Cathode Lifetime Research Articles

Longitudinal ion-clearing mechanism in space-charge neutralization of low energy beams

In electron guns, generating high-intensity beams comes with the cost of increasing the number of secondary ions produced by the residual gas ionization which can be a limiting factor in the cathode lifetime. However, it is often overlooked that the longitudinal potential generated by the beam can be the main factor influencing secondary ions to drift back toward the source and limit the maximum beam intensity. Using 3D simulations, this paper shows how the flux of trapped ions depends on the beam parameters. The results provide a practical framework that can be used as a guide in the design of the electron gun focal point to mitigate the ion back bombardment. Published by the American Physical Society 2024

Plume mode instability enhanced by emitter surface poisoning in hollow cathode

The unstable plume mode of hollow cathodes should be avoided in practical applications because it severely degrades the overall cathode lifetime. In this study, we investigate the spot-plume transition and plasma stability characteristics of an unused segmented lanthanum hexaboride emitter. The expansion of the unstable plume mode region is observed during a discharge experiment. Subsequently, the segmented emitter is retrieved, the inner surface of the emitter is observed, and the work function on the surface is measured at room temperature. The emitter surface exhibits color variations with oxygen and carbon detection. The downstream edge shows the original purple color and almost no degradation in the work function. The high temperature in this region promotes the desorption of carbon and oxygen. In the spot mode, this region mainly contributes to thermionic electron emission; therefore, the discharge voltage in the spot mode does not change during the discharge experiment. Carbon or carbide is detected in the middle of the axial direction on the emitter surface, where the surface temperature is not sufficiently high to desorb carbon during discharge. Based on the surface analysis results, the dominant substance in the region where carbon is detected was lanthanum carbide. An increase in the work function is indicated in the region, which appears to increase the plasma instability. According to previous studies, an increase in the work function results in a rise in the potential in the emitter, and an increase in the electron temperature in the outside plume region induces the plasma instability. Further investigation is needed to understand the mechanism connecting the rise in the work function and the rise in the electron temperature in the plume region.

A global model for evaluating discharge characteristics and performance of hollow cathodes

Hollow cathode, with the highest plasma density, current density, and temperature, becomes one of the most important components in the electro-thruster system. As the electric-propulsion thruster performance is directly related to the ionization rate, reliability, and lifetime of the hollow cathode, this paper develops a global model to study the effects of discharge current, gas flow rate, and gas species on the discharge characteristics in the insert and orifice regions of the hollow cathode. The emitter wall temperatures of hollow cathodes predicted by the global model are compared with experimental results from NSTAR thruster neutralization cathodes, confirming the model's validity. The influence of hollow cathode emitter material and structure sizes on the plasma parameters in the internal regions was also evaluated. The simulation results show that there is an optimal matching relationship between the discharge current and gas flow rate to guarantee the maximum ionization rate. The optimal working region for the hollow cathode has been determined under different energetic, regime and structural parameters. The global model established in this paper can quickly determine the key structure and operating parameters of hollow cathode at the design stage, and provide the theoretical basis for hollow cathode design and development.

Simulation of cathode catalyst durability under fuel cell vehicle operation –Effects of stack size and temperature

In this work, a fundamental cathode catalyst degradation model is developed in order to simulate fuel cell stack durability under realistic vehicle operation. A dynamic platinum degradation model featuring platinum dissolution and redeposition is developed to account for the Ostwald ripening phenomenon. The model is then calibrated for a variety of accelerated stress test data with different upper and lower potential limits and temperatures. Next, a case study of fuel cell transit bus operation in the city of Victoria, B.C., Canada is carried out to investigate the effects of stack size and temperature on the cathode catalyst durability. To this end, the required power density and cell voltage are calculated from the drive cycle considering vehicle dynamics and specifications. Larger stacks are found to experience higher upper potential limits; thus, showing lower lifetimes. The cathode lifetime is predicted to grow 379% at 80°C if the stack nominal power is lowered from 396 to 132 kW while the performance requirement is met. The temperature is also shown to have a tremendous impact on the cathode lifetime. A temperature drop from 80 to 60°C results in 154% cathode lifetime enhancement for a stack nominal power of 264 kW.

Heuristic Approach to Predict the Performance Degradation of a Solid Oxide Fuel Cell Cathode.

