Preparation,characterization, electrical properties and giant dielectric response in (In + Nb) co-doped TiO<Subscript>2</Subscript> ceramics synthesized by a urea chemical-combustion method

Pure rutile phase of nanocrystalline (In?+?Nb) co-doped TiO₂ (INTO) ceramics were prepared by a chemical combustion method using urea as fuel. Dense ceramic microstructure can be obtained by sintering INTO nanocrystalline powders. Good dispersion of In³⁺ and Nb⁵⁺ co-doping ions in the microstructure is observed. Notably, high dielectric permittivity (≈20,674) and low loss tangent (≈0.054) at a low frequency and 30?°C are achieved in the (In_1/2Nb_1/2)_0.015Ti_0.985O₂ ceramic. Using an impedance spectroscopy, the INTO ceramics are confirmed to be electrically heterogeneous, consisting of semiconducting and insulating phases. The giant dielectric response in INTO ceramics can suitably be explained by the interfacial polarization. The low value of the loss tangent of INTO ceramics is attributed to a large value of resistivity of insulating phase.     

Effect of sintering temperature on dielectric properties of Ba0.6Sr0.4TiO3–MgO composite ceramics prepared from fine constituent powders

Composite ceramics of Ba_0.6Sr_0.4TiO₃ + 60 wt.% MgO were prepared from fine constituent powders by sintering at 1200–1280 °C. The composite specimens sintered at the relatively low temperatures showed satisfactory densification due to fine morphology of the constituent powders. The elevation of sintering temperature promoted the incorporation of Mg²⁺ into the lattice of the Ba_0.6Sr_0.4TiO₃ phase and grain growth of the two constituent phases. The dependence of the dielectric properties on sintering temperature was explained in relation to the structural evolution. Controlling the sintering temperature of the composite was found to be important to achieve the desired nonlinear dielectric properties. Sintering at 1230 °C was determined to be preferred for the composite in terms of the nonlinear dielectric properties. The specimen sintered at the temperature attained a tunability of 17.3% and a figure of merit of 127 at 10 kHz and 20 kV/cm.     

Improvement in dielectric properties of Al2O3-doped Li0.30Cr0.02Ni0.68O ceramics

Li_0.30Cr_0.02Ni_0.68O giant dielectric ceramics doped with Al₂O₃ were prepared by solid-state reaction via sol-gel process. The sintered samples were characterized using X-ray powder diffraction and scanning electron microscopy, and dielectric properties were also investigated. All doped samples showed the single phase of cubic rock-salt structure NiO. With increasing Al₂O₃ content, the crystallite size and grain size decreased, possibly due to an occurrence of the secondary phases at grain boundaries which inhibit the grain growth. The sample with 0.2 wt.% Al₂O₃ showed nearly 7 times lower tanδ (2.37) and higher ε_r (7.25 × 10⁶) measured at 1 kHz and room temperature when compared to the pure sample.     

PTCR properties of Sb2O3-doped (Ba,Sr)TiO3

Perovskite barium–strontium titanate, (Ba,Sr)TiO₃ was prepared and effects of Sb₂O₃ additives on its PTCR properties were investigated. The (Ba,Sr)TiO₃ with 0.05∼0.25 mol% Sb₂O₃ showed semiconducting PTCR behavior and anomalous grain growth was also observed when sintered at 1360 °C. It was considered that charge compensation by doping Sb₂O₃ as well as anomalous grain growth by sintering leads to resistivity reduction from insulating to semiconducting transition.     

Studies on semiconductive (Ba0.8Sr0.2) (Ti0.9Zr0.1)O3 ceramics

In order to produce semiconductive (Ba_0.8Sr_0.2) (Ti_0.9Zr0.1)O₃ ceramics (BSZT), providing low resistivity for boundary-layer capacitor applications, a controlled valency method and a controlled-atmosphere method were applied and studied. In the controlled-valency method, trivalent ions (La³⁺ Sb³⁺) and pentavalent ions (Nb⁵⁺, Sb⁵⁺, Ta⁵⁺) were doped into BSZT ceramics, while in the controlled-atmosphere method, samples were sintered in air and a reducing atmosphere. The doped BSZT ceramics sintered in the reducing atmosphere showed much lower resistivities and smaller temperature coefficient of resistivity (TCR) than those sintered in air, indicating that low partial pressure of oxygen will increase the solubility of the donor dopant and enhance the grain growth. In addition, a small negative TCR at low temperature, as well as a small positive TCR at higher temperature, are also observed for specimens fired in a reducing atmosphere. The former is attributed to the semiconductive grain and the latter to the small barrier layer formed at the grain boundary.     

