Cesium Desorption Research Articles

Cesium and oxygen coadsorption on LaB 6 single crystal surfaces : II. Thermal desorption of cesium from LaB 6 (100)

Cesium and oxygen coadsorption on LaB 6 single crystal surfaces : II. Thermal desorption of cesium from LaB 6 (100)

Kinetics and equilibrium of cesium sorption and desorption by particulate nuclear graphite

Data and a consistent physical and mathematical model for the kinetic and equilibrium sorptive behavior of cesium with a particulate, nuclear-grade graphite (H-451) is given. The most important conclusions are: (1) The kinetics of absorption and desorption of cesium by H-451 graphite in particulate form (size range 44–74μm) is such that, in general, several days are required to reach a near equilibrium state. Accordingly, the radiometric isopiestic method, although time consuming, appears to be the best method for obtaining equilibrium data in the cesium vapor pressure range of about 10 down to 10 −4 Pa. (2) The kinetics data of the isopiestic experiment with graphite powder was found to be very well represented mathematically by an equation which is based on the site activation energy being approximately equal to the sorbate-sorbent site interaction energy, χ. In accordance with a theory for modified-exponential sorption which fits the equilibrium data well, the sites are taken to be non-uniformly distributed having a number which decreases exponentially with an interaction energy, χ, that has a finite upper limit χ L .

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10 15 cm −2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (∼1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10 15 oxygen atoms per cm 2. Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10 15 cm −2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.

Interaction of cesium and oxygen on W(110) I. Cesium adsorption on oxygenated and oxidized W(110)

Abstract Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10 15 cm −2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (∼1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10 15 oxygen atoms per cm 2 . Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10 15 cm −2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.

Desorption of radioactivity from nearshore sediment

Cesium-137, cerium-144 and ruthenium-106 discharged into the nearshore environment of the Bombay harbour, have been found to be readily sorbed by the sedimentary particles. On reaching peak sorption levels, however, the levels of all the three radionuclides in the sediment were found to drop exponentially with time (1971–1976) under environmental conditions, despite the continued discharges at low levels. From the distribution patterns of radioactivity in the harbour, the rate of decline of 137Cs (the sedimentary half-life) was found to be 1.86 years, significantly lower than its radioactive decay time (30 years). In contrast, those of 144Ce (0.59 year) and 106Ru (0.87 year) were found to bepractically the same as their radiological half-lives. The difference between the sedimentary and radiological half-lives of both cerium and ruthenium suggests about 20% influx of fresh sediment following monsoon runoff. The effective sedimentary half-life (1.86 years) of 137Cs was found to be significantly affected by other environmental parameters, in addition to that caused by the influx of fresh material. Of the various environmental factors, salinity (i.e. Co-ion, Na +, K +, Mg ++, concentrations) was found to affect the desorption of cesium significantly. The possible sorption-desorption mechanism of cesium, cerium, ruthenium and cobalt radionuclides both under laboratory and field conditions are discussed.

Cesium Sorption and Desorption Behavior of Kaolinites

AbstractData for three kaolinites show that Cs was preferentially sorbed by kaolinites in NaCl and CaCl2 solutions. Cesium desorption behavior of kaolinites revealed that a variable proportion, 0–3.5% was strongly held. The Cs desorption behavior as well as the amounts of Cs fixed by kaolinites correlated well with the nature of impurities in them.

A study of the sorption properties of natural calcium clinoptilolite by using radioactive indicators

The sorption of monovalent ions (cesium, silver, thallium, mercury), bivalent ions (strontium, barium, lead, copper, cobalt, zinc) and polyvalent ions (cerium) on calcium clinoptilolite under dynamic conditions has been studied. Both the dynamic exchange capacities in the different break-through of the ions and the degree of the exchanging ions in sorption of different metal cations have been determined. The good selectivity of calcium clinoptilolite in relation to cesium and strontium is displayed in the presence of sodium. The influence of various factors on the sorption the thermal and radiation treatment of the sorbent, the pH and concentration of solutions, equilibration time, presence of alkali and alkali earth ions deactivating agents—EDTA, citric acid tartaric acid, and boric acid in solution has been studied. The optimum conditions of sorption have been determined. Experiments for the desorption of cesium and strontium have been carried out. The possibility to use calcium clinoptilolite for the purpose of deactivation of radioactive wastes is shown. The better sorption properties of that sorbent, compared to calcium clinoptilolite, can be explained by the higher aluminium content, as well as by a prevalence of calcium and magnesium in its ion exchange complex.

Field Desorption of Cesium from a Cesium Covered Oxygenated Tungsten Surface

Field Desorption of Cesium from a Cesium Covered Oxygenated Tungsten Surface

Desorption and Migration of Cesium Adsorbed on Tungsten in the Presense of an Electric Field

Studies of the field desorption of cesium from tungsten and the equilibrium concentrations of cesium under electric fields are made by a field emission microscope (FEM). It is observed that, at small concentrations, the desorption begins from (211) vicinities and proceeds along cross-bone zones, while, at large concentrations, the desorption occurs instantaneously from the whole surface of the tip apex. The threshold field strength for desorption increases with the increase of the surface concentration of cesium. Surface migration of Cs atoms towards the tip apex from the tip shank following the field desorption is suppressed by an applied negative electric field, and the equilibrium concentration between the apex and the shank appears. These equilibrium concentrations under various electric fields are determined, and the results are used to calculate the dipole moments of the ad-atoms for many concentrations.

