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  • 1.
    Abd Alrahman, Chadi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Khodabakhsh, Amir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Qu, Zhechao
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Cavity-enhanced optical frequency comb spectroscopy of high-temperature H2O in a flame2014In: Optics Express, E-ISSN 1094-4087, Vol. 22, no 11, p. 13889-13895Article in journal (Refereed)
    Abstract [en]

    We demonstrate near-infrared cavity-enhanced optical frequency comb spectroscopy of water in a premixed methane/air flat flame. The detection system is based on an Er:fiber femtosecond laser, a high finesse optical cavity containing the flame, and a fast-scanning Fourier transform spectrometer (FTS). High absorption sensitivity is obtained by the combination of a high-bandwidth two-point comb-cavity lock and auto-balanced detection in the FTS. The system allows recording high-temperature water absorption spectra with a resolution of 1 GHz and a bandwidth of 50 nm in an acquisition time of 0.4 s, with absorption sensitivity of 4.2 x 10 (9) cm(-1) Hz(-1/2) per spectral element.

  • 2. Abdelsalam, UM
    et al.
    Moslem, WM
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Institut für Theoretische Physik IV, Fakultät für Physik und Astronomie, Ruhr-Universität Bochum, D-44780 Bochum, Germany; Nonlinear Physics Centre & Center for Plasma Science and Astrophysics, Ruhr-Universität Bochum, D-44780 Bochum, Germany; Max-Planck-Institut für extraterrestrische Physik, D-85741 Garching, Germany; GoLP/Instituto Superior Técnico, 1049-001 Lisbon, Portugal; CCLRC Centre for Fundamental Physics, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon 0X11 0QX, UK; SUPA Department of Physics, University of Strathclyde, Glasgow G 40NG, UK; School of Physics, Faculty of Science & Agriculture, University of Kwazulu-Natal, Durban 4000, South Africa; Department of Physics, CITT, Islamabad, Pakistan.
    Ion-acoustic solitary waves in a dense pair-ion plasma containing degenerate electrons and positrons2008In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 372, no 22, p. 4057-4061Article in journal (Refereed)
    Abstract [en]

    Fully nonlinear propagation of ion-acoustic solitary waves in a collisionless dense/quantum electron-positron-ion plasma is investigated. The electrons and positrons are assumed to follow the Thomas-Fermi density distribution and the ions are described by the hydrodynamic equations. An energy balance-like equation involving a Sagdeev-type pseudo-potential is derived. Finite amplitude solutions are obtained numerically and their characteristics are discussed. The small-but finite-amplitude limit is also considered and an exact analytical solution is obtained. The present studies might be helpful to understand the excitation of nonlinear ion-acoustic solitary waves in a degenerate plasma such as in superdense white dwarfs.

  • 3. Abdelsalam, UM
    et al.
    Moslem, WM
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Institut für Theoretische Physik IV, Fakultät für Physik und Astronomie, Ruhr-Universität Bochum, D-44780 Bochum, Germany; Nonlinear Physics Centre & Center for Plasma Science and Astrophysics, Ruhr-Universität Bochum, D-44780 Bochum, Germany; Max-Planck-Institut für Extraterrestrische Physik, D-85741 Garching, Germany; GoLP/Instituto Superior Técnico, 1049-001 Lisbon, Portugal; CCLRC Centre for Fundamental Physics, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon 0X11 0QX, UK; SUPA Department of Physics, University of Strathclyde, Glasgow G 40NG, UK; School of Physics, Faculty of Science & Agriculture, University of Kwazulu-Natal, Durban 4000, South Africa; Department of Physics, CITT, Islamabad, Pakistan.
    Localized electrostatic excitations in a Thomas-Fermi plasma containing degenerate electrons2008In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 15, no 5, article id 052303Article in journal (Refereed)
    Abstract [en]

    By using the Thomas-Fermi electron density distribution for quantum degenerate electrons, the hydrodynamic equations for ions, and the Poisson equation, planar and nonplanar ion-acoustic solitary waves in an unmagnetized collisionless plasma are investigated. The reductive perturbation method is used to derive cylindrical and spherical Korteweg-de Vries equations. Numerical solutions of the latter are presented. The present results can be useful in understanding the features of small but finite amplitude localized ion-acoustic solitary pulses in a degenerate plasma.

  • 4.
    Abdollahi, Elham
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Development of breath sampling system for detection of exhaled nitric oxide by Faraday modulation spectroscopy2013Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Technological developments over the years have resulted in many different techniques for detection of nitric oxide (NO) in both the atmosphere and from biological sources. One such technique is Faraday Modulation Spectroscopy (FAMOS), which is a laser-based spectroscopic technique for detection of paramagnetic molecules in gas phase. The technique uses a modulated magnetic field that introduce rotation of the polarization plane of linearly polarized laser light, which can be related to the concentration of the molecules. This enables sensitive and selective detection of paramagnetic gaseous compounds and the technique is thus well suited for detection of NO for biomedical applications in low concentrations which is essential for breath analysis.

    In this thesis, a system for breath analysis is developed and coupled to a Faraday modulation spectrometer for sensitive detection of NO at 5.33 μm based on a room temperature continuous wave distributed feedback (DFB) QCL. It also provides a theoretical model of FAMOS utilizing the most sensitive Q3/2 (3/2 ) transition in NO. The results from this study indicate that the mid-infrared FAMOS system, which was built around a continuous wave (cw) quantum cascade laser (QCL) emitting light with an output power of approximately 70 mW, is fully capable of detection of ppb levels of NO in exhaled human breath.

  • 5.
    Abdollahi, Elham
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Realization of an instrumentation for detection of acetylene in breath by the NICE-OHMS technique2014Independent thesis Advanced level (degree of Master (Two Years)), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Measurement is an important activity in nearly all branches of science and technology. A measurement technique provides an observer with a numerical value corresponding to the variable being measured. Researchers envision that laser spectroscopy will serve as a functional tool for measurement to detect molecules in gas phase. One such a laser spectroscopic technique for measurement is noise-immune cavity enhanced optical heterodyne molecular spectroscopy (NICE-OHMS), which is a highly sensitive laser-based spectroscopic technique for detection of molecules in gas phase. The technique was developed in the mid-1990s at the Joint Institute for Laboratory Astrophysics (JILA).

    In this thesis, a realization of instrumentation for detection of acetylene in breath by a fiber laser-based NICE-OHMS instrumentation working in the near-infrared wavelength region is obtained. The results of this study show that the NICE-OHMS system is fully capable of detection of parts-per-billion (ppb) levels of acetylene in exhaled human breath.

  • 6. Abdullah, Muhammad Imran
    et al.
    Janjua, Muhammad Ramzan Saeed Ashraf
    Mahmood, Asif
    Ali, Sajid
    Ali, Muhammad
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Quantum Chemical Designing of Efficient Sensitizers for Dye Sensitized Solar Cells2013In: Bulletin of the Korean Chemical Society (Print), ISSN 0253-2964, E-ISSN 1229-5949, Vol. 34, no 7, p. 2093-2098Article in journal (Refereed)
    Abstract [en]

    Density functional theory (DFT) was used to determine the ground state geometries of indigo and new design dyes (IM-Dye-1 IM-Dye-2 and IM-Dye-3). The time dependant density functional theory (TDDFT) was used to calculate the excitation energies. All the calculations were performed in both gas and solvent phase. The LUMO energies of all the dyes were above the conduction band of TiO2, while the HOMOs were below the redox couple (except IM-Dye-3). The HOMO-LUMO energy gaps of new design dyes were smaller as compared to indigo. All new design dyes were strongly red shifted as compared to indigo. The improved light harvesting efficiency (LHE) and free energy change of electron injection Delta G(inject) of new designed sensitizers revealed that these materials would be excellent sensitizers. The broken coplanarity between the benzene near anchoring group having LUMO and the last benzene attached to TPA unit in all new design dyes consequently would hamper the recombination reaction. This theoretical designing will the pave way for experimentalists to synthesize the efficient sensitizers for solar cells.

  • 7.
    Abdullah, Sarwin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Low-energy radar for handheld IR-cameras: Using mmWave radar as a complement for Multi Spectral Dynamic Imaging in handheld cameras2023Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The aim of this project is to assess whether a low energy radar, more specifically the mmWave radar from Acconer could be used as a complement to the handheld cameras from FLIR, in order to improve its performance, in particular the alignment of visual and infra-red image. The specific camera used for this project is a FLIR C5 IR-camera. It was found that if the distance is known, then one could align the images more easily and by the help of the radar this could occur automatically instead of doing it manually as it is implemented today. 

