碳纳米管薄膜场蒸发效应

在超高真空系统中对基于丝网印刷方法制备的碳纳米管薄膜的场蒸发效应进行实验研究. 实验发现, 碳纳米管薄膜样品存在场蒸发现象, 蒸发阈值场在10.0-12.6 V/nm之间, 蒸发离子流可以达到百皮安量级; 扫描电子显微镜分析和场致电子发射测量结果表明, 场蒸发会使碳纳米管分布变得更加不均匀, 会导致薄膜的场致电子发射开启电压上升(240→300V)、场增强因子下降(8300→4200)、蒸发阈值场上升(10→12.6V/nm), 同时使得薄膜场致电子发射的可重复性明显变好. 场蒸发也是薄膜自身电场一致性修复的表现, 这种修复并非表现在形貌上, 而是不同区域场增强因子之间的差距会越来越小, 这样薄膜场致电子发射的可重复性和稳定性自然会得到改善.     

A note on evaporation from heated spikes

We have investigated the effect of heat loss through evaporation on the surface temperature profile and the evaporation yield of an ion-induced spike. We derive a three-dimensional extension of a nonlinear integral equation first found by Mann and Wolf to describe the temperature profile in a semiinfinite medium in the presence of heat loss through the surface. The equation has been solved by perturbation expansion in powers of the evaporation rate. For heavy-ion induced, cylindrical elastic-collision spikes, noticeable but moderate corrections are found to evaporation yields estimated previously by neglecting heat loss due to evaporation. These results are relevant mainly to sputtering of metals by heavy atomic and molecular ion bombardment. Comments are also made on sputting of insulators both by heavy keV ions and by ionizing particles. Expressions for an effective sputter time and sputter area are derived for cylindrical geometry; both quantities turn out independent of the initial spike temperature. The sputter radius is normally greater than the depth of the crater formed; we conclude that the influence of crater formation on the evaporation yield is normally negligible.On leave from Institut für Theoretische Physik, Technische Universität, D-3300 Braunschweig, Fed. Rep. Germany     

Characteristic features of the emission properties of pulsed broad-beam ion and plasma sources utilizing the evaporation of metal by a vacuum arc

An experimental confirmation was obtained of the anode potential fall effect in pulsed broad-beam ion and plasma sources utilizing the evaporation of metal by a vacuum arc. An increase in the overall voltage across the arc discharge was discovered. The investigations demonstrated that the magnitude of the positive anode fall depends on the structural features of the ion source and are determined by the ratio of the plasma flux directed onto the lateral surface of the anode to the total plasma flux from the cathode spot. It was established that the anode fall effect is controlled and makes it possible to influence the homogeneity of the ion current distribution over the beam cross section, the efficiency of extracting ions from the plasma, and the charge composition of the ion flux.Scientific-Research Institute of Nuclear Physics, Polytechnic University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 82–92, February, 1994.     

Radiation-assisted formation of microprobes for scanning tunnel microscopes

The formation of tipped tungsten microprobes by ion bombardment in high electric fields with subsequent low-temperature field evaporation is reported. During irradiation, the current density was shown to rise nonmonotonically because of heavy particles present in the bombarding beam due to emitter erosion. It was found that the initially hemispheric working part of the probes turns into the axially symmetric complex-shaped surface. A correlation of these effects with inert gas ionization and tungsten sputtering under the action of super-high-density electron beams is discussed. The atomically smooth microprobes obtained by ion bombardment and field evaporation offer high stability and an atomic-level resolution.     

Electric-field-induced ion evaporation from dielectric liquid

A direct proof of ion field evaporation from dielectric liquids is presented. The flux of sodium ions ejected from the surface of an electrospray of formamide is measured using time-of-flight and retarding potential techniques. The electric field at the emitting surface is varied through the electrospraying parameters. We find that the evaporated ion current is a very steep function of the electric field.     

