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- Brandell, Daniel, 1975-
(författare)
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Understanding Ionic Conductivity in Crystalline Polymer Electrolytes
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymer electrolytes are widely used as ion transport media in vital applications such as energy storage devices and electrochemical displays. To further develop these materials, it is important to understand their ionic conductivity mechanisms. It has long been thought that ionic conduction in a polymer electrolyte occurs in the amorphous phase, while the crystalline phase is insulating. However, this picture has recently been challenged by the discovery of the crystalline system LiXF6∙PEO6 (X=P, As or Sb) which exhibits higher conductivity than its amorphous counterpart. Their structures comprise interlocking hemi-helical PEO-chain pairs containing Li+ ions and separating them from the XF6- anions. The first Molecular Dynamics (MD) simulation study of the LiPF6∙PEO6 system is presented in this thesis. Although its conductivity is too low for most applications at ambient temperature, it can be enhanced by iso- and aliovalent anion doping. It is shown that the diffraction-determined structure is well reproduced on simulating the system using an infinite PEO-chain model. The Li-Oet coordination number here becomes 6 instead of 5; minor changes also occur in the polymer backbone configuration. The crystallographic asymmetric unit and diffraction profiles are also reproduced. On simulating a shorter-chain system (n=22), more resembling the real material, the structure retains its double hemi-helices, but the polymer adopts a more relaxed conformation, facilitating the formation of Li+-PF6- pairs. Infinite-chain simulation shows the ionic conduction to be dominated by anion motion, in contrast to earlier NMR results. The effects of doping are also reproduced. Shortening the polymer chain-length has the effect of raising the transport number for lithium, thereby bring it into better agreement with experiment. It can be concluded that it is critical to take polymer chain-length and chain-termination into account when modelling ionic conductivity mechanisms in crystalline polymer electrolytes.
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| 2. |
- Fensch, Jeremy, et al.
(författare)
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Massive star cluster formation and evolution in tidal dwarf galaxies
- 2019
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The formation of globular clusters remains an open debate. Dwarf starburst galaxies are efficient at forming young massiveclusters with similar masses as globular clusters and may hold the key to understanding their formation.Aims.We study star cluster formation in a tidal debris - including the vicinity of three tidal dwarf galaxies - in a massive gas dominatedcollisional ring around NGC 5291. These dwarfs have physical parameters which differ significantly from local starbursting dwarfs.They are gas-rich, highly turbulent, have a gas metallicity already enriched up to half-solar, and are expected to be free of dark matter.The aim is to study massive star cluster formation in this as yet unexplored type of environment.Methods.We use imaging from theHubble Space Telescopeusing broadband filters covering the wavelength range from the near-ultraviolet to the near-infrared. We determine the masses and ages of the cluster candidates by using the spectral energy distribution-fitting code CIGALE, carefully considering age-extinction degeneracy effects on the estimation of the physical parameters.Results.We find that the tidal dwarf galaxies in the ring of NGC 5291 are forming star clusters with an average efficiency of∼40%,comparable to blue compact dwarf galaxies. We also find massive star clusters for which the photometry suggests that they wereformed at the very birth of the tidal dwarf galaxies and have survived for several hundred million years. Therefore our study showsthat extended tidal dwarf galaxies and compact clusters may be formed simultaneously. In the specific case observed here, the youngstar clusters are not massive enough to survive for a Hubble time. However one may speculate that similar objects at higher redshift,with higher star formation rate, might form some of the long lived globular clusters.Key words.galaxies: dwarf, galaxies: star clusters: general, galaxies: irregular, galaxies: star formation, galaxies: stellar content,galaxies: interactions
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