Sol-gel derived CaCu3Ti4O12 ceramics: Synthesis, characterization and electrical properties

The giant dielectric constant material CaCu₃Ti₄O₁₂ (CCTO) has been synthesized by sol-gel method, for the first time, using nitrate and alkoxide precursor. The electrical properties of CCTO ceramics, showing an enormously large dielectric constant ? ∼ 60,000 (100 Hz at RT), were investigated in the temperature range from 298 to 358 K at 0, 5, 10, 20, and 40 V dc. The phases, microstructures, and impedance properties of final samples were characterized by X-ray diffraction, scanning electron microscopy, and precision impedance analyzer. The dielectric permittivity of CCTO synthesized by sol-gel method is at least three times of magnitude larger than that synthesized by other low-temperature method and solid-state reaction method. Furthermore, the results support the internal barrier layer capacitor (IBLC) model of Schottky barriers at grain boundaries between semiconducting grains.     

Elastic modulus and hardness of CaTiO3, CaCu3Ti4O12 and CaTiO3/CaCu3Ti4O12 mixture

Three ceramic systems, CaTiO₃ (CTO), CaCu₃Ti₄O₁₂ (CCTO) and intermediate nonstoichiometric CaTiO₃/CaCu₃Ti₄O₁₂ mixtures (CTO.CCTO), were investigated and characterized. The ceramics were sintered at 1100 °C for 180 min. The surface morphology and structures were investigated by XRD and SEM. Elastic modulus and hardness of the surfaces were studied by instrumented indentation. It was observed that CCTO presented the higher mechanical properties (E = 256 GPa, hardness = 10.6 GPa), while CTO/CCTO mixture showed intermediate properties between CTO and CCTO.     

The non-ohmic and dielectric behavior evolution of CaCu3Ti4O12 after heat treatments in oxygen-rich atmosphere

CaCu₃Ti₄O₁₂ (CCTO) ceramics pellets were prepared using the solid-state reaction method, and then they were heat-treated at different temperatures in oxygen-rich atmosphere. The effect of heat treatments on the non-ohmic behaviors and dielectric properties were investigated. EDS analysis results indicate that the percent of oxygen at grain boundaries of CCTO ceramics heat-treated in oxygen-rich atmosphere increases markedly with the rise of temperature and approaches saturation state at about 850 °C. The breakdown voltage and nonlinear coefficient also exhibit an increase trend with the rise of temperature. In addition, the calculated results manifest that the height of Schottky potential barrier is closely related to the oxygen content at the grain boundaries. The permittivity and dielectric loss of samples heat-treated present a relatively intense decrease with the rise of temperature. But the permittivity has a behavior just reverse to the non-ohmic characteristics, which can be explained by the Schottky potential barrier theory.     

Microstructural evolution and dielectric properties of Cu-deficient and Cu-excess CaCu3Ti4O12 ceramics

The microstructural evolution and dielectric properties of CaCu_3−xTi₄O_12−x (3 − x = 2.8-3.05) ceramics were investigated. Normal grain growth behavior was observed at Cu/Ca ≤ 2.9, while abnormal grain growth was observed at Cu/Ca ≥ 2.95. A CuO-rich intergranular liquid phase at Cu/Ca ≥ 2.95 and angular grain morphology were the main reasons for abnormal grain growth. However, the abundant intergranular liquid at Cu/Ca = 3.05 significantly affected the relative dielectric permittivity and dielectric loss. The CuO composition is the key parameter that determines the microstructure and dielectric properties of CCTO ceramics.     

Effects of SrO–B2O3–SiO2 glass additive on the microstructure and dielectric properties of CaCu3Ti4O12

The effect of SrO–B₂O₃–SiO₂ glass additive (SBS) on the microstructure and dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics was investigated. This SBS–added CCTO ceramics were prepared by the solid state reaction. The undesirable impurity phases Ca₃SiO₅ started appearing in the XRD patterns, suggesting a possible chemical reaction between CaTiO₃ and SiO₂ (the devitrification production of SBS glass). The SBS glass additive promoted the grain growth and densification of CCTO ceramics. Cole–Cole plots of conductance suggested that the resistivity grain boundary decreased with increasing amount of SBS glass (when x = 0–2 wt%), then increased (when x = 2–3 wt%). The addition of SBS glass was desirable to increase the dielectric constants (up to 10⁴) and lowered the dielectric losses of CCTO over the frequency range of 450–40 kHz at the relatively lower sintering temperature for relatively shorter sintering time (1,050 °C, 12 h).     

