Microstructure-dependent giant dielectric response in CaCu3Ti4O12 ceramics

CaCu₃Ti₄O₁₂ ceramics were prepared at the sintering temperatures ranged from 1025 to 1125 °C, and the dielectric characteristics were evaluated together with the microstructures. The giant dielectric constant with the maximum of 53,120 was obtained in CaCu₃Ti₄O₁₂ ceramics at room temperature and 10 kHz, and strong processing and microstructure dependence of dielectric characteristics of the present ceramics was determined. The precipitation of the dispersed Cu-rich secondary phases of CuO and/or Cu₂O and their network structure provided the extrinsic origins of the enhanced giant dielectric response, and the present findings would offer the greater potential for enhancing the giant dielectric constant and controlling the dielectric loss in CaCu₃Ti₄O₁₂ ceramics by optimizing the microstructures.     

Post-sintering annealing effect on the diffuse phase transition in (Pb1−xBax)(Yb0.5Ta0.5)O3 ceramics

The effect of post sintering annealing on the dielectric response of (Pb_1−xBa_x)(Yb_0.5Ta_0.5)O₃ ceramics in the diffuse phase transition range (x=0.2) has been investigated. The samples are prepared by conventional solid-state reaction method. The samples are sintered at 1300 °C for 2 h and annealed at different temperatures (800, 900 and 1000 °C) for 8 h and at 800 °C for different time durations (8, 12 and 24 h). A significant change in the dielectric response has been observed in all the samples. The dielectric constant increases remarkably and the dielectric loss tangent decreases. The dielectric peaks of the annealed samples are observed to be more diffused with noticeable frequency dispersion compared to the as sintered sample.     

Dichroism of (CH3NH3)2CdCl4 in the farinfrared

The dielectric constant of (CH₃NH₃)₂ CdCl₄ has been measured as a function of temperature in the submillimeter range. Freshly grown samples with a wide domain structure at room temperature show a pronounced dichroism in this spectral range which is caused by the orientation of the molecules in the room temperature orthorhombic phase and their response to the electromagnetic wave.     

Vibrational and dielectric properties of La2Hf2O7 : Experiment and theory

The physical origin of the static dielectric constant and its relationship with lattice dynamics of La₂Hf₂O₇are studied by combining infrared spectroscopy and density functional perturbation theory (DFPT). Both La and Hf show obvious effective charge anomaly which is attributed to the hybridization between 2p states of the oxygen and 5d states of the cations, indicating a mixed covalent-ionic bonding between the cations and the oxygen. The dielectric response is determined by seven infrared phonon modes and the static dielectric constant extracted from infrared reflection spectrum is in close agreement with DFPT calculation. Both experiment and theory reveal that most of the contributions to the static dielectric constant are dominated by three infrared phonon modes at 137, 172 and 297 cm⁻¹. Two of them (172 and 297 cm⁻¹) are from the displacements of oxygen atoms inside HfO₆ octahedra and the other one (137 cm⁻¹) is from the bending of La₂O′ chain. This result indicates that the origin of the static dielectric constant of La₂Hf₂O₇ is directly connected with the two interpenetrating sub-networks of pyrochlore structure (HfO₆ octahedra and La₂O′ chain).     

Dielectric and AC conductivity behavior of BaBi2Nb2O9 ceramics

The complex dielectric and AC conductivity response of BaBi₂Nb₂O₉ relaxor ferroelectric ceramics were studied as a function of frequency (100 Hz-10 MHz) at various temperatures. The observed dielectric behavior was characterized by two types of relaxation processes which were described by the ‘universal relaxation law’. The frequency dependence of conductivity which showed a classical relaxor behavior followed the Jonscher's universal law σ(ω)=σ₀+Aωⁿ. The exponent n exhibited a minimum in the vicinity of temperatures of dielectric anomaly while the pre-factor A showed a maximum. The temperature dependence of n followed the Vogel-Fulcher relation with activation energy of about 0.14 eV.     

Magnetodielectric coupling in MnCr2O4 spinel

We have investigated the magnetic and dielectric properties of polycrystalline samples of the spinel MnCr₂O₄. Below the ferrimagnetic ordering temperature at T_N∼43 K, both magnetization and dielectric measurements show signatures of the onset of a conical structure at T_s∼17 K and a lock-in temperature at T_f∼14 K. These values are similar to those previously reported for single-crystal samples, where the spiral structure is short-range ordered (SRO) at low temperatures. The application of magnetic field suppresses the dielectric anomaly at T_f indicating that the coherence length of the ordering increases. MnCr₂O₄ exhibits a symmetrical magnetodielectric response between T_f and T_s that scales with the square of the magnetization. This suggests that the magnetodielectric coupling originates from the P²M² term in the free energy expansion. The magnetodielectric response becomes asymmetric with respect to field below T_f.     

