K/Na Ratio Dependence of the Electrical Properties of [(KxNa1−x)0.95Li0.05](Nb0.95Ta0.05)O3 Lead-Free Ceramics

[(K _x Na_{1− x} )_0.95Li_0.05](Nb_0.95Ta_0.05)O₃ (K _x NLNT) ( x= 0.40–0.60) lead-free piezoelectric ceramics were prepared by conventional solid-state sintering. The effects of K/Na ratio on the dielectric, piezoelectric, and ferroelectric properties of the K _x NLNT ceramics were studied. The experimental results show that the electrical properties strongly depend on the K/Na ratio in the K _x NLNT ceramics. The K _x NLNT ( x =0.42) ceramics exhibit enhanced properties ( d ₃₃∼242 pC/N, k _p∼45.7%, k _t∼47%, T _c∼432°C, T _o−t =48°C, ɛ_r∼1040, tanδ∼2.0%, P _r∼26.4 μC/cm², E _c∼10.3 kV/cm). Enhanced electrical properties of the K _x NLNT ( x =0.42) ceramics could be attributed to the polymorphic phase transition near room temperature. These results show that the K _x NLNT ( x =0.42) ceramic is a promising lead-free piezoelectric material.     

Prominent Nanocrystallization of 25K2O·25Nb2O5·50GeO2 Glass

Some K₂O-Nb₂O₅-GeO₂ glasses are prepared, and their crystallization behaviors are examined. 25K₂O·25Nb₂O₅·50GeO₂ glass with the glass transition temperature T _g= 622°3C and crystallization onset temperature T _x= 668°3C shows a prominent nanocrystallization. The crystalline phase is K_3,8Nb₅Ge₃O_20,4 with an orthorhombic structure. The sizes of crystals in the crystallized glasses heat-treated at 630° and 720°3C for 1 h are °10 and 20–30 nm, respectively, and the crystallized glasses obtained by heat treatments at 620°-850°3C for 1 h maintain good transparency. The density of crystallized glasses increases gradually with increasing heat-treatment temperature, and the volume fraction of crystals in the sample heat-treated at 630°3C for 1 h is estimated to be ∼35%. The usual Vickers hardness and Martens hardness (estimated by nanoindentation) of 25K₂O·25Nb₂O₅·50GeO₂ glass change steeply by heat treatment at T _g, i.e., at around 35% volume fraction of nanocrystals. The present study demonstrates that the composite of nanocrystals and the glassy phase has a strong resistance against deformation during Vickers indenter loading in crystallized glasses.     

Microstructure, Piezoelectric, and Ferroelectric Properties of Bi2O3-Added (K0.5Na0.5)NbO3 Lead-Free Ceramics

Lead-free piezoelectric (K_0.5Na_0.5)NbO₃– x wt% Bi₂O₃ ceramics have been synthesized by an ordinary sintering technique. The addition of Bi₂O₃ increases the melting point of the system and improves the sintering temperature of (K_0.5Na_0.5)NbO₃ ceramics. All samples show a pure perovskite phase with a typical orthorhombic symmetry when the Bi₂O₃ content <0.7 wt%. The phase transition temperature of orthorhombic–tetragonal ( T _{O − T} ) and tetragonal–cubic ( T _C) slightly decreased when a small amount of Bi₂O₃ was added. The remnant polarization P _r increased and the coercive field E _c decreased with increasing addition of Bi₂O₃. The piezoelectric properties of (K_0.5Na_0.5)NbO₃ ceramics increased when a small amount of Bi₂O₃ was added. The optimum piezoelectric properties are d ₃₃=140 pC/N, k _p=0.46, Q _m=167, and T _C=410°C for (K_0.5Na_0.5)NbO₃–0.5 wt% Bi₂O₃ 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.     

Piezoelectric Properties of (1−x)(Na0.5K0.5)NbO3–xAgSbO3 Lead-Free Ceramics

(1− x )(Na_0.5K_0.5)NbO₃– x AgSbO₃ lead-free piezoelectric ceramics were prepared by normal sintering. The effects of the AgSbO₃ on the phase structure and piezoelectric properties of the ceramics were systematically studied. These results show that the AgSbO₃-modified (K_0.50Na_0.50)NbO₃ lead-free piezoelectric ceramics form stable solution with orthorhombic structure, and the Curie temperature and the polymorphic phase transition of the ceramics decreased with increasing AgSbO₃. The result shows that the piezoelectric properties of the ceramics strongly depend on the AgSbO₃. The ceramics with x =0.05 possess optimum properties ( d ₃₃=192 pC/N, k _p=43%, T _c=348°C, T _o−t =145°C, ɛ_r∼632, and tan δ∼3.5%). These results indicate that the ceramic is a promising candidate material for lead-free piezoelectric ceramics.     

