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Publications (10 of 99) Show all publications
Flygare, M. & Svensson, K. (2019). Quantifying crystallinity in carbon nanotubes and its influence on mechanical behaviour. Materials Today Communications, 18, 39-45
Open this publication in new window or tab >>Quantifying crystallinity in carbon nanotubes and its influence on mechanical behaviour
2019 (English)In: Materials Today Communications, ISSN 2352-4928, Vol. 18, p. 39-45Article in journal (Refereed) Published
Abstract [en]

The different fabrication methods that have been developed for making carbon nanotubes will provide materials with different levels of crystallinity. As crystallinity is qualitatively known to have a profound influence on material properties, this raises the need for standardised quantitative analysis. Here we show how transmission electron microscopy can be used to provide quantitative information about effective crystallite sizes in individual nanotubes which we link to the mechanical behaviour of the tubes. The method relies on a thorough analysis of diffraction patterns and a careful extraction of instrumental and sample contributions to the peak shapes. We find that arc-discharge grown tubes have crystallite sizes that are comparable to the circumference of the outer tube walls, while commercial catalytically grown tubes have much smaller crystallites implying that each cylindrical nanotube wall can be thought of as a patchwork of small graphene-like grains. The clear differences in crystallite sizes are then compared to known differences in mechanical behaviour, such as a substantial disparity in stiffness and significantly different behaviours under bending stress.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Carbon nanotubes, Crystallite size, High resolution transmission electron microscopy, Transmission electron microscopy, Arc discharge, Bending stress, Crystallinities, Cylindrical nanotube, Fabrication method, Graphene likes, Mechanical behaviour, Quantitative information, Yarn
National Category
Materials Engineering Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-70410 (URN)10.1016/j.mtcomm.2018.11.003 (DOI)000456868200006 ()2-s2.0-85056626896 (Scopus ID)
Available from: 2018-12-05 Created: 2018-12-05 Last updated: 2019-02-14Bibliographically approved
Krakhmalev, P., Fredriksson, G., Svensson, K., Yadroitsev, I., Yadroitsava, I., Thuvander, M. & Peng, R. (2018). Microstructure, solidification texture, and thermal stability of 316 L stainless steel manufactured by laser powder bed fusion. Metals, 8(8), 1-18, Article ID 643.
Open this publication in new window or tab >>Microstructure, solidification texture, and thermal stability of 316 L stainless steel manufactured by laser powder bed fusion
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2018 (English)In: Metals, ISSN 2075-4701, Vol. 8, no 8, p. 1-18, article id 643Article in journal (Refereed) Published
Abstract [en]

This article overviews the scientific results of the microstructural features observed in 316 L stainless steel manufactured by the laser powder bed fusion (LPBF) method obtained by the authors, and discusses the results with respect to the recently published literature. Microscopic features of the LPBF microstructure, i.e., epitaxial nucleation, cellular structure, microsegregation, porosity, competitive colony growth, and solidification texture, were experimentally studied by scanning and transmission electron microscopy, diffraction methods, and atom probe tomography. The influence of laser power and laser scanning speed on the microstructure was discussed in the perspective of governing the microstructure by controlling the process parameters. It was shown that the three-dimensional (3D) zig-zag solidification texture observed in the LPBF 316 L was related to the laser scanning strategy. The thermal stability of the microstructure was investigated under isothermal annealing conditions. It was shown that the cells formed at solidification started to disappear at about 800 °C, and that this process leads to a substantial decrease in hardness. Colony boundaries, nevertheless, were quite stable, and no significant grain growth was observed after heat treatment at 1050 °C. The observed experimental results are discussed with respect to the fundamental knowledge of the solidification processes, and compared with the existing literature data.

Place, publisher, year, edition, pages
MDPI AG, 2018
Keywords
316 L stainless steel, Cellular solidification, Electron microscopy, Laser powder bed fusion, Solidification texture, Thermal stability of microstructure
National Category
Materials Engineering
Research subject
Materials Engineering
Identifiers
urn:nbn:se:kau:diva-69224 (URN)10.3390/met8080643 (DOI)000443616400079 ()2-s2.0-85052594962 (Scopus ID)
Available from: 2018-09-14 Created: 2018-09-14 Last updated: 2019-04-26Bibliographically approved
Kuzmenko, V., Wang, N., Haque, M., Naboka, O., Flygare, M., Svensson, K., . . . Enoksson, P. (2017). Cellulose-derived carbon nanofibers/graphene composite electrodes for powerful compact supercapacitors. RSC Advances, 7, 45968-45977
Open this publication in new window or tab >>Cellulose-derived carbon nanofibers/graphene composite electrodes for powerful compact supercapacitors
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2017 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 7, p. 45968-45977Article in journal (Refereed) Published
Abstract [en]

