Dielectric, ferroelectric, magnetic and magnetoelectric properties of PMN-PT based ME composites

A systematic study of magnetoelectric composite system (x) CoFe₂O₄+(1−x) Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ with x=0, 0.15, 0.30, 0.45 and 1 was carried out. The lattice strain was calculated using Williamson and Hall equation, which depends on the content of constituent phases in composites. The microstructure was studied using scanning electron microscopy. The ferroelectric transition temperature was independent of the content of individual phases, suggesting that the ferroelectric character is maintained in the composite. Observed P–E and M–H loops indicate that the multiferroic nature of magnetoelectric ceramics is dependent on the content of individual phases. The variation of magnetostriction with dc magnetic field was studied. The maximum magnetoelectric voltage coefficient of 7.2 mV/cm Oe is obtained for the synthesized composites. The magnetoelectric measurements are well explained with magnetostrictive behavior of the magnetic phase.     

Dielectric properties of ferroelectric diisopropylammonium iodide

Temperature dependences of dielectric constant, amplitude of the third harmonic and heat capacity for the organic ferroelectric of diisopropylammonium iodide (C₆H₁₆NI) have been investigated. The measurements were carried out through heating and cooling cycles in the range of 300–400?K. It was found that upon the first heating, only one phase transition occurred without the presence of the ferroelectric phase. For samples preheated over 420?K, two phase transitions at 363 and 378?K appeared in the heating process, and the ferroelectric state was also observed between them. Upon cooling, the ferroelectric phase was detected in the range of lower 361?K and persisted up to room temperature.     

氧化锌掺钡的电子结构及其铁电性能研究

运用基于密度泛函理论的第一性原理方法计算了不同原子百分比含量的Ba掺杂Zn0半导体体材料超晶胞的能带结构、电子态密度、极化率和相对介电值.计算结果表明:Ba掺杂的Zn0体系为直接带隙半导体材料,其禁带宽度随着Ba原子掺杂百分比的增加呈现出逐渐增大的趋势.体系铁电性能的计算表明:与纯Zn0相比,Zn0掺入Ba原子后的极化率与相对介电值发生了较为明显的变化,其极化率随着Ba原子掺杂百分比的增加而增大,相对介电值随着Ba原子掺杂百分比的增加而减小.对角化后的极化率分量的数值结果表明:在电场作用下超胞中可能存在微畴结构,并且由于畴间电偶极矩的强相互作用,使得超胞宏观上表现为几乎具有各向同性的极化率特征.     

钛酸钡基施主掺杂弛豫铁电体介电弥散的唯象分析

弛豫铁电体铁电相变的介电弛豫性涉及外来离子掺杂或原有离子的有序排列, 需要深入理解和有效统一的表征. 针对高浓度施主掺杂在晶粒中产生金属离子缺位, 降低居里温度并引起钛酸钡基铁电材料扩散相变的现象, 分析了高斯型的杂质分布、居里温度分布和金属离子缺位分布的等效对应关系, 得出了在介电常数峰值温区金属离子缺位浓度随温度降低而作用增大的结论, 认为金属离子缺位引起的单极电荷在电场作用下的跳跃运动有可能引起介电弥散和介电损耗. 提出了用‘弛豫度’表征铁电弛豫程度.     

Dielectric properties of electrolytic solutions

The variation in dielectric properties of water with the addition of ionic salts have been measured using automated frequency domain experimental microwave C-band. The dielectric properties, that is dielectric constant (ε′) and dielectric loss (ε″) of two electrolyte solutions for various concentrations have been measured at 5-GHz frequency at room temperature. It has been observed that for concentration between 0.2 to 1.0 mole the dielectric constant of water is smaller and some larger than that of pure water and dielectric loss increases with increasing concentration of these salts. It has been also observed that the variation in dielectric loss is different, though the ionic concentration of the two salts are equal.     

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.     

