Ion Current Ratio Research Articles

Study of optical characteristics of microdischarges in the micro-arc oxidation process

Micro-arc oxide coatings of aluminum alloy AD31 samples were formed in an electrolyte containing 2 g/l NaOH and 9 g/l Na2SiO3 for 2, 4, 8, 16 min in the anode and anode-cathode modes. During the processing, the forming curve, the time dependences of the charge passed through the galvanic cell, and the optical parameters of the microdischarges were measured using the optical synchronization method developed by the authors with subsequent image recognition. The thickness of the coatings was measured using a point autofocus probe surface texture measuring instrument Mitaka PF-60, the surface morphology was studied using a VEGA3 scanning electron microscope using SBH surface topography. The elemental composition of the coatings was determined using a scanning electron microscope JSM-6610LV. The analysis of the elemental composition and morphology of the surface revealed that the inner layer of synthesized coatings consists of Al2O3, the outer layer consists of Al2O3·SiO2 mullite, and the ratio of phases Al2O3 and mullite changes with changes in current density and oxidation mode. The formation of mullite is due to the presence of Na2SiO3 in the electrolyte. It is shown that with increasing processing time and current density, the thickness, roughness and porosity of coatings increase. The interrelation of the optical parameters of microdischarges with the morphology of the surface and the elemental composition of the formed coatings is substantiated; it is shown that the ratio of illuminated and non-illuminated sections of the sample surface by microdischarges can be used as a rough estimate of the ratio of electron and ion currents corresponding to microplasma and electrochemical processes. A mathematical model describing the dependence of the coating thickness on the oxidation time based on Faraday's laws for electrolysis and the results of measuring the optical parameters of microdischarges and the electrical parameters of the MAO process without taking into account microplasma processes that do not lead to an increase in the thickness of coatings is proposed. The error of adequacy of the proposed model does not exceed ±10 %. The results of the study can be used in the development of a digital twin of the micro-arc oxidation process.

Development and numerical investigation of Mach probe model in a hypersonic, low-temperature flowing plasma

This study conducted a numerical simulation around a Mach probe under hypersonic low-temperature plasma. The Mach probe has three ion collection planes: front, side, and back. Under a hypersonic flowing plasma, the front and side planes are practical ion collection areas, and the backplane collects no ion flux. The collected ion current density on the front plane is almost identical to that of the mainstream ion flux. By contrast, the ion current collected on the side plane is affected by the concentration of the electric field at the probe edge. As this edge effect has a different influence on the front and side planes, the ion current density ratio of the side to the front planes is dominated by a non-dimensional parameter—the ratio of electrostatic to kinetic flow energy. Based on this non-dimensional parameter, the calculated ion current density ratio can be fitted using a simple mathematical formula. Therefore, the proposed Mach probe model with non-dimensional parameters extends the conventional Mach probe model validated in sub-to-supersonic high-temperature plasma to hypersonic low-temperature flowing plasma, which is commonly observed in electric propulsions.

Functionalized nanopores based on hybridization chain reaction: Fabrication and microRNA sensing

Enzyme-free hybridization chain reaction (HCR) technology is often used as a signal amplification tool for the detection of different targets. In this study, an ultrasensitive and label-free method for detecting miRNA-21 was developed using the nanopore ionic current rectification (ICR) technology coupled with HCR technology. The probe oligonucleotide (DNA1) was combined with the gold-coated nanopore through the Au–S bond to form a DNA1-functionalized gold-coated nanopore (DNA1-Au-coated nanopore). Since miRNA-21 is partially complementary to DNA1, it can be selectively recognized by DNA1-functionalized gold-coated nanopores. The target (miRNA-21) can induce the opening of hairpin DNA and HCR reaction after the introduction of hairpin DNA H1 and H2. The concentration of miRNA-21 will affect the combination of H1 and H2 on the inner wall of the nanopore, and its surface charge will change with the internal modification, thereby changing the ion current rectification ratio. Under the condition that the concentration of H1, H2 and HCR reaction time are constant, the change of ICR ratio is linearly correlated with the logarithm of miRNA-21 concentration within a certain range, which shows that the sensing strategy we designed can achieve target miRNA-21 detection. This ultrasensitive miRNA holds great promise in the field of cancer diagnosis.

