Low Excitation Levels Research Articles

Kirigami auxetic structure for high efficiency power harvesting in self-powered and wireless structural health monitoring systems

Kirigami and auxetic topologies are combined to design an innovative metamaterial-based substrate (MetaSub) for piezoelectric energy harvesters. The proposed MetaSub piezoelectric energy harvester (MPEH) contains both advantageous metamaterial properties of negative Poisson’s ratio capability and enhanced planar stretchability. A computational parametric analysis is conducted to develop the optimum design for the MPEH to trap the maximum elastic energy. A finite element analysis (FEA) is employed to analytically and numerically validate the simulation model of the MPEH. Accordingly, two experimental results of conventional and auxetic strain energy harvesters are used to evaluate the power enhancement of the MPEH. The FEA results demonstrate the average power gained by the MPEH at a low level of frequency and strain excitation (10 Hz and 150 peak-to-peak) is 165 W which easily satisfies the minimum electric power amount required as a sensor node for self-powered wireless sensor networks. The harvested power output of the MPEH is 19.2 times more than power output produced by an equivalent conventional harvester with a plain substrate (8.6 W). The performance of the MPEH is investigated at different combinations of both low and high excitation frequencies. The creative design of the MetaSub can significantly improve the productivity of strain-induced devices whose efficiency is dependent on their deformation performance such as vibration energy harvesters, wearable sensors, flexible actuators, and micro electromechanical applications.

Improved stochastic multireference perturbation theory for correlated systems with large active spaces

We identify the dominant computational cost within the recently introduced stochastic and internally contracted FCIQMC-NEVPT2 method for large active space sizes. This arises from the contribution to the four-body intermediates arising from low-excitation level sampled determinant pairs. We develop an effective way to mitigate this cost via an additional stochastic step within the sampling of the required NEVPT2 intermediates. We find this systematically improvable additional sampling can reduce simulation time by 80% without introducing appreciable error. This saving is expected to increase for larger active spaces. We combine this enhanced sampling scheme with full stochastic orbital optimisation for the first time and apply it to find FCIQMC-NEVPT2 energies for spin states of an iron porphyrin system within (24,24) active spaces with relatively meagre computational resources. This active space size can now be considered as routine for NEVPT2 calculations of strongly correlated molecular systems within this improved stochastic methodology.

Comparison of linear, hyperbolic and double-hyperbolic models to assess the force–velocity relationship in multi-joint exercises

This study assessed the validity of linear, hyperbolic and double-hyperbolic models to fit measured force–velocity (F–V) data in multi-joint exercises and the influence of muscle excitation on the F–V relationship. The force–joint angle and F–V relationships were assessed in 10 cross-training athletes and 14 recreationally resistance-trained subjects in the unilateral leg press (LP) and bilateral bench press (BP) exercises, respectively. A force plate and a linear encoder were installed to register external force and velocity, respectively. Muscle excitation was assessed by surface EMG recording of the quadriceps femoris, biceps femoris and gluteus maximus muscles during the unilateral LP. Linear, Hill’s (hyperbolic) and Edman’s (double-hyperbolic) equations were fitted to the measured F–V data and compared. Measured F–V data were best fitted by double-hyperbolic models in both exercises (p < 0.05). F–V data deviated from the rectangular hyperbola above a breakpoint located at 90% of measured isometric force (F 0) and from the linearity at ≤45% of F 0 (both p < 0.05). Hyperbolic equations overestimated F 0 values by 13 ± 11% and 6 ± 6% in the LP and BP, respectively (p < 0.05). No differences were found between muscle excitation levels below and above the breakpoint (p > 0.05). Large associations between variables obtained from linear and double-hyperbolic models were noted for F 0, maximum muscle power, and velocity between 25% and 100% of F 0 (r = 0.70–0.99; all p < 0.05). The F–V relationship in multi-joint exercises was double-hyperbolic, which was unrelated with lower muscle excitation levels. However, linear models may be valid to assess F 0, maximal muscle power and velocity between 25% and 100% of F 0.

