Co-doping effects of A-site Y3+ and B-site Al3+ on the microstructures and dielectric properties of CaCu3Ti4O12 ceramics

Different doping elements have been used to reduce the dielectric losses of CaCu₃Ti₄O₁₂ ceramics, but their dielectric constants usually are undesirably decreased. This work intends to reduce their dielectric losses and simultaneously enhance their dielectric constants by co-doping Y³⁺ as a donor at A site and Al³⁺ as an acceptor at B site for substituting Ca²⁺ and Ti⁴⁺, respectively. Samples with different doping concentrations x = 0, 0.01, 0.02, 0.03, 0.05 and 0.07 have been prepared. It has been shown that their dielectric losses are generally reduced and their dielectric constants are simultaneously enhanced across the frequency range up to 1 MHz. The doped sample with x = 0.05 exhibits the highest dielectric constant, which is well over 10⁴ for frequency up to 1 MHz and is about 20% higher than the undoped sample. Impedance spectra indicate that the doped samples have much higher grain boundary resistance than the undoped one.     

Influence of TiO2 doping on thermo-optical properties of pyrochlore Sm2(Zr1–xTix)2O7 (0≤x≤0.15) ceramics

Sm₂(Zr_1–xTi_x)₂O₇ (0≤x≤0.15) ceramics have been fabricated by pressureless-sintering method at 1973 K for 10 h in air. The influence of TiO₂ doping on microstructure and thermo-optical properties of Sm₂(Zr_1–xTi_x)₂O₇ ceramics is investigated by X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy measurements. The partial substitution of Ti⁴⁺ for Zr⁴⁺ results in a significant increase in emissivity at low wavelengths contrasted with undoped Sm₂Zr₂O₇. Sm₂(Zr_0.85Ti_0.15)₂O₇ ceramic exhibits a high emissivity of above 0.70 at 1073 K in a wavelength range of 3–16 µm, where the highest value at this temperature is more than 0.90 especially in the wavelength range of 9–14 µm. FT-IR spectra and optical absorption spectra unveil the mechanisms of enhanced emissivity in Sm₂(Zr_1–xTi_x)₂O₇ (0.05≤x≤0.15) ceramics in the intermediate infrared range, especially at the wavelengths of 3–8 µm, due to Ti⁴⁺ ion substitution for Zr⁴⁺ ion.     

Colossal dielectric permittivity and electrical properties of the grain boundary of Ca1−3x/2YbxCu3−yMgyTi4O12 (x=0.05, y=0.05 and 0.30)

Dielectric properties of Ca_1−3x/2Yb_xCu_3−yMg_yTi₄O₁₂ (x=0.05, y=0.05 and 0.30) prepared using a modified sol–gel method and sintered at 1070 °C for 4 h were investigated. The mean grain sizes of the CaCu₃Ti₄O₁₂ and co-doped Ca_0.925Yb_0.05Cu_3−yMg_yTi₄O₁₂ (y=0.05 and 0.30) ceramics were ≈15.86, ≈3.37, and ≈2.32 μm, respectively. Interestingly, the dielectric properties can be effectively improved by co-doping with Yb³⁺ and Mg²⁺ ions to simultaneously control the microstructure and properties of grain boundaries, respectively. These properties were improved over those of single-doped and un-doped CaCu₃Ti₄O₁₂ ceramics. A highly frequency−independent colossal dielectric permittivity (≈10⁴) in the range of 10²–10⁶ Hz with very low loss tangent values of 0.018–0.028 at 1 kHz were successfully achieved in the co-doped Ca_0.925Yb_0.05Cu_3−yMg_yTi₄O₁₂ ceramics. Furthermore, the temperature stability of the colossal dielectric response of Ca_1−3x/2Yb_xCu_3−yMg_yTi₄O₁₂ was also improved to values of less than ±15% in the temperature range from −70 to 100 °C.     

