Dielectric response of Sb-doped CaCu3Ti4O12 ceramics

A series of Sb-doped CaCu₃Ti₄O₁₂ ceramics were fabricated by the conventional solid state method, and their crystalline structures, microstructures and dielectric properties were investigated systematically. All the ceramic samples exhibited perovskite-related structures in space group Im $\bar{3}$ . The grain size decreased slightly as Sb concentration increased; whereas the dielectric permittivity of the ceramics increased slightly. The giant dielectric response was considered to be closely related with a reduction in the potential barrier height at grain boundaries (GBs). The activation energy for the dc conduction process is comparable to that for conduction at GBs, indicating that the dc conduction process is associated with the electrical response of GBs.     

Dielectric properties of bismuth doped CaCu3Ti4O12 ceramics

Ca_1?3x/2Bi_xCu₃Ti₄O₁₂ (x = 0.0–0.3) ceramics were prepared by the conventional solid-state reaction method. X-ray powder diffraction analysis confirmed the formation of cubic CCTO phase except for subtle peaks of CuO. SEM micrographs suggested that the morphologies of doped CCTO ceramics had been sheet-like for high Bi-doping amount, and the dominant grain size decreasing could be seen for the small content of Bi-doping CCTO. Dielectric properties of pure and doped CCTO were investigated in a broad temperature range of 20–420 K. The results showed that bismuth doping could decrease the dielectric loss but suppress the dielectric temperature stability at the same time. Bi doped CCTO ceramics presented different relaxation properties. As to pure CCTO and BCCTO (x = 0.3) only one MW relaxation (Relaxation I) could be found, which moves to higher frequency with temperature increasing. However, two relaxation processes (Relaxation I and II) appear for BCCTO (x = 0.1–0.2). By means of complex impedance spectra analysis and Arrhenius fitting, we successfully separated the different conductive segments and explained the mechanisms of the two relaxation processes. Relaxation I appeared at low temperature could be attributed to the V_O doping energies inside CCTO grains which did not showed significant changing of activation energy after bismuth doping. For Relaxation II at higher temperature than Relaxation I, with activation energy obviously depending on the Bi-ion concentration, may be related with the V_O point defects at the grain boundaries.     

Giant dielectric and low voltage varistor behaviors of Ba-doped Bi1/2Na1/2Cu3Ti4O12 ceramics

Bi_1/2?xNa_1/2?xBa_2xCu₃Ti₄O₁₂ (x = 0, 0.025, 0.05, 0.075) polycrystalline ceramics are prepared by a modified Pechini method. Effect of Ba doping on the dielectric and nonlinear electrical behaviors are investigated in detail. All the samples exhibit giant dielectric effect and nonlinear electrical behavior. Compared with Bi_1/2Na_1/2Cu₃T_i4O₁₂ ceramics, the permittivity drops down and the nonlinear electrical behavior shifts to low voltage area by Ba doping. The permittivity reduces from 20,004 to 7,006 at 1 kHz. The breakdown voltage and nonlinear coefficient values decrease to 10.8 V/mm and 1.5, respectively. Additionally, the electrostatic barrier height drops down to 0.66 eV. Giant dielectric response and nonlinear electrical behavior can be interpreted in terms of internal barrier layer capacitors model. These results imply that Ba substitution can adjust the dielectric and nonlinear electrical behaviors of Bi_1/2Na_1/2Cu₃T_i4O₁₂ ceramics, which is a promising candidate for low voltage varistor materials.     

