314 research outputs found

    Use of Ramp Tests to Obtain Inverse Neural Network Process Models

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    Ionospheric effects on synthetic aperture radar (SAR) clutter statistics

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    Hydroclimate variability was the main control on fire activity in northern Africa over the last 50,000 years

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    North Africa features some of the most frequently burnt biomes on Earth, including the semi-arid grasslands of the Sahel and wetter savannas immediately to the south. Natural fires are fuelled by rapid biomass production during the wet season, its desiccation during the dry season and ignition by frequent dry lightning strikes. Today, fire activity decreases markedly both to the north of the Sahel, where rainfall is extremely low, almost eliminating biomass over the Sahara, and to the south where forest biomes are too wet to burn. Over the last glacial cycle, rainfall and vegetation cover over northern Africa varied dramatically in response to gradual astronomically-forced insolation change, changes in atmospheric carbon dioxide levels, and abrupt cooling events over the North Atlantic Ocean associated with the reorganisation of Meridional Overturning Circulation (MOC). Here we report the results of a study into the impact of these climate changes on fire activity in northern African over the last 50,000 years (50 kyr). Our reconstructions come from marine sediments with strong age control that provide an uninterrupted record of charcoal particles exported from the African continent. We studied three sites on a latitudinal transect along the northwest African margin between 21 and 9°N. Our sites exhibit a distinct latitudinal relationship between past changes in rainfall and fire activity. At the southernmost site (GeoB9528-3, 9°N), fire activity decreased during intervals of increasing humidity, while our northernmost site (ODP Site 658, 21°N) clearly demonstrates the opposite relationship. The site in the middle of our transect, offshore of the present day southern Sahel today (GeoB9508-5, 15°N), exhibits a “Goldilocks” relationship between fire activity and hydroclimate, wherein charcoal fluxes peak under intermediate rainfall climate conditions and are supressed by transition to more arid or more humid conditions. Our results are remarkably consistent with the predictions of the intermediate fire-productivity hypothesis developed in conceptual macroecological models and supported by empirical evidence of modern day fire activity. Feedback processes operating between fire, climate and vegetation are undoubtedly complex but temperature is suggested to be the main driver of temporal change in fire activity globally, with the precipitation-evaporation balance perhaps a secondary influence in the Holocene tropics. However, there is only sparse coverage of Africa in the composite records upon which those interpretations are based. We conclude that hydroclimate (not temperature) exerted the dominant control on burning in the tropics of northern Africa well before the Holocene (from at least 50 ka).publishedVersio

    Original biographies from the Dictionary of African Christian Biography

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    A publication of the Dictionary of African Christian Biography with U.S. offices located at the Center for Global Christianity and Mission at Boston University. The editors are pleased to offer the first annual cumulative volume of the Journal of African Christian Biography, the monthly scholarly publication that was launched in June of 2016. Since then, the life stories of twelve individuals who played vital roles in and through their faith communities have been published online as free downloads. But it is important that a selection of DACB stories be more readily available to those without access to the internet. As I mentioned in the fall 2016 newsletter of the DACB, each issue of the journal is available in its e-journal, on-line version, where it is configured either as A4 or 8.5 x 11 format printable as booklets, ready for local printing and binding or stapling. Our intention is to make it easy for academics and church leaders in various parts of Africa to make print copies of the journal available to their students, colleagues or church members. And so it is with this cumulative volume.This issue focuses on: 1. "Walatta Petros and Hakalla Amale, Pious Women of Ethiopia," with commentary by Dr. Jonathan Bonk, Project Director. 2. Walatta Petros. 3. Hakalla Amale. 4. Bishop Josiah Kibira of Tanzania, Ecumenical Statesman. 5. Josiah Mutabuzi Isaya Kibira. 6. Josiah Kibira. 7. David Lonkibiri Windibiziri. 8. Abiodun Babatunde Lawrence. 9. Dominic Ignatius Ekandem. 10. William Wadé Harris, Prophet-Evangelist of West Africa: His Life, Message, Praxis, Heritage, and Legacy. 11. William Wadé Harris. 12. Michael Timneng and Jeremiah Chi Kangsen: Christianity Beyond the Missionary Presence in Cameroon. 13. Michael Timneng. 14. Jeremiah Chi Kangsen. 15. Rainisoalambo, Ravelonjanahary, and Volahavana Germaine (Nenilava): Revival Leaders of Madagascar. 16. Rainisoalambo. 17. Ravelonjanahary. 18. Volahavana Germaine (Nenilava). 19. Recent Print and Digital Resourcews Related to Christianity in Africa