Low temperature preparation of CaCu3Ti4O12 ceramics with high permittivity and low dielectric loss

Low temperature preparation of CaCu₃Ti₄O₁₂ ceramics with large permittivity is of practical interest for cofired multilayer ceramic capacitors. Although CaCu₃Ti₄O₁₂ ceramics have been prepared at low temperatures as previously reported, they have rather low permittivity. This work demonstrates that CaCu₃Ti₄O₁₂ ceramics can not only be prepared at low temperatures, but they also have large permittivity. Herein, CaCu₃Ti₄O₁₂ ceramics were prepared by the solid state reaction method using B₂O₃ as the doping substance. It has been shown that B₂O₃ dopant can considerably lower the calcination and sintering temperatures to 870 °C and 920 °C, respectively. The relative permittivity of the low temperature prepared CaCu₃Ti_4−xB_xO₁₂ ceramics is about 5 times larger than the previously reported results in the literature. Furthermore, the dielectric loss of the CaCu₃Ti_4−xB_xO₁₂ ceramics is found to be as low as 0.03. This work provides a beneficial base for the future commercial applications of CaCu₃Ti₄O₁₂ ceramics with large permittivity for the cofired multilayer ceramic capacitors.     

Improved dielectric properties and grain boundary response of SrTiO3 doped Y2/3Cu3Ti4O12 ceramics fabricated by Sol-gel process for high-energy-density storage applications

A chemical solution processing method based on sol-gel chemistry (SG) was used to synthesize (1-x)Y_2/3Cu₃Ti₄O₁₂-xSrTiO₃ (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25) ceramics successfully. The 0.85Y_2/3Cu₃Ti₄O₁₂-0.15SrTiO₃ ceramics sintered at 1050 °C for 20 h showed fine-grained microstructure and high dielectric constant (ε′ ≈ 1.7 × 10⁵) at 1 kHz. Furthermore, the 0.85Y_2/3Cu₃Ti₄O₁₂-0.15SrTiO₃ ceramics appeared distinct pseudo-relaxor behavior. Two electrical responses were observed in the combined modulus and impedance plots, indicating the presence of Maxwell-Wagner relaxation. Sr vacancies and additional oxygen vacancies had substantial contribution to the sintering behavior, an increase in grain growth, and relaxation behaviors in grain boundaries. The contributions of semiconducting grains with the nanodomain and insulating grain boundaries (corresponding to high-frequency and low-frequency electrical response, respectively) played important roles in the dielectric properties of (1-x)Y_2/3Cu₃Ti₄O₁₂-xSrTiO₃ ceramics. The occurrence of the polarization mechanism transition from the grain boundary response to the electrode one with temperature change was clearly evidenced in the low frequency range.     

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.     

Mixed-valent structure,dielectric properties and defect chemistry of Ca1−3x/2TbxCu3Ti4−xTbxO12 ceramics

Single-phase Ca_1−3x/2Tb_xCu₃Ti_4−xTb_xO₁₂ (0.025≤ x≤0.075) (CTCTT) ceramics with a cubic perovskite-like structure and a fine-grained microstructure (1.6‒2.3 µm) were prepared using a mixed oxides method. The results revealed that mixed valence states of Cu²⁺/Cu⁺, Ti⁴⁺/Ti³⁺, and Tb³⁺/Tb⁴⁺ coexisted in CTCTT. A multiphonon phenomenon in the Raman scattering at 1148, 1323, and 1502 cm⁻¹ was reported for undoped and doped CTTO. Tb was mainly incorporated in the interior of the CTCTT grains rather than on the surface. The dielectric permittivity of CTCTT (ε_r'_RT =3590‒5200) decreased relative to CCTO (ε_r'_RT =10240) at f =1 kHz, but the dielectric loss of CTCTT (the minimum value of tan δ=0.12 at RT) increased as a result of Tb doping. The defect chemistry of CTCTT is discussed. The internal barrier layers capacitance (IBLC) model was adopted for impedance spectroscopy (IS) analysis. The activation energies of the grain boundaries (E_gb) and semi-conductive grains (E_g) for CTCTT were determined to be 0.52 eV and 104 meV, respectively. The IS and defect chemistry analyses confirmed that the decrease in the dielectric permittivity was mainly due to a decrease in conductivity in the semiconducting CTCTT grains caused by the acceptor effect of Tb⁴⁺ at the Ti site, which resulted in a decrease in the IBLC effect.     

