Effect Of Thermionic Emission Research Articles

Reduced graphene oxide/TiO2 hybrid synergistically enhanced polyimide nanofibers with high triboelectric output and thermal charge stability

The output performance of triboelectric nanogenerators (TENGs) can drop dramatically in high temperature environments because of the thermionic emission effect. To enhance the thermal charge stability and expand the working temperature of TENG, polyimide nanofibers (PINF) were synergistically enhanced by the electron trapping mechanism of reduced graphene oxide (rGO) and interfacial polarization of TiO2. With the multiple enhancement effect of polyimide/rGO/TiO2 composite nanofibers (PRTNF) on triboelectric output, PRTNF-TENG generated an open circuit voltage of 228.26 V and a short circuit current of 5.22 μA, which were 3.80 and 3.48 times higher than those of PINF-TENG. The effective output of polymer-based TENG was investigated from 25 − 300 °C. Compared to room temperature, the PRTNF-TENG can maintain 19.45 % (44.39 V) and 18.20 % (0.95 μA) of the voltage and current at 300 °C, respectively, and still drive 50 commercial LEDs. Furthermore, PRTNF-TENGs were applied as self-powered sensors for firefighters to provide real-time motion monitoring and ambient temperature detection. This PRTNF-TENG has broad application prospects for energy harvesting and smart electronic monitor in harsh environments such as fire and space.

A thermionic electron gun to characterize silicon drift detectors with electrons

The TRISTAN detector is a new detector for electron spectroscopy at the Karlsruhe Tritium Neutrino (KATRIN) experiment. The semiconductor detector utilizes the silicon drift detector technology and will enable the precise measurement of the entire tritium β-decay electron spectrum. Thus, a significant fraction of the parameter space of potential neutrino mass eigenstates in the keV-mass regime can be probed. We developed a custom electron gun based on the effect of thermionic emission to characterize the TRISTAN detector modules with mono-energetic electrons before installation into the KATRIN beamline. The electron gun provides an electron beam with up to 25 keV kinetic energy and an electron rate in the order of 105 electrons per second. This manuscript gives an overview of the design and commissioning of the electron gun. In addition, we will shortly discuss a first measurement with the electron gun to characterize the electron response of the TRISTAN detector.

Evidence of hot carrier extraction in metal halide perovskite solar cells

AbstractThe presence of hot carriers is presented in the operational properties of an (FA,Cs)Pb(I, Br, Cl)3 solar cell at ambient temperatures and under practical solar concentration. Albeit, in a device architecture that is not suitably designed as a functional hot carrier solar cell. At 100 K, clear evidence of hot carriers is observed in both the high energy tail of the photoluminescence spectra and from the appearance of a nonequilibrium photocurrent at higher fluence in light J–V measurements. At room temperature, however, the presence of hot carriers in the emission at elevated laser fluence is shown to compete with a gradual red shift in the PL peak energy as photoinduced halide segregation begins to occur at higher lattice temperature. The effects of thermionic emission of hot carriers and the presence of a nonequilibrium carrier distribution are also shown to be distinct from simple lattice heating. This results in large unsaturated photocurrents at high powers as the Fermi distribution exceeds that of the heterointerface controlling carrier transport and rectification.

Influence of Thermionic Emission on He Ionization and Plasma Enhancement in Thermionic Energy Conversion

In this paper, the influence of thermionic emission on He ionization and plasma enhancement in thermionic energy conversion (TEC) are studied by experiment and numerical simulation. A 1D unsteady plasma TEC model, which includes a He ionization model, plasma conservation equations, and a thermionic emission formula for the wall, is developed. A He plasma thermionic energy conversion device composed of a barium–tungsten cathode and a tungsten anode is established. The volt–ampere curves of the He plasma TEC device are measured at 1050 K, 1150 K, 1250 K, 1300 K, and 1350 K temperatures. Both important cathode parameters, work function and emission area, are estimated. Based on the modelling simulation and the experiment, the He ionization mechanism in plasma TEC is discovered. The effects of cathode temperature on several distributions of plasma reaction rates, particle number density, and potential in He plasma TEC are described. Some important parameters, including electron mobility, resistivity, and plasma equilibrium are analyzed. The relationship of thermionic emission on plasma enhancement to the output power of plasma TEC is presented. The output powers of plasma TEC and vacuum TEC are compared at various cathode temperatures. A dimensionless analyzing method concerning thermionic emission intensity and plasma enhancement power is proposed. A brief dimensionless relationship is deduced regarding thermionic emission intensity and the plasma enhancement contribution of TEC. The principles and methods for quantitative calculations concerning the output power of plasma TEC under the action of thermionic emission are established. It is possible to do quantitative research on the effects of thermionic emission on plasma-enhanced TEC.