Toward a theory of evaporation processes in liquid-solid systems

A theoretical analysis is presented of isothermal evaporation of a volatile component from a solid covered by a liquid layer. Binary compounds are considered, with the covering liquid produced by thermal decomposition of the solid material. It is shown that the relaxation time of the volatile concentration profile is much shorter than the characteristic time of motion of the melting interface; i.e., the instantaneous profile of volatile concentration at any time is a linear function of the spatial coordinate. A new nonlinear Stefan-type problem of evaporation in a solid-liquid-vacuum system is formulated that involves two moving phase transition interfaces: an evaporating interface and a melting interface. Exact analytical solutions to the problem are found. It is shown that the melting interface moves faster than the evaporating interface; i.e., the thickness of the liquid layer increases with time, its growth rate increasing with evaporation rate coefficient. It is demonstrated that the concentration profile evolves self-similarly in the course of time. An increase in evaporation rate coefficient leads to a steepening of the concentration gradient across the liquid layer, changing the volatile concentration at the evaporating interface, and the evaporative flux changes accordingly.     

Particle‐in‐cell simulation of the effect of curved magnetic field on wall bombardment and erosion in a hall thruster

A curved, convex towards the channel bottom magnetic field is an important feature of an advanced Hall thruster that allows confining the plasma flow in the channel center, reducing the divergence angle of the ejected ion beam, and improving the discharge performance. In this article, the discharge behaviour of a Hall thruster in magnetic fields with different degrees of curvature is simulated with a particle‐in‐cell numerical method, and the effect of curved magnetic field on the ion bombardment and wall erosion and the associated mechanisms are studied and analysed. The results show that, as the curvature of the magnetic field increases, the propellant ionization becomes more confined at the channel center, the potential drop inside the channel decreases, and the acceleration region shifts outside the channel, which lead to the attenuation of the ion energy bombarding the wall and the deviation of the bombardment angle from the optimal sputtering angle. Conversely, the ion flux bombarding the wall near the channel exit increases. Nevertheless, the bombardment energy and angle are the dominant factors for the wall erosion, and the wall erosion rate clearly decreases with the increasing curvature of the magnetic field. These findings are closely related to the behaviour of electron conduction under a curved magnetic field; the relevant mechanisms are clarified in this article.     

Collision induced dissociation of doubly charged silver clusters Agn 2+, n=21,22,23

Doubly charged silver clusters Ag_n ²⁺, n=21, 22, 23, are produced by electron bombardment of an Ag_n ⁺ ensemble stored in a Penning trap. After mass selection the clusters are subjected to collision induced dissociation. The fragmentation channels are determined by time-of-flight mass spectrometry after ejection of the resulting ion ensemble from the trap. Monomer evaporation is the only decay path observed. This revised version was published online in August 2006 with corrections to the Cover Date.     

A comparative study of the influence of different post-treatment methods on the properties of HfO2 single layers

Heat treatment, laser conditioning and ion post-treatment were used, respectively, on HfO₂ single layers deposited by electron beam evaporation (EBE). Optical, structural and laser-induced damage threshold (LIDT) properties of the films had been studied. It was found that all of the post-treatment methods are effective for improving the LIDT, but the mechanism for the improvement differs. Heat treatment in atmosphere was found to be effective for improving film stoichiometry. Laser conditioning was useful to decrease the defects by stress release. While in the ion post-treatment, the defects were removed by ion sputtering and the absorption was reduced at the effect of compression induced by ion bombardment. The most suitable method should be chosen for the required characteristics of the coatings.     

Field evaporation of silicon surfaces

Quantitative measurements on field evaporation of Si(111) surfaces in hydrogen imaging gas have been carried out by field ion microscopy. The field evaporation rate is found to increase exponentially with increase of the reciprocal of tip temperature in the range 80–103 K. The evaporation field strength increases with increase of tip temperature in the investigated range, 80–300 K. Within the applied pressure range, 5× 10^?6 to 2 × 10^?4 Torr of hydrogen gas, the evaporation rate linearly increases with the gas pressure. Similar effects of temperature and gas pressure on field evaporation of Si(111) surfaces have been observed also in silane imaging gas. A model, based on a field-induced formation of surface hydrides as a rate-determining step, is proposed, which accounts for all the experimental results of the field evaporation process.     

