First Principles Calculation of Field Emission from Nanostructures using Time-Dependent Density Functional Theory: a Simplified Approach

We introduce a new simplified method for computing the electron field emission current in short carbon nanotubes using ab-initio computation in periodic simulation cells. We computed the evolution of the wave functions using Time-Dependent Density Functional Theory, where we have utilized the Crank-Nicholson propagator. We found that in pristine carbon nanotubes, the emitted charge tends to emerge mostly from electrons that are concentrated at the nanotube tip region. The charge beam concentrates into specific channel structures, showing the utility of carbon nanotubes in precision emission applications.Comment: Submitted to Physica

Nuclear Structure of Positive Parity Levels in 15N As Studied by Inelastic Electron Scattering

High resolution measurements of inelastic electron scattering cross sections from the states in 15N (gas target of 99% 15N enrichment) up to 23.5 MeV excitation have been performed at NIKHEF-K Amsterdam. The measurements were carried out at 70, 130, 300, and 430 MeV incident electron energy with an angular range from 40 to 98.5 covering a momentum transfer region between 0.5 and 3.2 fm-1. The data of the present experiment together with the existing data obtained at a scattering angle of 18

Laboratory Evaluations Of Some Termiticides Against Subterranean Termite Coptotermes Gestroi (Wasmann) (Isoptera : Rhinotermitidae) [QL529. G978 2008 f rb].

Kesan bagi lima jenis racun anai-anai dinilai di dalam makmal, ke atas C. gestroi untuk menentukan kesesuaiannya di dalam mengawal anai-anai bawah tanah. The effects of five termiticides were evaluated in the laboratory against C. gestroi to determine their efficiency in controling subterranean termites

The optimal sequence of radiotherapy and chemotherapy in adjuvant treatment of breast cancer

<p>Abstract</p> <p>Background</p> <p>The optimal time sequences for chemotherapy and radiation therapy after breast surgery for patients with breast cancer remains unknown. Most of published studies were done for early breast cancer patients. However, in Egypt advanced stages were the common presentation. This retrospective analysis aimed to assess the optimum sequence for our population.</p> <p>Methods</p> <p>267 eligible patients planned to receive adjuvant chemotherapy [FAC] and radiotherapy. Majority of patients (87.6%) underwent modified radical mastectomy while, 12.4% had conservative surgery.</p> <p>We divided the patients into 3 groups according to the sequence of chemotherapy and radiotherapy. Sixty-seven patients (25.1%) received postoperative radiotherapy before chemotherapy [group A]. One hundred and fifty patients (56.2%) were treated in a sandwich scheme (group B), which means that 3 chemotherapy cycles were given prior to radiotherapy followed by 3 further chemotherapy cycles. A group of 50 patients (18.7%) was treated sequentially (group C), which means that radiotherapy was supplied after finishing the last chemotherapy cycle. Patients' characteristics are balanced between different groups.</p> <p>Results</p> <p>Disease free survival was estimated at 2.5 years, and it was 83.5%, 82.3% and 80% for patient receiving radiation before chemotherapy [group A], sandwich [group B] and after finishing chemotherapy [group C] respectively (p > 0.5). Grade 2 pneumonitis, which necessitates treatment with steroid, was detected in 3.4% of our patients, while grade 2 radiation dermatitis was 17.6%. There are no clinical significant differences between different groups regarded pulmonary or skin toxicities.</p> <p>Conclusion</p> <p>Regarding disease free survival and treatment toxicities, in our study, we did not find any significant difference between the different radiotherapy and chemotherapy sequences.</p

