Dielectric properties of (Ba,Nb) doped TiO2 ceramics: Migration mechanism and roles of (Ba,Nb)

Ultrahigh relative dielectric constants with stable frequency and temperature dependence as well as relatively low loss tangent are found in barium and niobium doped TiO₂ ceramics with a silver electrode. The roles of barium and niobium on the dielectric properties of the ceramics are explained after the migration mechanism and the influence of barium and niobium have been analysed. Both silver electrodes made from silver paste and evaporated aluminium electrodes are employed in the study, their effects are also discussed. The migration mechanism proposed is electron hopping among oxygen vacancies. Dissolving barium into TiO₂ ceramics creates an additional oxygen vacancy which results in an increase in conductivity and relaxation frequency of the loss tangent. In contrast, niobium tends to reduce the concentration of oxygen vacancies and causes the opposite effect. The effects of barium and niobium on the concentration of the oxygen vacancy are supported by results of densification and microstructural investigations.     

Characterization and dielectric properties of Ba5NdZnM9O30 (M=Nb, Ta) ceramics

Ba₅NdZnM₉O₃₀ (M=Nb, Ta) ceramics were prepared by the conventional high temperature solid state reaction route. The sintered samples were characterized by X-ray diffraction and scanning electron microscopy methods. They belong to paraelectric phases of filled tetragonal tungsten bronze structure at room temperature and have high dielectric constants of 282 and 85, combined with low dielectric losses of 0.0048 and 0.0081 at 1 MHz for Nb- and Ta-based ceramics, respectively.     

Microwave dielectric properties of Ba3La2Ti2Nb2−xTaxO15 ceramics

High dielectric constant and low loss ceramics in the system Ba₃La₂Ti₂Nb_2−xTa_xO₁₅ (x = 0–2) have been prepared by conventional solid-state ceramic route. Ba₃La₂Ti₂Nb_2−xTa_xO₁₅ solid solutions adopted A₅B₄O₁₅ cation-deficient hexagonal perovskite structure for all compositions. The materials were characterized at microwave frequencies. They show a linear variation of dielectric properties with the value of x. Their dielectric constant varies from 49.8 to 45.1, quality factor Q_u × f from 22,000 to 31,040 GHz and temperature variation of resonant frequency from + 6.9 to − 13.4 ppm/°C as the value of x increases. These low loss ceramics might be used for dielectric resonator (DR) applications.     

Abnormal grain growth and liquid-phase sintering in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics

Ceramics of Sr0.6Ba0.4Nb2O6 (SBN40) were prepared by the conventional mixed oxide route. Sintering at temperatures 1260 °C led to rapid, non-uniform grain growth and a duplex microstructure. Presintering at 1250 °C followed by higher temperature sintering (1350–1450 °C) controlled grain growth. Rapid cooling from 1450 °C froze-in second phases at grain boundaries. Scanning electron microscopy and transmission electron microscopy showed that the resulting grain-boundary phases were Nb2O5-rich and BaO-deficient, having low liquid-formation temperatures. In contrast, SBN40 ceramics prepared with excess BaO and a deficiency of Nb2O5 showed no enhancement of grain growth at the highest temperature. Sintering behaviour and microstructural development provide evidence for the existence of a liquid phase which assists abnormal grain growth. The effect of presintering in controlling grain growth is discussed, and a mechanism for abnormal grain growth in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics is proposed. © 1998 Chapman & Hall     

Direct-current field dependence of dielectric properties in B2O3-SiO2 glass doped Ba0.6Sr0.4TiO3 ceramics

The Ba_0.6Sr_0.4TiO₃ ceramics doped B₂O₃-SiO₂ glass prepared by a sol-gel process and the effect of glass content on the DC field dependence of dielectric characteristics in Ba_0.6Sr_0.4TiO₃ ceramics were studied. The samples were observed and analysed by SEM and X-ray diffraction. The dielectric constant of a BST sample with 1 mol% B₂O₃-SiO₂ sintered at 1250°C was as good as that of undoped BST sintered at 1340°C. The dielectric constant samples decreases as the applied DC field increases. The influence of the DC field on the loss factor is much less than that on the dielectric constant. With increasing of the applied voltage, the T _c was increased and the peaks were surpressed and broadened. With increasing of glass content, the peaks were also surpressed and broadened. The maximum dielectric constant and the percentage change of dielectric constant under a DC field increase.     

