Diffusion and desorption of barium and oxygen on tungsten dispenser cathode surfaces

Auger measurements have been made on the concentration of barium and oxygen diffusing out of a pore and slot onto the surface of a simulated tungsten dispenser cathode. Profiles of concentration vs distance from the source were obtained at different temperatures. It is found that at cathode operating temperatures very little gradient of oxygen concentration exists on the surface, and under steady state conditions Ba is the main diffusing species. Ba diffusion distances derived are consistent with previous measurements. However it is found that this distance increases with concentration in spite of the fact that the Ba surface lifetime decreases with concentration. Time dependent measurements made on a clean surface show that the rate of Ba and O build up is limited by the supply rate of oxygen to the surface. This supply rate is not diffusion limited but seems to be limited by the mechanism generating free oxygen. A comparison of emission microscope measurements on a tungsten matrix dispenser cathode with the simulated cathode data indicates that similar oxygen generation processes may be controlling the activation of these cathodes.     

“Alternative” auger analysis reveals important properties of M-type and scandate cathodes

It is shown that in the evaluation of Auger spectra, as measured on cathode surfaces, the elemental sensitivities have to be corrected for differences in elemental number density. The substrate and the surface covering material have to be treated differently in the analysis. The result for normal M-type cathodes is that — besides the Ba---O cover — there is an excess oxygen concentration on the surface of about two times the Ba---O concentration. This in spite of the fact that the oxygen to barium peak-to-peak height ratio in the measured spectra is about equal to 2. For a degraded M-cathode the excess oxygen level is found to be much higher than for the normal cathodes. A new type of scandate cathode is described and discussed. Its analysis shows more Ba and, relatively, much less excess oxygen than for the M-type cathodes. This may account for the substantially higher emission.     

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.     

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.     

Auger studies comparing the surface concentration of barium on tungsten impregnated and M-cathodes

A study comparing the surface concentration of barium on both an impregnated tungsten cathode surface and M-type cathode surface was made to determine whether the higher electron emission capability of the latter can be attributed to a higher average surface barium concentration. Relative quantitative data was obtained by the use of a computerized Auger microprobe spectrometer on the surface of the cathode, half of whose area was M-type, the other half being the tungsten impregnated type. The results indicate that both cathode surfaces have equivalent barium concentrations and that they are approximately equal to a barium monolayer after 3000 h of cathode life.     

Relative work function, surface composition and topography of “pedigreed” impregnated tungsten dispenser cathodes

A study has been made of the variation in work function, surface composition, and topography of 5:3:2 impregnated tungsten dispenser cathodes made under carefully controlled conditions (pedigreed cathodes). Despite these conditions several cathodes had unexpected deposits on their emitting surfaces, and one showed a variation in work function and composition across the surface during activation.     

Fast turn-on characteristics of tungsten-based dispenser cathodes. II

In the Proceedings of the 1982 Tri-Service Cathode Workshop, the authors described studies of the reactivations of tungsten-based dispenser cathodes following poisonings of the kind expected during shelf storage of a microwave tube [Appl. Surface Sci. 16 (1983) 73]. Further work on the problems of reactivation following such poisoning is described here. In addition to coated (“M”) and uncoated tungsten matrix cathodes, the tungsten-iridium mixed metal matrix (“MM”) cathode has been studied. In general reproducible results have been obtained from different examples of the same type of “M” and uncoated cathodes. However, although some “MM” cathodes have exhibited good reactivation characteristics, a large variation has been observed between different examples of “MM” cathodes. The composition of the impregnant in the dispenser cathodes has been found to be an important factor in determining the reactivation rate of a cathode. As the barium oxide concentration in the impregnant increases, the cathode will recover faster from a poisoning exposure. Studies of the poisoning caused by combinations of different gases suggest that poisoning will occur if there is a sufficient exposure of a poisoning agent, regardless of the total exposure. The exposures necessary to poison a cathode are so small, that poisoning of the cathode appears probable during shelf storage of a microwave tube. The reactivation results have been summarised in terms of the times and temperatures required to achieve both a given current density and a given degree of reactivation from a poisoned cathode. The studies also indicate that the limiting step during the reactivation process involves the dispensing of fresh material to the cathode surface rather than the desorption or conversion of a poisoned surface layer.     

Surface studies on the low work function surface complex of barium on an osmium-ruthenium substrate

Barium and oxygen on an osmium-ruthenium (Os/Ru) surface is the surface complex responsible for the excellent electron emission properties of commercial M-type thermionic cathodes. A computerized Auger technique was used to study the surface properties of this surface complex and compare it with results obtained earlier for Ba-O-W surfaces, which characterize the properties of commercial barium impregnated tungsten thermionic cathodes. Barium desorption, electron emission and barium, barium oxide evaporation were measured at elevated temperatures for this experimental surface study. Desorption measurements, at 1100°C, on the barium on osmium-ruthenium substrate show that the energy for desorption was higher than that for the lowest work function Ba-O-W surface (5.4 to 4.8 eV). Oxygen was always present on the sputtered Os/Ru substrate and contributed to the strong bonding of barium to the Os/Ru surface. The barium and barium oxide evaporative products from such a surface at 1100°C were found to be mainly barium with the Ba/BaO ratio equal to approximately 100.     

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.     

