Integrated photovoltaic-thermal solar energy conversion systems

A combined photovoltaic/thermal collector has been built and is now being tested. Initial tests have concentrated on evaluating the thermal efficiency of the collector before and after the silicon cells are mounted. With likely improvements in bonding between cells and receiver and in the absorptivity of the cells, thermal efficiencies greater than 50% can be expected for the combined receiver operating at 100 C

Evaluation of on-board hydrogen storage methods for hypersonic vehicles

Hydrogen is the foremost candidate as a fuel for use in high speed transport. Since any aircraft moving at hypersonic speeds must have a very slender body, means of decreasing the storage volume requirements below that for liquid hydrogen are needed. The total performance of the hypersonic plane needs to be considered for the evaluation of candidate fuel and storage systems. To accomplish this, a simple model for the performance of a hypersonic plane is presented. To allow for the use of different engines and fuels during different phases of flight, the total trajectory is divided into three phases: subsonic-supersonic, hypersonic and rocket propulsion phase. The fuel fraction for the first phase is found be a simple energy balance using an average thrust to drag ratio for this phase. The hypersonic flight phase is investigated in more detail by taking small altitude increments. This approach allowed the use of flight profiles other than the constant dynamic pressure flight. The effect of fuel volume on drag, structural mass and tankage mass was introduced through simplified equations involving the characteristic dimension of the plane. The propellant requirement for the last phase is found by employing the basic rocket equations. The candidate fuel systems such as the cryogenic fuel combinations and solid and liquid endothermic hydrogen generators are first screened thermodynamically with respect to their energy densities and cooling capacities and then evaluated using the above model

A combined theoretical and experimental study of the low temperature properties of BaZrO3

Low temperature properties of BaZrO3 are revealed by combining experimental techniques (X-ray diffraction, neutron scattering and dielectric measurements) with theoretical first-principles-based methods (total energy and linear response calculations within density functional theory, and effective Hamiltonian approaches incorporating/neglecting zero-point phonon vibrations). Unlike most of the perovskite systems, BaZrO3 does not undergo any (long-range-order) structural phase transition and thus remains cubic and paraelectric down to 2 K, even when neglecting zero-point phonon vibrations. On the other hand, these latter pure quantum effects lead to a negligible thermal dependency of the cubic lattice parameter below ~ 40 K. They also affect the dielectricity of BaZrO3 by inducing an overall saturation of the real part of the dielectric response, for temperatures below ~ 40 K. Two fine structures in the real part, as well as in the imaginary part, of dielectric response are further observed around 50-65 K and 15 K, respectively. Microscopic origins (e.g., unavoidable defects and oxygen octahedra rotation occurring at a local scale) of such anomalies are suggested. Finally, possible reasons for the facts that some of these dielectric anomalies have not been previously reported in the better studied KTaO3 and SrTiO3 incipient ferroelectrics are also discussed.Comment: 8 pages, 5 figures, submitted to Physical Review

Random local strain effects in homovalent-substituted relaxor ferroelectrics: a first-principles study of BaTi0.74Zr0.26O3

We present first-principles supercell calculations on BaTi0.74Zr0.26O3, a prototype material for relaxors with a homovalent substitution. From a statistical analysis of relaxed structures, we give evidence for four types of Ti-atom polar displacements: along the , , or directions of the cubic unit cell, or almost cancelled. The type of a Ti displacement is entirely determined by the Ti/Zr distribution in the adjacent unit cells. The underlying mechanism involves local strain effects that ensue from the difference in size between the Ti4+ and Zr4+ cations. These results shed light on the structural mechanisms that lead to disordered Ti displacements in BaTi(1-x)Zr(x)O3 relaxors, and probably in other BaTiO3-based relaxors with homovalent substitution.Comment: 5 pages, 4 figure

Two-year sealant survival in a high caries cohort at a graduate pedodontic clinic

