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-Loss Microwave Dielectric Ceramics Using (Mg1−xMnx)2TiO4 (x=0.02–0.1) Solid Solution

Composite ceramics in a solid solution of (Mg_{1− x} Mn _x )₂TiO₄ ( x =0.02–0.1) have been prepared by the mixed oxide route. Formation of the solid solution was confirmed by the X-ray diffraction, the EDX analysis, and the measured lattice parameters, which varied linearly from Mg₂TiO₄ ( a = b = c =8.4410 Å) to (Mg_0.9 Mn_0.1)₂TiO₄ ( a = b = c =8.4445 Å). The XRD analysis also confirmed the co-existence of a cubic-structured (Mg_{1− x} Mn _x )₂TiO₄ and an ilmenite-structured second phase (Mg_{1− x} Mn _x )TiO₃. The composition expected to have a maximum Q × f (276 200 GHz at 10.5 GHz) is (Mg_0.95Mn_0.05)₂TiO₄ with ɛ_r∼15.69 and τ_f∼−52.6 ppm/°C. The existence of the second phase, however, would lead to no significant variation in the dielectric properties of the specimen because it possesses compatible properties compared with that of the main phase.     

Formation and Transformation of ZnTiO3

A compound tentatively denoted as Zn₂Ti,₃O₈ is determined to be a low-temperature form of ZnTiO₃. At a heating rate of 10°C·min⁻¹ the low-temperature form crystallizes at 600° to 765° C from an amorphous material prepared by the simultaneous hydrolysis of zinc acetylacetonate and titanium isopropoxide. It has a cubic unit cell with a =0.8408 nm. The cubic-to-hexagonal transformation occurs slowly above 820°C; during transformation ZnTiO₃ decomposes into Zn₂TiO₄ and TiO₂ (rutile) at 965° to 1010°C. A single phase of the hexagonal form can be prepared by heating for 5 h at 900°C. The structure of both forms consists of octahedral TiO₆ groups.     

Phase Equilibria in the System ZnO—TiO2

The system ZnO-Ti0₂ has been investigated using quenching, hydrothermal, strip-furnace, and solid-state-reaction techniques. Two compounds were found: Zn₂Ti0₄, which melts congruently at 1549 C., and ZnTiO₂, which dissociates hydrothermally at 945° C. to give Zn₂Ti0₄and rutile. Two eutectics were found: one between ZnO and Zn₂Ti0₄ at 32 mole % TiO, and 1537°C. and the other between Zn₂TiO₄and TiO₂ at 58 mole % TiO₂ and 1418°C. Rutile was the only modification of TiO₂ observed. The reported melting points of ZnO and TiO₂ are 1975° and 1830°C. respectively; however, data exist which indicate the sublimation of ZnO at atmospheric pressure. Loss of ZnO by volatilization slightly decreased the accuracy of the liquidus relations. Reported solid solutions of TiO₂ in Zn₂Ti0₄ and ZnTiO₃ were not encountered, and explanations of this discrepancy are proposed.     

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.     

High-Q Microwave Dielectrics in the (Mg1−xCox)2TiO4 Ceramics

The microwave dielectric properties and the microstructures of (Mg_{1− x} Co _x )₂TiO₄ ceramics prepared by the conventional solid-state route were investigated. Lattice parameters were also measured for specimens with different x . The formation of solid solution (Mg_{1− x} Co _x )₂TiO₄ ( x =0.02–0.1) was confirmed by the X-ray diffraction patterns, energy dispersive X-ray analysis, and the lattice parameters measured. By increasing x from 0 to 0.05, the Q × f of the specimen can be tremendously boosted from 150 000 GHz to a maximum of 286 000 GHz. A fine combination of microwave dielectric properties (ɛ_r∼15.7, Q × f ∼286 000 GHz at 10.4 GHz, τ_f∼−52.5 ppm/°C) was achieved for (Mg_0.95Co_0.05)₂TiO₄ ceramics sintered at 1390°C for 4 h. Ilmenite-structured (Mg_0.95Co_0.05)TiO₃ was detected as a second phase. The presence of the second phase would cause no significant variation in the dielectric properties of the specimen because it possesses compatible properties compared with that of the main phase. In addition, only a small deviation in the dielectric properties was monitored for specimens with x =0.04–0.05 at 1360°–1420°C. It not only provides a wide process window but also ensures an extremely reliable material proposed as a very promising dielectric for low-loss microwave and millimeter wave applications.     

