Showing papers in "Journal of The Electrochemical Society in 1980"

Application of A-C Techniques to the Study of Lithium Diffusion in Tungsten Trioxide Thin Films

Abstract: The small signal a-c impedance of the cell Li]LiAsF6 (0.75M) in propylene carbonatel Lip V~O3 thin film on tin oxide covered glass substrate has been measured at room temperature as a function of frequency from 5  10 -4 Hz to 5 X l0 s Hz at various open-circuit voltages. The diffusion equations have been solved for the appropriate finite boundary conditions, and analysis of the impedance data by the complex plane method yields values for the chemical diffusion coefficient, the component diffusion coefficient, the partial ionic conductivity of lithium, and the thermodynamic enhancement factor for LiyWO3 as a function of y. The films of WO3 were prepared by vacuum evaporation and were largely amorphous to x-rays. The chemical diffusion coefficient has a value of 2.4  10 -12 cm2/sec at y _-- 0.1, increasing to 2.8 X 10 -11 cm2/sec at y ---- 0.26. At short times (t ~ 0.5 sec) the interracial charge transfer reaction is important, but at longer times the rate of lithium injection is determined by the diffusion kinetics.

Growth Kinetics and Polymorphism of Chemically Deposited CdS Films

Abstract: The kinetics of growth for chemical deposition of films from alkaline solutions of cadmium salts has been studied with respect to temperature of deposition and the relative concentrations of the various reactants in the solution. It has been established that the growth of the film takes place either by ion‐by‐ion condensation of Cd+2 and S−2 ions or by adsorption of colloidal particles of formed in the solution, depending on the various deposition parameters and the method of preparation. The former process of growth results in thin, hard, adherent and specularly reflecting films, whereas the latter results in thick, powdery and diffusely reflecting films. Occurrence of different polymorphic phases of (hexagonal and cubic) has been observed under different growth conditions. A model for growth mechanism has also been proposed.

Electrocatalytic Electrodes for the Polysulfide Redox System

Abstract: Porous electrocatalytic electrodes for the polysulfide redox system, containing one of various metallic sulfides (especially of Co, Cu, Pb) are described. Emphasis is placed on their use as counterelectrodes in photoelectrochemical cells employing polysulfide electrolytes. Their activity is measured as a function of electrolyte temperature and composition. The ratio of S to S2−, and through it the local redox potential of the solution, is shown to be an important factor controlling electrode activity. The short and long term stability of the electrodes, as cathodes, is discussed, and it is shown that, when used in conjunction with photoanodes, and may poison the photoelectrode surface, thereby reducing total cell efficiency.

A Mechanistic Study of O 2 Reduction on Water Soluble Phthalocyanines Adsorbed on Graphite Electrodes

Abstract: : Co and Fe tetrasulfonate phthalocyanines (M-TSP), adsorbed at monolayer levels on graphite surfaces, have been found to have a pronounced catalytic effect on the O2 reduction process in both acid and alkaline solutions. The kinetics have been examined with the rotating ring-disk electrode technique. Co-TSP promotes the O2 reduction process via 2-electrons to give peroxide whereas Fe-TSP promotes a 4-electron reduction to give water. (Author)

The Formation of Ferrous Monosulfide Polymorphs during the Corrosion of Iron by Aqueous Hydrogen Sulfide at 21°C

Abstract: The initial stages of corrosion of iron by unstirred saturated aqueous H/sub 2/S solutions at 21/sup 0/C and atmospheric pressure have been examined as a function of time, pH (from 2 to 7, adjusted by addition of H/sub 2/SO/sub 4/ or NaOH), and applied current. Detailed examination of the morphology and phase identity of the corrosion products has led to a qualitative mechanistic understanding of the corrosion reactions. Mackinawite (tetragonal FeS/sub 1-x/) is formed by both solid-state and precipitation processes. Cubic ferrous sulfide and troilite occur as precipitates between pH = 3 and pH = 5, subsequent to metal dissolution upon cracking of a mackinawite base layer formed by a solid-state mechanism. The corrosion rate, and the relative amounts of these phases produced, are controlled by pH, applied current, and the degree of convection. The corrosion rate increases with decreasing pH; the quantity of precipitated material peaks near pH = 4, below which dissolution becomes the dominant process as the solubilities of the sulfide solids increase. Significant passivation was observed only at pH = 7, when the initial mackinawite base layer remained virtually intact. The solid-state conversion of cubic ferrous sulfide to mackinawite at 21/sup 0/C was monitored by x-raymore » diffractometry. The resulting kinetics are consistent with the Avrami equation for a nucleation and growth process with a time exponent of 3.« less

