The effect of aqueous contaminants on the redox potential of chloride melts

The redox potential of a chloride melt contaminated with water is determined by the activities of its hydrolysis and dissociation products, H⁺, O²⁻, H₂ and O₂. The effects of changes in these were studied by changing the partial pressures of HCl, H₂, or O₂ above the melt, or adding Na₂O to the melt. Experiments were carried out in equimolar NaCl---KCl at 700°C.

Variations in O₂ partial pressure did not change the activity of O²⁻, but in the other three cases changes in redox equilibria in the melt were produced. In particular, hydrogen in contact with a chloride melt behaved like a metal and the H⁺ activity changed when hydrogen partial pressure was altered.

The upper limit of the range of redox potential over which platinum behaved as a redox electrode was found to be about −0.4 V (chlorine electrode scale).     

Voltammetric Study of Silver in a Glass Melt

The redox reaction and the oxidation state of silver in a glass melt were studied by cyclic voltammetry and chronopotentiometry between 800° and 1200°C. Silver ions in glass were reduced at a less noble potential than the rest potential of a platinum electrode. A reversible redox reaction of silver in glass was indicated from the analysis of the cyclic voltammogram. An analysis based on the electron transference number revealed that the solubility of reduced silver increased with increasing temperature. Ag⁺/Ag⁰ equilibrium in glass was suggested because the rest potential of the platinum electrode in glass was a mixed potential. No relation was observed between the redox behavior of silver in glass and optical basicity. O-type redox behavior of silver in glass was shown.     

TERNARY NaCl+LiCl+H2O MIXED ELECTROLYTE SYSTEM: DETERMINATION OF ACTIVITY COEFFICIENTS BASED ON POTENTIOMETRIC METHOD

The mean activity coefficients for NaCl in a ternary electrolyte system were determined by the potentiometric method, at 25°C, using a solvent polymeric (PVC) sodium-selective membrane electrode (Na⁺ ISE), containing N,N'-dibenzyl-N,N'-diphenyl-1,2-phenylenedioxydiacetamide as ionophore, and combined with an Ag/AgCl electrode. The potentiometric measurements were performed at the same ionic strengths in different series of mixed salt solutions, each characterized by a fixed salt molal ratio r (where r = m₁/m₂ = 1, 10, 50, 100). The nonideal behavior of the ternary NaCl(m₁) + LiCl(m₂) + H₂O electrolyte system was described based on the Pitzer ion-interaction model for mixed salts over the ionic strength ranging from 0.01 up to about 4 mol/kg. Two- and three-particle Pitzer interaction parameters for a mixed electrolyte system were determined based on potentiometric data, and the critical role of potentiometric selectivity coefficient (K₁₂) of ISE as limiting factor in the potentiometric measurements was analyzed.     

Electrodes de reference pour hautes temperatures

A reference electrode based on the system Ag/Ag⁺, capable of functioning very satisfactorily up to 960°C, the m.p. of silver, is described. Modifications permit the electrode to be applied up to 1300°C. Its principal properties are reviewed, and some anodic polarization curves are presented to illustrate its applications.

Zusammenfassung

Es wird eine Ag/Ag⁺ Bezugselektrode beschrieben, die bis zum Schmelzpunkt des Silbers (960°) in sehr befriedigender Weise zu arbeiten vermag. Einige neuartige Abänderungen haben es ermöglicht, den Anwendungsbereich der Elektrode bis auf 1300°C auszudehnen. Ihre wichtigsten Eigenschaften werden besprochen und zur Illustration der Anwendungsmöglichkeiten werden einige anodische Polarisationskurven wiedergegeben.     

A mixture of nitrogen dioxide and oxygen as a Lux acid in nitrate melts

The effect of an NO₂+O₂ gaseous mixture on a melt of sodium and potassium nitrates and the acid-base reactions of this mixture with some Lux bases were studied, measuring the potential of an oxygen electrode at a temperature of 260°C. It has been found that the NO₂+O₂ mixture acts as a Lux acid conjugated with NO⁻₃ anion as the base, even under the atmospheric pressure. The strengths of the NO₂ +O₂ acid and the NO⁻₃ base related to other studied conjugated acids and bases were qualitatively estimated from the shape of potentiometric curves of the acid—base reactions.     

