Dispenser Cathodes Research Articles

The effects of aluminate compounds on the free Ba generation and electron emission performance of dispenser cathode

Free barium (Ba) is critical to the formation of the active layer on the surface of dispenser cathode, which is contributed to the electron emission properties of the cathode. The Ba in the active layer of the dispenser cathode is generated by the chemical reaction between aluminate compounds and tungsten (W) under vacuum and high temperature. Nowadays, the effect of aluminate compounds on the generation of free Ba is not clear. In this work, aluminate impregnants called 411, 532 and 612 impregnants were prepared by coprecipitation method. The structure of the dominate compounds in 411, 532 and 612 impregnants were determined with density functional theory (DFT) calculation and Rietveld structure refinement. It was found that the dominate compounds of 411, 532 and 612 impregnants are Ba3.5Ca0.5Al2O7, Ba5CaAl4O12 and Ba3CaAl2O7, respectively. Then, the temperature dependence of the thermodynamic properties, such as Debye temperature, entropy, heat capacity and enthalpy, of the aluminate compounds were calculated. Based on the thermodynamic properties, the potential chemical reaction between aluminate compounds and W are energetically determined by deviation Gibbs energy. The effects of different aluminate compounds on the free Ba generation were evaluated with the equilibrium Ba(g) amounts that generated by the chemical reactions between aluminate compounds and W. It is shown that the equilibrium Ba(g) amounts generated by Ba3.5Ca0.5Al2O7 is higher than that by Ba5CaAl4O12 and Ba3CaAl2O7, implying the superiority of electron emission performance of Ba3.5Ca0.5Al2O7. The electron emission densities of the dispenser cathodes that impregnated with the 411, 612 and 532 impregnants are 5.06, 4.31 and 4.05 A/cm2, respectively. It is revealed that the Ba3.5Ca0.5Al2O7 has superiority in generating free Ba, which agreed well with thermodynamic simulation results.

Direct evidence of low work function on SrVO3 cathode using thermionic electron emission microscopy and high-field ultraviolet photoemission spectroscopy

Perovskite SrVO3 has recently been proposed as a novel electron emission cathode material. Density functional theory (DFT) calculations suggest multiple low work function surfaces, and recent experimental efforts have consistently demonstrated effective work functions of ∼2.7 eV for polycrystalline samples, both results suggesting, but not directly confirming, that some fraction of even lower work function surface is present. In this work, thermionic electron emission microscopy (ThEEM) and high-field ultraviolet photoemission spectroscopy (UPS) are used to study the local work function distribution and measure the work function of a partially oriented- (110)-SrVO3 perovskite oxide cathode surface. Our results show direct evidence of low work function patches of about 2.0 eV on the cathode surface, with a corresponding onset of observable thermionic emission at 750 °C. We hypothesize that, in our ThEEM and UPS experiments, the high applied electric field suppresses the patch field effect, enabling the direct measurement of local work functions. This measured work function of 2.0 eV is comparable to the previous DFT-calculated work function values of the SrVO-terminated (110) SrVO3 surface (2.3 eV) and SrO-terminated (100) surface (1.9 eV). The measured 2.0 eV value is also much lower than the work function for the (001) LaB6 single crystal cathode (∼2.7 eV) and comparable to the effective work function of B-type dispenser cathodes (∼2.1 eV). If SrVO3 thermionic emitters can be engineered to access domains of this low 2.0 eV work function, they have the potential to significantly improve thermionic emitter-based technologies.

Synthesis of impregnants for dispenser cathodes via homogeneous-lyophilization: Phase structure, optical response and emission property of four aluminates

