Microwave dielectric properties and microstructures of Ca(Nb1−xTax)2O6 ceramics

This study elucidates the microwave dielectric properties and microstructures of Ca(Nb_1?xTa_x)₂O₆ ceramics with a view to their potential for microwave devices. The Ca(Nb_1?xTa_x)₂O₆ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the Ca(Nb_0.93Ta_0.07)₂O₆ ceramics revealed no significant variation of phase with sintering temperatures. A dielectric constant (? _r ) of 17.7, a quality factor (Q × f) of 117,000 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?51 ppm/°C were obtained for Ca(Nb_0.93Ta_0.07)₂O₆ ceramics that were sintered at 1,400 °C for 4 h.     

Microstructures and microwave dielectric properties of (1 − y)Nd1−xYbx(Mg0.5Sn0.5)O3–yCa0.8Sr0.2TiO3 ceramics

The (1 ? y)Nd_1?xYb_x(Mg_0.5Sn_0.5)O₃–yCa_0.8Sr_0.2TiO₃ ceramics were prepared by the conventional solid-state method. The X-ray diffraction patterns of the Nd_1?xYb_x(Mg_0.5Sn_0.5)O₃ ceramics revealed that Nd_1?xYb_x(Mg_0.5Sn_0.5)O₃ is the main crystalline phase, which is accompanied by a little Nd₂Sn₂O₇ as the second phase. An apparent density of 6.87 g/cm³, a dielectric constant (? _r ) of 19.48, a quality factor (Q × f) of 117,300 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?61 ppm/°C were obtained when the Nd_0.96Yb_0.04(Mg_0.5Sn_0.5)O₃ ceramics were sintered at 1,600 °C for 4 h. The temperature coefficient of resonant frequency (τ _f ) increased from ?61 to ?3 ppm/°C as y increased from 0 to 0.6 when the (1 ? y)Nd_0.96Yb_0.04(Mg_0.5Sn_0.5)O₃–yCa_0.8Sr_0.2TiO₃ ceramics were sintered at 1,600 °C for 4 h. 0.4Nd_0.96Yb_0.04(Mg_0.5Sn_0.5)O₃–0.6Ca_0.8Sr_0.2TiO₃ ceramic that was sintered at 1,600 °C for 4 h had a τ _f of ?3 ppm/°C.     

Microstructure and microwave dielectric properties of xSm(Mg0.5Ti0.5)O3–(1 − x)Ca0.8Sr0.2TiO3 ceramics

xSm(Mg_0.5Ti_0.5)O₃–(1 ? x)Ca_0.8Sr_0.2TiO₃ (x = 0.50–0.95) ceramics are prepared by a conventional solid-state ceramic route. The microstructure and microwave dielectric properties are investigated as a function of the x-value and sintering temperature. The single phase solid solutions were obtained throughout the studied compositional range. The variation of bulk density and dielectric properties are related with the x-value. Increasing sintering temperature can effectively promote the densification and dielectric properties of xSm(Mg_0.5Ti_0.5)O₃–(1 ? x)Ca_0.8Sr_0.2TiO₃ ceramic system. With the content of Sm(Mg_0.5Ti_0.5)O₃ increasing, the temperature coefficient of resonant frequency τ _f value decreased, and a near-zero τ _f could be obtained for the samples with x = 0.80. The optimal microwave dielectric properties with a dielectric constant ε _r of 30.1, Q × f of 115,000 GHz (at 8.0 GHz), and τ _f of 8.9 ppm/°C were obtained for 0.80Sm(Mg_0.5Ti_0.5)O₃–0.20Ca_0.8Sr_0.2TiO₃ sintered at 1,550 °C for 3 h, which showed high density and well-developed grain growth.     

Microwave dielectric properties of (1 − y)Nd(1−2x/3)Bax(Mg0.5Sn0.5)O3–yCa0.8Sr0.2TiO3 ceramic

