Suchergebnisse

A constant supply of oxygen has been assumed to be necessary for the growth of titanium nanoparticles by sputtering. This oxygen supply can arise from a high background pressure in the vacuum system or from a purposely supplied gas. The supply of oxygen makes it difficult to grow metallic nanoparticles of titanium and can cause process problems by reacting with the target. We here report that growth of titanium nanoparticles in the metallic hexagonal titanium (alpha Ti) phase is possible using a pulsed hollow cathode sputter plasma and adding a high partial pressure of helium to the process instead of trace amounts of oxygen. The helium cools the process gas in which the nanoparticles nucleate. This is important both for the first dimer formation and the continued growth to a thermodynamically stable size. The parameter region, inside which the synthesis of nanoparticles is possible, is mapped out experimentally and the theory of the physical processes behind this process window is outlined. A pressure limit below which no nanoparticles were produced was found at 200 Pa, and could be attributed to a low dimer formation rate, mainly caused by a more rapid dilution of the growth material. Nanoparticle production also disappeared at argon gas flows above 25 sccm. In this case, the main reason was identified as a gas temperature increase within the nucleation zone, giving a too high evaporation rate from nanoparticles (clusters) in the stage of growth from dimers to stable nuclei. These two mechanisms are in depth explored in a companion paper. A process stability limit was also found at low argon gas partial pressures, and could be attributed to a transition from a hollow cathode discharge to a glow discharge. ; Funding Agencies|Knut and Alice Wallenberg foundation [KAW 2014.0276]; Swedish Research Council under the Linkoping Linneaus Environment-LiLi-NFM [2008-6572]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO Mat LiU) [2009 00971]; Swedish Research Council [2016-05137_4]

The effects of a positive pulse following a high-power impulse magnetron sputtering (HiPIMS) pulse are studied using energy-resolved mass spectrometry. This includes exploring the influence of a 200 mu s long positive voltage pulse (U-rev = 10-150 V) following a typical HiPIMS pulse on the ion-energy distribution function (IEDF) of the various ions. We find that a portion of the Ti+ flux is affected and gains an energy which corresponds to the acceleration over the full potential U-rev. The Ar+ IEDF on the other hand illustrates that a large fraction of the accelerated Ar+, gain energies corresponding to only a portion of U-rev. The Ti+ IEDFs are consistent with the assumption that practically all the TO-, that are accelerated during the reverse pulse, originates from a region adjacent to the target, in which the potential is uniformly increased with the applied potential U-rev while much of the Ar+ originates from a region further away from the target over which the potential drops from U-rev to a lower potential consistent with the plasma potential achieved without the application of U-rev. The deposition rate is only slightly affected and decreases with U-rev, reaching 90% at U-rev = 150 V. Both the Ti IEDF and the small deposition rate change indicate that the potential increase in the region close to the target is uniform and essentially free of electric fields, with the consequence that the motion of ions inside the region is not much influenced by the application of U-rev. In this situation, Ti will flow towards the outer boundary of the target adjacent region, with the momentum gained during the HiPIMS discharge pulse, independently of whether the positive pulse is applied or not. The metal ions that cross the boundary in the direction towards the substrate, and do this during the positive pulse, all gain an energy corresponding to the full positive applied potential U-rev. ; Funding Agencies|Swedish Research Council [VR 621-2014-4882]; Swedish Government Strategic Research Area in Materials Science on Functional materials at Linkoping University [2009-00971]