Effect of Temperature on the Work-Function Minimum of Cesiated Tungsten Surfaces

Work-function changes during the desorption of cesium from three tungsten surfaces, polycrystalline foil, (100), and (110) oriented single crystals, have been measured in the region of the work function minimum. A modification of the vibrating capacitor technique which made contact-potential difference on electron emitting surfaces possible was developed for these experiments. For different arrival rates of cesium, the minimum work function occurred at different temperatures and decreased in magnitude with temperature. Values at 295°K were 1.78 eV for polycrystalline foil, 1.795 eV for the (100), and 1.955 eV for the (110). For all specimens the decrease in magnitude with temperature was of the order of 10−3 eV·K0−1. These data were compared with thermionic values for the work function minimum and with theoretical predictions. We note especially that the sign of the variation is opposite to that predicted by the Rasor-Warner theory in every case.

Adsorption of cesium fluoride on tungsten

The ability of fluorine to greatly reduce the work function and increase the thermal stability of cesium on tungsten is demonstrated by these results. Work function values of φ = 1.0 eV, stable up to ∼ 530°K, have been obtained. Migration studies indicate complete dissociation of CsF on adsorption so that surface migration of cesium occurs in the usual range of 150–250 °K, while fluorine does not become mobile until T> 380 °K. Adsorption and desorption of cesium on an F/W layer, formed by impinging CsF on a tungsten surface at 1200°K, is completely reversible leaving the F/W layer unchanged, provided a positive field of 1.8 MV/cm is applied. However, application of zero or negative fields leads to permanent removal of fluorine during thermal desorption of cesium at ∼ 900 °K. For Cs/F/W layers exhibiting values of φ < 2.0 eV and at sufficiently high applied negative fields, the removal of F − by field desorption occurs. Complete restoration of the slightly field desorbed Cs/F/W surfaces occurs, however, at 115°K. The temperature coefficient of the average work function for the Cs/F/W system is −2.4 × 10 −3eV/deg.

Low-Energy Electron Interaction with Coadsorbates on Tungsten

The effect of low-energy electrons (20–300 eV) impinging on cesium, mercury, and oxygen adsorbed on clean and oxygenated tungsten has been studied by field-emission techniques. Desorption cross sections were determined from the work-function changes and the known relationships between work function and coverage for the various systems studied. For cesium on clean tungsten and for mercury on both clean and oxygenated tungsten, the cross sections are less than 10−21 cm2, the minimum level of sensitivity for these experiments. Cesium is desorbed from an oxygenated tungsten surface by the impact of 250 eV electrons without disturbing the underlying chemisorbed oxygen layer. Cross sections for the desorption of cesium from oxygenated tungsten increase with increasing cesium coverage to a maximum of 8×10−20 cm2, and are insensitive to the amount of underlying oxygen. These results suggest that an important change in the character of the surface bond is effected by the underlying oxygen layer, in particular, a greater localization of the bonding electrons in the cesium-oxygen bond as compared to the cesium-tungsten bond.

Field Emission Microscope Studies of Coadsorption of Oxygen and Cesium on Tungsten and Rhenium

The field emission microscope has been used to investigate the coadsorption of cesium and oxygen on retractory metals.The minimum work function value was 1. 33 eV for the coadsorption layer on tungsten, which was formed by deposition of cesium on tungsten covered with oxygen, in comparison with 1.45 eV for cesium on clean tungsten. For rhenium, the minimum work function value was 1. 25 eV for the coadsorption layer, in comparison with 1.40 eV for cesium on clean surface.In the case of adsorption of oxygen on tungsten and rhenium covered with cesium sufficiently, the temperature at which the activation phenomena occured was above 800°K for tungsten (O-Cs : W) but slightly higher temperature for rhenium (O - Cs : Re).With increasing the oxygen coverage, the field intensity and the temperature at which the build-up phenomena occur increased.The activation energy for both surface migration and thermal desorption of cesium increased with the oxygen coverage.

Neutral and Ionic Desorption of Cesium from Tungsten

Rates of neutral and ionic desorption of Cs from tungsten field emitters have been measured as functions of temperature and coverage, and the corresponding activation energies and frequency factors of desorption determined. The results for neutral desorption agree extremely well with the data of Taylor and Langmuir for coverages from near zero to 1.4×1014 atom/cm2, but fall below the heats of these authors at higher coverages. The combination of neutral and ionic heats yields a value of Φ=5.3 eV for the zero coverage work function of the regions from which ionic desorption occurs, in agreement with previous results for potassium. It is shown that surface equilibrium leads to a different averaging than independent evaporation, so that the above result is to be expected. The phenomenon of phase segregation observed by Taylor and Langmuir at very low coverages under steady-state conditions is discussed and shown to be a kinetic rather than a thermodynamic effect.

The effect of electric field on adsorbed layers of cesium on various refractory metals

A study of the effect of high positive and negative electric fields on surface diffusion, thermodynamic equilibrium coverage and thermal desorption of cesium on tungsten and molybdenum has been accomplished by means of field emission microscopy. The results show field dependent variations of both the pre-exponential factor and activation energy of the surface diffusion coefficient written as D = D 0 exp (− E d F / kT). At temperatures sufficient to induce mobility, an increase or decrease in cesium coverage in the high field region of the emitter occurs with positive or negative fields respectively. Thermal desorption rates are not greatly affected by negative fields; positive fields, on the other hand, produce large variations in desorption rates due to field desorption. Most of the field effects can be analyzed in terms of μ F and 1 2 αF 2 interactions except for field desorption which involve more complex field interactions.

Field Desorption of Cesium from Tungsten

The field desorption of Cs from tungsten has been investigated at low coverage. The results can best be explained on the basis of a polar groundstate and suggest that an adiabatic transition is involved in the desorption process. On this basis the polarizability of adsorbed Cs is found to be close to that of the free atom. The results are very similar to those obtained with Ba.