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.     

High-Temperature Transmission Electron Microscopy and X-Ray Powder Diffraction Studies of Polymorphic Phase Transitions in Ba4Nb2O9

Polymorphic phase transitions in Ba₄Nb₂O₉ were studied by thermal analyses, high-temperature transmission electron microscopy and X-ray powder diffractometry. Two stable polymorphs were isolated, low-temperature α-modification and high-temperature γ-modification, with the endothermic phase transition at 1176°C. The α→γ transformation is accompanied by the formation of a 120° domain structure, which is a consequence of hexagonal→orthorhombic unit cell reconstruction. Reheating the presintered γ-Ba₄Nb₂O₉ results in the formation of a metastable γ'-modification (formerly known as β-polymorph) in the temperature range between 360° and 585°C, before the γ→α transformation at 800°C. Above ∼490°C Ba₄Nb₂O₉ becomes moderately sensitive to a loss of BaO. In air the surface of Ba₄Nb₂O₉ grains decomposes to nanocrystalline Ba₅Nb₄O₁₅ and BaO, which instantly reacts with atmospheric CO₂ to form BaCO₃. Surface reaction delays γ→α transformation up to 866°C in air. In vacuum the loss of BaO is even more enhanced and consequently the formation of minor Ba₃Nb₂O₈ phase is observed above 1150°C.     

Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics: II. Compositional Variations in Pure BZN

The formation of non-stoichiometric cubic perovskite solid solutions based on BaZn_1/3Nb_2/3O₃ (BZN) was examined along 10 different directions in the BaO–ZnO–Nb₂O₅ ternary system. Limited ranges of non-stoichiometry were observed along several pseudo-binaries and the BZN structure can accommodate a variety of different types of defects. Although the deviations from stoichiometry are quite small, typically ∼1 mole%, they induce large changes in the extent and stability of the 1:2 B-site ordering, the sintering and microstructure, and the dielectric loss properties. The highest Q × f s (∼110 000 at 8 GHz) in the system, which coincide with the highest degree of order, were located in two regions along the BZN–Ba₅Nb₄O₁₅ and BZN–BaNb₂O₆ lines. The results of this study provide an explanation for the large variations in crystal structure and Q × f s previously reported for BZN and other related systems (e.g., Ba(Zn_1/3Ta_2/3)O₃), and demonstrate that non-stoichiometric starting compositions provide a route to the highest Q values.     

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.     

Low-Temperature Preparation of Ba5Nb4O15 Ceramics Through a Sol–Gel Process

Hexagonal Ba₅Nb₄O₁₅ nanorods and microdisks were synthesized by a sol–gel process at temperatures of 700°–900°C. The samples were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and UV–visible absorption spectroscopy. The visible light absorption edges of the Ba₅Nb₄O₁₅ nanorods and microdisks corresponded to the band gap energies of 3.63 and 3.70 eV, respectively. This shows that the nanorod and microdisk-type structures are promising candidates for application in the miniaturization of microwave components.     

Fabrication and Electrical Properties of Textured Sr0.53Ba0.47Nb2O6 Ceramics by Templated Grain Growth

Textured Sr_0.53Ba_0.47Nb₂O₆ ceramics with a relative density of >95% were fabricated using templated grain growth (TGG). Acicular KSr₂Nb₅O₁₅ template particles synthesized via a molten salt process were aligned by tape casting in a mixture of solid-state-synthesized SrNb₂O₆ and BaNb₂O₆ powders. The resulting ceramics possessed strong fiber texture along the polar axis ([001]) of the strontium barium niobate. Samples with 15.4 wt% templates attained a textured fraction of 0.82 after sintering at a temperature of 1450°C for 4 h. These materials showed peak dielectric constants of 7550 at 1 kHz, remanent polarizations of 13.2 μC/cm², saturation polarizations of 21 μC/cm² (60%–85% of the single-crystal value), piezoelectric strain coefficients of 78 pC/N (70%–85% of the single-crystal value), and room-temperature pyroelectric coefficients of 2.9 × 10⁻²μC·(cm²·°C)⁻¹ (52% of the single-crystal value). These results show that TGG is a viable option for accessing single-crystal properties in polycrystalline ceramics.     

