Emitter Electrode Research Articles

Mass spectrometric methods for monitoring redox processes in electrochemical cells

Electrochemistry (EC) is a mature scientific discipline aimed to study the movement of electrons in an oxidation–reduction reaction. EC covers techniques that use a measurement of potential, charge, or current to determine the concentration or the chemical reactivity of analytes. The electrical signal is directly converted into chemical information. For in-depth characterization of complex electrochemical reactions involving the formation of diverse intermediates, products and byproducts, EC is usually combined with other analytical techniques, and particularly the hyphenation of EC with mass spectrometry (MS) has found broad applicability. The analysis of gases and volatile intermediates and products formed at electrode surfaces is enabled by differential electrochemical mass spectrometry (DEMS). In DEMS an electrochemical cell is sampled with a membrane interface for electron ionization (EI)-MS. The chemical space amenable to EC/MS (i.e., bioorganic molecules including proteins, peptides, nucleic acids, and drugs) was significantly increased by employing electrospray ionization (ESI)-MS. In the simplest setup, the EC of the ESI process is used to analytical advantage. A limitation of this approach is, however, its inability to precisely control the electrochemical potential at the emitter electrode. Thus, particularly for studying mechanistic aspects of electrochemical processes, the hyphenation of discrete electrochemical cells with ESI-MS was found to be more appropriate. The analytical power of EC/ESI-MS can further be increased by integrating liquid chromatography (LC) as an additional dimension of separation. Chromatographic separation was found to be particularly useful to reduce the complexity of the sample submitted either to the EC cell or to ESI-MS. Thus, both EC/LC/ESI-MS and LC/EC/ESI-MS are common.

Studying the reducing potencies of antioxidants with the electrochemistry inherently present in electrospray ionization-mass spectrometry.

In this proof-of-principle study, the applicability of electrospray ionization-mass spectrometry (ESI-MS) to characterize the reducing potencies of natural antioxidants is demonstrated. The ESI source represents a controlled-current electrochemical cell. The interfacial potential at the emitter electrode will be at or near the electrochemical potential of those reactions that sufficiently supply all the required current for the ESI circuit. Indicator molecules prone to oxidation in ESI such as amodiaquine were used to visualize the impact of reducing compounds on the interfacial potential. The extent of inhibition of the oxidation of the indicator molecule was found to be dependent on the kind and amount of antioxidant added. Concentration–inhibition curves were constructed and used to compare reducing potencies and to rank antioxidants. This ranking was found to be dependent on the electrode material–indicator molecule combination applied. For fast and automated characterization of the reducing potencies of electrochemically active molecules, a flow-injection system was combined with ESI-MS. Liquid chromatography was used to process complex biological samples, such as red and white wine. Due to their high content of different polyphenols, red wine fractions were found to exhibit higher reducing potencies than the corresponding white wine fractions. Furthermore, for 14 important natural antioxidants, the results obtained with the controlled-current EC–ESI-MS assay were compared to those obtained with chemical antioxidant assays. Irrespectively of the kind of assay used to test the reducing potency, gallic acid, quercetin, and epicatechin were found to be potent reductants. Other antioxidants performed well in one particular assay only. This observation suggests that different kinds of redox and antioxidant chemistry were assessed with each of the assays applied. Therefore, several assays should be used to comprehensively study antioxidants and their reducing potencies. FigureFractions of a red wine sample were screened by ESI-MS for compounds showing reducing potency.

Growth of CuO Nanowires by Thermal Oxidation on Glass Substrate With Graphene as the Electrode

The authors report the growth of CuO nanowires (NWs) by thermal oxidation of copper film that was deposited on CuO/graphene/glass substrate. Monolayer graphene performs good adhesion after growing CuO NWs at 450 °C in air, and the morphology of CuO NWs were uniformly distributed on the sample. The field emission properties of these CuO NWs were also discussed that monolayer graphene was used as the emitter's electrode. The turn-on fields of CuO NWs were 16.7, 11.69, and 8.74 V/μm, and the NWs lengths were 0.3, 3.3, and 5.8 μm, respectively.

