Triple Junction Point Research Articles

High voltage performance of a dc photoemission electron gun with centrifugal barrel-polished electrodes.

The design and fabrication of electrodes for direct current (dc) high voltage photoemission electron guns can significantly influence their performance, most notably in terms of maximum achievable bias voltage. Proper electrostatic design of the triple-point junction shield electrode minimizes the risk of electrical breakdown (arcing) along the insulator-cable plug interface, while the electrode shape is designed to maintain <10 MV/m at the desired operating voltage aiming at little or no field emission once conditioned. Typical electrode surface preparation involves diamond-paste polishing by skilled personnel, requiring several weeks of effort per electrode. In this work, we describe a centrifugal barrel-polishing technique commonly used for polishing the interior surface of superconducting radio frequency cavities but implemented here for the first time to polish electrodes for dc high voltage photoguns. The technique reduced polishing time from weeks to hours while providing surface roughness comparable to that obtained with diamond-paste polishing and with unprecedented consistency between different electrode samples. We present electrode design considerations and high voltage conditioning results to 360 kV (∼11 MV/m), comparing barrel-polished electrode performance to that of diamond-paste polished electrodes. Tests were performed using a dc high voltage photogun with an inverted-geometry ceramic insulator design.

Pulsed Nanosecond Breakdown of a Hollow Cathode Discharge With Pseudospark Geometry

This paper reports the experimental results from the appearance of three discharge modes in a special geometry as a function of applied negative voltage pulses. The geometry is a four-gap pseudospark one with a gap distance of 4 mm, a metallic hollow cathode, and an anode. The applied pulsed voltage $U_{p}$ varies from −20 to −80 kV. The pulse shape is a rectangular one with a pulsewidth of 50 ns and a rise time of 4 ns. There is no breakdown at $U_{p} kV, and at $U_{p} > 27$ kV, a pseudospark discharge starts, which transforms at $U_{p} > 45$ kV into a surface discharge. The working gas is dry air at a constant pressure of 13 Pa. Using these parameters, electric field simulations of the triple point junctions are conducted and time-integrated pictures of the different discharge modes are presented. When the gas pressure is increased from 13 to 35 Pa, the voltage range of the pseudospark discharge mode grows from 13 to 65 kV.

Minimisation of electric field stress at triple junction of a functionally graded cone type spacer in a gas insulated busduct with metal inserts

Insulation integrity of supporting insulator plays a vital role for determining the continuity of service of the gas insulated substation which depends upon the electric field stress distribution on the surface of the spacer. Triple junction formed by the conductor, SF6 gas and support insulator is another critical area where the field stress is to be maintained at minimum value. Shape control is a technique employed for obtaining uniform field stress along the surface of the spacer but this technique may sometimes leads to very uneven shapes. In this study, a functional graded material is designed for the standard cone type spacer to obtain uniform field stress along the surface of spacer to overcome the above problem. The grading of spacer is done with different permittivities. Electric field is computed for three cases namely grading to high, grading to low and U Shape. The metal inserts were tuned and stress is obtained in account of lower fields at the triple point junction and an improved uniform field distribution along the surface is obtained.

A phase-field study on the peritectic phase transition in Fe-C alloys

A quantitative multi-phase-field model, with an anti-trapping current and involving diffusion in the solid, is proposed to simulate the peritectic phase transition in Fe-C alloys. An interface field method with a newly defined step function is adopted to formulate the governing equation of the multi-phase-field variables. The proposed model is applied to simulate the γ-platelet tip growth during the peritectic reaction and the subsequent peritectic transformation (γ-platelet thickness growth) in large computational domains of the experimental length scale. It is found that the local liquid concentration at the triple junction point L/δ/γ is slightly higher than the liquid concentration in equilibrium with the δ-phase, but lower than the liquid concentration in equilibrium with the γ-phase. This leads to slight melting of the δ-phase in the vicinity of the triple junction, while the γ-platelet growth continues. Higher tip velocities at higher undercoolings produce a steeper carbon concentration gradient along the γ-platelet's thickness direction, and thus yield a higher thickness growth velocity. It is also found that the ratio of tip and thickness growth velocity of the γ-platelets increases at higher undercoolings, leading to decreasing tip radius and platelet thickness. Good agreement between the simulations and the experimental data reported in literature is achieved, demonstrating the quantitative prediction capabilities of the proposed model, and confirming the diffusion control for the peritectic phase transition close to equilibrium conditions.

