Filters
Results 1 - 10 of 29
Results 1 - 10 of 29.
Search took: 0.024 seconds
| Sort by: date | relevance |
[en] This article summarizes improvements to the speed, simplicity and versatility of tapping mode atomic force microscopy (AFM). Improvements are enabled by a piezoelectric microcantilever with a sharp silicon tip and a thin, low-stress zinc oxide (ZnO) film to both actuate and sense deflection. First, we demonstrate self-sensing tapping mode without laser detection. Similar previous work has been limited by unoptimized probe tips, cantilever thicknesses, and stress in the piezoelectric films. Tests indicate self-sensing amplitude resolution is as good or better than optical detection, with double the sensitivity, using the same type of cantilever. Second, we demonstrate self-oscillating tapping mode AFM. The cantilever's integrated piezoelectric film serves as the frequency-determining component of an oscillator circuit. The circuit oscillates the cantilever near its resonant frequency by applying positive feedback to the film. We present images and force-distance curves using both self-sensing and self-oscillating techniques. Finally, high-speed tapping mode imaging in liquid, where electric components of the cantilever require insulation, is demonstrated. Three cantilever coating schemes are tested. The insulated microactuator is used to simultaneously vibrate and actuate the cantilever over topographical features. Preliminary images in water and saline are presented, including one taken at 75.5 μm/s - a threefold improvement in bandwidth versus conventional piezotube actuators
Journal
[en] Hydrogen is likely to be the most important future energy carrier, for many stationary and mobile applications, with the potential to make significant reductions in greenhouse gas emissions especially if renewable primary energy sources are used to produce the hydrogen. A safe transition to the use of hydrogen by members of the general public requires that the safety issues associated with hydrogen applications have to be investigated and fully understood. In order to assess the risks associated with hydrogen applications, its behaviour in realistic accident scenarios has to be predicted, allowing mitigating measures to be developed where necessary. A key factor in this process is predicting the release, dispersion and combustion of hydrogen in appropriate scenarios. This paper illustrates an application of CFD methods to the simulation of an actual hydrogen explosion. The explosion occurred on 3 March 1983 in a built up area of central Stockholm, Sweden, after the accidental release of approximately 13.5 kg of hydrogen from a rack of 18 interconnected 50 l industrial pressure vessels (200 bar working pressure) being transported by a delivery truck. Modelling of the source term, dispersion and combustion were undertaken separately using three different numerical tools, due to the differences in physics and scales between the different phenomena. Results from the dispersion calculations together with the official accident report were used to identify a possible ignition source and estimate the time at which ignition could have occurred. Ignition was estimated to occur 10 s after the start of the release, coinciding with the time at which the maximum flammable hydrogen mass and cloud volume were found to occur (4.5 kg and 600 m3, respectively). The subsequent simulation of the combustion adopts initial conditions for mean flow and turbulence from the dispersion simulations, and calculates the development of a fireball. This provides physical values, e.g. maximum overpressure and far-field overpressure that may be used as a comparison with the known accident details to give an indication of the validity of the models. The simulation results are consistent with both the reported near-field damage to buildings and persons and with the far-field damage to windows. The work was undertaken as part of the European Integrated Hydrogen Project-Phase 2 (EIHP2) with partial funding from the European Commission via the Fifth Framework Programme
Journal
[en] In this letter we present a theoretical study of the coherent diffraction of three-level atoms from a light field with a polarization gradient (counterpropagating crossed linearly) polarized beams) and a static magnetic field applied parallel to the laser propagation direction. We show that for a particular ratio of the laser field intensity and the magnetic-field strength, there occurs a resonance between the Larmor precession of the magnetic alignment and the Rabi oscillations. On resonance the atomic wave function is diffracted by an approximately triangular optical potential which leads to a very efficient coherent splitting of the atomic beam. The proposed configuration is particularly interesting in relation to atom interferometry, when efficient coherent beam splitters for atoms are required. (orig.)
