Emission Current Density Research Articles

Single-walled carbon nanotube thermionic electron emitters with dense, efficient and reproducible electron emission

Thermionic electron emitters have recently been scaled down to the microscale using microfabrication technologies and graphene as the filament. While possessing several advantages over field emitters, graphene-based thermionic micro-emitters still exhibit low emission current density and efficiency. Here, we report nanoscale thermionic electron emitters (NTEEs) fabricated using microfabrication technologies and single-walled carbon nanotubes (SWCNTs), the thinnest conducting filament we can use. The SWCNT NTEEs exhibit an emission current density as high as 0.45 × 105 A cm-2, which is superior to that of traditional thermionic emitters and five orders of magnitude higher than that of graphene-based thermionic emitters. The emission characteristics of SWCNT NTEEs are found to strongly depend on the electrical properties of the SWCNTs, with metallic SWCNT NTEEs showing a substantially lower turn-on voltage and more reproducible emission performances than those based on semiconducting SWCNTs. Our results indicate that SWCNT NTEEs are promising for electron source applications.

A moderate synthesis route of 5.6 mA-current LaB6 nanowire film with recoverable emission performance towards cold cathode electron source applications

The as-grown LaB6 nanowire arrays still remain a very large and stable emission current density over 16.7 mA cm−2 at high temperature as well as recoverable emission performances, which should have promising future in cold cathode electron sources.

Influence of copper nanowires grown in a dielectric layer on the performance of dielectric barrier discharge

This letter describes the fabrication of copper nanowires within a nanoporous anodic alumina oxide (AAO) layer by electrodeposition through an AAO template. The diameter of nanowires less than 100 nm is generally about 50 nm, which corresponds to the diameter of the nanopores, and can be controlled by the anodic oxidation voltage. Using the AAO with nanowires as the dielectric layer of the dielectric barrier discharge device enables a typical reduction in the breakdown and extinction voltage by about 12%–18%, and 22% at most, in the Neon pressure range of 10–90 kPa. The gas discharge intensity is also enhanced. The field emission property of the integrated nanowires is measured in a vacuum environment, and the resulting emission current density of 0.1 μA/cm2 at an electric field of 5.5 V/μm is three times larger than that of a device without nanowires. This indicates that the reduction in operating voltage is mainly due to additional electrons produced by the nanowires. Additionally, the resulting Fowler–Nordheim characteristic clearly demonstrates the field emission behavior of the copper nanowires.

Improvement of electron emission characteristics of porous silicon emitter by using cathode reduction and electrochemical oxidation

A new simple and convenient post-treat technique combined the cathode reduction (CR) and electrochemical oxidation (ECO) was proposed to improve the electron emission properties of the surface-emitting cold cathodes based on the porous silicon (PS). It is demonstrated here that by introducing this new technique combined CR and ECO, the emission properties of the diode have been significantly improved than those as-prepared samples. The experimental results showed that the emission current densities and efficiencies of sample treated by CR were 62μA/cm2 and 12.10‰, respectively, nearly 2 orders of magnitude higher than those of as-prepared sample. Furthermore, the CR-treated PS emitter shows higher repeatability and stability compared with the as-prepared PS emitter. The scanning electron microscope (SEM), atomic force microscope (AFM), energy dispersive spectrometer (EDS), furier transformed infrared (FTIR) spectroscopy results indicated that the improved mechanism is mainly due to the passivation of the PS, which not only improve the PS morphology by the passivation of the H+ but also improve the uniformity of the oxygen content distribution in the whole PS layer. Therefore, the method combined the CR treatment and ECO is expected to be a valuable technique to enhance the electron emission characteristics of the PS emitter.

