THE TRANSPORT OF CARBON DIOXIDE IN HIGH MOLECULAR WEIGHT BUFFER SOLUTIONS

The diffusion of carbon dioxide in biological media such as tissues and blood is an important physiological phenomenum. Transport of carbon dioxide in aqueous biological media causes, through chemical reactions, the simultaneous flux of several ionic species. The reversible reactions of CO₂ are coupled to amino acid dissociations of the protein species which have a large buffer capability. Due to the great difference in mobility of bicarbonate and protein, a diffusion potential evolves, which has a considerable influence upon the total CO₂ transport in the medium. The electrical potentials impede the carrier-facilitated CO₂transfer associated with the bicarbonate flux. New data on carbon dioxide transport in hemoglobin solutions are presented which clearly show the large reduction of CO₂ transport due to the electrical potentials. The experimental results correlate with diffusion potential data obtained previously. A theoretical model correctly predicts both the CO₂ transport and diffusion potential data as a function of The ionic composition of the solution. It is concluded that applied or electrical fields can have a significant effect on CO₂ transport in reactive biological media.     

Sharpening polycrystalline phase boundary for potassium sodium niobate ceramics with MnF2 modification

The influence of anion on structure and performance is unclear in potassium sodium niobate ((K,Na)NbO₃; KNN)-based ceramics, while cation doping has been widely researched. Here, the phase structure and electrical properties are explored in MnF₂-doped KNN ceramics. Significantly, sharp rhombohedral–orthorhombic (R–O) and orthorhombic–tetragonal (O–T) phase boundary as well as reduced diffusion degree is exhibited in the ceramics along with little changed phase transition temperatures due to the optimized F⁻ content at O site, which is different from that of cation replacement for A and B sites. Notably, the domain wall motion is facilitated due to the increased A vacancy and decreased O vacancy along with strengthened polarity, originating from the higher valence state and electronegativity of F⁻ with respect to O²⁻. And then, enhanced ferroelectricity is realized via MnF₂ modification, the piezoelectricity is elevated in turn. This work presents a new idea of anion doping for controlling structure and properties in perovskite materials.     

<Superscript>22</Superscript>Na diffusion and electrical conductivity of alkali-free silicate glasses

The diffusion of sodium ions in silica glasses produced by different methods and glasses in the Al₂O₃-R₂O₃-SiO₂ (R = La, Pr, Nd, Sm, Tb) systems has been investigated by the radioactive tracer method. The diffusion mobility of ²²Na ions in the aluminosilicate glasses containing rare-earth element oxides is close to that in the KSG silica glass prepared by high-temperature hydrolysis of silicon tetrachloride SiCl₄. A comparison of the diffusion coefficients with the electrical conductivity of the glasses has demonstrated that the conduction in the KI silica glass is due to the migration of sodium ions. In the KSG glass, as well as in the aluminosilicate glasses containing rare-earth element oxides, sodium ions are not charge carriers.     

The Diffusion of Divalent Europium into Single Crystals of Potassium Chloride

The diffusion of europium was studied in single crystals of potassium chloride by the spectrophotometric method. Measurements were carried out in the temperature range 418–550?C. The absorption spectra as well as the fluorescence spectra of crystals showed that europium diffused in its divalent state, being reduced by the crystal field of potassium chloride. The diffusion of europium can be represented by the equation D = 6.45. 10^?2exp(-29.400/RT). It is concluded that the activation energy consists of the sum of the jump energy of Eu²⁺ and of the enthalpy of solution of EuCl₂ in KCl.     

Reduced graphene oxide coated modified SnO2 forms excellent potassium storage properties

In this work, SnO₂ and Sn nanoparticles adhered to the surface of rGO (SnO₂/Sn/rGO) applied as potassium ion batteries (KIBs) anode materials were synthesized via thermal reduction. Preparing SnO₂ material into a nanostructure for modification can reduce ion diffusion distance to improve the number of active sites appropriate for K⁺ adsorption, and efficiently reduce the volume change which is conducive to enhancing the potassium storage capacity. Besides, layered rGO inhibits SnO₂/Sn aggregation, while increased surface area also reduces diffusion channel and electrolyte contact. However, larger specific surface area result in a lower initial Coulomb efficiency (ICE). The approach adopted here is to force the solid electrolyte interface (SEI) to fully emerge during the first charge-discharge cycle by using electrolytes containing 1 M KFSI in EC and DEC (1:1, v/v). According to density functional theory (DFT) analysis, the doping of rGO and SnO2 effectively enhanced the adsorption of potassium atoms and reduced the diffusion barrier of K⁺. Therefore, SnO₂/Sn/rGO nanocomposites have a high specific capacity (325.8 mAhg⁻¹ after 350 cycles at 0.1 Ag^-1), an excellent ICE (66.57%), and a long cycle life (203.6 mAhg⁻¹ after 1000 cycles at 0.5 Ag^-1).     

