Investigation of the cooperative-effects of Lewis- and Brønstedt acids in homogeneously catalyzed OME fuel synthesis by inline-NMR monitoring. - In: RSC Advances, ISSN 2046-2069, Bd. 14 (2024), 21, S. 14942-14948
Via inline-nuclear magnetic resonance measurements, the homogeneously catalyzed poly(oxymethylene dimethyl ether) fuel synthesis using trioxane and dimethoxy methane is investigated. Besides the Brønsted acid (BA) catalyst triflic acid (TfOH) different metal halides are studied as Lewis-acidic (LA) catalysts. Among the used LAs, MgCl2, the weakest based on electronegativity, reveals the highest catalytical activity. Additionally, the influence of the concentration of BA and LA is investigated. An increase in BA concentration leads to an exponential increase of the reaction rate, while increasing the concentration of the LA leads to a volcano plot with its optimum at a LA : BA ratio of 1 : 3. The influence of the LA on the electron density of the intermediate formaldehyde is concluded as the main factor for this behavior.
https://doi.org/10.1039/D4RA00744A
Nanocalorimetry of nanoscaled Ni/Al multilayer films: on the methodology to determine reaction kinetics for highly reactive films. - In: Advanced engineering materials, ISSN 1527-2648, Bd. 0 (2024), 0, 2302279, S. 1-10
Free-standing Ni/Al multilayer films with a planar morphology, a bilayer thickness of 20 nm, and an average composition of Ni50Al50 (at%) deposited by direct current magnetron sputtering are investigated by nanocalorimetry and conventional calorimetry. Both the novel fast differential scanning calorimeter (FDSC) Flash DSC 2+ from Mettler-Toledo (MT) and conventional calorimeter MT DSC 3 are used to cover a range of heating rates from 0.1 to 10^4 K s^−1. A quantitative kinetic study of the interdiffusion and phase reaction sequence is performed via a Kissinger analysis covering five orders of magnitude of heating rates. Using the calorimetric data, the derived apparent activation energies suggest monotonic reaction kinetics over the entire range of heating rates applied. To correct the thermal lag at the highest heating rates with the FDSC for nonadhered free-standing films, a new methodology for its correction is used. Overall, this work extends the application of commercial FDSC to nonadhered films.
https://doi.org/10.1002/adem.202302279
Low insertion loss interposer approach for RF applications based on commercial LTCC tape, alkaline-free glass and printed metallization. - In: Materials research bulletin, ISSN 0025-5408, Bd. 178 (2024), 112902, S. 1-5
Materials with high RF performance are in increasing demand due to the needs of modern telecommunications. Glass and low-temperature co-fired ceramics (LTCCs) are both interesting candidates for the design of complex signal transmission systems. Glass can be processed in large panels, and has suitable structure dimensions for silicon processing, but its multilayer capability is limited. LTCC is a mature technology for complex multilayer assemblies, but the structure dimensions and conductor line resolution are restricted by the need to use screen printing technology. The approach presented here combines the advantages of both technological domains: a thin glass sheet is bonded with no additional material to a LTCC multilayer ceramic and sintered to form a tight joint. Metallization of the glass is created using printable pastes and electroplating. Two fabrication routes are compared: etching of printed thick film metal layers, and semi-additive structuring. The results of RF measurements show a low attenuation per unit length for both types of metallization, indicating that our approach is a promising one for the integration of heterogeneous systems of RF transmission modules.
https://doi.org/10.1016/j.materresbull.2024.112902
Hexagonal ferrites for self-biasing circulators integrated in LTCC microwave modules. - In: Materials research bulletin, ISSN 0025-5408, Bd. 178 (2024), 112878, S. 1-8
Next-generation microwave LTCC modules for satellite communication systems require the integration of passive components including circulators to manipulate electromagnetic signals at high frequency. Substituted M-type hexagonal ferrites (Ba/Sr)ScxFe12−xO19 are applied as self-biasing microwave magnetic materials at frequencies of about 30 GHz. The ferrites are integrated into LTCC multilayer devices as sintered drop-in bulk samples. Sintered ferrites with preferential orientation of grains were fabricated by compaction of powders in a magnetic field and sintering at 1300 ˚C. Alternatively, screen-printed ferrite films were dried in an external magnetic field, and cofired at 900 ˚C. The magnetic texture of the ferrites is characterized using XRD, EBSD, and magnetic measurements. Integrated Y-type circulators were tested and exhibit non-reciprocal behavior. This indicates that hexagonal ferrites are promising candidates for self-biasing circulators embedded in LTCC microwave modules operating at Ka-band frequencies at around 30 GHz.