Densification and anisotropic grain growth in Sr2Nb2O7

Sr : Nb stoichiometry and donor-doping with La were found to affect densification behavior and anisotropic grain growth in Sr₂Nb₂O₇ ceramics. La-doping improved the high temperature a.c. resistivity, but inhibits grain growth by grain boundary pinning. The presence of excess Nb was found to promote anisotropic grain growth by forming a liquid at the grain boundaries in both undoped and doped Sr₂Nb₂O₇. Anisotropic grain growth in La-doped Sr₂Nb₂O₇ can be controlled by incorporating large template particles in a Nb-rich matrix. High sintered densities (98% of theoretical) were achieved in both undoped and La-doped samples.     

Enhancement of the Grain Boundary Conductivity in Ceramic Li0.34La0.55TiO3 Electrolytes in a Moisture‐Free Processing Environment

The grain boundary resistivity problem of highly conductive bulk Li_0.34La_0.55TiO₃ perovskite has been investigated by means of impedance spectroscopy and solid‐state NMR of samples processed in controlled atmospheres. The samples were sintered in air, synthetic air, and oxygen, in which the level of moisture varied. A dry atmosphere is critical to obtain dense ceramics with a low grain boundary resistivity. The grain boundary conductivity is five times higher for samples sintered in oxygen atmosphere due to the suppression of Li₂CO₃ secondary phase formation, which is responsible for low lithium ion diffusion at the grain boundary.     

Tunable and microwave dielectric properties of Ba0.5Sr0.5TiO3-BaWO4 composite ceramics doped with Co2O3

Co₂O₃ doped BaWO₄-Ba_0.5Sr_0.5TiO₃ composite ceramics, prepared by solid-state route, were characterized systematically, in terms of their phase compositions, microstructure and microwave dielectric properties. Doping of Co₂O₃ promoted grain growth, reduced Curie temperature and broadened phase-transition temperature range of BaWO₄-Ba_0.5Sr_0.5TiO₃, which were attributed mainly to the substitution of Co³⁺ for Ti⁴⁺ at B site in the perovskite lattice. Dielectric diffusion behaviors of the composite ceramics were discussed. The composite ceramics all had dielectric tunability of higher than 10% at 30 kV/cm and 10 kHz, with promising microwave dielectric properties. Specifically, the sample doped with 0.2 wt.% Co₂O₃ exhibited a tunability of 20%, permittivity of 225 and Q of 292 (at 1.986 GHz), making it a suitable candidate for applications in electrically tunable microwave devices.     

Fabrication and electrical properties of textured Ba(Zr0.2Ti0.8)O3–(Ba0.7Ca0.3)TiO3 ceramics using plate-like BaTiO3 particles as templates

Plate-like BaTiO₃ (BT) particles were used as templates to fabricate grain-oriented Ba(Zr_0.2Ti_0.8)O₃–(Ba_0.7Ca_0.3)TiO₃ (BZT–BCT) ceramics. The effects of the sintering temperature and the soaking time on the microstructure and electrical properties of the textured BZT–BCT ceramics were investigated. The results show that textured ceramics were obtained with orientation factor better than 0.5. The textured BZT–BCT ceramics have rhombohedral and tetragonal structures. Terrace morphology can be observed in the grains and the mechanism of grain growth is multi-nucleation multilayer growth. The T_R–T and T_C of BZT–BCT ceramics shift to higher temperature as the soaking time increases. Textured structures improve the dielectric, piezoelectric properties, and weaken the dielectric relaxor characteristics. When BZT–BCT ceramics sintered at 1,500 °C for 20 h, the maximum piezoelectric coefficient of 350 pC/N can be obtained.     

