Witold Kucza

Electrical properties of LiMn2O4−δ at temperatures 220–1100K

Abstract The work presents the results of low-temperature (220–300 K) studies of electrical conductivity and thermoelectric power for manganese spinel obtained by various methods. It has been shown that charge transport takes place through the small polaron mechanism. The results of high-temperature (300–1100 K) studies of electrical conductivity and thermoelectric power carried out in the thermodynamic equilibrium conditions as a function of temperature and oxygen pressure (10 −2 −1 atm) in the whole range of the LiMn 2 O 4− δ phase stability are also presented. It has been shown that an effective reaction related to the deviation from stoichiometry is the reaction Mn +4 +e − ⇔Mn +3 .

Time-dependent phenomena in the potential response of ion-selective electrodes treated by the Nernst-Planck-Poisson model. Part 2: Transmembrane processes and detection limit.

The detection limit of ion-selective electrodes (ISEs) is of great interest because of the many possible practical applications of ISEs in trace analysis. Existing theoretical interpretations of the detection limit of ISEs are restricted by severe assumptions such as steady-state and electroneutrality, which hamper theorizing on this problem. For this reason, the Nernst-Planck-Poisson (NPP) equations are used to predict and visualize the detection limit variability under nonequilibrium conditions. For the first time, the NPP model is applied to the so-called inverse problem: finding the optimal measurement time and inner solution concentration for lowering the detection limit.

Structural stability of MnO2 polymorphs and their reactivity vs. lithium

Abstract This paper describes the stability of dioxide structures, with ccp and hcp anion arrays, on cycling in lithium cells. Application of a regular solution formalism to transition metal dioxides has led to a potential-composition dependence, which is analogous to that derived from the lattice gas model. However, the present model offers a more integrated approach; it has been found a relationship between stability of MeO2 (Me=transition metal) polymorphs and their reactivity vs. lithium. On this ground, it has been explained why ternary oxides LiMeO2 attain a higher potential in relation to lithium than binary oxides MeO2. Structural modifications of initial electrode materials MeO2 and LiMeO2 and their influence on charge/discharge curves have been also investigated. Validity of presented considerations has been verified for different polymorphs of MnO2.

Modeling Non Equilibrium Potentiometry to Understand and Control Selectivity and Detection Limit

The majority of present theoretical interpretations of ion-sensor response focus on phase boundary potentials. They assume electroneutrality and equilibrium or steady-state, thus ignoring electrochemical migration and time-dependent effects, respectively. These theoretical approaches, owing to their idealizations, make theorizing on ion distributions and electrical potentials in space and time domains impossible. Moreover, they are in conflict with recent experimental reports on ion-sensors, in which both kinetic (time-dependent) discrimination of ions to improve selectivity, and non-equilibrium transmembrane ion-transport for lowering detection limits, are deliberately used. For the above reasons, the Nernst-Planck-Poisson (NPP) equations are employed here to model the non-equilibrium response in a mathematically congruent manner. In the NPP model, electroneutrality and steady-state/equilibrium assumptions are abandoned. Consequently, directly predicting and visualizing the selectivity and the low detection limit variability over time, as well as the influence of other parameters, i.e. ion diffusibility, membrane thickness and permittivity, and primary to interfering ion concentration ratios on ion-sensor responses, are possible. Additionally, the NPP allows for solving the inverse problem i.e. searching for optimal sensor properties and measurement conditions via target functions and hierarchical modeling. The conditions under which experimentally measured selectivity coefficients are true (unbiased) and detection limits are optimized are demonstrated, and practical conclusions relevant to clinical measurements and bioassays are derived.

Interdiffusion: Consistency of Darken's and Onsager's Methods

The Nernst-Planck flux formula is used in Darkens method to obtain the interdiffusion fluxes. The effective interdiffusion potentials, derived for the independent components in the system, allow obtaining the symmetrical matrix of the interdiffusion coefficients. The transport coefficients for 2, 3 and r-component system are presented. Interpretation of obtained matrixes in the light of Onsagers theory of irreversible thermodynamics is shown. Equation for the entropy production in the interdiffusion process is displayed. The presented approach allows calculation of entropy production during interdiffusion, as well as formulating Onsagers phenomenological coefficients for the interdiffusion in an explicit form, a form which is directly correlated with the mobilities of the atoms present in the system.

Flow injection analysis simulations and diffusion coefficient determination by stochastic and deterministic optimization methods

Stochastic and deterministic simulations of dispersion in cylindrical channels on the Poiseuille flow have been presented. The random walk (stochastic) and the uniform dispersion (deterministic) models have been used for computations of flow injection analysis responses. These methods coupled with the genetic algorithm and the Levenberg-Marquardt optimization methods, respectively, have been applied for determination of diffusion coefficients. The diffusion coefficients of fluorescein sodium, potassium hexacyanoferrate and potassium dichromate have been determined by means of the presented methods and FIA responses that are available in literature. The best-fit results agree with each other and with experimental data thus validating both presented approaches.

Dispersion in cylindrical channels on the laminar flow at low Fourier numbers

A numerical solution of the uniform dispersion model in cylindrical channels at low Fourier numbers is presented. The presented setup allowed to eliminate experimental non-idealities interfering the laminar flow. Double-humped responses measured in a flow injection system with impedance detection agreed with those predicted by theory. Simulated concentration profiles as well as flow injection analysis (FIA) responses show the predictive and descriptive power of the numerical approach. A strong dependence of peak shapes on Fourier numbers, at its low values, makes the approach suitable for determination of diffusion coefficients. In the work, the uniform dispersion model coupled with the Levenberg-Marquardt method of optimization allowed to determine the salt diffusion coefficient for KCl, NaCl, KMnO4 and CuSO4 in water. The determined values (1.83, 1.53, 1.57 and 0.90)×10(-9)m(2)s(-1), respectively, agree well with the literature data.