晶粒生长的控制及对铅钙铁铌陶瓷体系微波介质性能影响的研究

研究了晶粒生长的固有机制,并针对铅钙铁铌陶瓷体系的掺杂改性设计一组实验,实验结果表明工艺因素特别是烧结温度及保温时间对晶粒形貌有显著影响,随烧结温度及保温时间的增加晶粒尺寸不断增加,但超过一定值后,不断减小的气孔率又会增加。本文提出一种晶粒生长模型,将烧结分成烧结前期、烧结中后期以及临界点后烧结期3个阶段,并证明每种材料由于组成不同和基本晶粒的不同会有最佳的烧结临界点,把握该临界点是控制材料性能的关键。     

Nonlinear electrical characteristics and dielectric properties of (Ca, Ta)-doped TiO2 ceramics

TiO₂ ceramics doped with 1.0 mol% Ca and different concentrations of Ta were obtained by sintering processing at 1450°C. The microstructures, nonlinear electrical behavior and dielectric properties of the ceramics were investigated. The samples have nonlinear coefficients of = 2.0–5.0 and ultrahigh relative dielectric constants which is up to 10⁵. Especially, the effects of Ta dopant on the nonlinear electrical characteristics and dielectric properties of the (Ca, Ta)-doped TiO₂ ceramics were studied in detail. When the concentration of Ta is 2.0 mol%, the sample exhibits the highest nonlinear coefficient and a comparatively lower dielectric constant. By analogy to a grain-boundary atomic defect model, the effects of Ta and the nonlinear electrical behavior of the TiO₂ system were explained.     

Influence of synthesis methods on the dielectric properties of (Ba0.2Pb0.8)TiO3 ceramics

The dielectric properties of (Ba_0.2Pb_0.8)TiO₃ prepared by several methods related to dispersion have been studied. The sol-gel derived specimens without dispersion have lower density, and lower dielectric constant at the Curie temperature. The sol-gel specimens derived from a well-dispersed suspension with pH 11 and 13, and 2% PMMA show higher dielectric constant at the Curie temperature, and higher density, because the grains grow homogeneously in the well-dispersed system. The sol-gel derived specimens have a higher Curie temperature than that of calcined mixed oxides and molten salt synthesis derived specimens.     

制备工艺对 CaO - MgO - Nb2 O5-TiO2微波介质陶瓷结构与介电性能的影响

采用普通固相合成法和铌铁矿前驱体法合成了CaO-MgO-Nb2O5-TiO2微波陶瓷粉体,讨论了这两种粉体制备方法对CaO-MgO-Nb2O5-TiO2微波陶瓷微观结构和介电性能的影响.两种方法相比:前驱体法合成制得的CaO-MgO-Nb2O5-TiO2陶瓷粉体具有物相纯,粉体煅烧温度较低的特点;在1300℃下烧结保温5 h获得的陶瓷材料晶粒均匀、致密,并且其微波陶瓷的介电性能得到了改善.     

Ferroelectric,dielectric and piezoelectric properties of Sr0.6(BiNa)0.2Bi2Nb2O9 ceramics

Aurivillius-type ceramic, Sr_0.6(BiNa)_0.2Bi₂Nb₂O₉ (SBNBN), was synthesized by using conventional solid-state processing. Phase structure and microstructural morphology were confirmed by X-ray diffraction analyses (XRD) and the scanning electron microscopy (SEM). Dielectric, piezoelectric and electromechanical properties of the SBNBN ceramic were investigated in detail. Curie temperature (T_c), piezoelectric coefficient (d₃₃), electromechanical coupling coefficient k_p, k_t and quality factor Q_m of the SBNBN ceramic were found to be 586.5 °C, 22 pC/N, 5.0%, 8.7% and 651, respectively. In addition, the reasons for varieties of the resistivity and dielectric properties at high temperature were also discussed.     

