Electrocaloric response in a relaxor ferroelectric polymer at temperatures far below the dielectric maximum

ABSTRACT

Relaxor ferroelectric polymer poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) (P(VDF-TrFE-CFE)) and its blends have been shown to exhibit a giant electrocaloric effect (ECE) over a broad temperature range, e.g. from 0 to 50 °C. Here, a special calorimeter was designed to directly characterize the heat absorbed Q_ECE due to ECE cooling of the blend of P(VDF-TrFE-CFE) with P(VDF-TrFE) 65/35 mol% copolymer down to ?15 °C, which covers the temperature range for many refrigeration applications. From Q_ECE, the isothermal entropy change ΔS and adiabatic temperature change ΔT are deduced. The data reveal that at ?15 °C the relaxor terpolymer/copolymer blend generates ΔS = 23.0J kg^?1 K^?1 and ΔT = 5.1K under 100 MV/m, which are still more than 65% of the ECE at room temperature. This temperature is far below the dielectric peak temperature of the relaxor blend and the results reveal the promise of the relaxor polymers for a broad range of EC cooling applications.     

Relaxor ferroelectric behavior in high-energy electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymers

This paper presents experimental results showing that in a certain composition range and under a proper electron-irradiation treatment, a normal ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer can be converted into a material exhibiting many typical features of relaxor ferroelectrics, suggesting that this is a new class of relaxor ferroelectric material. Furthermore, the irradiated copolymer can generate giant electrostriction (ƽ%) with a high elastic energy density. The X-ray diffraction results obtained from the irradiated copolymer under electric field, indicate that the observed polarization and strain responses are mainly due to the local phase transformation from a non-polar phase to a polar phase.     

Pyroelectric and elastic properties of lead magnesium niobate-and barium titanate-based solid solutions near a phase transition

The pyroelectric, elastic, and electrostrictive properties of lead magnesium niobate-and barium titanate-based solid solutions were studied under a dc electric field near a phase transition. The results obtained for a relaxor transition (in a lead magnesium scandium niobate solid solution) are compared with those derived for a conventional ferroelectric transition (in a barium strontium titanate solid solution). The dielectric and electromechanical contributions to the induced pyroelectric effect are discussed.     

Dielectric and optical properties of Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)<Subscript>0.8</Subscript>Ti<Subscript>0.2</Subscript>O<Subscript>3</Subscript> (PMNT-0.2) ferroelectric relaxor single crystals

The dielectric and optical (optical transmission, small-angle light scattering, birefringence) properties of PMNT-0.2 single crystals and their variation induced by a dc electric field have been studied. The birefringence was found to increase anomalously at the transition from the rhombohedral ferroelectric to the inhomogeneous relaxor phase (the spontaneous ferroelectric transition temperature T_sp). Below T_sp, the dielectric and optical properties were observed to exhibit anomalies originating from reorientation and growth of domains in size. Unlike ferroelectric relaxors of the type of PbB′_1/3B′_2/3O₃ and PbB′_1/2B′_1/2O₃, in PMNT-0.2 neither induction of the ferroelectric phase by an electric field nor thermally stimulated destruction of the ferroelectric state occurs through the percolation mechanism (i.e., they are not accompanied by anomalously narrow maxima in small-angle light scattering). This is attributed to the inhomogeneous structure of the relaxor phase, as a result of which the phase transition does not take place simultaneously in various regions of the crystal.     

Evolution of domain processes during the transition from classical ferroelectric to relaxor ferroelectric

Dependences of the relative permittivity on the external bias electric field of ceramics of the yPZN-mPMN-nPNN-xPT (x = 0.25?C0.40) system have been studied. The evolution of domain processes during the concentration transition from classical ferroelectric to relaxor ferroelectric has been shown. Tentative conclusions on the mechanisms of electric field-induced transitions from the relaxor state to the ??normal?? ferroelectric state have been drawn.     

