A New Low-Loss Microwave Dielectric Ceramic Sr4LaTiNb3O15

A high dielectric constant and low-loss ceramic with composition Sr₄LaTiNb₃O₁₅ has been prepared by the conventional solid-state ceramic route. This compound adopts an A₅B₄O₁₅ cation-deficient hexagonal perovskite structure and crystallizes in the trigonal system with unit cell parameters a =5.6307(2), c =11.3692(3) Å, V =312.16(2) ų, and Z =1. The dielectric properties of dense ceramics sintered in air at 1460°C have been characterized at microwave frequencies. The results show that the material affords a relatively high dielectric constant ɛ_r∼43, a high quality factor Q × f ∼44 718 GHz, and a low temperature coefficient of resonant frequency TC_f∼13 ppm/°C.     

Sr3LaNb3O12: A New Low Loss and Temperature Stable A4B3O12-Type Microwave Dielectric Ceramic

The microwave dielectric properties of dense ceramics of a new A₄B₃O₁₂ type cation-deficient hexagonal perovskite Sr₃LaNb₃O₁₂ are reported. Single-phase powders can be obtained from the mixed-oxide route at 1320°C and dense ceramics (>96% of the theoretical X-ray density) with uniform microstructures (5–12 um) can be obtained by sintering in air at 1430°C. The ceramic exhibits a moderate dielectric constant ɛ_r∼36, a high quality factor Q × f ∼45 327 GHz, and a low temperature coefficient of resonant frequency τ _f of −9 ppm/°C.     

High-Energy Density Capacitors Utilizing 0.7 BaTiO3–0.3 BiScO3 Ceramics

A high, temperature-stable dielectric constant (∼1000 from 0° to 300°C) coupled with a high electrical resistivity (∼10¹²Ω·cm at 250°C) make 0.7 BaTiO₃–0.3 BiScO₃ ceramics an attractive candidate for high-energy density capacitors operating at elevated temperatures. Single dielectric layer capacitors were prepared to confirm the feasibility of BaTiO₃–BiScO₃ for this application. It was found that an energy density of about 6.1 J/cm³ at a field of 73 kV/mm could be achieved at room temperature, which is superior to typical commercial X7R capacitors. Moreover, the high-energy density values were retained to 300°C. This suggests that BaTiO₃–BiScO₃ ceramics have some advantages compared with conventional capacitor materials for high-temperature energy storage, and with further improvements in microstructure and composition, could provide realistic solutions for power electronic capacitors.     

Dielectric Behavior of Na0.5Bi0.5TiO3-Based Composites Incorporating Silver Particles

The dielectric properties of Na_0.5Bi_0.5TiO₃ (NBT) -based composites incorporating silver particles prepared by sintering at a low temperature of ∼900°C are reported. The dielectric constant increases with the amount of metal silver particles in the measured frequency range (150 Hz to 1 MHz), and could be enhanced up to ∼20 times higher than that of pure NBT ceramics, which was ascribed to the effective electric fields developed between the dispersed particles in the matrix and the percolation effect. Further investigation revealed that the dielectric constant of the composites has weak frequency and temperature dependence (−50°C to +50°C).     

Synthesis and Dielectric Properties of Solution Sol-Gel-Derived 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 Ceramics

A solution sol-gel method has been developed to prepare 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (0.9PMN-0.1PT) ceramics. During the processing the gel first converted to cubic pyrochlore phase at a calcination temperature of 600°C followed by the formation of pure perovskite phase at 775°C. The ceramics sintered at 1250°C for 4 h showed ≈98% of the theoretical density. The room-temperature dielectric constant of the pellets sintered at 1250°C showed a maximum value of 25035 at 1 kHz. Sintering studies at different temperatures revealed that the dielectric constant increased with increasing grain size in these ceramics.     

Preparation, Characterization, and Properties of Dy-Doped Small-Grained BaTiO3 Ceramics

The effects of Dy doping and sintering parameters on the properties of BaTiO₃ ceramics were studied. The average grain size decreases with increasing Dy content and is controlled at ∼1.5 μ m by 0.8 at.% Dy. The Curie temperature change, associated with ≤1.2 at.% Dy, is <3°C. The dielectric constant is ∼2600 for specimens doped with 0.8 at.% Dy, calcined at 1200°C, and sintered at 1450°C. The dielectric constant variation with ambient temperature is much less than that of conventional BaTiO₃ ceramics. Lattice constant c decreases with increasing Dy concentration whereas a increases slightly. The effects of grain size on dielectric constant, lattice parameters, and linear thermal expansion coefficient are more pronounced than the chemical effects of Dy doping in the ferroelectric state, whereas in the paraelectric state, these characteristics are almost independent of grain size as well as Dy concentration. The decrease in the frequency of occurrence of 90° twins with decreasing grain size implies that internal stress, which develops when BaTiO₃ ceramics are cooled below T_c , strongly influences the effects of grain size.     