Microstructure and Electrical Properties of [(K0.50Na0.50)0.95−xLi0.05Agx](Nb0.95Ta0.05)O3 Lead-Free Ceramics

[(K_0.50Na_0.50)_{0.95− x} Li_0.05Ag _x ](Nb_0.95Ta_0.05)O₃ (KNLNANT- x ) lead-free piezoelectric ceramics were prepared by normal sintering. Effects of the Ag content on the microstructure and electrical properties of KNLNANT- x ceramics were systematically investigated. It is found that the ceramics with x =0.03 exhibit relatively good electrical properties along with high Curie temperature: ( d ₃₃∼252 pC/N, T _c∼438°C, k _p∼45.4%, P _r∼30.1 μC/cm², E _c∼13.8 kV/cm, ɛ_r∼1030, and tan δ∼2.6%). The related mechanism for enhanced electrical properties of the ceramics was also discussed. These results show that KNLNANT-0.03 ceramic is a promising candidate material for high temperature lead-free piezoelectric ceramics.     

Structure and Properties of Low Phonon Antimony Glasses in the K2O–B2O3–Sb2O3–ZnO System

The monolithic glass-forming region of the low phonon and low softening point antimony glasses containing high Sb₂O₃ (40–75 mol%) in the novel quaternary K₂O–B₂O₃–Sb₂O₃–ZnO system has been found with the help of X-ray diffraction (XRD) analysis. The structure of a series of glasses with the general composition of (mol%) 15K₂O–15B₂O₃–(70− x )Sb₂O₃– x ZnO (where x =5–25) has been evaluated by infrared reflection spectral (FT-IRRS) analyses. All the glasses are found to possess a low phonon energy of around 600 cm⁻¹, as revealed by FT-IRRS. Their softening point ( T _s), glass transition temperature ( T _g), and coefficient of thermal expansion (CTE) have been found to vary in the ranges of 351°–379°C, 252°–273°C, and 195–218 × 10⁻⁷ K⁻¹, respectively. These properties are found to be controlled by their fundamental property, like the covalent character of the glasses, which is found to increase with an increase in Sb₂O₃ content. In addition, the devitrified glasses have been characterized by XRD and field emission scanning electron microscopy, which manifests the presence of nanozinc antimony oxide crystals with sizes of 21–43 nm. The exhibited properties have revealed that they are a new class of versatile materials.     

Phase Structure and Electrical Properties of (K0.48Na0.52)(Nb0.95Ta0.05)O3-LiSbO3 Lead-Free Piezoelectric Ceramics

(1− x )(K_0.48Na_0.52)(Nb_0.95Ta_0.05)O₃– x LiSbO₃ [(1− x )KNNT− x LS] lead-free piezoelectric ceramics were prepared by the conventional solid-state sintering method. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal phases was identified in the composition range of 0.03< x <0.05. The ceramics near the MPB exhibit a strong compositional dependence and enhanced electrical properties. The (1− x )KNNT– x LS ( x =0.04) ceramics exhibit good electrical properties ( d ₃₃=250 pC/N, k _p=45.1%, k _t =46.3%, T _c=348°C, T _{o − t} =74°C, P _r=25.9 μC/cm², E _c=10.7 kV/cm, ɛ_r∼1352, tan δ∼3%). These results show that (1− x )KNNT– x LS ceramic is a promising lead-free piezoelectric material.     

Low-Temperature Sintering of K0.5Na0.5NbO3 Ceramics

The objective of this work was to lower the sintering temperature of K_0.5Na_0.5NbO₃ (KNN) without reducing its piezoelectric properties. The KNN was sintered using 0.5, 1, 2, and 4 mass% of (K, Na)-germanate. The influence of the novel sintering aid, based on alkaline germanate with a melting point near 700°C, on the sintering, density, and piezoelectric properties of KNN is presented. The alkaline-germanate-modified KNN ceramics reach up to 96% of theoretical density at sintering temperatures as low as 1000°C, which is approximately 100°C less than the sintering temperature of pure KNN. The relative dielectric permittivity (ɛ/ɛ₀) and losses (tanδ), measured at 10 kHz, the piezo d ₃₃ coefficient, the electromechanical coupling and mechanical quality factors ( k _p, k _t, Q _m) of KNN modified with 1 mass% of alkaline germanate are 397, 0.02, 120 pC/N, 0.40, 0.44, and 77, respectively. These values are comparable to the best values obtained for KNN ceramics sintered above 1100°C.     

