Self-diffusion Coefficients Of Na Research Articles

KF-NaF-AlF<sub>3</sub>体系结构性质的分子动力学模拟

近年来低温铝电解质的研究已成为铝电解工业上节能减排的热点问题，对其基本物理化学性质的探索是一个必然的过程。本文运用密度泛函理论与经典分子动力学方法相结合的方法系统研究了1100 K下xKF-2.2NaF-AlF3熔盐体系的结构性质和输运性质。研究结果表明在平衡的高温钾冰晶石熔盐体系中六配位[AlF6]3−、五配位[AlF5]2−、四配位[AlF4]−络合离子共同存在，且六配位铝氟离子集团占主导地位(70%以上)，并会发生部分的离解，生成五配位或者四配位离子团；当KF浓度增大时，电解质体系中游氟原子含量会增加，桥氟含量逐渐降低，体系的聚合程度也会随之下降。此外，KF的加入使得桥氟离子解离，Na、Al的自扩散系数会先增大后降低，体系显示出低粘度高电导率的特性，但当KF浓度达到2%后，电导率会逐渐降低。 The study of low temperature aluminium electrolytes has become a hot issue for energy saving and emission reduction in the aluminium electrolysis industry in recent years, and the exploration of their basic physicochemical properties is an inevitable process. In this paper, the structural and transport properties of the nKF-2.2NaF-AlF3 molten salt system at 1100 K have been systematically investigated using a combination of density flooding theory and classical molecular dynamics methods. The results show that in the equilibrium high-temperature potassium cryolite molten salt system, hexacoordinated [AlF6]3-, pentacoordinated [AlF5]2- and tetracoordinated [AlF4]- complex ions co-exist, and the hexacoordinated aluminium-fluorine ion group dominates (more than 70%) and undergoes partial dissociation to produce pentacoordinated or tetracoordinated ion groups; As the KF concentration increases, the amount of free fluorine atoms in the electrolyte system increases, the bridge fluorine content gradually decreases and the degree of polymerisation of the system decreases. In addition, the addition of KF causes the dissociation of the bridging fluorine ions, the self-diffusion coefficients of Na and Al will first increase and then decrease, and the system shows the characteristics of low viscosity and high conductivity, but when the KF concentration reaches 2%, the conductivity will gradually decrease.

Counter-ions diffusion properties of silica embedded poly(perfluorosulfonic) acid membrane

To study the possibility of modifying diffusional-transport properties of the poly(perfluorosulfonic) acid (Nafion-117) membrane, the silica particles were incorporated in its matrix by in situ sol–gel method. The amount of silica incorporated in the membrane samples was varied by controlling the equilibrating time of the membrane sample in solution containing fixed proportion of precursor tetraethylorthosilicate (TEOS), methanol and water. The silica contents in the membrane samples were characterized by gravimetry, energy dispersive X-ray analysis (EDXRF) and transmission electron microscopy (TEM). Differential scanning calorimetry (DSC) studies showed that there were no significant difference in transition peak temperatures of both samples (Nafion-117 and Nafion/SiO 2) in same ionic form except that enthalpies were increased in Nafion/SiO 2 samples in all the ionic forms. The increase in enthalpy was due to higher water content in Nafion/SiO 2 as compared to Nafion-117 in given ionic form as enthalpies normalized with water content of the respective membrane were comparable. The ion-exchange capacity, water uptake capacity and ion-exchange isotherms of the Na + + Cs + and Na + + Ba 2+ binary systems in silica embedded membrane samples were experimentally measured using appropriate radiotracers, and compared with that of pristine Nafion-117 membrane. The specific conductivities of the membrane samples with and without silica were studied in Na +, Cs + and Ba 2+ ionic forms by electrochemical impedance spectroscopy (EIS). The self-diffusion coefficients of Na +, Cs + and Ba 2+ ions in the Nafion/SiO 2 were measured by the radiotracer method. It was observed from the experimentally measured specific conductivities and self-diffusion coefficients data that diffusion mobility of Cs + and Ba 2+ counter-ions were enhanced significantly after incorporation of SiO 2 in the Nafion-117 matrix. Contrary to this, diffusion mobility of Na + counter-ion decreased in Nafion/SiO 2 sample. The selectivity of Cs + and Ba 2+ counter-ions was reduced in Nafion/SiO 2 sample with respect to pristine Nafion-117.

