A comparative study of thermionic emission from vertically grown carbon nanotubes and tungsten cathodes

Thermionic emission from vertically grown carbon nanotubes (CNTs) by water-assisted chemical vapor deposition (WA-CVD) is investigated. I-V characteristics of WA-CNT samples exhibit strong Schottky effect leading to field proportionality factor β ∼ 10⁴ cm⁻¹in contrast to β ∼ 200 cm⁻¹ for the bare tungsten substrate. Non-contact atomic force microscopy imaging of CNT samples show propensity of nanoasperities over a scale of micron size over which the tungsten surface is seen to be atomically smooth. The values of root mean-square roughness for CNTs and W were found to be 24.2 nm and 0.44 nm respectively. The Richardson-Dushman plots yield work function values of Φ_CNT ? 4.5 and Φ_W ? 4.3 eV. Current versus time data shows that CNT cathodes are fifteen times noisier than tungsten cathode presumably due to increased importance of individual atomic events on the sharp CNT tips of bristle like structures. Power spectral density of current exhibited 1/f^ξ behavior with ξ ? 1.5, and 2 for W and CNTs. The former suggests surface diffusion whereas the latter indicates adsorption/desorption of atomic/molecular species as a dominant mechanism of noise generation.     

Theory of thermionic emission

A comprehensive theory of thermionic emission from clean metal surfaces is presented. The theory takes into account the energy band structure of the metal, inelastic scattering due to electron-electron collisions and the thermal vibration of the atoms. We applied the theory to thermionic emission from Cu(100). We calculated the thermally emitted current from this plane as a function of applied field. We find an almost periodic deviation from the Schottky line, similar in nature with that which is observed in emission from polycrystalline emitters [1]. We believe that accurate measurements of the amplitude and phase of these deviations from the Schottky line can, when analysed in the manner described here, provide valuable information on the surface optical potential. We have also calculated the total energy distribution of the emitted electrons for a typical value of the applied field. The dependence of the above measurable quantities on the parameters which enter the theory is analysed and demonstrated by explicit numerical calculations.     

Nano-textured W shows improvement of thermionic emission properties

Laser-assisted nano-texturing of W substrates cathodes via ablation in liquid environment is experimentally realized. Two laser sources are used, either a Ti:sapphire femtosecond laser or a Nd:YAG laser with pulse duration of 350 ps. Laser exposure of W results in the formation of hemi-spherical nanostructures situated on top of periodic ripples. Nano-textured thermionic W cathode demonstrates the decrease of the efficient work function by 0.3 eV compared to pristine surface.     

Effect of emission center density on the rate of degradation processes in thin-film MIM cathodes

The effect of the density of emission centers on the rate of degradation of these centers is studied for MIM systems with different materials of the upper electrode. It is shown that the degradation rate decreases sharply with increasing density of the emission centers. The obtained result is described by using the concepts about the nature of electronic moulding and the degradation mechanism of emission centers.Institute of Automatic Control Systems and Radioelectronics, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 95–98, June, 1992.     

Laser-photofield emission from needle cathodes for low-emittance electron beams

Illumination of a ZrC needle with short laser pulses (16 ps, 266 nm) while high voltage pulses (-60 kV, 2 ns, 30 Hz) are applied, produces photo-field emitted electron bunches. The electric field is high and varies rapidly over the needle surface so that quantum efficiency (QE) near the apex can be much higher than for a flat photocathode due to the Schottky effect. Up to 150 pC (2.9 A peak current) have been extracted by photo-field emission from a ZrC needle. The effective emitting area has an estimated radius below 50 microm leading to a theoretical intrinsic emittance below 0.05 mm mrad.     

多晶LaB6阴极的脉冲热发射特性

 采用多晶LaB₆材料制成平板二极管阴极，阳极采用钼材料，阴极采用热传导与热辐射加热，加热体为石墨。实验研究了不同阴极温度、不同真空度下的脉冲发射特性，并对热发射稳定性进行了分析。结果表明： 在动态真空系统中， 阴极发射面积为0.012 1 cm²， 工作真空度为2×10^-4 Pa， 阴极温度分别为1 600， 1 650和1 700 ℃，在脉冲宽度为40 ms、重复频率为107 Hz的条件下，最大脉冲发射电流密度分别为34.0，44.0和53.8 A/cm²; 2×10^-4，5×10^-4和2×10^-3 Pa压强下的发射能力没有明显的差异;脉冲宽度的变化不影响发射电流密度的变化。     

钡-钨阴极的组织特性与热电子发射性能

利用正交试验探讨了放电等离子技术工艺参数（温度、压力、保温时间）对钡-钨（Ba-W）阴极中的W的孔隙度的影响规律,获得了孔隙率在23%～30%内变化时所需要的最佳工艺参数。在此基础上,制备出了具有不同孔隙度的球形W基体和普通不规则的W基体。研究表明：球形多孔W颗粒间堆积、排列有序,无闭孔,孔径分布集中而均匀,在26.3%的孔隙度下中值孔径为1.41 μm;机械性能方面,球形钨粉基体维氏硬度低于传统普通不规则钨多孔体。在脉宽10 μs、频率1000 Hz的条件下,阴极脉冲发射电流密度随着孔隙度的增大,先增大后减小。当基体孔隙度为26.3%时,阴极电流发射密度最大,在1050 ℃,偏离点发射电流密度可达24.62 A/cm²,零场发射电流密度为7.62 A/cm²,功函数为1.95 eV。     

Time-dependent spectrum of thermionic emission from hot C60

Experimental measurements of time-dependent photoelectron spectra observed in thermionic emission of hot C₆₀ excited by multiphoton absorption are presented. Time resolved velocity-map imaging is used to record photoelectron spectra and to disentangle direct and delayed processes. The evolution of the kinetic energy distribution of thermal electrons as a function of the delay after multiphoton excitation is described within the general formalism of the detailed balance theory. Experimental spectra obtained in the near-UV are in excellent agreement with the assumption of thermal equilibrium.     

