Adsorption and desorption of oxygen on stepped tungsten surfaces

The adsorption and desorption of oxygen on stepped tungsten surfaces with orientations close to the (110) orientation and steps parallel to the most densely packed crystal direction ([111]) is studied with low energy electron diffraction, Auger electron spectroscopy, work function measurements and thermal desorption spectroscopy. With increasing deviation from the (110) orientation, an increasing preference for the formation of the p(2 × 1) domain with the densely packed direction parallel to the steps is noted. The adsorption kinetics does not differ markedly from that on the flat (110) surface, however the desorption behaviour at low coverages (θ < 0.3) is quite different. The results are interpreted in terms of the dissociation of a mobile precursor at terrace and step sites, the competition between the two domains during their growth and a step-induced premature transition to the complex structure observed on flat (110) surfaces at $\text{θ ? 8}$. The steps are believed to play also a significant role in desorption.     

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.     

The tungsten powder study of the dispenser cathode

The intercorrelation of tungsten powder properties, such as grain size, distribution and morphology, and porous matrix parameters with electron emission capability and longevity of Ba dispenser cathodes has been investigated for the different grain morphologies. It is shown that a fully cleaning step of the tungsten powder is so necessary that the tungsten powder will be reduction of oxide in hydrogen atmosphere above 700 °C. The porosity of the tungsten matrix distributes more even and the closed pore is fewer, the average granule size of the tungsten powder distributes more convergent. The porosity of the tungsten matrix and the evaporation of the activator are bigger and the pulse of the cathode is smaller when the granularity is bigger by the analysis of the electronic microscope and diode experiment.     

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.     

Diffusion of oxygen on tungsten (110)

The surface diffusion of oxygen on W(110) was studied in the temperature range 760–880°C and for coverages up to one monolayer. Coverage-distance plots after diffusion runs were determined via corresponding work function profiles measured by a newly developed, laterally resolving vibrating capacitor method. The coverage profiles were evaluated in terms of a concentration dependent diffusion coefficient (DC) using the Boltzmann-Matano analysis. The temperature dependence of the DC can be described by a constant activation energy of 27 ± 2 kcal/mole in the coverage range 0.4 < θ < 0.9. The concentration dependence of the DC shows a maximum near θ = 0.45and a decrease beyond θ = 0.6. This behavior is correlated with observed LEED structures and interpreted by a rather abrupt change of the chemical potential near θ = 0.5 and a decreasing hopping probability for higher coverages.     

Thermal desorption of silicon from polycrystalline tungsten surfaces

The adsorption of Si on polycrystalline tungsten surfaces was studied for the first time by means of thermal desorption. Instead of the main peak of Si ($\text{m}\text{e}\text{=28}$), the isotope ($\text{m}\text{e}\text{=30}$) contained naturally (～3%) was monitored by mass spectrometry. This method can reduce the contribution from CO to the mass signal of Si. Two desorption peaks were observed at 1480 and 1820 K. The activation energies for the desorption were estimated to be about 90 and 110 kcal/mol, respectively.     

Atomic imaging of oxygen desorption from tungsten trioxide

We report direct observations by high-resolution electron microscopy of oxygen desorption from tungsten trioxide. Clear evidence is found for layer epitaxial growth of metallic tungsten with (110)W|(100)WO₃ and [001]W and [001]WO₃ parallel to the electron beam, consistent with low-energy electron diffraction data on the low-temperature epitaxy of WO₃ on W. W[001] was always observed parallel to the electron beam independent of the surface normal. The results suggest that bulk and surface damage, and displacement processes, are similar.     

Lithium adsorption on hot oxygen covered tungsten surfaces

The mean residence times τ of lithium particles on oxygen covered tungsten surfaces were measured accurately over a wide temperature range (1200 < T < 1900 K) by the beam modulation technique with a lock-in analyzer. A predominant monocrystalline W(100) structure was obtained by recrystallization of a polycrystalline tungsten ribbon. The residence time was determined as a function of the oxygen coverage θ and the temperatureT of the surface. The desorption energy l and the preexponential factor τ₀, calculated from the Arrhenius equation, are not only dependent on the amount of adsorbed oxygen but also on the oxygen structure. Apparently the desorption parameters l and τ₀ are correlated. An increasing desorption energy l is connected to a decreasing “vibration period” τ₀ whereby the influence on the residence time is partly compensated.     