Piezoelectric non-linearity in PbSc0.5Ta0.5O3 thin films

Epitaxial (001)-oriented PbSc_0.5Ta_0.5O₃ (PST) thin films were deposited by pulsed laser deposition. Local piezoelectric investigations performed by piezoelectric force microscopy show a dual slope for the piezoelectric coefficient. A piezoelectric coefficient of 3 pm/V was observed at voltages up to 0.8 V. However, at voltages above 0.8 V, there is a steep increase in piezoelectric coefficient mounting to 23.2 pm/V. This nonlinear piezoelectric response was observed to be irreversible in nature. In order to better understand this nonlinear behavior, voltage dependent dielectric constant measurements were performed. These confirmed that the piezoelectric non-linearity is indeed a manifestation of a dielectric non-linearity. In contrast to classical ferroelectric systems, the observed dielectric non-linearity in this relaxor material cannot be explained by the Rayleigh model. Thus the dielectric non-linearity in the PST films is tentatively explained as a manifestation of a percolation of the polar nano regions.     

Dielectric response in ferroelectric BaTiO3

The far-infrared optical and dielectric properties of ferroelectric perovskite titanate powder BaTiO₃ are reported. The terahertz time-domain spectroscopy (THz-TDS) measurement reveals that the low frequency dielectric response of BaTiO₃ is closely related to the lowest pair of transverse optical (TO) and longitudinal optical (LO) modes near at 180 cm⁻¹, which is verified by Raman spectroscopy. This result provides a better understanding of the relation of low-frequency dielectric function with the optical phonon mode for ferroelectric materials. Combining terahertz TDS with Raman spectra, the overall low frequency optical phonon response of BaTiO₃ is presented in an extended spectral range from 6.7 to 1200 cm⁻¹.     

Dielectric parameters in Se70Te30 and Se70Te28Zn2 chalcogenide glasses

Temperature and frequency dependence of dielectric constant (ε′) and dielectric loss (ε″) are studied in glassy Se₇₀Te₃₀ and Se₇₀Te₂₈Zn₂. The measurements have been made in the frequency range (8-500 kHz) and in the temperature range 300 to 350 K. An analysis of the dielectric loss data shows that the Guintini's theory of dielectric dispersion based on two-electron hopping over a potential barrier is applicable in the present case.No dielectric loss peak is observed in glassy Se₇₀Te₃₀. However, such loss peaks exist in the glassy Se₇₀Te₂₈Zn₂ in the above frequency and temperature range. The Cole-Cole diagrams have been used to determine some parameters such as the distribution parameter (α), the macroscopic relaxation time (τ₀), the molecular relaxation time (τ) and the Gibb's free energy for relaxation (ΔF).     

杂质对K2ZnCl4晶体无公度结构相变的影响

对K₂ZnCl₄铁电晶体在无公度-公度转变的相变点附近的孤子行为进行了研究。发现在相变点附近杂质使介电函数明显偏离居里-外斯定律,并导致介电峰值的明显下降;同时还发现杂质对升温相变点T_c^h较之对降温相变点T_c^c有更大的影响,这与Rb₂ZnCl₄等晶体中的情况恰好相反。采用朗道自由能理论可较好地描述杂质在相变点附近对介电函数的影响。分 关键词：     

A comparative study of dielectric and Raman spectroscopy of Pb(Y b1/2Ta1/2)O3 and Pb(Y b1/2Nb1/2)O3

A comparative study of the properties of two highly ordered lead based complex perovskites Pb(Y b_1/2Ta_1/2)O₃ and Pb(Y b_1/2Nb_1/2)O₃ has been carried out through x-ray diffraction, dielectric and Raman scattering measurements. These two compounds differ significantly in their structure, dielectric response and phonon vibration although the ionic radii and valencies are same for Ta and Nb. The room temperature x-ray diffraction pattern and Raman spectra show that the symmetry of lead ytterbium tantalate is lower than that of lead ytterbium niobate. The Raman spectra of Pb(Y b_1/2Ta_1/2)O₃ also indicates the presence of local distortion in the lattice which may be one of the factors responsible for the existence of a secondary transition.     

Synthesis and dielectric characteristics of La0.5Bi0.5MnO3 ceramics

La_0.5Bi_0.5MnO₃ ceramics with a single phase were prepared by a solid-state reaction method, and their dielectric properties were characterized. Two dielectric relaxations with a giant dielectric constant were identified in the temperature range from 125 to 350 K. The electron hopping between Mn³⁺ and Mn⁴⁺ was found to be the origin of the dielectric relaxation at low temperatures (125–200 K) with an activation energy of 0.18 eV. The high temperature (200–350 K) dielectric relaxation can be attributed to the conduction.     

Magnetic and dielectric properties of Ni0.8Co0.1Cu0.1Fe2O4+PZT composites

Magnetoelectric composites of Ni_0.8Co_0.1Cu_0.1Fe₂O₄ and Lead Zirconate Titanate (PZT) were prepared by using conventional ceramic method. The measured values of saturation magnetization (M_s) and magnetic moments (μ_B) are in accordance with the volume fraction of ferrite content in the composite. The dielectric constant of the composites decreases with frequency. The plots of dielectric constant () against temperature (T) show a peak at their respective transition temperatures. The ME output was measured by varying dc bias magnetic field. A large ME output signal of 776 mV/cm was observed for 35% ferrite +65% ferroelectric composite. The magnetoelectric (ME) response is found to be dependent on the content of ferrite phase.     

