Dielectric Properties of Ba(Zr,Ti)O3-Based Ferroelectrics for Capacitor Applications

The purpose of the present investigation is to describe the influence of the processing parameters on the dielectric behavior of manganese-doped Ba(Ti_{1− x} Zr _x )O₃ceramics, particularly variations in the small-signal aging rate, temperature characteristic, and hysteresis. In this paper, the aging behavior of base-metal electrode materials above the temperature of the permittivity maximum ( T _M), including the influence of the zirconium content and annealing conditions, is described for the first time. The aging rate at temperatures greater than T _M decreases as the oxygen partial pressure increases during annealing and the zirconium content increases, whereas the aging rate exhibits a maximum at temperatures much less than T _M. The behavior is explained in terms of a diffuse phase transition. Hysteresis-loop deformation is observed during aging.     

Weak-Beam Dark-Field Microscopy of Complex Stress States in X7R-Type BaTiO3 Dielectric Core–Shell Structures

X7R-type BaTiO₃ materials were analyzed using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Powder XRD indicated that the materials had pseudocubic lattices, but core–shell grain structures predominated in bright-field (BF) TEM images. Electron diffraction patterns across the core–shell boundaries and convergent beam electron diffraction patterns of cores and shells indicated that coherent grain boundaries existed between cores and shells. The flat dielectric constant–temperature curves obtained from these materials can be interpreted in terms of the internal stress states in individual grains. The stress states were observed using weak-beam dark-field (WBDF) microscopy, and strain contours formed by distorted crystal planes were visible in the WBDF images. The contours observed were dependent on the stress state of the crystal instead of crystal symmetry and the stress distribution in individual grains was determined by both the thickness ratio of shell and core, and the geometrical relationship of the core and the shell. Twins observed in this material were determined to be growth rather than mechanical twins, through observation of the strain contour distribution.     

Influence of Stress and Chemical Homogeneity on Dielectric Properties of BaTi0.9Zr0.1O3

The influence of mechanical stress and chemical homogeneity on the permittivity of BaTi_0.9Zr_0.1O₃ ceramics prepared from mixed-oxide and hydrothermal powders was studied. To reduce stress, liquid-phase sintering was applied in conjunction with a low heating rate to stimulate the formation of large grains. The influence of chemical homogeneity was studied by variations in sintering temperatures and times. For both types of ceramics, the dielectric constant at the Curie temperature was influenced by both factors, but to a different extent. In the mixed oxide ceramic, chemical homogeneity played a more prominent role, while internal stress appeared to exert a larger influence in the hydrothermal ceramics. The dielectric constant at the Curie temperature could be increased by 5%–10% by an annealing treatment at 200°C, followed by slow cooling.     

Electrical Properties of Cerium-Doped BaTiO3

The high-temperature equilibrium electrical conductivity of Ce-doped BaTiO₃ was studied in terms of oxygen partial pressure, P (O₂), and composition. In (Ba_1−xCe _x )TiO₃, the conductivity follows the −1/4 power dependence of P (O₂) at high oxygen activities, which supports the view that metal vacancies are created for the compensation of Ce donors, and is nearly independent of P (O₂) where electron compensation prevails at low P (O₂). When Ce is substituted for the normal Ti sites, there is no significant change in conductivity behavior compared with undoped BaTiO₃, indicating the substitution of Ce as Ce⁴⁺ for Ti⁴⁺ in Ba(Ti_1−yCe _y )O₃. The Curie temperature ( T _c) was systematically lowered when Ce³⁺ was incorporated into Ba²⁺ sites, whereas the substitution of Ce⁴⁺ for Ti⁴⁺ sites resulted in no change in this parameter.     

Microwave Dielectric Properties of Low-Fired Ba5Nb4O15

The effect of B₂O₃ on the sintering temperature and microwave dielectric properties of Ba₅Nb₄O₁₅ has been investigated using X-ray powder diffraction, scanning electron microscopy, and a network analyzer. Interactions between Ba₅Nb₄O₁₅ and B₂O₃ led to formation of second phases, BaNb₂O₆ and BaB₂O₄. The addition of B₂O₃ to Ba₅Nb₄O₁₅ resulted in lowering the sintering temperature from 1400° to 925°C. Low-fired Ba₅Nb₄O₁₅ could be interpreted by measuring changes in the quality factor ( Q × f ), the relative dielectric constant (ɛ_r), and the temperature coefficient of resonant frequency (τ_f) as a function of B₂O₃ additions. More importantly, the formation of BaNb₂O₆ provided temperature compensation. The microwave dielectric properties of low-fired Ba₅Nb₄O₁₅ had good dielectric properties: Q × f = 18700 GHz, ɛ_r= 39, and τ_f= 0 ppm/°C.     