Piezoelectric, ferroelectric and dielectric properties of La2O3-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

La₂O₃ (0–0.8 wt.%)-doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (abbreviated as BNBT6) lead-free piezoelectric ceramics were synthesized by conventional solid-state reaction. The influences of La₂O₃ on the microstructure, the dielectric, ferroelectric and piezoelectric properties of the composites were investigated. X-ray diffraction (XRD) patterns indicate that 0.2-0.8 wt.% of La₂O₃ has diffused into the lattice of BNBT6 ceramics. Consequently, a pure perovskite phase is formed. SEM images show that the microstructure of the ceramics is changed with the addition of a small amount of La₂O₃. The temperature dependence of the relative dielectric constant shows that Curie point decreases with the increase of La₂O₃. At room temperature, the ceramics doped with 0.6 wt.% La₂O₃ show superior performance with high piezoelectric constant (d₃₃ = 167 pC/N), high planar electromechanical coupling factor (k_p = 0.30), high mechanical quality factor (Q_m = 118), high relative dielectric constant (ε_r = 1470) and lower dissipation factor (tanδ = 0.056) at a frequency of 10 kHz.     

The study on electrical behavior of Gd2O3-WO3 complex ceramics at high temperature

Gd₂O₃-WO₃ complex ceramics are fabricated by the conventional solid-state reaction process. The electrical characteristics and dielectric properties of the samples were measured at various ambient temperatures in a low electric field (E < 150 V/mm). As the temperature increases, the dielectric constant and the loss tangent show an obvious change at about 50 °C and 330 °C. When the temperature is above 200 °C, the samples display stable nonlinear electrical properties characterized by semiconductivity, and the nonlinearity increases along with increasing temperature. XRD analysis reveals that Gd₂W₂O₉ is the main phase and Gd₂O₃ is the secondary phase. Based on the phase transition of tungsten trioxide, these electrical properties of Gd₂O₃-WO₃ complex ceramics can be simply explained.     

Ferroelectric and dielectric properties of Bi4-xNdxTi3O12 thin films prepared by sol–gel method

Bi_4-xNd_xTi₃O₁₂ (BNT-x, x = 0, 0.25, 0.50, 0.75 and 1.0) thin films were prepared on Pt/Ti/SiO₂/Si substrates by a sol–gel method. The microstructure, ferroelectric and dielectric properties of BNT-x thin films were investigated. The single-phase BNT-x thin films were obtained. With increasing Nd content, the preferred orientation changed from random to (117) and surface morphologies changed from the mixture of rod- and plate-like grains to rod-like grains. The Nd substitution improved the ferroelectric and dielectric properties of BTO films. BNT-x films showed better electrical properties at x = 0.50—1.0. BNT-0.75 film exhibited the best electrical properties with remanent polarization (2P _r) of 26.6 μC/cm², dielectric constant (ε _r) of 366 (at 1 MHz), dielectric loss (tanδ) of 0.034 (at 1 MHz), leakage current density (J) of ±3.0 × 10⁻⁶ A/cm² (at ± 5 V) and fatigue-free characteristics.     

