Effect of cesium and cerium substitution on the dielectric properties of CaCu3Ti4O12 ceramics

In this study, CaCu₃Ti₄O₁₂ (CCTO) ceramics were doped with cesium and cerium atoms to possibly improve the electrical properties of these widely used ceramics. In all cases, pure phase perovskites were produced where cesium doping enhanced the grain growth and cerium doping produced grain growth inhibition. The cesium doping showed an improvement in loss tangent performance, in contrast to the cerium doping which showed a negative result. A high dielectric constant >15,000 with a dielectric loss lower than 0.06 was observed for cesium 2.0 mol% doped at high frequencies. These results were related to the change in microstructure and the properties of grain boundary after doping.     

Microstructural evolution and dielectric properties of SiO2-doped CaCu3Ti4O12 ceramics

The abnormal grain growth (AGG) behavior of undoped and SiO₂-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics were investigated. With the addition of 2 wt.% SiO₂, the AGG-triggering temperature decreased from 1100 to 1060 °C, and the temperature for obtaining a uniform and coarse microstructure decreased from 1140 to 1100 °C. The lowering of the AGG temperature by SiO₂ addition was attributed to the formation of a CuO-SiO₂-rich intergranular phase at lower temperature. The apparent dielectric permittivity of coarse SiO₂-doped CCTO ceramics was ∼10 times higher than that of fine SiO₂-doped CCTO ceramics at the frequency of 10³–10⁵ Hz. The doping of SiO₂ to CCTO ceramics provides an efficient route of improving the dielectric properties via grain coarsening. The correlation between the microstructure and apparent permittivity suggests the presence of a barrier layer near the grain boundary.     

Dielectric properties of CaCu3Ti4O12 improved by chromium/lanthanum co-doping

The dielectric properties of Cr + La co-doped CaCu₃Ti₄O₁₂ ceramics prepared by a solid-state reaction method were evaluated and compared to Cr-doped, La-doped, and parent CaCu₃Ti₄O₁₂ (CCTO). Their structure and grain size were evaluated by X-ray diffraction and scanning electron microscopy, respectively. No secondary phase was detected based on the XRD analysis. The results show that, the room temperature dielectric loss of the co-doped samples is reduced to 43% compared to CCTO and their dielectric permittivity is higher than the un-doped, Cr-doped, and La-doped samples at frequencies over 325 kHz, 30 kHz, and 12 Hz, respectively. Furthermore, the temperature stability of the co-doped sample is significantly more convenient than that of CCTO, and its dielectric loss is three times lower. The results also indicated that the co-doping method is effective in reducing the dielectric loss, still maintaining the high dielectric permittivity.     

Effects of LiF addition on microstructure and dielectric properties of CaCu3Ti4O12 ceramics

The effects of small amounts of lithium fluoride sintering aid on the microstructure and dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics were investigated. CCTO polycrystalline ceramics with 0.5 and 1.0 mol% LiF, and without additive were prepared by solid state synthesis. Good densification (>90% of the theoretical density) was obtained for all prepared materials. Specimens without the sintering aid and sintered at 1090 °C exhibit secondary phases as an outcome of the decomposition reaction. The mean grain size is controlled by the amount of LiF in specimens containing the additive. Impedance spectroscopy measurements on CaCu₃Ti₄O₁₂ ceramics evidence the electrically heterogeneous nature of this material consisting of semiconductor grains along with insulating grain boundaries. The activation energy for grain boundary conduction is lower for specimens prepared with the additive, and the electric permittivity reached 53,000 for 0.5 mol% LiF containing CCTO.     

High-temperature dielectric and relaxation behavior of Yb-doped Bi0.5Na0.5TiO3 ceramics

Microstructure, phase transition and dielectric properties of Yb-doped Bi_0.5Na_0.5TiO₃ (BNT) ceramics were investigated. It is found that ytterbium promotes the grain growth and densification of the ceramics while Ti-rich impurity appears due to the compensation of Ti-vacancy. The dielectric operational temperature range of the ceramics with a±15% tolerance was greatly broaden until 500 °C by ytterbium doping. Meanwhile, the diffuseness of the diffuse phase transition increases with the increase of doping Yb. BNT ceramics with 3 mol% Yb doping shows a near-plateau dielectric behavior in a broad temperature range from 147 to 528 °C and a low dielectric loss (<0.025) from 154 to 356 °C, indicating that it is a promising material for applications in high-temperature capacitor.     