Relationship between average and local crystal structure and the ferroelectric properties of a Sr-Bi-Ta-Si-O ferroelectric material

We investigated the relationship between the average and local crystal structures and the ferroelectric properties of Bi₂SiO₅, Bi₄Si₃O₁₂, or Bi₂O₃ added Sr_1−xBi_2+xTa₂O₉ (x=0, 0.2) produced by a solid-state reaction. By measuring the P-E hysteresis, we found that Sr_1−xBi_2+xTa₂O₉ (x=0, 0.2) has higher P_r and E_c than SrBi₂Ta₂O₉ (SBT). P_r increased and E_c decreased by adding Bi₂SiO₅, Bi₄Si₃O₁₂, or Bi₂O₃ to Sr_1−xBi_2+xTa₂O₉. The average crystal structures were determined by the Rietveld method. On the other hand, the local structure is important, because the ferroelectric property is related to the distortion, and ferroelectric complex oxides have domains. We also investigated the local crystal structure using atomic pair distribution function (PDF) analysis. Based on the results, the bond angle variance, σ², of each TaO₆ octahedron increased by substituting Si for the Ta site. The tilting angle, α_a,α_b, of each TaO₆ octahedron increased relative to that of the average structure, and the symmetry of the TaO₆ octahedron in the local structure deteriorated in comparison to that of the average structure. This distortion and symmetry of TaO₆ contributes to the remanent polarization.     

Hard mode Raman spectroscopy and its application to ferroelastic and ferroelectric phase transitions

The application of Raman spectroscopy for the investigation of phase transitions focused traditionally on the observation of soft modes in displacive systems. The present furthergoing study on displacive and order-disorder systems is based on the observation of systematic changes of the scattering profiles of hard modes during the phase transition. It reveals the temperature evolution of the order parameters, the role of order-parameter fluctuations and phonon density of states effects in model systems like improper ferroelastic Pb₃(P_1-x As _x O₄)₂, lead diluted ferroelastic (Pb_1-x Ba _x )₃(PO₄)₂, pure ferroelastic As₂O₅ and the relaxor ferroelectric Pb(Sc_0.5Ta_0.5)O₃. Allied with supplementary experimental techniques hard-mode Raman spectroscopy (HMRS) is an ideal method for the investigation of order-parameter coupling effects and the characterization of structural phase transitions.     

Dielectric properties of ultrathin films of molecular crystals

Dielectric properties of thin films (TF) of molecular crystals, including the effect of size and the boundary surfaces were analysed using Green's function method. Polarisability of molecules in various film layers and dielectric susceptibility of TF were calculated. A comparison with crystal bulk has shown that dielectric properties of TF are strongly influenced both by the sample dimensions and by the boundary conditions.The frequency dependence of the dielectric susceptibility has also been derived. One obtains the monotonous variation for the frequencies above and below exciton band. However, for the frequencies within the exciton band there appears to be complicated, non-monotonous dependence. The relationship between the dielectric susceptibility and the film width for the same frequency shows a complex, oscillatory behaviour. Furthermore, the amplitude of these oscillations increases with increasing film width, demanding the introduction of a damping factor. Finally, the thickness dependence of dielectric susceptibility was analysed in the Cole-Cole plot.     

Effect of Nb-doping on phase transitions and selected properties of (Ba0.90Pb0.10)TiO3 ceramics

Dielectric, pyroelectric characteristics as well as the changes of electric conductivity were investigated for Nb₂O₅-doped (Ba_0.90Pb_0.10)TiO₃ ceramics. The influence of this dopant on the ceramic microstructure and the lattice parameters were also studied. The analysis of the results of electric conductivity and Seebeck coefficient has led to the conclusion that Nb⁵⁺ ions reducing the conductivity by about two orders play the role of donors. The correlation between investigated electric characteristics and the dopant concentration was confirmed. Due to these findings some progress in understanding the influence of Nb-dopant was achieved.     

