Effect of Li2CO3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Mg2V2O7 Ceramics

Li₂CO₃ was added to Mg₂V₂O₇ ceramics in order to reduce the sintering temperature to below 900°C. At temperatures below 900°C, a liquid phase was formed during sintering, which assisted the densification of the specimens. The addition of Li₂CO₃ changed the crystal structure of Mg₂V₂O₇ ceramics from triclinic to monoclinic. The 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic was well sintered at 800°C with a high density and good microwave dielectric properties of ɛ _r=8.2, Q × f =70 621 GHz, and τ_f=−35.2 ppm/°C. Silver did not react with the 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic at 800°C. Therefore, this ceramic is a good candidate material in low-temperature co-fired ceramic multilayer devices.     

The Role of Li2CO3 and PbO in the Low-Temperature Sintering of Sr, K, Nb (SKN)-Doped PZT

It is demonstrated that the use of ∼0.9 mol% Li₂CO₃ (LC) as a sintering aid for Sr, K, Nb modified Pb(Zr_{1− x} ,Ti _x )O₃ (PZT) ceramics is effective only in the presence of excess PbO (∼2 mol%). It is shown that LC and PbO react to form the compound, Li₂PbO₃ (LPO) which has a melting temperature of ∼836°C. Using dilatometry, we were able to correlate shrinkage during heating of a green ceramic to the melting of the LPO. Consequently, complete densification and sizeable grain growth are achieved by solution-precipitation of the ceramic through the liquid phase. Importantly, sintering is not particularly effective with such small additions of either LC or PbO alone. In confirmation of this model, the LPO compound was presynthesized and used as the only sintering aid in the same PZT composition. The densification behavior of this mixture compared well with the case of separate additions.     

The Li2O–Nb2O5–TiO2 Composite Microwave Dielectric Ceramics with Adjustable Permittivities

A group of new y M-phase/(1− y ) Li_{2+ x} Ti_{1−4 x} Nb_{3 x} O₃ composite ceramics with adjustable permittivities for low-temperature co-fired ceramic applications was initially investigated in the study. The 0.5 M-phase/0.5 Li_{2+ x} Ti_{1−4 x} Nb_{3 x} O₃ ( x =0.01, 0.02, 0.04, 0.06, 0.081) composite ceramics were first investigated to find the appropriate "Li₂TiO₃ss" composition ( x value). The best dielectric properties of ɛ_r=40.1, Q × f values up to 9318 GHz, τ_f=25 ppm/°C, were obtained for the ceramics composites at x =0.02. Based on the good dielectric properties, the suitable "Li₂TiO₃ss" composition with x =0.02 was mixed with the Li_1.0Nb_0.6Ti_0.5O₃ powder as the ratio of y "M-phase"/(1− y ) "Li₂TiO₃ss" ( y =0.2, 0.4, 0.5, 0.6, 0.8). By adjusting the y values, the group of composite ceramics could exhibit largely are adjustable permittivities varying from ∼20 to ∼60, while Q × f and τ_f values relatively good. Nevertheless, in this study, because there are interactions between the M-phase and Li₂TiO₃ss during sintering process, their microwave dielectric properties could not be predicted precisely by the empirical model.     

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.     

Effect of V2O5 Doping on the Sintering and Dielectric Properties of M-Phase Li1+x−yNb1−x−3yTix+4yO3 Ceramics

The effect of the addition of V₂O₅ on the structure, sintering and dielectric properties of M -phase (Li_{1+ x − y} Nb_{1− x −3 y} Ti _x +4 _y )O₃ ceramics has been investigated. Homogeneous substitution of V⁵⁺ for Nb⁵⁺ was obtained in LiNb_{0.6(1− x )}V_{0.6 x} Ti_0.5O₃ for x ≤ 0.02. The addition of V₂O₅ led to a large reduction in the sintering temperature and samples with x = 0.02 could be fully densified at 900°C. The substitution of vanadia had a relatively minor adverse effect on the microwave dielectric properties of the M -phase system and the x = 0.02 ceramics had [alt epsilon]_r= 66, Q × f = 3800 at 5.6 GHz, and τ_f= 11 ppm/°C. Preliminary investigations suggest that silver metallization does not diffuse into the V₂O₅-doped M -phase ceramics at 900°C, making these materials potential candidates for low-temperature cofired ceramic (LTCC) applications.     

Congruent Composition and Li-Rich Phase Boundary of LiNbO3

The congruent composition of LiNbO₃ was determined precisely by correlating crystal growth compositions with the Curie temperatures of samples quenched from a melt before and after crystal growth and of various sections of crystals grown from that melt. The initial melt composition was determined by control of Li₂CO₃ and Nb₂O₅ contents. The melt compositions were varied from 47 to 49 mol% Li₂O. The variation of T_e with melt composition was found to follow T_e = 9095.2 − 369.05C + 4.228C², where C is mol% Li₂O. High-temperature DTA was used to determine T_e with a precision of ±2°C. Above 1000°C the Li-rich phase boundary was found to be a function of temperature by determining the weight gain after complete lithiation of congruently grown crystals. This boundary curves slightly toward lower Li₂O content as temperature increases. The congruent composition of LiNbO₃ contains 48.45 mol% Li₂O and has a measured Curie temperature of 1138°± 2°C.     

