Showing papers in "Journal of The Electrochemical Society in 1964"

High‐Temperature Oxidation II . Molybdenum Silicides

Abstract: The oxidation of , , and has been studied at temperatures of 1300°–2000°K, and at oxygen partial pressures of 2–20 Torr. Below 1970°K, at oxygen pressures around 10 Torr, measurements of the rate of total oxygen consumption vs. time show an initial period of high linear rate, an intermediate region where the rate declines sharply with time, and a final plateau region of virtually zero oxidation rate. For each of the silicides, the total oxygen consumed prior to the attainment of the plateau decreases with temperature, and under given experimental conditions is largest for , less by a factor of about six for , and less by another factor of six for . The oxidized samples display a smooth glassy outer oxide layer that bridges across the tops of the cracks in the alloys and an inner oxidized zone, consisting of several irregular phases. Electron micro‐probe analysis and x‐ray photomicrographs of the oxidized specimens show that the outer protective oxide is pure , within the limits of detectability, i.e., the outer oxide contains less than 0.1% Mo. The inner oxide layer contains , molybdenum rich phases, including the terminal solid solution phase, and the original alloy phase. Evidence is presented that suggests that the oxide layers grow by inward diffusion of oxygen to the oxide/alloy interface where preferential oxidation of silicon occurs. The excellent oxidation resistance of the molybdenum silicides derives from the formation of this continuous layer of pure silica. Above 1970°K, at oxygen pressures above 10 Torr, an increased oxidation rate is observed due to formation of at the alloy/oxide interface, which causes rupture of the protective oxide. Below 1970°K, at oxygen pressures near 2 Torr, the rate of oxidation of is linear at all times since the oxygen partial pressure is insufficient to maintain a protective layer on the alloy surface.

The Mechanism of Hydrogen Evolution on Iron in Acid Solutions by Determination of Permeation Rates

Abstract: The permeation of electrolytic hydrogen in Armco iron membranes has been investigated by the electrochemical technique described elsewhere. In purified solutions it has been found that the permeation rate follows the equations previously derived. This rate is inversely proportional to the thickness and to the surface coverage with atomic hydrogen θ. The intercept of the plot of reciprocal of the permeation rate against the thickness varies inversely as the coverage as also predicted by the equation. Anomalies in the thickness dependence often reported in the literature have been traced to the adsorption of impurities and to redeposited iron, which then limit the over‐all permeation rate. The variation of permeation with cathodic overpotential has been determined. It is found that at moderate overpotentials indicating a slow discharge rate‐determining step followed by Tafel recombination. At higher overpotentials , indicating a constant coverage brought about by a transition to electrochemical desorption. In confirmation of this mechanism attempts have been made to determine the pseudocapacitance of the electrode system by galvanostatic cathodic transients. The results show the complete absence of pseudocapacity in acid solutions, thus proving that discharge of hydrogen ions is the rate‐determining step.

Adsorption of n‐Decylamine on Solid Metal Electrodes

Abstract: The adsorption from solution of n‐decylamine on five metals has been measured using a new method, which consists of adsorbing radioactively labeled organic compounds on a metal tape electrode and measuring the reulting radioactivity from the tape. After calibration of the counters and determination of the roughness factor of the tapes by the B.E.T. method using krypton, the amount adsorbed per actual square centimeter is calculated. The coverage has been determined as a function of potential and bulk concentration, and the free energy of adsorption has been calculated for each system. A consideration of thermodynamic cycles for water and for the organic absorbate leads to a relationship between the free energy of adsorption and the solubility and vapor pressure of the organic compound. The adsorption arises largely from metal‐adsorbate dispersion interaction differences between water and the organic. These vary little with the metal. The potential dependence arises primarily from the field dependence of the metal‐water interaction.

The Kinetics of Anodic Oxidation of Iron in Neutral Solution I . Steady Growth Region

Abstract: Iron was anodically oxidized under both potentiostatic and galvanostatic conditions. It was found that the rate of film growth could be expressed by the over‐all equation where QT is the film thickness, E the potential, and κ', β, and B are constants. A model is proposed based on a "place‐ exchange" mechanism in which all the M‐O exchanges in a given row take place simultaneously. β is then related to the transfer of oxygen from the OH− in the solution to the oxide and B is related to the activation energy for an equivalent ion‐pair exchange. The activation energy for a single place exchange was calculated to be 3.5 kcal. This mechanism would lead to an increase of activation energy with thickness and a probable change of mechanism at some finite thickness.

Electrolytic Reductive Coupling I . Acrylonitrile

Abstract: The electrolysis of concentrated solutions of acrylonitrile in aqueous tetraethylammonium p‐toluenesulfonate at lead or mercury cathodes and at controlled pH yields adiponitrile in virtually quantitative yields and at current efficiencies close to 100%. If the catholyte has acrylonitrile concentrations much below 10% or contains alkali metal cations, increasing quantities of propionitrile appear as by‐product. An explanation is offered for this shift in product distribution. Variations in laboratory cell design and supporting electrolyte composition are discussed.

Fluorescent Properties of Some Europium‐Activated Phosphors

Abstract: The following europium‐activated phosphors were prepared: gadolinium oxide, gadolinium borate, gadolinium phosphate, lanthanum borate, and europium phosphate. The phosphors show an emission spectrum characteristic of the europium. From the excitation spectrum of some phosphors it is deduced that energy absorbed by gadolinium ions can be transferred to the europium ions. Most phosphors have a relatively high brightness at elevated temperatures. has a rather high luminous efficiency with cathode‐ray excitation.