Microwave Dielectric Properties of La5CrTi3O15 and La4MCrTi3O15 (M=Pr, Nd, and Sm) Ceramics

The La₅CrTi₃O₁₅ and La₄MCrTi₃O₁₅ (M=Pr, Nd, and Sm) microwave dielectric ceramics were prepared by the conventional solid-state ceramic route. The structure and microstructure of the ceramics were studied by X-ray diffraction and scanning electron microscopy methods. The dielectric properties of the ceramics were measured in the microwave frequency region using a network analyzer by the resonance method. The ceramics show a dielectric constant (ɛ_r) in the range of 37 to 39.5, a quality factor ( Q _u× f _o) 17,300 to 34,000 GHz, and a temperature coefficient of resonant frequency (τ_f) in the range from −22 to −38 ppm/°C.     

Microwave Dielectrics in the (La1/2Na1/2)TiO3–Ca(Fe1/2Nb1/2)O3 System

Dielectric properties of the system (1 − x)(La_1/2Na_1/2)TiO₃– _x Ca(Fe_1/2Nb_1/2)O₃, where 0.4 # x # 0.6, have been investigated at microwave frequencies. The temperature coefficient of resonant frequency (τ_f), nearly 0 ppm/°C, was realized at x = 0.58. These ceramics had perovskite structure and showed relatively low dielectric losses. A new dielectric material applicable to microwave devices having Q · f of 12000–14000 GHz and a dielectric constant (ε_r) of 59–60 has been obtained at 1300–1350°C for 5–15 h sintering.     

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.     

Effect of MnO on the Phase Development, Microstructures, and Dielectric Properties of 0.95Na0.5K0.5NbO3–0.05LiTaO3 Ceramics

Lead-free piezoelectric ceramics in the system 0.95Na_0.5K_0.5NbO₃–0.05LiTaO₃ were modified with ≤1 mol% MnO. Maximum densities occurred at a sintering temperature of 1050°C. Characteristic changes in the relative intensity of X-ray diffraction peaks were consistent with Mn ions substituting on the perovskite lattice to produce a change from orthorhombic to a mixture of tetragonal and orthorhombic phases. Grain growth during secondary recrystallization was also affected, leading to increased grain sizes. The dielectric constant increased from ∼600 in unmodified ceramics to ∼1040 in ceramics prepared with 0.5 mol% MnO.     

Microwave Dielectric Properties of Hexagonal 12R-Ba3LaNb3O12 Ceramics

The dielectric properties of dense ceramics of the n =0 member of a newly identified homologous series Ba_{3+ n} LaNb₃Ti _n O_{12+3 n} , where n =0, 1, and 2, are reported. Single-phase powders can be obtained from the mixed-oxide route at 1350°C and dense ceramics (>97% of the theoretical X-ray density) with uniform microstructures (3–5 μm) can be obtained by sintering in air at 1500°C. The ceramics are excellent dc insulators with a band gap >2.6 eV that resonate at microwave frequencies with a relative permittivity, ɛ_r∼44, a quality factor, Q × f _r, of ∼9000 at f _r∼5.5 GHz and a temperature coefficient of resonant frequency, TC_f,∼−100 ppm/K.     

Influence of Processing Parameters on the Sintering and Electrical Properties of Pb(Zn1/3Nb2/3)O3-Based Ceramics

Pb(Zn_1/3Nb_2/3)O₃-based ceramics have been prepared by two different processing methods: conventional (PZN-C) and reaction-sintering (PZN-RS). The conventionally prepared PZN-based ceramics densified at lower temperatures (950°C) than the reaction-sintered samples (1100°C), but the perovskite/pyrochlore ratio was always higher in PZN-RS. The presence of a substantial amount of pyrochlore phase in PZN-C ceramics caused a decrease in the electrical properties. The maximum dielectric constant values in PZN-C ceramics were 10%–15% lower than those of PZN-RS, despite a similar average grain size, 7 ± 0.2 μm. The temperature of the maximum of the dielectric constant ( T _max) was lower than that expected from the mixing rule because of the possible formation of Ba–Nb clusters. The higher chemical homogeneity in PZN-RS ceramics is the main reason for the higher dielectric constant, T _max and electromechanical response, as well as for the lower difference between T _max and the depolarization temperature ( T _d) and the lower diffusiveness parameter (δ).     