Effect of thermionic emission on the current in the circuit of a metal collector irradiated by an electron beam in the forevacuum pressure range

We describe the results of our exploration of current flow in the circuit of a metal collector irradiated by an electron beam at enhanced forevacuum pressures in the range of 5–30 Pa. We find that beam heating of the collector surface to a certain critical temperature results in a sharp decrease in the current measured in the collector circuit. It is shown that this decrease is mainly due to thermionic emission from the collector surface. Depending on the collector material, the thermionic current may be as high as 50% or more of the electron beam current.

A novel high-low-high Schottky barrier based bidirectional tunnel field effect transistor

In this work, we proposed a novel High-Low-High Schottky barrier bidirectional tunnel field effect transistor (HLHSB-BTFET). Compared with previous technology which is named as High Schottky barrier BTFET (HSB-BTFET), the proposed HLHSB-BTFET requires only one gate electrode with independent power supply. More importantly, take an N type HLHSB-BTFET as an example, different from the previously proposed HSB-BTFET, due to that the effective potential of the central metal is increased with the increasing of drain to source voltage (Vds), built-in barrier heights maintain at the same value when the Vds is increased. Therefore, there is no strong dependence between built-in barrier heights formed in the semiconductor region on the drain side and the Vds. Besides that low Schottky barrier formed on the interface between the conduction band of silicon regions on its both sides and the central metal (while high Schottky barrier formed between the valence band of silicon regions on its both sides and the central metal) have been designed for preventing the carriers in valence band from flowing into the central metal induced by thermionic emission effect. Thereafter, the proposed N type HLHSB-BTFET has a natural blocking effect on the carriers flowing in valence band, and this blocking effect is not significantly degraded with the increasing of Vds, which is a huge promotion from the previous technology. The comparison between the two technologies is carried out, which exactly agrees with the design assumptions.

Harsh Environmental-Tolerant and High-Performance Triboelectric Nanogenerator Based on Nanofiber/Microsphere Hybrid Membranes.

Triboelectric nanogenerator (TENG) can convert tiny mechanical energy into precious electrical energy. Constant improvements to the output performance of TENG is not only the driving force for its sustainable development, but also the key to expand its practical applicability in modern smart devices. However, most previous studies were conducted at room temperature, ignoring the influence of temperature on the output performance of TENG. Additionally, due to thermionic emission effect, the electrons transferred to a dielectric surface can be released into a vacuum after contact electrification. Therefore, TENG cannot maintain an effective electrical output under high-temperature conditions. Here, a series of high-temperature operatable flexible TENGs (HO-TENGs) based on nanofiber/microsphere hybrid membranes (FSHMs) was fabricated by electrospinning and electrospraying. The Voc of HO-TENG is 212 V, which is 2.33 times higher than that of control TENG. After 10,000 cycle stability tests, the HO-TENG shows excellent durability. Especially, this HO-TENG can maintain 77% electrical output at 70 °C compared to room temperature, showing excellent high-temperature operability. This study can not only provide a reference for the construction of advanced high-performance TENG, but also provide a certain experimental basis for efficient collection of mechanical energy in high-temperature environment and promote the application of TENG devices in harsh environments.

