Effects of Ta2O5/Y2O3 Codoping on the Microstructure and Microwave Dielectric Properties of Ba(Co0.56Zn0.40)1/3Nb2/3O3 Ceramics

The effects of Ta₂O₅/Y₂O₃ codoping on the microstructure and microwave dielectric properties of Ba(Co_0.56Zn_0.40)_1/3Nb_2/3O₃-xA-xB (A = 0.045 wt.% Ta₂O₅; B = 0.113 wt.% Y₂O₃) ceramics (x = 0, 1, 2, 4, 8, 16, 32) prepared according to the conventional solid-state reaction technique were investigated. The x-ray diffraction (XRD) results showed that the main crystal phase in the sintered ceramics was BaZn_0.33Nb_0.67O₃-Ba₃CoNb₂O₉. The additional surface phase of Ba₈CoNb₆O₂₄ and trace amounts of Ba₅Nb₄O₁₅ second phase were present when Ta₂O₅/Y₂O₃ was added to the ceramics. The 1:2 B-site cation ordering was affected by the substitution of Ta⁵⁺ and Y³⁺ in the crystal lattice, especially for x = 4. Scanning electron microscopy (SEM) images of the optimally doped ceramics sintered at 1340°C for 20 h showed a compact microstructure with crystal grains in dense contact. Though the dielectric constant increased with the x value, appropriate addition would result in a tremendous modification of the Q × f and τ _f values. Excellent microwave dielectric properties (ε _r = 35.4, Q × f = 62,993 GHz, and τ _f  = 2.6 ppm/°C) were obtained for the ceramic with x = 0.4 sintered in air at 1340°C for 20 h.     

Influence of Li-B-Si Additions on the Sintering and Microwave Dielectric Properties of Ba-Nd-Ti Ceramics

Li₂O-B₂O₃-SiO₂ (LBS) synthesized via a solid-state reaction process was chosen as a novel sintering aid for tungsten-bronze-type Ba₄Nd_9.3Ti₁₈O₅₄ (BNT) ceramic. The effects of LBS additions on the sintering behaviors, microstructures, and microwave dielectric properties of the BNT ceramic have been investigated, indicating that LBS addition obviously lowered the sintering temperature of the BNT ceramic without damaging its microwave dielectric properties. BNT ceramic doped with 3 wt.% and 4 wt.% LBS addition could be well sintered at 975°C and 950°C for 3 h and had excellent properties: ε _r = 65.99, Q × f = 4943 GHz (f = 4.4 GHz), τ _f = 19 ppm/°C, and ε _r = 64.56, Q × f = 4929 GHz (f = 4.3 GHz), τ _f = 11 ppm/°C, respectively.     

Microwave Dielectric Properties of BiCu<Subscript>2</Subscript>PO<Subscript>6</Subscript> Ceramics with Low Sintering Temperature

A BiCu₂PO₆ microwave dielectric ceramic was prepared using a solid-state reaction method. As the sintering temperature increased from 800°C to 880°C, the bulk density of BiCu₂PO₆ ceramic increased from 6.299 g/cm³ to 6.366 g/cm³; the optimal temperature was 860°C. The best microwave dielectric properties [permittivity (? _r ) = ～16, a quality factor (Q × f) = ～39,110 GHz and a temperature coefficient of resonant frequency (τ _f ) = ～?59 ppm/°C] were obtained in the ceramic sintered at 860°C for 2 h. Then, TiO₂ with a positive τ _f (～+400 ppm/°C) was added to compensate the τ _f value. The composite material was found to have a near-zero τ _f (+2.7 ppm/°C) and desirable microwave properties (? _r  = 19.9, Q × f = 24,885 GHz) when synthesized at a sintering temperature of 880°C. This system could potentially be used for low-temperature co-fired ceramics technology applications.     

