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1.
  • Grolig, Jan Gustav, 1986- (författare)
  • Coated Ferritic Stainless Steels as Interconnects in Solid Oxide Fuel Cells - Material Development and Electrical Properties
  • 2015
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Solid oxide fuel cells (SOFCs) are attracting increasing interest as devices with potentialuses in decentralized and clean electricity and heat production. Several challengeswith respect to materials have to be overcome to achieve efficiencies and life-spansthat are sufficient for long-term applications.An important element of an SOFC stack is the interconnect component, which connectstwo adjacent fuel cell elements. Interconnects, which are commonly composedof ferritic stainless steels, have to be corrosion-resistant, mechanically stable and costoptimized.This work aimed to investigate economic solutions for interconnect materials and tounderstand the underlying mechanisms of degradation and electrical conduction ofthese materials. Mainly two substrates, a commercially available steel (AISI 441) anda ferritic stainless steel that was optimized for an SOFC application (Sandvik SanergyHT) were combined with different barrier coatings and exposed to a cathode-sideatmosphere. A method was developed that allows for the electrical characterizationof promising material systems and model alloys, thereby facilitating a fundamentalunderstanding of the dominant electrical conduction processes linked to the oxidescales that grow on interconnects. The AISI 441 steel coated with reactive elementsand cobalt showed good corrosion and chromium evaporation profiles, while AISI 441coated with cerium and cobalt also had promising electrical properties. The SanergyHT steel was examined with coatings of copper and iron and copper and manganese,respectively. The corrosion and chromium evaporation profiles of Sanergy HT wereimproved by coating with copper and iron. The copper and iron-coated Sanergy HTshowed lower area specific resistance values than cobalt-coated Sanergy HT. Chromia,which is the main constituent of oxide scales, was synthesized using differentmethods. The electrical properties of chromia were found to be sensitive to not onlyimpurities, but also heat treatment. Finally the electrical properties of cobalt- andcobalt cerium-coated Sanergy HT steels were investigated. It was revealed that theaddition of cerium improved the conductivity of the interconnect by both slowingdown chromia growth and preventing the outward diffusion of iron into the spinel.
2.
  • Goel, Sneha, et al. (författare)
  • The Effect of Location and Post-treatment on the Microstructure of EBM-Built Alloy 718
  • 2018
  • Ingår i: Proceedings of the 9th International Symposium on Superalloy 718 & Derivatives: Energy, Aerospace, and Industrial Applications. - 2367-1181 .- 2367-1696. - 978-3-319-89479-9 ; s. 115-129
  • Konferensbidrag (refereegranskat)abstract
    • Alloy 718, usually referred to as Inconel 718, is a Ni-Fe based superalloy widely used in the aerospace industry by virtue of the excellent balance of mechanical properties and corrosion resistance that it affords at high temperature [1]. While Alloy 718 parts have been traditionally fabricated using conventional cast and wrought (C&W) techniques, there has been a growing interest in production of the complex high temperature service components through additive manufacturing (AM) processes due to the significant design freedom offered by the technology. Alloy 718 has been processed by laser (laser metal deposition, LMD; selective laser melting, SLM) and electron (Electron Beam Melting, EBM) beam based technologies [2]–[4]. AM, in general, involves high solidification rates and high thermal gradients, thus resulting in a finer microstructure compared to C&W products [5]. In EBM, a relatively high temperature is maintained throughout the process compared to SLM, thus resulting in lower residual stresses in as-manufactured parts [6]. However, as a relatively newer technology, the degree to which the EBM processing conditions (scanning strategies, preheat and melting parameters) affect the properties of EBM-built Alloy 718 material is less understood compared to the laser based techniques. Also, it has been challenging to date to build Alloy 718 parts that are free from issues such as defects, inhomogeneity, anisotropy, etc., thereby demanding post-treatment to achieve the required microstructure and intended part performance. Although post treatment of C&W Alloy 718 has been widely investigated over several decades [7]–[9], the inadequacy of conventional post-treatment protocols for EBM Alloy 718 [10]–[12], justifies further investigation of microstructural changes that occur in EBM-built Alloy 718 during post-treatment. A comprehensive study may eventually allow optimization of the combined build and post-treatment processes to obtain components with properties deployable in severe environments.
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3.