Spectroscopic Analysis of the Interface Between Si and Its Thermally Grown Oxide

Abstract: Spectroscopic ellipsometric data from 1.5–5.8 eV has been analyzed to determine the in situ optical response of the interface between Si and its thermally grown oxide. Results for , , and sample orientations show an interface of width consisting of atomically mixed Si and O of average stoichiometry . The optical data are not consistent with either microroughness at the interface or an abrupt transition between crystalline Si and or a significant accumulation of amorphous Si at the interface, but rather support a gradual transition region. The thickness and the average λ5461 refractive index of this region are in agreement with the fixed‐wavelength results, and , of Taft and Cordes. The oxide on the surface is shown to have a density about 1.2% less than that of corresponding oxides on and surfaces.

Electrostatic Binding of Metal Complexes to Electrode Surfaces Coated with Highly Charged Polymeric Films

Abstract: Previous reports in which metal complexes have been attached to electrode surfaces coated with polymeric molecules have depended upon the formation of covalent or coordination bonds in the attachment procedure (1-4) Such schemes can be quite successful but depending, as they do, on rather specific surface chemistry, they are not applicable to as wide a variety of metal complexes as might be desirable We have observed that coating graphite electrodes with polymers bearing charged ionic groups produces surfaces which strongly bind multiply-charged metal complexes bearing charges opposite to that on the attached ionic polymer By exploiting this observation it is entirely possible that virtually any desired metal ion can be attached in large quantities to electrode surfaces by coordinating the metal ion with ligands that produce a multiply-charged complex ion

On Predicting the Maximum Efficiency of Phosphor Systems Excited by Ionizing Radiation

Abstract: A model allowing theoretical prediction of the maximum possible efficiency for any cathode‐ray or x‐ray phosphor is proposed, based on an earlier statistical analysis of e‐h pair generation by van Roosbroeck. The model is independent of the detailed band structure of a particular solid, and predicts the limiting yield of e‐h pairs, Y, in terms of an energy loss ratio for energetic particles . The novelty of the model lies in the emphasis placed on competitive phonon losses during the carrier avalanching process, and it is probably successful because of the dominant role played by carriers with energies a few times greater than threshold where electrons and holes may be expected to behave similarly. A quantitative estimate of the average energy per e‐h pair in a particular phosphor can be made by reference to an index of efficiency, which is proportional to van Roosbroeck's parameter but which is defined in terms of accessible physical constants of the phosphor host lattice: ∈∞, ,, and . The predicted efficiencies are in good agreement with measured values for widely different phosphor types, suggesting that the model is quite generally applicable and may be used in a predictive fashion in the search for new and efficient phosphor systems. Finally, the relationships between the present theory and previous approaches involving plasmon excitation are discussed.

On High Temperature Oxidation of Chromium I . Oxidation of Annealed, Thermally Etched Chromium at 800°–1100°C

Abstract: Oxidation of annealed, thermally‐etched chromium has been studied at different oxygen pressures ranging from in the temperature range 800°–1100°C. A varied oxidation behavior is observed: at and 800°C the oxidation is approximately logarithmic, while above 900°C it is parabolic followed by repeated breakdowns and protective stages. At reduced oxygen pressures the oxidation behavior changes and becomes linear at the lower oxygen pressures. The different kinetics are correlated with properties of the scales which may crack, wrinkle, or balloon depending upon the reaction conditions. Large compressive stresses are built up in the scales during oxidation and the ability of the scale to deform increases dramatically with decreasing partial pressure of oxygen. Under most conditions the oxide scales become detached from the metal substrate. It is concluded that volume diffusion and transport along cracks and grain boundaries contribute to the growth of scales.

