Electrical behaviour of butyl acrylate/methyl methacrylate copolymer films irradiated with 1.5 MeV electron beam

Electrical conductivity and dielectric parameters of the (BuA/MMA) copolymer films irradiated with 1.5 MeV electron beam (EB) have been studied. The samples were irradiated with different doses of the electron beam: 5, 10, 50, 125 and 200 kGy. The electrical conductivity of the samples was found to decrease as the irradiation dose increases. The temperature dependence of the direct current (dc) conductivity for unirradiated and irradiated samples has been obtained over a temperature range from 293 to 373 K. The activation energy values were calculated for all samples. Moreover, measurements of the dielectric constant, dielectric loss and alternating current (ac) conductivity were performed at a frequency range from 100 Hz to 5 MHz at room temperature. The results indicated that the EB irradiation has formed some traps in the energy gap, which reduce the movement of the charge carriers. Furthermore, a direct proportional relationship between the activation energy and the irradiation dose was estimated in two regions: below and above the glass transition temperature of the polymer. Dipole relaxation was observed in the samples, and the dose effect was found to shift this relaxation towards higher frequencies.     

聚偏氟乙烯薄膜压电性的研究

本文研究了聚偏氟乙烯的压电性。聚偏氟乙烯薄膜是用DMA溶剂浇铸而成,然后进行单轴拉伸,并采用热驻极的方法使薄膜成极。实验结果表明,压电活性依赖于拉伸比、极化电场、极化温度和极化时间。此外,还研究了聚偏氟乙烯压电薄膜的稳定性和某些使用特性。     

Investigation on the growth of CaCu3Ti4O12 thin film and the origins of its dielectric relaxations

Using the radio frequency magnetron sputtering, CaCu₃Ti₄O₁₂ (CCTO) thin films were deposited on platinized silicon substrates. The influence of annealing temperature on structures and morphologies of the thin films was investigated. The high annealing temperature increased the crystallinity of the films. Temperature dependence of the dielectric constant revealed an amazing different characteristic of the dielectric relaxation at ∼10 MHz, whose characteristic frequency abnormally increased with the decrease of the measuring temperature unlike the relaxations due to extrinsic origins. Meanwhile, the dielectric constant at high frequencies was close to the value derived from the first principle calculation. All these gave the evidences to ascribe this relaxation to the intrinsic mechanism.     

Improving piezoelectric performance of lead-free polymer composites with high aspect ratio BaTiO3 nanowires

Flexible and lead-free piezoelectric nanocomposites were synthesized with BaTiO₃ nanowires (filler) and poly(vinylidene fluoride) (PVDF) (matrix), and the piezoelectric performances of the composites were systematically studied by varying the aspect ratio (AR) and volume fraction of the nanowire and poling time. BaTiO₃ nanowires with AR of 18 were synthesized and incorporated into PVDF to improve the piezoelectric performance of the composites. It was found that high AR significantly increased the dielectric constant up to 64, which is over 800% improvement compared to those from the composites containing spheroid shape BaTiO₃ nanoparticles. In addition, the dielectric constant and piezoelectric coefficient were also enhanced by increasing the concentration of BaTiO₃ nanowires. The piezoelectric coefficient with 50-vol% BaTiO₃ nanowires embedded in PVDF displayed 61 pC/N, which is much higher than nanocomposites with spheroid shape BaTiO₃ nanoparticles as well as comparable to, if not better, other nanoparticle-filled polymer composites. Our results suggest that it is possible to fabricate nanocomposites with proper mechanical and piezoelectric properties by utilizing proper AR fillers.     

Enhanced β‐phase in PVDF polymer nanocomposite and its application for nanogenerator