Journal
[en] 82Rb is a short-lived positron-emitting isotope (T1/2=75 s) that is increasingly being used in positron emission tomography for the evaluation of myocardial perfusion. The short half-life of 82Rb results in rapid decay of the administered radioactivity allowing repeat scans to be performed after a relatively short time interval. The 82Rb is available from a 82Sr/82Rb generator in which the 82Sr parent is immobilized on an ion exchange column and 82Rb eluted when required. Consequently, suitable ion exchangers for use in a generator must have a very high affinity for strontium and a negligible affinity for rubidium, allowing the pure 82Rb to be safely and efficiently eluted in high yields. This study evaluated a number of strontium-selective ion exchange materials using batch experiments and identified sodium nonatitanate as the best material for use in a 82Sr/82Rb generator
Journal
[en] We present recent improvements in assembly-level calculations for reactor analysis, including modifications that support core-level analysis by quasi-diffusion. Our main focus is on accurately approximating the effects that neighboring assemblies have on the few-group cross sections, assembly discontinuity factors, form factors, and other transport parameters of a given assembly. We show that we can do this by using albedo boundary conditions that are estimated with low computational cost. We also present an efficient way to tabulate these effects to permit accurate interpolation by the core-level algorithm. We describe our algorithms and present results from several difficult test problems containing mixed-oxide and UO2 assemblies. Our methodology significantly reduces the largest errors made by present-day methodology. For example, in our test problems it reduces the maximum pin-power error by a factor of ∼5
Journal
[en] A large body of information is available on the rates and mechanisms of inorganic electron-exchange processes. In contrast, purely organic systems have received only minor attention. The homogeneous electron-exchange rates (kexc) and the heterogeneous rate constants for the electrode reaction (kel) have been measured only for a few hydrocarbons. We have measured kexc for a variety of aromatic systems including hydrocarbons, quinones and nitro compounds. These measurements have been carried out via electron paramagnetic resonance (EPR) line broadening measurements on mixtures of radical ions and their parent compounds. We have been able to measure kexc with a precision that allows detection of small differences presumably due to molecular structure and environment. Hydrocarbon systems like anthracene/anthracene anion are very rapid with kexc values of ca. 108-109 litres mole-1 sec-1. Some substituted aromatics like quinones and nitriles are also quite rapid. However, when a strong electron acceptor function is present like a nitro group in nitrobenzene, the value of kexc decreases by a factor of 10. It is possible to correlate changes in kexc in the nitrobenzene series with the unpaired electron density in terms of the 14N coupling constants of the EPR spectra. Further, the nitro aromatic series show very large variations in kexc with the solvent system. These changes can be correlated with recent studies of the solvation effect on hyperfine coupling constants. Marcus has reviewed recently chemical and electrochemical electron-transfer theory and suggested correlations between kexc and kel. We have measured kel especially for the nitrobenzene system under conditions which are as nearly identical experimentally to the EPR studies as possible. The electrochemical investigations were carried out by a steady-state d.c. method to eliminate some of the uncertainties inherent in electrochemical relaxation techniques. Rotated disc electrodes at low temperatures were used. The solvent effects on kel (for nitrobenzene) are identical with those observed for kexc. There are, however, serious discrepancies between the values of kel observed and predicted by Marcus's theory. Some possible explanations of these discrepancies will be discussed. (author)