Electroconvection between coaxial cylinders of arbitrary ratio subjected to strong unipolar injection

Abstract This study deals with the electroconvection phenomenon that takes place in a dielectric liquid layer placed between two annular electrodes and subjected to the action of an electric field. The full set of governing equations describing the combined Electro-Hydro-Dynamic (EHD) flow is directly solved with the finite volume method. The development of electroconvective motion is investigated in details in the case of strong injection when the emitter electrode is the inner cylinder. We first examine the stability of such flow. As in the plane–plane configuration we have highlighted the existence of a linear and non-linear critical electric Rayleigh numbers giving rise to a hysteresis loop. The flow structure under the effect of the electric field is analyzed and the distribution of electric charge density is presented. In particular we investigate the effect of various parameters involved in this configuration, such as the radius ratio, injection strength and electric Rayleigh number. A multicellular convective pattern is particularly observed and it is shown that the number of cells is increased when the annular gap is narrower. Finally a significant increase of the rotational speed of the vortex with the electric Rayleigh number is recorded.

On the performance of electrohydrodynamic propulsion

Partially ionized fluids can gain net momentum under an electric field, as charged particles undergo momentum-transfer collisions with neutral molecules in a phenomenon termed an ionic wind. Electrohydrodynamic (EHD) thrusters generate thrust by using two or more electrodes to ionize the ambient fluid and create an electric field. We characterize the performance of EHD thrusters of single- (SS) and dual-stage (DS) configurations. SS thrusters refer to a geometry using one emitter electrode, an air gap and a collector electrode with large radius of curvature relative to the emitter. DS thrusters add a collinear intermediate electrode. SS thruster performance was shown to be consistent with a one-dimensional theory. Increasing the gap length requires a higher voltage for thrust onset, generates less thrust per input voltage, generates more thrust per input current and most importantly generates more thrust per input power. A thrust-to-power ratio as high as approximately 100 N kW −1 was obtained. DS thrusters were shown to be more effective than their SS counterparts at producing current, leading to a smaller total voltage necessary for producing equal thrust. However, losses involving ion collection at the intermediate electrode led to reduced thrust-per-power compared with the SS thruster of equal length.

Computational modeling and simulation of electro-hydrodynamic (EHD) ion-drag micropump with planar emitter and micropillar collector electrodes

Computational models can be used to simulate a prototype of electrohydrodynamic (EHD) ion-drag micropump with planar emitter and micropillar collector electrodes. In this study, a simple and inexpensive design of an ion-drag micropump was modeled and numerically simulated. A three-dimensional segment of the microchannel was simulated by using periodic boundary conditions at the inlet and outlet. The pressure and velocity distribution at the outlet and in the entire domain of the micropump was obtained numerically. The effect of the gap between the emitter and the collector electrode, width and the height of micropillar and flow channel height was analyzed for optimum pressure and output flow rate. The enhanced performance of micropump was compared with existing designs. It was found that the performance of micropump could be improved by decreasing the height of micropillar and the gap between both electrodes. The numerical results also show that a maximum pressure head of about 2350 Pa and maximum mass flow rate 0.4 g min−1 at an applied voltage 1000 V is achievable with the proposed design of micropump. These values of pressure and flow rate can meet the cryogenic cooling requirements for some specific electronic devices.

Triboelectric spray ionization

Triboelectric spray ionization (TESI) is a variation of electrospray ionization (ESI) using common instrumental components, including gas flow, solvent flow rate and heat, the only difference being the use of a high-voltage power supply for ESI or a static charge for TESI. The ionization of solvent or analyte is due to the electrostatic potential difference formed between the spray electrode and counter electrode. The ion source contains a pneumatic spray operated over a range of flow rates (0.15-1.5 µl/min) and gas pressures (0-100). This new design contains a standalone spray assembly and an optional metal mesh in front of the spray. There are several parameters that affect the performance during ionization of molecules including the flow rate of solvent, gas pressure, temperature, solvent acidity, distance and potential difference between emitter and counter electrode. A variable electrostatic potential can be applied for higher ionization efficiency. The new ionization method was successfully applied to solutions of various proteins under different conditions. The same charge-state distributions compared to other ESI techniques are observed for all the protein samples. The unique feature of TESI is very efficient spraying by using a natural electrostatic potential even at the potential that a human body can produce. This provides very gentle ionization efficiency of peptides and proteins in different solvents.