Resistivity‐Dopant Density Relationship for Phosphorus‐Doped Silicon

Abstract: New data for the resistivity‐dopant density relationship for phosphorus‐doped silicon have been obtained for phosphorus densities between 1013 and 1020 cm−3 and temperatures of 296°K (23°C) and 300°K. For dopant densities less than 1018 cm−3, results were calculated from resistivity and junction capacitance‐voltage measurements on processed wafers. For more heavily doped material, data were obtained from Hall effect and resistivity measurements on specimens cut from bulk silicon slices. The results differ by 5–15% from the commonly used Irvin curve, always in the direction of lower dopant density for a given resistivity. For comparison with the electrical measurements, phosphorus densities were also obtained by neutron activation analysis and the photometric technique. The values from these methods were within 10% of the electrical results. Analytical fits were determined for the resistivity‐dopant density product as a function of resistivity and dopant density for temperatures of 23°C and 300°K. Similar fits were obtained for the calculated electron mobility as a function of resistivity and electron density.

Solution Growth of CdSe and PbSe Films

Abstract: Cadmium selenide and lead selenide films have been deposited by a solution growth technique on single crystal germanium and silicon, glass, mica, and copper substrates. The effect of bath parameters (pH, temperature, and relative concentration of reactants) and the nature of the substrate on the rate of deposition and terminal thickness has been established. The structure of the films has also been studied. Based on the experimental results, a growth model has been proposed.

GaAs Oxidation and the Ga‐As‐O Equilibrium Phase Diagram

Abstract: The phases formed from the oxidation of have been considered in the light of an estimated equilibrium Ga‐As‐O condensed phase diagram. Thermal oxidation leads to phases consistent with the phase diagram. Plasma oxidation and anodic oxidation lead to phases inconsistent with the phase diagram. Thermal annealing of the plasma and anodic oxide layers leads to reactions producing phases consistent with the phase diagram. It is concluded that plasma and anodic oxides are produced far from equilibrium and that the estimated phase diagram is a satisfactory approximation to the equilibrium ternary phase diagram.

The Electrochemical Oxidation of Substituted Catechols

Abstract: The oxidation of substituted catechols was studied by cyclic voltammetry, chronoamperometry, rotating ring-disk electrode, and coulometry. The results showed that the quinones that were formed from the oxidation of substituted catechols reacted with the basic forms of the starting material to yield the dimeric product. These products were generally unstable and rapidly polymerized or underwent some other irreversible reaction to form an electroinactive product. For 3,4-dihydroxyacetophenone and propriophenone, the intermediate was stable long enough to be observed in cyclic voltammetry. The rate of the coupling reaction was found to correlate well with the Hammett P-q parameters and indicated that there was substantial negative charge in the transition state. Finally, an analysis of the coulometric n-values along with the iat1/2/C values indicated that the initial coupling product was a diphenyl ether. Analysis of the coulometry products showed extensive polymerization.

Factors Affecting the Electrochemical Responses of Metal Complexes at Pyrolytic Graphite Electrodes Coated with Films of Poly(4‐Vinylpyridine)

Abstract: Electrochemical responses from the reduction of RuIII (edta) coordinatedto films of high molecular weight poly(4-vinylpyridine) on pyrolytic graphiteelectrodes were studied as functions of film thickness, temperature, supportingelectrolyte composition, and solvent. Responses at filmed electrodes from metalcomplexes that do not coordinate to the films were also examined. With filmsthicker than ca. 1000A, the current responses are limited by the rates of molecularmotions within the films. Penetration of counterions, segmental motion ofsections of the polymer chains, and juxtapositioning of pairs of attached metalcomplexes to facilitate intercomplex electron transfer within the film or combinationsof the three are suggested as likely current limiting processes.

The Thermodynamics of Aqueous Water Electrolysis

Abstract: Precise definitions are given of three thermodynamic parameters which characterize the water‐electrolysis reaction: the enthalpic voltage, the higher‐heating‐value voltage, and the thermoneutral voltage. Expressions are derived for these parameters and for the reversible potential, as functions of temperature between 25° and 250°C, and of pressure between 1 and 100 atm. Heat losses due to radiation, convection, and conduction are also considered, and a thermal‐balance voltage is defined; representative values are calculated. Electrical‐energy efficiency is related to the characteristic parameters, and thermodynamic limitations on its value are discussed.

