Niobium Electrodes Research Articles

Anodic oxidation of wireless niobium electrode in bipolar electrochemistry

The anodic oxidation of a wireless niobium substrate serving as a bipolar electrode (BPE) in a direct or alternating current electric field was investigated in detail via spectrophotometry, electrochemical measurement, glow discharge–optical emission spectroscopy, and direct microscopic observation. Niobium oxide was a suitable material for the experiment because it is amorphous and can be evaluated through conventional electrochemical methods. It also has a clear interference color, enabling easy optical evaluation. The effect of the BPE configuration (e.g., vertical and horizontal alignments) in the cell on the anodic oxidation area was also investigated visually using the interference color. Transmission electron microscopy observation was also performed for an accurate measurement of the thickness of the anodic film formed on the niobium BPE. Although the basic film formation behavior and film composition were consistent with those in conventional anodization, even in wireless operation, the effective voltage directly contributing to film formation was found to be lower than the applied voltage (30%–70% of the applied voltage), depending on the electrolysis conditions.

Electroforming-free threshold switching of NbO x -based selector devices by controlling conducting phases in the NbO x layer for the application to crossbar array architectures.

Bipolar threshold switching characteristics, featuring volatile transition between the high-resistance state (HRS) at lower voltage than threshold voltage (Vth) and the low-resistance state (LRS) at higher voltage irrespective of the voltage polarity, are investigated in the Nb(O)/NbOx/Nb(O) devices with respect to deposition and post-annealing conditions of NbOx layers. The device with NbOx deposited by reactive sputtering with 12% of O2 gas mixed in Ar shows threshold switching behaviors after electroforming operation at around +4 V of forming voltage (Vf). On the other hand, electroforming-free threshold switching is achieved from the device with NbOx deposited in the reduced fraction of 7% of O2 gas and subsequently annealed at 250 °C in vacuum, thanks to the increase of the amount of conducting phases within the NbOx layer. Threshold switching is thought to be driven by the formation of a temporally percolated filament composed of conducting NbO and NbO2 phases in the NbOx layer, which were formed as a result of the interaction with Nb electrodes such as oxygen ion migration either by annealing or electrical biasing. The presence of a substantial amount of oxygen in the Nb electrodes up to ∼40 at%, named Nb(O) herein, would alleviate excessive migration of oxygen and consequent overgrowth of the filament during operation, thus enabling reliable threshold switching. These results demonstrate a viable route to realize electroforming-free threshold switching in the Nb(O)/NbOx/Nb(O) devices by controlling the contents of conducting phases in the NbOx layer for the application to selector devices in high-density crossbar memory and synapse array architectures.

An All Solid State Electrochemical pH Sensor Based on Niobium Modified Electrode

pH is an important chemical index. The measurement of pH is widely used in various fields. At present, the most common pH meter on the market is the glass electrode pH meter, which has many advantages, such as high accuracy, short response time and so on. However, when industrial wastewater and environmental water samples need long-time real-time monitoring, such as using glass electrode pH meter will lead to leakage of internal liquid, fragile glass and sodium difference. Therefore, it is urgent to design and develop a new type of pH meter with all solid, simple structure and no nanodeviation. In this study, an all solid state electrochemical modified pH electrode was developed based on the adsorption and storage of hydrogen by niobium. Hydrogen in hydrochloric acid was adsorbed and stored in niobium electrode by constant potential electrolysis. The results showed that the detection linear range was pH1-pH13, the response slope was −56.4 mV dec <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> , and the response time was 5 seconds. In the actual sample detection, the detection results are consistent with the glass electrode pH meter. In addition, cyclic voltammetry and electron microscopy were used to study the response mechanism.

