INITIAL EVOLUTION OF THE CATHODE DIRECTED STREAMER CHANNEL AFTER BRIDGING THE DISCHARGE GAP

The presence of water molecules in air introduces complexities to residual charge transports and energy relaxations that may provoke streamer discharge instabilities under repetitive pulses. Evolutions of pulse-periodic positive streamer dynamics were investigated in humid air. Pulse-sequence and temporally resolved diagnostics were implemented to capture discharge evolutions. The streamer development and evolutions of charged species in humid air are qualitatively analyzed based on a 2D–0D combined simulation. Evolution features of streamer behaviors in humid air include the faster filamentation of the primary streamer, pronounced propagation selectivity to previous secondary streamers, more branches of both primary and secondary streamers, and accelerated secondary streamer into stagnation. The repetitively pulsed breakdown is proceeded by the progressive axial prolongation of secondary streamers with bright heads and faint tails. Nonintuitively, the withstanding capability does not illustrate dramatic differences in dry air and humid air especially at high pulse repetition frequency. High-density residing hydrated ions with high electron bound energies (impeding streamer propagation) and the enhanced thermal release to cause higher reduced electric field (facilitating streamer propagation) may contrarily affect evolutions and discharge instability developments of positive streamer in humid air.

The experimental study was performed to clarify the basic behaviour of streamers traveling over the surface of oil-immersed polyethylene insulated wire under a standard lightning impulse voltage application. The effect of the grounded side electrode on the characteristics of creepage discharges, such as discharge inception voltage, discharge extension length, discharge current and charge, discharge figure has been studied. There is a remarkable difference in these characteristics between positive and negative polarities of the streamer. Though the approach of the grounded side electrode to the wire restrains the generation and progression of streamers, it leads to the reduction of a punch-through breakdown voltage of the wire insulator. The authors also measured the electric potential along the streamer channel and the streamer propagation velocity by means of a capacitive probe. It is indicated that the streamer continues to progress in a constant velocity and the streamer channel is not equipotential. The propagation of streamers appears to be stopped by a finite potential drop along the streamer channel.

We present a computational simulation study of non-equilibrium streamer discharges in a coaxial electrode and a corona geometry for automotive combustion ignition applications. The streamers propagate in combustible fuel-air mixtures at high pressures representative of internal combustion engine conditions. The study was performed using a self-consistent, two-temperature plasma model with finite-rate plasma chemical kinetics. Positive high voltage pulses of order tens of kV and duration of tens of nanoseconds were applied to the powered inner cylindrical electrode which resulted in the formation and propagation of a cathode-directed streamer. The resulting spatial and temporal production of active radical species such as O, H, and singlet delta oxygen is quantified and compared for lean and stoichiometric fuel-air mixtures. For the coaxial electrode geometry, the discharge is characterized by a primary streamer that bridges the inter-electrode gap and a secondary streamer that develops in the wake of the primary streamer. Most of the radicals are produced in the secondary streamer. For the corona geometry, only the primary streamer is observed and the radicals are produced throughout the length of the primary streamer column. The stoichiometry of the mixture was observed to have a relatively small effect on both the plasma discharge structure and the resulting yield of radical species.

SF 6 /N 2 mixtures is considered as a practical alternative proposal of SF 6 in GIS and GIL. Simulating particle transportations and chemical reactions during the positive streamer propagation in SF 6 /N 2 mixtures assists in the insulation performance optimization of SF 6 /N 2 mixtures. This paper presents a fluid dynamic model coupled with the chemical reaction system for the positive streamer propagation in SF 6 /N 2 mixtures. The Townsend coefficients of the 20%SF 6 /80%N 2 mixture derived from the proposed chemical reaction system are in accordance with the measurement results. Based on this model, the propagation of the positive streamer in a 1 mm needle-plate gap in the 20%SF 6 /80%N 2 mixture is simulated. Simulated results indicate that there exists a negative space charge region in the streamer channel. It can be attributed to the increasing attachment rates of SF 6 yielding SF 5 − and SF 6 −, and the reduction of the ionization rates of N 2 and SF 6 in the streamer channel. Besides, it is noted that the ionization activities of N 2 and SF 6 at the streamer head are equally important to support the continuous propagation of the positive streamer. The ionization of N 2 in the streamer channel serves as an important factor to supply electrons and avoid the accumulation of the negative space charge.

