Flat Cathode Research Articles

Singularity in the current density distribution at the boundary of emitting region on a flat cathode surface

The process of electron emission from the boundary of regions with infinite and zero emissivity on a flat cathode surface of a vacuum diode in the space-charge-limited current regime has been theoretically studied. The space charge density, electric field, and velocity field distributions in the interelectrode gap are determined using self-similar solutions of the equations of motion of charged particles. It is established that a singularity, where the emission current density tends to infinity according to a power law, appears at the boundary. From this special point, electrons are emitted at an angle of 103° relative to the electrode surface.

Electricity generation from model organic wastewater in a cassette-electrode microbial fuel cell

A new highly scalable microbial fuel cell (MFC) design, consisting of a series of cassette electrodes (CE), was examined for increasing power production from organic matter in wastewater. Each CE chamber was composed of a box-shaped flat cathode (two air cathodes on both sides) sandwiched in between two proton-exchange membranes and two graphite-felt anodes. Due to the simple design of the CE-MFC, multiple cassettes can be combined to form a single unit and inserted into a tank to treat wastewater. A 12-chamber CE-MFC was tested using a synthetic wastewater containing starch, peptone, and fish extract. Stable performance was obtained after 15 days of operation in fed-batch mode, with an organic removal efficiency of 95% at an organic loading rate of 2.9 kg chemical oxygen demand (COD) per cubic meter per day and an efficiency of 93% at 5.8 kg COD per cubic meter per day. Power production was stable during this period, reaching maximum power densities of 129 W m(-3) (anode volume) and 899 mW m(-2) (anode projected area). The internal resistance of CE-MFC decreased from 2.9 (day 4) to 0.64 Omega (day 25). These results demonstrate the usefulness of the CE-MFC design for energy production and organic wastewater treatment.

Organic light-emitting diodes with structured cathode

Organic light-emitting diode (OLED) devices were fabricated with a structured polymer-cathode interface. The devices have a layered structure indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/poly(2-methoxy-5-[(2′-ethylhexyl)oxy]-p-phenylenevinylene)/Al. The light-emitting polymer layers were patterned via a stamp imprinting process, followed by vapor deposition of the cathode. Devices employing the structured cathode showed improved electron injection and increased brightness when compared to conventional flat cathode devices. Electrostatic modeling of the structured cathode geometry revealed localized increase in electric field leading to enhanced electron tunneling through the polymer-cathode interface. Thus, improved OLED performance was demonstrated from cathode microstructuring.

Green’s function based space-charge field solver for electron source simulations

We present a theoretical formulation for calculating the electromagnetic space-charge fields within a simplified electron source geometry using time-dependent Green's function methods. The source geometry is assumed to be comprised of a flat cathode along with a pipe of arbitrary but uniform cross section. Under the assumption that the beam currents are parallel to the pipe axis, we derive exact solutions for the electromagnetic potentials in the Lorentz gauge. In addition, for the special case of a pipe with rectangular cross section, we present the exact solutions of the electromagnetic potentials for arbitrary beam currents. Finally, we show the results of an analytical benchmark study in which the electromagnetic fields that are solved using the Green's function method are in excellent agreement ($<1%$ error) with the benchmark fields.

Visualization of individual emission sites on flat broad-area field emission cathodes

We introduce a novel scanning projection field emission microscope (SPFEM) designed to study flat broad-area field emission cathodes. The instrument merges capabilities of measuring the electron field emission current from an individual emitting site and genuine projection of electrons onto a luminescent screen. This is achieved by an optimized shape of the anode probe having a 0.04 mm aperture which generates an uniform macroscopic electric field across the investigated area of the cathode. This fact also enables presentation of the relation between the current density and the applied electric field. The magnification of the electron–optical system alone was calculated by computational modeling for some cathode-probe distances and for some voltages. The unique SPFEM performance is demonstrated on smooth sulfur-doped nanodiamond films synthesized on molybdenum substrates.

Novel method of flat cold cathode formation from carbon–nitrogen nanofibers

A novel method of high-efficiency cold cathode formation is developed. The technique is based on the growth of nitrogenated carbon nanofibers in a high-pressure apparatus on a graphite substrate. An average nitrogen concentration up to 13% was achieved. The turn-on and threshold fields for such cathodes are substantially lower than those for cathodes based on other carbon materials. A special method of substrate preparation provides strong adhesion of carbon–nitrogen nanomaterial and its durability during long-term cathode operation. It is shown that due to high uniformity, emission efficiency and time reliability, the field emission cathodes based on carbon–nitrogen nanofibers (CNNs) are very promising for high-brightness flat indicators and displays.

