Ionic Regime Research Articles

Effect of Ag 2O on the conductive behaviour of silver vanadium tellurite glasses: Part II

In a previous work, the effect of the Ag 2O on the electrical conductivity of vanadium tellurite glasses of the form XAg 2O·(1− X)V 2O 5·2TeO 2 has been studied by using the impedance spectroscopy in a wide range of temperature and composition. The obtained results confirm the existence of a transition from a typically electronic (polaronic) conductive regime when the molar fraction ( X) of Ag 2O is equal to 0, to an ionic conductive regime when X tends to be 1. This transition is characterised by a deep minimum in the electrical conductivity of about three orders of magnitude. In the present paper, a complementary study of the system of the form 0.27Ag 2O·0.73[ YV 2O 5·(1− Y)TeO 2] is presented. In this system, there also exists such a transition, but now, from an ionic to an electronic conductive regime. The correlated behaviour between conductivity and the mean silver-silver and vanadium-vanadium distances indicates that a concentration-based explanation is appropriate.

Polyelectrolyte adsorption on charged particles: Ionic concentration and particle size effects—A Monte Carlo approach

The complexation between a charged polymer and an oppositely charged spherical particle is investigated using Monte Carlo simulations. Electrostatic interactions are described in the Debye–Hückel approximation. The influence of particle size and ionic concentration on the adsorption/desorption limit, interfacial structure of the adsorbed layer, amount of adsorbed polymer, and the overcharging issue is investigated. Attention is focused on polyelectrolyte adsorption on small spherical particles whose surface curvature effects are expected to limit the amount of adsorbed monomers, large particles that allow the polyelectrolyte to spread to the same extent as on a flat surface, and particles whose radius is close to the polyelectrolyte radius of gyration so that the chain can completely wrap around it. The formation of a polyelectrolyte/particle complex and the conformations of the adsorbed polyelectrolyte are found to result from two competing effects: the electrostatic repulsions between the chain monomers which force the polyelectrolyte to adopt extended conformations and limit the number of monomers which may be attached in particular to small particles, and the electrostatic attractive interactions between the particle and the monomers forcing the charged polymer to undergo structural transition and collapse at the particle surface. It is shown that adsorption is favored by increasing particle size and decreasing ionic concentration. Trains are favored at low ionic concentrations while loops (prior desorption) are favored more when increasing the ionic strength. Below a critical particle size, by decreasing the ionic strength, electrostatic repulsions between the adsorbed monomers force the polyelectrolyte to form protuding tails in solution, hence decreasing the amount of polyelectrolyte adsorption. By decreasing the particle size still further, the low ionic concentration regime is dominated by monomer–monomer repulsions; the polymer partially wraps around or becomes tangential to the particle and two tails extend in opposite directions. The complex may or may not exhibit charge inversion depending on the particle size and ionic concentration. We find that charge reversal increases with salt concentration and reaches a maximum when the polyelectrolyte is able to wrap around the particle completely.

Effect of Ag 2O on the conductive behaviour of silver vanadium tellurite glasses

The effect of the Ag 2O on the conductivity of vanadium tellurite glasses has been studied by using the impedance spectroscopy in a wide range of temperature. The obtained results confirm the existence of a transition from a typically electronic (polaronic) conductive regime when the molar fraction ( X) of Ag 2O is equal to 0, to a ionic conductive regime when X tends to 1. This transition is characterized by a deep minimum in the electrical conductivity of about 3 orders of magnitude.

Electrical conductivity and nonstoichiometry in doped Sr 3Ti 2O 7

A series of doped Ruddlesden–Popper phases, of general formula Sr 3Ti 2- x M x O 7- δ ( M= Al, Ga, Co) , were synthesized and their electrical conductivity characterized as a function of temperature and oxygen partial pressure. For fixed-valent dopants, p-type conductivity predominates at p(O 2)> 10 −5 atm, followed by a p(O 2)-independent electrolytic regime, and n-type electronic conductivity at very low p(O 2). The electrolytic regime exhibits activation energies in the range 1·7–1·8 eV. Doping with transition metals such as Co results in a very significant increase in total conductivity with a p-type conductivity at high p(O 2). Furthermore, an apparent ionic regime at intermediate p(O 2) is observed, characterized by high conductivity (> 10 −2 S/cm at 700°C) and low activation energy (0·6 eV). This interpretation is consistent with iodometric measurements as interpreted by a defect chemical model. Other measurements are in progress to confirm this conclusion.