Objectives: To assess the efficacy of resin sealants in children aged 6-12 years referred to one graduate pedodontic program located in the Middle East (2009-2010). Study Design: Data from a cohort of 110 patients with at least 2 erupted first permanent molars were included in this study. Dental health status was evaluated using the DMFs/t index. UltraSeal XT® Plus sealants (n=253 applications) were performed in presence of rubber dam isolation and survival carefully inspected and scored at 1 and 2 years. Cross-sectional examination of descriptives and bi/multivariate analysis followed. Results: Majority (63%) were males. Caries were diagnosed in 77.9% of children with only 1% deemed caries-free. Hence, mean DMFs/t was 9.3±6.1/7.2±5.1, respectively. At 2 years, 66% of sealants were completely retained and 11% completely lost. Analysis revealed higher sealant survival in mandibular teeth. Conclusions: Fluoride-releasing resin sealant is an effectual preventive tool especially in such a high carious population

Results of the post flash-flood disaster investigations in the Transylvanian Depression (Romania) during the last decade (2001–2010)

Flash-flood disasters are very rare in the Transylvanian Depression. In the last decades just three events were signalled in the study area, all of them during the last 10 years. The flash floods occurring in the study area during the last decade had a significant impact on several localities situated at the Transylvanian Depression border. Based on the post flash-flood investigation, the present study intends to find out the main characteristics of the flash floods and the causes that have led to disasters in a region rarely affected by such kinds of events. Analyzing the hydrological data, it has been seen that the maximum intensity of the flash floods was observed in the upper and middle basins. By comparing the unit peak discharges from the studied region with other specific peak discharges related to the significant flash floods from Romania, it was noticed that the events from the Transylvanian Depression have moderate to low intensity. On the other hand, the results showed that besides high stream power and unexpected character common to flash floods, the inappropriate flood risk management measures increased the dimension of the negative effects, leading to tens of lives lost and economical damages of tens of millions of dollars

First principles based atomistic modeling of phase stability in PMN-xPT

We have performed molecular dynamics simulations using a shell model potential developed by fitting first principles results to describe the behavior of the relaxor-ferroelectric (1-x)PbMg1/3Nb2/3O3-xPbTiO3 (PMN-xPT) as function of concentration and temperature, using site occupancies within the random site model. In our simulations, PMN is cubic at all temperatures and behaves as a polar glass. As a small amount of Ti is added, a weak polar state develops, but structural disorder dominates, and the symmetry is rhombohedral. As more Ti is added the ground state is clearly polar and the system is ferroelectric, but with easy rotation of the polarization direction. In the high Ti content region, the solid solution adopts ferroelectric behavior similar to PT, with tetragonal symmetry. The ground state sequence with increasing Ti content is R-MB-O-MC-T. The high temperature phase is cubic at all compositions. Our simulations give the slope of the morphotropic phase boundaries, crucial for high temperature applications. We find that the phase diagram PMN-xPT can be understood within the random site model.Comment: 27 pages, 9 figure

Impeachment of the Paraguayan President: Discursive Event and Cartoons

Indexación: Revista UNABEl presente artículo se ocupa de analizar caricaturas referidas al evento político “impeachment paraguayo”, que sucedió en el año 2012 en Paraguay. Varias noticias fueron transmitidas en diferentes blogs y sitios de internet, pero nos detuvimos a analizar tan solo las caricaturas humorísticas. Estas fueron seleccionadas por presentar características peculiares al género y, principalmente, por expresar dichos acerca del acontecimiento político. Para los análisis fueron utilizados conocimientos teóricos del Análisis del Discurso de línea francesa y fundamentación teórica de los estudios semióticos. Tenemos por objetivo presentar una forma de lectura que logre deconstruir los discursos estereotipados y artificiales que fueron naturalizados a lo largo del tiempo en la sociedad.This paper is concerned with analyzing political cartoons about the “Paraguayan impeachment” event, which took place in Paraguay in 2012. Many different news were diffused on blogs and websites, but we limited ourselves here to analyze only the humoristic cartoons. These ones were chosen because they have specific characteristics of this genre and they express specific sayings about that political event. For that, we searched theoretical knowledge for the analysis at the French Discourse theory of semiotics. We aim to present a way of reading which may deconstruct stereotypes and artificial discourses that somehow have been accepted naturally in society.http://revistahumanidades.unab.cl/wp-content/uploads/2015/01/Art%C3%ADculo-IMPEACHMENT-Carolina-Samara.pd