Interfacial tension between aluminum and cryolite melts during electrolysis of the systems Na3AlF6–AlF3 (NaF)–Al2O3

The interfacial tension between aluminum and cryolite melts containing different salt additions has been measured by the capillary depression method. The technique is based on the measurement of the capillary depression occurring when the capillary, which is moved vertically down through the molten salt layer, passes through the salt/metal interface. The depression is measured by simultaneous video recording of the immersion height of the alumina capillary. The interfacial tension was found to be strongly dependent on the n(NaF)/n(AlF₃) ratio (cryolite ratio, CR). At the cryolite ratio 2.28 (80 wt.% Na₃AlF₆ + 10 wt.% AlF₃ + 10 wt.% Al₂O₃ // Al, t = 1000 °C) the interfacial tension was 546 mN m⁻¹, while it was 450 mN m⁻¹ at the cryolite ratio 4.43 (80 wt.% Na₃AlF₆ + 10 wt.% NaF + 10 wt.% Al₂O₃ // Al, t = 1000 °C). Experiments under current flow conditions were also performed. During the electrolysis the interfacial tension at n(NaF)/n(AlF₃) ratio 2.28 decreased from 546 mN m⁻¹ at zero current to 518 mN m⁻¹ at 0.112 A cm⁻². The same trend was observed in the system with a cryolite ratio 4.43. The interfacial tension decreased from 450 mN m⁻¹ at zero current to 400 mN m⁻¹ at 0.112 A cm⁻². The consequent increase in interfacial tension of these systems caused by interruption of electrolysis was observed. Electrolysis of the system 25 wt.% NaF + 75 wt.% NaCl (eutectic mixture)/Al indicated no influence of applied current on the interfacial tension at 850 °C.     

A study of the voltage and current efficiencies of fused salt AlCl3 electrolysis using bench scale cells

The fused salt electrolysis of AlCl₃ dissolved in LiCl-NaCl melts at 700°C was studied using bench scale cells employing graphite electrodes. The effects of AlCl₃ concentration, forced convection, interelectrode spacing, and current density on the voltage, current, and energy efficiencies of the process were investigated. At a practical cell voltage of 3.05 V and current density of 15 kA·m^?2 the electrical energy consumption per kg of aluminum produced by the AlCl₃ process was about 32 MJ, compared with a typical value of 52 MJ·kg^?1 for the Hall-Héroult electrolysis.     

Electrochemical incineration of chloranilic acid using Ti/IrO2, Pb/PbO2 and Si/BDD electrodes

The electrochemical oxidation of chloranilic acid (CAA) has been studied in acidic media at Pb/PbO₂, boron-doped diamond (Si/BDD) and Ti/IrO₂ electrodes by bulk electrolysis experiments under galvanostatic control. The obtained results have clearly shown that the electrode material is an important parameter for the optimization of such processes, deciding of their mechanism and of the oxidation products. It has been observed that the oxidation of CAA generates several intermediates eventually leading to its complete mineralization. Different current efficiencies were obtained at Pb/PbO₂ and BDD, depending on the applied current density in the range from 6.3 to 50 mA cm⁻². Also the effect of the temperature on Pb/PbO₂ and BDD electrodes was studied.UV spectrometric measurements were carried out at all anodic materials, with applied current density of 25 and 50 mA cm⁻². These results showed a faster CAA elimination at the BDD electrode. Finally, a mechanism for the electrochemical oxidation of CAA has been proposed according to the results obtained with the HPLC technique.     

LiF对Na3AlF6-Al2O3熔盐电解阴极过程的影响

电解铝工业中铝在阴极上析出,铝析出反应的机理对电解铝生产具有理论指导意义。在运用循环伏安法研究的基础上,通过理论计算,对Na₃AlF₆-Al₂O₃和Na₃AlF₆-Al₂O₃-LiF体系中金属铝在钨电极上的电化学沉积行为以及铝钨金属间化合物的形成机理进行了研究。结果表明：两体系中铝钨金属间化合物在50 mV·s^-1≤ν≤150 mV·s^-1扫描速率下的形成过程是受扩散控制的准可逆过程。在化合物形成的过程中,两体系中Al³⁺的扩散系数从4.54×10^-9 cm²·s^-1增长到5.71×10^-9 cm²·s^-1,Al³⁺反应的活化能分别为11.14 kJ·mol^-1和10.47 kJ·mol^-1。在Na₃AlF₆-Al₂O₃-LiF体系的还原过程中,Li并没有还原析出,而在氧化过程中Al在金属间化合物中的氧化电流增大;在恒电流电解时,Al-W金属间化合物并不溶于熔盐中,会附着在工作电极表面,LiF的加入会使电极表面的WAl₄量变小,取而代之的是Al₂O₃的增加,说明LiF的加入使电解更加稳定,抑制了电极表面WAl₄的生长。     

