Emission Current Density Research Articles

Electron emission mechanism of scandium‑tungsten cathodes

Scandium‑tungsten (Sc-W) cathodes have garnered a lot of research attention due to their high emission current density. This study investigates the electronic properties of Sc-W cathodes using the VASP code to explain the operating mechanism of hot electron emission. The results reveal that the work function is decreased by 0.58 eV, whereas the charge density in the crystal structure, the number of high valence electrons and the energy of electrons increase when Sc content is increased from 1% to 2%. Moreover, the bandgap and energy required for electrons to jump from the valence band to the conduction band decreased with the increase of Sc content, enhancing the current emission density and reducing the work function of the cathode.

Investigation on a Self-Breakdown Repetitive Gap Switch Based on the Graphite Electrodes With TiC Surface Modification

In this article, a self-breakdown repetitive gap switch based on the graphite electrodes with surface modified by TiC is investigated to improve the scattering of the breakdown voltage. The self-breakdown repetitive gap switch was experimentally studied with the graphite electrodes and two kinds of TiC modified graphite electrodes (hereinafter referred to as TiC-I electrodes and TiC-II electrodes). Results show that the scatterings of the breakdown voltages of the graphite, TiC-I and TiC-II electrodes were 2.49%, 1.80%, and 1.54%, respectively. Compared with the graphite electrodes, the mass losses of TiC-I and TiC-II electrodes are reduced by 50% and 68%. The influence of electrode surface morphology on the switch stability is analyzed based on the mechanism of switch conduction process. Compared with the graphite electrodes, the electric field amplification coefficients of TiC-I and TiC-II electrodes are more evenly distributed, which leads to the more evenly distributed field emission current density. Thus, the breakdown areas of TiC-I and TiC-II electrodes are smaller and the gas breakdown processes are more stable than those of the graphite electrodes, resulting in smaller scattering of the breakdown voltage. The method that decreases the scattering of the breakdown voltage and reduces the mass loss of the electrodes by modifying graphite electrodes surface can greatly improve the stability of the switch, which is of significance to the application of the switch in the compact, long time stable operation of high-power pulse power devices.

Investigation of a novel ring-cusp magnetically confined plasma bridge neutralizer.

The plasma bridge neutralizer (PBN) based on a tungsten filament is a promising technique of a thermionic DC electron source where a hot filament is immersed in an inert gas flow and electrons are "bridged" from a small orifice to the ion beam. PBNs have been widely used in space propulsion and industrial applications due to their relatively simple structure and low power consumption. However, they have well-known disadvantages, namely, low emission current density and short lifetime. In this article, we propose a novel ring-cusp magnetically confined PBN (RCM-PBN) to address these issues. In the RCM-PBN, electrons are confined by a ring-cusp magnetic field, which improves the ionization efficiency and reduces the discharge chamber wall losses. Electrical insulation of the orifice plate from the chamber wall prevents a large number of electrons from being collected by the orifice plate, which greatly improves the extracted electron current. The effects of different operating parameters on the extracted electron current were studied through experiments. It was found that the increase in the extracted electron current with the extraction voltage was related to the anode spot formation. Analysis of the gas utilization factor and electron extraction cost shows that the optimal operating condition was obtained at an argon mass flow rate of 1.2 SCCM and a heater power of 45W. At its optimum, a stable electron current of 1.1A was extracted from the RCM-PBN with a gas utilization factor of 12.8 and an electron extraction cost of 143 W/A.

Enhanced field emission performance of Si nanopillars coated with LaB6 hats fabricated by microspheres lithography

LaB6 nanofilm is coated on the top surface of silicon nanopillars forming a field emission array (FEA). Such a structure will be referred to subsequently as a ‘LaB6 hat-coated Si-FEA’. The electron emission mechanism of the LaB6 hat-coated Si-FEA is analyzed by OPERA simulation software. It reveals that the electron emission mainly depends on the LaB6 film edge. Moreover, the LaB6 hat-coated Si-FEA sample is prepared by microsphere lithography, and the field emission properties are tested. The results show that when the anode voltage is 850 V, the field emission current density reaches 458 mA cm−2, which is in good agreement with the simulation results. This work demonstrates that the field emission cathodes with low cost, high integration and high current can be realized by using LaB6 as emitter material combined with microsphere lithography technology.

