Zeitschriftenaufsätze und Buchbeiträge

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Li, Feitao; Tan, Xinu; Flock, Dominik; Qiu, Risheng; Wang, Dong; Schaaf, Peter
Formation of CuO whiskers and facet-controlled oxidation during the oxidation of Au-Cu nanoparticles fabricated by solid-state dewetting. - In: Applied surface science, Bd. 610 (2023), 155547

The fabrication of cupric oxide (CuO) nanowires from Cu particles via thermal oxidation provides a simple and scalable method to produce hierarchical structures. A stress-induced growth mechanism is believed to account for the nanowire formation while a slow oxidation rate is favored to sustain the driving force. Here, CuO whiskers are grown from Au-Cu nanoparticles because the formation of Au-Cu phases decreases the Cu diffusion rate and in turn slows down the oxidation rate. The driving force for the whisker growth is attributed to the compressive stress imposed by the CuO shell on the Au-Cu core, which is induced by the significantdifference in their linear thermal expansion coefficients. The contribution of the compressive stress is proved by the calculation. Moreover, preferred oxidation is observed and it is related to the crystalline structures of different facets existing on the surface of Au-Cu nanoparticles. The more compact the plane, the slower the diffusion rate through the plane, resulting in the formation of thinner CuO on the relevant facet. The results open a cost-effect way to fabricate hybrid nanostructures consisting of Cu-based core-shell nanoparticles attached with CuO whiskers and bring new insights into the oxidation behaviors of Cu on different crystal planes.

Milanova, Margarita; Aleksandrov, Lyubomir; Yordanova, Aneliya; Iordanova, R.; Tagiara, Nagia S.; Herrmann, Andreas; Gao, G.; Wondraczek, Lothar; Kamitsos, Efstratios I.
Structural and luminescence behavior of Eu3+ ions in ZnO-B2O3-WO3 glasses. - In: Journal of non-crystalline solids, ISSN 0022-3093, Bd. 600 (2023), 122006

Structure and luminescence properties of glasses with compositions 50ZnO:40B2O3:10WO3:xEu2O3, 0 ≤ x ≤ 10 mol% were studied using infrared, Raman and photoluminescence spectroscopic techniques. Physical properties like density, molar volume, oxygen molar volume and oxygen packing density of the glasses were also determined. The overall results obtained indicate the efficiency of the 50ZnO:40B2O3:10WO3 glass structure for the luminescence performance of doped Eu3+. The most intense luminescence peak observed at 612 nm and the high integrated emission intensity ratio (R) of the 5D0&flech;7F2/5D0&flech;7F1 transitions at 612 and 590 nm of 5.77 suggest that the glasses are potential materials for red emission. The results are compared to measurements of a glass without WO3 addition (50ZnO:50B2O3:5Eu2O3) and results from other publications of similar glasses.

Link, Steffen; Dimitrova, Anna; Krischok, Stefan; Ivanov, Svetlozar
Electrochemical deposition of silicon in organic electrolytes. - In: Reference module in chemistry, molecular sciences and chemical engineering, (2023)

Electrodeposition is a versatile instrumental technique, already applied in many industrial fields. However, the deposition of silicon and other reactive elements is still challenging and requires further research and improvement. Accomplishing an efficient electrodeposition of silicon at room temperature is very attractive due to the high number of manufacturing technologies that would benefit from this approach. This work provides an overview of the electrochemical approaches for silicon deposition performed in organic electrolytes. The main factors that impact this process are individually discussed and exemplified with appropriately updated literature sources. Furthermore, the previously available research on characterization of electrodeposited silicon containing layers is provided. These studies are presented in the context of better understanding the structure, composition, and functional properties of the deposited silicon material, which may attract the attention of young academic scientists and process engineers.

Gholami-Kermanshahi, Mozhgan; Wu, Yu-Yan; Lange, Günther; Chang, Shih-Hang
Effect of alloying elements (Nb, Ag) on the damping performance of Cu-Al-Mn shape memory alloys. - In: Journal of alloys and compounds, Bd. 930 (2023), 167438, S. 1-11

This study investigates the damping properties of Cu-Al-Mn shape memory alloys (SMAs) with various chemical compositions and the effects of the addition of quaternary alloying elements Ag and Nb on the microstructure, martensitic transformation behavior, and damping capacity of SMAs. Compared to other Cu-12Al-xMn (x = 4-7wt. %) SMAs, Cu-12Al-5Mn has a more significant inherent and intrinsic internal friction (IFPT + IFI) peak above room temperature. The addition of Ag or Nb to Cu-12Al-5Mn reduced the grain size, thereby increasing the hardness of the alloys; however, the damping capacity and temperature of the IFPT + IFI peak decreased simultaneously. The addition of Ag to Cu-12Al-5Mn significantly reduced the damping capacity (IFPT+IFI peak) because of the notable decrease in the amount of transformed martensite. Moreover, the addition of Nb to Cu-12Al-5Mn caused the AlNb3 phase to precipitate, limiting the mobility of the martensite variant interfaces and slightly decreasing the damping capacity (IFPT + IFI peak). Among the Ag- and Nb-doped Cu-12Al-5Mn SMAs, Cu-12Al-5Mn-1Nb showed not only a significantly higher hardness but also a higher IFPT + IFI peak, with tan δ exceeding 0.01 at approximately 50 ˚C.

