3D printing of solid oxide fuel cell

Nowadays, 3D printing is booming as a processing technique, thanks to its versatility in the manufacture of complex geometries and with a quality finish, which cannot be obtained using traditional techniques and can reduce the costs of table-processing (such as, for example, surface finishing, etc.). With this technology, the aim is to bring science and society closer together, with the ultimate goal of developing new devices that are much more efficient than the current ones in order to produce clean energy. Specifically, in the field of energy and in particular in 3D printing of solid oxide fuel cells, since the fuel used is derived from hydrogen in contact with air to produce energy and water vapour. Therefore, this technology avoids the generation of greenhouse gases, such as CO2. The purpose of this final Bachelor’s project (TFG) is the combination of these two fields, focusing on the electrolyte printing of solid oxide fuel cells, with the main objective of obtaining the final material with microstructural and mechanical properties similar to those obtained by traditional techniques. Nevertheless, through this Bachelor’s project, a new research field will be implemented within the CIEFMA group (Centre for Structural Integrity and Reliability of Materials) of the Department of Materials Science and Metallurgic Engineering UPC's , with the future long term purpose of getting all the parts of the battery printed; electrolyte, cathode and anode. To carry out this study, two geometries have been chosen, tubular and hexagonal, where the proportions of the printing material, processing conditions, etc. have been modified in order to achieve materials with a relative density higher than 99% and mechanical properties similar to the theoretical values of the materials used, by means of technical characterization advances (for example: electron microscopy, nanoindentació, etc.). Subsequently, the cathode was deposited by dip-coating and adhesion was studied by means of nanometric scale scratch-out tests. A density greater than 99% has been obtained with a hardness and modulus of elasticity of the printed material comparable to the theoretical value obtained by conventional forming techniques. The cathode also has good adhesion to the electrolyte, since no cracks or other mechanisms of damage to the interface can be observed through the optical microscop

Fracture mechanics analysis of hardmetals by using artificial small-scale flaws machined at the surface through short-pulse laser ablation

Laser ablation has become an innovative treatment for cemented carbides, regarding edge rounding and surface modification, aiming to improve their tribomechanical performance. Meanwhile, the precision offered for this technique has also positioned it as an effective mean to generate micronotches used for evaluation of mechanical properties in structural materials. However, similar approach has not been attempted for hardmetals; thus, it becomes the main objective of this work. Dimple-like and elongated micronotches are introduced in one fine-grained WC-11%wtCo grade. In doing so, laser processing parameters are first optimized to attain micronotches with appropriated geometry and size, i.e. similar to critical flaws identified in broken pristine specimens. Success of the implemented approach is then validated through subsequent flexural testing, fractographic inspection and fracture mechanics analysis of the results attained on samples containing surface micronotches, as far as laser-induced residual stresses are taken into consideration. In this regard, elongated micronotches are found to exhibit lower residual stresses, and postulate themselves as the optimal option of the two micronotch types studied. The suitability of laser ablation for shaping artificial small-scale flaws opens a new route for introducing controlled defects, alike those intrinsic to processing or induced during service, key aspect for further understanding damage tolerance issues in cemented carbides.Peer ReviewedPostprint (published version

Testing length-scale considerations in mechanical characterization of WC-Co hardmetal produced via binder jetting 3D printing

The extreme versatility of additive manufacturing (AM) as processing technology results in “AMed pieces” with intrinsic characteristics linked to the shaping route followed, which are also key for defining mechanical integrity. The latter requires validation by measuring the mechanical properties, at both macroscopic (global) and microscopic (local) levels; and thus, consideration of specific testing length-scale aspects. This work aims to study the correlation between microstructure and mechanical properties for a WC-12%wt.Co hardmetal grade produced via binder jetting 3D printing (BJT) and subsequent sintering. In doing so, macro- and micro- Vickers hardness as well as scratch tests, using different loads and indenter tips, are conducted. It is found that studied samples processed by means of BJT exhibit a microstructure consisting of a relatively wide carbide size distribution, including a significant volume fraction (higher than 15%) of carbides larger than 3 µm. This is a direct consequence of the relatively high sintering temperature needed for getting full dense specimens, when manufactured following this AM route. Meanwhile, mechanical properties are found to be isotropic, with hardness and scratch resistance values falling within ranges of those expected for hardmetals with similar binder content and mean carbide grain size. Very interesting, length-scale effects on testing are observed in terms of dispersion of measured hardness value as applied load decreases. These findings, together with similar ones linked to length-scale influence on scratch response, point out that effective selection of mechanical testing parameters become critical for studying and understanding phenomena such as elastic/plastic and deformation/fracture transitions in AMed hardmetals.Peer ReviewedPostprint (published version

Micromechanical properties of yttria-doped zirconia ceramics manufactured by direct ink writing