Inclusion of silver nanoparticles in host poly(perfluorosulfonic) acid membrane using ionic and non-ionic reductants

Silver nanoparticles (Ag nps) in host poly(perfluorosulfonic) acid membrane (Nafion-117) were formed by in situ reduction of the Ag + ions with ionic (sodium borohydride) and non-ionic (dimethyl formamide, and formamide) reagents. Ag + ions were loaded in the membrane samples by the ion-exchange mechanism. Ag + ions were tagged with 110mAg radiotracer to monitor the process of Ag nps formation in the membrane matrix. Transmission electron microscopic analyses of cross-section of the reduced membrane samples were carried out to study the shape, size, and positioning of nps in the membrane. This study revealed that the shape of Ag nps in all the cases was spherical, but size and spatial distributions varied depending upon reducing conditions of the membrane. The bulk concentration of Ag nps having 15 ± 4 nm size was found to be located near the surface of BH 4 − ions reduced membrane sample. In case of slow dimethyl formamide reduction, the non-uniform and bimodal size distribution (5 ± 1 and 18 ± 5 nm) of Ag nps was observed in the membrane matrix. The reduction with formamide at 65 °C was found to produce Ag nps dispersed uniformly in the membrane matrix with narrow size distribution (9 ± 2 nm). EDXRF and radiotracer analyses were carried out to quantify Ag contents of the nanoparticles formed in the reduced membrane samples. The mechanism of formation of Ag nps in the membrane by ionic and non-ionic reductant was interpreted in terms of ion-exchange process operating during the reduction. The self-diffusion coefficients of Na +, Cs +, and Eu 3+ counterions in the Ag nps embedded membrane samples were measured by the isotopic-exchange method. It was observed that the presence of Ag nps enhanced the diffusion mobility of slow moving Cs + and Eu 3+ counterions ions in the membrane matrix by 2 and 5 times, respectively.

Self-Diffusion of <SUP>22</SUP>Na and <SUP>137</SUP>Cs in Simulated Nuclear Waste Glass

The knowledge of self-diffusion of various elements is important for understanding and elucidating the long-term dissolution of the nuclear waste glass. The self-diffusion coefficients of Na and Cs in P0798 simulated nuclear waste glass have been measured by an ion beam sputter-sectioning technique using the radioactive isotopes 22 Na and 137 Cs. The temperature dependence of the diffusion coefficients in each temperature range in P0798 glass below the glass transition temperature Tg can be expressed by the following equations: (428-574K) : D Na-belowTg = 2.7 x 10 -6 exp(-113±4kJ.mol -1 /RT)m 2 .s -1 (713-758K): D Cs-belowTg = 7.1 x 10 -5 exp(-241±10kJ.mol -1 /RT)m 2 .s -1 The diffusion coefficient of 22 Na was about ten orders of magnitude larger than that of 137 Cs at 573 K. The large difference in the diffusion coefficients is explained in terms of the ion size effect, the mixed-alkali effect and lower concentration of Cs compared with Na in P0798 glass.

Study of self-diffusion of alkali metal cations inside a Nafion membrane

A novel radiotracer method, based on the rotating electrode technique used in electrochemistry, was used for the study of the self-diffusion properties of ions in ion-exchange membranes; the self-diffusion coefficients of Na+ and Cs+ ions were determined in a Nafion 117 membrane. The influence of electrolyte concentration and temperature was investigated and an important influence of the former was observed. The results indicate that the diffusion process inside the membrane for Na+ ion depends linearly on the water volume fraction. The results for Cs+ may be a sign of stronger interactions than for Na+ ions with the membrane.

