Cold Cathode Research Articles

ZnO Nanowire Cold Cathode Hemispherical X‐Ray Sources

AbstractCurved or spherical X‐ray sources are significant for use in intraoperative radiotherapy, adaptive static medical imaging, and high‐throughput industrial inspection, but they are hard to achieve using traditional thermionic cathode point electron sources. In this study, copper (Cu)‐doped zinc oxide (ZnO) nanowires grown on a brass substrate with a designed shape are proposed to achieve cold cathode hemispherical X‐ray sources. The strain‐driven solid–liquid growth model of Cu‐doped ZnO nanowires is proposed, and the oxidation temperature‐dependent and time‐dependent growth characteristics are investigated to optimize the morphologies of ZnO nanowire cold cathodes with a typical turn‐on field of 7.36 MV m−1, a maximum current of 12.54 mA (4.93 mA cm−2) and a uniform field emission image with an area of 2.54 cm2. Hemispherical X‐ray sources formed by Cu‐doped ZnO nanowire field emitters grown on spherical brass alloy and an Al thin film transmission anode target deposited on a hemispherical quartz glass are successfully fabricated, achieving an operating voltage of 39 kV, a dose rate of 240 µGyair s−1 and a projection X‐ray imaging resolution of 2.8 lp mm−1, demonstrating their promising use in a variety of applications.

Modifying of surface properties and structural characteristics of low energy argon beam irradiated methylcellulose/TiO2 nanocomposite films

Flexible polymeric nanocomposites MC/TiO2 films, which consisting of titanium dioxide (TiO2) and methyl cellulose (MC) were fabricated in this research for applied in coating devices. The successful manufacturing of MC/TiO2 sheets were verified by FTIR, SEM and XRD methods, which demonstrated a uniform distribution of TiO2 in MC. Additionally, the chemical bonds of MC and TiO2 contribute for the broadening and decreasing of MC in the peaks intensity of XRD and FTIR with increasing TiO2, indicating the successful incorporation of TiO2 in MC. The impacts of argon beam bombardment on MC/TiO2 composites using cold cathode source with fluencies (2.5×1015, 5×1015 and 7.5×1015 ions/cm2 ). The contact angle, work of adhesion and surface free energy of MC/TiO2 were determined as a function of ion irradiation. The water contact angle is decreased from 70.32o to 43.34o by increasing ion fluence from 2.5×1015 ions/cm2 to 7.5×1015 ions/cm2 , while the surface free energy is enhanced from 38.83 mJ/m2 to 64.17 mJ/m2 . The collected data confirmed that the surface wettability of the irradiated MC/TiO2 films were improved to be can usage in coating and printing applications

Characteristics of Carbon Nanotube Cold Cathode Triode Electron Gun Driven by MOSFET Working at Subthreshold Region.

The carbon nanotube cold cathode has important applications in the X-ray source, microwave tube, neutralizer, etc. In this study, the characteristics of carbon nanotube (CNT) electron gun in series with metal-oxide-semiconductor field-effect transistor (MOSFET) were studied. CNTs were prepared on a stainless steel substrate by chemical vapor deposition and assembled with a mesh gate to form an electron gun. The anode current of the electron gun can be accurately regulated by precisely controlling the MOSFET gate voltage in the subthreshold region from 1 to 40 µA. The current stability measurements show the cathode current fluctuation was 0.87% under 10 h continuous operation, and the corresponding anode current fluctuation was 2.3%. The result has demonstrated that the MOSFET can be applied for the precise control of the CNT electron gun and greatly improve current stability.

Model of non-equilibrium near-cathode plasma layers for simulation of ignition of high-pressure arcs on cold refractory cathodes

The introduction of secondary ion-electron emission into an approximate model of non-equilibrium plasma layers on hot (thermionic) cathodes of high-pressure arc discharges allows extending the model to low cathode surface temperatures. Analysis of evaluation results shows that the extended model describes glow-like discharges on cold cathodes and thermionic arc discharges on hot cathodes, as it should. In the course of glow-to-arc transitions on cold cathodes, a transient regime occurs where a hot arc spot has just formed and a significant fraction of the current still flows to the cold surface outside the spot, so that the near-cathode voltage continues to be high. The power input in the near-cathode layer is very high in this regime, and so is the electron temperature in the near-cathode region. The mean free path for collisions between the atoms and the ions in these conditions exceeds the thickness of the layer where the ion current to the cathode is generated. A new method for evaluation of the ion current under such conditions is implemented. The developed model is applicable for cathode surface temperatures below the boiling point of the cathode material and may be used for multidimensional simulations of ignition of high-current arcs on refractory cathodes.