Pit morphology and depth after low-temperature hot corrosion of a disc alloy

Hot corrosion of the low solvus, high refractory (LSHR) disc alloy was studied at 700 °C. The purpose of this study was to determine the conditions which result in a discrete, isolated pit morphology and to examine the influence of SO2 gas additions and various salt concentrations on the depth of those pits. Three salts, pure Na2SO4 and two Na2SO4–MgSO4 compositions, were used. It was found that with a eutectic Na2SO4–MgSO4 salt, there was no significant increase in pit depth between 0 and 30 ppm SO2 when O2 was also present in the gas stream. Gas flow was observed to affect pit formation, but the variation in the position of the corrosion mounds/pits on the sample surface was unexpected. There was limited evidence that pit nucleation was not associated with grain boundaries or grain triple point junctions. An evolution from single, isolated pits, to coalesced pits, to overlapping pits on a single sample was observed. At higher SO2 concentrations, the extent of attack increased, resulting in a uniform type of attack morphology with significant metal loss across the sample surface. It was concluded that hot corrosion attack by pit formation for these conditions is not easily explained or predicted.

Dynamics of plume–triple junction interaction: Results from a series of three‐dimensional numerical models and implications for the formation of oceanic plateaus

AbstractMantle plumes rising in the vicinity of mid‐ocean ridges often generate anomalies in melt production and seafloor depth. This study investigates the dynamical interactions between a mantle plume and a ridge‐ridge‐ridge triple junction, using a parameter space approach and a suite of steady state, three‐dimensional finite element numerical models. The top domain boundary is composed of three diverging plates, with each assigned half‐spreading rates with respect to a fixed triple junction point. The bottom boundary is kept at a constant temperature of 1350°C except where a two‐dimensional, Gaussian‐shaped thermal anomaly simulating a plume is imposed. Models vary plume diameter, plume location, the viscosity contrast between plume and ambient mantle material, and the use of dehydration rheology in calculating viscosity. Importantly, the model results quantify how plume‐related anomalies in mantle temperature pattern, seafloor depth, and crustal thickness depend on the specific set of parameters. To provide an example, one way of assessing the effect of conduit position is to calculate normalized area, defined to be the spatial dispersion of a given plume at specific depth (here selected to be 50 km) divided by the area occupied by the same plume when it is located under the triple junction. For one particular case modeled where the plume is centered in an intraplate position 100 km from the triple junction, normalized area is just 55%. Overall, these models provide a framework for better understanding plateau formation at triple junctions in the natural setting and a tool for constraining subsurface geodynamical processes and plume properties.

Grain boundary, triple junction and quadruple point mobility controlled normal grain growth

Reduction in stored free energy provides the thermodynamic driving force for grain and bubble growth in polycrystals and foams. Evolution of polycrystalline networks exhibit the additional complication that grain growth may be controlled by several kinetic mechanisms through which the decrease in network energy occurs. Polyhedral boundaries, triple junctions (TJs), and quadruple points (QPs) are the geometrically distinct elements of three dimensional networks that follow Plateau’s rules, provided that grain growth is limited by diffusion through, and motion of, cell boundaries. Shvindlerman and co-workers have long recognized the kinetic influences on polycrystalline grain growth of network TJs and QPs. Moreover, the emergence of interesting polycrystalline nanomaterials underscored that TJs can indeed influence grain growth kinetics. Currently there exist few detailed studies concerned either with network distributions of grain size, number of faces per grain, or with ‘grain trajectories’, when grain growth is limited by the motion of its TJs or QPs. By contrast there exist abundant studies of classical grain growth limited by boundary mobility. This study is focused on a topological/geometrical representation of polycrystals to obtain statistical predictions of the grain size and face number distributions, as well as growth ‘trajectories’ during steady-state grain growth. Three limits to grain growth are considered, with grain growth kinetics controlled by boundary, TJ, and QP mobilities.

