| 1. |
- Johannes, A., et al.
(author)
-
Enhanced sputtering and incorporation of Mn in implanted GaAs and ZnO nanowires
- 2014
-
In: Journal of Physics D: Applied Physics. - Bristol : IOP Publishing. - 1361-6463 .- 0022-3727. ; 47:39
-
Journal article (peer-reviewed)abstract
- We simulated and experimentally investigated the sputter yield of ZnO and GaAs nanowires, which were implanted with energetic Mn ions at room temperature. The resulting thinning of the nanowires and the dopant concentration with increasing Mn ion fluency were measured by accurate scanning electron microscopy (SEM) and nano-x-Ray Fluorescence (nanoXRF) quantification, respectively. We observed a clearly enhanced sputter yield for the irradiated nanowires compared to bulk, which is also corroborated by iradina simulations. These show a maximum if the ion range matches the nanowire diameter. As a consequence of the erosion thinning of the nanowire, the incorporation of the Mn dopants is also enhanced and increases non-linearly with increasing ion fluency.
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| 2. |
- Virojanadara, Chariya, et al.
(author)
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Large homogeneous mono-/bi-layer graphene on 6H-SiC(0001) and buffer layer elimination
- 2010
-
In: JOURNAL OF PHYSICS D-APPLIED PHYSICS. - : Iop Publishing Ltd. - 0022-3727 .- 1361-6463. ; 43:37, s. 374010-
-
Journal article (peer-reviewed)abstract
- In this paper we discuss and review results of recent studies of epitaxial growth of graphene on silicon carbide. The presentation is focused on high quality, large and uniform layer graphene growth on the SiC(0 0 0 1) surface and the results of using different growth techniques and parameters are compared. This is an important subject because access to high-quality graphene sheets on a suitable substrate plays a crucial role for future electronics applications involving patterning. Different techniques used to characterize the graphene grown are summarized. We moreover show that atomic hydrogen exposures can convert a monolayer graphene sample on SiC(0 0 0 1) to bi-layer graphene without the carbon buffer layer. Thus, a new process to prepare large, homogeneous stable bi-layer graphene sheets on SiC(0 0 0 1) is presented. The process is shown to be reversible and should be very attractive for various applications, including hydrogen storage.
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| 3. |
- Kossyi, I.A., et al.
(author)
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Experimental and numerical investigation of multipactor discharges in a coaxial waveguide
- 2010
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 43:34
-
Journal article (peer-reviewed)abstract
- An experimental and numerical investigation is made of multipactor discharges in a coaxial waveguide. Particular attention is given to the determination of the multipactor threshold and the distribution of the impact energy of the electrons. Simulations are carried out for different parameters of the secondary emission coefficient of the electrode surfaces. This makes it possible to determine these parameters through a comparison between the numerical and experimental results. The comparison also shows that the observed multipactor is mainly of polyphase (non-resonant) nature and represents a mixture of single- and double-surface multipactor discharges.
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| 4. |
- Rasch, Joel, 1982, et al.
(author)
-
On the microwave breakdown stability of a spherical hot spot in air
- 2010
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 43:32
-
Journal article (peer-reviewed)abstract
- An analysis is made of the basic physical conditions under which a small local microwave-induced breakdown region in a gas may develop to extended 'global' breakdown. The analysis describes the different nonlinear stages of the microwave breakdown process. In the first stage, the increasing breakdown plasma density suppresses the electric field in the breakdown region to reach a quasi-stationary state with constant electron density. The subsequent heating of the gas due to absorption of microwave power in the breakdown plasma is then analysed and the corresponding steady state for the thermal evolution is found including the temperature dependence of the breakdown electric field. The stability properties of the stationary state are examined and it is found that there exists a critical (unstable) radius of the initial breakdown plasma region such that initial regions smaller than this critical dimension will shrink to ultimately vanish whereas plasma regions larger than the critical dimension will grow indefinitely and transform the local breakdown region into full scale 'global' breakdown. The practical implication of this model is to give an order of magnitude estimate for the critical size of hot spots, regions of enhanced field and intensified heating in rf systems.
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| 5. |
- Becerra, Marley, et al.
(author)
-
A self-consistent upward leader propagation model
- 2006
-
In: Journal of Physics D. - : Institute of Physics Publishing (IOPP). - 0022-3727 .- 1361-6463. ; 39:16, s. 3708-3715
-
Journal article (peer-reviewed)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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| 6. |
- Becerra, Marley
(author)
-
Glow corona generation and streamer inception at the tip of grounded objects during thunderstorms : revisited
- 2013
-
In: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 46:13, s. 135205-
-
Journal article (peer-reviewed)abstract
- The initiation of streamers prior to a lightning strike can be reportedly inhibited by glow corona discharges generated from tall objects. In contrast to previous studies based on a simplified one-dimensional model of glow corona, a two-dimensional evaluation of the corona ion drift from tall objects is used here to analyse this effect quantitatively. Proper estimates for the corona space charge distribution generated during both the charging process of a thundercloud and the descent of the downward stepped leader are thus calculated. It is found that the shielding effect of the corona space charge on the streamer inception is not as severe as previously reported. Estimations of the effective height of the downward leader tip at which streamer inception takes place are presented and discussed for lightning rods and dissipation array systems.
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| 7. |
- Becerra, Marley, et al.
(author)
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Time dependent evaluation of the lightning upward connecting leader inception
- 2006
-
In: Journal of Physics D. - : Institute of Physics Publishing (IOPP). - 0022-3727 .- 1361-6463. ; 39:21, s. 4695-4702
-
Journal article (peer-reviewed)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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| 8. |
- Festy, F., et al.
(author)
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Imaging surfaces with reflected electrons from a field-emission STM: image contrast mechanisms
- 2001
-
In: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; :34, s. 1849-
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Journal article (peer-reviewed)abstract
- Electrons backscattered from a scanning tunnelling microscope operating in the field emission mode have been collected to produce images of a rough Si(111) surface. We have obtained a spatial resolution of about 40 nm in such images. Comparison between backscattered electron images and topographic images reveals that edge enhancement and shadowing are important contrast mechanisms.
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| 9. |
- Schöldström, Jens, 1969-, et al.
