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

Factors moderating the risk of PTSD, emotional and behavioral problems amongst children in war zones and refugees escaping from warfare

Children who grow up in war zones are typically exposed to multiple stressors including physical harm, intimidation or other forms of psychological trauma. This can also lead to Post Traumatic Stress Disorder. On the other hand, resilient children show no psychiatric distress even when they are exposed to severe traumatic stress. Additionally, the number of refugee children due to warfare reasons is increasing. Past empirical studies have recognized that the process of migration and living life as a refugee is detrimental to the psychological health of young refugees. In this symposium we will examine the prevalence and determinants of resiliency among refugee children and children living in conditions of war and violence. The first study investigated the psychological, social and somatic effects of chronic traumatic experience on Palestinian children over six years (2000-2006). The sample consisted of 1,137 children who completed: Checklist of Traumatic Experiences, Symptoms of PTSD Scale, Network of Psycho-Social Support and Personality Assessment Questionnaire. It was found that 41% of the participants suffered from PTSD. From these 25% suffered from cognitive symptoms; 22% suffered from emotional symptoms; 22% suffered from social behavioral problems; 17% suffered from academic and 14% suffered from somatic symptoms. The support of family, friends, relatives and teachers, and positive personality traits were found to be strong protective factors aiding recovery from trauma and PTSD. The second study evaluated the relation of exposure to war traumas, and violence in the family, community, and school, to PTSD symptoms, emotional and behavioral problems amongst 330 Palestinian children. Results highlight the additive effects of exposure to war traumas and violence in different settings. In addition, it was found that psychosocial support reduced the effects of environmental factors in developing PTSD and behavioral problems. The third study included data from two refugee charity organizations in the UK. There were 200 refugee children coming from war zones and 210 control children (non-refugees). The study aimed to look at a range of factors to assess the differences between the above groups with regards to their well-being and peer and sibling relationships. Results showed that refugee children were significantly more likely to be in the clinical range for total difficulties and to have higher health and physical problems, negative friendship quality and low self esteem compared to the control group. Refugees who were bullied at home and at school were also more likely to develop PTSD symptoms. Protective factors are also discussed in this study. The above studies emphasize the fact that interventionists should consider the full range of sources of environmental risk for PTSD and emotional and behavioral problems and should strengthen the psychosocial support for children in or coming from war zones

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

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

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

Tailorable, visible light emission from silicon nanocrystals

J. P. Wilcoxon and G. A. Samara Crystalline, size-selected Si nanocrystals in the size range 1.8-10 nm grown in inverse micellar cages exhibit highly structured optical absorption and photoluminescence (PL) across the visible range of the spectrum. The most intense PL for the smallest nanocrystals produced This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. to induce a useful level of visible photoluminescence (PL) from silicon (Si). The approaches understood. Visible PL has been observed from Si nanocrystals, or quantum dots, produced by a variety of techniques including aerosols,2 colloids,3 and ion implantation.4 However, all of The optical absorption spectra of our nanocrystals are much richer in spectral features spectrum of bulk Si where the spectral features reflect the details of the band structure shown in nanocrystals estimated to have a Si core diameter of 1-2 nm. These measured quantum those in the spectrum of bulk Si in Fig. 1 are striking indicating that nanocrystals of this size 8-Room temperature PL results on an HPLC size-selected, purified 2 nm nanocrystals but blue shifted by -0.4 eV due to quantum confinement. Excitation at 245 nm yields the PL shows the PL spectrum for a similar sample excited at 490 nm (2.53 eV) trapped excitons at the surface of Si nanocrystals. The excitons are obtained for dimer bonds 1.8- 10 nm. These nanocrystals retain bulk-like optical absorption and an indirect bandgap Figure 1. The absorption spectrum of d = 2 nm Si nanocrystals compared to that of bulk7 Si. Figure 2. The extinction and PL (excitation at 490 nm) spectra ford= 8-10 nm Si nanocrystals

Hybrid Full-Duplex and Alternate Multiple Relay Selection and Beamforming in AF Cooperative Networks

In this paper, multiple relay selection and beamforming techniques are applied to a dual-hop full-duplex (FD) amplify-and-forward relaying network. We show that our proposed techniques allow the selection to be adaptive to the residual self-interference (SI) level for each of the available relays in the network. The adaptivity of our selection schemes is manifested through a hybrid system that is based on FD relaying and switching based on the overall channel conditions and the statistics of the residual SI channel for each of the relays. In particular, different proposed techniques are shown to be able to adaptively decide on when and how often the used relays should be switched in the case of overwhelming residual SI. Our results show that allowing such a fusion considerably improves the overall performance of the considered relaying scheme in terms of bit error rate compared with state-of-the-art relay selection schemes.This work was supported by the Qatar National Research Fund (a member of the Qatar Foundation) through GSRA under Grant #2-1-0601-14011.Scopu

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