Effect of Li and Bi co-doping and sintering temperature on dielectric properties of PLZT 9/65/35 ceramics

In the present study, lead-lanthanum-zirconate-titanate (PLZT) ceramics were prepared by a solid-state mixed oxide method. Different amount of lithium carbonate and bismuth oxide (0.15 mol%, 0.45 mol% and 0.75 mol%), where the ratio of Li:Bi =1:1 by mole, was added to PLZT to investigate the effect of Li and Bi co-doping. The ceramic samples were sintered at the temperatures of 1000, 1050, 1100, 1150 and 1200 °C for 4 h. After that, all samples were subjected to phase identification, physical property determination (sintered density and microstructure) and dielectric property measurement. It was found that doping of 0.15 mol% Li and Bi resulted in maximum dielectric constant (ε_r =7819) when sintered at 1200 °C. Grain size of PLZT ceramics was dependent on sintering temperature and dielectric properties were affected by the chemical composition rather than the grain size of the ceramics. Therefore, co-doping of Li and Bi was useful as it could improve the dielectric properties of PLZT ceramics.     

Effects of Nb-doping on the micro-structure and dielectric properties of (Bi0.5Na0.5)TiO3 ceramics

Bismuth sodium titanate [(Bi_0.5Na_0.5)TiO₃ or BNT] ceramics incorporated with 0, 1, 5, 10, 15 and 20 mol% niobium were prepared by conventional solid state reaction method. The green bodies were sintered at 1050 °C for 2 h to obtain dense ceramics. The effects of substitution of niobium ion for titanium ion in BNT ceramics on micro-structure and dielectric properties were investigated. X-ray diffraction analysis showed the presence of a secondary phase when more than 5 mol% niobium was added. Within the solubility limit, Nb doping caused the grain size of BNTNb to be smaller than the undoped sample. The investigation of the dielectric properties showed that the transition temperature (T_c) was found to shift towards lower temperature as the content of Nb increased. In this research, the donor-type behavior and induced charged defects had significant influence on the electrical properties of Nb-doped BNT ceramics.     

Decrease in the dielectric loss of CaCu3Ti4O12 at high frequency by Ru doping

CaCu_3−xRu_xTi₄O₁₂ (x=0, 0.03, 0.05 and 0.07) electronic ceramics were fabricated using a conventional solid-state reaction method. The microstructure, grain sizes and dielectric properties as well as the impedance behaviours of the ceramics were carefully investigated. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) results indicate that ruthenium (Ru) dopant inhibits the growth of grains during the sintering process by promoting the formation of high melting point oxides of Ca and Ti. The study on the frequency dependence of dielectric properties suggests that Ru doping shifts the dielectric loss peak of CCTO to a much lower frequency, thereby reducing the dielectric loss of CCTO at high frequency (f>1.0 MHz) accordingly. When doped with proper amount of Ru, the high frequency dielectric loss of CCTO is reduced to a very low value (tanδ<0.05). Our study conclusively suggests that Ru-doped CCTO, with sufficiently low dielectric loss and decent permittivity, presents potential applications at high frequency.     

Control of coring effect in BaTi4O9 microwave dielectric ceramics by doping with Mn4+

In the present work, we have attempted to reduce the effect of coring effect in the titanate ceramic system BaTi₄O₉ (BT4) by doping it with Mn⁴⁺. The microwave dielectric BaTi₄O₉ ceramics doped with 0, 0.5 and 1.0 mol% Mn⁴⁺ were synthesized by conventional ceramic processing route. The XRD studies confirmed a single phase crystalline structure for all the ceramic samples studied. The SEM micrographs of the ceramics reveal a microstructural change leading towards a more uniform grain size distribution as the Mn⁴⁺ content increases to 1.0 mol%. In the low frequency region (100 Hz to 1 MHz), the temperature stability of dielectric properties exhibits a marked improvement with the increasing amount of Mn⁴⁺ in the ceramic system. In the microwave frequency region (9.3 GHz), Q-factor increases from 11,625 GHz to 46,500 GHz for BaTi₄O₉ ceramic doped with 1.0 mol% Mn⁴⁺. The present paper reveals that the commonly observed degradation of dielectric properties due to coring effect in the BaTi₄O₉ ceramic system can be controlled by doping it with an appropriate quantity of Mn⁴⁺.     

