Non-linear dissolution mechanisms of sodium calcium phosphate glasses as a function of pH in various aqueous media

Phosphate glasses for bioresorbable implants display dissolution rates that vary significantly with composition, however currently their mechanisms of dissolution are not well understood. Based on this systematic study we present new insights into these mechanisms. Two-stage dissolution was observed, with time dependence initially parabolic and later linear, and a two-stage model was developed to describe this behaviour. Dissolution was accelerated by lower Ca concentration in the glass, and lower pH in the dissolution medium. A new dissolution mechanism is proposed, involving an initial stage where diffusion-controlled formation of a conversion layer occurs. Once the conversion layer is stabilised, layer dissolution reactions become rate-limiting. Under this mechanism the transition time is sensitive to the nature of the conversion layer and solution conditions. These results reveal the dependence of P₂O₅–CaO–Na₂O glass dissolution on solution pH, and provide new insight into the dissolution mechanisms, particularly regarding the transition between the two dissolution stages.     

Modelling and analysis of bacterial tracks suggest an active reorientation mechanism in Rhodobacter sphaeroides

Most free-swimming bacteria move in approximately straight lines, interspersed with random reorientation phases. A key open question concerns varying mechanisms by which reorientation occurs. We combine mathematical modelling with analysis of a large tracking dataset to study the poorly understood reorientation mechanism in the monoflagellate species Rhodobacter sphaeroides. The flagellum on this species rotates counterclockwise to propel the bacterium, periodically ceasing rotation to enable reorientation. When rotation restarts the cell body usually points in a new direction. It has been assumed that the new direction is simply the result of Brownian rotation. We consider three variants of a self-propelled particle model of bacterial motility. The first considers rotational diffusion only, corresponding to a non-chemotactic mutant strain. Two further models incorporate stochastic reorientations, describing ‘run-and-tumble’ motility. We derive expressions for key summary statistics and simulate each model using a stochastic computational algorithm. We also discuss the effect of cell geometry on rotational diffusion. Working with a previously published tracking dataset, we compare predictions of the models with data on individual stopping events in R. sphaeroides. This provides strong evidence that this species undergoes some form of active reorientation rather than simple reorientation by Brownian rotation.     

Place-making or place-masking? The everyday political economy of “making place”

As redevelopment and gentrification strategies globally continue to be aimed at attracting wealthier residents and consumers in an effort to drive economic growth, concerns for and interventions in the interests of social equity appear decreasingly relevant. Government, private sector and community organisations have of course worked together in different times and places to implement programs that are more rather than less inclusive – the variations always depending on the spatial politics of the context. This paper examines contemporary discourses and practices of place-making in Melbourne, and asks whether ways of thinking about urban redevelopment as place-making in this time and place are likely to enable the inclusion of social equity in these urban “improvements”.     

Complementing retrofit with engagement: exploring energy consumption with social housing tenants

Councils and social housing organisations are looking to retrofit as a way to make their housing more energy efficient. Previous studies on energy use in social housing have generally focussed on the technological aspects (such as the potential savings possible by retrofitting this class of housing across the UK) or have involved one‐off interventions or measures. During a 2‐year period, we worked with previously homeless people to reduce their energy consumption. The 32 participants lived in small blocks of flats (owned by a social housing organisation) that underwent retrofitting with air source heat pumps. We ran a three‐phase tenant engagement programme to compare a range of approaches aimed at energy reduction. It was found that education, social norms and self‐awareness are all key components when it comes to initiating environmentally responsible behaviours. The three approaches complemented each other, and these ought to be considered alongside technology provision if the aim is to reduce energy consumption. A number of reflections on the implementation of medium‐term tenant engagement programmes are also presented. Copyright © 2016 John Wiley & Sons, Ltd.     

Inverted polymer solar cells with a solution-processed cesium halide interlayer

We demonstrate the utility of a low-cost cesium iodide interlayer spun from an aqueous or 2-ethoxyethanol solution on ITO in inverted polymer solar cells of the structure ITO/CsI/P3HT:PCBM/MoO₃/Al, where P3HT is poly(3-hexylthiophene) and PCBM is [6,6]-phenyl-C₆₀-butyric acid methyl ester. The power conversion efficiency (PCE) of optimized cells was ～3.4%, comparable to that we obtained for inverted cells with Cs carbonate. The thickness of the CsI film was adjusted by varying the solution concentration. The concentration affected the surface morphology of P3HT:PCBM and the density of fractal-like aggregates (possibly related to the presence of Cs and film fabrication conditions) formed near the anode, as revealed by scanning electron microscopy. Auger analysis indicated a P3HT-rich surface. Optimization of the cells was achieved also by varying the thickness of the MoO₃ and the drying/annealing conditions of the active layer, as was evident from the current–voltage characteristics, external quantum efficiency spectra, and PCE. The cells with the CsI interlayer were compared additionally to cells with CsCl or CsF interlayers (with a PCE of up to ～2.7%), which were inferior to the comparable cells with Cs₂CO₃ or CsI. The surface concentrations of Cs and the halide on ITO were monitored using X-ray photoelectron spectroscopy. The iodine level was low with the Cs:I ratio exceeding 8:1. In contrast, the Cs:Cl ratio was ～1.4:1 and the Cs:F ratio was ～1:1; the Cs₂CO₃ decomposed partially, as expected. Therefore, for CsI, as is the case for Cs₂CO₃ but not for CsF, Cs–O bonds are formed at the surface. Such bonds on ITO are important in modifying the ITO work function, improving the cell performance. The results indicate that spin coating solutions of the high polarity CsI is a promising and easy approach to introduce Cs–O on ITO in inverted structures for increased electron extraction from PCBM and possibly hole extraction from the P3HT-rich surface at the anode.     

Improving the performances of earth air heat exchangers through Constructal design

Earth to air heat exchanger (EAHE) is a well‐known technique used to preheat or precool outdoor air before blowing it into a building. However, its geometry is often very simple as it consists in one or multiple straight pipes, while more complex arrangements can be found in heat exchangers design. In this paper, we explore the advantage of designing an EAHE as a network through the Constructal law point of view. A methodology is first proposed to design a single pipe EAHE when the need is defined in terms of cooling power, overall efficiency and enthalpy difference between the inlet air and the ground. Next, the single pipe EAHE is used as a reference for designing a tree‐shaped network under the constraint of identical fluid volume and cooling power. The geometrical features are allowed to change for the different branches of the network. The network coefficient of performance is found to increase significantly with the bifurcation level, illustrating the superior performances of the network. This approach was found to be robust as the improvements were not depending on the cooling demand or the environmental conditions. However, further work is needed to move from this theoretical result to practical considerations.     

Strength assessment of Al2O3 and MgAl2O4 using micro- and macro-scale biaxial tests

Journal of Materials Science - The application of ceramics in advanced functional applications often requires thicknesses below a few hundred micrometers, rendering an assessment of the...