Herein, we demonstrate a unique supercapacitor composite electrode material that is originated from a sustainable cellulosic precursor via simultaneous one-step carbonization/reduction of cellulose/ graphene oxide mats at 800 C. The resulting freestanding material consists of mechanically stable carbon nanofibrous (CNF, fiber diameter 50–500 nm) scaffolds tightly intertwined with highly conductive reduced graphene oxide (rGO) sheets with a thickness of 1–3 nm. The material is mesoporous and has electrical conductivity of 49 S cm 1, attributed to the well-interconnected graphene layers. The electrochemical evaluation of the CNF/graphene composite electrodes in a supercapacitor device shows very promising volumetric values of capacitance, energy and power density (up to 46 F cm 3, 1.46 W h L 1 and 1.09 kW L 1, respectively). Moreover, the composite electrodes retain an impressive 97% of the initial capacitance over 4000 cycles. With these superior properties, the produced composite electrodes should be the “looked-for” components in compact supercapacitors used for increasingly popular portable electronics and hybrid vehicles. 

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2017
Keywords
super capacitors, cellulose derived, carbon nanofibre, graphene
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-64426 (URN)10.1039/c7ra07533b (DOI)000412211300010 ()
Funder
Knut and Alice Wallenberg Foundation, 644378
Available from: 2017-10-02 Created: 2017-10-02 Last updated: 2018-12-10Bibliographically approved
Tofique, W., Bergström, J., Svensson, K., Johansson, S. & Peng, R. L. (2017). ECCl/EBSD and TEM analysis of plastic fatigue damage accumulation responsible for fatigue crack initiation and propagation in VHCF of duplex stainless steels. International Journal of Fatigue, 100, 251-262
Open this publication in new window or tab >>ECCl/EBSD and TEM analysis of plastic fatigue damage accumulation responsible for fatigue crack initiation and propagation in VHCF of duplex stainless steels
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2017 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 100, p. 251-262Article in journal (Refereed) Published
Abstract [en]

Fatigue test data of duplex stainless steel grades, LDX 2101 and 2304 SRG, in the Very High Cycle Fatigue (VHCF) regime is presented. Fatigue testing was conducted using ultrasonic fatigue test equipment operating at 20 kHz under fully reversed tension-compression load condition. Scanning Electron Microscope (SEM) analysis of the fracture surfaces and external surfaces of failed specimens was conducted. Electron Channelling Contrast Imaging (ECCI) and Electron Back Scattered Diffraction (EBSD) studies of the axially cut surface of the failed specimens was done to analyse the accumulation of plastic fatigue damage and fatigue crack growth in the grains adjacent to the external surface and crack initiation site. Transmission Electron Microscope (TEM) analysis of thin foils cut from failed specimens of LDX 2101 was carried out to examine the effect of fatigue loading on dislocation structure. SEM studies of the Crystallographic Growth Region (CGR) showed features like grain boundaries and fatigue striations on the fracture surfaces. SEM analysis of the external surfaces of fatigue loaded specimens showed inhomogeneous accumulation of plastic fatigue damage. ECCl/EBSD analysis showed Persistent Slip Bands (PSBs) in ferrite grains in LDX 2101 grade but no PSBs were observed in any grains of 2304 SRG specimens. The barrier effect of grain and phase boundaries on short fatigue crack propagation was observed. TEM analysis of thin foils cut from the failed specimens of LDX 2101 showed stacking faults in austenite grains and they were seen to stop at the grain and phase boundaries. (C) 2017 Elsevier Ltd. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Duplex stainless steel, High frequency testing, Crack initiation, Damage accumulation, Fatigue crack growth, Short cracks
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kau:diva-65533 (URN)10.1016/j.ijfatigue.2017.03.035 (DOI)000402217000024 ()
Available from: 2018-01-05 Created: 2018-01-05 Last updated: 2018-06-27Bibliographically approved
Andersson, S. & Svensson, K. (2017). Excitation and desorption of physisorbed H2 via the 2Σu electron scattering resonance.. Journal of Chemical Physics, 147, 114703-1-114703-11, Article ID 114703.
Open this publication in new window or tab >>Excitation and desorption of physisorbed H2 via the 2Σu electron scattering resonance.
2017 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 147, p. 114703-1-114703-11, article id 114703Article in journal (Refereed) Published
Abstract [en]

