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Item type:Publication, Access status: with files , Conversionless efficient and broadband laser light diffusers for high brightness illumination applications(Nature Publishing Group, 2020-03-18); ; ; ; Laser diodes are efficient light sources. However, state-of-the-art laser diode-based lighting systems rely on light-converting inorganic phosphor materials, which strongly limit the efficiency and lifetime, as well as achievable light output due to energy losses, saturation, thermal degradation, and low irradiance levels. Here, we demonstrate a macroscopically expanded, three-dimensional diffuser composed of interconnected hollow hexagonal boron nitride microtubes with nanoscopic wall-thickness, acting as an artificial solid fog, capable of withstanding ~10 times the irradiance level of remote phosphors. In contrast to phosphors, no light conversion is required as the diffuser relies solely on strong broadband (full visible range) lossless multiple light scattering events, enabled by a highly porous (>99.99%) non-absorbing nanoarchitecture, resulting in efficiencies of ~98%. This can unleash the potential of lasers for high-brightness lighting applications, such as automotive headlights, projection technology or lighting for large spaces.Publicationtype: Journal ArticleScopus© Citations 58 251 268 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Systematically Designed Periodic Electrophoretic Deposition for Decorating 3D Carbon-Based Scaffolds with Bioactive NanoparticlesThe coating of porous scaffolds with nanoparticles is crucial in many applications, for example to generate scaffolds for catalysis or to make scaffolds bioactive. A standard and well-established method for coating surfaces with charged nanoparticles is electrophoresis, but when used on porous scaffolds, this method often leads to a blockage of the pores so that only the outermost layers of the scaffolds are coated. In this study, the electrophoretic coating process is monitored in situ and the kinetics of nanoparticle deposition are investigated. This concept can be extended to design a periodic electrophoretic deposition (PEPD) strategy, thus avoiding the typical blockage of surface pores. In the present work we demonstrate successful and homogeneous electrophoretic deposition of hydroxyapatite nanoparticles (HAn, diameter ≤200 nm) on a fibrous graphitic 3D structure (ultralightweight aerographite) using the PEPD strategy. The microfilaments of the resulting scaffold are covered with HAn both internally and on the surface. Furthermore, protein adsorption assays and cell proliferation assays were carried out and revealed that the HAn-decorated aerographite scaffolds are biocompatible. The HAn decoration of the scaffolds also significantly increases the alkaline phosphatase activity of osteoblast cells, showing that the scaffolds are able to promote their osteoblastic activity.Publicationtype: Journal Article196 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Individual CdS-covered aerographite microtubes for room temperature VOC sensing with high selectivity(2019-09); ; ; ; The synthesis of new nanomaterials with a large surface-to-volume ratio is of high interest for sensing applications, and especially for gas sensors with high performances. In this work, a thin layer of CdS is deposited onto tubular graphitic/aerographite microstructures using RF magnetron sputtering for further integration into sensing devices using a FIB/SEM system. The quality of the deposited layers as well as microstructural features were analyzed by transmission electron microscopy and energy dispersive X-ray spectroscopy. The gas sensing measurements at room temperature demonstrated the excellent sensing properties towards vapors of volatile organic compounds (VOCs), such as ethanol, acetone, 2-propanol and n- butanol. The superior properties were attributed to the nanometer thickness (20–30 nm)and high surface-to-volume ratio of CdS thin layers as a result of the tubular structure of wrinkled Aerographite (AG). This causes a big number of gas sensitive potential barriers between particles, resulting in a high sensor response. This makes mesoporous graphitic/aerographite microtubes ideal construction blocks for the formation of hybrid materials and for their use in gas sensing applications. The presented strategy can be also applied to other materials with high performance gas sensing properties.Publicationtype: Journal Article190 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Theoretical Computational Fluid Dynamics Study of the Chemical Vapor Deposition Process for the Manufacturing of a Highly Porous 3D Carbon Foam(2019-06); ; ; ; Aerographite is a three-dimensional carbon foam with a tetrapodal morphology. The manufacturing of aerographite is carried out in a chemical vapor deposition (CVD) process, based on a