Theoretical analysis of 1D resonant tunneling behavior in ion-enhanced cold field and thermo-field emission

In cold field and thermo-field emission, positive ions or adsorbates very close to the cathode surface can enhance emission current by both resonant and non-resonant processes. In this paper, resonant tunneling behavior is investigated by solving the one-dimensional Schrödinger equation in the presence of an ion, and the enhancement due to resonant processes is evaluated. Results shows that as the applied electric field increases, the resonant states move from higher to lower energies as the ion energy levels are shifted down. Conversely, as the ion position moves closer to the cathode, the resonant states shift up in energy. Further, through a simplified perturbation analysis, the general scaling of these trends can be predicted. These shifts of resonant states directly impact the emission current density, and they are especially relevant when the applied field is on the order of a few volts per nanometer (∼0.5–3 V/nm) and the ion is a few nanometers (∼0.5–3 nm) away from the cathode. Further, when the energy level for resonant emission coincides with the Fermi level of a metallic cathode, the current density is particularly enhanced. The results of this study suggest that it may be possible to control (augment/inhibit) the resonant emission current by manipulating the supply function of a cathode relative to the operating conditions of the emitter in either ion-enhanced or adsorbate-enhanced field emission, which can be applied to various plasma and electron emission technologies.

Self-modulated field electron emitter: Gated device of integrated Si tip-on-nano-channel

We report the featured gated field electron emission devices of Si nano-tips with individually integrated Si nano-channels and the interpretation of the related physics. A rational procedure was developed to fabricate the uniform integrated devices. The electrical and thermal conduction tests demonstrated that the Si nano-channel can limit both the current and heat flows. The integrated devices showed the specialties of self-enhancement and self-regulation. The heat resistance results in the heat accumulation at the tip-apex, inducing the thermally enhanced field electron emission. The self-regulated effect of the electrical resistance is benefit for impeding the current overloading and prevents the emitters from a catastrophic breakdown. The nano-channel-integrated Si nano-tip array exhibited emission current density up to 24.9 mA/cm2 at a gate voltage of 94 V, much higher than that of the Si nano-tip array without an integrated nano-channel.

Study and Development of Yttrium-Doped Nanoscandate Cathode for High-Power VEDs

Nanoscandate cathode (NSC) has several advantages over a conventional scandate cathode in terms of emission uniformity, resistance to ion bombardment, and long life. In the present technology of “Yttrium-doped NSC,” yttrium oxide (Y2O3) is codoped with tungsten powder, scandium oxide, and other impregnant materials to make nanopowder using the solgel technique. The pellets, made out of powder, were integrated with potted heaters. The cathodes were analyzed in an analytical chamber containing: 1) Auger electron spectroscopy for surface analysis; 2) anode for emission measurements; and 3) thermionic emission microscope (THEM) for studying emission uniformity. It was found that by adding a small amount of yttrium oxide of 3%–5% by weight increases the emission uniformity of cathode significantly as observed through THEM. Emission current density of more than 80 A/cm2 was drawn from these cathodes under deep space charge conditions.

Formation of field-emission emitters by microwave plasma-chemical synthesis of nanocarbon structures

A nanocarbon-emitter technology for integrated field-emission elements has been developed. The modes in which various carbon film structures of diamond, graphite, and graphene-like types are produced were determined. The low-temperature method of obtaining ultradispersed diamonds was examined. It is shown that the high-emission properties of nanodiamond-graphite emitters result from the self-organization of diamond nanocrystals in a graphite film in the course of deposition from ethanol vapor at low pressure with the use of a strongly nonequilibrium microwave plasma. The following parameters of integrated field-emission diodes were obtained: emission threshold of 2.5 V/μm and emission current density of 1.75 A/cm2. The highest current density of more than 20 A/cm2 was obtained in a study of blade-type emitters.