Our high-resolution electron energy-loss measurements concern physisorbed H2 and comprise dif- ferential cross sections for the excitation of the internal H2 modes and the H2-surface bonding mode and their combinations and extend over the electron impact energy range of the classical low-energy H2 2Σu resonance. Comparison with corresponding data for the excitation of the internal modes of gas phase H2 reveals that strong elastic electron reflectivity from the Cu(100) substrate profoundly distorts the inelastic scattering pattern for physisorbed H2. We find that this influence can be corrected for and that the resulting peak cross sections agree with the H2 gas phase data, in accordance with theoretical predictions for the excitation of the internal H2 vibration. We have used corrected cross sections for the rotational mode spectra of physisorbed H2, HD, and D2 in a model concerning elec- tron induced desorption via rotation-translation energy conversion. These spectra include transitions from the ground state as well as excited levels of the physisorption potential well. H2 and HD can desorb from all levels while D2, for energetic reason, can only desorb from the excited levels. This model gives a satisfactory account of the observed desorption cross sections and predicts character- istic velocity distributions of the desorbing molecules. The cross section data for H2 and HD reveals that direct bound-free transitions also contribute to the electron induced desorption. 

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2017
Keywords
Hydrogen molecules, Physisorption, electron scattering resonances, HREELS
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-64425 (URN)10.1063/1.5003069 (DOI)000411462100030 ()
Available from: 2017-10-02 Created: 2017-10-02 Last updated: 2019-11-08Bibliographically approved
Tofique, M. W., Bergström, J. & Svensson, K. (2017). Very High Cycle Fatigue of cold rolled stainless steels, crack initiation and formation of Fine Granular Area. International Journal of Fatigue, 100(1), 238-250
Open this publication in new window or tab >>Very High Cycle Fatigue of cold rolled stainless steels, crack initiation and formation of Fine Granular Area
2017 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 100, no 1, p. 238-250Article in journal (Other academic) Published
Abstract [en]

Fatigue tests of cold rolled strip materials, a duplex stainless steel and a martensitic stainless steel, were performed using an ultrasonic fatigue test equipment operating at 20 kHz under a completely reversed tension compression load ratio R =-1. Fatigue test data (SN data) was generated in the VHCF regime and fracture surfaces of the failed specimens were investigated using a Scanning Electron Microscope (SEM). In the duple stainless steel grade, fatigue failures were found to be initiated at surface defects on the side surfaces or corners, created due to cold rolling, of the strip specimens. Features of a Fine Granular Area (FGA) were observed around the crack initiating surface defects on the fracture surfaces. In the martensitic stainless steel grade, fatigue crack initiation occurred due to aluminium-silicon oxide inclusions or surface defects created due to cold rolling of the material. In situ Focussed Ion Beam (FIB) technique was used to extract cross-sections from the FGA around the crack initiating defect on the fracture surface. Transmission Electron Microscope (TEM) investigations of the extracted cross-sections revealed FGAs in immediate vicinity of the crack initiating surface defects. The observed fine grained layers seemed to be composed of nano-sized grains and, thus, could be distinguished from the bulk material. The FGA around the surface crack initiating defects seems to have formed due to localized plastic deformation by stress concentration at the defects.

Place, publisher, year, edition, pages
oxford: Elsevier, 2017
Keywords
Duplex stainless steel, Martensitic stainless steel, Fatigue, High frequency testing, Crack initiation, Fine granular area
National Category
Other Materials Engineering
Research subject
Materials Engineering; Mechanical Engineering
Identifiers
urn:nbn:se:kau:diva-47126 (URN)10.1016/j.ijfatigue.2017.03.037 (DOI)000402217000023 ()
Funder
Knowledge Foundation
Available from: 2016-11-07 Created: 2016-11-07 Last updated: 2018-06-27Bibliographically approved
Kuzmenko, V., Saleem, A., Staaf, H., Haque, M., Bhaskar, A., Flygare, M., . . . Enoksson, P. (2016). Hierarchical cellulose-derived CNF/CNT composites for electrostatic energy storage. Journal of Micromechanics and Microengineering, 26(12), Article ID 124001.
Open this publication in new window or tab >>Hierarchical cellulose-derived CNF/CNT composites for electrostatic energy storage
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2016 (English)In: Journal of Micromechanics and Microengineering, ISSN 0960-1317, E-ISSN 1361-6439, Vol. 26, no 12, article id 124001Article in journal (Refereed) Published
Abstract [en]

