The role of new functional groups in the surface layer of LDPE during its high-voltage contact polarization

New functional groups containing Al-C bonds and Al₂O₃ molecules are formed in the surface layer of the polymer during depositing of aluminum on polyethylene films. This effect is absent when Au is deposited on the films. For capacitors with Al and Au electrodes, processes of high-voltage contact polarization and conductivity are studied at various configurations of the external electric field. If Al-C organometallic bonds and Al₂O₃ or, for example, Al-O-C bonds are formed in the surface layer during application of high electric fields, a number of dielectric “anomalies” are observed. They manifest themselves as the fact that an increase in the amount of cycles of a bipolar saw-tooth voltage is accompanied by a marked increase in the residual surface charge density. At such a configuration of the external field, a decrease in its frequency (for region, where dE/dt > 0) leads to a marked reduction in high-voltage dielectric permittivity to negative values. This phenomenon is explained by the fact that, along with the external field, an internal field appears owing to formation of the space charge near metal/polymer boundaries.     

Heterogeneous chemical crosslinking and high-voltage polarization in a ferroelectric copolymer of vinylidene fluoride and tetrafluoroethylene

The low-temperature chemical crosslinking of a copolymer of vinylidene fluoride and tetrafluoroethylene was studied through various physicochemical methods. The reaction was conducted in solution in the presence of diethylenetriamine as a crosslinking agent. The penetration of the mixture molecules and the crosslinking agent only in the amorphous phase of the copolymer was provided via selection of the ratio between a good solvent (dimethylformamide) and a poor solvent (ethanol). Owing to this, the crosslinking reaction occurred in the amorphous phase and hardly involved the crystals. This outcome was confirmed by wide-angle X-ray diffraction data. The structural and chemical changes in the amorphous phase during crosslinking were recorded with the use of IR spectroscopy and differential scanning calorimetry. It is shown that crosslinking is accompanied by the formation of C=C bonds in the copolymer chains. The study of high-voltage polarization and conductivity during exposure to bipolar rectangular pulses suggested that crosslinking leads to an increase in the carrier concentration. It was found that the surface potentials in the films increase with an increase in the number of high-voltage pulses applied to the sample. This circumstance is attributed to the fact that the double bonds formed in the copolymer chains can effectively trap negatively charged carriers.     

Molecular mobility and structuring in textured films of the ferroelectric copolymer of vinylidene fluoride with tetrafluoroethylene

The processes of molecular mobility in vinylidene fluoride–tetrafluoroethylene ferroelectric copolymer–based films textured via uniaxial drawing are studied. It is found that the cooperative mobility in the amorphous regions of the copolymer is described by the Vogel–Fulcher equation, whose parameters are sensitive to the drawing conditions. It is shown that a decrease in the drawing temperature causes a reduction in the effective activation energy of the above-mentioned motion. In accordance with the X-ray diffraction data, this circumstance may be explained by an increase in the packing density of chains in interlamellar spaces. This effect is partially provided by deformation-induced processes of formation of small and defective crystals of the paraelectric or antiferroelectric phase in spaces between neighboring lamellar crystals at the primary stage of crystallization. The relationship between the value of the orientational polarization in oriented films and the topology of structure formation in microfibrils is discussed. It is observed that enlargement of the interlamellar space (filled with a liquidlike amorphous phase) between neighboring crystals in microfibrils promotes an increase in the orientational polarization of chains in the amorphous phase.     

Polarization of block textured films of ferroelectric copolymers of vinylidene fluoride with tetrafluoroethylene in an external field

The polarization of oriented films of the ferroelectric copolymer of vinylidene fluoride and tetrafluoroethylene in a sinusoidal electric field is studied. In low fields, the copolymer behaves as a linear dielectric whose permittivity nonlinearly increases with the amplitude of the alternating field. This behavior is related to the preferential orientation of the dipole moment of the chain along the normal to the surface and to the monocrystal-structure formation. In higher fields, the hysteresis behaves formally coincidently with the behavior of an antiferroelectric. The important role of through stressed chains in the amorphous phase in the initiation of domains of a new direction in polar crystals is observed. Increased adsorption of water molecules on the surface of a polarized film is found and is attributed to the formation of the final density of the stable polarization charge on it.     