Structural and property changes in poly (vinylidene fluoride–trifluoroethylene) 70/30 mol % copolymer induced by proton irradiation

Poly(vinylidene fluoride–trifluoroethylene) 70/30 mol% copolymer has been irradiated with 3 MeV protons at doses ranging from 43 to 200 Mrad. The effects of irradiation on the polarization hysteresis, dielectric properties, lattice spacing, phase transition behavior and electric-field-induced strain have been studied. The irradiated copolymer exhibits the characteristic behavior of a relaxor ferroelectric, including frequency dispersion of the dielectric constant, which follows the Vogel–Fulcher rule. These results indicate that the proton irradiation breaks up the coherent polarization domains in the copolymer into nano-sized regions, thereby converting the copolymer to a relaxor ferroelectric. X-ray diffraction measurements show that the nano-sized regions are in the non-polar phase. Since the lattice spacing of the non-polar phase is substantially different from that of the polar phase, the local phase transformation between these two phases induced by an external electric field gives rise to a large lattice strain and hence a giant electrostrictive response. PACS 77     

Synthesis and electrical properties of Pb(Mg1/3Nb2/3)O3–PbTiO3 ceramics

Ceramic compositions of a combination between lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃, and lead titanate, PbTiO₃, were fabricated by using Mg₄Nb₂O₉ precursor technique. Their electrical properties with respect to temperature and frequency were examined and the effect of sintering conditions on phase formation, densification, microstructure and electrical properties of the ceramics were examined. It has been found that optimisation of sintering condition can lead to a highly dense and pyrochlore-free PMN–PT ceramics. The gradual decreasing of the physical properties of the sintered ceramics was related to the gradual decrease of density and inhomogeneous microstructure. The results also revealed that for the lower concentration of lead titanate a relaxor behavior is noticed with a high electrostrictive effect, which was almost hysteretic free. However, higher amount of lead titanate led to a normal ferroelectric behavior.     

基于弛豫型铁电三元共聚物聚(偏氟乙烯-三氟乙烯-氯代偏氟乙烯)热释电性能的能量采集研究

研究了利用弛豫型铁电三元聚合物薄膜P(VDF-TrFE-CFE)的热释电性质,以温度波动作为初始能量形式进行热电能量的采集。由于该聚合物薄膜在发生由温度变化诱导的纳米极性区极化机制转换时,介电常数表现出明显的非线性变化,所以可以结合Ericsson循环实现热电能量采集。实验结果显示,最佳能量采集温度区间为20~-20℃,利用不同温度下的单极性电滞回线进行Ericsson循环模拟,两种模拟方式分别实现能量采集最大值和最小值,并从微观角度给出了两种模式的解释。同时研究了温度波动和外加电场对能量采集的影响。在外加电场100 kV·mm^-1、温度波动为40℃的情况下,能量采集值达到3483 mJ·cm^-3。与单晶材料相比,能量采集值提高了10倍。当工作温度降低至室温时,材料具有柔性,在能量采集方面具有应用潜力。     

Low-frequency dielectric relaxation (1/<Emphasis Type="Italic">f</Emphasis> noise) in relaxors of the (1 − <Emphasis Type="Italic">x</Emphasis>)PMN-(<Emphasis Type="Italic">x</Emphasis>)PZT system

The dielectric properties of relaxor ferroelectric ceramics (1 ? x)PbMg_1/3Nb_2/3O₃-(x)PbZr_0.53Ti_0.47O₃ (PMN-PZT) have been studied at frequencies from 40 Hz to 1 MHz in the temperature range of the diffuse phase transition. It is established that the dielectric relaxation generates 1/f noise, whose spectral density depends on the relaxor composition and temperature.     

Chromium-and manganese-doped TlInS<Subscript>2</Subscript>—A new relaxor ferroelectric

The dielectric, pyroelectric, and polarization properties of TlInS₂ crystals doped with 0.1 mol % Cr and Mn are investigated. It is shown that TlInS₂ compounds doped with chromium and manganese can be assigned to the class of relaxor ferroelectric materials. The temperature range of existence of the stable relaxor (nanodomain) state and the temperature of the phase transition from this state to the ferroelectric (macrodomain) state are determined. The temperature dependence of the pyroelectric current is characterized by an anomaly in the range of the phase transition.     

