Fast Decay Research Articles

Yielding Is an Absorbing Phase Transition with Vanishing Critical Fluctuations.

The yielding transition in athermal complex fluids can be interpreted as an absorbing phase transition between an elastic, absorbing state with high mesoscopic degeneracy and a flowing, active state. We characterize quantitatively this phase transition in an elastoplastic model under fixed applied shear stress, using a finite-size scaling analysis. We find vanishing critical fluctuations of the order parameter (i.e., the shear rate), and relate this property to the convex character of the phase transition (β>1). We locate yielding within a family of models akin to fixed-energy sandpile (FES) models, only with long-range redistribution kernels with zero modes that result from mechanical equilibrium. For redistribution kernels with sufficiently fast decay, this family of models belongs to a short-range universality class distinct from the conserved directed percolation class of usual FES, which is induced by zero modes.

Bivalent metal ion co-doped methylammonium lead tribromide perovskite nanocrystal scintillator for X-ray imaging

Perovskite scintillators with low cost, easy preparation, environmental stability, and high-resolution have become candidates for X-ray imaging applications. Here, based on the calculation results of surface formation energy (Ef) and vacancy defect energy (Evac), a series of co-doped perovskite nanocrystals (NCs) with photoluminescence high quantum yield (PLQY) and fast luminescence decay time had been prepared by doping several bivalent metal ions into the B-site of methylammonium lead tribromide (MAPbBr3). In addition, the scintillator with flexibility was prepared using poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) as the carrier, showing excellent stability and scintillation performance, with a light yield of 21,200 photons/MeV, imaging resolution reached 11.7 lp/mm, and low detection limits of 462 nGyair s−1. Finally, we demonstrated the scintillator characteristic with high-quality imaging results and excellent tolerance under high dose rate X-ray (25 mGy s−1). Therefore, the combination of co-doped nanocrystals with excellent optical properties and PVDF-HFP further paves the way for the development of a scintillator.

Enhancing interfacial kinetics and stability of LiMn2O4 cathode by fabricating an artificial cathode electrolyte interphase of LiNbO3

LiMn2O4 (LMO) cathode material has received continuous attention due to its low price, environmental friendliness, high voltage platform, and high safety. However, the structure distortion is usually caused by the Jahn-Teller effect of Mn3+ in LMO, in which Mn3+ undergoes disproportionation reaction, resulting in the dissolution of Mn element and leading to the problems of fast capacity decay and short cycle life of battery. In this study, a thin (∼3 nm) artificial cathode electrolyte interphase (CEI) of LiNbO3 (LNO) is uniformly covered on the surface of LMO powder (LMO@LNO) by magnetron sputtering method. Compared with the bare LMO, LMO@LNO delivers a higher discharge capacity and better cycling stability. The capacity retention rate of bare LMO/Li cell reaches 80 % in 150 cycles, while LMO@LNO/Li cell can cycle up to 700 cycles, greatly prolonging the cycle life of battery. According to the experimental tests, it is found that the CEI layer of LNO not only improves the interfacial kinetics of electrode, but also inhibits the dissolution of Mn element and side reactions between electrolyte and electrode, thus effectively enhances the rate capability and cycling stability of batteries even at 55 °C and high-voltage of 4.8 V.

Numerical simulation of vortex rings in non-Newtonian fluids

The impulsive viscous flow through an orifice produces vortex rings that self-propagate until total dissipation of the vorticity. This work aims to study numerically such vortex rings for different types of non-Newtonian fluids, including the power-law, Carreau and simplified Phan-Thien-Tanner (PTT) rheological models, with particular focus on the post-formation phase. The simulations were carried out with the finite-volume method, considering small stroke ratios (L/D ≤ 4) and laminar flow conditions (ReG ≤ 103), while parametrically varying shear-thinning, inertia and elasticity. The vortex rings generated in power-law fluids revealed some peculiar features compared to Newtonian fluids, such as a faster decay of the total circulation, a reduction of the axial reach and a faster radial expansion. The behavior in Carreau fluids was found to be bounded between that of power-law and Newtonian fluids, with the dimensionless Carreau number controlling the distance to each of these two limits. The vortex rings in PTT fluids showed the most disruptive behavior compared to Newtonian fluids, which resulted from a combined effect between inertia, elasticity and viscous dissipation. Depending on the Reynolds and Deborah numbers, the dye patterns of the vortex rings can either move continuously forward or unwind and invert their trajectory at some point. Elasticity resists the self-induced motion of the vortex rings, lowering the axial reach and creating disperse patterns of vorticity. Overall, this work shows that the particular non-Newtonian rheology of a fluid can modify the vortex ring behavior typically observed in Newtonian fluids, confirming qualitatively some experimental observations.

