Universität Stuttgart
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Item Open Access Correlation between the microstructure of porous materials and the adsorption properties of H2 and D2(2011) Krkljus, Ivana; Roduner, Emil (Prof. Dr.)One of the most challenging tasks toward the full implementation of the hydrogen based economy is the reversible storage of hydrogen for portable applications. Three main approaches have been investigated to store the hydrogen, storage as a compressed gas or a liquid, or through a direct chemical bond between the hydrogen atom and the material. The alternative approach, the most recently investigated, is the storage of hydrogen at cryogenic conditions. Storage by physisorption within porous adsorbents has particular advantages of complete reversibility, the fast refueling time, the low heat evolution, and above all increased safety. The nature of interaction of hydrogen, deuterium, and gas mixtures with porous adsorbents was exploited by performing thermal desorption spectroscopy (TDS) measurements. This sensitive experimental technique gives qualitative information about the different adsorption sites, which show different desorption temperatures depending on the interaction energy. After an appropriate calibration the amount of gas desorbed may be quantified. To gain a more fundamental insight into the available adsorption sites multiple TDS spectra were recorded, corresponding to different surface coverages (in the pressure range of 1 to 700 mbar), and different heating regimes. Different kind of porous adsorbents, conventional carbon–based materials and novel Metal Organic Framework Materials (MOFs), were used to investigate the hydrogen/deuterium physisorption mechanism. For carbon materials an increase in the hydrogen interaction potential was observed for adsorbents with narrow pore size. The confined geometry, where hydrogen simultaneously interacts with all the surrounding adsorbent walls, strengthens the interaction potential with the adsorbate molecule, thus, maximizing the total van der Waals force on the adsorbate. Crystalline MOFs are a new class of porous materials assembled from discrete metal centers, which act as framework nodes, and organic ligands, employed as linkers. The material properties can be optimized by changing these two main components. Owing to their high porosity, high storage capacity at low temperature, and excellent reversibility kinetics, MOFs have attracted a considerable attention as potential solid–state hydrogen storage materials. This novel class of porous adsorbents has been extensively investigated within this thesis. The greatest challenge for porous adsorbents is to increase the strength of the H2 binding interaction, and bring adsorption closer to RT conditions. Several strategies, aimed at improving hydrogen adsorption potential in MOFs are closely investigated. These strategies comprise the inclusion of open metal sites and the optimization of the pore size and, thus, the adsorption energy by ligand modification. The influence of the coordinatively unsaturated metal centers, liberated by the removal of metal–bound volatile species, has been particularly investigated. As for carbon materials, the H2–MOF interaction potential is especially enhanced in materials with the pore size comparable to the kinetic diameter of the hydrogen molecule. Such effects may result from the overlap of the potential field due to the proximity of the pore wall, which strengthen the interaction potential with the adsorbate molecule. However, smaller pores prevent hydrogen penetration and induce diffusion limitations. Furthermore, the molecular transport in confined pores at low temperatures may be significantly affected by quantum effects.Item Open Access Chiral metamaterials(2016) Eslami, Sahand; Fischer, Peer (Prof. Dr.)Item Open Access Chemically active micromotors(2021) Yu, Tingting; Fischer, Peer (Prof. Dr.)Item Open Access Soft materials for acoustic applications(2022) Choi, Eunjin; Fischer, Peer (Prof. Dr.)Ultrasound finds wide application in imaging and testing because ultrasound can penetrate tissue and is benign. Gaseous microbubbles strongly scatter ultrasound and are therefore used as contrast agents. Ultrasound responsive materials can be used for many industrial and biomedical applications. Ultrasound can also be used to exert forces and manipulate particles solution and biological cells. In this thesis, material systems are developed for three application areas: 1) models of human organs for the quantitative evaluation of surgical procedures with ultrasound; 2) the fabrication of soft objects by assembling polymeric particles with ultrasound and the acoustic hologram; and 3) the characterization of antibubbles as novel contrast agents that can carry a fluid load. Organ phantoms serve as tools in medical fields to train and plan medical procedures. However, current organ phantoms miss important features or are not realistic. Current models tend to possess a Young’s modulus that is much higher than that of tissue. Furthermore, many of the current models do not show the correct contrast in a medical imaging setting. This thesis presents high fidelity organ phantoms that possess the correct elasticity, compliance, optical appearance, and correct ultrasound contrast. One model is developed for cystoscopy (CY) of the bladder. Another phantom for the transurethral resection of the prostate (TURP). The quality of the phantoms is validated by medical practitioners. For CY, the execution time of the medical practitioners is recorded to completely map the inside of the bladder phantom while localizing tumor models that have been embedded in the bladder wall. For TURP, the quality of the resection is compared with ultrasound imaging before and after the surgical simulation. Parameters are defined to quantify the success of the procedure. The phantoms developed as part of this thesis have received high satisfaction scores from medical practitioners. The parameters reflect the experience of the surgeons. In assembling soft matter, one challenge is that existing 3D printing methods are slow. In contrast, the use of ultrasound patterns shaped with a recently invented acoustic hologram allows objects to be built at once. In this thesis, polydimethylsiloxane (PDMS) particles have been assembled into two-dimensional shapes with ultrasound. To fix the assembly, the PDMS has been physically functionalized with an initiator using swelling. Suitable swelling solutions have been determined based on their solubility. The stability of the physisorbed initiators is evaluated, and the functionalized PDMS particles are fixed via photopolymerization after assembly in aqueous polyethylene glycol dimethacrylate (PEG-DMA) solutions. The fabrication steps can be repeated to increase the thickness of structures that are mechanically stable. The antibubble is an emerging ultrasound contrast agent. It has an inverse form to a conventional bubble in that a substance in the core is surrounded by a gaseous layer. The antibubble is acoustically responsive and, compared to conventional microbubbles, can carry a much greater load. In this thesis, the structure of antibubbles is examined. In particular, the volume of the load is quantified, and the amount of gas per bubble is estimated. The stability of the core substance against diffusion is investigated and shown to be stable for over 11 h.