Yttria-doped zirconia ceramics have many applications in a wide range of industries mainly due to their excellent mechanical properties, corrosion resistance and biocompatibility. In this study, micromechanical properties of yttria-doped zirconia produced by Direct-Ink Writing (DIW) were investigated and compared to the ones produced by Cold Isostatic Pressing (CIP). In doing so, mechanical response was assessed at different length scales, from macro- up to submicrometric-, by means of Vickers hardness, nanoindentation, and nanoscratch tests. Microstructure was also characterized by determining grain size, crystal structure and phase tetragonal to monoclinic phase transformation. Results revealed that printed samples displayed 20–25% lower hardness values compared to those exhibited by the respective CIP pairs. Differences in hardness between 3 and 8 mol% yttria content evaluated for CIP samples were slight for printed samples, due to the effect of microstructural defects like porosity, resulting from the processing parameters used. At the local level, such an effect was found to be lower. In this sense, hardness and elastic modulus achieved by nanoindentation were closer, when comparing printed and CIP samples. Scratch tests carried out from 0 to 250 mN revealed that 3 mol% Y2O3 samples developed micro-fracture events in the track length, being the printed samples the ones heavily deformed.Peer ReviewedPostprint (author's final draft

Influence of printing direction on the mechanical properties at different length scales for WC-Co samples consolidated by Binder Jetting 3D printing

Additive Manufacturing (AM) is rapidly growing as a revolutionary technique. It provides an interesting ability to produce complex geometries, a key feature for enhancing performance and widening application fields of hardmetal components. Within this context, all the samples produced by AM [AMed] are expected to exhibit characteristics linked to the shaping route followed, which are also vital for defining their mechanical integrity. This work aims to study the correlation of the printing direction to the final microstructure, mechanical properties and layer assemblage at different length scales for a 12%wtCo–WC grade hardmetals of medium grain size consolidated by binder jetting 3DP and subsequent SinterHIP. Vickers macro- and micro-hardness as well as scratch tests, using different loads and indenter tips, are conducted. The results are analysed and discussed in terms of printing orientation effects on microstructural variability, mechanical response determined, intrinsic physical behaviour of the material and feedstock used.Postprint (published version

Nanoscratch testing of 3Al2O3·2SiO2 EBCs: assessment of induced damage and estimation of adhesion strength

n this study, the structural integrity of mullite (3Al2O3·2SiO2) films, deposited on silicon carbide (SiC) substrates using chemical vapor deposition (CVD), was investigated via increasing load nanoscratch tests. The films were configured by mullite columns of stoichiometric composition growing from a silica-rich layer in contact with the SiC substrate. Controlled damage was induced in the 3Al2O3·2SiO2 films at relatively low scratch loads. Radial and lateral cracking were applied until final delamination and repeated chipping were achieved as the load increased. The intrinsic integrity of the 3Al2O3·2SiO2 film and the performance of the coated 3Al2O3·2SiO2/SiC system, regarded as a structural unit, were analyzed. With the aid of advanced characterization techniques at the surface and subsurface levels, the configuration and morphology of the damage induced in the coated system by the nanoscratch tests were characterized, and the scratch damage micromechanisms were identified. Finally, the adhesion of the film, in terms of energy of adhesion and interfacial fracture toughness, was determined using different models proposed in the literature. The results from this investigation contribute to the understanding of the mechanical performance and structural integrity of EBC/SiC-based systems, which over the past few years have increasingly been implemented in novel applications for gas turbines and aircraft engines.Peer ReviewedPostprint (published version

Corrosion evaluation of austenitic and duplex stainless steels in molten carbonate salts at 600 °C for thermal energy storage

Next-generation concentrated solar power (CSP) plants are required to operate at temperatures as high as possible to reach a better energy efficiency. This means significant challenges for the construction materials in terms of corrosion resistance, among others. In the present work, the corrosion behavior in a molten eutectic ternary Li2CO3-Na2CO3-K2CO3 mixture at 600 °C was studied for three stainless steels: an austenitic grade AISI 301LN (SS301) and two duplex grades, namely 2205 (DS2205) and 2507 (DS2507). Corrosion tests combined with complementary microscopy, microanalysis and mechanical characterization techniques were employed to determine the corrosion kinetics of the steels and the oxide scales formed on the surface. The results showed that all three materials exhibited a corrosion kinetics close to a parabolic law, and their corrosion rates increased in the following order: DS2507 < SS301 < DS2205. The analyses of the oxide scales evidenced an arranged multilayer system with LiFeO2, LiCrO2, FeCr2O4 and NiO as the main compounds. While the Ni-rich inner layer of the scales presented a good adhesion to the metallic substrate, the outer layer formed by LiFeO2 exhibited a higher concentration of porosity and voids. Both the Cr and Ni contents at the inner layer and the defects at the outer layer were crucial for the corrosion resistance for each steel. Among the studied materials, super duplex stainless steel 2507 is found to be the most promising alternative for thermal energy storage of those structural components for CSP plants.Peer ReviewedPostprint (published version

