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Thermal insulation and mechanical resistance play a crucial role for the performance of an intumescent coating. Both properties depend strongly on the morphology and morphological development of the foamed residue. Small amounts (4 wt%) of fiberglass, clay and a copper salt, respectively, are incorporated into an intumescent coating to study their influence on the morphology and Performance of the residues. The bench scale fire tests were performed on 75 x 75 x 2 mm³ coated steel plates according to the standard time–temperature curve in the Standard Time Temperature Muffle Furnace+ (STT Mufu+). It provided information about foaming dynamics (expansion rates) and thermal insulation. Adding the copper salt halved the expansion height, whereas the clay and fiberglass Change the height of the residue only moderately. The time to reach 500 °C was improved by 31% for clay and 15% for the other two fillers. Nondestructive micro computed tomography is used to assess the inner structure of the residues. A transition of the residue from a black, carbonaceous foam with closed cells into an inorganic, residual open cell sponge occurs at high temperatures. This transition is due to a loss of carbon; the change in microstructure is analyzed by scanning electron microscopy. Additional mechanical tests are performed and interpreted with respect to the results of the morphology analysis. Adding clay or copper salt improved the mechanical resistance tested by a factor 4. The additives significantly influence the thickness and foaming Dynamics as well as the inner structure of the residues, whereas their influence on insulation Performance is moderate. In conclusion, different modes of action are observed to achieve similar insulation performance during the fire test.
The aims of the Research Unit „Acting Principles of Nano-Scaled Matrix Additives for Composite Structures“ (DFG FOR 2021) are based on different synergetic pathways. Challenges are to achieve an improved damage tolerance combined with unchanged processability and a proof of the nano-based effect from molecular scale up to structural level. First of all, a comprehensive understanding of the acting mechanisms of nano-scaled ceramic additives onto polymer matrices of continuous fibre reinforced polymer composites with respect to improved matrix dominated properties is in focus. To proof of the nanoscopic and microscopic effects up to structural level; experimental investigations start on the functional correlation between the particle properties and the resulting properties of the epoxy as suspension and in the solid state. This includes tests for the resulting composite structures as well. Along the entire process chain different multi-scale simulations are performed from molecular modelling up to the macroscopic, structural level. The combination of experimental investigations and simulation methods enables a holistic understanding of the acting principles and basic mechanisms.
Specialized techniques based on Scanning Force Microscopy are the basis of our analysis of physicochemical properties of the boehmite nanoparticles and their polymer environment. A surface map of mechanical properties as an input for simulations facilitate a deeper understanding of such composites across all scales. This enables us to understand the macroscopic structure-property relationship and to predict failure mechanisms as well as routes for optimization.
The motivation to examine the influence of friction on surfaces of energetic materials (EM) has diverse backgrounds. On the one hand the very old hot spot theory predicts, that the size of such hot spot could be in the range of a molecule. The initiation of an EM could start by mechanical excitation, i.e. friction, and continues driven by an exothermal chemical reaction. Following such phenomena on the molecular scale with an imaging method such as AFM should enable us to separate several steps of ignition, if there are any. The experiments showed that HMX mainly undergoes a plastic deformation without further consequences. TNP however showed self healing in the wear track after scratching and simultaneously the destruction of a crystal edge outside the wear track. Additionally nanoparticles appear, tribologists call this "third body formation", which are proven to have a different chemical composition as the original TNP. The self healing effect on the surface is verified with experiments on self diffusion of TNP molecules to and fro the free edges of the crystal. The conclusion is that the formation of a hot spot can be shown to consist of several subsequent steps, separated temporally and locally. The goal to excite the thermal decomposition of a whole TNP crystal (nanoexplosion) was yet not reached due to unfavourable conditions related to thermal conductivity and build-up of pressure.
