Besser, Benjamin
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Item type:Publication, Fatigue behavior and damage analysis of PIP C/SiC composite(Elsevier Science, 2022-10-01); ; ; ; In this work, we study the fatigue behavior of a C/SiC composite produced by several cycles of polymer infil- tration and pyrolysis (PIP). Fatigue tests were performed with maximum stresses corresponding to 60–90% of the tensile strength of the composite. During the fatigue tests, acoustic emission (AE) monitoring was performed and the measured AE energy was utilized to quantify the damage and distinguish possible damage mechanisms. Most of the fatigue damage in the form of matrix cracking, interface damage and fiber breakage occurs in the first cycle. As loading cycles proceeded, damage in form of matrix crack re-opening and interfacial friction constantly accumulates. Nevertheless, all samples survived the run-out of 1,000,000 cycles. After the fatigue tests, an in- crease of the tensile strength is observed. This phenomenon is associated with the relief of process-induced in- ternal thermal stresses and the weakening of the fiber-matrix interface. In general, the studied material shows very high relative fatigue limit of 90% of its tensile strengthjournal articleBand:42Heft:1370 73 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, The influence of the functional group density on gas flow and selectivity: Nanoscale interactions in alkyl-functionalized mesoporous membranesMesoporous inorganic structures with mean pore diameters of 26 nm are prepared by extrusion based on a yttria stabilized zirconia nanopowder. The sintered capillary membranes serve as model structures to investigate the influence of an alkyl-chain (C16) surface functionalization on the gas diffusion kinetics of argon (Ar), nitrogen (N2) and carbon dioxide (CO2) in mesopores. The density of the C16 alkyl-chains immobilized on the membrane surface has an effect on both, gas flow as well as gas selectivity. For low functional group densities (<4 groups nm−2), the gas flow is reduced without having an effect on the selectivity. In contrast, for high alkyl-chain densities (>4 groups nm−2) the mean distance between the C16-chains is reduced to the order of magnitude of the gas molecules leading to a reduction in gas flow and a significant change of the gas selectivity. The selectivity is found to be influenced depending on the molecular diameter of the gas species, being more evident for CO2 compared to Ar and N2, suggesting a separation mechanism more comparable to molecular sieving than to surface diffusion.journal articleBand:237145 124 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Damage analysis of 2.5D C/C-SiC composites subjected to fatigue loadings(Elsevier {BV}, 2019-03-11); ; ; ; Damage analyses of a ceramic matrix composite during fatigue and quasi-static loads were performed by acoustic emission (A.E.) monitoring. The material studied was a 2.5D C/C-SiC composite produced by chemical vapor infiltration followed by liquid silicon infiltration. The analysis done during the first 200 cycles of a fatigue test showed that the number of A.E. hits is a good parameter for the quantification of damage. Furthermore, the A.E. hit energy was associated with the type of damage. In this sense, the damage developed during the fatigue loading was related to matrix crack initiation, propagation and re-opening, as well as fiber-matrix friction. Quasi-static tests on post-fatigue samples showed that the previous fatigue loadings increased the material`s damage threshold and hindered the development of new damage. Particular attention was given to the sample after 2,000,000 cycles as this sample showed distinct A.E. signals that could be related to fiber debonding.journal articleBand:39Heft:7110 103 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, A comparative experimental study on the deviation of the ideal selectivity in HDTMS-functionalized and untreated ceramic structures with pores in the upper mesoporous range(Elsevier, 2015-07-15); ; ; ; Mesoporous ceramic capillary membranes with mean pore sizes of about 20 nm are prepared as model structures to investigate the influence of an altered surface chemistry on the flow behavior of gases. To modify the membrane surface, a wet chemical silanization process with hexadecyltrimethoxysilane (HDTMS) is used to gain an alkyl-functionalized surface. Structural and surface characterizations show that the surface chemistry is altered without affecting the mean pore diameter. For the non-functionalized membrane, single gas permeation measurements at 20 °C reveal ideal permselectivities which are in good agreement with the Knudsen theory. In contrast, the HDTMS-functionalized membrane shows permselectivities regarding carbon dioxide (CO2) which deviate about 20% from Knudsen theory. The gas permeation measurements further indicate a relative flow enhancement for CO2 in comparison to nitrogen (N2), argon (Ar) and methane (CH4). Adsorption and desorption isotherms of CO2 and N2 at 20 °C show a decreased specific adsorption capacity for both gases, while the adsorption selectivity for CO2/N2 is increased. This indicates a weaker interaction of gas molecules and membrane surface due to HDTMS functionalization. This weaker gas–solid interaction along with the increased adsorption selectivity is proposed as reason for the experimentally observed deviation of the permselectivities from Knudsen theory.journal articleBand:217122 119 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Correction of second-order slip condition for higher Knudsen numbers by approximation