Microwave dielectric properties of Bi-substituted La(Mg1/2Ti1/2)O3

The system La(Mg_1/2Ti_1/2)O₃–Bi(Mg_1/2Ti_1/2)O₃ (LMT–BMT) was investigated in respect to formation of perovskite solid solutions based on lanthanum magnesium titanate. Single-phase perovskite (1 − x)LMT–xBMT ceramics (0 ≤ x ≤ 0.3) were prepared and their crystal structure and dielectric properties were studied. It has been found that within the solubility range the crystal structure of Bi-substituted LMT remains monoclinic, P2₁/n. Unit cell volume was evaluated to be almost independent on x, varying within the experimental error. Relative permittivity of the ceramics increases by almost a factor of 3 in the range 0 ≤ x ≤ 0.3 and its value is 40–45 at the compositional region where temperature coefficient of the resonant frequency passes a zero-value. Compositional and temperature variations of the dielectric parameters for LMT–BMT estimated at different frequency ranges are considered in comparison with those observed in other Bi-substituted ceramics based on LMT.     

The temperature-dependent piezoelectric and electromechanical properties of cobalt-modified sodium bismuth titanate

Lightly cobalt-modified, Aurivillius-type, sodium bismuth titanate (Na_0.5Bi_4.5Ti₄O₁₅, NBT) ceramics were synthesized by substituting a small amount of cobalt ions onto the Ti⁴⁺ sites using conventional solid-state reaction. X-ray photoelectron spectroscopy (XPS) analysis coupled with bond valence sum calculations show that the dopant cobalt ions substitute for Ti⁴⁺ ions in the form of Co³⁺. The resultant cobalt-modified NBT ceramics (NBT-Co) exhibit better piezoelectric and electromechanical properties by comparison with pure NBT. With only 0.3 wt% Co³⁺ substitution, the piezoelectric properties of the NBT-Co ceramics are optimal, exhibiting a high piezoelectric coefficient (d₃₃~33 pC/N), a low dielectric loss tan δ (~0.1% at 1 kHz), a high thickness planar coupling coefficient (k_t~34%) as well as a high Curie temperature (T_c~663 °C). Such NBT-Co ceramics exhibit nearly temperature-independent piezoelectric and electromechanical properties up to 400 °C, suggesting that these cobalt-modified NBT ceramics are promising materials for high temperature piezoelectric applications.     

Thermal stability improvement of dielectric properties and non-ohmic characteristic of CaCu3+xTi4O12 ceramics via a Cu-nonstoichiometric approach

In this work, the effects of Cu composition on the thermal stability of the dielectric and nonlinear properties of CaCu_3+xTi₄O₁₂ (?0.2 ≤ x ≤ 0.2) ceramics obtained via a polymer-pyrolysis chemical process were studied. The mean grain sizes of Cu-stoichiometric (x = 0), Cu-deficient (x < 0) and Cu-excess (x > 0) CaCu_3+xTi₄O₁₂ ceramics were found to be ~3.2, ~3.4 and ~3.7 μm, respectively. Interestingly, very good dielectric properties (0.020 ≤ tanδ ≤ 0.038 and 4000 ≤ ε′ ≤ 7065) were attained in CaCu_3+xTi₄O₁₂ (?0.2 ≤ x ≤ 0.1, excluding x = 0.2) ceramics. Moreover, the variation of dielectric constant (ε′) within a limit of ±15% (Δε′_{± 15%}) over a wide temperature range (T_R) of ?70 – 220 °C with low tanδ < 0.05 (tanδ_<0.05) over a T_R of ?70 to 80 °C were achieved in a CaCu_2.8Ti₄O₁₂ ceramic. These results suggest that this ceramic could be applicable for X9R capacitors and energy storage devices that require high thermal stability. Additionally, the nonlinear properties of Cu-nonstoichiometric ceramics could be improved when compared with those of the Cu-stoichiometric material. The incremental changes of dielectric and nonlinear properties of CaCu_3+xTi₄O₁₂ (?0.2 ≤ x ≤ 0.2) ceramics revealed the significant role of Cu composition on grain boundary resistance (R_gb), which was confirmed by impedance spectroscopy analysis. In addition, XANES results revealed the proper ratios of Cu⁺:Cu²⁺ and Ti³⁺:Ti⁴⁺ found in these ceramics, indicating the semiconducting behavior of these grains.     