Vibration energy harvesting enhancement exploiting magnetically coupled bistable and linear harvesters

Many common environmental vibration sources exhibit low and broad frequency spectra. In order to exploit such excitations, energy harvesting architectures utilizing nonlinearity, especially bistability, have been extensively explored as a promising energy source for self-powered small-scale electric devices. For such devices, the energetic interwell oscillations between their stable equilibria can provide enhanced power harvesting capability over a wider bandwidth compared to the linear counterpart. Yet, one of the limitations of these nonlinear architectures is that the interwell oscillation regime may not be readily activated for low excitation level that is not sufficient to overcome the potential energy barrier, thus resulting in low amplitude intrawell response, which provides poor energy harvesting performance. This research investigates a multi-degree of freedom (MDOF) vibration energy harvesting system that leverages magnetically coupled bistable and linear harvesters. It presents novel in-depth insights into capitalizing on a passive mechanism that not only facilitates the energetic interwell response for relatively low excitation amplitudes and frequencies than that may be required for conventional bistable harvester by passively and adaptively lowering the potential energy barrier level, but also effectively exploits the redistributed dynamic energy and the rich MDOF dynamic characteristics introduced by the magnetically coupled linear harvester. It is found that in addition to the enhanced power harvesting performance of bistable harvester with adaptive potential, the power captured from the redistributed energy and the higher harmonic resonances introduced by the passive mechanism further increase the energy harvesting performance especially at the lower frequency range. Analytical, numerical, and experimental investigations reveal that strategically incorporating a linear harvester magnetically coupled to a conventional bistable harvester provides an effective and easy to implement means for enhancing broadband energy harvesting performance.

A vibration‐based electromagnetic and piezoelectric hybrid energy harvester

This work presents the fabrication and testing of a vibration-based single energy hybrid harvester (VSEHH). Electromagnetic and piezoelectric transduction mechanisms are utilized for energy extraction in the developed harvester. Electromagnetic portion of the device composed of a permanent magnet, planar coil and wound coil, however, for piezoelectric portion polyvinylidene difluoride (PVDF) membrane is used. In the harvester the PVDF membrane having the magnet is kept loose in order to exploit the spring softening nonlinearity. At lower base excitation levels (less than 0.5g) the response of the VSEHH is linear, however, from 0.5 to 2.5g the harvester exhibits spring softening nonlinearity and at acceleration levels greater than 2.5g, the spring hardening nonlinearity is invoked. Because of nonlinear behavior of the harvester, the shift of resonant frequency, the sudden jump-up and jump-down phenomena result in the enhancement of the harvester's frequency bandwidth. Under sinusoidal excitation and at 123 Hz frequency and 4g acceleration, the electromagnetic portion of the harvester produced 40.6 mV load voltage and 212.7 μW with planar coil and 73.5 mV load voltage and 319.1 μW power with wound coil. Moreover, under the same vibrations condition a load voltage of 2930 mV and power of 57.6 μW is generated by the piezoelectric portion of the harvester. Collectively, the harvester is capable of producing a power of 589.4 μW and a power density of 334.13 μW/cm3. Furthermore, when subjected to broadband random vibrations, two central frequency peaks are produced, one is due to spring softening and the other corresponds to the spring hardening of the membrane.

