Analytical approaches for probing surface properties and reactivity

This thesis reports new analytical approaches involving the employment of various scanning probe microscopy techniques (along with other microscopy techniques), coupled -in particular- with numerical modelling to extract key information about surface properties and crystal dissolution. The set of techniques used include scanning electrochemical microscopy (SECM), scanning ion conductance microscopy (SICM), scanning electrochemical cell microscopy (SECCM), ion selective electrodes (ISEs), atomic force microscopy (AFM), and confocal laser scanning microscopy (CLSM). In general, the approaches involve the coupling of data from two techniques to enhance the amount of information that could be obtained in an experiment. A new bias modulated SICM technique is introduced as a powerful tool to map topography and surface charge density simultaneously. This significant advance takes SICM beyond its original use as a topographical technique and turns it into a method of greater scope and versatility. To further advance scanning electrochemical probe microscopy, the fabrication and application of dual function electrodes, coupling both SICM and SECM techniques is described. The SICM was used to map sample topography and for the local delivery of agents to the sample surface (here calcite microcrystals), while the SECM part was employed as an ion selective electrode to acquire ion activity profiles (Ca2+ and H+, respectively) in bulk solution and in proximity to the surface. The pH probe size was pushed down to the nanoscale, while the calcium ones were in the 1.5 - 3 μm across. A major aspect of this work was to analyse experimental measurements with numerical finite element method simulations enabling the determination of dissolution flux values, thus opening the door to many possible and interesting applications for these probes in the future. Acid attack on dental enamel surfaces is also considered using different approaches. In one approach SECM was coupled with CLSM to visualise the proton diffusion profile near enamel surfaces in a bid to extract highly temporal kinetic information about the acid attack on enamel. An attentive approach was to use the SECCM technique as a tool to probe acid-induced dissolution of enamel, by locally delivering protons with a well defined mass transport. Landing with the acidic droplet on the sample surface for different time periods generated etch pits, which were then analysed with AFM. A key aspect was to develop a numerical finite element method (FEM) model that was employed to extract dissolution kinetics for the different types of enamel samples (treated and untreated)

Combinatorial localized dissolution analysis : application to acid-induced dissolution of dental enamel and the effect of surface treatments

A combination of scanning electrochemical cell microscopy (SECCM) and atomic force microscopy (AFM) is used to quantitatively study the acid-induced dissolution of dental enamel. A micron-scale liquid meniscus formed at the end of a dual barrelled pipette, which constitutes the SECCM probe, is brought into contact with the enamel surface for a defined period. Dissolution occurs at the interface of the meniscus and the enamel surface, under conditions of well-defined mass transport, creating etch pits that are then analysed via AFM. This technique is applied to bovine dental enamel, and the effect of various treatments of the enamel surface on acid dissolution (1 mM HNO3) is studied. The treatments investigated are zinc ions, fluoride ions and the two combined. A finite element method (FEM) simulation of SECCM mass transport and interfacial reactivity, allows the intrinsic rate constant for acid-induced dissolution to be quantitatively determined. The dissolution of enamel, in terms of Ca2+ flux (jCa2+), is first order with respect to the interfacial proton concentration and given by the following rate law: jCa2+=k0[H+], with k0=0.099±0.008 cm s−1. Treating the enamel with either fluoride or zinc ions slows the dissolution rate, although in this model system the partly protective barrier only extends around 10–20 nm into the enamel surface, so that after a period of a few seconds dissolution of modified surfaces tends towards that of native enamel. A combination of both treatments exhibits the greatest protection to the enamel surface, but the effect is again transient

Fabrication and characterization of dual function nanoscale pH-scanning ion conductance microscopy (SICM) probes for high resolution pH mapping

The easy fabrication and use of nanoscale dual function pH-scanning ion conductance microscopy (SICM) probes is reported. These probes incorporate an iridium oxide coated carbon electrode for pH measurement and an SICM barrel for distance control, enabling simultaneous pH and topography mapping. These pH-SICM probes were fabricated rapidly from laser pulled theta quartz pipets, with the pH electrode prepared by in situ carbon filling of one of the barrels by the pyrolytic decomposition of butane, followed by electrodeposition of a thin layer of hydrous iridium oxide. The other barrel was filled with an electrolyte solution and Ag/AgCl electrode as part of a conductance cell for SICM. The fabricated probes, with pH and SICM sensing elements typically on the 100 nm scale, were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and various electrochemical measurements. They showed a linear super-Nernstian pH response over a range of pH (pH 2–10). The capability of the pH-SICM probe was demonstrated by detecting both pH and topographical changes during the dissolution of a calcite microcrystal in aqueous solution. This system illustrates the quantitative nature of pH-SICM imaging, because the dissolution process changes the crystal height and interfacial pH (compared to bulk), and each is sensitive to the rate. Both measurements reveal similar dissolution rates, which are in agreement with previously reported literature values measured by classical bulk methods

Keyhole mapping to enable closed-loop weld penetration depth control for remote laser welding of aluminium components using optical coherence tomography