Low temperature sintering of PLZST-based antiferroelectric ceramics with Al2O3 addition for energy storage applications

Antiferroelectric (AFE) materials have superior energy storage properties in high power multilayer ceramic capacitors (MLCCs). To adapt to the sintering temperature of inner metal electrodes with less palladium content, in this work, Al₂O₃ was added to Pb_0.95La_0.02Sr_0.02(Zr_0.50Sn_0.40Ti_0.10)O₃ (PLSZST) AFE ceramics, in an attempt to reduce the sintering temperature. Results of this study demonstrate that the optimal composition of PLSZST-0.8 wt% Al₂O₃ sintered at a lower temperature 1040 ℃, has a high recoverable energy density (W_re, 3.23 J/cm³) and a high efficiency (η, 90 %) at room temperature. It is also high in pulse discharge energy density (W_dis, 2.45 J/cm³), current density (1369 A/cm²), and has an extremely short period of discharge (less than 500 ns). In addition, both W_re and η demonstrate a good stability in temperature within a wide range of 30 ℃-100 ℃. In sum, this novel AFE composition has great potentials for energy storage applications such as high energy density MLCCs.     

Aluminium dissolution in NaF-AlF3-Al2O3 systems

The rate of dissolution of electrolytically deposited aluminium was determined by the method of current reversal chronopotentiometry at a tungsten electrode in NaF?AlF₃?Al₂O₃ melts of varying NaF/AlF₃ molar ratios or cryolite ratios (CR). The temperature was maintained at 1031±3°C and the alumina content at 4 wt%. More accurate data were obtained by introducing delay times of various lengths (at zero current) between the cathodic and anodic current pulses, compared to direct current reversal chronopotentiometry with varying forward (deposition) times. The rate of aluminium dissolution increased with increasing NaF/AlF₃ molar ratio, the curve showing an inflexion in the vicinity of CR=3. This inflexion indicates two dissolution mechanisms, one being predominant depending on the CR. The main reaction in acidic melts (CR<3) may be represented by $$2Al(l) + AlF_6^{3 - } \rightleftarrows 3Al(I)F_x^{1 - x} + (6 - 3x)F^ - $$ while in basic melts (CR>3) $$Al(l) + 3Na^ + \rightleftarrows 3Na(soln) + Al(III)$$ is the likely dominant mechanism. For 0.8?7 mol cm^?2s^?1.     

Electrochemical formation and control of chromium nitride films in molten LiCl-KCl-Li3N systems

Electrochemical formation and control of chromium nitride films have been investigated in molten LiCl-KCl-Li₃N systems at 723 K. Chromium nitride films were obtained by means of potentiostatic electrolysis of chromium electrodes in the melts. From XPS and XRD analyses, it was confirmed that all obtained films consisted of Cr₂N and CrN, and the composition of each nitride layer was effected by the applied potential value. For instance, a nitride layer consists mostly of Cr₂N on a chromium specimen after potentiostatic electrolysis at 1.0 V (vs. Li⁺/Li), and it consists mostly of CrN at 1.5 V. At the potential range from 1.0-1.5 V, the ratio of CrN-Cr₂N in the nitride layer increases, as the applied potential is more positive. Furthermore, the thickness of the nitride layer increases with increasing the electrolysis time. The obtained results suggest that compositions and thickness of nitride films can be controled by electrolytic conditions, e.g. applied potential and electrolysis time.     

