No abstract available

No abstract available

[en] This work studied an impact based frequency up-conversion mechanism via discontinuous dynamics analysis. The mechanism consists of a piezoelectric beam and a moving stopper. The moving stopper is excited by a sawtooth wave and impacts with the piezoelectric beam, which makes the beam vibrate with its national frequency repeatedly. In the system complex dynamics are induced by impacts, hence to better understand the energy harvesting performance of the piezoelectric beam, we first seek the periodic motions of the system. As the system parameters vary, the output voltage and power of the piezoelectric beam with periodic motions were obtained. The piezoelectric beam was modeled as a mass-spring-damper system, and the linear piezoelectric constitutive law was used to obtain the lumped model of the piezoelectric beam. Using discontinuous dynamics analysis, the generated power and voltage were obtained, and the effect of frequency-up-conversion was demonstrated by comparing the generated power of two cases at low excitation frequencies: (1) the piezoelectric beam was excited via impact with the stopper and (2) the piezoelectric beam was directly subjected to the sawtooth wave. In order to better understand the energy harvesting performance of the piezoelectric harvester, the stable and unstable periodic motions were obtained. The bifurcation diagram of the period-1 and period-2 motions were studied analytically with varying excitation frequency and the initial distance between the stopper and the beam.

[en] Frequency up-conversion is an effective approach to increase the output power of a piezoelectric energy harvester (PEH). In this work, we studied a piezoelectric energy harvesting system, which converts low-frequency vibration from ambient sources to the resonant vibration of the PEH to improve the energy harvesting efficiency. The PEH includes two beams, a pair of rack and pinions, and a slider crank mechanism to retrieve energy from ambient low-frequency vibrations through impacts. The soft driving beam is subjected to a sinusoidal base excitation along the transverse direction. The piezoelectric bimorph undergoes both slow longitudinal motion as well as transverse vibrations. The transverse vibration of the bimorph is induced by impacts which is the vibration source to generate output power. The longitudinal motion of the bimorph is used to control the impact with the soft driving beam, which guarantees the harvester to pump in more kinetic energy from the driving beam. Using the discontinuous dynamic theory, the energy harvesting performance of the impact-controlled system was studied in period one and period two motions. The stability of periodic solutions was investigated and bifurcation diagrams of impact velocities, times and displacements were obtained. The harvested power of the piezoelectric beam versus the based excitation frequency was also obtained, and the results were compared to the power generation of a piezoelectric beam directly subjected to the base excitation along the transverse direction.

[en] Highlights: • The ultra-stretchable bionic sensors based on FLM were integrated on a SCR to realize bionic sensing. • The TTS was sensitive to the unknown stimulus of various materials. The RSS was sensitive to the bending of SCR. • Closed-loop control enabled SCR to achieve intelligent sensing and automatic motion. Soft robots have significant advantages in terms of flexibility and adaptability, leading to potential applications in the bionics field. Inspired by the caterpillars in nature, this work proposed a soft caterpillar robot (SCR) by integrating two types of ultra-stretchable bionic sensors on a dual air-chamber pneumatic network structure. In order to realize self-powered tactile sensing, four triboelectric nanogenerator tactile sensors (TTSs) based on functional liquid metal (FLM) with thorny-structured bionic whiskers are developed and attached on the SCR. Meanwhile, two ultra-stretchable resistive strain sensors (RSSs) by using FLM are covered as the bionic skin to sense self-body deformation of the SCR. The TTS has a fast response time of 0.03 s and a minimum perception of 0.05 kPa, which can be very sensitive to the unknown stimulus of various materials. The RSS with a relatively high sensitivity of 2.94 and small hysteresis of 1.42% possess the ultra-stretchable ability of 180% strain, which helps to adapt and adjust its own body bending and crawling. The biological perception capabilities of the SCR play a crucial role in mimicking bionic actions and response in an unknown environment, such as escaping from unexpected attacks as well as adaptive crawling through an unknown tunnel environment.

[en] In this work the energy harvesting performance of a piezoelectric curved energy generator (THin layer UNimorph DrivER (THUNDER)) is studied via experimental and analytical methods. The analytical model of the THUNDER is created based on the linear mechanical electrical constitutive law of the piezoelectric material, the linear elastic constitutive law of the substrate, and the Euler–Bernoulli beam theory. With these linear modal functions, the Rayleigh-Ritz approach was used to obtain the reduced mechanical–electrical coupled modulation equations. The analytical model is verified by the experimental results. Both the experimental and analytical results of the THUNDER’s AC power output, DC power output with Rectifier Bridge and a capacitor, as well as the power output with a microcontroller energy harvesting circuit are reported. Based on the theoretical model, the analytical solution of the DC power is derived in terms of the vibration amplitude, frequency, and the electrical load. To harvest energy from low-frequency vibration source by a piezoelectric generator requires the piezoelectric device possessing low resonance frequency and good flexibility. The THUNDER developed by Langley Research Center exhibits high power when it is used as an energy generator and large displacement when it is used as an actuator. Compared to the less flexible PZT, although THUNDER is more difficult to model, THUNDER has better vibration absorption capacity and higher energy recovery efficiency. The effect of the THUNDER’s radius of curvature on energy harvesting efficiency is mainly investigated. We set the THUNDER’s radius of curvature as a dynamic tuning parameter which can tune the piezoelectric generators’ frequency with the source excitation frequency. (paper)

