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Paper des Monats
- ja (45)
The layerwise geometry build-up of additive manufacturing (AM) enables the possibility of in-situ process monitoring. The objective is the detection of irregularities during the build cycle, ensuring component quality and process stability. Focus of this work is the visual in-situ monitoring of the process of powder bed fusion with laser beam of metals (PBF-LB/M). Current state of the art visual monitoring systems for PBF-LB/M are limited by low resolution, allowing the detection of gross flaws. In this work a 65 Mpixel high-resolution monochrome camera is integrated into a commercial PBF-LB/M machine enabling a spatial resolution of approx. 17.2 µm/Pixel. The observed inhomogeneities are clustered into directly detectable irregularities, and indirectly detectable irregularities that can be inferred from the surface. In parallel, two different illumination techniques are realized in the process chamber and compared. The impact of the distinct illumination technique, direct light and dark field, on the identification of irregularities is evaluated.
In this letter, we demonstrate the robust presence of an elastic microstructure in a Zr-based metallic glass (MG) with a characteristic length-scale of the order of 100 nm. This length scale increases systematically towards the MG surface in differently sized casts, whereas thermal relaxation homogenizes both the internal length-scale gradient and the magnitude of the elastic fluctuations. Strongest changes during relaxation arise in the stiffest parts of the elastic microstructure. These findings indicate that the elastic microstructure emerges due to cooling constraints and may therefore be a microstructural manifestation of the internal residual stresses that arise during MG-solidification.
An attempt has been made in this work, to observe the influence on alloy aging by the sodium oxide (Na2O) in solar salt (60 wt% NaNO3 + 40 wt% KNO3). The accelerated aging was established by adding Na2O (0.005, 0.07, 0.135 and 0.2 wt%) to the solar salt and their effect on corrosion of 316L stainless steel (SS) at 600 ◦C in that oxide-rich solar salts for 168 h in synthetic air was investigated. Corrosion is significantly more in oxide-rich solar salt compared to pure solar salt. Strikingly, the oxide scale-base metal interface is wavy in solar salt containing 0.005 % Na2O clearly shows the oxide addition to salt melt influences Cr-rich inner oxide layer formation and its selective dissolution at early stage that leads to non-uniform corrosion. Interestingly, with increase of Na2O to 0.07 %, steel corrosion proceeded uniformly by accelerated disintegration of Cr-rich inner layer and subsequent dissolution. Severe scale spallation and weight loss in nitrate melt containing 0.2 % Na2O fostering more rapid corrosion, alarming that substantial tolerance of oxide content in solar salt is ≥ 0.135 % for an acceptable corrosion of 316L SS. Despite preferential dissolution of Cr and scale degradation/spallation with increased oxide content in solar salt, the corrosion layer in all cases comprised of sodium ferrite, and Cr-rich Cr-Fe mixed oxides with the Ni enrichment at the scale-metal interface. Competing processes between oxide scale growth, degradation and dissolution or even spallation has been discussed with an emphasis of Na2O addition to solar salt.
Photophysical and mechanistic studies, the comparison of different emitter classes, and the rational design of the next generation of molecular and nanoscale reporters require quantitative photoluminescence measurements and the reliable determination of the key performance parameter photoluminescence quantum yield (QY), i.e., the number of emitted per absorbed photons. This is of special importance for all photoluminescence applications in the life and material sciences in the UV/vis/NIR/SWIR. To improve the reliability and comparability of photoluminescence and QY measurements across laboratories, pitfalls, achievable uncertainties, and material-specific effects related to certain emitter classes must be explored. Also, suitable protocols and reference materials are needed which have been validated in interlaboratory comparisons for different wavelength regions and transparent and scattering luminophores.[1] Based on absolute and relative photoluminescence measurements of functional dyes and nanomaterials like semiconductor quantum dots and rods, spectrally shifting lanthanide upconversion nanocrystals, perovskites, and YAG:Cer converter materials, reliable methods for determining QY of transparent and scattering luminophores, nonlinear emitters, and solid luminescent nanomaterials have been developed.[2,3] Thereby, material- and method-related uncertainties of relative and absolute QY measurements and achievable uncertainties could be quantified for linear and nonlinear UV/vis/NIR/SWIR emitters and lately for also luminescent and scattering materials and solid phoshors, here in an interlaboratory comparison of three labs utilizing integrating sphere spectroscopy.[4,5] In addition, to provide simple tools for a better comparability of QY measurements, recently, a first set of UV/vis/NIR quantum yield standards has been developed and certified with complete uncertainty budgets.[6] In the following, the outcome of these studies will be presented, thereby addressing common pitfalls and providing recommendations on the performance of reliable QY measurements of linear and non-linear emitters in transparent, scattering, and solid samples.
