The demand for improved performances of power transmission devices requires ever smaller dimensions and higher working voltages which lead to increased risk of breakdown, for example, in satellite ...slip rings. Previous works are mostly limited to breakdown in simple geometries such as parallel plates or pin to plate. Here we discuss the effect of more complex geometries for dc breakdown in gases over a large pressure range (2 × 10−5 to 103 mbar). Experimental measurements of dc gas discharge breakdown in a ring assembly geometry are compared with a numerical simulation model for gas breakdown using a fluid model. Starting with parallel plates (1 and 100 mm gap width representing approximately the shortest and longest electric field path lengths in the ring assembly geometry) and extending to double gap and multi-gap geometries, an understanding of the overall shape of the breakdown voltage versus pressure curve is established. The high (low) pressure thresholds of gas discharge are determined by the shortest (longest) electric field path length in a complex geometry. Moreover, the availability of multiple path lengths leads to a breakdown voltage minimum over a wide range of intermediate pressure because breakdown can occur in the most favourable gap. Finally, the numerical simulation in the ring assembly shows the importance of parameters such as the secondary electron emission coefficient which play a major role in determining the breakdown voltage value.
Medical-grade poly(vinyl chloride) (PVC) was chemically modified to study how the incorporation of monovalent silver influences
Pseudomonas aeruginosa adhesion and colonization. The modification ...investigated consisted of a radio frequency-oxygen (RF-O
2) glow discharge pre-functionalization, followed by a two-step wet-treatment in sodium hydroxide and silver nitrate solutions. X-ray photoelectron spectroscopy (XPS) analysis and contact angle measurements were used to investigate the chemical nature and surface wettability of the films following each step of the modification. XPS analysis proved that the RF-O
2 plasma pre-functionalization of native PVC reproducibly increased the amount of functional groups representative of PVC additives, including ether/alcohol, esters and carboxyl groups. More specifically, we demonstrated that the
O–
C
̄
O
groups representative of the phthalic ester and zinc carboxylate additives identified for native PVC increased by two-fold following the RF-O
2 plasma pre-functionalization step. Although RF-O
2 pre-functionalization did not have an effect on the silver content of the NaOH/AgNO
3 treated substrates, such a modification was necessary for biomaterial products that did not have reproducible surfaces amongst production lots. XPS analysis also demonstrated that saponification with sodium hydroxide (NaOH) of esters, like those of the phthalic ester additives of PVC is a simple, irreversible method of hydrolysis, which produced sodium carboxylate and sodium phthalate salts. Exposure of native PVC to NaOH resulted in an increased surface hydrophilicity (from ca 90° to ca 60°) due to dechlorination. XPS analysis following further incubation in silver nitrate demonstrated that silver ions can be trapped when the sodium of sodium carboxylate is replaced by silver after performing a second treatment with a monovalent silver-containing solution. The creation of silver salt on native PVC resulted in an ultra-hydrophobic (>120°) surface. The chemical modifications using NaOH and AgNO
3 wet treatments completely inhibited bacterial adhesion of four strains of
P. aeruginosa to both native and oxygen-pre-functionalized PVC, and efficiently prevented colonization over longer periods (72
h). Our results suggest that surface modifications that incorporate silver ions would be extremely effective at reducing bacterial colonization to medical devices.
New geodetic data combined with seismicity, geologic and geochemical information document a major tectonic reorganization in the central Mediterranean around 0.8–0.5 Ma, when rapid trench migration ...and consequent Tyrrhenian back-arc extension which dominated the region's evolution since the Tortonian (10–8 Ma) essentially stopped. In response, the African convergence in Sicily was transferred to a back-thrust in the Southern Tyrrhenian, characterized by frequent M5–6 earthquakes in the last 20 years. A diffuse transform boundary formed across northeastern Sicily to connect the Sicilian and Calabrian plate boundaries, causing opening of the Messina Straits—home to large historical earthquakes—and unusual volcanism in the Aeolian Island arc and the intraplate Mount Etna. A divergence of the Ionian region from the main part of Africa, with northeastward motions that reflect the joint influence of African push and Hellenic slab pull, and are transmitted to Calabria and Adria, could explain motions observed in the Sicily Channel, Calabria and Apulia, as well as the high level of southeastern Sicilian seismic activity.
Southern Italy is a key area for understanding the tectonic processes in the Africa‐Eurasia collision zone. We analyze new GPS measurements carried out between 1994 and 2001. The results are ...presented in terms of time series, trajectories and velocities of crustal motion, as well as a geodetic strain rate field. While central Italy, Corsica, Sardinia and the Tyrrhenian Sea move like the Eurasian plate, the overall motion of the Sicily Rift Zone region matches African plate motion. Unexpected are the north‐northwest directed motions of northeastern Sicily and the Eolian islands. Most striking on the deformation field are a north‐south oriented compression along the northern Sicilian coast, compression between Apulia and northwestern Greece as well as extension in the Sicily Rift Zone and the interior of Sicily.