Optical and mechanical properties of hybrid nanocomposite light guide fibers

U uvodu istaknut je značaj istraživanja u oblasti nanokompozitnih materijali na bazi oksida aktiviranih jonima lantanida koji imaju veoma značajnu primenu za izradu lasera, laserskih i LED dioda, luminescentnih lampi, displeja, optičkih vlakana, biomedicinskih markera itd. Za primenu posebno su od interesa emisije uskog energetskog opsega, dugo vreme života pobuđenih elektronskih stanja (fosforescencija) i fotohemijska stabilnost. U zavisnosti od načina pobuđivanja, luminescencija se može podeliti na: a) fotoluminescenciju (PL) – posledica apsorpcije elektromagnetnog zračenja, b) elektroluminescencija – posledica apsorpcije električnog polja, c) radioluminescencija – posledica jonizujućeg zračenja, d) katodoluminescencija – posledica interakcije snopa elektrona velike brzine sa tankim slojem fluorescentne supstance, e) hemiluminescencija – posledica pretvaranja hemijske energije u svetlosnu, f) bioluminescencija. Predmet ove doktorske disertacije su luminiscentni materijali. Kada se fosforescentna čestica izloži dejstvu eksitacionog izvora, kao što je ultraljubičasta (UV) ili vacuum-UV (VUV) svetlost, ona prolazi kroz proces apsorpcije, relaksacije i emisije. Svetlost se apsorbuje od strane materijala domaćina i apsorbovana energija se prenosi sa materijala domaćina na aktivator (luminescentni centar). Aktivator na kraju emituje vidjivu svetlost preko mehanizma prenosa energije. Fotoekscitacija na određenoj talasnoj dužini u bliskoj infracrvenoj oblasti koja je praćena luminiscencijom na kraćim talasnim dužinama u vidljivoj svetlosti se naziva prelaz iz bliske infracrvene u vidljivu oblast. To je veoma neobična pojava jer se fotoni niže energije „pretvaraju“ u fotone sa višom energijom. Najmanje dva fotona u infracrvenoj oblasti su potrebna za generisanje jednog fotona u vidljivoj oblasti. Upkonverzija se uglavnom dešava u materijalima u kojima procesi relaksacije više fotona ne dominiraju, što omogućava više od jednog metastabilnog stanja. U jedinjenjima retkih zemalja, elektroni 4f i 5f su efikasno zaštićeni i samim tim nisu značajnije vezani- metal-ligand. Kao posledica toga, spoj elektron-fonon na f-f prelazima se smanjuje, a proces otpuštanja više fonona je manje konkurentan. Fenomen upkonverzije je zbog toga najčešće proučavan na materijalima koji sadrže jone lantanida. Kada neki medijum (na primer laser) emituje fosforescenciju kao posledicu ekscitacije sa upadnom svetlošću, talasna dužina je duža od ekscitovane svetlosti. To znači da se smanjuje energija fotona. Međutim, pod izvesnim okolnostima može doći do upkonverzije fluorescencije gde je talasna dužina emitovane svetlosti kraća. To je moguće putem ekscitacionih mehanizama koji uključuju više od jednog apsorbovanog fotona. Upconverzija fluorescencije se može potisnuti kada imamo mnogo prelaza fotona, jer oni mogu smanjiti život metastabilnih nivoa. Vlakna sa niskom maksimalnom energijom fotona, kao što su neka fluoridna vlakna, imaju mnogo slabije, više fononske procese, pa stoga i jači upkonverziju. U drugom poglavlju dat je prikaz luminiscentnih materijala. Luminescentni materijali, zvani fosfori konvertuju neki tip energije u elektromagnetno zračenje, najčešće u vidljivoj oblasti, mada se može javiti i u ultraljubičastoj i infracrvenoj oblasti. Ovi materijali su uglavnom neorganska jedinjenja u formi prahova ili tankih filmova, koji sadrže odgovarajuće jone, odnosno aktivatore. Aktivator apsorbuje pobuđujuće zračenje, prelazi u pobuđeno stanje, nakon čega se vraća u osnovno stanje...Dynamic Mechanical Analysis (DMA) is one of the most powerful tools to study the behavior of plastic and polymer composite materials and it is potentially very useful tool to simulate behavior of plastic optic fibers (POF) in real applications. Possibility of simultaneous measurements of some optical properties during DMA would significantly upgrade investigations of POF alone or embedded in some materials. In this work, single cantilever DMA of the POFs that was done simultaneously with measuring the transmitted optical signal intensity is described and discussed. In order to compare mechanical results of the same material for cylindrical and rectangular specimens, rectangular plates were prepared by melting POFs and the same kind of tests were performed. It is shown that changing the optical signal intensity corresponds to the changes of storage modulus of the POF during DMA, and the maximums in optical signals intensity indicate the beginning of glass transition processes in the POF material. The results of a study related to the processing and characterization of PMMA-Y2O3 (Eu3+) nanocomposites are presented herein. The nanocomposite samples were prepared using a laboratory mixing molder with different contents of Eu-ion doped Y2O3 nanophosphor powder. The influence of particle content on the optical and dynamic mechanical properties of the nanocomposites was investigated. The intensity of the luminescence emission spectra increased as the nanophosphor content in the composite increased. The results of dynamic mechanical analysis revealed that the storage modulus, loss modulus and glass transition temperature (Tg) of the polymer composites increased with increasing content of the nanophosphor powder. The microhardness data also confirmed that the hardness number increased with nanoparticles concentration in the PMMA nanocomposites. The obtained results revealed a relatively linear relationship between Tg and the Vickers hardness. This study reports research related to different preparation methods and characterization of polymer nanocomposites for optical applications. The Eu-ion doped Gd2O3 nanophosphor powder with different nanoparticle content was embedded in the matrix of PMMA. Preparation was carried out by mixing molding (bulk), electrospinning (nanofibers) and solution casting (thin films) with neat particles and particles coated with AMEO silane. Among the pros and cons for proposed methods, the mixing molding enables to avoid solvent use while the best deagglomeration and nanoparticle distribution is gained using the electrospinning method. The results of dynamic mechanical analysis (DMA) and nanoindentation revealed that the storage modulus of the composites was higher than that of pure PMMA and increased with nanophosphor content. Surface modification of particles improved the mechanical properties of nanocomposites

Performance de la gouvernance de l'eau d'irrigation par les groupements d'intérêt collectif, en périmètre public irrigué

