Bismuth/Samarium Cosubstituted Ba6−3xNd8+2xTi18O54 Microwave Dielectric Ceramics

Modification of the microwave dielectric properties in Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ ( x = 0.5) solid solutions by Bi/Sm cosubstitution for Nd was investigated. A large increase in the dielectric constant and near-zero temperature coefficient combined with high Qf values were obtained in modified Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ solid solutions where an enlarged solid solution limit of Bi in Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ was observed. Excellent microwave dielectric characteristics (ɛ= 105, Qf = 4110 GHz, and very low τ_f) were achieved in the composition Ba_{6−3 x} (Nd_0.7Bi_0.18Sm_0.12)_{8+2 x} Ti₁₈O₅₄.     

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.     

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).     

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.     

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.     

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.     

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.     

Structure and Microwave Dielectric Characteristics of Ca1+xNd1−xAl1−xTixO4 Ceramics

CaNdAlO₄ microwave dielectric ceramics were modified by Ca/Ti co-substitution, and their dielectric characteristics were evaluated along with their structure and microstructures. Ca_{1+ x} Nd_{1− x} Al_{1− x} Ti _x O₄ ( x =0, 0.025, 0.05, 0.10, 0.15, 0.20) ceramics with the relative density of over 95% theoretical density were obtained by sintering at 1400°–1450°C in air for 3 h, where the K₂NiF₄-type solid solution single phase was determined from the compositions of x <0.20, while a small amount of CaTiO₃ secondary phase was detected for x =0.20. With Ca/Ti co-substitution in CaNdAlO₄ ceramics, the dielectric constant (ɛ_r) increased with increasing x , and the temperature coefficient of resonant frequency (τ_f) was adjusted from negative to positive, while the Q × f ₀ value increased significantly at first and reached an extreme value at x =0.025 and the maximum at x =0.15. The best combination of microwave dielectric characteristics were achieved at x =0.15 (ɛ_r=19.5, Q × f ₀=93 400 GHz, τ_f=−2 ppm/°C). The improvement of the Q × f ₀ value primarily originated from the reduced interlayer polarization with Ca/Ti co-substitution, while the decreased tolerance factor, the subsequent increased interlayer stress, and the appearance of CaTiO₃ secondary phase brought negative effects upon the Q × f ₀ value.     

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.     

Preparation, Structure, and Dielectric Properties of the System Ba1−xLaxTi1−xNixO3

Attempts have been made to synthesize the compositions with x = 0.01, 0.05, 0.10, 0.20, 0.30, 0.40, and 0.50 in the valence-compensated solid-solution Ba_1–xLa_xTi_1–xNi_xO₃ by ceramic methods. Solid solutions formed in the compositions with x0.10. The structure of the composition with x = 0.01 is tetragonal, whereas samples with x = 0.05 and 0.10 are cubic. A ferroelectric-paraelectric transition is observed only in the composition with x = 0.01 at ∼ 350 K.     

Ba2Ti9O20 as a Microwave Dielectric Resonator

Microwave measurements of Ba₂Ti₉O₂₀ show that this ceramic is uniquely suited for dielectric resonators. (Suitable ceramics should have a high dielectric constant K , a low dielectric loss (high Q ), and a low temperature coefficient of resonant frequency, τ.) At 4 GHz, Ba₂Ti₉O₂₀ resonators have Q >8000, K = 39.8, and τ=2 ppm/°C. Measurements of Q and τ were made on unmetallized ceramic resonator disks positioned in a waveguide; K was measured using a dielectric post resonator technique. From 4 to 10 GHz, Q approaches that for a copper waveguide cavity, whereas the temperature coefficient is typically 8 times lower.     

High Q Microwave Dielectric Ceramics in (Ni1−x Znx)Nb2O6 System

(Ni_{1− x} Zn _x )Nb₂O₆, 0≤ x ≤1.0, ceramics with >97% density were prepared by a conventional solid-state reaction, followed by sintering at 1200°–1300°C (depending on the value of x ). The XRD patterns of the sintered samples (0≤ x ≤1.0) revealed single-phase formation with a columbite ( Pbcn ) structure. The unit cell volume slightly increased with increasing Zn content ( x ). All the compositions showed high electrical resistivity (ρ_dc=1.6±0.3 × 10¹¹Ω·cm). The microwave (4–5 GHz) dielectric properties of (Ni_{1− x} Zn _x )Nb₂O₆ ceramics exhibited a significant dependence on the Zn content and to some extent on the morphology of the grains. As x was increased from 0 to 1, the average grain size monotonically increased from 7.6 to 21.2 μm and the microwave dielectric constant (ɛ'_r) increased from 23.6 to 26.1, while the quality factors ( Q _u× f ) increased from 18 900 to 103 730 GHz and the temperature coefficient of resonant frequency (τ_f) increased from −62 to −73 ppm/°C. In the present work, we report the highest observed values of Q _u× f =103 730 GHz, and ɛ'_r=26.1 for the ZnNb₂O₆-sintered ceramics.     

