Spurious potential dependence of diffusion coefficients in Li insertion electrodes measured with PITT

Diffusion coefficients of Li⁺ in insertion electrodes determined by the potentiostatic intermittent titration technique (PITT) have been reported in the literature as being potential-dependent (and thus dependent on the degree of insertion). With this PITT method the diffusion coefficients D^PITT are determined from the current response of potential-step experiments using an approach based on the Cottrell equation. This equation assumes as boundary conditions infinitely fast kinetics and a linear system with semi-infinite thickness. Using the example of a spinel-type Li_δMn₂O₄ electrode it will be demonstrated that this potential dependence of D^PITT is not real but occurs, because the inadmissible boundary condition of infinitely fast kinetics which is not fulfilled has been adopted. This will be demonstrated with the help of numerically simulated PITT data calculated with a constant diffusion coefficient D. Diffusion coefficients D^PITT calculated from these simulated PITT data are potential dependent, even though the PITT experiments were simulated with a constant diffusion coefficient D. The inadmissible boundary condition of a linear system with semi-infinite thickness applied to a bed of spherical active particles with finite radii as represented by a Li_δMn₂O₄ electrode leads to further deviations of D^PITT which will be investigated.     

Non-structural cracking in RC walls: Part I. Finite element formulation

In this paper, a three-dimensional finite element model for the analysis of non-structural cracks occurring in reinforced concrete (RC) walls is introduced. The numerical model could take into account both time-dependent temperature variations due to hydration heat and non-uniform moisture distribution during drying, and the coupling effect between the heat transfer and the moisture diffusion. Calculation of the temperature and internal relative humidity variations of RC walls is followed by determination of stresses due to thermal gradients, differential drying shrinkage, and average drying shrinkage. The mechanical properties of early age concrete, determined from numerous experimental studies, are taken into consideration to improve the accuracy of the numerical results, and a discrete steel element derived using the equivalent nodal force concept is also used to simulate reinforcing steels embedded in a concrete matrix. The validity of the proposed procedure is verified by comparing the measured experimental data with the analytical results for RC walls.     

A test method for measuring chloride diffusion coefficients through nonsaturated concrete: Part I. The instantaneous plane source diffusion case

A test method is proposed for measuring chloride diffusion coefficients through nonsaturated concrete specimens with controlled water contents. The experimental setup used allows one to supply an initial limited amount of Cl⁻ to the tested concrete surface. The procedure consists of submitting the surface of concrete specimens to interaction with the products of combustion of PVC, which contain mainly gaseous hydrogen chloride. This interaction yields a limited Cl⁻ contamination of the concrete surface. After returning the specimens to their controlled humidity exposure conditions, the kinetics of Cl⁻ transport from the surface inwards may be studied. The experimental Cl⁻ concentration profiles determined at selected time intervals have been adjusted to a diffusion model of “instantaneous plane source,” which takes into account the particular initial and boundary conditions of the experimental procedure, for obtaining the corresponding diffusion coefficients.     

Theory for double potential step chronoamperometry for any potential values at spherical electrodes: Simultaneous determination of the diffusion coefficients of the electroactive species

We present a general and explicit analytical solution for double potential step chronoamperometry with any applied potential values (E₁, E₂) corresponding to a reversible charge transfer process at spherical electrode. This solution is essential to analyze double pulse electrochemical techniques such as RPV and DPV. We consider unequal diffusion coefficients, initial presence of both electroactive species and that the reaction product can dissolve in the electrolytic solution or in the electrode.From the analytical equation obtained it is possible to deduce interesting simplified expressions for some particular cases: both species soluble in the electrolytic solution with equal diffusion coefficients, planar electrodes, ultramicroelectrodes when both species are soluble in the electrolytic solution, and double potential step chronoamperometry with limit current potentials (E₁−E^°′→−∞, E₂−E^°′→+∞). In this last case, when reaction product is not initially present it is pointed that planar electrodes and ultramicroelectrodes cannot be used for determining both diffusion coefficients. This interesting practical consequence can be demonstrated by means of the analytical expression deduced here, which represents a notable advantage in front of numerical results.     

Technology update: Vacuum infusion

Vacuum, or resin, infusion techniques are becoming increasingly popular, especially in the marine industry where they can produce high quality boat hulls quickly and cleanly (see Reinforced Plastics' Marine Supplement, October 2003).     

