Dehydration-driven mass loss from packs of sintering hydrous silicate glass particles

Glass sintering involves the densification of packs of particles and the expulsion of the interparticle pore gas. The pore space begins as a convolute interconnected interparticle network, and ends as distributed isolated bubbles; two configurations that are separated by the percolation threshold. Here, we perform experiments in which (i) the particles are initially saturated in H₂O at 871 K, and (ii) they are then heated non-isothermally at different rates to temperatures in excess of 871 K. In step (ii), H₂O becomes supersaturated and the particles diffusively lose mass as they sinter together. We use thermogravimetry to track the loss of mass with time. We find that the mass loss is initially well predicted by solutions to Fick's second law in spherical coordinates with the appropriate material and boundary conditions. However, as the sintering pack crosses the percolation threshold at a time predicted by sintering theory, we find that the mass loss deviates from simple diffusional solutions. We interpret this to be the result of an increase in the diffusion distance from the particle-scale to the scale of the sintering pack itself. Therefore, we conclude that the open- to closed-system transition that occurs at the percolation threshold is a continuous, but rapid jump for diffusive and other transport properties. We use a capillary Peclet number Pc to parameterize for this transition, such that at low Pc diffusive equilibrium is achieved before the sintering-induced transition to closed system, whereas at high Pcthere is a “diffusion crisis” and disequilibrium may be maintained for longer relative timescales that depend on the system size.     

Spark Discharge Doping—Achieving Unprecedented Control over Aggregate Fraction and Backbone Ordering in Poly(3-hexylthiophene) Solutions

The properties of semiconducting polymers are strongly influenced by their aggregation behavior, that is, their aggregate fraction and backbone planarity. However, tuning these properties, particularly the backbone planarity, is challenging. This work introduces a novel solution treatment to precisely control the aggregation of semiconducting polymers, namely current-induced doping (CID). It utilizes spark discharges between two electrodes immersed in a polymer solution to create strong electrical currents resulting in temporary doping of the polymer. Rapid doping-induced aggregation occurs upon every treatment step for the semiconducting model-polymer poly(3-hexylthiophene). Therefore, the aggregate fraction in solution can be precisely tuned up to a maximum value determined by the solubility of the doped state. A qualitative model for the dependences of the achievable aggregate fraction on the CID treatment strength and various solution parameters is presented. Moreover, the CID treatment can yield an extraordinarily high quality of backbone order and planarization, expressed in UV–vis absorption spectroscopy and differential scanning calorimetry measurements. Depending on the selected parameters, an arbitrarily lower backbone order can be chosen using the CID treatment, allowing for maximum control of aggregation. This method may become an elegant pathway to finely tune aggregation and solid-state morphology for thin-films of semiconducting polymers.     

Photoswitchable Liquid-to-Solid Transition of Azobenzene-Decorated Polysiloxanes

Having external control over fundamental properties of polymers, such as their physical state, is a crucial yet challenging design criterion for smart materials. Liquifying polymers through photochemical events has significantly advanced various research lines. However, the opposite process of solidifying a polymer that is intrinsically in a liquid state reversibly with light is unattained. Herein, the light-controlled liquid-to-solid transition of polysiloxanes is reported, which are decorated with a small number of azobenzene-functionalized ureidopyrimidinone (Azo-UPy) pendants. The UPy moieties toggle between intra- and intermolecular hydrogen bonding via trans→cis photoisomerization of the azobenzene. This transformation on the molecular level leads to the formation of strong supramolecular cross-links, which, in turn, results in the macroscopic solidification of the material. The photoswitching event enables the post-synthetic tailoring of the polymers’ mechanical properties, thus providing an alternative to the addition of plasticizers or hardeners. Moreover, the adhesion strength of the photochromic material increases by a factor of 6 upon exposure to UV light. In situ illumination during rheological measurements reveals the delicate interplay between wavelength dependent penetration depth and photoswitching efficiency. This conceptually new (de)bonding on demand strategy paves the way for creating light-responsive materials with exciting applications in temporal adhesion, recycling, lithography, and material processing.     