Sorption of gases by crystalline and molten 12CaO.7Al2O3

In order to explain the role of gases in the stabilization of the structure of 12CaO.7Al₂O₃ a mass-spectrometric study was carried out in vacuum of 10⁻⁹ atm and in the temperature range of 900° – 1346°C, using polycrystalline samples obtained from the crystallization of melts soaked at 1500°C. It has been demonstrated by thermodynamic analysis of partial pressures of H₂O, CO₂, CO and O₂ measured over the samples that the gases CO₂, CO and O₂ were sorbed by the specimens in molten state and were encapsulated during its crystallization, while the water vapours further penetrated into the zeolitic structure of the crystals at the time of their cooling. High values of isosteric heat of desorption of gases CO₂, CO, and O₂, such as 70–80 K-cal/mole, show that the desorption of these gases takes place due to the breakdown of the crystal lattice, while the removal of water with isosteric heat of desorption of about 11 K-cal/mole occurs as a result of its escape through the ‘windows’ in the cellular structure of the crystal. Based on these experimental results a structural model of the melt formed from 12CaO.7Al₂O₃ crystals has been developed.     

Performance and transition mechanism from acidity to basicity of amphoteric oxides (Al2O3 and B2O3) in SiO2–CaO–Al2O3–B2O3 system: A molecular dynamics study

Amphoteric oxides (Al₂O₃ and B₂O₃) represent opposite effects on the structure and properties of silicate melts in different conditions, while the understanding about the transition from acidity to basicity is far from complete. Molecular dynamics simulation was adopted in the present study to investigate the performance and acidity-basicity transformation of Al₂O₃ and B₂O₃ in the SiO₂–CaO–Al₂O₃–B₂O₃ system. The results showed that, different from Ca²⁺ ions, excessive Al³⁺ or B³⁺ ions tend to destroy the bridge oxygen structures, showing the function of basic oxide. This is similar to the behavior of Ca²⁺ ions and other basicity ions. It was found that, on the one hand, B³⁺ ions tend to form [BO₃]^3- planar triangular structures with the increase of B³⁺ ions contents, on the other hand, B³⁺ ions could reduce the stability of Si–O bonds. Therefore, B³⁺ ions could make the system structure less stable, which is the reason why the B₂O₃ is a kind of active agent. In addition, because of the significant differences in lattice energy and atomic structure between Al₂O₃ and B₂O₃, the effects of Al₂O₃ and B₂O₃ on the thermodynamic properties of silicate melts are quite different.     

Study on the electrochemical behaviour of Al3+ in LiCl-KCl-AlCl3 melt

The electrochemical behaviour of Al³⁺ reduction on a tungsten electrode in LiCl-KCl-AlCl₃ molten salt was investigated by electrochemical techniques. Using cyclic voltammetry (CV) and square wave voltammetry (SWV), Al³⁺ was reduced to Al in one step, and Li was deposited on Al to form Al-Li alloys of different compositions. According to chronoamperometry (CA), co-deposition of Li-Al alloys can be obtained at cathode current densities greater than 0.045 A cm⁻² or cathode potentials lower than −2.28 V (vs. Ag/AgCl). The apparent standard potential was calculated using the equilibrium electrode potential obtained from the open-circuit chronopotentiometry. Al³⁺ diffusion coefficient calculated by CV at 703 K is 0.82 × 10⁻⁵ cm²/s. In addition, the diffusion activation energy of Al³⁺ in LiCl-KCl-AlCl₃ molten salt was also obtained using the Arrhenius equation. The linear polarization was used to calculate exchange current density at different temperatures. Based on the theoretical model of metal nucleation deposition developed by Scharifker and Hills, the nucleation process of Al³⁺ involves instantaneous nucleation in the range of 673~763 K.     

L'electrolyse de l'alumine dans la cryolithe fondue. Etude du mecanisme de decharge cathodique a l'aide de l'electrode a disque tournant

The use of a rotating disk platinum electrode, on which aluminium has been previously deposited, has allowed us to determine the mechanism of the cathodic reaction during the electrolysis of cryolite—alumina melts. This mechanism requires three stages within the Nernst's diffusion layer. The first of these is the dissociation of the ion AlF^3?₆, at δ′ from the electrode, forming the ions Al³⁺ and F^?. The second one is the discharge of ions Al³⁺ diffusing towards the cathode and the third one, at δ″ from the electrode, is the reaction of the ions F^? with the ions AlF^?₄ to give again the ions AlF^3?₆. The ions Na⁺, which transport most of the current serve to maintain electro-neutrality.     

Wetting,surface tension,and work of adhesion of As<Subscript>2</Subscript>S<Subscript>3</Subscript> and As<Subscript>2</Subscript>Se<Subscript>3</Subscript> glass melts to quartz glass

The contact angles of the quartz glass surface by the As₂S₃ and As₂Se₃ glass melts and surface tension of these glass melts in the temperature range of 325–370°С have been experimentally measured. The polytherms are linear and possess negative slope in the mentioned temperature range. The work of the adhesion of As₂S₃ and As₂Se₃ glass melts to the quartz glass surface has been calculated and compared to the data on the adhesion strength of the As₂S₃–SiO₂ and As₂Se₃–SiO₂ boundaries of the solid phases.     

