Effect of V2O5 additive to 0.4SrTiO3–0.6La(Mg0.5Ti0.5)O3 ceramics on sintering behavior and microwave dielectric properties

The effect of V₂O₅ addition on the microwave dielectric properties and the microstructures of 0.4SrTiO₃–0.6La(Mg_0.5Ti_0.5)O₃ ceramics sintered for 5 h at different sintering temperature were investigated systematically. It was found that the sintering temperature was effectively lowered about 200 °C by increasing V₂O₅ addition content. The grain sizes, bulk density as well as microwave dielectric properties were greatly dependent on sintering temperature and V₂O₅ content. The 4ST–6LMT ceramics with 0.25% V₂O₅ sintered at 1400 °C for 5 h in air exhibited optimum microwave dielectric properties of ɛ_r = 50.7, Q × f = 15049.6 GHz, T_f = −1.7 ppm/°C.     

Enhanced densification and microwave dielectric properties of Mg2TiO4 ceramics added with CeO2 nanoparticles

We report the study of the effects of processing parameters and additive concentration on the structure, microstructure and microwave dielectric properties of MTO–CeO₂ (x wt.%) ceramics with x = 0, 0.5, 1.0 and 1.5 prepared by solid-state reaction method by adding CeO₂ nanoparticles as a sintering aid. The pure Mg₂TiO₄ ceramics were not densifiable below 1450 °C. However, when CeO₂ nanoparticles were added to MTO, the densification achieved at 1300 °C along with the increase in average grain size with the uniform microstructure and improved microwave dielectric properties. This is mainly driven by the large surface energy of CeO₂ nanoparticles and their defect energy during the sintering process. While the addition of CeO₂ nanoparticles in MTO ceramics does not change the dielectric constant (?_r), the unloaded quality factor (Q_u) was altered significantly. MTO–CeO₂ (1.5 wt.%) ceramics sintered at 1300 °C exhibit superior microwave dielectric properties (?_r ～ 14.6, Q × f₀ ～ 167 THz), as compared to the pure Mg₂TiO₄ ceramics. The observed results are correlated to the enhancement in density and the development of uniform microstructure with the enhanced grain size.     

Gd2O3 doped 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 lead-free piezoelectric ceramics

Gd₂O₃ (0–0.8 wt.%)-doped 0.82Bi_0.5Na_0.5TiO₃–0.18Bi_0.5K_0.5TiO₃ (BNKT18) lead-free piezoelectric ceramics were synthesized by a conventional solid-state process. The effects of Gd₂O₃ on the microstructure, the dielectric, ferroelectric and piezoelectric properties were investigated. X-ray diffraction (XRD) data shows that Gd₂O₃ in an amount of 0.2–0.8 wt.% can diffuse into the lattice of BNKT18 ceramics and form a pure perovskite phase. Scanning electron microscope (SEM) images indicate that the grain size of BNKT18 ceramics decreases with the increase of Gd₂O₃ content; in addition, all the modified ceramics have a clear grain boundary and a uniformly distributed grain size. At room temperature, the ferroelectric and piezoelectric properties of the BNKT18 ceramics have been improved with the addition of Gd₂O₃, and the BNKT18 ceramics doped with 0.4 wt.% Gd₂O₃ have the highest piezoelectric constant (d₃₃ = 137 pC/N), highest relative dielectric constant (ε_r = 1023) and lower dissipation factor (tan δ = 0.044) at a frequency of 10 kHz. The BNKT18 ceramics doped with 0.2 wt.% Gd₂O₃ have the highest planar coupling factor (k_p = 0.2463).     

Structure and electrical properties of Er2O3 doped 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 lead-free piezoelectric ceramics

Er₂O₃ (0–0.8 wt.%)-doped 0.82Bi_0.5Na_0.5TiO₃–0.18Bi_0.5K_0.5TiO₃ (BNKT18) lead-free piezoelectric ceramics were synthesized by a conventional solid-state reaction method. The effects of Er₂O₃ on the microstructure and electrical properties were investigated. X-ray diffraction (XRD) data shows that Er₂O₃ in an amount of 0.2–0.8 wt.% can diffuse into the lattice of the BNKT18 ceramics and form the pure perovskite phase. Scanning electron microscope (SEM) images indicate that the grain sizes of BNKT18 ceramics decrease with the increase of Er₂O₃ content; in addition, the modified ceramics have the clear grain boundary and a uniformly distributed grain size. At room temperature, the electrical properties of the BNKT18 ceramics have been improved with the addition of Er₂O₃, and the BNKT18 ceramics doped with 0.6 wt.% Er₂O₃ have the highest piezoelectric constant (d₃₃ = 138 pC/N), the highest planar coupling factor (k_p = 0.2382), the highest remnant polarization (P_r = 25.2 μC/cm²), the higher relative dielectric constant (ε_r = 936) and lower dissipation factor (tanδ = 0.047) at a frequency of 10 kHz. Moreover, the T_m and T_d of the samples increase with the addition of Er₂O₃.     