Crystal Structure and Dielectric Properties of LaYbO3

The crystal structure and dielectric properties of LaYbO₃ ceramics prepared by the mixed-oxide route have been investigated. Rietveld refinements performed on X-ray and neutron diffraction data show the room-temperature structure to be best described by the orthorhombic Pnma space group [ a =6.02628(9) Å, b =8.39857(11) Å, and c =5.82717(7) Å; Z =4, and theoretical density, D _x =8.1 g/cm³] in agreement with electron diffraction experiments. LaYbO₃ ceramics fired at 1600°C for 4 h attain ∼97% of D _x and their microstructures consist of randomly distributed equiaxed grains with an average size of ∼8 μm. Conventional transmission electron microscopy shows densification to occur in the absence of a liquid phase and reveals domain-free grains. The relative permittivity, ɛ_r, of LaYbO₃ ceramics at radio frequencies is ∼26 in the range ∼10–300 K; however, a small dielectric anomaly is detected at ∼15 K. At room temperature and microwave frequencies, LaYbO₃ ceramics exhibit ɛ_r∼26, Q × f _r∼20 613 GHz (at 7 GHz), and τ_f∼−22 ppm/K. Q × f _r show complex subambient behavior, decreasing from a plateau value of ∼20 000 GHz between ∼300 and 200 K to a second plateau value of ∼6000 GHz at ∼90 K before decreasing to <1000 GHz at ∼10 K. The large decrease in Q × f _r at low temperature may be related to the onset of antiferromagnetism at ∼2.7 K. ¹     

Fabrication of Textured Bi3NbTiO9 Ceramics

Highly textured Bi₃NbTiO₉ ceramics are fabricated by normal sintering from molten salt-synthesized plate-like crystallites. Fine Bi₃NbTiO₉ plate-like crystallites (∼1 μm) not only facilitate the densification, but also enhance texture in Bi₃NbTiO₉ ceramics. Weak-agglomerated platelets exhibit higher sinterability and can be densified at a temperature as low as 1000°C, which is about 100°C lower than that of equiaxed powders prepared by directly calcining Bi₃NbTiO₉ precursor. Meanwhile, the orientation degree of textured Bi₃NbTiO₉ ceramics increases with sintering temperature. Highly oriented Bi₃NbTiO₉ (orientation degree of ∼0.91) ceramic with a relative density of ∼92% is obtained at 1150°C. Because of the oriented grain microstructure, textured Bi₃NbTiO₉ ceramic exhibits anisotropic electrical properties.     

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.     

Electrical Properties of Sr0.7Ba0.3TiO3 Ceramics Doped with Nb2O5, 3Li2O · 2SiO2, and Bi2O3

The electrical properties of Sr_0.5Ba_0.3TiO₃ in the presence of Nb₂O₅ as a donor, 3Li₂O · 2SiO₂ as a sintering agent, and Bi₂O₃ as a dopant have been studied. When the compositions of the ceramics were 1 mol Sr_0.7Ba_0.3TiO₃+ 0.5 mol% Nb₂O₅+ 2 mol% 3Li₂O · 2SiO₂+ 0.2 mol% Bi₂O₃, the ceramics were sintered at 1100°C and exhibited the following characteristics: apparent dielectric constant ɛ, 25000; loss factor tan δ, 2%; insulating resistivity ρ_j, 10¹⁰Ω· cm; variation of dielectric constant with temperature Δɛ/ɛ (−25° to +85°C), +10%, −14%. ɛ and tan δ show only small changes with frequency. The study shows this ceramic can be used in multilayer technology.     