(author)
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Determination of the optical constants for Cu(In,Ga)Se2 and CuxSe in the IR region
- 2012
-
In: Journal of Physics D. - : Institute of Physics Publishing (IOPP). - 0022-3727 .- 1361-6463. ; 45:11, s. 115101-
-
Journal article (peer-reviewed)abstract
- The complex refractive index of CuxSe and Cu(In,Ga)Se2 has been calculated from experimental data in the wavelength region from 0.3 to 20μm. For Cu(In,Ga)Se2, the imaginary part exhibits very low values in the infrared range, whereas the real part decreases gently from 3 to 2.5. The CuxSe film is found to be conducting with increasing real and imaginary parts. It is found not to follow the Drude model. Reflectance and transmittance measurements were performed on thin films grown on soda lime glass and double polished low-doped silicon as substrates. Solutions for the complex refractive index were extracted from the measurement data using an optical model for the film and substrate. A method to map all solutions for the complex refractive index was applied to select the physically correct one.
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| 10. |
- Widenkvist, Erika, et al.
(author)
-
Mild sonochemical exfoliation of bromine-intercalated graphite a new route towards graphene
- 2009
-
In: Journal of Physics D. - : IOP Publishing Ltd. - 0022-3727 .- 1361-6463. ; 42:11, s. 112003-
-
Journal article (peer-reviewed)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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| 11. |
- Bao, Jie, 1982, et al.
(author)
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Two-dimensional hexagonal boron nitride as lateral heat spreader in electrically insulating packaging
- 2016
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 49:July 2016, s. 265501-
-
Journal article (peer-reviewed)abstract
- The need for electrically insulating materials with a high in-plane thermal conductivity for lateral heat spreading applications in electronic devices has intensified studies of layered hexagonal boron nitride (h-BN) films. Due to its physicochemical properties, h-BN can be utilised in power dissipating devices such as an electrically insulating heat spreader material for laterally redistributing the heat from hotspots caused by locally excessive heat flux densities. In this study, two types of boron nitride based heat spreader test structures have been assembled and evaluated for heat dissipation. The test structures separately utilised a few-layer h-BN film with and without graphene enhancement drop coated onto the hotspot test structure. The influence of the h-BN heat spreader films on the temperature distribution across the surface of the hotspot test structure was studied at a range of heat flux densities through the hotspot. It was found that the graphene-enhanced h-BN film reduced the hotspot temperature by about 8–10°C at a 1000 W/cm2 heat flux density, a temperature decrease significantly larger than for h-BN film without graphene enhancement. Finite element simulations of the h-BN film predict that further improvements in heat spreading ability are possible if the thermal contact resistance between the film and test chip are minimised.
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| 12. |
- Barmann, P, et al.
(author)
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Spatially and temporally resolved electron density measurements in streamers in dielectric liquids
- 1997
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 30:5, s. 856-863
-
Journal article (peer-reviewed)abstract
- Spatially and temporally resolved spectroscopic measurements of light emitted from positive streamers in transformer oil are presented. Analyses of the measurements performed with a DC needle-plane gap yield electron densities and indications of the atomic excitation temperatures in the streamers. The hydrogen emission reveals an electron density below 10(16) cm(-3) during the main part of the streamer propagation time (80-90%). Later the light is also characterized by emission from a high-density plasma with electron densities in the range 10(18)-10(19) cm(-3). The electron density during this time increases approximately linearly with distance from the initiation point and a density factor of four higher has been measured at the streamer tip than at the root. Measurements with high spectral resolution detect both high and low electron densities simultaneously. A tentative model of the interior of the streamer plasma, spatially resolved, is presented.
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| 13. |
- Barmann, P, et al.
(author)
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Spectroscopic measurements of streamer filaments in electric breakdown in a dielectric liquid
- 1996
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 29:5, s. 1188-1196
-
Journal article (peer-reviewed)abstract
- Emission spectroscopy has been utilized to provide information about the electron density and temperature in streamers and breakdown arcs in transformer oil. Recorded spectra include strongly broadened hydrogen Balmer-alpha lines and vibration/rotation band profiles of the C-2 molecule. The origin of the observed broadening of hydrogen lines is discussed and it is concluded that it arises mainly from collisions with charged particles, so-called dynamic Stark broadening. By assuming that the broadening is due solely to dynamic Stark broadening, electron densities between 1 x 10(18) and 1 x 10(19) cm(-3) were obtained for the rear of positive streamer filaments during the later stages of propagation. For negative streamers we obtained an upper limit of 3 x 10(16) cm(-3) and for breakdown arcs electron densities up to 4 x 10(18) cm(-3). The temperature information in the C-2 emission profiles and the intensity ratio of the hydrogen Balmer lines are discussed. Rough estimations of the temperature are presented both for positive and for negative streamers.
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| 14. |
- Chen, W. H., et al.
(author)
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Nanophotonics-based low-temperature PECVD epitaxial crystalline silicon solar cells
- 2016
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 49:12
-
Journal article (peer-reviewed)abstract
- The enhancement of light absorption via nanopatterning in crystalline silicon solar cells is becoming extremely important with the decrease of wafer thickness for the further reduction of solar cell fabrication cost. In order to study the influence of nanopatterning on crystalline silicon thin-film solar cells, we applied two lithography techniques (laser interference lithography and nanoimprint lithography) combined with two etching techniques (dry and wet) to epitaxial crystalline silicon thin films deposited via plasma-enhanced chemical vapor deposition at 175 degrees C. The influence of nanopatterning with different etching profiles on solar cell performance is studied. We found that the etching profiles (pitch, depth and diameter) have a stronger impact on the passivation quality (open circuit voltage and fill factor) than on the optical performance (short circuit current density) of the solar cells. We also show that nanopatterns obtained via wet-etching can improve solar cell performance; and in contrast, dry-etching leads to poor passivation related to the etching profile, surface damage, and/ or contamination introduced during the etching process.
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| 15. |
- Ehn, Andreas, et al.