The present work aims to predict the degradation in the performance of a solid oxide fuel cell (SOFC) cathode owing to cation interdiffusion between the electrolyte and cathode and surface segregation. Cation migration in the (La0.60Sr0.40)0.95Co0.20Fe0.80O3-x (LSCF)-Gd0.10Ce0.90O1.95 (GDC) composite cathode is evaluated in relation to time up to 1000 h using scanning transmission electron microscopy (STEM)-energy-dispersive X-ray spectroscopy (EDXS). The resulting insulating phase formed within the GDC interlayer is quantified by means of the volume fraction using a two-dimensional (2D) image analysis technique. For the very first time, the amount of the insulating phase in the GDC interlayer is quantified, and the corresponding performance degradation of the LSCF cathode is predicted. Mathematical relationships are established for the estimation of degradation due to surface segregation of the cathode. The ohmic resistance between the cathode and the GDC interlayer/electrolyte interface and the polarization resistance of the cathode, characterized by electrochemical impedance spectroscopy (EIS), show an excellent match with the predicted results. The combined degradation analysis and modeling for the cathode lifetime prediction provide a systematic understanding of the time-dependent cation migration and segregation behavior.

Removal of disinfection byproducts through integrated adsorption and reductive degradation in a membrane-less electrochemical system

Proper control/removal of disinfection byproducts (DBPs) is important to drinking water safety and human health. In this study, a membrane-less electrochemical system was developed and investigated to remove DPBs through integrated adsorption and reduction by granular activated carbon (GAC)-based cathode. Representative DPBs including trihalomethanes and haloacetonitriles at drinking water concentrations were used for removal experiments. The proposed system achieved >70% removal of most DBPs in a batch mode. The comparison with control tests under either open circuit or hydrolysis demonstrated the advantages of electrochemical treatment, which not only realized higher DPBs removal but also extended GAC cathode lifetime. Such advantages were further demonstrated with continuous treatment. High dechlorination and debromination efficiencies were obtained in both batch (82.2 and 94.3%) and continuous (79.3 and 87.6%) reactors. DBPs removal was mainly contributed by the electrochemical reduction and adsorption by the GAC-based cathode, while anode showed little oxidizing effect on DBPs and halide ions. Dehalogenated products of chloroform and dichloroacetonitrile were identified with toxicity reduction. The energy consumption of the continuously operated system was estimated to be 0.28 to 0.16 kWh m−3. The proposed system has potential applications for wastewater reuse or further purification of drinking water.

Sludge Reduction and Surface Investigation in Electrochemical Machining by Complexing and Reducing Agents

Electrochemical machining (ECM) is widely applied to manufacture parts with complex geometries, used in electronic components and the automotive, military, and aeronautics industries. These parts have a surface shaped by controlled anodic dissolution at high current density levels, using a neutral solution of inorganic salts (i.e., NaCl or NaNO3) as the electrolyte. Such conditions generate a high amount of sludge that deposits onto the surfaces of equipment, devices, cathodes, and working pieces, requiring daily and complicated sludge management during the series production in the industry. Thus, the main goal of the present work is to propose a simple way to reduce sludge generation in the ECM industrial process. To do so, complexing (EDTA) or reducing (ascorbic acid) agents were added to the electrolyte composition, creating parallel reactions to keep the metallic ions from precipitating. The complexing agent EDTA resulted in a 30% reduction in sludge mass, using an alkaline solution (pH > 10.0). The reducing agent, ascorbic acid, resulted in a 90% reduction in sludge mass, using an acidic solution (pH < 5.0). This sludge reduction has the potential to contribute significantly to increasing equipment, devices, and cathode lifetime, as well as reducing costs associated with centrifuge or filter maintenance (sludge removal from electrolyte) and increasing the productivity of industrial ECM processes.

Sub-micron-sized hafnium (Hf) cathode with excellent thermal emission performance and superior hardness synthesized by powder metallurgy route

Here we report a sub-micron-sized hexagonal closest packed (HCP) hafnium (Hf) cathode with excellent thermal emission performance and superior hardness. Its thermal emission current density at the same applied voltage and temperature is about 355–392% of that of the traditional HCP Hf cathode, and its hardness is about 752% of that of the latter. These excellent thermal emission performances originate from the reduction of average work functions caused by the weak preferred orientation effect due to the nanoscale grain, leading to a greater thermal emission current density and cathode lifetime in comparison with face-centered cubic (FCC) Hf conterpart. These findings will provide a promising pathway to make much more efficient air and water vapor plasma torches.

