Microstructure and enhanced electrical properties of (1−x)(Na0.5K0.44Li0.06)NbO3–x(Ba0.85Ca0.15)(Zr0.10Ti0.90)O3 lead-free ceramics

Environment-friendly lead-free piezoelectric ceramics (1?x)(Na_0.5K_0.44Li_0.06)NbO₃–x(Ba_0.85Ca_0.15)(Zr_0.10Ti_0.90)O₃ doped with 1.0 mol% MnO₂ were synthesized by conventional solid-state sintering method. The phase transition behavior and electrical properties of the ceramics is systemically investigated. It was found that all the ceramics formed pure perovskite phase with 0.0 ≤ x ≤ 0.1, and the phase structure of the ceramics gradually transformed from orthorhombic to tetragonal phase with increasing x. Coexistence of the orthorhombic and tetragonal phase is formed in the ceramics with 0.04 ≤ x ≤ 0.06 at room temperature, and enhanced dielectric, ferroelectric and piezoelectric properties are achieved in the two phase’s region. The ceramics in the mixed phase region exhibits the following optimum electrical properties: d ₃₃  = 130–147 pC/N, ε _r  = 642–851, P _r  = 5.51–12.44 μC/cm². The Curie temperature of the ceramics with mixed phase region was found to be 353–384 °C. The significantly enhanced dielectric properties, ferroelectric properties and piezoelectric properties with high cubic-tetragonal phase transition temperatures (T _c ) make the KNLN–xBCZT ceramics showing the promising lead-free piezoelectrics for the practical applications.     

Microstructure, dielectric and piezoelectric properties of La-modified Bi0.5(Na0.84K0.16)0.5TiO3 lead-free ceramics

Lead-free ceramics (Bi_1?xLa_x)_0.5(Na_0.84K_0.16)_0.5TiO₃ were prepared by a conventional ceramic technique and the effects of La doping and sintering temperature on the microstructure, ferroelectric and piezoelectric properties of the ceramics were studied. All the ceramics possess a pure perovskite structure and La³⁺ diffuses into the Bi_0.5(Na_0.84K_0.16)_0.5TiO₃ lattices to form a solid solution with a rhombohedral symmetry. The addition of La leads to the significant change in the grain morphology and size for the (Bi_1?xLa_x)_0.5(Na_0.84K_0.16)_0.5TiO₃ and a number of rod grains with the length of 10–50 μm and the diameter of 1–2 μm are observed in the ceramic with x = 0.04 sintered at 1,140 °C for 2 h. However, as sintering temperature increases to 1,160 °C, the rod grains disappears and the uniform and rectangular grains are observed in the ceramics with x = 0.04. As x increases from 0 to 0.06, the coercive field E _c of the ceramics decreases from 4.33 to 2.81 kV/mm and the remanent polarization P _r of the ceramics retains the high values of 25.9–27.7 μm/cm². The depolarization temperature T _d decreases from 154 to 50 °C with x increasing from 0 to 0.10. All the ceramics exhibit the diffusive phase transition at high temperature (280–320 °C). The ceramic with x = 0.04 sintered at 1,150 °C for 2 h exhibit the optimum piezoelectric properties, giving d ₃₃ = 165 pC/N and k _p = 32.9 %. The optimum sintering temperature is 1,150 °C at which the improved piezoelectric properties (d ₃₃ = 165 pC/N and k _p = 32.9 %) are obtained. At the high La³⁺ level (x = 0.10 and 0.12), the ceramics exhibit weak ferroelectricity (P _r = 13.0–21.2 μm/cm²) and thus possess poor piezoelectricity (d ₃₃ = 17–27 pC/N).     

