Ingenieurwissenschaften und zugeordnete Tätigkeiten
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- Nanoparticle (11)
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- Additive manufacturing (7)
- KorroPad (7)
- SAXS (7)
- Small-angle X-ray scattering (7)
- Korrosionsschnelltest (6)
- NIR (6)
A combined methodical approach is tested for the first time with respect to the characterization of the inner structure of porous TiO2 layers infiltrated with ruthenium molecular sensitizer as typically used in modern dye sensitized solar cells (DSSC). Their performance is directly linked to the surface area ‘offered’ by the pore Network to the dye. The micrometer thick layer employed was manufactured by screen-printing of a starting TiO2 powder constituted of shape-controlled, bipyramidal anatase nanoparticles (NPs) on FTO/glass substrates. The
analytical methods exploited in our study are Focused Ion Beam (FIB) slicing followed by 3D reconstruction as well as the new approach transmission Kikuchi diffraction (TKD) technology in the scanning electron microscope (SEM). While the first method results in the visualization of the 3D pore network within the TiO2 layer, the second one can be successfully applied to analyze the crystal orientation of grains (i.e. NPs in our case) in nanometer resolution. Moreover, size and shape distribution of the TiO2 NPs within the layer can be extracted.
SEM in transmission mode and atomic force microscopy (AFM) have been used to verify the dimensional data obtained by the new combined methodical approach. Its analytical benefits but also the challenges and limitations
are highlighted.
Die Reduktion des Fahrzeuggewichts ist ein wesentlicher Ansatz zur Ver-ringerung des Energie- und Ressourcenverbrauchs und damit zur Senkung der CO2-Emissionen im Automobilbau. In der Karosserieentwicklung kann der vermehrte Einsatz von Aluminiumwerkstoffen einen bedeutenden Beitrag dazu leisten. Im preissensitiven Umfeld des Karosseriebaus etabliert sich das bei Stahlanwendungen genutzte Widerstandspunktschweißen zunehmend auch für Aluminiumverbindungen. Verfahrensbedingte Herausforderungen, wie verkürzte Elektrodenstandzeiten und mangelnde Kenntnis über den Einfluss von Imperfektionen auf die Festigkeit, begrenzen dennoch die Weiterverbreitung des Verfahrens und stellen die Prozessrobustheit insgesamt in Frage. Im Rahmen des hier vorgestellten Forschungsvorhabens wurden das Auftreten verschiedener Brucharten experimentell untersucht und Prognosefunktionen zur Abschätzung der Tragfähigkeit von Widerstandspunktschweißverbindungen unter verschiedenen Belastungsfällen erstellt. Anschließend wurde der Einfluss von Oberflächenrissen und Rissen in der Schweißlinse auf die Scherzugfestigkeit sowohl experimentell als auch simulativ analysiert.
In den letzten Jahren werden zunehmend Nanopartikel als Füllstoff für Polymere vorgeschlagen und auch erfolgreich in Elastomer-Nanocomposites eingesetzt. In dieser Arbeit wird Multilayergraphen (MLG) als Nanofüllstoff näher untersucht, der sich bereits bei geringen Konzentrationen als effizient erweist. MLG besteht aus nur etwa zehn Graphenlagen.
Chlorbutylkautschuk (CIIR)/MLG-Nanocomposites mit verschiedenen MLG-Gehalten wurden mit Hilfe eines ultraschallunterstützen Mischverfahrens in Lösung hergestellt und auf einem Walzwerk weiterverarbeitet. Das Einmischen von MLG führt zu einer deutlichen Verbesserung der rheologischen und mechanischen Eigenschaften, des Vernetzungsverhaltens sowie der Barrierewirkung gegenüber Gasen. Bereits der Zusatz von 3 phr MLG zu CIIR führt zu einem mehr als zweifach höheren E-Modul und zu einer Reduktion der Permeabilität von O2 und CO2 um 30 %. Höhere Konzentrationen an Nanofüllstoff resultieren in einer weiteren Verbesserung der Eigenschaften der Nanocomposites. Weiterhin zeigten die CIIR/MLG-Nanocomposites auch eine geringere Entflammbarkeit.