Pulsed Microplasma Using Carbon Nanotubes for Cathode

Using carbon nanotubes (CNTs) or a flat Pt film for the cathode, microplasma was produced in a scanning electron microscope (SEM) chamber containing Ar gas at nearly atmospheric pressure, with gap lengths from 10 to 800 µm. Field electron emission measurements were carried out in the SEM chamber, and gas discharges were conducted at the same configuration in a gas cell installed in the SEM chamber. The ignition voltages using the CNT cathode were much lower than those using the flat Pt cathode under the same conditions, because of the field electron emission. The ignition of gas discharge using the CNT cathode was reproducible every time, because of the ensured supply of primary electrons. It was suggested that the pulse discharge using the CNT cathode was a convolution of the discrete electron avalanche originating from the field emission.

Time-Resolved Spot Size Measurements From Various Radiographic Diodes on the RITS-3 Accelerator

Sandia National Laboratories is leading an intensive research effort into fielding and diagnosing electron-beam flash radiographic X-ray sources. Several X-ray sources are presently being studied, including the self-magnetic pinched diode, the immersed Bz diode, and the plasma-filled flat cathode (paraxial) diode. These studies are being carried out on RITS-3, an inductive voltage adder accelerator capable of delivering 140 kA at 5 MV with a radiation pulse of 70-ns full width at half maximum. The interactions of the electron beam with plasmas created at the anode and/or cathode, for the self-pinched and Bz diode or in the plasma cell for the paraxial diode, can greatly effect the temporal behavior of the radiation spot size. Measuring the dynamic behavior of the beam size and coupling this with theoretical models of the beam plasma interactions can lead to improvements that can be made in these sources. A time-resolved spot size diagnostic (TRSD) has been developed and fielded on RITS-3. This diagnostic consists of a linear array of scintillating fibers, shadowed by a tungsten rolled edge. The scintillating array is optically coupled to a streak camera, and the output is recorded on a charge-coupled device. This paper presents a description of this second-generation TRSD as well as data on the time history behavior of the spot sizes for these three diodes

Radial bunch compression: Path-length compensation in an rf photoinjector with a curved cathode

Electron bunch lengthening due to space-charge forces in state-of-the-art rf photoinjectors limits the minimum bunch length attainable to several hundreds of femtoseconds. Although this can be alleviated by increasing the transverse dimension of the electron bunch, a larger initial radius causes path-length differences in both the rf cavity and in downstream focusing elements. In this paper we show that a curved cathode virtually eliminates these undesired effects. Detailed numerical simulations confirm that significantly shorter bunches are produced by an rf photogun with a curved cathode compared to a flat cathode device. The proposed novel method will be used to provide 100 fs duration electron bunches for injection into a laser-driven plasma wakefield accelerator.

Optimization of field emission properties of carbon nanotubes cathodes by electrophoretic deposition

Cold cathodes of carbon nanotubes (CNTs) were deposited on the glass substrate by the electrophoretic deposition (EPD) method. The cathodes were tested in the diode construction with the cathode–anode gap of 170 μm in vacuum. The emission characteristics of the CNTs film cathodes have as good properties as those by screen printing and better emission uniformity. The influence of the voltage between electrodes in the electrophoretic process of flat cold cathode fabrication on the uniformity of the CNTs film distribution was studied. The results indicate that the uniformity of CNTs film cathode by EPD depends on the voltage between electrodes during the electrophoretic deposition. The uniformity of CNTs film and optimized emission properties of the cathode have been achieved when the voltage is 25 V.

Localized emission from flat diamond cathodes

We analyze steady-state field emission from an n-type semiconductor under the assumption that its surface presents no barrier for electron emission from the conduction band into the vacuum. We construct a classical Lagrangian field theory for the coupled electric and quasi-fermi potentials and use it to show that uniform emission is unstable against laterally nonuniform perturbations. We use a two-parameter model of normal emission current to show that the Lagrangian of the linearized system is minimal when all electrons are emitted from a single site. In addition to this intensely localized emission, we show the normal electric field at the surface is moderately enhanced at the solitary emission site even though the surface itself is planar. We use the result to explain the isolated emission sites observed in nanocrystalline n-type diamond films.