Spontaneous phase transition from relaxor to macrodomain ferroelectric state in single-crystal PbSc0.5Nb0.5O3-BaSc0.5Nb0.5O3 solid solutions

The cooling of Pb_1?x Ba_xSc_0.5Nb_0.5O₃ solid solutions with x≤0.04 leads to a spontaneous transition from a relaxor to a macrodomain ferroelectric state, accompanied by anomalous variation of the dielectric and optical properties of the material. As the barium content in the system increases, the relaxor state becomes more stable and eventually “freezes” at x≈0.05. The crystals with x=0.06 exhibited the appearance of a macrodomain ferroelectric phase induced both by an external electric field with a strength of 1.5 kV/cm and by an internal electric field formed in the course of dielectric aging.     

An aspect on relaxor behaviours and phase boundaries in ferroelectric mixed crystals

In order to search for Pb-free relaxor substances and to define relaxor behaviour, a theoretical aspect is presented from not only microscopic but also phenomenological viewpoint. In order–disorder type mixed crystals, the microscopic approach is attempted under conditions similar to those for relaxor perovskite-type oxides. Basing on the theory for the precursor to glass state, two kinds of technique of X-ray diffuse scattering (static properties) and quadrupole perturbed nuclear magnetic resonance (NMR-NQR) (dynamic properties) are discussed in ferroelectric (FE) -- antiferroelectric (AFE) mixed crystals of Rb_1?x(NH₄)_xH₂PO₄. After investigating both boundary regions between FE and glass (G) phases, and between AFE and G phases, relaxor-like behaviours as a large dispersion in dielectric constants are found in the AFE-G phase boundary region where the gradual growth of wave number to make the energy minimum and a discontinuous broad NMR-NQR spectrum appear in quite different forms from those of the FE-G phase boundary region. On the other hand, in perovskite-oxides, the conditions for relaxor appearance are discussed within the phenomenological theory by comparing with the microscopic analysis in hydrogen-bonded systems.     

Electric-field-induced phase transition in single-crystal lead zinc niobate

The behavior of lead zinc niobate, a relaxor ferroelectric, in electric fields has been studied by dielectric, optical transmittance, and small-angle light scattering techniques. A field-induced transition from an averaged cubic to macrodomain ferroelectric phase has been found. A comparison is made with the properties of the phase induced in lead magnoniobate, a classical relaxor. Fiz. Tverd. Tela (St. Petersburg) 40, 527–530 (March 1998)     

Divide line between relaxor,diffused ferroelectric,ferroelectric and dielectric

The dielectric spectroscopic studies of relaxor ferroelectric Ba(Ti_0.70Sn_0.30)O₃ (BTS30), diffused ferroelectric Ba(Ti_0.70Sn_0.20)O₃ (BTS20), classical ferroelectric BaTiO₃ (BT0), and normal dielectric Ba(Fe_0.50Ta_0.50O₃) (BFT) were carried out in the temperatures ～100 K–650 K, and frequencies ～100 Hz–1 MHz, which showed relaxor behavior at 130 K with typical 30 K frequency dispersion, perfect diffused ferroelectric character at 190 K, classical ferroelectric, and dielectric relaxation at 450 K–550 K respectively. A divide line was sketched between the relaxors, ferroelectrics and normal dielectrics based on the Maxwell–Wagner space charge model. The impedance spectroscopy and ac conductivity spectroscopy were carried out to explain the relaxor and pseudo relaxor behavior.     

Relaxor properties of Fe-doped TlInS<Subscript>2</Subscript>

The relaxor properties of Fe-doped TlInS₂ crystals are investigated. It is shown that Fe-doped TlInS₂ compounds exhibit all features inherent in relaxor ferroelectric materials. The temperature range of existence of the microdomain (relaxor) state and the temperature of the transition from this state to a macro-domain state are determined.     

Phase diagram of (1−x%)(0.89Bi0.5Na0.5TiO3–0.06BaTiO3–0.05K0.5Na0.5NbO3)–x%MnO2 lead-free anti-ferroelectric ceramics

A phase diagram for the lead-free ceramics in the (1?x%)(0.89Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–0.05K_0.5Na_0.5NbO₃)–x%MnO₂ (BNBKN-x%Mn) binary system is constructed for the first time based on the ferroelectric and dielectric measurements. The ferroelectric behaviors under different temperatures suggest that the ceramics are basically of relaxor anti-ferroelectric nature near room temperature. The temperature dependent dielectric properties show that when the addition of MnO₂ increases, the relaxor anti-ferroelectric phase can be stabilized to be close to the Curie point, which corresponds to a relaxor anti-ferroelectric to paraelectric phase transition.     

