Phase Evolution and Dielectric Properties of MgTiO3–CaTiO3-Based Ceramic Sintered with Lithium Borosilicate Glass for Application to Low Temperature Co-Fired Ceramics

Effects on phase evolution caused by the addition of a new sintering agent, lithium borosilicate, Li₂O·B₂O₃·SiO₂ (LBS) glass to 0.9MgTiO₃–0.1CaTiO₃ ceramic and resultant dielectric properties were investigated. The added LBS glass, a liquid phase sintering agent, significantly lowered the densification temperature from 1300° to about 950°C, while yielding decomposition of MgTiO₃ into MgTi₂O₅ and Mg₂TiO₄. At the same time, the by-products of the decomposition reaction, MgO and TiO₂, were dissolved into the glass network. Such phase evolution partly compensated the influence of deleterious glass addition so that the specimen demonstrated fairly good apparent dielectric properties.     

Microwave Characteristics of (Pb,Ca)(Fe,Nb,Sn)O3 Dielectric Materials

The dielectric properties of (Pb_{1− x} Ca _x ){(Fe_1/2Nb_1/2)_{1– y} Sn _y }O₃ solid solutions, where 0.4 lessthan equal tox ≤ 0.6, y = 0.05, 0.1, have been investigated at microwave frequencies. The replacement of Fe³⁺/Nb⁵⁺ by Sn⁴⁺ at the B–site of the perov-skite structure considerably improves the loss quality factor Q and does not remarkably affect the dielectric constant epsilon_r and the temperature coefficient of resonant frequency tau_f. The tau_f value of nearly 0 ppm/°C can be realized for x= 0.55. New high-quality dielectric ceramics having epsilon_r of 85.3-89.9,Qf values of 7510-8600 GHz, and τ_f of 0-9 ppm/°C were obtained at 1150°C for 3 h sintering in air. The influence of the sintering atmosphere on dielectric properties was also investigated.     

Low-Temperature Sintering and Microwave Dielectric Properties of Zinc Metatitanate-Rutile Mixtures Using Boron

Mixtures of zinc metatitanate and rutile (ZnTiO₃+ x TiO₂, where x = 0-0.5) have been prepared via the conventional mixed-oxide method. Centrifugal planetary milling with zirconia beads 1 mm in diameter produced very fine powders (mean particle size of 0.2 µm), which allowed the synthesis of ZnTiO₃ and sintering at temperatures <945°C, which is the decomposition temperature of ZnTiO₃. Sintering of the mixtures was enhanced further by the addition of B₂O₃. Densities of >94% of the theoretical density have been attained for the specimens that were sintered at 875°C for 4 h with B₂O₃ additions of <1 wt%. Microwave dielectric properties of the aforementioned compositions were as follows: dielectric constant of 29-31, normalized quality factor of 56000-69000 GHz, and a temperature coefficient of resonance frequency between -10 and +10 ppm/°C. Sintering was enhanced by the formation of a ZnO-B₂O₃ liquid phase, which affected the microwave properties, because of variation in the phase composition.     

Photoluminescent Properties of (Ca,Zn)TiO3:Pr,B Particles Synthesized by the Peroxide-Based Route Method

(Ca_{1− x} ,Zn _x )TiO₃:Pr, B red phosphor particles were prepared using the peroxide-based route and their photoluminescent (PL) properties were investigated by changing the sintering temperature, the concentration of the activator, the ratio of Ca to Zn, and the amount of H₃BO₃ flux. For the CaTiO₃:Pr phosphor, a pure perovksite-type CaTiO₃ phase was formed when the sintering temperature was 700°–800°C. It was found that the substitution of Zn atoms instead of Ca considerably enhanced the 614-nm red emissions. The PL intensity of (Ca_{1− x} ,Zn _x )TiO₃:Pr phosphor was also additionally improved by adding an H₃BO₃ flux. Finally, the optimized phosphor Ca_0.85Zn_0.15TiO₃:0.001Pr,0.1B showed the highest PL intensity.     