Harvesting energy from the ambient mechanical energy by using flexible piezoelectric nanogenerator is a revolutionary step toward achieving reliable and green energy source. Polyvinylidene fluoride (PVDF), a flexible polymer, can be a potential candidate for the nanogenerator if its piezoelectric property can be enhanced. In the present work, we have shown that the polar crystalline β‐phase of PVDF, which is responsible for the piezoelectric property, can be enhanced from 48.2% to 76.1% just by adding ZnO nanorods into the PVDF matrix without any mechanical or electrical treatment. A systematic investigation of PVDF‐ZnO nanocomposite films by using X‐ray diffractometer, Fourier transform infrared spectroscopy, and polarization‐electric field loop measurements supports the enhancement of β‐phase in the flexible nanocomposite polymer films. The piezoelectric constant (d₃₃) of the PVDF‐ZnO (15 wt%) film is found to be maximum of approximately −1.17 pC/N. Nanogenerators have been fabricated by using these nanocomposite films, and the piezoresponse of PVDF is found to enhance after ZnO loading. A maximum open‐circuit voltage ~1.81 V and short‐circuit current of 0.57 μA are obtained for 15 wt% ZnO‐loaded PVDF nanocomposite film. The maximum instantaneous output power density is obtained as 0.21 μW/cm² with the load resistance of 7 MΩ, which makes it feasible for the use of energy harvesting that can be integrated to use for driving small‐scale electronic devices. This enhanced piezoresponse of the PVDF‐ZnO nanocomposite film‐based nanogenerators attributed to the enhancement of electroactive β‐phase and enhanced d₃₃ value in PVDF with the addition of ZnO nanorods.     

Freezing the polarization of CH3NH3PbI3 and CH3NH3PbI3-xClx perovskite films

Thin films of methylammonium lead halides, CH₃NH₃PbI₃ and CH₃NH₃PbI_3-xCl_x, were deposited onto symmetrical microstructured electrode arrays of gold or platinum on Si/SiO₂ wafers. Polarization studies were carried out on perovskite films under vacuum in the dark. For poling, a constant voltage was applied to the samples while the temperature was cycled between 295 K and 4 K. The measured current densities depending on the temperature showed distinct characteristics relating strongly to the crystal phase and the dielectric properties of the perovskite films. Voltage sweeps were carried out at different scan rates at specific temperature intervals after poling. The polarization of the films due to the migration of iodide vacancies in direction of the blocking perovskite/metal interface was frozen almost up to room temperature. Charge carriers were only able to cross the blocking barrier and contribute to the current where the ions have accumulated during poling. All J-V curves showed hysteresis: inverted and regular hysteresis at room temperature and below, respectively. Inverted hysteresis originates from the slow accumulation of ions at the blocking barrier, while regular hysteresis arises from a distortion in the adjacent crystals which will be discussed.     

Dielectric study of post-irradiation effects in gamma-irradiated polyethylenes

The post-irradiation dielectric behaviour of different polyethylenes (PEs) has been studied by means of dielectric loss (tan δ) analysis over the wide temperature (25–325 K) and frequency (1 kHz–1 MHz) ranges. For this reason, low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) samples were previously gamma irradiated in air to absorbed dose of 300 kGy. The irradiated samples were divided into two groups, and for the first one annealing treatment which can substantially reduce the concentration of free radicals were employed. For the second group, e.g. samples stored in air at room temperature after irradiation, post-irradiation evolution in free radical concentration, dielectric relaxation spectra and carbonyl content was investigated as a function of storage time, up to 90 days. Dielectric relaxation behaviour is related to differences in the initial structures of PEs (such as branching, crystallinity, etc.) and to the radiation-induced effects; carbonyl groups that were introduced by irradiation and/or delayed (post-irradiation) oxidation were regarded as tracer groups. Electron spin resonance (ESR), differential scanning calorimetry (DSC), infrared (IR) spectroscopy and gel measurements were used to determine the changes in free radical concentration, crystal fraction, oxidation and degree of network formation, respectively.     

Optical and dielectric characteristics of photochromic hybrid sol-gel materials

Photochromic silica based organic-inorganic hybrid materials containing covalently linked cyanoazobenzene chromophores were investigated by optical and dielectric spectroscopy. These materials, obtained via sol-gel process, were deposited onto glass substrates by spin coating technique to achieve thin transparent films. To investigate photoinduced alignment, the UV-Vis absorption spectra of the sol-gel films were recorded under illumination with linearly polarized blue light. Dielectric relaxation spectroscopy revealed a variety of relaxation processes: the α-process related to the dynamic glass transition temperature located around 150°C, and an Arrhenius-type β-relaxation (activation energy 58–60 kJ/mol) that was assigned to orientational fluctuations involving the azobenzene group. The correspondence between dielectric and photochemical behavior was discussed.     