High-temperature field evaporation of rhenium

High-temperature field emission of Re, Pt, Ta, and W is studied by field-emission methods. Metal ions are found to evaporate mainly from the tops of thermal-field microprotrusions produced by high electric fields and temperatures on the emitter surface. For fi eld intensities of up to F=1–2 V/Å and temperatures of 1500–2000 K, the ion currents i are recorded from the entire emitter surface. They range from several tenths of nanoamperes to several nanoamperes. The activation energies of field evaporation determined from the Arrhenius plots logi=f(1/T) are found to be appreciably lower than those calculated within the charge exchange model for known parameters of the process and the metals evaporated. Reasons for such a difference in the activation energies and mechanisms of ion evaporation at high F and T are discussed.     

A determination of the rate and heat of evaporation by measuring the radius variations of a metal tip

A new material loss technique for the experimental determination of the free evaporation rate of a metal is presented. This technique is based on the study of tip profile changes. The tip radius variation is a consequence of the simultaneous action of free evaporation and surface self-diffusion. The rate of free evaporation is related to the tip radius evolution, so the vapor pressure and heat of evaporation can be determined by measuring in situ the radius changes with time. The method permits an easy control of the cleanliness of the surface by field electron microscopy, and should enable one to study the influence of adsorption on evaporation. The method is tested with molybdenum tips. The variation of the radius of the tips, heated in ultra-high vacuum, is determined by field electron and scanning microscope measurements. Vapor pressures and heat of evaporation values obtained are in agreement with values obtained by other techniques.     

Molecular flux distribution from the thermal evaporation source using Monte Carlo method: The importance of interparticle collision

Molecular flux distribution is critical information to obtain uniform thin films using the thermal evaporation technique in vacuum. However, the flux distribution is a quite complex function depending on the geometry of evaporation sources and the individual motion of evaporated molecules. This makes it difficult to establish a model describing the flux distribution that is directly associated with the distribution of film thickness on the substrate. In this study, we found that the conventional analytic model severely deviates from the experimental flux distribution except in the simple case. Thus, we developed a numerical model based on the Monte Carlo method to simulate the angular flux distribution from a thermal evaporation source. In particular, interparticle collisions have been handled rigorously. Evaporated molecules were assumed to be point particles that collide with each other and adsorbed (emitted) on (from) the nozzle wall of the source. The interparticle collision probability was addressed with the rarefied gas dynamics and the cosine law of emission was adopted to simulate the adsorption/emission process at the nozzle wall. Finally, we compared the simulated flux distribution with experiments on the cylindrical nozzle with various aspect ratios and deposition rates. The simulation results show excellent agreement with experimental data, indicating that interparticle collision is a key parameter for accurate simulation.     

Driving frequency effects on the mode transition in capacitively coupled argon discharges

A one-dimensional fluid model is employed to investigate the discharge sustaining mechanisms in the capacitively coupled argon plasmas, by modulating the driving frequency in the range of 40 kHz-60 MHz. The model incorporates the density and flux balance of electron and ion, electron energy balance, as well as Poisson's equation. In our simulation, the discharge experiences mode transition as the driving frequency increases, from the γ regime in which the discharge is maintained by the secondary electrons emitted from the electrodes under ion bombardment, to the α regime in which sheath oscillation is responsible for most of the electron heating in the discharge sustaining. The electron density and electron temperature at the centre of the discharge, as well as the ion flux on the electrode are figured out as a function of the driving frequency, to confirm the two regimes and transition between them. The effects of gas pressure, secondary electron emission coefficient and applied voltage on the discharge are also discussed.     

Spectroscopic investigations of Cr, CrN and TiCr anti-multipactor coatings grown by cathodic-arc reactive evaporation

