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Sökning: WFRF:(Bielecki Johan 1982) > (2010-2014)

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  • Sillrén, Per, 1982, et al. (författare)
  • A statistical model of hydrogen bond networks in liquid alcohols
  • 2012
  • Ingår i: Journal of Chemical Physics. - 1089-7690 .- 0021-9606. ; 136:9, s. 094514-094521
  • Tidskriftsartikel (refereegranskat)abstract
    • We here present a statistical model of hydrogen bond induced network structures in liquid alcohols. The model generalises the Andersson-Schulz-Flory chain model to allow also for branched structures. Two bonding probabilities are assigned to each hydroxyl group oxygen, where the first is the probability of a lone pair accepting an H-bond and the second is the probability that given this bond also the second lone pair is bonded. The average hydroxyl group cluster size, cluster size distribution, and the number of branches and leaves in the tree-like network clusters are directly determined from these probabilities. The applicability of the model is tested by comparison to cluster size distributions and bonding probabilities obtained from Monte Carlo simulations of the monoalcohols methanol, propanol, butanol, and propylene glycol monomethyl ether, the di-alcohol propylene glycol, and the tri-alcohol glycerol. We find that the tree model can reproduce the cluster size distributions and the bonding probabilities for both mono- and poly-alcohols, showing the branched nature of the OH-clusters in these liquids. Thus, this statistical model is a useful tool to better understand the structure of network forming hydrogen bonded liquids. The model can be applied to experimental data, allowing the topology of the clusters to be determined from such studies.
  • Fu, Yifeng, 1984, et al. (författare)
  • Selective growth of double-walled carbon nanotubes on gold films
  • 2012
  • Ingår i: Materials Letters. - 0167-577X. ; 72, s. 78-80
  • Tidskriftsartikel (refereegranskat)abstract
    • Growth of high-quality vertical aligned carbon nanotube (CNT) structures on silicon supported gold (Au) films by thermal chemical vapor deposition (TCVD) is presented. Transmission electron microscopy (TEM) images show that the growth is highly selective. Statistical study reveals that 79.4% of the as-grown CNTs are double-walled. The CNTs synthesized on Au films are more porous than that synthesized on silicon substrates under the same conditions. Raman spectroscopy and electrical characterization performed on the as-grown double-walled CNTs (DWNTs) indicate that they are competitive with those CNTs grown on silicon substrates. Field emission tests show that closed-ended DWNTs have lower threshold field than those open-ended.
  • Fu, Yifeng, 1984, et al. (författare)
  • Templated Growth of Covalently Bonded Three-Dimensional Carbon Nanotube Networks Originated from Graphene
  • 2012
  • Ingår i: Advanced Materials. - 0935-9648 .- 1521-4095. ; 24:12, s. 1576-1581
  • Tidskriftsartikel (refereegranskat)abstract
    • A template-assisted method that enables the growth of covalently bonded three-dimensional carbon nanotubes (CNTs) originating from graphene at a large scale is demonstrated. Atomic force microscopy-based mechanical tests show that the covalently bonded CNT structure can effectively distribute external loading throughout the network to improve the mechanical strength of the material.
  • He, Wenxiao, 1985, et al. (författare)
  • Biomimetic nanocrystalline apatite resembling bone
  • 2010
  • Ingår i: European Cells and Materials: 3rd International NanoBio Conference 2010, ETH Zurich, Switzerland August 24-27 2010. - 1473-2262. ; 20:suppl. 3
  • Konferensbidrag (refereegranskat)
  • Zhang, Zhfei, et al. (författare)
  • Characterization of CNT Enhanced Conductive Adhesives in Terms of Thermal Conductivity
  • 2012
  • Ingår i: ECS Transactions. - 1938-6737 .- 1938-5862. ; 44:1, s. 1011-1017
  • Konferensbidrag (refereegranskat)abstract
    • CNTs have excellent thermal, electrical and mechanical properties. They can be used in various ways. One researched field of application is CNT-polymer composites which combine common technologies with advanced materials. This paper will focus on the thermal property of CNT-Ag-filled adhesives and compares the new materials with conventional, electrical Ag-filled conductive adhesives. Several analytical methods, FTIR, Raman analysis, SEM and TEM have been carried out to examine the different surface conditions after physicval and chemical modification of CNTs. The thermal conductivities of composites containing different types of CNTs were investigated. The incorporation of CNTs into polymers resulted in enhancement of the thermal conductivity compared to Ag-filler. The increase of thermal conductivity with addition of CNT filler is obvious, especially for the purified CNT. The value of thermal conductivity is about two times higher than the Ag-filled conductive adhesive. However, improvement on thermal conductivity of the surface modified CNT-filled conductive adhesives is not so obvious. Since the thermal conduction in CNT is by phonon transfer, the nanometric size and the huge interface lead to strong phonon-scattering at the interface. Thus, a relatively low interfacial area, weak interfacial adhesion promotes the conduction of phonons and minimizes coupling losses. According to this, the non-treated MWCNTs seem to have the highest potential to improve the thermal conductivity of epoxies.