Titanium Incorporation in Zn2TiO4 Spinel Ceramics

Compositions in the Zn₂TiO₄+ x TiO₂ system ( x = 0–0.43) were synthesized via the solid-state reaction route, using high-purity (≥99.99%) metal-oxide powders. The incorporation of titanium, in the form of TiO₂, in Zn₂TiO₄ spinel ceramics was investigated by analyzing the crystal structure and measuring the dielectric properties. The results of the crystal structure analyses suggested that TiO₂ levels of x ≤ 0.33 could be incorporated into the Zn₂TiO₄ spinel at temperatures of T > 945°C, whereas the solid solubility of titanium in Zn₂TiO₄ decreased for T < 945°C. When x ≥ 0.28, the Zn₂Ti₃O₈ phase formed in the Zn₂TiO₄ grain interior while cooling after heat treatment. Measurement of the microwave dielectric properties also supported the conclusion that the solubility limit of titanium in Zn₂TiO₄ was close to x = 0.33, as determined through analysis of the crystal structure.     

Synthesis and Characterization of (Zn,Cd)TiO3 by a Modified Sol–Gel Method

Ilmenite-type (Zn_{1− x} Cd _x )TiO₃ (0≤ x ≤0.15 and 0.8≤ x ≤1.0) was synthesized by a modified sol–gel route including the Pechini process via two-step heat treatments. The thermal stability of (Zn_{1− x} Cd _x )TiO₃ depended on the amount of cadmium content. The as-synthesized (Zn_{1− x} Cd _x )TiO₃ (0≤ x ≤0.15 and 0.8≤ x ≤1.0) showed higher thermal stability than that of ZnTiO₃. The variation of the dielectric constant of all synthesized (Zn_{1− x} Cd _x )TiO₃ samples for all measurement frequencies showed a similar tendency. The dielectric constant of each (Zn_{1− x} Cd _x )TiO₃ sample decreased first with increasing frequencies and then increased slightly when the frequency was up to 10⁷ Hz. Moreover, the dielectric loss tangent of all synthesized (Zn_{1− x} Cd _x )TiO₃ samples for all measurement frequencies also changed in similar patterns. The dielectric loss tangent decreased with increasing measurement frequencies. The microwave dielectric properties of (Zn_{1− x} Cd _x )TiO₃ were changed with the cadmium doping content in the range of microwave frequency.     

Pyrochlore Phases in the System ZnO–Bi2O3–Sb2O5: I. Stoichiometries and Phase Equilibria

Two cubic pyrochlore phases exist in the system ZnO–Bi₂O₃–Sb₂O₅. Neither has the supposed "ideal" stoichiometry, Zn₂Bi₃Sb₃O₁₄. One, P ₁, is a solid solution phase, Zn_{2+ x} Bi_{2.96−( x − y )}Sb_{3.04− y} O_14.04+δ where 0< x <0.13(1), 0< y <0.017(2) and a =10.4285(9)−10.451(1) Å. The other, P ₂, is a line phase, Zn₂Bi_3.08Sb_2.92O_13.92 with a =10.462(2) Å. Subsolidus phase relations at 950°C involving phases P ₁ and P ₂ in the ZnO–Bi₂O₃–Sb₂O₅ phase diagram have been determined.     

High-Q Microwave Dielectric Materials Based on the Spinel Mg2TiO4

Composite ceramics based on the spinel Mg₂TiO₄ were prepared by a conventional mixed-oxide route. To achieve the temperature stabilization of the dielectric constant, each of the composites was added with 7 mol% CaTiO₃. The effect of the substitution of isovalent Co for Mg on the microstructure and the microwave dielectric properties of the composite ceramics was also investigated. A maximum Q × f value of around 150–160 THz was obtained for the undoped Mg₂TiO₄, whereas a reduced Q × f value was observed for an increase in the Co concentration in the system (1− x )Mg₂TiO₄− x Co₂TiO₄. Upon doping with 7 mol% CaTiO₃, the Q × f value passed through a maximum with increasing Co concentration. Adding ZnO–B₂O₃ to the composite system based on Co-doped Mg₂TiO₄ resulted in a reduction of the sintering temperature by 150°–200°C without any significant degradation in the Q × f value.     