Blue shifted phosphorescence (3LE) as compared to charge transfer singlet emission (1CT) in fluorenone-amine dyads under ambient conditions

In pure organics, room temperature phosphorescence (RTP) can be enhanced by effective inter system crossing (ISC, from S1→T1) and fast radiative decay of T1. According to El-Sayed rule, ISC is quite fast when a transition between singlet and triplet involves the change of molecular orbitals i.e. 1(π,π*)→3(n,π*) or 1(n,π*)→3(π,π*). Therefore, conjugated organics having non-bonding electrons from carbonyl group or heteroatoms (O, N, S, Se, etc.) are expected to facilitate strong spin-orbit coupling and faster ISC. In this work, we synthesized metal-free organics composed of fluorenone core-secondary amine (Flu-Ph-CBZ, Flu-PNA and Flu-DNA) for harvesting both singlet and triplet excitons. The carbonyl group in the rigid aromatic planar fluorene core provides non-bonding electrons and acts as an electron acceptor moiety. These donor-acceptor based materials showed blue shifted phosphorescence maxima (T1→S0) as compared to their fluorescence emission (1CT→So), a very uncommon phenomenon. From our photophysical studies, ΔE (1LE-3LE) for Flu-Ph-CBZ, Flu-PNA and Flu-DNA are found to be 0.52, 0.23, and 1.20 eV, respectively. Similarly, ΔE (1CT-3LE) for Flu-Ph-CBZ, Flu-PNA and Flu-DNA are found to be 0.24, 0.19 and 0.53 eV, respectively. Flu-Ph-CBZ shows TADF like emission in Me-THF and phosphorescence in PMMA doped film under ambient conditions. Powder samples of Flu-PNA and Flu-DNA are showing room temperature phosphorescence under air. Phosphorescence lifetimes in powder sample was found to be ∼15 and 56 µs, respectively for Flu-PNA and Flu-DNA.

Combined Charge Extraction by Linearly Increasing Voltage and Time-Resolved Microwave Conductivity to Reveal the Dynamic Charge Carrier Mobilities in Thin-Film Organic Solar Cells.

This article reports a purely experiment-based method to evaluate the time-dependent charge carrier mobilities in thin-film organic solar cells (OSCs) using simultaneous charge extraction by linearly increasing the voltage (CELIV) and time-resolved microwave conductivity (TRMC) measurements. This method enables the separate measurement of electron mobility (μe) and hole mobility (μh) in a metal-insulator-semiconductor (MIS) device. A slope-injection-restoration voltage profile for MIS-CELIV is also proposed to accurately determine the charge densities. The dynamic behavior of μe and μh is examined in five bulk heterojunction (BHJ) OSCs of polymer:fullerene (P3HT:PCBM and PffBT4T:PCBM) and polymer:nonfullerene acceptor (PM6:ITIC, PM6:IT4F, and PM6:Y6). While the former exhibits fast decays of μh and μe, the latter, in particular, PM6:IT4F and PM6:Y6, exhibits slow decays. Notably, the high-performing PM6:Y6 demonstrates both a balanced mobility (μe/μh) of 1.0-1.1 within 30 μs and relatively large CELIV-TRMC mobility values among the five BHJs. The results exhibit reasonable consistency with a high fill factor. The proposed new CELIV-TRMC technique offers a path toward a comprehensive understanding of dynamic mobility and its correlation with the OSC performance.