Circumventing boundary effects while characterizing epoxy/copper interphases using nanoindentation
(2017)
Characterization of the size and mechanical properties of interphases is essential when designing multicomponent materials. When nanoindentation is used to investigate the size and mechanical properties of an interphase, a common challenge is that the indenter or the stress zone formed around it are often restricted by the reinforcement, making it difficult to distinguish the mechanical property variations caused by the interphase itself from those caused by the boundary effect. In this work, a testing system was developed that allows determining the indent affected zone and accounting for it in the interphase measurements of an epoxy/Cu system. Using finite element analysis, we confirmed the validity of the proposed system. Nanoindentation was used to investigate the Interphase between copper and two different epoxy systems; amine-cured and anhydride-cured. Nanoindentation results showed that a copper layer that is only 10 nm thick still exhibits a constriction effect on the indentations in its vicinity. The amine-cured epoxy did not show any sign of interphase existence using the introduced method. However,
a soft interphase with a thickness of ~1.7 μm was measured on theanhydride-cured epoxy. Furthermore, we show that the proposed system can be used to determine the interphase thickness as well as its relative mechanical properties regardless of the indentation depth.
This system can be further used for investigating other polymer/metal interphases to better understand the factors influencing them, thus helping engineer the interphase size and properties to enhance composite performance.
Plasma polymers and electrospray-ionization (ESI) polymer layers are compared for most efficient adhesion promotion in carbon fiber-epoxy resin composites. The ultra-thin ESI layers (2–30 nm) of commercial poly(acrylic acid) and poly-(hydroxyethylmethacrylate) produce an significant increase of adhesion measured by single-fiber pull out tests.
However, plasma Treatment has also advantages, such as simultaneous activation of the fiber substrate. Chemical structure and composition are rather far from the regular structure of commercial polymers as deposited by ESI processing.
In this in vitro study, the protective qualities of different mouthguard types were examined during small hard object collisions. The aim was to investigate inconclusive aspects of hard inserts, nylon nets, and air spaces as reinforcements in the anterior region and the protection qualities of ethylene vinyl acetate (EVA).
Five different mouthguards with a labial thickness between 2 mm and 11 mm made of materials of varying stiffness were investigated. As a negative control, the same experiments were performed without a mouthguard. Different combinations of EVA and labial inserts ((polyethylene terephthalate glycol-modified [PETG]), nylon mesh, air space) were tested. Using a stainless steel pendulum device, blows of different energy (0.07-2.85 joules) were applied to the center of the crown of a pivoted tooth in a custom-built jaw model. A laser Doppler vibrometer measured the tooth deflection, while an acceleration sensor attached to the pendulum measured the braking accelerations.
Tooth deflection was reduced up to 99.7% compared to no mouthguard, and the braking acceleration was reduced up to 72.2% by increasing the mouthguards' labial thickness in combination with labial inserts of different stiffness and a built-in air space between the front teeth and the mouthguard. The mouthguards made of soft materials (EVA with nylon mesh) showed slightly better protection qualities than the more rigid mouthguards of similar thickness (PETG; P<.05). However, with increasing impact energy, their protective capacities decreased to a greater extent than the stiffer mouthguards.
The combination of increased labial thickness and labial inserts of varying stiffness and eventually an air space offers the best protection capacities for hard, small object collisions.
Control of organic polymorph formation: crystallization pathways in acoustically levitated droplets
(2017)
Theoretical and experimental studies indicate that crystal nucleation can take more complex pathways than expected on the ground of the classical nucleation theory. Among these pathways are the formation of pre-nucleation clusters and amorphous precursor phases. A direct in situ observation of the different pathways of nucleation from solution is challenging since the paths can be influenced by heterogeneous nucleation sites, such as container walls. Here, we provide insights into the crystallization process using the in situ combination of an acoustic levitator, Raman spectroscopy, and X-ray scattering. The contactless sample holder enables the observation of homogeneous crystallization processes and the detection of intermediates and final crystalline forms. We provide evidence for the existence of multiple pathways of nucleation based on the investigation of the crystallization of organic molecules from different solvents. Starting from a diluted solution, a supersaturation is reached during the experiment due to the evaporation of the solvent. The highly supersaturated solution reveals different pathways of crystallization. Depending on the degree of supersaturation either the thermodynamically stable or the metastable crystal form is observed.