    Hardware Design Improvements to the Major Constituent Analyzer

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    The Major Constituent Analyzer (MCA) onboard the International Space Station (ISS) is designed to monitor the major constituents of the ISS's internal atmosphere. This mass spectrometer based system is an integral part of the Environmental Control and Life Support System (ECLSS) and is a primary tool for the management of ISS atmosphere composition. As a part of NASA Change Request CR10773A, several alterations to the hardware have been made to accommodate improved MCA logistics. First, the ORU 08 verification gas assembly has been modified to allow the verification gas cylinder to be installed on orbit. The verification gas is an essential MCA consumable that requires periodic replenishment. Designing the cylinder for subassembly transport reduces the size and weight of the maintained item for launch. The redesign of the ORU 08 assembly includes a redesigned housing, cylinder mounting apparatus, and pneumatic connection. The second hardware change is a redesigned wiring harness for the ORU 02 analyzer. The ORU 02 electrical connector interface was damaged in a previous on-orbit installation, and this necessitated the development of a temporary fix while a more permanent solution was developed. The new wiring harness design includes flexible cable as well as indexing fasteners and guide-pins, and provides better accessibility during the on-orbit maintenance operation. This presentation will describe the hardware improvements being implemented for MCA as well as the expected improvement to logistics and maintenance

    Contamination and Surface Preparation Effects on Composite Bonding

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    Results presented here demonstrate the effect of several prebond surface contaminants (hydrocarbon, machining fluid, latex, silicone, peel ply residue, release film) on bond quality, as measured by fracture toughness and failure modes of carbon fiber reinforced epoxy substrates bonded in secondary and co-bond configurations with paste and film adhesives. Additionally, the capability of various prebond surface property measurement tools to detect contaminants and potentially predict subsequent bond performance of three different adhesives is also shown. Surface measurement methods included water contact angle, Dyne solution wettability, optically stimulated electron emission spectroscopy, surface free energy, inverse gas chromatography, and Fourier transform infrared spectroscopy with chemometrics analysis. Information will also be provided on the effectiveness of mechanical and energetic surface treatments to recover a bondable surface after contamination. The benefits and drawbacks of the various surface analysis tools to detect contaminants and evaluate prebond surfaces after surface treatment were assessed as well as their ability to correlate to bond performance. Surface analysis tools were also evaluated for their potential use as in-line quality control of adhesive bonding parameters in the manufacturing environment

    Utilizing energy transfer in binary and ternary bulk heterojunction organic solar cells

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    Energy transfer has been identified as an important process in ternary organic solar cells. Here, we develop kinetic Monte Carlo (KMC) models to assess the impact of energy transfer in ternary and binary bulk heterojunction systems. We used fluorescence and absorption spectroscopy to determine the energy disorder and Förster radii for poly(3-hexylthiophene-2,5-diyl), [6,6]-phenyl-C61-butyric acid methyl ester, 4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (DIBSq), and poly(2,5-thiophene-<i>alt</i>-4,9-bis(2-hexyldecyl)-4,9-dihydrodithieno[3,2-c:3′,2′-<i>h</i>][1,5]naphthyridine-5,10-dione). Heterogeneous energy transfer is found to be crucial in the exciton dissociation process of both binary and ternary organic semiconductor systems. Circumstances favoring energy transfer across interfaces allow relaxation of the electronic energy level requirements, meaning that a cascade structure is not required for efficient ternary organic solar cells. We explain how energy transfer can be exploited to eliminate additional energy losses in ternary bulk heterojunction solar cells, thus increasing their open-circuit voltage without loss in short-circuit current. In particular, we show that it is important that the DIBSq is located at the electron donor–acceptor interface; otherwise charge carriers will be trapped in the DIBSq domain or excitons in the DIBSq domains will not be able to dissociate efficiently at an interface. KMC modeling shows that only small amounts of DIBSq (<5% by weight) are needed to achieve substantial performance improvements due to long-range energy transfer

    Growth modification of seeded calcite using carboxylic acids: Atomistic simulations

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    Molecular dynamics simulations were used to investigate possible explanations for experimentallyobserved differences in the growth modification of calcite particles by two organic additives, polyacrylicacid (PAA) and polyaspartic acid (p-ASP). The more rigid backbone of p-ASP was found to inhibit the formationof stable complexes with counter-ions in solution, resulting in a higher availability of p-ASP comparedto PAA for surface adsorption. Furthermore the presence of nitrogen on the p-ASP backbone yieldsfavorable electrostatic interactions with the surface, resulting in negative adsorption energies, in anupright (brush conformation). This leads to a more rapid binding and longer residence times at calcitesurfaces compared to PAA, which adsorbed in a flat (pancake) configuration with positive adsorptionenergies. The PAA adsorption occurring despite this positive energy difference can be attributed to thedisruption of the ordered water layer seen in the simulations and hence a significant entropic contribution to the adsorption free energy. These findings help explain the stronger inhibiting effect on calcite growth observed by p-ASP compared to PAA and can be used as guidelines in the design of additives leading to even more marked growth modifying effects
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