  • Bielecki, Johan, 1982, et al. (författare)
  • Femtosecond optical reflectivity measurements of lattice-mediated spin repulsions in photoexcited LaCoO3 thin films
  • 2014
  • Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:3, s. art. no. 035129-
  • Tidskriftsartikel (refereegranskat)abstract
    • We present results on the temperature dependence of ultrafast electron and lattice dynamics, measured with pump-probe transient reflectivity experiments, of an epitaxially grown LaCoO3 thin film under tensile strain. Probing spin-polarized transitions into the antibonding e(g) band provides a measure of the low-spin fraction, both as a function of temperature and time after photoexcitation. It is observed that femtosecond laser pulses destabilize the constant low-spin fraction (similar to 63%-64%) in equilibrium into a thermally activated state, driven by a subpicosecond change in spin gap Delta. From the time evolution of the low-spin fraction, it is possible to disentangle the thermal and lattice contributions to the spin state. A lattice mediated spin repulsion, identified as the governing factor determining the equilibrium spin state in thin-film LaCoO3, is observed. These results suggests that time-resolved spectroscopy is a sensitive probe of the spin state in LaCoO3 thin films, with the potential to bring forward quantitative insight into the complicated interplay between structure and spin state in LaCoO3.
  • Bielecki, Johan, 1982 (författare)
  • Optical Spectroscopy on Correlated Electron Materials
  • 2010
  • Licentiatavhandling (övrigt vetenskapligt)abstract
    • Strongly correlated materials poses a great challenge to our microscopic understanding of matter, mainly due to the inability of perturbative methods to predict the properties of such materials. This inability stems from strong correlations between electronic, lattice and spin degrees of freedom, leading to a variety of complex and exotic phenomena such as (unconventional) superconductivity, metal-insulator transitions, non-Fermi liquid behavior and colossal magnetoresistance.In this thesis, optical spectroscopy investigations on two kinds of strongly correlated matter is presented: materials exhibiting colossal magnetoresistance (manganites) and superconductivity (iron-pnictides). The research has been done from a fundamental point of view, with the goal to advance our understanding of the fascinating interplay between electrons, phonons and spin fluctuation that is encountered in these materials. The electron-phonon coupling in the newly discovered iron-pnictide class of high temperature superconductors have been studied using Raman spectroscopy assisted by theoretical calculations with the goal to elucidate the role of phonons as the mediating boson between electrons in a Cooper-pair. Our results indicate that the phononic spectra can, to good accuracy, be explained by neglecting completely the electron-phonon coupling. This provides further credentials to the widespread opinion that magnetic fluctuations constitute the attractive force responsible for the destabilization of the Fermi-sea leading to the condensation into superconducting Cooper-pairs. Further analysis shows that, through F doping on the O site, ejecting electrons into the quasi-two dimensional Fe-As plane actually leads to hole-doping at the Fe site, in accord with previous first principle calculations.The fundamental dynamics between the electrons, lattice and spins in two manganite samples have been studied using sub-picosecond time resolved pump-probe reflectance spectroscopy: The parent compound LaMnO3 and the doped La0.7Ca0.3MnO3 exhibiting colossal magnetoresistance. Even though LaMnO3 is a semiconductor and La0.7Ca0.3MnO3 is a double-exchange metal at low temperatures, the observed femtosecond dynamics are very similar. We introduce a phenomenological model that can explain all observed features in both samples using a combination of thermal (>1ps) and non-thermal (
  • Bielecki, Johan, 1982, et al. (författare)
  • Short-range structure of the brownmillerite-type oxide Ba2In2O5 and its hydrated proton-conducting form BaInO3H
  • 2014
  • Ingår i: Journal of Materials Chemistry A. - 2050-7488 .- 2050-7496. ; 2:40, s. 16915-16924
  • Tidskriftsartikel (refereegranskat)abstract
    • The vibrational spectra and short-range structure of the brownmillerite-type oxide Ba2In2O6 and its hydrated form BaInO3H, are investigated by means of Raman, infrared, and inelastic neutron scattering spectroscopies together with density functional theory calculations. For Ba2In2O6, which may be described as an oxygen deficient perovskite structure with alternating layers of InO6 octahedra and InO4 tetrahedra, the results affirm a short-range structure of Icmm symmetry, which is characterized by random orientation of successive layers of InO4 tetrahedra. For the hydrated, proton conducting, form, BaInO3H, the results suggest that the short-range structure is more complicated than the P4/mbm symmetry that has been proposed previously on the basis of neutron diffraction, but rather suggest a proton configuration close to the lowest energy structure predicted by Martinez et al. [J.-R. Martinez, C. E. Moen, S. Stoelen, N. L. Allan, J. Solid State Chem., 180, 3388, (2007)]. An intense Raman active vibration at 150 cm(-1) is identified as a unique fingerprint of this proton configuration.