PVDF/TiO2 nanocomposites prepared by solution blow spinning: Surface properties and their relation with S. Mutans adhesion

Thermoplastic nanocomposite materials with potential bactericide properties were prepared and their surface properties and adhesion to Streptococcus mutans, S. mutans, were characterized. Solution blow spinning was successfully used to prepare films with a fiber-like structure on the surface of nanocomposites based on Polyvinylidene fluoride, PVDF, filled with well dispersed TiO₂ nanoparticles. PVDF/TiO₂ nanocomposites were prepared varying the nanoparticles content (0%, 1%, 2%, 5% and 10% by weight). In order to understand the influence of the presence of TiO₂ nanoparticles and the final surface properties on the adhesion of S. mutans to the materials, a deep characterization was carried out focusing on the morphology, roughness, surface free energy from contact angle measurements and cell adhesion by single cell force spectroscopy. It was observed that the uniform dispersion of the nanofiller arose from nanoparticles embedded in the polymer when fibers were created during the blow spinning process. TiO₂ content influenced the topography of the films probably due to a direct effect on the solvent evaporation rate. Although this factor greatly conditioned the roughness of the samples and therefore the surface free energy, S. mutant adhesion on the substrates under study was concluded to be more dependent on the specific interactions with the surface polar groups of the material.     

Pressure dependence of the β molecular relaxation process and dielectric properties of polyvinylidene fluoride

The effects of hydrostatic pressure to 20 kbar on the β molecular relaxation process of polyvinylidene fluoride (PVDF) and on the dielectric properties in the neighborhood of this relaxation have been investigated. This relaxation has a strong influence on the electrical and mechanical properties of PVDF. Pressure causes a large shift to higher temperatures (～ 10K/kbar) of the dielectric relaxation peak and a decrease in the width of the distribution of relaxation times. This slowing down of the relaxation process is discussed in terms of the Vogel–Fulcher equation and related models, and it results from an increase in both the energy barrier to dipolar motion and the reference temperature (T₀) for the kinetic relaxation process which represents the “static” dipolar freezing temperature for the process. The general applicability of the Vogel–Fulcher equation to relaxional processes in polymers and other systems is briefly discussed. The pressure dependence of the dielectric constant both above and below the relaxation peak temperature (T_max) is found to be dominated by the change in polarizability. The effect is larger above T_max because of the relatively large decrease in the dipolar orientational polarizability with pressure.     

The pockels coefficient of new polymers for nonlinear optics and its time dependence after poling,compared with dielectric measurements

Epoxy polymers with donor–acceptor type side groups were synthesized for application in nonlinear optics. The stability of the Pockels coefficient was measured in thin films after poling. The relaxation times and their temperature dependence seem to be correlated with dielectric measurements. Guest–host systems (polystyrene and polymethylmethacrylate with dimethylaminonitrostilbene) were investigated for comparison.     

Electrical impedance spectroscopy study of piezoelectric PVDF membranes

Piezoelectric poly (vinylidene fluoride) (PVDF) membranes were prepared from pre-fabricated membranes by electrically poling in an intense electric field. The electrical impedance of PVDF membranes measured over a frequency range of 10^?2–10⁵ Hz before and after poling is presented. The effect of pressure on the impedance characteristics of un-poled and poled PVDF membranes was also studied. A four element model circuit, including a constant phase element (CPE) was fitted to the impedance spectra. The elements of the circuit fitted to the poled sample were more conductive compared with those of the un-poled sample. Stronger CPE elements in the circuit were detected in the poled samples under pressure suggesting that the piezoelectric activity of PVDF is the major contributor to the constant phase angle seen at low frequencies.     

Evaluation of poly(4‐methyl‐1‐pentene) as a dielectric capacitor film for high‐temperature energy storage applications