NIOBIUM’S BEHAVIOR IN AQUEOUS HYDRO­FLUORIC ACID SOLUTION

Thanks to the unique combination of physicochemical properties, niobium and its compounds are widely used in various fields of science and technology. The main areas of niobium’s applications are the production of superconductors, nuclear energy, chemical engineering, metallurgy, manufacture of optically active materials, thin-film lithium batte­ries, fuel cells. The aim of this work is to study the processes that take place on the niobium electrode in aqueous solutions of hydrofluoric acid, as well as to establish the composition of niobium compounds that are formed. The paper presents the results of studies the behavior of the niobium electrode in aqueous solutions 0.25 N. hydrofluoric acid. The kinetic para­meters of the processes occurring at the phase boundary are determined. It was found that the anodic polarization of the niobium electrode is accompanied by the formation of a passive layer, the destruction of which is facilitated by increasing the polarization potential and fluorine anions, in the presence of which complex fluoroiobate anions [NbF7]2- and [NbOF5]2-are formed. Cathodic polarization of niobium is accompanied by the formation of hydrides on its surface, which causes an increase in the overvoltage of hydrogen evolution. The anodic polarization of the niobium electrode in a solution of hydrofluoric acid causes the formation on its surface of a passive layer, which is destroyed with increasing potential. In the Nbo–NbO2–0.25 –0.25 n HF system, [NbF7]2-anions are formed, as evidenced by bands in the region of 500 nm on the electron absorption spectra. The rate constants of [NbF7]2- and [NbOF5]2- formation are estimated at 3.78 • 10-3 s-1 and 5.18 • 10-3 s-1, respectively. The reduction of hydrogen at the niobium cathode from a solution of hydrofluoric acid is accompanied by the formation of hydrides, which causes an increase in the overvoltage of hydrogen evolution and high values of the angular coefficients of the Tafel dependence.

Surface processing and discharge-conditioning of high voltage electrodes for the Ra EDM experiment

The Ra EDM experiment uses a pair of high voltage electrodes to search for the atomic electric dipole moment of 225Ra. We use identical, plane–parallel electrodes with a primary high gradient surface of 200 mm2 to generate reversible DC electric fields. Our statistical sensitivity is linearly proportional to the electric field strength in the electrode gap. We adapted surface decontamination and processing techniques from accelerator physics literature to chemical polish and clean a suite of newly fabricated large-grain niobium and grade-2 titanium electrodes. Three pairs of niobium electrodes and one pair of titanium electrodes were discharge-conditioned with a custom high voltage test station at electric field strengths as high as +52.5 kV/mm and −51.5 kV/mm over electrode gap sizes ranging from 0.4 mm to 2.5 mm. One pair of large-grain niobium electrodes was discharge-conditioned and validated to operate at ±20kV/mm with steady-state leakage current ≤25 pA (1σ) and a polarity-averaged 98±19 discharges per hour. These electrodes were installed in the Ra EDM experimental apparatus, replacing a copper electrode pair, and were revalidated to ±20kV/mm. The niobium electrodes perform at an electric field strength 3.1 times larger than the legacy copper electrodes and are ultimately limited by the maximum output of our 30 kV bipolar power supply.

Josephson Effect in MgB2/Pd/Nb Trilayer Josephson Junctions

This paper reports fabrication techniques and results of MgB2/Pd/Nb trilayer Josephson junctions. The MgB2 bottom electrode was co-evaporated by molecular beam epitaxy (MBE) technique from both magnesium and boron sources at a low substrate temperature ~ 300 °C, while the interlayer and the top niobium electrode (Pd/Nb bilayer) were deposited ex-situ using RF sputtering. The junctions exhibited and Josephson effect as well as a modulation of the critical current in a magnetic field applied in a direction normal to the junction plane. Fractional and integer Shapiro steps were observed at voltages corresponding to the frequency of the applied microwave radiation field. The products of the junctions compare well with the previously reported values. The results suggest that it should be possible to fabricate all-MgB2 and MgB2 as one of the electrodes Superconductor/Normal/Superconductor (SNS), Superconductor/Insulator/Superconductor (SIS) or even Superconductor/Ferromagnet/Superconductor (SFS) tunnel junctions with interesting characteristics and for various applications.&#x0D; Keywords: MgB2; all-MgB2; Josephson Tunnel junctions; trilayer devices; Niobium

Electrospinning oxygen-vacant TiNb24O62 nanowires simultaneously boosts electrons and ions transmission capacities toward superior lithium storage

