Barium Layers Research Articles

PREPARING MICROCOMPOSITE POWDERS DOPED WITH ANTIMONY DIOXIDE AND PROPERTIES OF THICK FILMS BASED ON THOSE POWDERS

The goal of the work reported here was to develop a method of applying glass nanolayers of variable thickness to Sn0.9Sb0.1O2 particles and to investigate how this affects the electrical properties of thick-film resistors. We prepared Sn0.9Sb0.1O2 powders by calcinating coprecipitated tin and antimony hydroxides. Thin layers of aluminum, barium, and boron compounds were then precipitated from aqueous solutions onto powder particles. Nanolayers of glass in the BaO ― Al2O3 ― B2O3 system were obtained by dissolution followed by heat treatment. Resistive thick films made from such microcomposite powders have a higher resistivity than do those prepared by traditional methods and that resistivity changes little after repeated heat treatments. The thermal coefficient of electrical resistance decreases with increasing thickness of the glassy layer on the surface of the conducting particles.

Element specific imaging with high lateral resolution: an experimental study on layer structures.

Planar defects and individual layers in ceramic material are chemically imaged by high resolution energy-filtering TEM using a post-column imaging electron energy filter. Objects are barium layers in the cuprate superconductor NdBa2Cu3O7-delta (isostructual to YBa2Cu3O7-delta) as well as planar defects and precipitates of beta-WB in tungsten- and chromium-doped TiB2. The barium layers with a spacing of 0.42 nm in the cuprate are resolved in jump-ratio images using the Ba_N edge. In the boride system the beta-WB precipitates with thickness of 0.8 nm can be chemically imaged in elemental maps of B_K, Ti_L,Cr_L and W_M. The B as well as the Ti map show a decrease in intensity at the precipitates, whereas in the W map an increase in intensity is observed. The boron-deficient layers with a spacing of 0.38 nm in the beta-WB precipitate can be resolved in boron jump-ratio images. Additionally, defects containing single boron-deficient layers are chemically imaged. Hence structures in the dimension of interatomic distances can be imaged with respect to their elemental constituents. Although high resolution electron spectroscopic images contain strong interference contrast from elastic scattering, after normalization or background subtraction the element specific images are dominated by chemical contrast.

A New Ordered Oxygen-Deficient Manganite Perovskite: LaBaMn2O5.5. Crystal and Magnetic Structure

A new ordered oxygen-deficient perovskite, LaBaMn2O5.5, has been synthesized, using the topotactic deoxygenation of the ordered LaBaMn2O6 perovskite. The crystal structure of this manganite was solved in the space group Ammm (a = 3.86 Å, b = 8.19 Å, c = 15.47 Å) from neutron powder diffraction data. The [Mn2O5.5]∞ framework is build up of row of MnO6 octahedra and MnO5 pyramids running along a⃗. This structure is intermediate between that of the ordered LaBaMn2O6 perovskite and that of LaBaMn2O5: it consists of ordered barium and lanthanum layers stacked alternatively along c⃗, oxygen vacancies being located at the level of the La layers. The magnetic structure is original: manganese spins form ferromagnetic spin-ladders along the b⃗ axis that are antiferromagnetically coupled along the a⃗ and c⃗ axis. The HREM investigation shows the existence of 90° oriented domains and twinning phenomena.

YBaMn 2O 5: crystal and magnetic structure reinvestigation

The ordered oxygen-deficient perovskite YBaMn 2O 5 has been synthesized as a monophasic sample. A variable temperature neutron diffraction study of this phase has been carried out. The crystal structure of this manganite was solved in the space group P4/ nmm ( a = 5.547 Å and c = 7.649 Å). This oxide, which is closely related to the YBaCuFeO 5 structure, forms an alternate stacking of yttrium and barium layers along the c axis. It differs from the latter, however, by the existence of two distinct sites for manganese, corresponding to an ordering of the Mn 3+ and Mn 2+ cations, respectively. Elucidation of the magnetic structure of YBaMn 2O 5, from low temperature PND (powder neutron diffraction) data, leads to a G-type antiferromagnetic model; the superimposed Mn 2+/Mn 3+ charge order results in ferrimagnetic behavior for this phase and explains its magnetic properties, as obtained from susceptibility measurements.

