场致爆炸发射阴极表面微凸起热效应的数值研究

研究场致发射电流密度与阴极表面微凸起顶部的温度之间的高度非线性关系,用数值方法计算在外加宏观电场条件下,不同顶底半径比微凸起的热效应.结果表明:当微凸起顶底半径比值不同时,由于微凸起的微观电场增强因子不同,当微凸起顶部温度达到阴极材料的熔点时,微凸起内部温度分布差异显著;当外加宏观电场相同时,微凸起的顶底半径比越小,爆炸发射延迟时间越短;对于顶底半径比确定的微凸起而言,爆炸电子发射延迟时间随外加宏观电场强度的减小而成指数规律增长.     

空间电荷效应对阴极微凸起热不稳定性的影响

 由于金属微凸起爆炸电子发射的预发射电流密度一般都超过108 A/cm2，因此必须考虑其空间电荷效应的影响。基于金属微凸起爆炸电子发射起始过程模型，通过理论分析和数值模拟，给出了考虑预发射电流空间电荷效应的微凸起爆炸发射延迟时间随二极管平均电场的变化关系。与不考虑预发射电流空间电荷效应的结果进行对比表明，预发射电流的空间电荷效应可以显著增加金属微凸起的爆炸发射延迟时间。     

阴极微凸起形状对热不稳定性的影响

为了研究阴极微凸起形状对其热不稳定性的影响,采用数值模拟方法研究了不同外加电场条件下,圆柱、圆台和圆锥形等不同形状微凸起的热不稳定性发展过程。结果显示:对于不同形状的微凸起,当微凸起顶部温度达到阴极材料的熔点时,微凸起内部温度分布差异显著,随着微凸起形状由圆柱-圆台-圆锥形变化,微凸起内部温度接近材料熔点的部位越来越少;外加电场相同时,微凸起形状越接近圆锥形,爆炸电子发射延迟时间越长;在阴极表面电场强度高于11 GV/m时,爆炸电子发射延迟时间随着微凸起顶底半径比值的减小或阴极表面电场强度的下降近似成指数规律增长。     

碳纳米管场致发射中的空间电荷效应

采用微波等离子体化学气相沉积(MWPCVD)方法成功制备以碳纳米管束为单元的场致发射阵列,获得很好的场致发射电流发射特性,在电流密度较大时,发现I-V特性偏离由Fowler-Nordheim公式计算出的结果。采用Electron Beam Simulation(EBS)软件进行模拟分析发现:在电流密度较低时,I-V特性能很好与F-N公式吻合。但碳纳米管尖端电流密度大于106A/cm2时,碳纳米管尖端处的有效电场强度受空间电荷的影响比较明显,进而对碳纳米管的场致发射特性显现出不可忽略的影响,此时碳纳米管的发射电流密度开始受到空间电荷的限制。     

爆炸电子发射初期阴极表面电场的研究

为了研究二极管爆炸电子发射初始阶段阴极表面复杂的物理现象及规律, 建立了由场致电子发射阴极构成的一维平板真空二极管物理模型,通过自行编程数值求解泊松方程, 考虑了发射出的电子对阴极表面电场的非线性影响,自洽模拟得到了阴极表面电场随时间的变化情况. 模拟结果表明,爆炸电子发射初期,阴极表面电场随时间的增加而呈现出不断振荡的规律, 且振荡幅度越来越小,最终到达一个稳态的值,二极管两极板之间的外加电场越大, 阴极表面稳态电场的绝对值越大;电场增强系数越大,阴极表面稳态电场的绝对值越大. 在整个时间演变过程中,阴极表面的实际电场强度决定着阴极发射的电流密度大小, 反过来阴极发射的电流密度又会影响到阴极表面的电场.     

二极管间隙距离对场致发射过程中空间电荷效应的影响

在强电场条件下,由阴极通过场致发射产生的电子具有很强的空间电荷效应,因此真空二极管的空间电荷限制电流是设计高功率微波源等强流电子束器件时需要考虑的重要参数.场致发射电流密度只和阴极材料、阴极表面电场等有关,而空间电荷效应则会受二极管电压、间隙距离等因素的影响.为研究二极管间隙距离对场致发射过程中空间电荷效应的影响,建立了由场致发射阴极构成的一维平板真空二极管物理模型,利用第一性原理的粒子模拟方法,研究了二极管间隙距离和外加电压等参数变化时的阴极表面电场随时间的演变特性,得到了阴极表面稳态电场和二极管间隙距离之间的关系.结果表明,场致发射过程开始后,阴极表面电场先有个振荡过程,随后趋于稳定;在同一外加电场条件下,间隙距离越长,稳态电场的绝对值越小,且达到稳态所需的时间也越长;间隙距离越短,当阴极表面电场达到稳定状态时,二极管间隙区的电场分布变化越剧烈.     