(author)
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Setup for microwave stimulation of a turbulent low-swirl flame
- 2016
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 49:18
-
Journal article (peer-reviewed)abstract
- An experimental setup for microwave stimulation of a turbulent flame is presented. A low-swirl flame is being exposed to continuous microwave irradiation inside an aluminum cavity. The cavity is designed with inlets for laser beams and a viewport for optical access. The aluminum cavity is operated as a resonator where the microwave mode pattern is matched to the position of the flame. Two metal meshes are working as endplates in the resonator, one at the bottom and the other at the top. The lower mesh is located right above the burner nozzle so that the low-swirl flame is able to freely propagate inside the cylinder cavity geometry whereas the upper metal mesh can be tuned to achieve good overlap between the microwave mode pattern and the flame volume. The flow is characterized for operating conditions without microwave irradiation using particle imaging velocimetry (PIV). Microwave absorption is simultaneously monitored with experimental investigations of the flame in terms of exhaust gas temperature, flame chemiluminescence (CL) analysis as well as simultaneous planar laser-induced fluorescence (PLIF) measurements of formaldehyde (CH2O) and hydroxyl radicals (OH). Results are presented for experiments conducted in two different regimes of microwave power. In the high-energy regime the microwave field is strong enough to cause a breakdown in the flame. The breakdown spark develops into a swirl-stabilized plasma due to the continuous microwave stimulation. In the low-energy regime, which is below plasma formation, the flame becomes larger and more stable and it moves upstream closer to the burner nozzle when microwaves are absorbed by the flame. As a result of a larger flame the exhaust gas temperature, flame CL and OH PLIF signals are increased as microwave energy is absorbed by the flame.
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| 16. |
- Engström, Christian
(author)
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Bounds on the effective tensor and structural parameters for anisotropic two-phase composite material
- 2005
-
In: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 38, s. 3695-3702
-
Journal article (peer-reviewed)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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| 17. |
- Gelczuk, L., et al.
(author)
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Bi-induced acceptor level responsible for partial compensation of native free electron density in InP1-xBix dilute bismide alloys
- 2016
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 49:11
-
Journal article (peer-reviewed)abstract
- Deep level transient spectroscopy (DLTS) has been applied to study electron and hole traps in InPBi alloys with 2.2 and 2.4% Bi grown by molecular beam epitaxy. One donor-like trap with the activation energy of 0.45-0.47 eV and one acceptor-like trap with activation energy of 0.08 eV have been identified in DLTS measurements. For the reference sample (InP grown at the same temperature), the deep donor trap has also been observed, while the acceptor trap was not detected. According to the literature, the deep donor level found in InP(Bi) at 0.45-0.47 eV below the conduction band has been attributed to the isolated P-In defect, while the second trap, which is observed only for Bi containing samples at 0.08 eV above the valence band can be attributed to Bi clusters in InPBi. This acceptor level was proposed to be responsible for the observed partial compensation of native free electron density in InPBi layers. It is also shown that the deep donor traps are active in photoluminescence (PL). A strong radiative recombination between donor traps and the valence band are observed in PL spectra at energy 0.6-0.8 eV, i.e. similar to 0.47 eV below the energy gap of InPBi, which is determined by contactless electroreflectance.
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| 18. |
- Gylfason, Kristinn B., 1978-, et al.
(author)
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Ion-acoustic solitary waves in a high power pulsed magnetron sputtering discharge
- 2005
-
In: Journal of Physics D. - : Institute of Physics Publishing (IOPP). - 0022-3727 .- 1361-6463. ; 38:18, s. 3417-3421
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Journal article (peer-reviewed)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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| 19. |
- Kossyi, I.A., et al.
(author)
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Polyphase (non-resonant) multipactor in rectangular waveguides
- 2008
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 41:6, s. 065203 1-8
-
Journal article (peer-reviewed)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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| 20. |
- Kovacs, Andras, et al.
(author)
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Effect of post-growth annealing on secondary phase formation in low-temperature-grown Mndoped GaAs
- 2013
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 46:14, s. 7-145309
-
Journal article (peer-reviewed)abstract
- The microstructures of annealed GaAs layers containing 0.1%, 0.5% and 2% Mn are studied using aberration-corrected transmission electron microscopy (TEM). The layers were grown by molecular beam epitaxy at 270 ◦C. After heat treatment at 400, 560 and 630 ◦C, they are found to contain precipitate complexes of cubic or hexagonal (Mn, Ga) As, orthorhombic or rhombohedral As and voids. Information about the crystallographic structures and compositions of the phases is obtained using high-resolution TEM, scanning TEM and energy-dispersive x-ray spectroscopy. A phase diagram for secondary phase formation in annealed GaMnAs layers doped with low Mn concentrations is proposed.
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| 21. |
- Kusano, Yukihiro, et al.
(author)
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Water-cooled non-thermal gliding arc for adhesion improvement of glass-fibre-reinforced polyester
- 2013
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 46:13
-
Journal article (peer-reviewed)abstract
- A non-equilibrium quenched plasma is prepared using a gliding-arc discharge generated between diverging electrodes and extended by a gas flow. It can be operated at atmospheric pressure and applied to plasma surface treatment to improve adhesion properties of material surfaces. In this work, glass-fibre-reinforced polyester plates were treated using an atmospheric pressure gliding-arc discharge with air flow to improve adhesion with a vinylester adhesive. The electrodes were water-cooled so as to operate the gliding arc continually. The treatment improved wettability and increased the density of oxygen-containing polar functional groups on the surfaces. Double cantilever beam specimens were prepared for fracture mechanic characterization of the laminate adhesive interface. It was found that gliding-arc treatment significantly increases the fracture resistance in comparison with a standard peel-ply treatment.
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| 22. |
- Lucci, M., et al.
(author)
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1D Josephson quantum interference grids: diffraction patterns and dynamics
- 2016
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 49:6
-
Journal article (peer-reviewed)abstract
- We investigate the magnetic response of transmission lines with embedded Josephson junctions and thus generating a 1D underdamped array. The measured multi-junction interference patterns are compared with the theoretical predictions for Josephson supercurrent modulations when an external magnetic field couples both to the inter-junction loops and to the junctions themselves. The results provide a striking example of the analogy between Josephson phase modulation and 1D optical diffraction grid. The Fiske resonances in the current-voltage characteristics with voltage spacing Phi(0)((c) over bar /2L) , where L is the total physical length of the array, Phi(0) the magnetic flux quantum and (c) over bar the speed of light in the transmission line, demonstrate that the discrete line supports stable dynamic patterns generated by the ac Josephson effect interacting with the cavity modes of the line.
