Electric field dependence of topography in ferroelectric P(VDF/TrFE) films

The effect of electric fields on the topography and structures in P(VDF/TrFE) ferroelectric films was studied. The morphological changes after poling treatment at 25 °C, from previously chrysanthemum-like structures to regular and parallel slug-like domains, were observed by scanning probe microscope. Poling process at higher temperature induced even greater morphological changes. Imaging of local electrical properties showed that these changes could not be attributed to the influence of local charges injected into film surface during poling process. It was believed that this new morphology came from the reform of film structures induced by electric fields.     

Nanoscopic measurements of the electrostriction responses in P(VDF/TrFE) ultra-thin-film copolymer using atomic force microscopy

Atomic force microscopy (AFM) has been used as a new method to perform nanoscale measurements of the electrostriction coefficients in the lamellae structure of the ferroelectric P(VDF/TrFE) 73/27 copolymers. The result found shows that the electrostriction coefficient inside (in the middle of) the lamella crystals is 6×10^-19 (m²V^-2), which is three times larger than that at the boundary, 2×10^-19 (m²V^-2). To explain the dependence of the electrostriction coefficients with those two regions, some suggestions are proposed. By heat treatment at 140 °C during 2 h, the sample changed its morphology as well as its crystallinity; the amorphous phase is much reduced and the degree of the crystallinity inside the lamellae is higher than that in the border. Also, it is suggested that in the lamellae’s boundary the macromolecular chains come to an end, or one monolayer folds over the other layer. In this case, the electrostriction was suppressed due to the loss of surface energy in the lamellae’s boundary. The achievements will supply a guideline to develop new and better devices for electromechanical and actuator applications. Received: 23 June 2000 / Accepted: 23 August 2000 / Published online: 5 October 2000     

Influence of phase transitions on the spontaneous voltage in P(VDF/TrFE) copolymers

Spontaneous voltages were observed in a structure made of poly(vinylidene fluoride-trifluoroethylene) copolymers films sandwiched by metals evaporated onto both surfaces. Open voltages and short-circuit currents were measured as a function of the temperature using different combinations of evaporated metals as electrodes: aluminum, gold, nickel and copper. The ferroelectric-to-paraelectric transition was also observed in such structures for 60:40, 70:30 and 80:20 of VDF/TrFE molar ratios, including the temperature hysteresis characteristic of the first-order transition. Received: 9 February 2000 / Accepted: 28 March 2000 / Published online: 9 August 2000     

Properties and characteristics of P(VDF/TrFE) transducers manufactured by a solution casting method for use in the MHz-range ultrasound in air

Highly effective piezoelectric polymer transducers operating in air at high frequencies have been successfully made by casting a solution of ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) P(VDF/TrFE) directly on a backing metal plate, and their performance has been evaluated. By utilizing this method, it has been possible to develop the three kinds of transducers that operate respectively at 4, 6 and 10 MHz in air. For precise evaluation of the performance of the P(VDF/TrFE) transducers, the absorption loss in air was measured up to 10 MHz. It was confirmed that the empirical formula obtained from the measured absorption values in air at high frequencies was in alignment with its theoretical value. In addition, a high lateral resolution acoustic image of a ROM-Chip (amplitude-image) at 6 MHz in air was successfully displayed using an air coupled concave type P(VDF/TrFE) transducer by bonding an epoxy adhesive.     

Pyroelectric conversion—Effects of P(VDF–TrFE) preconditioning on power conversion

Large amounts of low-grade heat are emitted from various industries and wasted into the environment. This heat energy can be used as a free source for pyroelectric power generation. We first discuss the principle of pyroelectric conversion based on the differences in crystal structures of copolymer. Then we discuss the pyroelectric conversion of waste heat using copolymer films containing 60% vinylidene fluoride (VDF) and 40% trifluoroethylene (TrFE) and the impact of the preconditioning of the pyroelectric films on the net power output. We discovered that the pre-polarization (poling) of pyroelectric films has a significant beneficial effect by decreasing internal conduction during power conversion. This in turn increased net power output. We were able to reach 95–165 J/L of copolymer used in each cycle even including the internal leakage. We also showed it is critically important to operate the power conversion near the phase transition where electron discharge is the highest.     