Dispenser cathodes are the most widely used electron emission source in vacuum electron devices. The impregnant is the source for producing active substances in the dispenser cathodes, which play an irreplaceable role. However, impregnants’ phase structure and optical response properties have yet to be systematically investigated. Herein, four aluminates were prepared as impregnants via homogeneous-lyophilization followed by calcination. Due to the unique preparation process, the elemental distribution of aluminates is exceptionally uniform. Based on the first-principles DFT calculations, the primary phases of the two aluminates 532 and 411 were confirmed to be (Ba0.61Ca0.39)3Al2O6 (P213) and (Ba0.75Ca0.25)4Al2O7 (Cmca). At the same time, α-Ba2ScAlO5 and β-Ba2ScAlO5 are the only phases of aluminates 411-0.75Sc and 411-1.25Sc, respectively. XPS confirmed two chemical states of Ba and Sc in the aluminates. FT-IR and 27Al NMR indicate that the aluminum-oxygen skeleton in all aluminates exists mainly as [AlO4] tetrahedrons, accompanied by a small amount of distorted [AlOx] (x = 5, 6) polyhedrons. The UV–visible light absorption capacity, band gap and intra-structural electron transfer capacity of all aluminates are determined by UV–Vis DRS and PL spectra. The dispenser cathodes impregnated with aluminates 532 and 411 have zero-field current densities of 5.3 A/cm2 and 14.3 A/cm2 at 1100°Cb, respectively. The dispenser cathodes impregnated with aluminate containing scandium have different space-charge-limited current densities, i.e., 29.48 A/cm2 for 411-0.75Sc and 13.66 A/cm2 for 411-1.25Sc at 1000°Cb. This work will provide more options for synthesizing and applying impregnants.

Experimental and theoretical study on hybrid thermionic-photovoltaic energy converters with graphene/semiconductor Schottky junction

Both thermionic and thermophotovoltaic solid-state energy converters are being actively explored due to high output potential. Recently, hybrid thermionic-photovoltaic (TIPV) approaches was proposed to utilize both electrons and photons emitted by a high temperature cathode, whereas conventional two-terminal configuration is constrained by current-match. In this work, we develop a four-terminal TIPV converter with a dispenser cathode, a transparent indium-tin-oxide anode and tandem graphene/GaAs Schottky junction photovoltaics. The TI and PV sub-devices can operate independently without meeting the current-match constraint. The TIPV prototype yields a power density of 962 W/m2 at the cathode temperature of 1373 K. In addition, a three-terminal TIPV converter with Nano-scale electrode gap size is modelled with advantages of surface plasmon and evanescent wave tunnelling. Results demonstrate that Super-Planckian near-field enhancement guarantees a > 4 MW/m2 output power density for the converter. We also discuss the effects of electrode gap size and the cathode temperature. This work validates the TIPV converters with graphene/semiconductor Schottky junction and also paves a way to promising developments for the Super-Planckian near-field configuration.

Physics-Based Model for Nonuniform Thermionic Electron Emission from Polycrystalline Cathodes

A physics-based model that predicts the emitted current from thermionic cathodes is developed, which accurately spans from the temperature-limited (TL) to full-space-charge-limited (FSCL) regions. Experimental observations of thermionic electron emission demonstrate a smooth transition between TL and FSCL regions of the emitted-current-density-versus-temperature (J-T) (Miram) curve and the emitted-current-density-versus-voltage (J-V) curve. Knowledge of the temperature and shape of the TL-FSCL transition is important in evaluating the thermionic electron-emission performance of cathodes, including predicting the lifetime. However, there are no first-principles physics-based models that predict the smooth TL-FSCL transition region for real thermionic cathodes without applying a priori assumptions or empirical phenomenological equations that are physically difficult to justify. Previous work detailing the nonuniform thermionic emission found that the effects of three-dimensional space charge, patch fields (electrostatic potential nonuniformity on the cathode surface based on local work-function values), and Schottky barrier lowering can lead to a smooth TL-FSCL transition region from a model thermionic cathode surface with a checkerboard spatial distribution of work-function values. In this work, we construct a physics-based nonuniform emission model for commercial dispenser cathodes. This emission model is obtained by incorporating the cathode surface grain orientation via electron-backscatter diffraction and the facet-orientation-specific work-function values from density-functional-theory calculations. The model enables the construction of two-dimensional emitted-current-density maps of the cathode surface and corresponding J-T and J-V curves. The predicted emission curves show excellent agreement with experiment, not only in the TL and FSCL regions but, crucially, also in the TL-FSCL transition region. This model provides a method to predict the thermionic emission from the microstructure of a commercial cathode and improves the understanding of the relationship between thermionic emission and cathode microstructure, which is beneficial for the design of vacuum electronic devices.

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.

