Preparation of Ba6−3xNd8+2xTi18O54 via Ethylenediaminetetraacetic Acid Precursor

Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ ceramic powders were synthesized by the modified Pechini method using ethylenediaminetetraacetic acid (EDTA) as a chelating agent. A purplish red, molecular-level, homogeneously mixed gel was prepared, and transferred into a porous resin intermediate through charring. Single-phase and well-crystallized Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ powders were obtained from pulverized resin at a temperature of 900°C for 3 h, without formation of any intermediate phases. Meanwhile, the molar ratio of EDTA to total metal cation concentration had a significant influence on the crystallization behavior of Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄. The Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ ( x = 2/3) ceramics prepared via EDTA precursor have excellent microwave dielectric characteristics: ɛ= 87, Qf = 8710 GHz.     

A- and B Site Cosubstituted Ba6−3xSm8+2xTi18O54Microwave Dielectric Ceramics

Tin (Sn) substitution into the B-site and Nd/Sn cosubstitution into the A- and B-sites were investigated in a Ba _{6−3 x} Sm_{8+2 x} Ti₁₈O₅₄solid solution ( x = 2/3). A small amount of tin substitution for titanium improved the temperature coefficient of resonant frequency (τ_f) but led to a decrease of the relative dielectric constant (ɛ) and the quality factor ( Qf ). The Ba_{6−3 x} Sm_{8+2 x} (Ti_{1− z} Sn_z)₁₈O₅₄-based tungsten-bronze phase became unstable for compositions with a tin content of ≥10 mol%, where BaSm₂O₄and Sm₂(Sn,Ti)₂O₇appeared, and finally, these phases became the major phases. On the other hand, Nd/Sn cosubstitution led to a good combination of high ɛ, high Qf , and near-zero τ_f. Excellent microwave dielectric properties were achieved in Ba_{6−3 x} (Sm_{1− y} Nd _y )_{8+2 x} (Ti_{1− z} Sn _z )₁₈O₅₄ceramics with y = 0.8 and z = 0.05 sintered at 1360°C for 3 h: ɛ= 82, Qf = 10 000 GHz, and calculated τ_f=+17 ppm/°C. The tolerance factor and electronegativity difference exhibited important effects on the microwave dielectric properties, especially the Qf value. A large tolerance factor and high electronegativity difference generally led to a higher Qf value.     

Tin Substitution for Titanium in Ba6−3xNd8+2xTi18O54 Microwave Dielectric Ceramics

Tin (Sn) substitution for titanium (Ti) was investigated in Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ ( x =1/2, 2/3, and 3/4) ceramics. A small amount ( z <0.1) of Sn substitution resulted in Ba_{6−3 x} Nd_{8+2 x} (Ti_{1− z} Sn _z )₁₈O₅₄ solid solutions, and some secondary phases were observed with increasing Sn content. A small amount of Sn substitution improved the Q f value significantly, while, due to the formation of secondary phases, the Q f value degraded sharply for larger Sn content. The relative dielectric constant (ɛ_r) decreased with increasing Sn-content, while the temperature coefficient of resonant frequency (τ_f) generally decreased, although an obvious fluctuation was observed for x =3/4.     

Improved Microwave Dielectric Properties of Ba6Ti1−xSnxNb4O18 Ceramics

Single-phase polycrystalline microwave dielectric ceramics Ba₆Ti_{1− x} Sn _x Nb₄O₁₈, with x changing from 0 to 1, were synthesized by the solid-state reaction method. All the solid solutions fitted well with A₆B₅O₁₈ cation-deficient hexagonal perovskite structure. The substitution of Sn for Ti effectively enhanced the quality factor and controlled τ_f. With increasing Sn content, the dielectric constant decreased from ∼47 to ∼32, and the Q × f value increased significantly from 11 530 to 28 496 GHz, with τ_f varying from 64 to 0 ppm/°C. A zero τ_f was realized when Sn was fully replaced by Ti with the composition Ba₆SnNb₄O₁₈.     

Effect of Bismuth Borate/LiF on the Sintering, Thermal, and Microwave Dielectric Properties of Ba6−3xSm8+2xTi18O54 (x=2/3) Solid Solution

Ba_{6−3 x} Sm_{8+2 x} Ti₁₈O₅₄ ( x =2/3) (BST) was prepared by the solid-state method and the effect of bismuth borate (BB)/LiF on the sinterability, microstructure, and thermal and microwave dielectric properties were studied. BST+3.5 wt% BB+0.5 wt% LiF composite sintered at 1050°C has Q × f =4500 GHz, ɛ=52, and τ _f =+6 ppm/°C. Raman spectrum of the composite was compared with that of BST and the structural changes were investigated.     

