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Search Results: 1 - 10 of 39113 matches for " Chris Van Hoof "
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Multilayer Inorganic Electrets with and Layers for Applications on Heated Machinery
Vladimir Leonov,Chris Van Hoof
Smart Materials Research , 2012, DOI: 10.1155/2012/904168
Abstract: The physics and basic properties of electrets are discussed, namely, what happens during corona charging of dielectrics, why the surface potential and trapped charge show certain limits, where the trapped charge is stored, why and how the charge is released from traps at high temperatures. The experiments have been conducted on single-layer SiO2 and Si3N4 and on multilayer combination of these materials. A strong lateral mobility of charge trapped near the SiO2/Si3N4 interface was observed at elevated temperatures. The positively and negatively charged electrets are compared to each other. The experiments on charge retention at elevated temperatures have shown the studied electrets are suitable for devices working at temperatures of up to 200–300°C. 1. Introduction Electrets could serve as a quasipermanent source of polarization in emerging applications like electrostatic energy harvesters [1]. This would allow autonomous devices powered by energy harvesters in applications, where temperatures of up to 200–300°C are observed and, thus, batteries cannot be used as a source of power. Electrets also used, or can be used, in other applications, for example, sensors, transducers, and electrostatic microbearings and micromotors [2, 3]. The inorganic electrets composed of at least one SiO2 layer and one Si3N4 layer show superior charge retention at elevated temperatures. However, the physics of electrets is not understood well enough at this moment. For example, attempts to pattern them result in instability of trapped charge at small feature size of a pattern [3]. It is obvious that better understanding of the processes occurring during and after electret charging, influence of atmospheric ions and fringing field [4], effects of temperature [4–7], and humidity and relevant surface conduction [3, 8, 9] on charge retention is necessary. It would allow designing better microdevices with electrets suitable for the market with its requirements for the long lifetime of electrets (and therefore the devices), small feature size of photolithographic pattern, and the extended temperature range in applications. This research is primarily targeted at development of electrostatic energy harvesters. They must be able to work for at least 10 years in applications with temperatures of 150–200°C and survive occasional short overheating to about 300°C. Therefore, organic electrets that discharge at much lower temperatures are not suitable for the application. The single-layer electrets dramatically discharge at 300°C either [4, 10]. Therefore, the only remaining option in this
Batalin-Vilkovisky gauge-fixing of a chiral two-form in six dimensions
Chris Van Den Broeck,Kor Van Hoof
Physics , 1999, DOI: 10.1088/0264-9381/16/12/318
Abstract: We perform the gauge-fixing of the theory of a chiral two-form boson in six dimensions starting from the action given by Pasti, Sorokin and Tonin. We use the Batalin-Vilkovisky formalism, introducing antifields and writing down an extended action satisfying the classical master equation. Then we gauge-fix the three local symmetries of the extended action in two different ways.
An action for the (2,0) self-dual tensor multiplet in a conformal supergravity background
Kor Van Hoof
Physics , 1999, DOI: 10.1088/0264-9381/17/10/304
Abstract: We present the action for a self-dual tensor in six dimensions, coupled to a (2,0) conformal supergravity background. This action gives rise to the expected equations of motion. An alternative look upon one of the gauge symmetries clarifies its role in the supersymmetry transformation rules and the realisation of the algebra.
Measuring angular diameters of extended sources
P. A. M. van Hoof
Physics , 1999, DOI: 10.1046/j.1365-8711.2000.03281.x
Abstract: When measuring diameters of partially resolved sources often a technique called gaussian deconvolution is used. This technique yields a gaussian diameter which subsequently has to be multiplied with a conversion factor to obtain the true angular diameter of the source. This conversion factor is a function of the FWHM of the beam or point spread function and also depends on the intrinsic surface brightness distribution of the source. In this paper conversion factors are presented for a number of simple geometries: a circular constant surface brightness disk and a spherical constant emissivity shell, using a range of values for the inner radius. Also more realistic geometries are studied, based on a spherically symmetric photo-ionization model of a planetary nebula. This enables a study of optical depth effects, a comparison between images in various emission lines and the use of power law density distributions. It is found that the conversion factor depends quite critically on the intrinsic surface brightness distribution, which is usually unknown. The uncertainty is particularly large if extended regions of low surface brightness are present in the nebula. In such cases the use of gaussian or second moment deconvolution is not recommended. As an alternative, a new algorithm is presented which allows the determination of the intrinsic FWHM of the source using only the observed surface brightness distribution and the FWHM of the beam. Tests show that this implicit deconvolution method works well in realistic conditions, even when the signal-to-noise is low, provided that the beam size is less than roughly 2/3 of the observed FWHM and the beam profile can be approximated by a gaussian.