The nucleation and growth of pure titanium nanoparticles in a low-pressure sputter plasma has been believed to be essentially impossible. The addition of impurities, such as oxygen or water, facilitates this and allows the growth of nanoparticles. However, it seems that this route requires such high oxygen densities that metallic nanoparticles in the hexagonal alpha Ti-phase cannot be synthesized. Here we present a model which explains results for the nucleation and growth of titanium nanoparticles in the absent of reactive impurities. In these experiments, a high partial pressure of helium gas was added which increased the cooling rate of the process gas in the region where nucleation occurred. This is important for two reasons. First, a reduced gas temperature enhances Ti-2 dimer formation mainly because a lower gas temperature gives a higher gas density, which reduces the dilution of the Ti vapor through diffusion. The same effect can be achieved by increasing the gas pressure. Second, a reduced gas temperature has a more than exponential effect in lowering the rate of atom evaporation from the nanoparticles during their growth from a dimer to size where they are thermodynamically stable, r*. We show that this early stage evaporation is not possible to model as a thermodynamical equilibrium. Instead, the single-event nature of the evaporation process has to be considered. This leads, counter intuitively, to an evaporation probability from nanoparticles that is exactly zero below a critical nanoparticle temperature that is size-dependent. Together, the mechanisms described above explain two experimentally found limits for nucleation in an oxygen-free environment. First, there is a lower limit to the pressure for dimer formation. Second, there is an upper limit to the gas temperature above which evaporation makes the further growth to stable nuclei impossible. ; Funding Agencies|Knut and Alice Wallenberg foundation [KAW 2014.0276]; Swedish Research Council under the Linkoping Linneaus Environment-LiLi-NFM [2008-6572]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO Mat LiU) [2009 00971]; Swedish Research Council [2016-05137_4]

Sustaining a plasma in a magnetron discharge requires energization of the plasma electrons. In this work, Ohmic heating of electrons outside the cathode sheath is demonstrated to be typically of the same order as sheath energization, and a simple physical explanation is given. We propose a generalized Thornton equation that includes both sheath energization and Ohmic heating of electrons. The secondary electron emission yield gamma(SE) is identified as the key parameter determining the relative importance of the two processes. For a conventional 5 cm diameter planar dc magnetron, Ohmic heating is found to be more important than sheath energization for secondary electron emission yields below around 0.1. ; Funding Agencies|Swedish Research Council (VR) [621-2014-4882]; Icelandic Research Fund [130029]; Swedish Government Agency for Innovation Systems (VINNOVA) [2014-04876]

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. (C) 2015 AIP Publishing LLC. ; Funding Agencies|Icelandic Research Fund [130029-053]; Swedish Government Agency for Innovation Systems (VINNOVA) [2014-04876]; Belgian Government through the Pole dAttraction Interuniversitaire (PAI) [P7/34]

The plasma potential at a typical substrate position is studied during the positive pulse of a bipolar high-power impulse magnetron sputtering (bipolar HiPIMS) discharge with a Cu target. The goal of the study is to identify suitable conditions for achieving ion acceleration independent on substrate grounding. We find that the time-evolution of the plasma potential during the positive pulse can be separated into several distinct phases, which are highly dependent on the discharge conditions. This includes exploring the influence of the working gas pressure (0.3-2 Pa), HiPIMS peak current (10-70 A corresponding to 0.5-3.5 A cm(-2)), HiPIMS pulse length (5-60 mu s) and the amplitude of the positive voltage U (+) applied during the positive pulse (0-150 V). At low enough pressure, high enough HiPIMS peak current and long enough HiPIMS pulse length, the plasma potential at a typical substrate position is seen to be close to 0 V for a certain time interval (denoted phase B) during the positive pulse. At the same time, spatial mapping of the plasma potential inside the magnetic trap region revealed an elevated value of the plasma potential during phase B. These two plasma potential characteristics are identified as suitable for achieving ion acceleration in the target region. Moreover, by investigating the target current and ion saturation current at the chamber walls, we describe a simple theory linking the value of the plasma potential profile to the ratio of the available target electron current and ion saturation current at the wall. ; Funding Agencies|Swedish Research CouncilSwedish Research CouncilEuropean Commission [VR 2018-04139]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFOMat-LiU) [2009-00971]; Eurostars program (IonDrive) [E!114277]; Ministry of Education Youth and Sports of Czech RepublicMinistry of Education, Youth & Sports - Czech Republic [LM2018097]; Czech Science FoundationGrant Agency of the Czech Republic [GA19-00579S]; National Natural Science Foundation of ChinaNational Natural Science Foundation of China (NSFC) [51805102, 52165021]