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.     

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.     

Low-Fired (Zn,Mg)TiO3 Microwave Dielectrics

A dielectric ceramic comprised of (Zn_{1- x} Mg _x )TiO₃ ( x = 0 to x = 0.5) with low sintering temperature and promising microwave properties was prepared by applying a semichemical synthesis route and a microbeads milling technique. X-ray diffractometry and thermal analyses results indicated that the phase stability region of the hexagonal (Zn,Mg)TiO₃ extended to higher temperatures as the amount of magnesium increased. The dielectric properties in this system exhibited a significant dependence on the sintering conditions, especially near the phase decomposition temperature. From 950°C, the temperature compensation characteristics occurred as the phase composition changed from hexagonal (Zn,Mg)TiO₃ to two phases: (Zn,Mg)₂TiO₄ and rutile. The magnesium content for zero temperature coefficient (tau_f) was ~3 mol% at 950°C; however, tau_f increased with the sintering temperatures because of the shift of the decomposition temperature.     

Peculiar Dielectric Behaviors of (Na1/2Bi1/2)0.87Pb0.13TiO3 Thin Films

PbTiO₃-doped sodium bismuth titanate (Na_1/2Bi_1/2)_{1− x} Pb _x TiO₃ of perovskite structure is one of the best-known piezoelectrics/ferroelectrics. However, it has not been properly investigated in any thin-film forms. In this study, the dielectric properties of (Na_1/2Bi_1/2)_0.87Pb_0.13TiO₃ thin films synthesized via a sol–gel route were investigated. They exhibit a strong frequency dispersion of the dielectric permittivity at relatively high frequencies, which is shifted to lower frequencies with increasing temperature. The electrical behavior can be fitted using Jonscher's universal law for dielectric relaxation. The peculiar dielectric behaviors observed can be ascribed to the coexistence of two different dielectric phases in the films, which is believed to be associated with the growth of the local Pb²⁺TiO₃ nanoclusters upon substitution of Pb²⁺ for Na⁺/Bi³⁺ in the (Na_1/2Bi_1/2)_{1− x} Pb _x TiO₃ films.     

Molten-Salt Synthesis of Ba1–xPbxTiO3 in the System BaTiO3–PbCl2

Ba_{1– x} Pb _x TiO₃ powder with a fixed composition was prepared by the reaction of BaTiO₃ powders with molten PbCl₂at various PbCl₂/BaTiO₃ molar ratios at 600° and 800°C in a nitrogen atmosphere. When 0.1 μm powder was used, the reaction was finished when x = 0.9. Two phases of BaTiO₃and a solid solution of Ba_{1– x} Pb _x TiO₃ coexisted, but the final phase gave a solid solution of Ba_{1– x} Pb _x TiO₃ at 800°C. When 0.5 μm powder was used, the two phases coexisted in the products at 600°C at PbCl₂/BaTiO₃= 1.0. A sintered compact of Ba_{1– x} Pb _x TiO₃ powders solid solution was prepared by hot isostatic pressing, and its dielectric constant was measured in the temperature range 20°–550°C.     

Dielectric Properties of Ceramics in Ba (Co1/3 Nb2/3)O3–Ba(Zn1/3Nb2/3)O3 Solid Solution

The dielectric properties of the Ba (Co_1/3 Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ system were determined. Ba (Co_1/3 Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ has a complex perovskite structure, a high dielectric constant, a low dielectric loss, and a low temperature coefficient of the resonant frequency. A solid-solution ceramic with 0.7Ba (Co_1/3 Nb_2/3)O₃·0.3 Ba(Zn_1/3Nb_2/3)O₃ has a dielectric constant of K=33.5, Q=11000 at 6.5 GHz, and a temperature coefficient of the resonant frequency of τ_f=0 ppm/°C. The temperature coefficient of resonant frequency can be varied by changing the composition. The Q values of the ceramics can be increased by annealing in a nitrogen atmosphere. These ceramics can be used for resonant elements and stabilized oscillators.     

