| 61. |
- Ekeroth, Sebastian, 1988-
(författare)
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Plasma Synthesis and Self-Assembly of Magnetic Nanoparticles
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nanomaterials are important tools for enabling technological progress as they can provide dramatically different properties as compared to the bulk counterparts. The field of nanoparticles is one of the most investigated within nanomaterials, thanks to the existing, relatively simple, means of manufacturing. In this thesis, high-power pulsed hollow cathode sputtering is used to nucleate and grow magnetic nanoparticles in a plasma. This sputtering technique provides a high degree of ionization of the sputtered material, which has previously been shown to aid in the growth of the nanoparticles. The magnetic properties of the particles are utilized and makes it possible for the grown particles to act as building blocks for self-assembly into more sophisticated nano structures, particularly when an external magnetic field is applied. These structures created are termed “nanowires” or “nanotrusses”, depending on the level of branching and inter-linking that occurs.Several different elements have been investigated in this thesis. In a novel approach, it is shown how nanoparticles with more advanced structures, and containing material from two hollow cathodes, can be fabricated using high-power pulses. The dual-element particles are achieved by using two distinct and individual elemental cathodes, and a pulse process that allows tuning of individual pulses separately to them. Nanoparticles grown and investigated are Fe, Ni, Pt, Fe-Ni and Ni-Pt. Alternatively, the addition of oxygen to the process allows the formation of oxide or hybrid metal oxide – metal particles. For all nanoparticles containing several elements, it is demonstrated that the stoichiometry can be easily varied, either by the amount of reactive gas let into the process or by tuning the amount of sputtered material through adjusting the electric power supplied to the different cathodes.One aim of the presented work is to find a suitable material for the use as a catalyst in the production of H2 gas through the process of water splitting. H2 is a good candidate to replace fossil fuels as an energy carrier. However, rare elements (such as Ir or Pt) needs to be used as the catalyst, otherwise a high overpotential is required for the splitting to occur, leading to a low efficiency. This work demonstrates a possible route to avoid this, by using nanomaterials to increase the surface-to-volume ratio, as well as optimizing the elemental ratio between different materials to lower the amount of noble elements required.
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| 62. |
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| 63. |
- Eliasson, H., et al.
(författare)
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Modeling of high power impulse magnetron sputtering discharges with graphite target
- 2021
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Ingår i: Plasma sources science & technology. - : IOP Publishing Ltd. - 0963-0252 .- 1361-6595. ; 30:11
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Tidskriftsartikel (refereegranskat)abstract
- The ionization region model (IRM) is applied to model a high power impulse magnetron sputtering discharge in argon with a graphite target. Using the IRM, the temporal variation of the various species and the average electron energy, as well as internal parameters such as the ionization probability, back-attraction probability, and the ionized flux fraction of the sputtered species, is determined. It is found that thedischarge develops into working gas recycling and most of the discharge current at the cathode target surface is composed of Ar+ ions, which constitute over 90% of the discharge current, while the contribution of the C+ ions is always small (<5%), even for peak current densities close to 3 A cm(-2). For the target species, the time-averaged ionization probability is low, or 13-27%, the ion back-attraction probability during the pulse is high (>92%), and the ionized flux fraction is about 2%. It is concluded that in the operation range studied here it is a challenge to ionize carbon atoms, that are sputtered off of a graphite target in a magnetron sputtering discharge, when depositing amorphous carbon films.
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| 64. |
- Fischer, Joel, et al.
(författare)
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Insights into the copper HiPIMS discharge : deposition rate and ionised flux fraction
- 2023
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Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 32:12
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Tidskriftsartikel (refereegranskat)abstract
- The influence of pulse length, working gas pressure, and peak discharge current density on the deposition rate and ionised flux fraction in high power impulse magnetron sputtering discharges of copper is investigated experimentally using a charge-selective (electrically biasable) magnetically shielded quartz crystal microbalance (or ionmeter). The large explored parameter space covers both common process conditions and extreme cases. The measured ionised flux fraction for copper is found to be in the range from ≈10% to 80%, and to increase with increasing peak discharge current density up to a maximum at ≈ 1.25 A cm − 2 , before abruptly falling off at even higher current density values. Low working gas pressure is shown to be beneficial in terms of both ionised flux fraction and deposition rate fraction. For example, decreasing the working gas pressure from 1.0 Pa to 0.5 Pa leads on average to an increase of the ionised flux fraction by ≈ 14 percentage points (pp) and of the deposition rate fraction by ≈ 4 pp taking into account all the investigated pulse lengths.
