Browsing by Author "CABRERA, AL"

Now showing 1 - 9 of 9
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    A COMMERCIAL GAS ANALYZER ADAPTED FOR THERMAL-DESORPTION STUDIES
    (1993) CABRERA, AL; MORALES, E; ALTAMIRANO, L; ESPINOZA, P
    A system for thermal desorption studies can be implemented very straightforwardly from a commercial instrument designed for gas analyses. The commercial instrument incorporates a quadrupole mass spectrometer and the vacuum station to operate the spectrometer. The instrument is modified with a stainless steel six-way cross device which accommodates a sample manipulator, a leak valve and an Ar sputtering gun. The sample foil is resistively heated with very linear rates using a custom made power supply. Descriptions of the custom made sample manipulator and power supply are given. Results of hydrogen and carbon monoxide desorption from palladium foil are discussed.
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    ADSORPTION OF CARBON-DIOXIDE BY BARIUM-TITANATE - EVIDENCE OF ADSORPTION PROCESS MEDIATED BY A DIPOLE-DIPOLE INTERACTION
    (1994) CABRERA, AL; VARGAS, F; ZARATE, RA
    This work presents desorption curves of CO2 for powder of barium titanate with main particle sizes of 1000, 500, 200, 100 and 50 micrometers in diameter. The percentage of surface area occupied by CO2 molecules is calculated after measuring the total surface area of the samples using N-2 single point BET. Adsorption of CO2 and N-2 were measured with a Quantasorb instrument using flowing helium. Nitrogen or CO2 concentration changes in the flowing mixture, during desorption, were sensed by a specially designed thermal conductivity detector. The ratio of surface area covered by CO2 with respect to N-2 changes from 50% for particle sizes between 1000 and 200 mu m down to 19% for 50 mu m particles. Spontaneous polarization measurements for samples with main particle sizes of 50 an 100 mu m were made and they show a weak ferroelectric behavior, indicating that ferroelectricity is destroyed for these small particles. A tentative explanation of these results is the assumption that the adsorption of some CO2 molecules on the ferroelectric substrate is mediated by a dipole-dipole coupling between the CO2 molecules and the substrate.
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    ADSORPTION OF CARBON-DIOXIDE BY FERROELECTRIC LITHIUM-NIOBATE
    (1995) CABRERA, AL; VARGAS, F; ALBERS, JJ
    Desorption curves of CO2 are presented for powder of lithium niobate with main particle sizes of 1000, 500, 200, 100 and 50 mu m in diameter. The percentage of surface area occupied by CO2 molecules is calculated after measuring the total surface area of the samples using N-2 single point BET. Adsorption of CO2 and N-2 were measured with a Quantasorb instrument using flowing helium. The ratio of the surface area covered by CO2 with respect to N-2 changes from 11.5% for the 50 mu m particles down to 6.7% for the 100 mu m particles. Adsorption of CO2 for particles equal to or larger than 200 mu m could not be measured with our technique. XPS results on a single piece of lithium niobate indicated that CO2 is adsorbed on the surface when only Nb3+ ions are present. Induced crystallographic transformation of the crystal by an applied 2.5 kV field changes its surface composition from an initial mixture of Nb5+ and Nb3+ ions to pure Nb5+ ions and as a consequence of this change, adsorbed CO2 was released. A tentative explanation of these results is that the adsorption of the CO2 molecules by the ferroelectric substrate is due to a particular local electrostatic potential created by surface ions, in agreement with Pacchioni's work on other oxide surfaces.
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    MODIFICATION OF THE ELECTRICAL-RESISTANCE OF THIN COBALT FILMS UPON THE ADSORPTION OF CARBON-MONOXIDE
    (1995) CABRERA, AL; GARRIDOMOLINA, WH; VOLKMANN, UG
    Thin films of pure Co with thicknesses ranging between 10 and 50 nm were produced in UHV conditions. The films were deposited onto mica substrates with dimensions of 1 cm x 1 cm x 0.1 cm. Thin gold wires were attached to the ends of the films in order to monitor their electrical resistivity. The films were introduced into a UHV chamber which consists of a quadrupole mass spectrometer and an Ar-sputtering gun for cleaning. The films were exposed to 10(-6) torr of high-purity CO and the resistivity of the films was monitored during exposure. The resistivity changed in a ''sawtooth'' fashion similar to the changes observed by Pick in the Nb/H-2 system. The aim of these studies is to correlate changes in resistivity of the film with well-characterized adsorption states of CO on the Co surface. Carbon monoxide desorption from the cobalt films was studied (using a mass spectrometric method in an ultrahigh vacuum system) and two carbon monoxide desorption peaks were observed identical to prior work using Co foils. These two states correspond to molecular CO and presumably dissociated CO. Changes in resistivity of the thin Co films appear to be related only to the molecular CO adsorbed on the Co surface.