Microstructure and electrical properties of LaMnO3-CaCu3Ti4O12 composite ceramics

In this study, xLaMnO₃-(1???x)CaCu₃Ti₄O₁₂ (0?≤?x?≤?0.7) composite ceramics have been prepared by the conventional solid-state method. XRD results show that the composite ceramics are mainly composed of LaMnO₃ and CaCu₃Ti₄O₁₂ phases. Electrical property analysis shows that the resistivity ρ₂₅ gradually decreases from 2.17?×?10⁸ to 1.74?×?10⁵ Ω·cm with increasing LaMnO₃ content. The relationship between lnρ and 1000/T is not linear and changes around 350 °C. In the temperature range of 0–350 °C, the activation energy Ea_0–350 °C decreases gradually with the increase of the LaMnO₃ content. In the temperature range of 350–500 °C, the activation energy Ea_{350–500 °C} increases first and then decreases, which is related to the variation of oxygen vacancy. The compounding of LaMnO₃ gives rise to the decrease in B value, thus, providing the potential for the applications of wide temperature range thermistors. After annealing, the coefficient of aging (ΔR/R) of the composites decreases from 23.73 to 3.01%.

     

Sandwiched BaNd2Ti4O12/Bi4Ti3O12/BaNd2Ti4O12 ceramics prepared by spark plasma sintering

Spark plasma sintering (SPS) combined with heat treatment was investigated to prepare laminated ceramics from compositions having quite different sintering temperatures. Although the optimal sintering temperature of BaNd₂Ti₄O₁₂ (BNT) ceramics was much higher than that of Bi₄Ti₃O₁₂ (BIT) ceramics, sandwiched BaNd₂Ti₄O₁₂/Bi₄Ti₃O₁₂/BaNd₂Ti₄O₁₂ (BNT/BIT/BNT) composite ceramics were successfully prepared with this new method. The results of scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) showed that the BNT layers and the BIT layer were well bonded and no significant diffusion between them was observed. The temperature coefficient of dielectric constant of the laminated ceramic was found to be much smaller than that of BNT ceramic.     

Dielectric, magnetic and electrical properties of ZnFe2O4 ceramics

The polycrystalline sample of ZnFe₂O₄ was prepared by a high-temperature solid-state reaction technique. Preliminary X-ray diffraction studies of the compound showed the formation of a single-phase compound at room temperature. Studies of dielectric properties (ε_r, tan δ) of the above compound as a function of frequency in a wide temperature range show dielectric anomalies signifying existence of possible ferroelectric to paraelectric phase transition in the material. The confirmation of this assumption was made with observation of ferroelectric hysteresis loop at room temperature. Magnetic measurement exhibits anti-ferromagnetic nature of the sample. Studies of the zero-field cooled and the field-cooled magnetization in dc field provided the blocking temperature T_B. The temperature dependence of electrical parameters (impedance, modulus, conductivity, etc.) of the material exhibits a strong correlation between the microstructure (i.e., bulk, grain boundary, etc.) and electrical parameters of the material. Detailed studies of impedance parameters have provided an insight into the electrical properties and understanding of types of relaxation process in the material. The temperature variation of dc resistivity/conductivity exhibits negative temperature coefficient of resistance behaviour of the material. The frequency dependence of ac conductivity suggests that the material obeys Jonscher’s universal power law.     

Microstructural and dielectric properties of donor doped (La3+) CaCu3Ti4O12 ceramics

The effect of La³⁺ doping on Ca²⁺ sites in CaCu₃Ti₄O₁₂ (CCTO) was examined. Polycrystalline samples in the chemical formula Ca_(1-x)La_(2/3)x Cu₃Ti₄O₁₂ with x = 0, 0.5, 1 were synthesized via the conventional solid state reaction route. X-ray powder diffraction analysis confirmed the formation of the monophasic compounds and indicated the structure to be remaining cubic with a small increase in lattice parameter with increase in La³⁺ doping. The dielectric and impedance characteristics of Ca_(1-x)La_(2/3)x Cu₃Ti₄O₁₂ were studied in the 100 Hz–10 MHz frequency range at various temperatures (100–475 K). A remarkable decrease in grain size from 50 μm to 3–5 μm was observed on La³⁺ substitution. The dielectric constant of CaCu₃Ti₄O₁₂ decreased drastically on La³⁺ doping. The frequency and temperature responses of dielectric constant of La³⁺ doped samples were found to be similar to that of CaCu₃Ti₄O₁₂. The effects of La³⁺ doping on the electrical properties of CaCu₃Ti₄O₁₂ were probed using impedance spectroscopy. The conducting properties of grain decreased while that of the grain boundary increased on La³⁺ doping, resulting in a decrease of the internal barrier layer effect. A decrease in grain boundary capacitance and stable grain response in La³⁺ doped CCTO ceramics were unambiguously established by modulus spectra studies.     