Solid-State Preparation of BaTiO3-Based Dielectrics, Using Ultrafine Raw Materials

BaTiO₃ and Ba(Ti,Zr)O₃ dielectric powders have been prepared from submicrometer BaCO₃, TiO₂, and ZrO₂. By use of submicrometer BaCO₃ the intermediate formation of Ba₂TiO₄ second phase can be widely suppressed. Monophase perovskites of BaTiO₃ were already formed at 900°C and Ba(Ti,Zr)O₃ at 1050°C. Aggregates of very small subgrains could be easily disintegrated to particle sizes <0.5 μm.     

Insulating Properties of Lanthanum-Doped BaTiO3 Ceramics Prepared by Low-Temperature Synthesis

Samples of composition Ba_{1− x} La _x Ti_{1− x /4}O₃, x = 0, 0.003, 0.03, and 0.10, were prepared by an alkoxide sol–gel route with final firing of ceramics at 1100°C, 2 h in air. All samples showed bulk insulating behavior with no evidence of semiconductivity caused by either direct donor doping or oxygen loss.     

Dislocation Loop Formation in Nonstoichiometric (Ba,Ca)TiO3 and BaTiO3 Ceramics

Dislocation loops observed in nonstoichiometric and stoichiometric (Ba,Ca)TiO₃, and in stoichiometric BaTiO₃ sintered in a reducing atmosphere, were characterized by conventional transmission electron microscopy (TEM) under two-beam conditions and high-resolution TEM atomic structure analysis. Dislocation loops mostly lay on {100} planes with Burgers vectors of type 〈100〉. The dynamic behavior of these dislocation loops during the electron beam irradiation (EBI), however, was classified into two different types of dislocation loops: in A-site-excess (Ba,Ca)TiO₃, contrasts of dislocation loops faded completely away; in BaTiO₃ and B-site-excess (Ba,Ca)TiO₃, fine-line contrasts remained. Dislocation loops with Burgers vectors of type 1/2〈100〉 and the resultant crystallographic shear (CS) structure with a displacement vector of type 1/2〈110〉 after EBI were proposed to interpret residual line images. Disappearance of these line images in A-site-excess (Ba,Ca)TiO₃ strongly suggests preferential Ca ion site occupancy at the CS structure.     

Microstructure and Microwave Dielectric Properties of Ba(Zn1/3Ta2/3)O3 Ceramics with ZrO2 Addition

The effect of ZrO₂ on crystallographic order, microstructure, and microwave dielectric properties of Ba(Zn_1/3Ta_2/3)O₃ (BZT) ceramics was investigated. A small amount of ZrO₂ disturbed the 1:2 cation ordering. The average grain size of the BZT significantly increased with the addition of ZrO₂, which was attributed to liquid-phase formation. The relative density increased with the addition of a small amount of ZrO₂, but it decreased when the ZrO₂ content was increased. Variation of the dielectric constant with ZrO₂ addition ranged between 27 and 30, and the temperature coefficient of resonant frequency increased abruptly as the ZrO₂ amount exceeded 2.0 mol%. The Q value of the BZT significantly improved with the addition of ZrO₂, which could be explained by the increased relative density and grain size. The maximum Q × f value achieved in this investigation was ∼164 000 GHz for the BZT with 2.0 mol% ZrO₂ sintered at 1550°C for 10 h.     

Dielectric Properties of Pb(Fe2/3W1/3)O3–PbTiO3 Solid-Solution Ceramics

The dielectric properties of (1− x )Pb(Fe_2/3W_1/3)O₃· x PbTiO₃ solid solutions were investigated from 10² to 10⁶ Hz in the temperature range 150–600 K. The phase transition of Pb(Fe_2/3W_1/3)O₃ (PFW) was shifted by PbTiO₃ (PT) additions to higher temperatures at a rate of 6.3 K/mol% of PT. The temperature dependence of dielectric permittivity showed a sharper transition as the PT content increased. Dielectric measurements in a wide temperature range showed the presence of a second set of dielectric peaks at higher temperatures (350–600 K), besides the ferroelectric–paraelectric phase transition. This second set of peaks vanished when the samples were annealed in nitrogen. The activation energy values for the second relaxation varied between 0.50 and 0.63 eV, in agreement with the conduction activation energy determined for each sample. This relaxation is apparently related to electron holes.     

Hysteresis in the I–V Tunneling Characteristics of n-Type BaTiO3 Ceramics

Scanning tunneling microscopy (STM) and spectroscopy (STS) have been applied to study the surface electronic properties of n -type BaTiO₃ ceramics under ultrahigh vacuum and at various oxygen partial pressures. I – V tunneling characteristics of vacuum-annealed BaTiO₃ do not exhibit rectifying behavior, implying that the Fermi level is pinned at the surface. The surface band gap of BaTiO₃ annealed under vacuum at 540°C is equal to 1 eV. The top edge of the surface valence band is located 0.7 eV below the Fermi level. Hysteresis in the I – V characteristics has been observed at high oxygen partial pressures. Dosing of the BaTiO₃ with oxygen increases the surface band gap and unpins the Fermi level. As a result, the I – V characteristics acquire rectifying features similar to those observed for BaTiO₃ Schottky-type diodes. Hysteresis in the I – V spectra observed at high oxygen partial pressures is attributed to the changes of the surface potential barrier due to adsorption/desorption of oxygen modulated by the tip-sample potential difference.     