Melanised cell walls and extracellular polymeric matrices protect rock-inhabiting microcolonial fungi from hostile environmental conditions. How extracellular polymeric substances (EPS) perform this protective role was investigated by following development of the model microcolonial black fungus Knufia petricola A95 grown as a sub-aerial biofilm. Extracellular substances were extracted with NaOH/formaldehyde and the structures of two excreted polymers studied by methylation as well as NMR analyses. The main polysaccharide (~ 80%) was pullulan, also known as α-1,4-; α-1,6-glucan, with different degrees of polymerisation. Αlpha-(1,4)-linked-Glcp and α-(1,6)-linked-Glcp were present in the molar ratios of 2:1. A branched galactofuromannan with an α-(1,2)-linked Manp main chain and a β-(1,6)-linked Galf side chain formed a minor fraction (~ 20%). To further understand the roles of EPS in the weathering of minerals and rocks, viscosity along with corrosive properties were studied using atomic force microscopy (AFM). The kinetic viscosity of extracellular K. petricola A95 polysaccharides (≈ 0.97 × 10-6 m2 s-1) ranged from the equivalent of 2% (w/v) to 5% glycerine, and could thus profoundly affect diffusion-dominated processes. The corrosive nature of rock-inhabiting fungal EPS was also demonstrated by its effects on the aluminium coating of the AFM cantilever and the silicon layer below.
Statistical ethylene oxide (EO) and propylene oxide (PO) copolymers of different monomer compositions and different average molar masses additionally containing two kinds of end groups (FTD) were investigated by ultra high pressure liquid chromatography under critical conditions (UP-LCCC) combined with electrospray ionization time-of flight mass spectrometry (ESI-TOFMS).
Theoretical predictions of the existence of a critical adsorption point (CPA) for statistical copolymers with a given chemical and sequence distribution1 could be studied and confirmed. A fundamentally new approach to determine these critical conditions in a copolymer, alongside the inevitable chemical composition distribution (CCD), with mass spectrometric detection, is described. The shift of the critical eluent composition with the monomer composition of the polymers was determined. Due to the broad molar mass distribution (MMD) and the presumed existence of different end group functionalities as well as monomer sequence distribution (MSD), Gradient separation only by CCD was not possible. Therefore, isocratic separation conditions at the CPA of definite CCD fractions were developed. Although the various present distributions partly superimposed the separation process, the goal of separation by end group functionality was still achieved on the basis of the additional dimension of ESI-TOF-MS. The existence of HO-H besides the desired allylO-H end group functionalities was confirmed and their amount estimated. Furthermore, indications for a MSD were found by UPLC/MS/MS measurements. This approach offers for the first time the possibility to obtain a fingerprint of a broad distributed statistical copolymer including MMD, FTD, CCD, and MSD.
The mechanochemical synthesis of the zinc N-(phosphonomethyl)glycinate Zn(O₃PCH₂NH₂CH₂CO₂) H₂O is presented. The structure was solved from powder X-ray diffraction (PXRD) data. In the three-dimensional pillared structure, the Zn atoms are coordinated tetrahedrally. In situ investigations of the reaction process with synchrotron PXRD and Raman spectroscopy reveal a two-step process including the formation of an intermediate.
We present a series of remarkable structural transformations for a family of layered metal–organic frameworks (MOFs) in a three-step solid-state reaction cycle. The cycle represents new dynamic behavior of 2D coordination polymers and involves the sequence of reactions: {[Mn2(ina)4(H2O)2]·2EtOH}n (JUK-1) → {(NH4)2[Mn(ina)2(NCS)2]}n·xH2O (JUK-2) → {[Mn2(ina)2(Hina)2(NCS)2]}n (JUK-3) → JUK-1 (Hina = isonicotinic acid), each accompanied by rearrangement of intralayer coordination bonds and each induced by a different external stimulus. In situ investigation of the first step of the cycle by combined synchrotron X-ray diffraction and Raman spectroscopy reveals direct mechanochemical unzipping of JUK-1 bilayers to respective JUK-2 layers with reaction rates dependent on the milling conditions. In contrast, the reverse zipping of JUK-2 layers involves two steps and proceeds through a new MOF (JUK-3) whose structure was elucidated by powder X-ray diffraction. Magnetic measurements confirm conversions of manganese nodes in the reaction cycle. The findings indicate the possibility of developing coordination-based assemblies with large structural responses for use in smart stimuli-responsive systems and sensor technologies.