多晶LaB6阴极的脉冲热发射特性

采用多晶LaB6材料制成平板二极管阴极,阳极采用钼材料,阴极采用热传导与热辐射加热,加热体为石墨。实验研究了不同阴极温度、不同真空度下的脉冲发射特性,并对热发射稳定性进行了分析。结果表明： 在动态真空系统中, 阴极发射面积为0.012 1 cm2, 工作真空度为2×10-4 Pa, 阴极温度分别为1 600, 1 650和1 700 ℃,在脉冲宽度为40 ms、重复频率为107 Hz的条件下,最大脉冲发射电流密度分别为34.0,44.0和53.8 A/cm2; 2×10-4,5×10-4和2×10-3 Pa压强下的发射能力没有明显的差异;脉冲宽度的变化不影响发射电流密度的变化。     

A thermionic ring diode for high resolution Rydberg state spectroscopy

The design and optimum performance of a thermionic ring diode is described which increases the detection sensitivity and reduces the influence of electric stray fields considerably. It thus allows for high resolution spectroscopy of Rydberg states with principal quantum numbers n ? 200.     

重复频率爆炸发射阴极现状及展望

重复频率爆炸发射阴极(EEC)是产生高功率重复频率强流电子束的关键。阐述了爆炸发射的尖端场增强和闪络引发机制,分析了影响EEC重复频率运行的因素,综述了EEC研究现状并总结了几种EEC特性,指出实现重复频率发射阴极的方法在于探索新型材料和改进结构。提出采用六硼化镧(LaB6)作为EEC材料和适用于单焦斑多幅闪光照相工业冷阴极二极管(IXD)的新型阴极结构轮辐状金属-铁电陶瓷复合阴极,并对其前景进行展望。     

爆炸发射阴极特性的研究进展

文章对爆炸发射阴极的物理机制进行了阐述,着重对爆炸发射阴极的各种特性进行了介绍,包括阈值电压、发射均匀性、等离子体闭合速率、阴极寿命等.并且对研究爆炸发射阴极特性所采用的理论和测量方法进行了介绍,给出了目前阴极材料使用中存在的问题.     

钡钨阴极优化与热电子发射性能

分别从基体和铝酸盐两方面优化了钡钨阴极.在基体方面,首先采用窄粒度钨粉结合放电等离子体烧结获得了孔径分布窄的基体;再利用射频等离子体球化技术制备了球形钨粉,采用球形钨粉制备了多孔基体,获得了孔通道光滑、内孔连通性好、孔径分布更加窄的基体.与窄粒度钨粉基体相比,球形钨粉制备的阴极,空间电荷限制区的斜率由1.25增加至1.37,发射均匀性得到提高,拐点电流密度由6.6 A·cm^–2增至6.96 A·cm^–2.在此基础上,采用液相法改善了铝酸盐物相组成,发现空间电荷限制区的斜率增加至1.44,拐点电流密度增加至21.2 A·cm^–2.通过理论计算对钡钨阴极发射的物理本质进行了研究,发现钡钨阴极发射规律遵循偶极子理论.     

爆炸发射阴极特性的研究进展

文章对爆炸发射阴极的物理机制进行了阐述，着重对爆炸发射阴极的各种特性进行了介绍，包括阈值电压、发射均匀性、等离子体闭合速率、阴极寿命等．并且对研究爆炸发射阴极特性所采用的理论和测量方法进行了介绍，给出了目前阴极材料使用中存在的问题．     

天鹅绒与碳纳米管阴极强流脉冲发射特性

在2 MeV直线感应加速器注入器平台上开展了天鹅绒阴极与碳纳米管阴极的强流脉冲发射特性综合实验。研究结果表明:天鹅绒阴极与碳纳米管阴极均具有强流脉冲发射性能,在1.61 MV的二极管电压下,天鹅绒阴极与碳纳米管的发射电流密度分别为84,108 A/cm2;启动时间分别为21,40 ns;放气量分别为0.29,0.91 PaL;放出气体分子数目与发射电子数目之比分别为64,225;两种冷阴极强流脉冲发射时的放气质谱相似。     

天鹅绒与碳纳米管阴极强流脉冲发射特性

在2 MeV直线感应加速器注入器平台上开展了天鹅绒阴极与碳纳米管阴极的强流脉冲发射特性综合实验。研究结果表明：天鹅绒阴极与碳纳米管阴极均具有强流脉冲发射性能,在1.61 MV的二极管电压下,天鹅绒阴极与碳纳米管的发射电流密度分别为84,108 A/cm2;启动时间分别为21,40 ns;放气量分别为0.29,0.91 PaL;放出气体分子数目与发射电子数目之比分别为64,225;两种冷阴极强流脉冲发射时的放气质谱相似。     

Enhanced two temperature modeling of ultrashort laser ablation for the investigation of thermionic emission characteristics

Characteristics of thermionic electron emission during femtosecond laser ablation of gold film are studied numerically. For the rigorous calculation of electron and lattice temperatures, an enhanced two-temperature model with transient thermal and optical properties is developed and it is demonstrated that the model predicts the damage threshold fluences closely matching experimental data. From the calculated electron emission characteristics, quantum efficiency and electron current density are estimated.