Microwave and photoluminesent characterizations of (Ca<Subscript>2</Subscript>Mg<Subscript>3</Subscript>)(X<Subscript>1.75</Subscript>Sb<Subscript>0.25</Subscript>)TiO<Subscript>12</Subscript> [X = Nb and Ta] ceramics

(Ca₂Mg₃)(X_1.75Sb_0.25)TiO₁₂ [X = Nb and Ta] ceramics are prepared through the conventional solid-state route. The samples are calcined at 1,100 and 1,180 °C, and are sintered at 1,250 and 1,375 °C. The substitution of Sb decreases the calcination and sintering temperatures of pure (Ca₂Mg₃)(Nb/Ta)₂TiO₁₂. The structure of the samples is analyzed using X-ray diffraction method. The microstructure of the sintered pellet is studied using scanning electron microscopy. The dielectric properties such as dielectric constant (ε_r), quality factor (Q_uxf) and temperature coefficient of resonant frequency (τ_f) are measured in the microwave frequency region. By Sb substitution, thermal stability is achieved, with the increase in dielectric constant, without much change in the quality factor. The materials have intense emission lines in the wavelength region 500–700 nm. The compositions have good microwave dielectric properties and photoluminescence and hence are suitable for dielectric resonator and ceramic laser applications.     

Effects of sintering temperature on the microstructure and dielectric properties of titanium dioxide ceramics

Nanostructured (~200 nm grain size) titanium dioxide (TiO₂) ceramics were densified at temperature as low as 800 °C by pressureless sintering in a pure oxygen atmosphere. Phase transition and microstructural development of sintered samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Dielectric properties including d.c. conductivity, dielectric constant, loss tangent, and dielectric breakdown strength (BDS) were determined for samples sintered at various temperatures. The influence of sintering temperature on the microstructural development, defect chemistry, and dielectric properties of TiO₂ is discussed. Nanostructured TiO₂ ceramics with high sintering density (>98%) lead to improved dielectric properties; high BDS (~1800 kV/cm), low electrical conductivity (~5 × 10⁻¹⁵ S/cm), high dielectric constant (~130), and low loss tangent (~0.09% at 1 kHz), which is promising for application in high energy density capacitors.     

Effect of temperature on nonlinear electrical behavior and dielectric properties of (Ca, Ta)-doped TiO2 ceramics

TiO₂ ceramics doped with 0.75 mol% Ca and 2.5 mol% Ta were sintered at different temperatures ranging from 1300 to 1450°C. The effects of sintering temperature on the microstructure, nonlinear electrical behavior, and dielectric properties of the ceramics were studied. The sample sintered at 1300°C exhibits the highest nonlinear coefficient (5.5) and a comparatively lower relative dielectric constant.     

Phase formation, sintering behavior and microwave dielectric properties of bismuth and manganese co-doped [(Pb, Ca) La](Fe, Nb)O3+δ solid solution

The phase formation, sintering behavior and microwave dielectric properties of Bi₂O₃ and MnO₂ co-doped [(Pb, Ca) La](Fe, Nb)O_3+δ (PCLFN) ceramics were investigated. The Bi₂O₃ and MnO₂ binary dopants formed stable and low melting temperature solubilities at grain boundary which resulted in an effectively lowered sintering temperature by about 140 °C a more rapid sintering process and enhanced bulk densities. Sintering procedure has significant effect on grain size and porosities in ceramics. With high sintering temperature and time, the evaporation of PbO scaled up from surface toward the bulk and resulted in a Pb²⁺ deficient layer up to 0.25 mm depth under ceramic surface. Investigation of sintering dynamic revealed that either volume diffusion or second-order interface mechanism controlled the grain growth in present system. An optimal microwave dielectric properties of ε_r = 91.1, Q _f = 4,870 GHz and τ _f = 18.5 ppm/°C could be obtained in Bi₂O₃ and MnO₂ co-doped [(Pb, Ca) La](Fe, Nb)O_3+δ ceramics sintered at 1,050 °C for 4 h when the quality ratio of Bi₂O₃/MnO₂ was 1 and the doping content w = 1 wt%.     

Low temperature sintering and microwave dielectric properties of 7NiNb2O6–9TiO2 ceramics with CuO addition

The effects of CuO additive on sintering temperature and microwave dielectric properties of 7NiNb₂O₆–9TiO₂ ceramics prepared by solid-state reaction method have been investigated. The phases and microstructure have also been evaluated using X-ray diffraction and scanning electron microscopy. The pure 7NiNb₂O₆–9TiO₂ ceramics show a high sintering temperature of about 1,200?°C. However, the addition of CuO lowered the sintering temperature of 7NiNb₂O₆–9TiO₂ ceramics from 1,200 to 935?°C due to the CuO liquid-phase. The results showed that the microwave dielectric properties were strongly dependent on densification, crystalline phases and grain size. The 7NiNb₂O₆–9TiO₂ ceramics with the addition of 3.2 wt% CuO sintered at 935?°C afforded excellent dielectric properties of ε _r ?=?60.5, Q?×?f?=?10,039?GHz (at 3.8?GHz) and τ _f ?=?62?ppm/°C, which represented very promising candidates for LTCC dielectric materials.     