Semicrystalline polymers have to be described by a three phase model consisting of a mobile amorphous (MAF), a crystalline (CF), and a rigid amorphous fraction (RAF). For nanocomposites based on a semicrystalline polymer the RAF is due to both the crystallites (RAFcrystal) and the filler (RAFfiller). Polymer nanocomposite based on poly(L-lactide) and MgAl layered double hydroxide nanofiller were prepared.
Due to the low crystallization rate of PLA ist crystallization can be suppressed by a high enough cooling rate, and the RAF is due only to the nanofiller. The MAF, CF, and RAF were estimated by Temperature Modulated DSC. For the first time CF, MAF, RAFcrystal, and RAFfiller could be estimated. It was found, that RAFfiller increases linearly with the concentration of the nanofiller for this system. Furthermore, RAFcrystal is only slightly influenced by the presence of the nanofiller.
Kikuchi diffraction patterns can provide fundamental information about the lattice metric of a crystalline phase. In order to improve the possible precision and accuracy of lattice parameter determination from the features observed in Kikuchi patterns, some useful fundamental relationships of geometric crystal-lography are reviewed, which hold true independently of the actual crystal symmetry. The Kikuchi band positions and intersections and the Kikuchi band widths are highly interrelated, which is illustrated by the fact that all lattice plane trace positions of the crystal are predetermined by the definition of only four traces. If, additionally, the projection centre of the gnomonic projection is known, the lattice parameter ratios and the angles between the basis vectors are fixed. A further definition of one specific Kikuchi band width is sufficient to set the absolute sizes of all lattice parameters and to predict the widths of all Kikuchi bands. The mathematical properties of the gnomonic projection turn out to be central to an improved interpretation of Kikuchi pattern data, emphasizing the importance of the exact knowledge of the projection centre.
Non-isothermal heating rate kinetics was applied to two epoxy resin systems. In situ near-infrared (NIR) measurements were taken with a heatable NIR cell which allowed the cure to be monitored by characteristic absorption bands. An autocatalyzed reaction of the nth order was shown to describe the epoxy conversion curves. Differential Scanning Calorimetry (DSC) was used as a complementary method. The kinetic models developed by both NIR and DSC are in good accordance with experimental epoxy conversion in the in situ NIR setup for single and multiple cure temperature ramps.
A linear calibration curve of the characteristic absorption bands of epoxy normalized to aromatic vibrations was introduced. The curing degree of structural epoxies that were cured according to an industrial temperature cure profile was determined by NIR using the calibration curve. The epoxy conversions of the structural components showed good agreement with the experimental in situ NIR. Several degrees of cure for structural specimens were evaluated by NIR and residual reaction enthalpy by DSC. We present the non-destructive NIR spectroscopy as an alternative to determine fast and non-destructive epoxy conversion, particularly suitable for high degrees of cure on structural components.
Five new catalysts are prepared from dibutyltin oxide and catechol (HCa),
2,3-dihydroxynaphthalene (NaCa), 4-tert-butyl catechol (BuCa), 4-cyano
catechol (CyCa), and 4-benzoyl catechol (BzCa), but only BuCa gives useful
results. When benzyl alcohol is used as an initiator, linear chains having
benzyl ester end groups are formed in a slow polymerization process. In
contrast to cyclic or noncyclic dibutyltin bisalkoxides, neat BuCa yields
cyclic poly(l-lactide)s via a fast ring-expansion polymerization. Under certain
conditions, a high-melting crystalline phase (Tm = 191 °C) is obtained.