Microwave dielectric properties and microstructures of Nd(Mg0.5Sn0.5−xTix)O3 ceramics

This study elucidates the microwave dielectric properties and microstructures of Nd(Mg_0.5Sn_0.5?xTi_x)O₃ ceramics with a view to their potential for microwave devices. The Nd(Mg_0.5Sn_0.5?xTi_x)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the Nd(Mg_0.5Sn_0.4Ti_0.1)O₃ ceramics revealed no significant variation of phase with sintering temperatures. A dielectric constant (?_r) of 21.1, a quality factor (Q × f) of 50,000 GHz, and a temperature coefficient of resonant frequency (τ_f) of ?60 ppm/°C were obtained for Nd(Mg_0.5Sn_0.4Ti_0.1)O₃ ceramics that were sintered at 1550 °C for 4 h.     

Anti-reduction of Ti4+ in Ba4.2Sm9.2Ti18O54 ceramics by doping with MgO,Al2O3 and MnO2

The anti-reduction of Ti⁴⁺ ions in Ba_4.2Sm_9.2Ti₁₈O₅₄ (BST) ceramics at high sintering temperature over 1300 °C was investigated. MgO, Al₂O₃ and MnO₂ were added separately to suppress the reduction of Ti⁴⁺ ions so as to improve the microwave dielectric properties of BST ceramics. The microstructure of BST ceramics was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) was used to study the electroconductivity of BST ceramics and valency changes of Ti ions. The results showed that MgO or Al₂O₃, when acting as an acceptor, could effectively suppress the reduction of Ti⁴⁺ ions and significantly improve the Q × f values of BST ceramics at the cost of dielectric constant. Meanwhile, MnO₂ as an oxidant had also improved the Q × f values but with no decrease in dielectric constant. Excellent microwave dielectric properties were achieved in Ba_4.2Sm_9.2Ti₁₈O₅₄ ceramics doped with 0.2 wt.% Al₂O₃ sintered at 1340 °C for 3 h: ?_r = 76.9, Q × f = 10,120 GHz and τ_f  = ?22.7 ppm/°C.     

Microwave dielectric properties of BaxLa4Ti3 + xO12 + 3x (x = 0.0–1.0) ceramics

In the BaO–La₂O₃–TiO₂ system, the Ba_nLa₄Ti_3 + nO_12 + 3n homologous compounds exist on the tie line BaTiO₃–La₄Ti₃O₁₂ besides tungstenbronze-type like Ba_6 − 3xR_8 + 2xTi₁₈O₅₄ (R = rare earth) solid solutions. There are four kinds of compounds in the homologous series: n = 0, La₄Ti₃O₁₂; n = 1, BaLa₄Ti₄O₁₅; n = 2, Ba₂La₄Ti₅O₁₈; n = 4, Ba₄La₄Ti₇O₂₄. These compounds have the layered hexagonal perovskite-like structure, which has a common sub-structure in the crystal structure. These compounds have been investigated in our previous studies. In this study, we have investigated the phase relation and the microwave dielectric properties of Ba_xLa₄Ti_3 + xO_12 + 3x ceramics in the range of x between 0.2 and 1.0. With the increase in x, the dielectric constant ɛ_r locates around 45, the quality factor Q × f shows over 80,000 GHz at x = 0.2 and the minimum value of 30,000 GHz at x = 0.9, and the temperature coefficients of resonant frequency τ_f is improved from −17 to −12 ppm/°C. At x = 0.2, the ceramic composition obtained has dielectric constant ɛ_r = 42, the temperature coefficient of the resonant frequency τ_f  = −17 ppm/°C and a high Q × f of 86,000 GHz.     