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| 23. |
- Ludvigsson, Linus, et al.
(author)
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Investigations of initial particle stages during spark discharge
- 2015
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 48:31
-
Journal article (peer-reviewed)abstract
- The number of nanoparticle-based products on the market is expected to increase considerably during the coming decades. This forces the industry to have highly meticulous manufacturing of large amounts of nanoparticles using cheap and environmentally friendly methods. For the production of metal nanoparticles spark discharge generation is a promising route to fulfill these demands. The spark discharge generator can be easily scaled-up for mass production due to its simple design solely by placing several units in parallel. Before doing so, one first needs to optimize a single spark discharge generator unit. To optimize the spark discharge generator in a controlled way the first stage of nanoparticle formation needs to be understood. To improve this understanding we have constructed a customized nanoparticle sampler to enable sampling of the initial stages of particle formation for imaging in a TEM. In this article we present the design of the sampler and discuss optimal sampling parameters. We also present how the generation parameters can be tuned in order to affect the first stages of particle formation and hence the final nanoparticles.
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| 24. |
- Marucci, Mariagrazia, et al.
(author)
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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
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 40:9, s. 2870-2880
-
Journal article (peer-reviewed)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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| 25. |
- Mattisson, C, et al.
(author)
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Light deflection and convection in diffusion experiments using holographic interferometry
- 2001
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 34:20, s. 3088-3096
-
Journal article (peer-reviewed)abstract
- A study of the effect of light deflection during diffusion studies of ethanol into agarose gel using holographic laser interferometry is presented. Furthermore it also demonstrates how a diffusive flux could give rise to a convective flux in holographic laser interferometry experiments. The convective and diffusive mass transfer is also theoretically compared in both a liquid phase and a gel phase for the ethanol-agarose system used. The current study shows that errors due to light deflection in holographic laser interferometry are extremely small and can be neglected. It also shows the importance of designing the diffusion experiments to avoid natural convection. In gels the convective flow is cancelled by the friction forces between the liquid and the polymer network. However, in the liquid phase the natural convection could occur even though the density differences in the phase are small.
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| 26. |
- Ramprecht, Jörgen, et al.
(author)
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Magnetic Losses in Composite Materials
- 2008
-
In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 41:13, s. 135005-
-
Journal article (peer-reviewed)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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| 27. |
- Siderakis, K., et al.
(author)
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Influence of heat conductivity on the performance of RTV SIR coatings with different fillers
- 2005
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 38:19, s. 3682-3689
-
Journal article (peer-reviewed)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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| 28. |
- Sjöberg, Daniel
(author)
-
A modified Drude-Born-Fedorov model for isotropic chiral media, obtained by finite scale homogenization
- 2008
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 41:15
-
Journal article (peer-reviewed)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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| 29. |
- Sohl, Christian, et al.
(author)
-
Physical limitations on metamaterials: restrictions on scattering and absorption over a frequency interval
- 2007
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 40:22, s. 7146-7151
-
Journal article (peer-reviewed)abstract
- A limitation on the extinction cross section, valid for all scatterers satisfying some basic physical assumptions, is investigated. The physical limitation is obtained from the holomorphic properties of the forward scattering dyadic. The analysis focuses on the consequences for materials with negative permittivity and permeability, i.e. metamaterials. From a broadband point of view, the limitations imply that there is no fundamental difference between metamaterials and ordinary materials with respect to scattering and absorption. The analysis is illustrated by three numerical examples of metamaterials modelled by temporal dispersion.
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| 30. |
- van der Aa, Bart, 1985, et al.
(author)
-
The 2.5D MST for sound propagation through an array of acoustically rigid cylinders perpendicular to an impedance surface
- 2015
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 48:29
-
Journal article (peer-reviewed)abstract
- In this work a study of sound propagation through arrays of semi-infinitely long cylinders placed perpendicular to an impedance surface has been carried out. The cross sections of the structures are assumed to be invariant along the main axis of the cylinders, and the cylinders are considered rigid. It is further assumed that the structures are insonified by a monopole source placed above the impedance surface. To study such configurations, we introduce the two-and-a-half-dimensional multiple scattering theory (2.5D MST), which essentially solves the pressure in a three-dimensional domain by post-processing a set of precomputed solutions obtained in a two-dimensional domain. The total pressure can then be obtained by complex addition of four contributions: source-to-receiver, source-to-array-to-receiver, image source-to-receiver, and image source-to-array-to-receiver. The proposed method is validated using both analytical and numerical tools, showing very good agreement for all studied cases. Among other things, we show that a cylinder array placed on top of flat rigid ground can deteriorate the ground interference dips that exist without the array. In addition, we show that the characteristic response of the cylinder array, i.e. in terms of pass and stop bands, may be shifted up in frequency due to a projection phenomenon, which happens when the source or receiver is elevated along the main axis of the cylinders.
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| 31. |
- Xia, Y. X., et al.
(author)
-
Pyramid shape of polymer solar cells: a simple solution to triple efficiency
- 2013
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 46:30
-
Journal article (peer-reviewed)abstract
- Pyramid-shaped polymer solar cells fabricated on flexible substrates were investigated. Effective light trapping can be realized due to light reflection in all 360 degrees directions, and 100% space utilization is achieved when assembled into arrays. The power conversion efficiency is enhanced by 200% ([60] PCBM as the acceptor) and 260% ([70] PCBM as the acceptor) with a dihedral angle of 30 degrees between the opposite sides of the pyramid compared with a planar device, and a high V-oc of 3.5 V in series connection is obtained. Considering the material utilization, an angle of 90 degrees for pyramid-shaped polymer solar cells is proposed. Pyramid-shaped polymer solar cells are particularly suitable for installation on roof of vehicles and houses, which have limited surface area.
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| 32. |
- Yang, Xifeng, et al.
