Ray P. S. Han

Natural Frequencies and Normal Modes of a Spinning Timoshenko Beam With General Boundary Conditions

A free flexural vibrations of a spinning, finite Timoshenko beam for the six classical boundary conditions are analytically solved and presented for the first time. Expressions for computing natural frequencies and mode shapes are given. Numerical simulation studies show that the simply-supported beam possesses very peculiar free vibration characteristics: There exist two sets of natural frequencies corresponding to each mode shape, and the forward and backward precession mode shapes of each set coincide identically. These phenomena are not observed in beams with the other five types of boundary conditions. In these cases, the forward and backward precessions are different, implying that each natural frequency corresponds to a single mode shape.

The Dynamics of a Bouncing Ball with a Sinusoidally Vibrating Table Revisited

The dynamical behavior of a bouncing ball with a sinusoidally vibrating table is revisited in this paper. Based on the equation of motion of the ball, the mapping for period-1 motion is constructured and thereby allowing the stability and bifurcation conditions to be determined. Comparison with Holmess solution [1] shows that our range of stable motion is wider, and through numerical simulations, our stability result is observed to be more accurate. The Poincaré mapping sections of the unstable period-1 motion indicate the existence of identical Smale horseshoe structures and fractals. For a better understanding of the stable and chaotic motions, plots of the physical motion of the bouncing ball superimposed on the vibration of the table are presented.

Case-Encapsulated Triboelectric Nanogenerator for Harvesting Energy from Reciprocating Sliding Motion

Reciprocating motion is a widely existing form of mechanical motion in natural environment. In this work we reported a case-encapsulated triboelectric nanogenerator (cTENG) based on sliding electrification to convert reciprocating motion into electric energy. Patterned with multiple sets of grating electrodes and lubricated with polytetrafluoroethylene (PTFE) nanoparticles, the cTENG exported an average effective output power of 12.2 mW over 140 kΩ external load at a sliding velocity of 1 m/s, in corresponding to a power density of 1.36 W/m(2). The sliding motion can be induced by direct-applied forces as well as inertia forces, enabling the applicability of the cTENG in addressing ambient vibration motions that feature large amplitude and low frequency. The cTENG was demonstrated to effectively harvest energy from human body motions and wavy water surface, indicating promising prospects of the cTENG in applications such as portable and stand-alone self-powered electronics.

Self-powered thin-film motion vector sensor

Harnessing random micromeso-scale ambient energy is not only clean and sustainable, but it also enables self-powered sensors and devices to be realized. Here we report a robust and self-powered kinematic vector sensor fabricated using highly pliable organic films that can be bent to spread over curved and uneven surfaces. The device derives its operational energy from a close-proximity triboelectrification of two surfaces: a polytetrafluoroethylene film coated with a two-column array of copper electrodes that constitutes the mover and a polyimide film with the top and bottom surfaces coated with a two-column aligned array of copper electrodes that comprises the stator. During relative reciprocations, the electrodes in the mover generate electric signals of ±5 V to attain a peak power density of ≥65 mW m−2 at a speed of 0.3 ms−1. From our 86,000 sliding motion tests of kinematic measurements, the sensor exhibits excellent stability, repeatability and strong signal durability.

Modeling the size-dependent elastic properties of polymeric nanofibers

We present a strain gradient (SG) theory to explain the strongly inverse size dependence between the elastic modulus and fiber diameter in polymeric nanofibers. For centrosymmetric and isotropic materials we showed that the three length-scale parameters can be combined into a single parameter that can be used to predict the onset of the size-dependent trend when the fiber diameter is reduced past its critical size. To address the issue of whether the SG offers a plausible explanation of the size-dependent behavior we conducted a series of uniaxial tensile and static bending tests involving polycaprolactone nanofibers. Since the elastic modulus is highly sensitive to the fiber diameter, it is necessary to correct the experimental data to account for the lack of circularity in the cross-section of the real fiber. Additionally, we applied the SG model to study the size-dependent elastic properties of polypyrrole nanotubes. By approaching the SG theory from a dynamics point of view, our model is able to capture size-dependent effects in the mechanics of fine-scale materials for both static and dynamic responses.