Low-temperature firing and microwave properties of TiO2 modified Li2ZnTi3O8 ceramics doped with B2O3

The low-temperature firing and microwave properties TiO₂ modified Li₂ZnTi₃O₈ ceramics with B₂O₃ addition have been developed. B₂O₃ addition could reduce the sintering temperature of Li₂ZnTi₃O₈ ceramics from 1,150 to 900 °C, which is attributed to the formation of liquid phases during the sintering process observed by SEM. The 1.5 wt% B₂O₃ doped Li₂ZnTi₃O₈ ceramics sintered at 900 °C have ε _r = 23.3, Q × f = 48,817 GHz, and resonant frequency τ _f  = ?15.35 × 10^?6/°C. Further, due to the compensating effect of rutile TiO₂ (τ_f  = 450 ppm/°C), the temperature coefficient of τ_f for with TiO₂ was adjusted to near zero value. The 1.5 wt% B₂O₃ doped Li₂ZnTi₃O₈ ceramics with 3 wt% TiO₂ dielectrics sintered at 900 °C exhibited the optimal microwave properties: ε_r = 25.9, Q × f = 46,487 GHz, and τ_f = ?0.35 ppm/°C.     

Phase transition and piezoelectric properties of (1−x)(K0.42Na0.58)(Nb0.96Sb0.04)O3–x(Bi0.5Na0.5)0.90Mg0.10ZrO3 lead-free ceramics

(1?x)(K_0.42Na_0.58)(Nb_0.96Sb_0.04)O₃–x(Bi_0.5Na_0.5)_0.90Mg_0.10ZrO₃ [(1?x)KNNS–xBNMZ] lead-free ceramics have been prepared by the normal sintering, and effects of BNMZ content on their phase structure, microstructure, and electrical properties have been systematically investigated. These ceramics with 0.045 ≤ x ≤ 0.05 possess a rhombohedral–tetragonal (R–T) phase boundary, as confirmed by the temperature dependence of dielectric properties and X-ray diffraction patterns. The grain size of the ceramics first increases and then decreases as the BNMZ content increases, and the ceramic with x = 0.06 possesses much smaller grains (<1 μm), resulting in the abnormal electrical and phase transition behavior. In addition, the Mg²⁺ was homogenously distributed in the ceramic matrixes. These ceramics with R–T phase boundary show enhanced dielectric, ferroelectric, and piezoelectric properties as compared with a pure KNN, and optimum electrical properties (e.g., P _r ~ 16.23 μC/cm², E _C ~ 7.58 kV/cm, ε _r ~ 2,663, tan δ ~ 0.034, d ₃₃ ~ 434 pC/N, k _p ~ 0.47, and T _C ~ 244 °C) were found in the ceramic with x = 0.0475. We believe that the (1?x)KNNS–xBNMZ ceramic is a promising candidate for lead-free piezoelectric devices.     

Phase evolutions and electric properties of BaTiO3 ceramics by a low-temperature sintering process

BaTiO₃ (BT) nanoparticles were synthesized by a modified polymeric precursor method in a weak acid solution. The synthesized process of BT precursor with increasing calcination temperature was investigated through thermal analysis (DTA/TG), X-ray diffraction, transmission electron microscope and Fourier-transform infrared spectroscopy. Good dispersive and homogeneous cubic BT nanoparticles were calcined at 800 °C, whereas dense BT ceramics were sintered at ~1,160 °C. The present results showed that the dielectric, piezoelectric and ferroelectric properties of BT ceramics were dependent on the ceramics densification and crystallographic structure. The excellent electric properties (P _r = 10.5 μC/cm², d ₃₃ = 217 pC/N, k _p = 0.32 %) were found at a sintering temperature of 1,160 °C, which was due to the coexistence of tetragonal and orthorhombic phase. The depressed electric properties at higher sintering temperature were associated to oxygen vacancies and impurity phases. In addition, phase evolutions of BT nanoparticles and ceramics were all stated in detail.     

Sintering characteristic,crystal structure and microwave dielectric properties of a novel thermally stable ultra-low-firing Na2BiMg2V3O12 ceramic

A novel ultra low-firing microwave dielectric ceramic with the composition of Na₂BiMg₂V₃O₁₂ was fabricated by a solid state reaction method. The phase structure, sintering behavior and microwave dielectric properties of ceramics were investigated. The ceramic can be well densified at 660 °C for 4 h. X-ray diffraction data show that Na₂BiMg₂V₃O₁₂ has a cubic garnet structure with lattice parameters of a = 12.4929 Å, V = 1,949.80 Å³, Z = 8 and ρ = 4.42 g/cm³. The microwave dielectric properties are strongly related to the density and morphology of the samples. The ceramic sintered 660 °C exhibits good microwave dielectric properties with ε_r = 23.2, Q × f = 3,700 GHz and τ _f  = 8.2 ppm/°C. These results indicate that Na₂BiMg₂V₃O₁₂ ceramic is a candidate for low temperature co-fired ceramics devices, such as chip multi-layer LC filter, microstrip bandpass filter, multilayer antenna.     

