Synthesis and Characterization of a New Sillenite Compound—Bi12(B0.5P0.5)O20

A new sillenite compound, Bi₁₂(B_0.5P_0.5)O₂₀, was synthesized using a solid-state reaction method. The stoichiometry was confirmed by XRD analyses, microstructural investigations, and quantitative elemental analysis. An investigation of the dielectric properties at frequencies from 100 Hz to 1 MHz revealed a broad, highly frequency-dispersive, relaxor-like dielectric anomaly, which appeared in the temperature range of −80°–100°C. The permittivity, Q × f value, and temperature coefficient of the resonant frequency, measured at ∼5.5 GHz, were determined to be 37.4, 850 GHz, and −19 ppm/K, respectively.     

Effects of Calcium Substitution on the Structural and Microwave Dielectric Characteristics of [(Pb1−xCax)1/2La1/2](Mg1/2Nb1/2)O3 Ceramics

The effects of calcium substitution on the structural and microwave dielectric characteristics of [(Pb_{1− x} Ca _x )_1/2La_1/2](Mg_1/2Nb_1/2)O₃ ceramics (with x = 0.01–0.5) were investigated. All the materials were observed to have an ordered A(B_1/2^'B_1/2^")O₃-type perovskite structure; however, the space group of the structure changed from Fm 3 m to Pa 3 as the calcium content increased to x = 0.1, and then from Pa 3 to R 3¯ at the x = 0.5 composition. During the structural evolution, the lattice parameter of the perovskite cell decreased linearly, and the dielectric constant ( k ) also decreased, from k = 80 to k = 38. However, the product of the quality factor and the resonant frequency ( Q × f ) increased from 50 000 GHz to 90 000 GHz as the calcium content increased. Also, the temperature coefficient of resonant frequency (τ_ƒ) gradually changed from 120 ppm/°C to −40 ppm/°C as the calcium content increased. At the x = 0.3 composition, a combination of properties— k ∼ 50, Q × f ∼ 86 000 GHz, and τ_ƒ∼ 0 ppm/°C—can be obtained.     

Microwave Dielectric Properties of a New A5B4O15-Type Cation-Deficient Perovskite Ba2La3Ti3TaO15

High dielectric constant and low loss ceramics with composition Ba₂La₃Ti₃TaO₁₅ have been prepared by a conventional solid-state ceramic route. This compound adopts A₅B₄O₁₅ cation-deficient hexagonal perovskite structure. The dielectric properties of dense ceramics sintered in air at 1520°C have been characterized at microwave frequencies. It shows a relative dielectric constant of ∼45, quality factor Q _u× f of ∼26 828 GHz and temperature variation of resonant frequency of −0.97 ppm/°C.     

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.     

Topochemical Synthesis of Plate-Like Na0.5Bi0.5TiO3 from Aurivillius Precursor

Plate-like Na_0.5Bi_0.5TiO₃ (NBT) particles with perovskite structure were synthesized by topochemical microcrystal conversion from plate-like particles of layer-structured Na_0.5Bi_4.5Ti₄O₁₅ (NBIT) at 950°C in NaCl molten salt. As the precursors of NBT, plate-like NBIT particles were first synthesized by molten salt process by the reaction of Bi₄Ti₃O₁₂, Na₂CO₃, and TiO₂. After the topochemical reactions, layer-structured NBIT particles were transformed to the perovskite NBT platelets. NBT particles with a thickness of approximately 0.5 μm and a length of 10–15 μm retained the morphology feature of the precursor. High-aspect-ratio NBT platelets are suitable templates to obtain textured ceramics (especially NBT-based ceramics) by (reactive) template grain growth process.     

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.     

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.     