Characteristics of SiO2 etching by using pulse-time modulation in 60 MHz/2 MHz dual-frequency capacitive coupled plasma

60 MHz pulsed radio frequency (rf) source power and 2 MHz continuous wave rf bias power, were used for SiO₂ etching masked with an amorphous carbon layer (ACL) in an Ar/C₄F₈/O₂ gas mixture, and the effects of the frequency and duty ratio of the 60 MHz pulse rf power on the SiO₂ etch characteristics were investigated. With decreasing duty ratio of the 60 MHz pulse rf power, not only the etch rate of SiO₂ but also the etch rate of ACL was decreased, however, the etch selectivity of SiO₂ over ACL was improved with decreasing the duty ratio. On the other hand, when the pulse frequency was varied at a constant duty ratio, no significant change in the etch rate and etch selectivity of both materials could be observed. The variation of the etch characteristics was believed to be related to the change in the gas dissociation characteristics caused by the change in the average electron temperature for different pulsing conditions. The improvement in the etch selectivity with the decrease of duty ratio, therefore, was related to the decreased gas dissociation of C₄F₈ by the decrease of average electron temperature and, which resulted in a change in composition of the fluorocarbon polymer on the etched materials surface from C–C rich to CF₂ rich. With decreasing the duty ratio, not only the etch selectivity but also the improvement in the SiO₂ etch profile could be observed.     

Heat transfer enhancement using 2 MHz ultrasound

The present work focuses on possible heat transfer enhancement from a heating plate towards tap water in forced convection by means of 2 MHz ultrasound. The thermal approach allows to observe the increase of local convective heat transfer coefficients in the presence of ultrasound and to deduce a correlation between ultrasound power and Nusselt number. Heat transfer coefficient under ultrasound remains constant while heat transfer coefficient under silent conditions increases with Reynolds number from 900 up to 5000. Therefore, heat transfer enhancement factor ranges from 25% up to 90% for the same energy conditions (supplied ultrasonic power = 110 W and supplied thermal power = 450 W). In the same time cavitational activity due to 2 MHz ultrasound emission was characterized from mechanical and chemical viewpoints without significant results. At least, Particle Image Velocimetry (PIV) measurements have been performed in order to investigate hydrodynamic modifications due to the presence of 2 MHz ultrasound. It was therefore possible to propose a better understanding of heat transfer enhancement mechanism with high frequency ultrasound.     

Ultrasonic imaging of solid surfaces using a focussed air-coupled capacitance transducer

A variety of small solid objects have been imaged in atmospheric air using a focussed, micromachined air-coupled capacitance transducer. The transducer, which was capable of generating and receiving ultrasound in air over a large frequency bandwidth (< 100 kHz 1.5 MHz) was employed in a pulse-echo arrangement such that generated waves reflected off the surface of the object before returning to the same transducer for detection. By raster scanning the transducer in a plane and recording the detected ultrasonic echo amplitude as a function of transducer position, images of object surfaces were obtained. As the transducer had been fitted with a micromachined Fresnel zone-plate, the ultrasonic waves could be focussed to a spot-size of 680 μm so as to provide images of high lateral resolution. One of the key factors in making the Fresnel zone-plate function effectively in these imaging applications involved the inclusion of a second aperture in front of the zone-plate. This additional aperture blocked the zone-plate's side-lobes and reduced the appearance of multiple (diffracted) images.     

Effect of the mechanical stretching on the ferroelectric properties of a (VDF/TrFE) (75/25) copolymer film

The transition phase behaviour of a vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymer film was studied after being subjected to different mechanical stretching levels in both longitudinal (L) and transversal (T) directions relatively to the initial extrusion direction. Both ferroelectric-paraelectric (FE-PE) and melting transitions were detected in the films by differential scanning calorimetry, that were not affected by L stretching. This suggests that sliding mechanism along the c-axis of such films during plastic deformation influences slightly the all-trans chain conformations and the CF₂ dipole orientation. On the other hand, the FE-PE peak decreases in area and temperature upon T stretching, indicating a damage of the ferroelectric phase. However, the melting peak maintains unchanged. This fact provides evidence that the mechanical stretching do not change the total degree of crystallinity. The results suggest that the stretching induces a FE-PE phase transformation, without affecting the degree of crystallinity.     

Ultrasonic imaging in air using fan-beam tomography and electrostatic transducers

Air-coupled ultrasonic capacitance transducers operating at frequencies of up to 1 MHz have been employed in a fan-beam configuration for the cross-sectional tomographic imaging of temperature fields and flow fields in air, and the location of solid objects. Separate transmitter and receiver transducers were manufactured using thin polymer dielectric membranes and polished metal backplates, and used to acquire through-transmission data. The fan-beam reconstruction was developed in LabVIEW using a re-bin routine combined with a filtered backprojection algorithm and a difference technique to generate the cross-sectional images. The system was first used to reconstruct images showing the locations of solid objects positioned within the scanned region through interpretation of the arrival time of the transmitted ultrasound. The technique was then extended to image the temperature fields produced in air above a small heat source and the flow field produced by a nozzle connected to a regulated compressed air source. Reconstructed temperatures were within 4% of the measured background air temperature and 9% of the air temperature measured above the heat source. Reconstructed images of the flow field above a small nozzle were also presented, showing that the horizontal component of the flow velocity could be resolved using this method.     