Ultra-Low Loss Microwave Dielectric Ceramic Li<Subscript>2</Subscript>Mg<Subscript>2</Subscript>TiO<Subscript>5</Subscript> and Low-Temperature Firing Via B<Subscript>2</Subscript>O<Subscript>3</Subscript> Addition

Li₂Mg₂TiO₅, a rock-salt structured ceramic fabricated by a solid-state sintering technique, was characterized at the microwave frequency band. As a result, a microwave dielectric permittivity (ε_r) of 13.4, a quality factor of 95,000 GHz (at 11.3 GHz), and a temperature coefficient of resonance frequency (τ_f) of ? 32.5 ppm/°C have been obtained at 1320°C. Li₂Mg₂TiO₅ ceramics have low permittivity, a broad processing temperature region, and a low loss, making them potential applications in millimeter-wave devices. Furthermore, B₂O₃ addition efficiently lowered the sintering temperature of Li₂Mg₂TiO₅ to 900°C, which opens up their possible applications in low-temperature co-fired ceramics (LTCC) technology.     

Effect of CuO Addition on Microwave Dielectric Properties of 0.80Sm(Mg0.5Ti0.5)O3-0.20Ca0.8Sr0.2TiO3 Ceramics

The effects of CuO addition on phase composition, microstructure, sintering behavior, and microwave dielectric properties of 0.80Sm(Mg_0.5Ti_0.5)O₃-0.20 Ca_0.8Sr_0.2TiO₃(8SMT-2CST) ceramics prepared by a conventional solid-state ceramic route have been studied. CuO addition shows no obvious influence on the phase of the 8SMT-2CST ceramics and all the samples exhibit pure perovskite structure. Appropriate CuO addition can effectively promote sintering and grain growth, and consequently improve the dielectric properties of the ceramics. The sintering temperature of the ceramics decreases by 50°C by adding 1.00 wt.%CuO. Superior microwave dielectric properties with a ε _r of 29.8, Q × f of 85,500 GHz, and τ _f of 2.4 ppm/°C are obtained for 1.00 wt.%CuO doped 8SMT-2CST ceramics sintered at 1500°C, which shows dense and uniform microstructure as well as well-developed grain growth.     

Cu<Subscript>3</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript>: An Ultralow-Firing Microwave Dielectric Ceramic with Good Temperature Stability and Chemical Compatibility with Aluminum

An ultralow-firing microwave dielectric ceramic Cu₃Mo₂O₉ with orthorhombic structure has been fabricated via a solid-state reaction method. X-ray diffraction analysis, Rietveld refinement, Raman spectroscopy, energy-dispersive spectrometry, and scanning electron microscopy were employed to explore the phase purity, crystal structure, and microstructure. Pure and dense Cu₃Mo₂O₉ ceramics could be obtained in the sintering temperature range from 580°C to 680°C. The sample sintered at 660°C for 4 h exhibited the highest relative density (～ 97.2%) and best microwave dielectric properties with ε _r = 7.2, Q × f = 19,300 GHz, and τ _f = ? 7.8 ppm/°C. Chemical compatibility with aluminum electrodes was also confirmed. All the results suggest that Cu₃Mo₂O₉ ceramic is a promising candidate for use in ultralow-temperature cofired ceramic applications.     

Phase, Microstructure, and Microwave Dielectric Properties of NaCa4−x Sr x Nb5O17 (x = 0 to 4) Ceramics

A series of A₅B₅O₁₇-type NaCa_4?x Sr _x Nb₅O₁₇ (x = 0 to 4) compounds were processed through a solid-state mixed-oxide route. All the compositions formed dense single-phase ceramics within the detection limit of an in-house x-ray diffraction facility when sintered at 1300°C. The substitution of Sr for Ca changed the crystal symmetry from monoclinic (x = 0) to orthorhombic (x = 1 to 4) along with a slight increase in molar cell volume due to the relatively larger ionic radius of Sr. The relative permittivity (ε _r) and temperature coefficient of resonance frequency (TCF) increased from 46 to 84 and from ?117 ppm/°C to +377 ppm/°C, respectively, while the quality factor (Q × f) decreased from 11,063 GHz to 559 GHz with an increase in x from 0 to 4. Optimum properties were achieved for NaCa₃SrNb₅O₁₇, which exhibited ε _r = 57, Q × f = 4628 GHz, and TCF = ?41 ppm/°C. Compounds in the NaCa_4?x Sr _x Nb₅O₁₇ series exhibited high ε _r and Q × f with adjustable TCF; however, further work is required for simultaneous optimization of all three properties.     