Synthesis and characterization of low work function alkali oxide thin films for unconventional thermionic energy converters

In this work, we present the synthesis and the characterization of low work function thin films for Micro Thermionic Converters (MTC). The objective is producing a device operating at relatively low temperature (<1000 K). We aim at improving the MTC efficiency by reducing the work function of the electrodes and increasing the emitted current density by alkali metal oxides electrodes coating. In particular, in this work, we analyse and compare the performances of two alkali metal oxides: potassium and caesium oxides. Our choice to exploit those materials relies on their low work function and their abundance. For both materials, we present the results on the synthesis of the oxides under high vacuum and controlled temperature. The oxide thin films were characterized by X-ray photoelectron spectroscopy, photoemission, and thermionic emission measurements. By exploiting the latter technique, a quantitative evaluation of the current density, emitted by the heated oxides, is obtained as a function of temperature. Our results demonstrate that it is possible to decrease the silicon work function by almost 3 eV, enabling significant thermionic currents despite relatively low temperatures (below 850 K).

Enhanced performance of thermal-assisted electron field emission based on barium oxide nanowire

In this paper, thermal-assisted field emission properties of barium oxide (BaO) nanowire synthesized by a chemical bath deposition method were investigated. The morphology and composition of BaO nanowire were characterized by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SED), X-ray diffraction (XRD), and energy dispersive X-ray spectrometer (EDX) respectively. The turn-on field, threshold field and the emission current density could be affected relatively due to the thermal-assisted effect when the electric field was applied, in the meanwhile, the turn-on field for BaO nanowire was measured to be decreased from 1.12V/μm to 0.66V/μm when the temperature was raised from 293K to 593K, whereas for the threshold field was found to decrease from 3.64V/μm to 2.12V/μm. The improved performance was demonstrated due to the reduced work function of the BaO nanowire as the agitation temperature increasing, leading to the higher probability of electrons tunneling through the energy barrier and enhancement of the field emission properties of BaO emitters.

Morphological, structural and field emission characterization of hydrothermally synthesized MoS2-RGO nanocomposite

A few layered MoS2-RGO nanocomposite has been synthesized employing a facile hydrothermal synthesis route. The morphological and structural analysis performed using SEM, TEM, HRTEM and Raman spectroscopy clearly reveal formation of vertically aligned a few layer thick MoS2 sheets on RGO surface. Attempts have been made to reveal the influence of graphite oxide (GO) percentage on morphology of the nanocomposite. Furthermore, field emission (FE) investigations of as-synthesied MoS2-RGO nanocomposite are observed to be superior to the pristine MoS2 emitter. The values of turn-on field, defined at emission current density of 10 μA cm−2, are found to be 2.6 and 4.7 V μm−1 for the MoS2-RGO (5%) nanocomposite and pristine MoS2 emitters, respectively. The value of threshold field, defined at emission current density of 100 μA cm−2, is found to be 3.1 V μm−1 for MoS2-RGO nanocomposite. The emission current stability at the pre-set value of 1 μA over 3 h duration is found to be fairly good, characterized by current fluctuation within ±18% of the average value. The enhanced FE behavior for MoS2-RGO nanocomposite is attributed to a high enhancement factor (β) of 4128 and modulation of the electronic properties. The facile approach adopted herein can be extended to enhance various functionalities of other nanocomposites.

Calculation of photofield emission current by using the projection operator method of group theory

A calculation of photofield emission is discussed in which initial state wave function has been deduced by using the projection operator method of group theory. A spatial dependent vector field is used to evaluate the matrix element for calculating the photofield emission current density.

Pulsed laser-deposited nanocrystalline GdB6 thin films on W and Re as field emitters