Microwave Dielectric Properties of A6B5O18-Type Perovskites

Cation-deficient perovskites with the general formula A₆B₅O₁₈ (A=Ba, Sr, La; B=Nb, Ta, Ti, Zr, Mg, and Zn) have been synthesized and their microwave dielectric properties have been investigated. X-ray diffraction studies indicate the formation of monophase materials. The structures of Ba₆Ta₄TiO₁₈ and Ba₅SrTa₄TiO₁₈ are different from that of Sr₆Ta₄TiO₁₈. The A₆B₅O₁₈ have Q × f in the range 5600–51 000 GHz, dielectric constants 26–48, and the temperature coefficient of resonant frequency varies from −39 to +83 ppm/°C, depending on the composition. Scanning electron microscopy studies show that the grain size decreases with an increase in the Sr content.     

Processing and Characterization of BaTi4O9

BaTi₄O₉ powder prepared by calcining BaCO₃ and TiO₂ powders was sintered to over 97% of theoretical density. Less than 5% Ba₂Ti₉O₂₀ occurred as a second phase in "pure" BaTi₄O₉, and Al₂O₃ impurities from processing formed isolated hollandite (∼BaAl₂Ti₆O₁₆) grains, which were identified by fringes in bright-field TEM images. For pure BaTi₄O₉ at 1 MHz, a dielectric loss (tan δ) of 5 × 10⁻⁴ and dielectric constant of 39 were recorded. Hollandite impurities were found to increase tan δ by 2 orders of magnitude, whereas firing in oxygen decreased tan δ by an order of magnitude.     

Factors Affecting the Formation of Ba2Ti9O20

The phase development sequence based on a composition equivalent to Ba₂Ti₉O₂₀ during heating is found to be in the following order: BaTi₅O₁₁ > BaTi₄O₉ > Ba₂Ti₉O₂₀. The lowest rate of formation of Ba₂Ti₉O₂₀ is caused by its high surface energy and interface energy, which result in a low nucleation rate. The existence of BaTi₅O₁₁ in calcined powder helps to form Ba₂Ti₉O₂₀ in sintered compacts. The effect of BaTi₅O₁₁ on Ba₂Ti₉O₂₀ formation can be explained by their similar oxygen packing and by reduced volume change during transformation. The amount of BaTi₅O₁₁ formed during heating depends greatly on the compositional homogeneity of powders. The addition of SnO₂ aids the formation of Ba₂Ti₉O₂₀ by reduced strain energy at transformation and reduced surface energy.     

Tailoring the Microwave Dielectric Properties of MgNb2O6 and Mg4Nb2O9 Ceramics

The columbites MgNb₂O₆, MgTa₂O₆, and corundum-type Mg₄Nb₂O₉ ceramics were prepared by the conventional solid-state ceramic route. The structure and microstructure of the sintered samples were investigated by X-ray diffraction and scanning electron microscopic techniques. The microwave dielectric properties of the samples were measured by the resonance method in the frequency range 4–6 GHz. The dielectric properties have been tailored by forming a solid solution between MgNb₂O₆ and MgTa₂O₆ and by the substitution of TiO₂ for Nb₂O₅ in both MgNb₂O₆ and Mg₄Nb₂O₉ ceramics. The Mg(Nb_0.7Ta_1.3)O₆ has ɛ_r=29, Q _u× f =67 800 GHz, and τ_f=0.8 ppm/°C and the MgO–(0.4)Nb₂O₅–(1.5)TiO₂ composition has ɛ_r=34.5, Q _u× f =81 300 GHz, and τ_f=−2 ppm/°C.     

Phase Structure and Dielectric Properties of Bi2O3-ZnO-Nb2O5-Based Dielectric Ceramics

Phase structures and dielectric properties of the compounds with formulas BixZn_2/3Nb_4/3O_4+3x/2 (group M), Bi_xZn_8/3-x Nb_4/3O_6+x/2 (group V), and Bi_xZn_2-2x/3Nb_2-x/3O₇ (group W) have been investigated. Initial results indicate that a cubic pyrochlore structure is the predominant phase of these compound. Most of the measured ceramic specimens exhibit dielectric properties suitable as temperatures-stable and temperature-compensating dielectrics in the capacitor industry. The values of the dielectric constant K are 80-160, while those of the temperature coefficient are–500 to + 160 ppm/°C. The composition limits of the single pyrochlore phase are determined mainly by Bi₂O₃ additives.     