Microwave dielectric properties of (1 − x)(Mg0.95Co0.05)TiO3– xCa0.6La0.8/3TiO3 ceramics with V2O5 addition

The microstructures and the microwave dielectric properties of the (1 − x)(Mg_0.95Co_0.05)TiO₃–xCa_0.6La_0.8/3TiO₃ ceramic system were investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. Ca_0.6La_0.8/3TiO₃ has dielectric properties of dielectric constant ε_r ∼ 109, Q × f value ∼ 17,600 GHz and a large positive τ_f value ∼ 213 ppm/°C. (Mg_0.95Co_0.05)TiO₃ ceramics possesses high dielectric constant (ε_r ∼ 16.8), high quality factor (Q × f value ∼ 230,000 GHz), and negative τ_f value (−54 ppm/°C). As the x value varies from 0.1 to 0.8, (1 − x)(Mg_0.95Co_0.05)TiO₃–xCa_0.6La_0.8/3TiO₃ ceramic system has the dielectric properties as follows: 21.55 < ε_r < 75.44, 21,000 < Q × f < 90,000 and −10 < τ_f < 140. By appropriately adjusting the x value in the (1 − x)(Mg_0.95Co_0.05)TiO₃–xCa_0.6La_0.8/3TiO₃ ceramic system, zero τ_f value can be achieved. With x = 0.15, a dielectric constant ε_r ∼ 25.78, a Q × f value ∼ 84,000 GHz (at 9 GHz), and a τ_f value ∼ 2 ppm/°C were obtained for 0.85(Mg_0.95Co_0.05)TiO₃–0.15Ca_0.6La_0.8/3TiO₃ ceramics sintered at 1400 °C for 4 h. For practical application in communication systems, it is desirable to be able to sinter at lower temperatures. Therefore, V₂O₅ was as a sintering aid for lowering the sintering temperature of0.85(Mg_0.95Co_0.05)TiO₃–0.15Ca_0.6La_0.8/3TiO₃ ceramics. At the same time, the 0.85(Mg_0.95Co_0.05)TiO₃–0.15Ca_0.6La_0.8/3TiO₃ ceramic system with 0.5 wt% V₂O₅ can be obtained good properties at the microwave frequencies for 1200 °C.     

Improvements in the Sintering Behavior and Microwave Dielectric Properties of Geikielite-Type MgTiO3 Ceramics

Microwave dielectric ceramics based on geikielite-type MgTiO₃ were prepared by an aqueous sol–gel process. The precursor powders and dielectric ceramics were characterized by x-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and microwave methods. Highly reactive nanosized magnesium titanate powders with particle sizes of 20 nm to 40 nm were successfully obtained at 500°C as precursors. Sintering characteristics and microwave dielectric properties of MgTiO₃ ceramics were studied as a function of sintering temperature from 1100°C to 1300°C. With increasing sintering temperature, the density, ε _r, and Qf values increased, saturating at 1200°C with excellent microwave properties of ε _r = 17.5, Qf = 156,300 GHz, and τ _f  = ?44 ppm/°C. Correlations between the microstructure and dielectric properties of MgTiO₃ ceramics were also investigated.     

Investigation of Structural and Electrical Properties of B-Site Complex Ion (Mg1/3Nb2/3)4+-Modified High-Curie-Temperature BiFeO3-BaTiO3 Ceramics

B-site complex ion (Mg_1/3Nb_2/3)-modified high-temperature ceramics 0.71BiFeO₃-0.29BaTi_1?x (Mg_1/3Nb_2/3) _x O₃ (BF-BTMNx) have been fabricated by the conventional solid-state reaction method. The compositional dependence of the?phase structure, electrical properties, and depolarization temperature of the ceramics was studied. The main phase structure of BF-BTMNx ceramics is perovskite phase with pseudocubic symmetry. The experimental results show that the dielectric and piezoelectric properties, and temperature stability strongly depend on the (Mg_1/3Nb_2/3)⁴⁺ content. The optimum (Mg_1/3Nb_2/3) content enhances the piezoelectric properties, Curie temperature, and depolarization temperature. The ceramic with x = 1% exhibited enhanced electrical properties of d ₃₃ = 158 pC/N and k _p = 0.322, combined with high-temperature stability with Curie temperature of T _c = 453°C and depolarization temperature of T _d = 400°C. These results show that the ceramic with x = 1% is a promising lead-free high-temperature piezoelectric material.     