Low-Temperature Sintering and Microwave Dielectric Properties of Anorthite-Based Glass-Ceramics

TiO₂ nucleated anorthite-based glass-ceramics were fabricated from glass powders. After sintering and crystallization heat treatment, various physical properties, including apparent bulk density and water absorption, were examined to evaluate the sintering behavior of anorthite-based glass-ceramic. Results showed that the complete-densification temperature for specimens was as low as 900°C. Sufficient crystallization was achieved by subsequently raising the firing temperature to 950°C, and the dielectric quality factor was promoted to the maximum value. Contents of nucleating agent (TiO₂) played an important role in the dielectric constants. The crystallinity was controlled by raising the firing temperature at a constant heating rate. The degree of crystallization affected the dielectric properties of sintered glass-ceramics. At the resonant frequency of 10 GHz, anorthite glass-ceramics with 5 wt% TiO₂ possessed the lowest permittivity of 8 and exhibited appropriate dielectric properties as compared with those with B₂O₃ and 10 wt% TiO₂.     

Low-Temperature Fabrication of Cordierite Ceramics from Kaolinite and Magnesium Hydroxide Mixtures with Boron Oxide Additions

Thermal reactions of mixtures of ultrafine particles of magnesium hydroxide (Mg(OH)₂) and kaolinite in a composition of MgO:Al₂O₃:2SiO₂ were investigated to obtain dense cordierite ceramics at temperatures <1000°C. While heating the mixture of kaolinite and Mg(OH)₂ with the equivalent of 2 mass% of boron oxide (B₂O₃) (in the form of magnesium borate, 2MgOB₂O₃), an amorphous phase formed at a temperature of ∼850°C after thermal decomposition. Firing the mixture at a temperature of 900°C yielded dense ceramics with an apparent porosity of almost zero. The addition of B₂O₃ promoted the densification at 850°-900°C and accelerated the crystallization of alpha-cordierite. The specimen with 3 mass% of B₂O₃ that was fired at a temperature of 950°C showed a linear thermal expansion coefficient of ∼3 × 10⁻⁶ K⁻¹, a bending strength of >200 MPa, and a relative dielectric constant of 5.5 at 1 MHz. These cordierite ceramics may be used as substrate materials for semiconductor interconnection applications.     

Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 Ceramics via a Soft Mechanochemical Route

Single-phase perovskite 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (0.9PMN–0.1 PT) from a stoichiometric mixture of starting materials was synthesized by applying a mechanochemical technique to the stage of a precursor. A stoichiometric mixture of PbO, TiO₂, Mg(OH)₂, and Nb₂O₅ was milled for 60 min and heated at temperatures as low as 850°C for 4 h to obtain a single phase. The maximum dielectric constant of the samples from the milled mixture increased as the sintering temperature increased, with the remarkable grain growth, and attained 24600 at 1200°C. In contrast, poor densification and coexistence of the pyrochlore phase were observed on the samples from the nonmilled mixture. Further observation suggested that the pyrochlore phase concentrated near the surface during sintering and then migrated into the PbZrO₃ packing powder, leading to a pyrochlore–free phase at 1250°C. The dielectric constant of the latter ceramics was explained by the series mixing rule for the dielectric constant of a diphasic solid.     

Phase relations in the Barium Titanate—Titanium Oxide System

Phase relations in the BaTiO₃—TiO₂ system were studied at temperatures above 1300°C in air. Quenching experiments were performed with high-purity reagents, and a new equilibrium phase diagram was constructed. Results include redetermination of the liquidus boundaries, the eutectic temperature, the melting or decomposition temperatures of the stable compounds in the system, the cubic—hexagonal transition in BaTiO₃, and the solid solubility of TiO₂ in BaTiO₃.     

Characterization of Silver Interdiffusion into (Zn,Mg)TiO3+x:Bi:Sb Multilayer Ceramic Capacitor

In this study, (Zn,Mg)TiO_{3+ x} +Bi₂O₃+Sb₂O₅ multilayer ceramic capacitors (MLCCs) with different proportions of a silver (Ag)–palladium (Pd) mixture acting as the inner electrode were sintered at 925°C for 2 h to evaluate the effect of the inner electrode on reliability. The main results reveal that the lifetime is inversely proportional to the Ag content in the Ag/Pd inner electrode. Ag diffusion into the (Zn,Mg)TiO_{3+ x} +Bi₂O₃+Sb₂O₅ MLCC during cofiring at 925°C for 2 h and Ag migration at 140°C against 200 V are both responsible for the short lifetime of the (Zn,Mg)TiO_{3+ x} +Bi₂O₃+Sb₂O₅ MLCC, particularly the latter factor. A Bi,Sb-rich secondary phase was present at the triple junction and a small amount of Ag was detected from the second phase for a (Zn,Mg)TiO_{3+ x} +Bi₂O₃+Sb₂O₅ MLCC with a high Ag content in the inner electrode of Ag/Pd=99/01. However, this was not the case with a low Ag content in the inner electrode of Ag/Pd=90/10. This means that the Bi,Sb-rich second phase plays an important role in determining the degradation of insulation resistance due to the excessive formation of oxygen vacancies from the reaction of Ag⁺ for Bi³⁺ or Sb⁵⁺ substitution to lower the activation energy of the (Zn,Mg)TiO₃+Bi₂O₃+Sb₂O₅ dielectrics and to enhance markedly the effect of Ag migration.     