Spreading of a dielectric droplet through an interfacial electric pressure

A corona discharge-assisted technique for spreading of a gently deposited dielectric droplet into a uniform thin film over a dry isothermal conductive substrate is proposed. The surface charge was built up over the droplet interface through ion bombardment using a sharp emitter electrode. Interaction of the surface charge density and intense electric field generates an interfacial electrical pressure and leads to a uniform axisymmetric spreading of the droplet in the radial direction. It was shown that the droplet expansion process can be controlled through variation of the ion injection current and/or discharge exposure time. It is also demonstrated that the proposed technique can be analogous to the classical Stefan's squeezing liquid flow between two separated parallel discs. The dynamics of the film spreading owing to the corona discharge can be predicted through a simplified analytical model based on this analogy.

Fundamental investigation of charge injection type of electrostatic oil filter (effect of filter element on filtration speed)

Abstract This paper deals with the effect of filter element on the filtration speed of a charge injection type of electrostatic oil filter proposed by Yanada and his coworkers. The effect of the filter elements that are inserted between the emitter and collector electrodes on the filtration speed is examined using several types of oil. In order to discuss the effect of the filter elements on the filtration speed, numerical simulations of electrostatic field are conducted and ion drag flow patterns are measured using two-dimensional filter models. An optimal filter configuration is proposed.

Poly(3,4‐ethylenedioxypyrrole)‐modified emitter electrode for substitution of homogeneous redox buffer agent hydroquinone in electrospray ionization mass spectrometry

The electrolysis inherent to the operation of the electrospray ionization (ESI) source used with mass spectrometry (MS) is a well-known attendant effect of generating unipolar spray droplets and may affect the analysis of the analyte of interest. Undesirable electrolysis of an analyte may be prevented by limiting the emitter electrode current and/or the mass transport characteristics of the system. However, these ways to avoid analyte electrolysis may not be applcable in all ESI-MS experiments. For example, in the case of specific nanospray systems (e.g. the wire-in-a-capillary bulk-loaded or chip-based tip-loaded nanospray configurations), the solution flow rate is fixed in the 50-500 nL/min range and the electrode surface to volume ratio is large presenting a very effcient analyte to electrode mass transport configuration. In these situations, control over the interfacial potential of the working electrode via homogeneous or traditional heterogeneous (sacrificial metal) redox buffering is a possible way to prevent analyte electrolysis. However, byproducts of these redox buffering approaches can appear in the mass spectra and/or they can chemically alter the analyte. For example, the main reason for using hydroquinone as a homogeneous redox buffer, in addition to its relatively low oxidation potential, is that neither the original compound nor itsmore » oxidation product benzoquinone can be detected directly by ESI-MS. However, benzoquinone can alter analytes with thiol functional groups by reacting with those groups via a 1,4-Michael addition.« less

Compact lanthanum hexaboride hollow cathode

A compact lanthanum hexaboride hollow cathode has been developed for space applications where size and mass are important and research and industrial applications where access for implementation might be limited. The cathode design features a refractory metal cathode tube that is easily manufactured, mechanically captured orifice and end plates to eliminate expensive e-beam welding, graphite sleeves to provide a diffusion boundary to protect the LaB6 insert from chemical reactions with the refractory metal tube, and several heater designs to provide long life. The compact LaB(6) hollow cathode assembly including emitter, support tube, heater, and keeper electrode is less than 2 cm in diameter and has been fabricated in lengths of 6-15 cm for different applications. The cathode has been operated continuously at discharge currents of 5-60 A in xenon. Slightly larger diameter versions of this design have operated at up to 100 A of discharge current.