Nonisothermal Reaction Kinetics of SrNb2O6 and BaNb2O6 for the Formation of SrxBa1−xNb2O6

The solid-state reaction of SrNb₂O₆ and BaNb₂O₆ to form Sr _x Ba_{1− x} Nb₂O₆(SBN) at different temperatures and heating rates was investigated. The reaction kinetics were analyzed by X-ray diffractometry for quenched samples, and the internal-standard method was applied to quantify the extent of the reaction. A nonisothermal kinetic empirical model was proposed to evaluate the activation energy and rate constant of SBN with different Sr:Ba ratios. It was found that the kinetic form would change above and below a transition at a reaction fraction of ∼60%, which might be due to the change of the frequency factor. It was also verified that the model that was presented was more favorable to describe the nonisothermal reaction kinetics of SBN.     

Morphology-Controlled Synthesis and Characterization of Sr2Nb2O7 Nanocrystalline Powders by a Hydrothermal Process

Nanostructured Sr₂Nb₂O₇ powders with different morphologies were synthesized via a hydrothermal reaction at 200°C for 24 h in a Sr²⁺–Nb₂O₅· n H₂O–KOH system. X-ray powder diffraction characterization indicated that 0.3–5 M KOH solutions favored the formation of a Sr₂Nb₂O₇ pure phase. On increasing the concentration of KOH from 0.3 to 5 M , the morphology of Sr₂Nb₂O₇ samples changed from nanoneedles to nanorods, nanosheets, and finally nano-sized flakes, and their aspect ratios gradually reduced. Ultraviolet–Visible diffuse reflectance spectra analyses showed that the nanostructured Sr₂Nb₂O₇ powders had good light absorption properties in the ultraviolet light region.     

Phase Formation and Dielectric Characterization of the Bi2O3–TeO2 System Prepared in an Oxygen Atmosphere

Solid-state synthesis of compositions from the Bi₂O₃–TeO₂ system show that, under an oxygen atmosphere, Te⁴⁺ oxidizes to Te⁶⁺ and yields four room-temperature stable compounds: Bi₂Te₂O₈, Bi₂TeO₆, Bi₆Te₂O₁₅, and new a compound with the nominal composition 7Bi₂O₃·2TeO₂. Dense ceramics can be prepared from all these compounds by sintering between 650° and 800°C under an oxygen atmosphere. The permittivity of these compounds varies from ∼30 to ∼54, the Q × f value from 1.100 to 41.000 GHz (∼5 GHz), and the temperature coefficient of resonant frequency from −43 to −144 ppm/K. Bi₆Te₂O₁₅ and 7Bi₂O₃·2TeO₂ do not react with silver, and, therefore, they have the potential to be used for applications in low-temperature cofired ceramic (LTCC) technology.     

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.     

Low Temperature Sintering of Zn3Nb2O8 Ceramics from Fine Powders

A possibility to produce microwave (MW) dielectric materials by liquid-phase sintering of fine particles was investigated. Zn₃Nb₂O₈ powders with a grain size 50–300 nm were obtained by the thermal decomposition of freeze-dried Zn–Nb hydroxides or frozen oxalate solutions. The crystallization of Zn₃Nb₂O₈ from amorphous decomposition products was often accompanied by the simultaneous formation of ZnNb₂O₆. Maximum sintering activity was observed for single-phase crystalline Zn₃Nb₂O₈ powders obtained at the lowest temperature. The sintering of as-obtained powders with CuO–V₂O₅ sintering aids results in producing MW dielectric ceramics with a density 93%–97% of the theoretical, and a Q × f product up to 36 000 GHz at sintering temperature ( T _s)≥680°C. The high level of MW dielectric properties of ceramics was ensured by intensive grain growth during the densification and the thermal processing of ceramics.     