Air‐Stable Li2S Cathode for Quasi‐Solid‐State Anode‐Free Batteries with High Volumetric Energy

AbstractAnode‐free batteries can maximize the energy density but their development is hindered by a lack of Li‐rich cathodes for compensating the irreversible Li loss. Li2S cathode is particularly appealing to this desire due to 2.6–4.7 folds more Li content and 4.2–6.8 times higher capacity than conventional intercalation cathodes. But its practical application is hindered by poor stability against moisture attacking in the air. Herein, a facile expendable polymer sheathing strategy toward air‐stable Li2S cathodes with high capacities for developing high‐performance quasi‐solid‐state anode‐free batteries without risk of cell leakage is reported. Tight protection by dense polymer barrier dramatically prolongs the lifetime of Li2S cathode by 2,000 times at least in the air. Such air‐stable Li2S cathode allows for high compatibility of anode‐free battery production with commercial schemes. More attractively, the polymer protective layer can in situ transform to multifunctional gel polymer electrolyte for releasing ionic pathways and enhancing cell performance by inhibiting LiPS loss and smoothing Li plating. With air‐stable Li2S cathode, the quasi‐solid‐state anode‐free cells are assembled in ambient environment to deliver superb volumetric energy density of 1093 Wh L−1. This study may shed new light to push the commercialization of high‐energy and reliable anode‐free batteries forward.

Simulation of Cathode Catalyst Durability Under Fuel Cell Vehicle Operation - the Effect of Temperature

Fuel cell vehicles (FCVs) emerge to be promising candidates to produce clean power for the transportation sector in order to tackle climate change issues. Although the commercialization of polymer electrolyte membrane fuel cells (PEMFCs) has progressed in the past few decades, there are still obstacles to address, including high cost and limited hydrogen infrastructure. For heavy duty transportation applications, the PEMFC durability is also not yet proven, and extrapolating from lab data to real-world field operating conditions remains a significant challenge [1].In this work, the cathode catalyst degradation in PEMFCs is studied to make an estimation of fuel cell durability in the FCV application. The well-known Butler-Volmer approach is utilized to model platinum dissolution and redeposition, platinum oxidation and platinum ion formation during the fuel cell operation [2]. First, the model is calibrated with the results presented by Ferreira et al. [3] at 80℃; the calibrated model is then validated with Kocha’s results [4] to demonstrate the model’s capability of generating reliable results at different temperatures. In order to realistically predict the cathode degradation pattern and lifetime, the real-life fuel cell operating conditions should be obtained. Therefore, a drive cycle recorded based on a real-life transit bus operation in the city of Victoria is utilized to calculate the input fuel cell voltage profile. The procedure to calculate fuel cell operating voltage profile using the drive cycle has been thoroughly explained by Ahmadi and Kjeang [5]. This procedure employs Newton’s second law to calculate the required cell power density considering the air flow drag force counteracting the vehicle thrust. Finally, polarization curves representing the fuel cell performance are used to calculate the fuel cell voltage profile with the determined required cell power density. Polarization curves measured under a range of temperatures are employed to adequately reflect the effect of temperature on the fuel cell performance. The cell active area is considered to be 500 cm2 and the stack is assumed to contain 225 cells, representing a nominal stack power of 74 kW at 80℃. The fuel cell operation and degradation are simulated at three different temperatures: 60, 70, and 80℃. The change of remaining electrochemically active surface area (ECSA) with time is calculated as the output of the model.Temperature highly affects the electrochemical reactions of platinum dissolution in several ways. First, it affects the Tafel slope in the platinum dissolution reaction. Second, it substantially impacts on platinum degradation reaction rate constant [6]. The Arrhenius approach is taken in this study to apply the effect of temperature on the reaction rate constants. Fuel cell voltage loss over time is determined by assuming simple Tafel kinetics. The cathode lifetime is calculated at 0.6 A/cm2 and is estimated by considering 10% voltage drop as the failure criterion. Fig. 1 shows the simulated change of remaining ECSA over time and the resulting fuel cell lifetime for the three different cell temperatures. The results indicate that the cathode lifetime increases 168% when the cell temperature drops from 80℃ to 60℃ due to a significantly lower platinum degradation rate at 60℃. The results show relatively low cathode catalyst lifetimes for a transit bus. The reason is attributed to the bus drive cycle used as the model input. The drive cycle contains a great portion of deceleration and idling time which leads to a fuel cell operation close to open circuit voltage (OCV), thus experiencing a high degradation rate.In this regard, the present modeling framework could become a useful tool for fuel cell and FCV developers to predict lifetime for new products and systems prior to commercial release. Scientifically, the present model can also be used to further explore key influencing factors on fuel cell durability and develop more durable cell, stack, and system designs for a targeted FCV application. Acknowledgements This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Research Chairs, and Simon Fraser University Community Trust Endowment Fund.