(author)
-
Strain effect in determining the geometric shape of self-assembled quantum dot
- 2009
-
In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 42:125414
-
Journal article (peer-reviewed)abstract
- The geometric shape of a self-assembled quantum dot (QD) formed by the strain-induced Stranski-Krastanov mode has been studied as a function of strain energy by the short-range valence-force-field approach. It has been shown by dynamic bond relaxation through strain energy minimization that for the most commonly used InAs QD in GaAs and InP matrices and Ge QD in Si matrix, a pyramidal shape is energy favoured over an hemispherical shape when the QD is not capped due to the lattice relaxation at the QD surface. When the QD becomes totally embedded in the background material, the elastic strain energy of a hemispherical InAs QD is minimal. The results agree with experimental observations. We further studied the coupling of strain fields of QDs in adjacent QD layers which shows that QDs in multiply stacked QD layers can be aligned along the layer growth direction in order to minimize the strain energy.
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| 33. |
- Zhu, Jiajian, et al.
(author)
-
Dynamics, OH distributions and UV emission of a gliding arc at various flow-rates investigated by optical measurements
- 2014
-
In: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 47:29
-
Journal article (peer-reviewed)abstract
- We demonstrate a plasma discharge which is generated between two diverging electrodes and extended into a gliding arc in non-equilibrium condition by an air flow at atmospheric pressure. Effects of the air flow rates on the dynamics, ground-state OH distributions and spectral characterization of UV emission of the gliding arc were investigated by optical methods. High-speed photography was utilized to reveal flow-rate dependent dynamics such as ignitions, propagation, short-cutting events, extinctions and conversions of the discharge from glowtype to spark-type. Short-cutting events and ignitions occur more frequently at higher flow rates. The anchor points of the gliding arc are mostly steady at the top of the electrodes at lower flow rates whereas at higher flow rates they glide up along the electrodes most of the time. The afterglow of fully developed gliding arcs is observed to decay over hundreds of microseconds after being electronically short-cut by a newly ignited arc. The extinction time decreases with the increase of the flow rate. The frequency of the conversion of a discharge from glow-type to spark-type increases with the flow rate. Additionally, spatial distributions of ground-state OH were investigated using planar laser-induced fluorescence. The results show that the shape, height, intensity and thickness of ground-state OH distribution vary significantly with air flow rates. Finally, UV emission of the gliding arc is measured using optical emission spectroscopy and it is found that the emission intensity of NO gamma (A-X), OH (A-X) and N-2 (C-B) increase with the flow rates showing more characteristics of spark-type arcs. The observed phenomena indicate the significance of the interaction between local turbulence and the gliding arc.
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| 34. |
- Nilsson, Fritjof, et al.
(author)
-
Conductivity simulations of field-grading composites
- 2016
-
In: Journal of Physics D. - : Institute of Physics Publishing (IOPP). - 0022-3727 .- 1361-6463. ; 49:33
-
Journal article (peer-reviewed)abstract
- The electrical conductivity and the percolation threshold of field grading polymer composites intended for high voltage applications were examined with representative elementary volume simulation methods based on percolation threshold modeling (PTM) and electrical network modeling (ENM). Comparisons were made with experimental conductivity data for SiC-EPDM composites with spherical and angular particles, using different filler fractions and electrical field strengths. With a known conductivity of the filler particles (powder), the simulations could predict the percolation threshold and the composite conductivity as functions of the electrical field for a wide range of SiC-filler fractions. The effects of morphology, dispersion and filler shape were examined and the simulations were able to explain the experimental difficulty of reaching sufficient reproducibility when designing composites with filler fractions close to a percolation threshold. PTM of composites containing hard-core/soft-shell spheres revealed a y = (a + bx)((-1/c)) relationship (R-2 = 0.9997) between filler fraction and relative soft-shell thickness.
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| 35. |
- Willander, Magnus, et al.
(author)
-
Zinc oxide nanorods/polymer hybrid heterojunctions for white light emitting diodes
- 2011
-
In: JOURNAL OF PHYSICS D-APPLIED PHYSICS. - : Iop Publishing Ltd. - 0022-3727 .- 1361-6463. ; 44:22, s. 224017-
-
Journal article (peer-reviewed)abstract
- Zinc oxide (ZnO) with its deep level defect emission covering the whole visible spectrum holds promise for the development of intrinsic white lighting sources with no need of using phosphors for light conversion. ZnO nanorods (NRs) grown on flexible plastic as substrate using a low temperature approach (down to 50 degrees C) were combined with different organic semiconductors to form hybrid junction. White electroluminescence (EL) was observed from these hybrid junctions. The configuration used for the hybrid white light emitting diodes (LEDs) consists of two-layers of polymers on the flexible plastic with ZnO NRs on the top. The inorganic/organic hybrid heterojunction has been fabricated by spin coating the p-type polymer poly (3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT : PSS) for hole injection with an ionization potential of 5.1 eV and poly(9, 9-dioctylfluorene) (PFO) is used as blue emitting material with a bandgap of 3.3 eV. ZnO NRs are grown on top of the organic layers. Two other configurations were also fabricated; these are using a single MEH PPV (red-emitting polymer) instead of the PFO and the third configuration was obtained from a blend of the PFO and the MEH PPV. The white LEDs were characterized by scanning electron microscope, x-ray diffraction (XRD), current-voltage (I-V) characteristics, room temperature photoluminescence (PL) and EL. The EL spectrum reveals a broad emission band covering the range from 420 to 800 nm, and the emissions causing this white luminescence were identified.
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| 36. |
- Austgen, M, et al.
(author)
-
Sputter yield amplification by tungsten doping of Al(2)O(3) employing reactive serial co-sputtering : process characteristics and resulting film properties
- 2011
-
In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 44:34, s. 345501-
-
Journal article (peer-reviewed)abstract
- The deposition rate of reactively sputtered Al(2)O(3) coatings is demonstrated to increase by 80% upon tungsten doping of the used aluminium target. This effect is based on the recoil of the sputtering species at implanted dopants below the target surface and is termed sputter yield amplification. For the investigation of this effect, a novel type of magnetron sputter deposition system is employed that facilitates serial co-sputtering. In this technique doping of the elementary target is enabled by a dynamic sputtering process from an auxiliary cathode. In our case, the rotating aluminium target is dynamically coated with tungsten from this auxiliary cathode. Since the primary target rotates, the auxiliary cathode is placed in series with the primary erosion zone. The deposition rate of Al(2)O(3) can be considerably increased in this process already for very low concentrations of approximately 1% of tungsten in the resulting film. A characterization of the dynamics of reactive sputtering as a function of target rotation speed is performed.