[fr]
On dispose d'une importante documentation sur les vitesses et les mécanismes des processus d'échange d'électrons dans les systèmes inorganiques. En revanche, les systèmes purement organiques ont reçu jusqu'ici une attention moindre. Les vitesses d'échange homogène d'électrons (kexc) et les constantes de vitesses d'échange hétérogène (kel) pour la réaction à l'électrode n'ont été mesurées que pour quelques hydrocarbures. Les auteurs ont mesuré kexc pour divers systèmes aromatiques, notamment des hydrocarbures, des quinones et des nitrocomposés. Pour cela, ils ont mesuré l'élargissement desraies par résonance paramagnétique des électrons (RPE) sur des mélanges de radicaux ioniques et des composés ascendants. Ils ont pu mesurer kexc avec une précision tellequ'ellepermet de déceler de petites différences, lesquelles sont sans doute dues à la structure moléculaire et au milieu. Pour les systèmes d'hydrocarbures, comme le système anthracène/anion anthracène, la vitesse est très élevée: les valeurs de kexc sont d'environ 108 à 109 l * mole-1 * s-1. Les vitesses sont également élevées pour certains composés aromatiques substitués comme les quinones et les nitriles. Toutefois, lorsqu'un accepteur d'électrons fort est présent, comme par exemple un groupe nitro dans le nitrobenzene, la valeur de kexc diminue d'un facteur de 10. On peut trouver une corrélation entre les variations de kexc dans la série du nitrobenzène avec la densité des électrons célibataires, selon les constantes de couplage de 14N du spectre de RPE. La série nitro-aromatique accuse en outre de très fortes variations de kexc selon le système solvant. On peut rapprocher ces variations des récentes études de l'effet de solvatation sur les constantes de couplage hyperfines. Marcus a examiné récemment la théorie du transfert chimique et électrochimique d'électrons et a suggéré des corrélations entre kexc et kel. Les auteurs ont mesuré kel notamment pour le nitrobenzene dans des conditions expérimentales qui sont aussi proches que possible des études par RPE. Pour les recherches électrochimiques, onaappliqué une méthode à courant continu enrégimestationnaire, de manière à éliminer certaines des incertitudes inhérentes aux techniques de relaxation électrochimique. On a utilisé des électrodes à disques tournants à basse température. Les effets du solvant sur kel (pour le nitrobenzène) sont identiques à ceux qui ont été observés pour kexc- Mais o n relève des écarts considérables entre les valeurs de kel observées et celles qui ont été calculées par la théorie de Marcus, les auteurs discutent plusieurs explications possibles de ces écarts. (author)[es]
Se posee abundante información sobre las velocidades y mecanismos de los procesos de intercambio electrónico en sustancias inorgánicas. En cambio, se ha prestado menor atención a los sistemas puramente orgánicos. Las velocidades de intercambio homogéneo de electrones (kexc) y las constantes de velocidad de intercambio heterogéneo en los electrodos (kel) sólo se han medido en el caso de contados hidrocarburos. Los autores han determinado el valor de kexc en una serie de sistemas aromáticos que comprende hidrocarburos, quinonas y nitro compuestos. A tal efecto, han medido el ensanchamientode las lfneas por resonancia paramagnética de los electrones en mezclas de radicales iónicos y de sus precursores. Los autores han podido medir el valor de kexc con una precisión que permite observar ligeras variaciones debidas probablemento a la estructura molecular y al medio. En los sistemas de hidrocarburos como el antraceno/anión antraceno la velocidad de intercambio es muy elevada, siendo los valores de kexc del orden de 108 a 109 l mol-1 s-1. Las velocidades son también elevadas para algunos compuestos aromáticos sustituidos, como quinonas y nitrilos. Sin embargo, en presencia de un fuerte aceptor de electrones, como el grupo nitro del nitrobenceno, el valor de kexc es 10 veces menor. Es posible correlacionar las variaciones de kexc e n la serie del nitrobenceno con la densidad de electrones no apareados en función de las constantes de