Thermionic Electron Emission Research Articles

Non-equilibrium thermionic electron emission for metals at high temperatures

Stationary thermionic electron emission currents from heated metals are compared against an analytical expression derived using a non-equilibrium quantum kappa energy distribution for the electrons. The latter depends on the temperature decreasing parameter κ(T), which decreases with increasing temperature and can be estimated from raw experimental data and characterizes the departure of the electron energy spectrum from equilibrium Fermi-Dirac statistics. The calculations accurately predict the measured thermionic emission currents for both high and moderate temperature ranges. The Richardson-Dushman law governs electron emission for large values of kappa or equivalently, moderate metal temperatures. The high energy tail in the electron energy distribution function that develops at higher temperatures or lower kappa values increases the emission currents well over the predictions of the classical expression. This also permits the quantitative estimation of the departure of the metal electrons from the equilibrium Fermi-Dirac statistics.

Delayed electron emission from fullerene dianions: Evidence for a thermal tunneling detachment

Isolated multiply charged anions exhibit a repulsive Coulomb barrier (RCB) towards electron loss. In this paper, the role of tunneling through the RCB in the delayed electron emission is addressed. Electron kinetic energy distributions of photoexcited fullerene dianions ${\mathrm{C}}_{84}{}^{2\ensuremath{-}}$ and ${\mathrm{C}}_{90}{}^{2\ensuremath{-}}$ have been measured with a velocity-map imaging spectrometer for delays of about 150 ns after the nanosecond laser excitation. On such spectra, delayed electron emission is unambiguously separated from direct detachment. These measurements display significant delayed emission below the RCB. A detailed-balance model including electron tunneling through the RCB has been developed. The very good agreement between the measurements and the model provides quantitative evidence for a thermal tunneling electron emission.

Boundary conditions at the walls with thermionic electron emission in two temperature modeling of “thermal” plasmas

In this paper, we propose new boundary conditions for the electric potential, the electron energy equation, and the energy equation for heavy particles (ions and neutrals) at the hot walls with thermionic electron emission for two-temperature thermal arc models. The derived boundary conditions assume that the walls are made from refractory metals and, consequently, the erosion of the wall is small and can be neglected. In these boundary conditions, the plasma sheath formed at the electrode is viewed as the interface between the plasma and the wall. The derived boundary conditions allow the calculation of the heat flux to the walls from the plasma. This allows the calculation of the thermionic electron current that makes the model of electrode-plasma interaction self-consistent.

Deposition of TiN by plasma activated EB-PVD: Activation by thermal electron emission from molten niobium

This paper presents a new plasma activation process for electron beam evaporation. The plasma is generated by an interaction between the evaporated vapor (serving as anode) and the thermal electrons emitted from molten Nb (serving as cathode). Besides, an arc discharge burning in the anode vapor further enhances the plasma density. This process is demonstrated by means of the plasma activated deposition of TiN coating. Deposition rate of ~170nm/min has been measured. Contamination caused by the slight evaporation of Nb to the coating is ~0.1at.%.

Reflection of a probe pulse and thermal emission of electrons produced by an aluminum film heated by a femtosecond laser pulse

It is shown that an experimental decrease in the reflection of a probe femtosecond pulse from an aluminum film heated by a higher-power femtosecond pulse can be quantitatively described taking into account the inhomogeneous distribution of the laser pulse field in the film and the evolution of the electron and lattice temperature during absorption of the heating inhomogeneous field. Analysis of the electron temperature evolution on the heated film surface combined with modern concepts about the influence of a surface volume charge on thermal emission gave the relation between the amount of emitted electrons and experimental data on the heating of the aluminum film by the femtosecond pulse.

Thermal emission of hot electrons in a metal

The first reliable observation of emission from a hot gas of free electrons in a metal with a cold lattice is reported. The emission is observed upon the irradiation of the metal with femtosecond laser pulses. The distinctive feature of this emission as compared to the case of equilibrium heating is the dependence of the emission quantum yield on the parameters of the electron-phonon interaction.