Thermal desorption of metals from tungsten single crystal surfaces

After a short review of the experimental methods used to determine desorption energies E and frequencies v the assumptions underlying the theoretical analysis of the experimental data are discussed. Then recent experimental results on the flash desorption of Cu, Ag and Au from clean, well characterized W {110} and {100} surfaces are presented and analysed in detail, in particular with respect to the coverage dependence. The results obtained this way clearly reveal the limitations of previous analysis methods and of the experimental technique per se (such as structure and phase changes below and in the temperature region in which desorption occurs). Furthermore the need for more theoretical work to understand the large changes of v and E with coverage and the so-called “compensation effect”, i.e. the relation between In v and E, becomes evident.     

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.     

Diffusion and phase transitions in barium monolayers on the (011) plane of tungsten

The diffusion parameters of barium on W(011) are shown to correlate with phase transitions in the adlayer. A suprisingly strong difference is found both in absolute values and in the coverage dependence of the diffusion parameters for barium on the (011) plane of W and Mo. A considerably lower mobility of barium on molybdenum is attributed to a larger contribution of covalent bonding on this substrate. It is concluded that the electron structure, together with the atomic structure of a surface, plays an important role in surface diffusion.     

Diffusion and desorption of submonolayer platinum deposited on the multi-faceted surface of a tungsten micro-crystal

Diffusion and desorption of platinum on the tungsten micro-crystal in the form of the W(1 1 1) oriented emitter tip has been studied using the field electron microscopy (FEM) technique. Diffusion of small dose of platinum (average thickness about 0.18 geometrical ML after spreading) on the thermally clean W emitter tip was studied at temperatures 648-742 K. Average activation energy for diffusion E_diff was found to lie between 1.16 ± 0.08 eVand 1.30 ± 0.16 eV. During annealing at the diffusion temperatures Pt-induced faceting of the emitter surface was visible in the neighbourhood of the {1 1 1} pole. The layer equilibrated in the diffusion process was stable at temperatures up to 1100 K where reduction of the high voltage at a fixed emission current, characteristic of alloying of Pt with W, was detected. Submonolayer of platinum (Θ_Pt = 0.18 ML) started to desorb at tip temperature ≥1780 K. The measurements of average activation energy for desorption of ‘zero coverage’ Pt (0.03 ML ≤ Θ_Pt ≤ 0.06 ML) from the entire W emitter surface were carried out at temperatures 1990-2170 K and yield the value of E_des = 5.19 ± 0.22 eV to 5.33 ± 0.19 eV. The results are compared with data for diffusion of individual Pt atoms and small clusters and with data for adsorption of Pt atoms on a planar W(1 1 0) surface. In discussion the atomic surface structure of the substrate, modified by the strong interaction of Pt with the W micro-crystal, is also taken into account.     

Relaxation of tungsten adatoms on tungsten surfaces

The relaxation energy of a tungsten atom on tungsten surfaces is estimated to be about 1 eV. It seems difficult to neglect the variation of that energy in the interaction between adatoms.     

Diffusion on surfaces

A commentary is presented on current progress towards understanding diffusion on surfaces. Recent work by field ion microscopy shows that interactions between diffusing adatoms lead to correlated motions and add unexpected complexity to the elementary diffusion mechanisms. Work on high temperature surface self-diffusion also suggests the importance of interactions between diffusing atoms. Mass transport, catalysed surface diffusion, giant diffusivities, impurity, tracer and adsorbed layer diffusion, diffusion in sintering and other applications, are briefly discussed.     