Degradation factors of a new long life cathode

This paper analyses the degradation factors of a new long life coated impregnated cathode after accelerated life test. The surface state of the cathode is investigated with scanning electron microscope (SEM) as well as the content and variation of the various elements on the surface and the longitudinal section of the cathode are analyzed with Auger electron spectroscopy (AES) before and after the life test. The analyzing results with SEM show that the cathode coating shrinks at the life end and leads to a rise in its work function. The analyzing results with AES show that the percent of the W increases and the active materials Ba decreases on the cathode surface at the life end. Furthermore, there is less Ba underneath the cathode surface but still a lot of Ba in the tungsten matrix at the life end.     

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.     

Structural investigations of the Yb  Si(111) - 2x1, 5x1 and 3x1 overlayers

The YbSi interface has been investigated in the sub-monolayer regime employing Ion Scattering Spectroscopy (ISS), Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). Three different structures, YbSi(111) 2x1, YbSi(111) 5x1, and YbSi(111) 3x1, have been established by heat treatments of the interfaces. The structures consist of a stable overlayer of Yb atoms on the Si(111) surfaces. The distance of the Yb atoms to the uppermost layer of Si atoms has been estimated by comparing the YbSi ISS intensity ratio with the predictions of a model based on classical scattering theory and a Thomas-Fermi-Moliére potential. The height of the Yb atoms relative to the substrate toplayer was found to be 1.9 ± 0.3→.     

A study of H2S adsorption kinetics on the clean and oxygen covered (110) surface of copper

The very low pressure adsorption kinetics of H₂S on the clean and oxygen covered Cu(110) face have been examined by Auger Electron Spectroscopy (AES) and Mirror Electron Microscopy (MEM, used for continuous surface potential variations of the copper surface). The AES experimental curves on the clean copper face have been interpreted using a model of island growth by surface diffusion. The presence of an adsorbed oxygen layer on the copper surface changes notably the induction times observed on both AES and MEM measurements.     

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.     

Study of the chemical bonding of titanium with silicon and oxygen by AES and SEELS

Auger electron Spectroscopy (AES) and slow electron energy loss Spectroscopy (SEELS) have been employed to study the electronic structure of Ti, TiSi₂ and TiO₂. The changes in the Auger and loss spectra when Ti chemically binds with silicon to form TiSi₂ and with oxygen to form TiO₂ have been understood as manifestations of changes in electronic participation. AES spectra show distinct changes in line shapes of transitions involving the Ti valence electrons. The SEELS spectra provide information regarding shallow core levels, valence band and the collective excitation energies of the volume and surface plasmons. By monitoring the changes in the Auger peak at 387 eV and the 3p→ 3d quasiatomic transition (at about 45 eV), the role of d-orbital occupancies are studied in Ti and its compounds. The SEELS studies in the 0-80 eV range have enabled the authors to observe the behaviour of the 3p → 3d quasiatomic transition in Ti, which persists after oxidation but almost disappears during TiSi₂ formation. The values of the plasmon losses are related to the collective behaviour of conduction electrons.     

A study of Eu2O3, Sc2O3 co-doped tungsten matrix impregnated cathode

Eu₂O₃ and Sc₂O₃ co-doped W matrix impregnated cathodes have been prepared by the powder metallurgy method. The constitution of active elements on activated cathode surface is analyzed by in-situ Auger electron spectroscopy. It is found that although Eu exists in the matrix, no Eu is found on the cathode surface due to the formation of a stable Eu containing compound. Sc, Ba and O diffuse to the surface of the cathode and form an active surface layer during the activation period whereas the stable Eu-compound cannot liberate free Eu, which can diffuse from the cathode to the surface. The active substance of Sc, Ba and O on the cathode surface contribute to the emission property.     

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     

Behavior of zirconium in the ZrOW(100) system at high temperature, studied by ISS, AES and work-function measurements

The behavior of zirconium atoms at the W(100) surface associated with oxygen adsorption at different sample temperatures has been studied by Auger electron spectroscopy (AES), ion scattering spectroscopy (ISS), and the relative change of the work function (Δф) measured by the onset of the secondary electron energy distribution. The results have revealed: (i) adsorption of zirconium onto the W(100) surface followed by the elevation of the sample temperature up to 1710 K in an oxygen partial pressure of 2.7 × 10⁻⁴ induces complete diffusion of zirconium atoms into the W(100) substrate; (ii) further exposure of oxygen induces co-existence of oxygen and tungsten on the surface at 1710 K, resulting in a work function of 4.37 eV; (iii) keeping the sample temperature at 1710 K, simple evacuation of the system has resulted in surface segregation of zirconium atoms to the surface to form a zirconium atomic layer on the top-most surface, reducing the work function to 2.7 eV. The results have revealed that this specific behavior of zirconium atoms at high temperature assures, with very good reproducibility, the highly stable performance and long service life of Zr---O/W(100)-emitters in practical use, even in a low vacuum of 10⁻⁶ Pa.     

The bonding state of carbon segregated to α-iron surfaces and on iron carbide surfaces studied by electron spectroscopy

Segregated carbon on the Fe(100) surface has been studied by means of X-rayand ultraviolet photoelectron (XPS, UPS), Auger electron (AES) and electron energy loss spectroscopy (ELS). For comparison, the surfaces of polycrystalline graphite and of iron carbides stabilized by chromium or manganese additions have been investigated. On the iron surface, carbon exists as a chemisorbed state or graphitic multilayer. The two states exhibit different energy positions in XPS, and are different in energy positions and lineshapes in AES and ELS. During the transition of graphitic carbon to chemisorbed carbon on Fe(100) a novel coverage-dependent Auger feature is reported. The spectra of graphitic carbon on the iron surface always coincide with those of solid graphite. The carbon Auger transitions of chemisorbed carbon and of iron carbides exhibit very similar lineshapes, but the energy positions of both states differ in AES as well as XPS.