Dielectric properties of CaCu3Ti4O12 ceramics: effect of high purity nanometric powders

The dielectric properties of CaCu₃Ti₄O₁₂ ceramics fabricated with nano-size fine powders (~30 nm) are compared with that fabricated with micro-size coarse powders (0.1–0.3 μm). For the same sintering conditions, the ceramic samples with nano-size fine powders have more uniform and denser microstructures and higher room temperature dielectric constant (~10⁵, in the frequency range of 10^?1–10⁵ Hz) than that with micro-size coarse powders. That the use of nano powders facilitates the formation of Cu-rich amorphous phase in the grain boundary led to an increasing dielectric loss in low frequency range. Besides the common intrinsic defect structure of V _O ⁺ and V _O ⁺⁺ , the energy level of ~0.72 eV detected in high temperature range is attributed to the conduction relaxations, and the energy level of 0.30–0.40 eV which is only detected in the sample synthesized by common submicron powders is suggested to originate from the defect level of grain boundary related to Cu ions. This research provides a technical guidance for the application of this material.     

Bi(Mg_(1/2)Ti_(1/2))O_3-PbTiO_3压电陶瓷体系介电与居里温度特性研究

采用传统固相法工艺制备了(1-x)Bi(Mg1/2Ti1/2)O3-x Pb Ti O3(BMT-x PT,0.34≤x≤0.44)陶瓷。研究发现,随着PT含量增加,试样结构由三方相逐渐转变为四方相结构,当0.36x0.40时,试样结构处于准同型相界(MPB)区。研究表明BMT组元是一种具有非铁电体特征的组分,随着PT含量减少,BMT-PT体系的居里温度减小,介电峰变得越来越不明显。通过研究BMT-PT体系组分与居里温度(TC)的关系可以看出:(1)PT含量为0.34~0.44时,TC随BMT含量变化实验值和Stringer的经验值差异较小,变化趋势一致;(2)BMT-PT体系居里温度最大值可能在x=0.73的附近,其居里温度最大值TC max约为550℃。     

Dielectric and ferroelectric properties of Bi(Zn1/2Ti1/2)O3-PbTiO3-PbZrO3 ternary ceramics

Ceramics of a new ternary solid solution system, xBi(Zn(1/2)Ti(1/2))O(3-yPbTiO(3)z)PbZrO(3) (xBZT-yPT-zPZ), with compositions along the solubility limit curve are prepared by solid-state reaction and sintering technique. Two morphotropic phase boundaries (MPBs) separating the orthorhombic and tetragonal (MPB(O-T)) phases and the tetragonal and rhombohedral (MPB(T-R)) phases, respectively, are observed with increasing z (0.10 ≤ x ≤ 0.21; 0 ≤ y ≤ 0.49). It is found that the transition from the ferroelectric to paraelectric phase becomes more diffuse with the addition of BZT into the PZT solid solution. Enhanced dielectric and ferroelectric properties appear at MPB(R-T), which exists over a wide composition region (0.45 ≤ z ≤ 0.6), as revealed by X-ray diffraction and dielectric measurements. The dielectric constant reaches a maximum value (ε' = 1250) on the tetragonal majority side of the MPB. The highest remnant polarization (P(r) = 34.2 μC/cm(2)) is found in the composition at the center of the MPB, where the rhombohedral and tetragonal phases coexist in almost equal quantities.     