Luminescent Behavior of the Rare Earths in Yttrium Oxide and Related Hosts

Abstract: The absorption, excitation, and fluorescence spectra of Y 2O3 : Eu are presented and interpreted. The high luminescent efficiency of this material and its unusual performance at elevated temperatures are discussed. The comparative behavior of other rare earths in yttrium oxide and of the rare earths in certain other oxides of yttrium and lanthanum are also discussed briefly.

Anodic Oxidation of Lead at Constant Potential

Abstract: Lead and lead alloys were subjected to anodic oxidation in sulfuric acid solution under potentiostatic conditions. The current‐time transients reveal that, as a result of inhibited ionic diffusion across the corrosion film, a secondary corrosion reaction takes place below the lead sulfate layer built up in the primary phase. The experimental findings are explained in terms of a high local pH in the interior of the corrosion film. This hypothesis is shown to be in accord with self‐depassivation phenomena and cathodic constant‐current stripping experiments. The potential‐determining reactions in the corrosion film are discussed in the light of new pH‐potential diagrams, constructed under consideration of the formation of basic lead sulfates. A model for the composition of the corrosion film is developed which conforms to all experimental findings, including the phenomena relating to the ohmic resistance of the film.

The Transport of Gallium Arsenide in the Vapor Phase by Chemical Reaction

Abstract: The heat contents and entropies have been derived as a function of temperature for gallium arsenide and the gaseous species which exist in equilibrium with it, in the temperature range 800°–1200°K when iodine or hydrogen chloride and hydrogen are also present. The important gaseous species are shown to be , , , , , I2, I, , As4, and H2. Under certain conditions the species As2, , and Ga (liq.) may also be present. The free energy changes for reactions among the above species are derived. They are shown to be consistent with those isothermal equilibria that have been directly measured. The free energy changes are used, together with the conservation equations for nondepositing components (I, Cl, H, and As in excess of the stoichiometric ) to give the partial pressures of all the important species at any temperature in a temperature gradient in a constant pressure system. The results are applicable to the calculation of the flux of gallium arsenide in open flow or closed tube (diffusive transport) systems, and they are in good agreement with experimentally measured rates of transport in the system in an open flow apparatus.

Self‐Activated Luminescence of M2+ Niobates and Tantalates

Abstract: Among 22 compounds examined for photoluminescence, only Mg/sub 4/Nb/sub 2/O/sub 9/, CaTa/sub 2/O/sub 6/, and the metaniobates of Mg , Ca, Zn, and Cd show appreciable room temperature emission. With the exception of Mg/sub 4/Nb/sub 2/O/ sub 9/, all of these have the columbite structure; moreover, the only essentially nonluminescent columbite-type compound noted was ZnTa/sub 2/O/sub 6/. The absorption edges of the luminescent niobates lie at shorter wavelengths than those of otherwise similar compositions. This indicates a relationship between luminescence and the separation of energy states within the Nb--O bond, as influenced by structure. (auth)

Effect of Cold‐Work on Corrosion of Iron and Steel in Hydrochloric Acid

Abstract: Zone‐refined iron corrodes at the same rate whether cold‐worked or annealed. Cold‐working of iron containing 0.007–0.15% carbon increases the corrosion rate in , the rate increasing still more after heat treating the iron at 77°–100°C for 2 hr. Cold‐working of iron containing 0.01–0.02% nitrogen increases the corrosion rate only after heat treatment at 77°–200°C. Two maxima in the rate appear for heat treated N alloys but only one for C alloys, corresponding to precipitation of two different nitrides but only one iron carbide. Heat treatment above 200 °C reduces the corrosion rate for both alloys, but pure iron is not affected by any heat treatment schedule. Polarization measurements show that corrosion in all instances is controlled cathodically.These results are explained by lower H2 overvoltage of imperfection sites introduced by cold‐work, and associated with C or N atoms (Cottrell atmospheres). In absence of C and N, imperfection sites have the same apparent H2 overvoltage as iron, and their tendency to dissolve anodically is not pronounced. The C and N atoms segregated at imperfections come from interstitial sources as well as through dissociation of carbides and nitrides, the compounds being less stable than the corresponding Cottrell atmospheres. Increase in corrosion caused by cold‐work, especially when followed by heat treatment at 77°–200°C, is in part explained by annealing out some imperfections, such as lattice vacancies, causing re‐formation of carbides or nitrides in finely divided form. Increased peripheral area of the precipitate increases galvanic action. Carbides dissociate more rapidly at low temperatures than do nitrides, accounting for the observed difference in behavior of Fe‐N and Fe‐C alloys. Effect of cold‐work on corrosion of metals in general is greatest when a second phase precipitates to form active galvanic cells, whereas the increase in internal energy of a disarrayed metal lattice has little if any effect. Preferred grain orientation of surface metal sometimes resulting from cold‐work may either increase or decrease corrosion.

Evaluation of Passivated Integrated Circuits Using the Scanning Electron Microscope

Abstract: By examining passivated silicon integrated circuits in the scanning electron microscope, the surface contours of p‐n junctions have been mapped, potential drops across integrated resistors have been observed, and physical characteristics of the oxide surface, evaporated leads, and bonded gold wires have been determined. Typical faults discovered by this method of testing include poor registration, improperly masked diffusions, harmful and nonharmful surface scratches, poor evaporated interconnections, and defective passivation oxide layers. Most junctions in a 40 mil square integrated circuit can be delineated with 1μ resolution in approximately 1 min by this technique.