The intercalation pseudocapacitive anode material TiNbxOx+2.5x (x = 2,5,24) has attracted much attention owing to its high theoretical specific capacity (388–402 mAh g−1) and relatively safe working voltage. However, Ti4+/Nb5+ has a 3d/4d empty orbital that leads to a lower electronic conductivity, which limits its practical application. Therefore, reasonable design and adjustment of an effective electron/ion transfer path is the key to the realization of high-performance anode material, which is of great significance to improve the multiplier performance and cycle life for lithium-ion batteries. In this work, we employ a facile electrospinning and subsequent hydrogenation process to synthesize one-dimensional TiNb24O62 (H-TNO) nanowires with oxygen vacancies. The rapid Li+ diffusion path and high Li+ diffusion coefficient are achieved by nano engineering, besides, hydrogen treatment brings oxygen vacancy to improve the electronic conductivity, together with providing more Li+ active sites for TNO materials. As a result, the as-prepared H-TNO-1h (treatment with 1 h) electrode delivers a high reversible specific capacity (305.2 mAh g−1 at 0.1 A g−1), safer working voltage (~ 1.7 V vs. Li/Li +), high initial Coulombic efficiency (91.9%), excellent rate properties (180.5 mAh g−1 at 5 A g−1) and superior cycle stability, making it become one of the best choices in titanium niobium electrode material used for lithium-ion batteries.

Electrochemical Destruction of per- and Polyfluoroalkyl Substances (PFAS) in Complex Solution Matrices

Per- and Polyfluoroalkyl substances (PFAS) are a group of manmade chemicals which are persistent in the environment and have shown to be toxic to human health. They have accumulated in drinking water, groundwater, and wastewater around the globe for over six decades. Although the production of some chain lengths has ceased, remediation is critical. Current remediation techniques include adsorbents such as granular activated carbon (GAC) and Ion exchange (IX) columns. GAC is merely used to adsorb the PFAS, but struggles with capturing shorter chains and requires regeneration with incineration.1 IX has shown improvement in its ability to capture all PFAS chain lengths, and can be regenerated with a solution comprised of a salt or base and an alcohol.1 This regenerate solution desorbs all of the PFAS from the resin into a smaller concentrated solution of roughly a few hundred gallons. This solution is typically comprised of PFAS in the parts per million (ppm) range.2 These concentrated PFAS solutions can lead to further contamination of the environment through discharge of landfill leachate to wastewater treatment plants from landfill disposal. Due to the high salinity, low volume, and high concentration of PFAS in IX regenerate solutions, electrochemical oxidation is a promising treatment technology to destroy the PFAS and remove the chance of future human exposure.This study employs a boron-doped diamond (BDD) on niobium (Nb) electrode stack to investigate the role that current density has in the electrochemical destruction of PFAS in a complex solution matrix. The electrode stack was comprised of two anode plates sandwiched in between three cathode plates. This entire stack was then placed into a constantly stirred reactor comprised of 750 ml of solution. A constant current density of 50 mA/cm2 was investigated initially for a total of 12 hours. In order to save on energy consumption, further studies used a combined current density approach. This involved applying 50 mA/cm2 for only the first hour of the test before lowering the current density to 5 mA/cm2 for the remaining 11 hours. Multiple literature IX regenerate solutions were simulated and spiked with perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS).3–6 Multiple samples were taken over the course of the oxidation experiments and sent for PFAS analysis using the Environmental Protection Agency’s (EPA) modified 537 method. Additional process parameters were monitored over time including the pH, voltage, temperature, and the reduction in chemical oxygen demand (COD) and ammonia depending on the simulated regenerate solution used. Further analysis showed a significant reduction in energy with a minimal additional amount of PFAS remaining in the initial combined current density study compared to the traditional constant current density approach. 1. Woodard, S., Berry, J. & Newman, B. Ion exchange resin for PFAS removal and pilot test comparison to GAC. Remediation 27, 19–27 (2017).2. Liang, S., Pierce, R. “David”, Lin, H., Chiang, S. Y. D. & Huang, Q. “Jack”. Electrochemical oxidation of PFOA and PFOS in concentrated waste streams. Remediation 28, 127–134 (2018).3. Conte, L., Falletti, L., Zaggia, A. & Milan, M. Polyfluorinated Organic Micropollutants Removal from Water by Ion Exchange and Adsorption. in CHEMICAL ENGINEERING TRANSACTIONS 43, (2015).4. Zaggia, A., Conte, L., Falletti, L., Fant, M. & Chiorboli, A. Use of strong anion exchange resins for the removal of perfluoroalkylated substances from contaminated drinking water in batch and continuous pilot plants. Water Res. 91, 137–146 (2016).5. Chularueangaksorn, P., Tanaka, S., Fujii, S. & Kunacheva, C. Regeneration and reusability of anion exchange resin used in perfluorooctane sulfonate removal by batch experiments. J. Appl. Polym. Sci. 130, 884–890 (2013).6. Deng, S., Yu, Q., Huang, J. & Yu, G. Removal of perfluorooctane sulfonate from wastewater by anion exchange resins: Effects of resin properties and solution chemistry. Water Res. 44, 5188–5195 (2010).

