Hot Cathode Research Articles

Improvement of thin-film Ni-rich ALD cathode for microbatteries

Thin-film Li-ion batteries produced by precise thickness control atomic layer deposition (ALD) method are promising, safe, and high-energy density sources for miniature devices. The paper presents a detailed analysis of the multilayer thin film cathode for microbatteries. Structures were obtained by combining several technological parameters to increase capacity, uniformity, and lifetime. The protective function of a nano-sized coating was demonstrated. The influence of coating application on cathode capacity was explained. The studied structure consists of Ni-rich cathode layer modified with an amorphous Li-Ta-O functional layer on a steel substrate with a Cr coating. The cathode and the functional layer were obtained by the ALD multilayer approach followed by heat treatment at 800 °C for a minute. According to XPS and TEM data, the composition of the cathode after annealing was LiNi0.7Co0.3O2 well-defined structure without separation into Ni-rich and Co-rich layers, with texturing of crystallite pillars and with the absence of Fe. The Li-Ta-O films have a slight chemical composition gradient from Li3Ta1.2O4 on the surface to Ta1.7O5 in bulk. The electrochemical characterization showed that combining the functional layer and cathode heat treatment preserves the electrochemical capacity (32 µAh·µm−1·cm−2) and increases Coulomb efficiency.

Comparison of ionization vacuum gauges close to their low pressure limit

Parts of CERN’s accelerator complex and experiments, especially in the antimatter field, require a vacuum in the 10−12mbar range or better. Thus gauges are needed to reliably measure XHV during experimental operation and in order to study the vacuum science needed for those experiments. We therefore built a setup to reach 1⋅10−13mbar in order to simultaneously compare different hot cathode ionization gauges with the ability to measure high UHV and XHV close to their lower pressure limit: Barion extended, Extractor IE514, a modulated Bayard–Alpert gauge and two Improved Helmer gauges. All gauges but the Extractor behave similarly with respect to small pressure variations around the limit pressure, while the Extractor seems to overestimate high UHV hydrogen pressure. We show how gauge operation determines our ultimate achievable pressure due to outgassing, which was comparable for all gauges and in the order of Q∼10−10mbarls−1. Further we show the disturbances caused in the static system due to gauge pumping (visible only as electronic pumping), and report some of the possible difficulties and origins of noise when measuring pressures in the XHV range, including the thermoelectric effect.

Physical regimes of electrostatic wave-wave nonlinear interactions generated by an electron beam propagating in a background plasma.

Electron-beam plasma interaction has long been a topic of great interest. Despite the success of the quasilinear and weak turbulence theories, their validities are limited by the requirements of a sufficiently dense mode spectrum and a small wave amplitude. In this paper, we extensively study the collective processes of a mono-energetic electron beam emitted from a thermionic cathode propagating through a cold plasma by performing high-resolution two-dimensional particle-in-cell simulations and using analytical theories. We confirm that, during the initial stage of two-stream instability between the beam and background cold electrons, it is saturated due to the well-known wave-trapping mechanism. Further evolution occurs due to strong wave-wave nonlinear processes. We show that the beam-plasma interaction can be classified into four different physical regimes in the parameter space for the plasma and beam parameters. The differences between the regimes are analyzed in detail. We identify a new regime in the strong Langmuir turbulence featured by what we call electron modulational instability (EMI) that could create a local Langmuir wave packet growing faster than the ion plasma frequency. Ions do not have time to respond to EMI in the initial growing stage. On a longer timescale, the action of the ponderomotive force produces very strong ion density perturbations, and eventually, the beam-plasma wave interaction stops being resonant due to the strong ion density perturbations. Consequently, in this EMI regime, electron beam-plasma interaction occurs in a repetitive (intermittent) process. The beam electrons are strongly scattered by waves, and the Langmuir wave spectrum is significantly broadened, which in turn gives rise to strong heating of bulk electrons. Associated energy transfer from the beam to the background plasma electrons has been studied. A resulting kappa (κ) distribution and a wave-energy spectrum E^{2}(k)∼k^{-5} are observed in the strong turbulent regime.

