Surface composition and barium evaporation rate of “pedigreed” impregnated tungsten dispenser cathodes during accelerated life testing

A study has been made of the surface composition and barium evaporation rate of “pedigreed” impregnated tungsten dispenser cathodes. The effect of air exposure on coated cathodes was examined and was found to have no significant effect on barium evaporation rate although in some cases longer reactivation times were required. No changes in surface topography were apparent following air exposure and reactivation. Life testing was done at 100°C above the typical operating temperature for the cathode, where the typical operating temperature was taken to be 950°C for coated cathodes and 1050°C for uncoated cathodes. The cathodes were examined at different stages of life testing, up to 1200 h. Significant decreases in barium evaporation rates were found after as few as 500 h of life testing. After 1000 h the evaporation rate had decreased more than an order of magnitude. Changes in surface composition were also found. The effects of tungsten particle size, used in manufacture of the billet, on barium evaporation rate were also studied but no correlation was found.     

Changes in surface chemistry of an Ir---W mixed metal coated cathode during activation

A comparative study has been made between a mixed metal (60% Ir-40%W) coated cathode and a “B” cathode during activation and also in their respective steady states. The rate limiting factor in the activation of the coated cathode is the oxidation of the initial Ba type surface to a BaO type surface. Since on the “B” cathode Ba and O emerge together, its activation is faster than the coated cathode. In the steady state of operation, both cathodes exhibit a surface near BaO stoichiometry which is the optimum composition for the minimum work function. This work function is about 0.2 eV lower on the coated cathode than on the “B” cathode. An accelerated life test at 1575 K indicated a gradual decrease of the Ir concentration in the coating.     

Ion scattering spectroscopy studies of barium and oxygen on tungsten and tungsten-based dispenser cathodes

In order to use Ion Scattering Spectroscopy (ISS) for studies of tungsten dispenser cathodes, the relevant ISS sensitivities must be measured. Calibrations have been made using a polycrystalline tungsten ribbon with controlled coverages of oxygen, barium and combinations thereof. Auger Electron Spectroscopy (AES) was used to monitor these controlled surfaces and the escape depths of the tungsten Auger electrons in barium and oxygen have been measured. The absolute ISS sensitivities of all three elements were found to be strongly dependent on the barium coverage of the tungsten surface. This effect has been attributed to the lowering of the work function of the tungsten surface caused by the barium adsorption. However, the relative ISS sensitivities of the three elements are not affected in this way when both barium and oxygen (or oxygen alone) are present on the tungsten surface. ISS spectra of such surfaces have been analyzed quantitatively and found to be in reasonable agreement with AES measurements. The analysis has also been applied to ISS spectra of uncoated tungsten matrix dispenser cathodes in an active state and following exposure to oxygen. Compared to AES, these spectra indicate less oxygen on the active cathode surfaces as a result of the oxygen (associated with barium) not contributing to the oxygen ISS signal. Comparisons of the spectra from the active and oxygen poisoned cathodes suggest that oxygen adsorbed during the oxygen exposure sits on the topmost barium layer whereas the oxygen on the active cathode surface does not.     

A model of dispenser cathode activity

A semiquantitative model of dispenser cathode activity based on recent work on the co-adsorption of Ba and O onto W surfaces is presented. The co-adsorption studies have determined the shape of a three-dimensional surface of work function as a function of θ_O and θ_Ba, the surface coverages of O and Ba, respectively. Compositions of a variety of pedigreed dispenser cathodes were fitted to this surface and their composition changes during lifetime were modeled. Changes of surface composition with temperature and of workfunction, φ, with temperature were also found to fit these curves. The concept of a patchy surface implied by the co-adsorption measurements was used to explain earlier results on the shape of the X-ray excited Ba MNN Auger feature. Finally, SIMS measurements under UHV conditions was found to provide an extremely sensitive measurement of surface composition in the region of surface coverages of interest in the study of cathode phenomena. Extensions of this work to other types of cathodes such as M-types, and rhenium substrate cathodes is also discussed.     