Reversible polycondensations are polycondensations that include equilibration reactions at any stage of the process. Syntheses of Polyesters in bulk involving transesterification reactions and syntheses of polyamides in bulk involving transamidation are typical examples. In 1950, Jacobson and Stockmayer published a first theory of reversible polycondensations based on experimental studies with aliphatic polyesters.[1,2] They explained the reversibility by the reversible formation of cyclic oligomers and low molar mass polymers from an active chain end (so-called “back-biting”). The formation of cycles via end-to-end cyclization was excluded in agreement with Flory´s theory of irreversible polycondensations.[3] The reinvestigation of the Jacobson-Stockmayer experiments by the authors in combination with theoretical considerations shows that the JS theory is wrong. [4,5] It turned out that the experimental scenario is far from the reality. In real polycondensations, intermolecular equilibration is faster than “back-biting”, and end-to-end cyclization is quite normal. The revised theory of step-growth polymerization predicts that in the ideal case of 100% conversion, all reaction products are cycles, regardless, of whether the step-growth polymerization is reversible or not.
Photophysical and mechanistic studies, the comparison of different emitter classes, and the rational design of the next generation of molecular and nanoscale reporters require quantitative photoluminescence measurements and the reliable determination of the key performance parameter photoluminescence quantum yield (QY), i.e., the number of emitted per absorbed photons. This is of special importance for all photoluminescence applications in the life and material sciences in the UV/vis/NIR/SWIR.
UFP-Emission beim 3D-Druck
(2024)
Desktop-3D-Drucker haben in der letzten Dekade große Popularität in Bildungseinrichtungen, kleinen Unternehmen und Privathaushalten erlangt. Weit verbreitet ist mittlerweile die „Fused Filament Fabrication (FFF)“ Technologie. Hier wird ein thermoplastisches Filament geschmolzen, durch eine Metalldüse extrudiert und anschließend schichtweise so auf ein Druckbett aufgetragen, dass ein 3D-Objekt entsteht. Das Filamentmaterial wird dabei thermisch stark belastet, was zur Emission von Aerosolen sowie flüchtigen organischen Verbindungen (VOC) führt. Dabei werden hauptsächlich ultrafeine Partikel (UFP, dP < 100 nm) freigesetzt werden, die sogar in manchen Fällen im sub-4nm Größenbereich einen signifikanten Anteil ausmachen können (Tang und Seeger 2024).
Die gesundheitliche Relevanz eingeatmeter UFP ist durch sehr viele Studien gut belegt. Während eines i.d.R. mehrstündigen FFF-Druckvorgangs wird ein Anwender mit diesen Luftschadstoffen im Innenraum exponiert, häufig ohne eine Einschätzung des damit verbundenen Risikos zu haben. Die Exposition kann durch technische Faktoren (z.B. Druckerausstattung und -einstellung, Innenraumventilation), aber auch erheblich durch die Filamentauswahl beeinflusst werden. Unser Vorschlag zur Risikominderung besteht in der Auszeichnung emissionsarmer Filamentprodukte mit dem Umweltzeichen „Blauer Engel“, um so den Verbrauchern eine fundierte Auswahl zu ermöglichen. In unserem Projekt entwickeln wir dafür ein kammerbasiertes, standardisierbares und robustes Prüfverfahren zur vergleichenden Messung der Emission aus Filamenten. Mit dem Strangdruck-Verfahren (engl. Strand Printing Method, SPM) wird eine festgelegte Filamentlänge mit konstanter Rate extrudiert und auf dem Druckbett abgelegt, ohne dabei ein 3D-Objekt aufzubauen. Diese Vorgehensweise reduziert den Einfluss experimentell schlecht zu kontrollierender Faktoren sowie die Ausfallquote. Eine detaillierte Beschreibung und die Vorteile von SPM werden in Tang und Seeger (2022) erläutert. SPM wurde bereits für 44 Filamentprodukte aus unterschiedlichen Polymeren und Additiven als Vergleichstest angewendet. Alle Messungen wurden in einer klimatisierten 1 m³-Emissionsprüfkammer durchgeführt. Die Gesamtanzahl der emittierten Partikel (TP) dient als Beurteilungsmaß, in Anlehnung an die Vergabegrundlage DE-UZ-219 des Umweltzeichens Blauer Engel für Laserdrucker. Unter den getesteten Filamentprodukten variiert TP um ca. vier Größenordnungen (1E+9 ≤ TP ≤ 1E+13). Auch innerhalb jeder der untersuchten Polymergruppen variierte TP signifikant. Die Partikelemission kann daher nicht allein nach dem Basispolymer kategorisiert werden. Unsere Resultate belegen, dass die Stärke der Partikelemission eher eine charakteristische und individuelle Eigenschaft eines Filamentproduktes ist. Die Herkunft der Polymere, die herstellerspezifischen Additive sowie die nicht deklarierte Verunreinigungen können einen starken Einfluss auf die Emission haben. Die Auszeichnung von emissionsarmen Filament-produkten verspricht somit eine einfache, aber effektive Maßnahme zu sein, um Verbrauchern eine Minderung des Expositionsrisikos beim FFF-3D-Druck zu ermöglichen.