High-Temperature Strength and Creep Behavior of Melt-Infiltrated SiC-Mo≤5Si3C≤1 Composites

Molybdenum carbosilicide composites (SiC-Mo_≤5Si₃C_≤1) were fabricated via the melt-infiltration process. The fracture behavior of the composites was studied from room temperature up to 1800°C in 1 atm (∼10⁵ Pa) of argon. The bend strength of the composites slightly increased at ∼1200°C, because of the brittle-ductile transition of the intermetallic phase. The composites retained ∼90% of their room-temperature strength, even at 1700°C. Compressive creep tests were performed over a temperature range of 1760°-1850°C and a stress range of 200–250 MPa. The creep rate of the SiC-Mo_≤5Si₃C_≤1 composites was approximately an order of magnitude higher than that of reaction-bonded SiC.     

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.     

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.     

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.     

Chemical Solution Deposition of Transparent and Metallic La0.5Sr0.5TiO3+x/2 Films Using Topotactic Reduction

Metallic and transparent La_0.5Sr_0.5TiO_{3+ x /2} films were prepared by the chemical solution deposition (CSD) method using topotactic reduction processing. The use of Si powder as the reducing agent was facile and allowed easy manipulation. It was observed that metallic (resistivity at 300 K ∼2.43 mΩ cm) and transparent (∼80% transmittance at visible light) La_0.5Sr_0.5TiO_{3+ x /2} films could be obtained with an annealing temperature of 900°C, which was significantly lower than the hydrogen reduction temperature (∼1400°C). The successful preparation of metallic and transparent La_0.5Sr_0.5TiO_{3+ x /2} films using CSD has provided a feasible route for depositing other perovskite-structured functional layers on La_0.5Sr_0.5TiO_{3+ x /2} films using this low-cost all CSD method.     

Toughening and Strengthening of NiAl with Al2O3 by the Addition of ZrO2(3Y)

NiAl/10-mol%-ZrO₂(3Y) composites of almost full density have been fabricated via spark plasma sintering (SPS) for 10 min at 1300°C and 30 MPa. The former intermetallic compound, which contains a trace amount of Al₂O₃, has been prepared via self-propagating high-temperature synthesis. The composite microstructures are such that tetragonal ZrO₂ (∼0.2 μm) and Al₂O₃ (∼0.5 μm) particles are located at the grain boundaries of the NiAl (∼46 μm) matrix. Improved mechanical properties are obtained: the fracture toughness and bending strength are 8.8 MPa·m^1/2 and 1045 MPa, respectively, and high strength (>800 MPa) can be retained up to 800°C.     

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.     

Double Hysteresis Loop and Aging Effect in K0.5Na0.5NbO3–K5.4Cu1.3Ta10O9 Lead-Free Ceramics

In this work, the double-loop-like characteristics of K_0.5Na_0.5NbO₃+ x mol% K_5.4Cu_1.3Ta₁₀O₉ ceramic and its relationships with the transition temperature, aging, and switching have been investigated. Our results reveal that the phase transition temperature is an important parameter determining the aging requirement for the ceramics to exhibit the double-loop-like characteristics. For a ceramic with a high transition temperature, e.g. the ceramic with x =0.75 (tetragonal–orthorhombic phase temperature ∼206°C), the vacancies can migrate during the crystal transformation and settle in a distribution with the same symmetry as the crystal after the transformation. As a result, defect dipoles along the polarization direction are formed and provide restoring forces to reverse the switched polarizations, and thus producing a double polarization hysteresis ( P – E ) loop. On the other hand, aging is required for a ceramic with a low transition temperature, e.g. aging at 80°C for 30 days is required for the ceramic with x =1.5 (transition temperature ∼175°C). Our results also reveal that the defect dipoles can be switched under a slow-switching electric field (<1 Hz) or at high temperatures (>100°C), thus leading to an opening of the double P – E loop.     