Transport of potassium ions in niobium phosphate glasses

The influence of Nb₂O₅ on the structure and ionic conductivity of potassium phosphate glasses was investigated in glasses with composition xNb₂O₅–(100-x)[0.45K₂O–0.55P₂O₅], x = 10–47 mol%. The Raman spectra of glasses reveal a transition from predominantly orthophosphate to predominantly niobate glass network with increasing Nb₂O₅ content. In the glass structure, niobium forms NbO₆ octahedra which are interlinked with phosphate units for the glass containing 10 mol% Nb₂O₅, but for higher Nb₂O₅ content they become mutually interconnected via Nb-O-Nb bonds. The transport of potassium ions was found to be strongly dependent on the structural characteristics of the glass network. While the mixed niobate-phosphate glass network hinders the diffusion of potassium ions by providing traps that immobilize them and/or by blocking the conduction pathways, predominantly niobate glass network exhibits a rather facilitating effect which is evidenced in the trend of DC conductivity as well as in the features of the frequency-dependent conductivity and typical hopping lengths of potassium ions.     

Diffundierter Sauerstoff als dominierender flacher Akzeptor in p-Typ Kupferiodid-Dünnfilmen

The long-term stability of the optically transparent p-type semiconductor copper iodide is a current challenge. The electrical conductivity of CuI thin films depends critically on the environmental impact. Al₂O₃ cappings enhance the stability considerably. Systematic studies on Al₂O₃/CuI heterostructures in dependence of the N₂/O₂ growth pressure show the electrical conductivity of the CuI films being determined by the oxygen diffusion through Al₂O₃ und CuI. Oxygen seems to be a dominating acceptor in CuI. We traced the diffusion of atmospheric oxygen into CuI with ¹⁸O isotopes.     

The leaching kinetics and mechanism of potassium from phosphorus-potassium associated ore in hydrochloric acid at low temperature

The leaching kinetics of potassium from phosphorus-potassium associated ore in hydrochloric acid/fluorite (CaF₂) system was studied. HCl concentration, liquor/solid ratio, CaF₂ dosage, and temperature were found to be the main factors. The leaching rate of potassium can be reached more than 92% under the optimum operation conditions. A classic shrinking core model with the mixed chemically diffusion as the rate-controlling step was successfully modeled the leaching kinetics of potassium, and the activation energy was found to be 30.7 kJ·mol^?1. The leaching mechanism of potassium was also elucidated based on the experimental results.     

Reactive intermediate in the alkali-carbonate-catalysed gasification of coal char

Based on results from a variety of experimental measurements, a detailed mechanism is postulated for the action of the inorganic catalyst in char gasification. In this mechanism, a catalyst such as potassium carbonate in contact with char undergoes a chemical and physical transformation to form a molten potassium oxide film that covers the char surface. This film serves as an oxygen transfer medium between the gaseous reactant (H₂O or CO₂) and the char. At the catalyst/char interface, an oxidation-reduction reaction occurs and the anions in the catalyst react with the oxidized char to form a phenolate-type functional group that subsequently splits out CO. The anions are replenished by reaction between the oxidizing gas (H₂O or CO₂) and the oxide at the gas/catalyst interface. Net transport of oxygen from gas to char occurs by diffusion of the species in the molten catalyst film.     

MoO2 nanosheets embedded into carbon nanofibers with a self-standing structure for lithium ion and sodium ion batteries

Molybdenum dioxides have been investigated in alkali-ion batteries owing to their low electrical resistivity, large interlayer spacing and high theoretical capacity. However, the slow diffusion kinetics lead to the poor electrochemical performance of Molybdenum dioxides. Herein, MoO₂ nanosheets embedded into carbon nanofibers are designed and synthesized by multi-step methods. The composite delivers the high lithium/sodium ion storage performance and stable ion diffusion, ascribed to the synergistic effect of carbon nanofibers and MoO₂ nanosheets. MoO₂ nanosheets are major contributors to the capacity of composite and carbon nanofibers can provide an interconnected three-dimensional conductive and diffusion networks to improve the kinetics process of MoO₂. The proposed synthesis strategy may offer an effective route to construct the high-performance electrodes for the wearable devices.     