Microwave Dielectric Properties of Li2TiO3 Ceramics Doped with ZnO–B2O3 Frit

A type of new low sintering temperature ceramic, Li₂TiO₃ ceramic, has been found. Although it is difficult for the Li₂TiO₃ compound to be sintered compactly at temperatures above 1000°C for the volatilization of Li₂O, dense Li₂TiO₃ ceramics were obtained by conventional solid-state reaction method at the sintering temperature of 900°C with the addition of ZnO–B₂O₃ frit. The sintering behavior and microwave dielectric properties of Li₂TiO₃ ceramics with less ZnO–B₂O₃ frit (≤3.0 wt%) doping were investigated. The addition of ZnO–B₂O₃ frit can lower the sintering temperature of the Li₂TiO₃ ceramics, but it does not apparently degrade the microwave dielectric properties of the Li₂TiO₃ ceramics. Typically, the good microwave dielectric properties of ɛ_r=23.06, Q × f =32 275 GHz, τ_f = 35.79 ppm/°C were obtained for 2.5 wt% ZnO–B₂O₃ frit-doped Li₂TiO₃ ceramics sintered at 900°C for 2 h. The porosity was 0.08%. The Li₂TiO₃ ceramic system may be a promising candidate for low-temperature cofired ceramics applications.     

Crystal Chemistry and Phase Relations in the System Li2O-Fe2O3-TiO2

Above 755°C, compounds along the spinel join LiFe₅O₈-Li₄Ti₅O₁₂ form a complete solid solution and below that temperature a two-phase region separates the ordered LiFe₅O₈ and the disordered spinel phase. At 800° and 900°C, cubic LiFeO₂ ( ss ) and monoclinic Li_zTi0₃ ( ss ) exist on the monoxide join LiFeO₂-Li₂TiO₃. The distributions of cations in both the spinel and monoxide structures were calculated as a function of equilibrium temperature and composition. Sub-solidus equilibria in the system Li₂O-Fe₂O₃-TiO₂ at 800° and 900°C were determined for compositions containing ∼50 mol% Li₂O.     

Effect of Bi2O3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Zn2SiO4 Ceramics

Bi₂O₃ was added to a nominal composition of Zn_1.8SiO_3.8 (ZS) ceramics to decrease their sintering temperature. When the Bi₂O₃ content was <8.0 mol%, a porous microstructure with Bi₄(SiO₄)₃ and SiO₂ second phases was developed in the specimen sintered at 885°C. However, when the Bi₂O₃ content exceeded 8.0 mol%, a liquid phase, which formed during sintering at temperatures below 900°C, assisted the densification of the ZS ceramics. Good microwave dielectric properties of Q × f =12,600 GHz, ɛ_r=7.6, and τ_f=−22 ppm/°C were obtained from the specimen with 8.0 mol% Bi₂O₃ sintered at 885°C for 2 h.     

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.     

Pseudobinary Phase Relations of Li2Ti3O7

The Li₂O-TiO₂ pseudobinary phase diagram was determined from 50 to 100 mol% TiO₂ by DTA, microscopy, and X-ray analysis; Li₂Ti₃O₇ effectively melts congruently at 1300° and decomposes eutectoidally at 940°C. A solid solution based on Li₂TlO₃ from 50 to ∼65 mol% TiO₃ was observed to exist at >930°C. A new metastable phase was discovered with a composition of ∼75 mol% TiO₂ and with a hexagonal unit cell (8.78 by 69.86 × 10⁻¹nm). Discrepancies in the literature regarding some of these phase equilibria are reconciled.     

Low Temperature Coefficient of Resonance Frequency Composition in the System Pb(Zr,Ti)O3—Pb(Mn1/3 Nb2/3)O3

Pb(Zr,Ti)O₃–Pb(Mn_1/3 Nb_2/3)O₃ (PZT–PMnN) system has been studied for high-power piezoelectric applications. This study investigates this system to find out the composition with high-power density piezoelectric characteristics and low tem-perature coefficient of resonance frequency (TCF). It was found that the composition 0.9PZT–0.1PMnN (Zr/Ti = 0.51/0.49) modified with 6 mol% Sr exhibits a TCF of −8 ppm/°C (−20 to +80°C). Further, the dielectric and piezoelectric properties of this composition are as follows: k _p= 0.53; Q _m= 800; d ₃₃= 274; ε₃₃/ε₀= 1290 and tan δ=1.1%, which shows the suitability of this composition for ultrasonic devices used under fluctuating thermal environment.     