Study on high-pressure hydrogen leakage characteristics under different shapes of nozzles

In this paper, numerical simulations and experimental methods are utilized to investigate the near-field jet structures and far-field concentration distributions after the release of circular, equilateral triangular, square, and rectangular (AR2, AR4, AR8) nozzles. For all nozzle shapes, the hydrogen concentration along the jet centerline is inversely proportional to the distance from the nozzle. Circular nozzles exhibit the slowest decay of hydrogen concentration along the jet centerline, while rectangular nozzles show the fastest decay. Under the studied pressures, the concentration distributions along the jet centerline after the release of all nozzles are summarized into an equation. For the near-field jet structure, all nozzle except the circular nozzle experience varying degrees of deflection. When the aspect ratio is greater than or equal to 4, the Mach disk cannot be formed. An empirical formula for the position of the shock waves of the rectangular nozzle jet is proposed.

Minimal impact of nitrogen addition on bacterial and fungal communities during fungal necromass decomposition in a subalpine coniferous plantation

Fungal necromass is a vital source of stable carbon and available nitrogen in soils, especially in coniferous forests that are extensively colonized by ectomycorrhizal fungi. Here, we examined changes in the necromass microbial community during a 20-week incubation experiment in a subalpine coniferous plantation. We also explored how the necromass microbial community responded to simulated nitrogen deposition, a global change factor that substantially affects nitrogen-limited coniferous forests. The results showed that necromass microbial community was more dominated by copiotrophic bacteria with high community average rRNA operon copy number (3.1–6.4) and fast-growing fungi (moulds and yeasts) than surrounding soils. Ectomycorrhizal fungi also constituted a significant portion of the fungal community during the fast decay stage (first 5 weeks) of necromass decomposition, indicating that fungal necromass may serve as a nitrogen source for the nitrogen-limited trees. The shifts in microbial functional compositions (e.g., decreased proportions of copiotrophic bacteria) as decomposition progressed were partly due to alterations in substrate chemistry (i.e., the levels of aliphatic compounds including soluble carbohydrate, amorphous glucan polymers, and crystalline glucans and chitin). In contrast, nitrogen addition had limited influences on microbial diversity and composition, likely due to the nitrogen-rich nature of the fungal necromass. Overall, our findings reinforce the existence of a specific and dynamic core necrobiome in fungal necromass and enhance our understanding of how fungal necrobiome responds to environmental changes.

Stable lithium storage of hierarchical Zn2SiO4/C micro-/nanospheres enabled by in-situ introduction of an endogenous Zn4Si2O7(OH)2

Zn2SiO4 with high natural abundance and low cost has attracted a lot of attention in lithium-ion battery (LIB) anode realm, but its electrochemical performances are poor (such as short cyclic life and fast capacity decay). Herein, the electrochemical performances of Zn2SiO4 are significantly improved through a novel and simple strategy, including morphological engineering and especially the in-situ introduction of an endogenous Zn4Si2O7(OH)2 by simply regulating the carbonization temperature. The findings show that the conformal carbon-coated ultrathin Zn2SiO4/Zn4Si2O7(OH)2 hybrid nanosheets assembled hierarchical micro-/nanospheres (ZSO/ZSOH@C) are endowed with stable structure, enhanced conductivity, improved Li+ diffusion kinetics and a LiF-rich stable SEI layer during charge/discharge process. Therefore, the ZSO/ZSOH@C anode manifests superior electrochemical performance, with 475.5 and 372.1 mAh g−1 at 200 and 2000 mA g−1 after 350 and 400 cycles, respectively. The structure-property correlations of ZSO/ZSOH@C anode are systematically studied and demonstrate that the characteristically hierarchical structure and the synergistic effects between Zn2SiO4 and Zn4Si2O7(OH)2 are responsible for the superior electrochemical performances of the ZSO/ZSOH@C anode. Finally, this work may inspire ideas for the preparation of high-performance Zn2SiO4 or other silicates for LIB anodes.

