The abnormal grain growth and dielectric properties of (Nb,Ba) doped TiO2 ceramics

The effect of consolidation pressure and crystallite size of powders crystal phases of TiO₂ on sintered microstructure of TiO₂ ceramics doped with 0.25 mol % Nb and 1.0 mol % Ba were investigated. Also, the development sequence of abnormal grain growth of (niobium, barium) doped TiO₂ ceramics was proposed. The second phases of as-sintered surface were determined. The dielectric properties of Ag-electroded samples were correlated with the resistivity of the bulk (Nb, Ba) doped TiO₂ ceramics. Abnormal grain growth lowered the resistivity of bulk material of (Nb, Ba) doped TiO₂ ceramics, and moved the relaxation frequency of fan δ to high frequency region over 10⁵ Hz. Controlling the sintered microstructures can obtain reasonably good dielectric properties.     

Investigations of the electrical property, diffuse reflectance and ESR spectra of the La-(Fe,Mn)-codoped PTCR BaTiO3 annealed in reducing atmosphere

Electrical properties of La-(Fe,Mn)-codoped positive temperature coefficient of resistivity (PTCR) BaTiO₃ ceramics were studied by combining their diffuse reflectance measurements and electron spin resonance (ESR) spectroscopy. La-(Fe,Mn)-codoped samples showed high durability to reducing atmosphere. It is assumed that Fe and Mn ions segregated in the grain boundary contribute to the density of surface acceptor states, meanwhile localizing electrons in a form of Ti³⁺ and stabilizing the chemisorbed oxygens through La³⁺-Mn^3+,4+ or La³⁺-Fe³⁺ pairs. In addition, ESR signals of Fe³⁺ in annealed samples was intensified above Curie temperature (T_c), indicating that Fe ions still maintained its high valence states (Fe³⁺) in the grain boundary even after annealing in reducing atmosphere.     

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.     

The effect of Sb and Nb on the electrical conductivity of tin dioxide based ceramics

The electrical conductivity of Mn doped SnO₂ systems prepared by an organic route (Pechini’s method) has been investigated as a function of antimony and niobium concentration. The conductivity increases with the increase of both concentration ions, however, in a different manner. While the conductivity of niobium doped ceramics increases with the power of 1.6 for the entire range of concentrations studied (0.01–0.7 mol%), the conductivity of antimony doped ceramics increases with the power of 1.9 in the range 0.01–0.05 mol% of Sb; 3.7 in the range 0.05–0.30 mol% and 1.8 in the range 0.30–0.70 mol%. This behavior is attributed to the existence of two stable oxidation states for antimony: Sb³⁺ and Sb⁵⁺, while for niobium there is only one: Nb⁵⁺. The power of 3.7 for Sb would be related to the segregation of this ion on the grain boundary accompanied by an additional contribution coming from the substitution of Sn²⁺ by Sb³⁺ on the grain surface.     

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 condition and PTCR characteristics of porous (Ba,Sr)(Ti,Sb)O3

We fabricated porous (Ba,Sr)(Ti,Sb)O₃ ceramics by adding potato-starch (1–20 wt %) and investigated the effects of sintering temperature (1300–1450 °C) and time (0.5–10 h) on the positive temperature coefficient of resistivity characteristics of the porous ceramics. The room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics decreased with increasing sintering temperature, while that of the ceramics increased with increasing sintering time. For example, the room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics for the samples sintered at 1300 °C and 1450 °C for 1 h is 6.8×10³ and 5.7×10² cm, respectively, while that of the ceramics is 6.5×10² and 1.3×10⁷ cm, respectively, for the samples sintered at 1350 °C for 0.5 h and 10 h. In order to investigate the reason for the decrease and increase of room-temperature electrical resistivity of the samples with increasing sintering temperature and time, the average grain size, porosity, donor concentration of grains (N _d), and electrical barrier height of grain boundaries () of the samples are discussed.     

Effects of reducing and oxidizing atmospheres on the PTCR characteristics of porous n-BaTiO3 ceramics by adding polyethylene glycol

Donor doped BaTiO₃ (n-BaTiO₃) ceramics were fabricated by adding polyethylene glycol (PEG) at 20 wt %. The effects of reducing and oxidizing atmospheres on the PTCR characteristics of the porous n-BaTiO₃ ceramics were investigated. The PTCR characteristics of the porous n-BaTiO₃ ceramics is strongly affected by chemisorbed oxygen at the grain boundaries and are recovered as the atmosphere is changed from the reducing gas to oxidizing gas. The low room-temperature resistivity of the porous n-BaTiO₃ ceramics in reducing atmospheres may be caused by the decrease in potential barrier height, which originates from an increase in the number of electrons owing to the desorption of chemisorbed oxygen atoms at the grain boundaries. In addition, the high room-temperature resistivity of the porous n-BaTiO₃ ceramics in oxidizing atmospheres may be caused by the increase in potential barrier height, which results from the adsorption of chemisorbed oxygen atoms at the grain boundaries.     

