Paul C. Dastoor

Wear mechanisms in polymer matrix composites abraded by bulk solids

Abstract An experimental study of the wear of polymer matrix composite materials subjected to abrasion from bulk materials has been conducted. Three examples of vinyl ester resin systems were considered: (a) unreinforced, (b) reinforced with glass fibres, and (c) reinforced with particles of ultra high molecular weight polyethylene (UHWMPE). Soft and hard bulk materials used for abrasion were granular forms of coal and the mineral ignimbrite. The bulk material was presented to the wear surface on a conveyor belt in a novel wear tester. While UHWMPE reinforcement enhanced the wear resistance to both hard and soft abrasives, the situation for fibre reinforcement was more complicated. With coal as the abrasive, it was found that glass fibre reinforcement reduced the wear rate, whereas in the case of the harder ignimbrite, fibre reinforcement increased the wear rate. Microscopy indicated significant differences in the mechanism of wear in each surface/abrasive combination. Wear textures, consistent with both two and three-body wear, were observed with, respectively, soft and hard abrasive particles.

Organic Solar Cells: Understanding the Role of Förster Resonance Energy Transfer

Organic solar cells have the potential to become a low-cost sustainable energy source. Understanding the photoconversion mechanism is key to the design of efficient organic solar cells. In this review, we discuss the processes involved in the photo-electron conversion mechanism, which may be subdivided into exciton harvesting, exciton transport, exciton dissociation, charge transport and extraction stages. In particular, we focus on the role of energy transfer as described by Förster resonance energy transfer (FRET) theory in the photoconversion mechanism. FRET plays a major role in exciton transport, harvesting and dissociation. The spectral absorption range of organic solar cells may be extended using sensitizers that efficiently transfer absorbed energy to the photoactive materials. The limitations of Förster theory to accurately calculate energy transfer rates are discussed. Energy transfer is the first step of an efficient two-step exciton dissociation process and may also be used to preferentially transport excitons to the heterointerface, where efficient exciton dissociation may occur. However, FRET also competes with charge transfer at the heterointerface turning it in a potential loss mechanism. An energy cascade comprising both energy transfer and charge transfer may aid in separating charges and is briefly discussed. Considering the extent to which the photo-electron conversion efficiency is governed by energy transfer, optimisation of this process offers the prospect of improved organic photovoltaic performance and thus aids in realising the potential of organic solar cells.

Adsorption of organosilanes on iron and aluminium oxide surfaces

Thin films of a simple organosilane, propyltrimethoxysilane (PTMS), on polycrystalline aluminium and iron oxide substrates have been investigated using XPS. Comparison of the adsorption isotherms for the two substrates reveals significant differences between them. Although PTMS on iron oxide exhibits a Langmuir-like isotherm, PTMS on aluminium oxide shows an unexpected decrease in adsorption at intermediate exposures. The results demonstrate that even for the simplest organosilanes and single-component surfaces the formation of silane coatings on metals is complex and strongly dependent upon the nature of the surface. The implication of the results to silane coupling events (SCAs) is discussed briefly.

Conformational dynamics of γ-APS on the iron oxide surface: an adsorption kinetic study using XPS and ToF-SIMS

The formation and structure of thin films of γ-aminopropyltriethoxysilane (y-APS) on mechanically polished iron oxide surfaces have been investigated. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) have been used to study the time dependence of the surface interaction and the subsequent film structure, following adsorption from the liquid phase. It is shown, for the first time, that γ-APS exhibits the phenomenon of oscillatory adsorption, whereby the surface coverage of the silane species oscillates as a function of time. By analysing the time dependence of the known bonding moieties of the γ-APS molecule, the dominant molecular orientation of the adsorbate is shown to change as a function of time. A model of possible adsorbate conformations is presented and used to describe the observed adsorption behaviour.

Photoelectrochemical cells based on polymers and copolymers from terthiophene and nitrostyrylterthiophene

Abstract This research work was aimed at improving the photovoltaic efficiency of photoelectrochemical cells (PECs) made from terthiophene based conducting polymers. Two approaches were considered. One was to use poly(( E )-3″-( p -nitrostyryl)terthiophene) in expectation that the electron withdrawing nitro substituent would enhance charge separation. The other approach was to condition the polymers at different potentials before assembly into PECs. In this way, these polymers would have different conductivities that would be expected to have different charge transport properties. Thus, polymers and copolymer from terthiophene and nitrostyrylterthiophene have been electrochemically polymerised onto indium tin oxide (ITO) coated glass. These films were characterised by cyclic voltammetry. In addition, films were electrochemically conditioned at a series of different potentials before they were characterised by UV–VIS spectrophotometry and then fabricated into PECs using liquid electrolyte. The photovoltaic tests show that the electron withdrawing nitro group results in charge separation being the dominant factor in the photovoltaic behaviour of these polymers, as evidenced by their behaviour being independent of the conditioning potential. In contrast, the photovoltaic behaviour of polyterthiophene PECs shows that the best performance was obtained from the fully reduced polymers, which behave as p-type semiconductors for exciton formation and charge separation.