Microwave Dielectric Properties of Low-Fired ZnNb2O6 Ceramics with BiVO4 Addition

The BiVO₄ additive was found effective for low-temperature firing of ZnNb₂O₆ polycrystalline ceramics below 950°C in air without a serious degradation in their microwave dielectric properties. Dense BiVO₄-doped ZnNb₂O₆ samples of a relative sintered density over 95% could be prepared even at 925°C. An optimally processed specimen exhibited excellent microwave dielectric properties of Q · f = 55000 GHz, ɛ_r= 26, and τ_f=−57 ppm/°C. With increasing BiVO₄ addition up to 20 mol% relative to ZnNb₂O₆, while the quality factor Q · f was gradually decreased, the relative dielectric constant, ɛ_r, was linearly increased and the temperature coefficient of resonant frequency, τ_f, was slightly increased. The variations in Q · f and ɛ_r are surely attributable to the residual BiVO₄ in the ZnNb₂O₆ matrix. An unexpected slight increase in τ_f is probably due to the formation of the Bi(V,Nb)O₄-type solid solution.     

Microwave Dielectric Properties of CaTiO3-CaAl1/2Nb1/2O3 Ceramics Doped with Li3NbO4

The microwave dielectric properties of CaTi_{1− x} (Al_1/2Nb_1/2) _x O₃ solid solutions (0.3 ≤ x ≤ 0.7) have been investigated. The sintered samples had perovskite structures similar to CaTiO₃. The substitution of Ti⁴⁺ by Al³⁺/Nb⁵⁺ improved the quality factor Q of the sintered specimens. A small addition of Li₃NbO₄ (about 1 wt%) was found to be very effective for lowering sintering temperature of ceramics from 1450–1500° to 1300°C. The composition with x = 0.5 sintered at 1300°C for 5 h revealed excellent dielectric properties, namely, a dielectric constant (ɛ_r) of 48, a Q × f value of 32 100 GHz, and a temperature coefficient of the resonant frequency (τ_f) of −2 ppm/K. Li₃NbO₄ as a sintering additive had no harmful influence on τ_f of ceramics.     

Structure and Microwave Dielectric Properties of (Zn1−xCox)TiO3 Ceramics

Dielectric ceramics in the system (Zn_{1− x} Co _x )TiO₃ ( x = 0–1) were synthesized by the solid-state reaction route. The phase distribution, microstructure, and dielectric properties were characterized by using powder X-ray diffraction analysis, electron microscopy, and microwave measurement techniques. Three phase composition regions were identified in the specimens sintered at 1150°C; [spinel + rutile] at 0 ≤ x ≤ 0.5, [spinel + ilmenite + rutile] at 0.5 < x ≤ 0.7, and [ilmenite] phase at 0.7 < x ≤ 1. For the 0 ≤ x ≤ 0.5 region, the amount of Ti-rich precipitates incorporated into the spinel phase decreased with the Co content at 0 ≤ x ≤ 0.5, with a concomitant increase of the rutile phase. The ilmenite phase appeared for high Co content. The microwave dielectric properties depended on the phase composition and volume according to the three phase regions, where the relative amount of rutile to the spinel or ilmenite determined the dielectric properties. The dielectric constant as a function of Co addition was modeled with a Maxwell mixing rule. An optimum phase distribution was determined in this system with dielectric constant of 25, a Q * f 70 000 GHz, and a low temperature coefficient of the resonant frequency.     

Crystallization and Dielectric Properties of SrO–BaO–Nb2O5–SiO2 Tungsten-Bronze Glass-Ceramics

The crystallization and dielectric properties of SrO–BaO–Nb₂O₅–SiO₂ glass-ceramics have been investigated. Glass-ceramics that contain strontium barium niobate (SBN) as a primary crystalline phase, which has a tungsten bronze structure, are produced. The formation of crystalline secondary phases also has been studied. The SBN phase shows evidence of both surface nucleation and bulk nucleation, and the crystals have an average composition of Sr_0.47Ba_0.53Nb₂O₆. The dendritic morphology of the SBN crystals has been examined. The SBN content and composite dielectric constant each has been studied as a function of heating temperature/time. The highest SBN content and dielectric constant obtained in the present study are 42 vol% and 180, respectively. The dielectric constant of the glass-ceramics is determined primarily by the SBN content and the residual glass phase. The dielectric constant of the randomly oriented SBN crystal in the glass-ceramics is calculated, using dielectric mixture rules, to be ∼400.     