Room‐Temperature Ferroelectricity in an Organic Cocrystal

Ferroelectric materials exhibit switchable remanent polarization due to reversible symmetry breaking under an applied electric field. Previous research has leveraged temperature‐induced neutral‐ionic transitions in charge‐transfer (CT) cocrystals to access ferroelectrics that operate through displacement of molecules under an applied field. However, displacive ferroelectric behavior is rare in organic CT cocrystals and achieving a Curie temperature (T_C) above ambient has been elusive. Here a cocrystal between acenaphthene and 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane is presented that shows switchable remanent polarization at room temperature (T_C=68 °C). Raman spectroscopy, X‐ray diffraction, and solid‐state NMR spectroscopy indicate the ferroelectric behavior is facilitated by acenaphthene (AN) rotation, deviating from conventional design strategies for CT ferroelectrics. These findings highlight the relevance of non‐CT interactions in the design of displacive ferroelectric cocrystals.     

铁电极化对层状Bi2MoO6光催化性能的增强

通过对铁电光催化剂Bi_2MoO_6和聚甲基丙烯酸甲酯(PMMA)构建的有机-无机复合膜材料施加电场极化,来探究铁电极化对Bi_2MoO_6光催化剂活性提升的影响。未极化的Bi_2MoO_6在光照40 min时降解罗丹明B(RhB)的效率为57.6%,在光照150min时对双酚A(BPA)的降解效率为33.4%。在15 V电压下极化1.5 h的Bi_2MoO_6材料在相同条件下降解罗丹明B和双酚A的效率分别达到98.1%和79.2%,光催化活性得到了较大的提升。光催化活性提升的原因归因于内部电场的增强。未极化的Bi_2MoO_6的内电场的铁电畴是无序、分布不均匀的,光生载流子非常容易发生体内复合。当外加电场极化Bi_2MoO_6时,Bi_2MoO_6的铁电畴趋于有序,极化方向趋于同一方向,表面一侧(C+区)产生正电荷,在另一侧(C-区)产生负电荷,从C-区指向C+区的极化电场推动光生电子和空穴分别迁移到C+和C-区域。这一过程促使光生电荷载流子快速从体内迁移至表面,提高和延长了光生载流子的分离效率和寿命,导致光催化活性的提升。     

Structural dependence of phase transition and dielectric relaxation in ferroelectric poly(vinylidene fluoride-chlorotrifluoroethylene-trifluoroethylene)s

A series of ferroelectric poly(vinylidene fluoride-chlorotrifluoroethylene-trifluoroethylene)s, P(VDF-CTFE-TrFE), with systematically varied chemical compositions have been synthesized via a two-step approach consisting of copolymerization and dechlorination. The effect of polymer structure on polarization responses and dielectric properties has been investigated over a broad frequency and temperature range. As shown in the X-ray diffraction patterns, multiple phases coexist within the terpolymers as a result of the gauche conformation induced by the CTFE unit. The polarization hysteresis loops reveal the variation of remanent polarization and coercive electric field with the CTFE content due to the changes of crystallinity and crystalline phase. The observed broad dielectric constant peak with Vogel-Fulcher dielectric dispersion behavior suggests a transformation from a normal ferroelectric to a ferroelectric relaxor of the polymers. The relationship between the local relaxation process and relaxor ferroelectric behavior has been examined on the basis of the dielectric and mechanical loss tangents as a function of temperature.     

Glycol-based sol-gel process for the fabrication of ferroelectric PZT thin films

A new sol-gel system using ethylene glycol was developed for the fabrication of PZT thin films with compositions near the morphotropic phase boundary Pb(Zr_0.52Ti_0.48)O₃. Ethylene glycol was used as both a chelating agent and a solvent to replace the highly toxic methoxyethanol used in previous formulations. Thin films were deposited by spin coating the solutions onto platinized silicon substrates. Films were completely crystallized by about 600°C and contained the ferroelectric perovskite phase. A dielectric constant of about 750–800 at 1 KHz was obtained for thin films of 0.3 µm thickness. The hysteresis measurements revealed a remanent polarization of 15 mC/cm² with a coercive field of 60 kV/cm.     