Effect of micropillar electrode spacing on the performance of electrohydrodynamic micropumps

Electrohydrodynamic (EHD) micropumps with three-dimensional 50 μm × 50 μm micropillar electrodes were fabricated and tested in this study. Two basic electrode configurations were investigated: (i) micropillar emitter and collector electrodes (symmetric) and (ii) micropillar emitter and planar collector electrodes (asymmetric). The micropumps were fabricated by integrating chromium/gold planar electrodes with electroplated 3-D Nickel micropillars on a glass substrate with a 100 μm high PDMS microchannel. The effect of the spanwise micropillar spacing on the pump performance was determined. The pumps were tested using HFE-7100 as the working fluid for the maximum pressure generation under a no flow condition. The micropumps with the asymmetric electrode design generated a significantly higher pressure head than the corresponding micropumps with symmetric electrode configuration for the same applied voltage, with lower power consumption. A decrease in the spanwise spacing of the micropillar electrodes increased the pump performance for the symmetric configuration, while the performance decreased for the asymmetric configuration.

Minimizing analyte electrolysis in electrospray ionization mass spectrometry using a redox buffer coated emitter electrode

An emitter electrode with an electroactive poly(pyrrole) (PPy) polymer film coating was constructed for use in electrospray ionization mass spectrometry (ESI-MS). The PPy film acted as a surface-attached redox buffer limiting the interfacial potential of the emitter electrode. While extensive oxidation of selected analytes (reserpine and amodiaquine) was observed in positive ion mode ESI using a bare metal (gold) emitter electrode, the oxidation was suppressed for these same analytes when using the PPy-coated electrode. A semi-quantitative relationship between the rate of oxidation observed and the interfacial potential of the emitter electrode was shown. The redox buffer capacity, and therefore the lifetime of the redox buffering effect, correlated with the oxidation potential of the analyte and with the magnitude of the film charge capacity. Online reduction of the PPy polymer layer using negative ion mode ESI between analyte injections was shown to successfully restore the redox buffering capacity of the polymer film to its initial state.

1804 EHDマイクロポンプにおける電極の非対称性と流路高さの影響(OS18-2 ナノ・マイクロ流体のダイナミクス,オーガナイズドセッション)

The effect of asymmetry in the electrode geometry and channel height on the performance of electrohydrodynamic(EHD) micropumps was studied. The micropumps consisted of two glass substrate on which chromium/gold planar electrodes were designed. The pumps were tested using HFE-7100 as the working fluid. The micropumps with an asymmetric-electrode design, where the emitter and collector electrode widths were different, generated a lower pressure head than the corresponding micropumps with symmetric-electrode designs for same applied voltage. The micropumps with a channel height of 100 μm generated a higher pressure head than the corresponding micropumps with 50 μm and 150 μm channel height for same electric field. It was found that maximum static pressure was generated with the symmetric-electrode design and the channel height of 100 μm.

ELECTRICAL AND SPRAY CHARACTERISTICS OF A MULTIORIFICE CHARGE-INJECTION ATOMIZER FOR ELECTRICALLY INSULATING LIQUIDS

Electrostatic atomization of electrically insulating liquids has a number of potential uses with respect to combustion applications and the food processing industry. For charge-injection atomizers that generate well-dispersed spray plumes of this class of liquids, spray-specific charge is inextricably linked to the orifice diameter such that the product of these two variables is approximately constant. This, for a single-orifice atomizer, constrains the maximum flow rate against the mean spray drop diameter for a given pressure drop and spray-specific charge. A relaxation of this requirement is possible using a novel evolution of the high-voltage (HV) emitter electrode surface, permitting an orifice array to be used. This permits high spray charge densities at high flow rates without the precision alignment issues that face charge-injection atomizers that possess a pointed high-voltage electrode. Results show that current-voltage characteristics are very similar to a point-plane system, and both electrical and spray measurements revealed that the multiorifice atomizer is able to provide finely atomized jets even though there is a slight degradation in spray performance when introducing more holes.

The effect of corona discharge on free convection heat transfer from a horizontal cylinder

Abstract Free convection heat transfer from an isothermal horizontal cylinder in the presence of DC positive corona discharge with a blade edge emitter electrode has been studied experimentally and numerically. A Mach–Zehnder interferometer was used to determine the local Nusselt numbers. The effect of corona discharge on heat transfer from the cylinder was investigated at Rayleigh numbers in the range between 1500 and 5000. To find the details of the flow patterns and to further verify the experimental results, numerical simulations were also performed. It was found that the numerical results are in good agreement with experimental data. By increasing the applied voltage up to 15.5 kV, the corona discharge generates a recirculation zone around the blade and below the lower stagnation point of the cylinder. The effect of the recirculation zone becomes stronger near the breakdown voltage (17 kV) and it is responsible for a local decrease in the cooling of the cylinder around the lower stagnation point. The results indicate that corona discharge has a significant effect on the average Nusselt number at lower Rayleigh numbers whereas it has smaller effect at higher Rayleigh numbers.