Characterization of a dc atmospheric pressure normal glow discharge

Atmospheric pressure dc glow discharges were generated between a thin cylindrical anode and a flat cathode. Voltage–current characteristics, visualization of the discharge and estimations of the current density indicate that the discharge is operating in the normal glow regime. Emission spectroscopy and gas temperature measurements using the 2nd positive band of N2 indicate that the discharge forms a non-equilibirum plasma. Rotational temperatures are 700 K and 1550 K and vibrational temperatures are 5000 K and 4500 K for a 0.4 mA and 10 mA discharge, respectively. The discharge was studied for inter-electrode gap spacing in the range of 20 µm–1.5 cm. It is possible to distinguish a negative glow, Faraday dark space and positive column regions of the discharge. The radius of the primary column is about 50 µm and is relatively constant with changes in electrode spacing and discharge current. Estimations show that this radial size is important in balancing heat generation and diffusion and in preventing thermal instabilities and the transition to an arc.

Development of flat plate rechargeable alkaline manganese dioxide–zinc cells

This paper was focused on the development of prototypes for flat plate RAM™ (rechargeable alkaline manganese dioxide) cells. In contrast to cathodes used in cylindrical RAM™ cells, the mechanical stability is a significant issue for the preparation of flat plate cathodes. Therefore, the choice of an appropriate binder, e.g. Oppanol, is very important. In this work, an improved preparation process of flat plate RAM™ cathodes was developed by investigating the single steps of the preparation method. It was further demonstrated that the most critical factor of zinc electrode performance was the electrolyte content of the anode gel. Best overall cell performance was achieved at 40% zinc amount and a Zn/ZnO ratio of 5.0, in combination with an electrolyte content of 50.5%. In order to stabilise the γ-structure of manganese dioxide and to enhance rechargeability, the addition of barium compounds was also studied. Cell cycling has shown that flat plate RAM™ cells with BaSO 4-modified cathodes outperformed control cells by 24%, mainly because of the minimised fade of discharge capacity. Moreover, the admixture of barium manganate to the cathode yielded more than 15% capacity improvement after 25 cycles, compared to the barium sulfate additive.

V color centers in electrolytically colored hydroxyl-doped sodium chloride crystals

Hydroxyl-doped sodium chloride crystals were successfully colored electrolytically by using pointed anode and flat cathode at various temperatures and under various electric field strengths. V 2 and V 3 color centers were produced in the colored crystals. Current–time curves for the electrolytic colorations were given, and activation energy for the V 2 and V 3 color center migration was determined. Production of the V 2 and V 3 color centers and formation of current zones for the electrolytic colorations of the hydroxyl-doped sodium chloride crystals are explained.

Highly c-axis oriented ZnO film on silica glass using zinc naphthenate precursor

High transmittance and good electrical conductivity thin conducting oxides (TCOs) have potential use in a variety of technological applications including plasma display panels (PDPs), electroluminescent devices, flat cathode ray tube (CRTs) and solar cells [1–4]. Recently, zinc oxide (ZnO) thin films have emerged as one of the most promising oxide materials owing to their optical and electrical properties, together with their high chemical and mechanical stability. ZnO is a wide band gap (3.3 eV) semiconducting, piezoelectric, and photoconducting material. In particular ZnO films with preferential orientation along the c-axis have been shown to work as SAW devices because of large piezoelectric constant. Ohyama et al. [5] prepared ZnO films on silica glass substrates from zinc acetate sol containing 2-methoxyethanol and monoethanolamine (MEA) with extremely preferred orientation along (002) plane. Ohya et al. [6] reported transparent, electrically conductive ZnO thin films with polycrystalline structure from zinc acetate and alkoxide. Recently Wessler et al. [7] reported textured ZnO thin films on (0001) sapphire by spin-coating a sol of acetate and ethanolamine. In this letter, the preparing of highly c-axis oriented ZnO thin film on amorphous silica substrates from a zinc naphthenate precursor is reported. Crystallinity, surface morphology, surface roughness, and optical behavior were studied. A coating solution was prepared using zinc naphthenate (Nihon Kagaku Sangyo Co., Ltd., Japan) and by diluting the solution with toluene (concentration: 4 wt% metal/100 mL sol). Metal naphthenates are more advantageous than metal alkoxides as starting materials, in terms of cost, stability in air, and ease of handling. Preparation of the coating solution using zinc naphthenate was easily performed by the addition of toluene, while complicated procedures were needed for the preparation of sol from zinc alkoxide [6]. The starting solution was spin-coated onto the cleaned substrate at 1500 rpm for 10 s in air. The asdeposited film was prefired at 500 ◦C for 10 min in air. The coating process was repeated five times to prepare

Selective electrochemical recovery of gold and silver from cyanide aqueous effluents using titanium and vitreous carbon cathodes