Population densities of excited states of argon atoms in a high power impulse magnetron sputtering (HiPIMS) discharge are examined using a global discharge model and a collisional-radiative model. Here, the ionization region model (IRM) and the Orsay Boltzmann equation for electrons coupled with ionization and excited states kinetics (OBELIX) model are combined to obtain the population densities of the excited levels of the argon atom in a HiPIMS discharge. The IRM is a global plasma chemistry model based on particle and energy conservation of HiPIMS discharges. OBELIX is a collisional-radiative model where the electron energy distribution is calculated self-consistently from an isotropic Boltzmann equation. The collisional model constitutes 65 individual and effective excited levels of the argon atom. We demonstrate that the reduced population density of high-lying excited argon states scales with (p*)(-6), where p * is the effective quantum number, indicating the presence of a multistep ladder-like excitation scheme, also called an excitation saturation. The reason for this is the dominance of electron impact processes in the population and de-population of high-lying argon states in combination with a negligible electron-ion recombination. ; Funding Agencies|Free State of Saxony; European Regional Development FundEuropean Commission [100336119]; Icelandic Research Fund [196141]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [VR 2018-04139]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]

The magnetic field is a key feature that distinguishes magnetron sputtering from simple diode sputtering. It effectively increases the residence time of electrons close to the cathode surface and by that increases the energy efficiency of the discharge. This becomes apparent in high power impulse magnetron sputtering (HiPIMS) discharges, as small changes in the magnetic field can result in large variations in the discharge characteristics, notably the peak discharge current and/or the discharge voltage during a pulse. Here, we analyze the influence of the magnetic field on the electron density and temperature, how the discharge voltage is split between the cathode sheath and the ionization region, and the electron heating mechanism in a HiPIMS discharge. We relate the results to the energy efficiency of the discharge and discuss them in terms of the probability of target species ionization. The energy efficiency of the discharge is related to the fraction of pulse power absorbed by the electrons. Ohmic heating of electrons in the ionization region leads to higher energy efficiency than electron energization in the sheath. We find that the electron density and ionization probability of the sputtered species depend largely on the discharge current. The results suggest ways to adjust electron density and electron temperature using the discharge current and the magnetic field, respectively, and how they influence the ionization probability. ; Funding Agencies|Free State of Saxony; European Regional Development FundEuropean Commission [100336119]; Icelandic Research Fund [196141]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [VR 2018-04139]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

The hydrogen evolution reaction (HER) is a key process in electrochemical water splitting. To lower the cost and environmental impact of this process, it is highly motivated to develop electrocatalysts with low or no content of noble metals. Here, we report on an ingenious synthesis of hybrid PtxNi1-x electrocatalysts in the form of a nanoparticle-nanonetwork structure with very low noble metal content. The structure possesses important features such as good electrical conductivity, high surface area, strong interlinking, and substrate adhesion, which render an excellent HER activity. Specifically, the best performing Pt0.05Ni0.95 sample demonstrates a Tafel slope of 30 mV dec-1 in 0.5 M H2SO4 and an overpotential of 20 mV at a current density of 10 mA cm-2 with high stability. The impressive catalytic performance is further rationalized in a theoretical study, which provides insight into the mechanism on how such small platinum content can allow for close-to-optimal adsorption energies for hydrogen. ; Funding Agencies|Vetenskapsraodet [2017-04862, 2017-04380]; Energimyndigheten [45419-1, 50779-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO Mat LiU) [2009 00971]

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 (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. ; Funding Agencies|Free State of Saxony; European Regional Development FundEuropean Commission [100336119]; Icelandic Research Fund [196141]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [VR 201804139]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]