Analysis of Performance Degradation and Durability of the Air Cathode in an Alkaline Fuel Cell

Degradation of the air cathodes is one of the key issues affecting the lifetime and durability of alkaline fuel cells and metal-air batteries. To prevent the ingress of electrolyte in the air cathode, modifying the hydrophobicity and thickness of the AL has been reported [1,2]. It was reported that increased hydrophobicity/thickness of the AL resulted in a decrease in the air cathode performance [1,2]. In this work, experimental in-situ and post-test analyses of the air cathode were applied to investigate the mechanism of performance degradation. In-situ methods including performance / lifetime analysis using a half-cell setup [3, 4], polarization studies, and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical characteristics of the electrodes during operation. Post-mortem analysis methods included X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray computed tomography (X-ray CT), and Raman spectroscopy. The properties of pristine, conditioned, and failed air cathodes were characterized by these methods. Conditioning of air cathodes was performed by operating in half-cell utilizing 6 M KOH solution for 24 hours at 200 mA cm 2. The composition of the air cathode, operating conditions, and procedure for determining the performance and lifetime of the air cathode have been discussed in our previous work [3].Cross-sectional SEM-EDX analysis was conducted on the air cathode to determine the amount of flooding/penetration of the electrolyte inside the active layers for electrodes prepared with 15, 25, and 40 wt% PTFE in the AL. Air cathodes were operated in the half-cell battery at 200 mA cm -2. EDX maps of the air cathodes showed that increasing the PTFE content leads to increased hydrophobicity and decreased depth of KOH penetration inside the AL after 5 hours of operation. However, air cathodes with higher PTFE content of 40 wt% exhibited a lifetime of only 48 hours, compared to > 150 hours for those containing 15 wt% and 25 wt% (conducted in a limited test duration). This could be due to decreased electrolyte content in the AL or blocking of pore space and active reaction sites [5], leading to higher local current densities and more rapid degradation. Cross-sectional EDX analysis indicated that electrolyte penetration/flooding inside the macrostructure of the backing layer (BL) was not observed in any of the electrodes.XPS was performed to reveal further information of the chemical states of the conditioned and failed cathodes [6]. XPS analysis indicated changes in the surface functional groups, in particular increasing hydroxyl groups in the failed cathodes, which may be indicative of reduced hydrophobicity of the carbon support. XPS analysis also indicated other changes in the chemical states of the catalyst, oxygen, fluorine and carbon after conditioning and failure of the air cathode.In-situ galvanostatic EIS was conducted during long-duration and accelerated stress tests to determine ohmic resistance, charge transfer resistance, and mass transfer limitation. The EIS data indicates that mass transfer resistance increased significantly after failure of the air cathode, confirming that oxygen transport to the catalyst was the cause of the poor performance of failed cathodes. Raman spectra and mapping was carried out to obtain additional information about changes in the AL after degradation. X-ray CT were conducted on air cathodes to determine the changes of pore structure, distribution of catalyst, PTFE and potassium.

Simulation of Cathode Catalyst Durability Under Fuel Cell Vehicle Operation - the Effect of Fuel Cell Stack Size