Microstructure and properties of various fluorine-containing SiAlON ceramics synthesized by HIPing

An attempt was made to prepare various F-doped β-, O-, X-, and -SiAlONs from a mixture of Si₃N₄, SiO₂, Al₂O₃, AlN, or Y₂O₃ using AlF₃ or topaz as the fluorine source by HIPing at 1500–1800°C and 150 MPa. The phases were identified and the z, x, and m/n values determined for β-, O-, and -SiAlONs by X-ray diffraction. When AlF₃ was used, a single phase ceramic (O-SiAlON) was produced from a mixture of -Si₃N₄ and SiO₂ at 1500°C, with a mixture of O- and β-SiAlONs formed at 1700°C. A mixture of -Si₃N₄, AlN, and Y₂O₃ with AlF₃ produced β-/Y--SiAlON ceramics at 1730°C. The use of topaz produced the β-SiAlON ceramic with a trace of mullite from a mixture of -Si₃N₄ and AlN at 1770°C and mixed phase β-/O-SiAlON ceramics from -Si₃N₄ and SiO₂ at 1700°C. Single phase X-SiAlON could not be obtained under the present conditions. The microstructures of the single phase O- and β-SiAlON ceramics and the β-/Y--SiAlON mixture showed the growth of O- and β-SiAlON and Y--SiAlON crystals with hexagonal and/or long rod-like or platy shapes in a matrix of F-containing glassy phase. The compositions of the SiAlON crystals and the glass phase were semi-quantitatively determined by EDX; the total glass phase was estimated by a quantitative Rietveld XRD powder method. The F-doped β-SiAlON ceramics showed better corrosion resistance towards NaCl vapor and lower Vickers hardnesses.     

Development of solid oxide fuel cells based on a Ce(Gd)O2−x electrolyte film for intermediate temperature operation

Initial tests have been carried out with the fuel cell arrangement La_0.6Sr_0.4Co_0.2Fe_0.8O₃Ce_0.9Gd_0.1O_1.95Ni/YSZ, incorporating dense film (5–10 μm) Ce_0.9Gd_0.1O_1.95 electrolyte tape cast onto the supporting anode, to investigate the feasibility of intermediate temperature operation (500–700°C). A good open circuit voltage of approx. 0.8 V was obtained at 550°C using moist hydrogen as the fuel. Slightly lower open circuit voltages were found at higher temperatures, which may have been caused by minor gas leakage and the electronic conductivity of the electrolyte. Power outputs in excess of 100 mW/cm² were obtained at 650°C, and the cell resistance was 0.8Ω cm² at this temperature. This resistance, and the greater resistance at lower temperature, was predominantly due to the cathode according to AC impedance measurements. Experiments were also carried out at 600°C using direct methanol fuels at the anode; the maximum power output was approximately half of that obtained with hydrogen.     

Preparation of LiNiVO4 nano-powder using tartaric acid as a complexing agent

LiNiVO₄ was prepared from Li₂CO₃, Ni(CH₃COO)₂·4H₂O and NH₄VO₃ using tartaric acid as a complexing agent with 1:1–1:4 mole ratios of metals:tartaric acid and subsequent calcination at 350–700 °C for 6–12 h. Inverse spinel LiNiVO₄ was detected using XRD. FTIR and Raman analyses revealed the presence of stretching band of VO₄ tetrahedrons. Only Ni, V and O were detected by EDX. The 1:4 mole ratio for the product with 450 °C calcination for 6 h analyzed by SEM, TEM and electron diffraction (ED) composes of LiNiVO₄ nano-powder with 10–30 nm in diameter.     