New Mixed Conductors Based on Doped Layered Perovskites

ABSTRACTA series of doped Ruddlesden-Popper phases, of general formula Sr3Ti2−xMxO7−δ (M=Al, Ga, Co), were synthesized and their electrical conductivity characterized as a function of temperature and oxygen partial pressure. For fixed-valent dopants, p-type conductivity predominates at p(O2)>10−5 atm, followed by a p(O2)-independent electrolytic regime, and n-type electronic conductivity at very low p(O2). The electrolytic regime exhibits activation energies in the range 1.7-1.8 eV. Doping with transition metals such as Co results in a very significant increase in total conductivity with a p-type conductivity at high p(O2). Furthermore, an apparent ionic regime at intermediate p(O2) is observed, characterized by high conductivity (>10−2 S/cm at 700 °C) and low activation energy (0.7 eV). This interpretation is consistent with iodometric measurements as interpreted by a defect chemical model. Other measurements are in progress to confirm this conclusion.

The Mitigation of NORM Scale in the Gulf Coast Regions of Texas and Louisiana: A Laboratory and Field Study

ABSTRACT Some oil field scales have the potential to contain regulated levels of naturally occurring radioactive materials (NORM). It is estimated that between 300,000 and 1,000,000 tons of NORM scale are produced each year. In addition, scale deposition in producing facilities negatively impacts rates of production and is expensive to treat and remediate. The most common NORM containing scale is BaSO4, or barite. This paper presents the results of a Gas Research Institute study that investigates the causes of NORM scale formation and mitigation techniques employed in the field. Chemical threshold scale inhibitors are generally employed to inhibit scale formation in production systems. However, there is little agreement on which scale inhibitor is most effective with respect to differing water chemistries, temperatures and conditions encountered. Results using a GRI patented inhibitor evaluation apparatus can be used to determine the most effective inhibitor for a specific field application. Work in the Michigan Basin presented in the last conference indicated that a phosphinopolycarboxylate was most effective against NORM barium sulfate scale formation at low temperatures in relatively fresh water. Further work in this study identifies phosphonates as being more effective in the higher ionic strength (TDS) waters and higher temperature regimes that can be expected in the Gulf Coast. A matrix of ionic strength and temperatures with inhibition response is presented. Although more NORM fields were studied, two are presented in detail. In these fields, three causes of NORM scale were identified; 1) Incipient scale in a well due to production; 2) Scale formation due to previous seawater floods and; 3) The commingling of waters from different zones or wells. Field treatment techniques employed in the two fields studied are summarized in the paper. Inhibitor squeeze procedures were also studied in the laboratory and in the field. A squeeze simulation apparatus was constructed to research inhibitor squeeze practices in the laboratory. Results from this work resulted in successful inhibitor squeeze applications in the field. The inhibitor squeeze apparatus and the field results are discussed in the paper. Squeeze life has been extended from an average of two to six months to two to three years or more as a result of the work.

Oxide Ion Conductivity in the Solid Solutions: Ca1 − xNaxBi2Nb2 O 9 − x / 2 and CaBi2Nb2 − yTiy O 9 − y / 2: The Relative Importance of A Versus B Site Substitution

Oxide ion conductivity has been measured in the complete solid solution series , , and for comparison, in , . At , the conductivities of and are similar. In contrast, substitution of titanium for niobium to give the same oxygen vacancy concentration lowers the activation energy to 1.0 eV from the value of 1.7 eV measured for the parent compound. For compositions with , all samples display a discontinuity in the Arrhenius plots of the ionic conductivity at 850°C. Differential thermal analyses confirm the presence of a phase transition, attributed to an order‐disorder transition of the oxygen vacancies. A change in the activation energy also is observed at lower temperatures. Measurements of the electronic conductivity and of the influence of the oxygen partial pressure on the conductivity suggests that this transition is due to a change from an ionic extrinsic regime to an ionic intrinsic regime.