Microwave Dielectric Properties of ZnO-2TiO<Subscript>2</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> Ceramics with BaCu (B<Subscript>2</Subscript>O<Subscript>5</Subscript>) Addition

The influence of BaCu(B₂O₅) (BCB) addition on the sintering temperature and microwave dielectric properties of ZnO-2TiO₂-Nb₂O₅ (ZTN) ceramic has been investigated using dilatometry, x-ray diffraction, scanning electron microscopy, and microwave dielectric measurements. A small amount of BCB addition to ZTN can lower the sintering temperature from 1100°C to 900°C. The reduced sintering temperature was attributed to the formation of the BCB liquid phase. The ZTN ceramics containing 3.0 wt.% BCB sintered at 900°C for 2 h have good microwave dielectric properties of Q × f = 19,002 GHz (at 6.48 GHz), ε _r = 45.8 and τ _f  = 23.2 ppm/°C, which suggests that the ceramics can be applied in multilayer microwave devices, provided that Ag compatibility exists.     

Effect of bismuth addition on sintering behavior and microwave dielectric properties of zinc titanate ceramics

The influences of Bi₂O₃ addition on the sintering behavior and microwave dielectric properties of ZnO-TiO₂ ceramics were investigated. ZnO-TiO₂ ceramics were prepared with conventional solid-state method and sintered at temperatures from 950°C to 1,100°C. The sintering temperature of ZnO-TiO₂ ceramics with Bi₂O₃ addition could be effectively reduced to 1,000°C due to the liquidphase effects resulting from the additives. A proper amount of Bi₂O₃ addition could effectively improve the densification and dielectric properties of ZnO-TiO₂ ceramics. The temperature coefficient of resonant frequency could be controlled by varying the sintering temperature and lead to a zero τ_f value. At 1,000°C, 1ZnO-1TiO₂ ceramics with 1 wt.% addition gave better microwave dielectric properties ɛ_r of 29.3, a Q × f value of 22,000 GHz at 8.36 GHz, and a τ_f value of +17.4 ppm/ °C.     

Microwave Sintering and Optical Properties of Sm3+-Activated KSrPO4 Phosphors

The microwave sintering and photoluminescence properties of KSr_1?x PO₄:xSm³⁺ phosphors have been investigated. KSrPO₄ phosphates activated by various concentrations of Sm³⁺ ions (x = 0.007, 0.009, 0.01, 0.03) were microwave sintered at 1200°C for 3 h under air atmosphere. x-Ray diffraction patterns showed that all phosphor samples exhibited a single phase without any extraneous phases. Scanning electron microscopy images showed that the particle size increased with the Sm³⁺ concentration and that the particle morphology was fine and uniform. The photoluminescence results showed that a concentration quenching effect occurred when the concentration of Sm³⁺ ions reached x = 0.01. Decay time measurement results showed that the lifetime decreased gradually from 3.12 ms to 2.34 ms as the Sm³⁺ concentration increased. All the chromaticity (x, y) values of the microwave-sintered KSrPO₄:Sm³⁺ phosphors were located in the red region (0.57, 0.41).     