Study on low temperature sintering and electrical properties of CuO-doped Ca0.28Ba0.72Nb2O6 ceramics

Ca_0.28Ba_0.72Nb₂O₆ (CBN28) ceramics with different content of CuO were prepared by the conventional ceramic fabrication technique. The effects of CuO content on the phase structure, microstructure, dielectric and ferroelectric properties of obtained CBN28 ceramics were investigated. XRD results showed that pure tungsten bronze structure was obtained in all ceramics and CuO additive could accelerate the phase formation at lower temperatures. The CuO aid was effective for the uniform grain size in CBN28 ceramics, as it could facilitate the sintering behavior and suppress the anisotropic growth behavior obviously. The dielectric and ferroelectric properties of CBN28 ceramics depended greatly on the CuO content. Curie temperature T _c and dielectric loss tanδ both shifted downward, whereas the maximum dielectric constant ε _m and the dielectric constant around room temperature ε _r all increased initially and then decreased as x increased from 0.1 to 0.4 wt%. Normal ferroelectric hysteresis loops could be observed in all compositions, and the remnant polarization P _r decreased gradually. It was found that the comprehensive electric performance was optimized in CBN28-0.2 wt% CuO ceramics: ε _r = 453, ε _m = 3,371, T _c = 226 °C, tanδ = 0.048, P _r = 4.72 μC/cm² and E _c = 13.81 kV/cm, showed that CuO sintering aid could not only ameliorate the sintering behavior but also improve the electrical properties.     

Effects of cobalt and sintering temperature on electrical properties of Ba0.98Ca0.02Zr0.02Ti0.98O3 lead-free ceramics

Lead-free (Ba_0.98Ca_0.02)(Zr_0.02Ti_0.98)O₃-xmol% (x = 0–1.6) cobalt ceramics (BCZT-xCo) have been fabricated by the traditional solid-state reaction technique and the effects of Co and sintering temperature on ferroelectric, dielectric and piezoelectric properties of (Ba_0.98Ca_0.02)(Zr_0.02Ti_0.98)O₃ lead-free ceramics have been studied systematically. The orthorhombic–tetragonal (T _O–T) transition shift towards lower temperature with increasing Co addition, while Curie temperature (T _c) remained at relatively high value of 107 °C. And the Main piezoelectric parameters are optimized at x = 0.8 mol% with a high piezoelectric coefficient (d ₃₃ = 330 pC/N), a planar mode electromechanical coupling factor (k _p = 46.7 %), a high dielectric constant (ε _r = 2,675) and a low dielectric loss (tanδ = 0.90 %) at 1kHZ. Besides these, high remnant polarization (P _r) and low coercive field (E _c) of 11.5 μC/cm², 0.31 kV/cm are also obtained at (Ba_0.98Ca_0.02)(Zr_0.02Ti_0.98)O₃-0.8 mol%Co lead-free ceramics. Furthermore, greatly enhanced temperature stability of the piezoelectric properties was obtained in the temperature range from 20 to 90 °C. The above results indicate that BCZT-Co ceramics are promising lead-free materials for practical applications.     

Enhancing the microwave dielectric properties of Nd (Mg0.5Sn0.5)O3 ceramics by substituting Nd3+ with Ca2+

The microwave dielectric properties of Nd_(1?2x/3)Ca_x(Mg_0.5Sn_0.5)O₃ ceramics were examined to evaluate their exploitation for mobile communication. Nd_(1?2x/3)Ca_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the Nd_2.9/3Ca_0.05(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with the sintering temperature. Nd_2.9/3Ca_0.05(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1,550 °C for 4 h had the following properties: a density of 6.86 g/cm³, a dielectric constant (ε_r) of 19.3, a quality factor (Q × f) of 99,000 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?65 ppm/°C.     

