| 1. |
- Becerra, Marley, et al.
(författare)
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A self-consistent upward leader propagation model
- 2006
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Ingår i: Journal of Physics D. - : Institute of Physics Publishing (IOPP). - 0022-3727 .- 1361-6463. ; 39:16, s. 3708-3715
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Tidskriftsartikel (refereegranskat)abstract
- The knowledge of the initiation and propagation of an upward movingconnecting leader in the presence of a downward moving lightning steppedleader is a must in the determination of the lateral attraction distance of alightning flash by any grounded structure. Even though different models that simulate this phenomenon are available in the literature, they do not take into account the latest developments in the physics of leader discharges. Theleader model proposed here simulates the advancement of positive upward leaders by appealing to the presently understood physics of that process.The model properly simulates the upward continuous progression of thepositive connecting leaders from its inception to the final connection withthe downward stepped leader (final jump). Thus, the main physical properties of upward leaders, namely the charge per unit length, the injected current, the channel gradient and the leader velocity are self-consistentlyobtained. The obtained results are compared with an altitude triggeredlightning experiment and there is good agreement between the modelpredictions and the measured leader current and the experimentally inferredspatial and temporal location of the final jump. It is also found that the usualassumption of constant charge per unit length, based on laboratoryexperiments, is not valid for lightning upward connecting leaders.
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| 2. |
- Becerra, Marley, et al.
(författare)
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Time dependent evaluation of the lightning upward connecting leader inception
- 2006
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Ingår i: Journal of Physics D. - : Institute of Physics Publishing (IOPP). - 0022-3727 .- 1361-6463. ; 39:21, s. 4695-4702
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Tidskriftsartikel (refereegranskat)abstract
- The evaluation of the upward connecting leader inception from a grounded structure has generally been performed neglecting the effect of the propagation of the downward stepped leader. Nevertheless, field observations suggest that the space charge produced by streamer corona andaborted upward leaders during the approach of the downward lightning leader can influence significantly the initiation of stable upward positive leaders. Thus, a physical leader inception model is developed, which takes into account the electric field variations produced by the descending leader during the process of inception. Also, it accounts for the shielding effect produced by streamer corona and unstable leaders formed before the stable leader inception takes place. The model is validated by comparing its predictions with the results obtained in long gap experiments and in an altitude triggered lightning experiment. The model is then used to estimate the leader inception conditions for free standing rods as a function of tip radius and height. It is found that the rod radius slightly affects the height of the downward leader tip necessary to initiate upward leaders. Only an improvement of about 10% on the lightning attractiveness can be reached byusing lightning rods with an optimum radius. Based on the obtained results, the field observations of competing lightning rods are explained. Furthermore, the influence of the average stepped leader velocity on theinception of positive upward leaders is evaluated. The results obtained show that the rate of change of the background electric field produced by a downward leader descent largely influences the conditions necessary for upward leader initiation. Estimations of the leader inception conditions for the upper and lower limit of the measured values of the average downward lightning leader velocity differ by more than 80%. In addition, the striking distances calculated taking into account the temporal change of the background field are significantly larger than the ones obtained assuming a static downward leader field. The estimations of the present model are alsocompared with the existing leader inception models and discussed.
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| 3. |
- Widenkvist, Erika, et al.
(författare)
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Mild sonochemical exfoliation of bromine-intercalated graphite a new route towards graphene
- 2009
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Ingår i: Journal of Physics D. - : IOP Publishing Ltd. - 0022-3727 .- 1361-6463. ; 42:11, s. 112003-
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Tidskriftsartikel (refereegranskat)abstract
- A method to produce suspensions of graphene sheets by combining solution-based bromine intercalation and mild sonochemical exfoliation is presented. Ultrasonic treatment of graphite in water leads to the formation of suspensions of graphite flakes. The delamination is dramatically improved by intercalation of bromine into the graphite before sonication. The bromine intercalation was verified by Raman spectroscopy as well as by x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations show an almost ten times lower interlayer binding energy after introducing Br2 into the graphite. Analysis of the suspended material by transmission and scanning electron microscopy (TEM and SEM) revealed a significant content of few-layer graphene with sizes up to 30 µm, corresponding to the grain size of the starting material
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| 4. |
- Engström, Christian
(författare)
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Bounds on the effective tensor and structural parameters for anisotropic two-phase composite material
- 2005
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Ingår i: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 38, s. 3695-3702
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Tidskriftsartikel (refereegranskat)abstract
- This paper is concerned with the estimation of macroscopic properties such as the permittivity or the thermal conductivity of a composite material from the microstructure. A new method of estimating the microstructural parameters, such as the volume fraction of anisotropic two-phase composite material, is derived. The parameters are estimated using information from measurements of the random material or, in the periodic case, from numerical experiments. The method is used to derive new bounds on the effective tensor that incorporates information from measurements of a related parameter. These new bounds are called cross-property bounds. New tight bounds on low-order microstructural parameters are given in the anisotropic case.
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| 5. |
- Gylfason, Kristinn B., 1978-, et al.