Effects of graphite particle addition upon the abrasive wear of polymer surfaces

The abrasive wear performance of vinyl ester resins modified with various volume fractions (5, 10, 15, 20 and 30%) of graphite powder has been measured. Using a conveyor belt driven testing machine developed locally, it has been possible to realistically simulate the effect of three-body abrasive wear upon these graphite modified polymer samples. A comparison of the calculated dimensionless wear rates obtained for these surfaces reveals that the effect of the graphite powder depends strongly upon the volume fraction of particles in the resin matrix. It appears that, for intermediate volume fractions, the presence of graphite powder in the resin matrix reduces the abrasive wear of the polymer surface. Scanning electron microscopy has been used to probe the mechanisms of abrasive wear of the pure resin and graphite modified surfaces. It appears that the embedded graphite particles can act as a lubricant during the abrasion process thus reducing the wear rate. The effect of increasing graphite powder volume fraction upon the abrasive wear mechanism is discussed.

Low-temperature processed solar cells with formamidinium tin halide perovskite/fullerene heterojunctions

A new type of lead-free, formamidinium (FA)-based halide perovskites, FASnI2Br, are investigated as light-harvesting materials for low-temperature processed p–i–n heterojunction solar cells with different configurations. The FASnI2Br perovskite, with a band-gap of 1.68 eV, exhibits optimal photovoltaic performance after low-temperature annealing at 75 °C. By using C60 as electron-transport layer, the device yields a hysteresis-less power conversion efficiency of 1.72%. The possible use of an inorganic MoOx film as a new type of independent hole-transport layer for the present tin-based perovskite solar cells is also demonstrated.

A polyethylene-reinforced polymer composite abraded by bulk solids

The abrasive wear characteristics of polymer matrix composite materials reinforced with particles of ultra-high molecular weight polyethylene (UHMWPE) have been investigated. Granular ignimbrite (a hard mineral) was used as the abrasive. The effect of varying the volume fraction of UHMWPE reinforcement upon wear rate has been measured and scanning electron microscopy (SEM) used to study the wear mechanisms that operate. The results demonstrate that there is a distinct transition that occurs in the dominant wear mechanism as the volume fraction of UHMWPE reinforcement is increased. In particular, as the UHMWPE concentration in the resin is increased, friction at the surface decreases and the wear mechanism appears to change from predominantly three- to two-body wear, with a corresponding decrease in wear rate. This transition in mechanism appears to occur when the wall friction drops below the internal friction of the granular bed. At higher values of wall friction, the plane of shear failure is within the granular material so that the particles in contact with the surface have a rolling component across the surface. At lower values of wall friction, the plane of shear failure occurs at the surface.

Comparative analysis of Ti3SiC2 and associated compounds using x-ray diffraction and x-ray photoelectron spectroscopy

Ti3SiC2 exhibits a unique combination of ceramic and metallic properties suitable for both electrical and mechanical application. With high-temperature stability, high electrical and thermal conductivity and resistance to oxidation, Ti3SiC2 has proven promising as a contact layer for high power SiC semiconductors. However, until recently, synthesis of this material has proven difficult without appreciable quantities (<2 vol{%}) of impurity phases, namely TiC1-x and Ti5Si3Cx. As such, many properties of this compound are as yet unknown. In this paper, a comparable analysis of Ti3SiC2 and associated compounds, TiC and Ti5Si3Cx has been performed using both x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). Assessing impurity sensitivities for each technique, XRD was shown to readily identify impurities of TiC and Ti5Si3Cx within Ti3SiC2 at <2 wt{%}. Although XPS could not independently resolve these impurities, its use resulted in the detection of a complex oxide structure on Ti3SiC2. It was speculated that it was composed of mixed C-Ti-C-O and Si-Ti-C-O bond chemistries. In a comparison of TiC, Ti5Si3Cx and Ti3SiC2, differences in oxide states suggest that oxidation is chemically dissimilar for all the three compounds. However, upon etching, the binding energies of Ti3SiC2 and Ti5Si3Cx were shown to be very similar. It may be concluded that a concurrent analysis of both XRD and XPS was essential for identifying the overall surface chemistry of Ti3SiC2.

Influence of surface electrokinetics on organosilane adsorption

The time-dependent adsorption of propyltrimethoxysilane (PTMS) in aqueous solution on four substrates (aluminium, zinc, iron and chromium oxide) has been investigated using x-ray photoelectron spectroscopy. Oscillations in the adsorption isotherm as a function of adsorption time in solution have been observed for all of the substrates studied. The adsorption of PTMS is shown to depend strongly upon the isoelectric point of the surface oxide. The closer the isoelectric point is to the solution pH, the more rapid is the initial adsorption rate of silane on the surface. Models are proposed to explain the influence that the electrokinetics of the surface has upon the adsorption mechanism.