Dielectric relaxation in vinylidene fluoride–hexafluoropropylene copolymers

The molecular mobility in copolymers of vinylidene fluoride–hexafluoropropylene VDF/HFP of 93/7 and 86/14 ratios has been investigated by means of broadband dielectric relaxation spectroscopy (10^?1–10⁷ Hz), differential scanning calorimetry DSC (?100 to 150°C), and of wide angle X‐ray diffraction WAXS. Four relaxation processes and one ferroelectric‐paraelectric phase transition have been detected. The process of the local mobility β‐ (at temperatures below glass transition point) is not affected by chemical composition of the copolymer and the formed structure. Parameters of segmental mobility in the region of glass transition (α_a‐relaxation) depend on the ratio of comonomer with lower kinetic flexibility. α_c‐relaxation is clearly observed only in VDF/HFP 93/7 copolymer, which is characterized by a higher crystallinity and a higher perfection of crystals of α‐ (α_p‐) phase. Diffuse order–disorder relaxor type ferroelectric transition connected with the destruction of the domains in low‐perfect ferroelectric phase in the amorphous regions has been detected for both copolymers. An intensive relaxation process (α‐process) was observed for both copolymers in high‐temperature region. DSC data shows that it falls on the broad temperature region of α‐phase crystals melting. It is considered to be connected with the space charge relaxation. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Effect of recrystallization on the molecular mobility of a copolymer of vinylidene fluoride and hexafluoropropylene

Relaxation processes in copolymers of vinylidene fluoride with hexafluoropropylene (93/7) were studied by means of a dielectric method. The initial extruded film was recrystallized by free heating to temperatures above the melting point and by subsequent cooling. This increased both the perfection of the crystal phase and the degree of crystallinity. The impact of recrystallization on both the relaxation times (τ's) and the activation parameters of the local mobility (β process) and micro‐Brownian cooperative mobility in amorphous phase (α_a process) was almost negligible, whereas the τ's of the α_c relaxation were an order of magnitude higher after recrystallization. Qualitatively, it was predicted by the soliton model for the α_c relaxation. The recrystallization affected characteristics of the transition at the highest temperature (α) registered in the region of the melting of the crystals even more. The process is related to the relaxation of the space charge formed by ionogenic impurities, which migrate through the amorphous phase with high free volume. It was shown that the dynamics in the amorphous phase controlled the drift mobility of free charge carriers and, by that, determined the τ of the space charge relaxation process. The structuring processes during the recrystallization also affected the parameters of the order–disorder transition in the low‐perfect ferroelectric (antiferroelectric) phase. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Polarization and space charge performance in PVDF with MPB composition BCZT doped composite films

The morphotropic phase boundary composition 47(Ba_0.7Ca_0.3)TiO₃–0.53Ba(Zr_0.2Ti_0.8)O₃ (calcium barium zirconate titanate, BCZT) were doped in poly(vinylidene fluoride) matrix by solution casting method. The effect of BCZT particles on polarization characteristics of composite films were investigated through dielectric spectra, hysteresis, and thermally stimulated discharge current measurements. It reveals that relative dielectric constant, remnant polarization, and energy storage density gradually increase and reach the maximum value with 39 at 100 Hz, 1.17 μC/cm², and 3.69 J/cm³ at the concentration of 20 vol % BCZT. The Gaussian method was employed to decompose thermally stimulated discharge current composite peaks and an approximate model was used to analyze the activation energy levels of these films. The results indicated that the introduction of BCZT is effective for the charge carrier traps and activation energy distribution in the relaxation process of composite films. The decreased molecular movements, and increased interfaces formed between the BCZT grains and the poly(vinylidene fluoride) matrix may be responsible for these performances. And a new relaxation peak appeared at about 370 K in composites when BCZT concentration exceeded 20 vol %, which was considered to be caused by the ferroelectric‐paraelectric phase transition of BCZT. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45362.     

