Hybrid Filtration Membranes incorporating Nanoporous Silica within a Nanoscale Alumina Fibre Scaffold

Membranes were produced from fine (∼10 nm) alumina fibres, by dispersing them in liquid and using controlled sedimentation to produce two types of membrane – one a duplex structure (layers of well-dispersed fibres and micro-bundles) and the other entirely micro-bundles. Incorporation of silica, via a sol-gel route, produced “hybrid” forms. Filtration and separation efficiencies were assessed using two ionic dyes of similar molecular weight, but opposite charge. Successful separation of these is attributed to surface electrical effects within nano-pores. Hybrid duplex membranes give an excellent combination of fine scale filtration efficiency and high permeability.Financial support for this work has been provided by Trinity Hall College, Cambridge and the Cambridge Commonwealth TrustThis is the author accepted manuscript. The final version is available from Wiley at http://dx.doi.org/10.1002/adem.20150022

Cathodic discharges during high frequency plasma electrolytic oxidation

Using small area electrical data logging, high speed photography, sample mass gain monitoring, gas evolution measurement and microstructural examination, a study has been made of the formation and effect of cathodic discharges during PEO of Al substrates. Discharge formation during the cathodic half-cycle is promoted by high frequency, thick coatings and high pH. They form (under constant current conditions) when the voltage during cathodic polarization reaches a value (~250 V in the work described here) sufficient to cause dielectric breakdown across thin residual oxide layers on the substrate. This occurs when the normal cathodic process of proton flow through electrolyte channels in the coating can no longer deliver the required current. Cathodic discharges tend to carry higher currents, and emit more light, than anodic ones. Gas evolution rates during PEO are well above the Faraday yield level. This is due to water entering discharge plasmas, breaking down into ionized species and failing to recombine completely during subsequent collapse and quenching. It is reported here that rates of gas evolution rise as discharges start to take place in the cathodic part of the cycle, as well as in the anodic part. Rates of substrate oxidation (coating growth), however, drop off, rather than rise, when cathodic discharges start. Evidence is presented here suggesting that this is associated with their highly energetic nature, causing substantial amounts of oxide to be expelled into the electrolyte during cathodic discharges. This is also apparent in the coating microstructure, where recent cathodic discharge sites are identifiable as large, highly porous regions.EPSRC (grant number EP/I001174/1) Sims Scholarship (Cambridge University

Evaluation of residual stress levels in plasma electrolytic oxidation coatings using a curvature method

Experimental estimates have been made of typical levels of residual stress in plasma electrolytic oxidation (PEO) coatings formed on aluminium and magnesium alloy substrates. This has been done via measurement of the curvature exhibited by thin strip samples, coated on one side only, using coating stiffness values obtained in the current work. In order to obtain curvatures that were sufficiently large to be accurately measurable, it was necessary to produce relatively thick (~ 100 μm) coatings on relatively thin (~ 300–500 μm) substrates. In such cases, stress levels are significant in both constituents, and there are significant through-thickness gradients of stress. The relevant characteristics of the transformation (largely oxidation of the substrate) are therefore best expressed as a misfit strain. This was found to have a magnitude of about 0.6–0.9 millistrain for the Al substrate and 2–3 millistrain for Mg, with a positive sign (so that the stress-free in-plane dimensions of the coating are larger than those of the residual substrate). This puts the coating into residual compression and, on a thick substrate, typical stress levels would be around 40–50 MPa for Al and 130–150 MPa for Mg. These values should be regarded as approximate, although their order of magnitude is probably reliable. They are higher than those from the (very limited) previous work carried out using this type of technique. On the other hand, they are lower than many values obtained using X-ray diffraction. Explanations are proposed for these discrepancies.This work has been supported by EPSRC (grant number EP/I001174/1) and also by Keronite plc, from where contributions have been made by Steve Hutchins and Suman Shrestha.This is the final published version. It was originally published by Elsevier at http://dx.doi.org/10.1016/j.surfcoat.2014.11.00

Influence of the composition and viscosity of volcanic ashes on their adhesion within gas turbine aeroengines

