208 research outputs found
Aplicaciones del SiC biomórfico como reforzante estructural en hormigones refractarios
Una posible aplicación del SiC biomórfico (bioSiC) son los reforzante estructural en hormigones refractarios. En este caso
se han fabricado piezas de bioSiC con forma de cilindros alargados, 3-4 mm de diámetro y 30-35 mm de longitud, mediante
infiltración reactiva de Si líquido en piezas de carbón obtenidas por pirolización de madera de haya de calidad comercial.
Hemos estudiado las características microestructurales y las propiedades mecánicas de los reforzantes, como paso previo al
estudio de la aplicación mencionada, de la que se ofrece un avance en este trabajo. Para caracterizar la calidad del material
y del proceso de fabricación, la microestructura de las piezas se ha estudiado mediante microscopía electrónica de barrido.
Los reforzantes de bioSiC fueron ensayados a compresión uniaxial y flexión en cuatro puntos a temperatura ambiente y a alta
temperatura (1250-1400ºC) para la determinación de sus propiedades mecánicas, y se realizaron estudios fractográficos en el
segundo tipo de ensayos. Subsecuentemente, se prepararon ladrillos refractarios con un 3% en peso de reforzantes de bioSiC,
que fueron curados a diferentes temperaturas (máx. 1600ºC). Estos ladrillos se han ensayado en compresión y flexión en tres
puntos, a temperatura ambiente, comparándose los resultados con los obtenidos en ladrillos sin reforzantes y reforzados con
agujas metálicas de calidad comercial (acero refractario 306 ó 310), con la misma formulación y condiciones de curado.This work is devoted to the study of the time and temperature dependence of the static grain growth in YTZP 4 mol %, with
an average grain size within the submicrometric range ( > 0.1 µm). Also, the mechanical response in the temperature interval
between 1200 ºC and 1500 ºC is analysed. The grain growth is controlled by the yttria segregation at the grain boundaries,
which plays a key role in the cationic diffusion processes. Microstructural characterization of both as-received and deformed
samples allows to conclude that plastic deformation is due to grain boundary sliding (GBS), with stress exponents increasing
with the flow stress, but in all cases they are lower than n = 2
Magnetic properties of Ni nanoparticles dispersed in silica prepared by high-energy ball milling
We analyze the magnetic properties of mechanically ground nanosized Ni particles dispersed in a SiO_2 matrix. Our magnetic characterization of the as-milled samples show the occurrence of two blocking processes and that of non-monotonic milling time evolutions of the magnetic-order temperature, the high-field magnetization and the saturation coercivity. The measured coercivities exhibit giant values and a uniaxial- type temperature dependence. Thermal treatment carried out in the as-prepared samples result in a remarkable coercivity reduction and in an increase of the high-field magnetization. We conclude, on the basis of the consideration of a core (pure Ni) and shell (Ni-Si inhomogeneous alloy) particle structure, that the magnetoelastic anisotropy plays the dominant role in determining the magnetic properties of our particles
FeCo Nanowire-Strontium Ferrite Powder Composites for Permanent Magnets with High-Energy Products
Due to the issues associated with rare-earth elements, there arises a strong
need for magnets with properties between those of ferrites and rare-earth
magnets that could substitute the latter in selected applications. Here, we
produce a high remanent magnetization composite bonded magnet by mixing FeCo
nanowire powders with hexaferrite particles. In the first step, metallic
nanowires with diameters between 30 and 100 nm and length of at least 2 {\mu}m
are fabricated by electrodeposition. The oriented as-synthesized nanowires show
remanence ratios above 0.76 and coercivities above 199 kA/m and resist core
oxidation up to 300 {\deg}C due to the existence of a > 8 nm thin oxide
passivating shell. In the second step, a composite powder is fabricated by
mixing the nanowires with hexaferrite particles. After the optimal nanowire
diameter and composite composition are selected, a bonded magnet is produced.