Coexistence of induced superconductivity and quantum Hall states in InSb nanosheets

Hybrid superconducting devices based on high-mobility two-dimensional electron gases with strong spin-orbit coupling are considered to offer a flexible and scalable platform for topological quantum computation. Here, we report the realization and electrical characterization of hybrid devices based on high-quality InSb nanosheets and superconducting niobium (Nb) electrodes. In these hybrid devices, we observe gate-tunable proximity-induced supercurrent and multiple Andreev reflections, indicating a transparent Nb-InSb nanosheet interface. The high critical magnetic field of Nb combined with high-mobility InSb nanosheets allows us to exploit the transport properties in the exotic regime where the superconducting proximity effect coexists with the quantum Hall effect. Transport spectroscopy measurements in such a regime reveal an enhancement of the conductance at the quantum Hall plateaus, accompanied by a pronounced zero-bias peak in the differential conductance. We discuss that these features originate from the formation of Andreev edge states at the superconductor-InSb nanosheet interface in the quantum Hall regime. In addition to shedding light on the interplay between superconductivity and quantum Hall effect, our work opens a new possibility to develop hybrid superconducting devices based on 2D semiconductor nanosheets with strong spin-orbit coupling.

Supercurrent and Multiple Andreev Reflections in InSb Nanosheet SNS Junctions

In this study, the realization of mesoscopic Josephson junctions based on free‐standing InSb nanosheet grown by molecular‐beam epitaxy is reported. Below the critical temperature of superconducting aluminium electrodes (≈1.1 K), the high transparency of the contacts gives rise to proximity‐induced superconductivity. A dissipationless supercurrent which can be modulated by a gate voltage acting on the electron density in the nanosheet flows through the superconducting weak links. At finite bias voltage, subharmonic energy‐gap structures (SGS) originating from multiple Andreev reflections (MARs) are observed, indicating a highly transparent InSb nanosheet–superconductor interface. At last, a superconducting hybrid device with niobium electrodes is shown, suitable for further higher temperature and magnetic field transport measurements.

Comparative study on stability of boron doped diamond coated titanium and niobium electrodes

The service life of boron doped diamond (BDD) coated electrodes is closely related to the substrate material and preparation process of the BDD coatings. In this paper, the failure process of BDD coated titanium (Ti/BDD) and niobium (Nb/BDD) electrodes prepared by arc plasma chemical vapor deposition method was comparatively studied by accelerated life test. The results showed that the main failure mechanism of the two types of electrodes is delamination of the BDD coatings, accompanied by coatings' corrosion. The delamination of BDD coatings from both the substrates showed an incubation period, with the Ti/BDD electrode having a much shorter incubation period and a higher coating delamination rate than its Nb/BDD counterpart. By comparing growth process of diamond coatings and corrosion behavior of carbides formed on both substrates, it is found that on a Ti substrate, diamond coating is more liable to incorporate pore like defects, and carbide layer formed beneath the diamond coating is more susceptible to corrosion in electrolysis solution. It is believed that these two factors would be responsible for short service life for Ti/BDD electrodes as compared with that for Nb/BDD electrodes.

Electrical Transport between Superconducting Niobium and a Zinc-Oxide-based Electric Double-layer Transistor

We report electrical transport measurements between a metallic state induced on the surface of an insulating zinc oxide film by an electric double-layer transistor and superconducting niobium electrode. A superconductor/electric double-layer transistor channel/superconductor was created that can be a superconductor/metallic state/superconductor junction and is expected to function as a Josephson junction. The metallic state on the zinc oxide surface and a low contact resistance between niobium and zinc oxide were realized by high carrier doping arising from the electric field effect of the electric double-layer transistor. However, there was no supercurrent through the metallic state at the junction down to a temperature of 0.3 K. Nevertheless, temperature variations of the differential resistance of the junction below the superconducting transition temperature suggest the existence of a superconducting proximity effect between the superconducting Nb and the channel of the zinc oxide-based electric double-layer transistor.