Ion-induced electron emission cathode for a micro-newton HEMP thruster

An ion-induced electron emission cathode is applied for the first time in micro electric propulsion devices. In this investigation, ion-induced electron emission is operated on an annular LaB6 emitter for the neutralization and ion extraction of a microwave discharge HEMP Thruster (High Efficiency Multi-staged Plasma Thruster). A plume plasma diagnosis is performed with a Faraday probe and a retarding potential analyzer to analyze the discharge characteristics and plasma parameters. The results show that the ion-induced electron emission cathode can provides an emission current of over 0.4 mA. Compared with the condition with a thermionic cathode, there is a reduction in the thruster extracted ion current due to the deficiency of the emission current. The emission current shows different trends over anode voltage in different position configurations, which are affected by the incident ion flux and ion energy. Since the ion-induced electron emission cathode is free of power supply and propellant, it is of significant potential for low-power electric propulsion systems.

Single-step low-temperature synthesis routes for (Ba, Ce, Gd) hexaborides and their thermionic emission properties

Rare-earth/alkaline-earth hexaborides with high chemical stability and low work function have recently attracted much interest as promising thermionic cathodes. Their synthesis at low temperatures without post-synthesis purification treatments is still a significant challenge. The present work demonstrates a simple synthesis technique to produce pure barium, cerium and gadolinium hexaborides (BaB6, CeB6, and GdB6). For each hexaboride, a batch of samples was created by first milling highly pure precursor powders and then firing at various temperatures in a low-vacuum furnace. The synthesised products were characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron dispersive spectroscopy (EDS). Their thermionic properties were investigated utilising a Schottky device. Pure BaB6, CeB6, and GdB6 were successfully synthesised at lower temperatures (≤1400 °C). Nanocrystalline CeB6 was found to be produced at the lowest temperature of 1200 °C. The mean particle size of pure CeB6 and GdB6 synthesised is seen to be in the sub-micron range, with the exception of large BaB6 particles. The estimated work functions of the BaB6, CeB6, and GdB6 cathodes are 1.61 ± 0.18 eV, 2.50 ± 0.09 eV, and 2.42 ± 0.09 eV, respectively. These findings imply that hexaborides, as promising thermionic emitters, can be easily produced at low temperatures.

Deducing surface chemistry and annealing conditions from observed nanoparticle shapes: A study of scandate cathodes

Since the early 1900s it has been understood that knowledge of the surface energies of stable crystal facets under given chemical conditions allows prediction of the equilibrium shape of the crystal via the Wulff construction. Here, we demonstrate a practical application of an inverse Wulff construction: leveraging computed temperature- and chemical-environment-dependent surface energies to determine the chemical conditions under which particles exhibiting an experimentally observed equilibrated shape were formed. We apply this approach to reveal the chemical conditions required to fabricate high-performing scandate thermionic cathodes. Thermionic cathodes are critical components in vacuum electron devices, and scandate cathodes have exhibited dramatically enhanced performance compared to state-of-the-art cathodes. Despite this, manufacturing difficulties limit their integration into devices. In the following we show that fabrication of high-performing scandate cathodes exhibiting a characteristic W particle shape requires processing in narrow windows of temperature and oxygen partial pressure. We further show that small variations from these conditions result in large changes in cathode work function (and therefore thermionic emission) and that the role of Sc in enhancing emission performance may not be to directly modify surface work functions, but rather to control chemical conditions to stabilize (Sc-free) low work function surface configurations.

Demonstration of Low Work Function Perovskite SrVO3 Using Thermionic Electron Emission

AbstractEngineering a material's work function is of central importance for many technologies and in particular electron emitters used in high‐power vacuum electronics and thermionic energy converters. A low work function surface is typically achieved through unstable surface functional species, especially in high power thermionic electron emitter applications. Discovering and engineering new materials with intrinsic, stable low work functions obtainable without volatile surface species would mark a definitive advancement in the design of electron emitters. This work reports evidence for the existence of a low work function surface on a bulk, monolithic, electrically conductive perovskite oxide: SrVO3. After considering the patch field effect on the heterogeneous emitting surface of the bulk polycrystalline samples, this study suggests the presence of low work function (≈2 eV) emissive grains on SrVO3 surface. Emission current densities of 10–100 mA cm–2 at ≈1000 °C, comparable to commercial LaB6 thermionic cathodes, indicative of an overall effective thermionic work function of 2.3–2.7 eV are obtained. This study demonstrates that perovskites like SrVO3 may have intrinsically low work functions comparable to commercialized W‐based dispenser cathodes and suggests that, with further engineering, perovskites may represent a new class of low work function electron emitters.