The resulting magnet presents a 20% increase in remanence and an enhancement of
the energy product of 48% with respect to a pure hexaferrite (strontium
ferrite) magnet. These results put nanowire-ferrite composites at the forefront
as candidate materials for alternative magnets for substitution of rare earths
in applications that operate with moderate magnet performance
Negative magnetic relaxation in superconductors
It was observed that the trapped magnetic moment of HTS tablets or annuli
increases in time (negative relaxation) if they are not completely magnetized
by a pulsed magnetic field. It is shown, in the framework of the Bean
critical-state model, that the radial temperature gradient appearing in tablets
or annuli during a pulsed field magnetization can explain the negative magnetic
relaxation in the superconductor
The neovolcanic axis is a barrier to gene flow among Aedes aegypti populations in Mexico that differ in vector competence for Dengue 2 virus.
The Neovolcanic axis (NVA) traverses Mexico at the 19th parallel and is considered to be a geographic barrier to many species. We have demonstrated that the intersection of the NVA with the coast in Veracruz state is a barrier to gene flow in Ae. aegypti. This was unexpected because the intersection of the NVA with the Pacific Coast is not a barrier to gene flow. Further studies to identify the actual mechanism(s) that is(are) contributing to the lack of gene flow will provide important information on the trafficking potential of Ae. aegypti, which will be of great value to Ae. aegypti control programs. There are significant differences in vector competence for dengue virus between mosquitoes north and south of the NVA, but the epidemiological significance of these finding remains to be determined. Future studies will determine if, for example, the genes that condition midgut infection and vector competence of Ae. aegypti populations provide biomarkers for risk of dengue transmission. Such biomarkers could be of great value to control programs in resource limited environments by allowing targeting of vector control efforts to areas at most risk for epidemic dengue and dengue hemorrhagic fever
Pr 0.5 Ca 0.5 MnO 3 thin films deposited on LiNbO 3 substrates
Thin films of Pr 0.5 Ca 0.5 MnO 3 have been deposited on z-cut LiNbO 3 by pulsed laser ablation. The X-ray diffraction measurements showed that the films have grown highly oriented on LiNbO 3 , with a pseudocubic (111) preferred growth direction. The thicknesses of the films, measured by low angle X-ray reflectivity, are between 13 and 140 nm. Their electrical resistivity present a semiconducting-like behaviour with an anomaly around 240 K, that corresponds to the charge ordering transition. The temperature of the transition (T_CO) was estimated from ln(r) vs. (1/T) plots. The charge ordering temperature was found to be dependent on the strain induced by the lattice mismatch on the films.Fundação para a Ciência e a Tecnologia (FCT
Dense strontium hexaferrite-based permanent magnet composites assisted by cold sintering process
[EN] The use of rare-earth-based permanent magnets is one of the critical points for the development of the current technology. On the one hand, industry of the rare-earths is highly polluting due to the negative environmental impact of their extraction and, on the other hand, the sector is potentially dependent on China. Therefore, investigation is required both in the development of rare-earth-free permanent magnets and in sintering processes that enable their greener fabrication with attractive magnetic properties at a more competitive price. This work presents the use of a cold sintering process (CSP) followed by a post-annealing at 1100 °C as a new way to sinter composite permanent magnets based on strontium ferrite (SFO). Composites that incorporate a percentage ≤ 10% of an additional magnetic phase have been prepared and the morphological, structural and magnetic properties have been evaluated after each stage of the process. CSP induces a phase transformation of SFO in the composites, which is partially recovered by the post-thermal treatment improving the relative density to 92% and the magnetic response of the final magnets with a coercivity of up to 3.0 kOe. Control of the magnetic properties is possible through the composition and the grain size in the sintered magnets. These attractive results show the potential of the sintering approach as an alternative to develop modern rare-earth-free composite permanent magnets.This work has been supported by the Ministerio Español de Ciencia e Innovación (MICINN), Spain, through the projects MAT2017-86540-C4-1-R and RTI2018-095303-A-C52, and by the European Commission through Project H2020 No. 720853 (Amphibian). C.G.-M. and A.Q. acknowledge financial support from MICINN through the “Juan de la Cierva” program (FJC2018-035532-I) and the “Ramón y Cajal” contract (RYC-2017-23320). S. R.-G. gratefully acknowledges the financial support of the Alexander von Humboldt foundation, Germany. A.S. acknowledges the financialsupport from the Comunidad de Madrid, Spain, for an “Atracción de Talento Investigador” contract (No. 2017-t2/IND5395)