Improved phase-change properties of Sn–Zn–Sb alloys with a two-step crystallization process for multi-level data storage applications

We investigated Te-free ZnSb films for the potential applications in multi-level data storage. It was found that, the films could exhibit three phases during crystallization process, e.g., high amorphous phase, intermediate metastable ZnSb phase, and low stable ZnSb phase. However, the formation of the voids in the metastable ZnSb phase, high crystallization temperature and large amorphous resistance have an influence on the structure stability and SET power consumption during the repeated switching process. Interestingly, doping of Sn with a concentration of 11.1 at% into ZnSb can prevent the growth of metastable ZnSb grains, and different crystals like SnSb and ZnSnSb₂ can be precipitated out during heat treatment, but the two-step crystallization process is kept unchanged. Moreover, the chemical environment of Sn in Zn–Sb studied by X-ray photoelectron spectroscopy indicates the formation of the large number of Sn–Sb and Sn–Zn bonds, which in turn reduces the crystallization temperature, lowers crystallization activation energy as well as decreases the amorphous resistance. The in situ transmission electron microscopy measurements on the optimized (ZnSb)_88.9Sn_11.1 composition confirms a two-step phase transition process from amorphous to rhombohedral SnSb crystals first, and then to the stable tetragonal ZnSb₂Sn crystals. These excellent properties make Zn–Sb–Sn materials useful as a phase-change layer in low-power and high-density memory.     

The influence of the crystallization temperature on the reliability of PbTiO3 thin films prepared by chemical solution deposition

The preparation of quality ferroelectric PbTiO₃ (PT) thin films at the lowest possible temperature preserving its functional properties is necessary for their integration in microdevices. The crystallization below, close or above the para-ferrolectric transition temperature of the PbTiO₃ must produce an important effect on the microstructure, texture and residual stress state of the films and on functional properties. In this paper, the Chemical Solution Deposition method has been used to prepare PbTiO₃ films at different temperatures around that of the ferroelectric to paraelectric phase transition. The films were analyzed by X-ray diffraction, Optical and Scanning Electron Microscopy, and their microstructure, texture and residual stress correlated with their functional properties. The results show that these PT films prepared with crystallization temperatures close or below the phase transition develop a favorable microstructure and texture that lead to high remnant and saturation polarization values, making them good candidates for their integration in microdevices.     

Dielectric and microwave properties of barium strontium titanate (BST) thick films on alumina substrates

The dielectric and microwave properties of barium strontium titanate (BST) thick films on alumina substrates have been investigated. The BST films were screen printed and sintered at temperatures below 1300 °C. At temperatures below the Curie point the BST films exhibit tunability in the range 15–35% under a DC bias field of 2 kV/mm. The dielectric loss is critically dependent on film thickness with lower losses (<10⁻²) for the thicker films (>100 μm). A relaxation process appears to take place for the BST films in the MHz to GHz frequency regime. The variation of permittivity with bias field exhibits hysteretic behaviour in both the ferroelectric and paraelectric regions. This is believed to arise due to the non-uniform composition and existence of micro/nano-polar phases in the films.     

Metastable Tantalum Oxide Formation During the Devitrification of Amorphous Tantalum Thin Films

Microstructural evolution during the devitrification of amorphous tantalum thin films synthesized via pulsed laser deposition was investigated using in situ transmission electron microscopy (TEM) combined with ex situ isothermal annealing, bright‐field imaging, and electron‐diffraction analysis. The phases formed during crystallization and their stability were characterized as a function of the chamber pressure during deposition, devitrification temperature, and annealing time. A range of metastable nanocrystalline tantalum oxides were identified following devitrification including multiple orthorhombic oxide phases, which often were present with, or evolved to, the tetragonal TaO₂ phase. While the appearance of these phases indicated the films were evolving to the stable form of tantalum oxide—monoclinic tantalum pentoxide—it was likely not achieved for the conditions considered due to an insufficient amount of oxygen present in the films following deposition. Nevertheless, the collective in situ and ex situ TEM analysis applied to thin film samples enabled the isolation of a number of metastable tantalum oxides. New insights were gained into the transformation sequence and stability of these nanocrystalline phases, which presents opportunities for the development of advanced tantalum oxide‐based dielectric materials for novel memristor designs.     

Electrical characterizations of PZT ceramics in large frequency and temperature ranges

Electrical properties of PbZr_0.75Ti_0.25O₃ ceramics have been characterized. The measurements have been made in frequency ranging from 20 to 2.10⁹ Hz and between 20 and 730 °C for low and medium frequencies. Typically at room temperature, the dielectric constant ′ is higher than 500 at 1 MHz whereas the loss tangent is close to 0.01. From ′(T) measurements, the Curie temperature of our sample has been determined at 320 °C. In the paraelectric state, ′(T) follows the empirical Curie–Weiss law near the phase transition which is of second order type. The increase of ′ observed at high temperatures and low frequencies in the paraelectric state are explained: this abnormal behavior is due to the migation of oxygen ions towards the electrodes, creating an additional non-ferroelectric interface which generates a Maxwell–Wagner effect.     