Simulation of effect of thermionic emission on magnetized sheath near target plate of tungsten divertor

The high confinement mode (H-mode) is a preferred operation mode of tokamak devices in the future, but the burst of edge localized mode (ELM) will sharply increase the heat load deposited on the divertor target, raising the target temperature rapidly and strengthening surface thermionic emission. In this paper, a one-dimensional fluid model is used to simulate the influence of thermionic emission on the characteristics of the magnetized sheath. The results show that the amplitude of float potential and the electric field strength both decrease under the action of thermionic emission. Plenty of thermionic emission electrons leave the target, resulting in a region with negative charge density near the target plate, and the magnetized sheath is divided into two parts: ion sheath and electron sheath. In the electron sheath, with the rise of the target surface temperature, electrons accumulated in front of the target also increase, the potential distribution is non-monotonic, and a “virtual cathode” structure appears. The reverse electric field formed near the target will confine the thermionic emission electrons leaving the target and slow down the ion movement, leading to a decrease of the ion energy deposited on the target. With the increase of the angle between the magnetic field and the target normal, the potential of the magnetized sheath and the proportion of the thickness of the electron sheath in the magnetized sheath both increase. The virtual cathode potential decreases, the temperature of the target required to form the virtual cathode rises.

Electron Beam Emission System for Space Welding Based on Indirectly-Heated Bombardment Cathode

An electron beam emission system developed for space welding and consisting of an indirectly-bombardment cathode fabricated using LaB6 was studied to improve the working life of the space welding electron gun. The energy transfer model of the indirectly-heated bombardment cathode was established based on the cooling effect of thermionic emission and the bombardment heating structure. The efficiency of the energy transfer model was verified by conducting emission tests and the differences between the test and theoretical values were analyzed. The designed electron beam emission system could emit 500 mA beam current in a stable manner at an acceleration voltage of 20 kV, and the diameter of the beam spot was less than 400 μm. The structure, indirectly heated by electron bombardment, exhibited a stable performance, could stabilize the working temperature of the cathode by exploiting the bombardment beam, and the total heating efficiency of the system could reach 76.04%.

Triboelectric Polymer with High Thermal Charge Stability for Harvesting Energy from 200 °C Flowing Air

AbstractDue to the thermionic emission effect, the electron transferred to dielectric surface can be released into vacuum after the contact electrification (CE). Therefore, triboelectric nanogenerator (TENG) cannot maintain effective electrical output under high temperature conditions. In order to obtain high thermal charge stability, polyimide is modified with strong electron withdrawing groups like trifluoromethyl (CF3) and sulfone group (SO2) in backbone. The fluorinated polyimides (F‐PI) with a big band gap can provide a tribocharge density of 170 µC m−2 (4 times of common Kapton film) and become more negative than polytetrafluoroethylene in triboelectric series. In addition, BaTiO3 nanoparticles are doped in F‐PI film for inducing deep traps and interfacial polarizations for CE, which can further enhance the charge density (200 µC m−2) and thermal charge stability. Finally, a flutter‐driven TENG (FD‐TENG) is designed based on this BaTiO3‐doped F‐PI film to harvest wind energy and sense wind velocity. This FD‐TENG can maintain 32% of its output performance at 200 °C in comparison with room temperature, which is the highest thermal charge stability reported for triboelectric polymers. Therefore, this BaTiO3‐doped F‐PI has great application prospects for energy generation and motion detection in hot wind tunnel and many other harsh environments.

Possible mitigation of tokamak plasma–surface interactions using thermionic divertor plates with inverse sheaths

Recent studies demonstrate that when a plasma-facing surface emits a sufficient flux of electrons, it will form an inverse sheath. Here, we consider a possibility of using thermionic target plates with inverse sheaths as an innovative divertor operating scenario. We derive an electron heat flux boundary condition for inverse sheaths and show that for given power exhaust into a tokamak scrape-off-layer, an inverse sheath leads to a much lower target plasma electron temperature than a conventional sheath. Low enough target plasma temperatures for radiative divertor detachment could therefore be achieved using inverse sheaths instead of the usual need to inject neutral atoms that compromise the core plasma. Other advantages of inverse sheath detachment over conventional sheath operating scenarios include (a) ion impact energies are as low as possible, minimizing sputtering and tritium implantation, (b) surface recombination heat flux is reduced due to ion flux reduction, and (c) arcs are inhibited due to the sign of the surface electric field. This paper outlines the basic properties of inverse sheath detachment and considers the feasibility of implementation. We offer recommendations for future modeling efforts needed to better understand the effects of thermionic emission in tokamaks and whether inverse sheaths present a viable divertor solution.