Structural and magnetic properties of Dy3+ doped Y3Fe5O12 for microwave devices

The Dy³⁺ doped Y_3−xDy_xFe₅O₁₂ (x=0–3) nanopowders were prepared using microwave hydrothermal route. The structural and morphological studies were analyzed using transmission electron microscope, X-ray diffractometer and field emission scanning electron microscope. The nanopowders were sintered at 900 °C/90 min using microwave furnace. Dense ceramics with theoretical density of around 95% was obtained. Ferro magnetic resonance (FMR) spectrum and microwave absorption spectrum of Dy³⁺ doped YIG were studied, the signal exhibits a resonance character for all Dy³⁺ variations. It was observed that the location of the FMR signal peak at the field axes monotonically shifts to higher field with increasing Dy³⁺ content. The dielectric and magnetic properties (ε′, ε′′, µ′ and µ′′) of Dy³⁺ doped YIG were studied over a wide range of frequency (1–50 GHz). With increase of Dy³⁺ both ε′ and µ′ decreased. The low values of dielectric, magnetic properties and broad distribution of FMR line width of these ceramics are opening the real opportunity to use them for microwave devices above K- band frequency.     

Effect of Zr4+ Content on the Grain Growth,Dielectric Relaxation Behavior,and Ferroelectric Properties of Ba0.4Sr0.6Ti1−x Zr x O3 Nano-Ceramics Prepared by Different Methods Assisted by Fast Microwave Sintering

The effect of Zr⁴⁺ content on the grain growth, dielectric relaxation, and piezoelectric properties of Ba_0.4Sr_0.6Ti_1?x Zr _x O₃ (BSTZ; x = 0, 0.02, 0.04, 0.06) ceramics prepared by solid-state (SS) and sol–gel modified hydrothermal (SH) methods assisted by fast microwave sintering was investigated in this study. A combination of x-ray diffraction (XRD), scanning electron microscopy (SEM), impedance analysis, and ferroelectric analysis was used. All the ceramics had pure perovskite structures at room temperature, as seen from XRD patterns, indicating that Zr⁴⁺ was incorporated into Ba_0.4Sr_0.6TiO₃ lattices to form a solid solution. In the SEM micrographs, SH samples had higher densities and smaller and more homogeneous grain size than SS samples, which was in agreement with density measurements. Nano-ceramics were obtained by this method. When the temperature dependence of dielectric constant and dielectric loss was studied, SH samples had higher permittivity, better thermally activated relaxation, and lower dielectric loss at high temperature. Ferroelectric characteristics can still be detected in Ba_0.4Sr_0.6Ti_1?x Zr _x O₃ ceramics and residual polarization (P _r) decreased with increasing Zr⁴⁺ content.     

Structural and Magnetic Study of Cu x FeCr1−x O2 Oxides Under High External Magnetic Fields

The structural, electronic, and magnetic behaviors of Cu _x FeCr_1?x O₂ polycrystals are investigated. Investigations are conducted for increasing chromium substitution, according to varying x values in the formula versus copper, for x = 0, 0.2, 0.4, 0.6, 0.8, and 1. The magnetic response of polycrystalline samples under increasing external magnetic field from 0.4 T to 5 T is also studied. The partial crystal structure deformation/transition from delafossite CuFeO₂ structure to corundum-type FeCrO₃ structure containing CrO₂ and Cr₂O₃ blocks is determined. The change in the crystal structure geometry with increasing Cr substitution is observed. Besides, prominent changes in magnetic ordering are observed from antiferromagnetic (x = 1, 0.8, and 0.6) to ferromagnetic ordering (x = 0.4 and 0.2) for high applied external magnetic fields above 2 T.     

Preparation, Characterization, and Microwave Dielectric Properties of Sr2La3Nb1−x Ta x Ti4O17 (0 ≤ x ≤ 1) Ceramics

Sr₂La₃Nb_1?x Ta _x Ti₄O₁₇ (0 ≤ x ≤ 1) ceramics were processed via a solid-state mixed oxide route. Sr₂La₃Nb_1?x Ta _x Ti₄O₁₇ (0 ≤ x ≤ 1) solid solutions were single phase in the whole range of x values within the x-ray diffraction (XRD) detection limit. The microstructure comprised elongated and needle-shaped grains. The ceramics exhibit relative permittivity (ε _r) of 73 to 68.6, product of unloaded quality factor and resonant frequency (Q _u f ₀) of 7100 GHz to 9500 GHz, and temperature coefficient of resonant frequency (τ _f) of 78.6 ppm/°C to 56.6 ppm/°C.     