Remote Laser Welding (RLW) combines the positive features of tactile laser welding with additional benefits such as increased processing speed, reduced operational cost and service as well as higher process flexibility. A leading challenge preventing the full uptake of RLW technology in industry is the lack of efficient Closed Loop In-Process (CLIP) monitoring and weld quality control solutions. This underpins the need to fuse multiple sensor technologies, data analytics along with predictive engineering simulations. Although the development and integration of a variety of sensors, covering the radiation spectrum from ultra-violet to farinfrared, the flawless deployment of CLIP solutions is still challenged by the need for: signal de-noising in case of process instability; real-time data analytics; adaptive control engineering architecture to cope with process variations induced by manufacturing tolerances. This paper focuses on the aspect of the Weld Penetration Depth Control (WPDC) using Optical Coherence Tomography (OCT) as necessary step to enable adaptive penetration depth control during RLW of aluminium components in fillet lap joint configuration in consideration of part-to-part gap variation. The approach is decoupling the welding process parameters in two sub-sets: (1) in-plane control of the heat input on the upper part to facilitate the droplet formation; (2) outof-plane heat management to achieve the desired level of penetration control in keyhole mode. The paper presents the results of the keyhole mapping with variable part-to-part gap, that provide the insights for future research to enable the fully automatic closed-loop weld penetration depth control. Current limitations and next phases of research and development are highlighted based on the experimental study

Closed-loop gap bridging control for remote laser welding of aluminum components based on first principle energy and mass balance

Remote laser welding (RLW) has been successfully deployed for steel products, particularly doors, closures, and hang-on parts with overlap seam welding configurations. The growing demand for light-weight body structures has created interesting opportunities to apply RLW to fillet welding with the application to aluminum components. However, seamless migration from seam welding of steel to fillet welding of aluminum is limited by the following challenges: weld seam tracking capability to compensate trim edge variations; hot cracking resulting from the interaction between material chemistry and heat dissipation; and form error variations leading to unwanted part-to-part gaps, which in the absence of filling material must be bridged only by autogenous material. This paper focuses on the aspect of the part-to-part gap bridging and proposes a model to select and adjust welding process parameters to control the volume of the molten pool and achieve gap bridging. The proposed model is based on the observation that gap bridging is impaired by five distinct failure modes. Each mode is modeled by first-principle energy and mass balance criteria. Selection of welding parameters is presented by a set of gap bridging capability charts which helps to prevent failure modes and select feasible weld process parameters

Simultaneous Nanoscale Surface Charge and Topographical Mapping

Nanopipettes are playing an increasingly prominent role in nanoscience, for sizing, sequencing, delivery, detection, and mapping interfacial properties. Herein, the question of how to best resolve topography and surface charge effects when using a nanopipette as a probe for mapping in scanning ion conductance microscopy (SICM) is addressed. It is shown that, when a bias modulated (BM) SICM scheme is used, it is possible to map the topography faithfully, while also allowing surface charge to be estimated. This is achieved by applying zero net bias between the electrode in the SICM tip and the one in bulk solution for topographical mapping, with just a small harmonic perturbation of the potential to create an AC current for tip positioning. Then, a net bias is applied, whereupon the ion conductance current becomes sensitive to surface charge. Practically this is optimally implemented in a hopping-cyclic voltammetry mode where the probe is approached at zero net bias at a series of pixels across the surface to reach a defined separation, and then a triangular potential waveform is applied and the current response is recorded. Underpinned with theoretical analysis, including finite element modeling of the DC and AC components of the ionic current flowing through the nanopipette tip, the powerful capabilities of this approach are demonstrated with the probing of interfacial acid–base equilibria and high resolution imaging of surface charge heterogeneities, simultaneously with topography, on modified substrates

Simultaneous Nanoscale Surface Charge and Topographical Mapping

Nanopipettes are playing an increasingly prominent role in nanoscience, for sizing, sequencing, delivery, detection, and mapping interfacial properties. Herein, the question of how to best resolve topography and surface charge effects when using a nanopipette as a probe for mapping in scanning ion conductance microscopy (SICM) is addressed. It is shown that, when a bias modulated (BM) SICM scheme is used, it is possible to map the topography faithfully, while also allowing surface charge to be estimated. This is achieved by applying zero net bias between the electrode in the SICM tip and the one in bulk solution for topographical mapping, with just a small harmonic perturbation of the potential to create an AC current for tip positioning. Then, a net bias is applied, whereupon the ion conductance current becomes sensitive to surface charge. Practically this is optimally implemented in a hopping-cyclic voltammetry mode where the probe is approached at zero net bias at a series of pixels across the surface to reach a defined separation, and then a triangular potential waveform is applied and the current response is recorded. Underpinned with theoretical analysis, including finite element modeling of the DC and AC components of the ionic current flowing through the nanopipette tip, the powerful capabilities of this approach are demonstrated with the probing of interfacial acid–base equilibria and high resolution imaging of surface charge heterogeneities, simultaneously with topography, on modified substrates

Fabrication and Characterization of Dual Function Nanoscale pH-Scanning Ion Conductance Microscopy (SICM) Probes for High Resolution pH Mapping

The easy fabrication and use of nanoscale dual function pH-scanning ion conductance microscopy (SICM) probes is reported. These probes incorporate an iridium oxide coated carbon electrode for pH measurement and an SICM barrel for distance control, enabling simultaneous pH and topography mapping. These pH-SICM probes were fabricated rapidly from laser pulled theta quartz pipets, with the pH electrode prepared by <i>in situ</i> carbon filling of one of the barrels by the pyrolytic decomposition of butane, followed by electrodeposition of a thin layer of hydrous iridium oxide. The other barrel was filled with an electrolyte solution and Ag/AgCl electrode as part of a conductance cell for SICM. The fabricated probes, with pH and SICM sensing elements typically on the 100 nm scale, were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and various electrochemical measurements. They showed a linear super-Nernstian pH response over a range of pH (pH 2–10). The capability of the pH-SICM probe was demonstrated by detecting both pH and topographical changes during the dissolution of a calcite microcrystal in aqueous solution. This system illustrates the quantitative nature of pH-SICM imaging, because the dissolution process changes the crystal height and interfacial pH (compared to bulk), and each is sensitive to the rate. Both measurements reveal similar dissolution rates, which are in agreement with previously reported literature values measured by classical bulk methods