The concerns regarding climate change have made the researchers seek a clean alternative for the fossil fuel vehicles. Fuel cell vehicles (FCVs) are considered to be promising candidates owing to their efficient energy conversion and zero-carbon emission. However, a number of obstacles such as high cost and limited hydrogen infrastructure have made the FCVs commercialization process challenging. Polymer electrolyte membrane fuel cells (PEMFCs) have been proven promising for transportation applications. For heavy duty transportation applications, the PEMFC durability is also not yet proven, and extrapolating from lab data to real-world field operating conditions remains a significant challenge [1].In this work, the cathode catalyst degradation in PEMFC is studied to estimate the effect of stack size on fuel cell durability in the FCV application. Platinum dissolution and redeposition, platinum oxidation and platinum ion formation during the fuel cell operation are modeled using the Butler-Volmer approach presented in [2]. A drive cycle recorded based on a real-life transit bus operation in the city of Victoria is utilized to calculate the input fuel cell voltage profile based on the methodology presented by Ahmadi and Kjeang [3]. According to this methodology, the required cell power density is calculated using Newton’s second law considering the air flow drag force as a counteracting force against the vehicle movement. Then, the required voltage cycle is obtained by employing a polarization curve characterizing the fuel cell performance. Finally, the change of remaining electrochemically active surface area (ECSA) with time is calculated as the output of the model. The fuel cell is assumed to operate at 80 ℃ and the cell active area is considered to be 500 cm2. Simple Tafel kinetics is then used to determine the fuel cell voltage loss. A 10% voltage drop at 0.6 A/cm2 is considered as the failure criterion for the cathode lifetime. Moreover, the effect of the fuel cell stack size is studied. By increasing fuel cell stack size, the required cell power density drops, leading to a decrease in the voltage cycle amplitude while the voltage cycle period remains the same. According to the empirical kinetic rate equation, the catalyst degradation exponentially increases with increasing the voltage. Therefore, a higher degradation rate is observed for a catalyst operating on a voltage cycle with a lower amplitude while the period and the upper potential limit (UPL) are maintained the same, causing a significant platinum ion generation. Fig. 1 shows the change of remaining ECSA over time and resulting fuel cell lifetime for three stack sizes which are represented by the stack nominal powers. The results show that the fuel cell lifetime will be roughly doubled when the stack size is reduced by half. Stack sizing is thus an important consideration for fuel cell durability in the FCV application.In this regard, predicting fuel cell lifetime is a crucial step in commercializing FCVs. The present modeling framework could be utilized by FCV developers to predict lifetime for new products instead of carrying out time-consuming lifetime experiments. The factors influencing fuel cell durability can also be investigated using the present model framework to develop durables cells and stacks for a targeted FCV application. Acknowledgements This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Research Chairs, and Simon Fraser University Community Trust Endowment Fund.

Layer-by-layer laser cladding of crack-free Zr/Nb/Cu composite cathode with excellent arc discharge homogeneity

Cathode ablation shortens the cathode lifetime and hinders the development of arc heaters. Surface-modified Zr/Cu cathodes realize combinations of ablation-resistant surface and conductive matrix, while the cracking problem limits their applications. In this work, Nb cladding and Zr cladding were prepared on Cu matrix successively. The Nb cladding was effective in serving as a blocking layer against diluting of Cu and suppressing forming CuZr intermetallics. The surface Zr content reached as high as 85.6 at.% and no CuZr intermetallic was detected inside the cladding. Besides, the intermediate Nb cladding contributed to tight interlayer metallurgical bonding. The arc discharge behaviors of bi-layered Zr/Nb/Cu cathodes were investigated compared to Cu cathodes. The bi-layered Zr/Nb/Cu cathode exhibited an improved arc discharge homogeneity with shallow and smooth ablated areas. The cathode ablation rate (1.30 μg C−1) was 37.5 % lower than that of Cu cathodes. The Zr/Nb/Cu layered structure improved the cathodic arc ablation-resistance without cracking, which guaranteed its application in large-area manufacturing process.

Improvement of Q rr –I DSS and dynamic avalanche of field-plate MOSFET by local lifetime control on the cathode side

Reducing the reverse recovery charge (Q rr) is effective for reducing switching loss in field plate (FP)-MOSFETs. A lifetime killer is utilized to reduce Q rr while increasing the leakage current in the off-state. Device simulation shows that a local lifetime killer on the cathode side successfully improves the trade-off between Q rr and I DSS in comparison with that of a uniform lifetime killer. A known issue of cathode lifetime killers is overshoot voltage by hard recovery. However, the overshoot voltage of FP-MOSFET decreases with a cathode lifetime killer owing to an internal snubber, which is a feature of FP-MOSFETs. An internal snubber with a large series resistance causes a dynamic avalanche by both the increase of FP potential and excess carriers in high-speed operation. The cathode lifetime killer also improves dynamic avalanche by excess carriers. Consequently, the cathode lifetime killer is preferable for high-speed FP-MOSFETs because the Q rr–I DSS trade-off and the trade-off between dynamic avalanche and I DSS are effectively improved.