Crystal Growth Kinetics of 4K2O·CaO·10SiO2 from Its Own Melt and Two Other Melts in the K2O-CaO-SiO2 System

Crystal growth of 4K₂O·CaO·10SiO₂ from its own melt occurred with a faceted interface, while crystal growth from a melt ∼4 wt% richer in silica than the crystal showed dendritic morphology and growth from a melt ∼5 wt% leaner in silica showed dendritic growth at undercoolings <10°C and faceted growth at undercoolings >30°C. The crystal growth velocities in all cases were independent of time and fit well the empirical relation In [ U/(T _l- T) ^1.6] = A – Q/T , where U is velocity, A and Q are constants, and T_l , is the liquidus temperature of the melt. The crystal growth data for the three melt compositions lie together in a narrow swath when plotted using the above equation. An explanation for this behavior is proposed which suggests that the phase boundary reaction aided by nucleation at surface sites provided by screw dislocations determines the growth kinetics even when the melt and crystal have different compositions.     

The System K2O-BaO-SiO2

Phase equilibrium studies of compound formation and liquidus and solidus surfaces of the system K₂O-BaO-SiO₂ are presented. The system contains 3 ternary compounds: K₄BaSi₃O₉, K₈BaSi₁₀O₂₅, and K₂Ba₃Si₈O₂₀. Both high and low polymorphs of the third have fields on the ternary liquidus surface. Solid solution with SiO₂ depresses the high-low inversion from 1030°C at K₂Ba₃Si₈O₂₀ to 835°C at 70.2 mol% SiO₂. Data for 20 liquidus invariant points were found; 8 are thermal maxima and 12 are eutectics or peritectics. The isofracts of quenched glasses were determined.     

Dielectric Properties of (1−x)CaTiO3–xCa(Zn1/3Nb2/3)O3 Ceramic System at Microwave Frequency

The microwave dielectric properties of the (1− x )CaTiO₃– x Ca(Zn_1/3Nb_2/3)O₃ ceramic system have been investigated. The ceramic samples sintered at 1300°–1450°C for 4 h in air exhibit orthorhombic pervoskite and form a complete solid solution for different x value. When the x value increased from 0.2 to 0.8, the permittivity ɛ_r decreased from 115 to 42, the unloaded quality factor Q × f increased from 5030 to 13 030 GHz, and the temperature coefficient τ_f decreased from 336 to −28 ppm/°C. When x =0.7, the best combination of dielectric properties, a near zero temperature coefficient of resonant frequency of τ_f∼−6 ppm/°C, Q × f ∼10 860 GHz and ɛ_r∼51 is obtained.     

Phase Transitional Behavior and Piezoelectric Properties of Lead-Free (Na0.5K0.5)NbO3–(Bi0.5K0.5)TiO3 Ceramics

(1− x )(Na_0.5K_0.5)NbO₃–(Bi_0.5K_0.5)TiO₃ solid solution ceramics were successfully fabricated, exhibiting a continuous phase transition with changing x at room temperature from orthorhombic, to tetragonal, to cubic, and finally to tetragonal symmetries. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal ferroelectric phases was found at 2–3 mol% (Bi_0.5K_0.5)TiO₃ (BKT), which brings about enhanced piezoelectric and electromechanical properties of piezoelectric constant d ₃₃=192 pC/N and planar electromechanical coupling coefficient k _p=45%. The MPB composition has a Curie temperature of 370°–380°C, comparable with that of the widely used PZT materials. These results demonstrate that this system is a promising lead-free piezoelectric candidate material.     

Enhanced Ferroelectric Properties of Potassium Sodium Niobate Ceramics Modified by Small Amount of K3Li2Nb5O15

(1− x )K_0.5Na_0.5NbO₃– x K₃Li₂Nb₅O₁₅ (KNN–100 x KLN) ceramics were prepared by the solid-state reaction method. The results showed that small amount of KLN ( x ≤0.02) incorporated into the lattice and formed a single phase perovskite structure. The KLN modification lowered the phase transition temperature of orthorhombic–tetragonal ( T _O−T) and increased the Curie temperature ( T _c), but the transition temperatures remained almost unchanged in the range of x from 0.02 to 0.03. Small amount of KLN decreased the amount of defects, thus the remnant polarization increased and the coercive field decreased markedly, and the piezoelectric properties of KNN ceramics modified by small amount of KLN also enhanced markedly.     