Viscosity,fragility and structure of Na2O–CaO–Al2O3–SiO2 glasses of increasing Al/Si ratio

The research and development of a new float glass with higher content of Al₂O₃ is essential for the commercial flat glass. The study on the workability and kinetic fragility of Na₂O–CaO–Al₂O₃–SiO₂ glass melts with different Al/Si ratios has been linked with the structure. The viscosities as a function of temperature for glass melts were derived on the basis of Vogel–Fulcher–Tamman (VFT) equation. Some characteristic temperatures and four characteristic temperature intervals of forming process in tin bath were estimated. The results showed that: adding 12 wt% Al₂O₃ substitute for SiO₂, the melting point (Tm) increased about 35 K, entire temperature interval in tin bath narrowed down about 20 K, the shortening of workability was mainly reflected in the viscosity range of 10^5.75–10¹⁰ Pa s, the fragility index m increased by 15%. It reveals an inverse correlation between the workability and the fragility. The structural changes on the tetrahedron structural unit Qⁿ (n=1, 2, 3, 4) were obtained by using Raman spectroscopy. Our analysis indicates that: the number of NBO reducing and a more polymerized structure with adding Al/Si ratios are responsible for the increase of viscosity; the tetrahedral distortion, a decrease of Q³/Q² in the Qⁿ species, is responsible for the increase of fragility.     

Electron reflection spectra of Ce3+ in BeF2 and B2O3 melts

The electron spectra of the melts of BeF₂-CeF₃ and B₂O₃-Ce₂O₃ systems have been obtained via electron reflection spectroscopy in the near UV range. Coordination numbers of cerium(III) ions have been determined in these systems from spectral data. It has been found that the nephelauxetic shift of interconfigurational 4f → 5d electron transitions in cerium ions depends on the degree of covalence of the bonds and the valence of ligands in complex groups, CeF₆ and CeO₆. It has been shown that the character of spectrograms of the B₂O₃-Ce₂O₃ melts depends on the preliminary preparation of the components of the mixture. This is related to the structural heterogeneity of oxide complex groups of cerium(III) ions in borate melts.     

High breakdown strength and energy density in antiferroelectric PLZST ceramics with Al2O3 buffer

In this work, a new core-shell structure of antiferroelectric ceramic powder (Pb_0.97La_0.02Zr_0.85Sn_0.12Ti_0.03O₃-PLZST) coated with linear dielectric (Al₂O₃) has been successfully prepared to realize high energy density through tape-casting process. According to the experimental results of electron microscope, the sol-gel derived Al₂O₃ layer was uniformly coated on the PLZST particles and the Al₂O₃ layer can be taken as the buffer layer to effectively refine the grain growth as well. Therefore, the modified PLZST particles were fine and uniform compared with the pure PLZST. It was found that the buffer layer could undertake higher electric field and the electric field applied to PLZST particles was weakened based on finite element analysis, which can avoid the premature breakdown of PLZST. And the actual measured breakdown strength was significantly enhanced from 22.2 kV/mm to 35.5 kV/mm. Correspondingly, an extremely high recoverable energy storage density of 5.3 J/cm³ was obtained for PLZST with 0.5%wt Al₂O₃, an 204% enhancement over the pure PLZST ceramics (2.6 J/cm³), and the corresponding efficiency was up to 88.3%. In addition, impedance spectroscopy measurement was carried out to further confirm the better insulation of the ceramic with Al₂O₃ buffer.     

A Novel Multifunctional Upconversion Phosphor: Yb3+/Er3+ Codoped La2S3

Yb³⁺/Er³⁺ codoped La₂S₃ upconversion (UC) phosphors have been synthesized using high‐temperature solid‐state method. Under 971‐nm excitation, the maximum luminescence power can reach 0.64 mW at the excitation power density of 16 W/cm² and an absolute power yield of 0.36% was determined by an absolute method at the excitation power density of 3 W/cm², and the quantum yield of La₂S₃:Yb³⁺, Er³⁺ (green ~0.18%, red ~0.03%, integration ~0.21) was comparable to that of NaYF₄:Yb³⁺, Er³⁺ nanocrystals (integration ~0.005–0.30). Frequency upconverted emissions from two thermally coupled excited states of Er³⁺ were recorded in the temperature range 100–900 K. The maximum sensitivity of temperature sensing is 0.0075 K^?1. As the excitation power density increases, the temperature of host materials rapidly rises and the top temperature can reach to 600 K. Given the intense UC emission, high sensitivity, as well as good photothermal stability, La₂S₃:Yb³⁺/Er³⁺ phosphor can become a promising composite material for photothermal ablation of cancer cells possessing the functions of temperature sensing and in vivo imaging.     