Optimal design of sintered Ce9Nd21FebalB1 magnets with a low-melting-point (Ce,Nd)-rich phase

A systemic investigation was done on the chemistry and crystal structure of boundary phases in sintered Ce9Nd21FebalB1 (wt%) magnets. Ce2Fe14B is believed to be more soluble in the rare-earth (RE)-rich liquid phase during the sintering process. Thus, the grain size and oxygen content were controlled via low-temperature sintering, resulting in high coercivity and maximum energy products. In addition, Ce formed massive agglomerations at the triple-point junctions, as confirmed by elemental mapping results. Transmission electron microscopy (TEM) images indicated the presence of (Ce,Nd)O x phases at grain boundaries. By controlling the composition and optimizing the preparation process, we successfully obtained Ce9Nd21FebalB1 sintered magnets; the prepared magnets exhibited a residual induction, coercivity, and energy product of 1.353 T, 759 kA/m, and 342 kJ/m3, respectively.

Diffusion in an Ensemble of Intersecting Grain Boundaries

Grain boundary (GB) diffusion in an ensemble of three grain boundaries intersecting in the point of GB triple junction is described on the basis of quasi-steady Fisher’s model. Two versions of the configuration of the ensemble are considered, namely, with different number of GBs adjacent to the surface covered with a diffuser source and with different angle between GB and surface. Analytical expressions for the distribution of diffuser concentration along each GB of an ensemble are derived supposing that the GB diffusion fluxes are equal in the GB triple junction. The expressions for the diffuser concentration distribution along GBs in both ensembles include not only diffusion constants (like GB and bulk diffusion coefficients) but also structural characteristics of the ensemble of grain boundaries (i.e. the depth of the triple junction point under the surface and the angle between GBs in the triple junction point). The specific features of diffusion kinetics in the ensembles of different configuration with an angle of 120o(the equilibrium angle in a polycrystal) were revealed by comparing the diffuser concentration distributions in the ensembles and in the single GB.

Effect of Dopant and Impurity Segregation on the Ionic Conductivity of Doped Ceria Electrolytes

Doped ceria is a good electrolyte candidate for intermediate temperature solid oxide fuel cells (IT-SOFC) because of its enhanced ionic conductivity. Segregation of dopants and impurities at grain boundaries may degrade the performance of these materials. This grain boundary segregation can be mitigated through manipulation of processing conditions. Recent studies have shown that, at 550°C, Sm0.75Nd0.75CeO2-δ made by co-precipitation from nitrates followed by microwave sintering exhibits enhanced ionic conductivity (2.8 × 10-3 S·cm-1) as compared to the ball milled and conventionally sintered (1.9 × 10-3 S·cm-1) samples (1). Here we focus on the effects that the composition and processing have on the bulk and grain boundary contributions to ionic conductivity and gain a systematic understanding of the underlying processes. Scanning electron microscopy (SEM), electron energy dispersive spectroscopy (EDS) and electron backscattered diffraction (EBSD) studies indicate that the co-precipitated and microwave sintered samples exhibit smaller grain size, lower impurity content and less segregation at grain boundaries and triple point junctions as compared to conventional solid state processing route. These results are correlated with ionic conductivity data obtained from electrochemical impedance spectroscopy (EIS). REFERENCES:S. Omar, “Enhanced Ionic Conductivity of Ceria-Based Compounds for the Electrolyte Application in SOFCs”, Ph.D. Dissertation, University of Florida (2008).