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  • 8. Abou-Hamad, E.
    et al.
    Babaa, M. -R
    Bouhrara, M.
    Kim, Y.
    Saih, Y.
    Dennler, S.
    Mauri, F.
    Basset, J. -M
    Goze-Bac, C.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Structural properties of carbon nanotubes derived from (13)C NMR2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 16, p. 165417-Article in journal (Refereed)
    Abstract [en]

    We present a detailed experimental and theoretical study on how structural properties of carbon nanotubes can be derived from 13C NMR investigations. Magic angle spinning solid state NMR experiments have been performed on single-and multiwalled carbon nanotubes with diameters in the range from 0.7 to 100 nm and with number of walls from 1 to 90. We provide models on how diameter and the number of nanotube walls influence NMR linewidth and line position. Both models are supported by theoretical calculations. Increasing the diameter D, from the smallest investigated nanotube, which in our study corresponds to the inner nanotube of a double-walled tube to the largest studied diameter, corresponding to large multiwalled nanotubes, leads to a 23.5 ppm diamagnetic shift of the isotropic NMR line position d. We show that the isotropic line follows the relation d = 18.3/D + 102.5 ppm, where D is the diameter of the tube and NMR line position d is relative to tetramethylsilane. The relation asymptotically tends to approach the line position expected in graphene. A characteristic broadening of the line shape is observed with the increasing number of walls. This feature can be rationalized by an isotropic shift distribution originating from different diamagnetic shielding of the encapsulated nanotubes together with a heterogeneity of the samples. Based on our results, NMR is shown to be a nondestructive spectroscopic method that can be used as a complementary method to, for example, transmission electron microscopy to obtain structural information for carbon nanotubes, especially bulk samples.

  • 9. Abou-Hamad, E.
    et al.
    Kim, Y.
    Bouhrara, M.
    Saih, Y.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Luzzi, D. E.
    Goze-Bac, C.
    NMR strategies to study the local magnetic properties of carbon nanotubes2012In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 407, no 4, p. 740-742Article in journal (Refereed)
    Abstract [en]

    The local magnetic properties of the one dimensional inner space of the nanotubes are investigated using C-13 nuclear magnetic resonance spectroscopy of encapsulated fullerene molecules inside single walled carbon nanotubes. Isotope engineering and magnetically purified nanotubes have been advantageously used on our study to discriminate between the different diamagnetic and paramagnetic shifts of the resonances. Ring currents originating from the pi electrons circulating on the nanotube, are found to actively screen the applied magnetic field by -36.9 ppm. Defects and holes in the nanotube walls cancel this screening locally. What is interesting, that at high magnetic fields, the modifications of the NMR resonances of the molecules from free to encapsulated can be exploited to determine some structural characteristics of the surrounding nanotubes, never observed experimentally. (C) 2011 Elsevier B.V. All rights reserved.

  • 10.
    Abou-Hamad, Edy
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Goze-Bac, Christophe
    Université Montpellier II, France.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Schmid, Michael
    Physikalisches Institut, Universität Stuttgart, Germany.
    Aznar, Robert
    Université Montpellier II, France.
    Mehring, Michael
    Physikalisches Institut, Universität Stuttgart, Germany.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Electronic properties of Cs-intercalated single-walled carbon nanotubes derived from nuclear magnetic resonance2011In: New Journal of Physics, E-ISSN 1367-2630, Vol. 13, p. 053045 (1)-(9)Article in journal (Refereed)
    Abstract [en]

    We report on the electronic properties of Cs-intercalated singlewalled carbon nanotubes (SWNTs). A detailed analysis of the 13C and133Cs nuclear magnetic resonance (NMR) spectra reveals an increased metallization of the pristine SWNTs under Cs intercalation. The ‘metallization’ of CsxC materials where x =0–0.144 is evidenced from the increased local electronic density of states (DOS) n(EF)at the Fermi level of the SWNTs as determined from spin–lattice relaxation measurements. In particular, there are two distinct electronic phases called α and β and the transition between these occurs around x = 0.05. The electronic DOS at the Fermi level increases monotonically at low intercalation levels x <0.05 (α-phase), whereas it reaches a plateau in the range 0.05 < x < 0.143 at high intercalation levels (β-phase). The new β-phase is accompanied by a hybridization of Cs(6s) orbitals with C(sp2)orbitals of the SWNTs. In both phases, two types of metallic nanotubes are found with a low and a high local n(EF), corresponding to different local electronic band structures of the SWNTs.

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  • 11.
    Abou-Hamad, Edy
    et al.
    Universite Montpellier II.
    Kim, Y
    University of Pennsylvania.
    Talyzin, Alexandr
    Umeå University, Faculty of Science and Technology, Physics.
    Goze-Bac, Christophe
    Universite Montpellier II.
    Luzzi, David
    University of Pennsylvania.
    Rubio, Angelo
    University of Basque Country.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Hydrogenation of C-60 in Peapods: Physical Chemistry in Nano Vessels2009In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, no 20, p. 8583-8587Article in journal (Refereed)
    Abstract [en]

    Hydrogenation of C-60 molecules inside SWNT was achieved by direct reaction with hydrogen gas at elevated pressure and temperature. Evidence for the C-60 hydrogenation in peapods is provided by isotopic engineering with specific enrichment of encapsulated species and high resolution C-13 and H-1 NMR spectroscopy with the observation of characteristic diamagnetic and paramagnetic shifts of the NMR lines and the appearance of sp(3) carbon resonances. We estimate that approximately 78% of the C-60 molecules inside SWNTs are hydrogenated to an average degree of 14 hydrogen atoms per C-60 molecule. As a consequence, the rotational dynamics of the encapsulated C60Hx molecules is clearly hindered. Our successful hydrogenation experiments open completely new roads to understand and control confined chemical reactions at the nano scale

  • 12.
    Abou-Hamad, Edy
    et al.
    Universite Montpellier 2, France.
    Kim, Y.
    University of Pennsylvania, Philadelphia.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boesch, D.
    University of California at Berkeley, and Materials Sciences Division, Lawrence Berkeley National Laboratory.
    Aloni, S.
    University of California at Berkeley, and Materials Sciences Division, Lawrence Berkeley National Laboratory.
    Zettl, Alex
    University of California at Berkeley, and Materials Sciences Division, Lawrence Berkeley National Laboratory.
    Rubio, Angelo
    Universidad del Pas Vasco UPV/EHU.
    Luzzi, David E.
    University of Pennsylvania, Philadelphia.
    Goze-Bac, Christophe
    CNRS Universit Montpellier 2.
    Molecular dynamics and phase transition in one-dimensional crystal of C60 encapsulated inside single wall carbon nanotubes2009In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 3, no 12, p. 3878-3883Article in journal (Refereed)
    Abstract [en]

    One-dimensional crystals of 25% 13C-enriched C60 encapsulated inside highly magnetically purified SWNTs were investigated by following the temperature dependence of the 13C NMR line shapes and the relaxation rates from 300 K down to 5 K. High-resolution MAS techniques reveal that 32% of the encapsulated molecules, so-called the C60α, are blocked at room temperature and 68%, labeled C60β, are shown to reversly undergo molecular reorientational dynamics. Contrary to previous NMR studies, spin−lattice relaxation time reveals a phase transition at 100 K associated with the changes in the nature of the C60β dynamics. Above the transition, the C60β exhibits continuous rotational diffusion; below the transition, C60β executes uniaxial hindered rotations most likely along the nanotubes axis and freeze out below 25 K. The associated activation energies of these two dynamical regimes are measured to be 6 times lower than in fcc-C60, suggesting a quiet smooth orientational dependence of the interaction between C60β molecules and the inner surface of the nanotubes.

  • 13.
    Abrahamsson, Anton
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Three dimensional tracking of multiple objects using digital holographic microscopy2016Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
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  • 14.
    ABUDULIMU, ABASI
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Effectof Growth Time, Growth Temperature and Light  on Growth Mechanism of C60 nanorods2013Independent thesis Advanced level (degree of Master (Two Years)), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    In this thesis work C60 nanorods were produced by Liquid-Liquid Interfacial Precipitation method (LLIP) assisted with 10 s of weak sonication. Ethanol and m-dichlorobenzene were used as poor and good solvents of C60, respectively. Five different temperatures, 4, 10, 20, 30, 40 and 50                         , were chosen as growth temperatures of different samples to investigate the effect of temperature on the grown structures. Different samples were prepared in the dark and under the light with various growth time to determine the effect of light and growth time on growth of C60 nanorods. The characterization of the grown C60 nanorods were conducted by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The result of characterization indicated that the sonication introduced smaller C60 nanostructures; light irradiation and temperature increase (till 40 C0) during the growth time resulted in nanorods with smaller diameter, whereas the long growth time lead to the increase of the diameter of C60 nanorods. The as-grown C60 nanorods synthesized at different conditions possess an hcp crystal structure.    

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    Effect of Growth Time, Growth Temperature and Light on Growth Mechanism of C60 nanorods
  • 15.
    ABUDULIMU, ABASI
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Measuring the efficiency and charge carrier mobility of organic solar cells2012Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    P3HT single layer, P3HT/PCBM bilayer and P3HT/PCBM inverted bilayer devices were produced by spin coating organic layers onto ITO patterned glass in air, and clamping it with an Au coated silicon wafer, as top electrode, at the end (Figure13). Normal and inverted bilayer devices were also fabricated with and without PEDOT:PSS. All devices were divided into two groups by changing concentration of P3HT solution. The first group of devices contained 1.0 wt. % P3HT solution (P3HT in dichlorobenzene); the second group 0.56wt %. Power conversion efficiency, short circuit current, open circuit voltage, fill factor and maximum extracted power were measured on all produced devices.