Effects of oxygen irradiation on the electrical properties of polyethylene oxide/nickel oxide composite films

This study investigates the impact of the ion beam on the properties of composite PEO/NiO, which was fabricated using the solution casting method and applied in advanced dielectric applications. The samples were exposed to ion beam at different fluencies (5 × 1016, 10 × 1016, and 15 × 1016 ions/cm2) using cold cathode ion source. The structure of the pure and treated PEO/NiO films was studied using the XRD technique, which demonstrated the successful fabrication of the composite PEO/NiO. Moreover, the morphological changes were analyzed by SEM, which indicates the homogeneous distribution of NiO in PEO. Furthermore, the dielectric characteristics of PEO/NiO films were tested at a frequency range of 40–106 Hz. The dielectric constant enhanced from 22.8 for PEO/NiO to 128.5 for the irradiated 15 × 1016 ions/cm2, and the energy density enhanced from 1.1x10−4 to 5.6x10−4 J/m3. The results demonstrate that the irradiated PEO/NiO composite exhibits novel dielectric properties, allowing the use of the irradiated PEO/NiO composite in different devices as super-capacitors and batteries.

Influence of argon irradiations on modifying the surface properties and optical behavior of polymer composite films

ABSTRACT The composites PVA/CuO, which is made of copper oxide nanoparticles (CuONP) and polyvinyl alcohol (PVA) are created by the solution casting production process. The surface properties of the PVA/CuO composite were then altered using a cold cathode ion source for using these samples in optical devices. The PVA/CuO samples were exposed to argon fluencies of 3 × 1017, 6 × 1017, and 9 × 1017 ions.cm−2. By raising the fluence from 3 × 1017 to 9 × 1017 ions.cm−2, the contact angle is reduced from to 48.43° to 32.52°, while the work of adhesion increased from 119.93 mJ/m to a 132.86 mJ/m. Using the Wemple and Di-Domenico approach, the optical characteristics were computed. The absorption edge is reduced from 2.71 eV for PVA/CuO to 2.56, 1.99, and 1.82 eV, for the samples as exposed to 3 × 1017, 6 × 1017, and 9 × 1017 ions.cm−2 respectively. And the Urbach tail changed from 1.3 eV for the pure film to 1.6, 1.66, and 4.23 eV, and the bandgaps decrease from 3.67 eV to 3.56 eV, 3.48 eV, and 3.32 eV. The obtained results shows the irradiated PVA/CuO films are more applicable for use in optical devices.

Large size shielded metal-ceramic cathodes

The technology for manufacturing large size shielded metal-ceramic cold cathodes is described. It has been shown that the use of a shielded metal-ceramic cathode allows to significantly increase the impedance of the vacuum diode and to form an electron beam of constant cross-sectional size or of the required size (up to 400 mm wide) with a fairly uniform electron beam current density distribution (≤15%).It has been found that the emission decline of the shielded metal-ceramic cathode, like that of the metal-dielectric cathode operating at a high repetition rate (≥100 pps), is significant at the beginning of the process and then slows down, allowing continuous operation for up to 40 h. Both cathodes require periodic cleaning of the plates, which is more difficult for metal-ceramic plates.At the same time, a metal-ceramic cathode works better at low vacuum, allowing twice as much current to be generated (in the same geometry) as a metal-dielectric cathode, while reducing the accelerating voltage by ∼15%. However, metal-ceramic plates are about 100 times more expensive to manufacture than dielectric plates for a metal-dielectric cathode.Therefore, the choice of cathode type for vacuum diode accelerators is not a trivial problem, but depends on many factors that must be taken into account in each case and for each radiation technology.

Dependence of Ultrafast Electron Emission Characteristics of Graphene Cold Cathode on Femtosecond Photoexcitation Polarization Angle.

Ultrafast electron pulses, generated through femtosecond photoexcitation in nanocathode materials, introduce high-frequency characteristics and ultrahigh temporal-spatial resolution to vacuum micro-nano electronic devices. To advance the development of ultrafast electron sources sensitive to polarized light, we propose an ultrafast pulsed electron source based on a vertical few-layer graphene cold cathode. This source exhibits selective electron emission properties for varying polarization angles, with high switching ratios of 277 (at 0°) and 235 (at 90°). The electron emission of the graphene evolves from cosine to sine as the polarization angle increases from 0° to 90°. The variation of electron emission current with polarization angle is intrinsically related to light absorption, local field enhancement, and photothermal conversion efficiency. A physical mechanism model and semiempirical expression were presented to reveal the MPP and PTE mechanisms at different polarization angles. This tunable conversion between mechanisms indicates potential applications in tunable ultrafast optoelectronic devices.

