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With the advantages of superior wear resistance, mechanical durability, and stability, the liquid-solid mode triboelectric nanogenerator (TENG) has been attracting much attention in the field of energy harvesting and self-powered sensors. However, most reports are primarily observational, and there still lacks a universal model of this kind of TENG. Here, an equivalent circuit model and corresponding governing equations of a water-solid mode TENG are developed, which could easily be extended to other types of liquid-solid mode TENGs. Based on the first-order lumped circuit theory, the full equivalent circuit model of water-solid mode TENG is modeled as a series connection of two capacitors and a water resistor. Accordingly, its output characteristics and critical influences are examined, to investigate the relevant physical mechanism behind them. Afterward, a three-dimensional water-solid TENG array constructed from many single-wire TENGs is fabricated, which can not only harvest tiny amounts of energy from any movement of water, but also can verify our theoretical predictions. The fundamentals of the water-solid mode TENG presented in this work could contribute to solving the problem of electrical phenomena on a liquid-solid interface, and may establish a sound basis for a thorough understanding of the liquid-solid mode TENG.

This paper reports the noncontact manipulation of free-falling cylindrical streams of liquid metals into unique shapes, such as levitated loops and squares. Such cylindrical streams form in aqueous media by electrochemically lowering the interfacial tension. The electrochemical reactions require an electrical current that flows through the streams, making them susceptible to the Lorentz force. Consequently, varying the position and shape of a magnetic field relative to the stream controls these forces. Moreover, the movement of the metal stream relative to the magnetic field induces significant forces arising from Lenz's law that cause the manipulated streams to levitate in unique shapes. The ability to control streams of liquid metals in a noncontact manner will enable strategies for shaping electronically conductive fluids for advanced manufacturing and dynamic electronic structures.

Room-temperature liquid metal is discovered to be capable of penetrating through macro- and microporous materials by applying a voltage. The liquid metal penetration effects are demonstrated in various porous materials such as tissue paper, thick and fine sponges, fabrics, and meshes. The underlying mechanism is that the high surface tension of liquid metal can be significantly reduced to near-zero due to the voltage-induced oxidation of the liquid metal surface in a solution. It is the extremely low surface tension and gravity that cause the liquid metal to superwet the solid surface, leading to the penetration phenomena. These findings offer new opportunities for novel microfluidic applications and could promote further discovery of more exotic fluid states of liquid metals.

Recent developments of self-powered devices and systems have attracted much attention. Lead zirconate titanate (PZT) has been regarded as one of the most promising materials for building high-performance nanogenerators. Herein, vertically aligned PZT nanorod arrays were synthesized on a pre-oxidized Ti substrate in the presence of a surfactant by a one-step hydrothermal method. The PZT nanorod arrays consist of an initial layer of a PZT film and well aligned nanorods with (001)-orientated tetragonal single crystalline structures. The PZT nanorods exhibited a high piezoelectric response with a d33 value of up to 1600 pm V−1. A piezoelectric energy harvester was fabricated based on the PZT nanorod arrays, which exhibited outstanding energy harvesting performance with an open-circuit output voltage of 3.3 V and 8 V when the devices were pressed by a compressive 10 N force and a finger tapping motion, respectively. Moreover, the average power density generated by those two mechanical stimulations were up to 3.16 and 5.92 μW cm−2 with the external load of 1 MΩ.

With the increasing requirement of developing non-toxic piezoelectric materials, an alkaline niobate-based perovskite solid (K,Na)NbO3 (KNN) has been intensively studied. Promising piezoelectric properties are reported, which are mostly achieved by adding other elements, or simply varying the K/Na ratio. It is found that KNN nanorods grown on conductive Nb-doped SrTiO3 (STO) substrates show enhanced piezoelectric properties after annealing at 800 °C for 12 h [Y. He, Z. Wang, W. Jin, X. Hu, L. Li, Y. Gao, X. Zhang, H. Gu and X. Wang, Appl. Phys. Lett., 2017, 110, 212904]. However, the underlying mechanism for property enhancement at the atomic scale is not clearly revealed. In this study, comprehensive transmission electron microscopy techniques are utilized focusing on the atomic scale study of the interfacial composition, structures, strain, dipolar displacement vectors and their variations along the interface normal of the as-grown and annealed KNN nanorods. The results indicate phase transformation during annealing, and a larger spontaneous polarization within each unit cell of the annealed KNN nanorods, which lead to an overall enhancement of the piezoelectric properties. These results would be very beneficial for advanced nanogenerators and sensors with enhanced piezoelectric properties.

Lead-free piezoelectric one-dimensional (1D) nanostructures have exhibited great potential in building biocompatible micro/nano-energy harvesters and self-powered smart sensors. However, their low piezoelectric coefficient still remains as the biggest obstacle for practical applications. In this work, an ultrahigh piezoelectric coefficient with calibrated d33 from 400 pm V-1 to 814 pm V-1 is achieved in Li-doped (K,Na)NbO3 (KNLN) lead-free piezoelectric nanorod arrays (NRAs). The giant enhancement on the piezoelectric performance of the KNLN nanorods can be attributed to the manipulation on the phase transition behavior together with the relaxation of strain. The Li+ dopants induce large NbO6 octahedral distortion and result in the decreased phase transition temperature from orthorhombic (O) to tetragonal (T) phase, which leads to the construction of the O-T phase boundary at room temperature and improves the polarization performance of the products. The ultrahigh piezoelectric performance of the KNLN NRAs gives rise to the remarkably improved energy harvesting performance, which shows highly sensitive pressure sensing behaviors and can be utilized for monitoring the human-body motions.