The (1 ? y)Nd_(1?2x/3)Ba_x(Mg_0.5Sn_0.5)O₃–yCa_0.8Sr_0.2TiO₃ ceramics were prepared by the conventional solid-state method. The X-ray diffraction patterns of the Nd_(1?2x/3)Ba_x(Mg_0.5Sn_0.5)O₃ ceramics revealed that Nd_(1?2x/3)Ba_x(Mg_0.5Sn_0.5)O₃ is the main crystalline phase, which is accompanied by a little Nd₂Sn₂O₇ as the second phase. An apparent density of 6.89 g/cm³, a dielectric constant (ε _r ) of 19.1, a quality factor (Q × f) of 212,000 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?68 ppm/°C were obtained when the Nd_2.94/3Ba_0.03(Mg_0.5Sn_0.5)O₃ ceramics were sintered at 1,550 °C for 4 h. The temperature coefficient of resonant frequency (τ _f ) increased from ?68 to +55 ppm/°C as y increased from 0 to 0.7 when the (1 ? y)Nd_2.94/3Ba_0.03(Mg_0.5Sn_0.5)O₃–yCa_0.8Sr_0.2TiO₃ ceramics were sintered at 1,600 °C for 4 h. 0.4Nd_2.94/3Ba_0.03(Mg_0.5Sn_0.5)O₃–0.6 Ca_0.8Sr_0.2TiO₃ ceramic that was sintered at 1,600 °C for 4 h had a τ _f of ?7 ppm/°C.     

Improvement in microwave dielectric properties of La(Mg0.5Sn0.5)O3 ceramics by applying ZnO–B2O3–SiO2

The microwave dielectric properties of ZnO–B₂O₃–SiO₂ (ZBS)-doped La(Mg_0.5Sn_0.5)O₃ ceramics were investigated with a view to their application in microwave devices. ZBS-doped La(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method. The X-ray diffraction patterns of ZBS-doped La(Mg_0.5Sn_0.5)O₃ ceramics exhibited no significant variation of phase with sintering temperature. By adding 2.0 wt% ZBS, a dielectric constant of 19.14, a quality factor (Q × f) of 35,800 GHz, and a temperature coefficient of resonant frequency τ _f (?86 ppm/°C) were obtained when La(Mg_0.5Sn_0.5)O₃ ceramics were sintered at 1,400 °C for 4 h.     

Microwave dielectric properties of (1 ? x)CaO-xBaO-Li2O-(1 ? y) Sm2O3-yNd2O3-TiO2 ceramics system

The CaO-Li₂O-Sm₂O₃-TiO₂ system has been reported having a high dielectric constant and good temperature stability. Based on the composition of CaO : Li₂O : Sm₂O₃ : TiO₂ : = 16 : 9 : 12 : 63, the modifications of CaO and Sm₂O₃ were investigated. In this paper, CaO has been partially substituted by BaO and Sm₂O₃ by Nd₂O₃. It was found that the introduction of BaO leads the modified ceramics to be mixtures of two phases. The fQ value has been significantly promoted, while the dielectric constant can remain higher than 90. To obtain a higher dielectric constant, Sm₂O₃ has been further partially replaced by Nd₂O₃. The sintering temperatures for the property change were also studied. The temperature coefficient of resonant frequency could be varied from a positive value to a negative value according to the different sintering temperatures. Excellent dielectric properties of r = 103, fQ = 7200 and _f = + 2 ppm/°C were obtained with a composition of CaO : BaO : Li₂O : Sm₂O₃ : Nd₂O₃ : TiO₂ = 14 : 4 : 8 : 10 : 2 : 63 (molar ratio) and sintered at 1350°C for 3 h.     

Microwave dielectric properties of new SrLa4−xNdxTi5O17 ceramics

New microwave dielectric ceramics in the SrLa_4?xNd_xTi₅O₁₇ (0 ≤ x ≤ 4) composition series were prepared through a solid state mixed oxide route. All the compositions formed highly dense (～95%) single phase ceramics upon sintering at 1500–1580 °C. The molar volume and theoretical density decreased due to the substitution of small (1.27 Å) Nd ions for large (1.36 Å) La ions. This decrease was associated with a decrease in the dielectric constant (?_r), temperature coefficient of resonant frequency (τ_f) and quality factor (Q_uf_o). An analysis of properties achieved in the present study indicated that ceramics exhibiting nearly zero τ_f corresponding to ?_r ～ 54 could be fabricated in the SrLa_4?xNd_xTi₅O₁₇ composition series at x ～ 1.6; however, further work is required to improve Q_uf_o (～6000 GHz) for possible practical applications.     

Piezoelectric and dielectric properties of 0.98(Na0.5K0.5)NbO3–0.02Ba(ZrxTi(1−x))O3 ceramics

Lead-free 0.98(Na_0.5K_0.5)NbO₃–0.02Ba(Zr_xTi_(1?x))O₃ (0.98NKN–0.02BZT) ceramics with Zr contents were fabricated by a conventional mixed-oxide method. The results indicate that the Zr/Ti ratio significantly influences the structural, piezoelectric, dielectric, and ferroelectric properties of 0.98NKN–0.02BZT ceramics. For the 0.98NKN–0.02BZT (x = 0) ceramics sintered at 1090 °C, the bulk density increased as the Zr contents decreased and showed a maximum value at x = 0. The Curie temperature of the 0.98NKN–0.02BZT ceramics slightly decreased as the Zr contents increased. The dielectric constant, piezoelectric constant, and electromechanical coupling factor of samples were maximized at x = 0, which might be due to the increase in density. A high d₃₃ = 194 pC/N, k_p = 38% were obtained for the 0.98NKN–0.02BZT ceramics sintered at 1090 °C for 4 h.     