Compound Formation and Crystal Structure in the System ZnO-TiO2

In addition to the well-known zinc orthotitanate (Zn₂TiO₄) and zinc metatitanate (ZnTiO₃), the study of solid-state reactions in the system ZnO-TiO₂ disclosed a previously unrecognized compound, Zn₂Ti₃O₈. This compound was obtained when sulfate-containing hydrous titanium oxide of anatase structure was reacted with ZnO at 700° to 900° C. It crystallized in a defect spinel structure with eight Zn²⁺ ions occupying the tetrahedral positions and twelve Ti⁴⁺ ions distributed in sixteen octahedral positions (a₀= 8.395 ± 0.002 a.u.). Crystallographic data are given for zinc orthotitanate and metatitanate.
Formation of the latter compound appeared to be favored by rutile TiO₂ sources and those readily convertible to rutile under the reaction conditions.     

Characteristics of High-Q Microwave Dielectric Ceramics Nd(Co1/2Ti1/2)O3 With CuO Addition

We report the microwave dielectric properties and the microstructures of Nd(Co_1/2Ti_1/2)O₃ ceramics prepared by the conventional solid-state route. The prepared Nd(Co_1/2Ti_1/2)O₃ exhibits a mixture of Co and Ti showing a 1:1 order in the B site. Lowering the sintering temperature (as low as 1260°C) and promoting the densification of Nd(Co_1/2Ti_1/2)O₃ ceramics could be effectively achieved by adding CuO (up to 0.75 wt%). At 1350°C, Nd(Co_1/2Ti_1/2)O₃ ceramics with 0.5 wt% CuO addition possess a dielectric constant (ɛ_r) of 27.6, a Q × f value of 165 000 GHz (at 9 GHz), and a temperature coefficient of resonant frequency (τ_f) of −20 ppm/°C. By comparing with pure Nd(Co_1/2Ti_1/2)O₃ ceramics, incorporating additional CuO helps to render a dielectric material with a higher dielectric constant, a smaller τ_f value, and a 20% dielectric loss reduction, which makes it a very promising candidate for applications requiring low microwave dielectric loss.     

Synthesis of Phase-Pure Pb(ZnxMg1–x)1/3Nb2/3O3 up to x= 0.7 from a Single Mixture via a Soft-Mechanochemical Route

Phase-pure perovskite Pb(Zn _x Mg_{1– x} )_1/3Nb_2/3O₃ solid solution (PZ _x M_{1– x} N) is obtained for x ≦ 0.7 by heating a milled stoichiometric mixture of PbO, Mg(OH)₂, Nb₂O₅, and 2ZnCO₃·3Zn(OH)₂·H₂O at 1100°C for 1 h. Percent perovskite ( f _P) with respect to total crystalline phase decreases with increasing temperature of subsequent heating then increases to 900°C for the mixtures where x ≦ 0.8 and milled for 3 h. For mixtures with x = 0.9 and x = 1, f _P decreases monotonically. Curie temperature increases almost linearly with increasing x up to x = 0.7. The maximum dielectric constant at 1 kHz is 2×10⁴ and 1.7×10⁴ for the mixture with x = 0.4 and x = 0.7, respectively. The stabilization mechanism of strained perovskite is discussed.     

Phase Relations in the Na0.5Bi0.5TiO3–Li3xLa(2/3)−x□(1/3)−2xTiO3 System

The phase relations and the mechanism of solid-state synthesis for the Na_0.5Bi_0.5TiO₃–Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} TiO₃ system were investigated using X-ray powder diffraction, scanning electron microscopy, and thermal analysis. The study revealed that the extent of the homogeneity range—which is related to the A-site substitution between (Na_0.5Bi_0.5)²⁺ and (Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} )²⁺ pseudo cations of a perovskite structure—depends strongly on the ordering of the (Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} )²⁺ species. The solid-state reaction of the compounds in the homogeneity range is completed only after multiple high-temperature firings. However, the system is also subjected to a slow thermal decomposition; this is particularly so for the compounds with a high × value and an increased Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} TiO₃ concentration.