Thermal Expansion and High-Temperature Phase Transition of Ba6−3xLn8+2xTi18O54 (Ln=La, Nd, and Sm) Ceramics

Thermal expansion behaviors of Ba_{6−3 x} Ln_{8+2 x} Ti₁₈O₅₄ (Ln=La, Nd, and Sm, x =0.5, 0.67, and 0.75) ceramics were determined by dilatometric measurement. The samples of all investigated compositions expanded nearly linearly with increasing temperature in the range of 20°–1200°C. Their thermal expansion coefficients were determined to be 10.7–11.4 ppm/°C. A discontinuous change in sample size was observed at about 1350°C for each composition, indicating the existence of a phase transition. As determined by the high-resolution X-ray diffraction analysis, the phase constitution and the lattice parameters of Ba_{6−3 x} Sm_{8+2 x} Ti₁₈O₅₄ ceramics were maintained in the quenched samples. The origin of the phase transition was discussed thoroughly. Phase equilibrium in the BaTiO₃–Ln_2/3TiO₃ system was reviewed by considering the phase transition.     

Matrix Phase in Ceramics with Composition near BaO·Nd2O3·5TiO2

Phase structures of two ceramics of Compositions BaNd₂Ti₅O₁₄ and Ba_3.75Nd_9.5Ti₁₈O₅₄, both of which had earlier been reported to be single-phase materials, were investigated using electron microscopy and an electron probe microanalyzer. It was observed that the amount of secondary phases was much lower in ceramics prepared with the latter composition and that the composition of the matrix phase of both samples was near 4BaO·5 Nd₂O₃·18TiO₂. These results indicate that only the composition Ba_3.75Nd_9.5Ti₁₈O₅₄ is a single-phase composition.     

Space Group Determination of Ba6-3xNd8+2xTi18O54

The space group of the solid solution phase Ba_{6-3 x} -RE_{8+2 x} Ti₁₈O₅₄(RE = rare-earth cation) has been variously reported as Pba 2 (No. 32), Pbn 2₁(No. 33), Pbam (No. 55), or Pbnm (No. 62). New results are presented here which indicate that its correct space group assignment may be Pb 2₁ m (No. 26).     

Internal Oxidation of Ce3+-BaTiO3 Solid Solutions

The reoxidation process in highly Ce³⁺-doped BaTiO₃ ceramics was studied using TEM. Samples of two different types of solid solutions, Ba_1−XCe³⁺ _X Ti_1−X/4( V _Ti) X/4 O₃ and Ba_1−XCe³⁺ _X Ti⁴⁺_{1− X} Ti³⁺ _X O₃, were prepared by sintering oxide mixtures in air and in a reducing atmosphere, respectively. The solid solutions were reoxidized by annealing in air at high temperatures (1000°—1100°C). As a result of internal oxidation of Ce³⁺ and Ti³⁺, fluorite CeO₂ and monoclinic Ba₆Ti₁₇O₄₀ phases were precipitated in the perovskite matrix. In Ba_1−XCe³⁺ _X Ti_1−X/4( V _Ti)X/4O3 solid solution precipitates nucleate heterogeneously at grain boundaries and at extended defects inside the grains, whereas in Ba_1−XCe³⁺_XTi⁴⁺_1−XTi³⁺_XO₃ solid solution precipitates are nucleated mainly homogeneously inside reoxidized perovskite grains. The form of the precipitates and their orientational relationship with the matrix, as well as the mechanism of internal oxidation, are discussed.     

Crossover of Ba(Ti,Y)O3 Solid Solutions to Ba3Ti2YO8.5–BaTiO3 Composites and their Dielectric Properties

Ceramic samples with the nominal composition (1− x ) BaTiO₃+ x Ba₃Ti₂YO_8.5 ( x =0−0.535) were prepared by the mixed oxide method. X-ray diffraction (XRD) analysis shows that the materials are of single phase with a cubic symmetry as x ≤0.16. The compositions of the solid solutions ( x ≤0.16) can be expressed equivalently as Ba(Ti_{1− y} Y _y )O_3−δ ( y ≤0.122, y = x /(1+2 x )). For x >0.16, the materials are diphasic composites consisting of Ba(Ti_{1− y} Y _y )O₃ ( y =0.122) and Ba₃Ti₂YO_8.5. The microstructure observation by scanning electron microscopy supports the XRD result. The dielectric behavior and phase transitions of the solid solutions ( x ≤0.16) vary with different Y concentrations. The dielectric constant of the composites ( x >0.16) follows approximately the Lichteneker relation in a wide temperature range.     

Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics: I. Substitution of Ba3W2O9

A narrow region of Zn-vacancy-containing cubic perovskites was formed in the (1− x )Ba₃(ZnNb₂)O₉−( x )Ba₃W₂O₉ system up to 2 mol% substitution ( x =0.02). The introduction of cation vacancies enhanced the stability of the 1:2 B-site ordered form of the structure, Ba(Zn_{1− x} □ _x )_1/3(Nb_{1− x} W _x )_2/3O₃, which underwent an order–disorder transition at 1410°C, ∼35° higher than pure Ba(Zn_1/3Nb_2/3)O₃. The Zn vacancies also accelerated the kinetics of the ordering reaction, and samples with x =0.006 comprised large ordered domains with a high lattice distortion ( c/a =1.226) after a 12 h anneal at 1300°C. The tungstate-containing solid solutions can be sintered to a high density at 1390°C, and the resultant ordered ceramics exhibit some of the highest microwave dielectric Q factors ( Q × f =1 18 000 at 8 GHz) reported for a niobate-based perovskite.     

Barium Neodymium Titanate Electroceramics: Phase Equilibria Studies of Ba6–3xNd8+2xTi18O54 Solid Solution

A solid-solution phase with the general formula Ba_6-3x Nd_8+2x Ti₁₈O₅₄, where 0.25(5) ≤×≤ 0.75(5), has been characterized at 1250°C; this phase has been variously described as BaNd₂Ti₄O₁₂ and BaNd₂Ti₅O₁₄ in the literature. Variation in its stoichiometry is accommodated via the cation substitution mechanism, 3Ba²⇆2Nd³⁺. The location and extent of the solid solution were demonstrated by a combination of phase diagram studies and X-ray diffraction techniques, including lattice parameter measurements and electron microscopy. A combination of techniques was employed due to the insensitivity of secondary phase detection by X-ray diffraction in this system. Using this approach, a second possible solid-solution mechanism, Ba²⁺2Nd³⁺⇆2Ti⁴⁺, is discounted.     

Formation Mechanism and Synthesis of Apatite-Type Structure Ba2+xLa8−x(SiO4)6O2−δ

The formation process of Ba₂La₈(SiO₄)₆O₂ was clarified using thermogravimetry–differential thermal analysis (TG-DTA) and a high-temperature powder X-ray diffraction (HT-XRD) method. Phase changes identified from the HT-XRD data surprisingly corresponded to the weight loss and/or endothermic peaks observed in the TG-DTA curves. Raw material with the composition Ba₂La₈(SiO₄)₆O₂ was completely reacted at 1400°C and produced only an apatite-type compound without a secondary phase. Moreover, the synthesis of Ba_{2+ x} La_{8− x} (SiO₄)₆O_2−δ crystals with x = 0–2 was attempted using a solid-state reaction.     

Effect of V2O5 Doping on the Sintering and Dielectric Properties of M-Phase Li1+x−yNb1−x−3yTix+4yO3 Ceramics

The effect of the addition of V₂O₅ on the structure, sintering and dielectric properties of M -phase (Li_{1+ x − y} Nb_{1− x −3 y} Ti _x +4 _y )O₃ ceramics has been investigated. Homogeneous substitution of V⁵⁺ for Nb⁵⁺ was obtained in LiNb_{0.6(1− x )}V_{0.6 x} Ti_0.5O₃ for x ≤ 0.02. The addition of V₂O₅ led to a large reduction in the sintering temperature and samples with x = 0.02 could be fully densified at 900°C. The substitution of vanadia had a relatively minor adverse effect on the microwave dielectric properties of the M -phase system and the x = 0.02 ceramics had [alt epsilon]_r= 66, Q × f = 3800 at 5.6 GHz, and τ_f= 11 ppm/°C. Preliminary investigations suggest that silver metallization does not diffuse into the V₂O₅-doped M -phase ceramics at 900°C, making these materials potential candidates for low-temperature cofired ceramic (LTCC) applications.     

Structure-Property Phase Diagram of BaZrxTi1−xO3 System

In the course of searching environmental friendly lead-free relaxor ferroelectrics a complete phase diagram of barium zirconate titanate, Ba(Zr _x Ti_{1− x} )O₃ system with compositions 0.00≤ x ≤1.00 has been developed based on their dielectric behavior. It has been shown that BaZr _x Ti_{1− x} O₃ system depending on the composition, successively depicts the properties extending from simple dielectric (pure BaZrO₃) to polar cluster dielectric, relaxor ferroelectric, second order like diffuse phase transition, ferroelectric with pinched phase transitions and then to a proper ferroelectric (pure BaTiO₃). A comprehensive structure–property correlation of BaZr _x Ti_{1− x} O₃ ceramics has been studied to understand the various ferroelectric phenomena in the whole phase diagram.     