Solid state protonic conductor NH4PO3–(NH4)2Mn(PO3)4 for intermediate temperature fuel cells

A new proton-conductive composite of NH₄PO₃–(NH₄)₂Mn(PO₃)₄ was synthesized and characterized as a potential electrolyte for intermediate temperature fuel cells that operated around 250 °C. Thermal gravimetric analysis and X-ray diffraction investigation showed that (NH₄)₂Mn(PO₃)₄ was stable as a supporting matrix for NH₄PO₃. The composite conductivity, measured using impedance spectroscopy, improved with increasing the molar ratio of NH₄PO₃ in both dry and wet atmospheres. A conductivity of 7 mS cm⁻¹ was obtained at 250 °C in wet hydrogen. Electromotive forces measured by hydrogen concentration cells showed that the composite was nearly a pure protonic conductor with hydrogen partial pressure in the range of 10²–10⁵ Pa. The proton transference number was determined to be 0.95 at 250 °C for 2NH₄PO₃–(NH₄)₂Mn(PO₃)₄ electrolyte. Fuel cells using 2NH₄PO₃–(NH₄)₂Mn(PO₃)₄ as an electrolyte and the Pt–C catalyst as an electrode were fabricated. Maximum power density of 16.8 mW/cm² was achieved at 250 °C with dry hydrogen and dry oxygen as the fuel and oxidant, respectively. However, the NH₄PO₃–(NH₄)₂Mn(PO₃)₄ electrolyte is not compatible with the Pt–C catalyst, indicating that it is critical to develop new electrode materials for the intermediate temperature fuel cells.     

Devitrification of a glass obtained from porphyric sands, MgO (15%) and TiO2 (4%)

A glass of composition: SiO₂ = 59·84%; Al₂O₃ = 11·45%; MgO = 15·34%; TiO₂ = 4·23%; K₂O = 3·80%; Na₂O = 2·48%; CaO = 1·08%; Fe₂O₃ = 1·78%; was prepared from porphyric sands by addition of MgO and TiO₂. The quenched glass is demixed on a very fine scale. The non-isothermal devitrification has been studied. Three-dimensional crystal growth has been observed. The experimental data suggest a mechanism controlled by the crystal-glass interface reaction. The crystal growth activation energy E_c = 467 ± 20 kJ/mole has been evaluated. The temperature of most efficient nucleation is approximately T_N = 720°C.     

Polymerization of norbornene using novel palladium carboxylate/boron trifluoride etherate catalyst system

The vinyl polymerization of norbornene with Pd(carboxylate)₂/BF₃OEt₂ catalyst system has been investigated by varying the molar cocatalyst/metal ratio, the norbornene/metal ratio, the metal concentration and the reaction temperature. The effects on the catalyst activity were explained on the basis of complexation equilibrium for the active homogeneous complex. A “particular” activity of 154 100 kg NB per mole Pd for an hour has been achieved at B/Pd = 25 and 25 °C. The molecular weights M_w from 77 700 to 293 800 g/mol and glass transition temperatures T_g from 240 to 262 °C were observed for the representative samples of polynorbornene. The molar mass distribution indicates a single-site, highly homogeneous character of the active catalyst species. Catalytic activity and polymer molecular weight can be controlled by varying the reaction parameters over a wide range. NMR and IR spectroscopic studies of the polymer showed 2,3-enchained repeating units of polymer backbone with low diisotacticity. The simplicity of catalyst system composition might be of industrial importance.     

Electro-optical studies of submonolayers of lead formed on gold electrodes by faradaic adsorption in 1 M HClO4

Current-potential and specular reflectivity-potential curves of a Au electrode have been measured in 1 M CHlO₄ in the presence of 1·0 × 10⁻³ M Pb²⁺. Four peaks were found on a linear sweep voltammogram at potentials more positive than the Nernst reversible potential of Pb⁰/Pb²⁺ couple, which are attributed to the Faradaic adsorption of Pb²⁺.

Specular reflectivity-potential curves recorded simultaneously at varied wavelengths gave a clear indication of the adsorption which increased the reflectivity at shorter wavelengths while decreased at longer wavelengths.

From the analysis of the current-potential curve of four peaks in relation to the reflectivity change, the following results were obtained:

(1) The existence of four peaks on the voltammogram is the indication of four different adsorbed states.

(2) The Pb atoms in a submonolayer on Au electrode have rather similar optical properties to those of metallic Pb except for the adsorbed atoms formed at the most potential region.

