State Key Laboratory of Traction Power

Abstract Multifunctional micro‐force sensing in one device is an urgent need for the higher integration of the smaller flexible electronic device toward wearable health‐monitoring equipment, intelligent robotics, and efficient human–machine interface. Herein, a novel microchannel‐confined MXene‐based flexible piezoresistive sensor is demonstrated to simultaneously achieve multi‐types micro‐force sensing of pressure, sound, and acceleration. Benefiting from the synergistically confined effect of the fingerprint‐microstructured channel and the accordion‐microstructured MXene materials, the as‐designed sensor remarkably endows a low detection limit of 9 Pa, a high sensitivity of 99.5 kPa −1 , and a fast response time of 4 ms, as well as non‐attenuating durability over 10 000 cycles. Moreover, the fabricated sensor is multifunctionally capable of sensing sounds, micromotion, and acceleration in one device. Evidently, such a multifunctional sensing characteristic can highlight the bright prospect of the microchannel‐confined MXene‐based micro‐force sensor for the higher integration of flexible electronics.

Sensing devices with wearability would open the door to many advanced applications including soft robotics, artificial intelligence, and healthcare monitoring. Here, inspired by the configuration of the human epidermis, we present a flexible three-dimensional (3D) cellular sensor array (CSA) via a one-step thermally induced phase separation method. The CSA was framed by the 3D cellular electret with caged piezoelectric nanoparticles, which was ultrathin (80 μm), lightweight, and highly robust. For biomedical sensing, the 3D-CSA holds a decent pressure sensitivity up to 0.19 V kPa–1 with a response time of less than 16 ms. Owing to its rigid structural symmetry, the 3D-CSA could be identically operated from its both sides. It was demonstrated to successfully measure the human heartbeat, detect the eyeball motion for sleeping monitoring, and tactile imaging. Mimicking the functionalities of the human skin with a self-powered operation feature, the 3D-CSA was expected to represent a substantial advancement in wearable electronics for healthcare.

Abstract The power conversion efficiencies (PCEs) of the solar cells containing metal halide perovskites (MHPs) have rapidly increased and exceeded 25% during the past decade. The photovoltaic properties of these devices are extensively investigated in terms of their microstructures, environmental characteristics, and carrier dynamics, and the MHP structural evolution under high pressure is evaluated. In addition, the energy level structure, electron/hole dynamics, and optical/electronic properties of MHPs with anisotropic crystal structures are examined. However, the correlation between the structural anisotropy and material properties of these perovskites is rarely considered in the literature studies on their high‐pressure behavior. In this progress report, the optical/electronic properties of MHPs with anisotropic structures under thermal, mechanically imposed, and in‐service strains/stresses that have been previously neglected by researchers are summarized.

In this work, we experimentally investigate the impact of water droplets onto soft viscoelastic surfaces with a wide range of impact velocities. Several impact phenomena, which depend on the dynamic interaction between the droplets and viscoelastic surfaces, have been identified and analyzed. At low $\mathrm{We}$, complete rebound is observed when the impact velocity is between a lower and an upper threshold, beyond which droplets are deposited on the surface after impact. At intermediate $\mathrm{We}$, entrapment of an air bubble inside the impinging droplets is found on soft surfaces, while a bubble entrapment on the surface is observed on rigid surfaces. At high $\mathrm{We}$, partial rebound is only identified on the most rigid surface at $\mathrm{We}\ensuremath{\gtrsim}92$. Rebounding droplets behave similarly to elastic drops rebounding on superhydrophobic surfaces and the impact process is independent of surface viscoelasticity. Further, surface viscoelasticity does not influence drop spreading after impact---as the surfaces behave like rigid surfaces---but it does affect drop recoiling. Also, the postimpact drop oscillation on soft viscoelastic surfaces is influenced by dynamic wettability of these surfaces. Comparing sessile drop oscillation with a damped harmonic oscillator allows us to conclude that surface viscoelasticity affects the damping coefficient and liquid surface tension sets the spring constant of the system.