Order−Disorder Phenomena in New LaBaMn2O6-x CMR Perovskites. Crystal and Magnetic Structure

Three new perovskites in the LaBaMn2O6-x family have been synthesized by controlling the oxygen pressure, during both synthesis and postannealing. Structural determination from powder neutron diffraction (PND) data shows that one form of LaBaMn2O6 is cubic (a = 3.906 Å), with a disordered distribution of La3+ and Ba2+ cations, whereas a second form of LaBaMn2O6 is tetragonal (a = 3.916 Å; c = 7.805 Å), with an alternate stacking of lanthanum and barium layers along c. The same La/Ba cation order is observed for the ordered, oxygen-deficient perovskite LaBaMn2O5, which is also tetragonal (a = 5.650 Å; c = 7.808 Å) and adopts a YBaCuFeO5-related structure. Elucidation of the magnetic structure of LaBaMn2O5, from low-temperature PND data, leads to a G-type antiferromagnetic model; the superimposed Mn2+/Mn3+ charge order results in ferrimagnetic behavior for this phase and explains its magnetic properties, as obtained from susceptibility measurements. In both forms of LaBaMn2O6, the PND data show a ferromagnetic contribution. The CMR properties of the “O6” forms exhibit a remarkable feature: TC is increased from 270 K for the disordered phase to 335 K for the ordered one, probably owing to the La/Ba ordering.

Diffractive scattering of a helium molecular beam from a Ba monatomic layer on a W(110) surface

The detailed two-dimensional ordered structures of a monatomic layer of barium (barium oxide) on a W(110) surface has been investigated by using a molecular beam scattering technique which utilizes a high brightness helium supersonic beam with high coherence. We have successfully obtained, for the first time, diffractive scattering of a helium molecular beam from an ordered structure on the W(110) surface covered with a sub-monatomic layer of barium (barium oxide).

CNDO studies of Y-Ba-Cu-O superconductors III. The axial oxygen position

The dependence of electronic properties of [Cu3O12]−17, [Ba8Cu3O12]−1 and [Y8Cu3O12]+7 model clusters on the axial oxygen positions, O(2), are investigated using quasi-relativistic CNDO/1 molecular orbital method. No evidence of the double-well potential of the Jahn-Teller type is found. Barium layer lowers the sensibility of O(2) electron properties to the Cu-O(2) stretching.

Electrochemical preparation of YBaCu oxide layer on copper

A new electrochemical method to prepare an oxide layer of yttrium, barium and copper on a copper substrate has been developed. A nitrate solution of yttrium, barium and copper in the molar proportion 1:2:3 with citric acid and ethylene glycol is stirred and heated to form a gel. The gel is decomposed by heat and ground into fine powders. The copper substrate is anodized in 4 M sodium hydroxide solution at 80–85 °C to form a flexible copper oxide layer. The powders are coated or pressed on the substrate and then heat treated. The electrochemically formed oxide layer is well sintered to the coated powders. The resulting oxide layer was observed by scanning electron microscopy, analysed by X-ray diffractometry and characterized in terms of its resistance change as the temperature falls. The presence of copper oxide shifts the superconductivity to semiconductivity.

Magnetic interaction at oxygen sites of YBa 2Cu 3O 6.25 studied by TDPAD with 19F isomer

We measured the internal field at oxygen sites in a single crystal of the magnetic compound YBa 2Cu 3O 6.25 with the time differential perturbed angular distribution method using 19F isomer. The asymmetry of the rotation pattern in 1.37 T decreases to about half at low temperature from the value at 300 K. At the same time, the rotation amplitude decays considerably while the frequency is uncharged. The present results suggest that the magnetic interaction between Cu ions in the plane layers is the exchange via oxygen while the oxygens in the barium and chain layers feel the weak dipole field.

Electronic properties of stoichiometric Ba and O overlayers adsorbed on W(001).