(3) The adsorbed Pb formed at the most positive potential region have the shortest distance to the surface Au atom, and accordingly, have the strongest interaction with the surface.     

Determination of picomolar silver concentrations by differential pulse anodic stripping voltammetry at a carbon paste electrode modified with phenylthiourea-functionalized high ordered nanoporous silica gel

This study introduces the design of an anodic stripping voltammetric (ASV) method for the silver ion determination at a carbon paste electrode (CPE), chemically modified with phenylthiourea-nanoporous silica gel (Tu-SBA-15-CPE). The electroanalytical pro includes two steps: preconcentration of metal ions at an electrode surface, followed by quantification of the accumulated species by differential pulse anodic stripping voltammetric methods. Factors affecting the performance of the anodic stripping were investigated, including the modifier quantity in the paste, the electrolyte concentrations, the solution pH and the accumulation potential or time. The most sensitive and reliable electrode contained 10% Tu-SBA-15 and 90% carbon paste. The accumulation potential and time were set at, −200 mV and 300 s, respectively, and the scan rate at 50 mV s⁻¹ in the scan range of −200 to 700 mV. The resulting electrode demonstrated a linear response over range of silver ion concentration of 8.0-80 pmol/L with detection limit (S/N = 3) of 5 pmol/L. The prepared electrodes were used for the silver determination in sea and tap water samples and very good recovery results were obtained. The accuracy was assessed through recovery experiments and independent analysis by graphite furnace atomic absorption spectrometry.     

La–Si–O–N glasses: Part I. Extension of the glass forming region

The nitrogen-rich part of the glass forming region in the La–Si–O–N system has been the subject of a comprehensive study. Glasses were prepared by heating powder mixtures of La metal, Si₃N₄ and SiO₂ in a nitrogen atmosphere at 1650–1800 °C. By this new synthesis route, glasses containing up to 68 e/o of N and 62 e/o of La were prepared, showing that the glass forming region is significantly larger than previously reported. The glasses were characterized by elemental analysis, differential thermal analysis, X-ray powder diffraction, and scanning electron microscopy. They were found to be X-ray amorphous and homogenous, with the majority of them containing small amounts of crystalline La silicides and elemental Si. Glass transition temperatures (T_g) were found to vary between 900 and 1100 °C and crystallization to occur typically 120 °C above T_g. The forming of the glasses was investigated by characterizing samples taken out at various steps of the heating cycle. The results indicate that the glass formation is strongly dependent on reaction kinetics. A strong exothermal reaction occurs at temperatures 900–1100 °C, leading to the formation of assemblies of amorphous and crystalline (oxy)nitride phases that melt upon further heating at 1650–1800 °C.     

Electrochemical beaviour of the solid ionic conductor Ag7I4PO4

Electrochemical investigation of the solid superionic conductor Ag₇I₄PO₄ (0.8 AgI + 0.2 Ag₃PO₄) at room temperature and at 40°C were performed by means of cyclic voltammetry, cyclic chronoamperometry and cyclic chronocoulometry, normal pulse polarography and ac polarography. It was shown that the Ag⁺ ↔ Ag redox process on Pt and Ag working electrode occurs with a certain overvoltage, ie that for Ag⁺ → Ag⁺ oxidation and the return of Ag⁺ ions into the electrolyte a certain overvoltage is necessary. From the determined values of the exchange current one estimates the redox process as a rather fast one. The silver working electrode is electrochemically inactive, while only cathodically deposited silver is electrochemically active and can be oxidized to Ag⁺ ions. Chronoamperometry and chronocoulometry show that there is a certain difference in the behaviour of Pt and Ag working electrodes due to uneven passivating anodic processes. On the basis of measurements of faradaic and capacitance currents and their dependence on frequency, diagrams of complex impedance of the Pt/Ag₇I₄PO₄ interface at various anodic and cathodic polarizations of the Pt electrode were plotted. The dependence of the serial capacity of the interface on the dc potential and temperature are discussed.     