Effect of Functionally Graded Material of Disc Spacer with Presence of Multi-Contaminating Particles on Electric Field inside Gas Insulated Bus Duct

Solid insulators play a crucial role of electrical insulation in gas insulated power equipment. In order to improve the insulation performance of the solid insulators, two technical points should be considered, the first is the improvement of the insulation performance and the second is the control of the electric field distribution in and around the solid insulating spacers. Practically the insulation performance around the spacer can be improved by various techniques, such as; controlling the spacer shape, adding shield electrodes for electric field relaxation, the introduction of an embedded electrode, etc. These techniques lead to a more complicated structure of the equipment and increase the manufacturing cost. Thus, it is necessary to propose a new concept on solid spacers with keeping their simple structure and configuration. In this paper, a functionally graded material (FGM) is proposed to minimize the electric field distribution around the spacer, specially, on triple junction point, which was one of the important factors dominating a long-term insulating property of solid delectric. Finite Element Method (FEM) has been used throughout this work, for its favorable accuracy, to calculate the electric field distribution inside the bus duct. The Electric field distribution around earthed particle contamination which adhered to uniform and FGM of disc-spacer is presented. The effect of distance between particle and spacer on the electric field values is investigated. The effect of hemi-spherical radius and length of particle on maximum electric field at triple junction point is also discussed. Electric field relaxation effect (EFGM/Euniform) by introduction of the U-shape FGM spacer is also presented. The electric field distribution along the surface of FGM of disc spacer with presence of multi-contaminating particles at various positions is presented. DOI: http://dx.doi.org/10.11591/ijece.v3i6.4354

Experimental and Analytical Study on DC Breakdown Characteristics of Butt Gap Condition in $\hbox{LN}_{2}/\hbox{PPLP}$ Composite System

Due to ac loss in superconducting materials, high cryogenic costs are inevitable when superconducting devices are operated in ac power networks. Thus, dc electric power networks would be regarded as a better choice for the operation of superconducting devices. In order to develop superconducting devices for a dc network, the dc insulation characteristics, which are much different from the ac insulation characteristics, should be clarified. In this paper, in order to investigate the dc insulation characteristics of polypropylene laminated paper (PPLP), which is generally used for dc superconducting cable, a dc breakdown test and a dc electric field analysis were performed. For the dc breakdown test, specimens with three layers of PPLP with one butt gap were fabricated. In order to reveal the breakdown characteristic of PPLP, a dc electric fields calculation in the media at the moment of breakdown was performed considering capacitive and resistive field distributions. Consequently, the capacitive electric field and resistive electric field distributions were determined using dc field analysis techniques and it was found that the butt gap edge is affected enough by the high field strength to cause the breakdown. Furthermore, it was deduced that the butt gap edge acted as a triple-junction point which causes the breakdown.

Globule-rich lavas in the Razjerd district, Qazvin, Iran: a unique volcanic fabric

A hypocrystalline silica-rich (63–67 wt.% SiO2, dacitic composition) lava flow (called G-lava) in the subaerial eruptive sequence of the Alborz Mountains (Razjerd district, Qazvin Province) of northern Iran, contains abundant (40–50 vol.%) 0.1- to 5.0-cm globules set in a matrix of rather similar composition and microtexture. Numerous globules have coalesced, showing triple-point junctions with 120° angles. Both phases in the G-lava (globules and matrix) contain similar microphenocrysts (plagioclase, ortho- and clinopyroxene and magnetite) in a trachytic groundmass. However, their mesostasis differ in colour, in composition, in the amount of glass and their amount of volatiles and silica: in the globules the mesostasis is darker and richer in SiO2 but is volatile poor. Other volcanic materials in the same unit are very similar in composition to the G-lava. The globular fabric was formed with two phases: one poor in volatiles (the globules), the other rich in volatiles (the matrix). The globules are slightly more silicic (66.9 against 64.6 wt.% SiO2), more potassic (3.7 againt 2.8 wt.% K2O) and more viscous (of the order of 103 to 104) than the matrix outside the globules. It seems that the two phases (globules and matrix) with different silica and volatiles contents and thus different vesicularities, viscosities and densities, were produced in the dacitic melt due to temperature and pressure drop and magmatic degassing in the volcanic conduit involved fluid-melt exsolution processes. Some of the volatile-rich melt was probably frothy during eruption, producing volcanic bombs and scoria.