zinc oxide (ZnO) template, in which the morphology is replicated into a hollow carbon shell. During the replication process, the template is reduced by the simultaneous formation of the carbon structure. The CVD process is one of the most efficient methods for the manufacturing of various carbon nanostructures, such as graphene or carbon nanotubes (CNTs). Based on the growth mechanism of aerographite, a computational fluid dynamics model is presented for the fundamental investigations of the temperature and flow/microflow behavior during the replication process. This allows a deeper process understanding and further optimizations.Publicationtype: Journal Article131 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Processing, growth mechanism and thermodynamic calculations of carbon foam with a hollow tetrapodal morphology – Aerographite(2019-03-15); ; ; ; B.V. Aerographite is a 3D interconnected carbon foam with a tetrapodal morphology. The synthesis of Aerographite is based on a two-step process: first the production of a zinc oxide (ZnO) template in a flame transport synthesis (FTS) followed by the replication into the carbon structure in a chemical vapour deposition process (CVD). This study presents a growth model of this 3D carbon foam via analyzing the newly formed carbon structure in an interrupted synthesis by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. Moreover, the Gibbs free energy of the occurred replica CVD (rCVD) process, based on the reduction of ZnO and the formation of carbon layers, was calculated. During the CVD process the injected carbon deposits on the surfaces of the ZnO tetrapods, while simultaneously the replication into the carbon structure takes place, as a result of the reduction of ZnO into gaseous zinc and water vapour, which is due to the reaction of ZnO with the hydrogen (H 2 ) from the injected source. This replication of the ZnO template into a carbon structure is based on an epitaxial controlled process combined with a catalytic graphitization, whereby the morphology of the template structure is replicated by the carbon. Furthermore, the influence of the growth process on the arrangement of carbon in layers and formation of defects was explained.Publicationtype: Journal Article106 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Improving gas sensing by CdTe decoration of individual Aerographite microtubes(2019-02-08); ; ; ; Novel gas sensors have been realized by decorating clusters of tubular Aerographite with CdTe using magnetron sputtering techniques. Subsequently, individual microtubes were separated and electrically contacted on a SiO2/Si substrate with pre-patterned electrodes. Cathodoluminescence, electron microscopy and electrical characterization prove the successful formation of a polycrystalline CdTe thin film on Aerographite enabling an excellent gas response to ammonia. Furthermore, the dynamical response to ammonia exposure has been investigated, highlighting the quick response and recovery times of the sensor, which is highly beneficial for extremely short on/off cycles. Therefore, this gas sensor reveals a large potential for cheap, highly selective, reliable and low-power gas sensors, which are especially important for hazardous gases such as ammonia.Publicationtype: Journal Article180 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Biomimetic Carbon Fiber Systems Engineering: A Modular Design Strategy To Generate Biofunctional Composites from Graphene and Carbon Nanofibers(2019-02-06); ; ; ; Carbon-based fibrous scaffolds are highly attractive for all biomaterial applications that require electrical conductivity. It is additionally advantageous if such materials resembled the structural and biochemical features of the natural extracellular environment. Here, we show a novel modular design strategy to engineer biomimetic carbon fiber-based scaffolds. Highly porous ceramic zinc oxide (ZnO) microstructures serve as three-dimensional (3D) sacrificial templates and are infiltrated with carbon nanotubes (CNTs) or graphene dispersions. Once the CNTs and graphene coat the ZnO template, the ZnO is either removed by hydrolysis or converted into carbon by chemical vapor deposition. The resulting 3D carbon scaffolds are both hierarchically ordered and free-standing. The properties of the microfibrous scaffolds were tailored with a high porosity (up to 93%), a high Young's modulus (ca. 0.027-22 MPa), and an electrical conductivity of ca. 0.1-330 S/m, as well as different surface compositions. Cell viability, fibroblast proliferation rate and protein adsorption rate assays have shown that the generated scaffolds are biocompatible and have a high protein adsorption capacity (up to 77.32 ± 6.95 mg/cm3) so that they are able to resemble the extracellular matrix not only structurally but also biochemically. The scaffolds also allow for the successful growth and adhesion