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| 37. |
- Ehiasarian, Arutiun P., et al.
(author)
-
Distance-dependent plasma composition and ion energy in high power impulse magnetron sputtering
- 2010
-
In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 43:27, s. 275204-
-
Journal article (peer-reviewed)abstract
- The plasma composition of high power impulse magnetron sputtering (HIPIMS) has been studied for titanium and chromium targets using a combined energy analyser and quadrupole mass spectrometer. Measurements were done at distances from 50 to 300 mm from the sputtering target. Ti and Cr are similar in atomic mass but have significantly different sputter yields, which gives interesting clues on the effect of the target on plasma generation and transport of atoms. The Ti and Cr HIPIMS plasmas operated at a peak target current density of similar to 0.5 A cm(-2). The measurements of the argon and metal ion content as well as the ion energy distribution functions showed that (1) singly and doubly charged ions were found for argon as well as for the target metal, (2) the majority of ions were singly charged argon for both metals at all distances investigated, (3) the Cr ion density was maintained to distances further from the target than Ti. Gas rarefaction was identified as a main factor promoting transport of metal ions, with the stronger effect observed for Cr, the material with higher sputter yield. Cr ions were found to displace a significant portion of the gas ions, whereas this was less evident in the Ti case. The observations indicate that the presence of metal vapour promotes charge exchange and reduces the electron temperature and thereby practically prevents the production of Ar2+ ions near the target. The content of higher charge states of metal ions depends on the probability of charge exchange with argon.
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| 38. |
- Horwat, David, et al.
(author)
-
On the deactivation of the dopant and electronic structure in reactively sputtered transparent Al-doped ZnO thin films
- 2010
-
In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 43:13, s. 132003-
-
Journal article (peer-reviewed)abstract
- We report on the possible origin of electrical heterogeneities in 4 at% Al-doped ZnO (AZO) reactively sputtered films. It is found through the Zn L-3 and Al K edge x-ray absorption near-edge structure that a fraction of the Al dopant is deactivated by its positioning in octahedral conformation with oxygen. This fraction as well as the conductivity, optical bandgap and c-axis parameter of ZnO wurtzite are all found to depend on the sample position during deposition. The present results suggest the formation of a metastable Al2O3 (ZnO) m homologous phase that degrades the electrical conductivity.
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| 39. |
- Jiang, Kaiyun, et al.
(author)
-
Low temperature synthesis of alpha-Al(2)O(3) films by high-power plasma-assisted chemical vapour deposition
- 2010
-
In: Journal of Physics D. - : Institute of Physics. - 0022-3727 .- 1361-6463. ; 43:32, s. 325202-
-
Journal article (peer-reviewed)abstract
- In this study, we deposit Al(2)O(3) films using plasma-assisted chemical vapour deposition (PACVD) in an Ar-H(2)-O(2)-AlCl(3) atmosphere. A novel generator delivering approximately 4 times larger power densities than those conventionally employed in PACVD enabling efficient AlCl(3) dissociation in the gas phase as well as a more intense energetic bombardment of the growing film is utilized. We demonstrate that these deposition conditions allow for the growth of dense alpha-Al(2)O(3) films with negligible Cl incorporation and elastic properties similar to those of the bulk alpha-Al(2)O(3) at a temperature of 560 +/- 10 degrees C.
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| 40. |
- Kubart, Tomas, et al.
(author)
-
Modelling of low energy ion sputtering from oxide surfaces
- 2010
-
In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 43:20, s. 205204-
-
Journal article (peer-reviewed)abstract
- The main aim of this work is to present a way to estimate the values of surface binding energy for oxides. This is done by fitting results from the binary collisions approximation code Tridyn with data from the reactive sputtering processing curves, as well as the elemental composition obtained from x-ray photoelectron spectroscopy (XPS). Oxide targets of Al, Ti, V, Nb and Ta are studied. The obtained surface binding energies are then used to predict the partial sputtering yields. Anomalously high sputtering yield is observed for the TiO 2 target. This is attributed to the high sputtering yield of Ti lower oxides. Such an effect is not observed for the other studied metals. XPS measurement of the oxide targets confirms the formation of suboxides during ion bombardment as well as an oxygen deficient surface in the steady state. These effects are confirmed from the processing curves from the oxide targets showing an elevated sputtering rate in pure argon.
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| 41. |
- Rubino, Stefano, 1978-, et al.
(author)
-
Simulation of magnetic circular dichroism in the electron microscope
- 2010
-
In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 43:47, s. 474005-
-
Journal article (peer-reviewed)abstract
- As electron energy-loss spectroscopy (EELS) and x-ray absorption spectroscopy (XAS) probe the same transitions from core–shell states to unoccupied states above the Fermi energy, it should always be possible to apply the two techniques to the same physical phenomena, such as magnetic dichroism, and obtain the same information. Indeed, the similarity in the expression of the electron and x-ray cross-sections had been already exploited to prove the equivalence of x-ray magnetic linear dichroism and anisotropy in EELS, by noting that the polarization vector of a photon plays the same role as the momentum transfer in electron scattering. Recently, the same was proven true for x-ray magnetic circular dichroism (XMCD) by establishing a new TEM technique called EMCD (electron energy-loss magnetic chiral dichroism) (Schattschneider P et al 2006 Nature 441 486–8), which makes use of special electron scattering conditions to force the absorption of a circularly polarized virtual photon.The intrinsic advantage of EMCD over XMCD is the high spatial resolution of electron microscopes, which are readily available. Among the particular obstacles in EMCD that do not exist for synchrotron radiation, is the notoriously low signal and the very particular scattering conditions necessary to observe a chiral dichroic signal. In spite of that, impressive progress has been made in recent years. The signal strength could be considerably increased, and some innovations such as using a convergent beam have been introduced. EMCD has evolved into several techniques, which make full use of the versatility of the TEM and energy filtering, spectroscopy or STEM conditions (Rubino S 2007 Magnetic circular dichroism in the transmission electron microscope PhD Thesis Vienna University of Technology, Vienna, Austria).