Poly(4‐methyl‐1‐pentene) (P4MP) was characterized to evaluate its viability as a high‐temperature dielectric film for capacitors. Detailed investigation of thermal, mechanical, rheological, and dielectric properties was carried out to assess its high‐temperature performance and processability. P4MP was melt‐processable below 270 °C without degradation and application temperatures as high as 160–190 °C can be achieved. The dielectric constant and loss of melt‐processed P4MP films was comparable to biaxially oriented polypropylene (BOPP) capacitor films, although the dielectric strength was lower. Enhancements in dielectric strength up to 250–300% were achieved via solution‐processing P4MP films, which could be easily scaled up on a roll‐to‐roll platform to yield isotropic, free‐standing films as thin as 3–5 μm. The influence of crystal structure, crystallinity, and surface morphology of these films on the dielectric properties was examined. The dielectric strength was further increased by 450% through biaxial stretching of solution‐cast films, and a Weibull breakdown field of 514 V/μm was obtained. The dielectric constant was very stable as a function of frequency and temperature and the dielectric loss was restricted to <1–2%. Overall, these results suggest that BOP4MP is a promising candidate to obtain similar energy density as a BOPP capacitor film but at much higher operating temperatures. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1497–1515     

Time and poling history dependent energy storage and discharge behaviors in poly(vinylidene fluoride-<Emphasis Type="Italic">co</Emphasis>-hexafluoropropylene) random copolymers

 We studied cycle time (0.01–10 s with triangular input waves) and poling history (continuous versus fresh poling) dependent electric energy storage and discharge behaviors in poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-HFP)] films using the electric displacement ― the electric field (D-E) hysteresis loop measurements. Since the permanent dipoles in PVDF are orientational in nature, it is generally considered that both charging and discharging processes should be time and poling history dependent. Intriguingly, our experimental results showed that the charging process depended heavily on the cycle time and the prior poling history, and thus the D-E hysteresis loops had different shapes accordingly. However, the discharged energy density did not change no matter how the D-E loop shape varied due to different measurements. This experimental result could be explained in terms of reversible and irreversible polarizations. The reversible polarization could be charged and discharged fairly quickly (< 5 ms for each process), while the irreversible polarization depended heavily on the poling time and the prior poling history. This study suggests that it is only meaningful to compare the discharged energy density for PVDF and its copolymer films when different cycle times and poling histories are used.     

Piezoelectric properties and molecular motion of poly(β-hydroxybutyrate) films

Piezoelectric, elastic, and dielectric properties of films of poly(β-hydroxybutyrate) (PHB), an optically active natural polymer, were measured as functions of frequency and temperature. In mechanical properties, three relaxation processes were observed at 10 Hz: the α dispersion at 130°C, the β dispersion at room temperature, and the γ dispersion at ?120°C. It was concluded from x-ray diffraction and the thermal expansion coefficient that the α dispersion can be ascribed to thermal molecular motions in the crystalline phase, that the β dispersion is the primary dispersion due to the glass transition, and that the γ dispersion is related to local molecular motion of the main chains in the amorphous phase. Piezoelectric relaxations were also observed in these relaxation regions. It is proposed that the high-temperature process is due to ionic dc conduction. The piezoelectric relaxation at room temperature is ascribed to the increase of piezoelectric activity in the oriented noncrystalline phase, in which the sign of the piezoelectric modulus is opposite to that in the oriented crystalline phase.     

Elastic properties and mechanical relaxation behaviors of PVDF (poly (vinylidene fluoride)) at temperatures between −20 and 100 °C and at 2 MHz ultrasonic frequency

The elastic properties of PVDF have been investigated as a function of temperature. The propagation velocity and absorption of longitudinal and transverse ultrasonic waves have been measured at a constant frequency of 2 MHz and temperatures between –20 and 100 °C. Hence, the temperature dependences of storage and loss elastic constants have been obtained for temperatures between –20 and 100 °C. It has been seen that the relaxation behavior is affected from the form of mechanical disturbance. For the longitudinal mode, only one relaxation peak at 42 °C, but for transverse mode three relaxation peaks at 28 °C, 60 °C, and 94 °C have been observed. The results have been compared with the literature values obtained previously for PVDF. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2862–2873, 2005     

STUDIES ON THE CHAIN STRUCTURAL EFFECT OF PVDF AND VDF (52)/TrFE(48) COPOLYMER BY MEANS OF THE METHOD OF DIELECTRIC RELAXATION