Microsystem-assisted synthesis of carbon dots with fluorescent and colorimetric properties for pH detection
The present paper describes the use of a microfluidic system to synthesize carbon dots (Cdots) and their use as optical pH sensors. The synthesis is based on the thermal decomposition of ascorbic acid in dimethyl sulfoxide. The proposed microsystem is composed of a fluidic and a thermal platform, which enable proper control of synthesis variables. Uniform and monodispersed 3.3 nm-sized Cdots have been synthesized, the optical characterization of which showed their down/upconversion luminescence and colorimetric properties. The obtained Cdots have been used for pH detection with down and upconverison fluorescent properties as excitation sources. The naked eye or a photographic digital camera has also been implemented as detection systems with the hue parameter showing a linear pH range from 3.5 to 10.2. On the other hand, experiments on the cytotoxicity and permeability of the Cdots on human embryonic kidney cells revealed their adsorption on cells without causing any impact on the cellular morphology.This work was supported by Projects SGR 2009-0323 from Catalonia
Government and P10-FQM-5974 from the Junta de
Andalucía (Spain). These projects were partially supported by
European Regional Development Funds (ERDF). Our thanks to
“Reincorporacion de Doctores UGR” programs and Greib startu
Isolated nocturnal hypertension in individuals with human immunodeficiency virus
Introducción: La hipertensión nocturna aislada se asocia a mayor cantidad de eventos cardiovasculares y daño de órgano blanco por hipertensión arterial. La prevalencia en poblaciones especiales no se encuentra del todo descrita. El objetivo del siguiente estudio es describir la prevalencia de hipertensión nocturna aislada en población conviviendo con el virus de la inmunodeficiencia humana, y observar su relación con las categorías de presión arterial en el consultorio y los fenotipos de la medición ambulatoria de presión arterial de 24 h.
Metodología: Se realizó una cohorte retrospectiva en una población con el virus de la inmunodeficiencia humana en un hospital público de España, se registraron características clínico epidemiológicas, mediciones de presión arterial en consultorio y medición ambulatoria de presión arterial de 24 h (MAPA). Se realizó un análisis en función de los diferentes fenotipos de presión arterial por MAPA, así como también en función de las diferentes categorías de presión arterial de consultorio se calcularon los riesgos para la hipertensión nocturna aislada.
Resultados: Se incluyeron en el análisis 116 individuos, sin medicación antihipertensiva ni antecedentes de enfermedad cardiovascular establecida. Se describió una prevalencia de hipertensión nocturna del 23,3%. No se pudo demostrar diferencias significativas entre fenotipos por MAPA de ninguna variable propia del VIH. No hubo diferencias de riesgo ajustadas entre las diferentes categorías de normotensos en consultorio.
Conclusiones: La hipertensión nocturna aislada es más frecuente en pacientes con VIH, y los valores de presión arterial de consultorio en normotensos no son suficientes para predecir HTA nocturna aislada.Introduction: Isolated nocturnal hypertension is associated with a greater number of cardiovascular events and target organ damage due to arterial hypertension. It has been observed that patients in the general population with this entity do not have high blood pressure figures in the office; and it is necessary to perform an outpatient measurement to unmask it. The prevalence in special populations is not fully described. The objective of the following study is to describe the prevalence of isolated nocturnal hypertension in a population living with the human immunodeficiency virus and to observe its relationship with the categories of office blood pressure and the phenotypes of the 24-hour ambulatory blood pressure measurement.
Methodology: A retrospective cohort was carried out in a population with human immunodeficiency virus in a public hospital in Spain, clinical epidemiological characteristics, office blood pressure measurements and 24-hour ambulatory blood pressure measurement (ABPM) were recorded. An analysis was performed based on the different ABPM blood pressure phenotypes, as well as based on the different office blood pressure categories, the risks for isolated nocturnal hypertension were calculated.
Results: One hundred and sixteen individuals, without antihypertensive medication or history of established cardiovascular disease, were included in the analysis. A prevalence of nocturnal hypertension of 23.3% was described. It was not possible to demonstrate significant differences between phenotypes by ABPM of any variable specific to HIV. There were no adjusted risk differences between the different categories of office normotensives.
Conclusions: Isolated nocturnal hypertension is more frequent in patients with HIV and office blood pressure values in normotensive patients are not sufficient to predict isolated nocturnal hypertension.Facultad de Ciencias Médica
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