Beam extraction under Cs-free conditions in HUST negative ion source

A radio-frequency driven negative hydrogen ion source is under development at Huazhong University of Science and Technology, which aims to investigate the physics of production and extraction of negative ions. The source operational parameters are investigated to investigate the source plasma characteristics and to optimize the extraction properties before cesium injection, which means, in this study, that negative ions were mostly produced by reactions in the plasma volume. The magnetic filter field generated by a plasma grid (PG) current flowing through the PG can cool down the electron temperature below 1 eV, but simultaneously increased the plasma density in the extraction region. The plasma asymmetry in the direction perpendicular to the PG filter field, is discussed and compared with that in other ion sources. As the PG current increased, the asymmetry factors tended to increase first and then decrease. Biasing the PG positively with respect to the source body can reduce the co-extracted electrons effectively, resulting in reducing the electron to H− ion current ratio. An extracted negative beam with a current density of about 19 A m−2 and an energy of 12 keV was achieved under 20 kW RF power and 0.3 Pa filling pressure, with a ratio of electron current to ion current of about 54.

Multi-Layered Bipolar Ionic Diode Working in Broad Range Ion Concentration.

Ion current rectification (ICR) is the ratio of ion current by forward bias to backward bias and is a critical indicator of diode performance. In previous studies, there have been many attempts to improve the performance of this ICR, but there is the intrinsic problem for geometric changes that induce ionic rectification due to fabrication problems. Additionally, the high ICR could be achieved in the narrow salt concentration range only. Here, we propose a multi-layered bipolar ionic diode based on an asymmetric nanochannel network membrane (NCNM), which is realized by soft lithography and self-assembly of homogenous-sized nanoparticles. Owing to the freely changeable geometry based on soft lithography, the ICR performance can be explored according to the variation of microchannel shape. The presented diode with multi-layered configuration shows strong ICR performance, and in a broad range of salt concentrations (0.1 mM~100 mM), steady ICR performance. It is interesting to note that when each anion-selective (AS) and cation-selective (CS) NCNM volume was similar to each optimized volume in a single-layered device, the maximum ICR was obtained. Multi-physics simulation, which reveals greater ionic concentration at the bipolar diode junction under forward bias and less depletion under backward in comparison to the single-layer scenario, supports this tendency as well. Additionally, under different frequencies and salt concentrations, a large-area hysteresis loop emerges, which indicates fascinating potential for electroosmotic pumps, memristors, biosensors, etc.

Pyisotopomer: A Python package for obtaining intramolecular isotope ratio differences from mass spectrometric analysis of nitrous oxide isotopocules.

Obtaining nitrous oxide isotopocule measurements with isotope ratio mass spectrometry (IRMS) involves analyzing the ion current ratios of the nitrous oxide parent ion (N2 O+ ) as well as those of the NO+ fragment ion. The data analysis requires correcting for "scrambling" in the ion source, whereby the NO+ fragment ion obtains the outer N atom from the N2 O molecule. While descriptions exist for this correction, and interlaboratory intercalibration efforts have been made, there has yet to be published a package of code for implementing isotopomer calibrations. We developed a user-friendly Python package (pyisotopomer) to determine two coefficients (γ and κ) that describe scrambling in the IRMS ion source, and then to use this calibration to obtain intramolecular isotope deltas in N2 O samples. With two appropriate reference materials, γ and κ can be determined robustly and accurately for a given IRMS. An additional third reference material is needed to define the zero-point of the delta scale. We show that IRMS scrambling behavior can vary with time, necessitating regular calibrations. Finally, we present an intercalibration between two IRMS laboratories, using pyisotopomer to calculate γ and κ, and to obtain intramolecular N2 O isotope deltas in lake water unknowns. Given these considerations, we discuss how to use pyisotopomer to obtain high-quality N2 O isotopocule data from IRMS systems, including the use of appropriate reference materials and frequency of calibration.