ВОЗДЕЙСТВИЕ ТЕРМОЭЛЕКТРОННОЙ ЭМИССИИ УГЛЕРОДНЫХ ЧАСТИЦ НА СТЕПЕНЬ ИОНИЗАЦИИ ПЛАМЕНИ

ВОЗДЕЙСТВИЕ ТЕРМОЭЛЕКТРОННОЙ ЭМИССИИ УГЛЕРОДНЫХ ЧАСТИЦ НА СТЕПЕНЬ ИОНИЗАЦИИ ПЛАМЕНИ

Effects of a Si-doped InGaN Underlayer on the Optical Properties of InGaN/GaN Quantum Well Structures with Different Numbers of Quantum Wells.

In this paper we report on the optical properties of a series of InGaN polar quantum well structures where the number of wells was 1, 3, 5, 7, 10 and 15 and which were grown with the inclusion of an InGaN Si-doped underlayer. When the number of quantum wells is low then the room temperature internal quantum efficiency can be dominated by thermionic emission from the wells. This can occur because the radiative recombination rate in InGaN polar quantum wells can be low due to the built-in electric field across the quantum well which allows the thermionic emission process to compete effectively at room temperature limiting the internal quantum efficiency. In the structures that we discuss here, the radiative recombination rate is increased due to the effects of the Si-doped underlayer which reduces the electric field across the quantum wells. This results in the effect of thermionic emission being largely eliminated to such an extent that the internal quantum efficiency at room temperature is independent of the number of quantum wells.

Incorporation of MoS2 nanoflakes into poly(3-hexylthiophene)/n-type Si devices to improve the rectification behavior and optoelectronic performance

This study determines the effect of incorporating MoS2 nanoflakes into poly(3-hexylthiophene) (P3HT) on the electrical conduction mechanisms using the rectification current–voltage characteristics of P3HT/n-type Si devices. It is shown that the forward-voltage current for P3HT/n-type Si devices is limited by the combined effect of thermionic emission and space-charge-limited current conduction. However, carrier transport for P3HT:MoS2/n-type Si devices in the forward-voltage region is almost dominated by thermionic emission. Incorporation of MoS2 nanoflakes into P3HT modifies the P3HT-Si interface and the values for the carrier mobility in the P3HT layer and the external quantum efficiency of the P3HT/n-type Si devices are significantly increased, which improves the rectification and optoelectronic performance of P3HT:MoS2/n-type Si devices.

The Concept of the Combined Thermal Protection System for Leading Edges of Hypersonic Vehicles with Use of Thermionic Emission

This work represents a conceptual stage of the project on development of technology of the combined thermal protection system for hypersonic vehicles heat-stressed surfaces with use of technology of the thermionic power cell and thermal protection system with an external emission of electrons. The relevance of this work is to develop thermal protection system technology for aircraft, enabling prolonged controlled flight at hypersonic speeds, while providing low aerodynamic resistance and relative weight, the consistency of the geometric shape of the hypersonic vehicles leading edge. The various using types of thermal protection system are compared and the necessity to develop a new type of it using the effect of thermionic emission of electrons is proved. The scheme and the possible material composition of thermionic power cell with a reversed geometry of the electrodes are given. The problem of the choice of material for emission surface of the system with external electron emission and its manufacturing technology are discussed. Using cesium intercalated graphite as one of the possible coating materials is reviewed. A sequence of forthcoming studies is formulated at the stage of transition to the design basis for the operation of thermal protection of this type.

Absorptance enhancement in fs‐laser‐treated CVD diamond

Surface texturing by fs‐laser pulses has been performed in order to enhance optical absorptance of chemical vapour deposited diamond. The induced surface structures have been studied as a function of treatment dose D. Periodic structure with ripples of 170 nm has been observed for a D = 5.0 kJ cm−2, although not well defined texturing and damaged structures have been obtained for both lower and higher doses. Raman investigations point out negligible changes in crystal bulk for all investigated samples, thus suggesting that physical properties of the crystal were not changed by the treatment. Optical absorptance is strongly enhanced by fs‐laser texturing and it is an increasing function of the treatment dose. The absorptance of solar spectrum saturates up to values larger than 90%. The obtained outstanding enhancement of photon absorption represents a preliminary and promising step for the exploitation of synthetic diamond in future high‐efficient conversion devices for solar concentration based on photon‐enhanced thermionic emission effect.