Effect of TiO2 crystalline composition on the dielectric properties of TiO2/P(VDF‐TrFE) composites

In order to study the effect of the TiO₂ particle crystalline composition (with different proportions of rutile and anatase crystals) on the dielectric properties of the composite, titanium dioxide (TiO₂) particles and TiO₂/poly(vinylidene fluoride‐co‐trifluoroethylene) [P(VDF‐TrFE)] composites were synthesized by a reflux method and the solution route, respectively. The results indicated that the optimum TiO₂ particle crystalline composition is anatase content of 37% and rutile content of 63% for dielectric‐constant modifier applications. Furthermore, a dielectric constant of 25.7 with dielectric loss of 0.17 at 100 Hz at room temperature were obtained in the composite with 40 wt% TiO₂ particles. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A non-equilibrium atmospheric pressure Capacitively-Coupled-Plasma (CCP) driven at VHF (162 MHz) for plasma catalysis of CO2 into CO

The design of a Very-High-Frequency (VHF) 162 MHz driven atmospheric-pressure Capacitively-Coupled-Plasma (CCP), with top and bottom electrodes operated in push-pull configuration, powered via a Power-Splitting-Transmission-Line-Driver (PSTLD), is presented. Application to the reprocessing of carbon dioxide into carbon-monoxide in this "high” VHF atmospheric plasma is presented, demonstrating some behaviour of the plasma source. rf power in the system is characterized using measured current (~ 1′s Amps peak) and voltage (~ 10′s Volts peak) waveforms at the electrode; Both are sinusoidal confirming a glow-discharge operational condition. Analysis of Optical Emission Spectra results find a highly non-equilibrium plasma, with high vibrational temperatures (from N₂) in the range ~4000 K, while gas temperature, monitored by a thermocouple at the gas outlet, remains low ~300 K, and confirmed by analysis of the N₂ rotational bands. The relative density of CO produced, as a by-product of CO₂ dissociation, is measured optically using N₂ as an actinometer. The CO density increases with rf power and longer gas residence times in the plasma volume. The high VHF atmospheric plasma is found to operate in pure CO₂ flows (no helium) with minimal gas heating for the full range of power densities (specific energy input of 0.4–2 eV per molecule) investigated.     

Characterization of air-coupled ultrasound transducers in the frequency range 40 kHz-2 mHz using light diffraction tomography

The aim of this work was to show the applicability of light diffraction tomography on airborne ultrasound in the frequency range 40 kHz-2 MHz. Seven different air-coupled transducers were measured to show the method's performance regarding linearity, absolute pressure measurements, phase measurements, frequency response, S/N ratio and spatial resolution. A calibrated microphone and the pulse-echo method were used to evaluate the results. The absolute measurements agreed within the calibrated microphone's uncertainty range. Pulse waveforms and corresponding FFT diagrams show the method's higher bandwidth compared with the microphone. Further, the method offers non-perturbing measurements with high spatial resolution, which was especially advantageous for measurements close to the transducer surfaces. The S/N ratio was higher than or in the same range as that of the two comparison methods.     

Theoretical studies of Sm/Si(1 1 1)–n × 1 (n = 5, 7) and Sm/Si(1 1 1)–(8 × 2) structures

In this study we employ a state-of-the-art pseudopotential method to perform local density of states (LDOS) calculations of n × 1 (n = 5, 7) and (8 × 2) reconstructions induced by the adsorption of rare-earth samarium (RE) in the submonolayer range. We conducted a full comparison between images from scanning tunneling microscopy (STM) and theoretical LDOS. Images taken of both filled and empty states show the effects induced by honeycomb chains and Seiwatz chains. We conclude that LDOS calculations are consistent with the assignment of features observed experimentally by STM.     

Temperature dependence of thermal and electrical conductivity of Bi-based high-Tc (2 2 2 3) superconductor

Superconducting samples with nominal composition Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_δ were prepared by the conventional solid-state reaction technique. The samples have been characterized by X-ray diffraction, dc electrical resistivity, ac magnetic susceptibility and thermal conductivity. The X-ray diffraction studies were done at room temperature and the lattice constants of the material were determined by indexing all the peaks. All the above measurements show that, there exists two phases i.e. high-T_c (2 2 2 3) and low-T_c (2 2 1 2). The information obtained from dc electrical resistivity data agrees with ac magnetic susceptibility measurements. The onset temperature T_c (onset) and zero resistivity temperature T_c (R = 0) of the samples remains within the temperature 120 ± 1 K and 103 ± 1 K. Thermal conductivity has been measured with a transient plane source (TPS) technique in the temperature range 77–300 K. The estimation of the electrical resistivity change due to scattering by phonons and impurities has been discussed. An increase in thermal conductivity is observed above and below T_c (R = 0). The electron–phonon scattering time, phonon-limited mobility and the size of the electron–phonon constant are also calculated. Wiedemann–Franz law is applied to gain prediction about the magnitude of electronic and phonon contribution to the total thermal conductivity of the samples. It is observed that heat is mainly conducted by the phonons in this system.     