To study the mechanical interface behavior of single-walled carbon nanotubes (CNTs) embedded in a noble metal, we performed CNT-metal pull-out tests with in situ scanning electron microscope experiments. Molecular dynamics (MD) simulations were conducted to predict force-displacement data during pull-out, providing critical forces for failure of the system. In MD simulations, we focused on the influence of carboxylic surface functional groups (SFGs) covalently linked to the CNT. Experimentally obtained maximum forces between 10 and 102 nN in palladium and gold matrices and simulated achievable pulling forces agree very well. The dominant failure mode in the experiment is CNT rupture, although several pull-out failures were also observed. We explain the huge scatter of experimental values with varying embedding length and SFG surface density. From simulation, we found that SFGs act as small anchors in the metal matrix and significantly enhance the maximum forces. This interface reinforcement can lead to tensile stresses sufficiently high to initiate CNT rupture. To qualify the existence of carboxylic SFGs on our CNT material, we performed analytical investigation by means of fluorescence labeling of surface species and discuss the results. With this contribution, we focus on a synergy between computational and experimental approaches involving MD simulations, nano scale testing, and analytics (1) to predict to a good degree of accuracy maximum pull-out forces of single-walled CNTs embedded in a noble metal matrix and (2) to provide valuable input to understand the underlying mechanisms of failure with focus on SFGs. This is of fundamental interest for the design of future mechanical sensors incorporating piezoresistive single-walled CNTs as the sensing element.
A process was developed for graphite particle exfoliation in water to stably dispersed multi-layer graphene. It uses electrohydraulic shockwaves and the functionalizing effect of solution plasma discharges in water. The discharges were excited by 100 ns high voltage pulsing of graphite particle chains that bridge an electrode gap. The underwater discharges allow simultaneous exfoliation and chemical functionalization of graphite particles to partially oxidized multi-layer graphene. Exfoliation is caused by shockwaves that result from rapid evaporation of carbon and water to plasma-excited gas species. Depending on discharge energy and locus of ignition, the shockwaves cause stirring, erosion, exfoliation and/or expansion of graphite flakes. The process was optimized to produce long-term stable aqueous dispersions of multi-layer graphene from graphite in a single process step without requiring addition of intercalants, surfactants, binders or special solvents. A setup was developed that allows continuous production of aqueous dispersions of flake size-selected multi-layer graphenes. Due to the well-preserved sp(2)-carbon structure, thin films made from the dispersed graphene exhibited high electrical conductivity. Underwater plasma discharge processing exhibits high innovation potential for morphological and chemical modifications of carbonaceous materials and surfaces, especially for the generation of stable dispersions of two-dimensional, layered materials.
The paper presents the first complex study of Gamma radiation effects on a low-loss perfluorinated polymer optical fiber (PF-POF) based on Cytop® polymer. Influence of gamma radiation on fiber’s optical, mechanical and climatic performance is investigated. The radiation-induced attenuation (RIA) in the visible and near-infrared region (0.4 μm-1.7 mm) is measured and its origins are discussed. Besides attenuation increase, radiation is also shown to decrease the thermal degradation stability of the fiber and to increase its susceptibility to water. With regard to complex fiber transmission performance upon irradiation, the optimal operation wavelength region of PF-POF-based systems intended for use in Radiation environments is determined to be around 1.1 μm. On the other hand, the investigated fiber holds potential for low-cost RIA-based optical fiber dosimetry applications with sensitivity as high as 260 dBm⁻¹/kGy in the visible region.
In this paper we present results of our recent efforts to understand the mechanical interface behaviour of single-walled carbon nanotubes (CNTs) embedded in metal matrices. We conducted experimental pull-out tests of CNTs embedded in Pd or Au and found Maximum forces in the range 10 - 102 nN. These values are in good agreement with forces obtained from molecular Dynamics simulations taking into account surface functional Groups (SFGs) covalently linked to the CNT material. The dominant failure mode in experiment is a CNT rupture, which can be explained with the presence of SFGs. To qualify the existence of SFGs on our used CNT material, we pursue investigations by means of fluorescence labeling of surface species in combination with Raman imaging. We also report of a tensile test system to perform pull-out tests inside a transmission electron microscope to obtain in situ images of CNT-metal interfaces under mechanical loads at the atomic scale.