Electrochemical impedance spectroscopic studies on niobium anodic dissolution in HF

The dissolution of niobium electrode in anodic regime in hydrofluoric acid was studied using electrochemical techniques. In the active region, complex plane plots of the impedance spectra exhibited mainly two capacitive loops. In the passive region, the spectra exhibited a mid-frequency pseudo-inductive loop and a low-frequency negative resistance. X-ray photoelectron spectroscopic analysis showed that the surface contains Nb2+ and Nb5+. The impedance data were analysed using equivalent circuit with Maxwell elements as well as mechanistic modelling. A four-step mechanism with a chemical and electrochemical dissolution step is proposed to explain the results. The results show that Nb with an oxidation state of 2 is likely to be an intermediates species. The simulations predict that at low over potentials, the surface is covered mainly with bare metal, whereas at high over potentials, the surface is entirely covered by passivating Nb5+ film.

A Niobium and Nitrogen Co-Doped DLC Film Electrode and Its Electrochemical Properties

Niobium (Nb) and nitrogen (N) co-doped diamond-like carbon (DLC) film ((Nb:N)-DLC) was fabricated by reactive magnetron sputtering. Atomic force microscopy, Raman spectra and X-ray photoelectron spectroscopy were used to characterize the morphology, microstructure, and composition of the (Nb:N)-DLC film. The (Nb:N)-DLC film has typical DLC characteristics with enlarged and uniform specific surface area. Electrochemical tests show that the proposed co-doped DLC electrode not only presents fast apparent rate constant (1.92 × 10−2 cm s−1), low charge transfer resistance (13.86 Ω cm2), but also possesses excellent electrochemical reversibility and stability in K3Fe(CN)6 electrolyte. Furthermore, the (Nb:N)-DLC electrode was applied to the determination of dopamine (DA) and an apparent current response to a low concentration of 0.5 μM was obtained by differential pulse voltammetry.

On Interpretation of Constant-Phase Elements

Electrical circuits invoking constant-phase elements (CPE) are often used to fit impedance data arising from a broad range of experimental systems. The CPE is often used simply as a way to improve the fit of a model to impedance data, justified by a vague assertion that a distribution of time constants is present in the system under investigation. The development in the present work shows that the CPE parameters have a physical meaning. The CPE may be seen as a special case of time-constant dispersion in which the time constants follow a particular distribution. The interpretation of CPE parameters in terms of physically meaningful properties such as capacitance requires an understanding of the nature of the time-constant distribution. This presentation provides a review of recent work on the interpretation of constant-phase elements. A power-law distribution of resistivity in the direction normal to the electrode surface is shown to yield CPE behavior.[1-5] A formula developed by Brug et al. yields physical parameters for time–constant distributions along the surface of an electrode.[6] Work is underway to explore the physical origin of surface distributions that may give rise to a CPE.[7-9] It should be noted that not all time-constant distributions can be described by a constant-phase element. For example, the impedance of the passive layer for a stainless steel electrode presented by Frateur et al.[10] shows CPE behavior; whereas, the time-constant distribution associated with a niobium oxide covered niobium electrode [11] is not a CPE.References B. Hirschorn, M. E. Orazem, B. Tribollet, V. Vivier, I. Frateur, and M. Musiani, Journal of the Electrochemical Society, 157 (2010) C452–C457.B. Hirschorn, M. E. Orazem, B. Tribollet, V. Vivier, I. Frateur, and M. Musiani, Journal of the Electrochemical Society, 157 (2010) C458–C463.M. E. Orazem, B. Tribollet, V. Vivier, S. Marcelin, N. Pébère, A. L. Bunge, E. A. White, D. P. Riemer, I. Frateur, and M. Musiani, Journal of the Electrochemical Society, 160 (2013) C215–C225.S. Amand, M. Musiani, M. E. Orazem, N. Pébère, B. Tribollet, and V. Vivier, Electrochimica Acta, 87 (2013), 693-700.M. Musiani, M. E. Orazem, N. Pébère, B. Tribollet, and V. Vivier, Progress in Organic Coatings, 77 (2014) 2076–2083.G. J. Brug, A. L. G. van den Eeden, M. Sluyters-Rehbach, and J. H. Sluyters, Journal of Electroanalytical Chemistry, 176 (1984) 275–295.C. L. Alexander, B. Tribollet, and M. E. Orazem, Electrochimica Acta, 173 (2015) 416–424.C. L. Alexander, B. Tribollet, and M. E. Orazem, Electrochimica Acta, 188 (2016) 566–573.C. L. Alexander, B. Tribollet, and M. E. Orazem, Electrochimica Acta, (2016) in press.I. Frateur, L. Lartundo-Rojas, C. M´ethivier, A. Galtayries, and P. Marcus, Electrochimica Acta, 51 (2006) 1550–1557.S. Cattarin, M. Musiani, and B. Tribollet, Journal of the Electrochemical Society, 149 (2002) B457–B464.

