Inspired by the correlation between the propulsion efficiency of a flapping foil propeller and stability of the wake behind it (which leads to the optimal Strouhal number for propulsion), we ...numerically simulated a heaving/pitching foil in energy harvesting regime, and investigated the relation between wake stability and the energy harvesting efficiency. The base flow is computed using a Navier–Stokes algorithm and the stability analysis is performed via the Orr–Sommerfeld equation. The wake is found to be convectively unstable and the frequency of the most unstable mode fw is determined. The case when fw ~ f coincides with maximum energy harvesting efficiency of the system (f is the frequency of foil oscillation), suggesting that flow energy extraction is closely related to efficient evolution of the wake. This occurs at a frequency of f ~ 0.15 (f is normalized by the chord length and the flow speed), under the constraint that there is significant vortex shedding from the leading edge at sufficiently large effective angles of attack. Indeed, this ‘foil–wake resonance’ is usually associated with multi-vortex shedding from the leading edge. Furthermore, detailed examination of energy extractions from the heaving and the pitching motions indicates that near the optimal performance point the average energy extraction from the pitching motion is close to zero. This suggests the feasibility of achieving high-efficient energy harvesting through a simple fully passive system we proposed earlier in which no activation is needed.
Highly efficient and stable blue phosphorescent organic light‐emitting diodes are achieved by employing a step‐wise graded doping of platinum(II) ...9‐(pyridin‐2‐yl)‐2‐(9‐(pyridin‐2‐yl)‐9H‐carbazol‐2‐yloxy)‐9H‐carbazole (PtNON) in a device setting. A device employing PtNON demonstrates a high peak external quantum efficiency (EQE) of 17.4% with an estimated LT70 lifetime of over 1330 h at a brightness of 1000 cd m−2. PtNON is then investigated as a “triplet sensitizer” in an alternating donor–acceptor doped emissive layer to further improve the device emission color purity by carefully managing an efficient Förster resonant energy transfer from PtNON to 2,5,8,11‐tetra‐tert‐butylperylene as a selected acceptor material. Thus, such OLED devices demonstrate an EQE of 16.9% with color coordinates of (0.16, 0.25) and an estimated luminance (LT70) lifetime of 628 h at a high brightness of 1000 cd m−2.
A novel emissive layer is designed to enable a stable blue phosphorescent organic light‐emitting diode with an external quantum efficiency of 17.4% and an operational lifetime over 1300 h at 1000 cd m‐2. The technique is extended to a fluorescent/phosphorescent emitter system and demonstrates a deep‐blue emission with an external efficiency of 15% and an operational lifetime over 600 h at 1000 cd m‐2 with CIE coordinates of (0.16, 0.25).
By using a two-dimensional numerical approach, we investigate the response of a purely passive flapping foil flow energy harvester in a linear shear flow. Our focus is to prove that in such ...conditions the device is still capable of undergoing periodically oscillatory responses induced by flow-induced instability as it does in uniform incoming flows. Our simulations show that this regular and predictable response, essential for reliable energy harvesting, is achieved over a range of geometric and mechanical parameters when the shear rate is relatively small. It disappears when the shear rate is large. Other responses, such as tumbling motions and irregular motions, are also identified and mapped in the parametric space. The performance of the system, characterized by its energy harvesting efficiency (i.e., the percentage of the incoming flow energy flux captured by the system), is found to be comparable to that in the uniform-flow case.
Unprecedented dual aggregation‐induced emission (AIE) behavior based on a steric‐hindrance photochromic system is presented, with incorporation one or two bulky aryl groups, resulting in different ...flexibleness. The dual AIE behavior of open and closed isomers can be explained by restriction of intramolecular rotation (RIR), restriction of intramolecular vibration (RIV), and intermolecular stacking. The large bulky benzothiophene causes restricted rotation, enhancing the emission of open form in solution and weak π–π molecular packing, thereby efficiently enhancing the luminescence performance in the solid state. With incorporation of two large bulky benzothiophene groups, BBTE possesses the most outstanding AIE activity, undergoing highly efficient and reversible off‐to‐on fluorescence in film upon alternating UV and visible light irradiation along with excellent fatigue resistance. The off‐to‐on fluorescent photoswitch is successfully established in super resolution imaging.