Dielectric properties of BaTiO3 modified with ZrO2

The dielectric constant and loss tangent of BaTiO₈ with ZrO₂ additives have been studied. The loss tangent was less ranging between 0.16 for pure BaTiO₃ and 0.05 for samples containing 5% ZrO₂. The dielectric constant at Curie temperature decreased with increasing ZrO₂ concentration up to 0.4%. The volume resistivity measurements illustrate a peak value at a certain range of temperatures for each composition.     

First-principles calculations of the dielectric and vibrational properties of ferroelectric and paraelectric BaAl2O4

First-principles calculations have been conducted to study the structural, dielectric, and vibrational properties of ferroelectric and paraelectric BaAl₂O₄. High-frequency and static dielectric constants, and phonon frequencies at the Brillouin zone center are reported. Both BaAl₂O₄ polymorphs are promising infrared-transparent materials due to their low electronic dielectric constants. The ferroelectric and paraelectric BaAl₂O₄ have much smaller permittivity compared to the classical ferroelectric materials. From an atomic nanostructure standpoint, the abnormally low permittivity of BaAl₂O₄ polymorphs is mainly related to low coordination numbers of Ba (9) and Al (4).     

Influence of sintering conditions on structural,thermal, electric and ferroelectric properties of Na0.5Bi0.5TiO3 ceramics

Na_0.5Bi_0.5TiO₃ ceramics were prepared by a conventional solid-state reaction method and by a hot-pressing route. The influence of sintering conditions on structural, thermal, dielectric and ferroelectric properties of these ceramics was investigated. All obtained samples exhibited a single perovskite phase. It was shown that the sintering conditions significantly influence the properties under investigation. This includes changes in the value of the electric permittivity ? and dielectric loss tanδ, a shift of T_m and T_d and change of the ferroelectric properties. These effects are mainly related to volatility of the Na and Bi components during the sintering process along with formation of their compensating charge defects, which leads to local structure change.     

Effect of B-site isovalent doping on electrical and ferroelectric properties of lead free bismuth titanate ceramics

In the present work, zirconium modified bismuth titanate ceramics have been studied as potential lead-free ferroelectric materials over a broad temperature range (RT – 800 °C). Polycrystalline samples of Bi₄Ti_3−xZr_xO₁₂ (x=0.2, 0.4, 0.6) (BZrT) with high electrical resistivity were prepared using the solution combustion technique. The effect of Zr doping on the crystalline structure, ferroelectric properties and electrical conduction characteristics of BZrT ceramics were explored. Addition of zirconium to bismuth titanate enhances its dielectric constant and reduces the loss factor as it introduces orthorhombic distortion in bismuth titanate lattice which is exhibited by the growth along (0010) lattice plane. Activation energy due to relaxation is found to be greater than that due to conduction thus confirming that electrical conduction in these ceramics is not due to relaxation of dipoles. Remanent polarization of the doped samples increases as the Zirconium content increases.     

Dielectric behavior and PTCR effect in nanocrystallite PMN ferroelectric ceramics

The ac behavior of a nanocrystallite lead magnesium niobate (PMN) ceramic sample was studied over a wide range of temperatures and frequencies. The results revealed a diffuse phase transition and very high dielectric constant at lower frequencies. The high value of the dielectric constant at lower frequencies is shown to be due to barrier layer formation. A positive temperature coefficient of resistivity (PTCR) was observed in the temperature dependence of the ceramic resistivity. The results are explained on the basis of the Heywang and Jonker models. The Schottky barrier formed at grain boundary regions acts as a trap for the electrons available from oxygen vacancies in the ceramics. This provides PTCR characteristics from the transition temperature to about 208°C.     

Phase polymorphism and electro-optical properties of a ferroelectric liquid crystal containing the biphenyl system

ABSTRACT

In this article we present results concerning phase transitions and physical properties of the ferroelectric phase of the compound (S)-4-(1-methylheptyloxy)biphenyl–4'-(heptyloxy phenyl)-4-carboxylate (MHOBOPO7). The compound has the following phases: smectic ferroelectric C (SmC*), chiral nematic N*, and two defected phases, TGB_C and blue phase. The mesomorphic properties were investigated by means of three complementary methods: differential scanning calorimetry, polarizing light optical microscopy, and transmitted light intensity. The electro-optical measurements were carried out on an ordered sample in a middle electric field during very slow cooling from the nematic phase to the ferroelectric phase.     