Self-diffusion of sodium ions in hydrous stannic oxide from mixed-solvent solutions

Self-diffusion coefficients of Na +, D ̄ i , inside hydrous stannic oxide have been measured in aqueous and alcoholic solutions at different Na +-capacities. D ̄ i always displays the usual decrease with capacity, which seems to be much less profound at the relatively high capacities. There is apparently almost no effect of addition of methanol on D ̄ i , whereas a decrease of D ̄ i , which disappears towards relatively high capacities, occurs on addition of propan-1-ol. These results are in marked difference with previous results of the effective diffusion coefficients of Na + H + systems. This difference seems to indicate that the effect of alcohol addition in increasing the strength of electrostatic interaction of counter ions with the exchanger surface (a factor which, when predominating, decreases mobility) is higher with protons than with the Na + ions.

Water sorption and ionic diffusion in short side chain perfluorosulfonate ionomer membranes

Several properties of short side chain perfluorosulfonate ionomer membrane films, made by the Dow Chemical Company, have been measured as a function of equivalent weight of the polymer. Water and electrolyte sorption and the self-diffusion coefficients of Na +, Cs +, Cl −, and I − have been measured. along with activation energies of diffusion. Also, the near infrared combination band for OH has been used to develop insight into the nature of sorbed water in the ion clusters of these micro-phase separated materials. Results show that these polymers have properties which are similar to Nafion ® perfluorinated ionomer membranes, with their diffusional properties being intermediate between those of the sulfonate and carboxylate polymers. Equivalent weight was found to influence the spreads in diffusion coefficients among ions, which increase with increasing equivalent weight. The sorbed water for these polymers show significantly reduced hydrogen bonding strength compared to bulk water, and this property is counter-ion dependent.

Self-diffusion coefficients of ions in electrolyte solutions by nonequilibrium Brownian dynamics

The self-diffusion coefficients of the ions in a model electrolyte solution are calculated with a novel implementation of the nonequilibrium Brownian dynamics technique. The ions are coupled to an external color field E by color charges in such a way that each ionic species as a whole is electrically neutral to E. The ion–ion forces are not directly affected by the color charges or E. The method is tested on a model of a 1 M NaCl aqueous solution without hydrodynamic interactions and the results are compared with those of a previous equilibrium simulation for the same model system. The self-diffusion coefficients of Na+ and Cl− are determined with 2%–3% accuracy and, within this margin, agree with the results of the equilibrium simulation obtained with more than twice the computational effort. Furthermore, within the range of field strengths studied, the average color flows depend linearly on E.

Self-Diffusion of Sodium, Chloride and Iodide Ions in Methanol-Water Mixture

The self-diffusion coefficients of Na+, Cl- and I- in methanol-water solutions at 35 ± 0.01 °C have been measured in their dependence on the salt molarity in the range 1 · 10-4- 1 · 10-2 mol dm -3. The ionic self-diffusion coefficients in infinitely diluted solutions have been computed. The influence of the solvent composition on the solvation of the ions is discussed. A preferential hydration of Na+, Cl- and I- ions in water-methanol mixtures has been found.

Entropy and enthalpy of formation and migration of lattice defects in Na(I)

The self-diffusion coefficients of Na + and I −, as well as the ionic conductivity, are measured in Na(I) single crystals, pure and doped with Ca 2+ ions. The computer analysis of the whole set of data shows that the atom transport in Na(I) is ensured essentially by the free vacancies with a very small contribution (∼4%) of the vacancy pairs. The thermodynamic parameters of vacancy formation and migration are derived by taking the long range interactions into account (activity coefficients on the concentrations and relaxation factors on the mobilities). The respective entropies of Schottky defect formation, cation migration and anion migration are equal to 7.64, 3.23 and 2.91 k while the corresponding enthalpies are equal to 2.00, 0.58 and 0.77 eV.