In the recent days, coolant liquid plays a major role in the removal of heat from the heat exchanger. Past researchers analysed the stability and thermal conductivity of the coolant liquid with additives of various metal nanoparticles. In the present study, coolant liquid is prepared by using Multiwall Carbon Nanotubes (MWCNT) with water and surface modifiers. Six different compositions were prepared and tested the thermal conductivity through KD-2 Pro thermal analyser. Stability of the liquid was improved by sonication and two different surface modifiers. In this study thermo-physical properties of the prepared liquid were evaluated. The tested results of the coolant liquid reveal that the thermal conductivity value is found to be increased up to 24% for 0.2% with Cethyl Trimthyl Ammonium Bromide (CTAB) as surface modifier and the prepared coolant also shows better stability.KeywordsMultiwall carbon nanotubes (MWCNT)Volume fractionStabilityViscosityThermal conductivity

We investigate the effect of humidity on the propagation of streamers in air. We present a minimal set of chemical reactions that takes into account the presence of water in a nonthermal air plasma and considers ionization, attachment, detachment, recombination and ion conversion including water cluster formation. We find differences in streamer propagation between dry and humid air that we attribute mostly to an enhanced effective attachment rate in humid air, leading to higher breakdown electric field and threshold field for propagation. This higher effective attachment rate in humid conditions leads to a faster decay of the conductivity in the streamer channel, which hinders the accumulation of charge in the streamer head. In some cases a propagating streamer solution still exists at the expense of a smaller radius and lower velocity. In other cases a high humidity leads to the stagnation of the streamer. We finally discuss how all these statements may affect streamer branching and the dimensions and lifetime of a streamer corona.

Photo-ionization is the accepted mechanism for the propagation of positive streamers in air though the parameters are not very well known; the efficiency of this mechanism largely depends on the presence of both nitrogen and oxygen. But experiments show that streamer propagation is amazingly robust against changes of the gas composition; even for pure nitrogen with impurity levels below 1 ppm streamers propagate essentially with the same velocity as in air, but their minimal diameter is smaller, and they branch more frequently. Additionally, they move more in a zigzag fashion and sometimes exhibit a feathery structure. In our simulations, we test the relative importance of photo-ionization and of the background ionization from pulsed repetitive discharges, in air as well as in nitrogen with 1 ppm O2. We also test reasonable parameter changes of the photo-ionization model. We find that photo-ionization dominates streamer propagation in air for repetition frequencies of at least 1 kHz, while in nitrogen with 1 ppm O2 the effect of the repetition frequency has to be included above 1 Hz. Finally, we explain the feather-like structures around streamer channels that are observed in experiments in high purity nitrogen, but not in air.

Streamer properties such as their velocity, diameter, intensity and density, can be obtained by analysis of temporal and spatial resolved ICCD imaging. In this paper, experimental results on streamer generation and propagation as a function of several high-voltage pulse and reactor parameters are described. Experiments were performed on a large scale wire–plate reactor in ambient air. The set-up allows for independent variation of the parameters over wide ranges. The minimum gate time of the ICCD camera is 5 ns, allowing for a high temporal resolution. The camera can be triggered with a precision of 1 ns. Both negative and positive polarity pulses are investigated. The most important conclusions are as follows. (1) The streamer velocity ((0.5–2.5) × 106 m s−1) increases if the applied electric field and/or the voltage rise rate is increased. (2) The same is true regarding the velocity ((0.2–1.2) × 105 m s−1) with which the streamer diameter (0.7–3.0 mm) increases during propagation. (3) Typical properties (velocity, diameter, etc) of negative and positive polarity streamers vary less than 25%, especially when the applied electric field is high. (4) As long as the dc bias voltage is below the dc corona onset value it does not have a separate effect on the visual streamer properties. Only the total voltage (peak voltage + dc) is of importance. (5) A simple model was used to determine the electric field in the secondary streamer channel. It was found that in the light emitting part of the secondary streamer the electric field is approximately 21.5 kV cm−1. In the remainder (dark part) of the channel the electric field is around 6.5 kV cm−1. This paper shows mainly experimental findings. Not all observed relations and phenomena could be explained. This is partly caused by the fact that current theoretical and numerical models are not yet able to describe the experimental situation as used during this study.

A number of industries are developing new product-lines with innovative electronics packages that require low thermal resistance and high temperature stability, including hybrid electric vehicles, concentrator photovoltaic and wide bandgap RF amplifiers. One of the largest obstacles in the design and manufacture is the thermal management of the devices. Localized heat generation is the characteristic of the semiconductor chips used in these devices. For high power applications the thermal impedance of the die attach layer can play significant role in the thermal management and the operating temperature. Therefore, one would like to use the highest thermal conductivity and lowest thermal resistance die attach material that is capable for high volume manufacturing. In a typical electronic packaging process, chips are attached to substrates and electrically connected before they are encapsulated or sealed for protection. The attachment and electrical interconnections provide the chip with an infrastructure for the flow of electrical signals, mechanical support and heat removal. The die attach materials <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(1)</sup> used in the packaging of high performance power semiconductors are required to have high thermal conductivity. Lead solders, eutectic gold-tin, transient liquid phase sintering (TLPS) pastes <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> and nano-silver sintering technology <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(3)</sup> are typical materials used for the die attachment of power semiconductor. This paper will introduce a new die attach material using Hybrid Silver Sintering Technology (HSST).