空间电荷效应对热场致发射中诺廷汉效应的影响

热场致发射阴极所产生的强流电子束具有很强的空间电荷效应,为研究该效应对热场致发射过程中诺廷汉（Nottingham）效应的影响机理,在理论分析的基础上,用数值方法研究了不同逸出功和多个外加电场条件下考虑空间电荷效应对诺廷汉效应结果的影响,并与不考虑空间电荷效应时的情形进行了对比. 结果表明：空间电荷效应的强弱会显著影响到阴极表面的稳态电场,进而对诺廷汉效应产生不可忽略的影响;当逸出功在3.0–4.52 eV、外加电场在3×10⁹–9×10⁹ V/m范围内时,考虑空间电荷效应的影响后,热场致发射电子所带走的平均能量较不考虑空间电荷效应时增加0–2.5 eV,且温度越高或外加电场越大时,该增加值越大;考虑空间电荷效应对诺廷汉效应的影响后,热场致发射电子从阴极带走的平均能量随外加电场的增加呈非线性下降规律;当阴极表面温度较高时,诺廷汉效应中的冷却效应随二极管间隙距离的变大而增强. 关键词： 热场致发射 诺廷汉效应 空间电荷效应 阴极表面电场     

高温超导体超导态的场致发射

高温超导体处在临界温度以下时，能隙会引起体内电子分布的改变。本文在考虑到能隙对体内电子分布影响时，分析了高温超导体的场致发射电流密度和场致发射电子能谱。     

微波射频场在微波管中产生爆炸电子发射特性初步分析

 对微波射频场在微波管内引起的场致发射和爆炸电子发射及等离子体的产生进行了分析,推导了等离子体产生强度与微波振幅、材料的电阻率、热传导系数、质量密度和比热容之间的关系,得到了晶须温度分布的表达式,通过数值解析的方式总结出在远大于微波周期的时间尺度上晶须温度提高随时间线性上升。在模型所述材料特性下,温度的上升率达到了3.22×10¹⁰ ℃/s,在100 ns量级就可以使晶须发生气化形成等离子体。     

表面微结构对碳化硅晶须掺杂石墨阴极爆炸电子发射性能的影响

爆炸发射阴极已广泛应用于高功率微波源,但常规场致爆炸发射阴极存在使用寿命短或电子发射不均匀的问题,改善阴极材料是解决这一问题的有效途径.本文将碳化硅晶须掺杂到石墨中制备得到阴极,从二极管电流波形上升沿和输出微波脉宽产生的变化着手,分析了碳化硅晶须掺杂石墨阴极表面材料成分和微观形貌对其电子发射性能的影响机理.研究发现,碳化硅晶须的存在,不仅有利于阴极场发射的快速启动、发射微点数目增多,还有利于降低等离子体膨胀速度、抑制脉冲缩短现象,使得输出微波脉宽增大.随着脉冲发射数量增多,碳化硅晶须掺杂石墨阴极表面被等离子体不断"抛光",微凸起形状因子减小、均匀性提高,场发射启动速度减慢,但输出微波脉宽增大.     

丝网印刷制备碳纳米管阴极的强流脉冲发射特性研究

采用丝网印刷法制备了一种大面积的碳纳米管阴极,表征了阴极表面碳纳米管的形貌及分布.研究了该阴极在不同脉冲条件下的高压脉冲发射特性,分析了发射时阴极面等离子体产生和发射点的分布.研究表明：碳纳米管阴极的脉冲发射机制为爆炸电子发射,在平均场强为16.7V/μm的单脉冲电场下,阴极的最高发射电流密度为99 A/cm².在平均场强为15.4 V/μm的双脉冲电场下,阴极的最高发射电流密度为267 A/cm².碳纳米管阴极可以作为强流电子束源在高能微波器件中得到应用. 关键词： 强流脉冲电子束 碳纳米管 阴极 丝网印刷     

