A hybrid piezoelectric and triboelectric nanogenerator with lead-free BZT-BCT/PDMS composite and PVA film for scavenging mechanical energy.

A hybrid piezo/triboelectric nanogenerator (H/P-TENG) is designed for mechanical energy harvesting using polymer ceramic composite films; polydimethylsiloxane/Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (PDMS/BZT-BCT) and polyvinyl alcohol (PVA). A lead-free BZT-BCT piezoelectric ceramic was prepared via solid-state method and blended into PDMS to form a series of polymer-ceramic composite films, ranging from 5% to 30% by weight. The films were forward/reverse poled with corona poling and their electrical properties were compared to non-poled samples. The H/P-TENG constructed with forward-poled 15 wt% BZT-BCT in PDMS achieved the highest open-circuit voltage, V oc of 127 V, short-circuit current density, J sc of 67 mA m-2, short-circuit charge density, Q sc of 118 µC m-2, and peak power density of 7.5 W m-2, an increase of 190% over pristine PDMS-based TENG. It was discovered that incorporating BZT-BCT into the PDMS matrix improved the triboelectric properties of PDMS. The overlapping electron cloud (OEC) model was used to explain the enhancement and the effect of poling direction of the PDMS/BZT-BCT composite used in H/P-TENG, providing fundamental knowledge of the influence of piezoelectric polarisation on contact electrification.

Structural control of the dielectric, pyroelectric and ferroelectric properties of poly(vinylidene fluoride-co-trifluoroethylene) thin films.

The effects of solvents and temperature on the crystal formation were investigated for vinylidene fluoride/trifluoroethylene copolymer P(VDF-TrFE). Highly crystalline P(VDF-TrFE) thin films were fabricated by spin-casting using various polar solvents such as diethyl carbonate (DEC), methyl ethyl ketone (MEK), N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Ferroelectric polarisation reversal of the P(VDF-TrFE) was evidenced by a displacement (D)-electric field (E) hysteresis loop measurement and supported by the presence of pyroelectric activity. The samples which were annealed above 100 °C gave elongated rod-like crystalline structures and the highest crystallinity, Xc of 83% was formed at 120 °C. Consequently, remnant polarisation (85 mC m-2) and the pyroelectric coefficient (30 µC m-2 K-1) were enhanced. Thus, annealing plays a substantial role in controlling the crystalline structure of P(VDF-TrFE) films regardless of the choice of solvent. Although, the crystalline structure control is almost independent of the solvent's polarity, the choice of solvent is essential in preparing a smoother film surface. Molecular dynamics of P(VDF-TrFE) are discussed from the observation of two dielectric relaxation processes, ß and γ related to the segmental micro-Brownian and local modes, respectively. The complete ferroelectric polarization reversal caused a reduction in the dielectric polarization and reduced the lattice spacing of the 110/200 planes.

Molecular dynamics of anhydrous glycolipid self-assembly in lamellar and hexagonal phases.

The molecular dynamics of a synthetic branched chain glycolipid, 2-decyl-tetradecyl-ß-d-maltoside (C14-10G2), in the dry assemblage of smectic and columnar liquid crystal phases has been studied by dielectric spectroscopy as a function of frequency and temperature during the cooling process. Strong relaxation modes were observed corresponding to the tilted smectic and columnar phases, respectively. At low frequency (∼900 Hz to 1 kHz) in the smectic phase, Process I* was observed due to the tilted sugar bilayer structure. The process continued in the columnar phase (Process I) with an abrupt dynamic change due to phase transition in the frequency range of ∼1.3 kHz to 22 kHz. An additional process (Process II) was observed in the columnar phase with a broader relaxation in the frequency range of ∼10 Hz to 1 kHz. A bias field dependence study was performed in the columnar phase and we found that the relaxation strength rapidly decreased with increased applied dc bias field. This relaxation originates from a collective motion of polar groups within the columns. The results of dielectric spectroscopy were supported by a molecular dynamics simulation study to identify the origin of the relaxation processes, which could be related to the chirality and hydrogen bonds of the sugar lipid.

Hot plate annealing at a low temperature of a thin ferroelectric P(VDF-TrFE) film with an improved crystalline structure for sensors and actuators.

Ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer 70/30 thin films are prepared by spin coating. The crystalline structure of these films is investigated by varying the annealing temperature from the ferroelectric phase to the paraelectric phase. A hot plate was used to produce a direct and an efficient annealing effect on the thin film. The dielectric, ferroelectric and pyroelectric properties of the P(VDF-TrFE) thin films are measured as a function of different annealing temperatures (80 to 140 °C). It was found that an annealing temperature of 100 °C (slightly above the Curie temperature, Tc) has induced a highly crystalline ß phase with a rod-like crystal structure, as examined by X-ray. Such a crystal structure yields a high remanent polarization, Pr = 94 mC/m2, and pyroelectric constant, p = 24 µC/m2K. A higher annealing temperature exhibits an elongated needle-like crystal domain, resulting in a decrease in the crystalline structure and the functional electrical properties. This study revealed that highly crystalline P(VDF-TrFE) thin films could be induced at 100 °C by annealing the thin film with a simple and cheap method.

Phase sensitive molecular dynamics of self-assembly glycolipid thin films: a dielectric spectroscopy investigation.

Glycolipid, found commonly in membranes, is also a liquid crystal material which can self-assemble without the presence of a solvent. Here, the dielectric and conductivity properties of three synthetic glycolipid thin films in different thermotropic liquid crystal phases were investigated over a frequency and temperature range of (10(-2)-10(6) Hz) and (303-463 K), respectively. The observed relaxation processes distinguish between the different phases (smectic A, columnar/hexagonal, and bicontinuous cubic Q) and the glycolipid molecular structures. Large dielectric responses were observed in the columnar and bicontinuous cubic phases of the longer branched alkyl chain glycolipids. Glycolipids with the shortest branched alkyl chain experience the most restricted self-assembly dynamic process over the broad temperature range studied compared to the longer ones. A high frequency dielectric absorption (Process I) was observed in all samples. This is related to the dynamics of the hydrogen bond network from the sugar group. An additional low-frequency mechanism (Process II) with a large dielectric strength was observed due to the internal dynamics of the self-assembly organization. Phase sensitive domain heterogeneity in the bicontinuous cubic phase was related to the diffusion of charge carriers. The microscopic features of charge hopping were modelled using the random walk scheme, and two charge carrier hopping lengths were estimated for two glycolipid systems. For Process I, the hopping length is comparable to the hydrogen bond and is related to the dynamics of the hydrogen bond network. Additionally, that for Process II is comparable to the bilayer spacing, hence confirming that this low-frequency mechanism is associated with the internal dynamics within the phase.