Search

The second mode is the main instability factor in hypersonic flow and shows the characteristics of acoustic disturbance. The stabilization effect of a porous surface on the second mode in a hypersonic boundary layer is studied according to the acoustic disturbance characteristics—including the flow characteristics of the porous surface, the effects of porous parameters, and the effect of mechanics—by direct numerical simulation. A rectangle is the basic structure of pores (slots) with a two-dimensional porous surface. The results show that the porosity (φ) and the number of pores (n) are the primary and secondary primary factors affecting the inhibition effect on the second mode by the porous surface. By influencing the amplitude and phase distribution of the pressure disturbance, the porous surface affects the wall normal transport between the mainstream and the porous surface in the near wall region, and it finally inhibits the growth of the second mode. [ABSTRACT FROM AUTHOR]

Doping‐free organic light‐emitting diodes (OLEDs) are beneficial to reduce manufacturing costs for mass production, which, however, are limited by the ubiquitous aggregation‐caused quenching (ACQ) effect, especially for thermally activated delayed fluorescence (TADF) emitters with relatively long exciton lifetimes. Herein, two typical TADF emitters pACRS and pACRSO with different sulfur atom valence states are developed, where pACRS with a thioether unit exhibits suppressed intermolecular interaction, alleviated ACQ effect, and higher efficiency in a conventional doping‐free device, in contract to pACRSO with a sulfone unit. It is of interest that comparable efficiencies are achieved for both emitters in doping‐free OLEDs with an ultrathin emission layer (UEML), in which the emitter molecules are deposited onto the surface of the underlying organic layer of host material to form an ultrathin quasi‐host‐guest system with alleviated ACQ effect. Besides, the suppressed trap effect is the key to guarantee the efficiency of UEML‐OLEDs, while different intermolecular interactions of emitters that determine the optimized UEML thickness will affect the efficiency roll‐off. Furthermore, benefiting from the UEML structure, highly efficient doping‐free OLEDs are also successfully achieved for red TADF emitters with stronger intermolecular interactions. Such an UEML strategy meeting efficiency and cost demands holds great promise for doping‐free OLEDs. [ABSTRACT FROM AUTHOR]

Aimed at broadband microwave absorption, we present the linear polarization conversion metasurface (MS) by a novel design of a two-bit matrix-type coding method. The coding patterns are applied to a dielectric substrate and a magnetic substrate. The radar cross section (RCS) reduction curves of different matrices can fully verify the independence and correctness of the matrix-type coding method. As shown in simulation, the RCS reduction of 8 dB by a metal pattern with a dielectric layer has been achieved in the frequency range of 6–15 GHz. When the matrix-type coding pattern is placed in the middle of double-layered magnetic absorbing materials (MMs), a 10 dB RCS reduction can be achieved in the range of 8–16 GHz. Besides, the metal patterns can improve the high frequency reflection loss performance and broaden the absorption bandwidth of MMs almost without changing the thickness. The experimental results can nicely reproduce the simulated results. The correlative RCS reduction mechanism is further explored. This model provides a new idea in the design of coding MS, which implicates a potential application in stealth technology. [ABSTRACT FROM AUTHOR]

Thermally‐activated delayed fluorescence (TADF) emitters are usually constructed with twisted donor‐acceptor (D‐A) frameworks, while studies on the relationship between diverse D‐A structures are still in high demand to achieve high‐performance emitters. Herein, by adopting triphenylamine as electron donor and dibenzo[a,c]phenazine as electron acceptor, three TADF molecules are reported with different frameworks of D‐A (TPZ), D‐A‐D (DPZ) and D‐A‐A (APZ). Theoretical and experimental results demonstrate that different D‐A frameworks play significant effects on photophysical, horizontal dipole ratio, and electroluminescence properties of the TADF molecules. In comparison, the APZ‐OLED device achieves the best performance with a maximum external quantum efficiency of 27.5%, resulting from its low energy gap between the singlet and triplet, effective reverse intersystem crossing, high photoluminescence quantum yield, and horizontal dipole ratio. This work provides an insight into the relationship between efficient TADF emitters and rational molecular design engineering. [ABSTRACT FROM AUTHOR]

Metal-organic framework (MOF) crystals have promising applications in a number of different fields, but the growth of MOF crystals with controllable morphology and high uniformity remains a major challenge. By taking the advantage of the synergistic effects of water molecules in the mixed solvent and polyvinylpyrrolidone (PVP) surfactants, this work demonstrates an approach for the preparation of NH2-MIL-88B(Fe) MOF crystals with both highly customizable size and aspect ratios and high uniformity. The effects of the concentration of water in the mixed solvent and the amount of PVP surfactants on the morphology and size of the MOF crystals were investigated. The morphology and crystalline properties were characterized by scanning electron microscopy and X-ray diffraction techniques. The data showed that through controlling the concentration of water in the solvent and the amount of the PVP surfactant, the size of the MOF crystals can be tuned rather widely, over 344–5.14 μm, while the aspect ratio of the crystals can be varied widely over 1.0–7.1. Possible mechanisms for such high tunability were discussed. As an example of potential applications, the complex permeability and permittivity of MOF-derived composites (25 wt%) were measured over a wide frequency range of 2–18 GHz. The parameters were then used to evaluate the potential for electromagnetic wave absorption. The results indicate that the composite with a thickness of only 1.2 mm exhibits a "− 10 dB" absorption bandwidth of 4.1 GHz and a minimum reflection of − 20 dB. Such bandwidth and reflection properties provide inspiration for future design of lightweight, broadband electromagnetic wave absorbers from the perspective of fine morphology control. [ABSTRACT FROM AUTHOR]