Effect of dopants on wetting properties and electrochemical behaviour of graphite anodes in molten Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-cryolite melts

The effects of anode dopants and alumina bath content on the wetting properties of the anode/bath interface in an aluminium electrolysis cell have been investigated in this study. Increasing the wettability of the carbon anode maximises the anode surface area and prevents a sudden increase in current density that polarises the anode leading to the discharge of fluoride ions and the onset of anode effect. From the contact angle measurements presented in this study, it is clear that the presence of carbonate salts inside and at the surface of the substrate greatly increases the wettability of the graphite substrate/cryolitic melt interface. The higher the carbonate salt content of the substrate, up to a dopant levels of 6 wt%, the better the wetting properties. Furthermore, alumina appears to act as a surface-active component when present in the electrolyte in contact with a pure graphite anode, but these properties diminish when carbonate is present in the anode.     

Surface Energy and Crystallization Phenomena of Ammonium Dinitramide

Ammonium dinitramide (ADN) was characterized during recrystallization from the melt. The surface tension of molten ADN at 97 °C was measured to be 89 mN/m. The wetting angles between molten ADN and different solid surfaces (polytetrafluoroethylene, glass, steel, and aluminum) were determined. The wettability depends on the surface tension of molten ADN, the free surface energy of the solid surfaces and the interfacial tension between the solid and liquid. Observations of the recrystallization behavior of molten ADN showed that nucleation does not occur, even at super cooling rates of 70 K. Crystallization can be initiated by the application of seed crystals.     

Proprietes interfaciales et reactions anodiques sur electrodes de carbone dans les bains cryolithe—alumine

The influence of gaseous compounds adsorbed on a carbon anode in cryolite—alumina melts without aluminium metal is studied by chronopotentiometry. It shows that the active surface of the electrode depends on different factors such as the nature, the density and the porosity of carbon, the acidity or the basicity of the melt, the nature of alumina (α alumina, γ alumina, hydrated alumina).A schematic representation of these mechanisms is proposed taking into account the wetting properties and the interfacial tensions of the used materials. It is possible to obtain a more coherent representation of the experimental observations and this concept appears as an original way in studying the reactivity of carbons.This research had lead us to improve the experimental technique by using an ultrasonic desorption of the gases or a hollow cathode with vacuum inside. A rapid determination of the alumina content can be obtained in definite experimental conditions; however this method requires the exact knowledge of the active surface of the working electrode.     

Investigation of Alumina Wetting by Fe–Ti,Fe–P and Fe–Ti–P Alloys

Abstract

The wetting of alumina substrates by Fe–Ti, Fe–P and Fe–Ti–P alloys has been investigated using sessile drop experiments conducted under an inert gas atmosphere in the temperature range of 1550 to 1620°C. The surface and interfacial structures have been explored by scanning electron microscopy and energy dispersive X-ray spectroscopy. Substantial additions of titanium are known to induce steel melts to wet alumina due to the formation of a Ti-rich reaction product at the alloy/ceramic interface, but the present work has shown that even low Ti concentrations can induce a reactive wetting process leading to an improvement of the wettability of alumina by Fe alloys. The contact angle of molten steel containing phosphorus on alumina decreased with increasing P content. The improvement of the wetting behaviour in this system was attributed solely to the adsorption of P onto the surface of the Fe melt. The addition of P as a ternary alloying element to the system Fe–Ti proved to be beneficial to the wetting behaviour. The measured contact angles were much lower than those in the binary systems Fe–Ti and Fe–P. This effect was related to the fact that P enhances the activity of Ti in the Fe melt. According to experimental observations, it turns out that the wettability of liquid Fe-based alloys, when an Al2O3 surface is present, is not only a property of the metal/oxide couple but is also dependent on the oxygen partial pressure, whereas temperature variations bring about a comparatively small effect. This work is of interest in understanding the phenomena pertaining to inclusion engineering and steel– refractory interactions, such as the clogging of submerged entry nozzles by agglomerated alumina particles during the continuous casting process.     

The anode effect as a fluid dynamic problem

The so-called anode effect, particularly important in industrial alumina electrolysis, has mostly been interpreted as the consequence of altered wettability of the electrode surface by the melt. By means of a mathematical model assuming isolated large bubbles in contact with the electrode it is shown that the anode effect is the result of the combined action of fluid dynamics and wettability. The interpretation of the incipience of the anode effect obtained by means of a previous, completely different mathematical model is confirmed. The theoretical results are compared with experimental data by various authors.     