Dielectric properties of Pb[(1?x)(Zr1/2Ti1/2)?x(Zn1/3Ta2/3)]O3 ceramics prepared by columbite and wolframite methods

Polycrystalline samples of Pb[(1 − x)(Zr_1/2Ti_1/2) − x(Zn_1/3Ta_2/3)]O ₃ , where x = 0.1–0.5 were prepared by the columbite and wolframite methods. The crystal structure, microstructure, and dielectric properties of the sintered ceramics were investigated as a function of composition via X-ray diffraction (XRD), scanning electron microscopy (SEM), and dielectric spectroscopy. The results indicated that the presence of Pb(Zn_1/3Ta_2/3)O₃ (PZnTa) in the solid solution decreased the structural stability of overall perovskite phase. A transition from tetragonal to pseudo-cubic symmetry was observed as the PZnTa content increased and a co-existence of tetragonal and pseudo-cubic phases was observed at a composition close to x = 0.1. Examination of the dielectric spectra indicated that PZT–PZnTa exhibited an extremely high relative permittivity at the MPB composition. The permittivity showed a ferroelectric to paraelectric phase transition at 330 °C with a maximum value of 19,600 at 100 Hz at the MPB composition.     

Dielectric properties and electrical response of grain boundary of Na1/2La1/2Cu3Ti4O12 ceramics

The dielectric properties and electrical response of grain boundaries of Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics were investigated as a function of temperature. High dielectric permittivity (6.1–8.7 × 10³) and low loss tangent (0.032–0.038 at 10 kHz) were observed in Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics. Through analyses using complex impedance and electric modulus, it was found that the dielectric properties of Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics are closely related to the electrical response of grain boundaries. The investigation of non-Ohmic characteristics at various temperatures suggests that the potential barrier at the grain boundaries of Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics is due to the Schottky effect. The giant low frequency dielectric response in Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics is attributed to Maxwell–Wagner polarization at the grain boundaries.     

Abnormal dielectric behaviors in Mn-doped CaCu3Ti4O12 ceramics and their response mechanism

Mn-doped CaCu₃Ti₄O₁₂ (CCTO) polycrystalline ceramics have been prepared by the conventional solid state sintering. Our results indicate that 10% Mn doping can decrease the dielectric permittivity in CaCu₃Ti₄O₁₂ by about 2 orders of magnitude (from 10⁴ to 10²). The grain and grain boundary activation energies show an obvious increase from 0.054 eV to 0.256 eV, and decrease from 0.724 eV to 0.258 eV with increasing the Mn doping concentration, respectively, which may be caused by the variation of Cu and Ti valence states in the CCTO samples evidenced by the X-ray absorption spectra. The similar grain and grain boundary activation energies result in invalidation of the internal boundary layer capacitance effect for the 10% Mn-doped CCTO sample, and thus result in the dramatic decrease of dielectric permittivity.     

Low temperature sintering and microwave dielectric properties of ZnTiNb2O8 ceramics with BaCu(B2O5) additions

The phases, microstructure and microwave dielectric properties of ZnTiNb₂O₈ ceramics with BaCu(B₂O₅) additions prepared by solid-state reaction method have been investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The pure ZnTiNb₂O₈ ceramic shows a high sintering temperature of about 1250 °C. However, it was found that the addition of BaCu(B₂O₅) lowered the sintering temperature of ZnTiNb₂O₈ ceramics from above 1250 °C to 950 °C due to the BCB liquid-phase. The results showed that the microwave dielectric properties were strongly dependent on densification, crystalline phases and grain size. Addition of 3 wt% BCB in ZnTiNb₂O₈ ceramics sintered at 950 °C afforded excellent dielectric properties of ?_r = 32.56, Q × f = 20,100 GHz (f = 5.128 GHz) and τ_f = −64.87 ppm/°C. These represent very promising candidates for LTCC dielectric materials.     

Effect of Bi2O3 and Y2O3 doping methods on electrical properties and PTCR behavior of Ba0.95Ca0.05TiO3 ceramics

Lead-free PTCR ceramics based on Bi₂O₃ and Y₂O₃ doped Ba_0.95Ca_0.05TiO₃ were fabricated by the conventional mixed oxide method, while Bi₂O₃ and Y₂O₃ were doped directly or after pre-calcining, in the molar ratio of Bi₂O₃:Y₂O₃ = 1:1. There were two synthesizing route, i.e. the materials were pre-calcined at 900 °C to obtain BiYO₃ firstly and then doped into the basic materials, and the materials were directly doped into the starting materials, both of which could obtain samples with different electrical properties and PTCR behavior. The samples were characterized by using X-ray diffraction, scanning electron microscope, dielectric constant-temperature and resistivity-temperature measurement instrument. It was revealed that the perovskite lattice, the microstructure and the PTCR behavior of Ba_0.95Ca_0.05TiO₃ varied with different doping contents and methods. A further research was conducted so as to study the electrical properties of ceramics by impedance spectroscopy.     