At 160 °C and short reaction times even-numbered cycles are slightly
prevailing, but, surprisingly, at 120 °C, odd-numbered cycles are predominantly
formed. These results definitely prove that a ring-expansion mechanism
is operating.
L-Lactides were polymerized in bulk at 120 or 160 °C with cyclic dibutyltin catalysts derived from 1,2-dimercaptoethane or 2-mercaptoethanol. Only linear chains having one benzyl ester and one OH-end group were obtained when benzyl alcohol was added. When L-lactides were polymerized with neat dibutyl-2-stanna-1,3-dithiolane, exclusively cyclic polylactides were formed even at 120 °C. The temperature, time and monomer/catalyst ratio (M/C) were varied. These results are best explained by a combination of ring-expansion polymerization and ring-extrusion of cyclic oligo- or polylactides with Elimination of the cyclic catalyst. Neither syntheses of linear polylactides nor of cyclic lactides involved racemization up to 20 h at 160 °C.
L-Lactide is polymerized in bulk at 160 8C either with dibutyltin bis(benzylmercaptide) (SnSBzl), dibutyltin bis(benzothiazole 2-mercaptide) (SnMBT), or with dibutyltin bis(pentafluorothiophenolate) (SnSPF) as catalysts. SnSBzl yields linear polylactides having benzylthio-ester end groups in addition to cyclic polylactides, whereas SnMBT and SnSPF mainly or exclusively yield cyclic polylactides. This finding, together with model reactions, indicates that the SnS catalysts promote a combined ring-opening polymerization and polycondensation process including end-to-end cyclization. SnMBT caused slight racemization (3%–5%), when used at 160 8C. With SnSPF optically pure cyclic poly(L-lactide)s with high-molecular weights can be prepared at 160 8C.
Two classes of polyesters were prepared by irreversible polycondensations.
The dependence of the dispersities on the experimental parameters such as synthetic method, chemical structure, molecular weight and initial monomer concentration was determined. At first it was demonstrated that it is essential for a correct calculation of Mn and Mw to include all oligomers down to the dimers in the evaluation of SEC curves. Furthermore, it was demonstrated for poly(e-caprolactone)s and polylactides that reversible polycondensations and ring-opening polymerizations with equilibration yield identical products.
Finally, the dependence of the dispersity on various experimental Parameters was determined for equilibrated poly(e-caprolactone)s and polylactides.
The additive manufacturing technology of Selective Laser Melting (SLM) experiences a rapid development within an increasing marked of quite different application fields. The properties of SLM materials and structures are influenced by a number of tech-nological parameters such as the metal powder (particle size, homogeneity, cleanliness), the laser tool (power, beam diameter, pulse lengths), the scanning operation (speed, sequence and orientation of melting paths), parameters of the over-all equipment (design and preheating of the base plate, currents and turbulence in the protective gas atmosphere) and, last not least, the hatching strategy including the build-up direction of the structure with respect to the loading direction of the component.
For the perspective use of SLM structures as load carrying, safety-relevant components the knowledge of their mechanical properties is necessary. It is essential to understand these in the context of the manufacturing-related features and at the back-ground of the basic characteristics of metallic materials: crystal lattice, microstructure and material defects. The paper provides an overview on factors which affect the mechanical parameters stiffness, strength, ductility, toughness, fatigue crack propagation and fatigue strength in the context of selective laser melting.
The discussions on the nanoconfinement effect on the glass transition and glassy dynamics phenomena have yielded many open questions. Here, the thickness dependence of the thermal glass transition temperature of thin films of a PVME/PS blend is investigated by ellipsometry. Its thickness dependence was compared to that of the dynamic glass transition (measured by specific heat spectroscopy), and the deduced Vogel temperature (T0). While and T0 showed a monotonous increase, with decreasing the film thickness, the dynamic glass transition temperature () measured at a finite frequency showed a non-monotonous dependence that peaks at 30 nm. This was discussed by assuming different cooperativity length scales at these temperatures, which have different sensitivities to composition and thickness. This non-monotonous thickness dependence of disappears for frequencies characteristic for T0. Further analysis of the fragility parameter, showed a change in the glassy dynamics from strong to fragile, with decreasing film thickness.