Dielectric properties of bismuth doped Ba1−xCaxTiO3 ceramics

The dielectric properties of (Ba_1−xCa_x)_1−1.5yBi_yTiO₃ (x = 0.10, 0.20 and 0.30, y = 0.05) ceramics were investigated. XRD analysis shows that 5 at.% of Bi doping can be fully incorporated into the perovskite lattice of (Ba_1−xCa_x)TiO₃. The maximal dielectric constant K_m of (Ba_1−xCa_x)_1−1.5yBi_yTiO₃ ceramics decreases significantly with increasing x for all the compositions. Compared with undoped Ba_1−xCa_xTiO₃ ceramics [Mater. Chem. Phys. 77 (2002)], Bi doping remarkably shifts the temperature of the peak dielectric constants T_m to lower temperature and a broad dielectric peak exhibits strong frequency dispersion. With increasing frequency, K_m decreases and T_m shifts to higher temperatures in (Ba_1−xCa_x)_1−1.5yBi_yTiO₃ ceramics. A typical behavior to well-known relaxor ferroelectric is observed. The relaxation behavior observation is suggested due to a random electric field induced domain state.     

Temperature stable microwave dielectric ceramic 0.3Li2TiO3–0.7Li(Zn0.5Ti1.5)O4 with ultra-low dielectric loss

In the present work, the 0.3Li₂TiO₃–0.7Li(Zn_0.5Ti_1.5)O₄ ceramic was prepared via the conventional solid state reaction route, and the phase composition, microstructure, and sintering behavior were investigated. The ceramic sample sintered at 1100 °C for 2 h demonstrated high microwave dielectric performance with a relative permittivity of 23.5, a high quality factor (Qf) ~ 88,360 GHz (at 7.4 GHz), and near zero temperature coefficient of resonant frequency about ? 0.8 ppm/°C. These results indicate that the 0.3Li₂TiO₃–0.7Li(Zn_0.5Ti_1.5)O₄ ceramic might be a good candidate for dielectric resonators, filters and other microwave electronic device applications.     

Effect of BaCu(B2O5) on the sintering temperature and microwave dielectric properties of BaO–Ln2O3–TiO2 (Ln = Sm,Nd) ceramics

BaCu(B₂O₅) (BCB) ceramic powder was used to decrease the sintering temperatures of BaSm₂Ti₄O₁₂ (BST) and BaNd₂Ti₅O₁₄ (BNT) ceramics. The sintering temperature of the BST and BNT ceramics was reduced from approximately 1350 °C to 850 °C by the addition of BCB. The bulk density of the specimens increased and reached the saturated value with increasing BCB content. The variation of the dielectric constant (ɛ_r) was similar to that of the bulk density and, thus, the relative density plays an important role in determining the ɛ_r value of the specimens. The Q-value initially increased with the addition of BCB but decreased considerably when a large amount of BCB was added because of the presence of the liquid phase. Good microwave dielectric properties of Qxf = 4500 GHz, ɛ_r = 60 and τ_f = −30 ppm/°C were obtained for the 16.0 mol% BCB-added BST ceramics sintered at 875 °C for 2 h.     

Synthesis and dc conductivity of Nd2/3TiO2.988

The A-site deficient perovskite Nd_2/3TiO_3 − δ was synthesized under an H₂–CO₂ gas mixture. The sample was found to have slight oxygen deficiency of δ∼0.012. The crystal structure was assigned to a double perovskite structure with orthorhombic space group Pmmm, as in the case of La_2/3TiO_3 − δ. Electrical conductivity measurement has also been performed. The temperature dependence of conductivity shows that electronic transport in Nd_2/3TiO_2.988 is well described by Emin–Holstein adiabatic small polaron model. The polaron density extracted from the conductivity measurement is ∼1.96 × 10²⁰ cm^− 3. This result agrees well with nominal polaron density for Nd_2/3TiO_2.988, ∼2.1 × 10²⁰cm^− 3. We have also derived important quantities for transport in Nd_2/3TiO_2.988.     