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| 42. |
- Strömberg, F., et al.
(author)
-
Textured growth of the high moment material Gd(0001)/Cr(001)/Fe(001)
- 2011
-
In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 44:26, s. 265004-
-
Journal article (peer-reviewed)abstract
- By magnetic coupling of Fe and Gd via Cr interlayers, the large local moment of Gd can be combined with the high Curie temperature of Fe. The textured growth of a Gd film is studied here by preparing trilayer systems of Fe/Cr/Gd on MgO(1 0 0) substrates by molecular-beam epitaxy (MBE). The thickness of the Cr interlayer was varied between 3 and 5 monolayers. The structural quality of the samples was confirmed by in situ RHEED and ex situ XRD measurements. Epitaxial Cr(0 0 1)/Fe(0 0 1) growth was observed, as expected. By use of Fe-57-CEMS (conversion electron Mossbauer spectroscopy) in combination with the Fe-57 tracer layer method the Fe/Cr interface could be examined on an atomic scale and well separated Fe/Gd layers for all Cr thicknesses were confirmed. The unusual Gd/Cr crystallographic relationship of Gd(0 0 0 1)parallel to Cr(0 0 1), with domains of the hexagonal Gd basal planes randomly oriented in the sample plane and not in registry with the underlying Cr(0 0 1) lattice, was found from combined RHEED and x-ray measurements. Annealing of the samples resulted in a remarkable improvement of the crystalline structure of the Gd layers. On the other hand, the appearance of a single line in the CEM spectrum leads to the conclusion that during annealing a small amount of Fe diffuses into the Cr layer. The electronic structure and magnetism of this system are investigated by first-principles theory.
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| 43. |
- Wallin, Erik, et al.
(author)
-
Low-temperature alpha-alumina thin film growth : ab initio studies of Al adatom surface migration
- 2009
-
In: JOURNAL OF PHYSICS D-APPLIED PHYSICS. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 42:12, s. 125302-
-
Journal article (peer-reviewed)abstract
- Investigations of activation energy barriers for Al surface hopping on alpha-Al2O3 (0 0 0 1) surfaces have been carried out by means of first-principles density functional theory calculations and the nudged elastic band method. Results show that surface diffusion on the (most stable) Al-terminated surface is relatively fast with an energy barrier of 0.75 eV, whereas Al hopping on the O-terminated surface is slower, with barriers for jumps from the two metastable positions existing on this surface to the stable site of 0.31 and 0.99 eV. Based on this study and on the literature, the governing mechanisms during low-temperature alpha-alumina thin film growth are summarized and discussed. Our results support suggestions made in some previous experimental studies, pointing out that limited surface diffusivity is not the main obstacle for alpha-alumina growth at low-to-moderate temperatures, and that other effects should primarily be considered when designing novel processes for low-temperature alpha-alumina deposition.
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| 44. |
- Alami, J., et al.
(author)
-
On the phase formation of titanium oxide films grown by reactive high power pulsed magnetron sputtering
- 2009
-
In: Journal of Physics D. - : Institute of Physics. - 0022-3727 .- 1361-6463. ; 42:11, s. 115204-
-
Journal article (peer-reviewed)abstract
- High power pulsed magnetron sputtering is used for the growth of titanium dioxide (TiO(2)) films at different working pressures and orientations of the substrate with respect to the target surface. In the case of substrates oriented parallel to the target surface, the increase in the working pressure from 0.5 to 3 Pa results in the growth of crystalline TiO(2) films with phase compositions ranging from rutile to anatase/rutile mixtures. When depositions are performed on substrates placed perpendicularly to the target surface, rutile films that consist of TiO(2) nanocrystals embedded in an amorphous matrix are obtained at 0.5 Pa. Increase in the working pressure leads to the deposition of amorphous films. These findings are discussed in the light of the energetic bombardment provided to the growing film at the various deposition conditions.
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| 45. |
- Alami, J., et al.
(author)
-
On the relationship between the peak target current and the morphology of chromium nitride thin films deposited by reactive high power pulsed magnetron sputtering
- 2009
-
In: Journal of Physics D. - : Institute of Physics. - 0022-3727 .- 1361-6463. ; 42:1, s. 015304-
-
Journal article (peer-reviewed)abstract
- High power pulsed magnetron sputtering (HPPMS) is used to deposit CrN films without external heating at different peak target currents, while the average current is kept constant. Films are also grown by dc magnetron sputtering (dcMS), for reference. The plasma properties, the deposition rate and the morphology of the films are investigated. The plasma analysis reveals that HPPMS provides higher fluxes of ionized species (both gas and sputtered) to the growing film, as compared with dcMS. In addition, the ionic bombardment during HPPMS increases, when the peak target current is increased. The HPPMS films exhibit changes of the density and the surface roughness as the peak target current increased, while the deposition rate decreases drastically. Furthermore, it is found that different thin-film morphologies are obtained starting from a porous columnar morphology for the dcMS films, which turns to a dense columnar one at low peak target currents and ends up to a featureless morphology at high peak target currents for the films grown by HPPMS. A new structure zone model specific for high ionization sputtering is, therefore, outlined.
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| 46. |
- Amer, Eynas, et al.
(author)
-
Shock wave generation in laser ablation studied using pulsed digital holographic interferometry
- 2008
-
In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 41:21
-
Journal article (peer-reviewed)abstract
- Pulsed digital holographic interferometry has been used to study the shock wave induced by a Q-switched Nd-YAG laser (λ = 1064 nm and pulse duration 12 ns) on a polycrystalline boron nitride (PCBN) ceramic target under atmospheric air pressure. A special setup based on using two synchronized wavelengths from the same laser for processing and measurement simultaneously has been introduced. Collimated laser light (λ = 532 nm) passed through the volume along the target and digital holograms were recorded for different time delays after processing starts. Numerical data of the integrated refractive index field were calculated and presented as phase maps showing the propagation of the shock wave generated by the process. The location of the induced shock wave front was observed for different focusing and time delays. The amount of released energy, i.e. the part of the incident energy of the laser pulse that is eventually converted to a shock wave has been estimated using the point explosion model. The released energy is normalized by the incident laser pulse energy and the energy conversion efficiency between the laser pulse and PCBN target has been calculated at different power densities. The results show that the energy conversion efficiency seems to be constant around 80% at high power densities.