    In contrast, all devices with 1.0wt % P3HT concentration showed better result than the devices with 0.56wt %. The highest result was obtained for P3HT single layer devices in both cases with short circuit current 56uA/cm2, open circuit voltage 0.94mV, maximum power 11.4uW/cm2 and power conversion efficiency of 0.11%. Inverted bilayer devices performed better than the non-inverted one. The devices with PEDOT:PSS got slightly better performance than the non-PEDOT:PSS used one.

    Charge carrier mobility measurement was done for all fabricated devices with charge extraction by linearly increasing voltage (CELIV) and dark injected space charge limited current (DI-SCLC) methods. All devices showed same magnitude of charge carrier mobility 10-5 cm2/V.s, the highest value still belongs to P3HT single layer device. The charge carrier mobility in all devices observed by DI-SCLC technique is one order of magnitude higher than by CELIV technique. This may be due to DI-SCLC method`s restriction on ohmic contacts between material and electrode.

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    Abdilim
  • 16. Adhikary, N C
    et al.
    Misra, Amar P
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bailung, H
    Chutia, J
    Ion-beam driven dust ion-acoustic solitary waves in dusty plasmas2010In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 17, no 4, article id 044502Article in journal (Refereed)
    Abstract [en]

    The nonlinear propagation of small but finite amplitude dust ion-acoustic waves (DIAWs) in an ion-beam driven plasma consisting of Boltzmannian electrons, positive ions, and stationary negatively charged dust grains is studied by using the standard reductive perturbation technique. It is shown that there exist two critical values (γc1) and (γc2) of ion beam to ion phase velocity ratio (γ), above and below which the beam generated solitons are not possible. The effects of the parameters, namely, γ, the ratio of the ion beam to plasma ion density (μi), the dust to ion density ratio (μd), and the ion beam to plasma ion mass ratio (μ) on both the amplitude and width of the stationary DIAWs, are analyzed numerically, and applications of the results to laboratory ion beam as well as space plasmas (e.g., auroral plasmas) are explained.

  • 17.
    Adolfsson, Lena
    et al.
    Umeå University, Faculty of Science and Technology, Department of Science and Mathematics Education.
    Benckert, Sylvia
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wiberg, Marie
    Umeå University, Faculty of Social Sciences, Department of Statistics.
    Gapet har minskat: skillnader mellan hög- och lågpresterande flickors och pojkars attityder till biologi, fysik och kemi 1995 och 20072011In: NorDiNa: Nordic Studies in Science Education, ISSN 1504-4556, E-ISSN 1894-1257, Vol. 7, no 1, p. 3-16Article in journal (Refereed)
    Abstract [en]

    This article explores the change over time of boys’ and girls’ attitudes towards biology, physics and chemistry. We use data from the TIMSS studies for grade 8 in Sweden to investigate how the attitudes for high- and low performing pupils have changed between 1995 and 2007. The attitude is measured by four questions from the student questionnaire in the TIMSS study. The results indicate that there have been some changes in attitudes between 1995 and 2007. High-achieving pupils and especially boys have a more negative attitude towards all three subjects, biology, physics and chemistry, in 2007 compared to 1995. The low-achieving students think that they are performing better in all three subjects 2007 compared to 1995. The difference between the group that are most positive to physics and chemistry and the least positive group has diminished between the two years. The results are discussed in relation to the changes in Swedish schools during the period.

  • 18.
    Adranno, Brando
    et al.
    Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16C, SE-10691 Stockholm, Sweden.
    Renier, Olivier
    Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16C, SE-10691 Stockholm, Sweden.
    Bousrez, Guillaume
    Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16C, SE-10691 Stockholm, Sweden.
    Paterlini, Veronica
    Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16C, SE-10691 Stockholm, Sweden.
    Baryshnikov, Glib V.
    Department of Science and Technology, Laboratory of Organic Electronics, Linköping University, SE-60174 Norrköping, Sweden.
    Smetana, Volodymyr
    Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16C, SE-10691 Stockholm, Sweden.
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ågren, Hans
    Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden.
    Metlen, Andreas
    The QUILL Research Centre and School of Chemistry and Chemical Engineering The Queen’s University of Belfast Belfast, Northern Ireland BT9 5AG, UK.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Anja-Verena, Mudring
    Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16C, SE-10691 Stockholm, Sweden; Intelligent Advanced Materials (iAM), Department of Biological and Chemical Engineering and iNANO, Aarhus University, 8000 Aarhus C, Denmark.
    Rogers, Robin D.
    Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16C, SE-10691 Stockholm, Sweden; The QUILL Research Centre and School of Chemistry and Chemical Engineering The Queen’s University of Belfast Belfast, Northern Ireland BT9 5AG, UK.
    The 8-hydroxyquinolinium cation as a lead structure for efficient color-tunable ionic small molecule emitting materials2023In: Advanced Photonics Research, ISSN 2699-9293, Vol. 4, no 3, article id 2200279Article in journal (Refereed)
    Abstract [en]

    Albeit tris(8-hydroxyquinolinato) aluminum (Alq3) and its derivatives are prominent emitter materials for organic lighting devices, and the optical transitions occur among ligand-centered states, the use of metal-free 8-hydroxyquinoline is impractical as it suffers from strong nonradiative quenching, mainly through fast proton transfer. Herein, it is shown that the problem of rapid proton exchange and vibration quenching of light emission can be overcome not only by complexation, but also by organization of the 8-hydroxyquinolinium cations into a solid rigid network with appropriate counter-anions (here bis(trifluoromethanesulfonyl)imide). The resulting structure is stiffened by secondary bonding interactions such as pi-stacking and hydrogen bonds, which efficiently block rapid proton transfer quenching and reduce vibrational deactivation. Additionally, the optical properties are tuned through methyl substitution from deep blue (455 nm) to blue-green (488 nm). Time-dependent density functional theory (TDFT) calculations reveal the emission to occur from which an unexpectedly long-lived S-1 level, unusual for organic fluorophores. All compounds show comparable, even superior photoluminescence compared to Alq3 and related materials, both as solids and thin films with quantum yields (QYs) up to 40-50%. In addition, all compounds show appreciable thermal stability with decomposition temperatures above 310 °C.

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  • 19.
    Adranno, Brando
    et al.
    Physical Materials Chemistry, Department of Materials and Environmental Chemistry, Stockholm University, 10691 Stockholm, Sweden.
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Paterlini, Veronica
    Physical Materials Chemistry, Department of Materials and Environmental Chemistry, Stockholm University, 10691 Stockholm, Sweden.
    Smetana, Volodymyr
    Physical Materials Chemistry, Department of Materials and Environmental Chemistry, Stockholm University, 10691 Stockholm, Sweden; Intelligent Advanced Materials (iAM), Department of Biological and Chemical Engineering and iNANO, Aarhus University, 8000 Aarhus C, Denmark.
    Renier, Olivier
    Physical Materials Chemistry, Department of Materials and Environmental Chemistry, Stockholm University, 10691 Stockholm, Sweden.
    Bousrez, Guillaume
    Physical Materials Chemistry, Department of Materials and Environmental Chemistry, Stockholm University, 10691 Stockholm, Sweden; Intelligent Advanced Materials (iAM), Department of Biological and Chemical Engineering and iNANO, Aarhus University, 8000 Aarhus C, Denmark.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mudring, Anja-Verena
    Physical Materials Chemistry, Department of Materials and Environmental Chemistry, Stockholm University, 10691 Stockholm, Sweden; Intelligent Advanced Materials (iAM), Department of Biological and Chemical Engineering and iNANO, Aarhus University, 8000 Aarhus C, Denmark.
    Broadband white-light-emitting electrochemical cells2023In: Advanced Photonics Research, ISSN 2699-9293, Vol. 4, no 5, article id 2200351Article in journal (Refereed)
    Abstract [en]

    Emerging organic light-emitting devices, such as light-emitting electrochemical cells (LECs), offer a multitude of advantages but currently suffer from that most efficient phosphorescent emitters are based on expensive and rare metals. Herein, it is demonstrated that a rare metal-free salt, bis(benzyltriphenylphosphonium)tetrabromidomanganate(II) ([Ph3PBn]2[MnBr4]), can function as the phosphorescent emitter in an LEC, and that a careful device design results in the fact that such a rare metal-free phosphorescent LEC delivers broadband white emission with a high color rendering index (CRI) of 89. It is further shown that broadband emission is effectuated by an electric-field-driven structural transformation of the original green-light emitter structure into a red-emitting structure.

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  • 20.
    Agvik, Simon
    Umeå University, Faculty of Science and Technology, Department of Physics.
    A deformable terrain model in multi-domain dynamics using elastoplastic constraints: An adaptive approach2015Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Achieving realistic simulations of terrain vehicles in their work environment does not only require a careful model of the vehicle itself but the vehicle's interactions with the surroundings are equally important. For off-road ground vehicles the terrain will heavily affect the behaviour of the vehicle and thus puts great demands on the terrain model.

    The purpose of this project has been to develop and evaluate a deformable terrain model, meant to be used in real-time simulations with multi-body dynamics. The proposed approach is a modification of an existing elastoplastic model based on linear elasticity theory and a capped Drucker-Prager model, using it in an adaptive way. The original model can be seen as a system of rigid bodies connected by elastoplastic constraints, representing the terrain. This project investigates if it is possible to create dynamic bodies just when it is absolutely necessary, and store information about possible deformations in a grid.