Analyse des gaz et processus reactionnels anodiques dans les bains cryolithe—alumine

The composition of anode gas is an important indication for the mechanism of the electrolysis in cryolite-alumina melts. An original technique has been used for the captation of gas in situ (hollow graphite electrode under vacuum inside). The composition of anode gas was determined by mass spectrometry and shows that the nature of this gas depends on different factors such as the crystallographic structure of alumina (α-Al₂O₃, γ-Al₂O₃, hydrated alumina), the current density, the alumina concentration of the melt, the porosity and the B.E.T. area of carbon anode.The ratio [CO₂]/[CO] increases with increasing current density and descreases with increasing alumina content. The results suggest that CO₂ is the primary anode product which reacts partially with the anode following the Boudouard reaction.The structural water of the alumina (γ-Al₂O₃ or hydrated alumina) induces the formation of superficial COH complexes which lead to the passivation of the anode.A schematic representation of these mechanisms is proposed taking into account the kinetics of reaction during flow gas through the electrode walls. The contract time of gases with the carbon is an important factor in the mechanism of CO built up and the consumption of carbon by the anode process.     

Current efficiency studies for graphite and SnO2-based anodes for the electro-deoxidation of metal oxides

Studies were performed investigating the electrochemical reduction of chromium oxide (Cr₂O₃) by electro-deoxidation by utilising either a graphite anode or a tin oxide (SnO₂) based anode. Potentiostatic electrolysis was performed at 3.0 V for both a graphite and for a SnO₂-based anode, and also 2.0 V for a graphite anode. The cathode reduction purity, anode mass change, anode potential relative to a glassy carbon pseudo-reference and current efficiency were measured and compared. The key observations are that substituting a SnO₂-based anode for a graphite anode led to greater current efficiencies for electro-deoxidation. This was attributed to the lack of contamination of the melt by carbon and the lower cathode potential due to the higher anodic potential when using tin oxide based anodes for the same applied voltage. The current efficiency was also found to decrease with both anode materials when higher anode surface areas or lower current densities were used. Again this was attributed to a decrease in anodic potentials and a corresponding increase in the cathodic potential resulting in a greater number of parasitic reactions occurring at the cathode.     

Effect of aqueous phase pH on liquid–liquid extraction with impinging-jets contacting technique

The influence of aqueous phase pH on the rate of liquid–liquid extraction was investigated using a two impinging-jets contacting device with a high velocity of jet flows. The recommended chemical system of toluene–acetone–water was employed. Applying pH values from its natural value of 5.6–8, a range in which most industrial waters are lain, for the jets colliding force within 70.3–214.6 mN, leads a reduction in extraction efficiency and overall volumetric mass transfer coefficient up to about 18.9% and 35.2% respectively. A significant high variation is observed when the aqueous phase pH exceeds the water neutral value toward alkali conditions. These variations can be attributed to the adsorption of hydroxyl ions and lowering interfacial tension. A comparison between the results in this work and those for drop dispersion extraction indicates a high sensitivity in the impinging technique. As expected, increasing jets force improves the performance of the contacting device; however, simultaneously, the retarding effect of pH would be intensified. The variation of interfacial tension with pH was obtained and with the aid of dimensionless numbers, an empirical correlation was developed for the prediction of overall volumetric mass transfer coefficient.     

Assessment of the Wettability of Porous Electrodes for Lithium-Ion Batteries

The wettability of lithium cobalt oxide (LiCoO₂) and mesocarbon microbead electrodes in nonaqueous electrolyte is analyzed by a mathematical model of capillary liquid movement. Results show that wetting in the LiCoO₂ electrodes is difficult as compared with the MCMB electrodes at the same electrolyte composition. Wetting in the porous electrodes is controlled mainly by electrolyte penetration and spreading in pores. Electrolyte penetration is determined by viscosity. On the other hand, electrolyte spreading is controlled by surface tension. Organic solvent composition and lithium salt concentration may influence the wettability of porous electrodes due to changes in the viscosity and surface tension of the electrolyte. Increasing the amount of EC and/or lithium salts can cause poorer electrolyte spreading and penetration. Furthermore, careful pressure control has a positive effect on increasing the surface area of the solid–liquid interface. AC impedance data show that batteries with vacuuming prior to electrolyte filling may reach a maximum wetting in a few hours. If no vacuuming is applied, a few days are required to obtain sufficient wetting.     

Polarity effects on the wettability and interfacial chemistry at Cu–YSZ interface by applying a direct current

The application of a direct current, regardless of its polarity, can significantly improve the wettability of ZrO₂–8 mol% Y₂O₃ (YSZ) by molten Cu at 1373 K, and this effect was more pronounced when YSZ was connected to cathode. The mechanisms for the wetting improvement varied with the change in the current polarity. When YSZ was connected to the cathode, the oxygen ions in the YSZ body driven by the external electric power moved to the solid–liquid interface and dissolved into liquid Cu, forming OCu clusters. As a consequence, the solid–liquid interfacial energy and liquid surface tension were greatly reduced and wettability improved. On the other hand, when YSZ was connected to anode, the improvement in the wettability was mainly driven by the formation of nonstoichiometric zirconia and metallic Zr at the interface. Nevertheless, the newly formed ZrO₂ at the triple line region inhibited further spreading of liquid front.     