Glass-Free KxBa1−xGa2−xGe2+xO8 Ceramics for Low-Temperature Cofired Ceramics Technology: Synthesis, Phase Transitions, Sintering, and Microwave Dielectric Properties

K _x Ba_{1− x} Ga_{2− x} Ge_{2+ x} O₈ (0.6≤ x ≤1) polycrystalline ceramics are potential materials for glass-free low-temperature cofired ceramics (LTCC) substrates. We have made a comprehensive study of the kinetics of the monoclinic-to-monoclinic P 2₁/ a ⇔ C 2/ m phase transition. The low-temperature-stable P 2₁/ a phase with a high Q × f value was synthesized using a subsolidus method and was well sintered at the LTCC temperature with a H₃BO₃ additive. A good combination of low sintering temperature (910°–920°C), high Q × f values (96 700–104 500 GHz), low permittivities (5.6–6.0), and a small temperature coefficient of resonant frequency (∼−20 ppm/°C) was obtained for ceramics with x =0.67 and 0.9 and with 0.1 wt% of H₃BO₃.     

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.     

Crystal Structure of the Microwave Dielectric Resonator Ba2Ti9O20

A single-crystal X-ray study of dibarium nonatitanate, Ba₂Ti₉O₂₀, yielded the triclinic space group P 1 with a =0.7471(1), b= 1.4081(2), c= 1.4344(2) nm, α=89.94(2)°, β= 79.43(2)°, γ= 84.45(2)°, V = 1.476 nm³ Z = 4, and D_x= 4.61 Mg/m³. A refinement of atomic coordinates and isotropic thermal parameters led to a residual of 0.03. The structure consists of hexagonally closest-packed layers of Ba and O atoms in the sequence (hch)₃. All Ti atoms reside in octahedral interstices of this closest packing. The various Ti coordination octahedra share only edges and corners with each other. One-half of the Ba atoms is twelve-coordinated by oxygen atoms, the other half is eleven-coordinated.     

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.     

Microwave Properties of Ba2Ti9O20 Doped with Zirconium and Tin Oxides

The effects of solid-solution additives, their concentration, and the thermal processing schedule on the microstructure evolution and microwave properties of Ba₂Ti₉O₂₀ were studied. The solubility of tin in Ba₂Ti₉O₂₀ was higher than that of zirconium. Both elements facilitated the formation of phase-pure Ba₂Ti₉O₂₀ resonators. Ba₂Ti₉O₂₀ formed most easily with low dopant concentrations (0.82 mol%) (most impressively for ZrO₂ substitutions). Extended heat treatment (16 h versus 6 h at a temperature of 1390°C) resulted in volatilization of the grain-boundary liquid phase, which leads to more-porous resonators that have correspondingly lower permittivities. Increasing the dopant concentration resulted in minor increases in the quality factor; doping with zirconium led to slightly higher values (a maximum of 13900 at a frequency of 3 GHz). Increasing the measurement temperature degraded the quality factor (most precipitously for BaTi₄O₉). The temperature coefficient decreased as the ZrO₂ substitution increased but was largely unaffected by the SnO₂ concentration. Excess TiO₂ in a batch with no other dopants demonstrated degraded microwave properties.     

Effects of Silver Doping on the Sol–Gel-Derived Ba4(Nd0.7Sm0.3)9.33Ti18O54 Microwave Dielectric Ceramics

Tungstenbronze-type Ba₄(Nd_0.7Sm_0.3)_9.33Ti₁₈O₅₄ (BNST) microwave dielectric ceramics doped with 0–10 wt% silver (Ag) particles were successfully fabricated by a citrate sol–gel method. The influence of Ag doping on the sinterability, microstructure, bulk conductivity, and dielectric properties of BNST was investigated. The desired tungstenbronze-type phase was obtained at 900°–950°C. The sintering temperature of BNST decreased to 1100°C with the aid of a small amount of Ag addition (1 wt%). No chemical reaction between the tungsenbronze phase and Ag was detected. The particle size of the powders decreased with increasing Ag content up to 1 wt% and it then increased with a further increase in the Ag content. The dense fine-grained ceramics with submicrometer grains (∼300 nm) were obtained with 1 wt% Ag addition. The submicrometer-grained ceramics had excellent dielectric properties of ɛ_r∼81 and Q × f ∼11 000 GHz. Both the dielectric constant and dielectric loss significantly increased with large additions (>3 wt%) of Ag due to the percolation effect.     

Modification of MgAl2O4 Microwave Dielectric Ceramics by Zn Substitution

MgAl₂O₄ microwave dielectric ceramics were modified by Zn substitution for Mg, and their dielectric characteristics were evaluated, along with their structures. Dense (Mg_{1− x} Zn _x )Al₂O₄ ceramics were obtained by sintering at 1550°–1650°C in air for 3 h, and the (Mg_{1− x} Zn _x )Al₂O₄ solid solution was determined in the entire composition range. With Zn substitution for Mg, the dielectric constant ɛ of MgAl₂O₄ just varied from 7.90 to 8.56, while the Q × f value had significantly improved up to a maximal value of 106 000 GHz at x =1.0. Moreover, the τ_f of MgAl₂O₄ ceramics had declined from −73 to −63 ppm/°C.