Seismic hazard studies in Egypt

The study of earthquake activity and seismic hazard assessment of Egypt is very important due to the great and rapid spreading of large investments in national projects, especially the nuclear power plant that will be held in the northern part of Egypt. Although Egypt is characterized by low seismicity, it has experienced occurring of damaging earthquake effect through its history. The seismotectonic sitting of Egypt suggests that large earthquakes are possible particularly along the Gulf of Aqaba–Dead Sea transform, the Subduction zone along the Hellenic and Cyprean Arcs, and the Northern Red Sea triple junction point. In addition some inland significant sources at Aswan, Dahshour, and Cairo-Suez District should be considered. The seismic hazard for Egypt is calculated utilizing a probabilistic approach (for a grid of 0.5°×0.5°) within a logic-tree framework. Alternative seismogenic models and ground motion scaling relationships are selected to account for the epistemic uncertainty. Seismic hazard values on rock were calculated to create contour maps for four ground motion spectral periods and for different return periods. In addition, the uniform hazard spectra for rock sites for different 25 periods, and the probabilistic hazard curves for Cairo, and Alexandria cities are graphed. The peak ground acceleration (PGA) values were found close to the Gulf of Aqaba and it was about 220gal for 475year return period. While the lowest (PGA) values were detected in the western part of the western desert and it is less than 25gal.

SiAlON Bulk Glasses and Their Role in Silicon Nitride Grain Boundaries: Composition-Structure-Property Relationships

SiAlON glasses are silicates or alumino-silicates, containing Mg, Ca, Y or rare earth (RE) ions as modifiers, in which nitrogen atoms substitute for oxygen atoms in the glass network. These glasses are found as intergranular films and at triple point junctions in silicon nitride ceramics and these grain boundary phases affect their fracture behaviour. This paper provides an overview of the preparation of M-SiAlON glasses and outlines the effects of composition on properties. As nitrogen substitutes for oxygen in SiAlON glasses, increases are observed in glass transition temperatures, viscosities, elastic moduli and microhardness. These property changes are compared with known effects of grain boundary glass chemistry in silicon nitride ceramics. Oxide sintering additives provide conditions for liquid phase sintering, reacting with surface silica on the Si₃N₄ particles and some of the nitride to form SiAlON liquid phases which on cooling remain as intergranular glasses. Thermal expansion mismatch between the grain boundary glass and the silicon nitride causes residual stresses in the material which can be determined from bulk SiAlON glass properties. The tensile residual stresses in the glass phase increase with increasing Y:Al ratio and this correlates with increasing fracture toughness as a result of easier debonding at the glass/β-Si₃N₄ interface.

Formation of dense alumina nanowires from anodic alumina membranes

Dense alumina nanowire arrays were formed by a method that consists in a partial dissolution of the anodized aluminium oxide (AAO) membrane by chemical attack in 2M H3PO4 during 45 minutes, as a step immediately following the second anodizing. During this procedure the pore walls are partially dissolved by a controlled acid attack and the alumina nanowires arise as a consequence of this process, being the vertices of the hexagonal pores the origin of them. The thickness of them (≈40nm) is slightly smaller than the diameter corresponding to the triple junction point of the membrane. By controlling the dissolution time it is possible to obtain alumina nanowires covering virtually the entire surface with lengths similar to the thickness of the AAO membrane that exceeding 10μm. The steps of nanowires formation are discussed as follows.