Deviation From Fraunhofer-Type Modulation of Josephson Current Through Niobium Tunnel Junctions by Applying Vertical Magnetic Field

Anomalous modulation characteristics of Josephson current $I_{c}$ through a niobium tunnel junction have been measured by applying the external magnetic field in perpendicular direction. Josephson current $I_{c}$ through a superconducting niobium/aluminum-oxide/niobium tunnel junction is modulated by applying the parallel magnetic field $(H_{x},H_{y})$ and the vertical field $H_{z}$ to the junction plane using three pairs of Helmholtz coils. Modulation characteristics of the $I_{c}$ value upon the vertical field $H_{z}$ have hysteresis. Before applying the vertical field $H_{z}$ , the $I_{c}-(H_{x},H_{y})$ dependences are the product of the two Fraunhofer diffraction patterns in the $H_{x}$ and $H_{y}$ directions parallel to each edge of the square-shaped junction and show a square-shaped main peak area. After applying the vertical field $H_{z}$ more than 5000 A/m, during the removing process of this vertical field $H_{z}$ , the $I_{c}-(H_{x},H_{y})$ modulation pattern shows a triangle-shaped main peak area due to the remained trapping and shielding magnetic flux at niobium electrodes.

Graphene field effect transistors with niobium contacts and asymmetric transfer characteristics

We fabricate back-gated field effect transistors using niobium electrodes on mechanically exfoliated monolayer graphene and perform electrical characterization in the pressure range from atmospheric down to 10−4 mbar. We study the effect of room temperature vacuum degassing and report asymmetric transfer characteristics with a resistance plateau in the n-branch. We show that weakly chemisorbed Nb acts as p-dopant on graphene and explain the transistor characteristics by Nb/graphene interaction with unpinned Fermi level at the interface.

Metal Oxide Porous Coatings for Implantant Materials

The paper presents the results of survey, dedicated to synthesis of metal oxide coatings on porous materials applied for implant surgery (stainless steel 12X18H9T, titanium alloy and high-purity niobium VT5). This article examines kinetic features of electrochemical formation of anodic oxide coatings on steel, niobium and titanium. It is shown that for steel anodic treatment method does not provide a reliable surface passivation (no current decay, the surface indicates the transition to the passive state).Analysis of polarization dependences obtained on niobium and titanium in electrolytes with an activator (F-), indicates surface passivation (current slump), and at potentials above 2 on the surface of a transition curves in transpassive state (as evidenced by the current increase). Consequently, it can be concluded that presence of F- results surface activation of titanium and niobium electrode (F- oxide reacts to form water-soluble complexes) that promotes nucleation and formation of pores of the porous structure of the oxide coating.SEM results verify the presence of self-organized porous oxide film synthesized on titanium and niobium in solutions containing F-.

TiN coated aluminum electrodes for DC high voltage electron guns

Preparing electrodes made of metals like stainless steel, for use inside DC high voltage electron guns, is a labor-intensive and time-consuming process. In this paper, the authors report the exceptional high voltage performance of aluminum electrodes coated with hard titanium nitride (TiN). The aluminum electrodes were comparatively easy to manufacture and required only hours of mechanical polishing using silicon carbide paper, prior to coating with TiN by a commercial vendor. The high voltage performance of three TiN-coated aluminum electrodes, before and after gas conditioning with helium, was compared to that of bare aluminum electrodes, and electrodes manufactured from titanium alloy (Ti-6Al-4V). Following gas conditioning, each TiN-coated aluminum electrode reached −225 kV bias voltage while generating less than 100 pA of field emission (&amp;lt;10 pA) using a 40 mm cathode/anode gap, corresponding to field strength of 13.7 MV/m. Smaller gaps were studied to evaluate electrode performance at higher field strength with the best performing TiN-coated aluminum electrode reaching ∼22.5 MV/m with field emission less than 100 pA. These results were comparable to those obtained from our best-performing electrodes manufactured from stainless steel, titanium alloy and niobium, as reported in references cited below. The TiN coating provided a very smooth surface and with mechanical properties of the coating (hardness and modulus) superior to those of stainless steel, titanium-alloy, and niobium electrodes. These features likely contributed to the improved high voltage performance of the TiN-coated aluminum electrodes.