Phase transition,ferroelectric and piezoelectric properties of Bi(Mg0.5Zr0.5)O3-modified BiFeO3–BaTiO3 lead-free ceramics

(0.725 ? x)BiFeO₃–0.275BaTiO₃–xBi(Mg_0.5Zr_0.5)O₃ + 1 mol% MnO₂ lead-free ceramics (x = 0–0.08) were synthesized by a conventional solid state reaction method and the effects of Bi(Mg_0.5Zr_0.5)O₃ on phase transition, piezoelectric and ferroelectric properties of the ceramics were investigated. After the addition of Bi(Mg_0.5Zr_0.5)O₃, the crystal structure of the ceramics is transformed from rhombohedral to tetragonal phase and the morphotropic phase boundary (MPB) of rhombohedral and tetragonal phase is formed at x = 0.01. The grain size of the ceramics increases with x increasing from 0 to 0.02 and then decreases with x further increasing. The dielectric peak of the ceramics becomes diffusive with x increasing after the addition of Bi(Mg_0.5Zr_0.5)O₃. The ceramics with x = 0–0.08 exhibit much better electric insulation with the resistivity of 1.0 × 10⁹–5.0 × 10⁹ Ω·cm than pure BiFeO₃ ceramic with the resistivity of ~5 × 10⁷ Ω·cm. Due to the formation of the MPB, the ceramics with x = 0–0.02 possess good densification with the relative densities ρ _r of 94.9–96.3 %, strong piezoelectricity with the d ₃₃ of 129–135 pC/N and very high Curie temperature with the T _C of 559–610 °C.     

Piezoelectric and dielectric properties of Bi0.5Na0.5TiO3–Bi0.5Li0.5TiO3 lead-free ceramics

(1 − x)Bi_0.5Na_0.5TiO₃–xBi_0.5Li_0.5TiO₃ lead-free ceramics have been prepared by a conventional solid-state reaction method, and their piezoelectric and dielectric properties have been studied. X-ray diffraction studies reveal that Li⁺ diffuses into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure. The addition of Bi_0.5Li_0.5TiO₃ effectively lowers the sintering temperature of the ceramics and greatly assists in the densification of the ceramics. The ceramic with x = 0.075 possesses the optimum piezoelectric properties: piezoelectric coefficient d ₃₃ = 121 pC/N and planar electromechanical coupling factor k _P = 18.3%. After the partial substitution of Li⁺ for Na⁺ in the A-sites of Bi_0.5Na_0.5TiO₃, the ceramics exhibit more relaxor characteristic, which is probably resulted from the cation disordering in the 12-fold coordination sites. The depolarization temperature T _d shifts to low temperature with the substitution level x of Li⁺ for Na⁺ increasing.     

Microwave dielectric properties and low-temperature sintering of (Zn0.8Mg0.2)2SiO4–TiO2 ceramics with Li2O–B2O3

Effects of Li₂O–B₂O₃ on the sintering behavior and the microwave dielectric properties of (Zn_0.8Mg_0.2)₂SiO₄–TiO₂ ceramics were investigated as a function of Li₂O–B₂O₃ content and sintering temperature. The Li₂O–B₂O₃ combined additives successfully reduced the sintering temperature of (Zn_0.8Mg_0.2)₂SiO₄–TiO₂ ceramics from 1,250 °C to 900 °C. With the increase of Li₂O–B₂O₃ content, the TiO₂ phase decreased and the unknown second phase increased, which led to the dielectric constant (ε _r ) and the maximum Q × f value decrease, and the temperature coefficient of resonant frequency (τ _f ) shift to a negative value. The specimens with 3 wt%Li₂O–B₂O₃ sintered at 900 °C for 2 h showed ε _r of 8.84, Q × f value of 15,500 GHz, and τ _f of 17.8 ppm/°C. And the material was compatible with Ag electrodes, which made it a promising ceramic for low temperature co-fired ceramics technology application.     

