Anomalous Effect of Spinning Conditions on the Mechanical and Transport Properties of Mesophase Pitch-Based Carbon Fibers

Mesophase pitch, a nematic liquid crystal composed of disc-like polyaromatic hydrocarbons, has significant potential as an inexpensive and high-carbon-yielding precursor for structural carbon fibers. Before such potential is realized, the strength of mesophase pitch-based carbon fibers (MPCFs) must be enhanced while retaining their superior modulus and electrical/thermal conductivities. In MPCFs, lattice-related properties are favored by the three-dimensional graphitic development that originates from the preferential order that the liquid crystal molecules achieve during fiber formation. Carbon fibers derived from polyacrylonitrile (PAN), a long-chain polymer, do not possess graphitic microstructure; it is referred to as “turbostratic” carbon. Nevertheless, PAN-based carbon fibers constitute over 90% of global carbon fiber market, because of their higher tensile and compressive strengths (relative to those of MPCFs) that originate from less-perfectly formed carbon structure coupled with less aligned texture that prevents crack propagation. In MPCFs, it is believed that the large graphitic regions are also defect sensitive, and the tensile strength is lowered because it is a defect-limited property. Previous studies have demonstrated that fiber spinning/process conditions play an important role in the development of carbon fiber microstructure, which in turn determines the strength of the carbon fiber. MPCF processing consists of three main steps, namely melt spinning of mesophase pitch where fiber is formed, oxidative stabilization where fiber is rendered infusible, and high temperature treatment where carbon layer and crystallinity get developed. Studies on the relationships between fiber microstructure and carbon fiber properties have focused on fiber formation because it is in this step that the microstructure starts to develop. The melt spinning step consists of three main operations: melting of pitch, extrusion of molten pitch through spinnerets, and drawdown of filaments. Most discoveries relating the formation of fiber microstructure to MPCF properties have emphasized on processing conditions during the melting/extrusion steps, while the drawing step has not been investigated systematically. Therefore, the primary goal of this dissertation was to study how the microstructure and properties of MPCFs were influenced by changes in drawdown ratio (DDR). The specific objectives were to investigate the effects of DDR on (i) carbon fiber microstructure, (ii) fiber mechanical properties, and (iii) fiber transport properties. To accomplish these goals, precursor pitch fibers were melt-spun from a synthetic, naphthalene-based mesophase pitch using multiple sets of ultrafine diameter spinnerets with a range of capillary diameters of 50-150 µm. Pitch fibers were oxidatively stabilized in air (220-240 °C), and carbonized at 2100 °C under helium atmosphere. As a result, carbon fiber samples with average diameters in the range 8-16 µm were obtained, with drawdown ratios ranging from about 10 to 200. The microstructure of carbon fibers was studied by scanning electron microscopy (SEM), Raman spectroscopy and wide-angle x-ray diffraction (WAXD). SEM of the cross-section of fibers revealed a line-origin radial type of transverse microstructure across samples, which meant that transverse microstructure would not constitute a confounding variable in further analysis. Raman spectroscopy revealed that decreasing DDR led to an enhancement of crystallite coherence length along the fiber axis. The coherence length increased from ~ 60 nm for DDR of 190 to ~ 85 nm for DDR of 15, while at the same time reducing the defect density of carbon layer planes. WAXD revealed a 2θ peak position of ~ 26.0° for (002) planes indicating d002 spacing of 0.342 nm and Lc stacking thickness of approximately 20 nm, generally consistent with prior results on mesophase pitch-based carbon fibers produced at 2100 °C. However, significant differences in crystalline features resulting from different DDRs could not be detected, likely due to instrumental limitations. Tensile properties of carbon fibers were measured by single filament tensile testing. Between samples of equivalent fiber diameter and heat treatment temperature yet contrasting DDR, tensile strength was favored by decreasing DDR. Tensile strength was found to increase from 1.4 ± 0.2 GPa for DDR ~ 190 to 2.3 ± 0.3 GPa for DDR ~ 15, for carbon fibers with nominal diameter of 8 µm. Limited samples were tested by the tension-recoil testing to estimate compressive strength. The compressive strength displayed no measurable change, and remained about 800 MPa as the DDR was reduced from 190 to 15. The behavior of tensile strength with DDR is somewhat anomalous with that observed for carbon fibers derived from PAN for which a higher DDR is known to enhance tensile strength, via the improvement of molecular orientation along fiber axis caused by more pronounced drawing. Weibull analysis revealed that for samples of comparable fiber diameter, the Weibull modulus increases with low DDR, which means that the distribution of tensile strength is narrower. The electrical conductivity of single carbon fibers was experimentally measured, and the thermal conductivity was estimated using the Issi-Lavin electrical/thermal conductivity correlation. Limited measurements of thermal diffusivity of carbon fiber/polymer matrix composites were conducted using a laser/light flash analysis (LFA) technique. For the experimental carbon fibers the electrical resistivity was measured at ~ 6 µΩ∙m and the estimated thermal conductivity at ~ 200 W/m∙K. These measurements confirm that the conductivity of these MPCFs were a whole order of magnitude better than those of PAN-based fibers. However, no significant difference could be detected in the electrical or thermal conductivity as a function of the different DDRs. This is generally consistent with x-ray results that indicated no significant changes in graphitic crystallinity. In summary, while decreasing drawdown ratio used during pitch fiber spinning did not deteriorate electrical/thermal conductivity, the tensile strength was observed to increase with decreasing DDR