Effect of Ba6Ti17O40/BaTiO3 Interface Structure on {111} Twin Formation and Abnormal Grain Growth in BaTiO3

A structural transition of Ba₆Ti₁₇O₄₀/BaTiO₃ interfaces from faceted to rough was induced by reducing oxygen partial pressure in the atmosphere. As the oxygen partial pressure decreased, the number densities of {111} twins and abnormal grain decreased. TEM observation showed that the twin formation was governed only by the faceting of the interface. Experimental evidence of {111} twin-assisted abnormal growth of faceted BaTiO₃ grains was also obtained.     

Effect of Al2O3 and Bi2O3 on the Formation Mechanism of Sn-Doped Ba2Ti9O20

High-performance Ba₂Ti₉O₂₀ ceramics are attracting great attention, but their formation mechanism still is somewhat unclear. The present investigation shows that the formation of Ba₂Ti₉O₂₀ can be promoted strikingly by the participation of Bi₂O₃ and Al₂O₃. The effect of Bi₂O₃ on the formation of Ba₂Ti₉O₂₀ is attributed to the fact that migration of the involved reactants is accelerated by liquid which forms from the melting of Bi₂O₃ above 830°C. This migration, however, is not the only rate-limiting factor. A high potential-energy barrier, resulting from stress that arises along the crystal-structured layers, also heavily restricts the formation of Ba₂Ti₉O₂₀. The participation of Al₂O₃, on the other hand, can reduce the height of this potential-energy barrier and effectively improve the kinetics of the formation of Ba₂Ti₉O₂₀ by causing the formation of BaAI₂Ti₆O₁₆ crystals; these crystals intergrow with Ba₂Ti₉O₂₀ crystals and result in decreased stress.     

Ba8ZnTa6O24: A New High Q Dielectric Perovskite

The hexagonal perovskite, Ba₈ZnTa₆O₂₄, was prepared in single-phase form and was found to be a stable secondary phase, formed as a result of the loss of ZnO from Ba(Zn_1/3Ta_2/3)O₃ microwave dielectrics. The experimental and calculated X-ray patterns of Ba₈ZnTa₆O₂₄ indicate it is isostructural with Ba₈Ta₆NiO₂₄ with an 8H (cchc)₂ close-packed BaO₃ stacking sequence and the lattice parameters, a =10.0825(14), c =19.0587(38)Å. High-density ceramics of Ba₈ZnTa₆O₂₄ could be prepared at temperatures considerably lower (1400°C) than those used to sinter pure Ba(Zn_1/3Ta_2/3)O₃, and exhibit very good microwave dielectric properties with ɛ=30.5, Q _f=62 300, and τ_f=+36 ppm/°C at 8.9 GHz.     

Incorporation of Aliovalent Dopants into the Bismuth-Layered Perovskite-Like Structure of BaBi4Ti4O15

The solid solubility of the aliovalent dopants Fe³⁺ and Nb⁵⁺ in the BaBi₄Ti₄O₁₅ compound, a member of the family of Aurivillius bismuth-based layer-structure perovskites, has been studied using quantitative wavelength-dispersive spectroscopic microanalysis (SEM/EPMA) in combination with X-ray powder diffractometry (XRPD). The samples with nominal (starting) compositions corresponding to the chemical formulas BaBi₄Ti_{4–4 X} Fe_{4 X} O₁₅ and BaBi₄Ti_{4–4 X} Nb_{4 X} O₁₅ were prepared by hot forging a mixture of BaTiO₃ and Bi₄Ti₃O₁₂ with additions of Fe₂O₃ or Nb₂O₅ followed by a long annealing at 1100°C. The study showed that an excess charge introduced into the structure by the substitution of Ti⁴⁺ ions with aliovalent dopants was preferentially compensated by a change in the ratio of Ba²⁺ to Bi³⁺ ions in the host structure according to the general formulas of the solid solutions Ba_{1–4 X} Bi_{4+4 X} Ti_{4–4 X} Fe^'_{4 X} O₁₅ and Ba_{1+4 X} Bi_{4–4 X} Ti_{4–4 X} Nb^·_{4 X} O₁₅.