly discussed by measurement results. Therefore, it is possible to understand the origin of the measurement artifact and to give guidelines for its minimization. The EQE spectra with optimization of bias voltage and bias light to minimize the artifact are confirmed with the IV measurement under illumination. To further improve the accretion of EQE measurement, the heating due to the bias light illumination can be taken into consideration.

Phase modulation mode of scanning ion conductance microscopy

This Letter reports a phase modulation (PM) mode of scanning ion conductance microscopy. In this mode, an AC current is directly generated by an AC voltage between the electrodes. The portion of the AC current in phase with the AC voltage, which is the current through the resistance path, is modulated by the tip-sample distance. It can be used as the input of feedback control to drive the scanner in Z direction. The PM mode, taking the advantages of both DC mode and traditional AC mode, is less prone to electronic noise and DC drift but maintains high scanning speed. The effectiveness of the PM mode has been proven by experiments.

Effective CU size decision for HEVC intracoding.

In high efficiency video coding (HEVC), the tree structured coding unit (CU) is adopted to allow recursive splitting into four equally sized blocks. At each depth level (or CU size), it enables up to 35 intraprediction modes, including a planar mode, a dc mode, and 33 directional modes. The intraprediction via exhaustive mode search exploited in the test model of HEVC (HM) effectively improves coding efficiency, but results in a very high computational complexity. In this paper, a fast CU size decision algorithm for HEVC intracoding is proposed to speed up the process by reducing the number of candidate CU sizes required to be checked for each treeblock. The novelty of the proposed algorithm lies in the following two aspects: 1) an early determination of CU size decision with adaptive thresholds is developed based on the texture homogeneity and 2) a novel bypass strategy for intraprediction on large CU size is proposed based on the combination of texture property and coding information from neighboring coded CUs. Experimental results show that the proposed effective CU size decision algorithm achieves a computational complexity reduction up to 67%, while incurring only 0.06-dB loss on peak signal-to-noise ratio or 1.08% increase on bit rate compared with that of the original coding in HM.

Piecewise DC prediction in HEVC

In this paper, we propose an improved DC prediction method for high-efficiency video coding (HEVC) intra coding. The technique involves the application of pixel-wise predictors rather than block-based predictor in the DC mode. In HEVC, the pixels of neighboring reconstructed blocks are used to support multiple directional spatial prediction modes and reduce spatial redundancy. For lossless coding, pixel-by-pixel differential pulse code modulation (DPCM) is applied to directional predictions. Consequently, residuals are reduced and coding efficiency is improved. Since DC prediction still employs block-based coding, pixel-wise DC prediction is needed for better coding efficiency. When compared to HEVC lossless intra coding, the proposed algorithm reduces the bit rate by 5.95%. Furthermore, with pixel-by-pixel DPCM, the average bit rate reduction is 10.64% when compared to HEVC lossless intra coding.

Comparative analysis of Cr-B coatings deposited by magnetron sputtering in DC and HIPIMS modes

Surface coatings of the Cr-B system have been obtained by magnetron sputtering in the DC and high-power impulse (HIPIMS) regimes. It is established that the passage from the DC regime to HIPIMS leads to suppression of the columnar grain growth and a twofold increase in the resistance of coatings to plastic deformation, while the plasticity index and hardness of coatings increase by 29 and 18%, respectively.

Factors affecting the formation of the radiation pre-peak at the operation of a Grimm-type source in pulsed DC mode.

The plasma emission pre-peaks of many atomic and ionic spectral lines of Cu and Ar were systematically investigated in a Grimm-type pulsed glow discharge (PGD). To register the pre-peaks with sufficient time resolution, a monochromator with photomultiplier detection was used. When the applied power exceeded a specific threshold, pre-peaks were found in all spectral lines investigated, and it was revealed that the electrical pre-peak was the cause of the atomic emission pre-peak. The form and intensity of the pre-peak radiation were, however, found to be different for different atomic emission lines. The excitation energy of the upper energy level of the atomic line transition, and factors related to recombination and self-absorption, were found to affect the emission pre-peak. Pre-peaks observed when using pulsed DC and pulsed radio-frequency power were compared. This investigation provides insight into best practice when selecting spectral lines most suitable for analytical spectrometry using PGD.

Very large cathode current and long term stability of vacuum sealed tubes with engrafted-carbon-nanotube emitters

Abstract For the future commercial applications of carbon nanotubes (CNTs) in high power vacuum microwave amplifiers or compact X-ray tubes, we have attempted to fabricate engrafted CNT field emitters on a metallic substrate using both screen printing and chemical vapor deposition. Cobalt nano-grains are doped in the printed CNT paste and act as the catalyst for the engrafted growth of CNTs by the cold wall chemical vapor deposition. Stable cathode current (~ 30 mA) from a small area (~ 1.5 mm 2 ) of engrafted CNT emitters was measured in a vacuum-sealed diode tube. High current density (> 1.6A/cm 2 ) has also gotten in the vacuum sealed tube in which the emitters spread about 0.78 mm 2 after an aging process that lasts more than 12 h in DC mode with the water cooling of the anode.

A Combined AC+DC Distribution Network Interconnected with Renewable Energy

The combined AC+DC distribution network in this study provides a coupling and decoupling strategy of renewable energy in DC mode by using zig-zag transformer, which makes each line transmit AC electrical power and DC power simultaneously. The proposed scheme is digitally simulated with the help of Simulink software package. Simulation results indicate lower line voltage drop and less active and reactive power loss in steady state; almost similar or even better transient effects in transient state, which demonstrates the feasibility of combined AC+DC distribution network of single line interconnected with renewable energy.

Improving the Reliability of Particle Accelerator Magnets: Learning From Our Failures

There are many ways that a resistive particle accelerator electromagnet can fail, and most failure modes and their root causes are well known to more experienced magnet engineers. There are thousands of new electromagnets being fabricated every year and tens of thousands already operating in institutions worldwide. Yet some magnet engineers designing a new-style magnet still do not make the design choices that will lead to fewer failing magnets; fabricators still make errors as they assemble magnets and magnets still operate, for example, with low conductivity water (LCW) that corrodes or erodes the coils' conductor. One reason for these continuing problems is the lack of readily available information on the most reliable materials, fabrication techniques, and operating parameters. In order to learn from the experiences of other institutions running accelerators regarding their magnets' failure modes and how they have dealt with them, a web-based survey was created with 64 detailed questions. The survey was completed by 28 designers and operators of accelerator magnets worldwide covering conventional magnets 5 to 55 years old, being used in all kinds of accelerators, in dc, ramping, and pulsed modes. A detailed analysis of the survey's responses was carried out to find, for example, correlations between materials used and frequencies of related failure types. This paper describes the results of the survey analysis, leading to some more reliable design values, materials, fabrication techniques, and operating conditions, especially the properties of the LCW, and thus provides advice on how to improve the reliability of accelerator electromagnets.

Role of the cathode deposit in the carbon arc for the synthesis of nanomaterials

The atmospheric pressure carbon arc in helium is an important method for the production of nanomaterials. Typical arcs operate in a dc mode between a graphite anode, which is consumed, and a cathode which may be a lower melting point material. During arc operation, a carbon deposit is formed on the cathode surface. This deposit may contain different forms of the synthesised fullerenes. It is shown that this deposit plays a crucial role in conducting the arc current. Temperature measurements demonstrate that a sufficiently large area of the cathode deposit is hot enough for thermionic emission to be the source of most of the arc current. Due to the deposit’s low thermal conductivity, the cathode behind the deposit does not reach its melting point. The role of the deposit in emitting electrons can probably be generalized for other arc synthesis methods with consumed anodes.