Symposium CH
Porous Ceramics for Environmental Protection, Energy-related Technologies and Advanced Industrial Cycles
ABSTRACTS
CH-1:IL01 Increased Porosity and Inner Strut Accessibility in Reticulated Ceramic Foams
M. SCHEFFLER, Otto-von-Guericke University Magdeburg, Magdeburg, Germany
Even if additive manufacturing of cellular ceramics is of immense interest for future applications, the micro and macro structural features of reticulated porous ceramics in combination with low-cost manufacturing processes keep them in the focus of research and development as multifunctional materials. Compared to microporous materials, however, their surface-to-volume ratio is low, thus, an increase in their specific surface area may by a key factor for applications in sorption, catalysis, filtration and other novel-type applications. This paper deals with the increase of the specific surface area and the accessibility of the inner pore volume of the struts of reticulated porous ceramics. In order to achieve surface area enlargement and/or inner pore volume access several pore-forming processes such as freeze casting, pore former additions to the ceramic slurry or demixing of slurry components were combined with the well-established Schwartzwalder process for the manufacturing of reticulated porous ceramics. Within this context, the most relevant processing parameters and process-structure-properties relations are discussed, and an outlook will be given dealing with potential applications.
CH-1:IL03 Colloidal Processing of Reticular Ceramic Foams with Reaction Sintered Secondary Minor Phases
R. MORENO, C. Alcázar, A. PEREIRA, J.C. Fariñas, E. ROSADO, Institute of Ceramics & Glass - CSIC, Madrid, Spain
Ceramic foams are used in many applications including bone repair and catalyst supports. Open cell foams with complex shape can be obtained by additive manufacturing but simple structures can be produced much more easily by colloidal forming processes. Among them, the most popular are the replica method, the use of sacrificial templates, gelcasting and freeze casting. Colloidal processing also allows one to produce a scaffold that is subsequently functionalized impregnating the struts with a thin layer of an incompatible material, thus allowing the reaction with the substrate material to produce another phase forming a secondary phase by reaction sintering. The present work explores the possibility to produce open cell porous materials by colloidal shaping techniques such as freeze casting and replica method using aqueous suspensions of different oxides. In a second step, minor nanosized phases are infiltrated to produce coatings by reaction sintering. For example, ZrO2 substrates were infiltrated with colloidal TiO2 to produce ZrTiO4, and Al2O3 was infiltrated with silica to produce mullite-alumina composites. Such complex structures were produced optimizing the rheological behavior of the different suspensions. The same procedure was used to produce mullite nanoparticles on SiO2 reticulate foams produced by stereolitography infiltrating alumina slurries.
CH-1:IL04 Direct Ink Writing of Hierarchically Porous Geopolymeric Structures for Environmental Applications
G. FRANCHIN, R. Fuss Botti, K. Goulart De Oliveira, M. D’Agostini, G. Zangarini, P. Colombo, Dept. of Industrial Engineering, University of Padova, Padova, Italy
Geopolymers are inorganic materials that form long-range, covalently bonded, amorphous networks; they are usually synthesized through the reaction of alumino-silicates in an alkaline medium. They consolidate at low, even room temperature but can withstand temperatures above 1200°C. Their chemical composition is similar to zeolites, and they also possess intrinsic micro- and mesoporosity; for these reasons, they can be of interest for environmental applications as filters, adsorbers, catalysts and so on. Our group has been exploring Direct Ink Writing as a processing route for hierarchically porous components comprising geopolymers as active material and/or as a matrix for active fillers such as zeolites and activated carbons. Lattices with designed macroporosity above 50%vol were fabricated and characterized in terms of microstructure, mechanical properties, specific surface area and permeability. Specific compositions were developed for selected applications, including biodiesel production catalysis and water filtration from both cationic and anionic pollutants. Their performances will be presented and compared with more conventional materials and processing routes.
CH-1:IL05 Surface and Inner Structure Control in Electrospun Ceramic Nanofibers
G.S. GRADER, G.E. Shter, O. Elishav, Technion – Israel Institute of Technology, Haifa, Israel
Design of ceramic nanofibers with complex surface and inner structure and specific chemical and physical properties is required for material with improved performance. Porous or hollow structures reduce mass transport limitation to and from the fiber inner surface. Electrospinning is a cost-effective method to produce continuous nanofibers with controlled morphology of diverse materials. Typically, ceramic nanofibers are obtained after polymer removal by thermal treatment. During this stage, fibers shrinkage, deformation, and phase and morphology changes occur. Nanofibers with lamellar porous surface were synthesized. A general formation model for this morphology is suggested, where the final structure depends on the deformation processes. This mechanism is realized in several materials, providing similar morphologies. In addition, the fiber inner structure changes from solid to nanobelts under certain conditions. These results show the interplay between process conditions and the nanofibers’ complex surface and inner structure. The relation between synthesis conditions, morphology and performance will be discussed. The findings and proposed mechanisms open new research directions in other materials for diverse applications.