Field emission from two-dimensional (2D) CdSSe flake flowers structure grown on gold coated silicon substrate: An efficient cold cathode.

Field emission finds a vital space in numerous scientific and technological applications, including high-resolution imaging at micro- and nano-scales, conducting high-energy physics experiments, molecule ionization in spectroscopy, and electronic uses. A continuous effort exists to develop new materials for enhanced field emission applications. In the present work, two-dimensional (2D) well-aligned CdSSe flake flowers (CdSSe-FFs) were successfully grown on gold-coated silicon substrate utilizing a simple and affordable chemical bath deposition approach at ambient temperature. The time-dependent growth mechanism from nanoparticles to FFs was observed at optimized parameters such as concentration of precursors, pH (~11), deposition time, and solution temperature. The crystalline nature of CdSSe-FFs is confirmed by high-resolution transmission electron microscopy (HRTEM) results, and selected area electron diffraction (SAED) observations reveal a hexagonal crystal structure. Additionally, the CdSSe-FFs thickness was confirmed by TEM analysis and found to be ~20-30 nm. The optical, photoelectric, and field emission (FE) characteristics are thoroughly explored which shows significant enhancement due to the formation of heterojunction between the gold-coated silicon substrate and CdSSe-FFs. The UV-visible absorption spectra of CdSSe-FFs show enhanced absorption at 700 nm, corresponding to the energy band gap (Eg) of 1.77 eV. The CdSSe-FFs exhibited field emission and photosensitive field emission (PSFE) characteristics. In FE study CdSSe-FFs shows an increase in current density of 387.2 μ A cm-2 in an applied field of 4.1 V m-1 which is 4.08 fold as compared to without light illumination (95.1 μ A cm-2). Furthermore, it shows excellent emission current stability at the preset value of 1.5 μA over 3 h with a deviation of the current density of less than 5% respectively. RESEARCH HIGHLIGHTS: Novel CdSSe flake flowers were grown on Au-coated Si substrate by a cost-effective chemical bath deposition route. The growth mechanism of CdSSe flake flowers is studied in detail. Field emission and Photoluminescence study of CdSSe flake flowers is characterized. CdSSe flake flowers with nanoflakes sharp edges exhibited enhanced field emission properties.

Field-Emission Energy Distribution of Carbon Nanotube Film and Single Tube under High Current.

A narrow energy distribution is a prominent characteristic of field-emission cold cathodes. When applied in a vacuum electronic device, the cold cathode is fabricated over a large area and works under a high current and current density. It is interesting to see the energy distribution of the field emitter under such a working situation. In this work, the energy distribution spectra of a single carbon nanotube (CNT) and a CNT film were investigated across a range of currents, spanning from low to high. A consistent result indicated that, at low current emission, the CNT film (area: 0.585 mm2) exhibited a narrow electron energy distribution as small as 0.5 eV, similar to that of a single CNT, while the energy distribution broadened with increased current and voltage, accompanied by a peak position shift. The influencing factors related to the electric field, Joule heating, Coulomb interaction, and emission site over a large area were discussed to elucidate the underlying mechanism. The results provide guidance for the electron source application of nano-materials in cold cathode devices.

Investigating ion irradiation effects for modifying optical properties of CA/PANI films

In this study, flexible CA/PANI films, consisting of polyaniline (PANI) and cellulose acetate (CA), were created via a solution cast production process. Then, these films were irradiated using of handmade cold cathode ion device with argon fluence of 2x1017, 4x1017, and 6x1017 ions/cm2. The SRIM program was used to investigate the ion penetration depth, ion distribution in the composite and the energy stopping loss. Moreover, the FTIR technique was used to identify the changes of the treated CA/PANI. The UV-Vis technique was used to show the impact of argon beam on the optical characteristics of CA/PANI composite film. The oscillation energy (E0) dropped from 8.88 for CA/PANI to 7.83, 6.42, and 5.19 eV after exposed to 2x1017, 4x1017, and 6x1017 ions/cm2. The results showed that the ion altered the optical characteristics of the CA/PANI samples, which allow for applied these irradiated CA/PANI films in optoelectronic devices.