(100) MgAl2O4 as a lattice-matched substrate for the epitaxial thin film deposition of the relaxor ferroelectric PMN-PT

The (100) surface of MgAl₂O₄ is evaluated as a substrate for the thin film deposition of the relaxor ferroelectric PbMg_1/3Nb_2/3O₃(65%)–PbTiO₃(35%). With a lattice mismatch of less than 0.5%, this film-substrate combination presents a geometrical template for growth that is far superior to that formed with other commercially available oxide substrates. Films were deposited using the pulsed laser deposition technique and were characterized in terms of their crystallographic, microstructural, and dielectric properties. From a crystallographic perspective the films show excellent cube-on-cube epitaxy, are highly oriented, and show no evidence of the frequently observed parasitic pyrochlore phase. With the exception of a few faceted surface structures, the film’s microstructure is single-crystal-like, exhibiting a sharp film-substrate interface, a smooth top surface, and no discernable granularity. The dielectric response shows the frequency-dependent diffuse phase transition characteristic of a relaxor material, but with less frequency dispersion and a smaller maximum in the dielectric constant. Taken together, the results suggest that the (100) MgAl₂O₄ substrate could prove to be an effective substrate material, not only for the PbMg_1/3Nb_2/3O₃(65%)–PbTiO₃(35%) system, but also for a number of other important lattice-matched ferroelectric, relaxor, and ferroelectric superlattice systems.     

Influence of structure of poly(<Emphasis Type="Italic">o</Emphasis>-phenylenediamine) on the doping ability and conducting property

Poly(o-phenylenediamine) with ladder-type structure was formed in aqueous hydrochloric acid medium below pH 1, while open ring-type amine derivative, i.e., ?NH₂ functional group substitution of polyaniline structure, was obtained in 1:1 aqueous sulfuric acid medium from the chemical synthesis of the monomer o-phenylenediamine. However, poly(o-phenylenediamine) having structure like polyaniline derivative with free =NH functional groups was obtained by chemical synthesis in dimethyl sulfoxide medium. The ladder-type polymer was almost insoluble but the other two types of synthesized polymers having polar functional group substitutions were well soluble in polar organic solvent like dimethyl sulfoxide, N,N-dimethyl formamide, and tetrahydrofuran. The freestanding films were cast from dimethyl sulfoxide solution of both the soluble functional polymers. The polymers having different structures were doped with inorganic acid by solution doping technique and the doped polymers were characterized by various standard characterizations. In order to explore the electronic uses of the polymers like sensor and actuators, the influences of their structure on the doping ability as well as ionic properties of the sulfuric acid-doped polymers were compared.     

Crossover from ferroelectric to relaxor behavior in BaTi1− x Sn x O3 solid solutions

Dielectric relaxation of BaTi_{1? x} Sn _x O₃ ceramics is investigated by means of dielectric spectroscopy. The gradual crossover from ferroelectric to relaxor behavior is characterized by vanishing of the contribution due to domain walls and appearance of relaxation related to reorientation of polar nanosized regions. Typical behavior of relaxors is observed only in ceramics with x = 0.30, while the compositions with 0.175 ≤ x ≤ 0.25 show coexistence of both ferroelectric and relaxor features. The relaxor properties are supposed to be due to both weak random fields and disorder inherent in pure BaTiO₃.     

Electrocaloric effect in ferroelectric polymers

The electrocaloric effect (ECE) of poly (vinyledene fluoride?Ctrifluoroethylene) (P(VDF?CTrFE)) 55/45?mol% copolymers was directly measured, which confirms the results deduced from Maxwell relation. The adiabatic temperature change ??T under a given electric field peaks at the ferroelectric?Cparaelectric (FE?CPE) transition. Away from it, ECE becomes small. ??T versus applied electric field can be described well by a modified Belov?CGoryaga equation. The ECE in ferroelectric polymers, especially near FE?CPE transition where larger ECE is observed, are analyzed under different boundary conditions employing phenomenological theory and constitutive equations. The secondary pyroelectricity is found to play a significant role which enhances ECE in ferroelectric polymers.