Friction‐induced electroactive β polymorph of poly(vinylidene fluoride)

Poly(vinylidene fluoride) (PVDF) has been widely used in electric devices due to electroactive β polymorph. In this article, we probe the formation of β phase under friction by spectroscopy and thermal analysis. During continuous friction, entire sliding of PVDF is identified with two regimes, i.e., running‐in and steady‐state. At initial running‐in period, friction surfaces are dominated by plastic strain, which leads to striking formation of β phase from α polymorph (α→β). Subsequently, melting‐flow domains almost cover friction surfaces at steady‐state. Thus, formation of β crystal is correspondingly induced by shear crystallization. Nevertheless, β‐crystal content at steady‐state is lower than that at running‐in. With sliding proceeding, moreover, β‐crystal content exhibits a gradually decreasing tendency, attributed to rising surface temperature. Besides, the friction‐induced β phase is further confirmed by evaluation of wear debris. Overall, friction plays a crucial role as to the formation of β phase. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46395.     

Utilizing TiO2 amorphous precursors for polymorph selection: An in situ TEM study of phase formation and kinetics

Selective synthesis of metastable polymorphs requires a fundamental understanding of the complex energy landscapes in which these phases form. Recently, the development of in situ high temperature and controlled atmosphere transmission electron microscopy has enabled the direct observation of nucleation, growth, and phase transformations with near atomic resolution. In this work, we directly observe the crystallization behavior of amorphous TiO₂ thin films grown under different pulsed laser deposition conditions and quantify the mechanisms behind metastable crystalline polymorph stabilization. Films deposited at 10 mTorr chamber oxygen pressure crystallize into nanocrystalline Anatase at 325°C, whereas films deposited at 2 mTorr crystallize into significantly larger needle-like grains of Brookite and Anatase at 270°C. Increasing film deposition rate by a factor of 4 results in a 10× increase in the crystalline growth front velocity as well as a decrease in crystallization temperature from 270°C to 225°C. Engineering the amorphous precursor state through deposition conditions therefore provides routes to microstructure control and the accessibility of higher energy metastable phases.     

Enthalpy relaxation of photopolymerized multilayered thiol‐ene films

Multilayered thiol‐ene network films with two and three different components were fabricated by spin coating and photopolymerization. The distinctive glass transition temperatures of each layer component were observed at corresponding glass transition regions of each bulk sample. Sub‐T_g aging of 10‐, 21‐, and 32‐layered thiol‐ene films was investigated in terms of enthalpy relaxation. Enthalpy relaxation of each layer component occurred independently and presented the characteristic time and temperature dependency. Overlapped unsymmetrical bell‐shaped enthalpy relaxation distribution having peak maximum at T_g‐10°C of each layer component was observed, resulting in broad distribution of enthalpy relaxation over wide temperature range. In addition, enthalpy relaxation of each layer component in the multilayered thiol‐ene films was significantly accelerated comparing to that of bulk thiol‐ene samples. Dynamic mechanical thermal properties of multilayered thiol‐ene films also showed two and three separated glass transition temperature. However, for 32‐layered thiol‐ene film consisting of three different layer components, glass transition and damping region are overlapped and the width is extended more than 100°C. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Dynamic of polar nanoregions and its impact on the pyroelectric and dielectric properties in paraelectric KTN materials

The origin of the excellent properties of KTN-based materials around Curie temperature (T_C), which should be originated form the motion of polar nanoregions (PNRs), has attracted considerable research interest. In this paper, the relaxation of a KTa_0.63Nb_0.37O₃ single crystal is discussed with the temperature dependence of permittivity. Moreover, its pyroelectric effect above T_C is investigated. In detail, the pyroelectric coefficient decreases from ∼110.0 to ∼13.0 μC/(m² K), with the temperature increasing from 22 to 33°C, and finally reduces to 0 at 100°C with PNRs disappear. Moreover, the dynamic dielectric nonlinearity for the KTN single crystal is studied in the paraelectric phase. To investigate these mechanisms, the amplitude and phase angle of the first and third harmonics under various electric fields, frequencies, and temperatures are analyzed. As a result, the motions of PNRs induced by electric field, which is pinned and depinned by the defect, are presented to explain the nonlinear dielectric response observed in the paraelectric KTN single crystal.     