    Two methods used for transferring information between the dynamic bodies and the grid have been evaluated; an interpolating approach and a discrete approach. The test results indicate that the interpolating approach is preferable, with better stability to an equal performance cost. However, stability problems still exist that have to be solved if the model should be useful in a commercial product.

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  • 21.
    Ahlberg, Sebastian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ambjörnsson, Tobias
    Lizana, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Many-body effects on tracer particle diffusion with applications for single-protein dynamics on DNA2015In: New Journal of Physics, E-ISSN 1367-2630, Vol. 17, article id 043036Article in journal (Refereed)
    Abstract [en]

    30% of the DNA in E. coli bacteria is covered by proteins. Such a high degree of crowding affects the dynamics of generic biological processes (e.g. gene regulation, DNA repair, protein diffusion etc) in ways that are not yet fully understood. In this paper, we theoretically address the diffusion constant of a tracer particle in a one-dimensional system surrounded by impenetrable crowder particles. While the tracer particle always stays on the lattice, crowder particles may unbind to a surrounding bulk and rebind at another, or the same, location. In this scenario we determine how the long time diffusion constant D (after many unbinding events) depends on (i) the unbinding rate of crowder particles k(off), and (ii) crowder particle line density rho, from simulations (using the Gillespie algorithm) and analytical calculations. For small k(off), we find D similar to k(off)/rho(2) when crowder particles do not diffuse on the line, and D similar to root Dk(off)/rho when they are diffusing; D is the free particle diffusion constant. For large k(off), we find agreement with mean-field results which do not depend on k(off). From literature values of k(off) and D, we show that the small k(off) -limit is relevant for in vivo protein diffusion on crowded DNA. Our results apply to single-molecule tracking experiments.

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  • 22.
    Ahlman, Björn
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Coarse-Graining Fields in Particle-Based Soil Models2020Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In soil, where trees and crops grow, heavy vehicles shear and compact the soil, leading to reduced plant growth and diminished nutrient recycling. Computer simulations offer the possibility to improve the understanding of these undesired phenomena.

    In this thesis, soils were modelled as large collections of contacting spherical particles using the Discrete Element Method (DEM) and the physics engine AGX Dynamics, and these entities were analyzed.

    In the first part of the thesis, soils, which were considered to be continua, were subjected to various controlled deformations and fields for quantities such as stress and strain were visualized using coarse graining (CG). These fields were then compared against analytical solutions. The main goal of the thesis was to evaluate the usefulness, accuracy, and precision of this plotting technique when applied to DEM-soils. The general behaviour of most fields agreed well with analytical or expected behaviour. Moreover, the fields presented valuable information about phenomena in the soils. Relative errors varied from 1.2 to 27 %. The errors were believed to arise chiefly from non-uniform displacement (due to the inherent granularity in the technique), and unintended uneven particle distribution. The most prominent drawback with the technique was found to be the unreliability of the plots near the boundaries. This is significant, since the behaviour of a soil at the surface where it is in contact with e.g. a vehicle tyre is of interest.

    In the second part of the thesis, a vehicle traversed a soil and fields were visualized using the same technique. Following a limited analysis, it was found that the stress in the soil can be crudely approximated as the stress in a linear elastic solid.

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  • 23.
    Ahlman, Björn
    Umeå University, Faculty of Science and Technology, Department of Physics.
    The Physics and Image Processing of Cryo-ElectronTomography and the Estimation of Resolution using the Programs MOTIONCOR2 1.0.2 and CTFFIND 4.1.82019Independent thesis Advanced level (degree of Master (One Year)), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    We present a brief overview of the physics of cryo-electron tomography (cryo-ET) and the Image Processing related to this field. This serves as an introduction for readers unfamiliar with cryo-ET and as a background to the second part of the report, which concerns the programs CTFFIND and MOTIONCOR2. We correct tiltseries for sample motion with two different settings with MOTIONCOR2: with (only) Global Motion Correction and with Global+Local Motion Correction (5x5 patches). Then, using CTFFIND, we estimate the resolution in the tiltseries for the data sets without Motion Correction, with (only) Global Motion Correction, and with Global+Local Motion Correction, and make an assessment of the reliability of CTFFIND as a program for estimating the resolution. A total of 410 images were analyzed (10 tiltseries of 41 images each). When considering mean resolutions, we find CTFFIND to be useful and reliable, particularly when differences in means are of interest (e.g. differences between angles or defoci). However, due to e.g. the sensitivity of CTFFIND with respect to input parameters, we conjecture that CTFFIND is less reliable when individual images are analyzed, particularly when absolute resolutions are of interest. Provided that CTFFIND is accurate enough for a statistical analysis with respect to resolution, which we believe it is, the average increase in resolution when (only) Global Motion Correction is used for tiltseries from -60° to 60° with defocus -3 μm is around 0.9 Å (246 images analyzed). For defocus -5 μm, the equivalent average increase in resolution is around 1.1 Å (164 images analyzed). Results indicate that resolution is roughly linearly proportional to the magnitude of the tiltangle. The average difference in resolution between data sets with (only) Global Motion Correction and Global+Local Motion Correction for both defocus -3 μm and -5 μm is 0.0 Å, for two significant figures. This suggests that the sample motion is predominantly Global. For defocus -3 μm and -5 μm, there is a confidence level of 95% that if only Global Motion Correction is applied to a large set of tiltseries, a majority of the images will have a resolution that is higher than the images in the same set to which Motion Correction has not been applied (when corresponding images are compared). In an equivalent statistical analysis, we can not claim that either “(only) Global Motion Correction” or “Global+Local Motion Correction” is superior.

  • 24.
    Ahmed, Shibbir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Cadmium sulphide / Zinc sulphide quantum dot decoration of three-dimensional porous graphene2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
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  • 25.
    Ahmed, Shibbir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Micro-wave synthesis of co-doped transition metal oxides anchored on reduced graphene oxide and their implementation as catalysts for water oxidation2015Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Abstract: Artificial photosynthesis is a very attractive and a desirable way to solve the rising energy demand. In order to harvest energy directly from sunlight catalyst for oxygen reduction and evolution reaction are at the core of key renewable-energy technologies including fuel cells and water splitting. Herein, tungsten oxide-reduced graphene oxide (WO3-rGO), cobalt oxide-reduced graphene oxide (Co3O4-rGO) and tungsten oxide-cobalt oxide-reduced graphene oxide (WO3-Co3O4-rGO) nano-composites were synthesized via a facile, efficient, one-pot microwave assisted method as an efficient electro-catalyst for water oxidation. The structural and chemical compositions of the produced nano-structures were investigated by Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy as well as thremogravimetric analysis. Catalytic performances of the engineered nano hybrids for the oxygen evolution reaction (OER) were evaluated using a standard three electrode system in 0.1 M KOH aqueous solution, with a coiled Pt wire as the counter electrode and an Ag/AgCl as the reference electrode. In the produced nano-composites, WO3 and Co3O4 nano-particles are uniformly decorated on the reduced graphene oxide sheets without any agglomeration. The WO3-Co3O4-rGO hybrids afforded current density 1 mA/cm2 at 1.693 V vs RHE and the Co3O4-rGO reached the current density 1 mA/cm2 at 1.769 V vs RHE regarding the catalytic performance for OER in alkaline solution. The over potential was significantly reduced by 76 mV after co-doping of WO3 with Co3O4-rGO nano hybrids. The WO3-Co3O4-rGO gives current density 1 mA/cm2 at 1.658 V vs RHE. We also show that the over potential for oxygen evolution of the WO3-Co3O4-rGO was further reduced by illuminating the catalyst to visible light thus manifesting the photo-catalytic properties of the hybrid catalyst.

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  • 26.
    Aizawa, S.
    et al.
    IRAP, CNRS-CNES-UPS, Toulouse, France; Graduate School of Science, Tohoku University, Sendai, Japan.
    Griton, L.S.
    IRAP, CNRS-CNES-UPS, Toulouse, France; LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, Meudon, France.
    Fatemi, Shahab
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Exner, W.
    Institute for Geophysics and Extraterrestrial Physics, Technische Universität Braunschweig, Braunschweig, Germany; Institute for Theoretical Physics, Technische Universität Braunschweig, Braunschweig, Germany; School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, United States.
    Deca, J.
    Laboratory for Atmospheric and Space Physics (LASP), University of Colorado Boulder, CO, Boulder, United States; Institute for Modeling Plasma, Atmospheres and Cosmic Dust, NASA/SSERVI, CA, United States; Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Université de Versailles à Saint Quentin, Guyancourt, France.
    Pantellini, F.
    LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, Meudon, France.
    Yagi, M.
    RIKEN, Kobe, Japan.
    Heyner, D.
    Institute for Geophysics and Extraterrestrial Physics, Technische Universität Braunschweig, Braunschweig, Germany.
    Génot, V.
    IRAP, CNRS-CNES-UPS, Toulouse, France.
    André, N.
    IRAP, CNRS-CNES-UPS, Toulouse, France.
    Amaya, J.
    CmPA, Mathematics Department, KU Leuven, Belgium.
    Murakami, G.
    ISAS/JAXA, Sagamihara, Japan.
    Beigbeder, L.
    GFI, Toulouse, France.
    Gangloff, M.
    IRAP, CNRS-CNES-UPS, Toulouse, France.
    Bouchemit, M.
    IRAP, CNRS-CNES-UPS, Toulouse, France.
    Budnik, E.
    Noveltis, Toulouse, France.
    Usui, H.
    Kobe University, Kobe, Japan.
    Cross-comparison of global simulation models applied to Mercury's dayside magnetosphere2021In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 198, article id 105176Article in journal (Refereed)
    Abstract [en]