Effect of the structure of a ferroelectric vinylidene fluoride-tetrafluoroethylene copolymer on the characteristics of a local piezoelectric response

The surface topography and characteristics of a local piezoelectric-response signal in isotropic films of a ferroelectric vinylidene fluoride-tetrafluoroethylene copolymer with a composition of 94: 6 are studied via scanning probe microscopy. The X-ray diffraction and IR spectroscopy data indicate that crystallization occurs in a mixture of β and γ polymorphic modifications. The exposure of the original film to a dc field bipolar leads to the formation of polarized regions exhibiting low stability. Recrystallization is followed by structural transformations accompanied by conformational transitions. As a result, there is a decrease in the concentration of chain segments in the T₃GT₃G^? conformation and, conversely, an increase in the concentration of isomers in the planar zigzag conformation. In addition, the characteristics of the local piezoelectric response after polarization become more stable, while the signal undergoes phase reversal. A second-harmonic piezoelectric response signal induced by the electrostriction effect is detected for the studied ferroelectric polymer samples. It is found that the original film has two types of regions that contribute to the measured signal. A change in the pattern of the second-harmonic piezoelectric-response signal in the recrystallized film is attributed to a change in the contribution of the electrostriction effect to the macroscopic piezoelectric response.     

Synthesis and characterization of photochemical sol鈥揼el derived lanthanum doped Bi4Ti3O12 film and its micro-patterns

A unique photochemical sol–gel route was employed to prepare La-doped Bi₄Ti₃O₁₂ film (BLT) using bismuth nitrate, lanthanum nitrate, and titanium tetra-n-butoxide as starting materials. Through complex reaction between benzoyl acetone and Ti⁴⁺ ions, the synthesized La-doped Bi₄Ti₃O₁₂ sol and gel films exhibited photosensitivity. Micro-patterned BLT film was obtained by a photoresist-free micro-patterning technique. The phase microstructures, electrical and ferroelectric properties of the as-prepared BLT film were characterized. By doping Lanthanum, the strong (00l)-oriented BT phase was transformed to randomly-oriented orthorhombic phase, resulting in the enhanced remanent polarization and improved fatigue properties. The obtained BLT film showed good ferroelectric properties with a long fatigue cycles over 10¹⁰ cycles.     

Ferroelectric and piezoelectric properties of a quenched poly(vinylidene fluoride-trifluoroethylene) copolymer

The ferroelectric and piezoelectric properties of melt-quenched unoriented poly(vinylidene fluoride-trifluoroethylene) (73 : 27) copolymer films as a function of the number of poling cycles have been studied. The investigation revealed that quenched films exhibit a decrease in D-E hysteresis behavior as the number of poling cycles increases when the samples are poled at room temperature. Corresponding decreases in remanent polarization, Pr, as well as small increases in the coercive field, Ec, were observed as the material was subjected to successive poling cycles. The piezoelectric coefficients, d₃₁ and e₃₁, also decreased as the number of poling cycles increased. In addition, a clear reduction in the “apparent” Curie transition temperature between unpoled and poled material was observed. Preliminary evidence indicates that films quenched from the melt to below T_c do not form a stable ferroelectric crystal phase as previously believed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2671–2679, 1997     

A coloured ferroelectric side chain polymer

Abstract

Coloured ferroelectric liquid-crystalline side chain copolymers containing 5 per cent and 15 per cent of an azo dye were synthesized and characterized by DSC, GPC and optical microscopy. Order parameters, S, of the azo compound exceeding 0·8 were measured in the frozen smectic phase for the 5 per cent copolymer. The copolymers exhibit fast electrooptic switching in the range of a few hundred microseconds to milliseconds in the S_c* phase. Both linear (i.e. electroclinic-like) and ferroelectric switching have been observed. Tilt angles of ～19° and spontaneous polarization of ～300nC cm^?2 have been recorded in the S_c* phase.     