Morphology and Field Emission of ZnO Nanomaterials at Different Positions

By simply adjusting the temperature and the number of materials, rod-like ZnO with different morphology, such as ZnO nanoneedles, were synthesized by a flexible thermal evaporation method. The ZnO nanorod array has the lowest turn-on field, the highest current density, and the highest emission efficiency due to its good contact with the substrate and relatively weak field shielding effect. Experiments show that the morphology and orientation of one-dimensional ZnO nanomaterials have a great influence on its conduction field and emission current density, and the nanoarrays also contribute to electron emission. The research results have a certain reference value for the application of ZnO nanorod arrays as cathode materials for field emission devices.

Highly enhanced field emission from vertically aligned carbon nanotubes grown on a patterned substrate via non-lithographic method

In the present work, a silicon substrate has been patterned with a gold thin film of thickness 15 nm using thermal evaporation , followed by growth of carbon nanotubes (CNTs) using thermal chemical vapor deposition . The effect of temperature on the growth of CNTs on Au patterned substrate is studied using Raman and Field emission scanning electron microscope (SEM). A significant change in the growth of the CNTs grown on Au film patterned substrate with temperature can be observed using SEM images. However, no significant change in the micro crystallinity of the films was observed. Field emission properties of vertically aligned carbon nanotubes, grown at different temperatures, on gold film patterned substrate has been investigated. For the non-patterned substrate, for the CNT film grown at 900 °C, the current density comes out to be 8.0 mA/cm 2 @ 4.8V/μm, which increases to 14.6 mA/cm 2 in the case of CNT film grown on gold patterned substrate. The observed enhancement of nearly 180% in current density for the patterned substrate as compared to non-patterned silicon substrate is primarily attributed to the reduced electric field screening. The reduced screening is realized with the help of COMSOL simulation, showing a significant enhancement in the local electric field near edges. • Use of novel non- Lithographic technique to pattern CNT films. • Effect of temperature on the Growth of vertically aligned CNTs on patterned substrate. • Nearly 180% enhancement in Field emission current density for CNTs grown over patterned substrate at 900 °C. • Simulation based verification of enhancement in the Field emission properties due to reduced screening.

High-Performance Field Electron Emitters Fabricated Using a Free-Standing Carbon Nanotube Film

The carbon nanotube (CNT) field emitter was fabricated using a thin free-standing CNT film, indicating a line-shape CNT field emitter. Field emission properties of the CNT field emitter were investigated in both diode and triode configurations. The CNT field emitter showed a low turn-on electric field of 1.8 V/μm and a high emission current of 40.3 mA, corresponding to the emission current density of 96 A/cm in the diode configuration. It also exhibited a high anode current of 40 mA, corresponding to the anode current density of 95.2 A/cm in the triode configuration. In addition, the CNT field emitter showed a good electron beam transmittance of 86.4% and excellent emission stability without degradation for 15 h. The main reason for the high performance of our CNT field emitter is caused by the high density of emission sites at the edge of the CNT film.

Nanostructured SiC as a promising material for the cold electron emitters

In this paper, the novel cold electron emitters based on nanostructured SiC layers covering the Si(001) substrate have been proposed. Their main advantage is an extremely simple and cost-effective manufacturing process based on the standard microelectronics-grade silicon wafers with no ultra-high vacuum required and no complicated chemical deposition processes or toxic chemicals involved. It integrates within a single technological step both the SiC growth and nanostructuring the surface in the form of nanosized protrusions, which is extremely beneficial for cathode applications. A simple mathematical model predicts field emission current densities and turn-on electric fields, which would allow practical device applications. According to our estimations, emission currents in the milli-Amp range can be harvested from one square centimeter of the cathode surface with electric field close to 107 V/m. So, the nanostructured SiC can be the promising material for the cold electron emitters.

Preparations of lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6)

AbstractDue to strong anti‐poisoning ability, good emission stability, high emission current density, lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6) have been maturely applied in electron emission emitter. In this paper, a new manufacturing method for LaB6 (or CeB6) powder was proposed by using La2O3 (or CeO2), B4C, and Al as raw materials. After high‐temperature reaction in the range of 1673–1773 K and the following alkaline leaching at 90°C, LaB6 or CeB6 powder with particle size of about 10 μm was obtained. Furthermore, by Al metal flux method, the obtained powder was used to manufacture single crystal block with size of several millimeters.