Effect of bond valence on microwave dielectric properties of (1 − x)CaTiO3–x(Li0.5La0.5)TiO3 ceramics

The origins of microwave dielectric properties (1 ? x)CaTiO₃–x(Li_0.5La_0.5)TiO₃ (0.2 ≤ x ≤ 0.8) ceramics, prepared by a conventional solid-state reaction method, were investigated based on the theory of bond valence. The XRD and SEM results showed that complete solid solutions with orthorhombic perovskite structure were formed in the whole investigated compositional range. The dielectric constant (?_r), quality factor (Q × f) and temperature coefficient of resonant frequency (τ_f) were closely related to B-site, A-site and the difference between A-site and B-site bond valences of ABO₃ perovskite compounds, respectively. As x value increased from 0.2 to 0.8, the dielectric constant increased from 198.3 to 276.8, the Q × f value decreased from 4340 to 1880 GHz, and the τ_f value varied from +489.7 to ?178 ppm/°C. For practical applications, excellent microwave dielectric properties of ?_r = 245, Q × f = 2750 GHz and τ_f = +0.75 ppm/°C were obtained for 0.4CaTiO₃–0.6(Li_0.5La_0.5)TiO₃ ceramics.     

Low-temperature sintering and microwave dielectric properties of CaTi1−x(Fe0.5Nb0.5)xO3 ceramics with B2O3 addition

CaTi_1−x(Fe_0.5Nb_0.5)_xO₃ (0 ≤ x ≤ 1) dielectrics were synthesized via the solid state reaction route and structure analysis was performed together with the dielectric characterization. The substitution of Ti⁴⁺ by Fe³⁺/Nb⁵⁺ and developed phase were studied by X-ray diffraction. The dielectric constant and temperature coefficient of resonant frequency decrease rapidly with an increase of x. The influence of 1–5 wt.% B₂O₃ as a sintering additive investigated at CaTi_0.5(Fe_0.5Nb_0.5)_0.5O₃ solid solutions. The dielectric properties were found to strongly depend on the sintering conditions and contents of B₂O₃ additions. ɛ_r = 52.3, Q × f_o = 2930 GHz and T_f = 13 ppm/°C were obtained for CaTi_0.5(Fe_0.5Nb_0.5)_0.5O₃ specimen 3 wt.% B₂O₃ sintered at 900 °C for 2 h.     

Sol–gel-hydrothermal synthesis and sintering of K0.5Bi0.5TiO3 nanowires

The sol–gel-hydrothermal processing of K_0.5Bi_0.5TiO₃ (KBT) nanowires as well as their sintering behavior at 1000–1100 °C were investigated. The morphological analyses indicated that sol–gel-hydrothermal route led to the formation of KBT nanowires with diameters of 4 nm and lengths of 100 nm at low processing temperature of 200 °C with KOH concentration of 6 M. It is believed that the gel precursor and hydrothermal environment play an important role in the formation of the nanowires. The KBT ceramics with a relative density of more than 95% can be successfully fabricated from the high quality KBT nanowires even by a conventional sintering process. The KBT ceramics sintered at 1050 °C showed typical characteristics of relaxor ferroelectrics, and the dielectric properties were better than that prepared by all other methods reported previously.     

Effects of SrTiO3 on dielectric and piezoelectric properties of K0.48Na0.48Li0.04Nb0.96Ta0.04O3-based piezoceramics

In this study, (100 ? x) K_0.48Na_0.48Li_0.04Nb_0.96Ta_0.04O₃ ? xSrTiO₃ (0 ≤ x ≤ 10) ceramics were fabricated via normal sintering of synthesized powder by using solid state reaction. All ceramics revealed pure perovskite structure, indicating formation of solid solution between KNNLT and ST up to 10%. With increasing x, the crystal structure of ceramics changed from orthorhombic to tetragonal and finally pseudocubic symmetry when x = 10. Ceramic containing 1% ST had orthorhombic and tetragonal symmetries, simultaneously. Investigation of the variation of dielectric constant of ceramics versus temperature revealed that for ceramic with x = 1, polymorphic phase transition (PPT) temperature between orthorhombic and tetragonal is less than room temperature. Thus coexistence of two different structures in this ceramic is due to vicinity of its composition to morphotropic phase boundary (MPB). As a result, the maximum piezoelectric constant was measured for this ceramic. Ceramics containing 5 and 7.5% ST tend to appear relaxor ferroelectric behavior which is because of chemical inhomogeneities in both A- and B-sites of the ABO₃ perovskite structure.     