acoplamiento del 14N en los espectros de resonancia paramagnética de los electrones. Asimismo, las series nitroaromáticas presentan considerables variaciones de kexc según el solvente empleado. Estas variaciones pueden vincularse con los resultados de recientes estudios del efecto de solvatación sobre las constantes de acoplamiento hiperfinas. Marcus ha estudiado recientemente la, teoría de la transferencia quúmca y electroquímica de electrones y propuesto correlaciones entre kexc y kel. Los autores han medido kel, especialmente en el nitrobenceno, en condiciones experimentales lo más análogas posibles a las de los estudios por resonancia paramagnética de los electrones. Las investigaciones electroquímicas se efectuaron con corriente continua estabilizada para eliminar, algunas de las indeterminaciones inherentes a las técnicas de relajación electroquímica. Se utilizaron como electrodos discos giratorios a baja temperatura. Los efectos del solvente en el valor de kel (en el caso nitrobenceno) son idénticos a los observados respecto de kexc. No obstante, existen considerables discrepancias entre los valores de kel observados y los previstos según la teoría de Marcus. Se examinan algunas explicaciones plausibles de estas discrepancias. (author)[ru]
Imeetsja bol'shoe kolichestvo informacii otnositel'no skorosti mehanizma processov neorganicheskogo obmena jelektronov. V protivopolozhnost' jetomu chisto organicheskim sistemam udeljalos' dovol'no neznachitel'noe vnimanie. Skorost' gomogennogo jelektronnogo obmena (k0bm)i geterogennye konstanty skorosti dlja jelektrodnoj reakcii (kjel) byli izmereny lish' dlja neskol'kih ugle- vodorodov . Bylo izmereno k0bm dlja razlichnyh aromaticheskih sistem, v tom chisle uglevodorodov, hinonov i nitrosoedinenij. Jeti izmerenija byli provedeny posredstvom jelektronnogo para- magnitnogo rezonansa (EPR) s perenosom izmerenij na smesi radikal'nyh ionov i ih mate- rinskih soedinenij. Nam takzhe udalos' izmerit' k0bm s takoj tochnost'ju, kotoraja pozvo- ljaet obnaruzhit' nebol'shie razlichija, vyzvannye, verojatno; molekuljarnoj strukturoj i sre- doj. Uglevodorodnye sistemy, podobnye antracenovomu/antracenovomu anionu, javljajutsja ves'ma bystrymi pri velichine k0gM priblizitel'no 108-109 litr mol-i sek-1. Nekotorye zame- shhennye aromaticheskie soedinenija, podobnye hinonam i nitrilam, takzhe javljajutsja ves'ma bystrymi. Odnako, kogda imeetsja sil'naja funkcija jelektronnogo akceptora, podobno nitro- gruppe v nitrobenzole, velichina k0gM sokrashhaetsja na faktor 10. Predstavljaetsja vozmozhnym korrelirovat' izmenenija v kobm v serijah nitrobenzola s pomoshh'ju plotnosti neparnyh jelek- tronov po konstantam svjazi N14 spektra EPR. Krome togo, nitroaromaticheskie serii obna- ruzhivajut ochen' bol'shie razlichija v znachenijah k0b m s rastvorjajushhej sistemoj. Jeti izmene- nija mozhno korrelirovat' blagodarja nedavnim issledovanijam jeffekta sol'vatacii na sverh- tonkie konstanty svjazi. V poslednee vremja Markus rassmotrel teoriju himicheskogo i jelektrohimicheskogo pere- nosa jelektronov i predlozhil korreljacii mezhdu znachenijami kobm i kjel. Nami special'no byl izmeren kjel dlja sistemy nitrobenzola v uslovijah, kotorye jeksperimental'no byli - pochti identichny uslovijam pri izuchenii EPR. Elektrohimicheskie issledovanija provodilis' metodom tverdogo sostojanija de s tem, chtoby iskljuchit' nekotorye neopredelennosti, pri- sushhie metodam jelektrohimicheskoj relaksacii. Ispol'zovalis' pri nizkih temperaturah vrashhajushhiesja diskovye jelektrody. Rastvorjajushhie jeffekty po kje l (dlja nitrobenzola) iden- tichny jeffektam, nabljudavshimsja dlja k0bm - Odnako imejutsja ser'eznye rashozhdenija mezhdu velichinami nabljudavshegosja kJel i predskazannymi teoriej Markusa. Dajutsja nekotorye vozmozhnye raz'jasnenija jetih rashozhdenij. (author)Source
International Atomic Energy Agency, Vienna (Austria); 430 p; Oct 1965; p. 157-170; Symposium on Exchange Reactions; Upton, NY (United States); 31 May - 4 Jun 1965; IAEA-SM--64/21; ISSN 0074-1884; 
; 30 refs., 4 tabs., 2 figs.
; 30 refs., 4 tabs., 2 figs.
| 1 | 2 | 3 | Next |