Mechanisms of Degradation of the Air Cathode in an Alkaline Fuel Cell

Rechargeable zinc-air redox flow batteries have been reported as an energy storage technology that has high energy density, uses abundant low-cost raw materials, and has low environmental impact [1, 2]. Due to these characteristics, zinc-air batteries are attractive for both portable and smart-grid energy storage applications [1, 2]. However, the limited cycle life has been identified as a major problem for commercialization of zinc-air batteries [2].Flooding of the electrolyte within alkaline electrode pores is reported as one of the major causes of degradation and performance loss of the air cathode in these devices [1, 3]. To prevent the ingress of electrolyte in the zinc-air cathode, modifying the hydrophobicity and conductivity of the catalyst layer has been reported [4]. The ratio of carbon/binder has impact on ionic conductivity and gas transport in the electrode. In addition, it is observed that degradation of performance of the electrode without binder is faster than that with binder [4].In this study, degradation of the air cathode in the zinc air flow battery was investigated by measuring the variation in the cell voltage at a constant current density. The air cathode consisted of a sintered active layer (AL) composed of platinum nanoparticles supported on carbon (Pt/C), a carbon / PTFE backing layer (BL), and a nickel mesh current feeder. The experimental setup used a 6 M KOH electrolyte, with a zinc wire reference and nickel counter electrode. A control system for real time control of the operating temperature, pressure, and inlet air humidity was implemented to maintain stable operating conditions and record the data. Post-mortem (PM) analysis was conducted on the tested samples. The sample preparation method was carefully evaluated for the PM study to make sure that the KOH present in the pores is preserved during imaging.No significant degradation in the performance of the air cathode was observed after 148 hours at baseline operating conditions (current density of 200 mA cm-2 and 21% oxygen). In order to enable rapid evaluation of electrode durability, conditions for accelerated degradation of the air cathode were investigated. Under conditions of low oxygen stoichiometry (achieved by 50% drop in oxygen stoichiometry) and high current density (400 mA cm-2), the rate of degradation was accelerated and the lifetime of the air cathode was less than 24 hours. The lifetime of the air cathode was estimated based on significant increase in the potential loss ca. 0.3 V, while performing oxygen reduction from an air feed at baseline operating conditions. Polarization studies were performed on the air cathode after a significant performance loss during the accelerated degradation experiments. The results indicate that the air cathode degradation accelerated under conditions of oxygen mass transfer limitation. The increased potential loss is likely due to flooding of electrolyte inside the pores of the active layer of the electrode, which increases the mass transfer resistance for oxygen transport [5, 6].For evaluation of the impact of binder loading on air cathode degradation, three different loadings of polytetrafluoroethylene (PTFE) in the catalyst layer were used, and the performance and durability were compared. The morphology and microstructure of active and backing layers of the used air cathode was investigated by scanning electron microscopy (SEM). Cross sectional SEM images were carried out for PM analysis to investigate the penetration depth of KOH inside the active and backing layers after a significant potential loss was observed. Surface analysis of the active and backing layers were also conducted using Raman spectroscopy.

Pushing the capture limit of thermionic gun linacs

Although accelerator technology has matured sufficiently, state-of-the-art X-ray linacs for radiotherapy and cargo-scanning capture merely 30-50% of the electrons from a thermionic cathode, requiring a higher cathode current and leaving uncaptured electrons to cause problems due to back bombardment, shortening of cathode life, etc. Any solution to increase capture should be effective, simple, reliable, compact, and low cost in order to be adopted by industry. To address this, we present the design of a 6 MeV high capture efficiency S-band electron linac that captures 90% of the initial DC beam. This linac does not require any extra parts that would increase the cost as the high efficiency is achieved via a low-field-amplitude in the first bunching cell to decrease the number of backstreaming electrons, to velocity bunch the electron beam, and recapture backstreaming electrons. Under the low field amplitude, any electrons launched at decelerating phases travel backward with low speeds, thus most of them can catch the next RF cycle, and get re-accelerated/recaptured. As the electron speed is low, the cell length is also shorter than existing linacs. Such a short field is achieved by the use of asymmetric cells with differential coupling to the side-coupled cells. Our novel design has implications for all commercial high current thermionic gun linacs for increasing beam current and increasing cathode lifetime.