Aggregation-Induced Emission in a Flexible Phosphine Oxide and its Zn(II) Complexes—A Simple Approach to Blue Luminescent Materials

Easily accessible blue-emitting materials are in the focus of ongoing research, as they still lack the efficiency and lifetime of their red and green counterparts. The new multidentate phosphine oxide ligands and two respective ZnCl₂ complexes presented here combine a straightforward synthesis with high yields and show interesting luminescent properties. The free ligand exhibits blue luminescence in the crystalline state, but not in amorphous films or diluted solution. In contrast, the Zn(II) complexes shows intense blue luminescence in the crystalline state as well as in amorphous thin films and in solution. Fluorescence lifetime imaging microscopy measurements show luminescence lifetimes of 3–6 ns indicative of fluorescence. By combining the experimental data with quantum chemical calculations, we propose a model where the conformation of the molecule is restricted, either via the crystal environment, aggregation, or the steric fixation by the coordinating central atom, blocking the nonradiative relaxation from the excited into the ground electronic state. However, this nonradiative relaxation is still possible in the gas phase via elongation of a P C bond. These results may provide a general mechanism to explain the luminescence properties in a whole class of organic phosphine oxides.     

Cavitation Erosion Resistance of Additively Manufactured Materials

Cavitation-induced wear, also known as cavitation erosion, can be found in many fluid power components, especially in water hydraulics. Cavitation erosion leads to component damage and might even cause system failure. The resistance of various materials to cavitation erosion when using conventional manufacturing processes has already been investigated in the past. In this work, the effects of additively manufactured materials on the resistance to cavitation erosion are investigated and compared to effects after conventional manufacturing. Also, the influence of the build-up direction of the additively manufactured specimens on cavitation erosion is determined. As the main indicators of cavitation erosion, mass loss and the surface structure are determined for all specimens.     

Multi-Material Strain Mapping with Scanning Reflectance Anisotropy Microscopy

Strain-engineering of materials encompasses significant elastic deformation and leads to breaking of the lattice symmetry and as a consequence to the emergence of optical anisotropy. However, the capability to image and map local strain fields by optical microscopy is currently limited to specific materials. Here, a broadband scanning reflectance anisotropy microscope as a phase-sensitive multi-material optical platform for strain mapping is introduced. The microscope produces hyperspectral images with diffraction-limited sub-micron resolution of the near-normal incidence ellipsometric response of the sample, which is related to elastic strain by means of the elasto-optic effect. Cutting edge strain sensitivity is demonstrated using a variety of materials, such as metasurfaces, semiconductors, and metals. The versatility of the method to study the breaking of the lattice symmetry by simple reflectance measurements opens up the possibility to carry out non-destructive mechanical characterization of multi-material components, such as wearable electronics and optical semiconductor devices.     

Predicting Temperature-Dependent Activity Coefficients at Infinite Dilution Using Tensor Completion

Knowledge of thermodynamic properties of mixtures is essential in many fields of science and engineering. However, the experimental data is usually scarce, so prediction methods are needed. Matrix completion methods have proven to be very successful in predicting thermodynamic properties of binary mixtures. In this approach, the experimental data is organized in a matrix whose rows and columns correspond to the two components, and whose entries indicate the value of the studied thermodynamic property at fixed conditions. In the present work, we extend the concept to tensor completion methods (TCMs). This allows to account for the variation of the studied property depending on the chosen conditions. The feasibility is demonstrated by applying a TCM to predict activity coefficients at infinite dilution. The third dimension of the tensor is used to describe the influence of the temperature. The TCM is shown to yield better predictions than the well-established UNIFAC method. Furthermore, the proposed TCM is able to learn and unveil the physical law describing the temperature dependence of activity coefficients from the scarce experimental mixture data only.     

MO-LCAO-Berechnungen an Polymethinen. IX. Zur Farbigkeit gekoppelter Oxonol- und Cyaninfarbstoffe

MO-LCAO Calculations on Polymethines. IX. On the Chromaticity of Coupled Oxonol and Cyanine Dyes Streptopolymethines display low-energy electronic transitions with a high transition probability and possess, moreover, a high capability to accept or to release electrons. These properties account for the absorption colour of polymethine dyes consisting of two molecular fragments. Scope and limitations of a composite-molecule approach can be recognized by subjecting the electronic wavefunctions to the so-called configuration analysis. Colour-structure relationships are derived from inspection of key orbital interactions between the molecular fragments.