Dielectric and impedance properties of Nd3/2Bi3/2Fe5O12 ceramics

The polycrystalline sample of Nd_3/2Bi_3/2Fe₅O₁₂ was prepared by a high- temperature solid-state reaction technique. Preliminary X-ray structural analysis exhibits the formation of a single-phase tetragonal structure at room temperature. Microstructural analysis by scanning electron microscopy shows that the sintered sample has well defined grains. These grains are distributed uniformly throughout the surface of the sample. Detailed studies of dielectric response at various frequencies and temperatures exhibit a dielectric anomaly at 400 °C. The electrical properties (impedance, modulus and conductivity) of the material were studied using a complex impedance spectroscopy technique. These studies reveal a significant contribution of grain and grain boundary effects in the material. The frequency dependent plots of modulus and the impedance loss show that the conductivity relaxation is of non-Debye type. Studies of electrical conductivity with temperature demonstrate that the compound exhibits Arrhenius-type of electrical conductivity. Study of ac conductivity with frequency suggests that the material obeys Jonscher’s universal power law.     

Non-Ohmic and dielectric properties of Ba-doped CaCu3Ti4O12 ceramics

The microstructure, dielectric and electrical properties of Ca_1?x Ba _x Cu₃Ti₄O₁₂ (where x = 0, 0.025, and 0.05) ceramics were investigated. Our microstructural analyses revealed that Ba²⁺ doping ions preferentially form in a secondary phase, and are not introduced into the CaCu₃Ti₄O₁₂ lattice. Grain growth rate of CaCu₃Ti₄O₁₂ ceramics was significantly inhibited by the Ba-related secondary phase particles, resulting in a large decrease in their mean grain size. The dielectric permittivity of CaCu₃Ti₄O₁₂ ceramics decreased with increasing Ba content. Their loss tangent decreased after addition of CaCu₃Ti₄O₁₂ with 2.5 mol% of Ba²⁺, and increased with increasing Ba contents to 5.0 mol%. The nonlinear coefficient and breakdown field of the Ca_1?x Ba _x Cu₃Ti₄O₁₂ ceramics were significantly enhanced by adding 2.5 mol% of Ba²⁺, followed by a slight decrease as Ba²⁺ concentration was increased to 5.0 mol%. Using impedance spectroscopy analysis, it was revealed that variations in dielectric and non-Ohmic properties are associated with electrical response of grain boundaries. This supports the internal barrier layer capacitor model.     

Dielectric and piezoelectric properties of PbFe1/2Nb1/2O3-PbTiO3 ceramics from the morphotropic phase boundary compositional range

Dielectric, X-ray, and piezoelectric studies of highly-resistive Li-doped (1-x)PbFe(1/2)Nb(1/2)O(3)-(x)PbTiO(3) (PFN-xPT) ceramics from the 0 ≤ x ≤ 0.2 range fabricated by solid-state synthesis and usual sintering have been carried out. Distinct anomalies of dielectric and piezoelectric parameters, corresponding to the transition between rhombohedral (monoclinic) and tetragonal ferroelectric phases, have been observed in pure PFN and PFN-xPT compositions with PbTiO(3) content up to 8 mol.%. The x,T-phase diagram of the PFN-xPT solid solution system has been constructed using these data.     

Dielectric properties of La2O3-doped Ba0.8Sr0.2TiO3 ceramics

La₂O₃-doped Ba_0.8Sr_0.2TiO₃ dielectric ceramics were prepared by conventional solid state ceramic route. Scanning electron microscope was employed to observe the surface morphologies. The capacitance C and dielectric loss factor D of the samples were measured with automatic LCR Meter 4225 at 10 kHz respectively. The results show that: ε _r of the samples decreases and tgδ first decreases then increases with increasing amount of La₂O₃ doping. ε _r reaches better value, tgδ obtains the minimum value at 0.5 mol% La₂O₃. ε _r increases and tgδ decreases when sintering temperature increases. The samples doped with 0.5 mol% La₂O₃ sintered at 1,350 °C for 10 h exhibited attractive properties, including high relative dielectric constant (>4,000), low dielectric loss (16.8 × 10^?4), low temperature coefficient of relative dielectric constant(<±21 %) in the temperature range of +25 to +85 °C.