  • Bielecki, Johan, 1982 (författare)
  • Structure and Dynamics in Transition Metal Perovskites — An Optical Spectroscopy Study
  • 2012
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Transition metal oxides with the perovskite structure exhibit a vast number of exotic properties due to the large number of competing degrees of freedom at the Fermi surface. Together with couplings between and correlations within these degrees of freedom, this class of materials is of high interest from both a practical and theoretical point of view. In this thesis, the properties of four different perovskite systems and their relation to structure and dynamics is investigated: La1-x CaxMnO3, exhibiting the colossal magnetoresistance effect; LaCoO3, undergoing spin-state transitions influenced by complicated electron-lattice interactions; BiFeO3 that is of the only known room-temperature multiferroic materials; and the proton conductor BaInO3H.Two kinds of optical spectroscopy methods have been used to investigate the structure and dynamics of these transition metal perovskites. Raman spectroscopy measures low-energy excitations and is, in combination with group theoretical selection rules, a sensitive probe of electronic, atomic, and magnetic structure. Ultrafast time-resolved spectroscopy is used to study, in real time, fundamental dynamics and interaction mechanisms on the femtosecond timescale.Utilizing time resolved pump-probe reflectance spectroscopy, thermal and lattice contributions to the spin-state transition are temporarily decouple, and the first explicit observation of the high-spin repulsion in LaCoO3, first conjectured by Goodenough in the 1950's, is presented. This opens up a novel avenue for investigating the spin-state transition in LaCoO3. With the same experimental technique, the electron-lattice, lattice-spin, and electron-spin dynamics are investigated in La1-xCaxMnO3. Incorporating a two-component heat diffusion mechanism it is shown that the rate-equation based model captures all essentials of the >1 ps dynamics.Raman spectroscopy is used to investigate the structural phase-diagram of isovalently substituted BiFeO3. The vibrational frequencies as a function of substitution provides unique input into the controversial phonon assignments of BiFeO3 and the pronounced second-order scattering is explained in terms of LO-modes activated by the Fröhlich interaction. Further, the first phonon assignment is presented and the structural transition during dehydration is investigated for BaInO3H.Complementing the studies on perovskites, the electron-phonon interactions are investigated in a iron-pnictide high temperature superconductor by combining Raman spectroscopy and phonon-phonon calculations. Our results point towards weak electron-phonon couplings and supports the view that a non-phononic glue is responsible for the Cooper pair condensation in iron-pnictides.
  • He, Wenxiao, 1985, et al. (författare)
  • Formation of Bone-like Nanocrystalline Apatite Using Self-Assembled Liquid Crystals
  • 2012
  • Ingår i: Chemistry of Materials. - : The American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 24:5, s. 892-902
  • Tidskriftsartikel (refereegranskat)abstract
    • A two-step process using liquid crystalline phases combined with controlled postcrystallization for the preparation of bone-like apatite has been developed. First, amorphous calcium phosphate (ACP) spherules with a diameter of 10.8 +/- 1.4 nm and specific surface area (SSA) in the range of 150-170 m(2)/g were synthesized within a reverse hexagonal liquid crystalline (LC) phase. Second, the ACP spherules were dispersed and aged in Milli-Qwater, where they crystallized into poorly crystalline apatite (PCA). The addition of heparin during aging was explored, which was shown to retard the ACP - PCA conversion. The particle formation within the LC phase was monitored using synchrotron small-angle X-ray scattering, and the formed materials were characterized by X-ray diffraction, conventional and high-resolution transmission electron microscopy, nitrogen adsorption, thermogravimetry with infrared-coupled analysis, and Raman spectroscopy. The PCA formed using the LC aging route presented bone-resembling features, such as,Ca2+ and OH- deficiency, CO32- substitution, poor crystallinity; and ultrahigh SSA of 356 m(2)/g. The resulting particles were compared to hydroxyapatite synthesized via a conventional water-based precipitation method. The LC-aging route exhibited excellent controllability over the CaP crystallization, which enabled facile tailoring of the resulting material properties for different types of application.
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  • Resultat 1-10 av 12
  • [1]2Nästa
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Bielecki, Johan, 198 ... (12)
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Liu, Johan, 1960 (3)
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