Gadolinium hexaboride (GdB6) nanocrystalline thin films were grown on tungsten (W), rhenium (Re) tips and foil substrates using optimized pulsed laser deposition (PLD) technique. The X-ray diffraction analysis reveals formation of pure, crystalline cubic phase of GdB6 on W and Re substrates, under the prevailing PLD conditions. The field emission (FE) studies of GdB6/W and GdB6/Re emitters were performed in a planar diode configuration at the base pressure ~10−8 mbar. The GdB6/W and GdB6/Re tip emitters deliver high emission current densities of ~1.4 and 0.811 mA/cm2 at an applied field of ~6.0 and 7.0 V/µm, respectively. The Fowler–Nordheim (F–N) plots were found to be nearly linear showing metallic nature of the emitters. The noticeably high values of field enhancement factor (β) estimated using the slopes of the F–N plots indicate that the PLD GdB6 coating on W and Re substrates comprises of high-aspect-ratio nanostructures. Interestingly, the GdB6/W and GdB6/Re planar emitters exhibit excellent current stability at the preset values over a long-term operation, as compared to the tip emitters. Furthermore, the values of workfunction of the GdB6/W and GdB6/Re emitters, experimentally measured using ultraviolet photoelectron spectroscopy, are found to be same, ~1.6 ± 0.1 eV. Despite possessing same workfunction value, the FE characteristics of the GdB6/W emitter are markedly different from that of GdB6/Re emitter, which can be attributed to the growth of GdB6 films on W and Re substrates.

Oxide Coated Cathode Plasma Source of Linear Magnetized Plasma Device**supported by National Natural Science Foundation of China (No. 11275200)

Plasma source is the most important part of the laboratory plasma platform for fundamental plasma experimental research. Barium oxide coated cathode plasma source is well recognized as an effective technique due to its high electron emission current. An indirectly heated oxide coated cathode plasma source has been constructed on a linear magnetized plasma device. The electron emission current density can reach 2 A/cm2 to 6 A/cm2 in pulsed mode within pulse length 5–20 ms. A 10 cm diameter, 2 m long plasma column with density 1018 m−3 to 1019 m3 and electron temperature Te ≃ 3–7 eV is produced. The spatial uniformity of the emission ability is less than 4% and the discharge reproducibility is better than 97%. With a wide range of the plasma parameters, this kind of plasma source provides great flexibility for many basic plasma investigations. The detail of construction and initial characterization of oxide coated cathode are described in this paper.

Field emission properties and strong localization effect in conduction mechanism of nanostructured perovskite LaNiO3

We report the potential field emission of highly conducting metallic perovskite lanthanum nickelate (LaNiO3) from the nanostructured pyramidal and whisker shaped tips as electron emitters. Nano particles of lanthanum nickelate (LNO) were prepared by sol-gel route. Structural and morphological studies have been carried out. Field emission of LNO exhibited high emission current density, J = 3.37 mA/cm2 at a low threshold electric field, Eth = 16.91 V/μm, obeying Fowler–Nordheim tunneling. The DC electrical resistivity exhibited upturn at 11.6 K indicating localization of electron at low temperature. Magnetoresistance measurement at different temperatures confirmed strong localization in nanostructured LNO obeying Anderson localization effect at low temperature.

A Facile Approach towards Fabrication of GdB6‐ZnO Heteroarchitecture as High Current Density Cold Cathode

AbstractThree dimensional (3D) GdB6‐ZnO heteroarchitecture comprised of thin coating of GdB6 on self aligned ZnO urchins with pointed apex has been synthesized employing optimized Pulsed Laser Deposition (PLD) technique. The self aligned ZnO urchins on Zn substrate were obtained using hydrothermal route. The as‐synthesized GdB6‐ZnO heteroarchitecture was characterized using XRD, SEM, TEM XPS, and UPS in order to reveal its structural, morphological, chemical, and electronic properties. Interestingly, the GdB6‐ZnO heteroarchitecture exhibits superior field emission (FE) behviour in contrast to the pristine ZnO urchins envisaged by extraction of very high emission current density of ∼ 4.6 mA/cm2 at an applied field of ∼ 4.5 V/μm, against ∼1.5 mA/cm2 at an applied field of ∼ 5.6 V/μm from the pristine ZnO urchins emitter. Furthermore, the GdB6‐ZnOemitter exhibits good emission stability at pre‐set value of ∼5 μA over duration of more than 3 hours. The superior FE behaviour of the GdB6‐ZnO is attributed to low workfunction of GdB6 and presence of nanometric protrusions on the emitter surface, further enhancing the aspect ratio provided by the ZnO urchins. The present results demonstrate a facile approach towards fabrication of high current density cold cathodes due to rare earth hexaborides via designing heteroarchitectures comprised of their well adherent ultrathin coating on high aspect ratio metal oxide nanostructures.