Effect of the magnetic field on the operation of ionisation gauges

Hot cathode ionisation gauges are the only reliable pressure measurement devices suitable for both high and ultra-high vacuum measurement. These devices are characterized by low accuracy that is hardly better than 20%. The sources of loss of accuracy are documented in the literature although their quantification and details are not fully understood. In the present work, simulations of Bayard-Alpert and extractor gauges were performed under the influence of uniform magnetic fields up to 30 Gs. Results are in qualitative agreement with measurements taken previously with the Bayard-Alpert gauge. Both gauges are sensitive to low intensity magnetic fields which may be present in vacuum chambers. However, the extractor gauge proved to be more stable in the presence of a magnetic field due to its ring-shaped cathode.

Sc-Containing (Scandate) Thermionic Cathodes: Fabrication, Microstructure, and Emission Performance

Although thermionic emission has been studied for more than 100 years, recent interest in novel electron devices for military and civilian use has led to a surge in demand for cathodes with enhanced emission properties (e.g., higher current density, more uniform emission, lower operating temperatures, or extended in-service longevity). Sc-containing “scandate” cathodes have been widely reported to exhibit superior emission properties compared to previous-generation thermionic cathodes, including oxide, B-, and M-type cathodes. Despite extensive study spanning several decades, the mechanism by which the addition of Sc enhances cathode emission remains ambiguous, and nonuniform emission, low reproducibility, and inconsistent longevity continue to prevent widespread commercial integration of scandate cathodes into electron devices. This first part of a two-part review explores approaches that have been used for fabricating Sc-containing thermionic cathodes and the microstructures and emitting properties that result.

Sc-Containing (Scandate) Thermionic Cathodes: Mechanisms for Sc Enhancement of Emission

Although thermionic emission has been studied for more than 100 years, recent interest in novel electron devices for military and civilian use has led to a surge in demand for cathodes with enhanced emission properties (e.g., higher current density, more uniform emission, lower operating temperatures, or extended in-service longevity). Sc-containing “scandate” cathodes have been widely reported to exhibit superior emission properties compared to previous-generation thermionic cathodes, including oxide, B-, and M-type cathodes. Despite extensive study spanning several decades, the mechanism by which the addition of Sc enhances cathode emission remains ambiguous, and nonuniform emission, low reproducibility, and inconsistent longevity continue to prevent widespread commercial integration of scandate cathodes into electron devices. This second part of a two-part review explores the various mechanisms proposed for how Sc enhances electron emission and summarizes ongoing scientific challenges relating to the development and deployment of scandate cathodes.

Physics of thermionic orificed hollow cathodes: II. Scaling laws and design rules

Scaling laws for the total pressure, electron temperature, and attachment length within orificed hollow cathodes are derived from a theoretical zero-dimensional model combined with a charge-exchange-limited ambipolar diffusion model. These quantities are critical as they control the operational life of thermionic hollow cathode inserts. The underlying models were delineated and evaluated experimentally in a companion paper (Part 1). In the present paper, scaling laws are derived from first principles for the total pressure, and from a semi-analytical approach for the electron temperature and attachment length. The total pressure is found to scale with the sum of the square of the mass flow rate multiplied by a weak function of discharge current, and with the square of the discharge current. This scaling can be physically interpreted as due to the relative importance of the Lorentz force density and the gasdynamic pressure. Both electron temperature and attachment length are found to vary inversely with the neutral gas pressure-cathode diameter product. The predicted emission length is found to be between 0.6–1.4 times the insert radius for the experimental data considered, to scale weakly with the pressure-diameter product for Pd ⩾ 2 Torr-cm, and to be nearly independent of the orifice diameter. The analysis suggests that the diffusion-dominated nature of the insert plasma can account for the scaling of the emission length. A general cathode design rule is formulated based on the results of the analysis: the insert diameter should be chosen such that the attachment length is similar to the insert length to minimize the emission current density.

Experimental investigation of current jumps in linear geometry hot cathode ionization gauges in strong magnetic fields

In magnetic confinement nuclear fusion, neutral gas in the plasma edge region plays an important role for plasma fuelling and plasma wall interaction and thus influences the overall plasma behaviour. The ASDEX pressure gauge is a diagnostic instrument designed to provide local and temporally highly resolved measurements of the neutral gas flux density within the strong magnetic fields of fusion devices. It is a hot cathode ionization gauge with a linear geometry. Its function principle relies on the measurement of an ion current, caused by oscillating electrons in the gauge which ionize the neutral gas. Anomalous jumps in the ion current of up to 30% have been observed during gauge operation in the tokamak ASDEX Upgrade and the stellarator Wendelstein 7-X, which introduces an error in the measurement. This paper describes for the first time the characterization of these jumps over the broad operational space. Parameter dependencies on pressure, electron current, electrode potentials and magnetic field angle are discussed. The appearance of electrostatic oscillations with frequencies in the range of 160MHz to 350MHz which are associated with the current jumps has been discovered. The frequency of these oscillations was found to scale with the oscillation frequency of the electron motion inside the gauge. Potential physical mechanisms to explain the jumps are discussed.