Effect of alumina concentration on the incipience of the anode effect in aluminium electrolysis

The cause of the incipience of the anode effect in alumina reduction cells has long been a controversial issue. The most plausible interpretations are (i) an insufficient release of the gaseous phase from underneath the anode surface together with the action of diminished wettability and (ii) the depletion of oxygen-containing ions at the anode surface to result in the limiting current condition. It is shown by means of a mathematical model that each of these effects may be active. The predominant mechanism depends on the operational conditions. Theoretical results are compared with experimental data.     

Reactive wetting behavior and mechanism of AlN ceramic by CuNi-Xwt%Ti active filler metal

In order to propel the application of the developed CuNi-Xwt%Ti active filler metal in AlN brazing and get the universal reactive wetting mechanism between liquid metal and solid ceramic, the reactive wetting behavior and mechanism of AlN ceramic by CuNi-Xwt%Ti active filler metal were investigated. The results indicate that, with the increasing Ti content, surface tension for liquid CuNi-Xwt%Ti filler metal increases at low-temperature interval, but very similar at high-temperature interval, which influence the wetting behavior on AlN ceramic obviously. CuNi/AlN is the typical non-reactive wetting system, the wetting process including rapid wetting stage and stable stage. The wettability is depended on surface tension of the liquid CuNi filler metal completely. However, the wetting process of CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system is composed by three stages, which are rapid wetting stage decided by surface tension, slow wetting stage caused by interfacial reaction and stable stage. For CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system, although the surface tension of liquid filler metal is the only factor to influence the instant wetting angle θ₀ at rapid wetting stage, the reduced free energy caused by interfacial reaction at slow wetting stage plays the decisive role in influencing the final wettability.     

熔盐电解氧化铈相关物质的分解电压

分解电压是探索电极过程机理和实际电解过程中电位控制的重要依据。计算了氧化铈在CeF3-LiF-MF2（M=Ba,Ca）熔体中石墨阳极上的理论分解电压。在石墨阳极上,阳极生成碳氧化合物的理论分解电压较小,反应较易发生;在惰性阳极下,理论分解电压按氧化铈、三氟化铈、氟化锂、氟化钡、氟化钙的顺序依次增大,氧化铈将优先发生分解反应。两种电极条件下,氧化铈的分解电压随温度升高而降低,随氧化铈浓度降低而增大,当氧化铈浓度过低时,容易发生阳极效应。     

The role of an SiC interlayer at a graphite–silicon liquid interface in the solution growth of SiC crystals

SiC crystal growth using the top seeded solution growth (TSSG) method involves the precipitation of solid SiC from carbon that is dissolved in a silicon melt. The growth rate of SiC is strongly influenced by the solubility of C in liquid Si, which is quite low. In this study, the dissolution of C from graphite to the Si melt was explored by observing the formation of an SiC interlayer at a graphite – Si liquid interface. The SiC interlayer was observed to become thickened during the several hours needed to reach a certain thickness at 1500 °C. Assuming that the SiC interlayer is a direct C source, a pre-formed SiC layer was coated on the graphite crucible to evaluate its effect on the concentration of C in the Si melt. As a result, the concentration of C in the Si melt increased within a short time, especially at low temperatures. By applying the SiC coated crucible to the TSSG process for SiC crystal growth, we confirmed that the development of a pre-formed SiC layer enhanced the growth rate of SiC crystals, especially at the initial stage of crystal growth at low temperatures.     

Material aspects of the liquid feed direct methanol fuel cell

A study of a small scale Liquid Feed Direct Methanol Fuel Cell (LFDMFC), based on solid polymer electrolyte membrane, is reported. Two flow cell designs, one with a parallel flow channel arrangement and the other with a spot design of flow bed, are used. The structure of the DMFC comprises a composite of two porous electrocatalytic electrodes; Pt–Ru–carbon catalyst anode and Pt–carbon catalyst cathode, on either side of a solid polymer electrolyte (SPE) membrane. The performance of three Pt–Ru catalysts is compared. The influence of the degree of Teflon loading on the electrode structure is also reported. The effect of the following parameters: cell temperature, oxygen gas or air pressure, methanol liquid flow rate and methanol concentration on the power performance is described.     