Preparation,characterization, and giant dielectric permittivity of (Y3+ and Nb5+) co–doped TiO2 ceramics

The microstructure and giant dielectric properties of Y³⁺ and Nb⁵⁺ co–doped TiO₂ ceramics prepared via a chemical combustion method are investigated. A main rutile–TiO₂ phase and dense ceramic microstructure are obtained in (Y_0.5Nb_0.5)_xTi_1-xO₂ (x = 0.025 and 0.05) ceramics. Nb dopant ions are homogeneously dispersed in the microstructure, while a second phase of Y₂O₃ particles is detected. The existence of Y³⁺, Nb⁵⁺, Ti⁴⁺ and Ti³⁺ as well as oxygen vacancies is confirmed by X–ray photoelectron spectroscopy and X–ray absorption near edge structure analysis. The sintered ceramics exhibit very high dielectric permittivity values of 10⁴–10⁵ in the frequency range of 40–10⁶ Hz. A low loss tangent value of ≈0.08 is obtained at 40 Hz. (Y_0.5Nb_0.5)_xTi_1-xO₂ ceramics can exhibit non–Ohmic behavior. Using impedance spectroscopy analysis, the giant dielectric properties of (Y_0.5Nb_0.5)_xTi_1-xO₂ ceramics are confirmed to be primarily caused by interfacial polarization.     

Electrophoretic deposition of surface-modified titanate nanosheets via layer-by-layer assembly and deposited film properties

The surface of H₂Ti₄O₉·xH₂O titanate nanosheets was modified using the sulfonated tetrafluoroethylene-based polymer Nafion^®, via layer-by-layer assembly. The surface modification allowed the titanate nanosheets to be highly dispersed in hydrophobic organic solvents. Thick films of surface-modified nanosheets were prepared on indium tin oxide (ITO)-coated glass substrates as a negative electrode by electrophoretic deposition. The thickness of the films increased with increasing deposition time and grew to more than 8 μm in 600 s under potentiostatic conditions at 7.5 V. The electrophoretically deposited thick films showed significant hydrophobicity with contact angle for water 95°, and enhanced adsorption and higher photocatalytic activity for hydrophobic dyes such as thionine than those of thick films prepared from unmodified titanate nanosheets.     

Defects characterization of Dy-doped BaTiO3 ceramics via electron paramagnetic resonance

The point defects and the structural and dielectric properties of Dy-doped BaTiO₃ ceramics prepared at 1400 °C were investigated. The solubility of Dy in the self-compensation mode was determined to be x = 0.07 for (Ba_1−xDy_x)(Ti_1−xDy_x)O₃, and no EPR signals associated with the Dy³⁺ Kramers ion or the Ba and Ti vacancies were detected using the electron paramagnetic resonance (EPR) technique. As x increases, the dielectric behavior changed from a first-order phase transition to a diffuse phase transition to a Y7R dielectric-temperature stability. A strong EPR signal at g = 1.974, which is rare among rare-earth-doped BaTiO₃ ceramics appeared unexpectedly in the single-phase (Ba_1−xDy_x)Ti_1−x/4O₃ ceramics with deliberately designed Ti vacancies. This signal was attributed to ionized Ba vacancy defects. A preference for the self-compensation mode of Dy³⁺ ions is responsible for the appearance of Ba vacancies. The real formula of the nominal (Ba_1−xDy_x)Ti_1−x/4O₃ is expressed as (Ba_1−xDy_3x/4)(Ti_1−x/4Dy_x/4)O₃. In addition, the defect chemistry is discussed.     