Microstructure and electrical properties of Mn/Y codoped Ba0.67Sr0.33TiO3 ceramics

Pure and Mn/Y codoped Ba_0.67Sr_0.33TiO₃ (BST) ceramics were fabricated via the citrate–nitrate combustion technique, and the microstructure and electrical properties of BST ceramics were mainly investigated. The Mn/Y codoping concentration has a strong influence on the microstructure and electrical properties of BST ceramics. All BST ceramics possess a pure polycrystalline structure. The density, dielectric loss, leakage current, and ferroelectric properties are improved by codoping 0.5 mol% Mn and 1.0 mol% Y to BST. The relative density of 0.5 mol% Mn/1.0 mol% Y-codoped BST (BST0510) ceramics reaches 97.5% of the theoretical value. BST0510 ceramics have the lowest dielectric loss (tanδ < 0.0073 at 1 kHz) among all BST ceramics. BST0510 ceramics also demonstrate a low leakage current density (1.23 × 10^?7 A/cm²) at an applied field of 10 kV/cm, and excellent ferroelectric properties with a remanent polarization of 2P_r = 15.327 μC/cm² and a coercive field of 2E_c = 3.456 kV/cm. Therefore, the Mn and Y with optimum content help improve the electrical properties of BST materials.     

CaCu3Ti4O12 ceramics from co-precipitation method: Dielectric properties of pellets and thick films

Dielectric properties of CaCu₃Ti₄O₁₂ (CCTO)-based ceramics and thick films (e ～50 μm) prepared from powders synthesized by a soft chemistry method (co-precipitation) are presented and discussed. The characteristics of pellets and thick films are compared.The pellets exhibit high values of the dielectric permittivity (?_r ～1.4 × 10⁵) and relatively small dielectric losses (tan δ ～0.16) at 1 kHz and room temperature. These properties are independent of the nature of the metallization of the electrodes. In addition, the dielectric permittivity decreases when the diameter of the electrodes of the pellets increases, while the losses remain constant. This result, which is strongly related to the nature of the dielectric material in between the electrodes, constitutes a strong indication that the high dielectric permittivity values observed in this material are not related to an interfacial (electrode material) related mechanism but is an internal barrier layer capacitor (IBLC) type.Very high values of the dielectric permittivity of CCTO thick films are measured (?_r ～5 × 10⁴). The differences in dielectric permittivity between thick films and dense pellets may be attributed to the difference in grain size due to different CuO contents, and to the different reactivity of the materials.     

Doping behaviors of NiO and Nb2O5 in BaTiO3 and dielectric properties of BaTiO3-based X7R ceramics

Doping behaviors of NiO and Nb₂O₅ in BaTiO₃ in two doping ways and dielectric properties of BaTiO₃-based X7R ceramics were investigated. When doped in composite form, the additions rendered higher solubility than that doped separately due to the identical valence between the complex (Ni_1/3²⁺Nb_2/3⁵⁺)⁴⁺ and Ti⁴⁺. NiO–Nb₂O₅ composite oxide was more effective in broadening dielectric constant peaks which was responsible for the temperature-stability of BaTiO₃ ceramics. A reduction in grain size was observed in the specimens with 0.5–0.8 mol% NiO–Nb₂O₅ composite oxide, whereas the abnormal growth of individual grains took place in the 1.0 mol% NiO–Nb₂O₅ composite oxide-doped specimen. When the specimen of BaTiO₃ doped with 0.8 mol% NiO–Nb₂O₅ composite oxide was sintered at 1300 °C for 1.5 h in air, good dielectric properties were obtained and the requirement of (EIA) X7R specification with a dielectric constant of 4706 and dielectric loss lower than 1.5% were satisfied.     