Fast EPR imaging at 300 MHz using spinning magnetic field gradients

Electron paramagnetic resonance imaging (EPRI) technology has rapidly progressed in the last decade enabling many important applications in the fields of biology and medicine. At frequencies of 300-1200 MHz a range of in vivo applications have been performed. However, the requisite imaging time duration to acquire a given number of projections, limits the use of this technique in many in vivo applications where relatively rapid kinetics occur. Therefore, there has been a great need to develop approaches to accelerate EPRI data acquisition. We report the development of a fast low-frequency EPRI technique using spinning magnetic field gradients (SMFG). Utilizing a 300 MHz CW (continuous wave) EPRI system, SMFG enabled over 10-fold accelerated acquisition of image projections. 2D images with over 200 projections could be acquired in less than 3s and with 20s acquisitions good image quality was obtained on large aqueous free radical samples. This technique should be particularly useful for in vivo studies of free radicals and their metabolism.     

Reaction of tert-butyl isocyanate and tert-butyl isothiocyanate at the Ge(100) − 2 × 1 Surface

The adsorption of tert-butyl isothiocyanate and tert-butyl isocyanate at the Ge(100) ? 2 × 1 surface was probed using multiple internal reflection Fourier transform infrared (FTIR) spectroscopy X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) modeling. Results indicate that there are multiple surface products for each molecule. FTIR studies of tert-butyl isothiocyanate reveal adsorption through an S-dative bonded state, while XPS studies further suggest a reactive desorption product which leaves excess sulfur atoms at the surface. Studies of tert-butyl isocyanate indicate that the molecule dissociatively adsorbs at the surface, resulting in tert-butyl and germyl isocyanate groups, as the major pathway, in addition to forming several minor products, including a [2 + 2] cycloaddition product across the C=N bond. DFT was used to simulate vibrational spectra and map the reaction pathways, and confirms that the assigned products are energetically favorable.     

Interaction of NO with the O-rich RuO2(1 1 0) surface at 300 K

The interaction of NO with the O-rich RuO₂(1 1 0) surface, exposing coordinatively unsaturated O-bridge, O-cus, and Ru-cus atoms, was studied at 300 K by thermal desorption spectroscopy (TDS) and high-resolution electron energy-loss spectroscopy (HREELS). The conclusions are validated by isotope substitution experiments with ¹⁸O. During exposure to NO an O···N–O surface group (NO₂-cus) is formed with O-cus. Additionally, a smaller number of empty Ru-cus sites are filled by NO-cus. If one warms the sample to 400 K, NO₂-cus does not desorb but decomposes into O and NO again, the latter being either released into gas phase or adsorbed as NO-cus. With O-bridge such a surface group is not stable at 300 K. Our experiments further prove that O-cus is more reactive than O-bridge.     

Li3+ ion irradiation effects on ionic conduction in P(VDF–HFP)–(PC + DEC)–LiClO4 gel polymer electrolyte system

Swift heavy ion irradiation of P(VDF–HFP)–(PC + DEC)–LiClO₄ gel polymer electrolyte system with 48 MeV Li³⁺ ions having five different fluences was investigated with a view to increase the Li⁺ ion diffusivity in the electrolyte. Irradiation with swift heavy ion (SHI) shows enhancement of conductivity at lower fluences and decrease in conductivity at higher fluences with respect to unirradiated polymer electrolyte films. Maximum room temperature (303 K) ionic conductivity is found to be 2.2 × 10^− 2 S/cm after irradiation with fluence of 10¹¹ ions/cm². This interesting result could be ascribed to the fluence-dependent change in porosity and to the fact that for a particular ion beam with a given energy higher fluence provides critical activation energy for cross-linking and crystallization to occur, which results in the decrease in ionic conductivity. The XRD results show decrease in the degree of crystallinity upon ion irradiation at low fluences (≤ 10¹¹ ions/cm²) and increase in crystallinity at high fluences (> 10¹¹ ions/cm²). The scanning electron micrographs (SEM) exhibit increased porosity of the polymer electrolyte films after low fluence ion irradiation.