Abstract Low photoluminescence quantum yield (PLQY) and spectra instability, the two most difficult challenges in blue‐emitting CsPbBr x Cl 3− x NCs, have not yet been solved. Quickly controlling the reaction thermodynamics is crucial to enhance crystallinity, thus PLQY and spectra stability, but it has been ignored until now. An ultrafast thermodynamic control (UTC) strategy is designed by utilizing liquid nitrogen to instantaneously freeze the superior crystal lattices of CsPbBr x Cl 3− x NCs formed at high temperature. The average cooling rate exhibits a 33‐fold increase compared to conventional ice‐water cooling (from 1.5 to 50 K s −1 ). This UTC can make the reaction thermodynamic energy of the system lower than the threshold very quickly. Therefore, abrupt termination of further crystal growth can be achieved, which also avoids additional nucleation at low temperature. With the assist of defect passivation, the final blue‐emitting CsPbBr x Cl 3− x NCs exhibit an absolute PLQY of 98%, representing the highest value in Pb‐based blue perovskites to date. More importantly, they exhibit superior spectra instability. This UTC strategy not only represents a new avenue to synthesize perovskite NCs with excellent crystal quality and ultrahigh PLQY, but also provides a good reference to deal with the recognized bottleneck of spectra instability.

The impact of a liquid droplet on a solid surface is one of the most common phenomena in nature and frequently encountered in numerous technological processes. Despite the significant progress on understanding the droplet impact phenomenon over the past century, the impact dynamics, especially the coupling effects between the properties of a liquid and surface wettability on the impact process, is still poorly understood. In this work, we experimentally investigated the impact of viscous droplets on superamphiphobic surfaces, with the viscosity of liquids ranging from 0.89 to 150 mPa s. We showed that an increase in liquid viscosity will slow down the impact process and cause bouncing droplets to rebound lower and fewer times. The critical impact velocity, above which droplets can rebound from the superamphiphobic surface, was found to linearly increase with the liquid viscosity. We also showed that the maximum spreading factor increases with Weber number or Reynolds number but decreases with the liquid viscosity. Scaling analyses based on energy conservation were carried out to explain these findings, and they were found to be in good agreement with our experimental results.

Severe short-pitch rail corrugation was found to have occurred on four types of track on the same metro line. Field investigations found that, even with the same operation conditions, the corrugations had different wavelengths for the different types of track. Impact hammer tap tests were conducted to investigate the dynamic behaviour of the tracks. The test results showed that, in the investigated metro, short-pitch corrugations are associated with the resonance behaviour of the tracks. The test results also showed that the corrugations on the investigated tracks are not caused by torsional vibration due to the wheelsets. Numerical simulations were conducted to identify the resonance behaviour that is could not be observed in the impact hammer tests due to the limitations in the test method. Three-dimensional finite element models for the four types of track were established and they were used to study the dynamic characteristics of the different tracks. The resonance frequencies and modes that are related to the generation of the corrugation were clearly identified in the numerical modelling studies; this further verifies the relationship between the formation of corrugation and the resonance behaviour of the tracks. The effect of a low value of the fastener stiffness on the dissipation of wheel/rail vibration energy was investigated with the help of numerical simulations. Both experimental and numerical results showed that the resonance behaviour of track structures is of great importance in determining the initiation, characteristics and development of the short-pitch corrugation on the investigated tracks.

When a droplet impacts a solid surface, the entrapment of a submillimeter-sized bubble and the emission of a high speed jet can be observed at low impact velocities. In this work, we show that bubble entrapment occurs only on sufficiently hydrophobic surfaces within a narrow range of impact velocities. The bubble is entrapped on hydrophobic surfaces, whereas it is trapped into the top of the droplet on superhydrophobic surfaces. The collapse of the air cavity formed during droplet impact, which is dominated by inertia and influenced by surface wettability, is the cause of the bubble entrapment. The velocity of liquid jets emitted after cavity collapse for drop impact with and without bubble entrapment scales with their sizes according to different power laws, which is explained by simple scaling analyses.