Lead-free vanadium-substituted (K0.485Na0.5Li0.015)(Nb0.9Ta0.1)O3 piezoceramics synthesized from nanopowders

Lead-free, alkaline niobate-based piezoelectric ceramics substituted with vanadium (K_0.485Na_0.5Li_0.015)(Nb_0.9?x Ta_0.1V _x )O₃ (x = 0, 0.05, 0.10, 0.15 and 0.2) were synthesized from nanocrystalline powders by traditional solid state sintering technique. The base composition chosen is among those recently reported to show high piezoelectric properties. The nanocrystalline powders were produced by high energy ball milling. The crystalline phase of all the ceramics prepared was found to be perovskite with orthorhombic symmetry. Without any sintering aid, the bulk density of 97 % of the theoretical density was obtained for the ceramics with no vanadium. The optimum sintering temperature for all compositions was achieved at a low value of 1,050 °C. In the composition range studied, increasing V⁵⁺ content in the ceramics gives rise to a gradual decrease in room temperature dielectric constant (ε _r ) from 1,193 to 474, remnant polarization (P _r ) from 12.9 to 5.6 μC/cm², electromechanical coupling factor (k _p ) from 0.45 to 0.32, and piezoelectric charge constant (d ₃₃) from 156 to 53 pC/N. The decrease in these parameters is attributed to the associated decrease in density and grain size of the ceramics with increasing V⁵⁺ content. Increasing V⁵⁺ content from 0 to 0.15 results in an increase in the coercive field from 9.9 to 15.5 kV/cm, thereby, making the ceramics harder in this range of composition.     

Low temperature sintering of Li2(Mg0.3Zn0.7)Ti3O8-0.12TiO2 microwave dielectric ceramics with controllable grain size

The effects of BaO-B₂O₃-SiO₂ (BBS) frit on sinterability, microstructure and microwave dielectric properties of Li₂(Mg_0.3Zn_0.7)Ti₃O₈-0.12TiO₂ (LMZT) ceramics were systematically investigated. BBS frit can effectively lower the sintering temperature of LMZT ceramics to below 900 °C. Suitable BBS frit addition can accelerate the growth of the LMZT grains while inhibit the abnormal grain growth at the same time. The LMZT ceramics with 2 wt% BBS frit sintered at 900 °C for 3 h show homogeneous microstructure composed of 5–10 μm grains and excellent dielectric properties: ε _r  = 24.1, Q × f = 21,980 GHz, τ _f  = ?4.1 ppm/ °C. It is compatible with Ag electrodes, which makes it a potential candidate material for low temperature co-fired ceramics technology application.     

Effect of sintering temperature and time on microwave dielectric properties of CaNb2O6 ceramics

The microwave dielectric properties of CaNb₂O₆ ceramics were investigated with a view to their application in mobile communication. The CaNb₂O₆ ceramics were prepared by the conventional solid-state method with various sintering temperatures and sintering times. A maximum density of 4.67 g/cm³ was obtained for CaNb₂O₆ ceramic, sintered at 1,400 °C for 4 h. Dielectric constants (ε _r ) of 13.3–18.1 and quality factor (Q × f) of 12,200–50,000 GHz were obtained at sintering temperatures in the range 1,300–1,500 °C for 4 h. Dielectric constants (ε _r ) of 18.0–18.1 and quality factor (Q × f) of 44,300–50,000 GHz were obtained for sintering times in the range 2–6 h at a sintering temperature of 1,400 °C. A dielectric constant (ε _r ) of 18.1, a quality factor (Q × f) of 50,000 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?54 ppm/°C were obtained when CaNb₂O₆ ceramics were sintered at 1,400 °C for 4 h.     

Microwave dielectric properties of Li2ZnTi3O8 ceramics prepared by reaction-sintering process

Li₂ZnTi₃O₈ (LZT) microwave dielectric ceramics prepared by reaction-sintering process were investigated. Li₂CO₃, ZnO and TiO₂ powders were mixed, pressed and sintered directly into ceramic pellets without any calcination stage involved. Pure LZT phase was obtained after sintering at temperatures above 1,100 °C for 2 h. LZT ceramic with the maximum bulk density of 3.81 g/cm³ (96.2 % of the theoretical value) and excellent microwave dielectric properties of ? _r  = 25.8, Q × f = 78,216 GHz and τ _f  = ?10.5 ppm/°C was obtained after sintering at 1,100 °C for 2 h. The results show that the reaction-sintering process is a simple and effective method to produce LZT ceramics.     