Ion transport behavior in a vertically-oriented asymmetric Ti3C2Tx nanochannel membrane

Exceptional ion release and storage ability of biological ion channels are regarded as a prerequisite for various life activities. Constructing biomimetic nanochannels to modulate selective ion transport has received enormous attention. Herein, a study on ion transport behavior in a vertically-oriented three-sectional Ti3C2Tx membrane with asymmetric charge distribution was presented. The surface charge polarity of Ti3C2Tx nanosheet was changed from negativity to positivity by successfully grafting the silane molecules onto the surface. The heterogenous Ti3C2Tx nanochannel membrane consisting of three different charged sections was prepared to build up the nanofluidic device. The diode-like ion transport property was observed for the heterogenous membrane, and the maximum ion current rectification ratio reached approximately 32. The numerical simulation based on the Poisson-Nernst-Planck (PNP) equations indicated that the directional ion diffusion was attributed to the accumulation and depletion of ions in the nanochannels under different potential biases. Moreover, the application potential of desalination and ion storage of the heterogenous membrane was assessed. Our work paves the way for the design of heterogenous nanochannels for controllable ion transport and introduces a new perspective for ion sieving, energy conversion, and sensing applications.

Sensing with ion current rectifying solid-state nanopores

This review gives a current opinion on the state of the art of ion rectifying solid-state nanopore sensors, as well as on the recent directions and challenges of the field, focusing in particular on the progress made in the last two to three years. The review explains the phenomenon of ion current rectification in geometrically asymmetric nanopores and the principle of sensing with these systems. Aside from the conventional approach of analyte immobilization onto the pore surface, some intriguing sensing schemes that decouple the pore selectivity from the surface and promise a more flexible sensing approach are also presented. Lastly, an overview of the recent effort towards amplifying the ion currents and the rectification of these sensors is given, followed by a brief discussion of future perspectives for the field. • Decoupling the selectivity of detection from the pore surface can lead to universal sensors. • Maximizing ion current magnitude and rectification ratios is needed to enhance sensitivity. • Various mesopores and micropores have shown enhanced current magnitudes or rectification. • Examples of practical ion current rectifying nanopore sensors highlight real-world potential.

Identification of dominant factors determining droplet temperature in gas metal arc welding

In this study, to identify dominant factors of droplet temperature in a gas metal arc welding, the effects of polarity on droplet temperature were investigated. In particular, the droplet temperature, the wire heat input, and the wire melting rate with electrode positive (EP) polarity and electrode negative (EN) polarity using 100% Ar gas and 100% CO2 gas were measured. As a result, the droplet temperature with EP polarity was higher than EN polarity’s one using 100% Ar gas within the range of 130–230 A. This result showed the reverse tendency compared with the case of 100% CO2. Moreover, the wire heat input and the wire melting rate with EN polarity were larger than EP polarity’s ones regardless of the shielding gas type. The simplified calculation suggested that this was because the ion current ratio was about 80% or more. Besides, especially with EP polarity, the wire melting rate using 100% CO2 was larger than 100% Ar’s one. It was suggested that this was because the wire melting rate was made a difference by the wire preheating effect depending on the droplet frequency determined by the specific heat of the shielding gas. These results identified the dominant factors of droplet temperature as the welding current, the ion current ratio, and the specific heat of the shielding gas.

Difference of co-extracted electron current and beam acceleration in a negative ion source with hydrogen-isotope ions

Improvement of the performance on a hydrogen/deuterium negative ion source for a nuclear fusion device is reported. In particular, the suppression of the co-extracted electron current, Ie, is an important issue to ensure the stable beam acceleration. Improvement of the Ie has been confirmed by optimizing the magnetic field of the electron deflection magnet in the extraction grid. Two other new methods for reduction of the Ie were validated. The first was an electron fence whose rods were set between the rows of apertures on a plasma grid. The electron and negative ion current ratio, approximately Ie/I acc, was greatly improved from 0.7 to 0.25 in deuterium. The second was an outer iron yoke which enhanced the magnetic flux density 19% inside the arc discharge chamber. The Ie/Iacc using the outer yoke decreased by 0.1 compared with using a normal magnetic filter in a deuterium operation. These attempts have improved the total deuterium injection beam power of 8.4 MW by three negative ion based NBIs.

