You and the Time

A medic reflects on how things have changed in his life over time. Articles, stories, and other compositions in this archive were written by participants in the Mighty Pen Project. The program, developed by author David L. Robbins, and in partnership with Virginia Commonwealth University and the Virginia War Memorial in Richmond, Virginia, offers veterans and their family members a customized twelve-week writing class, free of charge. The program encourages, supports, and assists participants in sharing their stories and experiences of military experience so both writer and audience may benefit

Autoactivación de cemento de escorias con polvo de vidrio de descarte

The aim of this study is to evaluate the behavior of glass powder waste as alkaline activator of slag when both are used in blended cement. The slag blended cement was prepared with 70% slag, and then the slag was replaced with 10-20% glass powder by mass. The self-activation concept is here introduced. Ground glass releases alkalis through dissolution in a slow and sustained way over time. This would activate slag. Ground glass improves fluidity of mortars and the compressive strength overcomes dilution and increases with age due to the combined pozzolanic action of slag and glass. The proposed blended cement has better mechanical properties than slag-cement. The proposed blended cement has better mechanical properties than slag-cement and due to less use of slag, it becomes more sustainable and it contributes to waste management.El objetivo del presente trabajo es evaluar el comportamiento de un residuo molido de vidrio como activador interno de la escoria cuando ambos son usados en un cemento mezcla. El cemento de escoria fue preparado con un 70% de escoria y luego la escoria se reemplazó en un 10 y 20% en masa por vidrio molido. Este trabajo introduce el concepto de auto-activación. El vidrio molido libera álcalis por disolución, en forma lenta y sostenida en el tiempo que activarían la escoria. La presencia de vidrio molido mejora la fluidez de los morteros y la resistencia a compresión supera la dilución y crece con la edad por la acción puzolánica combinada de la escoria y el vidrio. El cemento mezcla propuesto tiene mejores propiedades mecánicas que el cemento de escorias pero con menor contenido de escoria por lo que es más sustentable y contribuye al manejo de residuos

Spectroscopic behaviour of some superconducting materials containing tetraoxoanions

A number of materials of composition YSr₂Cu₂.(XO₄)₀.₂Oy(X = S, P, V) and others in which Y (III) was partially replaced by Sr(II) and/or Ca(II) were prepared by solid state reactions. Their unit cell parameters were determined and refined from X-ray powder data. The infrared spectra of the different tetraoxoanions incorporated in these materials were analyzed and discussed in detail. The vibrational modes of the other structural units present in these lattices were also approximately assigned in comparison with known spectral data of related materials.Centro de Química Inorgánic

Quantum phase slips in superconducting Nb nanowire networks deposited on self-assembled Si templates

Robust porous silicon substrates were employed for generating interconnected networks of superconducting ultrathin Nb nanowires. Scanning electron microscopy analysis was performed to investigate the morphology of the samples, which constitute of polycrystalline single wires with grain size of about 10 nm. The samples exhibit nonzero resistance over a broad temperature range below the critical temperature, fingerprint of phase slippage processes. The transport data are satisfactory reproduced by models describing both thermal and quantum fluctuations of the superconducting order parameter in thin homogeneous superconducting wires.Comment: accepted for publication on Applied Physics Letter

Growth of dark matter perturbations during kination

If the Universe's energy density was dominated by a fast-rolling scalar field while the radiation bath was hot enough to thermally produce dark matter, then dark matter with larger-than-canonical annihilation cross sections can generate the observed dark matter relic abundance. To further constrain these scenarios, we investigate the evolution of small-scale density perturbations during such a period of kination. We determine that once a perturbation mode enters the horizon during kination, the gravitational potential drops sharply and begins to oscillate and decay. Nevertheless, dark matter density perturbations that enter the horizon during an era of kination grow linearly with the scale factor prior to the onset of radiation domination. Consequently, kination leaves a distinctive imprint on the matter power spectrum: scales that enter the horizon during kination have enhanced inhomogeneity. We also consider how matter density perturbations evolve when the dominant component of the Universe has a generic equation-of-state parameter w. We find that matter density perturbations do not grow if they enter the horizon when 01/3, their growth is faster than the logarithmic growth experienced during radiation domination. The resulting boost to the small-scale matter power spectrum leads to the formation of enhanced substructure, which effectively increases the dark matter annihilation rate and could make thermal dark matter production during an era of kination incompatible with observations