Dielectric properties of Y2O3 donor-doped Ba0.8Sr0.2TiO3 ceramics

Ba_0.8Sr_0.2TiO₃ ceramics doped with Y₂O₃ from 0 to 0.10 mol% exhibit normal ferroelectric phase transition, while the ceramics doped with Y₂O₃ from 0.20 to 0.30 mol% show a giant dielectric constant behavior with loss less than 0.15 at 1 kHz from −40 °C to 140 °C, which is suggested due to semiconductive grain and the Maxwell–Wagner effect by structure disordering in grain boundary. The analyses of unipolar charge for the semiconductive grain indicate three kinds of dielectric processes: thermally stimulated process of unipolar hopping, dispersion process of dielectric constant with frequency, and phase transition process accompanied with disappearance of giant dielectric constant in cubic phase. The XPS results confirm that some of the barium ions are in low energy state to form e-Ba²⁺ and to provide hopping sites for electrons. The ceramics doped with Y₂O₃ from 0.50 to 0.75 mol% recover the normal ferroelectricity. The possible mechanics are relevant to binding effect of cation vacancies on electrons.     

Crystallite structure,microstructure, dielectric,and piezoelectric properties of (Pb1.06−xBax)(Nb0.94Ti0.06)2O6 piezoelectric ceramics prepared using calcined powders with different phases

In an attempt to obtain dense lead metaniobate-based ceramics with improved dielectric and piezoelectric properties, the (Pb_1.06−xBa_x)(Nb_0.94Ti_0.06)₂O₆ (x = 0, 0.04, 0.08, 0.12) piezoelectric ceramics were prepared separately from the two kinds of calcined powders, i.e., the powders with the rhombohedral phase and orthorhombic phase. For obtaining the calcined powders with the different phases, two different calcination temperatures of 900 °C and 1250 °C were chosen. The calcined powders were characterized using X-ray diffraction, scanning electron microscope, laser particle size analyzer and differential scanning calorimetry. Effects of the phase structures of the calcined powders on crystallite structure, microstructure, dielectric and piezoelectric properties of the ceramics were studied in detail. The lattice parameters and grain size of the ceramics are related to the phase structures of the calcined powders. The doping of Ba²⁺ has an influence on the dielectric and piezoelectric properties of the ceramics. The ceramics with x = 0.08 fabricated from the calcined powders with the orthorhombic phase demonstrate the optimum dielectric and piezoelectric properties.     

Effect of grain size and Cu-rich phase on the electric properties of CaCu3Ti4O12 ceramics

The CaCu₃Ti₄O₁₂ ceramics were prepared by the traditional solid-state reaction method under different sintering conditions. The XRD patterns show that crystal structures of the samples are basically single-phase pseudo-cubic, except little second phases of CuO and Cu₂O in the samples sintered in air at 1050 and 1100 °C, respectively, for 12 h. The SEM results indicate that the pellet sintered at 1100 °C for 12 h possess larger grain size and more Cu-rich phases at the grain boundaries than the pellet sintered at 1050 °C for 12 h. It is interesting that the pellet sintered at 1050 °C under the pressure of 5 Gpa for 3 h shows smaller grain size (~1 μm) and no Cu-rich phases due to the higher pressure during the sintering process. The results show that the grain size has a reverse effect on the values of the permittivity and the values of breakdown electric field (E _b) and nonlinear coefficient. The pellet sintered at 1100 °C for 12 h exhibits a higher permittivity, but with a lower breakdown electric field (E _b) and a lower nonlinear coefficient due to larger grain size. The pellet sintered at 1050 °C under the pressure of 5 Gpa for 3 h exhibits a lower permittivity, but with a higher breakdown electric field (E _b) and a higher nonlinear coefficient due to smaller grain size. The Cu-rich phases at grain boundaries can raise the resistance of the grain boundary leading to the lower dielectric loss tangent, which has been supported by the results of impedance spectroscopy analysis.     