Carbon dioxide absorption kinetics in potassium threonate

The absorption of carbon dioxide in potassium threonate aqueous solutions is studied at concentrations ranging from 0.1 to 3 M and temperatures from 293 to 313 K. This study includes experimental density, viscosity, solubility of N₂O and absorption kinetics of CO₂ (using a stirred cell reactor) data obtained for the various potassium threonate solutions. The diffusion coefficients of CO₂ and potassium threonate in the absorption solutions are estimated using a modified Stokes-Einstein relation. N₂O solubility is interpreted using the Schumpe [1993. The estimation of gas solubilities in salt-solutions. Chemical Engineering Science 48(1), 153-158] model and CO₂ physical solubility estimated. Physical absorption experiments were performed in the stirred cell in order to determine the physical mass transfer coefficients. The kinetics results are interpreted using both the pseudo-first-order and the DeCoursey approaches. It was concluded that CO₂ absorption in the aqueous potassium threonate solutions is well represented by

     

Effects of tin doping on physicochemical and electrochemical performances of LiFe1−xSnxPO4/C (0 ≤ x ≤ 0.07) composite cathode materials

Nanocrystalline LiFe_1−xSn_xPO₄ (0 ≤ x ≤ 0.07) samples are synthesized using SnCl₄·5H₂O as dopant via an inorganic-based sol–gel method. The dependency of the physicochemical and electrochemical properties on the doping amount of tin are systemically worked out and regular changes are revealed. In the whole concentration range, the chemical valence of Fe²⁺ is not basically changed whereas tin is found in two different oxidation states, namely +2 and +4. The replacement of Fe²⁺ by supervalent Sn⁴⁺ would lead to electron compensation. Under the synergetic effects between the charge compensation and the crystal distortion, the electrical conductivities for the bulk samples first increase and then decrease with the increasing amount of Sn doping. Upon the doping amount, the apparent lithium-ion diffusion coefficient and the electrochemical performance also display the similar trends. The doping is beneficial to refine the particle size and narrow down the size distribution, however optimizing the doping amount is necessary. Compared with other samples, the sample with a doping amount of about 3 mol% delivers the highest capacities at all C-rates and exhibits the excellent rate capability due to the high electrical conductivity and the fast lithium-ion diffusion velocity.     

Preparation and electroanalytical characterization of polyaniline: Polyacrylonitrile composite films

Electrically conducting composite films of polyaniline:polyacyrlonitrile (PANI:PAN) prepared with varying composition ratios of aniline mixed with a fixed amount PAN. The films of optimum thicknesses (0.10 mm) were obtained using an electrically operated automatic pressure machine. The films polymerized by oxidative polymerization using 0.1M potassium persulphate (K₂S₂O₈), undoped in 1M aqueous ammonia (NH₄OH) and doped in 1M hydrochloric acid (HCl). The conductivity of composite films was studied by keeping it in 1M HCl for different time period using 4-in-line probe DC electrical conductivity measuring instrument and the temperature dependence of DC electrical conductivity was studied using isothermal technique. The PANI:PAN composite film is used as a working electrode in an electrochemical cell. Chemically doped composite film is used as cathode (working electrode), aluminum metal foil as anode (counter electrode) and platinum foil as reference electrode. The electrolyte is of 0.05M aluminum chloride (AlCl₃) in dimethyl sulfoxide (DMSO). The voltage of the working electrode is stabilized with respect to the reference electrode and current applied between the working and counter electrode through a 9-V battery. The change in voltage versus time is plotted as the discharge curve and reversing the cell processes results in the doping of the composite films. The diffusion coefficient of the dopant ion (Cl⁻) present in the fully doped films were estimated by the galvanostatic pulse technique and found to bedifferent in different samples in the range of 10⁻¹⁶ to 10⁻¹² cm² s⁻¹. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

A comparative study of the roles of KCN and NaCN as catalytic precursors in the Boudouard reaction

The rate of reaction between carbon dioxide and coke with or without added KCN and NaCN was studied using thermogravimetry. The chemical stability of KCN or NaCN during catalysis, the changes in the concentration of potassium or sodium in coke and the pore structure of coke were studied as a function of time. The influences of these variables on the rate of coke-CO₂ reaction were determined. Since the catalysis depends on the catalyst-carbon contact, the distribution of potassium in the coke structure was examined. A porous solid reaction model was used to examine the contributions of chemical reaction and pore diffusion in the overall rate of coke-CO₂ reaction.     

Oberflächendiffusion auf oxidiertem Nickel

A porous electrode of sintered nickel powder was used as a separating diaphragm between two electrolyte-solution spaces and its flow resistance for the solution and the electrical diaphragm resistance was measured. By galvanostatic or potentiostatic oxidation the electrode was oxidized on the surface to Ni(OH)₂ and NiOOH, causing a decrease of pore volume indicated by an increase of flow resistance. In contrast under certain conditions the electrical diaphragm resistance simultaneously diminishes. Thus the electric current is now carried not only by the ions of the electrolyte in the pores of the electrode; there is now an additional charge transport, which is assumed to be proton diffusion on the oxidized nickel surface. The thickness of the diffusion layer, the diffusion coefficient and the temperature dependence of proton diffusion were measured.     

