Adam Wojcik

Ammonia as a fuel in solid oxide fuel cells

The use of ammonia as a source of hydrogen for fuel cells has received little attention until now. Ammonia offers several advantages over hydrogen as a fuel and is produced commercially in massive quantities and as a biogas. This paper describes the results of a solid oxide fuel cell-based system running on ammonia and compares the performance with respect to hydrogen. A novel catalyst concept has been devised and employed with success. Results indicate that the ammonia performance, using the catalyst is comparable to hydrogen suggesting ammonia can be treated as an attractive alternative fuel.

Manufacture of small calibre quadruple lamina vascular bypass grafts using a novel automated extrusion-phase-inversion method and nanocomposite polymer

Long-term patency of expanded polytetrafluoroethylene (ePTFE) small calibre cardiovascular bypass prostheses (<6mm) is poor because of thrombosis and intimal hyperplasia due to low compliance, stimulating the search for elastic alternatives. Wall porosity allows effective post-implantation graft healing, encouraging endothelialisation and a measured fibrovascular response. We have developed a novel poly (carbonate) urethane-based nanocomposite polymer incorporating polyhedral oligomeric silsesquioxane (POSS) nanocages (UCL-NANO) which shows anti-thrombogenicity and biostability. We report an extrusion-phase-inversion technique for manufacturing uniform-walled porous conduits using UCL-NANO. Image analysis-aided wall measurement showed that two uniform wall-thicknesses could be specified. Different coagulant conditions revealed the importance of low-temperature phase-inversion for graft integrity. Although minor reduction of pore-size variation resulted from the addition of ethanol or N,N-dimethylacetamide, high concentrations of ethanol as coagulant did not provide uniform porosity throughout the wall. Tensile testing showed the grafts to be elastic with strength being directly proportional to weight. The ultimate strengths achieved were above those expected from haemodynamic conditions, with anisotropy due to the manufacturing process. Elemental analysis by energy-dispersive X-ray analysis did not show a regional variation of POSS on the lumen or outer surface. In conclusion, the automated vertical extrusion-phase-inversion device can reproducibly fabricate uniform-walled small calibre conduits from UCL-NANO. These elastic microporous grafts demonstrate favourable mechanical integrity for haemodynamic exposure and are currently undergoing in-vivo evaluation of durability and healing properties.

Improved adhesion in hybrid Si-polymer MEMS via micromechanical interlocking

A method for improving adhesion between polymer films on Si substrates is described involving mechanical interlocking. Isotropic reactive ion etching (RIE) is applied to form pits in the Si substrate which fill upon application of a polymer via spinning. After curing, interpenetrating polymer lobes form a robust bond with the Si substrate. Adhesion improvements over films on smooth substrates are demonstrated qualitatively through prolonged immersion in heated 40% potassium hydroxide (KOH) solution, and quantitatively through peel tests. Strips of SU-8 on smooth substrates separate completely within minutes of immersion in KOH, whereas mechanically interlocked strips remain attached throughout. Tests reveal significant improvements to peel resistance for strips with interpenetrating lobes, due to a combination of crack deflection and physical restraint at the bond interface. Surprisingly, lobes with a vertical profile offer better resistance to strip peeling compared with lobes having significant overhang. Stress concentrations at sharp bends in the latter raise local stresses to levels promoting failure in SU-8, limiting the nominal load-carrying capacity of the interpenetrating interface. For maximum peel resistance, the lobe profile needs to be optimized such that maximum pull-out stresses are close to the failure stress of SU-8.

Combination Rules for Multichamber Valveless Micropumps

The general design rules indicate that when identical macroscale pumps (each with a maximum flowrate Qm, and maximum pressure drop ΔPmax) are combined in series, the maximum flowrate is Qm, but the maximum pressure drop becomes 2ΔPmax, while combined in parallel the maximum flowrate becomes 2Qm, and the maximum pressure is ΔPmax. In this paper, we test whether these design rules apply to microscale valveless micropumps using highly resolved CFD calculations. The variation of flow with pump pressure drop is studied by varying the resistance of an external circuit. The analysis confirmed that the macroscale design rules for macroscale pumps are applicable to microscale pumps. The study also enabled the influence of different forcing strategies on the pump performance to be analyzed.