Cr, CrN, TiCr coatings have been investigated as potential anti-multipactor coatings. The coatings were synthesized by cathodic-arc reactive evaporation in Ar-N₂ atmosphere where the ion energy is controlled by substrate biasing. Chemical state analysis and surface composition were studied by X-ray photoemission spectroscopy (XPS), whereas bulk composition and depth profile were studied by glow discharge optical emission spectroscopy (GDOES). The surface morphology was studied by optical profilometry (OP) and scanning electron microscopy (SEM). The compositions of the coatings were CrN and Ti₄₀Cr₆₀ and they were homogeneous in depth. Surface oxidation was higher in Ti₄₀Cr₆₀ than in CrN. Coatings deposited at high negative bias show lower deposition rate and had lower surface roughness than those obtained at low bias. Secondary electron emission yield (SEY) was higher for CrN than for Ti₄₀Cr₆₀, both before and after low-energy Ar⁺ ion bombardment. The SEY of Ti₄₀Cr₆₀ (1.17 maximum) was clearly smaller than the others. The maximum yield, σ_m, and the first crossover electron energy, E₁, are the most important parameters, and (E₁/σ_m)^1/2 is a good figure of merit. This quantity was approximately 3 eV^1/2 for Cr and CrN and 4 eV^1/2 for Ti₄₀Cr. After Ar⁺ ion bombardment, the average value improved significantly to 8.9 eV^1/2 for Cr and CrN and 10.2 eV^1/2 for Ti₄₀Cr₆₀. The radio-frequency multipactor performance of these materials was simulated using the experimentally determined SEY parameters.     

Droplets evaporation: Problems and solutions

The evaporation of single droplets and sprays into gaseous atmosphere and the evaporation of sessile liquid droplets on solid substrates are here considered. We argue that if thermodynamics is augmented with Derjaguin’s (disjoining/conjoining) pressure to handle phenomena in a vicinity of the three-phase contact line, problems like the singularity of the evaporation flux and of the viscous stress at the three-phase contact line of a sessile droplet are ruled out.     

电阻热蒸发镀膜与电子束蒸发镀膜对纳米球刻蚀方法制备二维银纳米阵列结构的影响

在使用纳米球刻蚀法制备二维银纳米点阵的过程中,使用不同的镀膜方法在同样的模板上得到了不同形貌的银纳米阵列结构.使用电阻热蒸发镀膜方法获得了六角排列的银纳米三角形阵列结构,使用电子束蒸发镀膜方法则获得了六角排列的银纳米环阵列结构.研究表明,沉积纳米粒子的粒径、表面纳米曲率效应和热能是形成不同结构形貌的纳米阵列结构的关键因素. 关键词： 银纳米阵列 电阻热蒸发 电子束蒸发 纳米球刻蚀     

低能离子对高温超导YBa_2Cu_3O_(7-δ)薄膜的表面改性和机理

低能氩离子束轰击并后退火处理的离子束表面改性,会影响高温超导薄膜的表面结构和超导特性,但是其中的深刻微观机理不清楚.本文通过连续改变离子束轰击时间,系统研究了离子束表面改性对于超导膜结构和临界电流密度的影响.通过扫描电子显微镜、X射线衍射、J_(c-scanning)测试表征样品的结构特性和超导特性,并得出内应变、氧空位缺陷等参量.研究表明,经过表面改性的钇钡铜氧(YBa_2Cu_3O_(7-δ),YBCO)薄膜,随轰击时间增加表面形貌会变得更加均匀致密,a轴晶粒消失,并且临界电流密度有了显著的提高.由化学键收缩配对模型分析得出,临界电流密度的提高与薄膜内应变增大和引发的局部YBCO结构中Cu—O键收缩有关.     

Discrete-step evaporation of an atomic beam

We present a theoretical analysis of the evaporative cooling of a magnetically guided atomic beam by means of discrete radio-frequency antennas. First we derive the changes in flux and temperature, as well as in collision rate and phase-space density, for a single evaporation step. Next we show how the occurrence of collisions during the propagation between two successive antennas can be probed. Finally, we discuss the optimization of the evaporation ramp with several antennas to reach quantum degeneracy. We estimate the number of antennas required to increase the phase-space density by several orders of magnitude. We find that at least 30 antennas are needed to gain a factor 108 in phase-space density.     

Spatial and temporal distribution of potential in the pseudosparkswitch

The understanding of emission and transport in the pseudospark is complex because these are related to a transition between two fundamentally different modes of electron emission. In this paper probe measurements of plasma parameters are presented providing evidence for oscillatory behavior of the current and potential during this transition. It is shown that these fluctuations are related to a glow-to-arc transition wherein smooth current growth is interrupted by cathode spot formation inside of the hollow cathode resulting in very rapid rise in current. These results support a view that quenching observed in the pseudospark is a double-layer phenomenon that occurs when emission cannot exceed secondary emission produced by ion bombardment