Ultrasensitive detection of trypsin in serum via nanochannel device

A highly sensitive trypsin sensing system in serum was developed by using an anodic alumina oxide (AAO)-based, trypsin substrate-decorated hybrid ion permeation membrane. Owing to the trypsin-triggered peptide hydrolyzation reaction, the surface electrical feature of the peptide-decorated hybrid ion membrane changed. The electric double layer effect reduces the effective ion current diameter in the AAO nano unit, so that the ion current rectification ratio will be enhanced, realizing the quantitative detection of trypsin. The lowest detection concentration can be achieved as low as 0.1 pM. This method is no need for sample pre-preparation, easy to operate, highly sensitive, and also applicable to other enzyme evaluation systems by changing corresponding substrates. This study provides a new idea for selective measurements of proteases in complex biological samples.

Composition, structure and properties of Mo-N coatings formed by the method of vacuum-arc plasma-assisted deposition

Abstract A possibility in principle is shown for varying the elemental and phase compositions of nanocrystalline Mo-N coatings, synthesized using vacuum-arc plasma-assisted method, by changing the discharge parameters of the assisting gas plasma source at the constant pressure and composition of an Ar/N2 gas mixture. It is found out that an increase in the gas-to-metal ion current density ratio from 0 to 3 gives rise to a nearly twofold increase of nitrogen concentration in the coating of the Mo-N system. It is determined that as the discharge current is increased, the main metal (Mo) phase in the coating is replaced by a nitride phase with a nitrogen deficit (Mo2N), and then by a stoichiometric MoN phase. It is shown that the coatings formed in the plasma-assisted modes do not practically contain any metallic phase. With an increase in the nitrogen concentration the coating hardness increases by up to a factor of 1.3 (up to 36 GPa) and the wear resistance increases by about a factor of 2.4 (wear rate up to 2.1·10−7 mm3N−1 m−1) compared to the parameters of the coatings deposited in the regimes without plasma assistance. All coatings of the Mo-N system, synthesized in the plasma-assisted regimes, possess multiphase compositions: β-Mo2N, γ-Мо2N, δ-MoN. As the nitrogen concentration is increased, the volume fraction of δ-MoN phase increases to 67%. At the maximum nitrogen concentration, the Mo-N coating structure becomes columnar (plate-like) in the entire coating volume. The MoN and Mo2N plates have nanometric sizes, they are parallel to each other and combined into alternating packs. All Mo-N coatings are nanostructured, with the average Mo2N(β + γ) and MoN crystallite sizes being 3–4 and 3–5 nm, respectively.

First results of negative ion extraction with Cs for CRAFT prototype negative beam source

In order to understand the physics and pre-study the engineering issues for radio frequency (RF) negative beam source, a prototype source with a single driver and three-electrode accelerator was developed. Recently, the beam source was tested on the RF source test facility with RF plasma generation, negative ion production and extraction. A magnetic filter system and a Cs injection system were employed to enhance the negative ion production. As a result, a long pulse of 105 s negative ion beam with current density of 153 A m−2 was repeatedly extracted successfully. The source pressure is 0.6 Pa and the ratio of co-extracted electron and negative ion current is around 0.3. The details of design and experimental results of beam source were shown in this letter.