Using metal/organic junction engineering to prepare an efficient organic base-modulation triode and its inverter

In this study, we investigated the influence of a buffer layer of molybdic oxide (MoO 3) at the metal/organic junction on the behavior of organic base-modulation triodes. The performance of devices featuring MoO 3/Al as the emitter electrode was enhanced relative to that of corresponding devices with Au and Ag, presumably because of the reduced in the contact barrier and the prevention of metal diffusion into the organic layer. The device exhibited an output current of −16.1 μA at V B = −5 V and a current ON/OFF ratio of 10 3. Using this architecture, we constructed resistance–load inverters that exhibited a calculated gain of 6.

Effect of Electrode Asymmetry on Performance of Electrohydrodynamic Micropumps

The effect of asymmetry in the electrode geometry and interelectrode spacing on the performance of ion-drag-type electrohydrodynamic micropumps was studied. The micropumps consisted of chromium/gold planar electrodes on a glass substrate that was integrated within a polydimethylsiloxane microchannel that was 100 mum in height. The pumps were tested using HFE-7100 as the working fluid for the maximum-pressure generation under a no-flow condition and the maximum flow rate under a no-back-pressure condition. The micropumps with an asymmetric-electrode design, where the emitter and collector electrode widths were different, generated a significantly higher pressure head and flow rate than the corresponding micropumps with symmetric-electrode designs for the same applied voltage. The power consumption of the pumps with asymmetric electrodes was significantly lower than that with symmetric electrodes.

Theory of space charge limited regime of thermionic energy converter with negative electron affinity emitter

A theory of the space charge limited output current regime of a vacuum thermionic energy conversion device (TEC) employing a negative electron affinity (NEA) diamond emitter electrode is derived. The theory is developed by assuming that the electrons behave as a collisionless gas and self-consistently solving Vlaslov’s equation and Poisson’s equation. Special cases of the theory are addressed. Calculations are performed to model a TEC with a nitrogen doped diamond emitter material under various conditions. It is shown that the NEA material outperforms a similar positive electron affinity material in terms of output power and efficiency because the NEA lowers the electrostatic boundary condition at the emitter and therefore mitigates the negative space charge effect.

Emitter-metal-related degradation in InP-based HBTs operating at high current density and its suppression by refractory metal

We investigated the reliability of InP-based HBTs with a ledge structure, focusing on emitter-metal-diffusion-induced degradation. Bias-temperature accelerated tests under high temperatures and high current densities of up to 5 mA/μm 2 were conducted for HBTs with conventional emitter electrodes, whose metal configuration was Ti/Pt/Au, and for HBTs with refractory metal emitters of Ti/Mo–Ti/Pt/Au, Ti/W–Ti/Pt/Au, or W–Ti/Pt/Au. The emitter contact layer was analyzed by transmission electron microscopy, energy dispersive X-ray spectroscopy, and transmission electron diffraction. Severe damage and disruption of uniformity of the atomic composition were observed due to diffusion of Ti and Au in HBTs with conventional emitter, whereas suppression of those degradations was observed in HBTs with refractory emitter. Refractory metals were found to be advantageous for blocking upper metal diffusion. Interstitials of host species generated due to metal diffusion must cause a shift of atomic composition. The time-wise change in the emitter resistance was estimated to compare the speed of the contact layer degradation between different emitter electrode metals. The critical time, which we determined as an emitter resistance increases of 3% from the initial value, increased by one order for HBTs with refractory metal than for HBTs with conventional metal at the same junction temperature, despite the same activation energy of 2.0 and 1.65 eV for J c of 2 and 5 mA/μm 2, respectively, for all types of emitter electrode. The advantages of refractory metal for improving the reliability of InP HBTs were confirmed, especially for operation at high current densities.