Defects and oxygen diffusion in metasilicate melts: Molecular dynamics simulation

The specific features of the dynamics of oxygen ions in Me ₂O SiO₂ (Me = Li, Na, K, Cs) melts at a temperature of 2000 K are investigated by the molecular dynamics method in the partially ionic potential approximation. It is demonstrated that, as in the systems studied earlier, the formation of defect complexes is a necessary condition for an oxygen diffusion event to be successful. The methods for generating defect complexes are described, and the lifetimes of these complexes are calculated. The structure of the defect complexes is determined. It is found that, in the Li₂O SiO₂ melt, successful oxygen diffusion events occur predominantly with the participation of fivefold-coordinated silicon ions. The threefold-coordinated defects begin to predominate in the Na₂O SiO₂ melt, and their role becomes crucial in the diffusion in the potassium and cesium metasilicate melts. It is shown for the first time that free and threefold-coordinated oxygen ions can also be involved in the formation of defect complexes.Original Russian Text Copyright © 2005 by Fizika i Khimiya Stekla, Berezhnoi, Boiko.     

New NOx sensors based on hematite doped with alkaline and alkaline-earth elements

Alkaline (lithium, potassium, rubidium) and alkaline-earth (magnesium, barium) doped hematite materials were studied for NO₂ sensing application. The synthesized materials were characterized by laser granulometry, X-ray diffraction and scanning electronic microscopy. A temperature of 1300 °C was chosen as the optimal heat treatment in order to obtain the densest material. Humidity dependence of the electrical properties revealed a strong influence in the case of rubidium doped hematite material while the other doped materials were less sensitive.The AC impedance analyses underlined the n-type intrinsic semi-conduction of pure hematite. Alkaline-earth doped hematite materials showed two semi-conducting regions, below and above 500 °C, corresponding to extrinsic and intrinsic n-type semi-conduction, respectively. These electrical analyses associated with SEM observations suggested instability of the ferrites formed in rubidium and potassium doped materials.AC electrical measurements were performed in the 0-500 ppm NO₂ partial pressure range. The alkaline-earth doped hematite materials exhibited the most promising behavior.     

Influence of the Nature of an Alkali Cation on the Electrical Conductivity of Vitreous MePO3 (Me = Li, Na, or K)

The temperature dependence of the electrical conductivity of vitreous lithium, sodium, and potassium metaphosphates is investigated using active electrodes (amalgams of the relevant alkali metals). The decrease in the electrical conductivity in the sequence LiPO₃–NaPO₃–KPO₃ is explained by the fact that the electric current in lithium metaphosphate is carried by lithium ions, whereas the electricity transport in NaPO₃ and KPO₃ occurs through alkali metal ions together with protons formed upon dissociation of impurity water. The kink observed in the temperature dependence of the electrical conductivity of KPO₃ is interpreted in terms of the charge carrier type. It is demonstrated that high-temperature (>373 K) conduction is provided primarily by migration of potassium ions, whereas low-temperature conduction is due to migration of protons.     

NiO–ZnO based junction interface as high-temperature contact materials

Contact materials play a crucial role in an electronic device operating at moderate and elevated temperatures where chemical and thermal stability is of great importance. Oxide materials and their interfaces are potential candidates as high-temperature contact materials due to their high chemical and thermal stabilities. In this work, polycrystalline oxides of Ni_0.98Li_0.02O and Zn_0.98Al_0.02O were used to make junction interfaces, where the solid-state synthesis method was used to obtain the individual oxide materials. After assembly of the junction interfaces, properties such as electrical, chemical, and thermal stabilities of the interfaces were investigated. The electrical properties were assessed through current-voltage (I–V) and electrochemical impedance spectroscopy (EIS) measurements, where the interface revealed a transition from electrically rectifying to slightly ohmic contact within a temperature range from 500–1000 °C. After annealing the junction interfaces at these elevated temperatures, no secondary phase was observed at the junction interface, i.e., the interfaces remain chemically stable. Moreover, the effect of isothermal annealing on the I–V characteristics curve of the junction showed an increased reverse current output over long annealing time, attributed mainly to the increased effective contact area at the junction interface and cation inter-diffusion processes. Furthermore, an investigation of the cation inter-diffusion mechanism revealed mainly lattice diffusion of Zn²⁺ into Ni_0.98Li_0.02O, while Ni²⁺ diffusion into Zn_0.98Al_0.02O exhibited both lattice and grain-boundary diffusion mechanisms.