Sedimental Coal Ashes: Its pozzolanic effects in Portland cement clinker

Las centrales termoeléctricas generan una gran cantidad de residuos sólidos como producto de la combustión del carbón, conocidos como cenizas. Existen dos tipos de cenizas: volantes - de tamaño muy fino-, que son arrastradas por la corriente de humos de los sistemas de eliminación de partículas y, -las sedimentadas - más gruesas, que se funden y se aglomeran acumulándose en el fondo del homo o en los tubos de las calderas. Intentando resolver factores técnico-económicos y ecológicos al mismo tiempo, en este trabajo presentamos los resultados obtenidos de la incorporación de un 20% en peso de cenizas sedimentadas (Cs) como adición activa al cemento Portland. Se estudia la potencial puzolanidad de la adición y se muestran resultados de seguimiento de la hidratación a temprana edad, variación de color y resistencia mecánica del material compuesto obtenido por molienda conjunta del clinker portland y el material residual.The power plants generate a large amount of solid waste, known as ash, as a product of combustion of coal. There are two types of ash: fly - very fine in size- which are carried by the flow of smoke removal systems, and sedimentary particles -coarse particles - which are melted and agglomerated accumulated in the bottom of the furnace or in the boiler tubes. Trying to resolve technical-economic and ecologic factors at the same time, this work presents the results of adding a 20% weight of sediment ash (Cs) as an active addition to the Portland cement. The potential pozzolanity of the addition is studied and it is shown proceeding of the hydration at early age, color variation and mechanical strength of the composite material obtained by grinding together Portland clinker with the residual material.Centro de Tecnología de Recursos Minerales y CerámicaFacultad de Ingenierí

Blended Cements Elaborated with Kaolinitic Calcined Clays

In the clinker production process, the main component of Portland cement (PC), large amount of CO2 are emitted into the atmosphere. The use of calcined kaolinitic clays as supplementary material of cement is an alternative to mitigate the environmental impact. In this paper, the influence of the replacement of PC by two calcined kaolinitic clays (15 and 30%), with high content of metakaolinite and different reactivity was studied. Calcined kaolinitic clays were characterized, the pozzolanic activity using Frattini test, the compressive strength, rate of water absorption (sorptivity), the assemblage of hydration products and the pore size distribution were determined at 28 days. The results show that the replacement of high percentage (30%) of very reactive clays improves the mechanical behavior and durable parameters compared with those obtained for plain PC.Centro de Tecnología de Recursos Minerales y Cerámic

Electro-Thermal Parameters of Graphene Nano-Platelets Films for De-Icing Applications

This paper provides a study of some relevant electro-thermal properties of commercial films made by pressed graphene nano-platelets (GNPs), in view of their use as heating elements in innovative de-icing systems for aerospace applications. The equivalent electrical resistivity and thermal emissivity were studied, by means of models and experimental characterization. Macroscopic strips with a length on the order of tens of centimeters were analyzed, either made by pure GNPs or by composite mixtures of GNPs and a small percentage of polymeric binders. Analytical models are derived and experimentally validated. The thermal response of these graphene films when acting as a heating element is studied and discussed

Sedimental Coal Ashes: Its pozzolanic effects in Portland cement clinker

Las centrales termoeléctricas generan una gran cantidad de residuos sólidos como producto de la combustión del carbón, conocidos como cenizas. Existen dos tipos de cenizas: volantes - de tamaño muy fino-, que son arrastradas por la corriente de humos de los sistemas de eliminación de partículas y, -las sedimentadas - más gruesas, que se funden y se aglomeran acumulándose en el fondo del homo o en los tubos de las calderas. Intentando resolver factores técnico-económicos y ecológicos al mismo tiempo, en este trabajo presentamos los resultados obtenidos de la incorporación de un 20% en peso de cenizas sedimentadas (Cs) como adición activa al cemento Portland. Se estudia la potencial puzolanidad de la adición y se muestran resultados de seguimiento de la hidratación a temprana edad, variación de color y resistencia mecánica del material compuesto obtenido por molienda conjunta del clinker portland y el material residual.The power plants generate a large amount of solid waste, known as ash, as a product of combustion of coal. There are two types of ash: fly - very fine in size- which are carried by the flow of smoke removal systems, and sedimentary particles -coarse particles - which are melted and agglomerated accumulated in the bottom of the furnace or in the boiler tubes. Trying to resolve technical-economic and ecologic factors at the same time, this work presents the results of adding a 20% weight of sediment ash (Cs) as an active addition to the Portland cement. The potential pozzolanity of the addition is studied and it is shown proceeding of the hydration at early age, color variation and mechanical strength of the composite material obtained by grinding together Portland clinker with the residual material.Centro de Tecnología de Recursos Minerales y CerámicaFacultad de Ingenierí