Understanding the surface structure evolution and electron emission behaviors during the activation of Ir-coated dispenser cathodes

Metal coated dispenser cathode is of great interest due to its low operation temperature, high current density and long lifetime, however, the emission mechanism is still far from being fully understood. In this work, the tungsten dispenser cathodes were coated with iridium film of 300 nm in thickness. The phase transition and surface structure evolution during cathode activation was systematically investigated. The as-deposited iridium coating shows a columnar grain morphology with ultrafine size. Upon heating for 1 h at 1150 °C, a multiple-layer of Ir/Ir3W/IrW was detected and it fully converted to IrW layer after 4 h heat treatment. Such a structure evolution can be well explained based on thermodynamics. The Ir → Ir3W → IrW phase transition induces obvious film thickening and grain coarsening. The full activation could increase the emission current density from 17.35 A/cm2 to 30.92 A/cm2 at 1050 °C. Density function theory (DFT) simulation results demonstrate the enhanced emission of IrW-coated cathode may be due to its balanced adsorption behaviors with Ba and O atoms. Our findings will provide deeper insights into the design and development of metal coated cathodes with better performance.

Experimental and first-principles investigations on W-Ir mixed matrices cathodes with improved emission performance

Fundamental understanding of the intrinsic emission mechanism of mixed W matrix Ba type dispenser cathode is critical to rational design of advanced device with highly desired performance. In this work, experimental and theoretical work was conducted to investigate the underlying effect of Ir metal on the improved emission properties of mixed W cathodes. The results indicated that introduced Ir improves the stability of Ba-O dipole on the cathode surface by alloying with W rather than reaction with impregnants, which leads to higher atomic ratio of Ba/O on the surface emission layer with 50 wt% Ir adding, and therefore accounts for the enhanced emission current density of 11.90 A/cm2 with 2.2 times higher than W matrix dispenser cathode at 1000 ℃b. Furthermore, first-principles calculation was used to investigate the adsorption energy, work function, Bader charges, dipole moment and d-band center based on the high stable configuration of Ba0.25O0.5 adsorbing on IrW (111) and W (110), which explains the lowed work function of Ir-W cathode and in turn improved the emission property.

Experimental, analytical, and computational investigation of mesh grid thermal physics in an electron gun with dispenser cathode.

Gridded electron guns are key components of various electron beam based vacuum tubes. Mesh grids may be utilized for electron beam extraction and control. As part of the electron beam may be intercepted by the mesh grid, heating occurs, which could translate into performance degradation of the vacuum tube or even failure. This paper introduces an analytical model based on first physics principles for mesh grid heating in an electron gun, toward generating the upper bound for the intercepted electron beam power. 3D simulations and exploratory experiments for mesh grid heating in an electron gun directionally confirm the predictions of the analytical model. This analytical approach may be leveraged further when the upper bounds of mesh grid heating in electron guns are needed, as well as for adjusting mesh grid topology to increase its robustness against electron beam heating.

3-D Modeling and Experimental Studies for a Thermionic Dispenser Cathode Activation Process

The activation of dispenser cathodes while heating up to their brightness temperatures is a highly nonlinear and specific process. Since the cathode surface temperature is a crucial factor, controlling it and determining the physical and chemical factors affecting this parameter gives the opportunity to design dispenser cathodes more efficiently. Barium transport mechanism with temperature values can also be modeled. Thus, the extra time can be gained for activating cathode and it gives the advantages of determining cathode parameters that affecting directly emission quality and lifetime of the cathode. For this purpose, this article presents a 3-D modeling solution that is novel and very reliable for the analysis of this process. It has been obtained by the 3-D cathode model based on COMSOL Multiphysics simulations with heat transfer module environment. In addition to this, the degrees of temperature on the surface of our fabricated cathode have been measured experimentally as a function of delivered power for comparison with simulated data. The obtained results have been compared with MATLAB execution and also verified by values of a commercial-type cathode. The results show that the simulation data obtained by COMSOL have given a more realistic approach than theoretical ones. Thus, it has been observed that the activation process can be performed by the COMSOL model as a preliminary approach before heating a cathode physically.