Development of a K-band traveling wave tube based on carbon nanotube cold cathode

In this work, a K-band traveling wave tube (TWT) based on carbon nanotube (CNT) cold-cathode is developed, which achieves a large output power as well as a high power gain. Compared to traditional thermionic cathode TWT, the CNT-based cold-cathode TWT shows significant advantages in fast response speed, miniaturization, integration, and high efficiency. The CNT-cathode of the TWT can emit a current of 9.26 mA, and a high current of 7.80 mA is delivered to the TWT's electron beam tunnel. The maximum current of the collector is 5.90 mA, indicating that the current density of the electron beam in the helix circuit tunnel reaches 750 mA/cm2 and higher, and the maximum transmittance rate in the experiment can reach 92%. The demonstrated CNT-based TWT can output a maximum power of 10.90 W with a power gain of 15.60 dB, and the maximum power gain achieved is 18.97 dB. The above experiment results imply that the CNT-cathode has great potential for application in TWT devices.

Unified modeling and kinetic analysis of the near-cathode region and hot cathode in atmospheric-pressure arc discharges

The near-cathode region plays a crucial role in exploring the transport characteristics of the transition from arc column to the hot cathode in atmospheric-pressure arc discharges because of the existing non-equilibrium phenomena. A one-dimensional unified model, including the near-cathode region and the cathode body, is developed for an argon arc discharge with the tungsten cathode at atmospheric pressure in this paper. The electrostatic model coupled with an external circuit in the near-cathode region is solved based on the implicit particle-in-cell coupled Monte Carlo collision method without any assumptions of thermal or ionization equilibrium or quasi-neutrality. A detailed description of the arc plasma–cathode and cathode–gas interactions is obtained by calculating the nonlinear heat conduction equation in the cathode. It is shown that the space-charge sheath strongly affects particle transport in the near-cathode region and energy transport from arc plasma to the thermionic cathode. The total current density has significant effects on the kinetic characteristics of arc plasma by feedback-like mechanisms. The Joule heating by the external circuit and charged particles deposited into the cathode are dominating mechanisms of energy transfer from the near-cathode region to the cathode, while energy loss by radiation is more significant compared with natural convection.

Observation of non-thermal metastable ion velocity distributions in a miniaturized multi-dipole confined plasma device

Hot cathode discharges are common plasma sources for fundamental plasma physics studies and other applications due to their capability to produce quiescent plasma. This work presents experimental observations of presheath-associated non-thermal metastable ion velocity distributions in a miniaturized multi-dipole confined plasma device measured by laser-induced fluorescence. The intensity of this non-Maxwellian component is related to the collisions of these metastable ions with background particles. Additionally, the flow velocity of this component is lower than the Bohm velocity; thus, its energy is lower than the presheath potential drop kTe/2. This implies that these non-thermal metastable ion velocity distributions are formed via presheath acceleration and are associated with the source asymmetries of the miniaturized device. The strength decreases as the neutral pressure increases once the neutral pressure is adjusted, suggesting that the presheath length is the critical condition that determines whether these components can be observed.

Hybrid thermionic-photovoltaic converter with an In0.53Ga0.47As anode

Thermionic-photovoltaic (TIPV) converters exploit both electrons and photons emitted by a hot cathode to produce electric power. In this work, a TIPV converter structured with a tungsten cathode and an In 0.53 Ga 0.47 As photovoltaic (PV) anode (0.75 eV bandgap) is demonstrated to provide an increased output voltage with respect to the reference thermionic energy converter made of the same materials and operating under similar conditions. The higher voltage, measured to be from 0.3 to 0.5 V in the explored 1150–1450 °C temperature range, is explained by the additional contribution of the PV anode. Another work function engineered TIPV converter, obtained by coating the W cathode with LaB 6 and the In 0.53 Ga 0.47 As PV anode with BaF x , has led to an output power 2 orders of magnitude higher than the uncoated TIPV converter at 1200 °C cathode temperature. This opens the route towards future optimized TIPV electrodes. • Thermionic-photovoltaic (TIPV) devices with In 0.53 Ga 0.47 As anodes were assembled. • TIPV converter showed output voltages up to 0.5 V higher than the thermionic one. • A TIPV converter with low work function coatings (LaB 6 and BaF x ) was fabricated. • The coated TIPV energy converter provided an output power 100-fold than the uncoated one.