Morphology and properties of isotactic polypropylene modified with hydrocarbon resin MBG273. I. Binary blends

We investigated a system formed of isotactic polypropylene (iPP) and hydrogenated hydrocarbon resin MBG273 (up to 30 wt % resin) to study the influence of the composition on the morphology, structure, and properties of its blends and derived films. All the blends, after the mixing of the components in the melt and cooling at room temperature, were formed by a crystalline phase of iPP and by one homogeneous phase formed by amorphous iPP and the MBG273 resin. The presence of MBG273 did not influence the crystalline structure of iPP, which remained, for every blend, α‐monoclinic, but it reduced the crystallization temperature and nucleation density of iPP. Differential scanning calorimetry and dynamic mechanical thermal analysis showed an increase in the glass‐transition temperature with the resin content, confirming the formation of one amorphous phase. Tensile property analysis indicated an increase in Young's modulus and a decrease in the elongation at break of films as a function of the resin content in the blends. The water vapor permeability and tensile mechanical properties were related to an increase in the glass transition with the addition of MBG273. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3454–3465, 2004     

Metastable ferroelectric phase in lanthanum-doped lead zirconate titanate stannate antiferroelectric ceramics

Metastable ferroelectric phase induced by electric field in lanthanum-doped lead zirconate titanate stannate antiferroelectric ceramics near morphotropic phase boundary were studied as a function of electric field, temperature and hydrostatic pressure. Upon application of an electric field to the antiferroelectric ceramics, the rhombohedral FE phase can be induced from tetragonal AFE phase when the induced field was larger than the critical field E_AEF-FE, and the induced FE phase can remain metastable state when the electric field was removed. The metastable FE phase was unstable, which recovered to AFE phase again with the increasing of temperature at about 80 °C and the increase of hydrostatic pressure at 150 MPa, respectively. Large charge release from remanent polarization of metastable FE phase was accompanied with phase transition. The relationships of critical parameters of field-induced phase transition were also shown.     

Semi‐interpenetrating polymer networks based on polyurethane and polyvinylpyrrolidone. II. Dielectric relaxation and thermal behaviour

Semi‐interpenetrating polymer networks (semi‐IPNs) based on crosslinked polyurethane (PU) and linear polyvinylpyrrolidone (PVP) were synthezised, and their thermal and dynamic mechanical properties and dielectric relaxation behavior were studied to provide insight into their structure, especially according to their composition. The differential scanning calorimetry results showed the glass transitions of the pure components: one glass‐transition temperature (T_g) for PU and two transitions for PVP. Such glass transitions were also present in the semi‐IPNs, whatever their composition. The viscoelastic properties of the semi‐IPNs reflected their thermal behavior; it was shown that the semi‐IPNs presented three distinct dynamic mechanical relaxations related to these three T_g values. Although the temperature position of the PU maximum tan δ of the α‐relaxation was invariable, on the contrary the situation for the two maxima observed for PVP was more complex. Only the maximum of the highest temperature relaxation was shifted to lower temperatures with decreasing PVP content in the semi‐IPNs. In this study, we investigated the molecular mobility of the IPNs by means of dielectric relaxation spectroscopy; six relaxation processes were observed and indexed according the increase in the temperature range: the secondary β‐relaxations related to PU and PVP chains, an α‐relaxation due to the glass–rubber transition of the PU component, two α‐relaxations associated to the glass–rubber transitions of the PVP material, and an ionic conductivity relaxation due to the space charge polarization of PU. The temperature position of the α‐relaxation of PU was invariable in semi‐IPNs, as observed dynamic mechanical analysis measurements. However, the upper α‐relaxation process of PVP shifted to higher temperatures with increasing PVP content in the semi‐IPNs. We concluded that the investigated semi‐IPNs were two‐phase systems with incomplete phase separation and that the content of PVP in the IPNs governed the structure and corresponding properties of such systems through physical interactions. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1191–1201, 2003     

Space charge distribution and crystalline structure in low density polyethylene (LDPE) blended with high density polyethylene (HDPE)

The space charge distribution in polyethylene blends under direct current electrical field was measured by a pulsed electro‐acoustic method. It was found that blending LDPE with 0.5 wt% HDPE decreased the amount of accumulated space charges and improved their distribution. Small‐angle light scattering and differential scanning calorimetry showed that crystallization of LDPE/HDPE started at higher temperature than virgin LDPE, and the sizes of LDPE/HDPE spherulites were smaller than that of LDPE. HDPE plays a role on nucleation during the crystallization process. Crystalline form was investigated by wide‐angle X‐ray diffraction and the results indicate that the crystal form did not change after blending. The reduction of space charges in the blended sample can be explained as the result of the dissipation of charges through boundary regions of smaller spherulites. Copyright © 2004 Society of Chemical Industry     

Features of structure formation and electrophysical properties of poly(vinylidene fluoride) crystalline ferroelectric polymers