Nanopowders of Na0.5K0.5NbO3 Prepared by the Pechini Method

A Pechini-based chemical synthesis route was used to produce powders of Na_0.5K_0.5NbO₃ (NKN). The thermochemistry of the gel was investigated using thermogravimetric analysis-fourier transform infrared (TGA-FTIR) evolved gas analysis; in addition, powder FTIR was used to analyze the gel residues after different heat treatments. The final decomposition of the organic components occurred at ∼650°C. However, hydrated–carbonated secondary phase(s) were detected by FTIR in powders that had been heated at 700°C, indicating that the NKN nanopowders are susceptible to a reaction with atmospheric moisture and carbon dioxide. The NKN particle sizes were in the range 50–150 nm after decomposition at 700°C.     

Characterization of Bi0.5Na0.5TiO3-PbTiO3M Dielectric Materials

Bi_0.5Na_0.5TiO₃ (BNT) modified with Pb showed an increased and broadened dielectric constant and limited grain growth. Pb also lowered the first transition temperature. The phases of BNT were confirmed to be ferroelectric at room temperature which transformed to antiferroelectric above 220°C. When BNT was doped with 10% Pb, the first transition decreased to 140°C and abruptly disappeared in the composition with 17% Pb. The crystal structure of 17%-Pb-doped BNT at room temperature is tetragonal, which differs from the rhombohedral structure at lower Pb contents. Thus, the phase boundary between rhombohedral and tetragonal ferroelectric phase was determined to be between 15% to 17% Pb in this study.     

Solubility of Silver and Palladium in BaTiO3

Silver, palladium, and their alloys are frequently used as electrode materials for BaTiO₃ (BT) based dielectrics. However, the electrodes and dielectrics usually are cofired at high temperatures, and silver and palladium can dissolve into the BT during cofiring. In the present study, the solubility of silver and palladium into BT after cofiring was determined. Three measurement techniques were used to determine solubility: chemical analysis, structural analysis, and dielectric analysis. The solubility of the silver in the BT was low, 450 ppm, after cofiring at 1290°C for 2 h in air. The diffusion distance of the silver ions into the BT was >5 μm. The solubility of the palladium in the BT was even lower, 50 ppm at 1290°C, and the diffusion distance was ∼1 μm. The solubility of both the silver and the palladium in the BT decreased as the oxygen partial pressure of the sintering atmosphere decreased. These results demonstrated that both silver and palladium solutes act as acceptors for BT.     

Electric Field-Dependent Dielectric Properties and High Tunability of Porous Ba0.5Sr0.5TiO3 Ceramics

Porous Ba_0.5Sr_0.5TiO₃ (BST) ceramics were fabricated by the traditional solid-state reaction process, and their structural, microstructural, dielectric, and tunability properties were systemically investigated. Compared with the fully dense BST samples, porous samples exhibit smaller grain sizes, a more uniform microstructure, and much lower dielectric constants, while at the same time, exhibiting little increase in tunability, which is beneficial to the development of microwave-tunable applications. At a frequency of 10 kHz and a temperature of 18°C, as porosity increased from 0% to 28.8%, the dielectric constant of the BST ceramics (under zero bias field) decreased from ɛ_r(0)∼1690 to ɛ_r(0)∼990, while the dielectric losses were still less than 0.2%, and the tunability increased from 17.6% to 19.6% (2.6 kV/mm).     

Dense Al2O3/ZrO2 Particulate Composites by Free Sintering of Coated Powders

Composite powders, prepared by coating coarse ZrO₂ particles with fine Al₂O₃ powder using a chemical precipitation technique, were compacted and sintered freely at a constant heating rate of 4°C/min to ∼1600°C. Composites containing up to ∼30 vol% inclusions were sintered to nearly full density under the same conditions used for the unreinforced matrix. Furthermore, the sintering kinetics were not influenced significantly by the inclusion volume fraction. The sinterability of the composites formed from the coated powders was significantly better than that for similar composites formed from mechanically mixed powders. The present data provide a further demonstration that the use of coated powders may have widespread applicability for the fabrication, by free sintering, of dense ceramic particulate composites.