유한요소법을 이용한 X-선관 양극각도에 의존하는 전자빔 초점 특성 연구

The focal spot of electron beam depending on the anode angle in the structure and major parts of the X-ray tube was investigated by the OPERA-3D/SCALAR simulation program. The simulation worked on four spaces with with two spaces, including anode and cathode of X-ray tube, by applying the finite element method analysis. The analytical model and dimension for the emission orbit of thermal electrons made from one filament of the focused X-ray cathode is affected to the penumbra of detector for the X-ray depending on any real focal spot size. The model shape of focusing cap and focusing tube with an anode target angle and a cathode filament is analyzed by the current density distribution of thermal electrons. The focusing width of thermal electrons for the X-ray tube depended on the anode angle (θ). The focusing value of electron beams at a region of anode angle having 10 ~17 maintained to below value of 70 μm. The minimum focal size of the electron beam was 40 μm at an anode angle of 15 . The focused X-ray tube of many variables depended on the thermionic emission of hot electrons from the target trajectory. The focusing tube will contribute to

Vertical electron transport in van der Waals heterostructures with graphene layers

We propose and analyze an analytical model for the self-consistent description of the vertical electron transport in van der Waals graphene-layer (GL) heterostructures with the GLs separated by the barriers layers. The top and bottom GLs serve as the structure emitter and collector. The vertical electron transport in such structures is associated with the propagation of the electrons thermionically emitted from GLs above the inter-GL barriers. The model under consideration describes the processes of the electron thermionic emission from and the electron capture to GLs. It accounts for the nonuniformity of the self-consistent electric field governed by the Poisson equation which accounts for the variation of the electron population in GLs. The model takes also under consideration the cooling of electrons in the emitter layer due to the Peltier effect. We find the spatial distributions of the electric field and potential with the high-electric-field domain near the emitter GL in the GL heterostructures with different numbers of GLs. Using the obtained spatial distributions of the electric field, we calculate the current-voltage characteristics. We demonstrate that the Peltier cooling of the two-dimensional electron gas in the emitter GL can strongly affect the current-voltage characteristics resulting in their saturation. The obtained results can be important for the optimization of the hot-electron bolometric terahertz detectors and different devices based on GL heterostructures.

Planar geometry inertial electrostatic confinement fusion device

In the classic gridded inertial electrostatic confinement (IEC) fusion reactor, ion bombardment of the grid leads to heating, thermionic electron emission, significant power loss, and ultimately melting of the grid. Gridless IEC devices have sought to overcome these limitations. Klein reported a gridless device in which ions are circulated as a linear beam in an electrostatic analogue of an optical resonator. To overcome limits of stored ions due to space charge effects at the turning regions, the device employed multiple overlapping traps. The work reported here seeks to further increase the turning region space in a gridless trap by employing a planar geometry. Ion trapping in the planar device was examined by simulating trajectories of 2H+ ions with SIMION 8.1 software. Simulations were carried out using multiple potentials as in Klein's device and for a single potential trap as a planar analogue of the anharmonic ion trap. Scattering by background gas was simulated using a hard sphere collision model, and the results suggested the device will require operation at low pressure with a separate ion source.

Radiometric measurements of electron temperature and opacity of ionospheric perturbations

Changes in the sky noise spectrum are used to characterize perturbations in the ionosphere. Observations were made at the same sidereal time on multiple days using a calibrated broadband dipole and radio spectrometer covering 80 to 185 MHz. In this frequency range, an ionospheric opacity perturbation changes both the electron thermal emission from the ionosphere and the absorption of the sky noise background. For the first time, these changes are confirmed to have the expected spectral signature and are used to derive the opacity and electron temperature associated with the perturbations as a function of local time. The observations were acquired at the Murchison Radio-astronomy Observatory in Western Australia from 18 April 2014 to 6 May 2014. They show perturbations that increase at sunrise, continue during the day, and decline after sunset. Magnitudes corresponding to an opacity perturbation of about 1% at 150 MHz with a typical electron temperature of about 800 K were measured for the strongest perturbations.