Effects of SrO–B2O3–SiO2 glass additive on dielectric properties of Ba(Fe0.5Nb0.5)O3 ceramics

SrO–B₂O₃–SiO₂ (SBS) glass powders were prepared and employed as sintering aids to reduce the sintering temperature of Ba(Fe_0.5Nb_0.5)O₃ (BFN) ceramics. The effects of glass content on the dielectric properties and breakdown strength of BFN ceramics have been investigated. The volume density characterization results of (1 ? x) BFN ? x SBS ceramics indicate that the sintering temperature of BFN ceramics decreased by 200–350 °C with SBS glass addition (when x = 0, 0.01, 0.03 and 0.05). The XRD patterns show BFN ceramics indicate cubic crystal structure and without the formation of a secondary phase. The dielectric constant and dielectric loss decreased gradually with increasing glass content, and the dielectric loss decreased by one order of magnitude with SBS glass addition (when x = 0.05). The breakdown strength of (1 ? x) BFN ? x SBS ceramics increase with increasing glass content, in which is about 33.90 kV/cm with SBS glass addition (when x = 0.05). These improvements in the dielectric characteristics of BFN ceramics have great scientific significance for their applications.     

Structures and electrical properties of (Na0.5K0.5)NbO3–Li(Ta0.5Nb0.5)O3 lead-free piezoelectric ceramics

Solid solutions of (Na_0.5K_0.5)NbO₃ (NKN) and Li(Ta_0.5Nb_0.5)O₃ (LTN) were investigated as a potential candidate of lead-free piezoelectric ceramics. It was found that the Curie temperature of solid solutions increases slightly with increasing the LTN content and simultaneously the polymorphic phase transition temperature linearly decrease till below room temperature. An orthorhombic to tetragonal phase transformation at room temperature, or a morphotropic phase boundary, in NKN is induced by ~7 at% LTN addition, where the best dielectric, piezoelectric and electromechanical properties are achieved. The 0.94NKN–0.07LTN ceramics possess a dielectric constant of 765, a loss tangent of 0.04 at 1 kHz, a piezoelectric constant d₃₃ of 253 pC/N and an electromechanical coupling factor k_p of 48%.     

Microwave dielectric properties and low temperature sintering of Ba3Ti4?x(Mg1/3Nb2/3)xNb4O21 ceramics with BaCu(B2O5) addition

Microwave dielectric ceramics of Ba₃Ti_4?x(Mg_1/3Nb_2/3)_xNb₄O₂₁ solid solutions (BTMNN-x, x?=?0–4) were prepared via the conventional solid-state reaction method. The X-ray powder diffraction analysis revealed that the BTMNN-x ceramics formed complete solid solutions with hexagonal structure. The dielectric constant (ε_r) and the temperature coefficient of the resonant frequency (τ_f) of BTMNN-x ceramics decreased with the increase of x, while the quality factor (Q?×?f) enhanced with increasing the substitution content. In addition, a small amount of BaCu(B₂O₅) (BCB) additive can effectively lower the sintering temperature of BTMNN ceramics. The 1.5?wt% BCB doped BTMNN-2 ceramics can be sintered at 950?°C and have good microwave dielectric properties of ε_r?=?50, Q?×?f?=?10,500?GHz and τ_f?=?18?ppm/°C, which makes it possible to be a promising candidate for mid-permittivity low temperature co-fired ceramic materials.     

The electrostrictive effect and dielectric properties of lead-free 0.5Ba(ZrxTi1−x)O3–0.5(Ba0.75Ca0.25)TiO3 ceramics

Lead-free 0.5Ba(Zr_xTi_1?x)O₃–0.5(Ba_0.75Ca_0.25)TiO₃ (x = 0.25, 0.30, 0.35, 0.40) ceramics have been synthesized by a conventional solid state sintering method. The room temperature ferroelectric and electrostrictive properties of these ceramics were studied. Based on the measured properties, these ceramics showed a typical relaxor behavior. The Curie temperature of BZT–BCT ceramics decreases with increasing the Zr content. The largest electrostrictive strain and electrostrictive coefficient are founded in BZT–BCT ceramic with x = 0.25, the value is 0.16 % and 0.079 m⁴ C^?2, respectively. The polarization, electrostrictive strain and electrostrictive coefficient (Q ₁₁) decrease with increase in Zr concentration. For samples with low Curie temperature, which have large room temperature dielectric constant (ε), electrostrictive coefficient increases (Q ₁₁) is smaller. Because doping can disrupt the long range cation order, and electrostrictive (Q ₁₁) coefficient increases with cation order from disordered, through partially-ordered, simple relaxor and then ordered perovskites, ferroelectrics with a disordered structure have a huge permittivity, but a small electrostrictive coefficient (Q ₁₁).