TEMPO oxidized cellulose nanofibers-based heterogenous membrane employed for concentration-gradient-driven energy harvesting

The development and utilization of renewable clean energy has become a common way out for the world to solve energy crisis. The concentration gradient between sea water and river water is widely regarded as a very significant sustainable energy resource because of its easy availability and abundant reserves. Therefore, it is necessary for rapid technical breakthrough of membrane engineering in order to capture this energy existing in the fluidic system. Herein, we develop nanofluidic device that can harvest osmotic energy and rectify ionic transport by directly prepared with a nanoporous TOCNs membrane and a conical variable-channel porous polyethylene terephthalate (PET) substrate membrane. 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) oxidized cellulose nanofibers (TOCNs) nanofluidic device possesses the advantages of well-tunable geometry and high charges density, which develop an attractive material for the control of ion flow. The optimized TOCNs heterogeneous membrane shows prominent cation selectivity and ion current rectification ratio of 562. When applying this TOCNs heterogeneous membrane for a concentration-gradient-driven device, a high power density reaches 0.96 W/m2, which exhibits great promise for energy harvesting device. Overall, this work provides an effective way for devising cellulose nanofibers-based nanofluidic device and can promote the development of concentration-gradient-driven energy conversion system.

Effect of discharge chamber geometry on ion loss in microwave discharge ion thruster

The authors redesigned the discharge chamber for the μ10 microwave discharge ion thruster to improve its thrust performance and succeeded in enhancing the maximum beam current and thrust efficiency. However, it was found that the ion current ratio extracted from the discharge chamber with the redesigned configuration was lower than that obtained with the original configuration. To investigate the relationship between ion extraction and the magnetic field geometry, the ion loss current distribution in these two types of discharge chamber were measured by electrostatic probes. Using planar probes with a guard ring, the ion current that flowed into the wall was measured without disturbing the ion beam current. The results show that ionization occurs mainly near the upstream magnet. In addition, the ion flux on the sidewall in the redesigned discharge chamber is about 1.5-2 times larger than that in the original discharge chamber. This suggests that the distance between the edge of the plasmaproduction region and the chamber wall with consideration of the Larmor radius of ions is an important parameter in discharge chamber design. In addition, although the ion beam current showed a tendency to saturate at high microwave power, the ion loss to each part in the discharge chamber increased in proportion to input microwave power. The decrease in the extracted ion ratio in the redesigned discharge chamber is considered to be caused by a decrease in the electrostatic ion transparency of the screen grid. Therefore, in a well-tuned microwave discharge ion thruster, it is difficult to improve the thrust efficiency by increasing the discharge power. A design that suppresses the wall loss of ions is thus important.

Extension of high power deuterium operation of negative ion based neutral beam injector in the large helical device.

Second deuterium operation of the negative ion based neutral beam injector was performed in 2018 in the large helical device. The electron and ion current ratio improves to Ie/Iacc(D) = 0.31 using the short extraction gap distance of 7 mm between the plasma grid (PG) and the extraction grid (EG). The strength of the magnetic field by the electron deflection magnet installed in the EG increases by 17% at the PG ingress surface, which effectively reduces the electron component in the negative ion rich plasma in the vicinity of PG apertures. The reduction of the electron current made it possible to operate at a high power arc discharge and beam extraction. Then, the deuterium negative ion current increases to 55.4 A with the averaged current density of 233 A/m2. The thermal load on the EG using 7 mm gap distance is 0.6 times smaller than the thermal load using a 8 mm gap caused by the reduction of coextracted electron current. The injection beam power increases to 2.9 MW in the beam line BL3, and the total beam injection power increases to 7 MW by three beam lines in the second deuterium campaign.

Discharge characteristics of cathodic hollow-cathode plasma contactors and related similarity to ground simulations

This paper focuses on the main factors affecting the electron emission current–bias voltage (I–V) characteristics of cathodic hollow-cathode contactors and similarity conditions that should be satisfied in ground-based simulation experiments. These issues are of crucial importance for ensuring the reliability and credibility of the I–V data obtained in ground experiments, improving the I–V characteristics, and for reducing the mass flow consumption and dependence on the space environment. In this study, a 1-D spherically-symmetric model is first applied and compared with experimental data. We explore the influence of ion current, electron temperature, ion energy at the contactor exit, and environmental plasma density on the I–V curves, potential structure, and size of the plume area. The influence of the diameter of the vacuum chamber on the experimental I–V characteristics is investigated and the necessary conditions required for the diameter of the vacuum chamber is proposed. Moreover, an analysis is conducted of the effect of the background plasma source on the experimental I–V curves. The results suggest that the I–V behavior is sensitive to the exit ion current and flow rates. Another model is further employed to predict the exit ion-beam current for different net electron-emission currents and consumption flow rates at a given clamping current. The calculations show that a single-cathode contactor presents an extremely low utilization ratio. The exit ion current and utilization ratio should be effectively enhanced to improve the I–V characteristics.