Effect of Process Variables on Mechanical and Transport Properties of Carbon Fibers from Mesophase Pitch

To obtain mesophase pitch-based carbon fibers with improved mechanical strength while retaining superior electrical and thermal properties by i) Systematic investigation of the relationship between DDR versus longitudinal and transverse microstructure of mesophase MP fibers; and ii) Developing a novel type of microstructur

Aplicación del principio de imputación necesaria y su impacto en los requerimientos fiscales acusatorios, distrito judicial de Chepén, 2022

La presente investigación titulada: Aplicación del principio de imputación necesaria y su impacto en los requerimientos fiscales acusatorios en el distrito judicial de Chepén, 2022; ha tenido como objetivo general el de determinar el impacto de la aplicación del principio de imputación necesaria en los requerimientos fiscales acusatorios en el distrito judicial de Chepén; y ello debido a las numerosas devoluciones de acusaciones en la etapa de control de acusación. En relación a la metodológica esta es cualitativa, aplicada, descriptiva e interpretativa; y se ha empleado la teoría fundamentada; es así que luego de aplicar las entrevistas a especialistas y analizar la jurisprudencia y expedientes aleatorios del juzgado penal de investigación preparatoria de Chepén, se concluyó que los fiscales al presentar sus requerimientos acusatorios, en su mayoría han realizado una indebida aplicación de dicho principio al confundir la imputación con el correlato de la declaración de la agraviada o denunciante; cuando la imputación debe ser el desarrollo cognoscitivo que el fiscal realiza para imputar; siendo así esta indebida aplicación contraviene el principio acusatorio y vulnera el derecho de defensa y de contradicción

Raman microprobe characterization of electrodeposited S-rich CuIn(S,Se)2 for photovoltaic applications: Microstructural analysis

This article reports a detailed Raman scattering and microstructural characterization of S-rich CuIn(S,Se)2 absorbers produced by electrodeposition of nanocrystalline CuInSe2 precursors and subsequent reactive annealing under sulfurizing conditions. Surface and in-depth resolved Raman microprobe measurements have been correlated with the analysis of the layers by optical and scanning electron microscopy, x-ray diffraction, and in-depth Auger electron spectroscopy. This has allowed corroboration of the high crystalline quality of the sulfurized layers. The sulfurizing conditions used also lead to the formation of a relatively thick MoS2 intermediate layer between the absorber and the Mo back contact. The analysis of the absorbers has also allowed identification of the presence of In-rich secondary phases, which are likely related to the coexistence in the electrodeposited precursors of ordered vacancy compound domains with the main chalcopyrite phase, in spite of the Cu-rich conditions used in the growth. This points out the higher complexity of the electrodeposition and sulfurization processes in relation to those based in vacuum deposition techniques

Comparación técnico-económica entre pavimentos rigidos y felexibles en Nicaragua tomando como muestra el tramo de Carretera Santa Rita-Izapa