Effect of Crystallinity on the Field Emission Characteristics of Carbon Nanotube Grown on W-Co Bimetallic Catalyst.

Carbon nanotube (CNT) is an excellent field emission material. However, uniformity and stability are the key issues hampering its device application. In this work, a bimetallic W-Co alloy was adopted as the catalyst of CNT in chemical vapor deposition process. The high melting point and stable crystal structure of W-Co helps to increase the grown CNT diameter uniformity and homogeneous crystal structure. High-crystallinity CNTs were grown on the W-Co bimetallic catalyst. Its field emission characteristics demonstrated a low turn-on field, high current density, stable current stability, and uniform emission distribution. The Fowler-Nordheim (FN) and Seppen-Katamuki (SK) analyses revealed that the CNT grown on the W-Co catalyst has a relatively low work function and high field enhancement factor. The high crystallinity and homogeneous crystal structure of CNT also reduce the body resistance and increase the emission current stability and maximum current. The result provides a way to synthesis a high-quality CNT field emitter, which will accelerate the development of cold cathode vacuum electronic device application.

Ionization layer with collision-free atoms at the edge of partially to fully ionized plasmas

When a hot arc spot has just formed on the cathode surface, e.g. in the course of arc ignition on a cold cathode, a significant part of the current still flows in the glow-discharge mode to the cold surface outside the spot. The near-cathode voltage continues to be high at all points of the cathode surface. The mean free path for collisions between the atoms and the ions within the plasma ball near the spot is comparable to, or exceeds, the thickness of the ionization layer, which is a part of the near-cathode non-equilibrium layer where the ion current to the cathode is generated. The evaluation of the ion current to the cathode surface under such conditions is revisited. A fluid description of the ion motion in the ionization layer is combined with a kinetic description of the atom motion. The resulting problem admits a simple analytical solution. Formulas for the evaluation of the ion current to the cathode for a wide range of conditions are derived and the possibilities of using these formulas to improve the accuracy of existing methods for modeling high-pressure arc discharges in relation to glow-to-arc transitions are discussed.

Effect of beam and gate offset size on x-ray focal spot resolution of a cold cathode electron beam

In x-ray imaging, high resolution is essential, particularly in sectors such as medical and industries where the need for nondestructive defect detection is required. Previous research has shown that altering beam design and the number of gates offset holes has an impact on focal spot size (FSS). However, the specific effects of beam size and offset size were not thoroughly assessed. In the present study, the influence of beam size and gate offset size was evaluated by utilizing a cold cathode electron beam. Various beam sizes were employed to achieve a small FSS, and subsequently, the smallest beam was utilized to examine the impact of gate offset size. In doing so, the smallest FSS of 0.25 mm vertical and 0.33 mm horizontal was attained without the utilization of any additional focusing lens. This illustrates that by adjusting the beam size and gate offset size, it is possible to attain a small FSS, facilitating the development of an economically viable x-ray imaging beam.

Simulating vacuum arc initiation by coupling emission, heating and plasma processes

Vacuum arcing poses significant challenges for high-field vacuum devices, underscoring the importance of understanding it for their efficient design. A detailed description of the physical mechanisms involved in vacuum arcing is yet to be achieved, despite extensive research. In this work, we further develop the modeling of the physical processes involved in the initiation of vacuum arcing, starting from field emission and leading to plasma onset. Our model concurrently combines particle-in-cell with Monte Carlo collisions (PIC-MCC) simulations of plasma processes with finite element-based calculations of electron emission and the associated thermal effects. Including the processes of evaporation, impact ionization and direct field ionization allowed us to observe the dynamics of plasma buildup from an initially cold cathode surface. We simulated a static nanotip at various local fields to study the thresholds for thermal runaway and plasma initiation, identifying the significance of various interactions. We found that direct field ionization of neutrals has a significant effect at high fields on the order of 10 GV/m. Furthermore, we find that cathode surface interactions such as evaporation, sputtering and bombardment heating play a major role in the initiation of vacuum arcs. Consequently, the inclusion of these interactions in vacuum arc simulations is imperative.