https://doi.org/10.1016/j.materresbull.2024.112878
Correlation of rare earth coordination and spectral properties in Er3+ doped Na2O-Al2O3-SiO2 glasses with different Al2O3 concentrations by molecular dynamics simulations. - In: Journal of luminescence, ISSN 0022-2313, Bd. 273 (2024), 120676, S. 1-9
The molecular structure of Er2O3 doped Na2O-Al2O3-SiO2 glasses with varying Na2O/Al2O3 ratios is explored via molecular dynamics (MD) simulations using the so-called inherent structure sampling method, which allows the calculation of a large number of local structures of low concentration, as needed to determine the surrounding of low concentration dopants. General structural parameters, including radial distribution functions, coordination numbers and interatomic distances of all network forming and network modifying ions are reported. However, in this work, special attention is devoted to the effect of Al2O3 concentration on the local surrounding of the doped Er3+ ions. It is shown that the Er atoms coordinate 5-6 oxygen ions in their first coordination sphere in the investigated glasses. The Er-O coordination number increases monotonically with increasing Al2O3 concentration and decreasing Na2O/Al2O3 ratio. It is found that the Er atoms are preferably connected to non-bridging oxygen atoms (NBO) in all glasses, even in the peraluminous composition. Additionally, the MD simulation results are compared to the glasses spectral properties that were already investigated in detail by Tanabe and Hanada. The increasing Er-NBO coordination number derived by MD simulations could be correlated with the increased peak splitting of the Er3+ absorption peaks reported by Tanabe and Hanada. Furthermore, a correlation between the Judd-Ofelt parameters published by Tanabe and Hanada and the Er3+ coordination in the glass structure is discussed. It is shown that the Er-O coordination increases with increasing Ω2 parameter as the Al2O3 concentration increases in the glass composition. A correlation of the average overall Er-O coordination number with the symmetry of the local Er site is proposed.
https://doi.org/10.1016/j.jlumin.2024.120676
Impact of single-melamine tautomerization on the excitation of molecular vibrations in inelastic electron tunneling spectroscopy. - In: Nano letters, ISSN 1530-6992, Bd. 24 (2024), 24, S. 7195-7201
Vibrational quanta of melamine and its tautomer are analyzed at the single-molecule level on Cu(100) with inelastic electron tunneling spectroscopy. The on-surface tautomerization gives rise to markedly different low-energy vibrational spectra of the isomers, as evidenced by a shift in mode energies and a variation in inelastic cross sections. Spatially resolved spectroscopy reveals the maximum signal strength on an orbital nodal plane, excluding resonant inelastic tunneling as the mechanism underlying the quantum excitations. Decreasing the probe-molecule separation down to the formation of a chemical bond between the melamine amino group and the Cu apex atom of the tip leads to a quenched vibrational spectrum with different excitation energies. Density functional and electron transport calculations reproduce the experimental findings and show that the shift in the quantum energies applies to internal molecular bending modes. The simulations moreover suggest that the bond formation represents an efficient manner of tautomerizing the molecule.
https://doi.org/10.1021/acs.nanolett.4c00904
NMR relaxation dispersion of liquids adsorbed on modified surfaces of SBA-15 mesoporous silica. - In: The journal of physical chemistry, ISSN 1932-7455, Bd. 128 (2024), 21, S. 8785-8796
The NMR relaxation dispersion of 1H and 2H nuclei in water and alkanes was studied in mesoporous SBA-15 silica in its native state and with modified internal surfaces. By comparison with silica gel of comparable characteristic pore size, a qualitative agreement of the relaxation dispersion was found. In the absence of detectable amounts of paramagnetic centers, intramolecular relaxation is approximated by the model of Reorientations Mediated by Translational Displacements (RMTD), which assumes rigid molecules diffusing along curved surfaces and experiencing long-term memory of their relative orientation due to their polarity. For all liquids, significant relaxation dispersion is found so that the vanishing polarity of alkanes does not allow the assumption of a negligible surface interaction. The difference in dispersion shape between 1H and 2H nuclei, relaxing by dipolar and quadrupolar mechanisms, respectively, allows the reconstruction of the intermolecular contribution to relaxation, which has not yet been studied systematically in porous media. A model based on the relative contributions of intra- and intermolecular interactions as well as hydrogen exchange with OH- and NH2-groups is presented.