Effects of Heat-Treatment Temperature on the Microstructure, Electrical and Dielectric Properties of M-Type Hexaferrites

M-type hexaferrite BaCr _x Ga _x Fe_12?2x O₁₉ (x = 0.2) powders have been synthesized by use of a sol–gel autocombustion method. The powder samples were pressed into 12-mm-diameter pellets by cold isostatic pressing at 2000 bar then heat treated at 700°C, 800°C, 900°C, and 1000°C. X-ray diffraction patterns of the powder sample heat treated at 1000°C confirmed formation of the pure M-type hexaferrite phase. The electrical resistivity at room temperature was significantly enhanced by increasing the temperature of heat treatment and approached 5.84 × 10⁹ Ω cm for the sample heat treated at 1000°C. Dielectric constant and dielectric loss tangent decreased whereas conductivity increased with increasing applied field frequency in the range 1 MHz–3 GHz. The dielectric properties and ac conductivity were explained on the basis of space charge polarization in accordance with the Maxwell–Wagner two-layer model and Koop’s phenomenological theory. The single-phase synthesized materials may be useful for high-frequency applications, for example reduction of eddy current losses and radar absorbing waves.     

Preparation and Photoluminescence Properties of Pr3+ Ion-Doped Ca2LaTaO6 Phosphors

In this study, Pr³⁺ ion-doped Ca₂LaTaO₆ phosphors were synthesized using a vibrating milled solid-state reaction with metal oxides and calcined in air at 1100°C for 8 h. The crystal structure and photoluminescence properties were also investigated. The x-ray powder diffraction patterns show that all of the peaks can be attributed to monoclinic Ca₂LaTaO₆ phase with increasing Pr³⁺ ion doping. The scanning electron microscopy images show that the particles are irregular, with the most uniform distribution being obtained for Pr³⁺ ion concentration of 3 mol.%. The emission spectra of Ca₂(La_1?x Pr _x )TaO₆ phosphors showed a dominant green emission peak at 490 nm under excitation at 451 nm, which was due to the ³P₀ → ³H₄ transition. A series of weak emission peaks at 531 nm, 544 nm, 615 nm, 621 nm, and 652 nm were assigned to the ³P₀ → ³H₅, ¹D₂ → ³H₄, ³P₀ → ³H₆, and ³P₀ → ³F₂ transitions of Pr³⁺ ions, respectively. In addition, the emission intensities of the Ca₂(La_1?x Pr _x )TaO₆ phosphors increased then decreased as the Pr³⁺ ion concentration was increased, and the maximum emission intensity occurred for x of 0.03, corresponding to an average grain size of 41.5 nm with critical distance of 16.32 Å. The Commission Internationale de l’Eclairage color chromaticity coordinates for the Ca₂LaTaO₆:Pr³⁺ phosphors were all located in the green region, but shifted from (x = 0.145, y = 0.463) to (x = 0.119, y = 0.471) as the Pr³⁺ ion concentration was increased from 0.5 mol.% to 10 mol.%.     

High-Temperature Thermoelectric Properties of Compounds in the System Zn x In y O x+1.5y

Based on results obtained utilizing combinatorial chemistry techniques to screen the thermoelectric power factor of materials in the system Zn _x In _y O _x+1.5y , several multiphase candidates were down-selected and investigated in terms of their thermoelectric response from room temperature to 1050°C. While the screening experiments suggested that peaks in the power factor occur at relatively high indium oxide content, only the thermoelectric properties of zinc-oxide-rich homologous layered phases in the system (In₂O₃)(ZnO) _k have been well documented, since the phases where k < 3 cannot be easily formed. In the present study, indium-oxide-rich materials in the system In₂O₃–(In₂O₃)(ZnO)₃ were fabricated and their figures of merit were determined. The results suggest that the indium-oxide-rich phases have improved figures of merit, especially at elevated temperatures, relative to the best performing k phases by combining the high power factor of In₂O₃ and the low thermal conductivity of (In₂O₃)(ZnO) _k .