A Novel Temperature-Compensated Microwave Dielectric (1−x)(Mg0.95Ni0.05)TiO3−xCa0.6La0.8/3TiO3 Ceramics System

The microstructure and microwave dielectric properties of a (1− x )(Mg_0.95Ni_0.05)TiO₃− x Ca_0.6La_0.8/3TiO₃ ceramics system have been investigated. The system was prepared using a conventional solid-state ceramic route. In order to produce a temperature-stable material, Ca_0.6La_0.8/3TiO₃ was added for a near-zero temperature coefficient (τ_f). With partial replacement of Mg²⁺ by Ni²⁺, the dielectric properties of the (1− x )(Mg_0.95Ni_0.05)TiO₃− x Ca_0.6La_0.8/3TiO₃ ceramics can be promoted. The microwave dielectric properties are strongly correlated with the sintering temperature and the composition. An excellent Q × f value of 118,000 GHz can be obtained for the system with x =0.9 at 1325°C. For practical application, a dielectric constant (ɛ_r) of 24.61, a Q × f value of 102,000 GHz, and a temperature coefficient of resonant frequency (τ_f) of −3.6 ppm/°C for 0.85(Mg_0.95Ni_0.05)TiO₃−0.15Ca_0.6La_0.8/3TiO₃ at 1325°C are proposed. A parallel-coupled line band-pass filter is designed and simulated using the proposed dielectric to study its performance.     

Low-Temperature Sintering and Dielectric Properties of Ag(Nb,Ta)O3 Composite Ceramics

The sintering behavior and dielectric properties of perovskite Ag(Nb_{1− x} Ta _x )O₃ (0 < x < 1) solid solutions and two-phase composite assemblages were explored. A small amount of CuO (1 wt%) was used for liquid-phase sintering and led to high densification at temperatures <950°C. The temperature coefficient of capacitance, TCC, was adjusted by varying the Nb:Ta ratio within the solid-solution series and by creating composite microstructures. Two-phase assemblages consisting of Ag(Nb_3/4Ta_1/4)O₃ and Ag(Nb_1/4Ta_3/4)O₃ were synthesized to achieve a temperature-stable dielectric material for high-frequency applications. The composite dielectric with CuO addition had an average dielectric constant of 390 and a Q × f factor of 410 GHz at 2 GHz, with a stable TCC (0 to −180 ppm/°C) in the temperature range from −20° to +60°C. In addition, process compatibility with a silver conductor was confirmed by high-frequency ring-resonator measurements and microstructural characterization. The Ag(Nb_{1− x} Ta _x )O₃ solid solutions and composites are promising candidates as embedded capacitors for radio-frequency/microwave applications.     

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.     

Unique Dielectric Behavior of 0.6Pb(Ni1/2W1/2)O3·0.4PbTiO3 Derived from Mechanical Activation

0.6Pb(Ni_1/2W_1/2)O₃·0.4PbTiO₃(0.6PNW·0.4PT) of complex perovskite structure is successfully synthesized by mechanical activation of mixed oxide composition, followed by sintering at 950°C. It exhibits a considerably stable temperature dependence of dielectric constant over the wide temperature range of −120° to 20°C, although there occurs a dielectric peak at around 74°C. Raman spectroscopic studies show the coexistence of tetragonal and pseudocubic perovskite phases on sintering at 950°C, which are attributed to the inhomogeneous distribution of PbTiO₃ arising from mechanical activation. The dielectric behavior can be fine tuned by thermal annealing at 750°C, leading to phase redistribution in PNW-PT.     