Effect of Sm2O3 dopant on microstructure and electrical properties of ZnO-based varistor ceramics

ZnO-based varistor ceramics doped with fixed Y₂O₃ and different Sm₂O₃ have been prepared by the conventional solid-state reaction route, and the phase composition, microstructure and electrical properties have been investigated by XRD, SEM and a V–I source/measure unit. The XRD analyses show the presence of primary phase ZnO and some minor secondary phases. Doping appropriate contents of Sm₂O₃ decrease the leakage current and enhance nonlinearity characteristics of ZnO-based varistor ceramics markedly. The varistor ceramics with 0.25 mol% Sm₂O₃ sintered at 1,125 °C for 1 h exhibit reasonable electrical properties with the breakdown field of 446.4 V/mm, the nonlinear coefficient of 65.8 and the leakage current of 2.36 μA/cm². The results illustrate that doping Y₂O₃ and Sm₂O₃ may be a promising route for the production of ZnO-based varistor ceramics with good electrical properties.     

Dielectric properties of La2O3-doped Ba0.8Sr0.2TiO3 ceramics

La₂O₃-doped Ba_0.8Sr_0.2TiO₃ dielectric ceramics were prepared by conventional solid state ceramic route. Scanning electron microscope was employed to observe the surface morphologies. The capacitance C and dielectric loss factor D of the samples were measured with automatic LCR Meter 4225 at 10 kHz respectively. The results show that: ε _r of the samples decreases and tgδ first decreases then increases with increasing amount of La₂O₃ doping. ε _r reaches better value, tgδ obtains the minimum value at 0.5 mol% La₂O₃. ε _r increases and tgδ decreases when sintering temperature increases. The samples doped with 0.5 mol% La₂O₃ sintered at 1,350 °C for 10 h exhibited attractive properties, including high relative dielectric constant (>4,000), low dielectric loss (16.8 × 10^?4), low temperature coefficient of relative dielectric constant(<±21 %) in the temperature range of +25 to +85 °C.     

Microstructure,ferroelectric, piezoelectric and ferromagnetic properties of BiFeO3–BaTiO3–Bi(Zn0.5Ti0.5)O3 lead-free multiferroic ceramics

Multiferroic ceramics of (0.70?x)BiFeO₃–0.30BaTiO₃–xBi(Zn_0.5Ti_0.5)O₃ + 1 mol% MnO₂ with perovskite structure were prepared by a conventional ceramic technique and the effects of Bi(Zn_0.5Ti_0.5)O₃ doping and sintering temperature on the microstructure, multiferroic and piezoelectric properties of the ceramics were studied. All the ceramics possess a pure perovskite structure and no second phases can be detected. After the addition of a small amount of Bi(Zn_0.5Ti_0.5)O₃ (x ≤ 0.05), the ferroelectric and piezoelectric properties of the ceramics are improved and the grain growth is promoted. However, excess Bi(Zn_0.5Ti_0.5)O₃ (x ≥ 0.10) retards the grain growth, degrades the ferroelectricity and piezoelectricity, and induces two dielectric anomalies at high temperature. The ceramics can be well sintered at the very wide range of low sintering temperatures (880–980 °C) and exhibit good densification (relative density: 96.2–98.4 %) and strong electric insulation. The increase in the sintering temperature promotes the grain growth and improves the ferroelectricity of the ceramics. The ceramic with x = 0.05 sintered at 880–980 °C possesses improved ferroelectric and piezoelectric properties with remanent polarizations P _r of 21.9–28.1 μm/cm², piezoelectric constants d ₃₃ of 125–139 pC/N and planar electromechanical coupling factors k _p of 30.1–32.4 %, and high Curie temperatures T _C of 523–565 °C. A weak ferromagnetism with remanent magnetizations M _r of 0.0411–0.0422 emu/g and coercive fields H _c of 1.70–1.99 kOe were observed in the ceramics with x = 0–0.025.     