Today many applications require new effective approaches in energy delivery on demand. Supercapacitors are viewed as essential energy storage devices that can continuously provide quick energy. The performance of supercapacitors is mostly determined by electrode materials that can store energy via electrostatic charge accumulation. This study presents new sustainable cellulose-derived composite electrodes which consist of carbon nanofibrous (CNF) mats covered with vapor-grown carbon nanotubes (CNTs). The CNF/CNT electrodes have high electrical conductivity and surface area: two most important features that are responsible for good electrochemical performance of supercapacitor electrodes. The results show that the composite electrodes have fairly high values of specific capacitance, energy and power density and can retain excellent performance over at least 2 000 cycles. All of that makes us think that sustainable cellulose-derived composites can be extensively used in future as supercapacitor electrodes.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2016
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-45613 (URN)10.1088/0960-1317/26/12/124001 (DOI)000389022900002 ()
Available from: 2016-09-01 Created: 2016-09-01 Last updated: 2018-12-10Bibliographically approved
Anselmo, A. S., Dzwilewski, A., Svensson, K. & Moons, E. (2016). Photodegradation of the electronic structure of PCBM and C60 films in air. Chemical Physics Letters, 652, 220-224
Open this publication in new window or tab >>Photodegradation of the electronic structure of PCBM and C60 films in air
2016 (English)In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 652, p. 220-224Article in journal (Refereed) Published
Abstract [en]

Fullerenes are common electron acceptors in organic solar cells. Here the photostability in air of the electronic structures of spin-coated PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) and evaporated C60 films are studied using ultraviolet photoelectron spectroscopy (UPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. After exposing these materials in air to simulated sunlight, the filled and empty molecular orbitals are strongly altered, indicating that the conjugated π-system of the C60-cage has degraded. Even a few minutes in normal lab light induces changes. These results stress the importance of protecting fullerene-based films from light and air during processing, operation, and storage.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
Degradation; Fullerene; Photostability; Photovoltaics; NEXAFS
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-16111 (URN)10.1016/j.cplett.2016.04.003 (DOI)000377204100042 ()
Note

The article was still in manuscript form in anna Anselmos thesis. It had then the title: Light-induced modification of the electronic structure of PCBM and C60 films

Available from: 2012-12-07 Created: 2012-12-07 Last updated: 2019-05-20Bibliographically approved
Zhang, X., Lindberg, T., Svensson, K., Vyatkin, V. & Mousavi, A. (2016). Power Consumption Modeling of Data Center IT Room with Distributed Air Flow. International Journal of Modeling and Optimization, 6(1), 33-38
Open this publication in new window or tab >>Power Consumption Modeling of Data Center IT Room with Distributed Air Flow
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2016 (English)In: International Journal of Modeling and Optimization, ISSN 2010-3697, Vol. 6, no 1, p. 33-38Article in journal (Refereed) Published
Abstract [en]

Modern data centers are characterized by large sizes, high energy consumption and complexity involving IT, power supply, ventilation and cooling. Data center energy efficiency is a major concern for data center design and operation. To improve data center energy efficiency through efficient cooling and ventilation, advanced process control and optimization, process models to describe the process power consumption are required. In this work, data center power consumption models are investigated. A concept of distributed air flow control is presented. The objective is to develop a comprehensive data center power consumption model to describe IT room, computer room air handling (CRAH), data center ventilation and cooling characteristics as well as distributed air flow control. Data center operation scenarios with uneven IT load are simulated. Results show that the distributed air flow control can save the cooling energy significantly.

Place, publisher, year, edition, pages
International Association of Computer Science and Information Technology Press, 2016
Keywords
Server, rack, CRAH, model, power consumption
National Category
Other Physics Topics
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-44503 (URN)10.7763/IJMO.2016.V6.499 (DOI)
Available from: 2016-07-18 Created: 2016-07-18 Last updated: 2017-10-05
Jackman, H., Krakhmalev, P. & Svensson, K. (2015). Mechanical behavior of carbon nanotubes in the rippled and buckled phase. Journal of Applied Physics, 117(8), 084318
Open this publication in new window or tab >>Mechanical behavior of carbon nanotubes in the rippled and buckled phase
2015 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, no 8, p. 084318-Article in journal (Refereed) Published
Abstract [en]

We have studied the mechanical behavior of multi-walled carbon nanotubes for bending strains beyond the onset for rippling and buckling. We found a characteristic drop in the bending stiffness at the rippling and buckling onset and the relative retained stiffness was dependent on the nanotube dimensions and crystallinity. Thin tubes are more prone to buckle, where some lose all of their bending stiffness, while thicker tubes are more prone to ripple and on average retain about 20\% of their bending stiffness. In defect rich tubes the bending stiffness is very low prior to rippling but these tubes retain up to 70\% of their initial bending stiffness.

Keywords
carbon nanotubes, Young's modulus, bending stiffness, buckling, rippling, nonlinear behavior
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-35340 (URN)10.1063/1.4913701 (DOI)000351132500050 ()
Funder
Swedish Research Council, 2010-4324
Available from: 2015-03-07 Created: 2015-03-07 Last updated: 2019-07-11Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-1711-5595

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