Polarization processes in electrorheological fluids based on conductive polymers

Electrorheological (ER) fluids are composed of dielectric particles dispersed in an inert liquid of low electric permittivity. Upon the application of an electric field ER fluids rapidly solidify, or increase their viscosity. Characteristic increase of the viscosity of ER fluids is due to the formation of particle chains that bridge the electrodes. This process is greatly affected by polarization processes within the solid phase and at the surface of the grains. These phenomena are governed by dopants, functional groups, structure of the solid particles and the solid/liquid interface. To find relations between parameters of the ER effect and material properties of components of ER fluids, two main types of the materials were investigated: conjugated polymers [polyphenylene (PPP), pyrolyzed polyacrylonitrile (PAN) and polythiophene] and solid electrolytes based on polyacrylonitrile complexed with inorganic salts. It was found that the ER activity resulted from surface polarization processes due to the presence of polar species (PAN) or bulk polarization related to mobile ions (PPP). Polythiophene, despite the presence of a conjugated system of multiple bonds, showed only residual ER effect. Solid electrolyte‐based fluids exhibited relatively high activity originated from ionic polarization. Copyright © 2006 John Wiley & Sons, Ltd.     

The Role of Polarization in Photocatalysis

Semiconductor photocatalysis as a desirable technology shows great potential in environmental remediation and renewable energy generation, but its efficiency is severely restricted by the rapid recombination of charge carriers in the bulk phase and on the surface of photocatalysts. Polarization has emerged as one of the most effective strategies for addressing the above‐mentioned issues, thus effectively promoting photocatalysis. This review summarizes the recent advances on improvements of photocatalytic activity by polarization‐promoted bulk and surface charge separation. Highlighted is the recent progress in charge separation advanced by different types of polarization, such as macroscopic polarization, piezoelectric polarization, ferroelectric polarization, and surface polarization, and the related mechanisms. Finally, the strategies and challenges for polarization enhancement to further enhance charge separation and photocatalysis are discussed.     

Structure and electrical properties of Pb(Zr<Subscript>0.25</Subscript>Ti<Subscript>0.75</Subscript>)O<Subscript>3</Subscript> thin films on LaNiO<Subscript>3</Subscript>—Coated thermally oxidized Si substrates

Pb(Zr_0.25Ti_0.75)O₃ (PZT25) thin films were prepared on LaNiO₃-coated thermally oxidized silicon substrates by chemical solution deposition method, where LaNiO₃ electrodes were also prepared by a chemical solution deposition technique. The dielectric constant and dielectric loss of the PZT25 thin films were 570 and 0.057, respectively. The remanent polarization and coercive field were 20.11 μC/cm² and 60.7 kV/cm, respectively. The PZT25 thin films on LaNiO₃-coated thermally oxidized silicon substrates showed improved fatigue characteristics compared with their counterparts on plantium-coated silicon substrates.     

Structure of BaTiO<Subscript>3</Subscript> phases is studied by comparing neutron diffraction and Raman spectroscopy data