Microstructure and piezoelectric properties of NaF-doped K0.5Na0.5Nb0.95Ta0.05O3 lead-free ceramics

The effects of NaF on the microstructure and electrical properties of K_0.5Na_0.5Nb_0.95Ta_0.05 lead-free ceramics prepared by conventional sintering method were investigated in this study. The dopant NaF effectively lowers the sintering temperature and promotes the grain growth. Samples with a high relative density up to 96.3 % are achieved by adding 0.6 wt% NaF to improve the sinterability. The electric properties are also enhanced, and the optimum properties are achieved at the doping content of 1.0 wt% (d ₃₃ = 153 pC/N, k _P = 32.2 %, Q _m = 80.5, E _c = 0.89 kV/mm, and P _r = 16.5 μC/cm²). The improvement of ferroelectric and piezoelectric properties is suggested to be largely contributed to the compensation of sodium element from dopant NaF for the volatilization of A-site alkali elements.     

Low temperature sintering of Li1.1Nb0.58Ti0.5O3-xBi2O3 dielectric with adjustable temperature coefficient

Ceramics of Li_1.1Nb_0.58Ti_0.5O₃-xBi₂O₃ with low sintering temperature have been prepared by the solid-solution reaction method using B₂O₃ (2 wt% added) as sintering aid. For all compounds, the sintering temperature achieves 900 °C. Microstructure and dielectric properties of Li_1.1Nb_0.58Ti_0.5O₃-2 wt% B₂O₃-xBi₂O₃ (LNT-B-xBi) ceramics have been investigated. The X-ray diffraction patterns indicate for higher Bi₂O₃ content (x = 0.1 mol%) that the material is composed by two phases identified as M-phase and Bi₄Ti₃O₁₂. The Li_1.1Nb_0.58Ti_0.5O₃ + 0.15 mol% Bi₂O₃ composition sintered at 900 °C with B₂O₃ addition exhibits attractive dielectric properties (ε _r = 59.68, tan δ = 1.2×10^?4 and a temperature coefficient of the relative permittivity near zero) at 1 MHz. It is also shown that the introduction of Bi₂O₃ can tune the temperature coefficient of the relative permittivity. All dielectric properties lead this system compatible to manufacture sliver based electrodes multilayer dielectric devices.     

Influence of Ba2+ substitution on the microwave dielectric properties of Nd(Mg0.5Sn0.5)O3 ceramics

The microwave dielectric properties of Nd(Mg_0.5?xBa_xSn_0.5)O₃ ceramics were examined with a view to their exploitation in mobile communication. The Nd(Mg_0.5?xBa_xSn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the Nd(Mg_0.47Ba_0.03Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. A density of 6.91 g/cm³, a dielectric constant (ε _r ) of 19.14, a quality factor (Q × f) of 97,500 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?65.4 ppm/°C were obtained for Nd(Mg_0.47Ba_0.03Sn_0.5)O₃ ceramics that were sintered at 1,600 °C for 4 h.     

Microstructures and dielectric properties of Ba[(Co0.7Zn0.3)1/3Nb2/3]O3-based ceramics prepared by aqueous gelcasting-assisted solid-state method

The aqueous gelcasting-assisted solid-state method (AGAS) and traditional solid-state reaction method (TSSR) were used to prepare Ba[(Co_0.7Zn_0.3)_1/3Nb_2/3]O₃ (BCZN)-based ceramics. The effects of different powder-preparation methods on the microstructures and dielectric properties of the BCZN-based ceramics were investigated. X-ray diffraction analysis showed completely the same phase compositions regardless of the preparation method adopted. The relative permittivity (ε _r) did not significantly vary between the two methods. However, BCZN with CeO₂-added ceramics prepared by AGAS had higher and more uniform density (6.374 g/cm³) and high quality factor of resonant frequency (Q × f) value (75,843 GHz) than those prepared by TSSR because of the more uniform microstructures, as shown by scanning electron microscopy images. The temperature coefficients of resonant frequency (τ _f  = 7.4 ppm/°C) of the ceramics prepared by AGAS were also closer to zero than those prepared by TSSR.