Abstract Suppressing the naturally ultrafast nucleation and growth rates of perovskite nanocrystals is a big challenge to develop high‐performance deeply blue perovskite light‐emitting diodes. Here, a cryogenic temperature thermodynamically suppressed synthetic strategy using liquid nitrogen is designed to obtain ultrasmall CsPbBr 3 quantum dots (QDs; ≈3 nm). Due to its strong confinement effect, the as‐obtained CsPbBr 3 QDs present strong deeply blue emission (≈460 nm) with a high quantum yield value of up to 98%, a large exciton binding energy of 301.6 meV, and excellent spectra stability for 60 d under atmosphere environment. This unprecedented regime indicates that cryogenic temperature can eliminate pre‐existing trap states and suppress the nonradiative process. Besides, the resultant perovskite light‐emitting diodes based on ultrasmall CsPbBr 3 QDs show deeply blue emission (≈ 460 nm) with a Commission Internationale de l'Eclairage (CIE) color coordinate of (0.145, 0.054), better than the blue National Television Standards Committee (NTSC) standard. Evidently, this cryogenic temperature synthetic strategy will pave the way for the large‐scale synthesis of the strongly confined ultrasmall quantum dots systems and open the door for the development of next generation solid‐state lighting and displays.

Photoluminescence tunability is exceedingly crucial to all-inorganic halide perovskite (CsPbX3, X = Cl, Br, I) quantum dots (QDs) for high-resolution display. However, up to now, the wide-range color tunability could only be achieved by varying the halide ratio. A drawback in this approach is the color instability caused by inevitable halide ion exchange. Therefore, it is of great significance to exploit the new train of thought to tune the emission color in a wide range. Herein, we designed a novel exciton energy-transfer strategy by introducing the 1G4 energy level of Tm to construct the intermediate gradient energy levels between the conduction band of the host perovskite and the 4T1 energy level of Mn for efficiently promoting the exciton energy transfer from the host perovskite to Mn. Thus, the emission color could be accurately tuned from green to orange, and the corresponding correlated color temperature changed from 12 400 to 1800 K. Most strikingly, the single-component pure white light QDs with a record photoluminescence quantum yield (PLQY) of 54% were obtained based on this strategy. Additionally, white light-emitting diodes with a color rendering index up to 91 and a CIE color coordinate of (0.33, 0.34) are further fabricated. Therefore, we believe that this novel exciton energy-transfer strategy will possibly open up a new avenue for efficient emission controllability of all-inorganic perovskites, promoting the higher PLQY of single-component white light QDs and facilitating practical application.

In the long-distance operation of the high-temperature superconducting (HTS) magnetic levitation (maglev) systems, the dynamic characteristics of the vehicle are closely related to its security and stationarity, which require in-depth research to ensure its safe operation. Thus, we investigated the dynamic characteristics of the HTS maglev and assessed its safety and stationarity when running on a ring test line. In the experiments, the important parameters related to safety are lateral displacement (LD) and levitation height (LH). Results show that an appropriate low field-cooling height (FCH) is beneficial for its safe operation, and the maximum LD (MLD) should be considered in vehicle designs. Moreover, in other experiments, we tested the vibration acceleration of the HTS maglev vehicle using acceleration sensors and assessed its stationarity according to the Chinese National Standard GB5599-85, which is specifically published to assess the stationarity and security of railway vehicles by the China Ministry of Railways. The stationarity of the HTS maglev vehicle running on the ring test line is of good grade. When the secondary suspension and appropriate measures of vibration reduction are considered, the stationarity will be greatly improved.

