Analytical treatment of cold field electron emission from a nanowall emitter

This paper presents an elementary, approximate analytical treatment of cold field electron emission (CFE) from a classical nanowall. A simple model is used to bring out some of the basic physics of a class of field emitter where quantum confinement effects exist transverse to the emitting direction. A high-level methodology is presented for developing CFE equations more general than the usual Fowler-Nordheim-type (FN-type) equations, and is applied to the classical nanowall. If the nanowall is sufficiently thin, then significant transverse-energy quantization effects occur, and affect the overall form of theoretical CFE equations; also, the tunnelling barrier shape exhibits "fall-off" in the local field value with distance from the surface. A conformal transformation technique is used to derive an analytical expression for the on-axis tunnelling probability.Comment: 48 pages, 4 figure

Field electron emission characteristic of graphene

The field electron emission current from graphene is calculated analytically on a semiclassical model. The unique electronic energy band structure of graphene and the field penetration in the edge from which the electrons emit have been taken into account. The relation between the effective vacuum barrier height and the applied field is obtained. The calculated slope of the Fowler-Nordheim plot of the current-field characteristic is in consistent with existing experiments.Comment: 18 pages, 5 figures Copyright (2011) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. This article appeared in (J. Appl. Phys. 109 (2011) 044304) and may be found at (http://link.aip.org/link/?JAP/109/044304

Screening effects on field emission from arrays of (5,5) carbon nanotubes: Quantum-mechanical simulation

The simulation of field electron emission from arrays of micrometer-long open-ended (5, 5) carbon nanotubes is performed in the framework of quantum theory of many electrons. It is found that the applied external field is strongly screened when the spacing distance is shorter than the length of the carbon nanotubes. The optimal spacing distance is two to three times of the nanotube length, slightly depending on the applied external fields. The electric screening can be described by a factor that is a exponential function of the ratio of the spacing distance to the length of the carbon nanotubes. For a given length, the field enhancement factor decreases sharply as the screening factor larger than 0.05. The simulation implies that the thickness of the array should be larger than a value but it does not help the emission much by increasing the thickness a great deal

Atomic decoration for improving the efficiency of field electron emission of carbon nanotubes

The field electron emission from the single-walled carbon nanotubes with their open ends terminated by -BH, -NH, and -O has been simulated. The apex-vacuum barrier and the emission current have been calculated. It has been found that -BH and -NH suppress the apex-vacuum barrier significantly and lead to higher emission current in contrast to the -O terminated structure in the same applied field. The calculated binding energy implies that the carbon nanotubes terminated with -BH and -NH are more stable than those saturated by oxygen atoms or by hydrogen atoms.Comment: 8 pages, 9 figures, LaTeX; content changed, typos corrected, references adde

Simulation for field emission images of micrometer-long SWCNTs

The electron distribution of open-ended single-walled carbon nanotubes with chirality indexes (7,0) and (5,5) in the field emission conditions was calculated via a multi-scaled algorithm. The field emission images were produced numerically. It was found that the emission patterns change with the applied macroscopic field. Especially, the symmetry of the emission pattern of the (7,0) carbon nanotube is breaking in the lower field but the breaking is less obvious in the higher field. The enlargement factor increases with the applied macroscopic field.Comment: 8 pages, 4 figure

Using potassium catalytic gasification to improve the performance of solid oxide direct carbon fuel cells: Experimental characterization and elementary reaction modeling

The performance of a solid oxide electrolyte direct carbon fuel cell (SO-DCFC) is limited by the slow carbon gasification kinetics at the typical operating temperatures of cell: 650–850 °C. To overcome such limitation, potassium salt is used as a catalyst to speed up the dry carbon gasification reactions, increasing the power density by five-fold at 700–850 °C. The cell performance is shown to be sensitive to the bed temperature, emphasizing the role of gasification rates and that of CO production. Given the finite bed size, the cell performance is time-dependent as the amount of CO available changes. A reduced elementary reaction mechanism for potassium-catalyzed carbon gasification was proposed using kinetic data obtained from the experimental measurements. A comprehensive model including the catalytic gasification reactions and CO electrochemistry is used to examine the impact of the catalytic carbon gasification process on the device performance. The power density is maximum around 50% of the OCV, where carbon utilization is also near maximum. Results show that bed height and porosity impact the power density; a thicker bed maintains the power almost constant for longer times while lower porosity delivers higher power density in the early stages.National Natural Science Foundation (China) (20776078)National Natural Science Foundation (China) (51106085)Low Carbon Energy University Alliance (LCEUA) (Seed Funding

Industry-scale production of a perovskite oxide as oxygen carrier material in chemical looping

How to upscale the production of oxygen carrier particles from laboratory level to industrial level is still challenging in the field of chemical looping. The upscaled oxygen carrier must maintain its physical and chemical properties. In the present contribution, a spray drying granulation protocol was developed to produce a perovskite oxygen carrier (CaMn0.5Ti0.375Fe0.125O3-δ) at an industrial scale. The micro-fluidized bed thermogravimetric (MFB-TGA) experiments were performed to measure the oxygen uncoupling and redox reaction kinetics under the fluidization state with enhanced heat and mass transfer, and the obtained experimental data at different temperatures were fitted by a fluidized-bed reactor coupled with a semi-empirical kinetic model. The physical and chemical properties of granulates were compared with those of the same perovskite composition prepared at the laboratory level. The results show the volume fraction of particle size at 75–500 μm is greater than 90% for the upscaled granulats, and the particles show no degradation in reactivity and no agglomeration for more than 20 redox cycles at high temperatures. The heterogeneous reaction rates are high, especially for the oxidation, e.g. it only spent ∼ 5 s to achieve full oxidation. Low attrition index of 3.74 wt% was found after the five-hour attrition test. The industrial-scale particles possess similar chemical and physical properties as the laboratory-scale particles with regards to the reaction kinetics, attrition index, crystalline phase, etc. The required bed inventories and fan energy consumption were finally estimated and found to be lower than other oxygen carriers reported in the literature.acceptedVersio