The power density of power electronics in electric vehicles (EVs) is expected to be significantly enhanced, which would result in higher heat flux generation. Therefore, it is critical to find an efficient heat spreading and removal technology to reduce the junction temperature for high power density power electronics application. Vapor chamber using evaporation and condensation of the inside working fluid (i.e., water) as a heat spreader can remarkably improve the uniform heat spreading and heat removal for the power electronics compared to the traditional copper heat spreader. Furthermore, high thermal conductivity graphite foam (GF) would be embedded in the vapor core of the vapor chamber to further enhance the heat transfer performance. Numerical heat transfer simulations were performed for copper heat spreader, vapor chambers (with and without the presence of graphite foam) by using ANSYS. Various parametric effects on vapor chamber thermal performance were investigated, including the effects of heat flux from power electronics, heat transfer coefficient at heat sink, vapor chamber thickness, and graphite foam. Through the simulation studies, it was found that thinner vapor chamber (1.35 mm thickness) had better heat transfer performance than thicker vapor chamber (5 mm thickness) because of the extreme high effective thermal conductivities of ultra-thin vapor chamber. Furthermore, the effect of graphite foam on thermal performance improvement was very minor for ultra-thin vapor chamber, but significant for thick vapor chamber. The GF could help reduce the junction temperature by 15–30% in the 5-mm thick vapor chamber. Use of GF embedded vapor chamber could achieve 250–400 W/cm2 local heat removal for power electronics.

Heat removal by the divertor plate and limiter materials in fusion reactors

Aluminum nitride is used for the quick removal of exothermic heat from MPU (Micro Processing Unit) used in computers and from the power module for application to hybrid cars. It has high thermal conductivity, but this property varies with method of manufacturing material powder. Thermal conductivity of sintered body from direct nitrided powder is lower than that of sintered body from reduction nitrided powder. The decreasing of thermal conductivity caused by direct nitrided powder was larger than that of impurities in raw powder as oxygen and metal. Thus, the direct nitrided powder is thought to have another mechanism of the decreasing of thermal conductivity of AlN sintered body. We found that sintered body from direct nitrided powder has many dislocations in grains. These dislocations are thought to be the cause of the decreasing of thermal conductivity. In addition, we found that dislocations occurred in crashing process and occurred at the time of crystal breaking within 300 μm from cracks.

Uranium dioxide (UO2) is the most commonly used fuel in light water nuclear reactors and thermal conductivity controls the removal of heat produced by fission, therefore, governing fuel temperature during normal and accident conditions. The use of fuel performance codes by the industry to predict operational behavior is widespread. A primary source of uncertainty in these codes is thermal conductivity, and optimized fuel utilization may be possible if existing empirical models were replaced with models that incorporate explicit thermal conductivity degradation mechanisms during fuel burn-up. This approach is able to represent the degradation of thermal conductivity due to each individual defect type, rather than the overall burn-up measure typically used which is not an accurate representation of the chemical or microstructure state of the fuel that actually governs thermal conductivity and other properties. To generate a mechanistic thermal conductivity model, molecular dynamics (MD) simulations of UO2 thermal conductivity including representative uranium and oxygen defects and fission products are carried out. These calculations employ a standard Buckingham type interatomic potential and a potential that combines the many-body embedded atom method potential with Morse-Buckingham pair potentials. Potential parameters for UO2+x and ZrO2 are developed for the latter potential. Physical insights from the resonant phonon-spin scattering mechanism due to spins on the magnetic uranium ions have been introduced into the treatment of the MD results, with the corresponding relaxation time derived from existing experimental data. High defect scattering is predicted for Xe atoms compared to that of La and Zr ions. Uranium defects reduce the thermal conductivity more than oxygen defects. For each defect and fission product, scattering parameters are derived for application in both a Callaway model and the corresponding high-temperature model typically used in fuel performance codes. The model is validated by comparison to low-temperature experimental measurements on single crystal hyper-stoichiometric UO2+x samples and high-temperature literature data. Ongoing works include investigation of the effect of phase separation to UO2+U4O9 on the low temperature thermal conductivity of UO2+x, and modeling of thermal conductivity using the Green-Kubo method. Ultimately, this work will enable more accurate fuel performance simulations as well as extension to new fuel types and operating conditions, all of which improve the fuel economics of nuclear energy and maintain high fuel reliability and safety.

The aim of this paper is to develop simplified model for WiFi electromagnetic field propagation. The model can be used in preliminary estimation of WiFi transmitter localization for the purpose of WiFi-based indoor localization. It can be particularly useful when designing structural networks.We have developed a simplified linear model of WiFi electromagnetic field modelling and compared it with the most commonly used, more sophisticated models and with measurement results which were carried out in the laboratory. As demonstrated by the results of measurements conducted using various hardware, the accuracy of this simplified model introduced is similar to the commonly used models, but the number of parameters is lower. Therefore, our model easier to implement in real life conditions. The model presented in this paper enables WiFi electromagnetic field modelling when the exact values of propagation parameters and transmitter characteristics is unknown. This is usually the case at the early stage of structural network design,when exact parameters of building construction materials are not known. As the model is very simple, it does not require much effort to deploy, while its accuracy is sufficient for preliminary WiFi transmitter localization. Simplified models of WiFi electromagnetic field propagation are known, but no comparative research combined with measurements has been done in this field. This paper provides a comparison of different electromagnetic field models which can be applied to WiFi electromagnetic field propagation together with measurement results.

Kinetics of non-isothermal nucleation of thin films