Robocasting of dense 8Y zirconia parts: Rheology, printing, and mechanical properties

Advanced ceramics with complex geometry have become indispensable in engineering applications. Due to limitations of traditional ceramic fabrication processes, additive manufacturing represents a revolution for shaping and consolidation because of its unique capabilities for increasing shape complexity and reducing waste material. Among the additive manufacturing techniques, robocasting is often considered to yield fine and dense ceramic structures with geometrically complex morphology and high strength. Within this context, it is the objective to attain dense 8 mol% yttria-stabilized zirconia (8Y-ZrO2) by evaluating the influence of solid loading and filament orientation on the physical and mechanical properties of sintered parts. In doing so, a printable ink was developed using an inverse-thermoresponsive hydrogel. Results revealed that ceramic charges of 67.5 and 70 wt% achieved the best balance regarding density, hardness, and compression strength. Furthermore, rectilinear geometry with a filament orientation at 45º displayed higher mechanical response than 0/90º and cylindrical ones.Peer ReviewedPostprint (published version

3D Printing of Solid Oxide Fuel Cell

Nowadays, 3D printing is booming as a processing technique, thanks to its versatility in the manufacture of complex geometries and with a quality finish, which cannot be obtained using traditional techniques and can reduce the costs of table-processing (such as, for example, surface finishing, etc.). With this technology, the aim is to bring science and society closer together, with the ultimate goal of developing new devices that are much more efficient than the current ones in order to produce clean energy. Specifically, in the field of energy and in particular in 3D printing of solid oxide fuel cells, since the fuel used is derived from hydrogen in contact with air to produce energy and water vapour. Therefore, this technology avoids the generation of greenhouse gases, such as CO2. The purpose of this final Bachelor’s project (TFG) is the combination of these two fields, focusing on the electrolyte printing of solid oxide fuel cells, with the main objective of obtaining the final material with microstructural and mechanical properties similar to those obtained by traditional techniques. Nevertheless, through this Bachelor’s project, a new research field will be implemented within the CIEFMA group (Centre for Structural Integrity and Reliability of Materials) of the Department of Materials Science and Metallurgic Engineering UPC's , with the future long term purpose of getting all the parts of the battery printed; electrolyte, cathode and anode. To carry out this study, two geometries have been chosen, tubular and hexagonal, where the proportions of the printing material, processing conditions, etc. have been modified in order to achieve materials with a relative density higher than 99% and mechanical properties similar to the theoretical values of the materials used, by means of technical characterization advances (for example: electron microscopy, nanoindentació, etc.). Subsequently, the cathode was deposited by dip-coating and adhesion was studied by means of nanometric scale scratch-out tests. A density greater than 99% has been obtained with a hardness and modulus of elasticity of the printed material comparable to the theoretical value obtained by conventional forming techniques. The cathode also has good adhesion to the electrolyte, since no cracks or other mechanisms of damage to the interface can be observed through the optical microscop

Optimal feedstock composition to control the porosity in solid oxide fuel cell produced by additive manufacturing

Nowadays, the research of alternatives power resources has a huge importance in the society to develop new and eco-friendly systems to reduce the climate change. One good and studied option is the solid oxide fuel cells (SOFC), however, with the conventional shapes produced by using traditional processing routes, these systems present low efficiency. Within this context, one way to improve it is creating a high specific surface, and this can be achieved by means of the additive manufacturing (AM) technique. During the last decade, the AM has been the manufacturing technique of the future, thanks of the advantages that it provides; being one of the most important advantages the ability to print complex geometries like honeycomb among others. The purpose of this final Master’s project is the combination of these two fields, following the work developed by myself during my Bachelor’s project and some posterior work. In this case the idea is focused the project on the optimization of the resolution of the SOFC. As well as try to obtain the best feedstock composition to achieve the optimal porosity for each part. In addition, through this Master’s project, a continuity on this AM field implemented within the CIEFMA group (Centre for Structural Integrity and Reliability of Materials) of the Department of Materials Science and Engineering (CEM) UPC's , can be assured, opening new applications of AM combined with the energy field. To carry out this study, the used materials were: 8Y-TZP for the electrolyte, Lanthanum gallate strontium and magnesium doped for the cathode and gadolinium oxide for the anode. This study was divided in three parts; the first one consisted in an evaluation of the particle size, by using the laser diffraction particle size technique (also known as Mastersizer) and some scanning electron microscopy micrographs, with a step to process the powder and modify the size. In a second phase the best composition was search trying different ink compositions. The main rheological parameters for the optimal ceramic pastes (G’, G’’) will be studied. As a final step a cylindrical sample was printed by robocasting. Afterwards, the microstructural (e.g. density, phases, etc.) and micromechanical properties (e.g. hardness, elastic modulus, fracture mechanisms, etc.) will be determined by using advanced characterization techniques, like field emission scanning electron microscopy, focused ion beam, nanoindentation among others, to assure a minimal mechanical integrity of SOFC parts