Effects of K, O, and H adatoms on the adsorption kinetics of CO on Pt(111)

The sticking coefficient, S, of CO on clean and K-, O- and H-modified Pt(111) surfaces has been measured directly using a simple kinetic uptake method, as a function of CO and modifier coverage and surface temperature. A simple method is also introduced to determine the coverage-dependent desorption energy from these measurements without the need to know the pre-exponential factor or desorption order for those systems where S is only a function of coverage or only weakly dependent on temperature. The initial sticking coefficient of CO on clean Pt(111) is 0.9 and is independent of substrate temperature. The major influence of adsorbed K on the initial sticking coefficient of CO on Pt(111) at 320 K is site blocking, and the magnitude of this effect correlates well with the size of the adsorbed K ion or atom. The shape of S as a function of CO coverage for low K coverages in the ionic regime is similar to that of CO on clean Pt(111). A dramatic change occurs for K coverages near or greater than 0.24 ML, where S passes through a maximum at about case1 3 of the saturation CO coverage and coincident CO and K desorption is observed. This behavior of S, which has not been seen before, can be fit quite adequately by a simple model that incorporates the reduced site blocking effects of K due to the CO-induced transition of K from a metallic neutral to an ionic state and the accompanying size change of K atoms. These results are best described by a coverage-dependent alkali-CO interaction. In additional studies, we find that adsorption of 0.25 ML O on Pt(111) has no effect on S for low CO coverages at 100 K, and only the saturation CO coverage is reduced from 0.59 to 0.31 ML. In contrast, adsorbed H reduces not only the CO saturation coverage but also S even at low CO coverages.

NEUTRON SCATTERING MESOSCOPIC AND MICROSCOPIC STRUCTURE DETERMINATION OF BINARY LIQUID MIXTURES UNDERGOING NON-METAL TO METAL TRANSITION (APPLICATION TO METAL-AMMONIA AND METAL MOLTEN SALTS)

NEUTRON SCATTERING MESOSCOPIC AND MICROSCOPIC STRUCTURE DETERMINATION OF BINARY LIQUID MIXTURES UNDERGOING NON-METAL TO METAL TRANSITION (APPLICATION TO METAL-AMMONIA AND METAL MOLTEN SALTS)

Characterization of the solution properties of a 1.7% sulfonated polystyrene ionomer by fluorescence probing

The solution characteristics of the ionomer in toluene are studied. Ion pair/ion pair interactions were shown to give rise to ionic microdomains averaging 6-7 sulfonate groups per aggregate; in dilute solution ion aggregation is only 2-3. The ionic aggregates were shown to maintain constant dimensions over a wide concentration range, indicative of the integrity of the primary aggregates even at high concentrations where the ionomers begin to associate. It is shown that ionic material supported in these systems are effectively screened from the bulk solution phase and its attributed to a mantle of hydrocarbon polymer chain about the ionic regimes

The effect of dipole moment on diffusion controlled bimolecular reaction rates

Calculation of diffusion controlled bimolecular reaction rates for complicated macromolecular systems is made possible by a computer simulation approach based on the Brownian dynamics (BD) trajectory method. This is applied to the calculation of the diffusion controlled reaction rate between two charged spherical molecules, one of which represents a protein which possesses a strong dipole moment and asymmetric chemical reactivity. The role of the position of the dipole moment relative to reactive surfaces having various extents in media of varying ionic strength is assessed. No dipolar electrostatic effect on the diffusional rate constant of an isotropically reactive dipolar protein is observed. When the dipolar protein has a small axially symmetric reactive surface (e.g., of 10° extent), the dipole exerts a fivefold enhancement on the rate when the dipole vector is along the reactive patch axis, and retards the rate by a factor of 10 when oppositely disposed. A significant electrostatic effect persists through the physiological ionic strength regime. The positioning of dipolar charges relative to the protein surface at fixed dipole strength has an important effect on the electrostatic influence of the dipole. The variation of the protein dipole relative to its reactive surface can in fact be achieved in practice by chemical modification or site-directed mutagenesis.