Diffusion of tungsten clusters on tungsten (110) surface

Using molecular dynamics simulation and modified analytic embedded-atom method, we have investigated the self-diffusion of clusters on a tungsten (110) surface. As compared to the linear-chain configuration, the close-packed islands for tungsten clusters containing more than nine adatoms have been predicted to be more stable with the relatively lower binding energies. The migration energies show an interesting and oscillating behavior with increasing cluster size. The tetramer, hexamer and octamer have obviously higher migration energies than the others. The different atomic configurations and diffusion mechanisms have been determined during the diffusion processes. It is clear that the dimer-shearing mechanism occurs inside the hexamer, while it occurs at the periphery of heptamer. The successive hopping mechanism of individual atom is of critical importance in the migration of the clusters containing five or fewer adatoms. In addition, the diffusion of a cluster with nine adatoms is achieved through the changes of the cluster shape.     

Electron stimulated desorption study of oxygen adsorption on tungsten: Evidence for order—disorder transition

The adsorption of oxygen on a polycrystalline tungsten surface at ~300 K has been studied by means of electron stimulated desorption (ESD) Although precision gas dosing was not employed, the initial sticking probability for dissociative adsorption appears to be essentially unity, while the variation with coverage suggests that a high degree of order exists and that precursor state kinetics are significant. A most noticeable and reproducible discontinuity in ESD parameters occurs at a fractional coverage θ ~ 0.8 (exposure ~ 1.4 × 10¹⁵$\text{molecules}\text{cm}{}^2$ incident) which is interpreted as an order-disorder transition within a single (β₁) chemisorption state, and results in an increase in the ionic desorption cross-section by a factor of ~ 1.26. A discussion of the adsorption kinetics and the disorder transition is given in terms of current models of dissociative adsorption which include the effects of nearest neighbour lateral interactions.     

Diffusion effects on thermal desorption spectra of hydrogen from (111) surfaces of diamond-type crystals

We study the thermal desorption of atoms located in subsurface interstitial sites at the (111) surface of crystals with a diamond-type structure. From these sites they may either desorb or diffuse deeper into the crystal. A set of master equations is set up to describe the resulting random walk problem, and its solution yields both the desorption rate and total desorbed amount, as a function of time for constant temperature thermal desorption, or of temperature for flash desorption with a constant heating rate. Unequal values of the desorption and diffusion probabilities are allowed and the effect of initial occupation of deeper interstitial sites as well, is considered. In the latter case the desorption rate curve shows a double peak when the probabilities favour desorption, and in all cases a distinct long time or high temperature tail of the peak is found to result from diffusion. Based on this study, a model of H adsorption on Si(111) is presented and shown to explain several features of the experimental results of Schulze and Henzler.     

Diffusion mechanisms for iron on tungsten

Diffusion of iron atoms on clean W(1 0 0) and W(1 1 0) as well as on Fe/W(1 0 0) and Fe/W(1 1 0) surfaces was investigated by means of exhaustive first-principle calculations. Comparison of the activation energy barriers obtained for hopping and exchange migration processes shows that the surface diffusion proceeds via jumps to the nearest sites. The activation energies are higher for Fe adatom on clean tungsten than those for Fe adatoms moving on iron-covered tungsten. The magnetism of the underlying Fe/W(1 0 0) films has a pronounced influence on the diffusion, as evidenced not only by a reduced activation energy barrier but also by a change of the stable adsorption place. Fe atoms reaching step edges are trapped there and eventually diffuse along the steps more slowly than the adatoms on the terraces. The rate of diffusion increases upon depositing a row of Fe atoms along steps.     

Field desorption of a potassium-gold film on tungsten

The formation and destruction of alloys and chemical compounds of gold with potassium occurring in their coadsorption on tungsten have been studied by continuous-mode field-desorption microscopy, field-emission microscopy, and time-of-flight mass analysis of the products of field-induced desorption. The effect of an electric field on these processes was investigated. Monatomic and cluster ions of potassium and its compounds with gold were found to appear in fields too weak for field desorption to set in. Chemical reactions give rise to autooscillations in the field desorption of potassium.