https://doi.org/10.1021/acs.jpcc.4c00645
Investigating the suitability of various silver(I) complexes for use in a cyanide-free silver electrolyte. - In: Coatings, ISSN 2079-6412, Bd. 14 (2024), 5, 618, S. 1-16
The suitability of various nitrogen, sulfur, oxygen, and phosphorus compounds as complexing agents in a silver electrolyte was examined by using potentiometric titration under practical conditions. The setup consisted of three electrodes to measure the pH and the activity of the silver ions simultaneously. Different ratios of silver to complexing agent from 1:10 to 1:1 at a constant ionic strength of 0.2 mol/L were investigated. The type of the complexes and their corresponding critical stability constants were evaluated by fitting the measured data using a self-developed algorithm. The pH and Nernst potential curve were calculated for the assumed complexes based on the law of mass action to find the best approximation. The correct definition of the occurring species is challenging and can lead to significant changes in the calculation of stability constants. For this reason, the measured silver potential curves were primarily used for the rating of the complexing agents. An evaluation of the measurements shows that the donor atom of the complexing agent and its ligand field strongly affected the stability and type of the complexes. Only a few complexing agents were found to be suitable for use in the cyanide-free silver electrolyte.
https://doi.org/10.3390/coatings14050618
Miniaturized double-wing ∆E-effect magnetic field sensors. - In: Scientific reports, ISSN 2045-2322, Bd. 14 (2024), 11075, S. 1-12
Magnetoelastic micro-electromechanical systems (MEMS) are integral elements of sensors, actuators, and other devices utilizing magnetostriction for their functionality. Their sensitivity typically scales with the saturation magnetostriction and inversely with magnetic anisotropy. However, large saturation magnetostriction and small magnetic anisotropy make the magnetoelastic layer highly susceptible to minuscule anisotropic stress. It is inevitably introduced during the release of the mechanical structure during fabrication and severely impairs the device’s reproducibility, performance, and yield. To avoid the transfer of residual stress to the magnetic layer, we use a shadow mask deposition technology. It is combined with a free-free magnetoelectric microresonator design to minimize the influence of magnetic inhomogeneity on device performance. Magnetoelectric resonators are experimentally and theoretically analyzed regarding local stress anisotropy, magnetic anisotropy, and the ΔE-effect sensitivity in several resonance modes. The results demonstrate an exceptionally small device-to-device variation of the resonance frequency < 0.2% with large sensitivities comparable with macroscopic ΔE-effect magnetic field sensors. This development marks a promising step towards highly reproducible magnetoelastic devices and the feasibility of large-scale, integrated arrays.
https://doi.org/10.1038/s41598-024-59015-5
Unveiling the nature of atomic defects in graphene on a metal surface. - In: Beilstein journal of nanotechnology, ISSN 2190-4286, Bd. 15 (2024), S. 416-425
Low-energy argon ion bombardment of graphene on Ir(111) induces atomic-scale defects at the surface. Using a scanning tunneling microscope, the two smallest defects appear as a depression without discernible interior structure suggesting the presence of vacancy sites in the graphene lattice. With an atomic force microscope, however, only one kind can be identified as a vacancy defect with four missing carbon atoms, while the other kind reveals an intact graphene sheet. Spatially resolved spectroscopy of the differential conductance and the measurement of total-force variations as a function of the lateral and vertical probe–defect distance corroborate the different character of the defects. The tendency of the vacancy defect to form a chemical bond with the microscope probe is reflected by the strongest attraction at the vacancy center as well as by hysteresis effects in force traces recorded for tip approach to and retraction from the Pauli repulsion range of vertical distances.
https://doi.org/10.3762/bjnano.15.37