of free-molecular diffusionThe computational predictions of channel and pipe flows with classical models and no-slip condition at the wall reach excellent results for lower Knudsen numbers (Kn) only. Linear slip models reach a very good approximation of measurement results over the region of 10−3 < Kn < 10−1. The numerical results of higher-order slip models match experimental data up to Kn ≈ 1. The present work derives an analytical model for the transition from the slip regime to the free-molecular flows by the superposition of diffuse molecular boundary reflection and the molecular diffusion inside the bulk flow. The methodology of the present publication models the mass flow resulting from the molecular diffusion for the approximation of the mass flow in microchannels and micropipes for the regime of molecular mass flows (1 < Kn < 100) in an excellent way. The present model shows good agreement with the former models, measurement data, and direct simulation Monte Carlo results for the complete region from the transitional regime up to free-molecular flow (10−2 < Kn < 102).journal articleBand:32Heft:919 14 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Gas diffusion in functionalized mesoporous membranes(2017-02-07); ; ; This work investigates the fundamentals of gas diffusion in functionalized mesoporous structures with pore diameters of around 20 nm. For this purpose, an extrusion process based on a yttria stabilized zirconia nanopowder is optimized to shape defect-free capillary membranes. The membranes sintered at 1050 degC for 2 h show a highly homogeneous microstructure with an open porosity of around 40% and a monomodal pore size distribution with mean pore diameters between 23 and 26 nm. To investigate the influence of surface functionalizations on the gas flow, functional groups are covalently bond onto the pore walls using a wet-chemical silanization process. Hexadecyltrimethoxysilane, a silane with a C16 alkylchain as functional group, is chosen as functional model silane. After successful surface functionalization, the membranes show a functional group density between 2 and 4 groups nm-2 depending on the silane concentration during functionalization. Structural analysis reveal decreased open porosities (27 %) and slightly smaller mean pore diameters of around 20 nm which indicate a monolayer of immobilized C16-chains. The gas diffusion properties are analyzed via single gas permeation measurements using a setup operating in dead-end mode. Measurements are performed under different temperature conditions (0-80 degC) using nitrogen (N2), argon (Ar), methane (CH4) and carbon dioxide (CO2). Non-functionalized structures show ideal Knudsen diffusion behavior, independent of gas type and temperature. In contrast, the gas permeation of alkyl-functionalized structures is decreasing up to one order of magnitude with increasing alkyl-chain density on the pore walls. Furthermore, the ideal selectivities show an increased deviation from Knudsen theory, having the highest influence on CO2. These deviations are further increased with increasing operating temperature. It is assumed that sterical hinderance due to the long C16-chains on the material surface is responsible for the determined gas flow characteristics. In addition, it is hypothesized that the deviations in ideal selectivity are caused by the difference in molecular size of the gas species and that the increased deviation with increased temperature is caused by the temperature movement of the surface functional groups. The results contribute to the fundamental understanding of gas diffusion in functionalized structures as present in many applications, ranging from gas separation membranes to gas chromatography.doctoral thesis607 217 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Production of ceramic membranes with different pore sizes for virus retention(Elsevier, 2014-11-12); ; ; ; Abstract Porous ceramic capillary membranes made of yttria-stabilised zirconia (YSZ) are presented, which are conditioned for virus filtration by varying the initial YSZ particle size. Compared to polymeric membranes, ceramic membranes offer remarkable advantages for filtration processes as they show excellent chemical, thermal and mechanical stability and can easily be cleaned by backflushing. YSZ powders with different particle sizes (30 nm, 40 nm and 90 nm) are individually and mixed processed by extrusion, dried and finally sintered at 1050 °C for 2 h. The sintered YSZ capillaries are characterised by microstructural analysis including Hg-porosimetry, BET analysis and 3-point bending tests. By increasing the initial YSZ particle size, increased average membrane pore sizes ranging from 24 nm to 146 nm are obtained. Mechanically stable membranes are provided showing high open porosities of ∼45% and ∼36% for capillaries composed of single and mixed YSZ powders, respectively. By increasing the membrane pore size, reduced virus retention capacities in combination with increased water permeate fluxes are achieved. Capillaries made of YSZ-40 nm ensure both, log reduction values (LRV) ≥ 4 for small model bacteriophages MS2 and PhiX174 and high water permeate fluxes (∼30 L/(m2 hbar)), being suitable for sustainable virus filtration as requested by the World Health Organisation (WHO) and the United States Environmental Protection Agency (USEPA). Due to long-term virus filtration for two weeks, membrane pore plugging is successfully avoided by iterative backflushing and relatively high membrane fluxes in combination with requested LRV 4 level fulfilling the virus filter criterion are achieved.journal article109 115