Design, simulation and implementation of an efficient vacuum system and gas load calculations for a 6 MeV industrial LINAC

The design of vacuum system for a 6 MeV, commercial LINAC is presented. Unlike many commercial LINACs, the LINAC being developed has demountable electron gun, RF-waveguide and X-ray target assemblies. This design scheme is more efficient and economical. The LINAC has a total internal volume of 1.9 land surface area of ~3.2 × 103cm2, and is operated at Ultra High Vacuum. An iterative Simulation-Experimental approach is adopted to freeze a viable vacuum system for the LINAC. The vacuum simulations are performed in MOLFLOW+2.7.7, and the results are benchmarked experimentally. Due to compact geometry of the LINAC, reduced pumping speeds are experienced. The system was implemented using two 550 l/s commercially available turbo molecular pumps backed by rotary pumps, consuming ~2.4 kW. Particle dynamics simulations were performed using PARMELA 3. These simulations quantified the beam loss in the LINAC and predicted the sites of beam impingement. The system was experimentally verified for establishment of base pressure, activation of dispenser cathode, electron gun conditioning and the full power operation of LINAC at repetition rate of 50–110 Hz. Through all these regimes the vacuum system performed within permissible limits. The gas loads induced in the system were also evaluated using steady state and transient analysis. The pumping speed is optimized, and it is shown that base pressure of 1 × 10−9mbarcan be achieved at reduced pumping speed of ~100 l/s at the pumping ports. This reduced pumping speed also performed well for routine LINAC operation. Therefore, using benchmarked simulated results, an optimized and commercially economical vacuum system is proposed with reduced pumping speed of ~100 l/s consuming 6 times reduced power (0.4 kW) and ~40% reduced cost.

Structure and emission properties of barium calcium aluminates synthesized by room-temperature solid-state reaction approach

The two conventional aluminate precursor preparation methods are liquid-state co-precipitation and mechanical mixture, needing high temperature to fabricate thermionic cathodes for vacuum devices. Herein, we introduce the room temperature solid states (RTSS) approach to fabricate the precursors of barium calcium aluminates (BCA). For this, crystalline hydrates are used to prepare the precursor through mechanical grinding. The resulting product comprises of crystallized α-(Ba,Ca)CO3 solid solution and Al3+-contained amorphous precipitants. After sintering at 1200 °C, the active phases of Ba3CaAl2O7 and Ba3Al2O6 were obtained. The direct current (dc) emission measurements in a close-space diode configuration revealed that the divergent current density (Jdiv) of the test cathode reaches 7.2 A/cm2 at 1130 °CB, which is comparable to that of the co-precipitation liquid phase method. Considering the virtue of an environmentally-friendly and low-temperature preparation approach, the preparation of aluminates using the RTSS technique has great potential for the fabrication of dispenser cathodes for vacuum devices.

Studies of the Influence of ZrH2 Addition to the Impregnant on the Performance of Yttrium Oxide–Tungsten Matrix Ba Dispenser Cathodes

The cathode in the magnetron should have good secondary emission property and a certain thermionic property. In this article, ZrH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> was added as an activator to obtain rare earth-doped tungsten matrix impregnated cathode to improve the emission property. Cathodes with ZrH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (Y15-411-Zr cathode) or without ZrH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (Y15-411 cathode) were prepared by a spray drying method combined with high-temperature sintering in dry hydrogen atmosphere. The emission properties and emission mechanism have been studied. The results showed that thermionic emission performance and secondary emission performance of Y15-411-Zr cathode are 3.4 times and 1.2 times higher than those of Y15-411 cathode, respectively. With the addition of ZrH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , yttrium was produced during activating process, which contributed to the better emission properties of Y15-411-Zr cathode. Furthermore, the work function of barium and oxygen adsorbed on yttrium oxide and yttrium surface was calculated using the first principle. The results showed that the lowest work function of Ba-O absorbed on Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Y is 1.9 and 1.6 eV, respectively.

Surface excess free energies and equilibrium Wulff shapes in variable chemical environments at finite temperatures

Burgeoning interest in nanoparticles across a number of distinct fields has spurred broad investigation into the optimization of particle shape and surface chemistry. Here we demonstrate the application of DFT and DFPT calculations to compute bulk reference free energies and surface excess free energies accounting for vibrational and, where appropriate, configurational entropies of solid particles and particle surfaces. Using these results we construct “particle configuration maps”—graphical maps of the equilibrium shape and surface chemistry of particles over a range of temperatures and environmental conditions. Applied to W particles in O- or Ba/O-containing environments (a model system relevant to thermionic dispenser cathodes with critical applications in vacuum electronic devices) these maps highlight the critical role that Ba plays in controlling both the shape and surface chemistry of W particles in application-relevant conditions. These system-specific findings demonstrate the broad power of DFT+DFPT computed particle configuration maps: revealing connections between and insight into particle behavior and properties as a function of experimentally-relevant conditions.