Considering the structural unit of the rhombohedral BaTiO₃ phase in the form of Ti(-O...Ti)₆ which was constructed by neutron diffraction data, we revealed an atomic group in the form of a pyramidal complex anion TiO₃²⁻. Inspection of the Raman spectra of this phase showed symmetry group C _s for this complex anion. From the similarity of the Raman spectra of all ferroelectric BaTiO₃ phases, we inferred that the orthorhombic phase and tetragonal phase are also built of TiO₃²⁻ complex anions. In the paraelectric cubic phase, TiO₃²⁻ complex anions decompose to oxygen ions O²⁻ and Ti²⁺(O⁻)₂ molecules, which are randomly oriented over 12 possible positions. Average Ti-O distances derived from neutron diffraction data were used to calculate Ti-O⁻ and Ti=O bond lengths in TiO₃²⁻ complex anions of the ferroelectric BaTiO₃ phases. Two types of Ti-O...Ti bridges were found to exist; they belong to weak and strong intermolecular bonds, which are similar to weak and strong hydrogen bonds (H-bonds) in ferroelectrics. Weak bonds exist in all of the three ferroelectric phases; strong bonds are only in the orthorhombic phase and the tetragonal phase. As a result of a low potential barrier height in strong bonds, an oxygen atom persistently hops from one potential well to an adjacent one, which is responsible for bands appearing below 100 cm⁻¹ in the Raman spectra of the tetragonal phase and orthorhombic phase. The data obtained on the structure of the ferroelectric BaTiO₃ phases were used to interpret their spontaneous polarization, repolarization, and permittivity.     

Thermally stimulated depolarization current in electron‐irradiated poly(vinylidene fluoride‐trifluoroethylene) (56/44 mol %) copolymers

The effect of electron irradiation on poly(vinylidene fluoride‐trifluoroethylene) (56/44 mol %) copolymers was studied with dielectric constant measurements, differential scanning calorimetry (DSC), X‐ray diffraction, thermally stimulated depolarization current (TSDC) spectroscopy, and polarization hysteresis loops. The dielectric relaxation peaks, obeying the Vogel–Fulcher law, indicated that the copolymers were transformed from a normal ferroelectric to a relaxor ferroelectric. The X‐ray and DSC results showed that both the crystalline and polar ordering decreased after irradiation, indicating a partial recovery from trans–gauche bonds to local trans bonds (polar ordering). Moreover, the peak temperature decreased with the irradiation dose in the TSDC spectra; this demonstrated fewer dipoles and crystalline regions in the irradiated copolymer films during the ferroelectric–paraelectric transition and was consistent with polarization hysteresis loop measurements. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1099–1105, 2004     

Magnitude and direction of the spontaneous polarization of ferroelectric liquid crystals with several bond moments

The magnitude and direction of the spontaneous polarization in most ferroelectric liquid crystals have been confirmed to be determined by the location and magnitude of the bond moment around the chiral carbon and the core. In compounds with several bond moments their relative orientation is very important for obtaining a large spontaneous polarization. Compounds with benzene rings in the core substituted with OH also have a large spontaneous polarization, perhaps due to the formation of hydrogen bonds. Reversal of the direction of the spontaneous polarization with temperature has been found for EFPPOPB. This anomalous behaviour has been explained tentatively in terms of a conformation change due to the existence of a flexible -CH₂- unit between the chiral carbon and the dipole moment.     

Frequency dependence of hysteresis associated with the electromechanical performance of PVDF film

The thickness strains, the electric displacements, and the hysteresis exhibited by poly(vinylidene fluoride) films under sinusoidal applied electric fields were measured over a range of frequencies from 0.05 to 100 Hz. The loops of strain‐ and electric displacement versus electric field exhibit a shape and hysteresis that undergoes a continuous and reversible change with frequency. At lower frequencies (<5 Hz), the shape and large hysteresis are characteristic of a ferroelectric with degrees of remanent polarization. At higher frequencies (>5 Hz), the shape and slight hysteresis of the loops are representative of a nonremanently polarized ferroelectric. This dependence of shape and hysteresis on frequency is attributed to the difference in rates between the rapid rate by which the polarization of domains of aligned dipoles is reversed in a periodic field and the slower rate by which the polarization is made remanent. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3207–3214, 2007     

Methods and Results of Optical Investigations on Semiconductor Interfaces

Measurement of the state of polarization of reflected light (ellipsometry) permits the determination of the thickness and refractive index of thin layers formed on a surface. The IR absorption spectra of such thin layers, which can be measured by means of internal reflection spectroscopy (IRS), provide information about their chemical composition. These methods have been used to study adsorption processes and the formation of reaction layers at semiconductor interfaces, and may also be used for measurements of free charge carriers in the space-charge region and in surface states. Results of such investigations are given in this article.