Methods for evaluating the performance of emitters in plane parallel diodes

The curvature seen in Child's law plots of current-voltage characteristics measured using plane parallel diodes may be analysed in a number of ways to give information about the work function distribution of a cathode. This paper suggests a simple technique for characterising a cathode based on the assumption it has a “top-hat” work function distribution. The technique allows synthetic current/voltage characteristics to be generated which agree well with the practical characteristics from which the parameters of the work function distribution were derived. The parameters of the “top-hat” model may also be used to obtain an equivalent Gaussian work function distribution which gives almost identical synthetic characteristics and Schottky enhancement may be modelled in a rather empirical manner. Since either model gives good predictions, synthetic characteristics may be used to investigate other methods used to characterise cathodes. It is found that there is generally reasonable agreement which could be improved, for most practical work function distributions, by the choice of parameters slightly different from those normally used.     

Interpretation of AES data of impregnated cathodes

Many surface studies of impregnated cathodes involve a measurement of the Ba₅₉₀, O₅₁₀ and W₁₇₀ Auger peak heights. This paper shows how these measurements plotted in terms of O/Ba versus W/Ba concentration ratios can be used to interpret experimental results on impregnated B-type cathodes. Data are presented from actual cathodes and simulated cathode surfaces using well defined BaO films on W. These results as well topographic data will be compared to various calculated models. It will be shown that an active B-type cathode consists of nearly a monolayer of BaO on W with a slight excess of O compared to Ba. This excess O, however, is associated with W rather than the BaO. Ageing increases the O/Ba ratio and this takes place principally by the removal of Ba from the BaO surface, again with the excess O being associated with the W rather than the BaO. The work function increase during ageing corresponds only to the area-decrease of BaO. If the surface accumulates so much O that it exceeds the available empty Ba sites (i.e., all W is covered by Ba or O), then any additional O appears to sit on top of the remaining Ba and the surface will be poisoned. These conclusions are not only the result of work function observations but are also consistent with interatomic Auger analysis of Ba---O interactions as well as surface plasmon and ISS results. Toporgaphic data obtained from pore/impregnant regions are also in good agreement with calculated values of partial coverage of typical tungstate and impregnant residues.     

中间层Re的加入对覆膜钡钨阴极性能的改善

研究了一种新型的覆膜钡钨阴极——双层膜(Os-W/Re膜)钡钨阴极.对这种新型阴极的发射性能进行了测试，重点对其老炼前后表面薄膜的微观形貌进行了分析，表明中间层Re膜的加入使覆膜钡钨阴极的性能得到了改善.通过对Os-W双元合金膜钡钨阴极和Os-W/Re双层膜钡钨阴极发射特性的比较，发现Os-W/Re双层膜阴极的直流发射性能好于Os-W合金膜阴极.对两种阴极激活后发射表面的X射线光电子能谱分析表明，Os-W/Re双层膜阴极激活后表面形成的三元合金膜是其发射特性优于Os-W合金膜阴极的主要原因.应用扫描电子显微镜分析比较两种阴极激活老炼后的表面状态，结果表明：Os-W合金膜阴极在老炼一段时间后，其表面薄膜出现开裂，这会导致阴极发射均匀性下降；而Os-W/Re双层膜阴极在同样老炼条件下，发射表面薄膜均匀并保持完整，从而确保覆膜钡钨阴极发射均匀性和工作可靠性. 关键词： 双层膜钡钨阴极 Os-W/Re膜 Os-W膜 薄膜开裂     

Computer controlled techniques for high emission density mapping of thermionic cathodes

Some of the techniques commonly used (e.g. SLEEP and thermionic emission microscope) for measuring emission or work function uniformity of thermionic cathode surfaces require the use of very low or near zero current densities, thus the cathode is characterized at current densities and temperatures much lower than that of a normally operating cathode. The system reported on here uses a high voltage pulse technique and is capable of measuring emission densities in the range 1 to 80 A/cm² at normal cathode operating temperatures. The cathode surface is scanned with an anode having a 0.025 mm aperture whose position is controlled by computer operated stepping motors. The current through the aperture to a collector electrode is measured using a sample-and-hold amplifier. Pulsing and sampling are computer synchronized with the scanning, and data for each pulse are accumulated and can be processed and displayed in several ways using the computer, including a detailed “three-dimensional” map of either the electron emission density or work function variations. The entire surface of the cathode or any portion of it can be mapped in steps as small as 0.001 mm (1μm), but typically steps of 5–100 μm were used. Measurements are presented illustrating the uniformity or nonuniformity of the electron emission densities and work functions for type-B and type-M cathodes.     

