Ultrafast collinear scattering and carrier multiplication in graphene.

Graphene is emerging as a viable alternative to conventional optoelectronic, plasmonic and nanophotonic materials. The interaction of light with charge carriers creates an out-of-equilibrium distribution, which relaxes on an ultrafast timescale to a hot Fermi-Dirac distribution, that subsequently cools emitting phonons. Although the slower relaxation mechanisms have been extensively investigated, the initial stages still pose a challenge. Experimentally, they defy the resolution of most pump-probe setups, due to the extremely fast sub-100 fs carrier dynamics. Theoretically, massless Dirac fermions represent a novel many-body problem, fundamentally different from Schrödinger fermions. Here we combine pump-probe spectroscopy with a microscopic theory to investigate electron-electron interactions during the early stages of relaxation. We identify the mechanisms controlling the ultrafast dynamics, in particular the role of collinear scattering. This gives rise to Auger processes, including charge multiplication, which is key in photovoltage generation and photodetectors.

CO2-gasification of a lignite coal in the presence of an iron-based oxygen carrier for chemical-looping combustion

Chemical-looping combustion (CLC) has the inherent property of separating the product CO2 from flue gases. Instead of air, it uses an oxygen carrier, usually in the form of a metal oxide, to provide oxygen for combustion. All techniques so far proposed for chemical looping with solid fuels involve initially the gasification of the solid fuel in order for the gaseous products to react with the oxygen carrier. Here, the rates of gasification of coal were compared when gasification was undertaken in a fluidised bed of either (i) an active Fe-based oxygen carrier used for chemical looping or (ii) inert sand. This enabled an examination of the ability of chemical looping materials to enhance the rate of gasification of solid fuels. Batch gasification and chemical-looping combustion experiments with a German lignite and its char are reported, using an electrically-heated fluidised bed reactor at temperatures from 1073 to 1223 K. The fluidising gas was CO2 in nitrogen. The kinetics of the gasification were found to be significantly faster in the presence of the oxygen carrier, especially at temperatures above 1123 K. A numerical model was developed to account for external and internal mass transfer and for the effect of the looping agent. The model also included the effects of the evolution of the pore structure at different conversions. The presence of Fe2O3 led to an increase in the rate of gasification because of the rapid oxidation of CO by the oxygen carrier to CO2. This resulted in the removal of CO and maintained a higher mole fraction of CO2 in the mixture of gas around the particle of char, i.e. within the mass transfer boundary layer surrounding the particle. This effect was most prominent at about 20% conversion when (i) the surface area for reaction was at its maximum and (ii) because of the accompanying increase in porosity and pore size, intraparticle resistance to gas mass transfer within the particle of char had fallen, compared with that in the initial particle. Excellent agreement was observed between the rates predicted by the numerical model and those observed experimentally. ©2013 Elsevier Ltd. All rights reserved.

Different methods of manufacturing FE-based oxygen carrier particles for reforming via chemical looping, and their effect on performance

Chemical looping combustion (CLC) is a means of combusting carbonaceous fuels, which inherently separates the greenhouse gas carbon dioxide from the remaining combustion products, and has the potential to be used for the production of high-purity hydrogen. Iron-based oxygen carriers for CLC have been subject to considerable work; however, there are issues regarding the lifespan of iron-based oxygen carriers over repeated cycles. In this work, haematite (Fe2O3) was reduced in an N2+CO+CO2 mixture within a fluidised bed at 850°C, and oxidised back to magnetite (Fe3O4) in a H2O+N2 mixture, with the subsequent yield of hydrogen during oxidation being of interest. Subsequent cycles started from Fe3O4 and two transition regimes were studied; Fe3O4↔Fe0.947O and Fe 3O4↔Fe. Particles were produced by mechanical mixing and co-precipitation. In the case of co-precipitated particles, Al was added such that the ratio of Fe:Al by weight was 9:1, and the final pH of the particles during precipitation was investigated for its subsequent effect on reactivity. This paper shows that co-precipitated particles containing additives such as Al may be able to achieve consistently high H2 yields when cycling between Fe3O4 and Fe, and that these yields are a function of the ratio of [CO2] to [CO] during reduction, where thermodynamic arguments suggest that the yield should be independent of this ratio. A striking feature with our materials was that particles made by mechanical mixing performed much better than those made by co-precipitation when cycling between Fe3O4 and Fe0.947O, but much worse than co-precipitated particles when cycling between Fe3O 4 and Fe.

UK guidance on the initial evaluation of an infant or an adolescent with a suspected disorder of sex development

It is paramount that any child or adolescent with a suspected disorder of sex development (DSD) is assessed by an experienced clinician with adequate knowledge about the range of conditions associated with DSD. If there is any doubt, the case should be discussed with the regional team. In most cases, particularly in the case of the newborn, the paediatric endocrinologist within the regional DSD team acts as the first point of contact. The underlying pathophysiology of DSD and the strengths and weaknesses of the tests that can be performed should be discussed with the parents and affected young person and tests undertaken in a timely fashion. This clinician should be part of a multidisciplinary team experienced in management of DSD and should ensure that the affected person and parents are as fully informed as possible and have access to specialist psychological support. Finally, in the field of rare conditions, it is imperative that the clinician shares the experience with others through national and international clinical and research collaboration. © 2011 Blackwell Publishing Ltd.