Analysis of the kinetics of ion intercalation: Ion trapping approach to solid-state relaxation processes

This paper develops a new framework for the interpretation of impedance or capacitance spectroscopy of ion insertion processes in thin film electrodes, with a particular emphasis in electrochromic materials. The model distinguishes types of inserted ion charge regarding the energetics and kinetics properties of ion storage sites. The ion charge in a connected network of sites moves rapidly (fast sites) and is distributed according to equilibrium thermodynamic properties, while slower sites trap the ions and provide a variety of kinetic behaviors that dominate the response in the long time scale. We show that the assumption that a fraction of the intercalated ions have been immobilized at certain sites, and do not participate in diffusion, provides a suitable explanation for the observation of different capacitive components in the faradaic impedance of homogeneous systems (fast and slow charging modes). Moreover, ion trapping contributes a resistive component to the impedance, related to homogeneous charge-transfer between different types of sites (trap resistance). The model predicts an arc in the complex capacitance representation associated to the solid-state reduction step involved in the coloration by electroinsertion. The characteristic features of the relaxation in structurally disordered materials are studied in the kinetic framework of a multiple trapping scheme. We show that a wide distribution of trapping times provides a power law of frequency, which relates to the frequent observation of a constant phase element (CPE) impedance.     

Interphex 2004: Exhibitor focus

The exhibition component of Interphex 2004 has attracted more than 950 exhibitors from around the globe. There are representatives from every conceivable pharmaceutical sector, including ethical/proprietary drugs, cosmetics, R&D, the biotech field, and full-scale manufacturing support. The various companies have been attracted to New York by the thousands and thousands of visitors and industry decision-makers who are expected to attend this year's event. A number of companies will be showcasing their latest filtration and separation equipment systems, accessories and process solutions for application in the pharmaceutical market. Below Filtration+Separation presents a selection of new products and systems that will be on display in New York, covering everything from vertical centrifuges, membrane filters and sanitary sieves, to disposable filtration solutions, filter housings, depth filtration systems, and metal detection devices for process flows. We hope our “Exhibitor Focus” section helps you to select the companies that will be of most interest to you.     

WFC9: Exhibitor focus

The WFC9 exhibition is an ideal opportunity for industry professionals to see the latest products and developments in the fields of filtration and separation. Approximately 50 companies are expected to attend the international exposition event in New Orleans, USA, on Monday 19 April to Wednesday 21 April. The 2^1/2 day exhibition runs alongside the short courses and technical presentations that make up the rest of the programme of the ninth World Filtration Congress (WFC9). Below Filtration+Separation presents a selection of new products and systems that will be on display at the Hyatt Regency hotel in New Orleans, covering everything from new netting, radial cartridge filters, membranes, porous metals, and testing/validation equipment to filter presses and antimicrobial filter media. We hope that our Exhibitor Focus section will help you to select the companies and organisations that will be of most interest to you.     

Interphex 2004: Conference programme

Interphex 2004 is split into four concurrent events offering pharmaceutical technology solutions, under the banner of the exhibition, and educational opportunities, under the banner of the conference. The four segments, which will run alongside each other, are: Pharma Manufacturing; Pharma Discovery; Pharma IT; and Pharma Sourcing & Services. A separate conference programme has been organised for each strand providing industry professionals with the chance to learn about the latest trends or to advance their career with accredited short courses. Education providers include the International Society of Pharmaceutical Engineers (IPSE) and the Institute for International Research (IIR), amongst others.Below we highlight some of the more relevant presentations for professionals involved in the field of filtration and separation. For more detailed information and a full conference program please visit the Interphex 2004 website (www.interphex.com)     

Nucleation and diffusion-controlled growth of electroactive centers: Reduction of protons during cobalt electrodeposition

A theory is presented describing, for the first time, the temporal evolution of the fractional surface area, S(t), of 3D non-interacting nuclei growing at a rate limited by diffusion of electrodepositing ions onto substrates of a different nature. Likewise, an equation has been derived describing the potentiostatic current-time transients arising from the formation and growth of such nuclei with redox reactions occurring simultaneously on their surfaces. An equation is also proposed to describe the current due to redox reactions taking place on the surface of interacting growing nuclei. The latter is used to describe the experimental current transients recorded during nucleation and growth of cobalt at applied potentials where the proton reduction reaction occurs simultaneously with the electrocrystallization process.     

Experimental investigation and numerical modeling of carbonation process in reinforced concrete structures: Part I: Theoretical formulation

In this work, the mathematical-numerical model of carbonation process in reinforced concrete (RC) structures, which has been developed by the authors, is applied to different cases of study to take into account the probabilistic nature of durability assessment procedure even if within the framework of a rough and ready approach. In particular, the aim of this Part I of the work is to study how the variability of the parameters defining the differential equations in the model influence the assessment of the corrosion initiation time of RC structure. Comparison with experimental results and numerical simulations are undertaken in Part II of this work.     

Preview: JEC Composites Show 2004

This year's JEC exhibition takes place from 30 March to 1 April at the Paris Expo in France. This preview is designed to highlight some of the developments visitors to the show can expect to see, and gives those of you not going to JEC this year a taste of the event.     

Review: JEC Composites Show 2004

The JEC Composites Show took place on 30 March-1 April. In this feature we review a few of the developments which were on display in Paris over those three days.     

Show review: INCHEM TOKYO 2003

In the first week of November 2003, the Filtration and Separation Symposium was concurrently held with INCHEM TOKYO 2003 in Tokyo, Japan. A review of both events are presented below.