Analysis of Online Lyapunov-Based Adaptive State of Charge Observer for Lithium-Ion Batteries Under Low Excitation Level

The online estimation of the state-of-charge (SOC) of Li-ion battery using the adaptive Lyapunov-based observer is an attractive proposition due to the ensured stability, adaptability and reduced computing requirement. However, the observer requires the presence of the persistent excitation (PE) to guarantee the convergence of the battery model parameters to their correct values. The PE is satisfied using sufficiently rich (SR) signal that contains spectral components to excite the battery model. This paper revisits several important works that utilize such observer and highlights the absence of PE in their practical implementation. Since the previous works utilize dc excitation that lacks the SR characteristics, the validity of the published results is questionable. To rectify the problem, a scheme known as the forced excitation is proposed to estimate the battery parameters under dc or low excitation level. The SR signal is generated by chopping the battery current at a certain rate for specific interval. Moreover, the disruption of the load current (due to the chopping) is compensated using the supercapacitor. The concept is simulated by Matlab/Simulink and is realized experimentally using a Panasonic NCR18650B Li-ion battery. The forced excitation algorithm is implemented on the DS1104 dSPACE platform. The results show that the proposed method satisfies the PE condition and is able to correctly estimate the SOC even with low excitation signals.

Спонтанное и стимулированное излучение в тонких пленках твердых растворов Cu(In-=SUB=-1-x-=/SUB=-Ga-=SUB=-x-=/SUB=-)(S-=SUB=-y-=/SUB=-Se-=SUB=-1-y-=/SUB=-)-=SUB=-2-=/SUB=- в составе солнечных элементов

The results of a study of the emission spectra of thin nanocrystalline films of Cu(In1-xGax)(SySe1-y)2 (CIGSSe) direct-band-gap solid solutions in the structure of solar cells at ~ 0.5 W/cm2 continuous wave and nanosecond pulsed laser excitation in the range of excitation power density 0.1 - 53 kW/cm2 and temperatures of 10-300 K are presented. It was found that stimulated emission (SE) occurs in thin CIGSSe films in the temperature range from 10 K to 90 K in the spectral region h = 1.062 - 1.081 eV with a minimum threshold pump level of about 1 kW/cm2. It was shown, that, with increasing intensity of the exciting emission, the spontaneous emission bands shift toward higher energies. It was found that the photoluminescence bands at low excitation levels and the SE bands shift with increasing temperature toward higher energies, and the PL bands at high excitation levels shift toward low energies. Possible causes and mechanisms of the influence of temperature and excitation intensity on the spectral positions of spontaneous and SE of the films of solid solutions are discussed.

三稳态能量收集系统的同宿分岔及混沌动力学分析

Nonlinear dynamic performances such as homoclinic bifurcation and chaos were investigated for tristable vibration energy harvesting systems. The analytical expression of the symmetric and asymmetric homoclinic solution was obtained through the Padé approximation, which was consistent with the numerical solution. According to the Melnikov theory, the qualitative method of studying the homoclinic bifurcation of the energy harvesting system with a triple well was developed, and the necessary condition for the occurrence of homoclinic bifurcation was obtained. Numerical simulations yielded bifurcation diagrams and maximum Lyapunov exponents that demonstrated the inter-well responses predicted with the Melnikov method. Compared with the system with symmetric potential energy, the system with asymmetric potential energy has a lower threshold of homoclinic bifurcation. For a low excitation level, the system with asymmetric potential energy witnesses inter-well chaos, while the response of the system with symmetric potential energy still keeps trapped in a single well. The change of symmetry of the system potential energy function improves the output voltage due to the increase in the probability of generating a large periodical inter-well oscillation response. The research on the homoclinic bifurcation of nonlinear energy harvesting systems with symmetric and asymmetric triple potential wells provides an effective tool for the parametric design of high-performance energy harvesters.

Large-scale shell-model calculations near mass region 100-130

In this work, we have presented a microscopic shell-model description of the structure and collective behavior of intermediate-mass nuclei around doubly magic 100 Sn and 132 Sn nuclei. The Sn-isotopes lie between the two doubly magic nuclei and cover a range from exotic proton-rich N=Z nuclei to exotic neutron-rich nuclei with N/Z &gt; 1.6. The results obtained using BIGSTICK code for the low-level excitation states and transition probabilities for the studied Sn and Ba isotopic chains have been discussed in the radiance of available experimental data. We have used 100 Sn as a core for all the studied isotopes with the same valence space (i.e. 1d 5/2 , 2s 1/2 , 1d 3/2 , 0g 7/2 , 0h 11/2 or ‘sdgh’) for both protons (Z) and neutrons (N) between 50 and 82. These calculations are performed by means of globally optimized monopole effective interaction for the sdgh-shell.