Electron Thermionic Emission from Graphene and a Thermionic Energy Converter

In this paper, we propose a model to investigate the electron thermionic emission from a single-layer graphene (ignoring the effects of substrate) and to explore its application as the emitter of thermionic energy convertor (TIC). An analytical formula has been derived, which is a function of temperature, work function and Fermi energy level. The formula is significantly different from the traditional Richardson-Dushman (RD) law for which it is independent of mass to account for the supply function of the electrons in the graphene behaving like massless Fermion quasiparticles. By comparing with a recent experiment [Kaili Jiang et al., Nano Research 7, 553 (2014)] measuring electron thermionic emission from a suspended single layer graphene, our model predicts that the intrinsic work function of a single-layer graphene is about 4.514 eV with a Fermi energy level of 0.083 eV. For a given work function, a new scaling of $T^{3}$ is predicted, which is different from the traditional RD scaling of $T^2$. If the work function of the graphene is lowered to 2.5 to 3 eV, and the Fermi energy level is increased to 0.8 to 0.9 eV, it is possible to design a graphene cathode based TIC operating at around 900 K or lower, as compared with the metal-based cathode TIC (operating at about 1500 K). With a graphene based cathode (work function = 4.514 eV) at 900 K, and a metallic based anode (work function = 2.5 eV) like LaB$_6$ at 425 K, the efficiency of our proposed-TIC is about 45$\%$

Numerical model of the plasma formation at electron beam welding

The model of plasma formation in the keyhole in liquid metal as well as above the electron beam welding zone is described. The model is based on solution of two equations for the density of electrons and the mean electron energy. The mass transfer of heavy plasma particles (neutral atoms, excited atoms, and ions) is taken into account in the analysis by the diffusion equation for a multicomponent mixture. The electrostatic field is calculated using the Poisson equation. Thermionic electron emission is calculated for the keyhole wall. The ionization intensity of the vapors due to beam electrons and high-energy secondary and backscattered electrons is calibrated using the plasma parameters when there is no polarized collector electrode above the welding zone. The calculated data are in good agreement with experimental data. Results for the plasma parameters for excitation of a non-independent discharge are given. It is shown that there is a need to take into account the effect of a strong electric field near the keyhole walls on electron emission (the Schottky effect) in the calculation of the current for a non-independent discharge (hot cathode gas discharge). The calculated electron drift velocities are much bigger than the velocity at which current instabilities arise. This confirms the hypothesis for ion-acoustic instabilities, observed experimentally in previous research.

Synthesis and optical absorption properties of LaxCe1-xB6 submicron powders

According to original knowledge, lanthanum hexaboride (LaB6) as an excellent thermionic electron emitter is characterized by a low work function, a high emission density, and a high brightness. Recently, much attention has been drawn to its another excellent optical properties: strong light absorption in near infrared rays (NIR), and transparency in visible light (VL), which result from the free electron plasmon resonance. However, up to now the optical properties and syntheses of ternary rareflearth hexaborides have been very rarely reported in the literature. In this paper, ternary LaxCe1-xB6 submicron crystallines are successfully synthesized using a solid-state reaction, in which La2O3 and CeO2 are used as rareflearth sources and NaBH4 as boron sources in a continuous vacuum condition. Effects of La doping content on the LaxCe1-xB6 phase composition, microstructure, and optical absorption properties are investigated by X-ray diffraction, scanning electron microscope (SEM), and transmission electron microscope (TEM). It is found that all the synthesized samples are composed of CaB6-type single-phase alloy with a space group Pm3m at the reaction temperature of 1200 ℃ held by 2 h. The SEM results indicate that the cubic-shaped ternary LaxCe1-xB6 crystals with a mean size of 200 nm are obtained and the energy dispersive spectrometer results confirm that the La atoms are randomly distributed at the lattice sites of CeB6. High resolution transmission electron microscope images refleal the single-crystalline nature, and the FFT pattern indicates the lattice fringe d=0.42 nm which agrees well with the (100) crystal plane. EDS analysis of TEM also indicates the La element has been doped into CeB6. And the optical absorption result shows that the absorption valley of CeB6 is 62 nm. With increasing La doping content to x=0.6 and 0.8, the absorption valleys of La0.6Ce0.4B6 and La0.8Ce0.2B6 decrease to 613 and 610 nm respectively indicating the blueshifts of the wavelength of absorption valley From the view point of practical application, the tunable characteristic of LaxCe1-xB6 may extend the optical applications in improving the efficiency of organic photovoltaics, replacing the expensive gold and silver nanoparticles, which may have a good usage in optical filters.