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.     

Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics: I. Substitution of Ba3W2O9

A narrow region of Zn-vacancy-containing cubic perovskites was formed in the (1− x )Ba₃(ZnNb₂)O₉−( x )Ba₃W₂O₉ system up to 2 mol% substitution ( x =0.02). The introduction of cation vacancies enhanced the stability of the 1:2 B-site ordered form of the structure, Ba(Zn_{1− x} □ _x )_1/3(Nb_{1− x} W _x )_2/3O₃, which underwent an order–disorder transition at 1410°C, ∼35° higher than pure Ba(Zn_1/3Nb_2/3)O₃. The Zn vacancies also accelerated the kinetics of the ordering reaction, and samples with x =0.006 comprised large ordered domains with a high lattice distortion ( c/a =1.226) after a 12 h anneal at 1300°C. The tungstate-containing solid solutions can be sintered to a high density at 1390°C, and the resultant ordered ceramics exhibit some of the highest microwave dielectric Q factors ( Q × f =1 18 000 at 8 GHz) reported for a niobate-based perovskite.     

Fabrication of Thin-Film SrCe0.9Eu0.1O3−δ Hydrogen Separation Membranes on Ni–SrCeO3 Porous Tubular Supports

SrCe_0.9Eu_0.1O_3−δ thin-film (∼30 μm) tubular hydrogen separation membranes were developed in order to obtain high hydrogen fluxes. Fifteen centimeters long, one end closed, NiO–SrCeO₃ tubular supports were fabricated by tape casting, followed by rolling the green tape on a circular rod. SrCe_0.9Eu_0.1O_3−δ powders were prepared by the citrate process and coated on partially sintered NiO–SrCeO₃ tubular supports. Leakage-free hydrogen membrane cells were obtained by adjusting the presintering and final sintering temperatures to reduce the difference of linear shrinkage rates between SrCe_0.9Eu_0.1O_3−δ thin films and NiO–SrCeO₃ supports. A hydrogen flux of 2.2 cm³/min was obtained for the SrCe_0.9Eu_0.1O_3−δ on Ni–SrCeO₃ tubular hydrogen separation membranes at 900°C using 25% H₂ balanced with Ar and 3% H₂O as the feed gas and He as the sweep gas. Thus, a 40% single pass yield of pure H₂ was achieved with this membrane.     

Low-Temperature Firing and Microwave Dielectric Properties of Ca[(Li1/3Nb2/3)0.84Ti0.16]O3−δ Ceramics for LTCC Applications

The effects of LiF and ZnO–B₂O₃–SiO₂ (ZBS) glass combined additives on phase composition, microstructures, and microwave dielectric properties of Ca[(Li_1/3Nb_2/3)_0.84Ti_0.16]O_3−δ (CLNT) ceramics were investigated. The LiF and ZBS glass combined additives lowered the sintering temperature of CLNT ceramics effectively from 1150° to 880°C. The main diffraction peaks of all the specimens split due to the coexistence of the non-stoichiometric phase (A) and stoichiometric phase (B), which all possess CaTiO₃-type perovskite structures. The transformation from A into B became accelerated with the increase of LiF or ZBS content. ZBS glass restrained the volatilization of lithium salt, which greatly affected the microstructures and microwave dielectric properties. CLNT ceramics with 2 wt% LiF and 3 wt% ZBS sintered at 900°C for 2 h show excellent dielectric properties: ɛ_r=34.3, Q × f =17 400 GHz, and τ_f=−4.6 ppm/°C. It is compatible with Ag electrodes, which makes it a promising ceramic for low-temperature cofired ceramics technology application.     

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.     

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.     

Sodium Carbonate Flux Effects on the Luminescence Characteristics of (Y0.5Gd0.5)2O3:Eu Phosphor Particles Prepared by Spray Pyrolysis

Na₂CO₃ flux was introduced in the preparation of phosphor particles by spray pyrolysis to improve the photoluminescence (PL) characteristics of (Y_0.5Gd_0.5)₂O₃:Eu phosphor particles. The phosphor particles directly prepared by spray pyrolysis at 1300°C from solutions with 20 wt% Na₂CO₃ flux had the highest PL intensity, which corresponded to 130% of that of particles prepared from solution without flux. On the other hand, the maximum PL intensity of the annealed particles, which were as-prepared at 900°C and posttreated at 1200°C for 3 h, was obtained from a solution with 5 wt% Na₂CO₃ flux. The maximum PL intensity of particles directly prepared by spray pyrolysis without posttreatment was 86% of that of posttreated phosphor particles. Na₂CO₃ flux was also important in control of morphology of (Y_0.5Gd_0.5)₂O₃:Eu phosphor particles.     

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.