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    OXIDATION PROTECTION OF MILD-STEEL BY COATINGS MADE WITH ALUMINUM ALKYLS
    (1991) CABRERA, AL; ZEHNER, JE; ARMOR, JN
    The deposition of Al on 1010 steel from mixtures of trimethyl (TMA) and triethyl (TEA) aluminum alkyls in argon and hydrogen was studied. Unlike earlier work with SiH4/H2 no aluminum diffusion onto the steel was observed even when the samples were heated to 800-degrees-C. Coatings obtained with TEA at temperatures between 300-400-degrees-C resulted in overlays of elemental Al or Al oxide after exposure to ambient conditions. No deposition was obtained at temperatures higher than 400-degrees-C in a hot-wall reactor due to rapid decomposition of TEA on the walls of the reactor. Deposition with TMA at temperatures between 400-800-degrees-C resulted in overlays of Al carbide. For deposition at temperatures higher than 500-degrees-C, a cold-wall reactor was used. The Al overlay coatings resulting from TMA were dense, homogeneous, and adhered well to the steel. Both kinds of coatings provided oxidation protection to the steel in air at 800-degrees-C. The performance of the coatings produced with TMA was superior to the coatings produced with TEA and similar to the performance of an Alonized steel.
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    SIMULATION OF HYDROGEN THERMAL-DESORPTION FROM TRANSITION-METALS USING A MULTIPLE-SITE HOPPING MODEL
    (1994) CABRERA, AL; WEINKETZ, S; CABRERA, GG; MARSHALL, G
    A model to simulate thermal desorption is described. The model is applicable to systems where adsorbate-adsorbate lateral interactions are negligible and fast recombination occurs, as in the case of hydrogen desorption from transition metal surfaces. Thus, our model uses simple rules to simulate desorption. If two atoms are sitting in neighboring sites, then there is a high probability for the desorption of both to happen. If an atom does not have a neighbor, it will move to adjacent sites, in a random walk, until it finds a neighbor to recombine. The desorption rate plotted as a function of the logarithm of the number of random steps results in perfectly shaped Gaussian curves. Dimensionless curves of the rate versus the logarithm of the number of random steps were obtained for different coverages from 0.1 to 1.0 using this method. These curves are in perfect agreement with calculated curves for second-order desorption arising from the corresponding Polanyi-Wigner differential equation.
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    STRUCTURAL-CHANGES INDUCED BY HYDROGEN ABSORPTION IN PALLADIUM AND PALLADIUM-RUTHENIUM ALLOYS
    (1995) CABRERA, AL; MORALES, E; HASEN, J; SCHULLER, IK
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    STUDIES OF CARBON-MONOXIDE AND HYDROGEN ADSORPTION ON NICKEL AND COBALT FOILS AIMED AT GAINING A BETTER INSIGHT INTO THE MECHANISM OF HYDROCARBON FORMATION
    (1994) CABRERA, AL; GARRIDO, WH; VOLKMANN, UG
    Systematic studies of adsorption of hydrogen and carbon monoxide on polycrystalline surfaces of nickel and cobalt have been carried out. The aim of these studies was to pin a better insight into the catalyzed formation of hydrocarbons from H-2-CO mixtures. We have studied the adsorption of these gases by means of thermal desorption spectroscopy (TDS) on nickel foils and powders. More recently, we have obtained desorption spectra of hydrogen adsorbed on cobalt foils and powders. In this work we described desorption spectra of carbon monoxide on cobalt foils. Carbon monoxide desorption from cobalt foils was studied in a similar way as prior work, using a mass spectrometric method in an ultra high vacuum system. Two carbon monoxide desorption peaks were observed. These two states correspond to molecular CO and presumably dissociated CO, as it is observed in the case of stepped surfaces of Ni and Co single crystals. An activation energy of around 4.0 kcal/mol is obtained for the molecular state while for the dissociated state the energy is coverage-dependent with a value between 8.0 and 15.0 kcal/mol. The carbon monoxide desorption peaks were fitted with near Gaussian curves which facilitates the analysis of the data to obtain activation energies for desorption. Kinetic parameters for carbon monoxide and hydrogen desorption from nickel and cobalt foils are provided and compared with already published data involving single crystals.
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