Mathematical models for thermionic emission current density of graphene emitter

In this study, five mathematical models were fitted in the absence of space charge with experimental data to find a more appropriate model and predict the emission current density of the graphene-based thermionic energy converter accurately. Modified Richardson Dushman model (MRDE) shows that TEC's electron emission depends on temperature, Fermi energy, work function, and coefficient of thermal expansion. Lowest Least square value of S=sum {left({J}_{th}-{J}_{exp}right)}^{2}=0.0002 ,text{A}^{2}/text{m}^{4} makes MRDE most suitable in modelling the emission current density of the graphene-based TEC over the other four tested models. The developed MRDE can be adopted in predicting the current emission density of two-dimensional materials and also future graphene-based TEC response.

Energy density distribution of a modulated electron beam in a source with a plasma cathode based on a low pressure arc

The article presents the results of studies devoted to the study of the energy density distribution in the amplitude-modulated regime of electron beam generation. It is shown that in the first ≈ 50 μs of the duration of the beam current pulse, its spatial rearrangement occurs, due to the development of the arc discharge current. Thus, the rearrangement of the arc current, which develops from the axis of the system, leads to an axial diving of the emission current density and the beam current density on the target. With the development of the arc current, the energy density on the target on the axis of the system decreases and after ≈ 50 μs takes on a steady-state value, which can change only as a result of a change in the conditions for generating an electron beam or the transition to a modulated regime of electron beam generation. It has been experimentally shown using calorimetric measurements that the shape of the electron beam current pulse with its amplitude modulation with a pulse duration of more than 100 μs has little effect on the distribution of the beam energy density in the target region.

Runaway of electrons and initiation of explosive electron emission during pulse breakdown of high-pressure gases

We propose a scenario of the initiation of explosive electron emission on the boundary of the electrode and a high-pressure gas. According to this scenario, positive ions are formed due to the gas ionization by field-emission electrons and accumulated in the vicinity of protrusions of micron size at the cathode. The distance between the ion cloud and the emitting surface decreases with increasing pressure which results in a growth of the local field. As a consequence, an explosive growth of the emission current density occurs for a dense gas (the gas with the pressure of tens of atm). As a result, explosive-emission centers can be formed in dozens of ps. These centers give a start to plasma channels expanding towards the anode. Runaway electron flow generated near the channel heads ionizes the gas gap, causing its subnanosecond breakdown.

Research on the influence of cathode surface topography on spark discharge

To obtain the relation between spark discharges and the topography of electrode surfaces, some models, such as a micro-initial model of cone and spherical head cylinder, generated from the scanning of a scanning electron microscope are introduced. Based on the principles of energy conservation and thermal equilibrium, the second-order non-homogeneous differential equation is established. The relationships between the temperature of the cathode surface and the distance from the top of micro-protrusions, the cone angle, and the radius of the ball head are derived from a set of second-order non-homogeneous differential equations. When introduced electron emission theory, the main emission type at the initial stage of the discharge is electron field emission. As the discharge progresses, the Joule heat generated by the emission current will cause thermionic emission. During this progress, the emission current density gradually increases. When the temperature of the cathode surface reaches the melting point of the cathode metal, the current density will increase sharply. Observed from the discharge, the temperature of the cathode surface is inversely proportional to the cone angle and the radius of the ball head maintains a gradually decreasing slope. It is proportional to micro-protrusions maintaining a gradually increasing slope. When the cone angle is less than 12° and the radius of the ball head is less than 0.04 µm, the temperature of the cathode surface changes more dramatically. Through this study, a further understanding of factors that affect the density of cathode current emission can be obtained, which is of great significance to improve the capacity of cathode emission.