Low-temperature sintering and microwave dielectric properties of Li2TiO3 based ceramics

New low sintering temperature and temperature-stable low-loss ceramics based on Li₂TiO₃ with lithium zinc borate (LZB) glass and LiZnNbO₄ doping have been prepared by the conventional solid-state reaction route. The effect of LZB glass addition on the sinterability, phase purity, microstructure, and microwave dielectric properties of Li₂TiO₃ ceramics has been investigated. The XRD results suggest the presence of single Li₂TiO₃ phases for LZB glass-added Li₂TiO₃ ceramics. The addition of LZB glass can effectively lower the sintering temperature to 900 °C, and does not induce much degradation of the microwave dielectric properties. Typically, the 2.0 wt% LZB glass-added ceramic sintered at 900 °C has better microwave dielectric properties of ε_r = 23.2, Q × f = 38,909 GHz, and τ _f  = 30.1 ppm/°C. Meanwhile, LiZnNbO₄ compound is selected to tune the temperature coefficient of resonant frequency (τ _f ) to near zero. It is found that the 2.0 wt% LZB glass-added Li₂TiO₃ ceramics with 35 wt% LiZnNbO₄ sintered at 925 °C have good microwave dielectric properties of ε_r = 20.7, Q × f = 19,366 GHz, τ _f  = ?0.5 ppm/°C, which can find applications in microwave devices that require low sintering temperature.     

Low-temperature hydro/solvothermal synthesis of Ta-modified K0.5Na0.5NbO3 powders and piezoelectric properties of corresponding ceramics

K_0.5Na_0.5Nb_1–xTa_xO₃ (KNNTx, x = 0–0.4) powders were synthesized by a novel hydro/solvothermal method at a low reaction temperature (180 °C) and the corresponding ceramics were obtained by normal sintering. Compared with conventional solid-state reaction technique, the optimal sintering temperatures of these ceramics were reduced at least 150 °C. Crystalline structures and surface morphologies were analyzed by X-ray diffraction and scanning electron microscopy. The excellent piezoelectric properties could be obtained by selecting poling temperature near the orthorhombic–tetragonal polymorphic phase transition temperature region. Ta-modified KNN ceramics exhibited better piezoelectric properties than those of pure KNN, and the piezoelectric coefficient d₃₃ showed the maximum value of 156 pC/N for KNNT0.3 ceramics. In addition, the sintering temperature for maximum d₃₃ value differed from that for maximum density. The present hydro/solvothermal method provides a new potential route for preparing KNN-based materials at relatively low temperature.     

Dielectric properties of Sb2O3-doped Ba0.672Sr0.32Y0.008TiO3

Sb₂O₃-doped Ba_0.672Sr_0.32Y_0.008TiO₃ (BSYT) dielectric ceramics were prepared by conventional solid state method, and their dielectric properties were investigated with variation of Sb₂O₃ doping content and sintering temperature. The X-ray diffraction patterns indicated that all the BSYT specimens possessed the perovskite polycrystalline structure. The experimental results reveal that the introduction of Sb₂O₃ into Ba_0.672Sr_0.32Y_0.008TiO₃ can control the grain growth, reduce the relative dielectric constant and dielectric loss, shift the Curie temperature to lower temperature and significantly improve the thermal stability of the BSYT ceramics. The samples doped with 1.6 wt.% Sb₂O₃ sintered at 1320 °C for 2 h exhibited attractive properties, including high relative dielectric constant (> 1500), low dielectric loss (< 40 × 10^− 4), low temperature coefficient of capacitor(< ± 35%) over a wide temperature range from − 25 °C to + 85 °C.     

Preparation and characterization of the bismuth sodium titanate (Na0.5Bi0.5TiO3) ceramic doped with ZnO

This study investigates effects of the zinc oxide (ZnO) addition and the sintering temperature on the microstructure and the electrical properties (such as dielectric constant and loss tangent) of the lead-free piezoelectric ceramic of bismuth sodium titanate (Na_0.5Bi_0.5TiO₃), NBT, which was prepared using the mixed oxide method. Three kinds of starting powders (such as Bi₂O₃, Na₂CO₃ and TiO₂) were mixed and calcined. This calcined NBT powder and a certain weight percentage of ZnO were mixed and compressed into a green compact of NBT–ZnO. Then, this green compact of NBT–ZnO was sintered to be a disk doped with ZnO, and its characteristics were measured. In this study, the calcining temperature was 800 °C, the sintering temperatures ranged from 1000 to 1150 °C, and the weight percentages of ZnO doping included 0.0, 0.5, 1.0, and 2.0 wt%. At a fixed wt% ZnO, the grain size increases with increase in the sintering temperature. The largest relative density of the NBT disk obtained in this study is 98.3% at the calcining temperature of 800 °C, the sintering temperature of 1050 °C, and 0.5 wt% ZnO addition. Its corresponding dielectric constant and loss tangent are 216.55 and 0.133, respectively.     

