124 research outputs found
On-chip assembly of 3D graphene-based aerogels for chemiresistive gas sensing
Integration of the material preparation step into the device fabrication process is of prime importance for the development of high performance devices. This study presents an innovative strategy for the in situ assembly of graphene-based aerogels on a chip by polymerizationâreduction and annealing processes, which are applied as chemiresistive gas sensors for the detection of NO2.TU Berlin, Open-Access-Mittel - 201
Freeze Casting: From LowâDimensional Building Blocks to Aligned Porous StructuresâA Review of Novel Materials, Methods, and Applications
Freeze casting, also known as ice templating, is a particularly versatile technique that has been applied extensively for the fabrication of wellâcontrolled biomimetic porous materials based on ceramics, metals, polymers, biomacromolecules, and carbon nanomaterials, endowing them with novel properties and broadening their applicability. The principles of different directional freezeâcasting processes are described and the relationships between processing and structure are examined. Recent progress in freezeâcasting assisted assembly of low dimensional building blocks, including graphene and carbon nanotubes, into tailored microâ and macrostructures is then summarized. Emerging trends relating to novel materials as building blocks and novel freezeâcast geometriesâbeads, fibers, films, complex macrostructures, and nacreâmimetic compositesâare presented. Thereafter, the means by which aligned porous structures and nacre mimetic materials obtainable through recently developed freezeâcasting techniques and lowâdimensional building blocks can facilitate material functionality across multiple fields of application, including energy storage and conversion, environmental remediation, thermal management, and smart materials, are discussed.TU Berlin, Open-Access-Mittel - 202
Ceria-Based Dual-Phase Membranes for High-Temperature Carbon Dioxide Separation: Effect of Iron Doping and Pore Generation with MgO Template
Dual-phase membranes for high-temperature carbon dioxide separation have emerged as promising technology to mitigate anthropogenic greenhouse gases emissions, especially as a pre- and post-combustion separation technique in coal burning power plants. To implement these membranes industrially, the carbon dioxide permeability must be improved. In this study, Ce0.8Sm0.2O2âÎŽ (SDC) and Ce0.8Sm0.19Fe0.01O2âÎŽ (FSDC) ceramic powders were used to form the skeleton in dual-phase membranes. The use of MgO as an environmentally friendly pore generator allows control over the membrane porosity and microstructure in order to compare the effect of the membraneâs ceramic phase. The ceramic powders and the resulting membranes were characterized using ICP-OES, HSM, gravimetric analysis, SEM/EDX, and XRD, and the carbon dioxide flux density was quantified using a high-temperature membrane permeation setup. The carbon dioxide permeability slightly increases with the addition of iron in the FSDC membranes compared to the SDC membranes mainly due to the reported scavenging effect of iron with the siliceous impurities, with an additional potential contribution of an increased crystallite size due to viscous flow sintering. The increased permeability of the FSDC system and the proper microstructure control by MgO can be further extended to optimize carbon dioxide permeability in this membrane system.DFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische UniversitĂ€t Berli
Review of space resources processing for Mars missions: Martian simulants, regolith bonding concepts and additive manufacturing
Scientific exploration of extraterrestrial planets has gripped human imagination since the advent of space travel. Human missions to Mars could produce insight into the essential questions of how, when and where life began on Earth. Such missions would only be feasible using local space resources materials, a concept called in situ resource utilization (ISRU). In the absence of organic materials from plants, the globally available oxidic surface minerals (regolith) are the only viable resource for large-scale construction efforts such as habitats, greenhouses, landing pads and equipment building. This review provides the first comprehensive literature review of ISRU materials research employing Martian simulants. It gives a detailed overview of all Mars simulants, their history, properties, and challenges, introducing a generational concept for simulants development. The available Mars simulant processing literature (including selected work on lunar simulants) is categorized into seven regolith bonding concepts. The state-of-the-art on additive manufacturing (AM) in ISRU research is discussed. Detailed feasibility assessments for all processing approaches are given, including overview graphs comparing the mechanical performance of each fusion concept with feedstock availability on the surface of Mars. Finally, major open questions and future challenges of materials processing for early Mars missions is examined.DFG, 414044773, Open Access Publizieren 2021 - 2022 / Technische UniversitÀt Berli
Real-time direct transmission electron microscopy imaging of phase and morphology transformation from solid indium oxide hydroxide to hollow corundum-type indium oxide nanocrystallites
A time-resolved series of high-resolution transmission electron microscopy (HRTEM) images are used to monitor phase and morphology transformation of rod-like and spherical particles with the initial ortho-rhombic In OOH phase in situ under continuous illumination with high-energy electrons in a transmission electron microscope. For both particle types, the electron-beam irradiation induces a fast InOOH to rh-InâOâ decomposition accompanied by the formation of voids within the particle/rod center. After illu-mination time intervals of about 1â2 min (i.e.electron dose 6.3â12.6 Ă 10â·enmÂŻÂČ) for particles and8 min (4.3 Ă 108enmÂŻÂČ) for rods, respectively, several small empty cavities become visible in the particle/rod center. The cavities coalesce and form a large hollow space/canal after further illumination. Time-resolved in situ HRTEM unambiguously shows that the formation of internal voids in both nano particle types is a consequence of the structural InOOH-to-rh-InâOâ phase transition that starts at the surface of the corresponding particle. The as-formed oxide phase encapsulates the untransformed hydroxylated phase. Its decomposition does not follow the Kirkendall mechanism; the matter transferred outwards is removed in the form of water, leading to void formation inside without an increase of the particle size