Dual AIE behavior: The steric‐hindrance photochromic system efficiently regulates the intramolecular rotational/vibrational magnitude and intermolecular stacking to tune aggregation‐induced emission (AIE) activity. The off‐to‐on fluorescent photoswitch is successfully established in super resolution imaging.
Yellow-emitting YAG:Ce transparent ceramic is recognized as an ideal color converter in high-power blue LEDs and LDs, but the absence of scattering centers in its microstructure leads to a low light ...extraction efficiency and poor light uniformity. Here, taking advantage of the scattering effect and the transparency of YAG:Ce ceramics, Ce-free YAG phase was used as a second component to form a composite with YAG:Ce phosphor. The sintered YAG:Ce-YAG ceramic possessed a high transparency of ∼63 % and a thermal conductivity of 8.9 Wm−1 K−1. Due to its beneficial thermal properties and high external quantum efficiency of 70.2 %, the YAG:Ce-YAG ceramic could be excited under a high blue-laser flux density of up to 9.60 W/mm2 and showed a luminous emittance of 1220 lm/mm2. Due to light scattering arising from the slightly different refractive indices of the two phases, the designed YAG:Ce-YAG ceramic showed better lighting effects than a single-phase transparent YAG:Ce ceramic.
Flexible fiber‐shaped supercapacitors have shown great potential in portable and wearable electronics. However, small specific capacitance and low operating voltage limit the practical application of ...fiber‐shaped supercapacitors in high energy density devices. Herein, direct growth of ultrathin MnO2 nanosheet arrays on conductive carbon fibers with robust adhesion is exhibited, which exhibit a high specific capacitance of 634.5 F g−1 at a current density of 2.5 A g−1 and possess superior cycle stability. When MnO2 nanosheet arrays on carbon fibers and graphene on carbon fibers are used as a positive electrode and a negative electrode, respectively, in an all‐solid‐state asymmetric supercapacitor (ASC), the ASC displays a high specific capacitance of 87.1 F g−1 and an exceptional energy density of 27.2 Wh kg−1. In addition, its capacitance retention reaches 95.2% over 3000 cycles, representing the excellent cyclic ability. The flexibility and mechanical stability of these ASCs are highlighted by the negligible degradation of their electrochemical performance even under severely bending states. Impressively, as‐prepared fiber‐shaped ASCs could successfully power a photodetector based on CdS nanowires without applying any external bias voltage. The excellent performance of all‐solid‐state ASCs opens up new opportunity for development of wearable and self‐powered nanodevices in near future.
Wearable, fiber‐shaped, and all‐solid‐state asymmetric supercapacitors (ASCs) are successfully constructed with a maximum operating voltage of 1.5 V using ultrathin MnO2 nanosheets and graphene on carbon fibers as positive and negative electrode, respectively. The fabricated fiber‐shaped ASCs display excellent bendability and mechanical stability, and possess enough energy to power a CdS nanowire photodetector without applying any external bias voltage.
Absorbers with lightweight, low filler loading and broad absorption band are highly desirable for electromagnetic wave absorption field. Here, hollow Co1–xS microspheres constructed by nanosheets are ...fabricated via a facile synthetic method based on hydrothermal route. As an efficient wave absorber, the Co1–xS hollow spheres demonstrate excellent microwave absorption performance. With a weight content of only 3 wt%, the maximum reflection loss (RL) can reach as strong as −46.1 dB at 13.92 GHz and its qualified frequency bandwidth (with RL value over −10 dB) remarkably achieves 5.6 GHz, covering 35% of the entire measured bandwidth. In addition, compared with other cobalt sulfides (such as CoS2 and Co9S8), the Co1–xS microspheres with hollow structure exhibit more superior absorption intensity and broader qualified bandwidth. Therefore, this work provides a promising approach for the design and synthesis of hollow Co1–xS microspheres with lightweight and high‐performance microwave absorption.
The hollow Co1–xS microspheres with understanding microwave absorption performance are successfully fabricated through a facile hydrothermal route. The RLmax can reach to −46.1 dB at 13.92 GHz with an ultralow filler loading (3 wt%) and the effective frequency bandwidth is up to 5.6 GHz. Moreover, the possible wave absorption mechanism is also depicted comprehensively in this article.