Dynamics and high-efficiency of a novel multi-stable energy harvesting system

Scavenging energy from external vibration with low-frequencies and low-level intensities has always been a huge challenge for the energy harvesting. To remedy this key issue, this study is to present a novel multi-stable electromagnetic transduction energy harvesting system by adjusting the geometric parameters of linear springs. Here, we prove that the multi-stable energy harvesting system composed of a nonlinear restoring mechanism can significantly improve the efficiency of the energy converter. The electromechanical model is set up, and the corresponding equations are derived by the Extended Averaging Method and Fokker–Planck equation analysis. The response under harmonic excitation exhibits that, compared to a mono-stable and a bi-stable system, with a tri-stable or a quad-stable system, we can achieve a higher-energy interwell motion with large-amplitude periodic oscillation, leading to significant increases of the harvested current and power. Moreover, improved the expected value of the mean square current and mean harvested power can also be attained under the Gaussian white noise with a small intensity. The results show that the proposed multi-stable system is a feasible design that can improve the power capture performance of energy harvesting from low-level excitations.

Hardware-Software Complex for the Investigation of the Dynamic Characteristics of Separate Spectral Components of the Emission Spectrum of Leds

We present the description of an automated hardware-software complex intended for measuring the 3 dB frequency of modulation and evaluation of the quantum efficiency of separate spectral components of the full spectrum of electroluminescence of light-emitting diodes, including the range of low levels of excitation. The principle of operation of this complex is based on measuring the spectrum of radiation of a light-emitting diode with the help of an OceanOptics USB2000+ spectrometer in the signal-accumulation mode in the course of transmission of current pulses with given amplitude and repetition frequency through the light-emitting diode with subsequent selection of separate spectral components from the full spectrum of radiation. We present the results of testing the hardware-software complex performed by measuring, as an example, the frequency characteristics of separate spectral components of the full electroluminescence spectrum of commercial InGaN green light-emitting diodes. The analyzed hardware-software complex can be used for the diagnostics of the quality of light-emitting heterostructures with quantum wells.

Theoretical and Experimental Investigation of a Multi-stable Energy Harvester for Rotation Motion

Recently, different kinds of nonlinear energy harvesters in rotation environments were designed to achieve high-efficiency broadband energy harvester for low-level excitations. However, a mathematical model of the multi-stable energy harvester in rotation motion is not complete, and its dynamic performance remains experimentally uninvestigated due to the difficulty of the sensor layout in rotation motion. Based on these issues, in this paper, a multi-stable energy harvester used in rotation motion is proposed and a related mathematical model has been processed by Lagrangian equation to describe the dynamic performance and the output power of the harvester. Additionally, corresponding experiments between the bi-stable energy harvester (BEH) and the tri-stable energy harvester (TEH) are carried out to validate their performance under different rotation speeds. The experimental results demonstrate that the TEH can obtain high-efficiency energy harvesting performance in low rotation speed than the BEH.

A magnetically coupled nonlinear T-shaped piezoelectric energy harvester with internal resonance

This letter originally presents a magnetically coupled nonlinear T-shaped piezoelectric energy harvester with multiple resonance bands based on internal resonance principle. The proposed harvester consists of a clamped-clamped buckled beam and a vertical cantilever with tip magnetically coupled. The corresponding theoretical model is established. Two resonance bands with hardening nonlinear phenomenon are obtained. 1:2:3:4 internal resonance characteristics with the peak amplitude ratio of 1:1.81:0.56:0.31 are found for the generated voltage of the first resonance. The energy conversion efficiency can be enhanced by utilizing high frequency response under low-frequency excitations. The bandwidth of full width at half maximum output power reaches 13.8 Hz within the excitation between 5∼30 Hz. The response of random excitations is also investigated and an average power of 9.64 μW is achieved at the random excitation intensity of 0.001 g2Hz−1. Overall, the proposed harvester can achieve ultra-wide working frequency range under low-level excitations and is suitable for low-frequency random vibrations.