Corrections to “Study and Development of Active Sintered Controlled Porosity Dispenser Cathode”

We regret to inform that there had been some serious omission regarding the information of the concerned organization for supporting the work. The earlier published work [1] only mentions the Council of Scientific and Industrial Research–Central Electronics Engineering Research Institute, Pilani, India, through the Government of India under Grant PSC0101 for supporting the work. The addendum is that the work was also partly supported by the Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India, an academic unit of CSIR.

Single-phase (Ba,Ca)2ScAlO5 impregnant for dispenser cathodes with enhanced thermionic emission performance

(Ba,Ca)2ScAlO5 compounds are synthesized and improved emission performance is achieved by virtue of their single-phase structure and low work function. The substitution of Ca2+ for Ba2+ effectively lowers the melting points of (Ba,Ca)2ScAlO5 from over 1800 °C–1700 °C when the Ca2+ substitution reaches 20 at%. During sintering, the co-precipitation precursor first reacts to form (Ba,Ca)3Al2O6 near 912 °C before reacting with BaCO3 and Sc2O3 at the higher temperature of 1186 °C to generate (Ba,Ca)2ScAlO5. Emission tests reveal that the maximum full space charge current density is 9.6 A cm−2 at 1000 °CB (brightness temperature) for the cathode impregnated with single-phase (Ba,Ca)2ScAlO5. Low work function (~1.5 eV) and good emission uniformity make such a cathode promising for applications in high-power microwave devices.

A review on recent progress of thermionic cathode

As the performance of vacuum electron devices is essentially governed by the properties of their cathodes, developing efficient and durable thermionic cathode is necessary and highly desired to meet the boosting requirements of vacuum electron devices. This review summarized the progress made in the past decades with a detailed discussion on the occurred various thermionic cathodes and their features, and the understandings of the correlation between the emission properties and the composition, where structure and synthesis method are well illustrated. Furthermore, dispenser cathodes with novel structures and emission mechanism are highlighted to indicate the recent achievement in this area of research, and Sc-cathode is considered as a promising candidate for the next-generation vacuum electron devices due to the greatly improved efficiency. However, challenges still exist to meet the ever-growing demands of thermionic cathode with collaborative requirement of high performance, easy fabrication and inadequate reproducibility.

A self-oscillating electron beam experiment

A recently proposed scheme to persistently over-inject a vacuum electron diode so that it self-oscillates, releasing a periodic train of electron bunches [Leopold et al., Phys. Plasmas 24, 073116 (2017)] was only partially demonstrated [Siman-Tov et al., 26, 033113 (2019)] because of the presence of hot spots on the dispenser cathode surface. The results obtained utilizing a new dispenser cathode with uniform surface emission, produce self-oscillations close to the expected frequencies.

Металлопористые катоды, модифицированные наноуглеродом, с высокой долговечностью для применения в приборах СВЧ

The results of calculations of the physical and energy parameters of nanocarbon structures with components of the active substance of the dispenser cathode, which are in good agreement with experience, are presented. It was experimentally determined that the active substance modified by the sulfoadduct of carbon nanoclusters has an evaporation rate of 1.5 times lower than the typical one. Tests of cathodes modified by nanocarbon as part of space-based TWTs showed a durability of 1740800 hours with a current density of 0.645 A / cm2.

Intercepting Grids Made of Anisotropic Pyrolytic Graphite in Electron Guns with a Dispenser Cathode

The results of experimental studies into the application of anisotropic pyrolytic graphite (APG) as grid structures in power microwave devices with a dispenser cathode are detailed. The dependences of emission characteristics of molybdenum, hafnium, and APG in diodes and electron guns on the electron-flow power dissipated at the studied samples and on cathode temperature are presented. It is demonstrated that the power dissipation capacity of APG grid structures (without parasitic thermionic emission) is up to 20 and 9 times higher than that of molybdenum and hafnium grids, respectively.