Microwave dielectric ceramics in (Ca1− xBax)(Zn1 / 3Nb2 / 3)O3 system

(Ca_1−xBa_x)(Zn_1 / 3Nb_2 / 3)O₃ (x = 0–1.0) microwave dielectric ceramics were prepared and investigated. The Ba(Zn_1 / 3Nb_2 / 3)O₃-based solid solution was observed for x = 0.9, and the compositions with x = 0.1–0.7 resulted in the mixture of two phases. Dielectric constant ε_r and temperature coefficient of resonant frequency τ_f of the present ceramics varied anomalously and reached their maximum at x = 0.7–0.9, and these phenomena were originated from the partial substitution of small Ca²⁺ ions for larger Ba²⁺ at A-site. On the other hand, a good combination of microwave dielectric properties (ε_r = 36, Qf = 16,170 GHz, τ_f = − 12 ppm/°C) were obtained at x = 0.1, while the decreased Qf value was observed in other compositions.     

Microwave dielectric properties of ZnAl2O4–TiO2 spinel-based composites

The microstructures, phase compositions and microwave dielectric properties of ZnAl₂O₄–TiO₂ spinel-based composites have been investigated. It is found that ZnAl₂O₄ cannot form a solid solution with TiO₂. As TiO₂ content increases, the ε_r and τ_f values increase gradually, while the Q · f values degrade by degrees. Under the same amount of TiO₂ content, the ε_r and Q · f values increase initially and then decrease slightly with increasing sintering temperature, while the τ_f values increase slowly. The optimal microwave dielectric properties are achieved in (1 − x)ZnAl₂O₄–xTiO₂ (x = 0.21) sintered at 1500 °C for 3 h with ε_r value of 11.4, Q · f value of 71,810 GHz (at about 6.5 GHz), and τ_f value of − 0.5 ppm/°C.     

Sintering temperature dependence of microwave dielectric properties in Mg4(TaNb1−xVx)O9 compounds

The relationship between the sintering temperature and the microwave dielectric properties of Mg₄(TaNb_1−xV_x)O₉ (MTNV) compounds were investigated in this study in order to reduce the sintering temperature of the compound. A small amount of V₂O₅ doping lowered the sintering temperature of the MTNV compounds. The variations in the dielectric constant and the quality factor of the MTNV compounds depended on the amount of V₂O₅ doping and the sintering temperature; a small amount of V₂O₅ doping was effective in allowing low sintering temperatures without a detrimental effect on these dielectric properties. As a result, a dielectric constant of approximately 12 and a quality factor of approximately 200,000 GHz were obtained when the MTNV compounds with x = 0.2 was sintered at 1200 °C. The temperature coefficient of resonant frequency of the MTNV compound with x = 0.025 slightly changed from −63 to −73 ppm/ °C with an increased sintering temperature because of the presence of a secondary phase.     

Microwave dielectric properties of novel lithium containing pyrochlore type oxides: Li3Sm3 − xBixTi7Nb2O25 (x = 0, 1, 2 or 3)

Microwave dielectric properties of novel lithium ion containing pyrochlore type oxides: Li₃Sm_3 − xBi_xTi₇Nb₂O₂₅ (x = 0, 1, 2 or 3) have been reported in this paper. Powder X-ray diffraction patterns show that these oxides have cubic pyrochlore type structure. Ceramic microstructure of the sintered samples show well formed grains. They have relatively high dielectric constant (ε_r) in the range 80–137 at 1 MHz and ε_r, 45–83 at the resonant microwave frequency region. It is seen that the dielectric constant (ε_r) increases with the increase of Bi content. The best microwave dielectric properties obtained for fully substituted samarium compound, Li₃Sm₃Ti₇Nb₂O₂₅ are as follows: Q × f = 2007 and ε_r = 45 at the resonant microwave frequency, 3.78 GHz.