In order to understand better the mechanisms responsible for the lowering of the work function that is observed when surface layers containing barium and oxygen are adsorbed on refractory metals such as tungsten, we have initiated a series of full-potential linearized augmented-plane-wave (FLAPW) electronic-structure calculations on well-defined model systems representing the Ba/O/W interface. We report here our initial results for models in which c(2\ifmmode\times\else\texttimes\fi{}2) monolayers of barium and oxygen have been positioned on both sides of a five-layer W(001) film, with the adsorbate atoms being placed above the fourfold-hollow sites of the tungsten surface. Two different adsorbate configurations have been investigated: ``tilted,'' where the adsorbed monolayers have been arranged so that the barium and oxygen atoms each cover a different fourfold-hollow site in the c(2\ifmmode\times\else\texttimes\fi{}2) unit cell, and ``upright,'' where the overlayers are aligned vertically so that both adsorbates lie above the same site. We have minimized the total energy of the system to determine the optimal adsorbate positions within this set of configurations. We find that the calculated work function of the clean five-layer tungsten slab (4.65 eV) is lowered by approximately 2 eV by the adsorption of the barium and oxygen surface layers in either configuration, but the tilted structure has a significantly lower energy than does the upright. In addition, the position of the oxygen 2s state, which is very sensitive to the adsorbate geometry, strongly favors the tilted model. In both cases we find evidence of significant bonding between the d-like surface states of the tungsten substrate and both the Ba d and the oxygen 2p adsorbate levels. As a result, multiple dipoles are formed at the interface, and the competition between these polarized charge distributions leads to a net lowering of the work function.

Annealing treatment effects on structure and superconductivity in Y1Ba2Cu3O9−x

We report the effects of heat treatment and ambient on the structure and superconducting properties of Y1Ba2Cu3O9−x. The structure undergoes an orthorhombic-to-tetragonal transition on heating at about 700 °C, caused by oxygen loss and disordering of oxygen vacancies on the copper plane between the barium layers. Heat treatments that promote maximum ordering of the oxygen vacancies result in superior superconducting properties.

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.

An XPS investigation of the interaction of CH 4, C 2H 2, C 2H 4 and C 2H 6 with a barium surface

The generation and pumping of hydrocarbon gases by a barium getter layer in electronic vacuum devices has been investigated by characterizing a barium film in an ultra high vacuum equipment by means of XPS before, during and after exposures to respectively CH 4, C 2H 2, C 2H 4 and C 2H 6. The reaction conditions (temperatures and pretreatment of the surface, background pressure and exposure doses) closely resemble those in electronic vacuum devices. The probability that a barium layer will react with CH 4 and C 2H 6 was below the detection limit. C 2H 2 and C 2H 4 give rise to the formation of barium carbide compounds and with a high reaction probability. In addition, the interaction with C 2H 2 reveals the formation of carbon-containing surface complexes. Investigations by means of XPS on the C Is spectral features show the presence of at least two groups of carbon-containing surface complexes, which behave differently in response to moderate heating and to an exposure to water vapour. In cases where oxygen is present at the surface, oxygen-containing (hydro) carbon adsorbates are present too. XPS observations of the behaviour of these surface complexes show similarities with reaction steps in the mechanisms proposed for the hydrogenation of CO in the Fischer-Tropsch synthesis of hydrocarbons. Low-pressure hydrogenation of these adsorbates containing hydrocarbons and oxygen can led to the formation of hydrocarbon gases in electronic vacuum devices.

Preparation of barium and uranium targets on thick backings

Targets of 135Ba and 235U have been prepared by the technique of heavy ion sputtering. Rolled foils of 208Pb and 197Au were used to support 250–500 μg/cm 2 layers of barium. Uranium films have been prepared by sputtering UO 2 onto 1 mg/cm 2 titanium foils. Uranium deposit thicknesses of 300 to 1800 μg/cm 2 have been prepared.