Microstructural evolution under load and high temperature deformation mechanisms of a mullite/alumina fibre

A two-phase mullite alumina fibre, the 3M Nextel 720 fibre, has been studied in tension and creep. The fibre shows the highest creep resistance of all current commercial fine oxide fibres up to 1500 °C. The creep mechanisms involve progressive dissolution of mullite and simultaneous reprecipitation of alumina into elongated oriented grains and grain boundary sliding by a thin alumino-silicate liquid phase. The rate of grain growth in creep at a given temperature is dependant on the applied stress. The combination of sub-micron size mullite crystallites and alumina grains gives rise to a high sensitivity to alkaline contamination. Stress enhanced diffusion of the contaminants from the fibre surface results in crack nucleation, dissolution of mullite, formation of a liquid phase and slow crack growth. From 1200 °C, this process is coupled with a fast α-alumina grain growth at the fibre surface.     

Interaction of Y2O3-Stabilized Cubic ZrO2 with Sodium Silicate and Sodium Aluminosilicate Glass-Forming Melts

The kinetics of interaction of the cubic solid solution 92ZrO₂ · 8Y₂O₃ with sodium silicate and sodium aluminosilicate oxide glass-forming melts is investigated at a temperature of 1100°C. It is found that the stability of cubic -ZrO₂ against dissolution in an oxide melt is determined by the composition of the anionic matrix of the melt and depends on the difference between the acid–base characteristics of oxide systems being in contact. It is demonstrated that this process can be described by the contracting volume equation and is limited by the rate of motion of the reaction interface.     

Comparison of the photocatalytic degradation of 2-propanol in gas–solid and liquid–solid systems by using TiO2–LnPc2 hybrid powders

Photocatalytic degradation of 2-propanol was carried out as a probe reaction both in gas–solid and in liquid–solid systems in the presence of TiO₂ both bare and impregnated with lanthanide (Sm, Gd, Ho) bis-phthalocyanines (LnPc₂) used as sensitizers. Continuous and batch photo-reactors, irradiated with an equal flux of photons, were used in gas–solid and in liquid–solid systems, respectively. Propanone and acetaldehyde were the main intermediates found in both systems during 2-propanol oxidation, whereas carbon dioxide and water were the final oxidation products exclusively in the gas–solid regime. The photocatalysts exhibited significantly higher activity in the liquid–solid system than in the gas–solid one. Samples impregnated with the sensitizers were apparently more photoactive than bare TiO₂. The highest photoactivity was shown by the 1.85 HoPc₂–TiO₂ sample.     

Ni–YSZ SOFC anodes—Minimization of carbon deposition

An advantage of solid oxide fuel cells (SOFC) over some other types of fuel cells is the extended range of fuels they can use. Besides hydrogen, hydrocarbons are often used. A problem that may arise on a prolonged operation of cells using hydrocarbons is the formation of carbon deposits on the anodes. These deposits deteriorate the cell performance by blocking the access of reactants to the anode surface and into its pores, thus changing the micromorphology of the anode. The behavior of anodic material during the extended contact with such fuels must be tested and if needed modifications must be made in its composition in order to reduce the rate of carbon deposition.Ni–YSZ cermet materials prepared by different processes (sol–gel and combustion synthesis) with variations in composition and presence of dopants were tested by exposing these materials to methane at elevated temperatures. The thermal dissociation of methane, which leads to carbon deposits on the cermet surface, was studied by the analysis of the gas phase using mass spectrometry and gas chromatography. The influence of anode composition and its microstructure on carbon deposition was studied as well as the influence of some dopants. The amount of the deposited carbon was determined by temperature programmed oxidation.     

Mechanism of self-discharge in graphite-lithium anode

In order to evaluate the anode contribution to the lithium-ion battery self-discharge, three electrode coin cells composed of metallic lithium as reference and counter electrode, organic liquid electrolyte and graphite composite working electrode were constructed as test cells. They were first cycled for a dozen cycles and then stored in the full lithiated state of graphite, at 70 °C for periods from 1 to 8 days. The capacity loss was determined during the first delithiation following storage. The latter was found composed of two terms, a reversible and an irreversible one, where the relative amounts are storage time dependants. Electrochemical impedance spectroscopy (EIS) was used to investigate the changes in the cell interfacial characteristics. A model involving the formation of an absorbed electron-ion-electrolyte complex on the graphite surface is proposed as the mechanism of the reversible and irreversible capacity losses. It is also suggested that precipitation/dissolution reactions are taking place at the solid electrolyte interphase (SEI). Precipitation occurs with insoluble inorganic species such as, LiF and Li₂CO₃, whereas dissolution may concern the organic and/or polymer part of the SEI. The continuous growth of the inorganic (and most resistive) part of the SEI with the subsequent electrode isolation is proposed as the major mechanism of the electrode end of life.