Through improvements on a fully commercial and automatic system measuring frequency andtemperature spectra of the complex elastic, dielectric, and piezoelectric constants of polymerfilms, the precision for measuring complex dielectric constants achieved 2‰ from original 1%.The complex dielectric constants of PVDF, VDF (95 )/VF (5), VDF (52)/TrFE (48) and VDF(47. 5)/TrFE(47. 5)/HeFP (5) over a range of-120-140℃, 10-2-10~4Hz were measured bythe described system. The diminution of chain regularity due to 5mol% vinyl fluoride in the chainof PVDF led to disappearing of α-relaxation and increase in T_g (3℃) of VDF (95)/VF (5).Because of the spatial impediment of 5mol% HeFP in copolymer chain, the Curie point of VDF(52)/TrFE(48) decreased by 30℃ and T_g by 3℃. The behavior of dielectric relaxation indicatedthat 5mol% vinyl fluoride and HeFP made the relaxation strength of noncrystalline regionreduced, the activation energy of local relaxation increased and the relaxation time prolonged respectively.     

Dielectric behavior of some aromatic polyimide films

Aromatic polyimides were prepared by polycondensation reaction of two aromatic diamines, such as 4,4′-diaminodiphenylmethane (DDM) and 3,3′-dimethyl-4,4′-diaminodiphenylmethane (MDDM), with aromatic dianhydrides, such as 4,4′-isopropylidene-diphenoxy-bis(phthalic anhydride) (6HDA), benzophenonetetracarboxylic dianhydride (BTDA) and hexafluoroisopropylidene-bis (phthalic anhydride) (6FDA). These polymers are soluble in polar aprotic solvents and can be cast into thin films from such solutions. The polyimides show high thermal stability, with decomposition temperature being above 430 °C in air, and high glass transition temperature being in the range of 200–287 °C. The free standing films, having the thickness of tens of micrometers, exhibited good mechanical and electrical insulating properties. The dielectric constant, molecular mobility and AC conductivity of thin films prepared from these polymers were investigated in detailed. The study of their dielectric behavior evidenced low dielectric constant values, in the range of 2.88–3.48 at 1 Hz at room temperature, and three relaxation processes (γ,β₁ and β₂) were observed at sub-glass temperatures for polyimides based on 6HDA and 6FDA and only two (γ and β) relaxations were detected for polyimides based on BTDA. The cooperativity of the molecular motions associated with the relaxation processes was discussed.     

Orientation dependence of the dielectric and piezoelectric properties for the Ba0.98Ca0.02Ti0.96Sn0.04O3 thin films

Lead free Ba_0.98Ca_0.02Ti_0.96Sn_0.04O₃ (BCST) thin films with (110), (111) and (001) orientations were processed via chemical solution deposition, and effects of orientation on the dielectric and piezoelectric properties were investigated in some aspects. The (110) orientated BCST thin films exhibit highest Curie temperature (T _c of 85 °C) and lowest dielectric loss (tan δ of 0.02). While, the (111) orientated BCST thin films exhibit highest dielectric tunability (74 %) and largest piezoelectric coefficient (d ₃₃ of 78 pm/V), which indicate that it is a promising lead-free replacement for lead-based applications. The anisotropic dielectric and piezoelectric properties in the three kinds of oriented BCST films has been attributed to the difference of structure, in-plane stress and polarization rotation in orientation engineered BCST films. This work clearly reveals the dielectric and piezoelectric properties of BCST films exhibit a strong sensitivity to orientation.     

Microstructure,dielectric properties and optical band gap control on the photoluminescence behavior of Ba[Zr<Subscript>0.25</Subscript>Ti<Subscript>0.75</Subscript>]O<Subscript>3</Subscript> thin films

Ba[Zr_0.25Ti_0.75]O₃ (BZT) thin films were synthesized by the complex polymerization method and heat treated at 400 °C for different times and at 700 °C for 2 h. These thin films were analyzed by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, field emission gun-scanning electron microscopy (FEG-SEM) and atomic force microscopy (AFM), Ultraviolet–visible (UV–vis) absorption spectroscopy, electrical and photoluminescence (PL) measurements. FEG-SEM and AFM micrographs showed that the microstructure and thickness of BZT thin films can be influenced by the processing times. Dielectric constant and dielectric loss of BZT thin films heat treated at 700 °C were approximately 148 and 0.08 at 1 MHz, respectively. UV–vis absorption spectra suggested the presence of intermediary energy levels (shallow and deep holes) within the band gap of BZT thin films. PL behavior was explained through the optical band gap values associated to the visible light emission components.