CH-1:IL10 Additive Manufacturing of Porous Ceramics for Separation Applications
D. PENNER, ZHAW Zürich University of Applied Sciences; IMPE Institute of Materials and Process Engineering; Laboratory for Ceramic Materials; Winterthur, Switzerland
With the continuous progress in exploration and transfer of additive manufacturing technologies in the field of ceramic materials, new opportunities for specifically architected materials arise. This could be on different size scales or as a combination of different scales in terms of hierarchically ordered materials. Chemical engineering is an important industrial field, where such concepts might lead to new products with more controlled and improved properties. Different examples of controlled and architected ceramic structures will be presented to demonstrate the potential benefit of using additive manufacturing concepts for production of tailored porous ceramics. Typical applications are filter architectures, chromatography separation columns or fluid/gas exchange systems.
CH-1:IL11 Sepiolite Based Catalyst Supports Produced by Combined Freeze-robocasting Technique
J. GURAUSKIS, V. Gil Hernandez, Hydrogen Foundation of Aragon, Huesca, Aragon, Spain; B. Simonsen, DTU Energy, Technical University of Denmark, Kgs. Lyngby, Denmark
Structurally and functionally optimized porous materials can revolutionize many applications that rely on physical and chemical interactions. In general, the design of these materials is based on implementation of functional phases with huge surface area within as small as possible volume. Nevertheless, the real-world applications require to address the issues related to effective mass flow and structural robustness too. Having this in mind, strategies to increase surface area and permeability based on hierarchical porosity distribution and implementation of mass flow oriented porosity hold a great promise. Hierarchical porosity distribution delivers high surface area, whereas mass flow oriented porosity delivers high permeability at low penalty to structural robustness. In this study sepiolite ((Mg4Si6O15(OH)2·6H2O)) based porous scaffolds were shaped using a combination of directional freeze casting and robocasting techniques. Optimization of aqueous suspension rheological parameters in combination with optimized freeze/robocasting conditions permitted to obtain structural support scaffolds with hierarchical porosity distribution, where the main mass flow is guaranteed by robust and highly porous channeled microstructure. Detailed microstructural characterization and functionalization cases of produced titanium oxide based membranes will be presented.
CH-2:IL01 CO2 Absorption in Porous Geopolymers
V. MEDRI, E. Papa, E. Landi, CNR-ISTEC Faenza, Italy; A. Vaccari, M. Minelli, University of Bologna, Bologna, Italy
Alkali bonded ceramics are synthetic and amorphous alkali aluminosilicates, currently known as geopolymers. Since they can be regarded as the amorphous counterpart of zeolites, their application can be potentially extended in the chemical sector of molecular sieves. Geopolymers have a quite good CO2 adsorption capacity and selectivity up to 200 and 100 for CO2/N2 and CO2/CH4 separation, respectively, considerably higher than those of most of the adsorbent materials commonly accounted for such applications. The addition of zeolites or hydrotalcites as fillers can further improve the adsorption capacity for low and intermediate temperature CO2 capture applications. Na-based geopolymer-zeolite composites revealed a synergistic effect, as the CO2 capacity at low temperature was approximately 20% larger than the expected value. As well different types of hydrotalcites with different Mg/Al ratio can be used as fillers in geopolymer matrices for adsorption at intermediate temperature. Upon calcination, the structure of hydrotalcite changes, with loss of interlayered anions and water: the mixed oxide metaphase presents a large surface area and great affinity for CO2.