Effect of atmosphere on the PTCR characteristics of porous Ba(Ti,Sb)O3 ceramics produced by adding corn-starch

Porous Ba(Ti,Sb)O₃ ceramics were fabricated by adding corn-starch at 20 wt %. The effect of atmosphere on the PTCR characteristics of the porous Ba(Ti,Sb)O₃ ceramics and the role of oxygen on the grain boundaries in the PTCR characteristics of the Ba(Ti,Sb)O₃ ceramics were investigated. In air, O₂, N₂, and H₂ atmospheres, the electrical resistivity of Ba(Ti,Sb)O₃ ceramics below 150 °C was independent of atmosphere, while it was strongly dependent on atmosphere above 200 °C. The low electrical resistivity in reducing atmospheres was due to a decrease in potential barrier height, which originated from an increase in the number of electrons owing to the desorption of chemisorbed oxygen atoms at the grain boundaries. In a N₂ atmosphere, the electrical resistivity of Ba(Ti,Sb)O₃ ceramics during the cooling cycle was lower than that during the heating cycle, and then the electrical resistivity of the porous Ba(Ti,Sb)O₃ ceramics during subsequent heating and cooling cycles was increased again by exposure to an O₂ atmosphere.     

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.     

Enhanced dielectric and electrical properties in CaZrO3-doped X8R BaTiO3-based ceramics sintered at medium temperature

Lead-free X8R BaTiO₃-based ceramics were prepared by conventional solid-state method. The influence of the composition and procedures on the microstructures, lattice parameters and dielectric properties of ceramics materials were systemically studied. XRD results indicate that no secondary phase is formed in the CaZrO₃-doped samples after sintering. The content of Ca²⁺ ions and Zr⁴⁺ ions decrease from grain boundaries to grain interiors referring to EDS results, which indicate that the ions mainly locate in the grain boundaries. The Curie temperatures of all CaZrO₃ doped samples increase due to the increasing tetragonality caused by the addition of CaZrO₃. SEM micrographs reveal that the grain size of CaZrO₃-doped BaTiO₃-based samples are not uniform, ranging from 0.2 to 0.8 µm. Furthermore, samples doped with 1.0 wt % CaZrO₃ exhibit optimal dielectric properties with high temperature stability of capacitance, due to the fine-grain microstructures. Moreover, we obtained a lead-free high performance X8R dielectric ceramic composition sintered at medium temperature (1,135 °C).     

Effects of Nb5+ doping on sintering and electrical properties of lead-free (Bi0.5Na0.5)TiO3 ceramics

The Nb⁵⁺ doped (Bi_0.5Na_0.5)TiO₃ (BNT) ceramics were manufactured by a conventional solid state reaction method. The influence of Nb⁵⁺ doping on the sintering, microstructure and various electrical properties of BNT ceramics was investigated. The results of X-ray diffraction show that the solubility limit of Nb⁵⁺ in the BNT lattice is less than 3%. Additionally, Nb⁵⁺ doping produces significant effects on the densification and grain growth of BNT ceramics. Various electrical properties of BNT ceramics are obviously changed with doping a small amount of Nb⁵⁺. The ferroelectric and piezoelectric properties display enhanced values at a low doping level. The formation of A-site vacancies is considered as the reason for the changed ferroelectric and electromechanical behavior.     

High dielectric tunability of ferroelectric (Ba1−x,Srx)(Zr0.1,Ti0.9)O3 ceramics

(Ba_1?x,Sr_x)(Zr_0.1,Ti_0.9)O₃ (BSZT) ceramics with x = 0, 0.05, 0.15, 0.25, 0.35 and 0.45 were prepared by conventional solid state reaction method. The structural characterization with X-ray diffraction and scanning electron microscopy indicate a monotonical drop in lattice constants and grain size with the increase of Sr concentration. Consequently, the Curie temperature and remnant polarization of the ceramics exhibit a strong compositional dependence. A linear relationship between the Curie temperature and Sr concentration is revealed. At x = 0.45, the BSZT ceramics show substantially high tunability of over 55 % under 20 kV/cm dc electric field with very low dielectric loss value of 0.0025 at room temperature, suggesting the BSZT ceramics could be a promising alternative to traditional (Ba,Sr) TiO₃ ferroelectrics for developing high frequency tunable dielectric devices.     

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.     

Dielectric,ferroelectric, and piezoelectric properties of Sb2O3-modified (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 lead-free ceramics

The microstructures, phase structure, and electrical properties have been investigated for the Sb₂O₃-modified (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ lead-free piezoelectric ceramics. The grain size was strongly affected by Sb₂O₃, and the solid solutions show a single phase perovskite structure. The ceramics with x = 0.1 % exhibit enhanced electrical properties of d ₃₃ ～556 pC/N, k _p ～52 %, ε _r ～3,895, tan δ ～1.3 %, and P _r ～12.6 μC/cm² when sintered at a low temperature of ～1,350 °C. The high temperature stability of ferroelectric properties (P _r ～14.8 μC/cm²) was obtained in the temperature range from ?60 to 0 °C. And with the addition of small amount of Sb₂O₃, Curie temperature (T _c ) maintains virtually unaltered.     