We report a comprehensive study of magneto-transport properties in MoSi$_2$ bulk and thin films. Textured MoSi$_2$ thin films of around 70 nm were deposited on silicon substrates with different orientations. Giant magnetoresistance of 1000% was observed in sintered bulk samples while MoSi$_2$ single crystals exhibit a magnetoresistance (MR) value of 800% at low temperatures. At the low temperatures, the MR of the textured thin films show weak anti-localization behaviour owing to the spin orbit coupling effects. Our first principle calculation show the presence of surface states in this material. The resistivity of all the MoSi$_2$ thin films is significantly low and nearly independent of the temperature, which is important for electronic devices.

Rapid development of modern electronics promotes the research of high-efficiency self-powered devices and systems. Lead-free perovskite potassium sodium niobate (KNN) with high piezoelectric constant and biocompatibility are regarded as one of the most promising materials for building high-performance nanogenerators. In this work, well-aligned KNN nanorod arrays with [110]-orientation were directly grown on SrTiO3 substrates by a one-step hydrothermal method. The growth of the KNN nanorods could be attributed to the orientation of an initial KNN layer and the release of the compressive stress induced by the SrTiO3 substrate. The piezoelectric constant d33 along the [110] orientation of nanorods is ∼140 pm V−1, which is 2.5 times higher than the d33 measured along the [001]-orientation. Those results agree with the first-principles calculation results, where the spontaneous polarization of the orthorhombic KNN lattice is confirmed to be [110]-oriented. Furthermore, the KNN nanorod arrays exhibit an outstanding energy harvesting performance with the impulsive output voltage of 10 V. A laterally-integrated nanogenerator could also be fabricated by making the KNN nanorods laterally aligned in a PDMS layer. This device shows comparative energy harvesting performance to the nanorod arrays, and exhibits much higher flexibility which allows them to be applied in diversified conditions.

The rapid development of microscaled piezoelectric energy harvesters has provided a simple and highly efficient way for building self-powered sensor systems through harvesting the mechanical energy from the ambient environment. In this work, a self-powered microfluidic sensor that can harvest the mechanical energy of the fluid and simultaneously monitor their characteristics was fabricated by integrating the flexible piezoelectric poly(vinylidene fluoride) (PVDF) nanofibers with the well-designed microfluidic chips. Those devices could generate open-circuit high output voltage up to 1.8 V when a droplet of water is flowing past the suspended PVDF nanofibers and result in their periodical deformations. The impulsive output voltage signal allowed them to be utilized for droplets or bubbles counting in the microfluidic systems. Furthermore, the devices also exhibited self-powered sensing behavior due to the decreased voltage amplitude with increasing input pressure and liquid viscosity. The drop of output voltage could be attributed to the variation of flow condition and velocity of the droplets, leading to the reduced deformation of the piezoelectric PVDF layer and the decrease of the generated piezoelectric potential.

Lead-free (K,Na)NbO3 (KNN) nanorod arrays were synthesized with the assistance of a Nb: SrTiO3 single-crystal substrate through the hydrothermal process. The evolutions of the morphology, composition, and structure of the as-synthesized KNN nanorods with the increase in reaction time were investigated. The results confirmed that the increase in reaction time up to 3 h led to the increase in the length and aspect ratio of the well-aligned KNN nanorods. All samples have K-rich orthorhombic crystal structures, while the diffraction peaks shifted towards a higher degree. The peak shifts should be attributed to the increase in the Na content in the KNN lattice, which could decrease the lattice parameters owing to the small ionic radius of Na+ than that of K+. Moreover, the increase in reaction time also resulted in the suppression of oxygen vacancies on the surface of the KNN nanorods. These evolutions of the composition and crystal structure, as well as the decrease in the defect content, lead to great enhanc...

Orthorhombic (K,Na)NbO3 (KNN) nanorod arrays with the [110]-orientation were grown on SrTiO3 substrates by the hydrothermal method. The nanorods exhibited greatly enhanced piezoelectric performance, with the d33 piezoelectric coefficient increasing from 140 pm/V to 360 pm/V, after annealing in oxygen at temperatures ranging from 500 °C to 800 °C. The high temperature annealing process was accompanied by severe volatilization of K, which modified the K/Na ratio closer to 1:1. Thus, orthorhombic-tetragonal (O-T) phase boundaries appeared, which improved the piezoelectric property. Moreover, surface oxygen vacancies were passivated in the high-temperature oxygen atmosphere, which would reduce the charge density in the nanorods and contribute to the enhanced piezoelectricity. Therefore, excellent piezoelectricity in the KNN nanorod arrays was driven by the combination of the O-T phase boundaries and the oxygen vacancy passivation. This work demonstrates that KNN has great potential in piezoelectric materials ...

The development of self-powered vibration sensors using polymeric piezoelectric nanomaterials has attracted great attention owing to their outstanding flexibility and energy harvesting behaviours. In this study, ultra-long poly (vinylidene fluoride) (PVDF) nanofibres with optimised β-phase content were synthesised through electrospinning method with different DC voltages. The increase in the β-phase content of the PVDF nanofibres greatly enhanced their piezoelectric response with nearly tripled output voltage and current under the same strain condition. Moreover, the output voltage exhibited linear correlations with both the amplitude and frequency of the strain. Under a fixed frequency of 1.54 Hz, the output voltage exhibited a linear correlation to the strain amplitude with strain sensitivity up to 0.92 V rad−1 and 0.61 V mm−1. The frequency-dependent strain sensing behaviour also confirmed the necessity for frequency calibration to the measured results of vibration. Accordingly, the sensor can be used for self-powered monitoring of the vibration state of a metal foil and measuring the intrinsic resonance frequency of the objects without any powering source.