Sintering of Si3N4 with liquid in the system Ce2O3---AIN---SiO2

Liquid phase sintering of Si₃N₄ with melts from the system Ce₂O₃---AIN---SiO₂ has been studied. The glass forming region in this system and the reaction products formed during sintering at 1750–1800°C were analysed. Sintering of Si₃N₄ with two melt compositions selected from outside the glass forming region yields fully dense Si₃N₄. Post sintering treatment at 1300°C resulted in devitrification with consequent improvement of high temperature mechanical properties. The mechanical properties of Si₃N₄ sintered with liquids in the system Ce₂O₃---AIN---SiO₂ were found to be inferior to those of liquids selected from Y₂O₃---AIN---SiO₂, but superior to those selected from the system MgO---AIN---SiO₂.     

The acid-base reactions of mixtures of nitrogen dioxide and oxygen with polyacid lux bases

The dissolution of lux bases and acids and the acid-base reactions of solutions of polyacidic bases with a gaseous mixture of NO₂ + O₂ acting as a lux acid were studied by measuring the potential of an oxygen electrode at a temperature of 260°C. The strength of the conjugate pairs of acids and bases was compared qualitatively with respect to the nitrate system using the shapes of the potentiometric curves for the acid-base reactions. Some effects that were found for the first time on poteniometric curves were explained on the basis of theoretical treatment of acid-base reactions in systems where the poorly soluble acid is precipitated from the melt.     

Thermal evolution of Fe2O3-TiO2 sol-gel thin films

Fe₂O₃-TiO₂ thin films were deposited on silica glass slides using three sol-gel solutions containing Ti-butoxide and different iron oxide precursors. The thermal evolution of the coatings was followed by DTA-TGA and XRD from 200 to 1000°C. All the iron-containing samples were amorphous up to 700°C, when a phase separation between iron and titanium oxides was evidenced by the presence of crystalline Fe₂O₃. Above that temperature a titanate compound (pseudobrookite) formed according to the equilibrium phase diagram. The use of different precursors did not affect the thermal evolution and all the different samples exhibited the same trend.     

The anodic behavior of silver in chloride solutions—I. The formation and reduction of thin silver chloride films

The initial stages of growth and the subsequent reduction of AgCl films at a silver electrode in aqueous chloride solutions has been investigated using potentiodynamic and potentiostatic methods. The first step in the growth process involves the two-dimentional deposition of either a partial monolayer (50–75 μC cm⁻²) of AgCl film or the adsorption of chloride ions involving a partial charge transfer. Following this, at an overpotential of typically 20–40 mV, the nucleation and growth of three-dimensional AgCl films commences. Reduction of this type of AgCl film is characterized by two cathodic peaks, possibly depicting the reduction of AgCl nuclei formed by two nucleation mechanisms. Continuous potential cycling results in significant increases in the nucleation currents and charges (not due to conventional electrode roughening), indicating that active silver nuclei are being left on the electrode surface upon AgCl film reduction. However, by maintaining the potential at a value significantly negative of AgCl film growth for a period of time, the currents due to the three-dimensional nucleation and growth process diminish in magnitude. This is evidence for an active reconstruction of the silver surface leading to the loss of these silver nuclei.     

YSZ thin films deposited by spin-coating for IT-SOFCs

Dense and crack-free yttria stabilized zirconia (YSZ) thin films were fabricated using a spin-coating technique for intermediate temperature solid oxide fuel cells (IT-SOFCs). The film thickness was greatly affected by spinning speed and coating cycles. The morphology of the films was investigated with scanning electron microscope. With cathodes consisting of yttria-stabilized bismuth oxide and sliver, anodes of samaria-doped ceria and nickel, the supported YSZ films were characterized as electrolytes for single cells with humidified hydrogen as fuel and stationary air as oxidant. Open circuit voltage was 1.08 V at 700 °C, close to the theoretical value and power density was 535 mW/cm² at 750 °C and 400 mW/cm² at 700 °C. Impedance analysis indicates that the performances of the SOFCs are determined essentially by the interfacial resistances, suggesting that optimizing the electrode materials are especially important for IT-SOFCs.