Guy Denuault

Direct determination of diffusion coefficients by chronoamperometry at microdisk electrodes

The chronoamperometric response at a microdisk is used for the direct determination of the diffusion coefficient of an electroactive species. The method does not require knowledge of the bulk concentration and the number of electrons participating in the electrode reaction, and requires only a value for the disk radius. Subsequent determination of the number of electrons (n) for an electrode reaction or the concentration of electroactive species is also possible. This approach is demonstrated with the evaluation of the diffusion coefficient of Fe(CN)64− in KCl and that of borohydride ion in NaOH. In both cases, the values of n found remained constant over a wide time range and correspond to those expected for these processes.

Effect of oil quality on electrostatic charge generation and transport

This paper presents an experimental investigation into the effect of lubricating oil quality on tribocharging. The work forms part of a larger investigation into the causes of electrostatic charging events detected prior to severe scuffing during base oil lubricated wear experiments. A variable speed spinning disc charger was used for measurements of tribocharging, whilst controlling the oil viscosity, temperature, ageing and disc surface roughness. The results are discussed with reference to interactions between the double layer and fluid boundary layers and shear stress which could lead to charge separation at the disc-oil interface. Viscosity and surface roughness have been shown to affect the tribocharging of a base oil in a direct relationship, while temperature effects indicate a maxima due to transportation-recombination competition. Ageing experiments indicate an increase in conductivity, which will eventually lead to an increase in charging ability. Charge sign inversion is discussed in relation to specific adsorption of charged species (additives) on the disc surface, the presence of oppositely charged impurities and the differential of adsorption and/or diffusion rates for the negatively and positively charged species within the oils.

Scanning electrochemical microscopy: Probing the ingress and egress of protons from a polyaniline film

The electroactivity of polyaniline films in 0.01 mol/dm 3 HCl has been studied by probing amperometrically the variations of proton concentration at the film/solution interface. The influence of the absence and presence of the supporting electrolyte has been studied.

Development of the theory for the interpretation of steady state limiting currents at a microelectrode: EC' processes: first and second order reactions

Equations are developed to describe the radius dependence of the steady state limiting current at a microelectrode for the case where the electron transfer process is followed by a homogeneous chemical reaction regenerating the electroactive species. Following a brief discussion of systems where the coupled chemical reaction may be considered to be pseudo-first order, the theory is extended to the more general, second order case by seeking an asymptotic solution to the differential equation. This allows the study of fast chemical reactions and systems where the reactants are present in similar concentrations.

Investigation of electrostatic charging mechanisms in oil lubricated tribo-contacts

Fundamental studies using a spinning disc tribocharging rig have shown that disc surface roughness, oil temperature and oil composition have an effect on the charging ability of lubricating oils. Further investigations into the effect of oil composition on the charging nature have employed the oil droplet rig to investigate a range of base and formulated oils. Experimental results indicated that charging in base oils is controlled by levels of impurities, such as sulphur, and aromatic content. For formulated oils it has been determined that the additives are the major contributor to the charging nature and any effect that impurities and aromatics have is negligible. Studies into the sensitivity of the electrostatic sensor system to surface charge has shown that surface transformations that occur during scuffing in tribocontacts can lead to contact potential differences detectable by this system.

Electrostatic monitoring of oil lubricated sliding point contacts for early detection of scuffing

Electrostatic sensors have been used to investigate scuffing events in oil lubricated sliding point contacts. A pin-on-disc and a reciprocating laboratory wear rig have been used for this investigation in a fixed ball-on-disc configuration. Electrostatic charging events have been detected up to 1200 s prior to severe scuffing and have been shown to coincide with microstructural changes in the contact and regions of increased wear. The experiments presented demonstrate that the electrostatic detection technology has the potential to be used as the basis of an on-line (real-time) machinery condition monitoring system.