Measuring angular diameters of extended sources - I. Theory
P. A. M. van Hoof
Physics , 1999,
Abstract: When measuring diameters of partially resolved sources like planetary nebulae, H II regions or galaxies, often a technique called gaussian deconvolution is used. This technique yields a gaussian diameter which subsequently has to be multiplied with a conversion factor to obtain the true angular diameter of the source. This conversion factor is a function of the Full Width at Half Maximum (FWHM) of the beam. It also depends on the intrinsic surface brightness distribution of the source. In this paper the theory behind this technique will be studied. This study will be restricted to circularly symmetric geometries and beams. First an implicit equation will be derived, from which the conversion factor for a given surface brightness distribution and beam size can be solved. Explicit expressions for the conversion factor will be derived from this equation which are valid in cases where the beam size is larger than the intrinsic size of the source. A more detailed discussion will be given for two simple geometries: a circular constant surface brightness disk and a spherical constant emissivity shell with arbitrary inner radius. The theory is subsequently used to construct a new technique for determining the FWHM of an arbitrary observed surface brightness distribution. Usually the FWHM of the source and beam are measured using gaussian fits, but second moments can also be used. The alternative use of second moments in this context is studied here for the first time and it is found that in this case the conversion factor has a different value which is independent of the beam size. In the limit for infinitely large beam sizes, the values of the conversion factors for both techniques are equal.
Cold performance tests of blocked-impurity-band Si:As detectors developed for DARWIN
Stephan M. Birkmann,Jutta Stegmaier,Ulrich Groezinger,Oliver Krause,Tim Souverijns,Jan Putzeys,Deniz Sabuncuoglu Tezcan,Koen De Munck,Paolo Fiorini,Kiki Minoglou,Patrick Merken,Chris Van Hoof,Piet De Moor
Physics , 2008, DOI: 10.1117/12.789103
Abstract: We report first results of laboratory tests of Si:As blocked-impurity-band (BIB) mid-infrared (4 to 28 um) detectors developed by IMEC. These prototypes feature 88 pixels hybridized on an integrated cryogenic readout electronics (CRE). They were developed as part of a technology demonstration program for the future DARWIN mission. In order to be able to separate detector and readout effects, a custom build TIA circuitry was used to characterize additional single pixel detectors. We used a newly designed test setup at the MPIA to determine the relative spectral response, the quantum efficiency, and the dark current. All these properties were measured as a function of operating temperature and detector bias. In addition the effects of ionizing radiation on the detector were studied. For determining the relative spectral response we used a dual-grating monochromator and a bolometer with known response that was operated in parallel to the Si:As detectors. The quantum efficiency was measured by using a custom-build high-precision vacuum black body together with cold (T ~ 4 K) filters of known (measured) transmission.
Making and breaking the message
David A Mangus, Ambro van Hoof
Genome Biology , 2003, DOI: 10.1186/gb-2003-4-11-346
Abstract: Amid the Cold Spring Harbor Laboratory celebrations marking the 50th anniversary of the elucidation of the structure of DNA, researchers gathered to discuss recent advances in understanding the mechanisms regulating gene expression. The formation of a functional mRNA from the initial pre-mRNA synthesized by RNA polymerase II requires the addition of a cap and poly(A) tail as well as removal of introns. These individual reactions have traditionally been studied in isolation, leading to a perception that they are unconnected. It has, however, become clear that the events that transform a pre-mRNA into an mRNA are often carried out by large ribonucleoprotein (RNP) complexes and that these reactions are coupled and interdependent. Interestingly, these events and the links between them provide quality-control checkpoints to ensure that the message encoded in the genome is accurately communicated to the translation apparatus. These underlying concepts framed the presentations we describe below.The removal of introns from a pre-mRNA is catalyzed by a large set of RNAs and proteins collectively termed the spliceosome. Exciting advances have been made in the visualization of the spliceosome complexes within the last year. Cryoelectron micrographs presented by Melissa Jurica from the laboratory of Melissa Moore (Brandeis University, Waltham, USA), Cindy Will from Reinhard Lührmann's group (Max Planck Institute for Biophysical Chemistry, G?ttingen, Germany), and Melanie Ohi from the laboratory of Thomas Walz (Harvard Medical School, Boston, USA) all indicate that the active spliceosome (C complex) has three domains of density surrounding a central cavity. The spliceosome structures are roughly 270 × 250 × 240 ? in size, with resolutions ranging between 20 ? and 40 ?. The current challenges are to achieve greater resolution and to locate important proteins and RNAs within the structure. We expect that the generation of high-resolution structures of the spliceosome will be as re
La evolución y el futuro de la producción más limpia en Colombia
van Hoof,Bart; Herrera,Carlos Manuel;
Revista de Ingeniería , 2007,