Contact Effect on the Current–Voltage Characteristic of Graphene Nanoribbon Based Schottky Diode

This paper presents the study of the current-voltage characteristic of graphene nanoribbon based Schottky diode using analytical method. The work presents a simple model to analyse the current-voltage characteristic in the function of Schottky barrier properties such as the potential barrier and the Schottky barrier lowering effect of graphene nanoribbon contact. Thermionic emission effect and tunneling effect were considered in the development of the current-voltage characteristic. By using the analytical method, the analytical model for potential barrier, Schottky barrier lowering effect and the current-voltage characteristic of graphene nanoribbon Schottky diode are presented. Based on the obtained result, it is found that the potential barrier has a quadratic relationship relative to its depletion region width. Besides that, the Schottky barrier lowering effect is dependent on the applied voltage. Both of these two parameters, the potential barrier and the Schottky barrier lowering effect are temperature dependent. The presented current-voltage model was compared with silicon Schottky diode and germanium Schottky diode which shows different contact effect when different metal and semiconductor were used for Schottky contact. Moreover a comparison study between the presented model with experimental graphene Schottky diode shows that the presented model indicates similar leaning of the current-voltage characteristic with experimental work.

Photon-enhanced thermionic emission from heterostructures with low interface recombination

Photon-enhanced thermionic emission is a method of solar-energy conversion that promises to combine photon and thermal processes into a single mechanism, overcoming fundamental limits on the efficiency of photovoltaic cells. Photon-enhanced thermionic emission relies on vacuum emission of photoexcited electrons that are in thermal equilibrium with a semiconductor lattice, avoiding challenging non-equilibrium requirements and exotic material properties. However, although previous work demonstrated the photon-enhanced thermionic emission effect, efficiency has until now remained very low. Here we describe electron-emission measurements on a GaAs/AlGaAs heterostructure that introduces an internal interface, decoupling the basic physics of photon-enhanced thermionic emission from the vacuum emission process. Quantum efficiencies are dramatically higher than in previous experiments because of low interface recombination and are projected to increase another order of magnitude with more stable, low work-function coatings. The results highlight the effectiveness of the photon-enhanced thermionic emission process and demonstrate that efficient photon-enhanced thermionic emission is achievable, a key step towards realistic photon-enhanced thermionic emission based energy conversion.

A Physics-Based Effective Mobility Model for Polycrystalline Silicon Thin Film Transistor Considering Discontinuous Energy Band at Grain Boundaries

A physics-based effective mobility model for polycrystalline silicon (poly-Si) thin film transistor (TFT) is developed by considering the discontinuous energy band between grain and grain boundary and Gaussian energy distribution at grain boundary. Three conduction mechanisms, thermionic-emission effect, drift-diffusion effect and tunneling effect, are introduced to describe the mobility. It is found that the physical factors, such as the trapped state distribution at grain boundary, the additional potential barrier, the rate of carriers' thermal velocity to diffusion velocity and the in-grain quality, have a strong influence on the effective mobility of poly-Si TFT and would not be neglected or replaced by some fitting parameters. The proposed model provides an accurate and analytical description for the effective mobility of poly-Si TFT. The calculated results of the proposed model are verified by the experimental data.

Carrier capture into semiconductor quantum dots via quantum wire barriers: Localization and thermionic emission effects

Carrier transport and capture paths via barriers of different dimensionality in AlGaAs/GaAs quantum wire (QWR)/quantum dot (QD) heterostructures, grown in inverted pyramids, are studied by photoluminescence (PL) spectroscopy. Evidence for thermally activated diffusion related to potential disorder in the QWR barriers and thermionic emission of carriers from the QD into the QWR barrier is observed in temperature dependent PL spectra. Similar activation energies for the thermionic emission are derived from the continuous-wave and time-resolved PL spectroscopy.