This paper presents experimental investigations into adhesion characteristics of four types of (Icelandic) volcanic ash (VA). Firstly, powder (∼5–50 μm) was injected into a modified vacuum plasma spray set-up and the fractional mass of particles that adhered to a substrate was measured. Secondly, large (∼6 mm), dense pellets of each ash were heated and projected at a substrate, with their impact response monitored via high speed photography. The four ashes fall into two groups of two, one with high Si content (>20%) and the other containing less Si, but higher levels of lower valence cations (such as Ca, Mg & Fe). The glass transition temperatures were all relatively low (∼650–750 °C), favouring particle adhesion on surfaces in gas turbines. All of the ashes tended to adhere, especially with higher gas temperatures and impingement velocities. However, this tendency was much greater for the two ashes with high levels of the lower valence cations. The high speed photography confirmed that this was due to these two ashes having much lower viscosities (at high strain rates). This behaviour could not have been predicted solely on the basis of Tg or glass content values. However, these cations act as “network-modifiers” in silica-based glasses, effecting sharp reductions in melt viscosity, so inferences about the danger of specific VA may be possible from simple compositional analysis. In any event, it's clearly important for VA being generated during any particular eruption to be sampled (presumably by drones) and analysed, rather than relying solely on remote measurement of atmospheric ash levels.This work forms part of a research programme funded by EPSRC (EP/K027530/1). In conjunction with this project, a consortium of partners has been set up under the PROVIDA ("PROtection against Volcanic ash Induced Damage in Aeroengines") banner and information about its operation is available at http://www.ccg.msm.cam.ac.uk/initiatives/provida. The invaluable assistance of Kevin Roberts (Materials Department in Cambridge) with operation of the plasma spray facility is gratefully acknowledged. The authors are also grateful to Mr. Max Burley, of the Materials Science Department in Cambridge, for helpful contributions to the high speed photography and gas gun work, and to Dr. Margaret Hartley, of the University of Manchester, for kindly collecting the ashes during field trips to Iceland (funded by EasyJet) and also for extensive and valuable discussions related to the science of the specific eruptions concerned, and more generally concerning the complex relationships between geological and rheological characteristics of volcanic magma and ash. In compliance with EPSRC requirements, raw data in the form of selected video files are available at www.ccg.msm.cam.ac.uk/publications/resources, and are also accessible via the University repository at http://www.data.cam.ac.uk/repository.This is the final version of the article. It first appeared from Elsevier via https://doi.org/10.1016/j.actamat.2016.02.01

Limit case analysis of the “stable indenter velocity” method for obtaining creep stress exponents from constant load indentation creep tests

© 2016, The Author(s). This study concerns a commonly-used procedure for evaluating the steady state creep stress exponent, n, from indentation data. The procedure involves monitoring the indenter displacement history under constant load and making the assumption that, once its velocity has stabilised, the system is in a quasi-steady state, with stage II creep dominating the behaviour. The stress and strain fields under the indenter are represented by “equivalent stress” and “equivalent strain rate” values. The estimate of n is then obtained as the gradient of a plot of the logarithm of the equivalent strain rate against the logarithm of the equivalent stress. Concerns have, however, been expressed about the reliability of this procedure, and indeed it has already been shown to be fundamentally flawed. In the present paper, it is demonstrated, using a very simple analysis, that, for a genuinely stable velocity, the procedure always leads to the same, constant value for n (either 1.0 or 0.5, depending on whether the tip shape is spherical or self-similar). This occurs irrespective of the value of the measured velocity, or indeed of any creep characteristic of the material. It is now clear that previously-measured values of n, obtained using this procedure, have varied in a more or less random fashion, depending on the functional form chosen to represent the displacement–time history and the experimental variables (tip shape and size, penetration depth, etc.), with little or no sensitivity to the true value of n.EPSRC (grant RG62695), AW

The Permeability of Novel Hybrid Fiber Composite Material for Use as Diesel Particulate Filters