Progress toward a biosynthetic rationale of the aflatoxin pathway

Abstract

Heat capacity and internal motion of molten Bi+BiI3mixtures

Measurements of the heat capacities, Cp, of molten Bi+BiI3 mixtures at compositions up to around 50 mol.% Bi were made with an adiabatic calorimeter. The deviation Delta Cp,a from the Dulong-Petit law of the atomic heat capacities was calculated over the whole concentration range, including proviously published data for Cp above 50 mol.% Bi. A drastic increase of Delta Cp,a for the ionic liquid-semiconductor regime between around 20 and 50 mol.% Bi (i.e. it has a maximum value of around 2.7R at 50 mol.% Bi) is satisfactorily explained in terms of the internal motions (i.e. the vibrational and rotational modes) of the bismuth homopolyatomic ions, in addition to a contribution from the bismuth iodide complex ions. Between the onset ( approximately 50 mol.% Bi) and the end ( approximately 90 mol.% Bi) of the miscibility gap, Delta Cp,a decreases to less than half of its maximum value. This is because the metallic micro-regions, which are made up of the aggregation of a few of the polybismuth ions or more with the increase of the Bi content, no longer show any internal motion and do not give rise to the large value of Delta Cp,a. Since the molten mixtures between approximately 50 and approximately 90 mol.% Bi are microscopically inhomogeneous solutions consisting of the ionic liquid-semiconducting and metallic micro-regions, the so-called inhomogeneous cluster model is applied to the Delta Cp,a of these mixtures.

Polarization contributions to the vacancy binding energy in doped oxides

An effective medium theory due to Onsager is generalized to yield the macroscopic dielectric constant of a random assembly of associated vacancy-dopant bound pairs embedded in a dielectric host. The model is then further developed to give the vacancy-dopant binding energy, employing concepts long-established in electrolyte theory. The results of this analysis are used to compute the activation enthalpy for electrical conduction in the ionic conduction regime. The values so found are in reasonable accord with experimental data from various sources on Ce 1− x Ca x O 2− x , with x⩽0.02, assuming a vacancy migration enthalpy of 0.71 eV. The latter is the only adjustable parameter in the theory here developed. With this same value, the predicted variation of the low-temperature conduction activation enthalpy in Ce 1−2 x Y 2 x O 2− x and Ce 1−2 x Gd 2 x O 2− x (again at small x) are acceptably reproduced, although more experimental data would be desirable. The dielectric constant of the yttria-doped material is also described by the present model, again with no adjustable parameters. Several different features of the theory lead to its loss of validity in more concentrated mixtures. These are examined in detail.

Electrostatic screening contributions to the vacancy binding energy in doped oxides

Linear Response Theory is used to estimate the relative magnitudes of the electrostatic contributions to the binding energy of an oxygen vacancy to a dopant ion in mixed oxides. The theory is further developed to yield the electrical conductivity activation enthalpy and pre-exponential factor in the ionic conduction regime. Experimentally observed trends in these quantities are not reproduced. The discrepancy is not removed by inclusion of polarization effects associated with the field-induced displacement of the screening cloud around the mobile vacancy or ionized dopant. Both of these are estimated to produce negligible changes. The same applies to the enhancement of the local field at the vacancy due to nearby vacancy—dopant pairs. It is concluded that some other physical mechanism is required to account for the variation in the conductivity activation enthalpy with dopant concentration.

Low-temperature nuclear spin relaxation in β-aluminas

27A l NMR T 1 measurements for Na-, K-, and Na 0.5K 0.5β-alumina in the temperature range ∼5–180K are reported. Both Na- and Kβ-alumina exhibit a nearly linear temperature dependence of the spin-lattice relaxation rate T −1 1 up to ∼55K which is attributed to the presence of two-level system (TLS) tunneling modes. A shallow T 1 minimum at ∼30K is observed in Na 0.5K 0.5β-alumina. It is suggested that the origin of this minimum is associated with the distribution of TLS parameters, and may be a low-temperature manifestation of the mixed-alkali effect commonly observed in glasses in the higher temperature ionic hopping regime.