Laser‐driven white light exhibits exceptional brightness and directionality, making it particularly well‐suited for spotlight applications. Yet, attaining high‐quality white light remains challenging ...due to the sluggish heat dispersion and inconsistent light uniformity of color converters. In this study, an innovative phosphor‐in‐glass (PiG) film architecture is introduced, aiming to achieve superior quality laser‐driven light, where the traditional Y3(Ga,Al)5O12:Ce3+ (YAGG:Ce3+) PiG film is topped with a Al2O3‐in‐glass (AiG) film. The heightened thermal conductivity of the AiG film facilitates quicker dissipation of heat, consequently elevating the luminescence saturation threshold from 14 to 16 W mm−2 and luminous flux from 1243.8 to 1475.9 lm. The increased light scattering attributed to the AiG film also effectively disperses both laser and emitted light, significantly enhancing the angular consistency of color rendering index (Ra). This improvement is evident as the coefficient of variation (Cv) decreases dramatically from 16.69% to 0.25%. Moreover, by integrating CaAlSiN3:Eu2+ (CASN:Eu2+) into the PiG film and fine‐tuning the YAGG:Ce3+ and CASN:Eu2+ ratio, a high‐quality laser‐driven white light achieving an impressive color rendering index (Ra) of 88.7 and offering an adjustable correlated color temperature (CCT) has been developed. This innovation holds tremendous potential as a versatile spotlight solution for both indoor and outdoor settings.
An innovative phosphor‐in‐glass (PiG) film architecture is introduced, aiming to achieve superior quality laser‐driven light, where the traditional PiG film is topped with a Al2O3‐in‐glass film (AiG film). The heat dissipation performance and light uniformity of PiG film are enhanced due to the AiG film, enabling improved luminance and color quality.
In this paper, the zero sequence circulating current in open winding permanent magnet synchronous machine (OW-PMSM) drives with common dc bus is systematically analyzed for the first time. It is ...revealed that the zero sequence circulating current is affected by zero sequence back-electromotive force, cross coupling voltages in zero sequence from the machine side, pulse-width modulation induced zero sequence voltage, and inverter nonlinearity from the inverter side. Particularly, the influences from the cross coupling voltages in zero sequence and parasitic effect of inverter nonlinearity are investigated for the first time in this paper. Then, the synthetic model of the equivalent zero sequence circuit is proposed as well. Each cause is studied independently via analytical modeling, finite element analysis, and experiments. Meanwhile, to tackle this issue, the relevant suppression strategy using frequency adaptive proportional resonant controller is presented and tested on the 3 kW OW-PMSM platform.
Phosphorescent molecular aggregates show promise in realizing efficient and stable organic light‐emitting diodes (OLEDs) operating at high brightness level, which is highly desired for future ...lighting and display applications. Herein, four tetradentate Pd(II) complexes are prepared with judicious ligand design, and their electrochemical and photophysical properties are thoroughly examined. The studies indicate that slight structural changes of ligands can modify the hole and electron transporting capabilities, and alter the horizontal emitting dipole ratios of aggregates in amorphous film, the latter of which are sensitive to the thin‐film deposition conditions including the deposition rate and the choice of the templating layer. An optimized OLED device using Pd3O8‐Py5 aggregates exhibits a peak external quantum efficiency (EQE) of 37.3% and a reduced efficiency roll‐off with high EQEs of 36.0% and 32.5% at 1000 and 10 000 cd m−2, respectively. Moreover, such an efficient device demonstrates a long measured LT95 (time to 95% of the initial luminance) lifetime of over 500 h with an initial brightness of 17 304 cd m−2 corresponding to an estimated LT95 lifetime of 48 246 h at 1000 cd m−2.
A tetradentate Pd(II) complex, i.e., Pd3O8‐Py5, exhibiting efficient aggregate emission is developed through judicious ligand design. The optimized organic light‐emitting diode device using Pd3O8‐Py5 aggregates exhibits a peak EQEair of 37.3% and a peak EQEair+sub of 67.4% with reduced efficiency roll‐off, and a long measured LT95 lifetime (time to 95% of the initial luminance) of over 500 h with an initial brightness of 17 304 cd m−2.
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