Thermionic and Photo-Excited Electron Emission for Energy-Conversion Processes

This article describes advances in thermionic and photoemission materials and applications dating back to the work on thermionic emission by Guthrie in 1873 and the photoelectric effect by Hertz in 1887. Thermionic emission has been employed for electron beam generation from Edison’s work with the light bulb to modern day technologies such as scanning and transmission electron microscopy. The photoelectric effect has been utilized in common devices such as cameras and photocopiers while photovoltaic cells continue to be widely successful and further researched. Limitations in device efficiency and materials have thus far restricted large-scale energy generation sources based on thermionic and photoemission. However, recent advances in the fabrication of nanoscale emitters suggest promising routes for improving both thermionic and photo-enhanced electron emission along with newly developed research concepts, e.g., photonically enhanced thermionic emission. However, the abundance of new emitter materials and reduced dimensions of some nanoscale emitters increases the complexity of electron emission theory and engender new questions related to the dimensionality of the emitter. This work presents derivations of basic two and three-dimensional thermionic and photoemission theory along with comparisons to experimentally acquired data. The resulting theory can be applied to many different material types regardless of composition, bulk and surface structure.

Method for Approximating Electron Beam Currents Accelerated by a Piezoelectric Transformer

Piezoelectric transformers (PTs) are currently used to accelerate charged-particle beams for various applications. Beam interactions at the output of the PT can be treated as a parallel RC electrical load. The impedance of the load can affect the output voltage because of the small, finite amount of charge available in the PT; therefore, high-impedance diagnostics are required to characterize the PT. A thermionic electron emitter was used to provide a controllable beam current for testing the effects of electrical loading on the PT. The electron beam was operated in vacuum at pressures of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> torr. The input mechanical quality factor and the effective output electrical quality factor were used to approximate the electron beam current on the basis on the PTs equivalent electromechanical circuit. The output voltage needed for the output electrical quality factor was measured via bremsstrahlung interactions at the output electrode of the PT. Optical techniques for finding internal operating parameters such as electric field and stress were used to determine the load with curve fitting as a comparison with the quality factor diagnostic. Approximating the electron beam current with such methods will help determining the output power that such PTs can generate for future applications.

Modeling of X‐ray emissions produced by stepping lightning leaders

AbstractIntense and brief bursts of X‐ray emissions have been measured during the stepping process of both natural cloud‐to‐ground (CG) and rocket‐triggered lightning flashes. In this paper, we investigate theoretically the energy spectra of X‐rays produced by the bremsstrahlung emission of thermal runaway electrons accelerated in the inhomogeneous electric field produced around lightning leader tips. The X‐ray energy spectrum depends on the physical properties of the associated lightning leaders. Consequently, X‐ray measurements can be used for diagnostics of the electrical properties of lightning stepped leaders. We report simulation results of the photon energy spectra produced by 5 and 10 MV negative CG lightning discharges that would be measured from the ground using ideal detectors. We also quantify theoretically the radial dependence of X‐ray energy spectra received at ground level during the leader stepping process. Moreover, it is found that the ground radiation generated in this process is harmless to humans.

