| 1. |
- Bexell, Ulf, et al.
(författare)
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Scratch testing as a tool to evaluate the adhesion of thermally grown oxides on ferritic interconnect steel
- 2010
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Ingår i: 2010 Conference Memory Stick 9th European SOFC Forum. - Lucerne.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The attempt to decrease the temperature in solid oxide fuel cells has made it possible to use metallic materials as interconnect, i.e. the part that separates the anode and the cathode in a fuel cell stack. Besides being impermeable to gases the interconnect give good electron conduction between adjacent cells and to the external circuit. Thus, the unavoidable oxide scale must have good electron conductivity and therefore good adhesion to the metal since spalling of the oxide scale from the metal will give rise to higher resistance in the whole oxide scale metal system. Scratch testing is today a common technique in order to characterize the mechanical properties of thin hard coatings on various types of substrate materials. In this test the normal load applied on the scratching stylus is continuously increased while the stylus is moved relative the surface. The critical load, corresponding to a cohesive and/or adhesive coating failure is registered either by a change in the force, the acoustic emission signal or preferable by combining the information from signals with post-test characterisation of the scratch using scanning electron microscopy. The present study evaluates the possibilities to use scratch testing as a method to measure or at least quantitatively classify the adhesion characteristics of different types of oxide scales thermally grown on ferritic stainless interconnect steel. Both uncoated and interconnect steel pre-coated with thin metallic coatings are studied. The results show that all oxide layers investigated display a sufficient cohesive strength and adhesion to the underlying substrate and that the major scratching induced surface failure mechanisms are plastic deformation and cracking, i.e. no brittle like chipping or spalling could be observed.
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| 2. |
- Karlsson, Dennis, et al.
(författare)
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Binder jetting of the AlCoCrFeNi alloy
- 2019
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Ingår i: Additive Manufacturing. - : Elsevier. - 2214-8604 .- 2214-7810. ; 27, s. 72-79
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Tidskriftsartikel (refereegranskat)abstract
- High density components of an AlCoCrFeNi alloy, often described as a high-entropy alloy, were manufactured by binder jetting followed by sintering. Thermodynamic calculations using the CALPHAD approach show that the high-entropy alloy is only stable as a single phase in a narrow temperature range below the melting point. At all other temperatures, the alloy will form a mixture of phases, including a sigma phase, which can strongly influence the mechanical properties. The phase stabilities in built AlCoCrFeNi components were investigated by comparing the as-sintered samples with the post-sintering annealed samples at temperatures between 900 °C and 1300 °C. The as-sintered material shows a dominant B2/bcc structure with additional fcc phase in the grain boundaries and sigma phase precipitating in the grain interior. Annealing experiments between 1000 °C and 1100 °C inhibit the sigma phase and only a B2/bcc phase with a fcc phase is observed. Increasing the temperature further suppresses the fcc phase in favor for the B2/bcc phases. The mechanical properties are, as expected, dependent on the annealing temperature, with the higher annealing temperature giving an increase in yield strength from 1203 MPa to 1461 MPa and fracture strength from 1996 MPa to 2272 MPa. This can be explained by a hierarchical microstructure with nano-sized precipitates at higher annealing temperatures. The results enlighten the importance of microstructure control, which can be utilized in order to tune the mechanical properties of these alloys. Furthermore, an excellent oxidation resistance was observed with oxide layers with a thickness of less than 5 μm after 20 h annealing at 1200 °C, which would be of great importance for industrial applications.
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| 3. |
- Bexell, Ulf, et al.
