Plasmas & Ions最新文献

Unified self-consistent models representing the basic characteristics of the alkali fluids are reviewed. The self consistency consists of Monte Carlo simulations, in a classical lattice gas, to obtain atomic configurations which appear with a thermal probability e^F/k_BT, where the configurational free energy F is determined by that of a quantum mechanical electronic system with one valence electron per atom. The models allow calculation of the structural, thermodynamic, and electronic properties of the fluid. The results of such calculations have been compared with experimental data. The calculated properties which have been emphasized are the vapor–liquid coexistence curve and, at coexistence conditions, the electrical conductivity and the electronic paramagnetic susceptibility. The main conclusions which have been reached are the following. The asymmetric coexistence curves of these fluids arise from the effects of equilibrium density fluctuations which, due to the influence of delocalizable valence electrons, give rise to energies which are not pairwise additive. The nonmetal to metal transition in these fluids is predominantly due to the onset of percolation in the atomic structures driven by the electronic effects. Self consistency of atomic and electronic structures are crucial to the understanding of the properties of these fluids.

The mechanism of the electroreduction of K₂TiF₆ and KBF₄ in fused KCl–NaCl–NaF was determined from transient electrochemical techniques at temperatures ranging from 650 to 800 °C. The reduction of titanium occurred in a two-step reaction Ti⁴⁺ + e⁻→ Ti³⁺ and Ti³⁺ + 3e⁻→ Ti similar to that obtained in all-fluoride melts. The deposition of boron is obtained from the reduction of boron at oxidation state III: B³⁺ + 3e⁻→ B. Cyclic voltammetry showed that when KBF₄ was introduced in the KCl–NaCl–NaF–K₂TiF₆ melt a new peak appeared corresponding to the reaction Ti³⁺ + 2B³⁺ + 9e⁻→ TiB₂. Analysis by numerical simulation of the transient curves was used to obtain accurate values of the reaction parameters: rate constants, coverage factors, diffusion coefficients and standard potentials. From these measurements the thermodynamic properties of the solutions are calculated. It is shown that the Gibbs energy of formation of TiB₂ deduced from the present experiments is in good agreement with the values in the literature.

The electrode processes in the FLINAK–K₂TaF₇–KBF₄ melt were studied by the method of linear voltammetry with a silver working electrode at the temperature of 700–750$\text{°}$C. In order to eliminate possible errors, two approaches were used, namely KBF₄ addition to the FLINAK–K₂TaF₇ melt and K₂TaF₇ addition to the FLINAK–KBF₄ melt. During the experiment the oxygen content in the melt was constantly monitored by linear voltammetry with a glassy carbon electrode. It was shown that depending on the B/Ta molar ratio in the melt the voltammograms had at least one peak. So, at B/$\text{Ta}\text{=}\text{1}$ two cathodic peaks can be seen. Electrolysis of this melt at increasing current density leads to the formation of Ta_cub, then Ta_tetragon and a mixture of tantalum with borides. Further increase of B/Ta results in a greater number of cathodic peaks, as well as the appearance of TaB₂ and elemental boron in the cathodic deposit alongside with the aforementioned phases. In the electrolytes studied no corroboration of the existence of tantalum and boron heteronuclear complexes has been found. The addition of sodium oxide to the melt in question simplifies the shape of cyclic voltammograms of the FLINAK–K₂TaF₇–KBF₄ melt considerably.

The correlations between structural holes in the fluid phase of the one-component classical plasma near its freezing point at coupling strength Γ = 179 are studied by a statistical method using the Ornstein-Zernike relations for a partly quenched disordered system in combination with the hypernetted chain closure. The method involves inserting in the quenched structure of the plasma variable numbers of point-like charged particles, which on reaching equilibrium probe the holes in the matrix. When the probes carry the same charge as the plasma particles, the results may also be interpreted as describing the evolution of the correlations between annealed particles in a partly quenched disordered plasma upon varying the fraction of quenched particles at constant total density. Doubling the charge carried by the probes sharpens their correlations and improves the resolution that can be obtained in this method of structural analysis.

The electrochemical behavior of niobium in a 52:48 mole ratio AlCl₃:NaCl molten salt electrolyte was examined by linear sweep voltammetry, chronoamperometry and chronopotentiometry at 190 °C. Niobium oxidation begins at about 1.10 V versus Al but appears to be limited to a charge of about 3.7 mC/cm² before the electrode becomes inhibited. This inhibition is likely due to the limited solubility of the niobium oxidation product. The inhibition region extends from about 1.20 V to 1.40 V. At higher oxidation potentials, the current can be continuously increased up to the anodic limit of the chloroaluminate electrolyte. Aluminum-niobium alloys can be electrodeposited from the above electrolyte following Nb dissolution. Alloys containing a Nb atomic fraction of up to 13.4 % have been electrodeposited. X-ray diffraction and transmission electron microscopy examination of these electrodeposits indicate that the low Nb deposits are face centered cubic aluminum. As the Nb content increases, an amorphous phase is introduced into the structure. The exact composition of the amorphous phase as well as the phase distribution in the 3 % to 13.4 % Nb alloys have yet to be determined.

The liquid-phase organic reactions induced by a gaseous plasma, which was generated in a localized zone between an electrolytic solution and an anode by means of contact glow discharge electrolysis (CGDE), were investigated. Benzoic acids dissolved in a neutral phosphate buffer solution were efficiently oxidized and eventually decomposed to inorganic carbon. As the intermediate products, oxalic, formic and malonic acids were detected as well as hydroxybenzoic acids. On the basis of the detailed analysis of the hydroxylation products and kinetic consideration, a reaction pathway was proposed, where the attack of hydroxyl radicals on the benzene rings of starting materials might be a key step.

The particular properties of molten carbonates, oxyanionic salts, allow their application in the promising technological field of fuel cells. A general description of this class of molten salts is presented, outlining the importance of the oxoacidity concept and potential-acidity diagrams to understand the main features of this electrochemical device, dedicated to energy and heat generation. The main characteristics of the molten carbonate fuel cell (MCFC) are reviewed from the single cell, including the matrix electrolyte and the electrode mechanisms, to the stack, with a special attention to the bipolar plates. The factors limiting MCFC lifetime are discussed in order to analyze the potential improvements of the materials presently used. An overview of the principal demonstration prototypes all over the world is given. Finally, some conditions for the introduction of MCFC in the commercial energy market are briefly mentioned.

Vapour pressures of water over the three-component system NaNO₃-LiNO₃-H₂O are reported for five different compositions (100, 60, 40, 20, and 0 cationic % Na) at 179 °C. Water-activities calculated from these data are fitted to the equations of Pitzer [1], of Pitzer and Simonson [2], and Horsák and Sláma [3]. Excess thermodynamic properties are calculated from these models. A new vapour pressure apparatus is presented, which automatically records the water-activity of a salt-water mixture over the concentration range from a dilute solution to the fused salt during one experiment. The concentration of the liquid solution in the measuring cell can be changed by stepwise distillation of small portions of water, which are collected in a cup placed on a balance. The computer-based data acquisition makes it possible to collect the experimental data three times faster than on a discontinuous setup. The new distillation apparatus is tested against earlier data on NaNO₃-H₃O at 179 °C [4].