    We present the first comparison of multiple global simulations of the solar wind interaction with Mercury's dayside magnetosphere, conducted in the framework of the international collaborative project SHOTS - Studies on Hermean magnetosphere Oriented Theories and Simulations. Two 3D magnetohydrodynamic and two 3D hybrid simulation codes are used to investigate the global response of the Hermean magnetosphere without its exosphere to a northward-oriented interplanetary magnetic field. We cross-compare the results of the four codes for a theoretical case and a MESSENGER orbit with similar upstream plasma conditions. The models agree on bowshock and magnetopause locations at 2.1 ​± ​0.11 and 1.4 ​± ​0.1 Mercury planetary radii, respectively. The latter locations may be influenced by subtle differences in the treatment of the plasma boundary at the planetary surface. The predicted magnetosheath thickness varies less between the codes. Finally, we also sample the plasma data along virtual trajectories of BepiColombo's Magnetospheric and Planetary Orbiter. Our ability to accurately predict the structure of the Hermean magnetosphere aids the analysis of the onboard plasma measurements of past and future magnetospheric missions.

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  • 27. Ajuria, Jon
    et al.
    Arnaiz, Maria
    Botas, Cristina
    Carriazo, Daniel
    Mysyk, Roman
    Rojo, Teofilo
    Talyzin, Alexandr V.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Goikolea, Eider
    Graphene-based lithium ion capacitor with high gravimetric energy and power densities2017In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 363, p. 422-427Article in journal (Refereed)
    Abstract [en]

    Hybrid capacitor configurations are now of increasing interest to overcome the current energy limitations of supercapacitors. In this work, we report a lithium ion capacitor (LIC) entirely based on graphene. On the one hand, the negative-battery-type- electrode consists of a self-standing, binder-free 3D macroporous foam formed by reduced graphene oxide and decorated with tin oxide nanoparticles (SnO2-rGO). On the other hand, the positive-capacitor-type- electrode is based on a thermally expanded and physically activated reduced graphene oxide (a-TEGO). For comparison purposes, a symmetric electrical double layer capacitor (EDLC) using the same activated graphene in 1.5 M Et4NBE4/ACN electrolyte is also assembled. Built in 1 M LiPF6 EC:DMC, the graphene-based LIC shows an outstanding, 10-fold increase in energy density with respect to its EDLC counterpart at low discharge rates (up to 200 Wh kg(-1)). Furthermore, it is still capable to deliver double the energy in the high power region, within a discharge time of few seconds.

  • 28.
    Akhtari, Mohammad Mehdi
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Medicine, Department of Radiation Sciences.
    3D Structural similarity check between CT and SCT2014Independent thesis Advanced level (degree of Master (Two Years)), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The aim of the current work was to develop and test a method for comparing the geometrical representation of a patient using computed tomography (CT), and CT like images derived from magnetic resonance images (MRI). It would be beneficial to use MRI alone for both target delineation and treatment planning to save time and costs. This was first investigated in Umea university Hospital by introducing substitute computed tomography (SCT) obtained from MRI images and which can be used as CT equivalent information. The data that is used in this report are from five patients with intracranial tumors; A MATLAB code has been developed to compare DDRs based on s-CT data with CT based DRR’s for these five patients.

  • 29.
    Akhtari, Mohammad Mehdi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Accuracy of inverse treatment planning on computed tomography like images derived from magnetic resonance data2013Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Treatment planning for radiotherapy involves different types of imaging to delineate target volume precisely. The most suitable sources to get 3D information of the patient are the computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET)/CT modalities. CT is a modern medical imaging technique that allows three-dimensional treatment planning and conformal treatment techniques. By combining CT images with efficient dosimetry software, accurate patient positioning methods and verification and quality assurance good results can be achieved. The CT images show how the radiation interacts with the material based on each tissue has a different attenuation coefficient, so the data can be used for dose calculations in treatment planning.

    Radiation oncology is therapeutic modality, in which irradiating cancer cells as target is the main goal while always try to limit the dose to healthy tissues and organs. CT images have good potentials because they can provide high geometrical accuracy and electron density information. Having said that, however, using CT images alone for planning does not provide enough information in order to delineate the target volume accurately because the attenuation in soft tissue is fairly constant therefore the soft tissue contrast is poor. Here, (MR) imaging can be very useful since it has superior soft tissue contrast especially in conditions such as prostate cancer, brain lesions, and head and neck tumors. It should be noted that MR images cannot provide electron density information that is required for dose calculations.

    It has been hypothesized that since MRI images have certain benefits in comparison with CT images such as its superior soft tissue contrast which improves contrast resolution between different types of tissues, it would be beneficial to use MRI alone for both target delineation and treatment planning to save time and costs. This was investigated by introducing substitute computed tomography (SCT) which can be interpreted as CT equivalent information obtained by MRI images.

    We used data from five patients with intracranial tumors, and reviewed their initial dosimetric treatment plans that were based solely on CT images, that data was also used to evaluate the dosimetric accuracy of our research treatment plans. Optimization plans that are based on CT images and substitute CT (SCT) was compared with each other in the first step. On the second step the treatment plan that was based on SCT images was transferred to the CT images without any changes and comparisons between the dose calculations on both data sets were made. The delivered dose to planning target volume (PTV) and risk organs was compared.

    Gamma index results between SCT and transferred plan showed no difference in the dose distribution map in PTV. The maximum difference was in the outer contour to the skull. The average and median dose delivered to PTV was within 0.35% difference studying in all patients.

    In conclusion for patients with intracranial tumors the dosimetric accuracy of treatment plans based on SCT and MR images were very accurate, and we demonstrated that it was possible to reach the same dose volume histograms by SCT compared to CT with minimal differences, which were not significant. 

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  • 30.
    Akkerman, V'yacheslav
    Umeå University, Faculty of Science and Technology, Physics.
    Turbulent burning, flame acceleration, explosion triggering2007Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The present thesis considers several important problems of combustion theory, which are closely related to each other: turbulent burning, flame interaction with walls in different geometries, flame acceleration and detonation triggering.

    The theory of turbulent burning is developed within the renormalization approach. The theory takes into account realistic thermal expansion of burning matter. Unlike previous renormalization models of turbulent burning, the theory includes flame interaction with vortices aligned both perpendicular and parallel to average direction of flame propagation. The perpendicular vortices distort a flame front due to kinematical drift; the parallel vortices modify the flame shape because of the centrifugal force. A corrugated flame front consumes more fuel mixture per unit of time and propagates much faster. The Darrieus-Landau instability is also included in the theory. The instability becomes especially important when the characteristic length scale of the flow is large.

    Flame interaction with non-slip walls is another large-scale effect, which influences the flame shape and the turbulent burning rate. This interaction is investigated in the thesis in different geometries of tubes with open / closed ends. When the tube ends are open, then flame interaction with non-slip walls leads to an oscillating regime of burning. Flame oscillations are investigated for different flame parameters and tube widths. The average increase in the burning rate in the oscillations is found.

    Then, propagating from a closed tube end, a flame accelerates according to the Shelkin mechanism. In the theses, an analytical theory of laminar flame acceleration is developed. The theory predicts the acceleration rate, the flame shape and the velocity profile in the flow pushed by the flame. The theory is validated by extensive numerical simulations. An alternative mechanism of flame acceleration is also considered, which is possible at the initial stages of burning in tubes. The mechanism is investigated using the analytical theory and direct numerical simulations. The analytical and numerical results are in very good agreement with previous experiments on “tulip” flames.

    The analytical theory of explosion triggering by an accelerating flame is developed. The theory describes heating of the fuel mixture by a compression wave pushed by an accelerating flame. As a result, the fuel mixture may explode ahead of the flame front. The explosion time is calculated. The theory shows good agreement with previous numerical simulations on deflagration-to-detonation transition in laminar flows.

    Flame interaction with sound waves is studied in the geometry of a flame propagating to a closed tube end. It is demonstrated numerically that intrinsic flame oscillations coming into resonance with acoustic waves may lead to violent folding of the flame front with a drastic increase in the burning rate. The flame folding is related to the Rayleigh-Taylor instability developing at the flame front in the oscillating acceleration field of the acoustic wave.