Homogeneous Precipitation of TiO2 Ultrafine Powders from Aqueous TiOCl2 Solution

Crystalline TiO₂ powders were prepared by the homogeneous precipitation method simply by heating and stirring an aqueous TiOCl₂ solution with a Ti⁴⁺ concentration of 0.5 M at room temperature to 100°C under a pressure of 1 atm. TiO₂ precipitates with pure rutile phase having spherical shapes 200-400 nm in diameter formed between room temperature and 65°C, whereas TiO₂ precipitates with anatase phase started to form at temperatures >65°C. Precipitates with pure anatase phase having irregular shapes 2-5 µm in size formed at 100°C. Possibly because of the crystallization of an unstable intermediate product, TiO(OH)₂, to TiO₂ x H₂O during precipitation, crystalline and ultrafine TiO₂ precipitates were formed in aqueous TiOCl₂ solution without hydrolyzing directly to Ti(OH)₄. Also, formation of a stable TiO₂ rutile phase between room temperature and 65°C was likely to occur slowly under these conditions, although TiO₂ with rutile phase formed thermodynamically at higher temperatures.     

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.     

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.     

Low-Firing, Temperature-Stable Dielectric Compositions Based on Bismuth Nickel Zinc Niobates

Low-sintering dielectric materials with small temperature co-efficients were developed. The newly developed materials are based on the Bi₂(ZnNb₂)O₉ and Bi₃(Ni₂Nb)O₉ system with a bismuth-layered crystal structure. Preliminary results show that these materials have relatively large dielectric constants of 80 to 100, small temperature coefficients of ≤20 × 10⁻⁶/°C, and relatively large Q values of 2000 to 3000. Furthermore, these materials can be sintered at temperatures ranging from 850° to 950°C, and these low sintering temperatures might permit the use of a less expensive silver electrode. These materials compare favorably with the conventional NPO (small temperature coefficient) dielectric materials based on MgTiO₃.     

Decomposition of Al2TiO5 and Al2(1-x)MgxTi(1+x)O5 Ceramics

The decomposition of Al_{2(1- x )}Mg _x Ti_{(1+ x )}O₅ ceramics in air has been studied between 900° and 1175°C for 0 lessthan equal to x lessthan equal to 0.6. The decomposition temperature versus composition x predicted using a thermodynamic model based on the regular solution approach is in satisfactory agreement with the experimental results. The decomposition kinetics has been studied at 1100°C for x = 0, 0.1, and 0.2 and follows a nucleation and growth mechanism. Random nucleation of the reaction products is hindered by the high elastic stresses that result from the molar volume change related to decomposition because of the small chemical driving force available. Decomposition occurs only at a limited number of sites, probably associated with the presence of impurities and/or glassy phase. The decomposition products grow as nodules formed by an Al₂O₃ (+ MgAl₂O₄ for x > 0) core and a TiO₂ shell. The growth is parabolic for x = 0 and linear for x = 0.1 and 0.2. The rate-controlling step in the decomposition mechanism of pure Al₂TiO₅ ( x = 0) is the transport of Al³⁺ ions through the TiO₂-rutile phase.     

Low-Temperature Sintering (Ba0.6Sr0.4)TiO3 Thick Film Prepared by Screen Printing

(Ba_0.6Sr_0.4) TiO₃ thick films doped with glass slurry were fabricated by the screen-printing technique. The dielectric properties and the sintering mechanism were investigated. The films can be sintered at 600°C. The dielectric constant is 88 and the dielectric loss is 0.002 with a tunability of 23.86% under 100 kV/cm. Higher dielectric constant and tunability were obtained in the samples sintered at higher temperatures. The highest tunability is 61.12% under 150 kV/cm in the sample sintered at 800°C. The low sintering temperature and dielectric loss of the glass-doped thick films make them potential candidates for LTCC and microwave tunable devices.     

Oxidation Behavior of Titanium Boride at Elevated Temperatures

The oxidation behavior of dense TiB₂ specimens was investigated. Hot-pressed TiB₂ with 2.5 wt% Si₃N₄ as a sintering aid was exposed to air at temperatures between 800° and 1200°C for up to 10 h. The TiB₂ exhibited two distinct oxidation behaviors depending on the temperature. At temperatures below 1000°C, parabolic weight gains were observed as a result of the formation of TiO₂( s ) and B₂O₃( l ) on the surface. The oxidation layer comprised two layers: an inner layer of crystalline TiO₂ and an outer layer mainly composed of B₂O₃. When the oxidation temperatures were higher than 1000°C, gaseous B₂O₃ was formed along with crystalline TiO₂ by the oxidation process. In this case, the surface was covered with large TiO₂ grains imbedded in a highly textured small TiO₂ matrix.