Abstract: in 1997 the ministry of environmental affairs of colombia adapted its national cleaner production policy as a complementary environmental strategy to introduce prevention oriented approach into management practices of industry. since then a range of different initiatives have been developed, all focused on the improvement of the environmental performance of companies. the results of the demonstration projects, voluntary agreements, capacity buildings programs among others, sustain the potential of this strategy to combine the reduction of environmental contamination with cost reductions and competitiveness improvement. now, ten years after its expedition, cleaner production (cp) is still an emergent strategy for environmental management and industrial development in colombian, even when the context of the time period and the main drivers, have changed. when in the early year?s incentives to participate and contribute to cp initiatives were closely related to the new developed environmental regulation in colombia and responses of industry towards those. now, ten years later, the globalization of the economy and the integration of environmental variables in competitiveness make cp an important strategy to assume corporate social responsibility and respond to market and trade requirements. taking into account this importance, this article proposes new ideas and strategies for the renovation of the national cleaner production policy in colombia. the proposed ideas and strategies are based on a: (i) historical review of the different cp initiatives in colombia, (ii) the experience of the authors who have participated in a broad range of cp initiatives during the last ten years, and (iii) a literature review of cp initiatives and developments mainly published in the journal of cleaner production. the conclusions of this paper show how the cleaner production policy in colombia has to develop towards an innovative business development strategy, changing its early emphasis o
Adolescent Alcohol Consumption in Romania: A Blueprint for Measuring Alcohol (mis)Use
Joris J. van HOOF,Marit MOLL
Revista de Cercetare ?i Interven?ie Social? , 2012,
Abstract: In order to address the issues of adolescent alcohol (mis)use in Romanian cities and to develop local alcohol prevention policies comprised of interventions aimed at reducing alcohol consumption and alcohol related problems, information on the prevalence of alcohol use and relevant related topics is needed. This approach was pre-tested in the city of Piteoti, (within the first Romanian local alcohol policy project; DRAIN), but can be used in other Romanian cities or research projects as well. For this study we have constructed a compact questionnaire and easy to execute data collection method, that can be used and analysed by (for example) local universities in collaboration with their local council. We selected relevant questions from the international, and validated, ESPAD questionnaire 2003 and added specific Romanian and local topics. A stratified sample of 1,500 students was drawn, 50 classes in total and 3 classes per school (assuming that each class contains about 30 students), using type of school, grade and path as stratifyers. A total of 1,026 questionnaires were used for data analysis. Piteoti’s youth generally starts drinking at a young age and many of them are currently regular drinkers. This pattern is worrisome, since early onset of drinking and regular drinking as a teenager increases the chances of developing health risks and an addiction to alcohol later in life. This study supports the theory that peers influence teen drinking and also shows that the norms of parents are (too) liberal. Compared to Romanian youth, Pite[ti’s youth tends to drink a bit more. Parents should set the norm of ‘no alcohol until the (legal) age of 18’ for their children. Also no alcohol should be sold to or bought for adolescents under the age of 18 and no alcohol outlets should exist within 200 metres of high schools, as Romanian alcohol legislation stipulates. Within the alcohol prevention project DRAIN, the results of this questionnaire were used to develop ‘tailor-made’ alcohol legislation.
La evolución y el futuro de la producción más limpia en Colombia
Bart Van Hoof,Carlos Manuel Herrera
Revista de Ingeniería , 2007,
Abstract: En 1997, el Ministerio del Medio Ambiente de Colombia adoptó la Política Nacional de Producción más Limpia como una estrategia complementaria a la normatividad ambiental, para impulsar la nueva institucionalidad ambiental en el país. Desde entonces, diferentes iniciativas han sido desarrolladas por empresas, autoridades ambientales y universidades, entre ellas, los convenios de producción más limpias, programas demostrativos de asistencia técnica para la implementación de alternativas preventivas, centros de producción más limpia, guías ambientales, programas de autogestión ambiental. Los resultados de estas iniciativas muestran la efectividad de la estrategia de Producción más Limpia para mejorar el desempe o ambiental de las empresas y mejorar la competitividad empresarial a través la reducción de costos. Hoy diez a os después su expedición, la Producción más Limpia (PML) todavía es la estrategia emergente para afrentar los retos ambientales nacionales e internacionales en la industria. Los desarrollos relacionados con la globalización y la integración de la variable ambiental como un elemento central de la competitividad, hacen evidente la importancia del enfoque preventivo para asegurar un manejo responsable y competitivo de empresas ante los crecientes y variados requerimientos. Así, el presente artículo plantea orientaciones para dar un nuevo impulso a la Política Nacional de Producción más Limpia. Los planteamientos desarrollados, parten de un análisis del contexto actual de las tendencias ambientales nacionales e internacionales, un análisis de la evolución de las iniciativas de PML en Colombia y un análisis de avances de PML en la literatura internacional. Las conclusiones de este artículo muestran cómo la política de Producción más Limpia en Colombia debe evolucionar, desde un enfoque para la optimización y mejoramiento de procesos empresariales existentes hacia una estrategia de desarrollo empresarial que promueva la innovación tecnológica y la sostenibilidad de los negocios. La integración de la Estrategia Nacional de Mercados Verdes con la Política Nacional de Producción más Limpia es una de las recomendaciones planteadas para avanzar./ In 1997 the Ministry of Environmental Affairs of Colombia adapted its National Cleaner Production Policy as a complementary environmental strategy to introduce prevention oriented approach into management practices of industry. Since then a range of different initiatives have been developed, all focused on the improvement of the environmental performance of companies. The results of the demonstration projects
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