Microstructure and microwave dielectric properties of multi-oxide-doped (Zr,Sn)TiO4 ceramics

(Zr_0.8Sn_0.2)TiO₄ (ZST) ceramics were prepared by solid-phase method. The effects of MgO/La₂O₃/Nb₂O₅ doped on the phase composition, microstructure, sintering behavior, and microwave dielectric properties of ZST ceramics were investigated. XRD analysis showed that the major crystalline phase was ZST. Very small amounts of phases, Nb₂O₅ and Mg(Ti₂O₅), were observed when dopants were added, and Nb₂O₅ inhibited the formation of Mg(Ti₂O₅). The results showed that upon adding 7 wt% Nb₂O₅ and small amounts of MgO and La₂O₃ to the ceramics, the permittivity of the ceramics was greatly reduced compared to that of other oxide-doped (Zr_, Sn)TiO₄ ceramic materials, and the Q?×?f value was also increased. The coefficients of thermal expansion of ZST ceramics in this study were within the range reported in the literature. Optimal dielectric properties, ε_r?=?34.78, Q?×?f?=?55,190 GHz (f?=?5.8 GHz), τ_f?=?? 13.86 ppm/°C, and CTE?=?7.0 ppm/°C, were achieved for the sample with 7 wt% Nb₂O₅ sintered at 1330 °C for 2 h.

     

Studies of structural and dielectric properties of Ba5BiTi3Nb7O30 ceramics

The polycrystalline samples of Ba₅BiTi₃Nb₇O₃₀ (hereafter BBTN) belonging to ferroelectric oxide family of tungsten bronze structure were prepared by high temperature solid-state reaction method. Preliminary X-ray analysis of the samples provided the lattice parametersa=11·9331 Å,b=14·9684 Å, andc=7·0193 Å, and also formation of a single-phase orthorhombic structure at room temperature (303 K). Detailed studies of dielectric constant (ε) and loss (tanδ) as a function of frequency (500 Hz to 10 KHz) at room temperature and also as a function of temperature (liquid nitrogen to 160°C) show the dielectric anomaly and structural phase transition at 16·8°C.     

Ferroelectric, electromechanical, and dielectric properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 co-doped MnO2 and La2O3 lead-free ceramics

The modified (Na_0.5Bi_0.5)_0.94Ba_0.03La_0.02Ti_0.995Mn_0.005O₃ lead-free ceramics were prepared using a traditional solid-state reaction process, and their structures were characterized by X-ray diffraction, scanning electron microscope, and Raman spectroscopy, respectively. A double-like P–E loop was discussed at the room temperature for the co-doped ceramics by reason of the appearance of the anti-ferroelectric. The ferroelectric hysteresis loop and field-induced strain were studied at different temperatures. A large strain (S ₃₃ = 0.33 %, E = 50 kV/cm at 120 °C) was observed due to the transformation between ferroelectric and anti-ferroelectric phase at the phase transition temperature. Also, the depolarization temperature (T _d) of the co-doped ceramics was determined from the pyroelectric current density versus temperature plots, which was consistent with the result from the position of dielectric loss’s peak as a function of temperature.     

Enhanced dielectric temperature stability and energy-storage properties of (Y0.5Nb0.5)4+ co-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free relaxor ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06Ti_1?x(Y_0.5Nb_0.5)_xO₃ (abbreviated as BNTBT-100xYN) lead-free relaxor ceramics were designed and prepared using a traditional solid-state sintering technique. The influences of the introduction of (Y_0.5Nb_0.5)⁴⁺ complex ions for the dielectric properties and energy storage performances of BNTBT-100xYN ceramics were systematically studied. All samples exhibited a typical pseudo-cubic symmetry structure and obtained the dense microstructure with the uniform distribution of all elements. The ergodic relaxor behavior of all ceramics was observed and revealed a trend of increase as a function of composition. It accelerated the improvement of the temperature stability of the dielectric constant. All samples showed a single grain conduction mechanism and the activation energy decreased with the addition of composition. It is related to the generation of oxygen vacancies induced by the defect dipoles. BNTBT-6YN ceramic revealed excellent dielectric temperature stability within the temperature range from 87 to 479 °C and the loss tangent less than 0.05 between 25 °C and 474 °C. Besides, a high recoverable energy density of?~?0.91 J/cm³ with the corresponding efficiency of?~?78.5% at applied 115 kV/cm field was achieved for BNTBT-5YN ceramic. Hence, BNTBT-5YN and BNTBT-6YN ceramics will become one of the outstanding dielectric ceramics for the electronic components.