Improved ferroelectric,dielectric and magnetic properties with La- and Mn- doped Bi5Ti3FeO15 thin films

A series of La and Mn co-doped Bi₅Ti₃FeO₁₅ (BLTFMO) thin films were prepared by spin-coating deposition route. X-ray diffraction, atomic force microscopy and scanning electron microscopy were used to characterize the structures of these BLTFMO thin films. Ferromagnetic properties are obtained as the La-doping content is 0, 0.1, 0.2, and 0.3 with the transition temperature of 127.2 K, 65.1 K, 48.1 K, and 7.9 K, respectively. Well-defined ferroelectric loops are found in all these BLTFMO films, and a higher remnant polarization of 27.84, 24.21 and 24.02 μC/cm² is obtained in the 0.1, 0.2 and 0.3 La-doped films, respectively. A weak dielectric dispersion for the BLTFMO without La-doping, a strong one in 0.1, 0.2, 0.3 and 0.4 La doped films as indicated by the appearance of a dielectric loss peak, and a weak dispersion in 0.6, 0.8 and 1 La doped ones are demonstrated.     

Dielectric and ferroelectric properties of Sr_4CaSmTi_3Nb_7O_(30) with tetragonal tungsten bronze structure

Sr4CaSmTi3Nb7O30ceramics are synthesized and indexed as tetragonal tungsten bronze structure. The dielectric behavior and ferroelectric nature are investigated. Three dielectric anomalies are observed. The phase transition is a displacive phase transition with some diffusive characteristics, which indicates possible compositional variations within the materials on the microscopic scale. The weak distortion disappears in cooling process for differential scanning calorimetry measurement, and the large depression of Curie–Weiss temperature T0 indicates the difficulty in forming macroferroelectric domain. The ferroelectric nature in these filled tungsten bronze niobates originates from the off-center displacement of Bsite cations, but they are primarily dominated by A-site cation occupation. Both the radius and the valence of A1-site cations play an important role on ferroelectric properties of the filled tungsten bronze compounds. Existence of spontaneous polarization with a remanent polarization of 0.16 μC/cm2 a coercive field of Ec = 11.74 kV/cm confirms the room-temperature ferroelectric nature of Sr4CaSmTi3Nb7O30 ceramics.     

Dielectric, ferroelectric and pyroelectric properties of Pb[(Ni1/3Sb2/3)xTiyZrz]O3 ceramics

In the present study, various Pb[(Ni_1/3Sb_2/3)_xTi_yZr_z]O₃ where x+y+z=1, x=0.08 and y=0.44-0.49, ceramics in the morphotropic phase boundary (MPB) range were studied by dielectric and pyroelectric methods. The results of the investigations revealed an MPB composition range of y≅0.46. The study of the dielectric properties of these compounds as a function of temperature suggests that with increase in y the permittivity maximum increases and transition temperature shifts towards higher temperature. Well-saturated polarization versus electric field (P-E) hysteresis loops were obtained and values of P_r were calculated. The samples revealed good pyroelectric properties for y=0.44 and y=0.45 at room temperature with large figures of merit F_v=0.019 m²/C and F_D=1.34×10^-5Pa^1/2.     

Influence of lateral size on dielectric properties of ferroelectric thin films with structure transition zones

By taking into account structural transition zones near the lateral and thickness direction edges, this paper uses a modified transverse Ising model to study dielectric properties of a finite size ferroelectric thin film in the framework of the mean-field approximation. The results indicate that the influence of the lateral size on the dielectric susceptibility cannot be neglected and lateral structural transition zones could be a crucial factor that improves the mean susceptibility of the fixed size film.