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  • 31.
    Akkerman, Vyacheslav
    et al.
    West Virginia University, Morgantown, WV, United States.
    Bilgili, Serdar
    West Virginia University, Morgantown, WV, United States.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Demir, Sinan
    West Virginia University, Morgantown, WV, United States.
    Morella, Haley
    West Virginia University, Morgantown, WV, United States.
    Impacts of the Lewis and Markstein numbers effects on the flame acceleration in channels2016Conference paper (Other academic)
    Abstract [en]

    The effects of flame stretch and thermal/molecular diffusion on the flame acceleration in channels are quantified by means of the analytical and computational endeavours. The internal transport flame properties are accounted in the theory by means of the Markstein number, Mk. Being a positive or negative function of the thermal-chemical combustion parameters, such as the thermal expansion ratio and the Lewis and Zeldovich numbers, the Markstein number either moderates or promotes the flame acceleration. While Mk may provide a substantial impact on the flame acceleration rate in narrow channels, this effects diminishes with the increase of the channel width. The analysis is accompanied by extensive computational simulations of the Navier-Stokes combustion equations, which clarify the impact of the Lewis number on the flame acceleration. It is obtained that, for Le below a certain critical value, at the initial stage of flame acceleration, a globally-convex flame front is splits into two or more "fingers", accompanied by a drastic increase in the flame surface area and associated enhancement of the flame acceleration. Overall, the thermal-diffusive effects substantially facilitate the flame acceleration scenario, thereby advancing a potential deflagration-to-detonation transition. 

  • 32.
    Akkerman, Vyacheslav
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Physics.
    Flames with Realistic Thermal Expansion in a Time-Dependent Turbulent Flow2005In: Combustion, Explosion and Shock Waves, Vol. 41, p. 363-Article in journal (Refereed)
  • 33.
    Akkerman, V'yacheslav
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Velocity of weakly turbulent flames of finite thickness2005In: Combustion theory and modelling, ISSN 1364-7830, E-ISSN 1741-3559, Vol. 9, no 2, p. 323-351Article in journal (Refereed)
    Abstract [en]

    The velocity increase of a weakly turbulent flame of finite thickness is investigated using analytical theory developed in previous papers. The obtained velocity increase depends on the flow parameters: on the turbulent intensity, on the turbulent spectrum and on the characteristic length scale. It also depends on the thermal and chemical properties of the burning matter: thermal expansion, the Markstein number and the temperature dependence of transport coefficients. It is shown that the influence of the finite flame thickness is especially strong close to the resonance point, when the wavelength of the turbulent harmonic is equal to the cut off wavelength of the Darrieus-Landau instability. The velocity increase is almost independent of the Prandtl number. On the contrary, the Markstein number is one of the most important parameters controlling the velocity increase. The relative role of the external turbulence and the Darrieus-Landau instability for the velocity increase is studied for different parameters of the flow and the burning matter. The velocity increase for turbulent flames in methane and propane fuel mixtures is calculated for different values of the equivalence ratio. The present theoretical results are compared with previous experiments on turbulent flames. In order to perform the comparison, the theoretical results of the present paper are extrapolated to the case of a strongly corrugated flame front using the ideas of self-similar flame dynamics. The obtained theoretical results are in a reasonable agreement with the experimental data, taking into account the uncertainties of both the theory and the experiments. It is shown that in many experiments on turbulent flames the Darrieus-Landau instability is more important for the flame velocity than the external turbulence.

  • 34.
    Akkerman, V'yacheslav
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bastiaans, R. J. M.
    de Goey, L. P. H.
    van Oijen, J. A.
    Eriksson, L. E.
    Flow-flame interaction in a closed chamber2008In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 20, no 5, p. 055107-055121Article in journal (Refereed)
    Abstract [en]

    Numerous studies of flame interaction with a single vortex andrecent simulations of burning in vortex arrays in open tubesdemonstrated the same tendency for the turbulent burning rate$\propto U_{rms}\lambda^{2/3}$, where  $U_{rms}$ is theroot-mean-square velocity and $\lambda$ is the vortex size. Here itis demonstrated that this tendency is not universal for turbulentburning. Flame interaction with vortex arrays is investigated forthe geometry of a closed burning chamber using direct numericalsimulations of the complete set of gas-dynamic combustion equations.Various initial conditions in the chamber are considered, includinggas at rest and several systems of vortices of different intensitiesand sizes. It is found that the burning rate in a closed chamber(inverse burning time) depends strongly on the vortex intensity; atsufficiently high intensities it increases with $U_{rms}$approximately linearly in agreement with the above tendency. On thecontrary, dependence of the burning rate on the vortex size isnon-monotonic and qualitatively different from the law$\lambda^{2/3}$. It is shown that there is an optimal vortex size ina closed chamber, which provides the fastest total burning rate. Inthe present work the optimal size is 6 times smaller than thechamber height.

  • 35.
    Akkerman, Vyacheslav
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Physics.
    Eriksson, Lars-Erik
    Numerical Study of Turbulent Flame Velocity2007In: Combustion and Flame, ISSN 0010-2180, Vol. 151, no 3, p. 452-471Article in journal (Other (popular science, discussion, etc.))
  • 36.
    Akkerman, V'yacheslav
    et al.
    Department of Mechanical and Aerospace Engineering, West Virginia University, WV, Morgantown, United States.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kuznetsov, Mikhail
    Institute for Nuclear and Energy Technologies, Karlsruhe Institute of Technology, Karlsruhe, Germany.
    Law, Chung K.
    Department of Mechanical and Aerospace Engineering, Princeton University, NJ, Princeton, United States.
    Valiev, Damir
    Department of Mechanical and Aerospace Engineering, Princeton University, NJ, Princeton, United States.
    Wu, Ming-Hsun
    Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan.
    Fast flame acceleration and deflagration-to-detonation transition in smooth and obstructed tubes, channels and slits2013In: 8th US National Combustion Meeting 2013, Western States Section/Combustion Institute , 2013, p. 970-978Conference paper (Refereed)
    Abstract [en]

    This work is devoted to the comprehensive analytical, computational and experimental investigation of various stages of flame acceleration in narrow chambers. We consider mesoscale two-dimensional channels and cylindrical tubes, smooth and obstructed, and sub-millimeter gaps between two parallel plates. The evolution of the flame shape, propagation speed, acceleration rate, and velocity profiles nearby the flamefront are determined for each configuration, with the theories substantiated by the numerical simulations of the hydrodynamics and combustion equations with an Arrhenius reaction, and by the experiments on premixed hydrogen-oxygen and ethylene-oxygen flames. The detailed analyses demonstrate three different mechanisms of flame acceleration: 1) At the early stages of burning at the closed tube end, the flamefront acquires a finger-shape and demonstrates strong acceleration during a short time interval. While this precursor acceleration mechanism is terminated as soon as the flamefornt touches the side wall of the tube, having a little relation to the deflagration-to-detonation transition (DDT) for relatively slow, hydrocarbon flames; for fast (e.g. hydrogen-oxygen) flames, even a short finger-flame acceleration may amplify the flame propagation speed up to sonic values, with an important effect on the subsequent DDT process. 2) On the other hand, the classical mechanism of flame acceleration due to wall friction in smooth tubes is basically unlimited in time, but it depends noticeably on the tube width such that the acceleration rate decreases strongly with the Reynolds number. The entire DDT scenario includes four distinctive stages: (i) initial exponential acceleration at the quasi-incompressible state; (ii) moderation of the process because of gas compression; (iii) eventual saturation to a quasisteady, high-speed flames correlated with the Chapman-Jouguet deflagration; (iv) finally, the heating of the fuel mixture leads to the explosion ahead of the flame front, which develops into a self-supporting detonation. 3) In addition, we have revealed a physical mechanism of extremely fast flame acceleration in channels/tubes with obstacles. Combining the "benefits" of 1) and 2), this new mechanism is based on delayed burning between the obstacles, creating a powerful jet-flow and thereby driving the acceleration, which is extremely strong and independent of the Reynolds number, so the effect can be fruitfully utilized at industrial scales. Understanding of this mechanism provides the guide for optimization of the obstacle shape, while this task required tantalizing cut-and-try methods previously. On the other hand, our formulation opens new technological possibilities of DDT in micro-combustion.

  • 37.
    Akkerman, V'yacheslav
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Physics.
    Petchenko, Arkady
    Umeå University, Faculty of Science and Technology, Physics.
    Eriksson, Lars-Erik
    Accelerating flames in cylindrical tubes with nonslip at the walls2006In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 145, no 1-2, p. 206-219Article in journal (Refereed)
    Abstract [en]

    An analytical theory of flame acceleration in cylindrical tubes with one end closed is developed. It is shown that all realistic flames with a large density drop at the front accelerate exponentially because of the nonslip at the tube walls. Such acceleration mechanism is not limited in time and, eventually, it may lead to detonation triggering. It is found that the acceleration rate decreases with the Reynolds number of the flow. On the contrary, the acceleration rate grows with the thermal expansion of the burning matter. It is shown that the flame shape and the velocity profile remain self-similar during the flame acceleration. The theory is validated by extensive direct numerical simulations. The simulations are performed for the complete set of combustion and hydrodynamic equations including thermal conduction, diffusion, viscosity, and chemical kinetics. The simulation results are in very good agreement with the analytical theory.