Cassiterite Solubility in High-Silica K2O-Al2O3-SiO2 Liquids

The saturation surface of cassiterite, SnO₂, was determined for liquids in the system K₂O–Al₂O₃–SiO₂ as a function of bulk composition and temperature. At fixed K₂O/Al₂O₃ cassiterite solubility varies weakly with SiO₂ concentration (76 to 84 mol%), temperature (1350° to 1550°C), and log ( f _O2) (−0.7 to −5.3). Cassiterite solubility is also approximately independent of composition in liquids with molar ratios of K₂O/Al₂O₃ lessthan equal to 1 (peraluminous liquids). As K₂O/Al₂O₃ increases from 1 (peralkaline liquids), however, cassiterite solubility increases steeply and approximately linearly with K₂O in excess of Al₂O₃. It is proposed that potassium in excess of aluminum combines with Sn⁴⁺ to form quasi-molecular complexes with an effective stoichiometry of K₄SnO₄.     

Effect of Calcination Conditions and Excess Alkali Carbonate on the Phase Formation and Particle Morphology of Na0.5K0.5NbO3 Powders

Sodium-potassium niobate [Na_0.5K_0.5NbO₃] powders were prepared following the conventional mixed oxide method. An orthorhombic XRD pattern, consistent with single-phase Na_0.5K_0.5NbO₃, was obtained after calcination at 900°C for 6 h. Introducing 5 mol% excess Na₂CO₃ and K₂CO₃ into the starting mixture allowed milder calcination conditions to be used, for example 800°C for 2 h. Primary particles in 5 mol% excess samples were cuboid, with maximum sizes of ∼2.5 μm. Equiaxed 0.3–0.4-μm particles were formed for non-excess powders, and also for powders prepared with 1 and 3 mol% excess alkali carbonates. The results suggest liquid formation during calcination of the excess 5-mol% starting powders.     

High-Temperature Dielectrics in the BiScO3–BaTiO3–(K1/2Bi1/2)TiO3 Ternary System

Perovskite solid solution in the (1− x )[0.4BiScO₃–0.6BaTiO₃]+ x (K_1/2Bi_1/2)TiO₃ [BSBT–KBT x ] system was synthesized using conventional sintering and hot-isostatic pressing. Dielectric properties of BSBT ceramics with different dopant levels of KBT were characterized as a function of temperature and frequency for potential use of high-temperature capacitors. The BSBT ceramics with KBT exhibited high dielectric permittivities (ɛ_r) (>1700 at RT) and low dielectric loss over the temperature range from 100° to 300°C, with flat temperature coefficients of permittivity (TCɛs). In addition, BSBT ceramics with increasing KBT were observed to possess dielectric relaxation characteristics at temperatures (>RT) as observed in lead-based relaxors. Furthermore, high energy densities, being on the order of 4.0 J/cm³ at 220 kV/cm was observed for the BSBT–KBT20 ceramics from the electric-field polarization behavior.     

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.     

Oxidation of Ba2YCu3Ox at High Po2

Dense large-grain ceramic samples of Ba₂YCu₂O_x have been oxidized at 400° to 500°C in 10 to 30 MPa of oxygen for 1 to 10 d. The high-pressure treatment has increased the equilibrium content x slightly above 7 but there is no concomitant increase in T_c. At x = 7.03 the superconducting transition, measured by ac susceptibility, has an onset temperature of ∼91 K and ΔT∼4 K and no change in lattice parameter is observed. High oxygen pressure increases the rate of oxidation.     

The System K2SO4-Cs2SO4

The phase diagram for the system K₂SO₄-Cs₂SO₄ was determined by using DTA for melting relations and DTA and high-temperature X-ray diffractometry for subsolidus relations. At the solidus the system shows complete solid solubility, with a minimum at 940°C and 50 mol% Cs₂SO₄. Orthorhombic K₂SO₄ and Cs₂SO₄, the stable low-temperature forms, show mutual solid solubility and form a eutectoid at 50 mol% Cs₂SO₄ and 430°C, the lowest temperature of stability of the high-temperature hexagonal solid-solution phase. Isothermal plots of the a and c dimensions of this hexagonal phase vs composition show large positive deviations from linearity for c. These deviations are interpreted on the basis of the crystal structure of KNaSO₄ with a similar unit cell.