Improvement of the voltage-controlled negative resistance of a porous silicon emitter using cathode reduction and electro-pretreatment

A new composite treatment method combining cathode reduction (CR) and electro-pretreatment (EP) is proposed to improve the voltage-controlled negative resistance (VCNR) of porous silicon (PS) emitters. Four groups of PS emitters were treated, with nothing, CR, EP, and a combination of CR and EP, during different preparation stages. The experimental results indicate that both CR and EP improved the VCNR property and the emission characteristics of the PS emitter. The most favorable results occurred for the electron emitter treated with a combination of CR–EP. A peak-to-valley current ratio of 1.06 and an emission current density of 150 µA cm−2, which are the lowest value and approximately twice that of other samples, respectively, were obtained. In addition, the operating stability of the PS emitter also improved significantly compared with the two methods alone. Scanning electron microscopy, atomic force microscopy, and energy dispersive x-ray spectrometry results demonstrate that the improvements of the VCNR and the emission characteristics of PS emitters are due to the content variation of defects, impurities, and unstable microstructures in the PS layer under the influence of CR and EP. The mechanism of VCNR behavior is explained by a proposed energy band model, which is consistent with the experimental results.

Study on the compositions of emission active materials and the emission properties of dispenser cathodes made from Scandia-doped powders

Scandia-doped dispenser cathode provided significantly better performance compared to standard dispenser cathodes in the application of microwave devices. Emission active materials are the key to dispenser cathodes. Scandium doping can enhance the performance of dispenser cathodes. In this paper, active materials are fabricated by co-precipitation method. Comparison is made between the active materials undergoing different heat treatments. XRD and SEM methods are employed to analyze the compositions and microstructure of active material samples. The cathode matrix powders are prepared by the sol–gel method using different recipes of scandium content. The contents of scandium oxide are, respectively, 3.5wt%, 4wt% and 5wt%. Emission tests are carried out in dedicated test vehicles. The results show that there are different aluminates including Ba4Al2O7, CaAl2O4, Ba7Al2O10, Ba3CaAl2O7, and Ba3Al2O6 come into being after heating using different techniques. The cathode impregnated with self-developed emission materials which numbered LN4 can achieve an inflection point emission current density of 122A/cm2 at 1165°C. On the study of different cathode matrix powders, when impregnated with the conventional material (S), the cathode made from 3.5wt% Scandia-doped powders has the best emission performance. The 4wt% cathode sample performs better when the cathodes are impregnated with the self-developed emission materials.

High-voltage thermionic polaron emission in the presence of adsorbed nonmetal nanofilm on a cathode

The equation of thermionic emission has been derived, which takes into account the presence of thin films of adsorbed gas-environment molecules on a cathode under conditions of high-voltage gas discharge. It is shown that the consideration of electron polaron tunneling mechanism leads to a significant (by more than an order of magnitude) decrease in the emission-current density. A comparison with the classical Richardson–Schottky equation is performed. The role of the polaron effect is shown to strongly increase with an increase in the applied electric field strength and a decrease in temperature, which is due to an increase in the electron work function because of polaron nature.

Noise parameters of defects of the emitting surface of thermionic cathodes

A noise model in which fluctuation noise of output current is determined by weakly emitting fragments of thermionic cathode working in the saturation regime (defects on the emitting surface) is proposed. The measured noise characteristics of the shot and flicker components of noise can be used to determine distribution of defects with respect to emission current densities and the noise parameters of defects. The relation of noise parameters and the degree of degradation of cathode is analyzed using the pulsed emission current at relatively long working times of dispenser cathodes with the W–Re sponge and Os coating. Preliminary results show that that proposed noise parameters can be used for the development of working methods for estimation of cathode quality.