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| 47. |
- Aminlashgari, Nina, et al.
(author)
-
Characterization of degradation fragments released by arc-induced ablation of polymers in air
- 2016
-
In: Journal of Physics D. - : Institute of Physics Publishing (IOPP). - 0022-3727 .- 1361-6463. ; 49:5
-
Journal article (peer-reviewed)abstract
- Polymers exposed to high intensity arc plasmas release material in a process called arc-induced ablation. In order to investigate the degradation fragments released due to this process, two different polymeric materials, poly(oxymethylene) copolymer (POM-C) and poly(methyl methacrylate) (PMMA), were exposed to a transient, high-power arc plasma in air. A small fraction of the ablated material drifting away from the arcing volume was deposited on a fixed glass substrate during the total duration of a 2 kA ac current semicycle. In addition, another fraction of the released material was deposited on a second moving substrate to obtain a time-resolved streak 'image' of the arc-induced ablation process. For the first time, mass spectra of degradation fragments produced by arc-induced ablation were obtained from the material deposited on the substrates by using laser desorption ionization time-of-flight mass spectrometry (LDI-ToF-MS). It was found that oligomers with mean molecular weight ranging between 400 and 600 Da were released from the surface of the studied polymers. The obtained spectra suggest that the detected degradation fragments of POM could be released by random chain scission of the polymer backbone. In turn, random chain scission and splitting-off the side groups are suggested as the main chemical mechanism leading to the release of PMMA fragments under arc-induced ablation.
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| 48. |
- Arevalo, Liliana, et al.
(author)
-
Preliminary study on the modelling of negative leader discharges
- 2011
-
In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 44:31, s. 315204-
-
Journal article (peer-reviewed)abstract
- Nowadays, there is considerable interest in understanding the physics underlying positive and negative discharges because of the importance of improving lightning protection systems and of coordinating the insulation for high voltages. Numerical simulations of positive switching impulses made in long spark gaps in a laboratory are achievable because the physics of the process is reasonably well understood and because of the availability of powerful computational methods. However, the existing work on the simulation of negative switching discharges has been held up by a lack of experimental data and the absence of a full understanding of the physics involved. In the scientific community, it is well known that most of the lightning discharges that occur in nature are of negative polarity, and because of their complexity, the only way to understand them is to generate the discharges in laboratories under controlled conditions. The voltage impulse waveshape used in laboratories is a negative switching impulse. With the aim of applying the available information to a self-consistent physical method, an electrostatic approximation of the negative leader discharge process is presented here. The simulation procedure takes into consideration the physics of positive and negative discharges, considering that the negative leader propagates towards a grounded electrode and the positive leader towards a rod electrode. The simulation considers the leader channel to be thermodynamic, and assumes that the conditions required to generate a thermal channel are the same for positive and negative leaders. However, the magnitude of the electrical charge necessary to reproduce their propagation and thermalization is different, and both values are based on experimental data. The positive and negative streamer development is based on the constant electric field characteristics of these discharges, as found during experimental measurements made by different authors. As a computational tool, a finite element method based software is employed. The simulations are compared with experimental data available in the literature.
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| 49. |
- Azarov, A. Yu, et al.
(author)
-
Annealing of ion implanted CdZnO
- 2012
-
In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 45:23, s. 235304-
-
Journal article (peer-reviewed)abstract
- We have studied the effect of the Cd content on the recovery of ion-induced damage in wurtzite CdxZn1-xO (x <= 0.05) films and compared with that in pure wurtzite ZnO and rock-salt CdO.200 keV Au+ and 55 keV Ar+ ion implants were performed at room temperature in the dose range of 5 x 1014-6.5 x 1015 cm-2. Rutherford backscattering/channelling spectrometry was used to characterize the damage evolution in the course of annealing (600-900 degrees C in air). A complex defect annealing behaviour is revealed in CdZnO as a function of annealing temperature, Cd content and ion dose. In particular, defects in the low dose implanted CdZnO films can be effectively removed at 800 degrees C, while the high dose implantation results in the formation of defects stable at least up to 900 degrees C. Moreover, annealing of the CdZnO films is accompanied by Cd loss at the surface for temperatures exceeding 800 degrees C. In contrast, CdO exhibits a typical damage accumulation behaviour for metals and semiconductors with high degree of ionicity, resulting in saturation and extended defect formation at high ion doses. These extended defects in pure ZnO and CdO, formed either directly during implantation or by reconstruction during post-implant annealing, are substantially more stable compared with small defects which can be efficiently removed at 700 degrees C and 600 degrees C for ZnO and CdO, respectively.
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| 50. |
- Becerra Garcia, Marley, et al.
(author)
-
Laboratory experiments cannot be utilized to justify the action of early streamer emission terminals
- 2008
-
In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 41:8
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Journal article (peer-reviewed)abstract
- The early emission of streamers in laboratory long air gaps under switching impulses has beenobserved to reduce the time of initiation of leader positive discharges. This fact has beenarbitrarily extrapolated by the manufacturers of early streamer emission devices to the case ofupward connecting leaders initiated under natural lightning conditions, in support of thosenon-conventional terminals that claim to perform better than Franklin lightning rods. In orderto discuss the physical basis and validity of these claims, a self-consistent model based on thephysics of leader discharges is used to simulate the performance of lightning rods in thelaboratory and under natural lightning conditions. It is theoretically shown that the initiation ofearly streamers can indeed lead to the early initiation of self-propagating positive leaders inlaboratory long air gaps under switching voltages. However, this is not the case for positiveconnecting leaders initiated from the same lightning rod under the influence of the electricfield produced by a downward moving stepped leader. The time evolution of the developmentof positive leaders under natural conditions is different from the case in the laboratory, wherethe leader inception condition is closely dependent upon the initiation of the first streamerburst. Our study shows that the claimed similarity between the performance of lightning rodsunder switching electric fields applied in the laboratory and under the electric field producedby a descending stepped leader is not justified. Thus, the use of existing laboratory results tovalidate the performance of the early streamer lightning rods under natural conditions is not justified.
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