Papers by Keyword: Thermionic Emission

Abstract: Tungsten is the dominant resources in China, and its reserves and consumption ranks the first place in the world. The tungsten electrode material, as an important manufacture of tungsten product, widely used in metal inert-gas welding, plasma welding, cutting, plasma spraying, is a kind of important industrial material [1]. The radioactive thorium tungsten electrode material is now gradually substituted by rare earth tungsten electrode material since it is excellent in thermal electron emission property. Through many years' study, we have mastered the key technology for the tungsten electrode material. After the extension and application of the key preparation technology, the authors group has developed the rare earth tungsten electrode materials and rare earth tungsten products, and the adjustment of industrial structure and upgrading of the product been realized.

Abstract: The recently developed bombardment induced ion transport (BIIT) technique is reviewed. BIIT is based on shining an energy-selected alkali ion beam at the surface of a sample of interest. Attachment of these ions leads to the build-up of a surface potential and a surface particle density. This in turn generates the corresponding gradients which induce ion transport towards a single metal electrode connected to the backside of the sample where it is detected as a neutralization current. Two different versions of BIIT are presented, i.) the native ion BIIT and ii.) the foreign ion BIIT. The former is demonstrated to provide access to absolute ionic conductivities and activation energies, the latter leads to the generation of electrodiffusion profiles. Theoretical modelling of these concentration profiles by means of the Nernst-Planck-Poisson theory allows to deduce the concentration dependence of diffusion coefficients.

Abstract: Thermionic electron emitters based on doped diamond films have shown significant emission at less than 500°C. Results have established that it is necessary to control the electron affinity, doping levels and concentration, and band bending, and these properties have been achieved with engineered multilayered structures with controlled morphology, doping and substrate. Recently, visible light photo-electron emission has been demonstrated using the same diamond film emitters. This report presents a spectroscopic and surface electron microscopy study of photo-and thermionic emission from nitrogen doped diamond films with controlled morphology on metal substrates. Electron emission spectra were recorded to 500°C, while illuminated with sub diamond band gap light. Significant photo-induced emission was observed with an efficiency greater than metal photo cathodes.

Abstract: In this work, n-CdS/p-CuFeO₂ heterojunction diode was fabricated by thermal evaporating CdS thin films on 1 mm thick-CuFeO₂ ceramic substrate with substrate temperature kept at 373 K during evaporation process. The forward current-voltage characteristics of n-CdS/p-CuFeO₂ heterojunction in a temperature range of 100-300 K were investigated to determine the electrical parameters and conduction mechanism. It was found that, at forward bias below 0.5 V, the conduction mechanism of the diode is dominated by thermionic emission (TE) mechanism. At bias voltage above 0.5 V, the current transport is due to space charge limited current (SCLC) controlled by an exponential trap distribution in the band gap of CdS. The temperature dependence of the saturation current and ideality factor are well described by tunneling enhanced recombination at junction interface with activation and characteristic tunneling energy values as about 1.79 eV and E₀₀ = 86 meV, respectively. The value of interface state density (N_ss) evaluated from capacitance spectroscopy increases from 2.09x10¹¹ eV^-1cm^-2 (at 300 K) to 2.70x10¹¹ eV^-1cm^-2 (at 363 K). Free carrier concentration of 5.80x10¹³ cm^-3 at room temperature was estimated from capacitance-voltage measurements at 50 kHz.

Abstract: In this paper, a comparative study of temperature effect which introduces a thermionic current under a high applied electric field, on three different modes of field emission current, such as Tunneling current, Fowler-Nordheim current and Field emission current in between these two regions has been done. Moreover, an idea of micromechanical displacement sensor with high sensitivity, operating in Fowler-Nordheim current mode, has been proposed. The displacement sensitivity of proposed sensor in Fowler-Nordheim current domain is about 10-9 m/A. The displacement sensitivity has been shifted from its expected value due to thermal effect (at 700K temperature) at about 1010V/m applied electric field across tip gap.

Abstract: A new GaAs/InGaAs triangular barrier optoelectronic switch combined with tri-state characteristic is fabricated and demonstrated. Two GaAs/InGaAs barriers are employed to provide potential barriers for electron thermionic emission and hole confinement, respectively. Applying a sufficient DC voltage to this device, a double S-shaped negative differential resistance (NDR) phenomenon with nearly equal switching voltage difference is appeared at room temperature. This unique NDR property can be introduced to triple stable regions into the device circuit design. Based on a proper circuit design with suitable load line, the studied device has potential for triple-logic applications.

Abstract: With recent research, the author intends to outline the framework of the field emission of ZnO nanostructures. However, many groups’ reports ignored the thermionic emission process in the low electric field. A recently published field emission cathode parameter extraction method [X. He et al., J. Appl. Phys. 102, 056107(2007)] provided unambiguous and reliable cathode. The method utilized Richardson-Laue-Dushman law in low electric field and Fowler-Norheim equation in high electric field to solve a one-dimensional model including both thermionic and field emission. The model gave a much better agreement with the experimental data of ZnO cathode under the applied field and acquired a revised surface field enhancement factors and work function of ZnO cathode in the electron emission process.

Abstract: Rare-earth tungsten thermionic emission material was used in the high-temperature electrostatic dust removal technology. A static test facility was self-designed to explore the emission characteristic. XRD, SEM and EDS were used to analyze the microstructure and element distribution. Working in high-temperature and oxidizing condition, tungsten in the thermionic emission materials is oxidized to tungsten trioxide, but the rare-earth tungstate in the grain boundary has no chemical changes. In high-temperature and oxidizing ambience, there is oxidation and volatilization on the surface of rare-earth tungsten, resulting in materials loss. The degree of loss depends on the composition of gas.

Abstract: Vacuum thermionic energy conversion achieves direct conversion of heat into electrical energy. The process involves thermionic electron emission from a hot surface and collection of the electrons on a cold surface where the two surfaces are separated by a small vacuum gap. Results are presented which indicate that nanocrystalline diamond films could lead to highly efficient thermionic energy conversion at temperatures less that 700°C. A critical element of the process is obtaining a stable, low work function surface for thermionic emission. Results are presented which establish that N-doped diamond films with a negative electron affinity can exhibit a barrier to emission of less than 1.6 eV. Films can be deposited onto field enhancing structures to achieve an even lower effective work function. Alternatively, nanocrystalline diamond films prepared with S doping exhibit field enhanced thermionic emission and an effective work function of ~1.9 eV. The field enhanced structures can reduce the effect of space charge and allow a larger vacuum gap. The possibility of a low temperature nanocrystalline diamond based thermionic energy conversion system is presented.

Abstract: This paper presents a simple and novel model for low-frequency noise generation in polycrystalline-Si resistors within the number fluctuation model. The grain boundary in polycrystalline-Si thin films is the major source of noise and is modeled as independent symmetric Schottky barriers in series, face-to-face. It has been found that trapping and detrapping of the carriers at the traps in the space charge region of the grain boundary via thermal activation modulate the barrier height and generate the low-frequency noise. The model successfully explains the experimental data and gives useful information about the defects in the space charge region of the grain boundary. As a result, the Hooge parameter is interpreted in terms of defect density, among other parameters.