  • Jaladurgam, Nitesh Raj, 1993- (författare)
  • Heterogeneous deformation of multi-phase engineering materials - an in-situ neutron diffraction study
  • 2019
  • Licentiatavhandling (övrigt vetenskapligt)abstract
    • Gas turbines are complex power generation systems used in aerospace or land-based-power stations. Materials such as Ni-base superalloys are involved in the combustion zone of these machines, which continuously experiences harsh environments with loading at high temperatures. Moreover, the continuous demand for increasing operating temperature to achieve higher efficiencies and reduced emission levels opens the scene to new heat resistant materials like the state-of-the-art high entropy alloys (HEAs), which require a thorough understanding of the structure-process-property relationships. The microstructures of these advanced multi-phase, multi-component alloys are complex, and the deformation is generally heterogeneous both with respect to the different phases and to the crystallographic orientation within each phase. Hence, it is important to understand their behavior and performance during processing and service. In-situ neutron diffraction is a unique technique to probe the deformation behaviour during service/processing-like conditions, including plastic deformation at various temperatures, in order to provide insights into the structure-property relations. In the first part of this work the deformation mechanisms of a newly developed Ni-base superalloy was investigated using in-situ neutron diffraction and electron microscopy at room temperature. In addition, elasto-plastic self-consistent (EPSC) crystal plasticity simulations are used to obtain insights into the operating deformation mechanisms. In the second part, the as-cast eutectic high entropy alloy AlCoCrFeNi2.1 was studied using in-situ neutron diffraction at temperatures from 77 to 673 K. These investigations provide unique insights into the complex heterogeneous deformation behavior of these high-performance multi-phase engineering materials.
4.
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5.
  • André, Benny (författare)
  • Nanocomposites for Use in Sliding Electrical Contacts
  • 2011
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • <p>In this thesis nanocomposite materials for use in high performance electrical contacts are tested. Self mating silver as coatings on cupper substrates are the most used material combination in power connectors today. In this work two new concepts were tested. The first one was to change one of the mating surfaces to a hard thin coating and keep the other surface made of silver. Tested coatings were nanocomposites with hard carbides in a matrix of amorphous carbon. TiC/a-C and  Ti-Ni-C/a-C were tested both electrically and tribologically. The total amount of carbon and the amount of carbon matrix was important, both for the electrical and the tribological properties. The Ti-Ni-C coating also showed that substituting Ti in TiC with the weak carbide former Ni changed the stability of the carbides. The substitution resulted in more a-C matrix and less C in the carbides. Thin coatings of nc-TiC/a-C and  Ti-Ni-C/a-C showed high potential as material candidates for use in electrical contacts.</p> <p>The other tested concept was to modify the used silver instead of replacing it. This was done by embedding nanoparticles of solid lubricant IF-WS<sub>2</sub> in the silver. The results from reciprocating sliding displayed low friction and high wear resistance. The modified silver surfaces lasted for 8000 strokes with a friction of about 0.3 while at the same time allowing for a low contact resistance. The results for surfaces of pure silver coating displayed a friction of 0.8-1.2 and that the silver was worn through already after 300 strokes.</p> <p>A new method to investigate inherent hardness and residual stress of thin coatings, on complex geometries or in small areas, was also developed. An ion beam was used to create stress free coating as free standing micro pillars. Hardness measured on the pillars and on as-deposited coating were then used to calculate the residual stress in the coatings.</p>
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6.
  • He, Wenxiao, 1985- (författare)
  • Biomimetic Formation of Calcium Phosphate Based Nanomaterials
  • 2014
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • The intercellular material in bone is a nanocomposite of aligned “hard” inorganics—calcium phosphate (CaP) platelets embedded in the long-range ordered “soft” organic collagen matrix. This elaborate structural arrangement redeems the weaknesses of the individual components (being soft protein or brittle mineral) and gives bone its excellent mechanical properties for the protection and support of our bodies. The structural order and hierarchy in the soft matrix is organized via self-assembly of collagen molecules and is reinforced by intermolecular crosslinking. The subsequent growth of “hard” crystallites inside the “soft” matrix compartments, likely through the deposition of a transient amorphous calcium phosphate (ACP) phase, results in the interpenetrated composite structure.The aim of this thesis was to prepare synthetic mimetics of “hard” material (CaP) with well-defined nanostructures, soft organic matrices with long-range order and interpenetrated composites composing of the two. The work was inspired by the material deposition process in natural bone. Lyotropic liquid crystal (LC) phases self-assembled by block copolymers were used to mimic the structural order of the collagen matrix. Both the inorganic morphogenesis of CaP in LCs and the controlled crystallization of ACP were investigated. To explore ordered organic matrices, crosslinking of the LCs and the self-assembly of an amphiphilic peptide with designed sequence were performed. In addition, controlled mineralization within crosslinked LCs was examined for the formation of nanocomposites.ACP nanospheres, CaP nanowires and nanosheets were prepared from LCs via templated growth. The ACP nanospheres were capable of transforming into bone-like apatite by controlled aging in water and the prepared nanoparticles were shown to affect osteoblast gene expression. Dicalcium phosphate crystals (brushite and monetite) with structural hierarchy and distinct features were also grown in LCs through epitaxial overgrowth or a self-organization regime. Polymerized LCs were successfully prepared from a modified block copolymer (diacrylate derivative of Pluronic® F127), which served as a resilient matrix for the deposition of ACP nanospheres. A subsequent in situ crystallization of ACP into bone-like apatite resulted in mechanically stable composites retaining nanostructures that resembled that of natural bone. An amphiphilic peptide was designed using mainly natural amino acids and it was shown to self-assemble into distinct structures at different concentrations. Based upon the results presented in this thesis, nanomaterials with assorted structures can be further designed for bio-related applications.