阴极金属微凸起电场增强因子数值模拟

 利用有限元方法对无穷大平板二极管中阴极表面金属微凸起的微观电场增强因子进行了计算研究，给出了微凸起微观电场增强因子随凸起参数的变化规律，拟合得到了微观电场增强因子的简单实用的经验表达式，并与文献中的近似公式进行了比较。模拟的结果表明，对锥状球头微凸起的电场增强因子，文献给出的近似公式误差较大，应避免使用。     

Study of field-induced explosive electron emission mechanisms ofpseudospark superemission cathodes

Various mechanisms of electron emission, including the field, field-enhanced thermionic, and explosive electron emissions from pseudospark cathodes, are discussed and compared. The mechanism of the field-induced explosive electron emission due to microstructure on the cathode surface is considered to be more likely the pseudospark superemissive mechanism. A high-mean electric field up to 3-5 MV/cm on the cathode surface in the end of hollow cathode phase is enough to initiate the mechanism. The cathode spot initiation delay time (<10 ns) and explosive emission threshold current (~10⁸ A/cm² ) prior to the high current conducting phase are given by solving the initial value problem of the one-dimensional heat conduction equation, thus explaining the existing experimental data of the pseudospark cathode superemission. In the case of multigap discharge, the above mechanism occurs on nearly all cathode and interelectrode surfaces. Experimental evidence in single- and multigap pseudospark discharges supports the suggested explanation     

Thermionic field emission of electrons from metals and explosive electron emission from micropoints

We suggest a general approach to considering the thermionic, field, and thermionic field emissions of electrons from metals. For this purpose, based on the standard model of free electrons in a metal, we suggest a numerical method for determining the transmission coefficient through the potential barrier at the metal-vacuum interface suitable for an arbitrary barrier. This method is free both from the approximations based on the saddle-point approximation and characteristic of the analytical models for thermionic emission and from the approximations for the tunneling coefficient through the potential barrier characteristic of the models for field emission. Based on numerical simulations, we determine the thermal effect of the emission and ascertain that a very sharp transition from surface cooling by electron emission to heating occurs at certain electric field and temperature. We explain the triggering mechanism of the explosive electron emission observed during micropoint explosions by this phenomenon. The explosive emission is shown to begin when the level of the potential barrier at the micropoint tip drops below the Fermi level in the metal.     

Autoelectronic emission of metallic films induced by picosecond pulses of infrared laser radiation

The well-known defects of the surface of a solid, microscopic projections and spikes, play a decisive role in electron emission induced by an electric field. If there are mobile electrons of holes in the solid, then the electric field is enhanced by a factor of 10–100 at the tip of a microscopic projection. This effect was discovered in electrostatics more than a century ago. In turn, the probability of tunnel emission of an electron from a metal into a vacuum is an exponential function of the electric field strength. Correspondingly the electron emission current density at the tip of a microscopic spike can be larger than that on a smooth surface by an astronomical factor. This effect is particularly strikingly manifested when picosecond pulses of infrared laser radiation of moderate power are used to initiate autoelectronic emission. Relative to a smooth surface, the emission current density is enhanced by hundreds of orders of magnitude. These experimental conditions can be used to scan the surface of a conducting material with a laser beam and to detect all the microscopic projections, in order to male detailed observations with subpicosecond time resolution of the phase transition from autoelectronic emission to explosive emission. Polytechnic University, Tomsk. Institute of Electrophysics, Ural Branch of the Russian Academy of Sciences. French National Scientific Center, Saclay, France. Translated from Izvestiya Vysshikh Uchenbnykh Zavedenii, Fizika, No. 11, pp. 42–44, November, 1997.     

Field emission property of copper nitride thin film deposited by reactive magnetron sputtering

Copper nitride (Cu₃N) thin film was deposited on silicon (Si) substrate by reactive magnetron sputtering method. X-ray diffraction measurement showed that the film was composed of Cu₃N crystallites with anti-ReO₃ structure and exhibited preferential orientation of [1 0 0] direction. The field emission (FE) result showed that Cu₃N film had a turn-on electric field of about 3 V/μm at a current density of 1 μA/cm² and a current density of 700 μA/cm² was obtained at the electric field of 24 V/μm. The emission mechanism inferred by Fowler-Nordheim (FN) plot is shown as following: thermal electron emission at low field region and tunneling electron emission at high field region.