Mayer, Hannes; Tomaschko, Silvia; Mayer, Thomas; Lange, Günther
New method for evaluating and optimizing transient piston friction and cooling using a high-power laser in motored operation. - In: SAE International journal of engines, ISSN 1946-3944, Bd. 16 (2023), 4, 03-16-04-0025

The input of combustion heat in engines has a major impact on the piston friction and the resulting wear of the piston skirt. The new methodology presented here enables the simulation of combustion heat input during motored operation, and thus a detailed investigation of the piston friction under realistic piston temperature profiles of real engine operation is possible. For this purpose a standardized engine test bench for motored friction evaluations was expanded to include, among other things, a movable high-power diode laser with special defocusing optics. The setup of the test engine is based on the FEV teardown step methodology [1] and has open access to the engine piston from above due to a cylinder head dummy. Thus, the heat input by means of a high-power diode laser into the piston crown can be made. The reduced engine structure also enables a precise and highly accurate evaluation of the piston friction. A previously conducted validation process of the methodology ensures the most accurate possible replication of fired piston temperature profiles. The comparison between the piston temperatures measured in fired operation and those simulated in motored operation for a partial load operating point shows a maximum variance deviation of only 15˚C depending on the measuring point. The new methodology is also used in particular for the evaluation and detection of critical piston friction conditions. Experiments in this context are presented and discussed exemplary by using three measurement series at different operating temperatures and engine speeds. There is a gradual increase in the laser power for each series of measurements and thus in the heat input into the piston. The increase in heat input leads to a significant increase in friction at all operating points due to thermal expansion and the associated decrease reduction in piston clearance. Depending on the operating temperature and the engine speed, a critical piston friction condition is achieved and detected by the level of friction increase. The additional use of ultrasonic sensors and the knock sensor installed as standard makes a simultaneous measurement of the structure-borne sound signals possible. The increase in the acceleration levels of all sensors correlates here with the increase in piston friction. An evaluation of the noise, vibration, and harshness (NVH) measurement in both the frequency range and the crank angle (CA) range shows conspicuous high-frequency excitation levels that occur in the top dead center area. This correlation can be proven for all three measurement series. The results obtained here may open a path to an improved piston cooling strategy in the future.

Borkenhagen, Benjamin; Paszuk, Agnieszka; Knoop, Franz Niklas; Supplie, Oliver; Nandy, Manali; Lilienkamp, Gerhard; Kleinschmidt, Peter; Hannappel, Thomas; Daum, Winfried
Structure and origin of antiphase domains and related defects in thin GaP epilayers on As-modified Si(100). - In: Crystal growth & design, ISSN 1528-7505, Bd. 22 (2022), 12, S. 7040-7049

We study the origin and formation of antiphase domains (APDs) and related defects in 7 nm thin, lattice-matched GaP buffer layers deposited by metal-organic chemical vapor deposition (MOCVD) on well-defined, nearly single-domain, double-layer stepped, low-miscut Si(100) substrates obtained by specific treatment with arsenic. Using dark-field imaging modes in low-energy electron microscopy (LEEM), the minority reconstruction domains of Si(100):As and the APDs of the deposited GaP epilayer are identified, quantified, and compared. We show that residual (2x1)-reconstructed terraces of the minority domain on the Si substrate cause the formation of APDs and that the fraction of the minority domain of the substrate (≅0.07) entails a comparable fraction of APDs in thin GaP epilayers. The topographies of APDs are revealed by atomic force microscopy (AFM) and by scanning tunneling microscopy (STM). We observe two very different APD-related defects in the GaP epilayer, both pinned to residual monolayer steps of the substrate. GaP growth on minority domain terraces with widths in the range of 40-100 nm gives rise to APDs of comparable lateral dimensions. Minority domain terraces of the substrate with widths <20 nm cause the formation of 7-20 nm wide trenches in the GaP layer with rampart-like mounds along their rims. Using nanoscale Auger electron spectroscopy (AES), we provide evidence that these trenches extend through the GaP layer down to the exposed, uncovered Si substrate. We conclude that nucleation of GaP on small minority domain terraces is largely inhibited as most Ga and P atoms deposited on these terraces diffuse across the domain boundary and side walls of emerging trenches to adjacent majority domain terraces where they form the observed mounds. Nucleation of GaP does take place on minority domain terraces with widths ≥40 nm and leads to the growth of APDs.