Single-Calcination Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 and Pb(Mg1/3Nb2/3)O3 Ceramics Using a Coating Method

A coating approach for synthesizing 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃ (0.9PMN–0.1PT) and PMN using a single calcination step was demonstrated. The pyrochlore phase was prevented by coating Mg(OH)₂ on Nb₂O₅ particles. Coating of Mg(OH)₂ on Nb₂O₅ was done by precipitating Mg(OH)₂ in an aqueous Nb₂O₅ suspension at pH 10. The coating was confirmed using optical micrographs and zeta-potential measurements. A single calcination treatment of the Mg(OH)₂-coated Nb₂O₅ particles mixed with appropriate amounts of PbO and PbTiO₃ powders at 900°C for 2 h produced pyrochlore-free perovskite 0.9PMN–0.1PT and PMN powders. The elimination of the pyrochlore phase was attributed to the separation of PbO and Nb₂O₅ by the Mg(OH)₂ coating. The Mg(OH)₂ coating on the Nb₂O₅ improved the mixing of Mg(OH)₂ and Nb₂O₅ and decreased the temperature for complete columbite conversion to ∼850°C. The pyrochlore-free perovskite 0.9PMN–0.1PT powders were sintered to 97% density at 1150°C. The sintered 0.9PMN–0.1PT ceramics exhibited a dielectric constant maximum of ∼24 660 at 45°C at a frequency of 1 kHz.     

Preparation and Characterization of Relaxor Ferroelectric 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 by a Polymerizable Complex Method

A modified polymerizable complex (PC) method for the preparation of the relaxor ferroelectric 0.65Pb(Mg_1/3Nb_2/3)O₃–0.35PbTiO₃ (PMN–PT) ceramics has been developed using a novel water-soluble Nb precursor. The effects of Pb content and sintering temperature on the structure, morphology, composition, and electrical properties of PMN–PT powders and ceramics were investigated systematically. It was found that the modified PC method could effectively reduce the initial crystallization temperature of the perovskite phase to 500°C. For PMN–PT samples with 15% excess Pb content sintered at 600°C for 2 h, the 87% perovskite phase can be achieved, which is much higher than that in conventional solid-state reactions and other solution-based methods at the same temperature. On further increasing the sintering temperature to 1100°C, the perovskite phase content basically remains constant. This is attributed to the Pb-deficient pyrochlore phase formation. On increasing the sintering temperature to 1250°C, the dielectric constant and remnant polarization of PMN–PT ceramics significantly improved due to the larger grain sizes, enhanced density, and the decreasing pyrochlore phase. PMN–PT ceramics with a 98.5% content of the perovskite phase have been fabricated at 1250°C. It displays typical ferroelectric relaxor characteristics with a remnant polarization of 18 μC/cm², a coercive field of 9.6 kV/cm, a piezoelectric coefficient of d ₃₃=360 pC/N, and room-temperature and maximum dielectric constants of 3600 and 10 500 at 1 kHz, respectively.     

Microstructures and Microwave Dielectric Characteristics of Ca(Zn1/3Nb2/3)O3 Complex Perovskite Ceramics

Ca(Zn_1/3Nb_2/3)O₃ microwave dielectric ceramics were prepared using a solid-state reaction process, and their microwave dielectric properties were evaluated as functions of sintering and postdensification annealing conditions. The relationship between microwave dielectric properties and processing was interpreted through the variation of microstructures. The dielectric constant showed slight variation with sintering and annealing conditions, but the Q × f value increased at first and then decreased with increased sintering temperature, and annealing in oxygen indicated significant improvement in Q × f , especially for the specimens sintered at higher temperatures. The good microwave dielectric properties were obtained in the ceramics sintered at 1225°C in air for 3 h and annealed at 1100°C in oxygen for 8 h: ɛ= 34.1, Q × f = 15 890 GHz, τ_f=−48 ppm/°C.     

Preparation of PbZrO3–PbTiO3 Ferroelectric Thin Films by the Sol–Gel Process

Ferroelectric films, PbZr _x Ti_{1− x} O₃ ( x = 0 to 0.6), have been prepared from corresponding metal alkoxides partially stabilized with acetylacetone through the sol-gel process. The films dip-coated in an ambient atmosphere were heat-treated at 400°C for decomposition of residual organics and then at temperatures between 500° and 700°C for crystallization of the films. The perovskite phase precipitated at temperatures above 560°C, accompanied by an increase in dielectric constant. The dielectric constant of the films, which was comparable with that of sintered bodies, showed a maximum (∼620) at around x = 0.52 in PbZr _x Ti_{1− x} O₃. These films showed D – E hysteresis, with slightly higher values of coercive field, compared with those of sintered bodies.