© 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim An investigation is presented into modeling of gas flow through particle/fiber hybrid composite materials designed for use as diesel particulate filters (DPFs). The work leads to the creation of a modified version of the Carman–Kozeny equation, specifically tailored for such structures. Details of computational fluid dynamics (CFD) simulation of gas flow through modeled structures are provided, leading to formulations to account for the loss of surface area associated with sintering and a tortuosity effect dependent on the porosity level. Based on these outcomes, a simple analytical expression is derived, and it is shown that predicted permeability values obtained with it are in good agreement with experimental data

Nano-terracing on polycrystalline palladium induced via simple heat treatment

This paper concerns formation of terraces on polycrystalline Pd, via heat treatments followed by quenching with gas jets. The driving force for terrace formation is the crystallographic anisotropy of the surface energy. Information is presented regarding the surface topography of the terraces and of the grain boundary regions. Typically, the step heights are about 50 nm and the widths of the faces between them are around 1 μm. It is shown that a measure of control can be exercised over the structures produced, although they are determined by a complex interplay of related effects.This research was supported by the EPSRC (EP/E025862/1) and the European Research Council (grant no. 240446)

Extraction of plasticity parameters from a single test using a spherical indenter and FEM modelling

A methodology is presented for obtaining plasticity characteristics of bulk metallic materials from single run indentation data. It involves repeated FEM modelling, with the predicted outcome (load-displacement plot) being systematically compared with experiment. The “correct” property values are found by searching for the combination giving the maximum value for a “goodness of fit” parameter (g) measuring the agreement between experimental and predicted outcomes (ranging from 0 for no agreement to 1 for perfect agreement). A matrix of property values are used as input data for the FEM model. The key issue is that of promoting convergence on the “correct” parameter combination. It is becoming accepted that use of more than one indenter shape will assist in this operation and the paper includes an exploration of this issue. It is emphasized that the strain field beneath an indenter affects the relationship between stress-strain curve and load-displacement plot, so use of shapes that create different strain fields adds extra degrees of freedom that facilitate convergence. However, there are various problems associated with use of indenters having “sharp” points or edges, and a spherical shape is much preferred. It is highlighted here that, provided the indenter shape is not self-similar (so that the nature of the strain field changes with increasing penetration depth), analogous benefits to those arising from multiple shapes can be obtained by carrying out “g-screening” operations on multiple sections of a single load-displacement plot. This is an entirely novel approach that offers considerable promise for the tractable characterization of plasticity via a single indentation run with a spherical indenter. It has been employed in the present work to obtain values of three plasticity parameters from such a run for an extruded copper sample. In fact, the stress-strain curve for this material is not one that conforms closely to a simple analytical formulation, imposing a limit on the fidelity of the inferred stress-strain curve, but it is nevertheless shown that the proposed procedure is viable and potentially very accurate.Engineering and Physical Sciences Research Council (Grant ID: RG62695), AW

A critical assessment of the "stable indenter velocity" method for obtaining the creep stress exponent from indentation data

A technique for evaluating the (steady-state) creep stress exponent (n) from indentation data has come into common use over recent years. It involves monitoring the indenter displacement history under constant load and assuming that, once its velocity has stabilised, the system is in a quasisteady state, with Stage II creep dominating the behaviour. The stress field under the indenter, and the way in which the creep strain field is changing there, are then represented by "equivalent stress" and "equivalent strain rate" values. These are manipulated in a similar manner to that conventionally employed with (uniaxial) creep test data, allowing the stress exponent, n, to be obtained as the gradient of a plot of the logarithm of the equivalent strain rate against the logarithm of the equivalent stress. The procedure is therefore a very simple one, often carried out over relatively short timescales (of the order of an hour or less). However, concerns have been expressed about its reliability, regarding the neglect of primary creep (after a very short initial transient) and about the validity of representing the stress and strain rate via these "equivalent" values. In this paper, comprehensive experimental data (both from a conventional, uniaxial loading set-up and from instrumented indentation over a range of conditions) are presented for two materials, focussing entirely on ambient temperature testing. This is supplemented by predictions from numerical (FEM) modelling. It is shown that the methodology is fundamentally flawed, commonly giving unreliable (and often very high) values for n. The reasons for this are outlined in some detail. An attempt is made to identify measures that might improve the reliability of the procedure, although it is concluded that there is no simple analysis of this type that can be recommended.RCUK, Othe