Synthesis of [Ca24Al28O64]4+(4e−) single crystal through xenon lamp melting combined with Ti vapor deoxygenation

In this paper, the high-quality [Ca24Al28O64]4+(4e−) (C12A7:e−) single crystal was synthesized by xenon lamp melting combined with Ti vapor treatment. The relative density of the sample is over 99.7%, and the sample's electron density is 1021 cm−3. The results of electron emission performance show that the sample's emission current density is 1.86 A/cm2 when the applied voltage is 3500 V at 1100 °C, and the fluctuation does not exceed 5%. Theoretical calculation results show that C12A7:e− single crystal has a lower work function (2.36 eV). Also, the free electrons in the cage cavity are mainly distributed around the frame oxygen, which is in a weakly bound state. This makes C12A7:e− single crystal have excellent electron emission characteristics. Theoretical calculations provide support for experiments. These laid the foundation for its application in electron emission.

A new multiscale approach to rapidly determine the local emission current density of nanoscale metallic field emitters

We advocate the use of lookup tables in the development of extremely fast and accurate multiscale models based on the coupling of a quantum-mechanical wave impedance approach and finite-element simulations to determine the local emission current density (LECD) from a metallic emitter of arbitrary shape. The lookup tables are prepopulated with numerical solutions of LECD that can be adjusted to accommodate any form of higher order physics, which is critical for current state-of-the-art emitters. Results show that the use of lookup tables can speed up numerical simulations of the field emission current from metallic cathodes by a factor of about 1000× while retaining high precision, with a maximum error of less than 1% when compared to direct numerical solutions. Implementation of nanoscale emitter physics into lookup tables is discussed and used to assess the validity of the Kemble approximation for nanoscale metallic cathodes. The use of lookup tables is illustrated through a calculation of the LECDs of a metallic field emitter with a rugged surface and from an array of ellipsoid-on-a-post emitters. Section V contains our conclusions and suggestions for future work.

The role of secondary species emission in vacuum facility effects for electrospray thrusters

Theoretical, analytical, and experimental investigations of electrospray operation in vacuum facilities show that secondary species emission (SSE) plays a significant role in the behavior of electrospray thrusters during ground testing. A review of SSE mechanisms, along with an analysis of onset thresholds for electrospray thruster conditions, indicates that secondary species (e.g., electrons, anions, cations, etc.) must be carefully considered for accurate measurements and determination of performance and life. Presented models and experiments show that SSE-induced thruster-to-facility coupling can lead to considerable measurement uncertainty but can be effectively mitigated with an appropriate beam target design. The Electrospray SSE Control-volume Analysis for Resolving Ground Operation of Thrusters model is applied to experimental data to analyze SSE behavior. A heat and mass flux analysis of the Air Force Electrospray Thruster Series 2 (AFET-2) shows that SSE-induced Ohmic dissipation can cause performance limitations in ionic liquid ion source thrusters. The presented analytical models show that backstreaming current density contributing to less than 0.1% of measured emitter current density can cause substantial variation in propellant properties. Additionally, backstreaming current density contributing to less than 3% of emitted current can cause the 0.86 μg s−1 neutral loss rate estimated during AFET-2 testing. Arguments are presented to support the notion that glow discharges observed in electrospray thrusters during vacuum operation are a consequence of secondary species backstreaming to the emission site, rather than a process intrinsically caused by ion evaporation. Recommendations for general best practices to minimize the effects of SSE on electrospray thruster operation are provided.

Study the electron field emission properties of plasma-based reduction of graphene oxide (GO): An ex-situ plasma approach

Vacuum-based electron emission devices requires fast and low turn-on field processes. Graphene oxide (GO) can be a perfect candidate for field electron emission applications. In this work, we have prepared graphene oxide (GO) via modified Hummer's method and treated it with dense plasma of argon gas (Ar). The surface morphology of the prepared GO reveals the growth of the stacked-layer and large surface area flakes of GO. Raman spectra reveals the high defective growth of GO. The impurities in the GO are due to the incorporation of oxygen moieties. For enhancing the field emission properties, the GO thin films were treated with a dense plasma of Ar gas at 50, 100, 150 and 200 W. Raman spectra strongly indicates the removal of oxygen moieties and formation of defects and vacancies in the sheet of plasma-treated GO. The field emission properties drastically improved in the scenario of plasma-treated GO field emitters. The turn-on and threshold fields were reduced after plasma treatment in comparison with pristine GO field emitters. The reduction in the turn-on and threshold fields is due to the enhancement in the edge effects, protrusion formation and modification in structural properties. The emission current density was drastically enhanced in the 200 W plasma-treated GO field emitters. The emission stability test was performed for a 600 min at 3 V / μ m applied electric field. The stability was enhanced with plasma power and shows the minimum current fluctuation. The field enhancement factor also improved after plasma treatment and for 200 W treated β becomes 3 times the pristine GO. We have also implemented the orthodox emission hypothesis test, which was qualified by the prepared pristine and plasma-treated GO field emitters due to the f e x t r values belonging in the acceptable range. Conclusively, Ar gas plasma-treated GO field emitters can be useful for emission applications in vacuum devices.