Effect of holding time on the dielectric properties and non-ohmic behavior of CaCu3Ti4O12 capacitor-varistors

Giant dielectric ceramics CaCu₃Ti₄O₁₂ (CCTO) with non-ohmic electrical properties were prepared by a sol–gel processing method. Crystal structure and microstructure of the ceramics have been characterized using X-ray diffraction and Scanning electron microscopy. The effects of sintering duration and cooling rate on electrical properties of the ceramics were investigated by measuring the properties of permittivity, I–V and grain-boundary barriers. Prolonging holding time led to substantial improvement in permittivity, furthermore, the quenched sample showed larger dielectric permittivity than the furnace-cooling one. The non-ohmic behavior relating current density (J) to the applied electric field (E) at different temperatures was characterized. A low-voltage and giant dielectric permittivity CCTO varistor with breakdown voltages in the range of E _b = 0.2–3 kV cm^?1, nonlinearity coefficient α = 2–6 and dielectric permittivity ε = 4,000–30,000 was obtained. A linear relationship between ln(J) and E ^1/2 indicated that a Schottky barrier should exist at the grain boundary.     

Influence of sintering temperature on piezoelectric properties of (K0.4425Na0.52Li0.0375)(Nb0.8925Sb0.07Ta0.0375)O3 lead-free piezoelectric ceramics

Microstructure characteristics, phase transition, and electrical properties of (K_0.4425Na_0.52Li_0.0375) (Nb_0.8925Sb_0.07Ta_0.0375)O₃ (KNLNST) lead-free piezoelectric ceramics prepared by normal sintering were investigated with an emphasis on the influence of sintering temperature. The microstructure and piezoelectric, ferroelectric, and dielectric properties were investigated, with a special emphasis on the influence of sintering temperature from 1,100 to 1,140 °C. Orthorhombic phases mainly exist in the ceramics sintered at 1,100–1,130 °C, whereas the tetragonal phase becomes dominant when sintering temperature is above 1,130 °C. Because of the existence of MPB-like transitional behavior, the piezoelectric coefficient (d ₃₃), electromechanical coupling coefficient (kp), and dielectric constant (ε) show peak values of 304pC/N, 0.48, and 1,909, respectively, which are obtained in the sample sintered at 1,120 °C, and its Curie temperature (T _C) is about 271 °C.     

Microwave dielectric properties of BaNb(2?x)TaxP2O11 (x?=?0, 0.5, 1, 1.5 and 2) ceramics

BaNb_(2−x)Ta_xP₂O₁₁ (x = 0, 0.5, 1, 1.5 and 2) ceramics were prepared by the conventional solid state ceramic route. The relative permittivity (ε_r) of the ceramics decreased from 25.3 for x = 0 to 12.9 for x = 2. BaTa₂P₂O₁₁ (x = 2) sintered at 1250°C showed good microwave dielectric properties with Q_u × f = 28,900 GHz and τ_f = − 29 ppm/°C. The addition of 5 wt% TiO₂ lowered the sintering temperature to 1225°C and improved τ_f to −6 ppm/°C with ε_r = 13.4 and Q_u × f = 17,200 GHz.     

Effects of Y2O3 addition on microwave dielectric characteristics of Al2O3 ceramics

Microwave dielectric characteristics of alumina ceramics with yttria addition were investigated. The sintering temperature was lowered, and the dielectric constant (ε_r) did not remarkably change by adding yttria. The microwave dielectric loss (tan δ) increased from 8.4 × 10^− 5 to 2.2 × 10^− 4, due to the presence of Al₅Y₃O₁₂ secondary phase. The grain size had significant effects on the dielectric loss, and there was an optimum grain size where the dielectric loss reached the minimum.