International audienceDepuis la décennie 1980-1990 et au cours de la mise en oeuvre du programme d'ajustement structurel, la Tunisie a ajouté à l'objectif de modernisation de l'agriculture, d'autres objectifs de productivité et de compétitivité sur les marchés extérieurs. Ces transformations nationales ont été accompagnées d'un changement des politiques et du rôle de l'Etat qui se sont traduits par des reformes institutionnelles remarquables en matière de la gestion de l'eau d'irrigation dans les périmètres irrigués. Aussi, la raréfaction des ressources en eau, l'augmentation de la demande en eau, le maintien d'objectifs nationaux, notamment la diminution des charges de l'Etat la compétitivité modifié les objectifs de la gestion des ressources en eau, en termes de maîtrise de la demande et de l'efficacité de l'irrigation en particulier. Cette situation aura des conséquences sociales et spatiales importantes, qui pourraient être marquées par des difficultés de la petite agriculture irriguée familiale et l'émergence de conflits d'intérêts entre les différents acteurs. Notre étude propose de comprendre comment, en Tunisie, la politique hydraulique a réussi à s'adapter aux différentes transformations économiques et sociales. A partir d'un échantillon de groupements d'intérêt collectif et d'usagers, nous faisons une appréciation de la performance de la gouvernance de l'eau d'irrigation. Nous nous intéressons, d'une part, aux groupements d'intérêt collectif en tant qu'organisation disposant d'un objectif d'efficience à atteindre et d'autre part, aux déterminants de l'efficience de la gouvernance de l'eau d'irrigation par les groupements. La théorie des coûts de transaction et la théorie positive d'agence constituent le cadre d'analyse, afin d'identifier les coûts occasionnés par les conflits entre les acteurs lors de la transaction de l'eau et qui influencent leur comportement. La performance des groupements est appréciée par des variables reflétant la qualité de la gestion administrative, technique et financière. Une analyse factorielle de correspondance a permis de visualiser les relations de cause à effet, en termes de coûts de transaction et de coûts d'agence ayant une incidence sur la performance des groupements et sur l'efficacité d'utilisation de l'eau par les usager

Production structure and Cost efficiency of irrigated agriculture: A shadow cost approach

International audienceThe improvement of irrigated agriculture needs sound performance assessment based on economic principles. This is particularly relevant to design adequate policies in terms of water allocation. In this paper we endeavour to assess irrigation efficiency using a behavioural approach in which the cost function (shadow cost) is based on shadow prices of inputs. This approach is useful to derive price distortion coefficients to indicate the level of allocative efficiency in two irrigated areas: Jendouba and Kairouan, in Tunisia. Results show that in the case of Jendouba price ratio shows an even distribution around one with 50% of farmers having values below one and 50% greater than one. In the case of Kairouan the same distribution is observed except that the magnitude of the distortion is greater (values around Two). This shows that inefficiencies are greater in Kairouan than in Jendouba. Such difference is explained by the prevalence of irrigation tradition which is older in Jendouba than in Kairouan

Experimental Evaluation of Medium-Voltage Cascode Gallium Nitride (GaN) Devices for Bidirectional DC–DC Converters

As renewable energy sources, such as photovoltaic (PV) cells and wind turbines, are rapidly implemented in DC microgrids, energy storage systems play an increasingly significant role in ensuring uninterrupted power supply and in supporting the reliability and stability of microgrid operations. Power electronics, especially bidirectional DC–DC converters, are essential parts in distributed energy storage and alternative energy systems because of their grid synchronization, DC power management, and bidirectional power flow capabilities. While there is increasing demand for more efficient, compact, and reliable power converters in numerous applications, most existing power converters are hindered by traditional silicon (Si) based semiconductors, which are reaching their theoretical and material limits as there is an insignificant possibility for further improvements. Wide bandgap (WBG) semiconductors, such as gallium nitride (GaN) and silicon carbide (SiC), exhibit superior physical properties and demonstrate great potential for replacing conventional Si semiconductors with WBG technology, pushing the boundaries of power devices to handle higher switching frequencies, output power levels, blocking voltages, and operating temperatures. However, tradeoffs in switching performance and converter efficiency when substituting GaN devices for Si and SiC counterparts are not well defined, especially in a cascode configuration. Additional research with further detailed investigation and analysis is necessitated for medium-voltage GaN devices in power converter applications. Therefore, the objective of this research is to experimentally investigate the impact of emerging 650/900 V cascode GaN switching devices on bidirectional DC–DC converters that are suitable for energy storage and distributed renewable energy systems. Dynamic characteristics of Si, SiC, and cascode GaN power devices are examined through the double-pulse test (DPT) at different gate resistance values, device currents, and DC-bus voltages. Furthermore, the switching behavior and energy loss as well as the rate of voltage and current changes over the time are studied and analyzed at different operating conditions. A 500 W experimental converter prototype is designed and implemented to validate the benefits of cascode GaN devices on the converter operation and performance. Comprehensive analysis of the power losses and efficiency improvements for Si- based, SiC-based, and GaN-based converters are performed and evaluated as the switching frequency, working temperature, and output power level are increased. The experimental results reveal a significant improvement in switching performance and energy efficiency from cascode GaN power devices used in the bidirectional converters