Copper Nanowire Arrays: Growth and Properties

Cu nanowire arrays of three different diameters have been synthesized via template assisted electrode position technique. Morphological, structural, optical, electrical and field emission properties were examined for these three nanowire arrays. Morphological study reveals the nanowire arrays are of uniform diameter throughout the length and of desired dimensions. Structural study shows the face centred cubic structure of nanowires. The crystallite size has been calculated using the Debye-Scherrer relation and micro strain has been calculated using Williamson-Hall analysis. The surface plasmon resonance absorption peak shifts towards red end with increase in diameter of nanowires. I-V characteristics show the ohmic behaviour. Electron field emission properties of Cu nanowire arrays have been studied using Fowler Nordheim theory. The field emission parameters like maximum emission current density, turn on field and field enhancement factor have been calculated for nanowire arrays of different diameters. Cu nanowire arrays of diameter 50 nm exhibit better field emission properties, indicating that thin nanowire arrays have great potential to be used as field emitters.

Numerical design and frequency response of MQW transistor lasers based entirely on group IV alloys

A theoretical model is developed for n–p–n mid-infrared transistor lasers (TLs) with a strain-balanced Ge0.85Sn0.15 multiple quantum well (MQW) structure in their base. The variation of the optical confinement factor, the modal gain, and the threshold current density is rigorously investigated for different numbers (N) of QWs in the MQW structure. The results show that overall the optical confinement factor and the modal gain increase with N. The frequency response of the MQWTL in the common-base (CB) configuration is estimated from the small-signal relationship between the photon density and the emitter current density by solving the laser rate equation and the continuity equation, considering the virtual states as a conversion mechanism. Increasing N causes the modulation bandwidth first to increase then to decrease with N. This reveals a shift in the nature of the device for higher values of N. The results also suggest that judicious selection of N will enable the proposed device to become a viable monolithic light source.

Synthesis, physico-chemical characterization and field emission behaviour of 3D chrysanthemum like pristine ReS2, and ReS2-rGO nanocomposite

3D chrysanthemum like pristine ReS2 and nanocomposite of ReS2 with reduced graphene oxide (rGO) have been synthesized using facile one step hydrothermal method, followed by physico-chemical characterizations to reveal their phase, structural and electronic properties. Efforts have been made to reveal the influence of process parameters on morphology and growth of the as-synthesized products. From application point of view, field emission (FE) behavior of pristine ReS2 and ReS2-rGO nanocomposite emitters has been investigated at base pressure of 1 × 10–8 torr. The ReS2 and ReS2-rGO nanocomposite emitters showed values of turn-on field (corresponding to emission current density of 1μA cm−2) as 2.10 and 1.66 V μm−1, and field enhancement factor (β) as ∼965 and 1176, Furthermore, the ReS2-rGO nanocomposite emitter delivered maximum emission current density of ∼1472 μA cm−2 at applied field of 3.1 V μm−1. Both the emitters exhibited good emission current stability at pre-set value ∼5 μA over more than 3 h duration. The superior FE properties of the ReS2-rGO nanocomposite emitter are attributed to optimized morphology offering high field enhancement factor coupled with modulation of electronic properties reflected as lowering of the work function. The value of work function of ReS2-rGO nanocomposite, measured using a retarding field analyzer, is found to be 4.29, lower than that of the pristine sample (4.49 eV). The results signify that functionality of pristine nanostructures is greatly improved via formation of nanocomposites and desirable nanocomposites possessing unique morphology can be synthesized under optimized experimental conditions using a facile and inexpensive hydrothermal route.