On the theory of temporal aberrations for cathode lenses

A new approach to the theory of temporal aberration for cathode lenses is given in the present paper. A definition of temporal aberration is given in which a certain initial energy of electron emission along the axial direction ε_z1 (0ε_z1ε_0max) is considered. A new method to calculate the temporal aberration coefficients of cathode lenses named “direct integral method” is also presented. The “direct integral method” gives new expressions of the temporal aberration coefficients which are expressed in integral forms. The difference between “direct integral method” and “τ-variation method” is that the “τ-variation method” needs to solve the differential equations for the three of temporal geometrical aberration coefficients of second order, while the “direct integral method” only needs to carry out the integral calculation for all of these temporal aberration coefficients of second order.All of the formulae of the temporal aberration coefficients deduced from “direct integral method” and “τ-variation method” have been verified by an electrostatic concentric spherical system model, and contrasted with the analytical solutions. Results show that these two methods have got identical solutions and the solutions of temporal aberration coefficients of the first and second order are the same as with the analytical solutions. Although some forms of the results seem different, but they can be transformed into the same form. Thus, it can be concluded these two methods given by us are equivalent and correct, but the “direct integral method” is related to solve integral equations, which is more convenient for computation and could be suggested for use in practical design.     

Work function distribution for W-Ir mixed metal matrix cathodes

Mixed metal matrix cathodes have inherent non-uniformity and patchiness of emission due to the presence of two-alloy phase structure on the surface. I-V characteristics of cathode studied in a close spaced diode configuration is one of the easy and cost effective methods to estimate the variation of work function on the cathode surface. Tungsten iridium mixed metal matrix dispenser cathodes of Ø1.4 mm (80 wt.% W-20 wt.% Ir) have been fabricated in the laboratory and their I-V characteristics have been investigated in diode configuration. In this paper the model suggested by Tonnerre et al. has been used to find out the work function distribution of W-Ir cathodes from I-V characteristics. An attempt has been made to correlate the microstructure with the work function values.     

Multi-pulsed intense electron beam emission from velvet,carbon fibers,carbon nano-tubes and dispenser cathodes

The experimental results of studies of four kinds of cathode emitting intense electron beams are demonstrated under multi-pulsed mode based on an experimental setup including two multi-pulse high voltage sources.The tested cathodes include velvet,carbon fibers,carbon nano-tubes(CNTs)and dispenser cathodes.The results indicate that all four are able to emit multi-pulsed beams.For velvet,carbon fiber and CNTs,the electron induced cathode plasma emission may be the main process and this means that there are differences in beam parameters from pulse to pulse.For dispenser cathodes tested in the experiment,although there is a little difference from pulse to pulse for some reason,thermal-electric field emission may be the main process.     

Novel comprehension of “common path” principle

The idea of “common path” has been widely applied in optical instrument design for 30 years and even today. But the meaning of “common path” has not yet been explained clearly and sometimes confusion has been created. In this paper an “adaptive principle” is proposed and recommended on optical instrument system. It suggests that the designer not only arranges the measurement system to obtain measurement signal but also sets a channel to give prediction of noise or disturbance in real time or short term. Such a recommendation is based on the recent studies on nonlinear dynamics and atmospheric disturbance by means of experiments as well as theoretical analysis.     

Evolution Strategy Optimization for Selective Pulses in NMR

We present a first set of improved selective pulses, obtained with a numerical technique similar to the one proposed by Geen and Freeman. The novelty is essentially a robust and efficient “evolution strategy” which consistently leads, in a matter of minutes, to “solutions” better than those published so far. The other two ingredients are a “cost function,” which includes contributions from peak and average radiofrequency power, and some understanding of the peculiar requirements of each type of pulse. For example, good solutions for self-refocusing pulses and “negative phase excitation pulses” (which yield a maximum signal well after the end of the pulse) are found, as may have been predicted, among amplitude modulated pulses with 270° tip angles. Emphasis is given to the search for solutions with low RF power for selective excitation, saturation, and inversion pulses. Experimental verification of accuracy and power requirements of the pulses has been performed with a 4.7 T Sisco imager.     