Na0.25K0.25Bi0.5TiO3无铅压电陶瓷的介电特性研究

用固相反应法制备了Na_0.25K_0.25Bi_0.5TiO₃ (NKBT50)陶瓷，研究了该陶瓷在室温至400℃温度范围内的介电性能.发现该陶瓷的介电温谱与烧结气氛、极化状态有关.在空气中烧结的未极化样品在70℃附近存在介电和损耗峰，而极化后及在氧气氛中烧结的样品并不存在该介电、损耗峰.分析认为70℃的介电和损耗峰与氧空位形成的缺陷偶极子的极化弛豫有关.热激电流显示，陶瓷的去极化温度为225℃，与此相对应的介电、损耗峰也 关键词： 介电性能 氧空位 极化弛豫 钛酸铋钠钾     

Diffuse dielectric anomaly of （Na0.5K0.5Bi）0.5TiO3 crystal at high temperature

This paper investigates the dielectric properties of (Na_0.5K_0.5Bi)_0.5TiO₃ crystal at intermediate frequencies (1kHz \le f \le 1MHz) in the temperature range of 30--560℃. A pronounced high-temperature diffuse dielectric anomaly has been observed. This dielectric anomaly is shown to arise from a Debye-like dielectric dispersion that slows down following an Arrhenius law. The activation energy E_r obtained in the fitting process is about 0.69eV. It proposes that the dielectric peak measured at low frequency above 400℃ is not related to the phase transition but to a space-charge relaxation.     

Effect of sintering temperature on structure and nonlinear dielectric properties of Ba0.6Sr0.4TiO3 ceramics prepared by the citrate method

Ba_0.6Sr_0.4TiO₃ ceramics were prepared by a citrate precursor method. The structure and nonlinear dielectric properties of the resulting ceramics were investigated within the sintering temperature range 1200-1300 °C. Adopting fine Ba_0.6Sr_0.4TiO₃ powder derived from the citrate method was confirmed to be effective in reducing the sintering temperatures required for densification. The ceramic specimens sintered at 1230-1280 °C presented relative densities of around 95%. A significant influence of sintering temperature on the microstructure and nonlinear dielectric properties was detected. The discrepancy in nonlinear dielectric behavior among the specimens sintered at different temperatures was qualitatively interpreted in terms of the dielectric response of polar micro-regions under bias electric field. The specimens sintered at 1230 and 1250 °C attained superior nonlinear dielectric properties, showing relatively low dielectric losses (tan δ) of 0.24% and 0.22% at 10 kHz together with comparatively large figure of merits (FOM) of 121 and 142 at 10 kHz and 20 kV/cm, respectively.     

Oxygen-vacancy-related high-temperature dielectric relaxation and electrical conduction in 0.95K0.5Na0.5NbO3-0.05BaZrO3 ceramic

The crystal structure and dielectric properties of 0.95K_0.5Na_0.5NbO₃-0.05BaZrO₃ (KNN-BZ) ceramic have been investigated by X-ray diffraction and dielectric measurement. A rhombohedral distortion was caused and the dielectric permittivity near Curie temperature was significantly enhanced by introducing BZ into KNN. The dielectric and conductivity properties of the sample were studied by using AC impedance spectroscopy and universal dielectric relaxation law in detail. The typical high-temperature dielectric relaxation process was confirmed to be related to the oxygen vacancies inside the ceramic. The effect of lattice distortion on the activation energy for oxygen vacancy migration in KNN-BZ was discussed by comparing with KNN and KNN-BaTiO₃.     

Dielectric properties for the ordered-perovskite cuprate Sr2Cu(Re0.69Ca0.31)O6

Dielectric properties are reported on polycrystalline cubic ordered-perovskite cuprate Sr₂Cu(Re_0.69Ca_0.31)O₆ in the frequency range 10 Hz-100 kHz at temperature from 300 to 500 K. Both the dielectric permittivity and dielectric loss factor are found to be frequency and temperature dependent. The enhanced value of the low frequency dielectric permittivity is associated to ionic polarization and interfacial phenomena. The material is found to possess significantly high dielectric permittivity. The calculated ac conductivity suggests semiconducting behaviour for the Sr₂Cu(Re_0.69Ca_0.31)O₆.     

Reduced dielectric loss and leakage current in CaCu3Ti4O12/SiO2/CaCu3Ti4O12 multilayered films

The CaCu₃Ti₄O₁₂/SiO₂/CaCu₃Ti₄O₁₂ (CCTO/SiO₂/CCTO) multilayered films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition method. It has been demonstrated that the dielectric loss and the leakage current density were significantly reduced with the increase of the SiO₂ layer thickness, accompanied with a decrease of the dielectric constant. The CCTO film with a 20 nm SiO₂ layer showed a dielectric loss of 0.065 at 100 kHz and the leakage current density of 6×10⁻⁷ A/cm² at 100 kV/cm, which were much lower than those of the single layer CCTO films. The improvement of the electric properties is ascribed to two reasons: one is the improved crystallinity; the other is the reduced free carriers in the multilayered films.