In situ reactivation of low-temperature thermionic electron emission from nitrogen doped diamond films by hydrogen exposure

Nitrogen doped, hydrogen terminated diamond films have shown a work function of less than 1.5eV and thermionic electron emission (TE) has been detected at temperatures less than 500°C. However, ambient exposure or extended operation leads to a deterioration of the emission properties. In this study thermionic electron emission has been evaluated for as-received surfaces and for surfaces after 18months of ambient exposure. The initial TE current density of the freshly deposited diamond film was ~5×10−5A/cm2 at 500°C. In contrast, the initial TE current density of a film aged for 18months was ~1.8×10−9A/cm2 at 500°C. The decreased emission current density is presumed to be a consequence of oxidation, surface adsorption of contaminants and hydrogen depletion from the surface layer. In situ reactivation of the aged film surface was achieved by introducing hydrogen at a pressure of 1.3×10−4mbar and using a hot filament of a nearby ionization gauge to generate atomic and/or excited molecular hydrogen. After 2h of exposure with the sample at 500°C, the surface exhibited a stable emission current density of ~2.3×10−6A/cm2 (an increase by a factor of ~1300). To elucidate the reactivation process thermionic electron energy distribution (TEED) and XPS core level spectra were measured during in situ hydrogen exposure at 5×10−8mbar. During the isothermal exposure it was determined that atomic or excited hydrogen resulted in a much greater increase of the TE in comparison to exposure to molecular hydrogen. During exposure at 400°C the surface oxygen was substantially reduced, the TEED cut-off energy, which indicates the effective work function, decreased by ~200meV, and the TE intensity increased by a factor of ~100. The increase in thermionic emission with hydrogen was ascribed to the reactivation of the surface through the formation of a uniform surface dipole layer and a reduction of the surface work function.

Effect of boundary conditions on the heat flux to the wall in two-temperature modeling of ‘thermal’ plasmas

In this paper we propose new boundary conditions at floating and biased walls for two-temperature thermal arc models. In the derived boundary conditions the walls are assumed to be cold and therefore there is no thermionic electron emission and erosion of the wall. The boundary conditions previously used in modeling high-pressure thermal arcs ignore the formation of a plasma sheath at floating walls. As shown in the paper, using the sheath boundary conditions lead to a drastic increase in the heat flux to the wall that has to be taken into account in modeling thermal arcs and heat transfer.

Photo- and thermionic emission of MWPECVD nanocrystalline diamond films

Nanocrystalline diamond (NCD) films with and without a diamond buffer layer (BL) have been grown on p-type silicon substrates by microwave plasma enhanced chemical vapor deposition technique at different values of deposition temperature (652–884°C). The photo- and thermionic electron emission properties of NCD films have been investigated, illustrated and explained by analyzing the surface morphology and the grain shape determined by atomic force microscopy, the chemical-structural properties by Raman spectroscopy and nanocrystallites size by X-ray diffraction. The NCD films with BL grown at the highest deposition temperature have shown the highest photo- and thermionic emission currents.

Thermionic emission from phosphorus (P) doped diamond nanocrystals supported by conical carbon nanotubes and ultraviolet photoelectron spectroscopy study of P-doped diamond films

Materials with low work function values (<2eV) are highly in demand for low temperature thermionic electron emission, which is a key phenomenon for waste heat recovery applications. Here we present the work function reduction of phosphorus (P) doped (i) diamond nanocrystals grown on conical carbon nanotubes (CCNTs) and (ii) diamond films grown on silicon substrates. Thermionic emission measurements from phosphorus doped diamond crystals on CCNTs resulted in a work function value of 2.23eV. The CCNTs provide the conducting backbone for the P-doped diamond nanocrystals and the reduced work-function is interpreted as due to the presence of midband-gap state and no evidence for negative electron affinity was seen. However, ultraviolet photoelectron spectroscopy studies on phosphorus doped diamond films yielded a work function value of ~1.8eV with a negative electron affinity (NEA) value of 1.2eV. Detailed band diagrams are presented to support the observed values for both cases.