Chemical Vapor Deposition of Single Crystalline β ‐ SiC Films on Silicon Substrate with Sputtered SiC Intermediate Layer

Abstract: A single crystal of was grown by chemical vapor deposition using an system on a silicon substrate with a sputtered layer. The grown layer of 4 μm thickness was confirmed as a single crystal by examination with reflection electron diffraction and x‐ray diffraction. To reduce the large mismatch between and a silicon substrate, a sputtered layer was employed as a buffer layer. Even though the sputtered layer was polycrystalline, the subsequent layer deposited by CVD was a single crystal. The crystallinity of the deposited layer was strongly affected by the thickness of the sputtered layer, the substrate temperature during sputtering, and the temperature of chemical vapor deposition.

Oxygen Evolution on La1 − x Sr x Fe1 − y Co y O 3 Series Oxides

Abstract: series oxides with perovskite‐type structure were synthesized and were examined as the electrode for the oxygen evolution reaction in alkaline solution. The oxides except for , , and their neighbors were suitable for the electrode with respect to high conductivity. The catalytic activity for the oxygen evolution reaction increases along the direction from to in composition. It is concluded that the following two conditions are desirable for the perovskite‐type oxide with high catalytic activity for the oxygen evolution reaction: (i) the oxide has a broad band and (ii) the transition metal cation in the oxide exists as the higher oxidation state. In addition, it was found that the catalytic activity of is the highest in the oxides prepared in this study.

Structure and Properties of LPCVD Silicon Films

Abstract: Silicon films deposited by low pressure chemical‐vapor deposition over the temperature range from 525° to 725°C were investigated. It was found that polycrystalline films are formed above 600°C and are more stable than the amorphous films deposited at lower temperatures. Their crystal structure is a strong function of the deposition temperature and a weaker function of the deposition rate. Either the {110} or the {100} texture may dominate the structure, depending primarily on the deposition temperature. The electrical resistance obtained on doping the LPCVD films that are polycrystalline as deposited is maximum for films deposited at the lower temperatures (near 600°C), although this dependence on deposition temperature decreases after annealing at higher temperatures. Dopant atoms reversibly segregate to the grain boundaries during lower temperature heat‐treatments subsequent to doping and are dispersed at higher temperatures, with corresponding changes in resistivity. The oxidation rate is only a weak function of the deposition temperature, although the initially amorphous films may oxidize somewhat more rapidly. The index of refraction of amorphous films is significantly higher than that of polycrystalline films.

Mechanism of Na2 SO 4 Induced Corrosion at 600°–900°C

Abstract: The kinetics of corrosion were measured by accelerated oxidation tests on Co‐30Cr and Ni‐30Cr as a function of temperature from 600°–900°C, in the environment and deposit composition. The alloys were rapidly attacked at temperatures between 650° and 750°C when a liquid sulfate phase was obtained from an initially pure solid deposit. The rapid rate of attack resulted from sulfation of the transient surface nickel or cobalt oxides and the dissolution of these transition metal sulfates into to yield a liquid phase. This retarded the formation of a protective scale. The exposure conditions under which liquids could form from mixtures were calculated from thermodynamic considerations.

Properties of Thin Polyimide Films

Abstract: Polyimide is receiving increasing interest as a possible insulator on semiconductor devices, replacing . Several commercially available polyimides have been characterized by testing films of them for electric breakdown strength, conductivity, adhesion, and other properties. The results of these tests are summarized, and the polyimides are compared to sputtered .

Studies in Selenious Acid Reduction and CdSe Film Deposition

Abstract: The reaction sequence for the reduction of in at various surfaces and the formation of selenide films in this medium with and without metal ions present have been investigated by rotating disk and ring‐disk electrode techniques. The system is complicated by competitive paths leading to selenium formation from Se(−2)‐Se(+4) chemical reactions and the result is sensitive both to the nature of the substrate with the possibility of metal selenide formation and to the concentration of solution species. The implications for cathodic deposition of semiconductors like , from voltammetric and photospectral data, are discussed.