Western Greece is one of the seismotectonically most active regions in Europe. The main tectonic structures are the West Hellenic Arc (WHA) and the Kephalonia Fault Zone. In order to monitor and ...understand the crustal movements in space and time, a continuous GPS network was installed. In this paper we present results of 6 yr (1995–2001) of measurements. To ensure a consistent reference frame, 54 mainly European IGS and EUREF sites were included in the processing. A selected subset was used to estimate an Euler pole for the rotation of Eurasia. In order to obtain coordinate time-series of high precision that are representative for crustal deformation, special emphasis was given to the elimination of non-tectonic effects. Four steps of improvement were pursued, including a reprocessing after exclusion of poor data, the removal of remaining outliers, the correction of unknown phase centre offsets after antenna changes and weighted common-mode filtering. With this procedure, non-tectonic irregularities were reduced significantly, and the precision was improved by an average of 40 per cent. The final time-series are used as a base for depicting trajectories of crustal motion, interpreting the temporal behaviour of the sites and for estimating velocities. For the first time, height changes in the WHA area were detected and quantified by GPS. Sites that are located near the epicentres of the 1997 Strofades (Mw = 6.6) and the 1999 Athens (Mw = 6.0) earthquakes are particularly considered.
The fluctuating behavior of a Sulzer Metco F4 DC plasma gun has been investigated by simultaneous measurement of the time dependencies of the are voltage and of images from the nozzle interior. An ...end-on imaging arrangement using a mirror and a mask in the optical path from the are to the camera allows visualization of the anodic arc attachment by strongly attenuating the bright emission from the are column. With the torch operating in the restrike mode, sequences of images have been acquired in synchronization with several typical features of the are voltage fluctuations showing that the attachment nature changes during a restrike cycle. Multiple attachments which coexist at least during the 1 /spl mu/s exposure time of the camera have been evidenced and are interpreted as a continuous process of creation/vanishing of successive arc roots with a smooth transfer of the current from one to the other. The anode wear is shown to have a strong effect on the root position over the anode periphery, with a preference for attachment in eroded regions. The effects of operation parameters such as current, gas flow and injector type on the attachment nature and position are also presented.
The Hellenic plate boundary region, located in the collision zone between the Nubian/Arabian and Eurasian lithospheric plates, is one of the seismo-tectonically most active areas of Europe. During ...the last 15 years, GPS measurements have been used to determine the crustal motion in the area of Greece with the aim to better understand the geodynamical processes of this region. An extended reoccupation network covering whole Greece has been measured periodically in numerous GPS campaigns since the late eighties, and a continuous GPS network has been operated in the region of the Ionian Sea since 1995. In this paper, we present a new detailed high-quality solution of continuous and campaign-type measurements acquired between 1993 and 2003. During the GPS processing, a special effort was made to obtain consistent results with highest possible accuracies and reliabilities. Data of 54 mainly European IGS and EUREF sites were included in the GPS processing in order to obtain results which are internally consistent with the European kinematic field and order to allow for a regional interpretation. After an overview of the results of the IGS/EUREF sites, the results from more than 80 stations in Greece are presented in terms of velocities, time series, trajectories and strain rates. Previous geodetic, geological and seismological findings are generally confirmed and substantially refined. New important results include the observation of deformation zones to the north and to the south of the North Aegean Trough and in the West Hellenic arc region, arc-parallel extension of about 19 mm/yr along the Hellenic arc, and compression between the Ionian islands and the Greek mainland. Due to continuous long-term observations of 4–8 years, it was possible to extract height changes from the GPS time series. In Greece, we observe a differential subsidence of the order of 2 mm/yr between the northern and central Ionian islands across the Kefalonia fault zone. The differential subsidence of the central Ionian islands with respect to the northwestern Greek mainland amounts to 4 mm/yr.
Electrical discharge machining (EDM) is a well-established non-conventional manufacturing process. The underlying working principles of EDM relay on interactions of electric discharge plasmas with ...the electrodes material and dielectric in micrometer gaps. Key aspects of the process, such as material removal mechanisms, are strongly related to this complex interaction. Therefore, observation of small gaps plasma-material interactions can help to better understand this particular EDM process, supply new boundary conditions for existing models, and lead to more sophisticated processing. Different methods of analysis have been used to study and explain the physics of EDM discharges. In the present work, spatially-resolved high-speed optical emission spectroscopy is used to analyze emission spectra from small gap discharges under different conditions. Results on the plasma expansion in function of time as well as time resolved profiles of plasma components and of different plasma parameters are obtained and interpreted. Optical emission spectra supported by emission spectra simulations suggest that the center of the micro discharge is dominated by highly ionized species, whereas single ionized and atomic species are more uniformly distributed over the whole plasma. Furthermore, the spatially-resolved measurements shows that the center of the plasma is not in thermodynamic equilibrium, probably field emission leads to an ion-enhanced FE driven Townsend discharge in the small gaps.
PDF