Dielectric behaviour of Hf-doped CaCu3Ti4O12 ceramics obtained by conventional synthesis and reactive sintering

CaCu₃(Ti_4?xHf_x)O₁₂ ceramics (x = 0.04, 0.1 and 0.2) were prepared by conventional synthesis (CS) and through reactive sintering (RS), in which synthesis and sintering of the material take place in one single step. The microstructure and the dielectric properties of Hf-doped CCTO (CCTOHf) have been studied by XRD, FE-SEM, AFM, Raman and impedance spectroscopy (IS) in order to correlate the structure, microstructure and the electrical properties. Samples prepared by reactive sintering show slightly higher dielectric constant than those prepared by conventional synthesis in the same way than the pure CCTO. Dielectric constant and dielectric losses decrease slightly increasing Hf content. For CCTOHf ceramics with x > 0.04 for CS and x > 0.1 for RS, a secondary phase HfTiO₄ appears. As expected, the reactive sintering processing method allows a higher incorporation of Hf in the CCTO lattice than the conventional synthesis one.     

Enhancement of giant dielectric response in Ga-doped CaCu3Ti4O12 ceramics

The influences of Ga³⁺ doping ions on the microstructure, dielectric and electrical properties of CaCu₃Ti₄O₁₂ ceramics were investigated systematically. Addition of Ga³⁺ ions can cause a great increase in the mean grain size of CaCu₃Ti₄O₁₂ ceramics. This is ascribed to the ability of Ga³⁺ doping to enhance grain boundary mobility. Doping CaCu₃Ti₄O₁₂ with 0.25 mol% of Ga³⁺ caused a large increase in its dielectric constant from 5439 to 31,331. The loss tangent decreased from 0.153 to 0.044. The giant dielectric response and dielectric relaxation behavior can be well described by the internal barrier layer capacitor model based on Maxwell?Wagner polarization at grain boundaries. The nonlinear coefficient, breakdown field, and electrostatic potential barrier at grain boundaries decreased with increasing Ga³⁺ content. Our results demonstrated the importance of ceramic microstructure and electrical responses of grain and grain boundaries in controlling the giant dielectric response and dielectric relaxation behavior of CaCu₃Ti₄O₁₂ ceramics.     

Dielectric properties of manganese and cobalt doped lead iron tantalate ceramics

This paper reports the results of synthesis and sintering studies as well as dielectric properties of Pb(Fe_1/2Ta_1/2)O₃ (PFT) relaxor ferroelectric ceramics. Influence of doping with MnO₂ and Co₃O₄ (0.1–1 mol%) on resistivity and dielectric characteristics were investigated. The dielectric permittivity and dissipation factor of the ceramics were determined as a function of temperature in the range from −55 to 500 °C at frequencies 10 Hz to 1 MHz. DC resistivities of the samples were measured in the temperature range 20–500 °C. Two maxima in dielectric permittivity versus temperature curves were observed, dependent on frequency and the content of dopants. The investigated PFT ceramics were characterized by high dielectric permittivity of 3500–6700 at the transition temperature and 900–17,000 at the second maxima.     

Influence of cobalt and sintering temperature on structure and electrical properties of BaZr0.05Ti0.95O3 ceramics

Pure and Co-modified BaZr_0.05Ti_0.95O₃ ceramics were fabricated by the traditional solid state reaction technique. The influence of cobalt and sintering temperature on structure, dielectric, ferroelectric properties and diffuse phase transition of BZT ceramics were investigated systematically. 1300 °C was the optimal sintering temperature for BZT ceramics. The solid solubility limit of Co ions in BZT matrix was determined to be 0.4 mol%. The introduction of a moderate amount of Co ions was believed to benefit the microstructure development and make the grain size more uniform. Compared with undoped counterparts, 0.4 mol% Co-modified ceramics showed equivalent ferroelectric properties with a high remnant polarization (P_r=9.6 μC/cm²) and a low coercive field (E_c=0.21 kV/mm). Besides these, a relative high dielectric coefficient (ε_r=2030) and a low dielectric loss (tan δ=1.85%) were also obtained on this composition. The degree of diffuse phase transition was enhanced by the addition of Co ions. The related mechanism of the diffused phase transition behavior was discussed.     