  • 38.
    Akkerman, V'yacheslav
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Physics.
    Petchenko, Arkady
    Umeå University, Faculty of Science and Technology, Physics.
    Eriksson, Lars-Erik
    Flame oscillations in tubes with nonslip at the walls2006In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 145, no 4, p. 675-687Article in journal (Refereed)
    Abstract [en]

    A laminar premixed flame front propagating in a two-dimensional tube is considered with nonslip at the walls and with both ends open. The problem of flame propagation is solved using direct numerical simulations of the complete set of hydrodynamic equations including thermal conduction, diffusion, viscosity, and chemical kinetics. As a result, it is shown that flame interaction with the walls leads to the oscillating regime of burning. The oscillations involve variations of the curved flame shape and the velocity of flame propagation. The oscillation parameters depend on the characteristic tube width, which controls the Reynolds number of the flow. In narrow tubes the oscillations are rather weak, while in wider tubes they become stronger with well-pronounced nonlinear effects. The period of oscillations increases for wider tubes, while the average flame length scaled by the tube diameter decreases only slightly with increasing tube width. The average flame length calculated in the present work is in agreement with that obtained in the experiments. Numerical results reduce the gap between the theory of turbulent flames and the experiments on turbulent combustion in tubes.

  • 39.
    Akkerman, V’yacheslav
    et al.
    Nuclear Safety Institute of Russian Academy of Sciences B. Tulskaya 52, 115191 Moscow, Russia.
    Ivanov, Mikhail
    Department of Physics and Power Engineering, Moscow Institute of Physics and Technology, 141700 Dolgoprudny, Moscow Region, Russia.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Turbulent flow produced by Piston Motion in a Spark-ignition engine2009In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 82, no 3, p. 317-337Article in journal (Refereed)
    Abstract [en]

    Turbulence produced by the piston motion in spark-ignition engines is studied by 2D axisymmetric numerical simulations in the cylindrical geometry as in the theoretical and experimental work by Breuer et al (Flow Turb. Combust. 74 (2005) 145, Ref. [1]). The simulations are based on the Navier-Stokes gas-dynamic equations including viscosity, thermal conduction and non-slip at the walls. Piston motion is taken into account as a boundary condition. The turbulent flow is investigated for a wide range of the engine speed, 1000-4000 rpm, assuming both zero and non-zero initial turbulence. The turbulent rms-velocity and the integral length scale are investigated in axial and radial directions. The rms-turbulent velocity is typically an order-of-magnitude smaller than the piston speed. In the case of zero initial turbulence, the flow at the top-dead-center may be described as a combination of two large-scale vortex rings of a size determined by the engine geometry. When initial turbulence is strong, then the integral turbulent length demonstrates self-similar properties in a large range of crank angles. The results obtained agree with the experimental observations of [1].

  • 40.
    Akkerman, Vyacheslav
    et al.
    Princeton University.
    Law, Chung
    Princeton University.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Self-similar accelerative propagation of expanding wrinkled flames and explosion triggering2011In: Physical Review E, Statistical, nonlinear and soft matter physics, ISSN 1539-3755 (print), 1550-2376 (online), Vol. 83, p. 026305-Article in journal (Refereed)
    Abstract [en]

    The formulation of Taylor on the self-similar propagation of an expanding spherical piston with constant velocity was extended to an instability-wrinkled deflagration front undergoing acceleration with RF∝tα, where RF is the instantaneous flame radius, t the time, and α a constant exponent. The formulation describes radial compression waves pushed by the front, trajectories of gas particles, and the explosion condition in the gas upstream of the front. The instant and position of explosion are determined for a given reaction mechanism. For a step-function induction time, analytic formulas for the explosion time and position are derived, showing their dependence on the reaction and flow parameters including thermal expansion, specific heat ratio, and acceleration of the front.

  • 41.
    Akkerman, Vyacheslav
    et al.
    Princeton University.
    Law, Chung
    Princeton University.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Eriksson, Lars-Erik
    CTH.
    Analysis of flame acceleration induced by wall friction in open tubes2010In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, p. 053606-Article in journal (Refereed)
    Abstract [en]

    Spontaneous flame acceleration leading to explosion triggering in open tubes/channels due to wall friction was analytically and computationally studied. It was first demonstrated that the acceleration is affected when the thermal expansion across the flame exceeds a critical value depending on the combustion configuration. For the axisymmetric flame propagation in cylindrical tubes with both ends open, a theory of the initial (exponential) stage of flame acceleration in the quasi-isobaric limit was developed and substantiated by extensive numerical simulation of the hydrodynamics and combustion with an Arrhenius reaction. The dynamics of the flame shape, velocity, and acceleration rate, as well as the velocity profile ahead and behind the flame, have been determined.

  • 42.
    Akkerman, V'yacheslav
    et al.
    Center for Alternative Fuels, Engines and Emission (CAFEE), Department of Mechanical and Aerospace Engineering, West Virginia University, WV, Morgantown, United States.
    Sadek, Jad
    Center for Alternative Fuels, Engines and Emission (CAFEE), Department of Mechanical and Aerospace Engineering, West Virginia University, WV, Morgantown, United States.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Theory of flame acceleration in open/vented obstructed pipes2016In: 2016 Spring Technical Meeting of the Eastern States Section of the Combustion Institute, ESSCI 2016, Eastern States Section of the Combustion Institute , 2016Conference paper (Refereed)
    Abstract [en]

    A shockless, conceptually-laminar formulation on extremely fast flame acceleration in semi-open obstructed pipes [Physical Review Letters 101 (2008) 164501; Combust. Flame 157 (2010) 1012], Refs. [8-9] is extended to pipes with both ends open/vented. The acceleration is devoted to a powerful jet-flow produced by delayed combustion in the pockets between the obstacles, and it leads to a prompt deflagration-to-detonation transition event. Starting with inviscid approximation, the analysis subsequently incorporates the viscous forces (hydraulic resistance). The theory is validated by the recent experiments [http://arxiv.org/abs/1208.6453], Ref. [11]. It is shown that hydraulic resistance is not required to drive the flame acceleration. In contrast, this is a supplementary effect, which actually moderates the acceleration rate. On the other hand, hydraulic resistance plays an important role: it is responsible for the initial delay, before the flame acceleration onset, observed in the experiments. It is demonstrated that flames accelerate strongly in open/vented obstructed pipes, and the acceleration mechanism is qualitatively the same as that in the semi-open ones. However, because of the flame-generated flow distributed upward and downward of the flame front, the acceleration rate in open pipes is noticeably less than that in the semi-open ones.

  • 43. Aksenova, N.A.
    et al.
    Isakina, A.P.
    Prokhvatilov, A.I.
    Strzhemechny, M.A.
    Soldatov, Alexander V.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Structure studies of C60 polymerized at low pressures1997In: Fullerenes: Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, vol. 5, Pennington, NJ: The Electrochemical Society , 1997, p. 687-694Conference paper (Refereed)
    Abstract [en]

    The structure and lattice properties of a polymerized C60 sample were investigated by X-ray powder diffraction over the range from liquid-nitrogen to room temperature. The aim of these studies was to establish the effect of low-pressure compression (up to 1.1 GPa) on the phase composition of the sample, to determine the structure of the phases involved, to obtain the variation of the lattice parameters with temperature, to observe eventual phase transformation in polymerized C60, and to elucidate the evolution of the orientational order with varying temperature. Analysis of the room-temperature diffraction patterns indicates that the sample contained at least two major phases, namely rhombohedral and and tetragonal with the polymerization array close to two-dimensional. Our data obtained on samples annealed at 300 C in the air allow us to conclude that polymerized C60 converts to a mixture of the fcc structure plus a new phase, presumably C60O.

  • 44.
    Aksnes, Dag W.
    et al.
    Nordic Institute for Studies in Innovation, Research & Education (NIFU), Norway.
    Blöcker, Christopher
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Colliander, Cristian
    Umeå University, Faculty of Social Sciences, Department of Sociology. Umeå University, Umeå University Library.
    Nilsson, Lena Maria
    Umeå University, Arctic Research Centre at Umeå University. Umeå University, Faculty of Medicine, Department of Epidemiology and Global Health.
    Kullerud, Lars (Contributor)
    UArctic.
    Larson, Keith (Contributor)
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    SCITE team, (Contributor)
    Arctic Research Trends: Bibliometrics 2016-20222023Report (Other academic)
    Abstract [en]

    This work was conducted by the UArctic Thematic Network on Research Analytics and Bibliometrics. It was supported by Global Affairs Canada through the Global Arctic Leadership Initiative.

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  • 45. Aladi, M.
    et al.
    Bolla, R.
    Cardenas, D. E.
    Veisz, László
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Foldes, I. B.
    Cluster size distributions in gas jets for different nozzle geometries2017In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 12, article id C06020Article in journal (Refereed)
    Abstract [en]

    Cluster size distributions were investigated in case of different nozzle geometries in argon and xenon using Rayleigh scattering diagnostics. Different nozzle geometries result in different behaviour, therefore both spatial- and temporal cluster size distributions were studied to obtain a well-characterized cluster target. It is shown that the generally used Hagena scaling can result in a significant deviation from the observed data and the behaviour cannot be described by a single material condensation parameter. The results along with the nanoplasma model applied to the data of previous high harmonic generation experiments allow the independent measurement of cluster size and cluster density.