Nanoparticle ZnS:Ag/6LiF—a new high count rate neutron scintillator with pulse shape discrimination

A neutron sensitive scintillator employing nanoparticles of ZnS:Ag has been developed for the first time. Pulse shape differences between neutron and gamma signals were observed in this material and neutron-gamma discrimination was applied. With initial signal processing parameters, gamma sensitivities of 8.5×10−6 to 60Co gammas were achieved. The average primary decay of neutron scintillation in nanoparticle ZnS:Ag/6LiF was measured to be 18ns, and afterglow was significantly suppressed in comparison to standard ZnS:Ag/6LiF scintillators that employ micron sized ZnS:Ag. Fast decay times and minimal afterglow indicate potential for use in high count rate capability applications. Prospective count rate capabilities were investigated here as proof of concept, with rates of 1.12Mcps measured for a single readout channel with less than 3.5% count loss. This is approximately 70 times greater than the count rate capability of the current standard ZnS:Ag/6LiF scintillation detectors. With improvements to signal processing and scintillator composition, nanoparticle ZnS:Ag/6LiF could be a promising candidate for future high rate capability neutron detectors.

Ultrafast Surface Plasmon Probing of Interband and Intraband Hot Electron Excitations.

Upon the interaction of light with metals, nonthermal electrons are generated with intriguing transient behavior. Here, we present femtosecond hot electron probing in a noveloptical pump/plasmon probe scheme. With this, we probed ultrafast interband and intraband dynamics with 15 nm interface selectivity, observing a two-component-decay of hot electron populations. Results are in good agreement with a three-temperature model of the metal; thus, we could attribute the fast (∼100 fs) decay to the thermalization of hot electrons and the slow (picosecond) decay to electron-lattice thermalization. Moreover, we could modulate the transmission of our plasmonic channel with ∼40% depth, hinting at the possibility of ultrafast information processing applications with plasmonic signals.

Fast energy decay for wave equation with a monotone potential and an effective damping

We consider the total energy decay of the Cauchy problem for wave equations with a potential and an effective damping. We treat it in the whole one-dimensional Euclidean space [Formula: see text]. Fast energy decay like [Formula: see text] is established with the help of potential. The proofs of main results rely on a multiplier method and modified techniques adopted in [R. Ikehata and Y. Inoue, Total energy decay for semilinear wave equations with a critical potential type of damping, Nonlinear Anal. 69(4) (2008) 1396–1401].

Numerical investigation of the vortical structures in the near wake of a model wind turbine

It is well known that the vortex system in a horizontal axis wind turbine wake is highly relevant in terms of fatigue loads and performance of wind turbines located in the wake of other wind turbines. The breakdown process of tip vortices particularly influences the mixing process of the low-speed wake region with the undisturbed flow outside the wake. As a collaboration with the Technische Universität Berlin (TUB), a major goal in a joint research is to study the effects involved in tip vortex breakdown. TUB designed a model turbine that will be towed through a large water tank to analyse and to control the tip vortex decay. To accommodate the measurement equipment, the ratio of blade length to nacelle length is unconventionally small, leading to uncertainty regarding the effect on the breakdown of the tip vortex. Due to the dimensions of the model wind turbine the root vortex system and the nacelle wake are not comparable to former studies and should therefore be investigated in detail. To do so computational fluid dynamics, in particular delayed detached eddy simulations, are conducted for the model turbine with the compressible flow solver FLOWer and the two-equation Menter-SST turbulence model. Two simulations are conducted with and without the nacelle. The results indicate that the root vortices propagate downstream and interact with each other. Additionally, these vortical structures are also influenced by the geometry of the turbine nacelle which causes faster decay of the root vortices compared to a configuration without the nacelle. However, the root vortex breakdown does not influence the tip vortices as the turbulent intensity matches for the area where tip vortices are present. In short, this investigation shows that the influence of the nacelle geometry and root vortex system on the tip vortices is negligible. Thus, the model wind turbine designed by TUB is suitable for the investigation of tip vortices.