Time-tuned ZnO(x)/MWCNTs hybrid cold cathodes for next-generation electron emission

Time-tuned ZnO(x)/MWCNTs hybrid cold cathodes for next-generation electron emission

Emission characteristics of pulse CNT cold cathode X-ray source combined with channel electron multiplier

Carbon nanotube (CNT)-based field emission X-ray source with the advantage of fast start-up response offers the chance to achieve high-frequency X-ray emission. In this study, a high-frequency random pulse X-ray source of CNT cold cathode combined with a channel electron multiplier (CEM) was built, and its direct current (DC) and pulse emission characteristics were tested. The DC measurement results were used for parameter selection for performing pulse experiments. During the DC test, with the conditions of 2.2 kV CEM bias voltage and 25 kV anode voltage, the anode currents are 141, 250, and 300 μA at grid voltages of 290, 387.6, and 432.2 V, respectively; the corresponding grid field values are 1.45, 1.94, and 2.16 V/μm. During the pulse test, the amplitude–frequency response of the X-ray source reaches 3.58 MHz at 3 dB. The developed pulse X-ray source was introduced into the X-ray communication (XCOM), and the experimental communication rate reached 6 Mbps with the bit-error-rate of 1.1 × 10−3. The developed high-frequency pulse CNT-CEM X-ray source has potential applications in XCOM, high-speed X-ray imaging, and other fields.

Influence of ion beam on the surface characteristics and structural properties of PVA/TiO2 films

This work used the solution-casting preparation method to create flexible polymer composite materials including titanium dioxide (TiO2) and poly(vinyl alcohol) (PVA). Then, using a wide-beam homemade cold-cathode ion generator, the samples were irradiated with oxygen fluences of 4 × 1016, 8 × 1016 and 12 × 1016 ions/cm2. Using the energy-dispersive X-ray spectroscopy, contact angle and X-ray diffraction techniques, the successful production of PVA/titanium dioxide composite films was evaluated. The effects of ion exposure on the optical behavior were determined using the ultraviolet–visible spectroscopy technique. The PVA/titanium dioxide dispersion energy was 1.75 eV. It increased to 2.21, 2.66 and 3.12 eV with irradiation of 4 × 1016, 8 × 1016 and 12 × 1016 ions/cm2, respectively. Moreover, the refractive index (n 0) of PVA/titanium dioxide was 1.18, which increased to 1.25, 1.32 and 1.40, respectively. Moreover, the oscillation energy E 0 reduced from 4.24 eV for the PVA/titanium dioxide composite to 3.84, 3.64 and 3.27 eV, respectively. Consequently, PVA/TiO2 films are treated with ions to change their optical and structural properties, which are then used in a variety of optoelectronic applications.

Impact of electrical aging on x-ray image quality and dose rate with vertically aligned carbon nanotube based cold cathode electron beam (C-beam)

The adoption of vertically aligned carbon nanotubes (VACNTs) as electron emitters in x-ray generation has opened a new path for medical imaging technology advancement. With their outstanding electron emission capabilities, VACNTs provide a distinct advantage in miniaturizing and improving the performance of x-ray devices. This research focuses on the effect of electrical aging on x-ray imaging quality and the dose rate while using VACNTs as the electron source. The study includes a thorough examination of the electrical aging effects on VACNT-based x-ray systems, with an emphasis on changes in emission characteristics, beam stability, and the resulting variations in x-ray output. Experiment results show that electrical aging has a considerable impact on the performance of VACNT-based x-ray sources, with visible changes in electron emission parameters and subsequent consequences on x-ray imaging quality. Furthermore, the study investigates the relationship between electrical aging and the x-ray dose rate, providing vital insights into radiation exposure optimization in medical diagnostics.

Evaluating the Field Emission Properties of N-Type Black Silicon Cold Cathodes Based on a Three-Dimensional Model.

Black silicon (BS), a nanostructured silicon surface containing highly roughened surface morphology, has recently emerged as a promising candidate for field emission (FE) cathodes in novel electron sources due to its huge number of sharp tips with ease of large-scale fabrication and controllable geometrical shapes. However, evaluating the FE performance of BS-based nanostructures with high accuracy is still a challenge due to the increasing complexity in the surface morphology. Here, we demonstrate a 3D modeling methodology to fully characterize highly disordered BS-based field emitters randomly distributed on a roughened nonflat surface. We fabricated BS cathode samples with different morphological features to demonstrate the validity of this method. We utilize parametrized scanning electron microscopy images that provide high-precision morphology details, successfully describing the electric field distribution in field emitters and linking the theoretical analysis with the measured FE property of the complex nanostructures with high precision. The 3D model developed here reveals a relationship between the field emission performance and the density of the cones, successfully reproducing the classical relationship between current density J and electric field E (J-E curve). The proposed modeling approach is expected to offer a powerful tool to accurately describe the field emission properties of large-scale, disordered nano cold cathodes, thus serving as a guide for the design and application of BS as a field electron emission material.