Vortex-induced vibrational tristable energy harvester: Design and experiments

Wind energy harvesters have been widely studied for their great application potential to power small wireless sensors. Meanwhile, the unique dynamic characteristics of vibrational tristable energy harvesters have been theoretically and experimentally verified. More importantly, such vibrational tristable energy harvesters have excellent broadband energy harvesting performance under low-frequency and low-level excitations. This paper aims to develop a new kind of aeroelastic energy harvesters for enhancing wind energy harvesting performance. In detail, a vortex-induced vibrational tristable energy harvester is designed by using the magnetic force to realize the tristable configuration. A mathematical model of the presented harvester is provided. Experimental results verify that the presented vortex-induced vibrational tristable energy harvester performs better than the traditional linear vortex-induced vibration energy harvester.

Improvement of the Time Resolution of Radiation Detectors Based on Gd3Al2Ga3O12 Scintillators With SiPM Readout

Coincidence time resolution (CTR) of scintillation detectors based on Ce- and Mg-codoped Gd3Al2Ga3O12 (GAGG) scintillation crystals and high-density silicon photomultipliers (SiPMs) is shown to be 165 ps (full width at half maximum) for 511-keV $\gamma $ -quanta, approaching that achieved by using LSO scintillators. To study the prospective for further improvement of the time resolution, the population of the emitting Ce centers was investigated by optical pump and probe technique using selective photoexcitation and probing by a white light continuum with subpicosecond time resolution. The importance of free electron trapping for excitation transfer to emitting Ce ions was revealed. The influence of transfer delay on the scintillation response time is described, and the dynamics of electron relaxation to the lowest excited level of Ce ion is studied experimentally and analyzed by taking into account intracenter relaxation and relaxation via conduction band. The influence of electron diffusivity on the rise time of the population of the emitting level is described. It is shown that codoping of GAGG:Ce by magnesium even at the level as low as 10 ppm efficiently decreases the scintillator response time by enhancing the electron diffusivity.

Experiments and simulations of an industrial assembly with different types of nonlinear joints subjected to harmonic vibrations

The structure Harmony-Gamma is a metallic assembly representative of an industrial structure for which the vibratory response is influenced by the apparition of nonlinear phenomena within two specific types of joints, the first corresponding to friction joints and the second to elastomer joints. Firstly, the paper presents the experimental procedure and results obtained for this structure. Secondly, a global methodology for modeling and simulation of the nonlinear vibrational response is set up.The experiments are performed for longitudinal and transverse swept sine experiments. First of all, swept sine experiments are performed at low excitation levels on the structure in order to update a linear finite-element model. Then, the evolution of the frequency response function is studied at increasing excitation levels in order to identify the contribution of the nonlinear effects on the global vibrational response of the system.The methodology for modeling and simulation consists of five steps. Firstly, a finite-element model of the structure is presented and updated in order to be representative of the structure when it is excited at low excitation levels. This model is then reduced using a hybrid sub-structuring technique. Thirdly, the nonlinear models of the friction and elastomer joints are added. The resulting optimization problem is solved by means of the Harmonic Balance Method (HBM) coupled with a Newton-Raphson continuation and a condensation process. Lastly, the simulation results are compared to the experimental results in order to validate the development of the finite element model for the nonlinear industrial structure Harmony-Gamma with nonlinear joints of different natures and to achieve a refined understanding of the nonlinear phenomena and their origin.