CH-2:IL02 Structured Porous Ammonia Carriers for the Automotive Selective Catalytic Reduction (SCR) System
F. AKHTAR, Division of Materials Science, Luleå University of Technology, Luleå, Sweden
Nitrogen oxides (NOx) are one of the most harmful air pollutants, resulting in severe environmental problems. The NOx level has been reported as one of the key contributors to the fatality of coronavirus disease 2019 (COVID-19). The restriction on NOx emissions has been increasingly stringent. The automotive selective catalytic reduction (SCR) system is one of the common approaches to eliminate NOx by ammonia (NH3). Therefore, a reliable and stable structured ammonia carrier is required to meet the emission standard. In this regard, we will present the structuring of porous sorbents and alkaline earth metal halides (AEMHs) as ammonia carriers with superior sorption kinetics and structural stability. We show that the ammonia adsorption-desorption kinetics in the metal-organic frameworks (MOFs) [M2(adc)2(dabco)] (M = Co, Ni, Cu, Zn) are 4 times faster in the ammonia sorption and 5 times faster in ammonia desorption in the first 10 min, compared to MgCl2.1 AEMHs can be structured using carbon materials to overcome issues of melting and volume swing during sorption. We show that the incorporation of 20 wt. % carbon materials into (AEMH) MgCl2, the melting spread of the MgCl2 can be prevented during the ammonia sorption to reduce the structural risk of using alkaline earth metal halides (AEMHs) as ammonia carriers. Moreover, the ammonia kinetics of the 20 wt.% graphene nanoplatelets aggregates (GNA) -80 wt.% MgCl2 composite show an increase of 83% and 73% in the ammonia sorption and first-2-minute of desorption, respectively.2 The volume expansion and shrinkage of the AEMHs as ammonia carriers is as large as 400%. We show novel design and fabrication of porous SrCl2 porous structures, scaffolded by reduced graphene oxides (rGO) networks. The porous SrCl2 structure maintains the macro- and micro-structure accommodating the volume swing after 20 ammonia absorption–desorption cycles. Moreover, the porous 80 wt% SrCl2-rGO composite possesses rapid ammonia absorption–desorption kinetics, 1.4 times faster in absorption and 5.4 times faster in desorption compared with pure SrCl2 pellet.3 Notably, our method is also verified with other AEMHs, including MgCl2, CaCl2. These enhancements solve the long-time problems of the AEMHs as ammonia carriers in both structure and kinetics aspects in applications and offer reliable and safer operation.
(1) Cao, Zhejian; Narang, Kritika; Akhtar, F. Rapid Ammonia Carriers for SCR Systems Using MOFs [M2 (adc) 2 (dabco)](M= Co, Ni, Cu, Zn). Catalysts 2020, 10 (12), 1444.
(2) Cao, Z.; Osorio, N. G.; Cai, X.; Feng, P.; Akhtar, F. Carbon-Reinforced MgCl2 Composites with High Structural Stability as Robust Ammonia Carriers for Selective Catalytic Reduction System. J. Environ. Chem. Eng. 2020, 8 (1), 103584.
(3) Cao, Zhejian; Akhtar, F. Porous SrCl2 Scaffolded by Graphene Networks as Ammonia Carriers. Adv. Funct. Mater. 2021, 008505.
CH-2:IL03 Capillary Transport in Freeze Cast SiOC Ceramics
D. Schumacher1, P. Braun1, Huixing Zhang1, M. Dreyer2, K. Rezwan1, 3, M. Wilhelm1, 1Advanced Ceramics, University of Bremen, Germany; 2Applied space technology and microgravity, University of Bremen, Germany; 3MAPEX-Centre for Materials and Processes, University of Bremen, Germany
The diverse possibilities of freeze casting in terms of pore morphology, pore size and porosity can be used to generate components for capillary transport applications. Preceramic polymers offer the interessting feature that they can be processed in suspension-based freeze casting with preceramic particles and water as well as in solution-based freeze casting with organic solvents as liquid phase. Both processes provide distinct characteristics which can be useful in capillary transport applications. For a water-based process the hydrophobic polysiloxane has to be transformed to hydrophilic preceramic particles at first. For that, a polysiloxane was pre-pyrolyzed to partially decompose the polymer to hydrophilic particles. In a second approach, 3-Aminopropyl)triethoxysilane (APTES) was added to a polysiloxane and introduced hydrophilic aminopropyl groups. After freezing, a hierarchical lamellar micro/meso/macroporous structure is obtained for both approaches. While the samples showed promising thermal conductivity for capillary transport at cryogenic conditions, the mechanical strength remains relatively low. Structures could be further improved by using solution-based freeze casting where no pre-treatment of the polysiloxane is necessary. Cyclohexane and tert-butyl alcohol as solvents lead to a dendritic and prismatic pore structure, respectively. Besides altering the surface characteristic, the addition of preceramic and ceramic filler particles significantly improves the mechanical strength. Further adaptions of the pore structure were achieved by changing the freezing conditions: from radial to unidirectional and from a constant freezing temperature to a constant freezing velocity. This allows to generate a fully aligned pore structures with a constant porosity and pore size. Wicking of water-based structures revealed a good agreement with a prediction via the Lucas-Washburn equation. Wicking experiments of cyclohexane and tert-butyl alcohol based structures showed that the prismatic pore morphology wicks faster than the dendritic one. Additionally, aligned structures wick considerably faster than clustered structures. Wicking of these complex structures can’t be reliably predicted by the Lucas-Washburn equation. The gained comprehensive understanding of the relation between process parameters, pore structure and wicking performance allows for the precise tailoring of components for various capillary transport applications.