Chemical etching method shows potential for large-scale and low-cost processed MXenes but incorporates surface terminations such as F&OH that probably deteriorate the lithium storage characteristics. Herein, we propose that tailoring appropriate surface functionalization and the intrinsic electrical properties can dramatically enhance the lithium storage capability of Ti3C2Tx (T stands for F, OH, and O) MXene materials. By carefully controlling the annealing process, the Ti3C2Tx films possess fewer F&OH elements so that with higher conductivity, they are still freestanding and flexible. Density functional theory computations of the low-F&OH-containing Ti3C2Tx show low-ion-diffusion barrier values of 0.34–0.43 eV and a significant increase of Li adsorption energy by 6–30 times compared to those of high-F&OH-containing Ti3C2Tx, suggesting high Li-ion storage and transfer capabilities can be achieved in low-F&OH-containing Ti3C2Tx MXenes. Dramatically, the heteroatom-controlled Ti3C2Tx MXene films show reversible capacities of 221 mAh g–1 for lithium-ion batteries at a current density of 0.1 C (1 C = 320 mAh g–1), which is the highest up to now for pure Ti-based MXenes. These results demonstrate the importance of surface chemistry and electronic structure of MXene in the lithium storage capability, which provides valuable information on designing high-performance MXene-based materials for energy storage, optoelectronic, thermoelectric, and magnetic applications.

The photoluminescence quantum yields (PLQYs) of all-inorganic halide perovskites in the green and red spectral ranges have approached over 90%, overwhelmingly arousing burgeoning interests for creating a revolution in next-generation high-definition displays. However, obtaining pure blue-emitting perovskites with high PLQYs still remains a challenge. Herein, we designed a novel strategy to pre-optimize CsPbCl3 quantum dots (QDs) using praseodymium(iii) chloride (PrCl3), and then efficient blue-emitting CsPbBrxCl3-x QDs were obtained through halide exchange between the optimized CsPbCl3 and efficient CsPbBr3 QDs. Specifically, the PrCl3 optimization simultaneously and efficiently passivated the surface vacancy defects and appropriately reduced the surface long-chain organic ligands of the CsPbCl3 QDs, synergistically eliminating the deep trap states, and hence considerably suppressing nonradiative recombination. As a result, the radiative recombination rate was enhanced by more than one order of magnitude from 4.3 to 79 μs-1. Benefiting from this, the blue-emitting CsPbBrxCl3-x QDs exhibited an admirable PLQY of up to 89%, which is competitive compared with that of the state-of-the-art red and green-emitting perovskites. This strategy provides a unique understanding regarding the low PLQY of blue-emitting perovskites and an efficient method to boost it, which is especially attractive for constructing efficient blue and white light-emitting diodes.

In this study, a method regarding frame lateral vibration control based on the state feedback of an additional oscillator is proposed, so as to improve the bogie hunting stability. The multi-objective optimisation method (MOOP), with two objective functions of the stability index and control effort, is solved by the NSGA-II algorithm to obtain the feedback gains. The frame lateral vibration control can effectively improve the bogie hunting stability according to the linear and non-linear analysis of a high-speed train bogie, in which a fault of the yaw damper and time delay in the control system are considered. The effect of the oscillator suspension parameters and time delay on the system stability and robustness are analysed. The results show that the damped vibration frequency of the oscillator should be equal to the bogie hunting frequency, but a harder oscillator suspension can be used to improve the hunting critical speed margin of the bogie control system. However, just as how the feeding the frame states back directly, a hard oscillator suspension will lead to instability in the control system at a certain time delay. Therefore, the improvement of bogie hunting stability and reduction of control system stability must be considered when optimising the oscillator parameters. For the 350 km/h train bogie covered in this study, the optimal mass, natural frequency and damping ratio of the additional oscillator are acquired.

, which focuses on their synthetic methods and the positive effects of metal doping on structure, optical properties, morphology control, carrier behavior and related optoelectronic and photovoltaic devices. Finally, we put forward a few opportunities and challenges about the further investigation of metal-doped perovskites, which may help researchers explore new research directions.