  • 46.
    Alakpa, Enateri V.
    et al.
    Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
    Bahrd, Anton
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wiklund, Krister
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andersson, Magnus
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Novikov, Lev N.
    Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
    Ljungberg, Christina
    Umeå University, Faculty of Medicine, Department of Surgical and Perioperative Sciences, Hand Surgery.
    Kelk, Peyman
    Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
    Bioprinted schwann and mesenchymal stem cell co-cultures for enhanced spatial control of neurite outgrowth2023In: Gels, E-ISSN 2310-2861, Vol. 9, no 3, article id 172Article in journal (Refereed)
    Abstract [en]

    Bioprinting nerve conduits supplemented with glial or stem cells is a promising approach to promote axonal regeneration in the injured nervous system. In this study, we examined the effects of different compositions of bioprinted fibrin hydrogels supplemented with Schwann cells and mesenchymal stem cells (MSCs) on cell viability, production of neurotrophic factors, and neurite outgrowth from adult sensory neurons. To reduce cell damage during bioprinting, we analyzed and optimized the shear stress magnitude and exposure time. The results demonstrated that fibrin hydrogel made from 9 mg/mL of fibrinogen and 50IE/mL of thrombin maintained the gel&rsquo;s highest stability and cell viability. Gene transcription levels for neurotrophic factors were significantly higher in cultures containing Schwann cells. However, the amount of the secreted neurotrophic factors was similar in all co-cultures with the different ratios of Schwann cells and MSCs. By testing various co-culture combinations, we found that the number of Schwann cells can feasibly be reduced by half and still stimulate guided neurite outgrowth in a 3D-printed fibrin matrix. This study demonstrates that bioprinting can be used to develop nerve conduits with optimized cell compositions to guide axonal regeneration.

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  • 47.
    Alam, Md Khorshed
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fabrication of surface enhanced Raman spectroscopy (SERS) active substrates based on vertically aligned nitrogen doped carbon nanotube forest2015Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This thesis work describes the fabrication and surface enhanced Raman spectroscopy (SERS) characterization of vertically aligned nitrogen (N) doped multi walled carbon nanotube (MWCNT) forests coated by silver (Ag) and gold (Au) nanoparticles. In the present work, the CNT forests were grown from a catalyst metal layer by the chemical vapor deposition (CVD) process at temperature of 800 oC and a physical vapor deposition (PVD) and annealing processes were applied subsequently for the evaporation and diffusion of noble metal nanoparticles on the forest.

    Transistor patterning of 20, 50 and 100 μm were made onto the silicon-oxide (SiO2) wafers through the photolithography process with and without depositing a thickness of 10 nm titanium (Ti) buffer layer on the Si-surfaces. Iron (Fe) and cobalt (Co) were used together to deposite a thickness of 5 nm catalyst layer onto the Single Side Polished (SSP) wafers. As carbon and nitrogen precursor for the CNT growth was used pyridine. Two different treatment times (20 and 60 minutes) in the CVD process determined the CNT forest height. Scanning Electron Microscopy (SEM) imaging was employed to characterize the CNT forest properties and Ag and Au nanoparticle distribution along the CNT walls.

    The existence of “hot spots” created by the Ag and Au nanoparticles through the surface roughness and plasmonic properties was demonstrated by the SERS measurements. Accordingly, the peak intensity at wave number of 1076 cm-1 was picked up from each SERS spectra to establish the Ag- and Au-trend curves with different concentrations of 4-ATP solution. The SERS mapping was also carried out to study the Ag- and Au-coated CNT surface homogeneity and “hot spots” distribution on the CNT surface. The SERS enhancement factors (EF) were calculated by applying an analyte solution of ethanolic 4-ATP on the CNT surface. The calculated values of EF from Ag- and Au-coated CNT forests were 9×106 and 2.7×105 respectively. 

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  • 48.
    Alammar, Tarek
    et al.
    Department of Chemistry, College of Science, King Faisal University, P.O Box 400, Al-Ahsa, Saudi Arabia.
    Mudring, Anja-Verena
    Umeå University, Faculty of Science and Technology, Department of Physics. Department of Biological & Chemical Engineering, Intelligent Advanced Materials, Aarhus University, Aarhus, Denmark.
    Synthesis and exploration of Barium Stannate-Zirconate BaSn1-xZrxO3 (0 ≤ X ≤ 1) solid solutions as photocatalysts2024In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 63, no 14, p. 6132-6140Article in journal (Refereed)
    Abstract [en]

    Employing ionic liquid-assisted microwave synthesis and moderate heat treatment allows for the preparation of otherwise difficult-to-obtain perovskite-type BaSn1-xZrxO3 solid solutions (0 ≤ x ≤ 1). The impact of substituting Sn for the crystal structure, crystallinity, morphology, and photocatalytic performance was investigated. The obtained materials are characterized by X-ray diffraction, scanning electron microscopy, Brunauer-Emmett-Teller (BET) surface area analysis, X-ray photoelectron spectroscopy, UV-Vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, and Raman and IR spectroscopy. SEM images show that the morphology of the samples varies from rods for x = 0, 0.2 to spherical for x = 0.5, 0.8, 1. Upon Zr for Sn substitution, the band gap changes from 3.1 to 5.0 eV as the valence and conduction bands move to lower and higher energies. The photocatalytic activities of the BaSn1-xZrxO3 samples in the hydroxylation of terephthalic acid (TA) follow the order BaSn0.5Zr0.5O3> BaSn0.8Zr0.2O3> BaSnO3> BaSn0.2Zr0.8O3> BaZrO3. The superior photocatalytic activity of BaSn0.5Zr0.5O3 can be attributed to the synergistically favorable combination of a suitable band structure, band gap size, and increased surface area-to-volume ratio, resulting in a diminished crystalline particle size unattainable from samples prepared via traditional synthetic routes or without ionic liquid.

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  • 49.
    Albano, Amanda
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Exploratory statistical study of long-term variability in echocardiographic indices (echocardiovariability) in healthy and diseased1987Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Heart rate variability, HRV, has been well researched for some decades. The oscillations of the heart rate is studied over a time period of some minutes up to 24 hours, it is measured with electrocardiography, ECG. From this one has concluded that the heart rate signal oscillates in accordance with the respiration, the resistance in the vessels etc.

    The most frequently used examination method of the heart is done with ultrasound, called echocardiography. One interesting variable at a time is measured and it is measured for a single heartbeat. With inspiration ofthe HRV studies this project focuses on some of the variables measured with ultrasound but over time andsimultaneously. The variables of interest are the myocardial motion and the blood flow in the left part ofthe heart, they are measured over two minutes. To complement these variables the well known variables HRand Resp are measured with ECG and added to the analysis.

    The methods used for analysing the variables are first of all descriptive statistics like mean and standard deviation. Secondly spectral analysis is performed to investigate in which frequencies the variables oscillates. Through coherence this is compared with the spectrum for HR where the three peaks have known origin. Finally principal component analysis, PCA, is performed as a method to compare all variables at the same time.

    The analyses are performed on seven measurements from five (5) healthy persons and five measurementsfrom four (4) patients with the disease FAP (“Skelleftesjukan”). The variables are investigated and described for the healthy persons first, then the healthy persons and patients are compared.

    The result from the study shows that most of the echo-variables oscillate in accordance with the respirationand the heart rate. For a healthy person the oscillations are within normal values and the relative deviation isaround 10%. The patients with FAP are most affected in the variables connected to the myocardium apartfrom HR, which is known since before.

    The coherence between the echo-variables and HR is low in one of VLF, very low frequency, or LF, lowfrequency, region and high in the other. In HF, high frequency, region the coherence is high for all variables.

    Finally the PCA was conducted on measurements from all healthy persons as one data set, from one ofthe healthy persons and from one of the patients with FAP. The analysis showed that for healthy personsrespiration is the process causing most variation and all of the echo-variables have a correlation to therespiration. For a patient with FAP the respiration is not as salient. A PCA over blocks of data at different time points however show that the signals are not oscillating in the same way multivariately over the wholetime series.

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    AmandaAlbano_MScthesis
  • 50. Aleksandrovskii, A. N.
    et al.
    Dolbin, A.V.
    Esel'son, V.B.
    Gavrilko, V.G.
    Manzhelii, V.G.
    Udovidchenko, B.G.
    Bakai, A.S.
    Gadd, G.E.
    Moricca, S.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Low-temperature thermal expansion of pure and inert gas-doped fullerite C602003In: Fizika Nizkikh Temperatur (Kharkov), ISSN 0132-6414 (Print) 1816-0328 (On-line), Vol. 29, no 4, p. 432-442Article in journal (Refereed)
    Abstract [en]

    The low temperature (2–24 K) thermal expansion of pure (single-crystal and polycrystalline) C60 and polycrystalline C60 intercalated with He, Ne, Ar, and Kr has been investigated using the high-resolution capacitance dilatometer. The investigation of the time dependence of the sample length variations deltaL(t) on heating by deltaT shows that the thermal expansion is determined by the sum of positive and negative contributions, which have different relaxation times. The negative thermal expansion usually prevails at helium temperatures. The positive expansion is connected with the phonon thermalization of the system. The negative expansion is caused by reorientation of the C60 molecules. It is assumed that the reorientation is of a quantum character. The inert gas impurities affect the reorientation of the C60 molecules very strongly, especially at liquid helium temperatures. A temperature hysteresis of the thermal expansion coefficient of Kr– and He–C60 solutions has been revealed. The hysteresis is attributed to orientational polyamorphous transformation in these systems.

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