Scanning electrochemical microscope (SECM) study of the relationship between proton concentration and electronic charge as a function of ionic strength during the oxidation of polyaniline

The SECM has been used to investigate the influence of ionic strength on the ingress and egress of protons to and from a polyaniline (PANI) film. This was done in solutions containing 0.01 mol dm(-3) HCl, with concentrations of NaCl ranging from 0 to 0.5 mol dm(-3), by measuring the steady-state faradaic current for H+ reduction on a 18 mu m diameter Pt microdisc located within few micrometres of the PANI film while cycling the potential of the film through the different oxidation states. The microelectrode response was calibrated using microdiscs of various sizes in different solutions of HCl, with and without the presence of supporting electrolyte. This investigation shows the influence of ionic strength on the film mechanism and in particular on the relationship between the behaviour of H+ at the film\solution interface and the behaviour of electrons at the film\electrode interface.

Three-dimensional random walk simulations of diffusion controlled electrode processes: (I) A hemisphere, disc and growing hemisphere

The model presented in this article is based on a statistical approach where diffusion controlled electrode processes are simulated in terms of the random walk of an ensemble of particles. The algorithm simulates diffusion in three dimensions and yields data which are directly related to real world quantities. The validity of the model was tested by simulating diffusion controlled current and charge transients for a hemispherical electrode, disc electrode and for a growing hemispherical electrodeposit. In addition, the algorithm was used to generate concentration profiles. In all cases the results of the simulation were compared with analytical equations

Scanning electrochemical microscopy: Amperometric probing of diffusional ion fluexes through porous membranes and human dentine

Abstract The scanning electrochemical microscope (SECM) was used to probe highly localised fluxes of electroactive species diffusing through the channels of porous materials. The validity and feasibility of the approach were demonstrated with the help of model experiments carried out on a track-etched polycarbonate membrane. This sample was chosen for its low porosity (5%) and its quasi-perfect 10 μm diameter pores. While diffusion of redox species was induced by a large concentration gradient across the sample, other mass transport mechanisms were eliminated. The arrival of redox species, [Fe(CN) 6 ] 4− , was detected amperometrically by the SECM tip positioned a few micrometres above the mouth of the pores. Two basic SECM techniques were considered. Approach curves (tip current versus tip-substrate distance Z ) were recorded to assess the extent of diffusion in the solution close to the substrate and to establish the optimum tip-substrate distance for the detection of the pores. Maps (tip current versus planar coordinates X , Y ) were recorded to assess the distribution of diffusional fluxes. Images of localised diffusional fields were obtained and analyzed in terms of single ion fluxes associated with individual pores. More complex experiments were carried out with human dentine samples which had been characterised by atomic force microscopy (AFM). Dentine has a very high porosity (75%) with tubules varying from 2 to 5 μm in diameter. It was chosen to challenge the ability of the SECM to image the fluxes of electroactive species diffusing through closely spaced pores and because ion transport through dentinal tubules is thought to be one of the major mechanisms for nerve stimulation in dentine hypersensitivity. As expected the diffusion fields from individual channels overlapped and showed the limitation of the SECM. However, the tip response was highly sensitive to the presence of obstruction inside the pores. Spatially resolved low current regions were analyzed in terms of blocked pores. Numerical simulations of diffusion through model porous substrates were used to predict the time dependence and spatial distribution of diffusional patterns in the presence and absence of the SECM tip. The experimental approach reported forms the basis of novel investigations for the assessment of the treatment of dentine hypersensitivity.

Scanning electrochemical microscopy: potentiometric probing of ion fluxes

The scanning electrochemical microscope has been used to probe the diffusion layer by measuring the potential difference between a large reference electrode located in the bulk of the solution and a microindicator electrode positioned a few µm away from a substrate electrode. Ag microdiscs (10 and 50 µm diameter) have been used as potentiometric sensors to probe the concentration profile due to the diffusion of Ag+ to and from a planar Ag electrode. A similar experiment was carried out with an Ag microcylinder (50 µm diameter) as substrate electrode. In a separate set of experiments, Ag/AgCl microdiscs (10 and 50 µm diameter) were used to monitor the flux of Cl– consumed and generated by a film of polyaniline electrodeposited on Pt. The tip potential was recorded while cycling the potential of the PANI film past the first oxidation peak. Direct measurement of the ingress and egress of Cl– ions supports previously reported mechanisms for the oxidation of PANI films.