3D stretchable electronics attract growing interest due to their new and more complex functionalities compared to 1D or 2D counterparts. Among all 3D configuration designs, a 3D helical structure is commonly used as it can be designed to achieve outstanding stretching ratios as well as highly robust mechanical performance. However, the stretching ratio that mainly focuses on the axis direction hinders its applications. Inspired by hierarchies in a tendon, a novel structural design of hierarchical 3D serpentine-helix combination is proposed. The structural design constructed by a sequence with repeating small units winding in a helical manner around the axis can enable large mechanical forces transferred down to a smaller scale with the dissipation of potentially damaging stresses by microscale buckling, thereby endowing the electronic components made from high-performance but hard-to-stretch materials with large stretchability (≥200%) in x-, y-, or z-axis direction, high structural stability, and extraordinary electromechanical performance. Two applications including a wireless charging patch and an epidermal electronic system are demonstrated. The epidermal electronic system made of several hierarchical 3D serpentine-helix combinations allows for high-fidelity monitoring of electrophysiological signals, galvanic skin response, and finger-movement-induced electrical signals, which can achieve good tactile pattern recognition when combined with an artificial neural network.

Tire–pavement friction plays a key role in traffic safety. With the development of auto vehicle industry, most of the new vehicles are equipped with Anti Braking System (ABS). Therefore, a prediction model representing the braking process of vehicles equipped with ABS is deemed necessary. In this paper, the tire–pavement friction is measured by Dynamic Friction Tester (DFT) and Hand Friction Tester. The surface texture of asphalt pavement is acquired using a recently developed programme named as 2-Dimensional Image Texture Analysis Method (2D-ITAM). Tire–pavement friction at optimum design slip speed corresponding to the maximum tire–pavement friction is calculated with a widely used model. Then a prediction model correlating the tire–pavement friction at optimum design slip speed with the macro-texture and micro-texture of pavement is established using multivariate non-linear regression analysis. This prediction model is validated through laboratory test indicating its effectiveness of predicting the tire–pavement friction. The model is anticipated to be an improved tool which can be considered by practitioners in an optimised asphalt mixture design including the evaluation of skid resistance of pavement.

UV/ozone oxidation provides a simple and efficient method to prepare super-hydrophilic SiO_x films for tribochemistry-induced nanofabrication on Si substrates.

Understanding excitonic behavior in light absorption and recombination process is urgently needed to regulate luminescent properties of two-dimensional (2D) perovskite; however, such information is still unexplored. Here, first, we demonstrate that pure phenethylamine lead bromine (PEA2PbBr4) nanosheets (NSs) possess strong exciton absorption and emission spectrum with high exciton binding energy (up to 310 meV) and prove that free excitons (FEs) play a decisive role in the optical transition process. Then, we design Cd-doped PEA2PbBr4 NSs to induce self-trapped excitons (STEs). Doping with isoelectric Cd naturally induces an impurity-driven lattice distortion and a strong exciton–phonon coupling, resulting in the STEs broadband extrinsic luminescence spectra. Further, the corresponding exciton fluorescent components of FEs and STEs can be effectively distinguished from the emission spectrum through spectral line-shape analysis, revealing the synergistic effect of exciton states in the recombination process. Unambiguously, this work will provide a deeper understanding of the exciton behavior and look forward to regulating exciton composition to obtain ideal luminescence properties.

To explore the effects of holes and other defects on the dynamic mechanical properties of frozen soil under impact loading, frozen soil samples with three types of circular hole defects and different apertures were developed, and a dynamic compression experiment was performed with a split Hopkinson pressure bar (SHPB) device. The results indicated that the existence of holes weakens the dynamic compressive properties of frozen soil. The effects of temperature and holes on the defective samples were evident. The trend observed in the stress–strain curve was consistent with that observed in the samples without holes. The SHPB experiment using frozen soil samples with prefabricated holes was numerically simulated using LS-DYNA software and the Holmquist–Johnson–Cook material model; the simulation results showed good agreement with the experimental results. Using the postprocessing software LS-Prepost, the specific failure mode of the samples with holes was identified. The samples were found to have an X-type shear failure that was consistent with the theoretical research and the same as the failure rule of rock with holes. The dissipation energy per-unit volume was used to represent the energy-dissipation efficiency of the sample in the experiment. As the failure of a sample with holes led to more cracks, the energy-dissipation rate of a sample with holes was higher than that of a sample without holes, and it increased as the aperture increased.