CH-3:IL02 Ceramic Foams with High Phonon Conductivity
U. BETKE, D. Chazaro Mendoza, M. Scheffler, Otto-von-Guericke-University Magdeburg, Magdeburg, Germany
Open-celled ceramic foams are promising candidates for applications in catalysis and heat transformation processes exploiting their high specific surface area and fluid permeability. A high thermal conductivity of the strut material is mandatory for the transportation of heat generated in the aforementioned processes. Ceramic materials possessing a high thermal conductivity frequently belong to the group of „adamantine compounds“ adopting a crystal structure closely related to diamond. Well known examples are SiC and AlN, but also more exotic representatives like BeO or ZnO belong to the group of adamantine materials. For all of these compounds a strong (significantly covalent) bonding is present, which is responsible for their good thermal properties on the one hand, but also for a low sintering activity, on the other. Cellular structures made of adamantine materials are predominantly limited to SiC. Within the present work the manufacturing of open-celled ceramic foams made of more wurzite-type materials such as AlN and ZnO following the Schwartzwalder sponge replication technique is presented. Their thermal conductivity is investigated as a function of the processing conditions and correlated to their respective strut microstructure.
CH-3:L03 Fabrication of Ultra High Porosity Nanofibrous Ceramic Structures in Zirconia-Titania System
R. YAGER, S. Nealy, R. Day, C. Severino, A. Stanishevsky, University of Alabama at Birmingham, Birmingham, AL, USA
Highly porous nanofibrous ceramics (NFCs) are important for catalysis, filtration, thermal insulation, biomedical scaffolds and other applications. Fabrication of NFCs with tailored microstructure and properties represents the major challenge in their technology. The use of free-surface electrospinning methods can adress those challenges and increase the efficiency of nanofiber production. In this work, nanocrystalline zirconia/titania NFCs with different compositions were prepared from precursor fibers obtained by an uncommon, high-throughput alternating force electrospinning process. Calcination and partial sintering of fibers at temperature up to 1200 °C results in flexible NFC meshes with 0.1–5 mm thickness and porosity up to 99 %. Nanofibers in those meshes had 50–400 nm diameters with the crystalline structure depending on the temperature, heating rate, and composition of precursor. Compression strength of NFC meshes varied from 0.05 to 23 MPa when porosity changed from 99 to 72 %, respectively. The results fit the power law model for the scaling of mechanical properties of open cell porous structures. This study shows that highly porous, nanofibrous zirconia/titania ceramics can be prepared at relatively low sintering temperatures and exhibit good mechanical durability.
CH-3:IL05 Mechanical Properties of Porous Ceramics: New Insights in Fracture and Crack Propagation Behaviour
S. MEILLE, Université de Lyon, INSA de Lyon, Univ. Lyon 1, CNRS, MATEIS UMR5510, Villeurbanne, France
Porous ceramics show multifunctional properties, gathering the chemical stability, the high temperature resistance and the biocompatibility of ceramics with a capacity for gas or liquid filtration, cells access, thermal and acoustic insulation bring by the porous phase. A strong limitation in their use is their low mechanical properties due to the large amount of porosity. This hinders the development of more efficient materials and limits their service life. The presentation will focus on the use of new approaches to characterize the mechanical properties of porous ceramics and eventually optimize their microstructure for a better performance. Such approaches include experimental aspects, such as micromechanical testing (pillars and cantilever beams) and in situ testing, as well as numerical aspects with the use of specific models for the study of fracture and crack propagation simulation in brittle and porous materials. Applications of interest are numerous, from energy production (solid oxide cells, nuclear fuel), to building materials and biomaterials.
CH-4:IL02 Shrinkage and Distribution of Water during Drying of Ceramic Green Bodies
B. NAIT-ALI, S. Oummadi, N. LAURO, A. Alzina, D.S. Smith, University of Limoges, IRCER, UMR CNRS 7315, Limoges, France
For many ceramic processes which involve water, the drying step has to be performed carefully to avoid defects in the final object. The first stage of drying during which shrinkage occurs is the most important to avoid cracks, unwanted body warping and also to ensure a dimensional control of the final object. The microstructure of the porous green body at the end of drying is also related to shrinkage. A method to evaluate shrinkage in two directions, using optical cameras, will be presented and tested with regards to different shaping methods (pressing, extrusion) and different ceramic materials (alumina and clay based ceramics). >From shrinkage curves, useful information can be extracted for a better understanding of the paste behaviour during drying. In particular, information related to the porous microstructure can be deduced as porosity content and orientation of anisometric grains. We will present different situations where the variations in the porous microstructure are revealed by shrinkage curves. As a final part we will present an example to illustrate how optical devices can be used to obtain real time information on shrinkage in a view to implement a feedback loop in the drying process to ensure a uniform shrinkage.