Neitzel, Benedikt; Puch, Florian
Optical detection of void formation mechanisms during impregnation of composites by UV-reactive resin systems. - In: Journal of composites science, ISSN 2504-477X, Bd. 6 (2022), 11, 351, S. 1-15

During the impregnation of reinforcement fabrics in liquid composite molding processes, the flow within fiber bundles and the channels between the fiber bundles usually advances at different velocities. This so-called “dual-scale flow” results in void formation inside the composite material and has a negative effect on its mechanical properties. Semi-empirical models can be applied to calculate the extent of the dual-scale flow. In this study, a methodology is presented that stops the impregnation of reinforcement fabrics at different filling levels by using a photo-reactive resin system. By means of optical evaluation, the theoretical calculation models of the dual-scale flow are validated metrologically. The results show increasingly distinct dual-scale flow effects with increasing pressure gradients. The methodology enables the measurability of microscopic differences in flow front progression to validate renowned theoretical models and compare simulations to measurements of applied injection processes.

Böttcher, René; Bund, Andreas; Ispas, Adriana
Abscheidung von Aluminiumlegierungen aus ionischen Flüssigkeiten. - In: Galvanotechnik, ISSN 0016-4232, Bd. 113 (2022), 10, S. 1299-1304

Umweltfreundliche Schichten aus Aluminiumlegierungen werden gegenüber Schichten aus bedenklichen Metallen wie Cadmium zunehmend interessanter. Allerdings können sie mittels galvanischer Verfahren nur aus aprotischen Elektrolyten, wie ionischen Flüssigkeiten, abgeschieden werden. Um eine hohe Prozessstabilität zu erzielen, können Multianoden eingesetzt werden.

Gambaro, Sofia; Nascimento, M. Lucia; Shekargoftar, Masoud; Ravanbakhsh, Samira; Sales, Vinicius; Paternoster, Carlo; Bartosch, Marco; Witte, Frank; Mantovani, Diego
Characterization of a magnesium fluoride conversion coating on Mg-2Y-1Mn-1Zn screws for biomedical applications. - In: Materials, ISSN 1996-1944, Bd. 15 (2022), 22, 8245, S. 1-18

MgF2-coated screws made of a Mg-2Y-1Mn-1Zn alloy, called NOVAMag® fixation screws (biotrics bioimplants AG), were tested in vitro for potential applications as biodegradable implants, and showed a controlled corrosion rate compared to non-coated screws. While previous studies regarding coated Mg-alloys have been carried out on flat sample surfaces, the present work focused on functional materials and final biomedical products. The substrates under study had a complex 3D geometry and a nearly cylindrical-shaped shaft. The corrosion rate of the samples was investigated using an electrochemical setup, especially adjusted to evaluate these types of samples, and thus, helped to improve an already patented coating process. A MgF2/MgO coating in the µm-range was characterized for the first time using complementary techniques. The coated screws revealed a smoother surface than the non-coated ones. Although the cross-section analysis revealed some fissures in the coating structure, the electrochemical studies using Hanks’ salt solution demonstrated the effective role of MgF2 in retarding the alloy degradation during the initial stages of corrosion up to 24 h. The values of polarization resistance (Rp) of the coated samples extrapolated from the Nyquist plots were significantly higher than those of the non-coated samples, and impedance increased significantly over time. After 1200 s exposure, the Rp values were 1323 ± 144 Ω.cm2 for the coated samples and 1036 ± 198 Ω.cm2 for the non-coated samples, thus confirming a significant decrease in the degradation rate due to the MgF2 layer. The corrosion rates varied from 0.49 mm/y, at the beginning of the experiment, to 0.26 mm/y after 1200 s, and decreased further to 0.01 mm/y after 24 h. These results demonstrated the effectiveness of the applied MgF2 film in slowing down the corrosion of the bulk material, allowing the magnesium-alloy screws to be competitive as dental and orthopedic solutions for the biodegradable implants market.

Lucero Lucas, Gisella Liliana; Romanus, Henry; Ispas, Adriana; Bund, Andreas
Hollow platinum-gold and palladium-gold nanoparticles: synthesis and characterization of composition-structure relationship. - In: Journal of nanoparticle research, ISSN 1572-896X, Bd. 24 (2022), 12, 245, S. 1-15

Hollow palladium-gold (PdAu) and platinum-gold (PtAu) alloy nanoparticles (NPs) were synthesized through galvanic replacement reactions. PdAu NPs denoted PdAu-99.99 and PdAu-98 were produced using palladium precursors with different purity degree: Na2PdCl4 ≥ 99.99% and Na2PdCl4 98%, respectively. The effect of the addition time of the gold palladium precursor solution on the size of the generated NPs was evaluated. Two types of particles, with a rough and a smooth surface, were identified in the suspensions of PtAu and PdAu NPs by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). The atomic percentage of gold, platinum, palladium, and cobalt (atomic %) in the nanoparticles was determined by energy dispersive X-ray spectroscopy (EDX). PtAu NPs (26-42 nm) contain Pt (41 at%), Au (36 at%), and Co (23 at%). Two groups of hollow palladium gold NPs (30-50 nm) with a different residual cobalt content were produced. PdAu-99.99 NPs consisted of Pd (68 at%), Au (26 at%), and Co (6 at%), whereas PdAu-98 NPs were composed of Pd (70 at%), Au (22 at%), and Co (8 at%). The hollow structure of the NPs was confirmed by EDX line scanning. Selected area electron diffraction analysis (SAED) revealed the formation of PtAu and PdAu alloys and it was used in estimating the lattice parameters, too.