(författare)
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Ion-acoustic solitary waves in a high power pulsed magnetron sputtering discharge
- 2005
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Ingår i: Journal of Physics D. - : Institute of Physics Publishing (IOPP). - 0022-3727 .- 1361-6463. ; 38:18, s. 3417-3421
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Tidskriftsartikel (refereegranskat)abstract
- We report on the creation and propagation of ion-acoustic solitary waves in a high power pulsed magnetron sputtering discharge. A dense localized plasma is created by applying high energy pulses (4-12 J) of length; approximate to 70 mu s, at a repetition frequency of 50 pulses per second, to a planar magnetron sputtering source. The temporal behaviour of the electron density, measured by a Langmuir probe, shows solitary waves travelling away from the magnetron target. The velocity of the waves depends on the gas pressure but is roughly independent of the pulse energy.
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| 6. |
- Kossyi, I.A., et al.
(författare)
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Polyphase (non-resonant) multipactor in rectangular waveguides
- 2008
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 41:6, s. 065203 1-8
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Tidskriftsartikel (refereegranskat)abstract
- The polyphase multipactor, i.e. the non-resonant form of secondary electron emission rf discharges in vacuum, has been analysed and studied experimentally. The multipactor discharge was observed in an evacuated standard rectangular waveguide through which pulsed high-power microwave radiation in the decimeter wavelength range was transmitted. The power interval in which the two-sided (between the wide walls of the waveguide) multipactor appeared has been determined. It is found that there is a characteristic delay time for the onset of the multipactor breakdown as compared with the time at which the microwave power is applied. The dependence of this delay time on the microwave power has been established. The experimental results are compared with results of numerical simulations which make it possible to estimate the secondary emission properties of the waveguide walls. Reasons for some observed discrepancies between numerical results and experimental data are discussed as well as the nature of the observed multipactor delay.
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| 7. |
- Marucci, Mariagrazia, et al.
(författare)
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Determination of a diffusion coefficient in a membrane by electronic speckle pattern interferometry: a new method and a temperature sensitivity study
- 2007
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 40:9, s. 2870-2880
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Tidskriftsartikel (refereegranskat)abstract
- In this work, a method has been developed to easily determine the effective diffusion coefficient (D-e) of a solute in a permeable membrane using electronic speckle pattern interferometry. Fringes are introduced parallel to the direction of diffusion during the diffusion process and D-e can be calculated by simple measurements on the interference pattern. For a fast and convenient determination of D-e, a mathematical expression has been derived from the analytical solution of diffusion in two media separated by a resistance. The D-e obtained when fringes are introduced is in agreement with that obtained when fringes are not introduced. The effect of temperature variation on the optical path of the reference and the object beams has also been investigated. The error introduced into the calculation of D-e, when the temperature oscillation is not taken into account, has been compared for the case when fringes are not introduced during the diffusion experiment and the case when fringes are introduced. In the first case, the relative error can be greater than 100%. Interestingly, in the latter case, the error caused by temperature oscillation is considerably reduced, and no error is introduced if the temperature changes homogeneously over the whole diffusion cell used for the diffusion experiment.
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| 8. |
- Ramprecht, Jörgen, et al.
(författare)
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Magnetic Losses in Composite Materials
- 2008
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 41:13, s. 135005-
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Tidskriftsartikel (refereegranskat)abstract
- We discuss some of the problems involved in homogenization of a composite material built from ferromagnetic inclusions in a nonmagnetic background material. The small signal permeability for a ferromagnetic spherical particle is combined with a homogenization formula to give an effective permeability for the composite material. The composite material inherits the gyrotropic structure and resonant behaviour of the single particle. The resonance frequency of the composite material is found to be independent of the volume fraction, unlike dielectric composite materials. The magnetic losses are described by a magnetic conductivity which can be made independent of frequency and proportional to the volume fraction by choosing a certain bias. Finally, some concerns regarding particles of small size, i.e. nanoparticles, are treated and the possibility of exciting exchange modes are discussed. These exchange modes may be an interesting way to increase losses in composite materials.
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| 9. |
- Siderakis, K., et al.
(författare)
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Influence of heat conductivity on the performance of RTV SIR coatings with different fillers
- 2005
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 38:19, s. 3682-3689
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Tidskriftsartikel (refereegranskat)abstract
- Room temperature vulcanized silicone rubber (RTV SIR) coatings are employed in order to improve the pollution performance of high voltage ceramic insulators by imparting surface hydrophobicity. In this paper, the performance of three RTV SIR coatings containing different fillers is investigated in a salt-fog test. Alumina trihydrate (ATH) and silica are the fillers included in the formulation, aiming to increase the material endurance to the energy supplied by the surface electrical activity during periods of hydrophobicity loss. The primary action of these fillers is to increase the material heat conductivity, i.e. the amount of energy conducted to the substrate. In addition, in the case of ATH relief is also achieved due to particle decomposition. The results indicate that for the compositions commercially available, where low amounts of fillers are used, and under the conditions of the test, ATH filled coatings performed better than the silica filled ones. This is attributed to ATH decomposition which further relieves the material structure and therefore decelerates material aging.
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| 10. |
- Sjöberg, Daniel
(författare)
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A modified Drude-Born-Fedorov model for isotropic chiral media, obtained by finite scale homogenization
- 2008
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 41:15
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Tidskriftsartikel (refereegranskat)abstract
- Isotropic chiral media can be modelled in several ways. We show that when realizing such a medium from spiral inclusions with real permittivities and permeabilities in a periodic structure, a modified Drude–Born–Fedorov model is obtained. This model equals the original Born model for small wave vectors, but the strength of the chirality depends on the propagation direction. The effective parameters are calculated by an unbiased finite scale homogenization method, which in principle could generate any model.
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