Dielectric characteristics of Ba(Mg1/3Ta2/3)O3 ceramics sintered at low temperatures

Dielectric characteristics of Ba(Mg_1/3Ta_2/3)O₃ ceramics (BMT ceramics) sintered at low temperatures with 2–3 wt% NaF additives were determined. A dielectric constant of 25 and extremely low dielectric loss (< 0.0001) were measured at 100 kHz and 1 MHz in BMT ceramics sintered under these conditions, and no frequency dependence of the dielectric constant was observed. This suggested that NaF as sintering additive had no harmful influence on the dielectric properties of the ceramics.     

Influence of different dopant sites by lanthanum on the dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

The effects of La³⁺ doped in calcium copper titanate (CCTO) at Ca²⁺ site and Cu²⁺ site were examined. The doped compositions, La_0.1Ca_0.85Cu₃Ti₄O₁₂ (LCCTO) ceramics and CaLa_0.1Cu_2.85Ti₄O₁₂ (CLCTO) ceramics were prepared by the solid-state method. The microstructure, dielectric properties, complex impedance and nonlinear I–V characteristics were studied. And it was found that La³⁺ doped at Ca²⁺ site achieved lower sintering temperatures than that doped at Cu²⁺ site in CCTO ceramics. The dielectric loss (tan δ) of LCCTO ceramics was about 0.05 at 40 kHz when the sample was sintered at 1080 °C. Dielectric constant (ε′) of LCCTO ceramics was about 3.2 × 10⁴ when the sample was sintered at 1100 °C, which was larger than CLCTO ceramics examined under the same process condition with sintering temperatures vary. The impedance analysis revealed that LCCTO ceramics had an influence of resistance of grain boundaries, which was stronger than that of CLCTO ceramics. Meanwhile, both LCCTO ceramics and CLCTO ceramics had a nonlinear-Ohmic property.     

Sintering and properties of highly donor-doped barium titanate ceramics

The electrical properties of donor (La³⁺) doped BaTiO₃ samples with a donor concentration in the range from 0.3 to 1.5 mol.% of La were studied. Samples were sintered at a low partial pressure of oxygen in order to facilitate anomalous grain growth and donor incorporation. In order to optimise the PTCR anomaly, the samples were annealed in air at 1100°C. Results show that with the use of a specific sintering profile PTCR ceramics containing an amount of donor dopant >0.3 mol.%, can be prepared. Heavily doped samples which do not exhibit anomalous grain growth show a core shell structure.     

Structural and Electrical Properties of Bismuth Ferrite Ceramics Sintered in Different Atmospheres

Bismuth ferrite (BiFeO₃) ceramics were synthesized by the solid-state reaction method followed by rapid liquid phase sintering. The effect of sintering atmosphere (N₂, air and O₂) on the structure and electrical properties of BiFeO₃ multiferroic ceramics were investigated. XRD analysis revealed that N₂ sintering was effective in reducing impurity phases and improving the crystallization behavior. XPS analysis showed that fewer Fe²⁺ ions but more oxygen vacancies were involved in the N₂ sintered ceramics. The SEM investigations suggested that the grain size of the BiFeO₃ ceramics sintered in nitrogen are larger than those sintered in air and O₂. Electrical measurements revealed that the ceramics sintered in N₂ showed lower leakage current, superior dielectric and ferroelectric properties.     

Influence of zirconium addition on final properties of K0.5Na0.5NbO3-based ceramics

Microstructure, electrical and dielectric properties of K_1/2Na_1/2NbO₃ (KNN) modified with ZrO₂ were investigated. Powders were obtained by a conventional solid-state method. Samples doped with 0–2 mol% of ZrO₂ were sintered at 1,125 °C for 2 h. Through XRD spectra, the perovskite structure was observed, in addition to small peaks corresponding to secondary phases. It was also determined that zirconium drastically changed the microstructure and grain size of KNN ceramics. The addition of upto 1.0 mol% of Zr⁴⁺ produced a softening effect in the ferroelectric properties of the material, and increased its density. Conversely, samples prepared with contents higher than 1.0 mol% reduced piezoelectric and dielectric properties.     

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.