Correlation between Arrangement of Dielectric Layers and Microwave Dielectric Properties of Mg0.93Ca0.07TiO3−(Ca0.3Li0.14Sm0.42)TiO3 Ceramics

Effect of geometrical shape and induced thermal strain on the microwave dielectric properties of the layered structure ceramics of Mg_0.93Ca_0.07TiO₃ with (Ca_0.3Li_0.14Sm_0.42)TiO₃ was investigated as a function of the number of dielectric layers. The dielectric constant and the temperature coefficient of resonant frequency (TCF) were not changed significantly with the number of dielectric layers but only depended on the net compositional ratio. However, the dielectric loss quality was affected by the number of interfaces between dielectric layers, which had a partial composition inhomogeneity due to the diffusion of Mg²⁺ and Ti⁴⁺ ions. The dielectric loss quality also decreased with an increase of thermal strain induced to each dielectric layer.     

Incorporation of Er3+ into BaTiO3

The incorporation of Er³⁺ into BaTiO₃ ceramics was investigated on samples containing 0.25, 0.5, 1, 2, 8, and 10 at.% of dopant, after sintering at 1350–1550°C in air. For Er³⁺ concentrations ≤1 at.%, dense and large-grained ceramics with low room-temperature resistivity (10²–10³Ω·cm) were obtained. The observed properties are largely independent of stoichiometry. Simultaneous substitution of Er³⁺ at both cation sites, with higher preference for the Ba site, is proposed. The behavior of heavily doped ceramics depends on stoichiometry. When Ba/Ti < 1, the electrical properties change from slightly semiconducting to insulating as Er concentration increases from 2 to 8 at.%. The ceramics have tetragonal perovskite structure and contain a large amount of Er₂Ti₂O₇ pyrochlore phase. On the other hand, when Ba/Ti > 1, the ceramics are insulating, fine-grained, and single phase. In this case, incorporation of Er³⁺ predominantly occurs at the Ti site, with oxygen vacancy compensation. Incorporation is accompanied by a significant reduction of tetragonality and by expansion of the unit cell. The different results indicate that Er³⁺ solubility at the Ba site does not exceed 1 at.%, whereas solubility at the Ti site is at least 10 at.%. However, the incorporation of Er³⁺ and the resulting properties are also strongly affected by sintering conditions.     

Structure and Dielectric Properties of Pb(Sc2/3W1/3)O3–Pb(Zr/Ti)O3 Relaxors

The structure and dielectric properties of (1− x )Pb(Sc_2/3W_1/3)O₃–( x )Pb(Zr/Ti)O₃ ceramics have been investigated over a full substitution range. All compositions with x < 0.5 adopt a cubic perovskite structure; however, for x ≤ 0.25 a doubled cell results from a 1:1 ordered distribution of the B-site cations. The structural order in Pb(Sc_2/3W_1/3)O₃ (PSW) can be described by a random-site model with one cation site occupied by Sc³⁺ and the other by a random distribution of (Sc_1/3³⁺W_2/3⁶⁺). The ordering is destabilized in solid solutions of PSW with PbZrO₃ (PSW–PZ), but stabilized by PbTiO₃ in the (1− x )PSW–( x )PT system. The changes in order are accompanied by alterations in the dielectric response of the two systems. For PSW–PZ the temperature of the permittivity maximum ( T _ɛ,max) increases linearly with x ; however, for PSW–PT T _ɛ,max decreases in the ordered region (up to x = 0.25) and then increases rapidly as the order is lost. Similar effects were produced by modifying the degree of order of (0.75)PSW–(0.25)PT; when the order parameter was reduced from ∼1.0 to ∼0.65, T _ɛ,max increased by more than 60°C.     

Atmospheric Dependence on Dielectric Loss of 1/6Ba5Nb4O15· 5/6BaNb2O6 Ceramics

Samples of 1/6Ba₅Nb₄O₁₅·5/6BaNb₂O₆ along with the pure end members, Ba₅Nb₄O₁₅ and BaNb₂O₆, were sintered under low oxygen partial pressure. The degradation mechanisms of dielectric loss in this reducing atmosphere have been studied. We found that the degradation occurred primarily due to the formation of oxygen vacancies caused by the reduction of Nb⁵⁺. This was determined by measuring the electrical conductivity, and through X-ray photoelectron spectroscopy. More importantly, the dielectric loss of 1/6Ba₅Nb₄O₁₅·5/6BaNb₂O₆ samples with higher temperature stability was further decreased on sintering in a reducing atmosphere. This observation has been explained by considering the increased porosity and formation of a reduced second phase, Ba_0.65NbO₃.