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.     

Dielectric properties of Sb2O3-doped Ba0.672Sr0.32Y0.008TiO3

Sb₂O₃-doped Ba_0.672Sr_0.32Y_0.008TiO₃ (BSYT) dielectric ceramics were prepared by conventional solid state method, and their dielectric properties were investigated with variation of Sb₂O₃ doping content and sintering temperature. The X-ray diffraction patterns indicated that all the BSYT specimens possessed the perovskite polycrystalline structure. The experimental results reveal that the introduction of Sb₂O₃ into Ba_0.672Sr_0.32Y_0.008TiO₃ can control the grain growth, reduce the relative dielectric constant and dielectric loss, shift the Curie temperature to lower temperature and significantly improve the thermal stability of the BSYT ceramics. The samples doped with 1.6 wt.% Sb₂O₃ sintered at 1320 °C for 2 h exhibited attractive properties, including high relative dielectric constant (> 1500), low dielectric loss (< 40 × 10^− 4), low temperature coefficient of capacitor(< ± 35%) over a wide temperature range from − 25 °C to + 85 °C.     

Effects of A/B cation ratio on the microstructure and lifetime of (Ba1-xCax)z(Ti0.99-yZryMn0.01)O3 (BCTZM) sintered in reducing atmosphere

Microstructure and dielectric properties of (Ba_1-xCa_x)_z(Ti_0.99-yZr_yMn_0.01)O₃ (BCTZM) with various cation ratios (A/B) sintered in reducing atmosphere and then annealed to reoxidize the ceramic bodies were investigated. With decreasing A/B cation ratio, concurrent with the grain size reduction, the insulation resistance and lifetime of the BCTZM are both increased. The degradation of insulation resistance is closely related to oxygen partial pressure used during annealing and the resulting microstructure of the ceramics. Within the same volume, the specimen with fine grain size provides more grain boundaries, in comparison to large grain size specimens, which helps in providing an efficient diffusion pathway for oxygen during annealing. The result of oxidation of the Mn, that is, a change in the valence state of Mn from Mn⁺² to Mn⁺³ is confirmed from thermogravimetric analysis and electron paramagnetic resonance analysis.     

Influence of the sintering process on the PTCR effect of chip-type Ba1.022–x Sm x TiO3 ceramics prepared by the reduction sintering-reoxidation method

We investigated the effects of sintering temperature and reoxidation annealing on the positive temperature coefficient of resistance (PTCR) effect of Ba_1.022–x Sm _x TiO₃ ceramics that were sintered at 1,180–1,260 °C for 30 min in a reducing atmosphere and reoxidized at 800 °C for 1 h. Results indicated that the room-temperature (RT) resistivity and resistance jump of the ceramics decreased with increasing sintering temperature; moreover, the samples exhibited a remarkable PTCR effect with a resistance jump of 3.3 orders of magnitude and achieved a low RT resistivity of 374.4 Ω cm at a lower sintering temperature. Furthermore, the higher grain-boundary resistivity of the ceramics obtained at a high reoxidation temperature after sintering at low temperature was estimated using an impedance analyzer. In addition, the voltage versus current behavior was investigated in present study.     

Fabrication of high Tc lead free (1???x)BaTiO3�CxBi0.5K0.5TiO3 positive temperature coefficient of resistivity ceramics using reoxidation method

(1 ?C x)BaTiO₃?xBi_0.5K_0.5TiO₃ (abbreviated as BT?CBKT, where x = 0, 0.1 and 0.2) ceramics were prepared by solid state reaction method. All ceramic samples were sintered in a pure N₂ flow atmosphere, subsequently reoxidized at a temperature range of 800?C1,100 °C in air for several hours. The influences of BKT content and reoxidation on the positive temperature coefficient of resistivity (PTCR) behavior of ceramic samples were investigated. BT?CBKT ceramic samples sintered in N₂ possessed relatively low room temperature resistivity (??_RT) and showed weak PTC effect. Through an appropriate reoxidation, the ceramic samples re-obtained PTC effect of almost three orders of magnitude. With the addition of BKT, the Curie temperature (Tc) was enhanced by ~50 °C than the pure BT ceramics.     

Preparation and dielectric properties of microwave ceramics based on doped zinc titanates

We have studied the dielectric properties of microwave ceramic materials in the binary system Zn₂TiO₄-TiO₂ and examined the effect of doping with Sn²⁺, Sb³⁺, Bi³⁺, and Mg²⁺ on the dielectric properties and microstructure of the ceramics. The doped Zn-Ti-O ceramics are thermally stable and have ? = 18–33 and tan δ from 0.0001 to 0.007. Our results demonstrate that the doped Zn-Ti-O ceramics are potentially attractive for microwave applications, in particular for use as ceramic components (resonator substrates) of miniature antennas.