El presente trabajo comprende el análisis económico de alternativas de pavimentos flexibles (concreto asfáltico) y rígidos (concreto hidráulico) en Nicaragua. Estas alternativas se generaron definiendo rangos característicos de tráfico y calidad soporte de los suelos de terracería, y diversas estrategias de mantenimiento aplicadas a éstas, dejando constantes a valores reales observados la distribución vehicular, el peso y coeficiente de daño promedio de cada tipo de vehículo y la tasa crecimiento anual de tránsito. El tema se desarrolla en ocho capítulos; el primer capítulo explica las condiciones actuales que se encuentran los pavimentos en el país (Nicaragua), el segundo y tercer capítulo muestra los elementos básicos que se deben de tomar en la construcción de las carreteras, el cuarto capítulo pauta sobre los materiales que se deben de usar en cada capa existente en un pavimento y a su vez la compactación, el quinto capítulo relata los métodos que comúnmente son ocupados en el diseño de un determinado tipo de pavimento, el sexto capítulo habla del estudios de impacto ambiental que se es necesario realizar antes, durante y después de una construcción, el séptimo capítulo toma el caso en estudio como es la carretera Santa Rita – Izapa, y por último y no menos importante el octavo capítulo, presenta el cálculo minucioso del diseño mismo de los pavimentos rígidos y flexibles en el tramo de carretera Santa Rita – Izapa. Los resultados demuestran que si solamente se toma en cuenta en la selección de alternativas de pavimento el costo inicial de construcción, la alternativa escogida sería el pavimento flexible, sin embargo, al incluir todos los costos a lo largo de la vida útil de la vía en cuestión, la alternativa de concreto hidráulico resulta ser la opción de costo relativamente bajo para volúmenes medios y altos. En tanto el pavimento a base de asfalto predomina en tráficos medios y terracería con calidad buena. Así mismo se evidencia lo conveniente de ejercer intervenciones oportunas para garantizar la vida útil del pavimento con el consiguiente menor costo descontado de mantenimiento y menores costos de operación

Numerical and experimental investigation of the acoustic black hole effect for vibration damping in beams and elliptical plates

Flexural waves in beams and plates slow down if their thickness decreases. Such property was successfully used for establishing the theory of acoustic black holes (ABH). In fact, in the case of a sharpened edge having a power-law profile, it can be shown that the refection coefficient of a wave propagating towards the sharpened edge can be equal to zero. However, manufacturing such profiles is always related to truncations and imperfections that undermine ABH. It is known though that the use of a thin absorbing film drastically improves the damping effect of ABH. The aim of the current paper is to show numerically and experimentally the capability of ABH to provide structural damping without introducing additional mass. The dynamic behaviour of a non uniform Euler-Bernoulli beam is described using a Riccati equation for the beam impedance, which leads to the reflection matrix of the sharpened edge of the beam. The influence of length of the profile, thickness and length of the absorbing film are evaluated as realistically as possible and optimised numerically in order to reduce wave reflection from the edge. Keeping in mind the numerical results, an elliptic plate with a pit of power law profile placed at one of its focuses has been designed and tested. As a result, both numerical simulations and experimental measurements show significant reduction of vibration levels

Perpetration of intimate partner violence and mental health outcomes: sex- and gender-disaggregated associations among adolescents and young adults in Nigeria.

BACKGROUND: The association between intimate partner violence (IPV) victimisation and poor mental health outcomes is well established. Less is known about the correlation between IPV perpetration and mental health, particularly among adolescents and young adults. Using data from the nationally representative Violence Against Children Survey, this analysis examines the association between IPV perpetration and mental health for male and female adolescents and young adults in Nigeria. METHODS: Multivariate logistic regression models were used to examine associations between ever-perpetration of IPV and four self-reported mental health variables: severe sadness, feelings of worthlessness, suicide ideation, and alcohol use. Models were sex-disaggregated, controlled for age, marital status, and schooling, and tested with and without past exposure to violence. Standard errors were adjusted for sampling stratification and clustering. Observations were weighted to be representative of 13-24 year-olds in Nigeria. RESULTS: Males were nearly twice as likely as females to perpetrate IPV (9% v. 5%, respectively; P < 0.001), while odds of perpetration for both sexes were higher for those ever experiencing IPV (adjusted odds ratio (aOR) = 4.60 for males; aOR = 2.71 for females). Female perpetrators had 2.73 higher odds of reporting severe sadness (95% confidence interval CI = 1.44, 5.17; P = 0.002) and 2.72 times greater odds of reporting suicide ideation (1.28, 5.79; P = 0.010) than non-perpetrating females, even when controlling for past-year violence victimisation. In contrast, male perpetrators had 2.65 times greater odds of feeling worthless (1.09, 6.43; P = 0.031), and 2.36 times greater odds of reporting alcohol use in the last 30 days (1.50, 3.73; P < 0.001), as compared to non-perpetrating males. CONCLUSIONS: Among adolescents and young adults in Nigeria, IPV perpetration and negative mental health outcomes are associated but differ for males and females. Mindful of the cross-sectional nature of the data, it is possible that socially determined gender norms may shape the ways in which distress from IPV perpetration is understood and expressed. Additional research is needed to clarify these associations and inform violence prevention efforts