Determination of the SiO2 Activity in Melts of the CaO–Al2O3–SiO2 System by High-Temperature Mass Spectrometry

This paper reports on the results of systematic investigations into the thermodynamic properties of melts lying along the joins with constant SiO₂ contents of 0.5 and 0.6 mole fractions in the CaO–Al₂O₃–SiO₂ system at mole fraction ratios CaO : Al₂O₃ varying from 0.25 to 4.0. The activities of SiO₂ in melts of this system at a temperature of 1905 ± 10 K are determined by high-temperature mass spectrometry. It is found that, in the concentration range studied, the activity of SiO₂ remains constant at SiO₂ contents of 0.5 and 0.6 mole fractions.     

Polarization measurements on solid platinum—molten sodium sulphate—sodium chloride interfaces

Electrochemical studies in sodium sulphate—sodium chloride melts at 900° are reported. The limiting reactions which occur during electrolysis of fused sodium sulphate are suggested to be the SO₃ reduction to S^2? and O^2? and the SO^2?₄ oxidation to SO₂ and O₂ at the cathode and anode, respectively. Additions of sodium chloride into the sulphate do not reduce its stability range.A relevant finding is that platinum is anodically passivated in sulphate melts. Voltammetric scans made to elucidate its passivation mechanism show that the anodically polarized platinum is continuously modified and passes through three states of passivation. Moreover, it is observed that the platinum samples used as cathodes are attacked by the sulphate reduction products.     

Aluminium dissolution in the NaF-AlF3-Al2O3 system: Effects of alumina,temperature, gas bubbling and dissolved metal

Current reversal chronopotentiometry, with and without a delay time between the forward and reverse current pulses, was employed to evaluate the effects of temperature, alumina content, gas bubbling (argon and carbon dioxide) and dissolved metal on the rate of aluminium dissolution in NaF–AlF₃–Al₂O₃ molten bath. The working electrode was a tungsten wire electrode and the temperature range studied was 824–1040°C. The effect of the alumina content was determined in melts with CR=1.45 and CR=4.3 at 1029±3°C (CR = mol NaF/mol AlF₃). The experiments involving gas bubbling and dissolved metal were carried out in melts similar to industrial compositions, i.e. CR=2.4, 4.8 wt. % Al₂O₃ at 980°C. In general, the dissolution rate of aluninium increased with increasing temperature, decreased slightly with increasing alumina content in acidic melts (CR<3) but changed little in basic melts (CR>3), increased with bubbling and decreased in the presence of dissolved metal. The rate of Al dissolution is thus mass transport controlled.     

Coincidence Doppler broadening spectroscopy in polyvinyl chloride after doping with Al2O3

Coincidence Doppler broadening of annihilation radiation (CDBAR) and Vickers hardness techniques were performed to study pure Al₂O₃, pure polyvinyl chloride (PVC), and doped PVC with different concentrations of Al₂O₃ (10–50%). The CDBAR ratio curves with respect to pure PVC were presented and reflect the momentum distribution of all the samples. The peak around 14.5 ×10^?3 m_oC in the CDBAR ratio curves suggests a large contribution of positron annihilation with the Al₂O₃. There is a linear correlation between the height of this peak and the Al₂O₃ concentration. The S‐ and W‐parameters were extracted from the CDBAR spectra and increase with increasing the Al₂O₃ concentration showing discontinuity at 30% of Al₂O₃ concentration on PVC. The present data confirmed that there is no positronium formation in pure Al₂O₃ as a result of smaller S‐parameter. The Vickers hardness increases with increasing the Al₂O₃ concentration in PVC showing a linear dependence with two different slopes depend on the Al₂O₃ concentration range. A correlation between the Vickers hardness (macroscopic data) and the W‐parameter (microscopic data) was observed. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008