Role of marangoni tension effects on the melt convection in directional solidification process for multi-crystalline silicon ingots

We carried out global simulations to investigate the marangoni tension effect on the thermal and flow fields in the silicon melt of the directional solidification process for multi-crystalline silicon ingots. The argon flow rate was varied to provide different solidification conditions and to change the relative values between the argon shear stress and the marangoni tension at the melt free surface. We found that the marangoni tension together with the shear stress mainly influences the upper layer melt convection while the thermal buoyancy force dominates the bulk flow of the melt. At low argon flow rates, the argon shear stress can be neglected and the marangoni tension alone enhances the melt convection intensity near the gas–melt–crucible triple junction point. The marangoni tension is so weak that it cannot modify the melt flow pattern in this case. For medium flow rate, the marangoni tension can significantly weaken the shear stress effect at the outer part of the melt free surface, leading to a distinctive flow pattern in the silicon melt. With further increase in argon flow rate, the shear stress sharply increases and dominates the upper layer melt flow, limiting the marangoni tension effect to the triple point. The numerical results are helpful for better understanding and controlling of the directional solidification process for high quality multi-crystalline silicon ingots.

Microstructural Evolution of Mo-Si-B Ternary Alloys through Heat Treatment at 1800°C

First of all, the as-cast microstructures of Mo-rich Mo-Si-B ternary alloys were investigated around the triple junction point of the primary Mo solid solution, Mo5SiB2 and Mo2B in this work, based on the liquidus projections of the Mo-Si-B system which have been reported in earlier studies. Subsequently, their microstructural evolution through heat treatment was investigated. Since Mo2B crystallizes out during solidification into a primary or secondary phase even though the alloy composition lies in the triangle of Mo-Mo5SiB2-Mo3Si in the Mo-Si-B equilibrium phase diagram, the as-cast microstructures include the non-equilibrated Mo2B in wide compositional ranges. However, Mo2B was completely decomposed during heat treatment at 1800 °C for 24 h and this contributed to the development of homogeneous, fine microstructures. On the other hand, since Mo2B was not decomposed perfectly during 24 h of 1600 °C heat treatment, as-cast microstructures largely remained. Therefore, it is realized that the heat treatment at 1800 °C is necessary to obtain well-developed microstructures of Mo-Si-B alloys.

Curie depth map for Sinai Peninsula, Egypt deduced from the analysis of magnetic data

Sinai Peninsula is considered as a unique region in the world due to its geographical location, tectonic and thermal activities. It is located geographically at the crossroads of Europe, Asia, and Africa constituting a triple junction point between the three continents. It is also characterized by thermal manifestations represented by several hot springs with varied temperatures (30–70 °C). Most of these hot springs are located along the shoreline of the Gulf of Suez. In this study, we aim to map the Curie depth isotherm surface for Sinai Peninsula based on the analysis of ground magnetic data. Spectral analysis technique will be used to estimate the boundaries (top and bottom) of the magnetized crust. The depths obtained for the bottom of magnetized crust are assumed to correspond to Curie point depths where the magnetic layer loss its magnetization. Results of this study indicate that the shallow Curie depths (~ 15–18 km) are located at the southern part of Sinai Peninsula and along the shoreline of the Gulf of Suez and depths increase (22–25 km) towards the central and north western portions of Sinai Peninsula. The whole average Curie depth point of Sinai Peninsula is about 20 km. Generally, the shallow depths to Curie isotherm indicate that Sinai Peninsula is a promising area for further geothermal exploration particularly near the eastern side of the Gulf of Suez.

Effect of UV illumination on surface flashover characteristics of epoxy resin under pulse

Surface flashover in vacuum is a primary limitation to pulsed-power devices. Recently, there are more studies on the pulsed vacuum surface flashover focused on the flashover mechanism. In this paper, the UV illumination is used to study the mechanism of vacuum surface flashover. The effects of different illumination positions, different illumination durations and different illumination intensities are investigated. The results show that the pulsed vacuum surface flashover characteristic is affected by the UV illumination significantly. When the UV light illuminates the cathode triple-point junction, the flashover voltage is reduced; this reduction is almost unaffected by the illumination duration, but significantly increased with the enhancement of illumination intensity. When the UV light illuminates the sample surface, the flashover voltage reduction is increased both by the increases of illumination duration and illumination intensity. Moreover, UV illumination can also change the surface potential and thus influence the surface flashover characteristic.