Data-driven materials design aims to predict and optimize material properties, such as stability and thermal conductivity, which are influenced by vibrational behavior. Approaches like DFT are computationally demanding and have limitations for phonon calculations. Machine learning-driven interatomic potentials (MLIP), like the Gaussian approximation potential (GAP), offer a more efficient alternative.1–8 We developed a Python workflow to automate MLIP generation using the Materials Project database.9 DFT computations, MLIP fitting and benchmark steps are automated.10,11 This approach accelerates phonon calculations and supports testing different data generation strategies and hyperparameters, and further validation12 is planned. Our goal is to provide open-source code and share these potentials.
Resistance to carbonation and chloride migration are critical durability properties in cementitious construction materials. The ingress of CO2 and chlorides can lead to concrete deterioration and reinforcement corrosion, underscoring the importance of ensuring high resistance to these durability-reducing factors. Traditional methods, such as determinating the carbonation coefficient using standard procedures, are time-consuming and resource-intensive. These tests typically require 42 days of preconditioning, followed by 70 days before the final results can be determined using a phenolphthalein test, for a total duration of 112 days. In addition, some test chambers can only accommodate four samples at a time.
In this study, we investigate whether 1H NMR relaxometry can effectively predict not only the carbonation resistance, KAC,3%, but also the chloride migration coefficient, DRCM, and the compressive strength, fc, of concrete, providing a method that could potentially streamline and accelerate the material development process by directing us quickly to potentially well-suited formulations. We performed measurements on non-carbonated fully saturated concrete and mortar samples at 28, 56, and 92 days using the NMR tomograph at BAM (8.9 MHz). Notably, the NMR measurements taken at 28 days showed no significant differences, suggesting that this timeframe is sufficient for meaningful results. The samples analyzed were drill cores, each 70 mm in diameter and approximately 120 mm in height. The NMR features we focused on included the initial amplitude E0, the x and y values of the dominant T2 relaxation time, and the logarithmic mean of the T2 relaxation time distribution. Another key feature was the surface relaxivity, which was determined by comparing the NMR curves with results from mercury intrusion porosimetry. We examined the correlation of these NMR features with the aforementioned durability properties, which were determined in a laboratory on sister samples using standard procedures. No chloride migration tests were performed on the mortar samples, and the carbonation procedure was carried out unter atmospheric conditions.
The preliminary results show that the correlation factors for the NMR features with the carbonation coefficients of concrete were particularly remarkable. The strongest correlations were observed for the dominant relaxation time and the logarithmic mean, with values approaching 1. The preliminary results also indicate that there are high correlations between the compressive strength and amplitude-related features. In contrast, no strong correlations are observed for the DRCM. Although the results related to the mortar samples are still being analyzed, these initial correlations suggest that 1H NMR relaxometry could be a valuable tool for early assessment of material properties. Furthermore, the rapid and non-destructive nature of NMR measurements, requiring only a few minutes per sample, suggests that this technique has the potential to significantly accelerate the process of evaluating durability properties in cementitious materials. This capability also paves the way for the use of NMR features as input for AI-driven predictive models.
The control of stress development in cast ceramics during drying is usually one of critical steps in ceramic processes, which is important also for additive manufacturing technologies using a suspension as feedstock. This work introduces a method based on the cantilever deflection method, to simultaneously quantify the kinetics of solvent evaporation, the shrinkage and the intensity of in-plane stresses developed during drying. Particular attention is given here to the experimental limits of the method and to the optimization of the experimental conditions to suitably measure the intensity of in-planar stress in the coating. The optimized method is applied to
four alumina slurries for the water-based additive manufacturing technology LSD-print. Four stages of drying are identified and discussed in relation with the granulometry and morphology of the alumina ceramic particles.