Nanocrystalline high-entropy hexaboride ceramics enable remarkable performance as thermionic emission cathodes

The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential, yet relevant studies have been rarely reported. In this work, we report nanocrystalline (La0.25Ce0.25Nd0.25Eu0.25)B6 high-entropy rare-earth hexaboride (HEReB6-1) ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time. The as-fabricated samples exhibited a highly dense (96.3%) nanocrystalline (94 nm) microstructure with major (001) fiber textures and good grain boundaries without any impurities, resulting in a remarkable mechanical, electrical, and thermionic emission performance. The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures; these were greater than the average values of corresponding binary rare-earth hexaborides, such as a Vickers hardness of 23.4 ± 0.6 GPa and a fracture toughness of 3.0 ± 0.4 MPa•m1/2 at room temperature. More importantly, they showed high emission current densities at elevated temperatures, which were higher than the average values of the corresponding binary rare-earth hexaborides. For instance, the maximum emission current density reached 48.3 A•cm−2 at 1873 K. Such superior performance makes the nanocrystalline HEReB6-1 ceramics highly suitable for potential applications in thermionic emission cathodes.

Study on Morphology and Chemical States of Surface Active Layer of Th-W Cathode.

The surface morphology and chemical states of W-2%ThO2 thermionic cathode during vacuum high-temperature treatment were investigated in this research. The W-2%ThO2 thermionic cathode was prepared by a solid-liquid doping method combined with high-temperature sintering. The morphology and distribution of thorium oxide were observed using a transmission electron microscope and scanning electron microscope. The chemical states of elements at different temperatures were analyzed by X-ray photoelectron spectroscopy. Results indicate that the surface morphology and chemical form of the alloy evolve with the increase of temperature. The matrix had a lamellar structure at low temperatures, and the surface was relatively flat. The samples were heated to 500 °C, 1100 °C, and 1300 °C for 1 h. During the heating process, thorium oxide changed from granular to spherical, and the matrix was recrystallized. As the heating temperature rises, diffusion channels appear inside the cathode. As the temperature increases, the high-priced tungsten gradually decreases, and the zero-valent tungsten content increases. The adsorbed oxygen left the cathode surface, and the lattice oxygen increased. The surface oxygen content decreased, and the thorium and tungsten content increased.

Power law fitting of the ion saturation current and the three-temperature Maxwellian EEDF in a multi-dipole confined hot cathode discharge: an experimental revisitation

Langmuir probe diagnostic in a multi-dipole confined hot cathode discharge has been revisited with an improved sheath expansion model with fitted expansion parameters. The results show a three-temperature Maxwellian EEDF composition also observed in previous studies using a very similar system. The dependence on neutral pressure and discharge current of each electron species are consistent with known physical processes in a multi-dipole confined hot cathode discharge. This shows that the employment of the new ion saturation fitting method does not eliminate the contribution of the high energy degraded primaries to the Langmuir probe I–V trace, as sheath expansion and high energy electron current contributes differently on an I–V trace. The result remains supportive to the existence of a very hot (T dp > 15 eV) degraded primary electron species separate from the typical hot electron (T hot ∼ 3 eV) species. Thus it is recommended to sweep the Langmuir probe beyond the bias voltage of the hot cathode, whenever possible, in order to properly resolve the contribution to the I–V trace of the degraded primaries.

A cold electron-impact ion source driven by a photo-cathode – New opportunities for the delivery of radioactive molecular beams?

The thick-target ISOL (Isotope mass Separation OnLine) method provides beams of more than 1000 radionuclides of 74 elements. The method is well established for elements with sufficiently high volatility at ca. 2000 °C. To extract non-volatile elements the formation of a volatile molecule is required. While successful in some cases (e.g. carbon or boron), most of these elements are not yet available as ISOL beam. A variety of volatile carrier molecules has been proposed for all elements produced in the target material, but their probability of survival during the extraction and ionization process is often limited by the high temperatures required for isotope diffusion in the thick targets and for ion source operation. While cold target concepts have already been proposed, the normal mode of operation of the typically used Versatile Arc Discharge Ion Source (VADIS) with a hot cathode is not well suited. Here, we report about first measurements with an electron-impact ion source operated at ambient temperature using electrons that were liberated via the photo-electric effect from a copper cathode.