Investigations on barium getter films by AES and XPS

AES and XPS techniques are used to study the chemical behaviour of a vapour deposited barium layer before, during and after exposure to common residual gases in a picture tube and to electrons under conditions that closely resemble those of a barium getter in electronic vacuum devices. Measurement of the surface composition of the barium layer by these techniques during exposure provide data on the kinetics of the reaction with these gases. The XPS spectra of O 1s and C 1s are used to distinguish between oxide, hydroxide, carbide and carbonate formation. Investigations on the higher order reactions of the barium film with residual gases reveal remarkable differences between the two techniques, primarily due to electron bombardment effects. Thin Ba(OH) 2 and BaCO 3 films are found to change in chemical composition for electron doses (at 2 keV) as low as 2 × 10 -3 C/cm 2 and 2 × 10 -2 C/cm 2, respectively.

Auger studies of the initial reaction probability of a barium film with oxygen, water, carbon monoxide and carbon dioxide

The reaction of oxygen, water vapour, carbon monoxide and carbon dioxide with films of barium evaporated on sputter-cleaned nickel has been studied at room temperature. The initial reaction probability and the chemical state of the exposed film were investigated by means of Auger electron spectroscopy. The initial increase of oxygen and carbon Auger signals was found to be (within the experimental error) linearly dependent on degree of exposure up to pressures of 6 × 10 −6 Pa. Its dependence on film thickness (up to thicknesses of 25 A) showed that the initial reaction occurs apparently homogeneously through the barium layer. Assuming a flat surface the initial reaction probability for oxygen is 0.4, for water vapour 0.6, for carbon monoxide 0.2 and for carbon dioxide 0.6.

A proposed physical model for the impregnated tungsten cathode based on Auger surface studies of the Ba-O-W system

Auger and work function measurements have been made on a tungsten surface containing absorbed layers of (a) barium, and (b) barium and oxygen. The experimental techniques employed enabled one to distinguish the quantitative effects of oxygen on the barium-oxygen- tungsten surface reaction. It was found that the barium on an impregnated tungsten cathode is not oxidized barium (BaO), nor is it present as metallic barium chemisorbed on tungsten; rather it seems to be in an intermediate state probably as a coadsorbed (barium and oxygen) layer on tungsten. As a result of these studies a physical model for the impregnated tungsten cathode is proposed. The model assumes that the surface of the cathode, during life, has an adsorbed surface layer of a monolayer or less of both barium and oxygen on the surface. With life the barium is depleted leading to a higher work function surface. End of life occurs when the barium partial monolayer is so low that the resultant high cathode work function cannot sustain the required emission level. Recent NASA life test results on TWT type tubes are reported and explained by the proposed model.

Investigation of electron interaction in adsorption by static skin effect method

The reflection of the conduction electrons from surfaces of a thin ( d = 100 μm) monocrystalline plate of tungsten [(110) face] is investigated by the method of static skin effect. It is found that by heating the crystal in vacuum up to 2500 °K, the oxide layer is removed and the specular reflection of the conductive electrons increases. The adsorption of the monomolecular layers of barium and copper atoms markedly increases diffuse reflection. A resonance mechanism of carrier interaction in the metal with adsorbed atoms is discussed.

Photosensitized oxidation through stearate monomolecular films.

Abstract— The photosensitized oxidations of rubrene by methylene blue and of diphenylan‐thracene by eosin have been studied where the sensitizer and the oxidizable substrate are separated by oxygen permeable layers of barium or cadmium stearate monomolecular films. The rate of the photosensitized oxidation reaction was followed by measurements of the hydrocarbon fluorescence, as a function of the stearate layer thickness. It has been found that the sensitized reaction is observable for thickness up to 500 Å and depends only on the deactivation process of the excited species (singlet oxygen) diffusing through the layers. It has been found also that half of the excited oxygen molecules are deactivated after a diffusion path of 115 A. The starting hydrocarbon can be regenerated by heating the sample after the photoreaction to approximately 120°C showing the transannular peroxide nature of the oxidation product.

The crystal structure and refinement of ferrimagnetic barium ferrite, BaFe12O19*

The crystal structure of barium ferrite, BaFe12019,has been determined and refined with the use of three-dimensional counter data. The symmetry is P63/mmc; the unit cell whose dimensions are a = 5.893 Aand c = 23.194 A, contains two formula units. The structure is built up of ten close-packed (both cubic and hexagonal) layers of barium and oxygen atoms. The sequence of the layering in