Porous Glass-Ceramics with a Skeleton of the Fast-Lithium-Conducting Crystal Li1+xTi2−xAlx(PO4)3

Porous glass-ceramics with a skeleton of the fast-lithium-conducting crystal Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ (where x = 0.3–0.5) were prepared by crystallization of glasses in the Li₂O─CaO─TiO₂─Al₂O₃–P₂O₅ system and subsequent acid leaching of the resulting dense glass-ceramics composed of the interlocking of Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ and β-Ca₃(PO₄)₂ phases. The median pore diameter and surface area of the resulting porous Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ glass-ceramics were approximately 0.2 μm and 50 m²/g, respectively. The electrical conductivity of the porous glass-ceramics after heating in LiNO₃ aqueous solution was 8 × 10⁻⁵ S/cm at 300 K or 2 × 10⁻² S/cm at 600 K.     

Pyrochlore Phase Formation in the System Bi2O3–ZnO–Ta2O5

The subsolidus phase diagram of the system Bi₂O₃–ZnO–Ta₂O₅ in the region of the cubic pyrochlore phase has been determined at 1050°C. This phase forms a solid solution area that includes the ideal composition P, Bi₃Zn₂Ta₃O₁₄; possible solid solution mechanisms are proposed, supported by density measurements of Zn-deficient solid solutions. The general formula of the solid solutions is Bi_{3+ y} Zn_{2− x} Ta_{3− y} O_{14− x − y} , based on the creation of Zn²⁺, O²⁻ vacancies in Zn-deficient compositions and a variable Bi/Ta ratio.     

Effects of Ba6Ti17O40 on the Dielectric Properties of Nb-Doped BaTiO3 Ceramics

The dielectric properties, including the DC breakdown strength, of 1 mol% Nb⁵⁺-doped BaTiO₃ ceramics with different quantities of excess TiO₂ have been investigated. The breakdown strength was found to decrease with increasing TiO₂ content, but could not be readily explained by relative density and grain size effects. The decrease in the breakdown strength from a stoichiometric BaTiO₃ composition to samples with excess TiO₂ is believed to be due to the field enhancement effect (up to a factor of 1.40) at the BaTiO₃ matrix because of the presence of a Ba₆Ti₁₇O₄₀ second phase. The thermal expansion coefficient mismatch between the BaTiO₃ matrix phase and the Ba₆Ti₁₇O₄₀ phase may also result in a low breakdown strength. The dielectric properties of the pure Ba₆Ti₁₇O₄₀ phase were also investigated and are reported herein.     

The Li2O–Nb2O5–TiO2 Composite Microwave Dielectric Ceramics with Adjustable Permittivities

A group of new y M-phase/(1− y ) Li_{2+ x} Ti_{1−4 x} Nb_{3 x} O₃ composite ceramics with adjustable permittivities for low-temperature co-fired ceramic applications was initially investigated in the study. The 0.5 M-phase/0.5 Li_{2+ x} Ti_{1−4 x} Nb_{3 x} O₃ ( x =0.01, 0.02, 0.04, 0.06, 0.081) composite ceramics were first investigated to find the appropriate "Li₂TiO₃ss" composition ( x value). The best dielectric properties of ɛ_r=40.1, Q × f values up to 9318 GHz, τ_f=25 ppm/°C, were obtained for the ceramics composites at x =0.02. Based on the good dielectric properties, the suitable "Li₂TiO₃ss" composition with x =0.02 was mixed with the Li_1.0Nb_0.6Ti_0.5O₃ powder as the ratio of y "M-phase"/(1− y ) "Li₂TiO₃ss" ( y =0.2, 0.4, 0.5, 0.6, 0.8). By adjusting the y values, the group of composite ceramics could exhibit largely are adjustable permittivities varying from ∼20 to ∼60, while Q × f and τ_f values relatively good. Nevertheless, in this study, because there are interactions between the M-phase and Li₂TiO₃ss during sintering process, their microwave dielectric properties could not be predicted precisely by the empirical model.     

Microstructural Changes during the Reduction/Reoxidation Process in Donor-Doped BaTiO3 Ceramics

Microstructural changes occurring during oxidation of the reduced form of donor-doped BaTiO₃ (Ba_{1− X} D _X ^.Ti_{1− X} ⁴⁺Ti _X ³⁺O₃) and during reduction of the oxidized form of donor-doped BaTiO₃ (Ba_{1− X} D _X ^.Ti_{1− X /4}⁴⁺( V _Ti^) _{X /4}O₃) were studied using TEM. Samples of both types of solid solutions, containing different La concentrations (from 2 to 20 mol% La), were prepared by sintering under reducing conditions and in air, respectively. The reduced form of donor-doped BaTiO₃ was oxidized by annealing at high temperatures (1150° and 1350°C) in air, while the oxidized form was reduced by annealing under reducing conditions. Because of oxidation of the reduced phase of donor-doped BaTiO₃, the Ti-rich phases Ba₆Ti₁₇O₄₀ and BaLa₂Ti₄O₁₂ were precipitated. Reduction of the oxidized form caused precipitation of the Ba-rich phase Ba₂TiO₄ preferentially inside the matrix grains. All precipitates had well-defined orientational relationships with the perovskite matrix.