The structure and properties of 0.95MgTiO3–0.05CaTiO3 ceramics co-doped with ZnO–ZrO2

The (Mg_0.93Ca_0.05Zn_0.02)(Ti_1?xZr_x)O₃ ceramics were prepared by conventional solid-state route. The dielectric properties and structure of (Mg_0.93Ca_0.05Zn_0.02)(Ti_1?xZr_x)O₃ ceramics were investigated. It has been found that MgTiO₃ and CaTiO₃ are the main phases and a second phase CaZrTi₂O₇ appeared in 95MCT ceramics co-doped with Zn–Zr. With Zn–Zr additive, the sintering temperature of 95MCT ceramics can be reduced to 1300 °C, and adjust the temperature coefficient of dielectric constant. With the increasing of Zr content, dielectric constant ?_r decrease from 22.6 to 19.91 and the temperature coefficient of dielectric constant α_c from 5.93 to 2.52 ppm/°C when x = 0.01, 0.02, 0.03 and 0.04 mol respectively. The 95MCT ceramics with x = 0.02 has a dielectric constant ?_r of 22.02, a dielectric loss of 2.78 × 10^?4 and a temperature coefficient of dielectric constant α_c value of 2.98 ppm/°C.     

A novel strategy to enhance dielectric performance and non-Ohmic properties in Ca2Cu2−xMgxTi4O12

A novel strategy to improve the dielectric and non-Ohmic properties of CaCu₃Ti₄O₁₂ ceramics that deliberately created a binary-phase system of CaCu_3−xMg_xTi₄O₁₂/CaTiO₃ was proposed and can be performed with a starting nominal formula of Ca₂Cu_2−xMg_xTi₄O₁₂. Mg²⁺ doping ions were preferentially incorporated only into the CaCu₃Ti₄O₁₂ phase. Substitution of Mg²⁺ into CaCu₃Ti₄O₁₂/CaTiO₃ can cause a significant increase in dielectric permittivity and a large reduction of the loss tangent to <0.015 at 1 kHz; while, retaining excellent temperature dielectric-stability. Sintering time had a slight influence on the dielectric properties, but remarkable effects upon the nonlinear electrical properties of CaCu_3−xMg_xTi₄O₁₂/CaTiO₃ ceramics. Degradation of nonlinear properties with increased sintering time is suggested to be the result of the dominant effect of oxygen vacancies. Impedance spectroscopy analysis demonstrated that improved dielectric and nonlinear properties could be attributed to the enhanced electrical responses of CaCu₃Ti₄O₁₂–CaTiO₃ and CaCu₃Ti₄O₁₂–CaCu₃Ti₄O₁₂ interfaces resulting from Mg²⁺ doping ions.     

Influence of donor and acceptor substitutions on the extrinsic behaviour of PZT piezoceramics

A study of the substitution influence of Nb⁵⁺ and Fe³⁺ impurities on piezoelectric behaviour has been carried out. Different substitution concentrations as well as undoped materials have been performed for ceramics Pb(Zr_1 − xTi_x)O₃ with x = 47 and 40. The different impurity concentrations, and their type, produce variations in the domain wall mobility related to the extrinsic coefficient. This dependence between extrinsic behaviour and impurity concentration is studied for different material structures: the rhombohedral and morphotropic phase boundary (MPB). Two methods are used to characterize the extrinsic and intrinsic behaviours. Elastic and piezoelectric non-linear characterization at resonance has been made in isothermal conditions by impedance and vibration velocity measurements. In order to obtain the intrinsic coefficients, linear characterization at very low temperature has been carried out.     

Effects of mechanical milling on preparation and properties of CuAl1−xFexO2 thermoelectric ceramics

CuAl_1?xFe_xO₂ (x = 0, 0.1, and 0.2) thermoelectric ceramics produced by a reaction-sintering process were investigated. Pure CuAlO₂ and CuAl_0.9Fe_0.1O₂ were obtained. Minor CuAl₂O₄ phase formed in CuAl_0.8Fe_0.2O₂. Addition of 10 mol% Fe lowered the sintering temperature obviously and enhanced the grain growth. At x = 0.1, electrical conductivity = 3.143 Ω^?1 cm^?1, Seebeck coefficient = 418 μV K^?1, and power factor = 5.49 × 10^?5 W m^?1 K^?2 at 600 °C were obtained. The reaction-sintering process is simple and effective in preparing CuAlO₂ and CuAl_0.9Fe_0.1O₂ thermoelectric ceramics for applications at high temperatures.