Numerical investigation of plasma properties for the microwave discharge ion thruster μ10 using PIC-MCC simulation

This paper reports the numerical investigation of plasma properties for the microwave discharge ion thruster μ10. The model consists of a particle in cell simulation and a Monte Carlo collision simulation. The results indicate that the plasma density and the electron temperature in the confined region are larger than those in other regions and are qualitatively consistent with probe measurements. Moreover, we traced the trajectories of charged particles to investigate the plasma generation and transport. The electron trajectories indicate that electrons are strongly confined by the mirror magnetic field and the sheath, which indicates that the confinement depends on the electron energy. As a result, the electron energy distribution function is a combination of two Maxwellian distributions. Although the hot electrons account for 3.4% of all electrons, they account for 50.1% of the ionization and can generate plasma with an excitation loss of 1/3 of that of cold electrons. The ion trajectories indicate that they are affected by the magnetic field. To investigate the effect of the magnetic field on the transport, we evaluate the ion and electron current percentage toward the wall and compare with the wall surface percentage. The ion and electron current ratios differ because of diffusion with respect to the magnetic field. The ion current percentage is larger than the surface area percentage in the grid, which indicates that ions are transported to the grid more efficiently due to the magnetic field. Therefore, the effect on ions by the magnetic field is one of the most important criteria for microwave discharge ion thrusters.

Exploring deuterium beam operation and the behavior of the co-extracted electron current in a negative-ion-based neutral beam injector

The achievements of the deuterium beam operation of a negative-ion-based neutral beam injector (N-NBI) in the large helical device (LHD) are reported. In beam operation in LHD-NBIs, both hydrogen (H) and deuterium (D) neutral beams were generated by changing the operation gas using the same accelerator. The maximum accelerated deuterium negative-ion current () reaches 46.2 A from two beam sources with the averaged current density being 190 A m−2 for 2 s, and the extracted electron to accelerated ion current ratio () increases to 0.39 using 5.6 V high bias voltage in the first deuterium operation in 2017. An increase of electron density in the vicinity of the plasma grid (PG) surface, which is considered the main reason for the increase of co-extracted electrons in a beam, is confirmed by the half-size research negative-ion source in the neutral beam test stand at the National Institute for Fusion Science (NIFS). The deuterium negative-ion density is also larger than the hydrogen negative-ion density in the vicinity of the PG surface using the same discharge conditions. In the latest experimental campaign in 2018, increases to 55.4 A with the averaged current density being 233 A m−2 for 1.5 s using the shot extraction gap length. The low of 0.31 can be maintained by using high discharge power. The various parameters mentioned above are defined in detail below.

Optimisation of fully depleted SiGe channel with raised source/drain buried oxide nMOSFET

A fully depleted silicon-germanium (SiGe) n-channel heterojunction MOSFET with raised buried oxide in the source/drain is reported under the consideration that channel is made of SiGe and source/drain regions is made of silicon (Si). Due to the raised buried oxide source/drain region, it provides better current due to the improvement of mobility in the channel region and also reduce the surface scattering effects. Bandgap engineering has been done to improve the electrical behaviour of the device. Simulation work for different parameters on the device has been carried out and presented in the paper. The electrical characteristics of the proposed device are optimised by varying the device dimensions. Effect of mole fraction on threshold voltage (VTH), Subthreshold swing (SS), ION and IOFF current ratio is analysed. It is observed that due to presence of SiGe channel and the raised source/drain, the proposed device shows enhancement in electrical characteristics. A CMOS inverter through proposed device has been implemented and the effect of mole fraction on its characteristic is reported. Average delay increases as mole fraction increases.