Niobium Speciation in NaCl-KCl Based Melts: An Electrochemical and Spectroelectrochemical Study

Molten chlorides can be used for niobium electrowinning and electrorefining. Understanding niobium electrochemical properties and speciation in chloride melts is essential for designing or optimizing the industrial electrochemical process. Apart from the technological value, the information on the kinetics of the electrode reactions can also be used for verifying theoretical predictions concerning the nature of the heterophase electrochemical reactions.In the present study a variety of electrochemical techniques (polarization curves measurements (PCM), chronoamperometry (CA), chronopotentiometry (CP), linear (LVA), cyclic (CVA) and square-wave voltammetry (SWVA), impedance spectroscopy) were applied for studying electrode processes involving niobium species in NaCl-KCl-based melts. In addition the most valuable and reliable information about niobium speciation was obtained by combining electrochemical and spectroscopic (measuring electronic absorption spectra, EAS) methods.The results of spectroelectrochemical experiments showed that the major product of niobium anodic dissolution in chloride melts at current densities up to 40 mA/cm2 were niobium(III) species, NbCl6 3-. In the potentiostatic regime of anodic dissolution of niobium metal niobium(III) complex ions were formed at potentials below -1.15 V vs.chlorine reference electrode. The polarization curves characterizing the anodic dissolution of niobium consist of three or more waves. The first wave is associated with reaching limiting current density of niobium ionization according to the three-electron scheme. Further increase of applied current leads to the oxidation of Nb(III) to Nb(IV) ions. These conclusions were confirmed by determined values of potentials of niobium or indifferent electrodes in the equilibrium with the melts containing Nb(III) and Nb(IV) ions.When studying anodic dissolution of niobium we found that at high current densities there is sudden increase of the ohmic part of voltage drop sometime accompanied by increase of the anode potential. As a result stationary and non-stationary polarization dependencies exhibit unstable behavior. This is explained by the salt passivation, SP. The product of the SP of niobium is unstable NbCl4, which decomposes to NbCl3.13 and volatile NbCl5.The results of electrochemical experiments (PCM, CVA, LVA, CA, CP and SWVA) showed that soluble niobium(V) species were formed during Nb(IV) electrooxidation and represented the highest oxidation state of niobium in NaCl-KCl melts. We noticed that peak potential value (both cathodic and anodic) on CV changes with scan rate, whereas the ip/ν1/2 value remains constant. The formation of Nb(V) species was confirmed by the disappearance of absorption bands in the visible part of the spectrum due to Nb(V) species formation during elecrooxidation of niobium-containing melts at potentials up to -0.5 V. The quasireversibility of Nb4+↔Nb5+reaction was explained by the instability of niobium(V) species.Niobium(IV) ions were formed when potentiostatic electrolysis of NbCl3-NaCl-KCl melts (prepared by anodic dissolution of niobium metal) was performed at -(1.0÷0.8) V vs.chlorine reference electrode on a glassy-carbon anode, Fig. 1.The electroreduction of niobium on a tungsten electrode in NbCl3-NaCl-KCl melts at potentials below -1.45 V led to the formation of metallic niobium (Fig. 2), no evidence of any intermediate product formation was obtained.Linear voltammetry measurements showed that intermediate products can be formed during elecroreduction of NaCl-KCl-NbCln (n=3.8-4.1) melts. Most likely the electroreduction of Nb(IV) results in the formation of niobium(III) ions. The shape of cyclic voltammograms recorded in niobium containing melts depended on the scan rate. Similar sets of cyclic voltammograms were obtained at different temperatures and niobium concentrations. SWVA measurements performed on different electrodes also confirmed that there is more than one step of electroreduction. The obtained results were explained by overlapping of the following electrode reactions: Nb(IV) → Nb(III), Nb(IV) → Nb(0) and Nb(III) → Nb(0).