Dynamic analysis of bi-stable hybrid symmetric laminate

The resulting oscillation of bi-stable hybrid symmetric laminate could enable broadband energy harvesting via piezoelectric transduction. The aim of this work is to investigate its dynamic behaviors to provide the basis for designing broadband energy harvester. This paper proposes the dynamic analysis of this bi-stable laminate, focusing on the intra-well dynamics around its stable states and the inter-well dynamics between two stable states. Two types of stacking sequence are performed for this bi-stable laminate, and experimental testing with different harmonic excitations is carried out for each type of this bi-stable laminate. The strain responses at three positions are monitored to investigate the dynamic responses. A finite element model is also developed to analyze the static strain distribution and capture the dynamics. The results show that the laminates exhibit intra-well response under low-level excitation, and the inter-well response at a particular range frequency appears when excitation level increases. Different types of inter-well response mode involving the snap-through behavior are obtained in experiments, such as intermittent inter-well vibration and chaotic vibration. The presented results essentially highlight the need for considering the stacking sequence associated with the dynamics while designing the hybrid bi-stable symmetric laminate to obtain the desirable nonlinear response.

Time-resolved photoluminescence from n-doped GaN/Al0.18Ga0.82N short-period superlattices probes carrier kinetics and long-term structural stability

Heavily n-doped GaN/Al0.18Ga0.82N short-period superlattices with and without SiN protection layers are studied in spectrally and temporally resolved photoluminescence (PL) experiments. The room-temperature PL from a protected sample displays a nonexponential decay with an initial decay time of 150 ps for low excitation levels and an exponential decay with a time constant of ∼300 ps for higher excitation. The PL decays are governed by nonradiative carrier relaxation into deep defect states which are partially saturated at high excitation densities. PL measurements at low sample temperature reveal a marked influence of carrier cooling on the PL kinetics in a time range up to 50 ps and a significant radiative decay component. SiN coatings are shown to provide long-term stabilization of surface morphology while increasing nonradiative carrier relaxation rates.

Phonon Coupling between a Nanomechanical Resonator and a Quantum Fluid.

Owing to their extraordinary sensitivity to external forces, nanomechanical systems have become an important tool for studying mesoscopic physics and realizing hybrid quantum systems. While nanomechanics has been widely applied in solid-state systems, its use in liquid receives less attention. There it finds unique applications such as biosensing, rheological sensing, and studying both classical and quantum fluid dynamics in unexplored regimes. In this work, we demonstrate efficient coupling of a nano-optomechanical resonator to a bosonic quantum fluid, superfluid 4He, through ultrahigh-frequency phonons (i.e., sound waves) approaching gigahertz frequencies. A high phonon exchange efficiency >92% and minimum excitation rate of 0.25 phonons per oscillations period, or equivalently kB T/ hfm Qm = 0.044 ≪ 1, are achieved. Based on our experimental results, we further predict that strong coupling between a nanomechanical resonator and superfluid cavity phonons with cooperativity up to 880 can be achieved. Our study opens new opportunities in controlling and manipulating superfluid at the nanoscale and low-excitation level.

Terahertz Detection with Perfectly-Absorbing Photoconductive Metasurface.

Terahertz (THz) photoconductive devices are used for generation, detection, and modulation of THz waves, and they rely on the ability to switch electrical conductivity on a subpicosecond time scale using optical pulses. However, fast and efficient conductivity switching with high contrast has been a challenge, because the majority of photoexcited charge carriers in the switch do not contribute to the photocurrent due to fast recombination. Here, we improve efficiency of electrical conductivity switching using a network of electrically connected nanoscale GaAs resonators, which form a perfectly absorbing photoconductive metasurface. We achieve perfect absorption without incorporating metallic elements, by breaking the symmetry of cubic Mie resonators. As a result, the metasurface can be switched between conductive and resistive states with extremely high contrast using an unprecedentedly low level of optical excitation. We integrate this metasurface with a THz antenna to produce an efficient photoconductive THz detector. The perfectly absorbing photoconductive metasurface opens paths for developing a wide range of efficient optoelectronic devices, where required optical and electronic properties are achieved through nanostructuring the resonator network.