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| 65. |
- Fälthammar, Carl-Gunne, et al.
(författare)
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Magnetosphere-ionosphere interactions as a key to the plasma Univers
- 1995
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Ingår i: IEEE Transactions on Plasma Science. - : Institute of Electrical and Electronics Engineers (IEEE). - 0093-3813 .- 1939-9375. ; 23, s. 2-9
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Tidskriftsartikel (refereegranskat)abstract
- Almost all known matter in the universe is in a state, the plasma state, that is rare on Earth, and whose physical properties are still incompletely understood. Its complexity is such that a reliable understanding must build on empirical knowledge. While laboratory experiments are still an important source of such knowledge, Earth’s magnetospere-ionosphere system, made accessible by space technology, vastly widens the parameter ranges in which plasma phenomena can be studied. This system contains all three main categories of plasma present in the universe. Furthermore, the interaction between the magnetosphere and the ionosphere excites a wealth of plasma physical phenomena of fundamental importance. These include, among others, formation of magnetic-field aligned electric fields, acceleration of charged particles, release of magnetically stored energy, formation of filamentary and cellular structures, as well as unexpected chemical separation processes. What has been learned, and what stilt remains to be learned, from study of the magnetosphere-ionosphere system should therefore provide a much improved basis for understanding of our universe.
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| 66. |
- Gudmundsson, Jon Tomas, 1965-, et al.
(författare)
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An ionization region model of the reactive Ar/O-2 high power impulse magnetron sputtering discharge
- 2016
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Ingår i: Plasma sources science & technology. - : Institute of Physics (IOP). - 0963-0252 .- 1361-6595. ; 25:6
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Tidskriftsartikel (refereegranskat)abstract
- A new reactive ionization region model (R-IRM) is developed to describe the reactive Ar/O-2 high power impulse magnetron sputtering (HiPIMS) discharge with a titanium target. It is then applied to study the temporal behavior of the discharge plasma parameters such as electron density, the neutral and ion composition, the ionization fraction of the sputtered vapor, the oxygen dissociation fraction, and the composition of the discharge current. We study and compare the discharge properties when the discharge is operated in the two well established operating modes, the metal mode and the poisoned mode. Experimentally, it is found that in the metal mode the discharge current waveform displays a typical non-reactive evolution, while in the poisoned mode the discharge current waveform becomes distinctly triangular and the current increases significantly. Using the R-IRM we explore the current increase and find that when the discharge is operated in the metal mode Ar+ and Ti+ -ions contribute most significantly (roughly equal amounts) to the discharge current while in the poisoned mode the Ar+ -ions contribute most significantly to the discharge current and the contribution of O+ -ions, Ti+ -ions, and secondary electron emission is much smaller. Furthermore, we find that recycling of atoms coming from the target, that are subsequently ionized, is required for the current generation in both modes of operation. From the R-IRM results it is found that in the metal mode self-sputter recycling dominates and in the poisoned mode working gas recycling dominates. We also show that working gas recycling can lead to very high discharge currents but never to a runaway. It is concluded that the dominating type of recycling determines the discharge current waveform.
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| 67. |
- Gudmundsson, Jon Tomas, 1965-, et al.
(författare)
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Are the argon metastables important in high power impulse magnetron sputtering discharges?
- 2015
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 22:11
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Tidskriftsartikel (refereegranskat)abstract
- We use an ionization region model to explore the ionization processes in the high power impulse magnetron sputtering (HiPIMS) discharge in argon with a titanium target. In conventional dc magnetron sputtering (dcMS), stepwise ionization can be an important route for ionization of the argon gas. However, in the HiPIMS discharge stepwise ionization is found to be negligible during the breakdown phase of the HiPIMS pulse and becomes significant (but never dominating) only later in the pulse. For the sputtered species, Penning ionization can be a significant ionization mechanism in the dcMS discharges, while in the HiPIMS discharge Penning ionization is always negligible as compared to electron impact ionization. The main reasons for these differences are a higher plasma density in the HiPIMS discharge, and a higher electron temperature. Furthermore, we explore the ionization fraction and the ionized flux fraction of the sputtered vapor and compare with recent experimental work.
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| 68. |
- Gudmundsson, J. T., et al.