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7.
  • Persson, Henrik, 1983-, et al. (författare)
  • A simple way of improving graphite nanoplatelets (GNP) for their incorporation into a polymer matrix
  • 2012
  • Ingår i: Express Polymer Letters. - 1788-618X. ; 6:2, s. 142-147
  • Tidskriftsartikel (refereegranskat)abstract
    • A simple method of solvent exfoliation/refining of direct-graphite nanoplatelets for their better incorporation into a polymer matrix is presented. We demonstrate the method for polystyrene. The method relies on sonication in N-methyl-2-pyrrolidone solvent, with surfactant assistance. A small amount of polystyrene is added to the solvent in order to increase the viscosity, this enhancing the exfoliation process and resulting in formation of a polymeric layer on graphene for its better incorporation in the polymer matrix. Polystyrene-coated thin graphene stacks form a stable dispersion, while thicker graphite nanoplatelets settle out. Thus bulk separation of thin and thick graphene stacks takes place.The polystyrene-coated thin graphene stacks are studied using Transmission Electron Microscopy in two ways: (i) we calculate the number of graphene layers forming thin graphene stacks, and (ii) we employ Selected Area Diffraction to confirm our image analysis results by checking the intensity ratio (1100) and (2100) deflections in the diffraction patterns. Five layers is found to be the cut-off number, that is there are no stacks >5 layers, and 3 layer stacks are dominantly present. The average largest in-plane dimension is found to be approximately 2.5 mu m (reduction by 50%).
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8.
  • Goel, Sneha, et al. (författare)
  • Effect of post-treatments under hot isostatic pressure on microstructural characteristics of EBM-built Alloy 718
  • 2019
  • Ingår i: Additive Manufacturing. - 2214-8604. ; 28, s. 727-737
  • Tidskriftsartikel (refereegranskat)abstract
    • Electron beam melting (EBM) has emerged as an important additive manufacturing technique. In this study, Alloy 718 produced by EBM was investigated in as-built and post-treated conditions for microstructural characteristics and hardness. The post-treatments investigated were hot isostatic pressing (HIP) and combined HIP + heat treatment (HIP + HT) carried out as a single cycle inside the HIP vessel. Both the post-treatments resulted in significant decrease in defects inevitably present in the as-built material. The columnar grain structure of the as-built material was found to be maintained after post-treatment, with some sporadic localized grain coarsening noted. Although HIP led to complete dissolution of δ and γ′′ phase, stable NbC and TiN (occasionally present) particles were observed in the post-treated specimens. Significant precipitation of γ′′ phase was observed after HIP + HT, which was attributed to the two-step aging heat treatment carried out during HIP + HT. The presence of γ′′ phase or otherwise was correlated to the hardness of the material. While the HIP treatment resulted in drop in hardness, HIP + HT led to ‘recovery’ of the hardness to values exceeding those exhibited by the as-built material.
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9.
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10.
  • Henrysson, Erica, 1987-, et al. (författare)
  • Conceptual Design and Analysis of Membrane Structures
  • 2016
  • Ingår i: Proceedings of 29th Nordic Seminar on Computational Mechanics – NSCM29.
  • Konferensbidrag (övrigt vetenskapligt)abstract
    • In this work one approach for formfinding and analysing tension membranestructures is described. Focus has been on the conceptual stage. For this the computersoftware SMART Form has been further developed, enabling the possibility to do real-timeformfinding and analysis of fabric structures. The software is based on a method where theorthotropic membrane is modeled with a triangular mesh, where the mass is lumped on thenodes. As a computational tool dynamic relaxation is used to find the static equilibriumconfiguration for the structure. The advantage with this is that there is no need for formulationand manipulation of matrices common in the finite element method.
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