Dielectric properties and diffuse phase transition behavior of CuO-doped lead-free Ba(ZrxTi1−x)O3 ceramics

CuO-doped lead-free Ba(Zr_0.2Ti_0.8)O₃ (BZT20) ceramics were prepared through a solid state processing technique, and the effects of CuO on microstructure, dielectric properties and diffuse phase transition behavior were investigated. The average grain sizes were increased by CuO doping. The temperature and frequency dependences of the dielectric constant revealed that CuO-doped BZT20 ceramics exhibited broad diffuse phase transition behavior. The dielectric constant increased with increasing CuO concentration. The value of T_m and degree of diffusion (γ) changed regularly in the studied compositional ranges. The BZT20 samples with 1.0 mol% CuO doping, sintered at 1310 °C, showed excellent dielectric property and lower diffusivity with ε_m=21,371 and γ=1.87. These results can be explained by the disordered distribution of Cu ions in the B sites and the weakened bonding force with oxygen ions in Cu substituted BZT20 structure.     

B-site donor and acceptor doped Aurivillius phase Bi3NbTiO9 ceramics

The electrical properties of B-site donor and acceptor doped Aurivillius phase Bi₃NbTiO₉-based ceramics have been investigated. The effect of donor and acceptor doping on the dielectric constant, coercive field, dc conductivity and piezoelectric constant are presented. The band gap of Bi₃NbTiO₉ (BNTO) is about 3.4 ± 0.2 eV, determined from high-temperature dc conductivity measurements. All of the ceramics are ferroelectrics with high Curie points (∼900 °C). In acceptor doped ceramics, a low-temperature peak in the dielectric loss tangent is explained in terms of a Debye-type relaxation that results from an oxygen ion-jump mechanism. The activation energy for the relaxation is calculated as 0.93 ± 0.05 eV. The reduction of the piezoelectric constant below 500 °C is produced by depolarization, which is produced by the switching of thermally unstable non-180° domain walls.     

Dielectric behaviors of Nb2O5–Co2O3 doped BaTiO3–Bi(Mg1/2Ti1/2)O3 ceramics

Nb₂O₅ and Nb–Co doped 0.85BaTiO₃–0.15Bi(Mg_1/2Ti_1/2)O₃ (0.85BT–0.15BMT) ceramics were investigated. From XRD patterns, undesired phase was observed when the (Nb₂O₅/Nb-Co) doping levels exceed 3 wt.%/2 wt.%, giving rise to the deteriorate dielectric constant. The 0.85BT–0.15BMT ceramics doped with 2 wt.%Nb₂O₅ was found to possess a moderate dielectric constant (? ～ 1000) and low dielectric loss (tan δ = 0.9%) at room temperature and 1 kHz, showing flat dielectric behavior over the temperature range from ?55 to 155 °C. It was found that the formation of core–shell structure in the BT based ceramics is controlled by the doping sequence of Nb- and Bi-oxides.     

Evolution of the intergranular phase during sintering of CaCu3Ti4O12 ceramics

CaCu₃Ti₄O₁₂ (CCTO) ceramics have been processed by solid state reaction and sintered at 1100 °C for different times. A clear increase of the dielectric constant of the material up to values of 6 × 10⁴ has been observed with the sintering time. This increase is accompanied by a limited grain growth and intergranular Cu-oxide phase thickness reduction. The disappearance of the Cu-oxide phase is caused by the incorporation of Cu cations into the grains, contributing to the increase of the dielectric constant. Raman spectroscopy shows the decrease of TiO₆ octahedral rotational modes with the sintering time due to the incorporation of Cu cations into the CCTO grains. XANES measurements show that the Cu main oxidation state is Cu²⁺ and does not change with the sintering time. The fitting of the experimental dielectric constant to the Internal Barrier Layer Capacitance (IBLC) model reveals the change of the intergranular phase dielectric constant, caused by a compositional change due to the incorporation of Cu into the CCTO grains.