(författare)
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High power impulse magnetron sputtering discharge
- 2012
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Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 30:3, s. 030801-
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Forskningsöversikt (refereegranskat)abstract
- The high power impulse magnetron sputtering (HiPIMS) discharge is a recent addition to plasma based sputtering technology. In HiPIMS, high power is applied to the magnetron target in unipolar pulses at low duty cycle and low repetition frequency while keeping the average power about 2 orders of magnitude lower than the peak power. This results in a high plasma density, and high ionization fraction of the sputtered vapor, which allows better control of the film growth by controlling the energy and direction of the deposition species. This is a significant advantage over conventional dc magnetron sputtering where the sputtered vapor consists mainly of neutral species. The HiPIMS discharge is now an established ionized physical vapor deposition technique, which is easily scalable and has been successfully introduced into various industrial applications. The authors give an overview of the development of the HiPIMS discharge, and the underlying mechanisms that dictate the discharge properties. First, an introduction to the magnetron sputtering discharge and its various configurations and modifications is given. Then the development and properties of the high power pulsed power supply are discussed, followed by an overview of the measured plasma parameters in the HiPIMS discharge, the electron energy and density, the ion energy, ion flux and plasma composition, and a discussion on the deposition rate. Finally, some of the models that have been developed to gain understanding of the discharge processes are reviewed, including the phenomenological material pathway model, and the ionization region model.
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| 69. |
- Gudmundsson, Jon Tomas, 1965-, et al.
(författare)
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ON ELECTRON HEATING IN MAGNETRON SPUTTERING DISCHARGES
- 2017
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Ingår i: 2017 IEEE International Conference on Plasma Science (ICOPS). - : IEEE.
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Summary form only given. The magnetron sputtering discharge is a highly successful tool for deposition of thin films and coatings. It has been applied for various industrial applications for over four decades. Sustaining a plasma in a magnetron sputtering discharge requires energy transfer to the plasma electrons. In the past, the magnetron sputtering discharge has been assumed to be maintained by cathode sheath acceleration of secondary electrons emitted from the target, upon ion impact. These highly energetic electrons then either ionize the atoms of the working gas directly or transfer energy to the local lower energy electron population that subsequently ionizes the working gas atoms. This leads to the well-known Thornton equation, which in its original form is formulated to give the minimum required voltage to sustain the discharge. However, recently we have demonstrated that Ohmic heating of electrons outside the cathode sheath is typically of the same order as heating due to acceleration across the sheath in dc magnetron sputtering (dcMS) discharges. The secondary electron emission yield γsee is identified as the key parameter determining the relative importance of the two processes. In the case of dcMS Ohmic heating is found to be more important than sheath acceleration for secondary electron emission yields below around 0.1. For the high power impulse magnetron sputtering (HiPIMS) discharge we find that direct Ohmic heating of the plasma electrons is found to dominate over sheath acceleration by typically an order of magnitude, or in the range of 87 - 99 % of the total electron heating. A potential drop of roughly 100 - 150 V, or 15 - 25% of the discharge voltage, always falls across the plasma outside the cathode sheath.
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| 70. |
- Gudmundsson, Jon Tomas, et al.
(författare)
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The current waveform in reactive high power impulse magnetron sputtering
- 2016
-
Ingår i: 2016 IEEE International Conference on Plasma Science (ICOPS). - : Institute of Electrical and Electronics Engineers (IEEE). - 9781467396011
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Konferensbidrag (refereegranskat)abstract
- Summary form only given. The understanding of the current waveform for the non-reactive HiPIMS discharge is now rather well established [1,2]. It is described by a rise in the current to an initial peak and then a drop followed by a stable plateau. The drop is a result of a strong gas compression due to the sudden large flux of atoms from the target. For the reactive HiPIMS discharge striking differences are observed and those seem to depend on the mode of operation, the reactive gas and the target material. The discharge current waveform changes in shape as well as in the peak value when the target surface enters the poisoned mode. For Ar/O2 discharge with Ti target the discharge current waveform varies with oxygen partial pressure and pulse repetition frequency [3]. For the higher repetition frequencies the familiar nonreactive current waveform is observed. As the repetition frequency is lowered there is an increase in the current which transits into a different waveform as the repetition frequency is decreased further. The waveform observed at low repetition frequency is similar to the one observed at high reactive gas flow rate. Similarly, the current waveform in the reactive Ar/N2 HiPIMS discharge with Ti target is highly dependent on the pulse repetition frequency and the current is found to increase significantly as the frequency is lowered [4]. However, the discharge current keeps its shape and it remains as for the non-reactive case as the current increases. These findings will be compared with results for various combinations of gas mixtures and targets found in the literature [5]. Furthermore, we explore the current waveform in reactive HiPIMS using the ionization region model (IRM) [6] of the reactive Ar/O2 discharge with a Ti target. We discuss the current waveform development and how the discharge composition varies between metal and poisoned mode.
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