[en] Lead(IV) dioxide (PbO₂) has been used as the electron injection layer (EIL) to realize high-efficiency inverted top-emitting organic light-emitting diodes (I-TOLEDs). It can be seen that the inserting of the PbO₂ EIL significantly reduces operational voltage, thus greatly improving the current efficiency and power efficiency of fabricated I-TOLEDs. The 10-(2-benzothiazolyl)-1, 1, 7, 7-tetramethyl-2, 3, 6, 7-tetrahydro-1H, 5H, 11H-[1] benzopyrano [6, 7, 8-ij] quinolizin-11-one (C545T)-based I-TOLEDs with the PbO₂ EIL exhibit a maximum current efficiency of 31.6 cd A⁻¹ and a maximum power efficiency of 14.3 lm W⁻¹, which are both higher than 22.5 cd A⁻¹ and 5.4 lm W⁻¹ of the I-TOLEDs with LiF as the EIL respectively. A detailed analysis with respect to the role mechanism of PbO₂ in electron injection has been presented. The improvement in EL performance is attributed to the formation of the interfacial dipoles at the electrode interface due to charge transfer between PbO₂ and Alq₃

[en] This paper presents the analysis of the local and codimension-3 degenerate bifurcations in a simply supported flexible beam with quintic nonlinear terms subjected to a harmonic axial excitation for the first time. The quintic nonlinear equation of motion with parametric excitation is derived using the Hamilton's principle. The parametrically excited system is transformed to the averaged equations using the method of multiple scales. Numerical method is used to compute the bifurcation response curves based on the averaged equations. The investigations are made on the effects of quintic nonlinear terms and parametric excitation on the local bifurcations. The stability of trivial solution is analyzed. With the aid of normal form theory, the explicit expressions are obtained for normal form associated with a double zero eigenvalues and Z₂-symmetry of the averaged equations. Based on normal form, the analysis of codimension-3 degenerate bifurcations is performed for a simply supported quintic nonlinear beam with the focus on homoclinic and heteroclinic bifurcations. It is found from the analysis of homoclinic and heteroclinic bifurcations that multiple limit cycles may simultaneously exist for quintic nonlinearity. In particular, the number of limit cycles can be precisely determined analytically. New jumping phenomena are discovered in amplitude modulated oscillations

[en] To fabricate thorium-based fuel kernel for solid fuel molten salt reactor, a component of tri-structural isotropic fuel particles is mostly based on sol-gel method. The preparation of thorium sol is an important step for fabrication of thorium-based fuel kernels, such as thorium carbide and thorium oxide. The gel quality affects the gel particle dispersion and the final products. In this work, thorium sols were prepared using Th(NO₃)₄ and NH₃·H₂O by sol-gel method. The effects of thorium concentration, mole ratio of NH₄⁺/NO₃^- and reaction temperature on the pH, viscosity, turbidity, particle size and the thorium sol distribution were investigated. The results show that the viscosity and turbidity increased with the NH₄⁺/NO₃^- ratio; the turbidity and colloidal particle size increased with the reaction temperature, which affected little the sol viscosity; the sol viscosity increased with the thorium concentration, which virtually did not change the turbidity; and the particle size decreased and the size distribution narrowed with increasing thorium concentration. The sol could be stored at room temperature for one day without significant property changes. Thorium gel spheres of good quality were prepared at 60 ℃ with the NH₄⁺/NO₃^- ratio of 75-85% and the thorium concentration of 1.2-1.4 mol/L. (authors)

[en] Here, we report our study results of a flexible piezoelectric tensile strain sensor which is fabricated by synthesizing 0.5Ba (Zr_0.2Ti_0.8) O₃–0.5(Ba_0.7Ca_0.3) TiO₃ (0.5BZT–0.5BCT) nanofibers via an electrospinning process. Our nanofibers show an ultrahigh d₃₃ of 275 pm V⁻¹. 0.5BZT–0.5BCT nanofibers and MW-CNTs are dispersed in polydimethylsiloxane (PDMS) to fabricate a highly stretchable and flexible tensile sensor, and the multiple roles of the MW-CNTs are probed and demonstrated. This nanofiber-based piezoelectric tensile strain sensor shows great resolution and sensitivity under external mechanical deformation. It is suitable for applications in complex environments. (technical note)