Conductibilite, nombres de transport et autodiffusion des ions dans differents monocristaux de chlorure de sodium

The measurements of conductivity, transport numbers and self diffusion coefficients of Na + and Cl − have been made with an accuracy of 1 per cent in single crystals of pure and Sr 2+-doped NaCl. The contributions of cation-vacancies, anion-vacancies and vacancy-pairs have been separated; this has allowed to reach, from the whole of the experimental data, the true values of the enthalpies and entropies of defects formation and migration. The Nernst-Einstein relation has then been strictly studied; this relation is verified with an accuracy of 2 per cent, whereas the correlation theory had forecast a multiplicative factor equal to 0,78. It seems therefore that in the studied system, the rigid lattice model, the consequence of which is the existence of correlation effects, is worth being reconsidered.

Ion exchange properties of and rates of ionic diffusion in beta-alumina

The sodium ion in beta-alumina has been exchanged in molten salts with a number of univalent and divalent ions. The sodium and potassium concentrations in the solid beta-alumina in contact with various binary NaK nitrate compositions have been carefully determined and the free energy difference between K + and Na + beta-alumina calculated. The self diffusion coefficients of Na +, Ag +, K +, Rb +, and Li + in single crystals of beta-alumina have been determined as a function of temperature. Sodium ion has the largest self diffusion coefficient (1 × 10 −5 cm 2/sec at 300°C and 4·0 × 10 −7 cm 2/sec at 25°C) followed by Ag +. L +, Li +, and Rb +. Data are also presented for interdiffusion conefficients and dielectric loss measurements, and some speculations are made as to the mechanism of the diffusion.

Self-diffusion Coefficients of Na<sup>+</sup> Ion in NaPO<sub>3</sub>-Al(PO<sub>3</sub>)<sub>3</sub> Glasses about Transition Temperatures

Self-diffusion coefficients of Na+ ion in NaPO3-Al(PO3)3 glasses at temperatures about transition points were measured by means of tracer technique using radio-isotope 22Na.The counting of radio-activity at successively ground surfaces gave better results than that of powders removed by grinding because of less experimental errors.The activation energies for the diffusion of Na+ ion above and below transition points were 30kcal/mole and 20kcal/mole respectively, which were in accord with the values in silicate glasses and were also agreed with the activation energies for the electrical conductivity of these glasses.According to a corrected Nernst-Einstein equation, the relation between diffusion coefficient, Di, and the electrical conductivity, σ, of solid is given byDi=σtikT/niZi2e2⋅fwhere ti is the transfer number, k is Boltzmann constant, T is the absolute temperature, ni is the number of ions per cubic centimeter, Zi is the valence, e is the electronic charge, f is the correlation factor named by Bardeen and Herring.The f-value obtained from the author's experiment was about 0.15-0.25, and this value was maintained at temperatures above and below the transition point. It was interpreted, therefore, that the transport mechanism of Na+ ion in these glasses about transition temperatures is one of the indirect interstitial (interstitialcy) transport rather than the simple vacancy transport.

Self-Diffusion of Exchangeable Cations in Bentonite

AbstractA special integral form of the solution of Fick's law was applied to study the self-diffusion of Na, Cs, and Ca ions in Wyoming bentonite plugs by means of a radioactive tracer technique. A simple procedure for measuring diffusion coefficients was developed by depositing a thin radioactive tracer film on the surface of clay mineral plugs at controlled temperatures. Thin sections of the clay plugs were removed and radioactivity measurements made on the portion of the plugs remaining. The self-diffusion coefficients of the ions were evaluated by a graphical method from the solution of Fick's law at special boundary conditions. Under oven-dry conditions at 105°C diffusion of Na ion in a bentonite film was observed, while in the same experiment, SO42− ion did not diffuse. In a clay-water system, however, the anion diffused much more rapidly than the cation. Based on the experimental results of observing Na22- and S35-tagged Na2SO4 diffusion, different diffusion paths were proposed using a picture of the clay matrix as a structure with “channels.” Diffusion coefficients of exchangeable cations decreased with increasing clay contents. From the results of activation energy and self-diffusion coefficients of Na and Ca ions at various clay-water concentrations, the relative “hopping distances” of diffusing ions were calculated and discussed. It is suggested that in the self-diffusion of exchangeable Na ion in bentonite the ion could be hopping from one individual clay plate to another, and the Ca ion from one “package” of plates to another “package” of plates.