Nucleation and growth mechanism for flame synthesis of MoO2 hollow microchannels with nanometer wall thickness

The growth and morphological evolution of molybdenum-oxide microstructures formed in the high temperature environment of a counter-flow oxy-fuel flame using molybdenum probes is studied. Experiments conducted using various probe retention times show the sequence of the morphological changes. The morphological row begins with micron size objects exhibiting polygonal cubic shape, develops into elongated channels, changes to large structures with leaf-like shape, and ends in dendritic structures. Time of probe–flame interaction is found to be a governing parameter controlling the wide variety of morphological patterns; a molecular level growth mechanism is attributed to their development. This study reveals that the structures are grown in several consecutive stages: material “evaporation and transportation”, “transformation”, “nucleation”, “initial growth”, “intermediate growth”, and “final growth”. XRD analysis shows that the chemical compositions of all structures correspond to MoO₂.     

Phonetographic profiles and F0-SPL characteristics of untrained versus trained vocal groups

The phonetogram has been recommended as an international tool for voice analysis. However, the capability of this technique to distinguish between different vocal groups has not been clearly established. The purpose of this study was to examine untrained versus trained vocalists using the phonetogram and the fundamental frequency by intensity (F₀/SPL) information derived through that method. In this study, “musical” or “controlled” ranges of phonation were stressed rather than “physiological” ranges. Results indicated that (a) characteristic phonetographic profiles may be established for untrained versus trained vocalists, and (b) trained vocalists show significantly increased capability in terms of F₀ range and maximum, minimum, and comfortable SPL production. Elicitation of “controlled” phonations may be the key to revealing the underlying vocal capabilities of seemingly different vocal groups.     

Physical operating principles of scandate cathodes for microwave devices

The electronic structure of barium oxide crystallites determining the emission properties of both dispenser and scandate cathodes has been studied using electron spectroscopy for chemical analysis, electron energy loss spectroscopy, and optical spectroscopy. It has been established that the other elements (calcium, aluminum, scandium, and tungsten) contained in cathode materials are diluted in barium oxide and significantly affect its electronic structure and, consequently, emission properties. The obtained results give an idea about the physical and physicochemical mechanisms of the effect of scandium on the reduction of the work function of scandate cathodes relative to that of the cathodes of other types.     

Work function variation versus alloy concentration for dispenser cathodes

The dependence of the work function (∅) of dispenser cathodes on the concentration of Ir and Re alloyed with the tungsten emitting surface has been determined for controlled porosity dispenser (CPD) cathodes and matrix type cathodes. ∅ versus concentration curves show a minimum with a distinct cusp near 50/50 concentration for CPD cathodes and near 40% tungsten for matrix cathodes. Our matrix cathode data (using Ir) is consistent (relative to the concentration at which a minimum occurs) with Os/W data derived from other work [6,15]. For all alloys the concentration at which the minimum occurs is independent of which noble metal is involved, but does depend on the nature of the tungsten substrate, and perhaps the deposition technique used to deposit the noble metal film. The data is interpreted in terms of a previously proposed model [7] involving enhanced bonding between Ba-Ir and O-W compared to Ba-W and O-Ir bonds. In addition we suggest a model which explains the cusp as well as other characteristics of the work function concentration curves in terms of a postulated surface phase transition.     

Vague concept of “reversible cleavage” in the theory of the surface tension of solids

Numerous derivations of the well-known Shuttleworth equation have been based on the unclear concept of “reversible cleavage” leading to the decisive step in any derivation - equalization of the surface free energy and surface stress. This is the key concept in contemporary surface thermodynamics of solids. But “cleavage” is not a surface process and, in this field, it cannot be a reversible operation. Besides, the “reversible cleavage” has no formal definition in the domain of the surface tension of solids that is an abnormal for any exact science. Consequently, this concept and all its corollaries including the Shuttleworth and generalized Lippmann equations have to be recognized as incorrect.