In this paper, a different approach for the electrochemical recovery of gold-rich alloys is presented. A filter-press-type electrochemical flow reactor was used and low adherent metal layers were obtained on highly polished vitreous carbon (VC) and titanium (Ti) flat cathodes. On VC, the metallic coating peeled off, being recovered as small flakes outside the reactor. On Ti, the metallic coating could be easily scraped off with a plastic spatula. Gold, silver and copper ions were recovered from real cyanide effluents in conditions such that reduction reactions were mass transfer controlled. Cyclic voltammetry was used for the identification of the potential range in which reduction of Au(I) was the main process in order to improve selectivity. Subsequently, controlled potential electrolysis was carried out at potentials from −1.1 to −1.6 V vs. the saturated calomel electrode (SCE) on VC and from −1.0 to −1.2 V vs. SCE on Ti cathode. Lower potentials favored greater selectivity and alloys containing 76% and 84% of gold were obtained on Ti and VC, respectively. Concentration decay profiles, as a function of potential and electrolysis time, presented pseudo-first-order kinetic and mass transfer coefficients for Au(I) in the order of magnitude of 10 −5 m s −1 on both titanium and VC, in the region of potential in which electro-recovery presented greater selectivity for gold. Cathodic efficiencies were higher on the Ti cathode in which 23% was observed for a gold-rich alloy recovered at −1.0 V, and 15% when only gold is considered.

Field electron emission from flat metal cathodes covered by thin polymer films

We have studied the physical properties of a new class of field electron emitters representing metal cathodes covered by thin polymer films based on a soluble imide-siloxane copolymer. Polymer coating leads to an increase in the emission current and provides for a quite stable field electron emission of flat polished molybdenum and niobium cathodes.

Influence of the interelectrode distance in electrophoretic cold cathode fabrication on the emission uniformity

In the present work, we studied the influence of the distance between electrodes in the electrophoretic process of flat cold cathode fabrication on the uniformity of the emission centers distribution. As the emitting material, we used single-wall carbon nanotubes. We found that the uniformity of field emission strongly depends on the distance between electrodes during the electrophoretic deposition. The best results have been achieved when the distance was smallest within the limits of the studied range (0.3–1.8 cm). For the high distances (1.1–1.8 cm), most of the emission sites were located on the edges of the cathodes. Good uniformity of field emission current has been achieved for the cathodes deposited with interelectrode distance equal to 0.3 cm. The cathodes were tested in the diode construction with the cathode–anode gap of 200 μm. Uniform light emission was obtained after the short training in the vacuum chamber. The turn-on voltage was 600 V, and the maximum achieved current was 850 μA at an applied voltage of 1.3 kV.

Characteristics of a self-initiating pulse-periodic discharge in a Kr/SF6 gas mixture

The characteristics are given of a low-pressure, volume, pulse-periodic discharge in a krypton/SF6 gas mixture, accompanied by the formation of a domain in a “spherical anode–flat cathode” system of electrodes. It is shown that, when a dc voltage of Uch ≤ 1.0 kV is applied to the anode, a volume pulse-periodic discharge with a pulse repetition rate of 0.1 to 120 kHz is ignited in the interelectrode gap. The static and dynamic current–voltage characteristics of the discharge are investigated, as well as the plasma radiation spectra in the range from 130 to 350 nm, oscillograms of voltage across the discharge gap, of the current and radiation of plasma, the dependence of the radiation brightness of the 249-nm KrF(B–X) band on the mean discharge current, and the resource characteristics of radiation on the electronic-vibrational transitions of an excimer molecule. It is demonstrated that the domain plasma may be used in a low-pressure pulse-periodic excimer lamp emitting in the 230 nm KrF(D–X)/249 nm KrF(B–X) band system. The active medium proper plays the role of the commutator in the lamp, and a dc voltage source is used to ignite the discharge. Such lamps can be used in polymer chemistry, biology, potable water purification systems, for the sterilization of medical instruments, and in micro- and quantum electronics.

A hollow cathode discharge modification as a source of sputtered atoms and their ions

The sputtering–atomising properties of a conventional flat bottom hollow cathode discharge (FBHCD) are analyzed. Furthermore, a modification of the HCD grounded on a conical bottom (CB) cylindrical cathode is reported. This CBHCD enhances the main HCD property, i.e. the sputtering of the surface of cathode or inserted probe. Comparative polarization measurements with a FBHCD show a stronger narrowing of the Hanle signal width for the sputtered Cu atoms with the CBHCD. Thus, their density is higher in CBHCD. This result is confirmed by the opto-galvanic radial profile. It contains two peaks of the mentioned density, i.e. near the cylindrical cathode surface and over the cone peak. Some preliminary examinations of CBHCD as an ion source in three arrangements are performed and discussed.