3D-printed channel-aligned core-sheath architecture with high sulfur loading for kinetics-enhanced Li–S battery

Sluggish kinetics in thick cathode with high sulfur (S) loading will lead to low active material utilization and fast capacity decay, thus hampering the practical applications of Li–S batteries. To tackle such issue, a slightly thickness-dependent cathode of activated carbon (AC)-S/single-walled carbon nanotube (SWNT)@SWNT with fast reaction kinetics is constructed via continuously printing core-sheath filaments. Such micro-filaments are composed of vertically aligned “thin AC-S/SWNT” in core layer and porous SWNT-strengthened shell layer, which constructs an appealing well-interconnected porous 3D architecture ensuring fast ion/electron/mass transport throughout the whole printed matrix, effectively expediting electrochemical kinetics even under a high S loading. Moreover, the SWNTs located in shell layer and intercalated in-between AC-S phase in core layer crosslink well for acting as the functional interlayer to suppress polysulfide shuttling, thereby enabling high S utilization. As a result, the 3D-printed thick AC-S/SWNT@SWNT cathode delivers outstanding gravimetric/areal capacity (752 mAh g−1/6.41 mAh cm−2 at 0.5C) and cycling performance under high S loading of 8.6 mg cm−2.

Autoencoders for discovering manifold dimension and coordinates in data from complex dynamical systems

While many phenomena in physics and engineering are formally high-dimensional, their long-time dynamics often live on a lower-dimensional manifold. The present work introduces an autoencoder framework that combines implicit regularization with internal linear layers and L 2 regularization (weight decay) to automatically estimate the underlying dimensionality of a data set, produce an orthogonal manifold coordinate system, and provide the mapping functions between the ambient space and manifold space, allowing for out-of-sample projections. We validate our framework’s ability to estimate the manifold dimension for a series of datasets from dynamical systems of varying complexities and compare to other state-of-the-art estimators. We analyze the training dynamics of the network to glean insight into the mechanism of low-rank learning and find that collectively each of the implicit regularizing layers compound the low-rank representation and even self-correct during training. Analysis of gradient descent dynamics for this architecture in the linear case reveals the role of the internal linear layers in leading to faster decay of a ‘collective weight variable’ incorporating all layers, and the role of weight decay in breaking degeneracies and thus driving convergence along directions in which no decay would occur in its absence. We show that this framework can be naturally extended for applications of state-space modeling and forecasting by generating a data-driven dynamic model of a spatiotemporally chaotic partial differential equation using only the manifold coordinates. Finally, we demonstrate that our framework is robust to hyperparameter choices.

Impact of nuclear effects on the ultrafast dynamics of an organic/inorganic mixed-dimensional interface

Electron dynamics at weakly bound interfaces of organic/inorganic materials are easily influenced by large-amplitude nuclear motion. In this work, we investigate the effects of different approximations to the equilibrium nuclear distributions on the ultrafast charge-carrier dynamics of a laser-excited hybrid organic/inorganic interface. By considering a prototypical system consisting of pyrene physisorbed on a MoSe2 monolayer, we analyze linear absorption spectra, electronic density currents, and charge-transfer dynamics induced by a femtosecond pulse in resonance with the frontier-orbital transition in the molecule. The calculations are based on ab initio molecular dynamics with classical and quantum thermostats, followed by time-dependent density-functional theory coupled to multi-trajectory Ehrenfest dynamics. We impinge the system with a femtosecond (fs) pulse of a few hundred GW cm−2 intensity and propagate it for 100 fs. We find that the optical spectrum is insensitive to different nuclear distributions in the energy range dominated by the excitations localized on the monolayer. The pyrene resonance, in contrast, shows a small blue shift at finite temperatures, hinting at an electron-phonon-induced vibrational-level renormalization. The electronic current density following the excitation is affected by classical and quantum nuclear sampling through suppression of beating patterns and faster decay times. Interestingly, finite temperature leads to a longer stability of the ultrafast charge transfer after excitation. Overall, the results show that the ultrafast charge-carrier dynamics are dominated by electronic rather than by nuclear effects at the field strengths and time scales considered in this work.