Quantification of the Internal Void Network upon Optimization of Synthesis Conditions for Lithium-Ion Cathode Materials

Current industrial methodologies for lithium-ion battery cathode materials are suited to the production of well-defined, dense, spherical secondary particle structures assembled from nanocrystalline primary particles. Specifically, LiNixMnyCozO2 (LNMC) cathode material is synthesized by first growing the hydroxide precursor, NixMnyCoz(OH)2 (NMC), through a coprecipitation method using a stoichiometric ratio of mixed transition metal sulfate solutions, sodium hydroxide, and ammonia hydroxide pumped into a continuous stirred tank reactor (CSTR). Each new composition of cathode material requires an extensive synthesis parameter optimization study to refine the even formation of metal species, density, and uniformity of size and shape of the final cathode material. The characterization and visualization of the three-dimensional secondary cathode structure, internal morphology, and void network is an elusive task using traditional scanning electron microscopy (SEM) because it is 2D technique that requires the use of mechanical cross-sectioning which can cause the deformation of the internal structure. Yet the variation of the internal structure and density changes within LNMC secondary particles can have large effects on both the cycling kinetics, capacity, and lifetime of the final cathode material.In this study, we present a five-parameter optimization study of Ni0.25Mn0.25Co0.50(OH)2 (NMC112) using our lab-built CSTR. The optimization and morphological changes of the primary and secondary particles during high temperature lithiation is investigated further. Using synchrotron-based transmission X-ray microscopy tomography (TXM) in combination with a novel deep learning semantic segmentation process, the three-dimensional cathode structure and internal pore network of LNMC112 particles was mapped across different lithiation temperature and reaction times. The deep learning semantic segmentation allowed for the quantification of void volume, shape, size, and final overall density of the cathode secondary particles. The results of these morphological differences show effects in both the kinetics and cyclability through galvanostatic cycling. The quantification of the three-dimensional cathode microstructure shown in this study is agnostic of the specific cathode composition. Therefore, it is highly suited to better inform optimization targets of the coprecipitation synthesis, as well as illuminating the importance the internal pore network of secondary particles.

Operating modes of a magnetized direct-current discharge in helium at pressures ∼10 Pa

A high-pressure direct current magnetron discharge is analyzed using a self-consistent two-dimensional fluid model. This discharge has potential interest in plasma switching applications due to its ability to generate rather dense (plasma density of ∼1019 m−3) stable nonequilibrium plasmas at pressures of ∼13.3 Pa (100 mTorr). This discharge has several advantages over nanosecond pulsed high pressure discharges used in conventional plasma switchers since it operates at much lower voltages and lower gas pressures, with the advantage of much longer cathode lifetime. In the present paper, we analyze the operating modes of this magnetron discharge for conditions close to that of Sommerer et al. [J. Phys. D: Appl. Phys. 52, 435202 (2019)]. We observe the subnormal and normal modes of the magnetron operation whose physics differs from that typical for unmagnetized plasmas.

Sustainable and homogeneous arc ablation behavior of zirconium cathodes improved by in situ formation of zirconium oxides

Discharge homogeneity is one of the dominant factors affecting cathode lifetime. In this work, the arc ablation behavior of a zirconium cathode is investigated in air and argon atmospheres. Homogeneous discharge processes are observed in air, while poor discharge homogeneity with scattered cathode spots is found in argon. The homogeneous discharge process is related to the layered structure at the discharge center. The surface zirconium oxide layer stabilizes the arc foot and reduces the effective work function. A dual-phase layer underneath, consisting of zirconium oxides and zirconium, enables the transition from the zirconium matrix to the surface oxide layer. The sustainable discharge behaviors are realized by the balance in the cathode, which is created by the ablation of surface zirconium oxides and the oxidization of zirconium in the dual-phase layer.

Development of Pulsed TEM Equipped with Nitride Semiconductor Photocathode for High-Speed Observation and Material Nanofabrication

The development of pulsed electron sources is applied to electron microscopes or electron beam lithography and is effective in expanding the functions of such devices. The laser photocathode can generate short pulsed electrons with high emittance, and the emittance can be increased by changing the cathode substrate from a metal to compound semiconductor. Among the substrates, nitride-based semiconductors with a negative electron affinity (NEA) have good advantages in terms of vacuum environment and cathode lifetime. In the present study, we report the development of a photocathode electron gun that utilizes photoelectron emission from a NEA-InGaN substrate by pulsed laser excitation, and the purpose is to apply it to material nanofabrication and high-speed observation using a pulsed transmission electron microscope (TEM) equipped with it.