CH-5:IL02 Mass and Heat Transport in Monolithic Ceramic Catalysts during Gas Phase Reactions for Chemical Energy Storage
J. Thöming, K. Kuhlmann, M. Sadeghi, C. Sinn, G. Pesch, University of Bremen, Chemical Process Engineering, Bremen, Germany
In processes such as the conversion of CO2 into fuels the involved reactions are strongly exothermic. High space-time yields that are required for an economic operation, however, easily lead to detrimental or even dangerous hot-spots in fixed-bed reactors. Therefore, an excellent heat transport from the catalyst to the cooled reactor walls is mandatory. The rising relevance of open-cell foams (OCFs) in reaction technology as catalyst supports is attributed to a combination of beneficial properties. Their potentially high surface area determines the achievable catalyst inventory. On the other hand, they still show a continuous solid phase that can be used for creating sufficient heat transport properties. Yet, the higher the catalyst inventory, the lower the effective thermal conductivities and the higher the pressure losses. In order to improve their properties and to make the OCFs well suited as an alternative for packed beds of pellets, a good understanding of the processes within the OCFs is key. Here we demonstrate operando measurement techniques for analysing mass and heat transfer in the opaque OCF catalyst supports placing a tube reactor inside a nuclear magnetic resonance (NMR) device. Results are compared with those obtained by computational fluid dynamics (CFD) simulations. Finally we demonstrate that the choice of both porosity and pore window diameter of OCFs allows to solve the trade-offs. For that purpose we used a pareto-optimisation that adjusts open porosity and window diameter to face the tradeoff problem between high catalyst inventory, low pressure drop and high thermal conductivity. It is shown that, at production scale, commonly reported low pressure drops are only achievable at the price of reduced space-time yield. This is demonstrated using the strongly exothermic conversion of CO2 into methane as case study.
CH-5:L04 Atomistic Simulations of Mechanical Properties of Hybrid Core-shell Ceramic Nanoparticles
K. Kayang, A.N. Volkov, Department of Mechanical Engineering, University of Alabama, Tuscaloosa, AL, USA
Porous materials composed of a scaffold of nanoparticles (NPs) covered by ceramic coatings have a potential to transform engineering practice and help to make critical advances in areas such as the design of ultra-light-weight and multifunctional aerospace structures, energy absorbing and damping, energy generation, conversion and storage, and biomedical applications. The ceramic coatings are used to increase the mechanical integrity and durability of NP films or to change physical and chemical properties of NP interfaces. In this work, we perform a systematic atomistic simulations of hybrid NPs with TiO2 and SiC cores and Al2O3 shells. The atomistic simulations are targeted at revealing fundamental microscopic mechanics of fracture and the load transfer between core/shell NPs. For individual spherical NPs, the simulations of stretching and compression are performed with periodic boundary conditions, enabling modelling of the periodic lattices of NPs. The computational results are obtained for both crystalline and amorphous materials, as well as for overlapping, e.g., as a result of material deposition, and non-overlapping shells around interacting NPs. The key characteristics of the NPs, such as elastic moduli, yield and fracture stress, and toughness are determined.
CH-6:IL02 Porous Ceramics - From Processing to Novel Applications
T. FEY, FAU, Institute of Glass and Ceramics, Erlangen, Germany
Cellular materials offer a wide spectrum of applications such as catalyst support structures, lightweight materials, energy adsorption or energy storage materials. Due to several ways of processing and different materials, a wide range of material properties e.g. thermal conductivity, mechanical strength or damping can be adjusted, measured and verified, with regard to the expected properties. Various techniques for processing porous ceramics and their corresponding composites independent of material are presented. Especially in heterogeneous and homogeneous porous structures and their composites, only global effective material properties can be determined and measured. For example, the knowledge on the predominating influence of the microstructure on the global properties is the key for designing materials with desired properties. To fill this gap and enable a "look-in" a microstructure model derived from µ-CT measurements carried out at certain processing steps can be used as model for FEM-calculations. Combining estimated material properties by experiment with microstructure models offers the possibility to carry out different simulations over different hierarchical levels in order to design the structures for future applications of porous ceramics.