Fluid–structure coupling in valveless micropumps

Valveless micropumps work on the principle that the reciprocal forcing of fluid by an actuator through diffuser/nozzle elements generates a rectified mean flow. The pump characteristics depend on the coupling between the fluid and structure. We analyze the characteristics of valveless micropumps by developing a coupled model for the area-averaged membrane displacement and the rectified flow through the diffuser/nozzle elements. Analytical expressions are derived for the pump flow rate Q and pressure drop ΔP characteristics as functions of the forcing frequency. The predicted natural angular frequency is compared against published results and the agreement is reasonable. The model shows that the maximum flow rate Qmax satisfies Qmax ~ ω for ω/ωN 1, which is supported by published experimental data. The maximum pressure ΔPmax satisfies ΔPmax ~ ω2 for ω/ωN 1. The available experimental data are sparse (in comparison to Qmax). The general model is applicable to all valveless micropumps which incorporate diffuser/nozzle elements.

The use of the potential drop technique for creep damage monitoring and end of life warning for high temperature components

Abstract Electrical potential drop (EPD) is a well-established technique for the measurement of crack initiation/growth in metals. Two variants exist, one using AC excitation, the other using DC. EPD provides crack dimensions (principally depth) in contexts such as fracture/fatigue testing, and in-field NDE. Whilst it has been employed for on and off-line assessment of creep damage, use within a non-lab (i.e. industrial) context is limited by connection issues and, significantly, data interpretation – especially with regard to detecting subtle changes in EPD over general background ‘noise’. We describe, here, a methodology where high sensitivity detection of creep damage can be achieved by looking for a characteristic ‘signature’ within data. This is based on the combination of AC-EPD with its DC equivalent so as to generate a synergistic approach to damage detection. The methodology has been successfully applied to a semi-industrial context to provide prior warning of failure in excess of several weeks.

Remote monitoring of bi-axial loads on a lifting surface moving unsteadily in water

A system of measuring the bi-axial load on a lifting surface (blade) which is freely moving and operates submerged in water at the laboratory scale is described. A blade with a span of 500 mm, a chord of 60 mm and a thickness of 9 mm (15% of the chord) was employed and the lift/drag forces were measured using a bespoke strain-gauge based load cell located at the mid-span of the blade, measuring bending moments in two independent directions. The requirement to move freely dictated that the load cell was encapsulated within the blade, along with signal conditioning circuitry, power supply and a data logger with wireless transmission. Submerged operation in water resulted in very short transmission distances, meaning that data were recorded and subsequently transferred using an aerial placed close to the blade while it was stationary. Assumptions based on Euler–Bernoulli beam bending theory were used to infer the total load from measurements of the bending moment at the mid-span and example data from a freely moving aerofoil on a Darrieus-type tidal energy extraction device are presented. The novelty of this system lies in its combination of free movement, submerged operation and small scale.

A Parametric Study of a Diode-Resistor Contrast Model for SEM-REBIC of Electroceramics

The observation of terrace contrast in REBIC studies of polycrystalline electroceramics such as ZnO has been reported previously, and a theoretical model postulated for its formation. This developed from a simple model containing purely resistive elements, to one comprising both resistive and non-linear″diodic″ ones. Presented here are the results of theoretical and experimental work centred around this latter model. A parametric study has been performed to examine the sensitivity of the predicted contrast to the relative magnitudes of the model′s resistor and diode elements. By varying the values of these, the overall nature of the contrast response was shown to alter significantly, as well as the superimposed, and finer, local terrace contrast. The model delivered ‘linescans’ representing the contrast responses obtained. The model also showed that the linescans were sensitive to the injected beam current level. These findings suggested a route that could eventually allow the extraction of local grain/grain boundary characteristics, including breakdown behaviour, from global REBIC contrast data.