Hydrogenation of Carbon Dioxide to Dimethyl Ether on CuOâZnO/ZSM-5 Catalysts:Comparison of Powder and Electrospun Structures
The promising direct dimethyl ether (DME) production through CO2 hydrogenation was systematically analyzed in this research by synthesizing, characterizing, and testing several catalytic structures. In doing so, various combinations of precipitation and impregnation of copper- and zinc-oxides (CuOâZnO) over a ZSM-5 zeolite structure were applied to synthesize the hybrid catalysts capable of hydrogenating carbon dioxide to methanol and dehydrating it to DME. The resulting catalytic structures, including the co-precipitated, sequentially precipitated, and sequentially impregnated CuOâZnO/ZSM-5 catalysts, were prepared in the form of particle and electrospun fibers with distinguished chemical and structural features. They were then characterized using XRD, BET, XPS, ICP, TGA, SEM, and FIB-SEM/EDS analyses. Their catalytic performances were also tested and analyzed in light of their observed characteristics. It was observed that it is crucial to establish relatively small-size and well-distributed zeolite crystals across a hybrid catalytic structure to secure a distinguished DME selectivity and yield. This approach, along with other observed behaviors and the involved phenomena like catalyst particles and fibers, clusters of catalyst particles, or the whole catalytic bed, were analyzed and explained. In particular, the desired characteristics of a CuOâZnO/ZSM-5 hybrid catalyst, synthesized in a single-pot processing of the precursors of all involved catalytically active elements, were found to be promising in guiding the future efforts in tailoring an efficient catalyst for this system.</p
Fabrication and Characterization of Ice Templated Membrane Supports from Portland Cement
Porous ceramic membranes for aqueous microfiltration and ultrafiltration processes suffer from the high-costs of material and processing. The latter is mainly due to the high-temperature sintering step. In this work, cement-based membrane supports from ultrafine Portland cement are studied as a low-cost alternative to traditional oxidic ceramic supports. An environmentally friendly freeze-casting fabrication route is applied for the fabrication of porous membrane supports. Cement membrane supports are becoming mechanically stabile after hydration reaction of cement with water, which does not require any high-temperature sintering step as in a conventional ceramic membrane fabrication process. This fabrication route, which is sintering-free, decreases the cost and environmental impact of the membrane fabrication process by eliminating extra energy consumption step during sintering. The Archimedes method, scanning electron microscopy (SEM), micro-computed tomographic (”CT), and mercury porosimetry characterize the membrane supports in respect to open porosity, pore size distribution, morphology, and connectivity. The flexural strength of the 3 mm thick membranes is in the range from 1 to 6 MPa, as obtained by the ring-on-ring tests. The obtained membrane supports possess porosity in the range between 48 and 73% depending on fabrication conditions (cooling rate and the solid content, as determined by Archimedes method enabling water flux in the range between 79 and 180 L/(h·m2) at 0.5 bar transmembrane pressure difference and 3 mm membrane thickness.TU Berlin, Open-Access-Mittel â 202
CrabNet for explainable deep learning in materials science: Bridging the gap between academia and industry
Despite recent breakthroughs in deep learning for materials informatics, there exists a disparity between their popularity in academic research and their limited adoption in the industry. A significant contributor to this âinterpretability-adoption gapâ is the prevalence of black-box models and the lack of built-in methods for model interpretation. While established methods for evaluating model performance exist, an intuitive understanding of the modeling and decision-making processes in models is nonetheless desired in many cases. In this work, we demonstrate several ways of incorporating model interpretability to the structure-agnostic Compositionally Restricted Attention-Based network, CrabNet. We show that CrabNet learns meaningful, material property-specific element representations based solely on the data with no additional supervision. These element representations can then be used to explore element identity, similarity, behavior, and interactions within different chemical environments. Chemical compounds can also be uniquely represented and examined to reveal clear structures and trends within the chemical space. Additionally, visualizations of the attention mechanism can be used in conjunction to further understand the modeling process, identify potential modeling or dataset errors, and hint at further chemical insights leading to a better understanding of the phenomena governing material properties. We feel confident that the interpretability methods introduced in this work for CrabNet will be of keen interest to materials informatics researchers as well as industrial practitioners alike.TU Berlin, Open-Access-Mittel - 202
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AlF3-assisted flux growth of mullite whiskers and their application in fabrication of porous mullite-alumina monoliths
Mullite is a promising material with its competitive thermochemical and mechanical properties. Although mullite could be obtained by several synthesis methods, the flux method emerges with its advantages over other methods. However, obtaining mullite whiskers with a high aspect ratio and length for ceramic reinforcements is still challenging. In this work, mullite whiskers were grown from AlF3-assisted flux. The addition of AlF3 to flux salt not only decreases the formation temperature of mullite to as low as 700 â°C and suppresses the formation of corundum side phase, but also increases the length and aspect ratio of the whiskers. The obtained mullite whiskers were used as reinforcement for porous alumina monoliths prepared by the freeze casting route and subsequent sintering at 1500 â°C. The fabricated mullite-alumina monoliths show competitive compressive strength of 25.7 âMPa while having as high as 70.6% porosity, which makes them a potential candidate for membrane applications.DFG, 414044773, Open Access Publizieren 2021 - 2022 / Technische UniversitĂ€t Berli
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