Piezoelectricity describes interconversion between electrical charge and mechanical strain.As expected for lattice ions displaced in an electric field, the proportionality constant is positive for all piezoelectric materials. The exception is poly(vinylidene-fluoride) (PVDF), which exhibits a negative longitudinal piezoelectric coefficient. Reported explanations consider exclusively contraction with applied electric field of either the crystalline or the amorphous part of this semi-crystalline polymer. To distinguish between these conflicting interpretations, we have performed in-situ dynamic X-ray diffraction measurements on P(VDF-TrFE) capacitors. We find that the piezoelectric effect is dominated by the change in lattice constant but, surprisingly, it cannot be accounted for by the polarization-biased electrostrictive contribution of the crystalline part alone. Our quantitative analysis shows that an additional contribution is operative, which we argue is due to an electromechanical coupling between the intermixed crystalline lamellae and amorphous regions. Our findings tie the counterintuitive negative piezoelectric response of PVDF and its copolymers to the dynamics of their composite microstructure. 3 Piezoelectricity describes the conversion of electrical charge to mechanical strain and vice versa. The direct piezoelectric effect is observed as a change in surface charge density of a material in response to an external mechanical stress. The effect is reversible; the thermodynamic equivalent is a change in dimension upon applying an electric field.A large piezoelectric coefficient, describing the change in spontaneous electrical polarization with applied mechanical stress, is obtained for ferroelectric materials. When an electric field is applied in the direction of the polarization most ferroelectric materials will expand. However, there is one well-known exception. The ferroelectric polymer poly(vinylidene-fluoride) (PVDF) and its copolymers with trifluoroethylene P (VDF-TrFE) show an unusual negative longitudinal piezoelectric effect. Counterintuitively, these polymers contract in the direction of an applied electric field. The two opposite behaviours are schematically represented in Fig. 1.It has been shown that the strain in PVDF varies with the polarization squared. [1] Hence the origin of piezoelectricity is electrostriction biased by the spontaneous polarization. A negative piezoelectric coefficient was extracted. Presently, two contradicting microscopic models have been proposed; the piezoelectric response is attributed to either the crystalline or the amorphous part of the semi-crystalline polymer.Quantum chemical calculations for the ferroelectric β−phase of PVDF have shown that for a single-crystal the piezoelectric effect is negative.[2] When an electric field is applied perpendicularly to the PVDF chain, the backbone stretches and its height is compressed. The lattice constant is reduced. The calculated coefficient agrees with the value experimentally determined on bulk samples, imp...

Electrochromic (EC) device reversibly changes color and optical state by applying electric potential. EC materials shows color change due to redox process and electron transfer across various states under influence of electric field. EC devices are categorized based on various classes of EC materials inorganic metal oxide and polyoxometalates (POMs), metal complexes, hybrid materials, metal plasmonics–metal/alloy and organic molecules/conjugated polymers. EC materials and its viability for device application are presented herein considering various performance parameter indices. The performance of EC devices is monitored by its switching time between transparent‐bleached state to colored state and vice versa, cycling stability, coloration efficiency, applied voltage, electro‐optical properties, electrochemical stability and breakdown potentials of EC materials and electrolytes. Recent advances in the area of EC devices and materials, its operation mechanism in various categories of EC materials along with existing challenges and recommendations to improve performances and reliability are summarized.

A good thermoelectric material usually has a high power factor and low thermal conductivity for high figure of merit (ZT), and is also environmentally friendly and economical.

Boron nitride nanotubes (BN-NTs) were synthesized by using excimer laser ablation at 1200 °C in different carrier gases. The main characteristic of the BN-NTs produced by this method is that nanotubes are of only one to three atomic layers thick, which could be attributed to the dominance of the axial growth rate over the radial growth rate. The diameter of the BN-NTs ranged from 1.5 to 8 nm. The tips of the BN-NTs are either a flat cap or of polygonal termination, in contrast to the conical ends of carbon nanotubes. The atomic ratio of boron to nitrogen as measured by means of parallel electron energy loss spectroscopy is 0.8, which is within the experimental error of the stoichiometry of hexagonal BN structure.

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Depolarization in ferroelectric materials has been studied since the 1970s, albeit quasi-statically. The dynamics are described by the empirical Merz law, which gives the polarization switching time as a function of electric field, normalized to the so-called activation field. The Merz law has been used for decades; its origin as domain-wall depinning has recently been corroborated by molecular dynamics simulations. Here we experimentally investigate domain-wall depinning by measuring the dynamics of depolarization. We find that the boundary between thermodynamically stable and depolarizing regimes can be described by a single constant, P r / ε 0 ε ferro E c . Among different multidomain ferroelectric materials the values of coercive field, E c , dielectric constant, ε ferro , and remanent polarization, P r , vary by orders of magnitude; the value for P r / ε 0 ε ferro E c however is comparable, about 15. Using this extracted universal value, we show that the depolarization field is similar to the activation field, which corresponds to the transition from creep to domain-wall flow.

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The polarization of the ferroelectric polymer P(VDF-TrFE) decreases upon prolonged cycling. Understanding of this fatigue behavior is of great technological importance for the implementation of P(VDF-TrFE) in random-access memories. However, the origin of fatigue is still ambiguous. Here we investigate fatigue in thin-film capacitors by systematically varying the frequency and amplitude of the driving waveform. We show that the fatigue is due to delamination of the top electrode. The origin is accumulation of gases, expelled from the capacitor, under the impermeable top electrode. The gases are formed by electron-induced phase decomposition of P(VDF-TrFE), similar as reported for inorganic ferroelectric materials. When the gas barrier is removed and the waveform is adapted, a fatigue-free ferroelectric capacitor based on P(VDF-TrFE) is realized. The capacitor can be cycled for more than 108 times, approaching the programming cycle endurance of its inorganic ferroelectric counterparts.