of fibroblast cells, showing that we provide a novel, highly scalable modular design strategy to generate biocompatible carbon fiber systems that mimic the extracellular matrix with the additional feature of conductivity.Publicationtype: Journal Article181 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Tailored crystalline width and wall thickness of an annealed 3D carbon foam composites and its mechanical propertyCarbon nanostructures in form of 3D carbon foams are mainly popular in materials community because of their ultralow densities, variable morphologies, and remarkable properties, etc. One of these foams is Aerographite, which exhibits a tetrapodal interconnected morphology. Similar to other synthetic carbon structures, the lattice defects are formed during the synthesis of Aerographite, which can be healed by a post-thermal treatment. Aerographite shows a property dependency on wall thickness (number of graphitic layers), which affects both, the electrical and mechanical properties. In this study, the wall thickness is tailored by varying of the total reaction time during the replication process. The influence of the thermal treatment of Aerographite on its mechanical performance in an Aerographite-epoxy nanocomposite, by determining the fracture toughness (K 1C ) in three-point bending tests (SEN-3PB), is investigated. An increase of the fracture toughness with increasing wall thickness is observed for untreated Aerographite. The graphitization of Aerographite leads to a reduction of the mechanical properties, by increasing the crystalline width. Consequently, the measured fracture toughness is dependent on the graphitization, the calculated crystalline width and the wall thickness of tubes in the hollow Aerographite tetrapodal network. Finally, based on these relations, a phenomenological mechanical failure model is developed and briefly discussed.Publicationtype: Journal Article204 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Fundamentals of the growth mechanism, tailored properties and applications of 3D hollow carbon foams(2019); The discovery of 1D carbon nanostructures in the 20th century and subsequently of 2D carbon nanostructures in the past century, a competition for possible applications of this material with outstanding physical properties began. Due to the geometric restriction of 1D and 2D carbon structures, a variety of manufacturing techniques have been developed around of the millennium to synthesise 3D carbon structures, which are also referred to as carbon aerogels or carbon foams. Based on their morphology, these structures offer decisive advantages over the existing structures and thus potentially new fields of application arise in the field of energy storage systems and catalysts. The growth mechanism of carbon structures has a property-defining influence on their properties. During their growth, carbon atoms arrange in ideal hexagonal lattices, which can also lead to defects that adversely affect the properties. These defects can be reduced by annealing processes such as a high-temperature treatment. Besides to defects, it was also shown that the number of carbon layers significantly affect the properties. This work concerned with the analysis of the growth mechanism of the 3D carbon structure Aerographite. This carbon foam has a tetrapodal morphology and is distinguished by its high mechanical and electrical properties at a density down to ~2 mg/cm3. Here, the influence of the growth mechanism via ex-situ studies and the tailoring of the properties such as electrical conductivity is investigated. The wall thickness of the tetrapods can be specifically varied by means of a targeted process control during the synthesis of Aerographite in the CVD process. A stepwise increase in the electrical conductivity occurs because of a subsequent temperature treatment of Aerographite, as a function of the temperature and its VI holding time. Additionally, the healing of Aerographite by a thermal treatment and the influence of the graphitisation on the properties of Aerographite is in focus in this work. Furthermore, this study aims a variation of the morphology and the manufacturing process for 3D carbon foams and the identification of possible applications. The focus here is on an application-related morphology and a comparatively simplified manufacturing process of the carbon structure. The developed carbon foam has a hierarchical globular morphology and is produced based on a porous ceramic template. Based on thermodynamic calculations, a wide range of catalytically active materials can be added to the green body, which are remains after the CVD process. Finally, a carbon foam with incorporated catalysts can be used for several applications.Publicationtype: Doctoral Thesis440 1162 - Some of the metrics are blocked by yourconsent settings
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