Nanogenerator as an active sensor for vortex capture and ambient wind-velocity detection

In this paper, we report a simple and practical composite structure for a nanogenerator (NG). The composite design using two kinds of piezoelectric materials, zinc oxide and poly(vinylidene fluoride), requires no more system size and complexity than for a single material, but improves the power density and sensitivity of the NG significantly. With no need of an external power source or batteries, the composite NG can efficiently convert the vortex motion in the atmosphere into electricity. Based on the Karman vortex street principle, ambient wind-speed measurements with the NG are demonstrated. Due to the simple structure, high sensitivity and good environment-friendly properties, the NG as an active sensor should play an important role in wireless environmental monitoring networks.

In situ grown graphene-encapsulated germanium nanowires for superior lithium-ion storage properties

Alloying anode materials (Si, Ge, Sn etc.) in lithium-ion batteries usually suffer from a remarkable loss of capacity during the charge–discharge cycling. Herein, homogeneous in situ-grown graphene-encapsulated Ge nanowires are successfully achieved by a simple, completely catalyst-free route via arc-discharge. The Ge@G composite is composed of a graphene sheath and a metallic Ge nanowire core. This unique composite of graphene-encapsulated Ge nanowires is an ideal anode material for lithium ion storage. It can exhibit excellent electrochemical performance with a reversible specific capacity of 1400 mA h g−1 after 50 cycles at a current density of 1600 mA g−1 (the theoretical specific capacity of Ge is 1624 mA h g−1). These encouraging results demonstrate that arc-discharge synthesis provides efficient graphene encapsulation of Ge nanowires, and graphene encapsulation is a feasible solution to protect electrode materials for lithium-ion storage.

Selective adsorption of CO2/CH4 and CO2/N2 within a charged metal–organic framework

Presented here is a new ultramicroporous metal–organic framework (MOF) formulated as [Zn3L2(HCOO)1.5][(CH3)2NH2]1.5·xDMF, 1 (H3L = 9-(4-carboxy-phenyl)-9H-carbazole-3,6-dicarboxylic acid), DMF = N,N-dimethylformamide, consisting of an anionic framework and two types of interlaced one-dimensional channels with 0.42 and 0.79 nm diameters respectively, in which the larger channels accommodate protonated dimethylamine as the counter cations. Gas sorption analysis of N2, CO2 and CH4 was investigated and the isotherms exhibit reversible thermodynamic behaviours without hysteresis desorption, evidencing framework rigidity and permanent porosity of solvent-free 1. The synergistic effect of the open ultramicropores and dimethylamine cations may lead to high efficiency separation of CO2 from CH4 and N2. According to the Toth model, the selectivity of CO2/CH4 and CO2/N2 was calculated to be 96 and 37, respectively. This effort will give rise to a new conception to tailor the charged MOF for high efficiency CO2 adsorption and separation.

Dynamic Response of a Spinning Timoshenko Beam With General Boundary Conditions and Subjected to a Moving Load

Solution of the problem is achieved by formulating the spinning Timoshenko beams as a non self adjoint system. To compute the system dynamic response using the modal analysis technique, it is necssary to determine the eigenquantities of both the original and adjoint systems. In order to fix the adjoint eignvectors relative to the eigenvectors of the original system, the biorthonormality conditons are invoked

Controlled Synthesis of Core–Shell Carbon@MoS2 Nanotube Sponges as High-Performance Battery Electrodes

Heterogeneous inorganic nanotube structures consisting of multiwalled carbon nanotubes coated by long, continuous MoS2 sheets with tunable sheet number are synthesized using a carbon-nanotube sponge as a template. The resulting 3D porous hybrid sponges have potential applications as high-performance freestanding anodes for Li-ion batteries with excellent specific capacity and cycling stability.