The influence of intramolecular dipole–dipole interaction changes on structure formation peculiarities and some electrophysical properties were investigated with example of copolymers of vinylidene fluoride with tetrafluoroethylene and hexafluoropropylene with different compositions. The decrease of such dipole–dipole interactions in vinylidene fluoride/tetrafluoroethylene copolymers leads to an increase of the a and b parameters of the ferroelectric phase lattice and were accompanied by a shift of the Curie point to lower temperatures. The presence of peak‐halo at angles near 2θ = 18° were attributed to a paraelectric phase localized in the interfacial domains at the crystal–amorphous phase boundaries. Similar peak‐halos for vinylidene fluoride/hexafluoropropylene copolymers crystallizing into the nonpolar α phase were associated with the presence of an antiferroelectric phase formed by the chains in the planar zigzag conformation. The temperature range where dielectric anomalies were detected was characterized by conformational changes at which the decrease in planar zigzag conformation isomers took place. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

High‐Temperature Dielectric Relaxation in Pb(Mg1/3Nb2/3)O3–PbTiO3 Single Crystals

We reported the dielectric properties of Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ single crystal in the temperature range of 300–1073 K and the frequency range of 100 Hz–10 MHz. Our results showed the coexistence of both true‐ and pseudo‐relaxor behaviors in the crystal. The true relaxor behavior related to the paraelectric‐ferroelectric phase transition occurs at~423 K. The pseudo‐relaxor behavior appearing at~773 K was found to be related to oxygen vacancies. Further investigation reveals that the pseudo‐relaxor behavior has fine structure: it contains two oxygen‐vacancy‐related relaxation processes. The low‐temperature relaxation process is a dipolar relaxation created by the hopping motions of the oxygen vacancies, and the high‐temperature relaxation process is a Maxwell‐Wagner relaxation caused by the sample/electrode contacts.     

Preparation and properties of porous BaTiO3 nanostructured ceramics produced from cuboidal nanocrystals

The preparation and properties of BaTiO₃ nanostructured ceramics with porosity level in the range of percolation limit (33% and 37% porosity) produced by partial sintering of cubic nanoparticles are presented. Hydrothermally synthesized cuboid-like particles were produced by using Field-Assisted Sintering Technique facility in which temperature and pressure were selected to ensure the consolidation of mechanically stable porous nanoceramics, while preserving as much as possible the starting grain shape. Nanosized grains in the range of (10–40) nm and multiscale porosity ranging from a few nm to hundreds of nm were observed in the sintered ceramics. The dielectric constant of porous nanoceramics assumes low values of ~280–320 and shows a flat thermal response typical to nanostructured ceramics, without a net ferroelectric-paraelectric peak, followed by a Curie-Weiss dependence in the paraelectric state, with negative Curie Weiss temperatures and lowered Curie constant, as result of porosity and ultrafine grain size. A strong conductivity relaxation around room temperature related to air-ceramic interface phenomena indicated a possible sensitivity of these ceramics for gas sensing. Preliminary qualitative tests with saturated acetone vapours have shown a good response of both resistive and reactive components of such porous BaTiO₃ nanoceramics and possible gas sensing interface-related mechanisms were discussed.     

Enhanced magnetic and dielectric properties of Ti‐doped YFeO3 ceramics

Polycrystalline YFeO₃ (YFO) and YFe_1?(4/3)xTi_xO₃(YFTO) ceramics were prepared using the powder synthesized from the sol‐gel route. X‐ray diffraction analyses of the polycrystalline ceramics revealed the crystallization of the phase in orthorhombic crystal structure associated with the space group Pnma. The magnetization versus magnetic field hysteresis loops were obtained at room temperature for YFO and YFTO ceramics. The magnetic property changes from weak ferromagnetic in YFO to ferromagnetic in YFTO ceramics. The dielectric constant recorded at room temperature for YFTO ceramics was six times higher than that of YFO, whereas the dielectric loss gets reduced to 0.06 from 0.3 for YFO at 1 kHz. Impedance spectroscopy study carried out on YFO and YFTO ceramics confirmed the existence of non‐Debye‐type relaxation. Observed single semicircle in Z′ vs ?Z′′ plot established the incidence of intrinsic (bulk) effect and ruled out any grain boundary or electrode effects. The mechanism for the dielectric relaxation and electrical conduction process observed in YFO and YFTO ceramics was discussed by invoking electric modulus formalisms. Activation energy obtained by ac conductivity study suggested that the conduction process in YFO was linked up with the existence of the polaron and oxygen vacancies, whereas only oxygen vacancies contribute to the conduction process in YFTO ceramics.