(författare)
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Chromium evaporation from plastically deformed pre-coated Sandvik Sanergy HT 22% Cr ferritic interconnect steel
- 2010
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Ingår i: 2010 Conference Memory Stick 9th European SOFC Forum. - Lucerne.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Ferritic stainless steel has attracted a great deal of attention for its use as an interconnector in solid oxide fuel cells (SOFCs). The ferritic Sandvik Sanergy HT chromium steel is specially developed for interconnectors in SOFC with a unique chemical composition, which gives the alloy a good high temperature corrosion resistance as well as good surface conductivity in the formed chromium oxide scale. However, chromium evaporation from metallic interconnectors in SOFC fuel cells tends to poison the cathode of the fuel cell. Furthermore, the evaporation of chromium species from the oxide surface tends to increase the oxidation rate resulting in increased contact resistance. It is nowadays well known that thin coatings of e.g. cobalt can substantially reduce the chromium evaporation from the interconnectors. In this study, a 22% Cr ferritic steel, Sandvik Sanergy HT has been coated with a thin metallic cobalt film. The coated material was plastically deformed and the effect of chromium evaporation was studied as a function of the degree of deformation. Coated samples are also compared to uncoated material. The results show that the metallic cobalt film cracks when the steel sheet is formed but the chromium evaporation is more or less unaffected.
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| 4. |
- Karlsson, Dennis, et al.
(författare)
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Elemental segregation in an AlCoCrFeNi high-entropy alloy : A comparison between selective laser melting and induction melting
- 2019
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 784, s. 195-203
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Tidskriftsartikel (refereegranskat)abstract
- Additive manufacturing of a high-entropy alloy, AlCoCrFeNi, was studied with selective laser melting from gas atomized powder. A wide process parameter window in the SLM process was investigated but it was impossible to produce crack-free samples, attributed to stresses that originate during the building processes. The microstructure and elemental segregation in the SLM samples were compared with induction-melted AlCoCrFeNi. The induction-melted sample crystallizes in randomly oriented large grains (several hundred microns). Dendritic and inter-dendritic areas with slightly different chemical composition can be observed. Within these areas a spinodal decomposition occurs with a separation into FeCr- and NiAl-rich domains. Further spinodal decomposition within the FeCr-rich regions into Cr- and Fe-rich domains was observed by atom probe tomography.In contrast, the SLM-samples crystallizes in much smaller grains (less than 20 μm) with a dendrite-like substructure. These dendrite-like features exhibit distinct chemical fluctuations on the nm-scale. During annealing more pronounced chemical fluctuations and the formation of Cr-rich and Cr-poor regions can be observed. The difference in microstructure and spinodal decomposition between the induction-melted and SLM samples is attributed to the significantly higher cooling rate for SLM. This study shows that, by using different synthesis pathways, it is possible to modify the microstructure and segregation of element within alloys. This can be used to tune the materials properties, if the cracking behavior is handled e.g. by change of alloy composition to minimize phase transformations or use of a heating stage.
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| 5. |
- Schuisky, Mikael
(författare)
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CVD and ALD in the Bi-Ti-O system
- 2000
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Bismuth titanate Bi4Ti3O12, is one of the bismuth based layered ferroelectric materials that is a candidate for replacing the lead based ferroelectric materials in for instance non-volatile ferroelectric random access memories (FRAM). This is due to the fact that the bismuth based ferroelectrics consists of pseudo perovskite units sandwiched in between bismuth oxide layers, which gives them a better fatigue nature.In this thesis thin films of Bi4Ti3O12 have been deposited by chemical vapour deposition (CVD) using the metal iodides, BiI3 and TiI4 as precursors. Films grown on MgO(001) substrates were found to grow epitaxially. The electrical properties were determined for films grown on Pt-coated silicon and good properties such as a high dielectric constant (ε) of 200, low tan δ of 0.018, a remnant polarisation (Pr) of 5.3 μC/cm2 and coercive field (Ec) as high as 150 kV/cm were obtained. Thin films in the Bi-Ti-O system were also deposited by atomic layer deposition (ALD) using metalorganic precursors.In addition to the ternary bismuth titanates, films in the binary oxide systems i.e. bismuth oxides and titanium oxides were deposited. Epitaxial TiO2 films were deposited both by CVD and ALD using TiI4 as precursor. The rutile films deposited by ALD were found to grow epitaxially down to a temperature of at least 375 ¢ªC on α-A12O3(0 1 2) substrates. The TiO2 ALD process was also studied in-situ by QCM. Different bismuth oxides were deposited by halide-CVD using BiI3 as precursor on MgO(0 0 1) and SrTiO3(0 0 1) substrates and the results were summarised in an experimental CVD stability diagram. The Bi2O2.33 phase was found to grow epitaxially on both substrates.
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