Solar-blind UV detector with fast response speed and an ultrahigh detectivity based on SrTiO3 (111) substrate grown β-Ga2O3 thin film

Wide band gap semiconductor β-Ga2O3 thin film is widely used in photodetectors. Along the β-Ga2O3 [102] direction, the lattice mismatch between (2‾ 01) β-Ga2O3 and (111) SrTiO3 (STO) is approximately 2.4 %. STO can be used as a substrate for heterostructure fabrication. Here, taking advantage of the symmetry and interatomic super-cellular matching between β-Ga2O3 and the STO substrate, β-Ga2O3 epitaxial films were cultivated on STO (111) substrates via an LPCVD technique. Photoconductive solar blind ultraviolet (UV) detectors have been fabricated based on cultivated Ga2O3 thin films, and the intrinsic linkage of the detector performance with temperature variation has been analyzed in detail. The composition, surface morphology and electrical properties of the films were characterized. The optimized device showed a response rate of 66.64 A/W. The fast rise time (τr1/τr2) and decay time (τd1/τd2) at 20 V bias were 0.15 s/0.18 s and 0.20 s/0.25 s, respectively, with the detectivity of 4.1x1012 Jones.

Oxidized Graphite Nanocrystals for White Light Emission

We investigated the formation of graphite nanocrystals covered with graphite oxide for white light generation. The nanoparticles were formed using cost-efficient oxidation of a carbon-based dye pigment at different temperatures and verified using X-ray diffraction and Raman measurements. Formation of the graphite nanoparticles via thermal annealing was observed, while their light emission increased at higher oxidation temperatures. This was associated with a higher amount of oxygen defect groups. The time-resolved photoluminescence measurements showed linearly faster decays at shorter wavelengths and similar decays at different annealing temperatures. Broadband and linear vs. excitation emission spectra of the particles were found to be suitable for white-light-emitting devices and phosphor markers. The fast photoluminescence decay opens the possibility for the application of nanoparticles in optical wireless communication technology.

High-light-yield and fast-response β-Ga2O3–Al2O3 thick-film scintillators epitaxially grown via chemical vapor deposition

β-gallia (β-Ga2O3) has been investigated for adoption in power semiconductor wafers; however, it is a promising scintillating material for X-ray detection screens. Thus, the research on methods for growing β-gallia crystals has attracted considerable attention. Herein, 15-μm thick Ga2O3–Al2O3 solid solution films with β-gallia structure were synthesized using metal–organic chemical vapor deposition (CVD) at deposition rate of 15 μm h−1. A high scintillation light yield of 5400 photons per 5.5 MeV and a fast decay response of 5.1 ns of the produced films were observed, which were comparable or superior to those of the β-Ga2O3 single crystals. CVD method proved advantageous for the rapid synthesis of micrometer-thick Ga2O3-based scintillation screens.

Effect of Hydrophobic Side-Group Chemistry on Conformation, Intermolecular Structure and Dynamics of Cationic Poly(Vinyl Amine) in Aqueous Solution

Atomistic molecular dynamics simulations of the single chains of cationic polyelectrolytes of atactic poly(vinyl amines) (PVAm) modified by hydrophobic side-groups, poly(N-ethyl-n-vinyl amine) (ethyl-PVAm) and poly(N-benzyl-n-vinyl amine) (benzyl-PVAm), in aqueous solution, were carried out over the entire range of degree-of-ionization, f. These polymers, to our knowledge, were studied here by molecular simulations for the first time. The interplay of electrostatic interactions, excluded volume, hydrophobic effect, hydrogen bonding and counter-ion interaction with hydrophobic-polyions, is analyzed. The conformational transition of the polyelectrolyte chain from a coiled to an extended state was gradual and was observed for 0.3 < f < 0.7. Ethyl-PVAm and benzyl-PVAm exhibited similar values of radius-of-gyration Rg and end-to-end distance R for f > 0.5, in agreement with available experimental data. The more hydrophobic benzyl-PVAm showed a more extended conformation with a preference for trans states in the backbone. The change in chain expansion with respect to f was more for benzyl-PVAm as compared to ethyl-PVAm. The relaxation time of polymer-water H-bonds increased with f and the hydrophobicity of the polymer chain, with faster decay for ethyl-PVAm. The water molecules solvating the hydrophobic PVAm’s showed dynamical restriction due to bulky hydrophobic side-groups resulting in long-lived H-bonds. The number of polymer-water H-bonds showed a transition at a certain charge, which influenced the complex interplay of steric effect, hydrophobic effect and solvation.